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Copper Gallium Diselenide Thin Film Absorber Growth for Solar Cell Device Fabrication


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1 COPPER GALLIUM DISELENIDE THIN FILM ABSORBER GROWTH FOR SOLAR CELL DEVICE FABRICATION By RYAN KACZYNSKI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

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2 2007 Ryan Kaczynski

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3 To my family I love you

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4 ACKNOWLEDGMENTS First, I would like to thank Pr of. A. Brad Anton at Cornell University for encouraging me to pursue my doctorate degree when I had no idea what I wanted to do in the future. I express my sincerest gratitude to Dr. Oscar Crisalle for taking me und er his guidance. It has been a pleasure working for him. His relaxed attitude has been very benefi cial to our working relationship. I would like to acknowledge the many members of the CIS solar cell team at the University of Florida: Dr. Timo thy Anderson and Dr. Sheng Li for your expertise in the solar cell field and bringing these many excellent graduate students together, Billy Stanbery for starting this project, Serkan Kincal for training me on the PMEE react or, Suku Kim for training me on film deposition, Ryan Acher for helping with react or maintenance (by far the toughest job) and film characterization, Woo Kyoung Kim and Seokhyun Yoon for growth support, Jiyon Song and Xuege Wang for device characterization, and Andre Baran and Wei Liu for device fabrication. Each one of you was very integral to my success on this project. I would also like to recognize the staff at MICROFABRITECH, espe cially Scott Gapinski and Diane Badylak.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES................................................................................................................ .......12 ABSTRACT....................................................................................................................... ............15 CHAPTER 1 INTRODUCTION..................................................................................................................17 System Description............................................................................................................. ....18 PMEE Reactor.................................................................................................................18 Chamber........................................................................................................................ ..20 Load-lock...................................................................................................................... ...21 Chalcogen Zone...............................................................................................................21 Heater Zone.................................................................................................................... .22 Metals Zone.................................................................................................................... .22 Control........................................................................................................................ .....23 Problem Statement.............................................................................................................. ....24 2 SOLAR CELLS.................................................................................................................... ..26 Energy......................................................................................................................... ............26 Sunlight....................................................................................................................... .....26 Costs.......................................................................................................................... ......28 Photovoltaic Systems.......................................................................................................29 Future......................................................................................................................... ......30 Solar Cell History............................................................................................................. ......31 Solar Cell Device Physics...................................................................................................... .32 Band Gap....................................................................................................................... ..32 Electric Field................................................................................................................. ..33 Recombination.................................................................................................................33 Defects........................................................................................................................ .....34 Thin Films..................................................................................................................... ..........35 Direct vs Indi rect Band Gap............................................................................................36 Absorption Length...........................................................................................................36 Diffusion Length.............................................................................................................36 CuInSe2-Based Solar Cells.....................................................................................................37 Material Properties..........................................................................................................37 Defects........................................................................................................................ .....39 Gallium Addition.............................................................................................................39

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6 CuGaSe2............................................................................................................................... ...40 Defects........................................................................................................................ .....41 Recombination.................................................................................................................41 Type inversion.................................................................................................................42 Sulfide-based Chalcopyrites...................................................................................................43 Deposition Processes........................................................................................................... ...44 Multijunctions................................................................................................................. ........45 Theoretical Multijunctions..............................................................................................46 Tandem Structure............................................................................................................47 Monolithic vs Mechanical...............................................................................................48 3 ABSORBER GROWTH AND DE VICE FABRICATION...................................................52 Growth Calibration............................................................................................................. ....52 Standard Growth Procedure....................................................................................................53 Growth Schemes................................................................................................................. ....56 Absorber Characterization......................................................................................................59 ICP............................................................................................................................ .......59 SEM............................................................................................................................ .....60 XRD............................................................................................................................ .....60 Device Fabrication............................................................................................................. .....61 Substrate and Back Contact.............................................................................................61 Post Absorber Deposition................................................................................................63 Buffer Layer................................................................................................................... .63 Alternative Buffers..........................................................................................................65 Window Layers...............................................................................................................66 Metallization.................................................................................................................. ..67 Anti-Reflective Coating...................................................................................................67 Device Characterization........................................................................................................ ..67 Current-Voltage...............................................................................................................67 I-V Measurement Technique...........................................................................................69 Quantum Efficiency.........................................................................................................69 QE Measurement Technique...........................................................................................70 4 COPPER GALLIUM DISELENIDE ABSORBER GROWTH.............................................73 Growth Matrix.................................................................................................................. ......73 Absorber Characterization......................................................................................................82 Conclusions.................................................................................................................... .........90 5 COPPER GALLIUM DISELENIDE DEVICE FABRICATION........................................120 Best Devices in the Literature...............................................................................................120 Device Fabrication............................................................................................................. ...121 Device Characterization........................................................................................................123 Conclusions.................................................................................................................... .......129

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7 6 CIGS ABSORBER GROWTH AND DEVICE FABRICATION.......................................134 Best Devices in the Literature...............................................................................................134 Growth Matrix.................................................................................................................. ....134 Absorber Characterization....................................................................................................138 Orientation.................................................................................................................... .138 Morphology...................................................................................................................140 Device Fabrication............................................................................................................. ...140 Device Characterization........................................................................................................141 Conclusions.................................................................................................................... .......143 7 DYNAMIC REACTOR MODEL........................................................................................150 Flux Modeling.................................................................................................................. ....150 PMEE Reactor Modeling......................................................................................................152 Conclusions.................................................................................................................... .......156 8 CONCLUSIONS AND FUTURE WORK...........................................................................159 Conclusions.................................................................................................................... .......159 Future Work.................................................................................................................... ......160 APPENDIX A GROWTH RUN DATA.......................................................................................................161 B REACTOR MODEL............................................................................................................236 Input_PMEE.m................................................................................................................... ..236 PMEE_model.m................................................................................................................... .238 LIST OF REFERENCES.............................................................................................................243 BIOGRAPHICAL SKETCH.......................................................................................................250

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8 LIST OF TABLES Table page 2-1. Efficiencies of copper chalcopyrites..................................................................................50 4-1. First CGS growth series................................................................................................. ....91 4-2. Second CGS growth series................................................................................................ .91 4-3. Third CGS growth series................................................................................................. ..91 4-4. Fourth CGS growth series................................................................................................ ..91 4-5. Fifth CGS growth series................................................................................................. ....92 4-6. Sixth CGS growth series................................................................................................. ...92 4-7. Seventh CGS Growth Series..............................................................................................92 4-8. Eighth CGS growth series................................................................................................ ..93 5-1. Device parameters of reco rd CGS cells produced at NREL............................................130 5-2. Device parameters for the second CGS absorber growth series......................................130 5-3. Device parameters for the fourth CGS absorber growth series.......................................130 5-4. Device parameters for the fi fth CGS absorber growth series..........................................130 5-5. Device parameters for the sixth CGS absorber growth series.........................................130 5-6. Device parameters for the seventh CGS absorber growth series.....................................131 5-7. Device parameters for the eighth CGS absorber growth series.......................................131 6-1. CIGS growth series...................................................................................................... ....144 6-2. Device parameters for the CIGS growth series................................................................148 A-1. Reactor conditions for Growth Run #443........................................................................161 A-2. Reactor conditions for Growth Run #444........................................................................163 A-3. Reactor conditions for Growth Run #445........................................................................164 A-4. Reactor conditions for Growth Run #446........................................................................165 A-5. Reactor conditions for Growth Run #447........................................................................166

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9 A-6. Reactor conditions for Growth Run #452........................................................................167 A-7. Reactor conditions for Growth Run #453........................................................................168 A-8. Reactor conditions for Growth Run #454........................................................................169 A-9. Reactor conditions for Growth Run #455........................................................................170 A-10. Reactor conditions for Growth Run #456........................................................................171 A-11. Reactor conditions for Growth Run #457........................................................................172 A-12. Reactor conditions for Growth Run #458........................................................................173 A-13. Reactor conditions for Growth Run #459........................................................................174 A-14. Reactor conditions for Growth Run #472........................................................................175 A-15. Reactor conditions for Growth Run #474........................................................................176 A-16. Reactor conditions for Growth Run #475........................................................................177 A-17. Reactor conditions for Growth Run #476........................................................................178 A-18. Reactor conditions for Growth Run #477........................................................................179 A-19. Reactor conditions for Growth Run #478........................................................................180 A-20. Reactor conditions for Growth Run #479........................................................................181 A-21. Reactor conditions for Growth Run #480........................................................................182 A-22. Reactor conditions for Growth Run #510........................................................................183 A-23. Reactor conditions for Growth Run #511........................................................................184 A-24. Reactor conditions for Growth Run #512........................................................................185 A-25. Reactor conditions for Growth Run #513........................................................................186 A-26. Reactor conditions for Growth Run #514........................................................................187 A-27. Reactor conditions for Growth Run #515........................................................................188 A-28. Reactor conditions for Growth Run #516........................................................................189 A-29. Reactor conditions for Growth Run #521........................................................................190 A-30. Reactor conditions for Growth Run #522........................................................................191

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10 A-31. Reactor conditions for Growth Run #523........................................................................192 A-32. Reactor conditions for Growth Run #524........................................................................193 A-33. Reactor conditions for Growth Run #525........................................................................194 A-34. Reactor conditions for Growth Run #535........................................................................195 A-35. Reactor conditions for Growth Run #536........................................................................196 A-36. Reactor conditions for Growth Run #537........................................................................197 A-37. Reactor conditions for Growth Run #538........................................................................198 A-38. Reactor conditions for Growth Run #540........................................................................199 A-39. Reactor conditions for Growth Run #541........................................................................200 A-40. Reactor conditions for Growth Run #542........................................................................201 A-41. Reactor conditions for Growth Run #569........................................................................202 A-42. Reactor conditions for Growth Run #575........................................................................203 A-43. Reactor conditions for Growth Run #578........................................................................204 A-44. Reactor conditions for Growth Run #579........................................................................205 A-45. Reactor conditions for Growth Run #582........................................................................206 A-46. Reactor conditions for Growth Run #586........................................................................207 A-47. Reactor conditions for Growth Run #587........................................................................208 A-48. Reactor conditions for Growth Run #588........................................................................209 A-49. Reactor conditions for Growth Run #628........................................................................210 A-50. Reactor conditions for Growth Run #629........................................................................211 A-51. Reactor conditions for Growth Run #630........................................................................212 A-52. Reactor conditions for Growth Run #634........................................................................213 A-53. Reactor conditions for Growth Run #635........................................................................214 A-54. Reactor conditions for Growth Run #636........................................................................215 A-55. Reactor conditions for Growth Run #637........................................................................216

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11 A-56. Reactor conditions for Growth Run #638........................................................................217 A-57. Reactor conditions for Growth Run #639........................................................................218 A-58. Reactor conditions for Growth Run #640........................................................................219 A-59. Reactor conditions for Growth Run #641........................................................................220 A-60. Reactor conditions for Growth Run #647........................................................................221 A-61. Reactor conditions for Growth Run #648........................................................................222 A-62. Reactor conditions for Growth Run #649........................................................................223 A-63. Reactor conditions for Growth Run #652........................................................................224 A-64. Reactor conditions for Growth Run #653........................................................................225 A-65. Reactor conditions for Growth Run #654........................................................................226 A-66. Reactor conditions for Growth Run #655........................................................................227 A-67. Reactor conditions for Growth Run #656........................................................................228 A-68. Reactor conditions for Growth Run #657........................................................................229 A-69. Reactor conditions for Growth Run #658........................................................................230 A-70. Reactor conditions for Growth Run #659........................................................................231 A-71. Reactor conditions for Growth Run #660........................................................................232 A-72. Reactor conditions for Growth Run #661........................................................................233 A-73. Reactor conditions for Growth Run #662........................................................................234 A-74. Reactor conditions for Growth Run #666........................................................................235

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12 LIST OF FIGURES Figure page 1-1. Top view of the PMEE reactor..........................................................................................25 2-1. Spectral irradiance versus wave length under AM0 and AM1.5 conditions......................49 2-2. Photovoltaic system....................................................................................................... ....49 2-3. Chalcopyrite structure of CuInSe2.....................................................................................50 2-4. CIGS/CGS monolithic tandem device structure................................................................51 3-1. UF growth recipes......................................................................................................... .....71 3-2. Typical CIGS device structure...........................................................................................72 4-1. Morphologies of films grown at lower growth temperatures by similar growth recipes........................................................................................................................ ........94 4-2. Morphology of a film grown at a higher growth temperature...........................................94 4-3. Morphologies of the Cu-rich domain regi on of CGS films grown by the same recipe at different growth temperatures........................................................................................95 4-4. Morphologies of the Ga-rich matrix regi on of CGS films grown by the same recipe at different growth temperatures........................................................................................95 4-5. Morphologies of the Cu-rich domain re gion of CGS films gr own with different growth recipes at 491C.....................................................................................................95 4-6. Morphologies of the Ga-rich matrix re gion of CGS films grown with different growth recipes at 491C.....................................................................................................96 4-7. Morphologies of CGS films grown by the emulated 3-stage process at 491C (X30,000)...................................................................................................................... .....96 4-8. Morphology of a Cu-rich film (#542) w ith large grains and a uniform surface................96 4-9. Diffraction patterns of films grown at diffe rent temperatures with the same modified three-stage process............................................................................................................ .97 4-10. Diffraction patterns of films grown at diffe rent temperatures with the same modified three-stage process featuring an initial GaSe layer............................................................98 4-11. Diffraction patterns of films grow n at different rotational speeds.....................................99 4-12. Diffraction patterns of films grown at di fferent levels of overall Cu-richness................100

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13 4-13. Effect of KCN-etch on the diffr action pattern of a Cu-rich film.....................................101 4-14. Diffraction patterns of films grown by the Constant Cu Rate Process............................102 4-15. Diffraction patterns of f ilms grown with varying levels of peak Cu-richness.................103 4-16. Diffraction patterns of films grown by the Emulated 3-Stage Process............................104 4-17. Diffraction pattern of a film grown by th e Emulated 3-Stage Process that was never Cu-rich........................................................................................................................ .....105 4-18. Surface morphology of films grown by th e Constant Cu Rate process (X100)..............106 4-19. Surface morphology of a Cu-rich film gr own by the Constant Cu Rate Process (X5000)........................................................................................................................ ....107 4-20. Surface morphology of a Cu-rich film gr own by the Constant Cu Rate Process (X10,000)...................................................................................................................... ...108 4-21. Surface morphology of a Ga-rich film grow n by the Constant Cu Rate Process............109 4-22. Effect of KCN-etch on the surface morphol ogy of the island region of a Cu-rich film..110 4-23. Effect of KCN-etch on the surface morphol ogy of the field region of a Cu-rich film....111 4-24. Surface morphology of a Ga-rich f ilm with rings around the islands..............................112 4-25. Distinct grain structure of a Ga-ri ch film with rings around its islands...........................113 4-26. Surface morphology of a film grown by the Emulated 3-Stage Process.........................114 4-27. Surface morphology of a Cu-rich film gr own by the Emulated 3-Stage Process............115 4-28. Surface morphology of a Ga-rich film gr own by the Emulated 3-Stage Process............116 4-29. Diffraction pattern of a Ga-rich film grown by the Constant Cu Rate process...............117 4-30. Diffraction patterns of films grown by the Constant Cu Rate process............................118 4-31. Diffraction patterns of films grown by 3-stage process...................................................119 5-1. Dark and illuminated I-V curves for Device #523...........................................................132 5-2. Spectral response curves comparing Device #523 and #452...........................................132 5-3. Photo I-V curve for Device # 640....................................................................................133 5-4. Photo I-V curve for Device # 655....................................................................................133

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14 6-1. Diffraction pattern of CIS film #582...............................................................................144 6-2. Diffraction patterns of CIGS films grown to different thicknesses.................................145 6-3. Diffraction patterns of CIGS films grown with different Cu/III ratios............................146 6-4. Diffraction patterns of CIGS films gr own with different Ga/III ratios............................147 6-5. Illuminated I-V curve for Device #582............................................................................148 6-6. Comparison of illuminated I-V curves of Device #575 and #588...................................149 6-7. Comparison of the illuminated I-V curv es of Device #588 and the calibration cell.......149 7-1. Metal source crucible..................................................................................................... ..157 7-2. Deposition flux FS(r) (atoms/cm2-s) on the substrate at nine different melt levels.........157 7-3. Positioning of the sources in the reactor..........................................................................158 7-4. Cryoshroud................................................................................................................ .......158

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15 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy COPPER GALLIUM DISELENIDE THIN FILM ABSORBER GROWTH FOR SOLAR CELL DEVICE FABRICATION By Ryan Kaczynski May 2007 Chair: Oscar Crisalle Major: Chemical Engineering A custom-built migration-enhanced epitaxy reactor originally optimized for CuInSe2 (CIS) deposition was modified to grow gallium-contai ning compound semiconductor thin films, such as CuGaSe2 (CGS) and CuIn1-xGaxSe2 (CIGS). The addition of gallium allows for the manufacturing of solar cell absorber layers with wider band gaps. Three distinct growth recipe s under several growth temperat ures and a wide range of metal-composition ratios are used to deposit po lycrystalline CGS thin films. The surface morphology of gallium-rich films is typically very uniform, with long needle-like grains when grown by the first recipe, a consta nt copper-rate process. In c ontrast, copper-rich films grown by this same recipe or by a modified three-stage process have island struct ures with very large grains embedded in a matrix region that pos sesses small grains. The surface morphology becomes more uniform and the grains in the matr ix region become larger when a higher growth temperature is used. The third recipe, an emulated three-stag e process, does not produce films with an island-matrix structure, and the grains are uniformly large. The highest conversion efficien cy achieved for solar cells based on CGS is 5.3%, delivered by a copper-rich absorber deposited at the highest sustainable growth temperature of 491C. This device has a large fill factor of 66 %, but the open-circuit voltage of 0.48 V is lower than

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16 what is expected from a wide band-gap absorber A set of CIGS solar cells was completely fabricated and characterized inhouse. This led to the most efficient device produced from an absorber grown in our reactor, in the form of a 9 % CIS solar cell featuring a one-micron film deposited at 491C. Finally, a dynamic reactor model was created to describe the deposition environment in our epitaxial reactor. All relevant physical features are incorporated, including the cyclic motion of a rotating platen and the spatial distribution of th e flux produced by three metal effusion sources. Reaction occurs under an excess of selenium, and operational variables such as rotational speed and melt height can be simulated. The output s are predicted film thickness and composition. Further work is proposed to identify the values of adjustable sticking coefficients using experimental data.

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17 CHAPTER 1 INTRODUCTION This first chapter is in tended to give the reader an overa ll description of the system under study. This information will be followed by the st atement of the objectives of the project and finally by the proposed solution strategy to the problems. Not many details will be covered in this chapter; the purpose is to give the reader an overall idea of the rationale behind the remaining sections of the report. The second chapter is an introdu ction to photovoltaics and thin film solar cells, especially those based on copper chalcopyrites. Its purpose is to familiarize th e reader with energy production, solar cells and the important parameters that affect their efficiency. This chapter may be skipped by readers who are already familia r with the field without a loss of continuity. The third chapter is an in-depth description of the absorber growth and device fabrication procedures. It is complimented by an explanation of the various t echniques used to characterize the respective films and devices. Our technique s are compared to those in the literature. Chapter 4 is an account of Copper Gallium Dise lenide absorber growth in our modified molecular beam epitaxy reactor. The main motiva tion is the characteriza tion of the films grown under various processing conditions. This ch apter is accompanied by Appendix A, which includes the growth conditions for each absorber film described w ithin the course of this work. The fifth chapter is very similar in structure to the previous one, with the focal point shifting to the device fabrication of CuGaSe2 solar cells. The thin films grown in the PMEE reactor that are discussed in Chapter 4 are used as the absorber layers in solar cell devices. Most cells were finished at the National Renewa ble Energy Laboratory (NREL), except for the final set which was completely fabricated in-house at the University of Florida.

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18 In Chapter 6, low gallium content Cu(In,Ga)Se2 absorbers are grown and completely fabricated within our facilities. The as-g rown absorbers and subsequent devices were characterized to determine the e ffect of Ga composition on films grown by a simple single-stage process at a substrate temper ature below 500C. Appendix A also contains the reactor conditions pertaining to each of the growth runs. In the seventh chapter, the focus is shifted to modeling of the reactor. A flux model had already been developed and needed to be incorpor ated into overall dynamic reactor model. This chapter is complemented by appendix B, which includes the details of model development. The final two sections of the manuscript are conclusions and the list of references. The conclusions chapter also contains a list of possible future direc tions this research can take. System Description This project was initiated after the Boeing Co mpany decided to terminate its photovoltaics research program and donated some research equi pment to the University of Florida. Billy Stanbery, who was part of the Boeing Team, decide d to enroll at the University of Florida to pursue a PhD degree. This jump-started a co mprehensive, multi-faceted and multidisciplinary CIS solar cell research effort at the University of Florida. PMEE Reactor Physical Vapor Deposition (PVD) describes se miconductor thin film growth in a reactor whose high vacuum conditions cause material to fl ow in the molecular regime. Molecular Beam Epitaxy (MBE) describes this deposition process when epitaxial growth results. The main attributes of MBE compared to other techniques are a low growth temperature that limits diffusion, a slow growth rate that ensures two-dimensional growt h, a simple growth mechanism, and compatibility with in situ analysis. Because of its unpreced ented control down to the atomic

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19 scale, MBE has been employed for the growth of many novel devices that require band gap engineering. Migration Enhanced Epitaxy (MEE) is a vari ant of MBE based on se quential rather than simultaneous exposure of the substrate to source fluxe s. Rather than using shutters to control the material deposition on the substrates, the substr ates rotate on a donut-shaped platen that takes them through the different deposition zones as well as fluxless relaxation steps in between. Each substrate is sequentially exposed during a co mplete cycle to Cu+In+Ga, background vacuum ambient, Se, and the background vacuum ambient again [1]. Chalcopyrite films have been grown by MBE for nearly 30 years [2], but the rotating platen, which is the main concept of MEE, makes our work unique comp ared to other research groups. Our own reactor has been named with the acronym PMEE (plasma-assisted migration enhanced epitaxy) because of the incorporation of a plasma cracker for selenium or sulfur deposition. This reactor was originally designed to deposit CuInSe2 (CIS) absorber layers and then was modified to support the growth of CuGaSe2 (CGS) and CuIn1-xGaxSe2 (CIGS) films as well. The PMEE reactor can deposit Cu, In, Ga, Se, S, and Na, allowing for the deposition of a wide-variety of Cu-chalcopyrit e thin films. It can support device manufacturing based on polycrystalline co-deposited CIS-based films, as well as an assortment of studies such as single crystal growth and bilayer precursor design for RT P studies. This process is low throughput and thus not economically feasible, but this res earch is concerned with investigating the film properties, not large-scale produc tion. The ultra-high vacuum ( UHV) creates an extremely clean condition and makes it possible to generate the mol ecular beam of each source so that the growth system can be used to grow epitaxial CIS thin fi lms of high crystalline qu ality. It can overcome,

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20 to a certain extent, a disadvantage of MBE, which is low productivity by processing nine samples in one batch. There are some disadvantages of the system. Due to the rotational movement of the platen and thus the substrates, direct in-situ measurement of the subs trate temperature is virtually impossible. The thermocouple is currently located in the gap between the platen and the heater and reading sort of an average value of those two temperatures. The localized heater position creates non-uniformity of the temperature distribu tion on the substrates. The growth rate is significantly limited by the selenium flux de livery. Even with a high Se flux rate ([Se]/([Cu]+[In])>5), it is hard to obtain sufficient Se incorpor ation into a growing film under a high temperature condition since th e Se deposition zone is confined, and the high vapor-pressure material is easily re-evaporated from the surface. As a result, maximum flux rates of Cu and In are limited, which makes it difficult to achieve high growth rate. A time-consuming and costly problem is that the PMEE reactor has been cu stom designed and therefore requires extensive customizations to the regular sources available from manufacturers. Chamber The reactor can be divided into four zones as is shown in Figure 11: load-lock, chalcogen, heater, and metals. Materials are sequentially deposited as the substr ate passes through their respective zones rather than co -deposited. The total flux is hi ghly enriched in the specie evaporating from the nearer of the two sources as the substrate initially enters the metals deposition zone from either side. Hence, substr ate rotation direction can result in substantially different compositions within the fi rst metal layer. Counterclockwis e rotation results in an initial metal flux during each MEE cycle th at is substantially Cu-enriched. The high-vacuum chamber is divided vertically into two zones by the cryoshroud and is maintained at a base pressure of around 10-8 torr by a series of diffusion and mechanical pumps.

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21 Above the cryoshroud is the growth zone, where all the sources and substrates are located. Liquid nitrogen, circulated in the cryoshroud, further reduces the pres sure in the growth zone to a base pressure around 10-9 torr. Pressure during depos ition is in the range of 10-7 to 10-8 torr depending on the operating conditions. If the ch amber is brought to atmospheric pressure, it takes a few days to get the appropriate low pressure back. Load-lock A load-lock, attached to a port at the substrate platen level of the chamber, allows the system to remain under vacuum for months duri ng operation. The load-lock is independently pumped with a small turbomolecular pump (TMP) and isolated by a gate valve from the loading chamber. Chamber venting uses argon gas and the load-lock is equipped with a Venturi pump and a liquid nitrogen sorption pump for rough-pump ing down to the TMPs crossover pressure of 10-3 torr. Up to nine 2 diameter wafers or 2 x 2 square substrates can be loaded onto the donut-shaped molybdenum platen via a two-prong fork, which is then rotated so that each substrate travels through all four zones. Chalcogen Zone The chalcogen zone is where a thermal cracker for Selenium (Se) and a low-capacity plasma cracker that can be charge d with Selenium or Sulfur (S) are installed. Since Se does not evaporate as a very reactive species, it must be fu rther cracked to be incor porated into the film. The thermal cracker breaks the large molecules in to smaller, more reactive molecules by heating them to very high temperatures in a doubleoven reactor before deposition. Hence the temperature in the evaporation z one of this double oven controls th e flux of the material and the temperature in the cracking zone controls the species distribution [3]. The plasma cracker accelerates particles to make them more effectively reactive, whic h is an alternative to the high temperature of the thermal cracker. No sensors are used to measure the flux since the Se is

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22 deposited in excess; only the temperatures of the sources are measured. Excess Se deposits everywhere so a chalcogen zone shield was incorpor ated to isolate this zone from the rest of the reactor. Heater Zone After passing through the chalcogen zone, subs trates are brought to the desired operation temperature in the heater zone with radiative heating by a boron nitride-coated radiation heater. Most of the substrate platen hea ting is provided here. In other z ones, the substrates are slowly cooled down since there is no di rect heating there. Some ex tent of non-uniformity of the temperature distribution on th e platen is expected due to the complex design [4]. Metals Zone Finally, the substrates enter the metal depositio n zone where they are sequentially exposed to Copper (Cu), Indium (In), and Gallium (Ga) fl uxes. The Cu, In, and Ga sources are thermal evaporation sources with conical shaped cruc ibles and free-evaporating surfaces. Deposition uniformity is improved significantly with a conical instead of a cylindrical crucible [5]. The effusion sources are identical in structure: 7.5 tapered angle, 30 cc ca pacity, and constructed with Pyrolithic Boron Nitride (PBN). Cu and Ga have dual filament heating structures due to their properties, whereas In has only one. The tip filament keeps the tip of the crucible hot so no impurities can condense on the surface. Shielding prevents deposition on each individual substrate except during that portion of each rotation cy cle of the substrate plat en when it is inside the shield. The platen then rotates back into the load-loc k zone where the entire cycle restarts. The dopant source is located in this ar ea to introduce small quantities of impurities. It is charged with a very small amount of NaF. The metals zone was expanded into the previously allocated loadlock area so that the Gall ium source could be added.

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23 Control Every material source is equipped with ther mocouples for monitoring the temperature. The source temperature is actually measured i ndirectly by putting the th ermocouple in thermal contact with the crucible. Th ese are mainly c-type thermoc ouples, with the exception of the evaporation zone in the two crack ers which are fitted by k-type thermocouples due to the lower temperatures involved. The substrate temperature is measured indirectly by means of a c-type thermocouple suspended in between the rotating pl aten and the substrate heater. The rotating nature of the platen prevents getting direct temperature readings on the substrate [6]. Rate control is provided by a Leybold-Heraeu s Inficon Sentinel III with both EIES (Electron Impact Emission Spectroscopy) sensor s for monitoring and controlling the metals deposition process and quartz crystal monitors (QCM) for calibration. Closed-loop feedback control is conducted along with the in-situ rate measurement em ployed by the EIES sensors for Cu and In sources. EIES sensors are calibrated by a QCM that is locate d right over the source cells whenever source material is reloaded. A single QCM is used to monitor the Ga flux because the reactor modifications were limited by space and the current setup. The dopant flux is monitored by QCM because it doesnt need to be controlled tightly. No instrument is used to measure the Se flux rate in-situ so closed-loop fe edback control based on temperature has been adopted. An instrumentation and control interface for th e PMEE reactor was designed to enable the implementation of advanced control strategies en visioned for the local sources as well as the supervisory control structure. A human-m achine interface is pr ogrammed on a LABVIEW platform so that real time control of the PM EE reactor can be admini stered through a central computer [6]. Using microproc essor control, one can ensure run-to-run repeatability by constantly monitoring and adjustin g the various growth parameters.

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24 Problem Statement The purpose of this project is to explore the key processing issues a ssociated with growing copper chalcopyrite films containi ng gallium in a migration enhan ced epitaxy reactor. Copper Gallium Diselenide is theoretically a good candida te as the top cell in a tandem solar device, owing to its nearly ideal band gap of 1.7 eV and maximum theoretical efficiency of 26%. Practical use in a tandem cell will require efficienci es greater than the current best cell efficiency of approximately 10%. A low temperat ure process for the growth of CuGaSe2 absorber layers should be developed to avoid the degradation of the junctions lo cated underneath th e top cell in a monolithic tandem structure. To grow high-quality CGS absorbers, several st eps were required. First, a gallium source needed to be retro-fit into a custom-built MEE reactor used for the deposition of CuInSe2 absorber films. Then it needed to be shown th at this reactor could gr ow polycrystalline CuGaSe2 thin films that produced working solar cell device s. Various growth reci pes were investigated, along with varying material compositions, growth temperatures, and post-deposition processes. The structural and morphological properties of the films were characterized along with electrical properties of the subsequently fabricated devices. Another goal was to achieve complete in-hous e fabrication and char acterization of CGS devices. This was intended to decrease the feed back time needed in investigating the effect of processing changes on electrical properties of the solar cell. This required a team effort of several graduate students. Finally, the flux models of the effusion sources needed to be incorporated into a dynamic reactor model. This model will be the basis for a control feedback scheme that will correlate film properties and hence, device properties with the input conditions of the reactor.

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25 Figure 1-1. Top view of the PMEE reactor.

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26 CHAPTER 2 SOLAR CELLS Energy Rapid industrialization combined with an expanding population is driving the worlds demand for energy, which is projecte d to triple by the end of the century (from 13 TW to 46 TW) [7]. The fossil fuel reserves that currently fu rnish power to the globe will fall short of this demand over the long term, and their continued use produces harmful side effects such as pollution. Finding sufficient supplies of clean ener gy for the future may be civilizations most difficult challenge. Alternative renewable fuels ar e currently not competitive with fossil fuels in cost and production capacity, but solar cells ha ve the potential to become a key part of the solution to this problem. Sunlight Photovoltaic (PV) systems exploit an inexha ustible resource that is free to use and available anywhere in the world. More energy fr om sunlight reaches the earths surface in one hour (4.3 x 1020 J) than is consumed by civiliz ation in an entire year (4.1 x 1020 J) [7]. If 0.16% of the land on Earth was covered with 10% effi cient solar cells, 20 TW of power would be provided, which is more than the worlds current consumption rate of fossil energy [7]. This illustrates the impressive magnitude of the solar resource and the potential harbored by solar cell technology. The sun emits energy as a blackbody radiator at a temperature of approximately 6000 K with a spectrum ranging from the ultraviolet (3.5 eV), through the visible, into the infrared (0.5 eV). The energy of the visible region ranges from 3.1 eV (violet) to 1.8 eV (red) with the peak power of the sun occurring at approximately 2.25 eV. This distribution of photons in the spectrum is one of the greatest limiting factors on solar cell performance. Under monochromatic

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27 light, a typical PV cell might be able to convert 60% of the light to electricity, but under the multicolored solar spectrum, the same cell would only be able to convert 10% of the lights energy to electricity [8]. Diffuse light is the portion of sunlight that has been refracted or scattered in the atmosphere before it reaches th e Earths surface. When the sky is completely clear, about 10% of the sunlight is diffuse. Most losses in th e spectrum at lower energies are caused by light being absorbed by molecules of wa ter vapor while at energies higher than 3 eV, almost all sunlight is absorbed by ozone [8]. The AM1 solar spectrum represents the sunlig ht on the Earths surface when the sun is at its peak. At a solar zenith angle of 48.2, th e equivalent of 1.5 of these noontime air masses (AM1.5) is diminishing the intensity of the sunl ight [8]. The AM1.5 condition has an incident power of 84.4 mW/cm2 and is the most appropriate for calcu lating the conversion efficiency of a solar cell in the terrestrial environment [9 ]. The irradiant power of the sun under AM0 conditions is 135.3 mW/cm2, which is the spectrum measured outside the earths atmosphere. AM1.5 is compared to AM0 in Figure 2-1 [9]. The peak watt (WP) rating is the power (in watts) produced by a solar cell module illumina ted under the following standard conditions: 100 mW/cm2 intensity, 25C ambient temperature, and AM1.5. Because of day/night and time-ofday variations and cloud cover, the average electrical power produ ced by a solar cell over a year is about 20% of its WP rating [7]. The two most important variab les controlling the amount of annual sunlight are latitude and local cloud cover. Latitude is important to the amount of annual su nlight for two reasons: the angle of the sun and the length of the days. The Northeast and Northwest United States are the cloudiest regions, while the Sout hwest usually has the clearest sk ies. In Buffalo, NY, there is about 60% and in Sacramento, CA, about 85% of the solar energy available in Albuquerque, NM

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28 [10]. Almost 90% of the count ry gets between 6 and 8 kWh/m2 daily, which is enough for the effective use of PV [8]. Ther e is also a seasonal effect on the energy produced by PV modules because in the summer the sun spends more time at high elevation angles (low air mass) than in the winter (high air mass) Costs Clearly, solar energy can be exploited on the needed scale to meet the global energy demand. Sunlight is readily available and its use does not harm the environment through pollution or the climate through greenhouse gases. Yet, U.S. electricity production by solar cells currently represents a tiny fracti on (<0.02%) of the total electricity supply [7 ]. The wide-spread use of PV has been hampered by the relatively hi gh price of the solar cell module. The huge gap between our present consumption of solar en ergy and its enormous undeveloped potential defines a grand challenge in energy research. Solar cells typically have a lifet ime of at least 30 years and in cur no fuel expenses, but they do involve a capital cost. The cost for the electricity produced by th e cell is estimated by spreading the total capital cost over the entire lifetime of the cell while considering the total electrical energy that will be produced during that time. Hi gher conversion efficiency thus directly impacts the overall electricity cost be cause higher efficiency cells will produce more electrical energy per unit cell area over the cell lifetime. The most useful cost calculation for PV cell modules ($/WP) is determined by the ratio of module cost per unit area ($/m2) divided by the maximum amount of electric power delivered per unit of area (module efficiency multiplied by 1000 W/m2, the peak isolation power) [7]. In additi on to module costs, a PV system also has costs related to the non-photoactive parts of the system, called balance of system (BOS) costs. To compete with electricity produced from fossil fu els, solar cell costs must eventually approach $0.40/WP [7].

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29 Photovoltaic Systems power out Efficiency = power in (2-1) For a solar cell, this is the ratio of the electric power produced by the cell at any time versus the power of the sunlight arriving at the cell. Efficiencies do not fluctuate much over the life of a cell unless it is degrading. By defin ition, the higher a photovoltaic devices efficiency, the more electricity it produces for a given e xposed area. Besides device efficiency, the definition of performance must also include uniformity, reproducibi lity, throughput, materials utilization, and yield [11]. A single cell is only useful when powering wr istwatches and calculators so many cells are connected within a module. The module serves two purposes: it protects the solar cells from the outside environment and it deliver s a higher voltage than a single cell. Individual small-area cells must be connected in series so some part of the module area is lo st for the interconnection of cells. Another important effect which re duces the performance of modules is the nonuniformity of voltage and current density over a large area, which is mainly related to the compositional variation of the absorber layer [12]. A critical issue that must be resolved is the large gap in efficiency between small-ar ea laboratory devices a nd large-area modules. The basic components of a photovoltaic system are the modules, support structures, land, and possibly sun trackers. There are two di fferent kinds of modules: flat plates and concentrators. Flat plates are large panels that can be assembled into even larger arrays. To maximize energy production, a module can be mounted onto a two-axis solar tracker so that it always points directly at the s un [10]. Concentrators use large, concentrating lenses to focus sunlight onto small cells. These lenses replace large areas of expensive semiconductor material, reducing the total module cost. A concentrator do es not produce any energy when the weather is

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30 cloudy because it cannot focus diffuse light. Hen ce, they are geographically limited to sunny locations as opposed to flat plates, which can be useful in cloudy areas. There are two major market sectors: grid-conn ected and stand-alone systems. The former delivers power directly to the grid by convert ing the direct current of solar modules into alternating current by an inverter. The latter supplies power to is olated sites and to small-scale consumer products. Since solar energy is not alwa ys available, effective storage and distribution are critical to matching supply with demand. Stor age drives up the cost of solar cells. A simplified photovoltaic system is shown Figure 2-2. Besides the terrestrial market, there is also the space market, which has different material and cost requirements. Future The primary objective of worldwide solar cell research and development is to reduce the cost of photovoltaics to a level that will be competitive wi th conventional ways of generating power. The world market grew from less than 10 MWP/yr in 1980, to 80 MW in 1996 sold at prices close to $10/WP [13], to finally exceeding 1000 MW in 2004 at less than $7/WP [14]. Costs need to be reduced even more for solar ce lls to become competitive with the likes of oil, coal, and natural gas. Otherwise, future large-scale use of PV might depend more on environmental concerns rather than economic competitiveness. Behind the progress of photovoltaics as a technol ogy is a loyalty to PV as an idea. There are technical obstacles that must be overcom e and their solutions depend on the funding of fundamental research. As these issues are reso lved, the costs will continue to fall. Solar electricity is part of Americas present and its future as is the R&D that enables it Larry Kazmerski, one of the pioneers of thin film solar cells [15].

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31 Solar Cell History Photovoltaics had its beginnings in the ninet eenth century. French Scientist AlexandreEdmond Becquerel discovered the PV effect in 1839. He observed that a voltage and a current were produced when two electrodes in a beaker fu ll of fluid were exposed to sunlight. And in 1873, Willoughby Smith found that the element selenium conducted far more electricity when it was illuminated than it did when it was dark [8]. Many consider the work accomplished during the 1950s at Bell Laboratories to be the true origin of photovoltaics. Cal Fuller, Darryl Ch apin, and Gordon Pearson made a silicon cell that was able to convert 6% of sun light into electricity [16], wh ich was a large improvement over selenium cells. Fuller and Chapin eventually reached 10% conversion efficiency, but compared to traditional electricity in the 1950s, the cost of PV-produced power was a thousand times as much [8]. From the mid 1950s to the early 1970s, PV research and development was directed primarily toward space applications where it is th e conventional power source. In space, payload weight is critical and solar cells weigh very little compared to the power that they produce. Nearly every communications satellite, military sate llite, and scientific probe is powered by PV. Satellite manufacturers can endure a high cost because it is a small fraction of their total cost, but they cannot tolerate an unreliable power source that may jeopardize their entire investment. Then in 1973, a greatly increased level of resear ch and development on solar cells was initiated following the oil embargo in that year, which le d to the creation of the U.S. Department of Energy, along with its PV program, a few years later. Researchers at Bell Labs showed three decades ago that the I-III-VI2 semiconductor CuInSe2 (CIS) allows for efficient solar-to-elect rical energy conversion. A single crystal CIS cell of 12% efficiency was made at Bell Laborator ies in 1975 [17] and the University of Maine

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32 produced a polycrystalline CuInSe2 cell of nearly 6% efficiency in the following year [18]. Although Bell Labs and Maine initia ted the study of CIS, the most important early research was done by a small group at Boeing Aerospace Corporati on in Seattle. Boeings team, led by Reid Mickelsen and Wen Chen, achieved over 10% effici ency with CIS cells in 1982 using elemental co-evaporation of the source materials in vacuum [19]. Solar Cell Device Physics Photovoltaics (PV) is the direct conversion of sunlight into electricity; solar cells absorb sunlight and change it continuous ly into electricity. Accordin g to quantum theory, light can behave either as waves or as particles. Discre te particle-like packets of light are called photons, and each photon has a well-defined energy and wavelength. Band Gap The valence and conduction bands of an i norganic semiconductor are separated by a forbidden energy range that the electrons cannot occupy [20]. This minimum threshold energy is called the energy gap or band gap (Eg), and it varies for different materials because each of them has a different bond strength. Semiconductors with weak bonds have small energy band gaps. Conduction is only possible if we can impart kine tic energy to an electr on. When photons with energies h > Eg impinge upon a pn junction solar cell, th ey are absorbed and the rate of generation of electron-hole pairs as a function of distance x from the surface of the solar cell is given by the following equation [9]: gE(x) = o(1-R)ex (2-2) where is the optical absorption coefficient, o is the incident photon flux density per unit bandwidth per second, and R is the reflection coefficient. Photons with energies below the band gap pass through the absorber without being absorbed.

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33 Electric Field Most semiconductor devices incorporate both positive and negative regions, and it is the space-charge region formed between them that leads to their useful electrical characteristics [20]. After a certain number of electrons and holes have flowed from one region to the other, an electric field will be built up, preventing further ne t flow of the carriers. The greater the density of free carriers initially on each si de of the interface, the greater will be the electric field that forms when they mix. This electric field is not tenuous; it does not come and go. The area of the field is also called the depletion region because in that region there are no fr ee carriers. They all are either in bonds or swept away by the field. Under equilibrium conditi ons, electron-hole pairs are continuously generated ever ywhere within the semiconducto r and in the absence of an applied voltage, the electron-ho le pairs recombine and therefore no current flow results. However, when a positive voltage is applied to the n-region of a diode with respect to the pregion, the electron-hole pairs, once generate d, will be separated a nd their probability of recombination is diminished. The most important attribute of these p-n j unctions is that they rectify, i.e. they permit the passage of elect ric current in only one direction [20]. The electric field that drives the current is proportional to the materials band gap. A semiconductor with a small band gap produces an insignificant voltage. As the band gap is increased, more solar photons within the spectr um will lack the energy to produce electrons and holes so a very large band gap would produce a high voltage with a tiny current. For a semiconductor to be a sufficient abso rber layer in a solar cell, it mu st have a band gap that allows for both reasonable curre nt and voltage. Recombination Radiative recombination, in wh ich a hole reacts with an elect ron and produces a photon, is exactly the reverse of absorption; it is the spontaneous transition of an electron from the

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34 conduction band to an unoccupied state in the valence band [21]. In real solar cells, recombination via impurities is the predominant recombination process. Recombination centers located within the electric field can severely re duce the fields strength and thus the voltage. These are called shunts [8]. Defects located at the interface can also be efficient recombination centers because they introduce deep trap levels into the band gap [22]. Defects In compound semiconductors, intrinsic point de fects are introduced to compensate for deviations from the stoichiometry [23]. The simplest point defect is the vacancy in which a single atom is missing from the lattice. An inte rstitial is an extra atom occupying space between the normal lattice sites. A component atom may also occur on a site intended for another. This is called an antisite defect. Point defects influence the bulk properties of a semiconductor as opposed to grain boundaries or interfaces th at only affect the film locally [22]. A grain boundary is the complete fracturing of bonds along an entire surface. They occur when a lattice takes shape in such a way that a defect spreads and makes it impossible for nearby atoms to bond together. According to the gr ain boundary carrier-tr apping theory, grain boundaries work as trapping centers and therefore hi nder the transport of ch arge carriers towards the pn interface [24]. Cell performance would suffe r considerably because these lost electrons would not contribute to the cell current. It is very co mmon for crystal defects to cause a polycrystalline structure where th e grain boundaries separate regions of different crystallographic orientation. Many of the electri cal and optical properties of th ese materials are determined by the corresponding properties of the grain boundaries and these may differ considerably from a single crystal [22]. When grain boundaries cannot be avoided, they need to be passivated to minimize their effect. This consists of adding some extra ma terial, usually oxygen, that can make the grain

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35 boundary defects less harmful. Most added ma terials diffuse preferentially down grain boundaries so it goes directly to the region where it can have the most effect. The added oxygen might passivate defects in the grain bounda ry by grabbing loosely bound electrons, removing many of them as undesirable recombination cente rs. Some materials have relatively harmless grain boundaries like CuInSe2; they may be self-passivating or some growth step may passivate them. They can be made inexpensively and yet be have almost as if they were single crystals. Others have very harmful grain boundaries, like gallium arsenide (GaAs) and silicon (Si ) [8]. Thin Films Crystalline silicon currently dominates the photovoltaic market despite a complicated manufacturing process and a high production cost. Its advantages are a readily available raw material, mature processing technology, and non-t oxicity. Si-based pr oducts are also very reliable and are capable of achieving high effici encies [14]. The preeminence of the element Si, in its amorphous or crystalline form, within the market is an overwhelming 99% [25]. A strategy for reducing costs is to use thin film materials that have a very high absorptivity for solar photons. The leading th in film technologies, a-Si, CdTe, and CIS, offer the potential for significant manufacturing advantag es over crystalline Si. They have a lower consumption of materials, independence from Si shortages, fe wer processing steps, and a monolithic circuit design so no assembly of individual solar cells into a final product is needed [26]. Less material usage leads to lower material costs, thinner laye rs leads to faster processes and lower capital costs, and the processing of larg e-area devices leads to reduced handling costs. They can also be made into flexible and light-weight modules on alternative substrates, which provide multiple advantages in processing [27]. The most serious threat to largescale deployment of existing thin films is materials availa bility of key elements such an In, Te, and Ge. Thus, Si-based technology may always be relevant.

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36 Direct vs Indirect Band Gap Si has an indirect band gap so light absorption is much weaker than for thin films, which are direct band gap semiconductors. The differen ce in absorption strength between direct and indirect band gap semiconductors comes from th e different processes by which they absorb individual photons. Direct transitions are transitions in which the momentum of the electronhole pair does not change. Light of sufficient energy to free an electron from its fixed state is absorbed by the electron, which is then freed. Light-absorption is more complicated in an indirect band gap material. Promotion of an electron to the conduction band requires the simultaneous interaction of a photon and a therma l vibration of the crys tal lattice, called a phonon. When a photon and a phonon of the proper en ergies are both absorbed, at the same time, by a bound electron, a free electron-hole pair re sults. Almost all of the energy needed to generate the electron-hole pair is carried by the p hoton; the phonon just ac ts to catalyze the process. Absorption Length The absorption length of light in a semiconduc tor is determined by the likelihood that a photon will be absorbed. The absorption length for crystalline Si is approximately 30 microns, while it is about 0.3 microns for CIS. Highe r energy photons have a greater probability of interacting with a boun d electron and being absorbed than lower energy photons, even though both have more than the band gap energy. For instance, in CIS (Eg = 1.0 eV), the absorption length of high-energy photons (>2.5 eV) is le ss than 0.1 microns, but a beam of low-energy photons (1.1 eV) might require 1 micr on to be equally satisfied [8]. Diffusion Length Free electrons on the p-side move around randoml y before recombining. During this short period of time, called the lifetime, they have a fi nite chance of encounteri ng the electric field and

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37 being sent to the n-type side of the device. This separation is a resu lt of diffusion and the average distance that minority carriers can move to ward the built-in field before they return to their fixed states is called the diffusion length. Materials with longer di ffusion lengths are able to produce more current. In solar cells, they can vary from less than a micron to over 100 microns in some single-crystal semiconductors. Large diffusion lengths are a necessity for indirect band gap materials like crystalline si licon, but direct band ga p materials absorb enough light within their depletion region that their performance is not criti cal to diffusion length. If the ratio of diffusion length to absorp tion length is greater than one, most carriers will be separated. Crystal quality is the most important factor in determining a materials diffusion length because diffusion is constrained by the propens ity of free carriers to recombine. CuInSe2-Based Solar Cells CuInSe2-based solar cells are the most promising of the thin film solar cells and are the basis for the investigations within this wor k. Cu-chalcopyrites, of the general composition Cu(In1-xGax)(SySe1-y)2, offer a wide range of band gaps from 1.0 eV for CuInSe2 to 2.4 eV for CuGaS2. Recently, new world record total-ar ea efficiencies of 15.0, 19.5, and 10.2% for CdS/CIGS solar cells have been achieved fo r x=0 (CIS), x~0.28 (CIGS) and x=1 (CGS), respectively [28]. Table 2-1 s hows these record efficiencies alongside their theoretical output. Material Properties The direct optical band ga p of single crystal CuInSe2 has a value near 1 eV at room temperature [29]. The ab sorption coefficient of 105 cm-1 for light greater than the band gap means the thickness of the absorber can be reduced theoretically to less th an 1 micron [30]. CISbased materials belong to the group of I-III-VI2 semiconducting compounds. Their lattice elements are tetrahedrally coordinated sim ilar to diamond-like semiconductors [31]. The chalcopyrite cell cons ists of two zinc blende ce lls with Cu and In occupying the same lattice sites

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38 in the upper and lower cell, alternately as seen in Figure 2-3 [ 32]. In the chalcopyrite structure, each I (Cu) or III (In, Ga) atom has four bonds to the VI atom (Se). In turn, each Se atom has two bonds to the Cu and two to the In (or Ga). Because the strengths of the I-VI and III-VI bonds are different, the ratio of th e lattice constants c/a is not exactly 2; it varies from 2.01 for CIS [33] to 1.96 in CGS [34]. Doping in chalcopyrites is mainly controll ed by compositional variation and thereby induced defects, which lead to strong compensa tion [35]. This makes the systematic study of their electronic properties more difficult than in extrinsically dopable materials. The band gap can be engineered, which offers a greater pos sibility of finding the optimum photovoltaic material with respect to co st, efficiency, and stability. Four different phases have been found to be relevant: -phase (CuInSe2), -phase (CuIn3Se5), -phase (high temperature sphalerite phase), and Cu2-ySe [36]. The existence range of single-phase CuInSe2 is very small and does not include the stoichiometric composition of 25% Cu. The Cu content of absorbers for efficien t thin-film solar cells typically varies between 22 and 24% (at.) Cu. At the grow th temperature of 500-550C, this region lies within the singlephase region of the -phase. Some interesting features of CIS-based solar cells include low open-ci rcuit voltage to band gap ratio compared to Si and III-V devices, in sensitivity of the conversion efficiency to the [Cu]/[III] ratio over a wide range, exceptiona l tolerance to grain boundaries, and loss of performance for CIGS alloys with x greater than 0.3 [37] Single crystal CIGS efficiencies also lag behind those of polycrystalline CIGS. Optimal sodium incorpor ation is beneficial to device performance, and excess sodium is detrimental. Na depth profiles typically exhibit some

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39 qualitative features: enrichment at the CIS surf ace and a relatively lower concentration in the bulk of the CIS with concentrati on increasing toward a maximum at the CIS/Mo interface [38]. Defects For Chalcopyrite compounds, intrinsic defect s are introduced to ma intain the crystal structure for non-stoichiometric composition. There are twelve intrinsic point defects that can be formed in the ABX2 chalcopyrite lattice: three vacancies (VA, VB, VX), three interstitials (Ai, Bi, Xi), and six antisite defects. In addition to the an tisite defects that can be formed by an exchange of anions (X) and cations (A ,B) as in binary compounds (AX, BX, XA, XB), one can also form antisite defects on the same sublattice by an exchange of the cations (AB, BA) [22]. The Cu vacancy, VCu, is considered to be the dominant accepto r in Cu-poor p-type material, while the Se vacancy, VSe, is considered to be the do minant donor in n-type material [36]. Most of the offstoichiometry defects must be electronically inactive to allo w for large deviations from stoichiometry without the deterioration of the electronic quality of the film [39]. A critical issue in regard to CIS-based solar cells is the control of these point defects, which are responsible for recombination in the space char ge region of the devices [40]. Gallium Addition When indium is replaced by gallium, the band gap increases. This is an effect of the smaller size of the Ga atom, when compared to In, and the subsequent formation energies involved. With the addition of gallium, some of the material properties also change. These include structural properties li ke lattice constants, film morphology, and adhesion, and chemical changes such as defect levels, affinities, a nd carrier concentrations. The band gap of CuIn1xSe2GaxSe2 can be tuned from 1.0 to 1.7 eV by adjus ting the Ga content (x) from 0 to 1. Although the Ga/(Ga+In) ratio of roughly 0.65 would provide the optimal band gap by theory, the actual Ga content in the current r ecord device is 28%, which corresponds to a band

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40 gap of approximately 1.2 eV [41]. This dispar ity implies that there are factors effecting conversion efficiency other than band gap. The in crease in efficiency in the range of 0 to 28% Ga is due mainly to the increase of the band gap, and the potential on th e grain boundary in this gallium content range stays strong. However, at higher Ga content, the ab sence of the potential on the grain boundary seems to be significant in comparison to the effect of the band-gap widening [42]. CuGaSe2 Copper Gallium Diselenide, as a member of the I-III-VI2 compound semiconductors, has a direct energy band gap of 1.7 eV [43] a very high absorption coefficient = 3 x 104 cm-1 at 1.7 eV [44], and an easily controllable elect rical resistivity in a wide range of 10-1 to 105 ohm-cm [45]. The band gap of CGS decreases with in creasing Cu/Ga ratio [46]. The temperature coefficient is lower for wide gap materials, wh ich means that the efficiency loss at operating temperature is less than for smaller band gap semi conductors [47]. To date device efficiency of greater than 9.5% is reported using less than 2 microns of CG S absorber film by NREL [48]. CuGaSe2 has been investigated for more than 25 years, but in comparison to the rapid progress made for CuInSe2 and Cu(In,Ga)Se2, the efficiency of CGS-based solar cells is still relatively low. CGS did not overcome a limitation of 6.2% until fairly recently when efficiencies of up to 9.3% for thin films and 9.7% for single crystal devices were achieved [49]. These reported values lie well below the theoretical limit for CGS of = 26% [50]. The most critical drawback of CGS solar cells is the low open-circuit voltage (VOC) compared to band gap. In the case of CIS or CIGS, the VOC follows a relationship with the band gap according to VOC ~ Eg/q500 mV, where q is the elementary charge. A VOC of 1.2 V should therefore be possible for CGS-based devices, but Cu-rich abso rbers have been limited to a VOC of 750 mV [51], while high efficiency Ga-rich absorbers have peak ed at 900 mV [48]. The improved device

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41 performance of Ga-rich films results from decrea sed defect densities in the bulk and a decrease of tunneling-enhanced recombination [52]. Cell performance may be fundamentally constrained since it seems that the Fermi level might be limited to values less than 800 meV above the valence band edge, which would always make CGS p-type [53]. Defects Wide gap chalcopyrites, such as CGS seem to have extensive materi al and growth-related problems that limit the device performance. The tetragonal structure is capable of sustaining a large concentration of vacancy a nd antisite defects in CGS [54]. Devices based on slightly Cupoor CuGaSe2 absorbers have shown better performances than stoichiometric ones. In Cu-poor material, the deviation from stoich iometry is not facilitated by the formation of a second phase, as in the Cu-rich case, but the material develops a high density of defects. This leads to a high degree of compensation, which in tu rn causes lateral potential fluctu ations in the concentrations of the charged defects [55]. This density of defects is higher at the increased Ga contents because the lattice mismatch in that composition range is anticipated to be larger. Better lattice matching between the surface and the bulk leads to better performance [56]. Optimal growth conditions of CGS thin films are realized by a tr ade-off between growing Ga-rich films, reducing the density of states in the ba nd gap, and growing stoichiometric CGS to achieve optimal grain size and crystallinity [55]. Recombination The actual path of recombination, which limits open circuit voltage, is important; it can take place at the interface or within the space charge region, or it can be tunneling supported [57]. Increasing the Ga content in CIGS inte nsifies the contribution of tunneling, which is observed as a larger char acteristic tunne ling energy, Eoo. This facilitates recombination and in turn increases the recombination loss. For CGS, Rau et al. obtained significant values of Eoo and

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42 found that the devices with the highest VOC are those with the lowest charge density and the lowest tunneling currents [58] At room temperature, the tunneling contribution to recombination is insignificant for low gallium content CIGS [59]. Hence, there is no fundamental difference between CIS and CGS with regard to recombination path, but recombination losses in CGS are enhanced due to a higher contribu tion of tunneling [57]. Cu-rich CGS devices are controlled by high Eoo values due to tunneli ng enhanced interface recombination. The efficiency gain achieved by th e use of Ga-rich absorbers is mainly explained by a reduced doping level and the decreased t unneling rate. All the beneficial device modifications like air-annealing or the increase of CdS deposition temperature lead to a further decrease of Eoo. The increased Cd diffusion into the ab sorber material duri ng CBD explains the reduction of tunneling [51]. Type inversion The efficiency of a CuInSe2 device is aided by the fact that its surfaces can be inverted to become n-type even though the bulk of the sample is p-type. This is done via deposition of CdS, which leads to band bending. The amount of band bending equals the shift in Fermi level with respect to the valence ba nd maximum from the p-type to the n-type region. A large amount of band bending results in a Fermi level close to the conduction band minimum, which is needed to get n-type conditions. The beneficial effects of this weakly n-type surface layer include the reduction of the recombination rate and the enha ncement of the carrier collection efficiency by shifting the electrical junction away from the interface betwee n CdS and the absorber [60]. The photovoltaic performance of CIGS devices deteri orates for x>0.3. Due to the lack of type inversion for CIGS with x>0.3, the pn j unction moves to the r eal CdS/CIGS interface causing higher recombination losses [61]. CGS ha s been reported p-type for all compositions, for thin films as well as single crystals; this is true for all deviations from stoichiometry and

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43 molecularity. The doping-pinning ru le predicts both nand p-t ype behavior for CIS and only ptype behavior for CGS. The maximum Fermi le vel positions are basically the same for both these materials [57]. Thus, it de pends on the relative position of the band edges with respect to those maximum Fermi level positions whether the ma terial can be p-type or n-type or both. As one applies to CGS the same process that conver ts CIS to n-type, the Fermi level does not rise towards the conduction band minimum [62]. The larger band gap of CGS leads to a higher difference between the Fermi level and the upper edge of the valence band at the interface [63]. In CGS, type inversion cannot be achieved under normal conditions, but it can be reached due to doping via non-equilibrium effects [64]. Schon et al. have demonstrated that n-CGS can be obtained using ion-implanta tion and Zn, Ge co-doping [65]. As-grown p-type CGS single crystals are first doped by Ge-implantation and th en heated in vacuum. Finally, annealing of the implanted samples in Zn atmosphere re sults in n-type conduction of CGS [66]. Sulfide-based Chalcopyrites CuInS2 has an optical band gap (Eg) of approximately 1.5 eV [ 67], which is an excellent match for the solar spectrum. Its high absorption coefficient, = 104 cm-1 (at = 500 nm) is also very good for solar cell devices [68]. Masse et al. calculated a maximum solar energy conversion efficiency of 28% for a CuInS2 homojunction [69], but to date, the highest reported efficiency for a solar cell is about 12% [70]. Although CuInS2 solar cells are theoretically expected to possess higher effici encies than CIGS, sulfides have reached only about 60% of the performance of selenides so far. Selenide-based chalcopyrite solar cells that are slightly copper poor have shown the best efficiencies, but sulfid e absorbers must be prepared Cu-rich to produce a working device [57]. CuGaS2 has a band gap Eg = 2.5 eV [69], but it has not been used in the PV industry to date [71].

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44 Deposition Processes The progress of the solar industry depends not only on conversion efficiency, but also on the development of techniques, which must be conducive to producing large-area devices at a low cost. The absorber deposition method gene rally has a significant impact on the resulting film properties as well as on production cost. Common thin film deposition methods for CISbased solar cells are co-evaporation from elemen tal sources, selenization of metallic precursor layers, evaporation from compound sources, ch emical vapor deposition, closed-space vapor transport, and low-temperature liquid phase me thods like electrodepositio n, spray pyrolysis, and particle deposition techniques. Siemens Solar Industries (SSI) was the first co mpany in the world to produce CIS modules using the selenization process. Low temperatur e (200C) CIS precursor deposition is followed by high temperature selenization [72]. At 500C, a complete recrystalliza tion of the precursor film occurs [73]. The material qua lity of the absorber is determin ed by the structural features of the precursor and the experiment al conditions during se lenization. Some disadvantages of this process are that it involves complicated interm ediate phases, interdiffusion, and reaction, which can affect the controlla bility of the film quality. Usually high-quality I-III-VI2 absorber thin films are prep ared on a laboratory scale by physical vapor deposition (PVD) or molecular b eam epitaxy (MBE). These techniques require high temperatures for the source metal evaporati on [74]. Molecular beam epitaxy is used to grow CGS epilayers [45] while two and threestage co-evaporation are the benchmark methods for depositing polycrystalline CIGS. Although th ese growth methods are relatively easy to implement on a small R&D scale, scale-up to a co mmercial level proves to be challenging. High-quality CIGS deposition requires a high subs trate temperature (>500C) which limits the selection of substrate material s and decreases throughput due to heat-up and cool down periods.

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45 Co-deposition of the elements requires precis e control of the flux of each element and an overpressure of chalcogens (Se or S) during deposition, which result s in low material utilization and high equipment maintenance costs. An alte rnative low temperature route to the formation of CIS is the rapid thermal processing of stacked metal/Se layers [75]. Most PVD deposition techniques are quite wa steful of materials, but printing or electrodeposition techniques are quite efficient in depositing materials. The co-electrodeposition technique [76], where Cu-In-Ga-Se species are pres ent in the same chemical bath, is a simple process to prepare low-cost thin films. It is crucial to control the de position parameters like pH, chemical bath composition, deposition time, depos ition temperature, and the applied potential, owing to their influence on th e film properties and quality. Printing, spraying, or coating of inks involves the depositi on of particulate precursor materials onto substrates at low temperatures and the subsequent sintering under chalcogen overpressure. The reaction kinetics of nanopartic le-derived CIGS precursor films, typically 0.52.0 microns, is much different than those prep ared by evaporation [77]. ISETs non-vacuum process utilizes nanoparticles of mixed oxides of Cu, In, and Ga with a fixed Cu/(In+Ga) ratio that are synthesized into precursor inks [78]. Multijunctions The major reason for losses in simple photovoltaic devices is the inefficient use of the solar spectrum by cells that have only one built-in electric field. Some proportion of the sunlight is not used because certain photons do not have enough energy to be absorbed and to free electrons. For those photons that do have enough energy, ther e is no distinction betw een them; they are all treated as if they have just enough energy to free an electron. The band gap at which these spectrum-driven losses are smallest in single-junc tion cells is about 1.4 eV [8]. A range of band gap values at which losses are still manag eable extends from about 1.0 eV to 1.8 eV.

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46 Theoretical Multijunctions The single junction thermodynamic limit for solar cell conversion efficiency was determined to be 32% by Shockley and Qu eisser [79], but the pr actical lab limit of polycrystalline thin film solar cells is about 20% under 1-sun illumination. The major assumption in the calculation of the theoretical limit is that electrons and holes created by the absorption of photons with energies above th e band gap lose thei r excess energy by phonon emission. One way to achieve efficiencies above the Shoc kley-Queisser limit is to use a series of semiconductor pn junctions arranged in tandem configuration. The efficiency of solar cells can be significantly increased by stack ing several cells with different band gaps such that the gap energy decreases from top to bottom. This multijunction cell uses more than one electric field to separate electrons and holes. Light is inci dent on the top cell wh ich has a high band gap. Photons with energies greater than the band gap of the top cell are absorbed while those with lower energies pass through to the next semiconducto r where they are absorbed if their energy is greater than the band gap of that cell. Thus, the solar spectrum is split so that photons are used more efficiently; losses due to the mismatch be tween the energies of the photons and the cells band gap are reduced. Two cells in series connection have a maxi mum theoretical efficiency of 41.9% and with a larger number of cells, 50 % efficiency can be exceeded [8]. The thermodynamic limit for solar energy conversion is significantly higher s till, 66% at 1-sun and 86% at full solar concentration (46,200 suns), for an infinite tandem [21]. The grand challenge is to push solar cell efficiency towards its theoretical limit while maintaining low cost. The efficiency benefit of a tandem solar cell to that of a single junction has been known for quite some time, but it has only been practica lly observed in expensive crystalline III-V materials. Multijunction cells under concentr ated light have just recently exceeded 40%

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47 efficiency (Spectrolab). Traditionally, they have been used to pow er satellites and other spacecraft. The use of multijunction cells to genera te clean energy for terrestrial applications has been sought because, when combined with hi gh concentration, multijunction cell modules have the potential of producing the lowest $/wa tt amongst solar cell technologies [80]. Coutts et al. identified optimum band gaps for two-junction tandem thin film solar cells. A current-matched, 28% efficient tandem is possibl e with a top cell absorber of 1.72 eV and a bottom cell absorber of 1.14 eV [81]. These ba nd gaps are ideally matched to the CIS-CGS material system. Low Ga content CIGS has th e band gap and performance to be the low gap cell. The wide band gap top cell material of the tandem is critical; it is estimated that approximately two-thirds of the tandem cell effi ciency originates ther e [82]. A high band gap, transparent top cell with efficiency greater th an 17% is needed to form a tandem with an efficiency of at least 25% [83]. Tandem Structure In typical CIGS thin film solar cells, meta llic Mo back contacts are used, which makes it impossible for light to pass through this layer. Ho wever, a semitransparent solar cell is required for the top cell of tandem devices. Nakada et al. report that the cell performances of CIGS devices incorporat ing tin oxide (SnO2) and indium tin oxide (ITO) b ack contacts are similar to those using molybdenum [84]. The superstrate c onfiguration, where the glass substrate is not only used as a supporting structure, but also as a window for illumination, has an advantage of easy and reliable encapsulation. Since the diffu sion of sodium from the soda-lime glass is strongly inhibited by the front cont act in this design, Na-doping is necessary. The addition of Na from co-evaporated Na2Se has been reported to more than double the efficiency in superstrate cells [85].

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48 Monolithic vs Mechanical There are pros and cons to monolithic or mechanical tandems. In a two terminal device, the stacked cells are connected at a common boundary where the bottom contact of one is the top contact of another. Current flows continuously between the cells under illumination. Using this monolithic approach, only one thic k transparent conducting oxide (T CO), one grid, and one antireflective coating (ARC) would be needed. Ho wever, current-matching and thermal stability issues arise. The lowest current will limit the entire device so band gaps must be chosen that split the spectrum equally: half of the sunlight absorbed on the top and half transmitted to the bottom cell and absorbed there. Several difficult technical issues need to be addressed in order for high efficiency monolithic tandem cells to be developed. The botto m, first deposited, cell must not be destroyed by the processing conditions of the top cell. Hi gh-efficiency CIGS devices are vulnerable to temperatures greater than 200 C where diffusion destroys th e pn junction. Therefore, a successful tandem device fabrication procedure will require a bottom cell that is not affected by the processing conditions of the top cell or a top cell that can be grown at a much lower processing temperature [86]. Some clever tandem structures are being investigated because of the need to grow thin films at temperatures greater than 500C to obt ain high-quality absorbers [87]. Another critical issue for a monolithically interconnected tandem cell is providing a transparent interconnect between the top and bottom cells [86]. The mechanical stack may appear much simpler, but there are other i ssues involved. In a four-terminal device, each cell has a top and bottom contact connected to an external circuit so their output is taken off separate ly. Performance of each cell is independent so the spectrum doesnt need to be split between them. More materials (ARCs, TCOs, and glass) are needed for the overall structure, which increases the cost [82].

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49 Figure 2-1. Spectral irradiance vers us wavelength under AM0 and AM1.5 conditions. Figure 2-2. Photovoltaic system.

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50 Table 2-1. Efficiencies of copper chalcopyrites. Material Band gap (eV) Theor. (%) Achieved (%) CuInSe2 1.0 [29] 25 [57] 15.0 [28] CuIn0.72Ga0.28Se2 1.1 [28] 27.5 [57] 19.5 [28] CuInS2 1.5 [67] 28.5 [71] 12.2 [70] CuGaSe2 1.7 [43] 26 [49] 10.2 [28] Cu Se In a c Figure 2-3. Chalcopy rite structure of CuInSe2.

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51 Figure 2-4. CIGS/CGS monolithic tandem device structure.

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52 CHAPTER 3 ABSORBER GROWTH AND DEVICE FABRICATION Growth Calibration Fabricating and testing a working solar cell requires multiple steps including equipment calibration, the deposition of multiple layers, an d film and device characterization. When the source materials are depleted, the reactor mu st be shut down and brought to atmospheric conditions. Copper (Cu) is annealed in a hydroge n furnace to remove the oxide film present on the Cu pellets received from the supplier. The oxides have a higher melti ng point than copper so their presence in the source may cause sputte ring and thus non-uniformity in the flux distribution. The indium (In) and gallium (Ga) s ource materials are introduced into the vacuum system in the same condition that they are receive d from the manufacturer. To add Cu or In to the reactor, the respective source shutter must be disconnected while an optical port must be removed to add Ga. After the source material is replenished, the Sentinel III rate controller, using Electron Impact Emission Spectroscopy (EIES) sensors, is calibrated by Quar tz Crystal Monitors (QCM). Both the Cu and In sources are equipped with an EIES optical sensor located adjacent to the rotating platen at substrate level and a QCM sensor located directly above the source center at a fixed distance above the substrates EIES is a system of evapor ant excitation by electrons that uses the optical intensity of th e subsequent de-excitation as a m eans of process control. These sensors are used for online measurement while th e QCMs provide an absolute value of the flux for the calibration of the optical sensors and can not be used online because they are located directly above the substrates. The Ga source is equipped with a single sensor, which is a QCM that is in an identical position to the EIES sens ors used for the Cu and In sources. QCM rate control of gallium was inadequate so a temperature control scheme was implemented. Once the

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53 Sentinels parameters are adjusted, films can be deposited based upon the deposition rate sensed by the EIES sensors. Before a growth run series is initiated, the Cu and/or In deposition ra tes must be calibrated with a specific Ga source temperature. Assume d Cu-rich, Ga-rich, and near-stoichiometric thin films, typically 0.25-0.5 microns, are grown a nd the composition is measured through ICP analysis. It is assumed that the Ga deposition ra te is fixed throughout a ru n since the temperature is manually controlled to a specific value. Cu an d/or In rates can be ad justed based on the ICP results of the previous run. For example, an aver age Cu rate is determined from the previous run and is divided by the Cu/Ga ratio verified from ICP to give the Cu rate needed to produce stoichiometric CGS. The Cu rate for the curren t run can be adjusted a ppropriately to give the desired overall composition. ICP feedback re sults must be maintain ed throughout a growth series because reactor conditions may change This procedure is only as good as the repeatability of reactor cond itions between successive runs. Standard Growth Procedure System startup is a lengthy process during whic h stringent guidelines must be followed to ensure proper operation of the reac tor. A cryotrap is filled with liquid nitrogen so that the reactor chamber reaches a certain crossover pressure to safely switch the pumping to the diffusion pump, which is necessary to get to high vacuum. As th e trap fills, samples can be loaded into and unloaded out of the reactor through the load-lock. After switching the sy stem into high vacuum, the ionization gauges are degassed for a minute to re move any deposits from them. The platen is then started to its desired rotating speed, which is typically 12 rotations per minute. Platen rotation must be started before the substrate heat er is engaged so as to not warp it. The PMEE reactor Supervisory Control Pa nel is then opened on the att ached system PC. Film predeposition parameters are set for the metals such as soak power, rise time, and soak time.

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54 Heating layers are also set to determine the or der in which the heaters are turned on and how much power should be supplied. When the appropriate parameters are ente red into the LABVIEW program, heating can begin. The pyrolytic boron nitrid e (PBN) substrate heater is st arted first and brought up to the growth temperature. The inputte d temperature is actua lly the temperature in the gap between the heater and the platen as a thermocouple cannot be directly placed on the ro tating platen. As the PBN heaters power is increased, the Cu tip is turned on and the power is gradually increased manually. Before turning on the selenium source heaters, the cryoshrou d that surrounds the metal sources is filled with liquid nitrogen. The cryoshroud helps keep the excess Se in its designated reactor zone. First the cracker is heat ed and then the crucible. As the Se crucible approaches its final temperature set point, th e metals primary heaters are switched on. The practice for source preparation in this MBE system before initiating deposition is to hold the sources at a particular soak power for a set pe riod of time. The metal sources first go through a period of rising temperature and then a soaking period so that the solid metal sources become melts. Since the Ga source is manually temperat ure controlled, control is taken over manually after an initial rise time and the power is adjusted to reach the Ga temperature set point before deposition. The Ga shutter needs to be manually opened when the other metals shutters open automatically after their soak period. If a certain metal source is not bein g deposited in the film, its heaters are not turned on a nd its shutter remains closed th roughout the run. Selenium is depositing on the substrates as they pass th rough the chalcogen zone as the Se crucible temperature approaches its final value prior to the start of metals deposition. Startup time leading up to deposition is approximately two hours.

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55 The deposition time begins once the metal shu tters are opened and metal beam fluxes are impinging upon the rotating substrates. Cu and In rates are controlled by adjusting the local set point and corresponding offset. Desired element deposition rates are set in advance and layer thickness is controlled by adjusti ng open shutter time. The average deposition rate over a certain period is calculated and these parameters can be adjusted appropriately to achieve the desired composition. The Se source is kept at a constant temperature during evaporation. The power of the Ga source is manually adjusted to maintain temperature control sin ce QCM rate control was ineffective. The reactor conditi ons are closely monitored with the cryoshroud periodically being filled throughout the run. Different growth stra tegies can be administered by closing certain metal shutters during the growth run; selenium is supplied in excess while the Cu and In rates can be adjusted. Dopant NaF can be added at a c onstant rate for a set pe riod of time. When the appropriate thickness is reached, the metals shutte rs are closed and the heaters are shut down. Se can be deposited in an annealing procedure un der the designated growth temperature for a set period of time as the metal heaters cool down. Otherwise, power to the Se crucible and the substrate heater are decreased at the end of metals deposition. When all heaters are cooled to below 200C, which typically takes at least an hour and a half, the system is taken out of Hi-Vac. Once the system cools completely, the grown films can be removed from the reactor by way of the load-loc k. Films to be used for device fabrication are immediately vacuum sealed to isolate them fr om the atmospheric cond itions prior to buffer deposition. The films are also re-sealed after the buffer laye r is added and prior to ZnO sputtering. One sample can be cut up and used for absorber characterization. Sometimes, the samples were exposed to a normal room temp erature air ambient for over a month between

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56 deposition and analysis. This wa s also the case for absorbers used for device fabrication prior to the purchase of a vacuum sealing system. Growth Schemes High quality absorber layers th at are well-controlled are essential to the fabrication of high efficiency solar cells. Thermal evaporation pr ocesses have been mainly designed on a basis of experience and intuition to grow polycrystalline thin film layers [88]. Therefore a fundamental understanding of CIGS film deposition is n ecessary to design the best absorbers. Single Stage In the simplest single-step process, all rates as well as the substrate temperature are kept constant during the whole process. A one-stage process typically produc es low-quality material when compared to the bilayer or three-stage processes. In CIGS growth, three-stage coevaporation leads to an absorber with a graded band gap, while single step co-deposition results in a uniform band gap [89]. Bilayer The first growth strategy used to synthesize highly-efficient CuInSe2 films was developed at Boeing by Mickelesen and Chen. In the Boei ng bilayer process, a two-phase film containing CIS and Cu2-xSe is first deposited at low temperature a nd then reacted with a Cu-deficient flux of co-evaporated Cu, In, and Se vapors at a higher temperature. The precursor deposition and regrowth chemistry are shown in the following equations [90]: Cu(v) + In(v )+ Se(v) CuInSe2(s) + Cu2-xSe(s) (3-1) CuInSe2(s) + Cu2-xSe(s) + Cu(v) + In(v )+ Se(v) CuInSe2(s) (3-2) Copper-rich material tends to form larger grains. This is typically true above approximately 525C because of a liquid phase assi sted re-growth process due to the melting of Cu2-xSe in the presence of excess Se. Thus, by fi rst depositing a layer containing excess copper,

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57 larger CIS grains are formed. In-rich layers generally have smaller grains, but when grown on top of Cu-rich layers they ar e inclined to conform to the same growth pattern [8]. Three-Stage In the mid 1990s, NREL developed the threestage process to grow high-quality CIGS films [91]. Indium, gallium, and selenium are evaporated at 260C to form a (In,Ga)2Se3 precursor. The temperature is then ramped up to 550C within a Se flux. At this point, sufficient Cu is co-deposited with Se to make the film Cu -rich. Additional In, Ga, and Se are added to bring the overall compositi on back to Cu-poor. The amount of In and Ga deposited in the third stage is usually 10% of the total in the first and th ird stages combined. The film is finally cooled down within a flux of Se at about 350C. Th e three-stage process is based on the following reaction chemistry (precursor deposi tion, re-growth, and titration) [90]: In(v )+ Se(v) In2Se3(s) (3-3) In2Se3(s) + Cu(v )+ Se(v) CuInSe2(s) + Cu1-xSe(l) (3-4) CuInSe2(s) + Cu1-xSe(l) + In(v )+ Se(v) CuInSe2(s) (3-5) The intermediate Cu-rich growth stage has b een shown to be beneficial to the morphology and electronic quality of CIGS layers. The Cu2-xSe secondary phase has a higher emissivity in the IR range than Cu-deficient CIGS, and the in creased emission of heat radiation leads to a lowering of the substrate temperature. Cu2-xSe segregations begin to appear once the film reaches stoichiometric composition, i.e. [Cu]/[III] =1.00. This is reflected in a drop of the substrate temperature, which is recorded by the thermocouple on the sample rear side. When the substrate temperature is ramped up to 550C after the completion of the first stage, the PID temperature controller is cut off a nd a constant heating power is s upplied [92]. The third stage is

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58 terminated when the temperatur e reading reaches the value reco rded before the film became stoichiometric during the second stage. Growth Strategies in the PMEE reactor Three different growth recipes were i nvestigated for the deposition of CuGaSe2 absorber layers. Another growth strategy was employed in th e reactor during the earlie st investigations of CGS growth, an initial Cu-rich layer followed by a Ga-rich layer similar to the Boeing bilayer process, but it never re sulted in quality devices. We refe r to each strategy as follows: Constant Copper Flux Process (Figure 3-1A) Modified Three-Stage Process (Figure 3-1B) Emulated Three-Stage Process (Figure 3-1C) The Constant Cu Flux Process is illustrated in Figure 3-1a. The selenium crucible is maintained at a constant temperature, typically 2 65C, so that selenium is provided in excess for all film growth in our PMEE reactor. Since temp erature control is used for gallium deposition, we maintained a specific gall ium temperature for each growth run. Thus, we can change the overall composition of the absorber by manipula ting the copper flux. Calibration runs were performed to determine the relationship between the average copper deposition rate and the Cu/Ga ratio. This strategy simply keeps the same Cu flux over the entire growth run so that the absorber maintains either Cu-richness or Ga-r ichness throughout the deposition of the film. Figure 3-1b shows the Modified Three-Stage Process. This growth recipe starts by depositing GaSe for a set period of time, followe d by a Cu-rich layer, and ending with a Ga-rich layer. Since the Ga temperature is maintained for the complete growth run, the Cu flux must be adjusted to achieve either a Cu-rich layer or a Ga-rich layer. The overa ll composition and peak

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59 Cu-richness can be adjusted by varying the durat ion of growth and the Cu flux employed for each step. The Emulated Three-Stage Process is illustra ted in Figure 3-1c. In contrast to NRELs approach, our Emulated Three-Stage Process does not use end-point detection, but is based on the composition results of previous runs. The thermocouple for substrate temperature measurement is placed in the gap between the heater and the plat en for temperature measurement since it could not be placed in contact with the substrate because of the rotating nature of the platen. An additional possibility to control the composition is to monitor the emissivity of the films, but problems with pyrometry were pres ented by selenium condensation on the optical ports. In this recipe, we deposit GaSe for a certain amount of time and then deposit CuSe until we reach the desired thickness. The greatest coppe r-richness is reached at this point, and then GaSe is deposited until the overall composition becomes Ga rich. The gallium temperature remains constant throughout the first and third st ages, while the same Cu rate is maintained during the second stage. Absorber Characterization Characterization of the absorber film is integr al to the production of high quality devices. Many research groups have implemented in-situ t echniques to observe the growing film, but this is not possible inside our reactor. Typically, one absorber is set aside strictly for characterization purposes. The techniques described below are used extensively within this research. ICP Inductively Coupled Plasma-Optical Emissi on Spectroscopy (ICP-OES) is most commonly used for bulk analysis of liquid samples or solid s dissolved in liquids [93]. The ICP operates on the principle of atomic emission by atoms ionized in an argon plasma. Photons are emitted as electrons return to the ground st ate of the ionized elements, wh ich allows for the quantitative

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60 identification of the species that are present. The strengths of ICP-OE S include its speed, low detection limits, and relatively small interference effects, but it is a destructive technique that provides only elemental compos ition. Calibration curves must be made using a series of standards to relate emission inte nsities to the concen tration of each elemen t of interest. The Perkin-Elmer Plasma 3200 ICP system used in this study is located in th e Particle Engineering Research Center, University of Florida. This system is capable of analyzing materials with a detection limit range of less than 1 part per million. Known concentrations of Cu-In-Ga-Se dissolved in solution (0, 1, 5, and 10 ppm) are used to create a calibration curve fo r ICP characterization. A small piece of the characterization absorber, typically 2 cm x 1 cm, is dissolved in a 10 mL nitric acid solution. After the film reacts for a few hours, the solution is diluted with 50 mL of deionized water. The overall composition determined from this small sample may not be representative of the entire film if it does not have uniform morphology. SEM Scanning Electron Microscopy (SEM) can be used to determine the grain size and shape of absorber films [94]. The SEM is commonly used for image analysis by focusing a source electron beam into a fine probe and rastering ove r the surface of the samp le. Secondary electron and backscattered images are obtained to provi de the surface topographical information. SEM, using the SEM JEOL JSM 6400, characterizatio n measurements were done at the Major Analytical Instrumentation Center Department of Materials Scien ce and Engineering, University of Florida. XRD X-ray diffraction (XRD) is a powerful technique used to uniquely id entify the crystalline phases present in materials and to measure the structural properties of these phases [95].

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61 Polycrystalline thin films can have a distribution of orientations, which in fluences the thin-film properties. When sizes of crys tal grains are less than about 1 00 nm, x-ray diffraction lines will become broadened. Hence, grain size can be estimated by measuring the broadening of a particular peak. XRD is noncontact and nondestruc tive, which makes it ideal for in-situ studies. Characterization measurements, using the XRD Philips APD 3720, were done at the Major Analytical Instrumentation Center Department of Materials Scien ce and Engineering, University of Florida. The most important use of thin-film XRD is phase identification. XRD provides positive phase identification by comparing the measured d-spacings in the diffrac tion pattern with known standards in the JCPDS Powder Di ffraction File. Some thin film s have a preferred orientation, but the JCPDS file contains measurements for fi lms with random orientations so there can be some disagreement between the measured values and the standard. For films possessing several phases, the proportion of each phase can be determ ined from the integrat ed intensities in the diffraction pattern. Device Fabrication The most commonly used structure for CIS-based solar cells is the substrate configuration; the absorber layer is evaporated on Mo-coated glass, and on top of this is a thin CdS buffer layer and a transparent ZnO front contact. The univers ally accepted device design for fabricating high efficiency, thin-film CIGS solar cells: MgF2/ZnO/CdS/CIGS/Mo/SLG is shown in Figure 3-2 [96]. Substrate and Back Contact Soda-lime glass (SLG) is commonly used as th e substrate for high-e fficiency solar cells, but it deforms at the temperatures used for hi ghest device efficiencies, 550-600C. Soda-lime glass contains significant amounts (15.6 wt %) of sodium in the form of Na2O [97], and it is

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62 typically coated with molybdenum, which serves as the back contact. When the substrate temperature approaches the softening point of th e glass, Na ions diffuse from the glass through the Mo back contact into the grow ing CIGS film. The extent of Na diffusion is related to the Mo sputtering pressure. At low pressure, the amount of Na out-diffused from the SLG is low, while at the highest pressure, the amount of Na out -diffused exceeds the optimal value required for high-quality devices due the formation of microvoids and microcracks [97]. For good electronic device properties, the forma tion of an ohmic contact for the majority carriers (holes) from the p-type CIGS and a lo w recombination rate for the minority carriers (electrons) at the CIGS/back contact interface is e ssential. The back contact should be inert to the highly corrosive environment during depos ition and it must impede the diffusion of impurities from the substrate into the absorber. Fina lly, a high optical reflectance is necessary to minimize optical losses. Molybdenum, the historical back contact material for CIGS solar cells, complies well with most requirements; it is iner t during deposition, allows for the growth of large grains, and forms an ohmic contact via an intermediate MoSe2 layer [98]. CIGS can also be deposited onto various substrat es other than glass, even flexible ones. Stainless Steel (SS) can be heated up over 500 C which is necessary to achieve high-quality absorbers, but Na-doping is needed since SS doe snt contain sodium like soda-lime glass. Unlike metal foils, polymer substrates are electrically insulating, which simplifies monolithically-integrated module fabrication. Ho wever, polymer substrates have a limited maximum operating temperature and can cause a dhesion problems between the CIGS film and the Mo back contact [99]. St ainless steel substrates have generated CIGS films with 17% efficiency [100] while applying a low temper ature, 450C, CIGS deposition process and a

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63 reliable method for controlled Na incorporation on polyimide substrates has yielded cells with 14% efficiency [101]. Post Absorber Deposition Exposure of CIGS absorbers to the atmosphere for significant amounts of time prior to the buffer layer deposition step leads to surface oxi dation. Yamada et al. observed large oxidation rates for polycrystalline films; the surface of the film is likely to be oxidized to a depth of a couple nm in a brief time after removal from the growth chamber [102]. Ox idation of the surface can also lead to large changes in resistivity of up to three orders of magnitude in just a few days [103]. This shows clearly that it would indeed ma ke a difference if the application of the buffer layer and the completion of the solar cells is done immediately after absorber deposition, a few days, or even a few hours later. Nadenau reported a dramatic decrease in cell performance if the CGS layers were exposed to air for one day prior to the CdS deposition [104]. CIGS grown under a large Cu-excess condition contains copper seleni de phases at the surface and along the grain boundaries. A cyanide-ba sed chemical treatment is used to remove any secondary phases while being inert to the CI GS phase [105]. The remaining material after potassium cyanide (KCN) etching is expected to be the stoichiometric phase, but a rough surface may remain [102]. This type of morphology may affect the device performance adversely because it can lead to poor metallurg ical contact at the interface [106]. Buffer Layer The main purpose of the buffer layer in a solar cell device is to act as barrier to diffusion of impurities from the transparent conducting oxide la yer into the absorber. Other benefits may include interface passivation and the establishment of an inverted region in the absorber [107]. The properties of a buffer layer often depend on th e deposition technique us ed in its fabrication and the ability to control the grow th parameters for each technique.

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64 Most high-efficiency CIGS device structures employ a high-resistivity CdS buffer layer deposited by chemical bath deposition (CBD). Because the band gap of CdS is too low (2.4 eV) to permit transmission of all useful light, a balanc ing act is employed to optimize the structure. If the CdS layer is too thick, unacceptable absorp tion occurs, leading to the reduction in short circuit current (JSC). If the CdS layer is too thin, shunt paths are generated, which leads to a decrease in the open-circuit voltage (VOC) [108]. The high efficiencies that have been achieved by the CBD process are the result of a set of cr itical interactions that can produce n-type doping or inversion, compositional grad ing, and interface pa ssivation [109]. The direct diffusion of cadmium into the CIGS layer has been observed a nd this may lead to the formation of a buried pn-junction [110]. A CBD bath temperature at 80C instead of 60C, which is the common standard for CIGS, is employed for CGS devices. The growth sp eed increase due to the elevated temperature so the concentrations in the solution are modi fied. The quality of the buffer layer and the interface with the absorber are improved by the 80 C procedure [111]. Chemical bath deposition also affects the defects in the bulk of the absorb er material so tunneling recombination is reduced in samples grown with the highe r CdS deposition temperature. There are a few problems associated with cadmium sulfide technology. The band gap of the CdS layer is still low enough to limit the short wavelength part of the solar spectrum that can reach the absorber, and this leads to a reduction in the current that can be collected. This current reduction becomes proportionally more severe fo r higher band gap cells [112]. The substitution of the heavy metal compound, CdS, is also desi rable from an environmental and economical point of view. A large-scale CBD-CdS buffer deposition process creates extra costs for the

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65 necessary safety precautions needed for the handli ng and disposal of toxic material, especially for such an inefficient and exceedingly wasteful process [113]. Alternative Buffers An alternative buffer layer such as ZnS is at tractive due to its wi de optical band gap and reduced ecological issues. By widening the Eg beyond that of CdS, a higher short-wavelength quantum efficiency is expected in CIGS solar cells, thereby increasing the short-circuit current [114]. Using a ZnS CBD buffer, a champion ce ll of 18.6% was achieved by Hariskos et al. [115]. Based on Andersons model of heterojunctions, the electron affinities of both layers should match in order to obtain the maximum built-in po tential and hence a high open-circuit voltage. The electron affinity of CdS (4.5 eV) is larger than that of CGS (3.9 eV) so current loss can be minimized by mixing ZnS with CdS to form a ternary compound ZnxCd1-xS in which the electron affinity can be varied from 3.7 eV to 4.5 eV. Electron affinities are e qual at about x = 0.78 and any variation in the buffer layer composition from this optimum value results in a deterioration of the cell performance [105]. VOC increases with increasing Zn concentration whereas the JSC decreases. Ramakrishnu et al. produced a CGS cell with moderate efficiency ( ~ 5 %) with a Zn composition fixed at x = 0.5 [105]. Due to good lattice matching and to ideal elect ronic band offsets, ZnSe is expected to provide a perfect buffer layer for CGS [116]. For the CdS/CGS interface, the CdS conduction band minimum is below that of CGS resulting in a cliff structure. Devi ces with this type of band alignment show interface recombination domi nated behavior, and hence suffer from a loss in VOC. In order to avoid this eff ect, a material with a smaller valence band offset and a larger gap is required. ZnSe has a band gap of 2.7 eV and the lattice constant s of ZnSe and CGS are closely matched, which should result in an almost strain-free interface [117]. Rusu et al.

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66 produced ZnSe/CGS heterojunctions with very hi gh voltages, but poor conversion efficiency due to very low current (VOC ~0.96 V and JSC ~2 mA/cm2) [118]. Window Layers From the aspect of band alignment, the II-VI buffer layer could be omitted, but solar cells prepared with a direct CIGS/ZnO heterocontact show only poor efficiencies [119]. Ramanathan et al. observed that the direct sputtering of ZnO on CIGS typi cally yields only 2-5% devices [120]. These cells are characterized by enhanced current losses probably due to tunneling or recombination processes via trap levels associated with impurities that diffuse into the absorber during transparent conducting oxide (TCO) deposition [107]. The basic properties for making high quality tr ansparent conductors are high conductivity, high optical transmission, minimal surface roughn ess, thermal stability to withstand the processing temperature, chemical stability, and cr ystallinity. Typical TCOs used in solar cell fabrication have band gaps in the range of 3.3-3.8 eV, carrier concentratio ns in the range of 10201021 cm-2, a conductivity of 104 (ohm-cm)-1, a sheet resistance of about 5-10 ohms/square, and an optical transmission greater than 85% over the visi ble part of the spectrum [121]. Almost all of the well-known TCOs that are used in solar cell devices, such as ZnO, In2O3, and SnO2 have ntype conductivity [122]. In the fabrication of CIGS solar cells, it is customary to use a high/low resistivity grading of the ZnO layer. An undoped layer of ZnO (high resistivity) is first deposited on CdS, followed by the deposition of a doped layer. Ramanathan reports that solar ce lls made without the undoped ZnO layer are identical to those made w ith the bilayer so the undoped ZnO layer may be unnecessary even when CdS is very thin [89].

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67 Metallization To collect the current, contacts are placed acro ss the entire surface of a PV cell. This is normally done with a grid of metal strips. Howeve r, placing a large, opa que grid on top of the cell shades the active parts of the cell from the sun so they are designed with many thin, conductive fingers spreading to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well with low resistance, but thin en ough not to block much of the incoming light. Ni/Al grids are deposited by e-beam evaporation using a mask. Cell areas are then delineated by mechanical scribing to give individual cell areas of 0.429 cm2 and a final In contact is soldered on after the film is scratche d away to reveal the Mo back contact far away from the grid. Anti-Reflective Coating Bare solar cells can reflect about 30% of the sunlight. Since the power output is proportional to the amount of sunlight that is ab sorbed, these losses are detrimental to the device performance. Surface reflection loss can be reduced by adding an ti-reflection coatings (ARC) to the solar cell. An ARC is typically deposited onto CIGS devices by the E-beam evaporation of MgF2 to a thickness of 800-1200 Angstroms. The gain in short-circuit curr ent is typically 4-8% with a corresponding enhancement in conversion efficiency [106]. Device Characterization Current-Voltage Current-voltage (I-V) analysis is a critical tool used to st udy solar cell performance. The electrical parameters, includi ng the conversion efficiency ( ), open-circuit voltage (VOC), short circuit current density (JSC), fill factor (FF), series resistance (RS), shunt resistance (RSH), diode ideality factor (n), and sa turation current density (Jo), of a device can be determined from the measured illuminated and dark I-V curves. The conversion effici ency is defined by

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68 OCSC INFF V I = P (3-6) PIN is the total power of incident light. Considering the general expressions for VOC and ISC, the key material parameters that determine the e fficiency of the solar cell are the lifetime and mobility of the minority charge carriers a nd the surface recombination velocities [22]. The power that a cell provides is a product of its operating current and voltage. Under short-circuit conditions, current is maximum, but voltage is near ly zero so almost no power is provided to the circuit. Under ope n-circuit conditions, voltage is highest, but no current flows so power is again zero. Fill factor is the percen tage of maximum power as compared to the product of open-circuit voltage and short-circuit current. So me cells can have good VOC and good JSC, but a poor FF, which results in not much power a nd a low efficiency. The series resistance can affect the shape of the photo I-V curve, mainly the FF [9]. To make a good measurement, two other paramete rs are controlled: total power in the light and the temperature of the cell. Standard power is 100 mW/cm2, which is approximately the power density of sunlight at the Earths surface at noon on a cloudless day. The cell is held at a fixed temperature of 25C because cell voltage and thus power output varies with temperature. To get an accurate efficiency, the precise cel l area must be known because the amount of input sunlight depends on the cell area. Total area is th e total area of the top surface of the cell, while active area is the surface area of the cell without counting metal contacts even if those are on top of the active portion of the cell. The solar cell to be measured is exposed to simulated sunlight, and as a resistive load is varied from open-ci rcuit voltage through shor t-circuit conditions, the cells I-V characteristics are measured [8].

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69 I-V Measurement Technique The reference cell method, which basically uses a reference cell to adjust the illumination level of the solar simulator, is employed in the performance measurement of CIGS (and CGS) solar cells in this study. The solar simulato r intensity is adjusted by changing the distance between the tungsten-halogen lamp and th e test plane so that the measured JSC of the reference cell is equal to its cal ibrated value at the standard m easurement intensity of 100 mW/cm2. We use a CIGS solar cell calibrated against a pr imary reference cell and the global reference spectrum by NREL to set the illuminati on level of the solar simulator. The open-circuit voltage of CIGS solar cells decreases with increas ing temperature at 100 mW/cm2. The temperature of the test cell is ma intained at 25C 1C by a temperature controller that circul ates cooling water through the asse mbly during the illuminated I-V measurement. The temperature controller of the cooling system is set at 20C to keep the reading of the thermocouple and hence the temperat ure of the test cell at 25 1C. The semiautomated I-V measurement system is contro lled by a personal computer with the data acquisition and data analysis software LabVIEW [123]. Quantum Efficiency Quantum efficiency (QE) is defined as the number of electron-hol e pairs generated per absorbed photon and is a measure of the effectiv eness of a cell in converting light of various energies into electricity [9]. The cell is illumi nated with monochromatic light while its electrical output is being recorded. We know the number of photons in the monochromatic light and we can measure the resulting electric current, whic h tells us how many elect rons are being produced by the cell. By slowly changi ng the monochromatic light to va rious energies, we can measure the cells response to the spectrum of solar photons. If the photons making up the monochromatic light have less en ergy than the cells band gap, th ey will pass through it without

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70 producing any current (QE = 0). Just above the ba nd gap of the cell, light will be very weakly absorbed and unless the materials diffusion length is very large, the quantum efficiency will be small. QE will begin to rise sharply as the ener gy of the incident photons is increased. In very good cells, quantum efficiency of over 90% can be reached across most of the solar spectrum [8]. QE Measurement Technique A spectral response measurement system empl oying a grating monochrom ator is used to analyze the quantum efficiency of our solar cells. The monochromator, which is controlled via a computer program written in LabVIEW, scans th e spectral range from 400 to 1400 nm using 10 nm incremental steps. Two order sorting filter s are use to block the undesired harmonic terms from the monochromator; one is applied for th e range from 630 to 1000 nm and the other for 1000 to 1400 nm. The incident power density on th e test plane is first measured by calibrated silicon and germanium photodetectors, and it is saved on the hard disk of the computer. The measured spectral response is calculated from the data stored in the computer previously and the measured photocurrent of the test cell (Itest cell( )). Finally, the external quantum efficiency as a function of the wavelength can be converted fr om the spectral response using the following equation [123] test cell detectortest cellhcI QE100% qpower densityArea (3-7) where h, c, q, and are Plancks constant, the speed of light, the electronic charge, and the photon wavelength, respectively.

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71 0 5 10 15 20 25 050100150200250300 Time (minutes)Rate (A/s)0 200 400 600 800 1000 1200Temp (C)Ga Cu Se A 0 5 10 15 20 25 050100150200 Time (minutes)Rate (A/s)0 200 400 600 800 1000 1200Temp (C)Ga Cu Se B 0 5 10 15 20 25 0100200300400 Time (minutes)Rate (A/s)0 200 400 600 800 1000 1200Temp (C)Ga Cu Se C Figure 3-1. UF growth reci pes. A) Constant Cu-Flux. B) Modified 3-Stage. C) Emulated 3Stage.

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72 Figure 3-2. Typi cal CIGS device structure.

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73 CHAPTER 4 COPPER GALLIUM DISELENIDE ABSORBER GROWTH Like other I-III-VI2 compounds, CuGaSe2 has a wide phase stability region. The existence range of CGS extends down to a Cu/Ga ratio of approximately 0.7 [53]. Optical and electrical properties are greatly affected by film composition because intrinsic point defects exist in the material as its composition deviates from stoichio metry. It is not surp rising that different research groups have presented slightly differe nt results depending on th eir specific preparation method since the defects present are strongly dependent on the growth method and thermal treatment [124]. Growth Matrix During the last few years, CGS films were gr own in order to optimi ze the performance of devices. Eight sets of CGS films were grown on Mo coated soda-lime glass substrates in the PMEE system under different growth conditions. The specific PMEE reactor conditions for each growth run described within th is document are available in A ppendix A. Growth temperature and recipe were adjusted for the films and the growth rate fluctuated from about 0.4 to 0.9 /s. The thickness estimated for each run was varied from 1.0 to 1.5 m based upon the total Cu thickness sensed by the Sentinel III. This estimation method ha s been a better predictor of thickness for films containing indium rather than gallium. ICP analysis of film composition gives the parts per million of each species in the solution. Knowing the area of the characterization sample and the density of each species allows you to determine a thickness estimate of the film. There is of course er ror involved in the ex act measurement of the characterization samples area, but this proce dure estimates the actual thickness range to be approximately 0.6 to 1.3 m. Each table related to the specific growth run series summarizes the growth process, as-grown composition and if ava ilable, the composition of the film after a KCN

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74 etch for each of the films. The column la beled growth process shows the intended composition of each layer of the film. The first set of films, samples 443 through 447 shown in Table 4-1, was deposited at 386oC. They were grown mostly gallium rich by following a procedure similar to Boeings bilayer process: Cu-rich deposition followed by a Ga-rich layer. Three different compositional stages were incorporated: an initial Cu-rich stage, an intermediate stage that varied from less Curich, to near stoichiometric, to Ga-rich dependi ng on the desired overall Cu/Ga ratio, and a final Ga-rich layer. The layers were approximately the same thickness with each constituting onethird of the total thickness. The growth rate for these films was approximately 0.5 /s and the film thickness was estimated to be about 1.2 m. Films with Cu-rich initial stages seem to give poor quality absorber films when grown in the PMEE reactor. We believed this may have to do with poor adhesion of these films to the molybdenum, like in Stacked Elemental Layer pro cessing where only gallium as the first layer led to good adhesion of the absorber layer to the Mo back contact [103]. Klenk et al. suggest that Cu is not useful as the first material deposited onto the Mo as it caused severe adhesion problems [125]. Films that star t and finish with Ga sequences should adhere better to the substrate and result in a more uniform morphology [126]. We intended to test this by changing our growth procedure to either include a Ga-r ich CGS initial layer or a thin GaSe layer. The second set of films, samples 452-459 show n in Table 4-2, were also grown at 386oC. The first part of this growth series consiste d of films grown overall copper rich followed by gallium rich samples. The intended growth pr ocedure was a Modified Three-Stage Process. A rather thin initial Ga-rich layer was deposited fo llowed by a Cu-rich layer. Most of the films had a thicker final Ga-rich layer deposited on them except for 452, which had only 2 stages, and 453,

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75 whose final layer was composed of GaSe. This Ga-rich layer was typically half the total film thickness. These films were grown at a similar rate to the previous set, but they were much thinner, approximately 0.8 m. Selenium annealing was performed for 30 minutes after metal deposition was complete at the growth temperature for all the samples. The Se flux remained the same as it was during metals deposition. Copper selenide is a degenera te p-type solar cel l found in CGS cells with overall Cu/Ga ratios greater than one. If present, the Cu2-xSe phase will tend to short-circuit the device, not allowing for a measure of device performance that is representative of the underlying absorber material quality. It must be removed before depo sition of the buffer layer [127]. A cyanide etch should remove any Cu2-xSe phase at the surface and between the network of grains in the film so we dipped the CuGaSe2 film in a 10% KCN solution for five minutes. After the removal of the Cu-Se surface phase, the composition of Cu-rich CGS films has been found to be near stoichiometry [35]. It has also been shown to increase the photoluminescence intensity of Curich grown CIGS film up to five times by Keye s [128]. However, the re moval of the secondary phase did not result in the Cu-rich sample becoming more like the (In, Ga)-rich samples since the dominant defects and recombina tion processes are inherent to the CIGS phase. This result should be analogous to CuGaSe2 samples. The third set of films, samples 472 to 480 shown in Table 4-3, consisted of films that were mostly gallium rich or near-stoichiometric. For many of the films, an initial layer of gallium selenide was grown. This was done to promote adhesion of the film to the substrate. The initial GaSe layer was followed by a Cu-rich CGS laye r, and a thin GaSe or Ga-rich layer was deposited on top. For two of these films, the Emulated Three-Stage growth recipe was used. The final GaSe layer was very thin so it is assu med that since the final composition is slightly

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76 Ga-rich for these two samples th at the film may have never b een Cu-rich during the growth procedure. The gallium primary temperature wa s increased to 1007C, compared to 975C that was used for the previous growth series, to increa se the growth rate. The growth rate for these samples ranged from 0.8 to 0.9 /s, except for the emulated three-stage process, which had a growth rate of about half that value. The film thickness was approximately 0.9 m. The same final Se vapor treatment was performed after deposition. In the PMEE system, the substrates are radi atively heated by a resistive heater located above the rotating substrate platen. This platen carries the substrates through each of the four zones in the system, the metals zone, the fluxl ess load-lock zone, the chalcogen zone, and the heater zone. A thermocouple is placed in the ga p between the heater and the rotating platen for temperature measurement. This temperature is then controlled during gr owth. Previous work had been done to determine the relationship between the gap thermocouple temperature, Tg, and the actual temperature, Ts, of the substrates, yielding the relationship [6]: Ts = 0.5247 Tg + 18.856 (4-1) It had been previously believed th at for the PMEE system, using a Tg greater than 700C may result in damage to the heater. However, the efficiencies of devices grown at this temperature were relatively low. It was decide d that a higher growth te mperature was needed to improve device efficiencies. After much invest igation, it was concluded that the system could likely be used safely at a higher temperature. CGS solar cells with the highest performance were for a long time based on Cu-rich composition, but the current record cells incorpor ate absorber films with an overall as-grown composition that is gallium rich. The disadvantages of an overa ll as-grown Cu-rich composition are a high doping density and a high concentra tion of electrically act ive traps [111]. Air

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77 annealing mainly diminishes the density of deep tr aps in Cu-rich CGS [52]. It is well known that the grain size of Cu-rich chalcopyrite films is la rger as compared to Cu-poor films [129]. The Cu-content of the film determin es the activation energy for gr ain boundary motion. Higher Cucontents lead to lower activation energies for grain boundary motion and therefore to the formation of larger grains [88]. A Cu-rich growth period has been deemed to have beneficial effects on device performance by some, but others have defined thes e benefits as limited to depositions at reduced temperatures or times. Comparing different fl ux profiles, it was shown that at 400C, Cu-rich growth is necessary to achiev e good performance in CIGS by Shafarman et al. At higher substrate temperatures, device pe rformance is insensitive to gr owth sequence allowing greater process flexibility [130]. Two-st age, rather than three-stage gr owth was utilized there. The simultaneous deposition of group I and III atoms dur ing the two-stage process may provide more time for the necessary reactions and lessen the be nefit of the Cu-rich gr owth period since the benefit of the Cu-rich growth period has been surmised to come from a fluxing of the CIGS grains by excess liquid Cu2Se [131]. The lack of quality absorbers obtained by deposition in the PMEE from overall Ga-rich composition is a perplexing phenomenon. It is po ssible that the films were not grown Ga-rich enough, but many were in the compositional existe nce range of high quality absorbers. It is possible that the processing conditi ons arrived at after years of op timizing the performance of the low-Ga absorber material are not optimal for use with higher Ga-contai ning films, especially when the Cu/III ratio falls much below 1 [128]. Since previous CGS absorber layers grow n by PMEE with overall Cu-rich compositions had shown higher efficiencies, subsequent film gr owth series were grown copper rich. Table 4-4

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78 displays the fourth set, which consisted of se ven CGS films. Samples 510-516 were grown at Tg = 750C, which corresponds to a substrate temper ature of 412C. These samples used growth recipes that were similar to the ones that resulted in our previous best cells at a slightly elevated growth temperature. The growth rate was si milar to the previous runs, about 0.4 to 0.5 /s, and the films were slightly thinner at 0.6-0.8 m. Cu-rich samples became nearly stoichiometric after etching them in a 10% KCN solution for 5 mi nutes to remove the unwanted copper selenide secondary phase. Se annealing wa s also performed for these films. High substrate temperatures may be even mo re important for CGS deposition than for CIS or low gallium content CIGS growth. Purwins et al. found that the formation of CIS is finished before CGS starts to form at an elevated temper ature of approximately 390 C [132]. Most of our film growth occurred at a substrate temperature near this lower limit of CuGaSe2 constitution. Growth temperature seemed to be a limiting factor in producing hi gh-quality films so a much higher gap temperature of Tg = 900oC, corresponding to a growth temperature of 491oC, was used for the fifth set of films, 521-525, s hown in Table 4-5. These films had a similar growth rate and thickness to the previous set. Film #523 was grown at a very low growth rate of 0.4 /s to a thickness of only about 0.6 m. The final selenium vapor treatment for 30 minutes occurred at the new elevated growth temperature of 491C. The growth temperature used while depositi ng CGS films in our PMEE reactor is lower than ideal resulting in lower quality films. This may be a result of inadequate sodium incorporation into the film. Na presence is due to diffusion from the soda-lime glass. During absorber growth and potential annealing steps, Na diffuses through the Mo film into the absorber, improving the doping conc entration of the absorber. Ideally, a higher Na amount is found closer to the Mo contact and the concentration gradually decrea ses in the bulk. Rusu et al

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79 found a sodium concentration of 1 atomic percent at th e surface [74]. A m oderate level of Na improves the efficiency of the cells by enhancing the p-type conductivity. It incorporates into the lattice of the CIGS by reacting with Se and forming Na-Se compounds [97]. These compounds slow the growth of CIGS and facilitate the Se incorpor ation into the film. Excessive Na diffusion may limit the efficiency of the cells because of the introduction of additional deep states. Table 4-6 shows a sixth set of films, samples 535-542, which were mostly grown copper rich at 491oC. Growth runs 535-540 used an elevated gallium primary crucible temperature of 1005C to increase the Ga flux. Previous attempts at growing quality absorbers by the Emulated Three-Stage Process were unsuccessful so the pr ocedure was attempted again for some of these films while maintaining an overall Cu-rich compos ition. The initial GaSe layer was grown at 450oC and the final two stages were deposited at 491oC. Films #541 and #542 used the same processing sequence as our best absorber to date #523. Se annealing, as described above, was only performed on absorbers #541 and #542. The thic kness of the films ranged from 0.6 to 0.8 m while employing a growth ra te of approximately 0.4 /s. Three-stage co-evaporation imposes stringen t limits on the parameter space if highly efficient devices are to result. The growth ki netics, substrate temperat ure profile, and reaction time will make the outcome of local equilibria unique to the growth process. The Ga and Se delivered in the third stage reacts with the CuxSe to form additional CuGaSe2 until the CuxSe is consumed. Cu must diffuse out of the CGS grains to react with new Ga and Se while some Ga will diffuse into the bulk grains to bring them to more Cu-poor compositions. By varying the temperature during this stage, the counterdiffusi on process can be enhanced or inhibited such that the thickness and/or composition of surface Cu-poor phases can be controlled [91]. The

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80 evolution of the intrin sic defects depends on the dynamics of the reaction pathway, i.e. the composition changes that occur when the film transitions from Cu-rich to Ga-rich [133]. The degree of overall Cu-richness or lack of Cu-richness that the film has after the second stage may have a large effect on the absorber quality. The three growth strategies described in the preceding chapter were incorporated into a seventh set of runs, 628-641, that produced CGS ab sorber layers. The second column of Table 4-7 identifies the growth process utilized. For example, Sample #628 was grown using the Constant Cu Flux Process descri bed previously. When the Modi fied Three-Stage Process was utilized, the second column of th e table indicates the target com position of each sublayer of the resulting CGS film. For example, Samp le #634 has a growth process denoted as /1.2/1.62/1.2. This indicates that the absorber f ilm in this sample is composed of four layers. In the first layer, the ratio of the copper flux to the gallium fl ux is zero, indicating that the effective metal flux reaching the substrate was co mposed only of gallium. In the second layer the ratio of the copper flux to the gallium flux is 1.2, indicating that th ere was a 20% excess of copper relative to gallium reaching the substrat e and thus this sublay er was grown under copperrich conditions. In an anal ogous fashion, the ratio values of 1.62 and 1.2 describe the relationship between the fluxes imposed when grow ing the third and fourth sublayers. Finally, Sample #640 incorporates an Emulated Three-St age Process designated as GaSe/CuSe/GaSe. This indicates that the growth was done with th e intent of defining three sublayers, where the bottom most and the top sublayers are grown under gallium and selenium fluxes, while the middle sublayer is grown under co pper and selenium fluxes. The third column of Table 4-7 indicates the overall ratio of copper to gallium content in the final CGS film, as measured via ICP, and when ever applicable, the fourth column gives the

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81 copper to gallium ratio in the film after a 5 mi n etch procedure in a 10% KCN solution carried out to eliminate surface CuSe ma terial. For example, Sample #629 had a copper-to-gallium ratio of 1.17 as determined by ICP, and hence was a copper-rich film. The last column shows that after the KCN etch procedure the ratio of copper to gallium in Sample #629 was reduced to 1.00, putting the film in a stoich iometric Cu:Ga composition. In every growth run the gallium source prim ary temperature was maintained at 970C for each run, and the substrate temperature was esti mated to be approximately 440C. Although an elevated substrate temperature of approximately 490C gave higher quality CGS absorbers, the heat being generated was also warping the rotating platen, whic h led to scraping and erratic rotational movement of the substrates. A gap temperature of 800C, corresponding to a substrate temperature of 440C, was deemed safe so this is the maximum growth temp erature that is used in the PMEE reactor. Each absorber layer was estimated to be grown to a thickness of 1.5 microns except for Sample #641, which was grow n to a thickness of 2 microns. The actual thickness is more likely to be in the range of 1.1 to 1.3 m for those grown to an estimated thickness of 1.5 m by analysis of the component masses over a defined film surface area. A thickness of 1 micron is sufficient for the abso rption of photons up to 750 nm, however thicker layers result in a better performa nce of the solar cell [134]. The f ilm growth rate is estimated to be about 0.8 /s, except for those using the emulated th ree-stage process where the growth rate is about half of this value. Three samples were grown under the constant copper flux strategy: one was grown under near a 1:1 (i.e., stoichiometric) Ga:Cu fluxes (Sample #628), one under Cu-rich conditions (Sample #629), and one under Ga-rich conditions (Sample #630). Thicker versions of the process that resulted in our best absorber, name ly #523, were grown. Overall Ga-rich and near-

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82 stoichiometric films were grown incorporating the modified three-stage process with varying levels of peak copper richness. For this process, a thin initial GaSe la yer was deposited followed by Cu-rich layer and then a Ga-rich layer. The final two layers had similar thicknesses incorporating half of the total film thickness. Gallium rich samples in this growth series were slightly more selenium rich than KCNetched copper rich samples. Etched sample s ranged from 0.490 to 0.494 selenium composition while the overall gallium rich samples varied fr om 0.494 to 0.500. Epitaxial CGS films grown at a substrate temperature of 500C by Gu et al. showed a similar tr end. The Se content value was slightly higher than 50 at.% in the Ga-rich region and slightly lowe r than 50 at.% for the Cu-rich region [45]. A final set of films, 647-662, were grown at 440C by the Constant Copper Flux Process over a range of Cu/Ga ratios from approximate ly 0.9 to 1.25. Films #648, grown by the procedure that produced our best absorber to date, and #649, grown by the three-stage process, were included for comparison. All of the films were grown at a rate of 0.7 to 0.9 /s, except for #649, which was grown at a rate around 0.45 /s. Approximate film thickness varied from about 1.0 to 1.3 m. A growth series of bilayer precursors wa s started, represented by growth run #666 in Appendix A, but the Ga source wa s damaged during or following Ga Se deposition. It is likely that the gallium crucible cracked and the metal le aked out and shorted the source. The intent was to grow a low temperature CuSe/GaSe stack that was to be rapidly ther mally processed (RTP). Absorber Characterization The effect of different growth conditions on the film morphology such as growth temperature, overall Cu to Ga ratio and growth recipe were studied by using Scanning Electron Microscopy (SEM). All CG S films having overall Cu to Ga ratio greater than one at some point

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83 during the growth showed the morphology that ha d matrix and domain structure. In this structure, the domain region showed highly Cu -rich composition and large grains while the matrix region had small grains and stoichiometric or Ga rich composition. It means at the point we had the overall Cu-rich composition for the film, there was a formation of liquid-like CuSe secondary phase in the film and it made the grain si ze in the domain region larger than that in the matrix region. As this kind of inhomogeniety was observed in Cu-rich films with large grains that lead to better efficiencies, the growth temperature was increased to get more uniform films. Figure 4-2 shows that film #523 has better uni formity than the films grown by a similar process at a lower growth temperature as seen in Figure 4-1. More unifo rm films are more likely to produce high-quality CGS absorbers. Figure 4-3 and 4-4 shows the morphologies of films grown with the similar Cu to Ga ratio profile but at different growth temperatures during the growth. As seen in those figures, the grain size in the domain region appears to be largest at the lowest growth temperature, while the grain size in the matrix region is the smallest. The grain size in domain region was smaller at an intermedia te growth temperature and got bigger at the highest growth temperature. It appears that th e grains in the matrix region got bigger as the growth temperature increased. This means th ere might have been possible phase separation between the Cu-rich domain region and Ga-rich matrix region at the lowest growth temperature, and the film got more homogeneous as growth temperature increased This improved bulk crystal quality may be due to a more ideal incorp oration of sodium into the film from the sodalime glass substrate at an elevated temperature [135]. Shafarman et al. showed that with Cu-rich gr owth of CIGS, the mean lateral grain area decreases from 1.8 to 0.3 squared microns as substrate temperature is reduced from 550 to 400C, but only at the highest substrate temperat ure does the grain size de pend on growth recipe

PAGE 84

84 [130]. Films deposited at 400 C have a greater average sodi um concentration than those deposited at higher substrate te mperatures. Thus, improved device performance with increased substrate temperature cannot be ex plained by greater availability of Na. Films with smaller grain size or a greater density of gr ain boundaries may have greater average sodium concentration since nearly all Na probably resides along those boundaries. In figure 4-5 and 4-6, the mor phologies of films grown at th e highest growth temperature were compared for three different growth pro cesses. Film #521 was grown by using a reverse Boeing process, which used Ga-rich and then Cu-rich conditions. Film #523 was grown by the Modified Three-Stage Process to utilize the liquid-like CuSe s econdary phase to get larger grains. Finally, film #525 was made by a proces s similar to Boeings. As shown in those figures, absorbers #521 and #523 have similar mor phologies, both for the matrix and the domain regions. And the grain size in domain region is only slightly larger than that in the matrix region for those two films. For film #525, we could see th at the grain sizes in tw o regions appear to be much different from each other and there might have been possible phase separation again. In figure 4-7, the morphologies of Cu-rich and Ga-rich films grown by the Emulated 3Stage Process were compared. Even though both films showed good homoge neity, the grains of Cu-rich film are much larger than that of Ga-ri ch film. The Ga-rich f ilm (#536) has very small grains (~100 nm) while the Cu-rich film has larg er grain size (300 ~ 900 nm). Films #541 and #542 have a more uniform morphology, as seen in Figure 4-8, than those grown at lower substrate temperature. They al so didnt show the domain (large grain size region) and matrix structure so it is assumed that devices ma de from both absorbers #541 and #542 will show good performance.

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85 Preference for a certain orientation seems to be dependent upon the growth recipe that was followed in the PMEE reactor. Films grown by simila r growth recipes, but at different substrate temperatures, exhibit nearly identical XRD pa tterns. Shafarman et al. claim that XRD measurements did not show any significant difference in the f ilm orientation for different processes or substrate temperatur es. All their films had nearly random orientation [130]. Figure 4-9 shows that films #511 and #522 both have a pr eference for the (112) orientation of CGS while Figure 4-10 shows that #515 and #523 have co mparatively less of a preference for this orientation. Absorbers #515 and #523 were grow n with an initial GaSe layer while #511 and #522 had a Ga-rich initial layer and a final layer of GaSe. The f ilms that had no initial Cu flux during deposition had much more intense (220) peaks compared to the (204) peaks, whereas films without this good adhesion la yer seem to have slightly more intense (204) peaks than (220) peaks. The full width half maximum of the ( 112) diffraction peak of film #523 is sufficiently small indicating that the cr ystalline quality is fairly good. Some groups claim that there is a clear correlation between higher (112) orientation and smoothness of the films [136], but that does not seem to be the case for CGS films grown by PMEE at lower than ideal substrate temperatures. Figure 4-11 shows two films with the same growth conditions, but grown at different rotational speeds. The XRD patterns are very sim ilar, but the film grown at the higher rotational speed, #479, has slightly sharper p eaks. A higher rotationa l speed may lead to larger grains. The film grown at the lower speed #476, also seems to have a stronger preference for (112) orientation. XRD patterns also show which secondary pha ses are present. Films #452 and #455 were grown at the same low growth temperature, but film #455 has an as-grown Cu/Ga ratio of about 1.4 while #452 has a ratio of 1.1. Figure 4-12 shows that the Cu2-xSe peaks are much more

PAGE 86

86 intense for the very Cu-rich film. After the 10 % KCN etch for five minutes, these Cu2-xSe peaks disappear as shown in Figure 4-13. As shown earlier, the growth recipe can have an effect on the preferre d film orientation. Films grown by the Constant Cu Rate Proce ss have similar XRD patterns no matter the composition. Figure 4-14 shows that the peak s for the KCN-etched Cu-rich film, #629, are nearly identical to the Ga-rich film, #630. The extent to which a film goes copper rich in the Modified Three-Stage Process can also have an affect on the pattern. Films #635 and #636 have nearly identical compositions and similar gr owth processes, but #636 becomes more Cu-rich during deposition. Figure 4-15 sh ows a greater (220) peak intensit y for the film that has a lower copper peak composition. The Emulated Three-Stage Process produced ab sorbers that favored the (204) orientation of CGS rather than (112), which is the more prevalent configuration fo r the other two growth recipes. Figure 4-16 shows that Cu-rich and Ga-rich films employing GaSe deposition followed by CuSe deposition have (220) CGS p eaks that are more intense than even the (112) peaks. The preference for these orientations may be due to the fact that the Emulated Three-Stage Process deposits copper and gallium in separate layers, wh ile they are deposited concurrently in the other growth strategies. Yet, the XRD pattern for film #478 shown in Figure 4-17, which was also grown by a three stage process, but at a lower growth temperature does not show these same characteristic peaks. Due to the very thin GaSe final layer and overall Ga-rich composition, film #478 may not have ever been Cu-rich and hen ce may have had different growth kinetics. The surface morphology of films #628 through #641, grown at 440C by various growth processes, was investigated by SEM. Figure 4-18 shows the Constant Cu Rate Process of a Curich (#629) and a Ga-ric h film (#630) at 100X magnification. Both Samples were grown under a

PAGE 87

87 constant Cu flux throughout the entire deposi tion run with no Cu:Ga pr ofile grading in any sublayers. The Ga-rich film is very uniform wh ile the Cu-rich film has many island structures. Figures 4-19 and 4-20 show the Cu -rich films grain structure in the field region and the island region at 5000X and 10,000X, respectively. The fi eld region shows very small grains and the island structures have large grains up to a micr on in size. The Ga-rich film morphology that is shown in Figure 4-21 exhibits long needle-like grains. The Cu-rich films have Cu2-xSe secondary phase on their surface. Figure 4-22 shows the island region and Figure 4-23 shows the field region of film #634 before and after the 10 % KCN etch. The gaps left by the etching of the copper nodules are very appare nt in the island region while the field region appears to be unchanged. Co-evaporation of Ga-rich samples in an in-lin e deposition process revealed that a bilayer process yields large, columnar grains, whereas a single layer process leads to absorbers with very small grains. The bilayer-like process had a Cu -rich growth regime at the beginning and the single-layer like process had consta nt rates throughout [137]. Shaf arman et al. showed that at lower temperatures, the uniform flux process ap pears to give more columnar grains and a smoother surface than CIGS films with a Cu-rich growth period. There is no apparent difference between films grown with Cu-rich flux at either the beginning or middle of deposition [130]. The morphology of CGS films is strongly dependent on composition. Haug et al. observe long and needle-like grains that are of small si ze for Cu/Ga~0.3. Grains of somewhat Ga-rich CGS layers with Cu/Ga ratios between 0.9 and 0. 7 are triangular, and la yers deposited at higher temperatures have an increased grain size. The Cu-rich layer consists of grains of irregular shapes with a typical grain size of 1-3 microns (Cu/Ga ~1.1) [134].

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88 Orsal et al. observed slightly different mor phology [138]. For slightly Ga-rich, there are grains in the background with thin and long shaped grains starting to grow on the surface. With an even higher Ga-content, the morphology is again homogenous and constituted of plateletshaped grains that are tilted on the surface. Cu -rich films exhibit small and homogenous grains on the bottom with large polyhedral and packed grai ns on the surface. While it is not the case for Cu-rich or Ga-rich films, the morphology is very sensitive near stoichiometric composition and seems to depend on growth temperature and kind of substrate. Triangular crystallites are more evident at 450C with a grain size of appr oximately 0.6 microns. At 400C, grains are polyhedral whereas the layer is composed of a melt of triangular and polyhedral grains at 500C. Columnar growth is observed at each growth temperature. Films 635-637 and #639 show some very peculia r surface morphology. The growth recipe involved an initial GaSe layer followed by a Cu -rich layer and a Ga-rich layer. The overall composition was either Ga-rich or near stoichiometric. The surface has islands surrounded by rings that show a grain transition of large grains to smaller grains to long thin grains as can be seen in Figure 4-24. Figure 4-25 shows the l ong needle-like grains pres ent in the field region that are the same as those in films grown Ga-rich by the Constant Cu Rate Process and the large grains in the island region that are of similar size and shape to the ones found in the island region of Cu-rich films grown by the Constant Cu Rate Pr ocess. It is likely that these films may have copper selenide phases in the island region although the overall composition may be Ga-rich. The sample taken for ICP compositional characteriz ation may not have been representative of the entire film since it is very non-uniform. This could be th e case for film #639 since the Cu/Ga ratio changed to 0.95 after the KCN etch from 1.04. Typically the film becomes nearly stoichiometric, usually very slightly Ga-rich after the cyanide etch The pieces of the

PAGE 89

89 characterization absorber used for the as-grown and post-etch composition analysis may have started with drastically different overall composit ions because of the non-uniformity of the film. Smooth surface morphology is characteristic of films deposited by the three-stage process [139]. The Emulated Three-Stage Process produced very uniform films as can be seen in Figure 4-26. Figure 4-27 shows that the Cu-rich process that did not incorporate a final GaSe layer has long tubular grains that have a la rger axial diameter than the n eedle-like grains of the Ga-rich films. A SEM picture on a 45 tilt in Figure 4-26 gives a clear view of these tubular grains. The Ga-rich absorber film, #640 shown in Figure 4-28 seems to have somewhat triangular-shaped grains that have not been produced by any ot her growth recipe used in the PMEE reactor. Films 647-662 were mostly grown with a Consta nt Copper Rate Process to investigate the difference in the absorber film properties base d on composition. Figure 4-29 shows a slightly different XRD pattern for the films grown the mo st Ga-rich like #647. The peak intensity for (220) CGS is slightly greater than that of (204) CGS. Nearer stoichiometric Ga-rich films and Cu-rich films, as can be seen in Figure 4-30, de monstrate the characteristics peaks of a constant copper rate process that were seen in the previous growth series. Figure 4-31 also shows that the Modified and Emulated Three-Stag e Processes exhibit the same orie ntation as those in previous growth series. In all the diffraction patterns of the CGS films grown in the PMEE reactor, the (220) and (204) peaks are clearly separa ted, showing that the films have the chalcopyrite type crystallographic structure. This was true fo r the near-stoichiometric and Cu-rich CGS films grown by MBE by Yamada et al., but they also obse rved sphalerite crystallite structure for films with Ga-rich composition [140]. But this was for very Ga-rich (Cu/Ga~0.66) films, which we never grew in these growth run series.

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90 Conclusions Growth temperature, growth recipe, a nd overall Cu/Ga ratio each had varying yet substantial effects on the film mo rphology and orientation. Growth temperature seems to be the most critical variable in achievi ng high-quality absorber films. The fact that Cu-rich absorbers grown by PMEE have been more successful than Ga-rich ones is likely due to the lower deposition temperature. Ga-rich absorbers pr oduce the highest efficiency CGS cells in the literature, but they are also grown at an elevated temperature of at least 550C. The intrinsic defects produced at different pr ocessing conditions results in abso rbers with distinct properties. For the final growth series, we were able to maintain very consistent growth conditions between runs in our reactor, which gave us grea t confidence in the compositional results for each run, even though we lack in-situ measurement techniques. A packaging system allows us to vacuum seal the absorbers after growth to lesse n any degradation that ma y occur before the cell has been processed. The biggest drawback in using the PMEE reactor to grow polycrystalline films is the limit that we must observe on the maximum substrate growth temperature. A more effective technique may be to grow CGS bilaye r precursors at a low substrate temperature and then utilize an RTP system to ra pidly raise the temperature for a brief period of time. Klenk et al. performed a post-growth rapid thermal treatme nt at 550C for 6 minutes on stacked elemental layers that were deposited by evaporation at a low deposition temperature [103]. Our research group has been successful in the past employing an analogous strategy to grow CIS films via the RTP processing of a bilayer [141] ; we anticipate that the RTP processing route is likely to produce similar satisfactory results for the CGS material.

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91 Table 4-1. First CGS growth series. Film # Process Cu/Ga ratio 443 1.1/0.9/0.7 0.89 444 1.15/0.95/0.75 1.00 445 1.15/0.95/0.75 0.96 446 1.3/1.1/0.9 1.13 447 1.15/0.95/0.75 0.98 Note: Process refers to the intended Cu/Ga ratio of each graded layer. Table 4-2. Second CGS growth series. Film # Process Cu/Ga ratio 452 0.8/1.25 1.11 453 0.8/1.4/0 1.00 454 0.85/1.4/1.1/0.87 1.11 455 0.85/1.4/1.1/0.87 1.40 456 0.85/1.4/1.05/0.81 1.04 457 0/0.85/1.5/1.1/0.75 0.97 458 0.85/1.4/1.2/0.8 0.91 459 0.85/1.2/1.05/0.75 0.85 Table 4-3. Third CGS growth series. Film # Process Cu/Ga ratio Cu/Ga after KCN-etch 472 0/1.3/0 0.93 -74 0/1.4/0.96 1.00 0.98 475 0/1.3/0.90 0.97 -476 0/1.3/0 1.01 -477 0/1.3/0 0.97 -478 GaSe/CuSe/GaSe 0.98 -479 0/1.3/0 1.01 0.99 480 GaSe/CuSe/GaSe 0.95 -Table 4-4. Fourth CGS growth series. Film # Process Cu/Ga ratio Cu/Ga after KCN-etch 510 0.8/1.25 1.14 0.99 511 0.8/1.4/0 1.59 1.01 512 0/1.4/0.8 1.07 0.99 513 1.15/0.95/0.75 0.97 0.96 514 0.8/1.25 1.19 0.99 515 0/0.9/1.45/0.9 1.19 0.99 516 Constant 1.06 1.01

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92 Table 4-5. Fifth CGS growth series. Film # Process Cu/Ga ratio Cu/Ga after KCN-etch 521 0.8/1.25 1.14 -522 0.8/1.4/0 1.23 1.00 523 0/0.9/1.45/0.9 1.36 -524 0/1.4/0.8 1.28 0.97 525 1.15/0.95/0.75 1.09 -Table 4-6. Sixth CGS growth series. Film # Process Cu/Ga ratio 535 GaSe/CuSe/GaSe 1.04 536 GaSe/CuSe/GaSe 0.98 537 GaSe/CuSe/GaSe 1.12 538 0/1.3 1.58 540 GaSe/CuSe/GaSe 1.54 541 0/1/1.6/1 1.80 542 0/1.2/1.6/1 1.67 Note: These samples were etched by KCN, but the composition of the etched samples was not measured. Table 4-7. Seventh CGS Growth Series. Film # Process Cu/Ga ratio Cu/Ga after KCN-etch 628 Constant 0.97 -629 Constant 1.17 1.00 630 Constant 0.89 -634 0/1.2/1.62/1.2 1.37 0.98 635 0/1.3/0.7 0.89 -636 0/1.6/0.7 0.90 -637 0/1.3/0.87 0.98 -638 GaSe/CuSe 1.26 1.01 639 0/1.6/0.8 1.04 0.95 640 GaSe/CuSe/GaSe 0.91 -641 0/0.8/1.37/1.35 1.49 0.99 Note: Ga-rich samples were not KCN-etched.

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93 Table 4-8. Eighth CGS growth series. Film # Process Cu/Ga ratio 647 Constant 0.92 648 0/1.2/1.6/1.2 1.41 649 GaSe/CuSe/GaSe 0.95 652 Constant 0.98 653 Constant 0.99 654 Constant 1.23 655 Constant 0.95 656 Constant 1.17 657 Constant 0.98 658 Constant 1.17 659 Constant 1.12 660 Constant 1.04 661 Constant 1.00 662 Constant 1.23

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94 A B Figure 4-1. Morphologies of films grown at lower growth temperatures by similar growth recipes (X1000). A) Tsub = 386C. B) Tsub = 412C. Figure 4-2. Morphology of a film gr own at a higher growth temperature (X3000). Tsub = 491C.

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95 A B C Figure 4-3. Morphologies of the Cu-rich domain region of CGS films grown by the same recipe at different growth temperatures (X30,000). A) Tsub = 386C. B) Tsub = 412C. C) Tsub = 491C. A B C Figure 4-4. Morphologie s of the Ga-rich matrix region of CGS films grown by the same recipe at different growth temperatures (X30,000). A) Tsub = 386C. B) Tsub = 412C. C) Tsub = 491C. A B C Figure 4-5. Morphologies of the Cu-rich domain region of CGS film s grown with different growth recipes at 491C (X30,000). A) (.8/1.25). B) (0/.9/1.45/.9). C) (1.15/.95/.75).

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96 A B C Figure 4-6. Morphologies of the Ga-rich matrix region of CGS films grown with different growth recipes at 491C (X30,000). A) (.8/1.25). B) (0/.9/1.45/.9). C) (1.15/.95/.75). A B Figure 4-7. Morphologies of CGS films grown by the emulated 3-stage process at 491C (X30,000). A) Ga-rich. B) Cu-rich. A B Figure 4-8. Morphology of a Cu-rich film ( #542) with large grains and a uniform surface. A) X3000. B) X30,000.

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97 511K0 2000 4000 6000 8000 10000 12000 14000 16000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) A 522K0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) B Figure 4-9. Diffraction pa tterns of films grown at different temperatures with the same modified three-stage process. A) Tsub = 412C. B) Tsub = 491C.

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98 515K0 2000 4000 6000 8000 10000 12000 14000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220) CGS(312/116) CGS(204) A 523K0 2000 4000 6000 8000 10000 12000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220) CGS(312/116) CGS(204) B Figure 4-10. Diffraction patte rns of films grown at different temperatures with the same modified three-stage process featurin g an initial GaSe layer. A) Tsub = 412C. B) Tsub = 491C.

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99 4760 2000 4000 6000 8000 10000 12000 14000 16000 18000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) A 4790 2000 4000 6000 8000 10000 12000 14000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) B Figure 4-11. Diffraction patterns of films grown at different rota tional speeds. A) 12 RPM. B) 20 RPM.

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100 4520 2000 4000 6000 8000 10000 12000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220) CGS(204) CGS(312) CGS(116) Cu2-xSeCu2-xSeCu2-xSe A 4550 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) Cu2-xSeCu2-xSeCu2-xSe B Figure 4-12. Diffraction patterns of films grown at different leve ls of overall Cu-richness. A) Cu/Ga = 1.11. B) Cu/Ga = 1.40.

PAGE 101

101 6340 1000 2000 3000 4000 5000 6000 7000 8000 9000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220) CGS(204) CGS(312) CGS(116) Cu2-xSeCu2-xSeCu2-xSe A 634K0 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220) CGS(204) CGS(312) CGS(116) B Figure 4-13. Effect of KCNetch on the diffraction pattern of a Cu-rich film. A) Before. B) After.

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102 629K0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220/204) CGS(312/116) A 6300 2000 4000 6000 8000 10000 12000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220/204) CGS(312) CGS(116) B Figure 4-14. Diffraction pattern s of films grown by the Constant Cu Rate Process. A) KCNetched Cu-rich. B) Ga-rich.

PAGE 103

103 6350 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220/204) CGS(312/116) A 6360 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220/204) CGS(312/116) B Figure 4-15. Diffraction pattern s of films grown with varying levels of p eak Cu-richness. A) Peak Cu/Ga = 1.3. B) Peak Cu/Ga = 1.6.

PAGE 104

104 638K0 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220)CGS(204)CGS(312) CGS(116) A 6400 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)CountsCGS(112) Mo(110) CGS(220)CGS(204)CGS(312) CGS(116) B Figure 4-16. Diffraction pattern s of films grown by the Emulated 3-Stage Process. A) Cu-rich. B) Ga-rich.

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105 4780 2000 4000 6000 8000 10000 12000 14000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) Figure 4-17. Diffraction patte rn of a film grown by the Emul ated 3-Stage Process that was never Cu-rich.

PAGE 106

106 A B Figure 4-18. Surface morphology of films grown by the Constant Cu Rate process (X100). A) Cu-rich. B) Ga-rich.

PAGE 107

107 A B Figure 4-19. Surface morphology of a Cu-rich f ilm grown by the Constant Cu Rate Process (X5000). A) Field Region. B) Island Region.

PAGE 108

108 A B Figure 4-20. Surface morphology of a Cu-rich f ilm grown by the Constant Cu Rate Process (X10,000). A) Field Region. B) Island Region.

PAGE 109

109 A B Figure 4-21. Surface morphology of a Ga-rich film grown by the Constant Cu Rate Process. A) X5000. B) X10,000.

PAGE 110

110 A B Figure 4-22. Effect of KCN-etch on the su rface morphology of the is land region of a Cu-rich film (X10,000). A) Before. B) After.

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111 A B Figure 4-23. Effect of KCN-etch on the su rface morphology of the fi eld region of a Cu-rich film (X10,000). A) Before. B) After.

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112 A B Figure 4-24. Surface morphology of a Ga-rich f ilm with rings around the islands A) X100. B) The ring region magnified (X5,000).

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113 A B Figure 4-25. Distinct grain structure of a Ga-rich film with rings around its islands (X10,000). A) Field region. B) Island region.

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114 A B Figure 4-26. Surface morphology of a film grow n by the Emulated 3-Stage Process. A) X100. B) at a 45 tilt.

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115 A B Figure 4-27. Surface morphology of a Cu-rich film grown by the Emulated 3-Stage Process. A) X5000. B) X10,000.

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116 A B Figure 4-28. Surface morphology of a Ga-rich film grown by the Emulated 3-Stage Process. A) X5000. B) X10,000.

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117 6470 2000 4000 6000 8000 10000 12000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) Figure 4-29. Diffraction patte rn of a Ga-rich film grown by the Constant Cu Rate process.

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118 6570 2000 4000 6000 8000 10000 12000 14000 16000 18000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) A 6580 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220/204) CGS(312/116) Cu2-xSeCu2-xSeCu2-xSe B Figure 4-30. Diffraction patte rns of films grown by the Consta nt Cu Rate process. A) Stoichiometric. B) Cu-rich.

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119 6480 1000 2000 3000 4000 5000 6000 7000 8000 9000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220) CGS(204) CGS(312) CGS(116) Cu2-xSeCu2-xSeCu2-xSe A 6490 2000 4000 6000 8000 10000 12000 14000 102030405060702 theta (degrees)countsCGS(112) Mo(110) CGS(220)CGS(204)CGS(312) CGS(116) B Figure 4-31. Diffraction pattern s of films grown by 3-stage pro cess. A) Modified 3-Stage Curich film. B) Emulated 3-Stage Ga-rich film.

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120 CHAPTER 5 COPPER GALLIUM DISELENIDE DEVICE FABRICATION It can be difficult to determine what causes sm all, but significant, changes in the solar cell parameters when comparing devices. A summary of the properties of th e absorbers and devices can pave the way for more carefu l and controlled experimentation, and the testing of key ideas. Any unique properties or features that have been identified should be noted. Best Devices in the Literature A record total area efficiency of 10.2% [142] for CuGaSe2 was achieved at NREL using a thicker absorber and a more Cu-ri ch condition than the previous best cell [48]. The biggest difference was the addition of a very small amount of In at the end of the growth with In/(In+Ga) < 0.8%. This surface-modified CGS is grown so that the surface region is similar to that of CIGS to minimize defects in the material. The VOC and FF of this new cell were lower compared to the previous record cell, but the JSC increased. This indicates that the recombination mechanisms are different in the two devices. Table 5-1 compares the surface modified CGS I-V results to the previ ous record cell [142]. Record efficiency of ZnO/CdS/CGS cells we nt from 6.2 to 9.3% by making the following improvements: slightly Ga-rich absorber compos ition, the use of Na-cont aining substrates, and a CdS buffer layer deposition temperature optimi zed for CGS [111]. Unlike low Ga-content CIGS, the efficiency of single crystals has been higher than polycrystalline CGS solar cells. Saad et al. produced a single crystal CGS so lar cell with an efficiency of 9.7% (VOC = 946 mV, JSC = 15.5 mA/cm2, and FF = 66.5) [143]. The enhancement of the FF seems to be the mo st important issue in order to achieve highefficiency CGS cells. CIGS solar cells reach 90 % average QE while CGS so far has been limited to about 70% [57]. Current transport th rough CGS cells is dominated by interface

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121 recombination; the diode ideality factor and the saturation current change drastically under illumination. This may be the main difference to the heterojunctions based on low gallium content CIGS where a relatively small diode fact or of 1.5 indicates current transport dominated by recombination in the depleti on layer; interface recombination only plays a minor role [63]. Device Fabrication After the first seven sets of growth runs, sa mples containing CGS absorber film were sent to James Keane at the National Renewable Energy Laboratory (NREL) to be processed. To eliminate the copper selenide which is known to form in the Cu-rich CGS, all Cu-rich CGS absorber films were etched with a 10% KCN solu tion for five minutes before being shipped. Also, the 2 x 2 samples were cut in half before being sent. The first si x growth series had the misfortune of being shipped while not being prot ected from the elements. A vacuum sealer was purchased to remedy this situation. Device fa brication completed at NREL included CdS grown by a CBD process followed by the deposition of a high/low resistivity ZnO bilayer film and Ni/Al grid deposition. The first and third grow th series had CBD CdS bu ffer layers deposited on them in-house before being sent to NREL for fu rther processing. Once the devices were sent back to the University of Florida, I-V measuremen ts were taken. For the seventh set of samples, 628-641, the devices were also characterized at NR EL and never sent back to the University of Florida. The final CGS growth series was fabricated completely in-house. World-record surface-modified CGS films ar e processed with 500-600 Angstroms of CdS using CBD at a bath temperature of ~ 65C. A ZnO bilayer was then added, containing an undoped ZnO layer (90 nm thick) followed by an Al-doped ZnO layer (about 120 nm thick). A 100 nm thick MgF2 antireflective coating was de posited using sputtering [142]. The films fabricated completely at the University of Florida had ZnxCd1-xS buffer layers, instead of the traditional CdS buffers, deposited on them after absorber growth. The CGS films

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122 were cut into 1 x 1 samples and vacuum-sealed after their removal from the vacuum chamber, and they remained so until the buffer was deposite d by CBD. The Cu-rich films were etched in a 10% solution of potassium cyanide for five minu tes while the Ga-rich films were fabricated asgrown. The addition of Zn to the most widely used CdS buffer layer material decreases the lattice constant with a lattice match to CGS absorb er and produces a more favorable conduction band alignment. Adding Zn enhances both VOC and JSC of the device to yi eld a higher conversion efficiency for CGS. An aqueous solution of 1.20 x 10-3 M CdCl2 2(1/2)H2O, 1.39 x 10-3 M NH4Cl, 1.19 x 10-2 M thiourea (H2NCSNH2), 6.27 x 10-4 M ZnCl2, and 5.27 x 10-4 M NH3 was used. The following conditions tend to give the maximum increases in solar-cell conversion efficiencies: CBD bath temperature of 85 C, duration of bath-deposition tdep = 45-50 minutes, and xbath = 0.2-0.3 (i.e., bath solu tion containing rela tive amounts of Zn ranging from 20% to 30%). The chemical bath is kept at the target temperature of 85 C at all times during film growth. These parameters produce good films on glass and on CGS/CIGS substrates that are visually shiny, uniform, and have no powdery pa tches. Structural, surface morphology, optical, and electrical analyses show that CdZnS films with a 30% Zn composition should be a successful buffer layer for CGS [144]. The window layer consisted of a 600 nm Al-doped ZnO layer, which was magnetron sputtered. The sheet resistance of the characterization TCO film grown along with the fabricated devices ranged from 38 /square to 45 /square. Cells were completed by evaporation of a Ni (50 nm)/Al (0.3 m) contact grid and finally defining the cells (0.429 cm2) by mechanical scribing. No anti-reflective coating is added. For I-V measurement purposes, part of the CGS layer away from the grid was scratched off a nd an In contact was added to the Mo surface.

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123 Device Characterization The photo current density voltage (I-V) char acteristics were meas ured under AM1.5, 100mW/cm2 illumination at 25C in our own electrical measurement lab. Dark I-V measurements were taken for a few select samples. The I-V results for the selected devices are summarized in the respective tables for each growth series. The CGS absorbers for the first growth seri es, 443-447, had a CdS buffer layer added by CBD in-house at the University of Florida a nd then were shipped off to NREL for further processing. These films were deposited at a fa irly low growth temperature of approximately 386C. The only device that had an efficiency of at least 1% was #446, which also was the only Cu-rich film in the series. It had a reas onably good short circuit current of 14.4 mA/cm2, but the cell was severely limited by its extremely low open-circuit voltage of 0.2 V. Since the first growth series was unsuccessful with a Cu-rich initial stage, we decided to make the initial stage of the next set of runs Ga -rich using our Modified Three-Stage Process. These films were completely processed at NREL a nd sent back to the University of Florida for electrical characterization. Th e device parameters are compiled in Table 5-2. An unsuccessful absorber from the previous series, namely #447, and #455-F with a buffer layer added by UF were also sent to NREL to be processed to ch eck the effectiveness of our CBD CdS process. Device #447 showed an efficiency of over 2% with the NREL buffer layer and #455-F had less than half the efficiency of #455, which was completely processed at NREL (1.5 vs 3.3%). No Ga-rich cells were effective; only near stoichiome tric and Cu-rich cells showed efficiency greater than 1%. The best cell, #452, had a similar JSC to #446 and a marginally better FF (39% compared to 34%), but its VOC (0.64 V) was much larger resulting in a cell with an efficiency of 3.4%. The slightly Ga-rich cell with a Cu/Ga ratio of 0.97 had the highest JSC of 17.6 mA/cm2,

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124 but suffered from a lower VOC (0.44 V) and FF (33%) resulting in a lower efficiency of 2.6%. This was also the only absorber that incorporated an initial GaSe layer rather than a Ga-rich layer. The device made from absorber #455 had very similar results to #452 with the only difference being a slightly lower VOC. All films in the third growth series were either near-stoichiometric or Ga-rich. This growth run series represented the first attempt at us ing a three-stage growth process in the PMEE reactor. All films received a CBD CdS buffer laye r at the University of Florida and then were sent to NREL for processing. All of thes e devices had efficiencies less than 1%. Since all successful absorber films grown in the PMEE reactor up to this point were Curich or nearly stoichiometric, we decided to investigate Cu-rich f ilms for device fabrication. This is less than ideal since it requi res a cyanide etch to remove th e copper selenide secondary phase before buffer layer deposition or the heterojuncti on will be shorted. The same film processes that had been investigated previously that gave efficiencies greater than 2% were repeated at a slightly higher growth temperature of 412C. An absorber (#516) that employed the constant copper rate process was also added to see if Ga grading was necessary at a low growth temperature. Table 5-3 shows that all the cells ranged from 2 to 3% efficiency, except for device #513, which started Cu-rich and had a slightly Ga-ri ch overall composition. It had the lowest JSC and VOC for any of the cells. All the other cells we re Cu-rich and had initial GaSe or Ga-rich stages. The most efficient device, #514 with = 2.9%, did not have the highest VOC or JSC in the series, but did have the highest fill factor (64%) of any CGS cell grown by PMEE up to that point. The VOC or JSC of these cells were not better than be st of the previous growth series, but across the board these devices all had higher fill factors than had previously been achieved.

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125 Shafarman et al. observed an increase in VOC and FF with an elevated substrate temperature for three different deposition pro cesses. The uniform deposition process gave poorer device performance at 400 C than the Cu-rich growth processes, despite comparable grain sizes in the films. At higher temperatur es, there is no advantage to the Cu-rich growth even with increased grain size at 550C [130]. In general, lower device efficiency with reduced substrate temperature cannot be simply described by changes in grain size, surface morphology, or the availability of sodium The lower open-circuit voltage and increased recombination current indicate a greater density of intra-grain trap states. Since high quality copper gallium diselenide films are typically grown at substrate temperatures above 500C, we felt that we need ed to increase the maximum growth temperature employed in the PMEE reactor. We felt that a ga p temperature of 900C, which correlated to a substrate temperature of 491C, wo uld be a safe maximum operati ng temperature in our system. This elevated temperature helped us achieve a cell efficiency of greater than 5%. The device parameters of the best CGS cell (#523) grown in our PMEE reactor are shown in Table 5-4 along with the other devices in this growth series. Th is cell showed an efficiency of 5.3% with Jsc = 16.9 mA/cm2, Voc = 0.476 V and F.F. = 65.5 %. These results are quite g ood considering the approximated thickness of this sample, possibly only 0.6 m. Device #521 was very similar to 452 with the highest VOC within this growth series at an el evated temperature, but also suffered from a very low FF. The film that started Cu-r ich with a Ga-rich layer added afterwards showed very poor performance because of an extremely lo w FF. The dark and illuminated I-V curves for device #523 are shown in Figure 5-1. Rockett et al. showed that the efficiency of devices is improved significantly by the presence of Na during growth. Sodium does not seem to have a specific ongoing effect in

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126 devices; it is only important during film growth to improve the crystal quality. The amount of sodium that is incorporated into a film is depe ndent on the temperature of the substrate. There can be as much as a 50% increase in device e fficiency with Na addition, decreasing at both higher and lower concentrations. Primary improvements are in VOC and often in FF with little or no change in collect ed current [135]. The quantum efficiency (QE) characteristics of two high-efficiency ZnO/CdS/CGS solar cells measured at UF are shown in Figure 5-2. Both absorbers of #452 and #523 were grown in our PMEE reactor and the CdS buffer layer, ZnO window layer and metallization were prepared by NREL. The comparison of QE characteristic s of the two devices shows that the spectral response of #523 in the range of short wavelength (<500 nm) is similar to th at of #452, but in the range between 500 and 740 nm, it is much better than that of #452. This means that the improvement of device efficiency (5.3%) in #523 is mainly caused by the improvement of absorber quality. Also, the cut-off wavelength (~ 740 nm) indicates the band gap of the CGS absorber is around 1.7 eV. The three-stage process may be more ideal in the PMEE reactor at an elevated temperature so we attempted to grow Cu-rich and near-stoic hiometric absorber films by the Emulated ThreeStage Process at 491C. All the films grown by Emulated Three-Stage Process resulted in devices with efficiencies less than 2% as show n in Table 5-5. They suffered from very low short-circuit currents (all less than 10 mA/cm2), but a VOC of 0.57 V was achieved by device 540. For films #541 and #542, the same process that was used for #523 was re peated, but with the goal of an even higher Cu/Ga ra tio to see if an extremely Cu -rich absorber can make a good device after being etched. De vice #542, which was grown at 465C, had a smaller FF than #523

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127 and this resulted in a reduced effi ciency while #541 had a larger VOC, but a much smaller FF leading to an efficiency of only 3.1%. We intended to replicate the process used to grow our most efficient CGS absorber for Sample #634 and Sample #641, but with a thicke r absorber layer of 1.5 and 2.0 microns, respectively. The actual thickness was more likely to be about 1.2 and 1.7 m. We also needed to reduce the substrate temperatur e to 440 C to avoid the apparent warping of the rotating platen that seems to be taking place at the higher grow th temperature. Table 5-6 shows that the efficiency results for these two cel ls were unexpectedly low. This may have been the result of limiting the substrate temperature, but they still we re not as efficient as the devices that used CGS absorbers grown at 386C. So it is more lik ely that the poor film quality demonstrated in the previous chapter is the real reason for the low efficiency. We were able to produce Ga-rich films that led to photovoltaic cells of modest efficiency (see Device #628 and Device #630 in Table 5-6), which generated the highest opencircuit voltages for our CGS cells to date. Device #634 showed the highe st shunt resistance, RSHo = 669 -cm, but it also suffered from a high series resistance of 25 -cm. The best device (#630) had a series resistance of 16 cm and a shunt resistance of 334 -cm. Device #636, with the lowest RSo = 8 -cm, was also destroyed by shunts (RSHo = 12 -cm). The goal is to get a very low series resistance and a very high shunt resistance, for example like RS = 1.1 -cm and RSH = 3000 -cm in the world record 15% CIS cell [142]. The loss of power due to seri es resistance is manifested as a drop in FF. These measurements were done at NREL and they also performed air annealing of two of the devices at 200C for 2 minutes. The devi ce parameters of #639 improved upon annealing. The VOC improved greatly to 0.60 V, while the FF increased slightly to 56% and the JSC remained unchanged to give an improved efficien cy of 2.3%. Device #628 saw its FF decrease

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128 slightly with an increase in VOC to nearly 0.8 V resulting in an efficiency that was nearly the same. A less efficient cell on the same device (#628) saw its VOC increase from 0.77 V to 0.82 V. The JSC for this cell is extremely low at about 5 mA/cm2. This is likely due to a very high series resistance of 31 -cm. The shunt resistance, RSHo = 477 -cm, was one of the highest of our CGS devices that were tested at NREL. Kniese et al. observed, using co-evaporation of single elemen ts in an in-line deposition process at a high growth temper ature, that Ga-rich CGS grown by a single layer process (7.2%) produced better efficiencies than that grown by a bilayer-like process (6 %). The single layer process exhibited a slightly larger VOC and a much larger FF [137]. The reduced JSC and increased FF indicate that there is a difference in the minority carrier collection behavior in the two cells. The final CGS absorber growth series was comp letely fabricated into devices in-house at the University of Florida by the procedures descri bed earlier. Table 5-7 shows the results from the I-V characterization of these cells. Only the absorbers with Cu/Ga ratios of greater than one were etched with KCN. None of the as-grown Cu-rich or stoichiometric absorbers produced working solar cells, but we were able to produce our most efficient Ga-rich CGS device. Device #640 featured an absorber grown by the Emulated 3-Stage Process resultin g in a 4.9 % efficient cell. We also produced a 4.2 % efficient solar cell from a Ga-rich absorber (#655) grown by the Constant Cu Rate Process. The photo I-V curves for these two devices are shown in Figure 5-3 and Figure 5-4, respectively. The short-circuit current and op en-circuit voltage of device #640 are comparable to our best CGS device (#523), but the FF is slightly smaller (57% as compared to 66%). The VOC of each of these cells is still too low for a wide band gap absorber like CGS.

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129 Improvement in this area would likely produ ce devices with a much higher conversion efficiency. Conclusions The best CGS cell produced in the PMEE reactor was fabricated from an absorber grown at the highest allowable substr ate temperature, 491C. Alt hough substrate temperature was not the only factor that limited device performance, it was a key problem. The best cell was rather thin, approximately 0.6 m, so a thicker absorber under the same conditions should produce a more efficient device. Device annealing shoul d improve the low open-ci rcuit voltage of our devices as can be seen in the gains achieved by the devices that were annealed at NREL. We also demonstrated the ability to completely manuf acture CGS devices within our own facilities at the University of Florida. This research effort also produced the most efficient cell fabricated from a Ga-rich CGS absorber. Thin films de posited by both the Emulated Three-Stage Process and the Constant Copper Rate Process were made into devices with conversion efficiencies greater than 4%. Maximum device parame ters were achieved on different cells: Jsc = 17.6 mA/cm2 (#457), Voc = 0.82 V (annealed #628), F.F. = 65.5 % (#523), and = 5.3 (#523).

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130 Table 5-1. Device parameters of record CGS cells produced at NREL. Device VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) Surface-modified 0.823 18.6 66.8 10.2 Previous record 0.905 14.9 70.8 9.5 Table 5-2. Device parameters for th e second CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 447 0.536 9.8 40.7 2.1 452 0.640 14.3 38.8 3.4 453 0.355 8.7 32.0 1.0 454 0.310 8.0 33.7 0.8 455-F 0.265 16.7 34.8 1.5 455 0.589 14.5 38.8 3.3 457 0.437 17.6 33.2 2.6 Note: Film #447 was from the first grow th series and 455-F had a CdS buffer layer deposited at UF. #456, #458, and #459 showed very low efficiencies. Table 5-3. Device parameters for the fourth CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 510 0.413 11.9 40.8 2.0 511 0.323 15.0 49.3 2.4 512 0.343 11.3 50.1 2.0 513 0.244 7.2 47.6 0.8 514 0.383 12.0 63.7 3.0 515 0.292 13.7 54.0 2.2 516 0.417 9.4 59.1 2.3 Table 5-4. Device parameters for th e fifth CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 521 0.530 12.6 33.5 2.2 522 0.396 11.4 65.2 2.9 523 0.476 16.9 65.5 5.3 524 0.436 13.7 58.2 3.5 525 0.367 10.9 15.4 0.6 Table 5-5. Device parameters for th e sixth CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 535 0.444 6.9 45.8 1.4 536 0.421 7.8 44.9 1.5 537 0.478 6.5 45.8 1.4 538 0.454 11.9 35.8 1.9 540 0.571 8.3 37.6 1.8 541 0.557 14.5 38.4 3.1 542 0.481 17.2 54.0 4.5

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131 Table 5-6. Device parameters for the seventh CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 628 0.702 6.2 56.0 2.4 629 0.307 3.5 46.9 0.5 630 0.621 10.1 47.8 3.0 634 0.435 4.0 51.8 0.9 635 0.201 7.2 41.9 0.6 636 0.075 7.3 27.9 0.2 637 0.095 4.2 31.2 0.1 638 0.172 8.2 39.7 0.6 639 0.423 6.7 50.0 1.4 641 0.287 7.4 45.5 1.0 Note: I-V characteriza tion was done at NREL. Table 5-7. Device parameters for th e eighth CGS absorber growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Efficiency (%) 640 0.502 16.9 57.4 4.9 647 0.480 13.7 55.7 3.7 649 0.426 16.8 46.3 3.3 655 0.516 14.2 57.8 4.2 Note: Device fabrication was done at UF.

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132 Figure 5-1. Dark and illuminated I-V curves for Device #523. Figure 5-2. Spectral response curves comparing Device #523 and #452.

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133 Figure 5-3. Photo I-V curve for Device # 640. Figure 5-4. Photo I-V curve for Device # 655.

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134 CHAPTER 6 CIGS ABSORBER GROWTH AND DEVICE FABRICATION So far, achievements in optimizing absorber growth have mainly been directed by intuition; the understanding of CI GS film growth seems to be limited. Voltage differences that could not be accounted for by band gap variations were found to have excel lent correlation with trap densities within the CIGS [142]. The pe rformance disparities betw een devices made using absorber layers fabricated in various facilities ma y be explained by this assortment of defects. The three-stage process currently leads to the best solar cells produced in the literature. Methods that are used to form epitaxial films, such as MBE or MOCVD, have revealed interesting features for fundamental studies, such as phase segregation and defect formation, but cannot be used to form the base material fo r high-efficiency solar cells [36]. Best Devices in the Literature The world record Cu(In1-xGax)Se2 small-area device fabricated at NREL has an efficiency of 19.5% (VOC = 0.694 V, JSC = 35.2 mA/cm2, FF = 79.7%) [146]. The band gap for this absorber is 1.14 eV (x~0.3). NREL also had th e first research team break the 20% efficiency barrier for thin film solar cells: a CIGS cell with = 21.1% at 14X concentration [106]. The most efficient CIS cell fabricat ed at UF from an absorber grown in the MEE reactor had previously been 7.1% (VOC = 0.376 V, JSC = 31.0 mA/cm2, FF = 0.61%) [3]. Growth Matrix Maximum performance of Cu(In1-xGax)Se2 is attained within an optimum atomic compositional range. This is true for the devia tion from stoichiometry (Cu/III ratio) and for the Ga content (x), which directly affects the band ga p. Increasing the Ga content mainly increases the band gap by shifting the conduction band position. The composition window is 0.88 < [Cu]/([In]+[Ga]) < 0.95 and [Ga]/([In]+[Ga]) ~ 0.3. Small variations in band gap (or Ga content)

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135 can lead to measurable changes of performan ce (Contreras, CIGS11). In cell designs with homogenous composition, the optimum Ga/(In +Ga) ratio is a trade-off between VOC and JSC [147]. The use of Ga/(Ga+In) profiles has been found to increase the efficiency of CIGS thin film solar cells [136]. The obtainable composition profiles are strongly di ctated by the deposition method. The objective is to vary the band ga p in a manner to simultaneously design a lower band gap to enhance the absorption along with a higher band gap to improve the voltage in the same device. By introducing a Ga gradient in the film with higher Ga concentration near the back contact, a back surface field is created. Th e effect of this back surface field has been observed as an increase mainly in the VOC of the solar cells. A dditionally, when thinning down the CIGS film, the back surface field has been found to be efficient in reducing VOC and FF losses caused by the limited thickness of the sample. In order to optimize the Ga profiles, the interdiffusion and intermixing pr operties of In-rich and Ga-rich CIGS compounds must be taken into account. An absorber with normal profiling is one wher e the band gap is increased from front to back [106]. To utilize normal profiling in CIGS, higher Ga content must be added toward the back of the CIGS absorber. This will result in an increase in the conduction band edge and the attainment of an effective force field repelling minority carriers fr om the back contact. Linear graded absorbers show a correlation between VOC, JSC, and the slope of the grading [148]. The VOC improves with higher Ga contents in th e front part of the absorber, while the JSC is affected by the minimum Ga content, but also strongly by the electric fields that determine the current collection.

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136 The double profiling structure incorporates normal profiling plus an inverse profiling in the region adjacent to the surface of the film. A particular case of this double profiling is a notch structure: a low band gap material sandwiched between two wider gap materials. The possibility of attaining higher open-circu it voltages due to the presence of a wider band gap absorber material at the junction struct ure has potential for optimized photon absorption. The two energy gaps at the front of the double pr ofile structure can be engineered to match certain bands of the terrestrial solar spectrum in order to capture more efficiently from th e blue and red spectral regions. The improved quantum efficiency in such a device will translate into enhanced current generation. An additional enhancement of current generation could also come from a fieldassisted collection where the increasing conduction band edge found towards the back of the absorber can provide an effective force field fo r electrons drifting toward the back contact. Devices incorporating a double profi ling absorber preserve the high VOC values attained in the normal profiling devices and yield higher JSC for comparable effective band gaps. This can be explained from the enhanced spectral response of the double-profiling structure. Enhancements in VOC and FF could be explained by the reduced dark -current due to the grading in the absorber [106]. In multi-graded absorbers, the absorpti on and by this, the current, is dominated by the minimum band gap if proper current collection is guaranteed. Furthermore, it has been shown that VOC is correlated to the band gap in the space charge region [148]. In most ungraded solar cells, there is no drif t force outside the depletion region to help carriers move toward the junction to be collected. They must rely on diffusion to collect carriers generated outside the depletion region and if the carriers are generated much further than a diffusion length away from the edge of the depl etion region, they have only a small probability of being collected. Modeling of an increased band gap in the depletion region showed an

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137 increase in VOC [91]. The uniform incorporation of Ga into the absorber raises the band gap and increases the VOC with this higher VOC being the primary goal of any alloying attempts. Device performance may be further enhanced by nonuniform incorporation. However, graded structures that significantly degrade performance may just as easily result. Therefore, it is important to understand how different compositiona l profiles arise from different sequential proc essing steps, and what effects these profiles have on device performance. Material and device parameters other than the band gap vary with composition su ch as carrier concentration, mobility, diffusion lengths, types of intrinsic defect s, and band offsets with CdS. The price and availability of indium will become a dominant concern for the photovoltaics industry when larger scale pr oduction of CIS-based solar cells gets under way. Most highefficiency CIGS absorbers are currently grown to a thickness of about 2 to 2.5 microns [89]. One way to lessen the need for In would be to reduce the thickness of th e absorber layer by at least 50%. Lundberg et al. f ound that approximately 1% abso lute efficiency was lost by reducing the CIGS thickness from 1.8 microns down to 1 micron at the same growth rate [149]. A small growth series was implemente d to test the feasibility of CuIn1-xGaxSe2 growth in the PMEE reactor. These growth runs were accomp lished at a time when the reactor was able to withstand a substrate temperature of 491C. The details are located in Table 6-1. The Constant Cu Rate Process was used for the deposition of these films because of its simplicity and this leads to ungraded CIGS absorbers. Four di fferent CIGS films were grown with varying thickness with x ~0.3. They ranged from about 0.8 to 1.9 m. Then x was varied from 0 to 0.4 for films grown slightly thinner than 1 m (about 0.9 for the CIGS films and about 1.0 for the CIS film). The growth rate varied from about 0.9 to 1.6 /s. All of the absorbers were exposed

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138 to the cyanide etch to ensure that no copper selenide secondar y phases would be present on the surface. Absorber Characterization Orientation The X-ray diffraction pattern for film #582, as seen in Figure 6-1, showed a slightly stronger (204)/(220) double peak than (112) peak and a much w eaker double peak of (116)/(312) orientation. Several additional peaks, which could be clearly as cribed to the CIS structure as well, were very weak and therefore not used for evaluation. The orientation of single step CIGS films did not seem to be dependent upon any of the growth parameters: Cu/(Ga+In) ratio, Ga/(In+Ga) ratio or thickness. The films grown at various thicknesses displayed different diffraction patterns (Figure 6-2), but the orientation did not seem to be related to the thickness. The thinnest film, #575, showed a strong (112) peak, while #578 had a strong (204)/(220) double peak. The film with the greates t thickness, #579, had a very str ong (112) peak. The Ga content also seemed to have no specific effect on orientati on as can be seen in Figure 6-4. CIGS films with Ga/(Ga+In) ratios from 0 to 0.4 had different diffraction pattern, but no specific trend was followed. This was also true for the Cu/(Ga+I n) ratio where a Cu-poor and near-stoichiometric sample are compared in Figure 6-3. As will be revealed below from the literature, orientation strongly depends on substrate t ype, Se flux ratio, and growth temperature, but preferred orientation in the absorber is not a necessa ry requirement for attaining high-efficiency. The high-efficiency CIGS absorbers produced at NREL display a <220/204> type of preferred orientation [150]. They had a ratio of XRD intensity of the <220/204> peak over the <112> peak of ~15.4. Previous absorbers grow n at NREL and from other groups have typically been either randomly oriented or, in some cases (112) oriented [151]. The <220/204> preferred orientation in CIGS is obtained when using relatively high Se ove rpressures, such as an atomic

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139 flux of Se that is at least four times the flux of the metals. The <112> preferred orientation was obtained using Se overpressure s below this value [150]. Precise control of the elemental composition of Se in the vapor flux is not easy to obtain due to the low sticking coefficient of Se. The sticking coefficient of Se onto the substrate increases significantly when adding even small me tal fluxes. Umeno et al. have shown that the Se/metal flux ratio affects grain growth and the orientation of the bulk region in CIGS. Precise control of the Se/metal flux ratio is important for improving the crystallinity and thus the device performance [152]. Hanna et al. showed that th e Se to metal flux ratio (SMR) has an effect on films deposited at different growth rates. The best devices obtained from fast processes (25 minutes) had an SMR of 4.5, while a higher SM R of 13.5 was required in order to obtain comparable results from slow processes (75 mi nutes) [153]. All samples were going towards the (220/204) orientation with incr easing SMR, which is in agre ement with the observations by Contreras. In order to compensate for the lack of Se in the vapor phase, growing CIGS films form additional metal-rich segregations, which lead to shunts in the solar cell. In addition, layers prepared at low SMRs lose indium by way of the formation of the volatile compound, In2Se. Growth temperature can also have an effect on the preferred orientat ion of films grown by the three-stage process. Contrera s et al. showed that when Cu is incorporated using a substrate temperature of ~400-500C, the resulting CIS fi lms consistently present a (220/204) preferred orientation. On the other hand, when Cu depos ition occurs at a substrate temperature greater than 500C, the films begin to show random orie ntation or somewhat of a (112) texture [154]. At low temperatures, the Na diffusion from th e glass to the CIGS layer is suppressed. Lammer et al. observed that a reduction of depositi on temperature leads to a total loss of (112) preferential orientation. Na co-evaporation leads to a more pronounced (220/204) orientation, as

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140 normally seen in films for high-efficiency devi ces [155]. Hanna noted a high Na supply at the beginning by a NaF precursor layer or a str ongly reduced Na supply by a Na barrier layer promotes (112) orientation. A fair amount of Na at the beginning of the process as supplied by the standard substrates leads to a (220/204) or ientation, for all SMRs [153]. But Contreras believes that sodium, when present at a critical level, can hinder the at tainment of the (220/204) preferred orientation. The NREL group says that (220/ 204) preferred orientation can be attained for Cu-poor materials, but Cu-rich films grow n under similar growth conditions tend to be randomly oriented [154]. Morphology Shafarman et al. showed that at lower temp eratures, the uniform flux process appears to give more columnar grains and a smoother surface than CIGS films with a Cu-rich growth period. There is no apparent difference between films grown with Cu-rich flux at either the beginning or middle of deposition [130]. Typical characteristics of the high-efficiency absorbers grown by the three-stage process are a compound grain structure observed in the cross-section view and faceted grains that ar e visible in plan view [156]. Device Fabrication The typical device fabrication procedure for highly-efficient CIGS devices at NREL uses CdS deposition where the CIGS thin film is imme rsed at room temperature and the temperature of the bath is increased to 60C [156]. 50-60 nm were deposited in 16 minutes. A 90 nm thick undoped ZnO layer and a 120 nm Al2O3-doped ZnO layer were deposited. Ni/Al grids were deposited by e-beam evaporation. The total cell area was 0.408 cm2. A 100 nm MgF2 film is deposited as an anti-reflective coating. Our CIGS films were fabricated completely at the University of Florida and had ZnxCd1-xS buffer layers, instead of the traditional CdS buffe rs, deposited on them after absorber growth.

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141 The CIGS films were cut into 1 x 1 samples and vacuum-sealed after their removal from the vacuum chamber, and they remained so until the buffer was deposited by CBD. All the CIGS films were etched in a 10% solu tion of potassium cyanide for five minutes. The procedure for the CBD deposition of ZnxCd1-xS is exactly the same as the one that we used for CGS as described in Chapter 5. The window layer consisted of a magnetron sp uttered 600 nm Al-doped ZnO layer just like the in-house fabricated CGS devices. The sheet resistance of the characterization TCO film grown along with the fabricated devices is approximately 20 /square. Cells were completed by evaporation of a Ni (50 nm)/Al (0.3 m) contact grid and finally defining the cells (0.429 cm2) by mechanical scribing. No anti-reflective coat ing is added. For I-V measurement purposes, part of the CIGS layer away from the grid was scratched off and an In contact was added to the Mo surface. Device Characterization The reference cell method of cal culating the illuminated I-V cu rve described in Chapter 3 was implemented to measure the electrical prope rties of the CIGS devices. One problem was that the controller that manipulated the opera ting temperature of the cell was broken. The temperature controller of the cool ing system is usually set at ~ 20C to keep the reading of the thermocouple and hence the temperature of the test cell at 25 1C. To r ectify the situation, the system was allowed to cool down between measur ement runs so that the thermocouple reading was maintained between 25 and 27 C throughout the testing period. The calibration cell, namely S2049-A1 fabricat ed in November of 2002, with a total cell area of 0.408 cm2 was measured on 3/22/04 at NREL with the following parameters for cell #2: VOC = 0.661 V, JSC = 33.73 mA/cm2, FF =77.47 %, and = 17.28 %. The goal of the reference method is to match the short-circuit current of the reference cell in our measurement system.

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142 The distance between the lamp and the test cell was decreased when the measured JSC was below 33 mA/cm2. An acceptable solar simulator intensity was reached giving the following measured calibration cell parameters: VOC = 0.642 V, JSC = 33.62 mA/cm2, FF =76.09 %, and = 16.42 %. After the lengthy measurement period of all the cells on eight different CIGS devices with an area of 0.429 cm2, the calibration cell was measured again to make sure there was no drift in the solar simulator intensity. The following parameters were measured for cell #2: VOC = 0.643 V, JSC = 33.56 mA/cm2, FF =76.24 %, and = 16.24 %. These results were satisfactory. This initial CIGS absorber series produced so me interesting results. The most efficient devices ever created in the PMEE reactor were the result of this growth plan. Table 6-2 shows the results of the IV character ization of the illuminated solar cells. Device #582, the CuInSe2 absorber, showed an efficiency of 8.9%. It had a very high open circuit voltage of 0.46 V. Its JSC = 30.5 mA/cm2 and FF = 64% were also quite good. Figure 6-3 shows the illuminated I-V curve for #582. The highest fill factor (67%) for any device produced from an absorber grown in the PMEE reactor was achieved by #588 resulting in an 8.6 % efficient cell. Both of these films were grown to thicknesses around 1 m. Figure 6-6 shows how the increased FF of device #588 compared to #575 leads to a higher conversion ef ficiency despite the fact that #575 has an elevated open circuit voltage. The illuminated I-V curve of device #588 is also compared to the CIGS calibration cell provided by NREL (Figure 67), which had a FF greater than 75 % and an efficiency greater than 16 %. Troubling, t hough, is the relatively poor efficiency of #586 and #587 since they were grown under almost identical conditions as #588. Solar cell performance generally drops w ith substrate temperature [157]. Devices fabricated at low substrate temperatures, however have realized efficiencies that do not fall below the minimum efficiencies obtained at 550C [158]. Although the observed differences in

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143 defect response between films grown at various temperatures may provide valuable insight into the differences between these materials, they ar e only one factor affec ting device performance. High-efficiency CIGS absorbers typically requ ire the incorporation of 0.1 at% Na, which enhances the FF and VOC. Rudmann et al. obtained a maximum cell efficiency of 13.8% using the post-deposition of NaF at a substrate temper ature of 400C, while a maximum efficiency of 14.9% was achieved for CIGS cells prepared on SLG at 580C [159]. Rudmanns group also showed that solar cells with CIGS absorber s deposited on soda-lime glass with an alkali diffusion barrier and external Na incor poration showed a better efficiency ( =14%) than those grown on SLG without the barrier and no outside source of Na ( =12.8%). Cells without any Na showed much lower efficiencies ( =9.5%) [160]. Conclusions A device with the highest efficiency for any absorber grown in the PMEE reactor was the result of this study. This nearly 9% cell was co mpletely fabricated at the University of Florida revealing our improved processing capabilities. The best cell was produced at a less than ideal growth temperature, with a thickness of approx imately one micron, and with no absorber or device annealing. The highest quality absorber al so had no gallium content, but a CIGS absorber did result in a device with a slightly lower efficiency ( = 8.6%). This absorber was grown with the Constant Cu Rate Process resulting in no band gap grading, which means there is further room for improvement for CIGS cells.

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144 Table 6-1. CIGS growth series. Film # Cu/III ratio Ga/(In+Ga) ratio Thickness ( m) 569 0.97 0.30 0.8 575 0.97 0.33 0.85 578 0.97 0.32 1.3 579 1.00 0.30 1.9 582 0.91 0 1.0 586 0.90 0.25 0.9 587 0.99 0.41 0.85 588 0.98 0.21 0.9 Note: All films were KCN-etched before device fabrication. 5820 1000 2000 3000 4000 5000 6000 7000 8000 9000 102030405060702 theta (degrees)countsCIS(112) Mo(110) CIS(220/204) CIS(312/116) Figure 6-1. Diffrac tion pattern of CIS film #582.

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145 5750 1000 2000 3000 4000 5000 6000 7000 8000 9000 102030405060702 theta (degrees)countsCIGS(112) Mo(110) CIGS(220/204) CIGS(312/116) A 5780 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)countsCIGS(112) Mo(110) CIGS(220/204) CIGS(312/116) B Figure 6-2. Diffraction patte rns of CIGS films grown to differe nt thicknesses. A) 0.9 m. B) 1.3 m.

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146 5860 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)countsCIGS(112) Mo(110) CIGS(220/204) CIGS(312/116) A 5880 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)countsCIGS(112) Mo(110) CIGS(220/204) CIGS(312/116) B Figure 6-3. Diffraction patterns of CIGS film s grown with different Cu/III ratios. A) Cu/III = 0.90. B) Cu/III = 0.98.

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147 5870 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)countsCIGS(112)Mo(110) CIGS(220/204) CIGS(312/116) A 5880 1000 2000 3000 4000 5000 6000 7000 8000 102030405060702 theta (degrees)countsCIGS(112) Mo(110) CIGS(220/204) CIGS(312/116) B Figure 6-4. Diffraction patterns of CIGS film s grown with different Ga/I II ratios. A) Ga/III = 0.41. B) Ga/III = 0.21.

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148 Table 6-2. Device parameters for the CIGS growth series. Device # VOC (V) JSC (mA/cm2) FF (%) Eff. (%) 569 0.457 24.3 39.4 4.4 575 0.476 26.3 55.5 7.0 578 0.325 26.6 34.2 3.0 582 0.457 30.5 64.0 8.9 586 0.317 21.3 26.8 1.8 587 0.308 18.1 41.7 2.3 588 0.437 29.3 67.0 8.6 Note: Device #579 gave no I-V characteristics. Figure 6-5. Illumina ted I-V curve for Device #582.

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149 Figure 6-6. Comparison of illumi nated I-V curves of Device #575 and #588. Figure 6-7. Comparison of the illuminated I-V curves of Device #588 and the calibration cell.

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150 CHAPTER 7 DYNAMIC REACTOR MODEL The prospect of real-time process control fo r the fabrication of CI S-based modules would accelerate the introduction of a profitable product. Continuous processing of material requires real-time control of the critical growth parameters. Active cont rol in the deposition zone is required to prevent the drift in flux rates that would otherwise occur due to material depletion and dynamic heat redistribution in evaporative sources [161]. Process optimization, e.g. by design of experiments, requires re liable processing conditions, which in turn are only attainable if robust sensors and real-time, in-situ process co ntrol strategies are avai lable. A high level of control would lead to a substantial increase in manufacturing yield and further optimization of cell efficiency [162]. The process to be modeled is a rotating disc molecular beam epitaxy reactor. As the high purity raw materials required for MBE growth beco me more costly, the run-to-run repeatability of the process, as well as the improved uni formity over larger su rface areas, becomes increasingly important. The behavior of the th ermal effusion sources used in such systems is very non-linear due to the e xponential relationship between th e rate of evaporation and temperature. Under high vacuum conditions, the case in a MBE reactor, the mean free path of particles is very large and thus molecular flow conditions prevail through most of the reactor. The process dynamics of this reactor should corr elate well with an in-line vacuum process. Flux Modeling The three sources in the PMEE r eactor whose performance is cri tical to the quality of the films produced are the Cu, In, and Ga effusion s ources. These three sources are thermal effusion cells with free evaporating surfaces that have the geometry shown if Figure 7-1. The crucible is

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151 radiatively heated by the filament s through which a regulated current flows. The shutter can be opened or closed in order to precisely contro l the exposure of the melt to the substrates. The spatial flux distribution on the substrate as a function of melt height and temperature has already been characterized by Serk an Kincal [163]. The only net s ource of flux is from the melt surface, in which the mass flow ra te is expressed as the produc t of the flux leaving the surface and the surface area of the melt. The evapor ation rate from the surface is governed by Knudsens effusion equation [163] i Vm ii22 mm i imPT FT3.5110 AWT (7-1) where iAW = Atomic weight of species i i mT = melt temperature of species i The vapor pressure, PV, in Equation (7-1) is determined by Antoines equation i i Vmi i mB PTexpA+ T (7-2) where A and B are material specific constants. Finally, the surface area of th e melt can be described by 2 mmA = rt (7-3) where mr = the radius of the melt In all of the calculations by Kincal, the evaporation rate from the melt surface, Fm(T), was set at unity without a loss of generality since the final results can be scaled to any evaporation rate by simply multiplying by the desired value of Fm(T). This normalization makes the results applicable to any material that satisfies An toines Law at any temperature of interest. The flux incident on the substrate is a function of the radial position. The fill level can be varied from a completely full crucible (hm = 9.0 cm) to an almost empty condition (hm = 1.0 cm). The deposition rate varies significantly with the melt height and posit ion on the substrate,

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152 making it impossible to use a single value for the flux to estimate the enti re flux distribution. These two requirements conflict with the need to have real-time deposition profiles since the calculations invol ved are numerically intensive. Fast calculation of fluxes at any melt height can be done on-line by first creating a look-up table with the results shown in Figure 7-2 [163]. Intermediate flux distribu tions can be estimated very quickly by inter polation from a set of deposit ion profiles at a series of reference heights. As material exits the crucible duri ng deposition, the melt height is continuously decreasing. Kincal determined that the time rate of change of the melt height (in cm/s) is given by [6] 32 iiiii i mmi3m2m1m0 m 2 i 2i2i mA BBmm-FTAWhhh dh dtN R2mRhmhaaaa (7-4) where i m = melt density of species i AN= Avogadros number i mh = melt height of species i BR= bottom radius of the crucible EBRR m = H (ER= exit radius and H = crucible height) 1234,,,aaaa are constants related to the geometry of the crucible and are not materials specific. PMEE Reactor Modeling The previously described flux model can be incorporated into an overall reactor model that describes the dynamics of thin film depos ition. The PMEE reactor can be modeled by a set of differential equations accompanied by a set of algebraic equations. The substrates are not stationary due to the rotating natu re of the platen so this dynamic must be incorporated into the model. The change in linear position of the substrate over time is defined by O2R dx dt (7-5) where = frequency of rotation OR= radius of the center of the platen

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153 The circumference of the platen centerline is given by OL = 2R (7-6) Finally, the position on the platen, x(t), is determined by the equation x't = modx,L (7-7) where mod is the modulus function. For example, mod10,31 Position x(0) = 0 was set as the point directly under the substrat e heater as shown in Figure 7-3. A mass balance on the film for metal species i gives an equation for the differential mass of species i. ,'ii i SSmiidmass AFhtxtAW dt (7-8) where SA = the area of the deposited film i SF = the atomic flux as a function of melt height (htm ibg) and platen position (xt'b g) i = sticking coefficient FS was calculated by Kincal [163] and it represen ts the flux hitting the substrate depending upon the melt height and the position of the substrate in relation to the center of the crucible. The platen rotates countercloc kwise at a constant rate wher e it travels th rough all of the deposition zones. In each metal deposition z one, flux strikes the subs trate leading to an accumulation of mass and reaction between the assorted elements The differential masses of each species are then summed to give the total mass balance around the deposited film. totidmassdmass dtdt (7-9) Algebraic equations complement the preceding di fferentials to give a total description of the system. The film compositi on of species i (mole fraction) depends on the mass and atomic weight of all the species present in the film.

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154 i i i j ik ijkmass AW x mass massmass AWAWAW (7-10) The mole fraction of one of the species can be determined from the others because they all must add up to 1, as follows xxxSeCuIn 1 (7-11) The total mass of a deposited CIGS film is depe ndent upon the total thickn ess of that film ( ), the density of the film ( ), and the deposition area. totCIGSCIGSSmassttA (7-12) Computer code The model is coded in MATLAB. An M-file (the name given to subroutines in MATLAB) named PMEE_model.m outputs film composition and thickness based on the following input parameters: platen radius, crucib le dimensions, substrate area, rotations per minute, deposition time, normalized flux matrix de pending on melt height and substrate position, initial melt heights, atomic wei ght of each species, sticking co efficients, melt temperature of each effusion cell, Antoine coefficients A and B of each species to determine vapor pressure, the density of each species, and the density of the final film. The printout of the m-file is given in Appendix B, which is commented to explain how the program works. SIMULINK is used to solve the differential equations; th e total time, the step change in time, and the method of solving ordinary differential equati ons, such as Euler or Runge -Kutta must be chosen. Two different models could be developed: one that takes into acc ount the selenium flux or one that assumes the Se flux is distributed in great excess with respect to film stoichiometry. Since the Se flux has not been modeled like the metal effusion sources, and Se is supplied in

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155 excess in the PMEE reactor, the model using exce ss Se was investigated. This model can be modified in a straightforward fashi on to include seleni um flux modeling. A look-up table must be created for the meta l flux based on the various melt heights that are possible in the source crucible. The normali zed flux on the substrate varies with the radial position of the substrate in relation to the center point of the crucible. Since each metal source has shielding around it, the radial distance away fr om the middle of the crucible that metal flux can still reach the substrate is limited. As is shown in Figure 7-4 of the cryoshroud, each metal zone sweeps out 40 of the circumference of the r eactor. This pertains to approximately 6.2 cm on each side of the crucible center point. So the look-up tables are only relevant up to this distance. The look-up table for the flux only contains values at cert ain melt heights so bicubic interpolation is used to calculate the flux at inte rmediate melt heights. Since the melt height is decreasing throughout the growth run, a new melt heig ht is being continuously calculated. This new melt height is used to calculate the subse quent interpolated flux. The normalized flux for each metal species is then converted into a ma ss, which accumulates over time on the substrate. Since the PMEE reactor incorporates a rotating platen, a specific flux distribution hits the substrate during each rotation. The amount of tim e the substrate remains in the metal deposition zone depends upon the rotational speed of the platen so the mass reaching the substrate per turn is time-dependent. Over a long growth run, the total mass will be the same, but the amount of mass accumulated per rotation differs. A faster ro tational speed allows the substrate to see the center of the metal crucible more times during a deposition period, but accumulates less mass per rotation.

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156 This model can be used for any material grow th that uses thermal effusion cells with free evaporating surfaces in a rotati ng disc reactor. Typical PMEE reactor parameters and film properties relevant to Cu-chalcopyrite growth are implemented into the input m-file named input_pmee.m. For the condition of excess Se flux, the composition of Se was set to xSe = 0.50. Conclusions The PMEE reactor has now been successfu lly modeled under the condition of excess selenium flux distribution. To represent our system without this stipulation, modeling of the selenium flux from the crucible must be done. One over-simplified solution to this is to model the selenium source as a therma l effusion source with an evap orating free surface. Once film properties are related to the performance paramete rs of the device, a true predictive model would be accomplished. The next step is to take known experimental parameters and incorporate them into the model. Knowing initial melt he ights, along with melt temperatur es and deposition time allows for the modeling of the experimental conditions within the PMEE reactor Theoretical film composition and thickness can be compared to the actual results to determine the sticking coefficients of each species. These sticking coe fficients can be calculated for several different reactor conditions to obtain an error bar of acceptable values.

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157 Figure 7-1. Metal source crucible. 0 5 10 15 20 25 0 0.002 0.004 0.006 0.008 0.010 r radial distance on substrate ( cm ) Normalized Flux (atoms/cm2.s)hm=9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 Figure 7-2. Deposition flux FS(r) (atoms/cm2-s) on the substrate at ni ne different melt levels, hm (cm). The results are normalized to un ity evaporation rate from the melt.

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158 Figure 7-3. Positioning of the sources in the reactor. Figure 7-4. Cryoshroud.

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159 CHAPTER 8 CONCLUSIONS AND FUTURE WORK Conclusions Growth temperature, growth recipe, and overall Cu/Ga ratio were varied over eight different sets of CGS growth runs. The subseq uent effect on film morphology and orientation was noted. The trends observed during film gr owth became more noticeable when the absorbers were used in the fabrication of solar cell devices. Growth temperature seems to be the most critical variable in achieving high-quality absorber films. The fact that Cu-rich absorbers grown by PMEE have been more successful than Ga-ric h ones is likely due to the lower deposition temperature. Ga-rich absorbers produce the high est efficiency CGS cells in the literature, but they are also grown at an elevated temperature of at least 550C. The best CGS cell produced in the PMEE reactor was grown at the highest growth temperature available, namely 491C. The abso rber used in this de vice was rather thin, approximately 0.6 m, so a thicker absorber under the same conditions should produce a more efficient device. Annealing under the standa rd conditions at 200C for 2 minutes should improve the low open-circuit voltage of our be st devices. The maximum device parameters achieved by different cells produced from absorb ers grown in the PMEE reactor are as follows: Jsc = 17.6 mA/cm2, Voc = 0.82 V, F.F. = 65.5 %, and = 5.3. Low Ga-content CIGS growth by PMEE was also st udied and resulted in the most efficient solar cell ever produced by our gr oup. This nearly 9% CuInSe2 cell was completely fabricated at the University of Florida revealing our improved pr ocessing capabilities. S till, the best cell was produced at a less than ideal growth temperature, with a thickness of onl y 1 micron, and with no absorber or device annealing. An 8.6% CIGS ce ll was produced in addition to the CIS cell. Our

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160 CIGS absorbers were grown with the Constant Cu Rate Process, resulting in an ungraded band gap, so further improvements are still possible. The PMEE reactor has now been successfu lly modeled under the condition of excess selenium flux distribution. The effusion source flux model developed previously was incorporated into a dynamic reactor model. The integral design issu e is the rotating nature of the substrates. Film parameters such as compositi on and thickness were re lated to the operation conditions of the reactor and the intrinsic prope rties of the materials involved in deposition. Future Work Since the biggest drawback in using the PMEE r eactor to grow polycrystalline films is the limit that we must observe on the maximum subs trate growth temperature, a more effective technique may be to grow Cu-Se and Ga-Se bilaye r precursors at a low substrate temperature and then anneal the samples at a high temperatur e for a short period of time to produce CuGaSe2. Successful CGS growth by RTP coul d lead to the fabrication of tandem cells that use a CGS layer in the design of the top cell. The growth of Ga-rich CGS absorbers should be investigated more thoroughly. The improvement in device efficiency in the literature was a result of optimal Na incorporation, an optimal buffer layer deposition process, and de vice annealing in an oxygen atmosphere. The effects of these different processes should be investigated at lower growth temperatures. Once film properties are related to the perf ormance parameters of the device, a true predictive model would be accomplished. Incorpor ation of this model into the existing control structure would improve the production efficiency of the PMEE reactor. This would eliminate the need for in-situ measurements of the film properties that are expensive and sometimes difficult to implement.

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161 APPENDIX A GROWTH RUN DATA The following tables in this appendix show the PMEE reactor conditions for each growth run described within Chapter 4 and Chapter 6. The film numb er, along with the growth run objective is presented with the r eactor operator and date of experiment in th e first part of the table. Target values, such as the metal deposition rates for each sublayer, the gallium primary temperature, and the gap temperature of the substr ate heater, are shown in the next section of the table. The values used for the pre-deposition he ating of the metal sour ces can be found in the following segment. Finally, the operation result s are displayed; these include temperatures, growth rates, pressures, and the deposition time.

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162 Table A-1. Reactor conditions for Growth Run #443. Run # 443 Date: 03/22/03 Film: CGS Operator: Suku Kim, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: To grow thick graded CGS with overall Cu/Ga~0.9 Target Value Metal Se Substrate Heater Cu Rate (A/s) 8.3/6.8/5.3 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 255 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.2 -11.8-12.0 14.8 9.2 35.5 Tip (C) 1133-1141 ----Primary (C) 1089-1096 -974-976 255.1 500.1 700.1 LSP 8.3/6.8/5.3 -975 255 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.2-15.5 -10 14.9 9.5 35.9 Thick. (kA) 150 -----Run time (hr:min:sec): 6:20:55 Headspace P (torr): Run time (seconds): 22,855 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 50 8.29 50 100 6.79 100 150 5.28

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163 Table A-2. Reactor conditions for Growth Run #444. Run # 444 Date: 03/23/03 Film: CGS Operator: Woo Kyoung Kim, Seokhyun Yoon Substrate: 2 Mo, 1 Glass Objective: To grow thick graded CGS with overall Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 8.6/7.1/5.6 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 255 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.2 -12.3-12.4 15.9-16.2 9.2-9.3 35.5-36.5 Tip (C) 1129 -937 ---Primary (C) 1078 -974-976 255.2 500.3 700 LSP 8.6/7.1/5.6 -975 255 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16/14.7/13.4 -10 16.4 10.0 35.4 Thick. (kA) 150 -----Run time (hr:min:sec): 6:00:22 Headspace P (torr): Run time (seconds): 21,622 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 50 8.63 50 100 -100 150 -

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164 Table A-3. Reactor conditions for Growth Run #445. Run # 445 Date: 03/24/03 Film: CGS Operator: Suku Kim, Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: To grow thick graded CGS with overall Cu/Ga~0.95 and a Se temp. change Target Value Metal Se Substrate Heater Cu Rate (A/s) 8.6/7.1/5.6 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 230/255 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15.5/15/--12.1 10.5/15.5 9.1 35 Tip (C) 1131-1133 -936 ---Primary (C) 1051-1079 -975 230.4/254.6 500.5 700 LSP 8.6/7.1/5.6 -975 230/255 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15.5/14.5/12.6 -10 16.4 9.5 35.9 Thick. (kA) 150 -----Run time (hr:min:sec): 6:02:30 Headspace P (torr): Run time (seconds): 21,750 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 50 8.68 50 100 7.12 100 150 5.57

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165 Table A-4. Reactor conditions for Growth Run #446. Run # 446 Date: 03/25/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: To grow thick graded CGS with overall Cu/Ga~1.1 Target Value Metal Se Substrate Heater Cu Rate (A/s) 9.8/8.3/6.8 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 230/255 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.5/15.3/--12.0-12.2 9.2/13.1 9.0 34.8-36.2 Tip (C) 1138-1146 -936-937 ---Primary (C) 1091/1082/1072 -974-976 230.2/255.2 500.4 699.8-700.5 LSP 9.8/8.3/6.8 -975 230/255 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16/14.9/--10 9.8/13.6 10.0 35.7 Thick. (kA) 150 -----Run time (hr:min:sec): 5:07:50 Headspace P (torr): 3.5 x 10-8 Run time (seconds): 18,470 Growth P (torr): 6.0 x 10-8 Cu rate Thickness (kA) Rate (A/s) 0 50 9.80 50 100 8.29 100 150 6.81

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166 Table A-5. Reactor conditions for Growth Run #447. Run # 447 Date: 03/26/03 Film: CGS Operator: Woo Kyoung Kim, Seokhyun Yoon Substrate: 1 p-GaAs, 1 Mo, 1 Glass Objective: To grow thick graded CGS with overall Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 8.8/7.3/5.8 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15.9/15.2/14.0 -11.9-12.0 9.2/13.1 8.7-9.3 37.8-37.9 Tip (C) 1143/1138/1130 -935-936 ---Primary (C) 1089/1080/1064 -974-976 259.8/260.3 500.9 699.5-700.6 LSP 8.8/7.3/5.8 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15.7/14.95/13.4 -10 13.8 10.3 37 Thick. (kA) 150 -----Run time (hr:min:sec): 5:53:53 Headspace P (torr): Run time (seconds): 21,233 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 50 8.83 50 100 7.31 100 150 5.72

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167 Table A-6. Reactor conditions for Growth Run #452. Run # 452 Date: 04/05/03 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Modified 3-stage CGS with overall Cu/Ga~1.1 Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.54/8.66 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 12.5-13.5 -12.0-12.1 15.2 9.0 35.0-37.2 Tip (C) 1127/1144 -----Primary (C) 1054/1089 -974-976 260.1 500.3 699.4-700.5 LSP 5.54/8.66 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12-13/16 -10 15.4 9.7 36.1 Thick. (kA) 110 -----Run time (hr:min:sec): 4:00:34 Headspace P (torr): 4.0 x 10-8 Run time (seconds): 14,434 Growth P (torr): 1.7 x 10-8 Cu rate Thickness (kA) Rate (A/s) 0 26.72 5.55 26.72 110 8.66

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168 Table A-7. Reactor conditions for Growth Run #453. Run # 453 Date: 04/06/03 Film: CGS Operator: Woo Kyoung Kim, Seokhyun Yoon Substrate: 3 Mo, 1 Glass Objective: CGS with overall Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.5/9.7/0 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) --/17 --14 9 35-37 Tip (C) --/1143 -----Primary (C) --/1099 --260.2 500.5 700.0 LSP 5.54/9.7 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset --/16 -10 15 10 36.1 Thick. (kA) 110 -----Run time (hr:min:sec): 4:13:44 Headspace P (torr): Run time (seconds): 15,224 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 100 8.37 *GaSe was deposited for ~ 54:30 after CG S deposition and befo re Se annealing

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169 Table A-8. Reactor conditions for Growth Run #454. Run # 454 Date: 04/07/03 Film: CGS Operator: Woo Kyoung Kim, Seokhyun Yoon Substrate: 3 Mo, 1 Glass Objective: CGS with overall Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 6/9.7/9.6/6 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) --/18/17 --14.8-14.9 9.1 35.5-37.5 Tip (C) --/1148/1138 -----Primary (C) --/1104/1091 --260.3 500.4 699.5-700.5 LSP 5.9/9.7/9.6/6.0 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.2/17.2/16.5 -10 15.4 10.0 36.1 Thick. (kA) 100 -----Run time (hr:min:sec): 4:00:04 Headspace P (torr): Run time (seconds): 14,404 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 10 5.9 10 23.71 9.7 23.71 45 7.6 45 100 6.0

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170 Table A-9. Reactor conditions for Growth Run #455. Run # 455 Date: 04/08/03 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: CGS with overall Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 6/9.7/9.6/6 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15.5-18.4 -12.1-18.4 16 9.3 35.2-37.4 Tip (C) 1134-1150 -930 ---Primary (C) 1080-1110 -974-976 260.0 500.2 699.5-700.5 LSP 5.9/9.7/9.6/6.0 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15/19/17/15.7 -10 16.0 9.7 36.2 Thick. (kA) 100 -----Run time (hr:min:sec): 4:11:55 Headspace P (torr): 4.0 x 10-8 Run time (seconds): 15,115 Growth P (torr): 1.7 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 10 5.89 10 23.71 9.67 23.71 45 7.56 45 100 5.99

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171 Table A-10. Reactor conditions for Growth Run #456. Run # 456 Date: 04/09/03 Film: CGS Operator: Seokhyun Yoon Substrate: 3 Mo, 1 Glass Objective: CGS with overall Cu/Ga~0.9 Target Value Metal Se Substrate Heater Cu Rate (A/s) 6/9.7/7.3/5.6 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.8-17.5 -11.9 14.8-15 9.05 35-37 Tip (C) 1128-1140 -932.5 ---Primary (C) 1075-1144 -974-976 259.8 500.4 699.5-700.5 LSP 5.9/9.7/7.3/5.6 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.7-17 -10 14.8 9.7 36 Thick. (kA) 100 -----Run time (hr:min:sec): 4:23:20 Headspace P (torr): Run time (seconds): 15,800 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 10 5.9 10 19.4 9.7 19.4 45 7.3 45 100 5.6

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172 Table A-11. Reactor conditions for Growth Run #457. Run # 457 Date: 04/10/03 Film: CGS Operator: Ryan Acher Substrate: 3 Mo, 1 Glass Objective: CGS with overall Cu/Ga~0.96 Target Value Metal Se Substrate Heater Cu Rate (A/s) 0/6/10.3/7.6/5.2 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) --/17.5/--11.8-12.0 14.7 9 36 Tip (C) 1130-1143 -932 ---Primary (C) 1059-1106 -974-976 260.1 500.3 699.5-700.5 LSP 5.9/10.3/7.6/5.2 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.5-17.2 -10 15 10 36 Thick. (kA) 100.4 -----Run time (hr:min:sec): 4:13:25 Headspace P (torr): 4.9 x 10-8 Run time (seconds): 15,205 Growth P (torr): 1.9 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 10 5.9 10 47.3 10.3 47.3 73.8 7.6 73.8 100.4 5.2 *GaSe was deposited first for 20 minutes

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173 Table A-12. Reactor conditions for Growth Run #458. Run # 458 Date: 04/11/03 Film: CGS Operator: Ryan Kaczynski, Woo Kyoung Kim Substrate: 4 Mo, 1 Glass Objective: Modified 3-stage CGS with overall Cu/Ga~0.96 Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.2/8.5/7.8/4.9 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 12.5-13.5 -12.0-12.1 15.2 9.0 35.0-37.2 Tip (C) 1127-1144 -932 ---Primary (C) 1054-1089 -974-976 260.1 500.3 699.4-700.5 LSP 5.2/8.5/7.8/4.9 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12-16.2 -10 15.4 9.7 36.1 Thick. (kA) 100 -----Run time (hr:min:sec): 4:43:39 Headspace P (torr): 4.4 x 10-8 Run time (seconds): 17,019 Growth P (torr): 1.8 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 10 5.17 10 38.8 8.50 38.8 50 7.79 50 100 4.88

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174 Table A-13. Reactor conditions for Growth Run #459. Run # 459 Date: 04/12/03 Film: CGS Operator: Ryan Acher Substrate: 3 Mo, 1 Glass Objective: Modified 3-stage CGS with overall Cu/Ga~0.86 Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.5/7.8/6.8/4.9 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.1-16.8 -11.8-12.1 15.0 9.1 36.5 Tip (C) 1118-1130 -930 ---Primary (C) 1056-1085 -974.8-975.2 260.0 500.3 699.5-700.5 LSP 5.5/7.8/6.8/4.9 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.6-16.3 -10 15 9.7 36.3 Thick. (kA) 100 -----Run time (hr:min:sec): 4:57:30 Headspace P (torr): Run time (seconds): 17,850 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 10.5 5.5 10.5 31.2 7.8 31.2 46.2 6.8 46.2 100 4.9

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175 Table A-14. Reactor conditions for Growth Run #472. Run # 472 Date: 05/15/03 Film: CGS Operator: Ryan Kaczynski, Seokhyun Yoon Substrate: 1 Mo, 1 Glass Objective: GaSe/CGS with Cu/Ga~1.25/GaSe Target Value Metal Se Substrate Heater Cu Rate (A/s) 13.9 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 250/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 22.5-22.7 -13.3-13.5 16/23 9.5 35.8-36 Tip (C) 1133 -959 ---Primary (C) 1130 -1006-1008 250.6/276 501 699.6 LSP 13.9 -1007 250/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 22.3 -10 16.3 12.0 35.8 Thick. (kA) 110 -----Run time (hr:min:sec): 2:52:01 Headspace P (torr): Run time (seconds): 10,321 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 110 13.99 *GaSe was deposited for 36 minutes befo re CGS deposition and 5 minutes after

PAGE 176

176 Table A-15. Reactor conditions for Growth Run #474. Run # 474 Date: 05/19/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: GaSe/Cu-rich CGS/Ga-rich CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 0/14.9/10.3 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 21/19 -13.2-13.35 16.8/21.5 9.3 35.5-37.0 Tip (C) 1151/1141 -958 ---Primary (C) 1135/1113 -1006-1008 260/276 500.3 699.5-700.2 LSP 14.9/10.3 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 20.6/18.4 -10 16.5/19.0 10.0 36.0 Thick. (kA) 110 -----Run time (hr:min:sec): 3:19:09 Headspace P (torr): 4.7 x 10-8 Run time (seconds): 11,949 Growth P (torr): 3.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 90 14.9 90 110 10.3 *GaSe deposited for 33 minut es before Cu deposition

PAGE 177

177 Table A-16. Reactor conditions for Growth Run #475. Run # 475 Date: 05/20/03 Film: CGS Operator: Seokhyun Yoon Substrate: 2 Mo, 1 Glass Objective: GaSe/Cu-rich CGS/Ga-rich CGS, overall Cu/Ga~0.96 Target Value Metal Se Substrate Heater Cu Rate (A/s) 0/14/9.7 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 20.6/--13.2 15/16.5 9.3 36-37 Tip (C) 1155/--958-959 ---Primary (C) 1132/--1006-1008 260/276 500.4 699-700 LSP 14/--1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 20.3/--10 16.0/19.0 10.0 36.0 Thick. (kA) 110 -----Run time (hr:min:sec): 2:56:48 Headspace P (torr): 5.0 x 10-8 Run time (seconds): 10,608 Growth P (torr): 3.3 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 90 14.0 90 110 9.86 *GaSe deposited for 33:50 before Cu deposition

PAGE 178

178 Table A-17. Reactor conditions for Growth Run #476. Run # 476 Date: 05/21/03 Film: CGS Operator: Ryan Acher Substrate: 1 Mo, 1 Glass Objective: GaSe/Cu-rich CGS/GaSe, overall Cu/Ga~0.96 Target Value Metal Se Substrate Heater Cu Rate (A/s) 14 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 21.1 -12.9-13.1 17.6/19 9.1 36.5 Tip (C) 1153-1159 -958-960 ---Primary (C) 1135-1140 -1006-1008 260/277 500.4 699.7-700.3 LSP 14 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 20.6 -10 16/19 10.0 36.5 Thick. (kA) 110 -----Run time (hr:min:sec): 2:57:30 Headspace P (torr): 3.2-4.1 x 10-8 Run time (seconds): 10,650 Growth P (torr): 3.5-4.1 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 13.98 *GaSe was deposited for 39:40 befo re CGS deposition and 6:40 after

PAGE 179

179 Table A-18. Reactor conditions for Growth Run #477. Run # 477 Date: 05/22/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: GaSe/Cu-rich CGS/GaSe, overall Cu/Ga~0.96 Target Value Metal Se Substrate Heater Cu Rate (A/s) 14 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 21.4 -13.02 15.5/16 9.0 37 Tip (C) 1161 -958.3 ---Primary (C) 1140 -1006-1008 260/276 500.5 700.0 LSP 14 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 20.8-21 -10 15.6/18.8 10.0 36.1 Thick. (kA) 110 -----Run time (hr:min:sec): 2:57:33 Headspace P (torr): 5 x 10-8 Run time (seconds): 10,653 Growth P (torr): 5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 14 *GaSe was deposited for 42:54 befo re CGS deposition and 3:33 after

PAGE 180

180 Table A-19. Reactor conditions for Growth Run #478. Run # 478 Date: 05/23/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: 3-stage emulation (GaSe/CuSe/GaSe) Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.8 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 19.9 -12.9-13.1 15.1/18.3 9.0 36-38 Tip (C) 1152 -958-961 ---Primary (C) 1125 -1006-1008 260/277 500.5 699.4-700.4 LSP 10.8 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 19.5 -10 16.0/18.5 10.0 36.8 Thick. (kA) 110 -----Run time (hr:min:sec): 5:48:00 Headspace P (torr): 3.8-4.9 x 10-8 Run time (seconds): 20,880 Growth P (torr): 1.3-4.2 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 10.73 *GaSe was deposited for 2:50:07 befo re CGS deposition and 7:05 after

PAGE 181

181 Table A-20. Reactor conditions for Growth Run #479. Run # 479 Date: 05/24/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: GaSe/Cu-rich CGS/GaSe, overall Cu/Ga~0.96 at 20 RPM Target Value Metal Se Substrate Heater Cu Rate (A/s) 14 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 22.7 -13.4 15.2/20.8 9.0 36.8 Tip (C) 1142 -957.4 ---Primary (C) 1132 -1006-1008 260.3/275 500.5 700.0 LSP 14 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 22.3 -10 16.0/17.5 10.0 36.8 Thick. (kA) 110 -----Run time (hr:min:sec): 2:57:33 Headspace P (torr): 4 x 10-8 Run time (seconds): 10,653 Growth P (torr): 4 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 14 *GaSe was deposited for 42:54 befo re CGS deposition and 3:49 after

PAGE 182

182 Table A-21. Reactor conditions for Growth Run #480. Run # 480 Date: 05/26/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: 3-stage emulation (GaSe/CuSe/GaSe) at 20 RPM Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.8 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260/275 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1007 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 21.8 -13.4-13.6 14.6/18.1 9.0 36.8-37.7 Tip (C) 1133-1140 -954-958 ---Primary (C) 1116-1118 -1006-1008 260/276 500.5 699.8-700.2 LSP 10.8 -1007 260/275 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 21.05 -10 15.5/18.5 10.0 37.2 Thick. (kA) 110 -----Run time (hr:min:sec): 5:48:00 Headspace P (torr): 3.5-4.8 x 10-8 Run time (seconds): 20,880 Growth P (torr): 1.4-4.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 10.84 *GaSe was deposited for 2:50:00 befo re CGS deposition and 7:05 after

PAGE 183

183 Table A-22. Reactor conditions for Growth Run #510. Run # 510 Date: 08/01/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 452 with T = 700C, Cu/Ga~1.1 Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.25/8.2 Cracker (C) 500 Temp. (C) 700 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 700C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14/16.5 -16.3-17 15-16 9-11 36-38 Tip (C) 1118/1125 -933-936 ---Primary (C) 1047/1078 -974-976 260.1-260.6 499.5-500.5 699.5-700.5 LSP 5.25/8.21 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.2/15.3 -10 16.5 10.3 37.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:12:21 Headspace P (torr): 4 x 10-8 Run time (seconds): 15,141 Growth P (torr): 2 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 26.65 5.25 26.65 110 8.28

PAGE 184

184 Table A-23. Reactor conditions for Growth Run #511. Run # 511 Date: 08/04/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 453 with T = 750C, Cu/Ga~1.1 Target Value Metal Se Substrate Heater Cu Rate (A/s) 6.55/11.45 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14/19 -17.1-17.4 15.5-16.5 10-11.5 40-42 Tip (C) 1120/1140 -930 ---Primary (C) 1058/1100 -974-976 255-265 499.5-500.5 749-750.3 LSP 6.55/11.45 -975 260 500 750 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14/18.5 -10 15 10.4 40.0 Thick. (kA) 110 -----Run time (hr:min:sec): 3:22:18 Headspace P (torr): 4.5 x 10-8 Run time (seconds): 12,138 Growth P (torr): 3.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 15.7 6.58 15.7 110 11.43 *GaSe deposited for 25 minutes after Cu deposition

PAGE 185

185 Table A-24. Reactor conditions for Growth Run #512. Run # 512 Date: 08/05/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: Reverse process of Run# 511 Target Value Metal Se Substrate Heater Cu Rate (A/s) 8/4.55 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.5/13.9 -16.9-17.6 16.5 9.8 40-42 Tip (C) 1128/1108 -925-927 ---Primary (C) 1078/1047 -974-976 259.5-260.5 500.3 749-750.5 LSP 8/4.5 -975 260 500 750 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16/13.5 -10 16.5 10.4 40.5 Thick. (kA) 110 -----Run time (hr:min:sec): 4:53:51 Headspace P (torr): 4.5 x 10-8 Run time (seconds): 17,631 Growth P (torr): 3.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 85.4 7.97 85.4 110 4.54 *GaSe deposited for 25 minut es before Cu deposition

PAGE 186

186 Table A-25. Reactor conditions for Growth Run #513. Run # 513 Date: 08/06/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 447 with T = 750C, Cu/Ga~0.95 Target Value Metal Se Substrate Heater Cu Rate (A/s) 6.7/5.5/4.4 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.1-15.6 -17.5-18.1 16 9.7 40-42.5 Tip (C) 1110-1123 -930 ---Primary (C) 1048-1068 -974-976 260-261 500.4 749-750.5 LSP 6.68/5.52/4.3 -975 260 500 750 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.5-15.0 -10 17 10.4 40.5 Thick. (kA) 110 -----Run time (hr:min:sec): 5:32:04 Headspace P (torr): 6.0 x 10-8 Run time (seconds): 19,924 Growth P (torr): 3.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 44.4 6.7 44.4 81 5.5 81 110 4.4

PAGE 187

187 Table A-26. Reactor conditions for Growth Run #514. Run # 514 Date: 08/08/03 Film: CGS Operator: Woo Kyoung Kim, Seokhyun Yoon Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 452 with T = 750C, Cu/Ga~1.1 Target Value Metal Se Substrate Heater Cu Rate (A/s) 4.65/7.25 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 13.7/------Tip (C) 1111/------Primary (C) 1044/------LSP 4.65/7.25 -975 260 500 700 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.0/--10 ---Thick. (kA) 110 -----Run time (hr:min:sec): 4:45:45 Headspace P (torr): Run time (seconds): 17,145 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 26.7 4.71 26.7 110 7.25

PAGE 188

188 Table A-27. Reactor conditions for Growth Run #515. Run # 515 Date: 08/11/03 Film: CGS Operator: Seokhyun Yoon Substrate: 2 Mo, 1 Glass Objective: Graded Cu-rich CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 0/5.3/8.4/5.2 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.7/--/--18.05 15-17 8.8-9.2 39-41 Tip (C) 1118/--/--926 ---Primary (C) 1055/1092/--974-976 260-261 500-501 749-750 LSP 5.3/8.3/5.3 -975 260 500 750 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.1/17.5/--10 18 11 42 Thick. (kA) 110 -----Run time (hr:min:sec): 4:37:29 Headspace P (torr): Run time (seconds): 16,649 Growth P (torr): Cu rate Thickness (kA) Rate (A/s) 0 18.8 5.30 18.8 98.9 8.42 98.9 110 5.59 *GaSe deposited for 26:40 before Cu deposition

PAGE 189

189 Table A-28. Reactor conditions for Growth Run #516. Run # 516 Date: 08/12/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: Cu-rich CGS with consta nt Cu rate throughout growth run Target Value Metal Se Substrate Heater Cu Rate (A/s) 6 Cracker (C) 500 Temp. (C) 750 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 260 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 750C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15 -17.8-18 15-16 10 40-42 Tip (C) 1120 -925 ---Primary (C) 1065 -974.5-975.5 260-261 500.0 750.0 LSP 6 -975 260 500 750 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.3 -10 17.3 10.0 41 Thick. (kA) 110 -----Run time (hr:min:sec): 5:05:33 Headspace P (torr): 5-9 x 10-8 Run time (seconds): 18,333 Growth P (torr): 5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 110 6.0

PAGE 190

190 Table A-29. Reactor conditions for Growth Run #521. Run # 521 Date: 09/29/03 Film: CGS Operator: Ryan Kaczynski Substrate: 1 Mo, 1 Glass Objective: Repeat Run# 452 with T = 900C (0.8/1.25) Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.4/8.4 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.5/17.7 -17.7-17.8 19-20 11.9 53-57.8 Tip (C) 1120/1138 -928-931 ---Primary (C) 1056/1090 -977-979 265.0 500.3 898-900.5 LSP 5.4/8.4 -978 265 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.5/16.8 -10 20.0 12.5 54.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:07:20 Headspace P (torr): 1.8 x 10-7 Run time (seconds): 14,840 Growth P (torr): 2.5 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 26.72 5.43 26.72 110 8.40

PAGE 191

191 Table A-30. Reactor conditions for Growth Run #522. Run # 522 Date: 09/30/03 Film: CGS Operator: Seokhyun Yoon Substrate: 1 Mo, 1 Glass Objective: Repeat Run# 453 with T = 900C (0.8/1.4/0) Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.4/8.4/0 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15/18.6 -17.1 19.9 12.1-12.3 55-57 Tip (C) 1118/1130 -928 ---Primary (C) 1050/1097 -977-979 264.5-265.5 500.3 897-900 LSP 5.4/8.4 -978 265 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13/16 -10 19.5 12.5 54.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:06:01 Headspace P (torr): 2.1 x 10-7 Run time (seconds): 14,761 Growth P (torr): 1.6 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 13 5.4 13 110 8.4 *GaSe deposited for 35 minutes after Cu deposition

PAGE 192

192 Table A-31. Reactor conditions for Growth Run #523. Run # 523 Date: 10/01/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 1 Mo, 1 Glass Objective: Repeat Run# 515 with T = 900C (0/0.9/1.45/0.9) Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.85/9.43/5.85 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) --/18.8/--17.5-17.6 20-20.5 12.2 55-57 Tip (C) --/1136/--928 ---Primary (C) --/1100/--974-976 264.5-265.5 500.2 897-900 LSP 5.85/9.43/5.85 -975 265 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15/17.75 -10 19.5 12.5 55.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:11:10 Headspace P (torr): 1.8 x 10-7 Run time (seconds): 15,070 Growth P (torr): 1.8 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 17.5 5.85 17.5 99.5 9.43 99.5 110 5.85 *GaSe deposited for 25 minut es before Cu deposition

PAGE 193

193 Table A-32. Reactor conditions for Growth Run #524. Run # 524 Date: 10/02/03 Film: CGS Operator: Seokhyun Yoon Substrate: 1 Mo, 1 Glass Objective: Repeat Run# 453 with T = 900C (0/1.4/0.8) Target Value Metal Se Substrate Heater Cu Rate (A/s) 0/9.1/5.2 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 258 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 19.3/15.3 -17.9 19.9-20.3 12.2-12.5 55-58 Tip (C) 1139/1125 -924 ---Primary (C) 1101/1064 -977-979 256-258 500.1 897-900 LSP 9.1/5.2 -978 258 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 18.4/14.8 -10 19.5 12.5 55.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:13:03 Headspace P (torr): 1.0 x 10-7 Run time (seconds): 15,183 Growth P (torr): 1.0 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 88.7 9.1 88.7 110 5.2 *GaSe deposited for 25 minut es before Cu deposition

PAGE 194

194 Table A-33. Reactor conditions for Growth Run #525. Run # 525 Date: 10/03/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 513 with T = 900C (1.15/0.95/0.75) Target Value Metal Se Substrate Heater Cu Rate (A/s) 7.5/6.2/4.9 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15-17.3 -17.7-18.1 19.5 12.1 55-60 Tip (C) 1116-1133 -924 ---Primary (C) 1060-1083 -977-979 265.2 500.1 898-900 LSP 7.5/6.2/4.9 -978 265 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.2-16.3 -10 20.0 12.5 56.0 Thick. (kA) 110 -----Run time (hr:min:sec): 4:57:40 Headspace P (torr): 1.0-1.8 x 10-7 Run time (seconds): 17,860 Growth P (torr): 1.0-1.4 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 44.4 7.46 44.4 81.1 6.17 81.1 110 4.85

PAGE 195

195 Table A-34. Reactor conditions for Growth Run #535. Run # 535 Date: 12/05/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: 3-stage emulation (GaSe/CuSe/GaSe) at T = 822/900C Target Value Metal Se Substrate Heater Cu Rate (A/s) 9.83 Cracker (C) 500 Temp. (C) 822/900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1005 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 19-19.4 -18.5-19.3 22/18 12.1 52/58-62 Tip (C) 1144 -943-947 ---Primary (C) 1107-1115 -1004-1006 265.0 500.0 822.5/900.5 LSP 10 -1005 265 500 822/900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 18-19.5 -10 22/18.7 12.0 51/59 Thick. (kA) 100 -----Run time (hr:min:sec): 5:47:52 Headspace P (torr): 7.5 x 10-8 1.0 x 10-7 Run time (seconds): 20,872 Growth P (torr): 3.0 x 10-7 1.0 x 10-6 Cu rate Thickness (kA) Rate (A/s) 0 100 9.84 *GaSe was deposited for 2:49:30 before CuSe deposition and 8:55 after

PAGE 196

196 Table A-35. Reactor conditions for Growth Run #536. Run # 536 Date: 12/12/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: 3-stage emulation (GaSe/CuSe/GaSe) at T = 822/900C Target Value Metal Se Substrate Heater Cu Rate (A/s) 10 Cracker (C) 500 Temp. (C) 822/900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1005 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17.3-18.5 -18.7-18.9 21/18 12.3 50-53/58-62 Tip (C) 1153-1156 -940-945 ---Primary (C) 1108-1113 -1004-1006 265.9 499.9 823/901 LSP 10.0 -1005 265 500 822/900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16.5-17.7 -10 21.5/18.5 12.0 51/60 Thick. (kA) 100 -----Run time (hr:min:sec): 6:01:33 Headspace P (torr): 7.5 x 10-8 1.5 x 10-7 Run time (seconds): 21,693 Growth P (torr): 2.8 x 10-7 9.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 100 10.00 *GaSe was deposited for 3:05:12 before CuSe deposition and 9:44 after

PAGE 197

197 Table A-36. Reactor conditions for Growth Run #537. Run # 537 Date: 12/13/03 Film: CGS Operator: Ryan Kaczynski, Ryan Acher Substrate: 2 Mo, 1 Glass Objective: Thin 3-stage emulation (GaSe/CuSe/GaSe) at T = 822/900C Target Value Metal Se Substrate Heater Cu Rate (A/s) 10 Cracker (C) 500 Temp. (C) 822/900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1005 Total Film Thickness ( m): 0.5 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 19.5 -18.8-19.3 21 12 52/58-62 Tip (C) 1136-1150 -941 ---Primary (C) 1101-1109 -1004-1006 265.3 500.0 822.5/901 LSP 10.0 -1005 265 500 822/900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 17.3-19.5 -10 21.5/18.5 12.0 51.5/62 Thick. (kA) 50.0 -----Run time (hr:min:sec): 3:01:15 Headspace P (torr): 4.4-8.3 x 10-8 Run time (seconds): 10,875 Growth P (torr): 1.9-7.8 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 100 9.95 *GaSe was deposited for 1:32:36 before CuSe deposition and 4:52 after

PAGE 198

198 Table A-37. Reactor conditions for Growth Run #538. Run # 538 Date: 12/15/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: Cu-rich, constant-rate CGS with GaSe start Target Value Metal Se Substrate Heater Cu Rate (A/s) 13 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1005 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 22.1-22.5 -18.6-18.9 18.5 12.1 58-62 Tip (C) 1142-1148 -941-943 ---Primary (C) 1124-1130 -1004-1006 265.5 500.0 899-901 LSP 13.0 -1005 265 500 822/900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 21.3-22.0 -10 18.5 12.0 60.0 Thick. (kA) 100.0 -----Run time (hr:min:sec): 2:17:26 Headspace P (torr): 6.5 x 10-8 1.0 x 10-7 Run time (seconds): 8,246 Growth P (torr): 3.6 x 10-7 6.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 100 12.98 *GaSe deposited for 9 minut es before Cu deposition

PAGE 199

199 Table A-38. Reactor conditions for Growth Run #540. Run # 540 Date: 12/17/03 Film: CGS Operator: Ryan Kaczynski Substrate: 2 Mo, 1 Glass Objective: 3-stage emulation (GaSe/CuSe/GaSe) at T = 822/900C Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.6 Cracker (C) 500 Temp. (C) 822/900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 1005 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 20.5-21.5 -19.0-19.3 21.3/19.2 12.1 50/58-62 Tip (C) 1140-1144 -935-943 ---Primary (C) 1116-1122 -1004-1006 265-266 499.9 823/901 LSP 12.6 -1005 265 500 822/900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 20.0-21.5 -10 21.5/18.5 12.0 51/63 Thick. (kA) 100 -----Run time (hr:min:sec): 4:34:45 Headspace P (torr): 3.2 x 10-8 1.4 x 10-7 Run time (seconds): 16,485 Growth P (torr): 1.8 x 10-7 9.3 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 100 12.60 *GaSe deposited for 2:12:17 before CuSe deposition and 10:10 after

PAGE 200

200 Table A-39. Reactor conditions for Growth Run #541. Run # 541 Date: 12/18/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 523 with T = 822/900C (0/1/1.6/1) Target Value Metal Se Substrate Heater Cu Rate (A/s) 5.8/9.4/5.8 Cracker (C) 500 Temp. (C) 822/900 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 975 Total Film Thickness ( m): 1.1 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 900C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17.4/19.9/--17.8 18.1 12.2 59-62 Tip (C) 1114/1132/--912 ---Primary (C) 1066/1101/--974-976 264.5-265.5 500.2 897 LSP 5.8/9.4/5.8 -975 265 500 900 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 17/19.2/16.5 -10 18 12.5 55.0 Thick. (kA) 110.5 -----Run time (hr:min:sec): 4:11:10 Headspace P (torr): 1.0 x 10-7 Run time (seconds): 15,070 Growth P (torr): 5-7 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 10.5 5.8 10.5 94.75 9.4 94.75 110.5 5.8 *GaSe deposited for 25 minut es before Cu deposition

PAGE 201

201 Table A-40. Reactor conditions for Growth Run #542. Run # 542 Date: 12/19/03 Film: CGS Operator: Woo Kyoung Kim Substrate: 2 Mo, 1 Glass Objective: Repeat Run# 523 with T = 850C (0/1.2/1.6/1.0) Target Value Metal Se Substrate Heater Cu Rate (A/s) 7/9.4/5.8 Cracker (C) 500 Temp. (C) 850 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 89.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.15 Film Set Value Cu In Ga Soak Power (%) 19 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 *Annealing for 30 minutes in Se atmosphere at T = 850C Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17-19.4 -18.1 18.4 12.1 55-56 Tip (C) 1125-1137 -----Primary (C) 1077-1170 -977-979 265.5-265.7 500.2 849.5-850.4 LSP 7.0/9.4/5.8 -978 265 500 850 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16.4-18.9 -10 19.5 12.5 55.0 Thick. (kA) 115.6 -----Run time (hr:min:sec): 4:06:22 Headspace P (torr): 9.0 x 10-8 Run time (seconds): 14,782 Growth P (torr): 3.6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 12.2 7.0 12.2 93 9.36 93 115.6 5.85 *GaSe deposited for 25 minut es before Cu deposition

PAGE 202

202 Table A-41. Reactor conditions for Growth Run #569. Run # 569 Date: 03/19/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 2 Mo Objective: Constant rate CIGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.5 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 7.5 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 25.3 36 19.0 20.5-22 10.7-10.8 62-64 Tip (C) 1127 -907 ---Primary (C) 1139 922 977-979 264.7-265 500.7 899-900.5 LSP 12.5 7.5 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 22.2 35.4 10 21 12.0 62.0 Thick. (kA) 100 59.92 ----Run time (hr:min:sec): 2:13:22 Headspace P (torr): 6.3-8.7 x 10-8 Run time (seconds): 8002 Growth P (torr): 1.7-2.5 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 12.50 0 59.9 7.49

PAGE 203

203 Table A-42. Reactor conditions for Growth Run #575. Run # 575 Date: 04/26/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 4 Mo, 1 Glass Objective: Constant rate CIGS, x~0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.5 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 8 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 22-24 36 19-19.3 22-22.5 10.7-10.8 58.5-63.5 Tip (C) 1132-1136 -927-934 ---Primary (C) 1133-1139 942-966 977-979 264.7-265.3 501.3 898-901 LSP 12.5 8.0 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 21.8-22.0 35.5-37.0 10 23 12.0 61.0 Thick. (kA) 100 64.1 ----Run time (hr:min:sec): 2:13:33 Headspace P (torr): 1.2 x 10-7 Run time (seconds): 8013 Growth P (torr): 7.8-9.0 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 12.48 0 64.1 8.00

PAGE 204

204 Table A-43. Reactor conditions for Growth Run #578. Run # 578 Date: 05/03/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Thick, constant rate CIGS, x~0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.5 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 8 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 19 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 22-23.5 36-37.5 19.5-20.1 22-22.5 9-9.5 59-63.5 Tip (C) 1129-1140 -916-927 ---Primary (C) 1133-1140 941-971 977-979 264.8-265.4 501.7 899-901 LSP 12.5 8.0 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 22-22.8 35.8-36.5 10 23 12.0 61.0 Thick. (kA) 150 96.13 ----Run time (hr:min:sec): 3:20:24 Headspace P (torr): 1.2 x 10-7 Run time (seconds): 12,024 Growth P (torr): 8.5-9.5 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 12.48 0 64.1 8.00

PAGE 205

205 Table A-44. Reactor conditions for Growth Run #579. Run # 579 Date: 05/04/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Thick, constant rate CIGS, x~0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.5 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 8 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 2.0 Film Set Value Cu In Ga Soak Power (%) 19 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 23-25 37-37.5 20.4-20.7 22.6-23.6 9-9.7 59.5-64 Tip (C) 1129-1145 -914-920 ---Primary (C) 1133-1150 945-975 977-979 264.7-265.3 501.3 899-901 LSP 12.5 8.0 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 22.5-23.5 36.0-36.7 10 23 12.0 61.0 Thick. (kA) 200 128 ----Run time (hr:min:sec): 4:26:26 Headspace P (torr): 8.0 x 10-8 1.2 x 10-7 Run time (seconds): 15,986 Growth P (torr): 7.0 x 10-7 1.0 x 10-6 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 12.51 0 64.1 8.00

PAGE 206

206 Table A-45. Reactor conditions for Growth Run #582. Run # 582 Date: 05/11/04 Film: CIS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Constant rate CIS, x=0 Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.5 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 12.25 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) -Direction CCW Ga primary T (C) -Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 19 39 -Rise Time (min) 5 5 -Soak Time (min) 13 13 -Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 21.5-25 38.3-39.5 -23.8-24 9.5-10.4 59-63.5 Tip (C) 1137-1150 -----Primary (C) 1140-1148 965-992 -264.5 500.8 898-901 LSP 12.5 12.25 -265 500 900 Kc 2 2 -2 2 2 Ti 2 2 -2 2 2 Td 0 0 -0 0 0 Offset 22.3-23.0 37.8-38.2 -23 12.0 61.0 Thick. (kA) 100 128 ----Run time (hr:min:sec): 2:13:10 Headspace P (torr): 6-9 x 10-8 Run time (seconds): 7990 Growth P (torr): 4-7 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 12.52 0 97.6 12.22

PAGE 207

207 Table A-46. Reactor conditions for Growth Run #586. Run # 586 Date: 05/17/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Constant rate CIGS, x<0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 15.0 Cracker ( C) 500 Temp. (C) 900 In Rate (A/s) 11.5 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 23 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 23.5-24.5 37-38 20.6-20.9 23.3-23.8 12.1 59-64 Tip (C) 1143-1148 -916-919 ---Primary (C) 1149-1158 965-987 977-979 264.6-264.9 500.0 898-901 LSP 15.0 11.5 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 22.8-24.5 36.6-38.0 10 23 12.0 61.0 Thick. (kA) 100 76.52 ----Run time (hr:min:sec): 1:51:06 Headspace P (torr): 5-9 x 10-8 Run time (seconds): 6666 Growth P (torr): 3-4 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 15.0 0 76.5 11.48

PAGE 208

208 Table A-47. Reactor conditions for Growth Run #587. Run # 587 Date: 05/18/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 3 Mo, 1 Glass Objective: Constant rate CIGS, x>0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.0 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) 4.85 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 20 35 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 20.5-22.1 34-34.8 20.4-20.6 22.5-22.7 11.8 60.7-65 Tip (C) 1125-1134 -917-920 ---Primary (C) 1120-1130 920-933 977-979 265.2-265.5 500.1 898-900.5 LSP 10.0 4.85 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 21-21.4 33.7-34.5 10 23-23.5 12.0 61.5 Thick. (kA) 100 48.3 ----Run time (hr:min:sec): 2:46:27 Headspace P (torr): 4-8 x 10-8 Run time (seconds): 9987 Growth P (torr): 2.5-5 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 10.01 0 48.3 4.84

PAGE 209

209 Table A-48. Reactor conditions for Growth Run #588. Run # 588 Date: 05/19/04 Film: CIGS Operator: Ryan Kaczynski Substrate: 4 Mo, 1 Glass Objective: Constant rate CIGS, x<0.3 Target Value Metal Se Substrate Heater Cu Rate (A/s) 18 Cracker (C) 500 Temp. (C) 900 In Rate (A/s) 14.8 Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 84.9 Crucible (C) 265 RPM 12 Ga tip power (%) 22 Direction CCW Ga primary T (C) 978 Total Film Thickness ( m): 1.0 Film Set Value Cu In Ga Soak Power (%) 23.5 39 7 Rise Time (min) 5 5 5 Soak Time (min) 13 13 0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 25-26 39.3-40 20.7-20.9 23.6-20.9 11.9-12.1 60-65 Tip (C) 1147-1150 -916-918 ---Primary (C) 1166-1171 980-1005 977-979 264.7 500.0 898-901 LSP 18.0 14.8 978 265 500 900 Kc 2 2 2 2 2 2 Ti 2 2 2 2 2 2 Td 0 0 0 0 0 0 Offset 21.8-22 38.5-41.0 10 23.0 12.0 61.5 Thick. (kA) 100 82.14 ----Run time (hr:min:sec): 1:32:40 Headspace P (torr): 4-7 x 10-8 Run time (seconds): 5560 Growth P (torr): 2-3 x 10-7 Rate Cu In Thickness (kA) Rate (A/s) Thickness (kA) Rate (A/s) 0 100 17.99 0 82.1 14.77

PAGE 210

210 Table A-49. Reactor conditions for Growth Run #628. Run # 628 Date: 07/18/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.7 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 14 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 12.3-13.1 -9.4-9.6 21-21.4 11.9-12.1 56.0-61.0 Tip (C) 1105-1118 -958-963 ---Primary (C) --969-971 265.3-265.6 500.0 798-800.5 LSP 10.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.6-16.3 -10 22.0 12.0 57-58 Thick. (kA) 150 -----Run time (hr:min:sec): 3:53:56 Headspace P (torr): 9.0 x 10-8 1.0 x 10-7 Run time (seconds): 14,036 Growth P (torr): 4 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 11.57 10 11.12 100 10.74 150 10.69

PAGE 211

211 Table A-50. Reactor conditions for Growth Run #629. Run # 629 Date: 07/20/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 13.5 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 14.8-15.9 -9.5-9.6 19.8-21.4 11.9-12.0 56.5-60.5 Tip (C) 1114-1126 -958-963 ---Primary (C) --969-971 265.7-266.1 500.0 798-800.5 LSP 13.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 13.0 -10 22.0 12.0 57.5 Thick. (kA) 150 -----Run time (hr:min:sec): 3:05:01 Headspace P (torr): 5-8 x 10-8 Run time (seconds): 11,101 Growth P (torr): 2.7-3.7 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 14.40 10 14.00 100 13.54 150 13.51

PAGE 212

212 Table A-51. Reactor conditions for Growth Run #630. Run # 630 Date: 07/22/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.5 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 13 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 13.1-14.2 -9.5-9.7 21.0-21.5 11.9-12.0 57.0-60.6 Tip (C) 1105-1117 -957-963 ---Primary (C) --969-971 265.3-265.6 500.0 798-800.5 LSP 10.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 11.4-12.8 -10 22.0 12.0 57.5 Thick. (kA) 150 -----Run time (hr:min:sec): 3:58:08 Headspace P (torr): 5.2-7.7 x 10-8 Run time (seconds): 14,288 Growth P (torr): 2.4-3.6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 10.42 10 10.59 100 10.52 150 10.50

PAGE 213

213 Table A-52. Reactor conditions for Growth Run #634. Run # 634 Date: 08/03/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich growth based on Run# 523 at T = 800C Target Value Metal Se Substrate Heater Cu Rate (A/s) 14/19/14 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15/18/16 -9.3-9.5 20.7-21.0 11.8-12.1 58.0-61.5 Tip (C) 1124/1147/1134 -955-961 ---Primary (C) --969-971 265.5-265.7 500.0 798.2-800 LSP 14/19/14 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12.5/15/13 -10 22.0 12.0 58.0 Thick. (kA) 150 -----Run time (hr:min:sec): 2:37:45 Headspace P (torr): 7.5 x 10-8 1.0 x 10-7 Run time (seconds): 9,465 Growth P (torr): 3.5-4.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 23 14.13 23 136.5 18.99 136.5 150 14.06 *GaSe deposited for 15 minut es before Cu deposition

PAGE 214

214 Table A-53. Reactor conditions for Growth Run #635. Run # 635 Date: 08/04/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich CGS by modified 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 15.3/8.2 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16/12 -9.2-9.4 21.0-22.0 11.8-12.0 59.0-62.0 Tip (C) 1134/1114 -952-962 ---Primary (C) --969-971 265.0-265.5 500.1 798.0-800 LSP 15.3/8.2 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12.5/10 -10 22.0 12.0 58-59 Thick. (kA) 150 -----Run time (hr:min:sec): 3:55:24 Headspace P (torr): 5.0 x 10-8 1.0 x 10-7 Run time (seconds): 14,124 Growth P (torr): 2.5-4.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 91.5 15.27 91.5 150 8.20 *GaSe deposited for 16:40 minutes before Cu deposition

PAGE 215

215 Table A-54. Reactor conditions for Growth Run #636. Run # 636 Date: 08/05/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich CGS by modified 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 19.0/8.22 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17/12 -9.3-9.5 21.2-21.8 11.9-12.0 59.0-63.0 Tip (C) 1150/1118 -952-960 ---Primary (C) --969-971 265.1-265.5 500.1 798.0-800 LSP 19.0/8.2 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15/11 -10 22.0 12.0 60.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:53:27 Headspace P (torr): 4.5 x 10-8 1.0 x 10-7 Run time (seconds): 14,007 Growth P (torr): 2.5-4.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 76 18.99 76 150 8.22 *GaSe deposited for 16:40 minutes before Cu deposition

PAGE 216

216 Table A-55. Reactor conditions for Growth Run #637. Run # 637 Date: 08/10/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Stoichiometric CGS by modified 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 15.27/10.16 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.7/13.5 -9.3-9.5 21.5-22.3 12.0-12.2 60.0-63.0 Tip (C) 1141/1127 -952-958 ---Primary (C) --969-971 264.8-265.2 500.0 798.0-800 LSP 15.3/10.2 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.5/12 -10 22.0 12.0 60.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:36:36 Headspace P (torr): 5.5 x 10-8 1.0 x 10-7 Run time (seconds): 12,996 Growth P (torr): 2.5-5.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 84 15.27 84 150 10.16 *GaSe deposited for 16:40 minutes before Cu deposition

PAGE 217

217 Table A-56. Reactor conditions for Growth Run #638. Run # 638 Date: 08/11/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich using 2-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 14.6 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.8-17.5 -9.5-9.6 21.5/24.5 12.1-12.7 59-62 Tip (C) 1136-1139 -945-957 ---Primary (C) --969-971 264.0-265.5 499.6 798-800 LSP 14 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14.7-14.9 -10 22.0-23.5 12.0 60.0 Thick. (kA) 150 -----Run time (hr:min:sec): 5:56:57 Headspace P (torr): 5.0 x 10-8 1.0 x 10-7 Run time (seconds): 21,417 Growth P (torr): 2.5-5.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 150 14.02 *GaSe deposited for 2:58:35 before CuSe deposition

PAGE 218

218 Table A-57. Reactor conditions for Growth Run #639. Run # 639 Date: 08/12/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Stoichiometric CGS by modified 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 19/9.8 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 18.0/14.0 -9.6-9.7 21.7-22.2 11.8-12.0 59.0-62.0 Tip (C) 1150/1125 -952-957 ---Primary (C) --969-971 264.9-265.1 500.0 798.0-800 LSP 19.0/9.8 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 16/13 -10 22.0 12.0 60.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:29:10 Headspace P (torr): 4.5 x 10-8 1.0 x 10-7 Run time (seconds): 12,550 Growth P (torr): 2.2-5.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 91.5 15.27 91.5 150 8.20 *GaSe deposited for 16:40 minutes before Cu deposition

PAGE 219

219 Table A-58. Reactor conditions for Growth Run #640. Run # 640 Date: 08/15/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich using 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 19.0 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17.6-18.5 -9.6-9.7 21.7/24.0 12.0-12.7 60-65 Tip (C) 1142-1145 -946-958 ---Primary (C) --969-971 264.0-265.1 499.6 798-800 LSP 19 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15.5 -10 22.0 12.0 60-61 Thick. (kA) 150 -----Run time (hr:min:sec): 6:20:00 Headspace P (torr): 5.0 x 10-8 1.0 x 10-7 Run time (seconds): 22,800 Growth P (torr): 2.0-6.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 150 19.17 *GaSe deposited for 3:24:35 before CuSe deposition and 45 minutes after

PAGE 220

220 Table A-59. Reactor conditions for Growth Run #641. Run # 641 Date: 08/17/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Thick Cu-rich growth based on Run# 523 at T = 800C Target Value Metal Se Substrate Heater Cu Rate (A/s) 14/19/14 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 2.0 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 25/29/22 -9.4-9.6 21.3-21.8 11.9-12.1 61.7-64.7 Tip (C) 1162/1190/1155 -942-958 ---Primary (C) --969-971 265.1-265.3 500.0 798.0-799.6 LSP 14/19/14 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 25/26/19 -10 22.0 12.0 61.0 Thick. (kA) 200 -----Run time (hr:min:sec): 3:31:01 Headspace P (torr): 5.0 x 10-8 1.0 x 10-7 Run time (seconds): 12,661 Growth P (torr): 2.0-5.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 31 13.95 31 182 18.99 182 200 14.00 *GaSe deposited for 20 minut es before Cu deposition

PAGE 221

221 Table A-60. Reactor conditions for Growth Run #647. Run # 647 Date: 12/07/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 8.5 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 10 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 13.0-14.5 -9.4-9.7 22.0-22.3 12.1-12.2 61.8-64.1 Tip (C) 1105-1119 -945-952 ---Primary (C) --969-971 264.7-265.0 500.0 798-799.8 LSP 8.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 11.0-13.0 -10 22.0 12.0 61.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:54:18 Headspace P (torr): 5.5-6.5 x 10-8 Run time (seconds): 17,658 Growth P (torr): 3.2-5.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 7.53 10 8.13 80 8.57 150 8.50

PAGE 222

222 Table A-61. Reactor conditions for Growth Run #648. Run # 648 Date: 12/08/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich growth based on Run #523 at T = 800C Target Value Metal Se Substrate Heater Cu Rate (A/s) 11/14.5/11 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 13 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 15.5/17.5/16 -9.4-9.7 21.4-21.8 11.9-12.1 62.0-63.8 Tip (C) 1121/1139/1129 -943-953 ---Primary (C) --969-971 265.5-265.7 500.0 798-799.5 LSP 11/14.5/11 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12.5/15.2/14 -10 22.0 12.0 61.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:24:43 Headspace P (torr): 4.0-6.7 x 10-8 Run time (seconds): 12,283 Growth P (torr): 2.2-3.6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 0 21.3 11.04 21.3 128.7 14.50 128.7 150 10.94 *GaSe deposited for 16:40 before Cu deposition

PAGE 223

223 Table A-62. Reactor conditions for Growth Run #649. Run # 649 Date: 12/10/05 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich using 3-stage process Target Value Metal Se Substrate Heater Cu Rate (A/s) 15.0 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 17 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 17-18 -9.6-9.9 21.0-24.0 12.1-12.7 63.0-65.0 Tip (C) 1146-1150 -942-952 ---Primary (C) 1109-1112 -969-971 265.5-265.7 499.7 798-799 LSP 15.0 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 14-16 -10 22.0 12.0 61.0 Thick. (kA) 150 -----Run time (hr:min:sec): 7:53:48 Headspace P (torr): 5.0 x 10-8 1.0 x 10-7 Run time (seconds): 28,428 Growth P (torr): 2.5-6.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 14.0 100 15.05 150 15.06 *GaSe deposited for 4:37:47 before CuSe deposition and 30 minutes after

PAGE 224

224 Table A-63. Reactor conditions for Growth Run #652. Run # 652 Date: 02/07/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.9 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 10 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 10.3-13.4 -9.1-9.4 23.0-24.0 12.2 62-65 Tip (C) 1110-1122 -966-969 ---Primary (C) 1054 -970-971 263.9-264.3 499.9 798.5-800.0 LSP 10.9 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 8.2-11.0 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:48:58 Headspace P (torr): 6-8 x 10-8 Run time (seconds): 13,738 Growth P (torr): 4.5-6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 13.3 20 11.5 80 11.1 150 10.92

PAGE 225

225 Table A-64. Reactor conditions for Growth Run #653. Run # 653 Date: 02/09/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.9 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 10.5-11.5 -9.1-9.4 23.0-24.0 12.1-12.2 62-65 Tip (C) 1114-1125 -966-969 ---Primary (C) 1057 -969-971 264.0-264.5 499.9 798.0-799.7 LSP 10.9 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 8.5-10.0 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:48:58 Headspace P (torr): 5.5-8 x 10-8 Run time (seconds): 13,738 Growth P (torr): 4-6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 11.0 20 10.9 100 10.94 150 10.92

PAGE 226

226 Table A-65. Reactor conditions for Growth Run #654. Run # 654 Date: 02/14/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.7 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 10 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 12.0-13.0 -9.2-9.3 21.3-21.8 12.2-12.3 62-65 Tip (C) 1121-1132 -965-968 ---Primary (C) 1071-1073 -969-971 265.1-265.4 499.9 798.0-799.7 LSP 12.7 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 9.5-10.8 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:17:42 Headspace P (torr): 4.8-7.0 x 10-8 Run time (seconds): 11,682 Growth P (torr): 2.7-3.7 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 12.75 30 12.71 100 12.67 150 12.65

PAGE 227

227 Table A-66. Reactor conditions for Growth Run #655. Run # 655 Date: 02/23/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.3 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 9 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 9.0-10.3 -9.2-9.5 24.0 12.2 64-65 Tip (C) 1115-1127 -962-966 ---Primary (C) 1053-1055 -969-971 264.0 499.7 798.0-799.3 LSP 10.3 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 7.0-8.5 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:03:17 Headspace P (torr): 5.5-6.5 x 10-8 Run time (seconds): 14,597 Growth P (torr): 3.0-4.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 11.47 50 10.36 100 10.28 150 10.28

PAGE 228

228 Table A-67. Reactor conditions for Growth Run #656. Run # 656 Date: 03/14/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Cu-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 12.1 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 10 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 10.3-11.8 -9.1-9.4 22.0-23.0 12.1 64-66 Tip (C) 1126-1168 -966-969 ---Primary (C) 1067-1073 -969-971 264.8-265.3 499.9 798.5-800.0 LSP 12.1 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 8.0-9.7 -10 23.0 12.0 63.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:26:23 Headspace P (torr): 9 x 10-8 1 x 10-7 Run time (seconds): 12,383 Growth P (torr): 4.3-5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 13.3 20 12.5 100 12.11 150 12.11

PAGE 229

229 Table A-68. Reactor conditions for Growth Run #657. Run # 657 Date: 03/16/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Ga-rich, constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.0 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 9.6-10.9 -9.3-9.6 22-23 12.3 64-66 Tip (C) 1117-1128 -963-963 ---Primary (C) 1057-1059 -969-971 264.8-265.0 499.9 798.0-799.2 LSP 10.0 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 8.0-8.8 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:10:06 Headspace P (torr): 4.7-6.3 x 10-8 Run time (seconds): 15,006 Growth P (torr): 2.4-3.7 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 10.27 20 10.18 100 10.01 150 10.00

PAGE 230

230 Table A-69. Reactor conditions for Growth Run #658. Run # 658 Date: 03/24/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 11.0 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 11.0-13.0 -9.3-9.6 21.5-22.2 12.2-12.3 64-66 Tip (C) 1121-1133 -960-964 ---Primary (C) 1070-1072 -969-971 264.9-265.2 499.9 797.8-799.0 LSP 11.0 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 9.0-11.0 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:48:02 Headspace P (torr): 5-7 x 10-8 Run time (seconds): 13,682 Growth P (torr): 3-4 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 10.73 20 10.91 100 10.93 150 10.96

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231 Table A-70. Reactor conditions for Growth Run #659. Run # 659 Date: 04/14/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.5 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 11.0-14.0 -9.3-9.6 22.6-22.8 12.2-12.3 64-65 Tip (C) 1115-1130 -960-964 ---Primary (C) 1067-1070 -969-971 264.6-264.7 499.9 798.0-798.7 LSP 10.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 9.0-11.5 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 3:59:19 Headspace P (torr): 5.4-6.7 x 10-8 Run time (seconds): 14,359 Growth P (torr): 3.9-4.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 10.08 20 10.36 100 10.41 150 10.45

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232 Table A-71. Reactor conditions for Growth Run #660. Run # 660 Date: 05/09/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 9.5 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 11.8-13.4 -9.2-9.4 23.0 12.2-12.3 65-66 Tip (C) 1113-1123 -958-962 ---Primary (C) 1067 -969-971 264.5 499.9 796.0-799.0 LSP 9.5 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 10-11.8 -10 22.0 12.0 63.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:24:00 Headspace P (torr): 5.5-8.0 x 10-8 Run time (seconds): 15,840 Growth P (torr): 4.5-6.0 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 9.63 20 9.46 100 9.46 150 9.47

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233 Table A-72. Reactor conditions for Growth Run #661. Run # 661 Date: 05/11/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 9.0 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 8 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 12.4-13.1 -9.2-9.5 22.5-22.7 12.1-12.2 65-66 Tip (C) 1115-1122 -956-963 ---Primary (C) 1062-1063 -969-971 264.6-264.7 499.9 795.0-798.5 LSP 9.0 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 10-11.2 -10 22.0 12.0 62.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:37:40 Headspace P (torr): 4.5-6.0 x 10-8 Run time (seconds): 16,660 Growth P (torr): 4.0-5.5 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 8.49 10 8.97 100 9.03 150 9.00

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234 Table A-73. Reactor conditions for Growth Run #662. Run # 662 Date: 05/12/06 Film: CGS Operator: Ryan Kaczynski Substrate: 3 Mo Objective: Constant rate CGS Target Value Metal Se Substrate Heater Cu Rate (A/s) 10.2 Cracker (C) 500 Temp. (C) 800 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 970 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 9 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 13.9-14.6 -9.3-9.5 20.9-21.5 12.1 65-66 Tip (C) 1120-1130 -955-961 ---Primary (C) 1075 -969-971 264.5 499.9 798.5-799.2 LSP 10.2 -970 265 500 800 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 12-13 -10 22.0 12.0 63.0 Thick. (kA) 150 -----Run time (hr:min:sec): 4:04:07 Headspace P (torr): 3.8-5.5 x 10-8 Run time (seconds): 14,647 Growth P (torr): 2.5-3.6 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 9.54 20 10.19 100 10.26 150 10.24

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235 Table A-74. Reactor conditions for Growth Run #666. Run # 666 Date: 07/21/06-07/22/06 Film: CGS Operator: Ryan Kaczynski Substrate: 4 Mo Objective: Bilayer CGS (CuSe/GaSe) Target Value Metal Se Substrate Heater Cu Rate (A/s) 14.0 Cracker (C) 500 Temp. (C) 440/250 In Rate (A/s) -Ramp (C/s) 0.5 Ramp (C/s) 0.3 Cu tip power (%) 76.9 Crucible (C) 265 RPM 12 Ga tip power (%) 24 Direction CCW Ga primary T (C) 980 Total Film Thickness ( m): 1.5 Film Set Value Cu In Ga Soak Power (%) 15 -7 Rise Time (min) 5 -5 Soak Time (min) 13 -0 Operation Results Cu In Ga Se Crucible Se Cracker Heater Power (%) 16.2-16.7 -11.3-11.7 23 12.9 24/10 Tip (C) 1127-1135 -956-958 ---Primary (C) 1091-1096 -979-982 264.5 499.5 440.5/252 LSP 14.0 -980 265 500 440/250 Kc 2 -2 2 2 2 Ti 2 -2 2 2 2 Td 0 -0 0 0 0 Offset 15.0 -10 22.0 12.0 25.0/15.0 Thick. (kA) 150 -----Run time (hr:min:sec): 5:58:22 Headspace P (torr): 2.6-3.9 x 10-8 Run time (seconds): 21,502 Growth P (torr): 2.4 x 10-8 1.2 x 10-7 Cu rate Thickness (kA) Rate (A/s) 5 14.5 20 14.4 100 14.05 150 14.02

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236 APPENDIX B REACTOR MODEL This appendix includes the MATLAB code fo r two M-files needed to run the SIMULINK program pmee_sim: an input file named pmee_input that creates a vector of all the growth and reactor related parameters and an S-file na med pmee_model that generates compositions and film thickness based on the growth conditions. Input_PMEE.m % Input file % Defines the parameters and other % configuration elements needed to % run PMEE_sim % by R. Kaczynski 3/23/07 % Assigns a value to "film" to determine which metal sources are "on" film = input( 'Film deposition (0=CIGS, 1=CIS, 2=CGS)?' ) ; hm_Cu0 = input( 'Initial Copper melt height (cm)?' ) ; Tm_Cu = input( 'Melt temperature of Copper (C)?' ) ; % If the film is CGS, there will be no In flux if film == 2 hm_In0 = 0 ; Tm_In = 0 ; else hm_In0 = input( 'Initial Indium melt height (cm)?' ) ; Tm_In = input( 'Melt temperature of Indium (C)?' ) ; end % If the film is CIS, there will be no Ga flux if film == 1 hm_Ga0 = 0 ; Tm_Ga = 0 ; else hm_Ga0 = input( 'Initial Gallium melt height (cm)?' ) ; Tm_Ga = input( 'Melt temperature of Gallium (C)?' ) ; end RPM = input( 'Rotations per minute?' ) ; % PMEE reactor dimensions in centimeters (cm) % Platen centerline radius Ro = 17.78 ; % Crucible bottom radius Rb = 0.26 ; % Crucible exit radius Re = 1.44 ;

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237 % Crucible height H = 9 ; % Platen circumference L = 2*pi*Ro ; % Dimensionless unit m = (Re-Rb)/H ; % Geometric constants relating to the crucible's dimensions gc0 = -0.14417 ; gc1 = 0.47195 ; gc2 = -0.10846 ; gc3 = 0.015233 ; % Dimension of a square substrate (cm) w = 5 ; % Substrate area (cm^2) As = w^2 ; % Avogadro's constant NA = 602200000000000000000000 ; % Atomic weights (g/mol) AW_Cu = 63.546 ; AW_In = 114.82 ; AW_Ga = 69.72 ; AW_Se = 78.96 ; % Constants A and B for Antoine's equation A_Cu = 9.284 ; B_Cu = -17073 ; A_In = 8.073 ; B_In = -12136 ; A_Ga = 8.558 ; B_Ga = -14658 ; % Densities (g/cm^3) rho_Cu = 8.9 ; rho_In = 7.31 ; rho_Ga = 5.91 ; rho_film = 5.6 ; % Sticking Coefficients (value of 0 to 1) sigma_Cu = 1 ; sigma_In = 1 ; sigma_Ga = 1 ; % Evaporation rate of Cu Fm_Cu = 35100000000000000000000*exp(A_Cu+(B_Cu/(Tm_Cu+273))) ... /sqrt(AW_Cu*(Tm_Cu+273)) ;

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238 % Evaporation rate of In % If the film is CGS, the In shutter is closed if film == 2 Fm_In = 0 ; else Fm_In = 35100000000000000000000*exp(A_In+(B_In/(Tm_In+273))) ... /sqrt(AW_In*(Tm_In+273)) ; end % Evaporation rate of Ga % If the film is CIS, the Ga shutter is closed if film == 1 Fm_Ga = 0 ; else Fm_Ga = 35100000000000000000000*exp(A_Ga+(B_Ga/(Tm_Ga+273))) ... /sqrt(AW_Ga*(Tm_Ga+273)) ; end % Degrees swept out by each metal zone in the reactor deg = 40 ; % Distance from the crucible center point to the zone edge edge = L*(deg/360)*0.5 ; % Load normalized flux matrix NF load NF % Build parameters vector p = [ hm_Cu0, hm_In0, hm_Ga0, RPM, L, ... Fm_Cu, Fm_In, Fm_Ga, AW_Cu, AW_In, ... AW_Ga, AW_Se, rho_Cu, rho_In, rho_Ga, ... rho_film, NA, gc0, gc1, gc2, ... gc3, Rb, m, edge, As, ... sigma_Cu, sigma_In, sigma_Ga ] ; PMEE_model.m % PMEE model % function that outputs Cu/(Ga+In) ratio, % Ga/(Ga+In) ratio, and thickness as well % as the new melt heights after a growth % run simulation in PMEE_sim % by R. Kaczynski 3/23/07 function [sys, x0] = pmee_model(t,x,u,flag,p,NF) % Assign physical parameter values from p hm_Cu0 = p(1) ; Fm_Cu = p(6) ; AW_Cu = p(9) ; rho_Cu = p(13) ; hm_In0 = p(2) ; Fm_In = p(7) ; AW_In = p(10) ; rho_In = p(14) ; hm_Ga0 = p(3) ; Fm_Ga = p(8) ; AW_Ga = p(11) ; rho_Ga = p(15) ; AW_Se = p(12) ; rho_film = p(16) ;

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239 RPM = p(4) ; gc0 = p(18) ; NA = p(17) ; sigma_Cu = p(26) ; L = p(5) ; gc1 = p(19) ; edge = p(24) ; sigma_In = p(27) ; Rb = p(22) ; gc2 = p(20) ; As = p(25) ; sigma_Ga = p(28) ; m = p(23) ; gc3 = p(21) ; % Vectors that define the rows and columns of NF Height = 9:-0.5:0 ; Dist = 0:0.5:30.5 ; if flag == 0 size_states = 7 ; size_disc_states = 0 ; size_outputs = 6 ; size_inputs = 1 ; size_disc_roots = 0 ; size_feedthrough = 0 ; sys = [ size_states, size_disc_states, size_outputs, ... size_inputs, size_disc_roots, size_feedthrough ] ; % Initial condition vector % The initial mass of In is set to a very small number % greater than 0 to avoid "division by zero" errors x0 = [ 0; hm_Cu0; hm_In0; hm_Ga0; 0; 0.00000000000000000001; 0; ] ; elseif abs(flag) == 1 % Differential position exdot = (RPM/60)*L ; % Position of the substrate on the platen exprime = mod(x(1),L) ; % Differential melt height of Cu hm_Cudot = -((Fm_Cu*AW_Cu)/(pi*rho_Cu*NA))*((gc3*(x(2))^3+gc2*(x(2))^2 ... +gc1*x(2)+gc0)/(Rb^2+2*m*Rb*x(2)+(m^2)*(x(2))^2)) ; % If the substrate is in the first half of % the Cu deposition zone if exprime > L/4-2*edge & exprime < L/4-edge % Position of the substrate within the deposition zone pos = edge-(exprime-(L/4-2*edge)) ; % Interpolated normalized Cu flux reaching the substrate Fs_Cu = interp2(Dist,Height,NF,pos,x(2), 'cubic' ) ;

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240 % If the substrate is in the second half of % the Cu deposition zone elseif exprime > L/4-edge & exprime < L/4 pos = exprime-(L/4-edge) ; Fs_Cu = interp2(Dist,Height,NF,pos,x(2), 'cubic' ) ; % If the substrate is outside the Cu deposition zone, % there is no Cu flux reaching the substrate else Fs_Cu = 0 ; end % Differential melt height of In hm_Indot = -((Fm_In*AW_In)/(pi*rho_In*NA))*((gc3*(x(3))^3+gc2*(x(3))^2 ... +gc1*x(3)+gc0)/(Rb^2+2*m*Rb*x(3)+(m^2)*(x(3))^2)) ; % If the substrate is in the first half of % the In deposition zone if exprime > L/4 & exprime < L/4+edge pos = edge-(exprime-(L/4)) ; % Interpolated normalized In flux reaching the substrate Fs_In = interp2(Dist,Height,NF,pos,x(3), 'cubic' ) ; % If the substrate is in the second half of % the In deposition zone elseif exprime > L/4+edge & exprime < L/4+2*edge pos = exprime-(L/4+edge) ; Fs_In = interp2(Dist,Height,NF,pos,x(3), 'cubic' ) ; % If the substrate is outside the In deposition zone, % there is no In flux reaching the substrate else Fs_In = 0 ; end % Differential melt height of Ga hm_Gadot = -((Fm_Ga*AW_Ga)/(pi*rho_Ga*NA))*((gc3*(x(4))^3+gc2*(x(4))^2 ... +gc1*x(4)+gc0)/(Rb^2+2*m*Rb*x(4)+(m^2)*(x(4))^2)) ; % If the substrate is in the first half of % the Ga deposition zone if exprime > L/2-2*edge & exprime < L/2-edge pos = edge-(exprime-(L/2-2*edge)) ; % Interpolated normalized Ga flux reaching the substrate Fs_Ga = interp2(Dist,Height,NF,pos,x(4), 'cubic' ) ;

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241 % If the substrate is in the second half of % the Ga deposition zone elseif exprime > L/2-edge & exprime < L/2 pos = exprime-(L/2-edge) ; Fs_Ga = interp2(Dist,Height,NF,pos,x(4), 'cubic' ) ; % If the substrate is outside the Ga deposition zone, % there is no Ga flux reaching the substrate else Fs_Ga = 0 ; end % Differential mass of each metal species on the substrate mass_Cudot = As*Fm_Cu*Fs_Cu*(AW_Cu/NA)*sigma_Cu ; mass_Indot = As*Fm_In*Fs_In*(AW_In/NA)*sigma_In ; mass_Gadot = As*Fm_Ga*Fs_Ga*(AW_Ga/NA)*sigma_Ga ; sys = [ exdot ; hm_Cudot ; hm_Indot ; hm_Gadot ; ... mass_Cudot ; mass_Indot ; mass_Gadot ] ; elseif flag == 3 % Position ex = x(1) ; % Melt heights hm_Cu = x(2) ; hm_In = x(3) ; hm_Ga = x(4) ; % Deposited mass mass_Cu = x(5) ; mass_In = x(6) ; mass_Ga = x(7) ; % Se flux is supplied in excess so the mass of Se % is set so that comp_Se = 0.5 mass_Se = ((x(5)/AW_Cu)+(x(6)/AW_In)+(x(7)/AW_Ga))*AW_Se ; % Composition comp_Cu = (mass_Cu/AW_Cu)/ ... ((mass_Cu/AW_Cu)+(mass_In/AW_In)+(mass_Ga/AW_Ga)+(mass_Se/AW_Se)) ; comp_In = (mass_In/AW_In)/ ... ((mass_Cu/AW_Cu)+(mass_In/AW_In)+(mass_Ga/AW_Ga)+(mass_Se/AW_Se)) ; comp_Ga = (mass_Ga/AW_Ga)/ ... ((mass_Cu/AW_Cu)+(mass_In/AW_In)+(mass_Ga/AW_Ga)+(mass_Se/AW_Se)) ;

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242 comp_Se = (mass_Se/AW_Se)/ ... ((mass_Cu/AW_Cu)+(mass_In/AW_In)+(mass_Ga/AW_Ga)+(mass_Se/AW_Se)) ; % Cu/(Ga+In) ratio Cu_III = comp_Cu/(comp_In+comp_Ga) ; % Ga/(Ga+In) ratio Ga_III = comp_Ga/(comp_In+comp_Ga) ; % Film thickness (microns) thick = ((mass_Cu+mass_In+mass_Ga+mass_Se)/(rho_film*As))*10000 ; sys = [ hm_Cu; hm_In; hm_Ga; ... Cu_III; Ga_III; thick ] ; else sys = [] ; end

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250 BIOGRAPHICAL SKETCH Ryan Michael Kaczynski was born in Amherst, NY on January 11, 1979. He grew up in the small Western New York town of Newfane and was named valedictor ian of his graduating senior class in June 1997. He then attended Corn ell University in Ithaca, NY where he received his Bachelor of Science degree in chemical engineering in May 2001. In August 2001, he joined the graduate program in the Chemical Engineering department at the Univer sity of Florida (UF) to pursue the degree of Doctor of Philosophy. While at UF, he became a member of the multidisciplinary Copper Indium Disele nide (CIS) solar cell group head ed by Dr. Oscar D. Crisalle (his advisor), Dr. Timothy J. Anderson, and Dr. Sheng S. Li.


Permanent Link: http://ufdc.ufl.edu/UFE0019804/00001

Material Information

Title: Copper Gallium Diselenide Thin Film Absorber Growth for Solar Cell Device Fabrication
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0019804:00001

Permanent Link: http://ufdc.ufl.edu/UFE0019804/00001

Material Information

Title: Copper Gallium Diselenide Thin Film Absorber Growth for Solar Cell Device Fabrication
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0019804:00001


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COPPER GALLIUM DISELENIDE THINT FILM AB SORBER GROWTH
FOR SOLAR CELL DEVICE FABRICATION



















By

RYAN KACZYNSKI


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007

































O 2007 Ryan Kaczynski



































To my family I love you









ACKNOWLEDGMENTS

First, I would like to thank Prof. A. Brad Anton at Cornell University for encouraging me

to pursue my doctorate degree when I had no idea what I wanted to do in the future. I express

my sincerest gratitude to Dr. Oscar Crisalle for taking me under his guidance. It has been a

pleasure working for him. His relaxed attitude has been very beneficial to our working

relationship.

I would like to acknowledge the many members of the CIS solar cell team at the

University of Florida: Dr. Timothy Anderson and Dr. Sheng Li for your expertise in the solar cell

Hield and bringing these many excellent graduate students together, Billy Stanbery for starting

this proj ect, Serkan Kincal for training me on the PMEE reactor, Suku Kim for training me on

fi1m deposition, Ryan Acher for helping with reactor maintenance (by far the toughest j ob) and

fi1m characterization, Woo Kyoung Kim and Seokhyun Yoon for growth support, Jiyon Song

and Xuege Wang for device characterization, and Andre Baran and Wei Liu for device

fabrication. Each one of you was very integral to my success on this project. I would also like to

recognize the staff at MICROFABRITECH, especially Scott Gapinski and Diane Badylak.













TABLE OF CONTENTS




page


ACKNOWLEDGMENT S .............. ...............4.....


LI ST OF T ABLE S ........._... ......___ ...............8....


LIST OF FIGURES .............. ...............12....


AB S TRAC T ............._. .......... ..............._ 15...


CHAPTER


1 INTRODUCTION ................. ...............17.......... ......


System Description............... ..............1
P MEE Reactor ................. ................. 18..............
Chamber .............. ...............20....
Load-lock................ ...............2

Chalcogen Zone ................. ...............21.................
Heater Zone .............. ...............22....
Metal s Z one ................. ...............22.......... ......
Control ................ ...............23.......... ......
Problem Statement ................. ...............24.................


2 SOLAR CELLS .............. ...............26....


Energy ................. ...............26.......... ......
Sunlight ................. ...............26.................
C osts .............. .. ...............28...

Photovoltaic Systems............... ...............29
Future ................. ...............30.................

Solar Cell History ................. ...............31.......... ....
Solar Cell Device Physics............... ...............32
Band Gap .............. ...............3 2....
Electric Field .............. ...............33....
Recombination............... .............3
Defects ................. ...............34.................
Thin Films............... .... .. .... ..........3

Direct vs Indirect Band Gap ................ ...............36...............
Ab sorption Length ................. ...............36........... ....
Diffusion Length .............. ...............36....
CulnSe2-Based Solar Cells .............. ...............37....
Material Properties .............. ...............37....
Defects ................. ...............39.................
Gallium Addition............... ...............39















CuGaSe2............... ...............40
Defects ................. ...............41........._.....
Recombination............... .............4

Type inversion ......_. ................ .......__. ..........4
Sulfide-based Chalcopyrites ................ ...............43.................

Deposition Processes .............. ...............44....
M ultij uncti ons ................. ........... ...............45......
Theoretical Multijunctions .............. ...............46....
Tandem Structure .............. ...............47....
Monolithic vs Mechanical .............. ...............48....


3 ABSORBER GROWTH AND DEVICE FABRICATION .................. ................5


Growth Calibration .............. ...............52....
Standard Growth Procedure............... ...............5
Growth Schemes ................. ...............56.................
Absorber Characterization ................. ...............59.................
ICP ................ ...............59.......... ......
SE M ................ ...............60.......... ......
X RD ................ ...............60.......... ......
Device Fabrication..................... ...........6
Substrate and Back Contact............... ...............61

Post Absorber Deposition ................. ...............63........... ....
Buffer Layer .............. ...............63....
Alternative Buffers .............. ...............65....

W indow Layers .............. ...............66....
M etal liz ati on ................. ...............67........... ...

Anti-Reflective Coating............... ...............67
Device Characterization............... ............6

Current-Voltage ................. ...............67.................
I-V Measurement Technique ................. ...............69........... ....

Quantum Efficiency............... ...............6
QE Measurement Technique .............. ...............70....


4 COPPER GALLIUM DISELENIDE AB SORBER GROWTH.............__ .........___.......73


Growth M atrix .............. ...............73....
Absorber Characterization ............. ...... .__ ...............82...
Conclusions............... ..............9


5 COPPER GALLIUM DISELENIDE DEVICE FABRICATION............. ............._ ..120


Best Devices in the Literature................ ..............12
Device Fabrication............... ..............12
Device Characterization............... ...........12
Conclusions............... ..............12













6 CIGS ABSORBER GROWTH AND DEVICE FABRICATION ................... ...............13


Best Devices in the Literature................ ..............13
Growth M atrix .............. ...............134....
Absorber Characterization ................. ...............138................
Ori entati on ........._.__....... .__ ...............138...

M orphology ................. ...............140......... ......
Device Fabrication............... ..............14
Device Characterization............... ...........14
Conclusions............... ..............14


7 DYNAMIC REACTOR MODEL .............. ...............150....


Flux M odeling .............. ...............150....
PMEE Reactor Modeling ................. ...............152................
Conclusions............... ..............15


8 CONCLUSIONS AND FUTURE WORK ................. ...............159........... ...


Conclusions............... ..............15
Future Work............... ...............160.


APPENDIX


A GROWTH RUN DATA ................ ...............161...............


B REACTOR MODEL .............. ...............236....


InputPM EE.m .............. ...............236....
PMEE model.m ................. ...............238................


LIST OF REFERENCES ................. ...............243................


BIOGRAPHICAL SKETCH .............. ...............250....











LIST OF TABLES


Table page


2-1. Efficiencies of copper chalcopyrites. .............. ...............50....

4-1. First CGS growth series. .............. ...............91....

4-2. Second CGS growth series............... ...............91.

4-3. Third CGS growth series. ............. ...............91.....

4-4. Fourth CGS growth series............... ...............91.

4-5. Fifth CGS growth series............... ...............92.

4-6. Sixth CGS growth series ................. ...............92........... ...

4-7. Seventh CGS Growth Series ................. ...............92........... ...


4-8. Eighth CGS growth series............... ...............93.

5-1. Device parameters of record CGS cells produced at NREL............._ .........._ ..... 130

5-2. Device parameters for the second CGS absorber growth series. .................. ...............130

5-3. Device parameters for the fourth CGS absorber growth series. .................. ...............130

5-4. Device parameters for the fifth CGS absorber growth series. ............. .....................13

5-5. Device parameters for the sixth CGS absorber growth series. ............. .....................13

5-6. Device parameters for the seventh CGS absorber growth series. .........._... ........._.....13 1

5-7. Device parameters for the eighth CGS absorber growth series. .................. ...............131

6-1. CIGS growth series. .............. ...............144....

6-2. Device parameters for the CIGS growth series............... ...............148

A-1. Reactor conditions for Growth Run #443 ....._ .....___ ........__ ...........6

A-2. Reactor conditions for Growth Run #444 ................. ...............163........... ..

A-3. Reactor conditions for Growth Run #445 ................ ...............164......___..

A-4. Reactor conditions for Growth Run #446 ................. ...............165........... ..

A-5. Reactor conditions for Growth Run #447 ................. ...............166........... ..












A-6. Reactor conditions for Growth Run #452 ................. ............_ .....167.........


A-7. Reactor conditions for Growth Run #453 .....___.....__.___ .......____ ...........6


A-8. Reactor conditions for Growth Run #454 ................. ............_ .....169.........


A-9. Reactor conditions for Growth Run #455 ...._. ......_._._ .......__. ...........7


A-10. Reactor conditions for Growth Run #456 ................. ............_ .....171.........


A1.Reactor conditions for Growth Run #457 ................. ............_ .....172.........


A-12. Reactor conditions for Growth Run #458 ................. ............_ .....173.........


A-13. Reactor conditions for Growth Run #459 ................. ............_ .....174.........


A-14. Reactor conditions for Growth Run #472 ................. ............_ .....175.........


A-15. Reactor conditions for Growth Run #474 ................. ............_ .....176.........


A-16. Reactor conditions for Growth Run #475 ....__. ...._.._.._ ......._.... ...........7


A-17. Reactor conditions for Growth Run #476 ................. ............_ .....178.........


A-18. Reactor conditions for Growth Run #477 ................. ............_ .....179.........


A-19. Reactor conditions for Growth Run #478 ................. ............_ .....180.........


A-20. Reactor conditions for Growth Run #479 ................. ............_ .....181.........


A-21. Reactor conditions for Growth Run #480. ................ ...._.._ .....__.. .......18


A-22. Reactor conditions for Growth Run #510. ................ ...._.._ ...............183.


A-23. Reactor conditions for Growth Run #511 .........._._ ... ........ ....._.... ......18


A-24. Reactor conditions for Growth Run #512. ................. ....._.. ............ ......18


A-25. Reactor conditions for Growth Run #513 .........._._ ... ........ ....._.._..........8


A-26. Reactor conditions for Growth Run #514 ................. ..........._._.....187...... ..


A-27. Reactor conditions for Growth Run #515 ....._.. ................ .. ........ ...... ... 18


A-28. Reactor conditions for Growth Run #516. ................. ....._.. ............ ......18


A-29. Reactor conditions for Growth Run #521 .........._._ ........... ......_.. ........19


A-30. Reactor conditions for Growth Run #522 ................. ............_ .....191.........












A-31. Reactor conditions for Growth Run #523 ................ ...............192....._._. .


A-32. Reactor conditions for Growth Run #524 ................. ............_ .....193.........


A-33. Reactor conditions for Growth Run #525 ................ ...............194........... ..


A-34. Reactor conditions for Growth Run #535 .........._._ ........... ......_.. ........19


A-3 5. Reactor conditions for Growth Run #536. ................. ....._.. ............ ......19


A-36. Reactor conditions for Growth Run #537 ................. ............_ .....197.........


A-37. Reactor conditions for Growth Run #53 8. ................ ....___ ....___ .........19


A-38. Reactor conditions for Growth Run #540 ................. ..........._._.....199...... ..


A-39. Reactor conditions for Growth Run #541 ...._.._.._ ................._._. .........20


A-40. Reactor conditions for Growth Run #542. .............. ...............201....


A-41. Reactor conditions for Growth Run #569. .............. ...............202....


A-42. Reactor conditions for Growth Run #575. ............. ...............203....


A-43. Reactor conditions for Growth Run #578. ............. ...............204....


A-44. Reactor conditions for Growth Run #579. .............. ...............205....


A-45. Reactor conditions for Growth Run #582. .............. ...............206....


A-46. Reactor conditions for Growth Run #586. .............. ...............207....


A-47. Reactor conditions for Growth Run #587. .............. ...............208....


A-48. Reactor conditions for Growth Run #588. ............. ...............209....


A-49. Reactor conditions for Growth Run #628. ............. ...............210....


A-50. Reactor conditions for Growth Run #629 ................. ............_ .....211.........


A-51. Reactor conditions for Growth Run #630. .............. ...............212....


A-52. Reactor conditions for Growth Run #634. .............. ...............213....


A-53. Reactor conditions for Growth Run #63 5 ................ ...............214.............


A-54. Reactor conditions for Growth Run #636. .............. ...............215....


A-55. Reactor conditions for Growth Run #637. .............. ...............216....












A-56. Reactor conditions for Growth Run #63 8 ...._.__._ ......_._. ....._._. .........21


A-57. Reactor conditions for Growth Run #639. .............. ...............218....


A-58. Reactor conditions for Growth Run #640. .............. ...............219....


A-59. Reactor conditions for Growth Run #641 ...._.._.._ ................._._. .........20


A-60. Reactor conditions for Growth Run #647. .............. ...............221....


A-61. Reactor conditions for Growth Run #648. ............. ...............222....


A-62. Reactor conditions for Growth Run #649. .............. ...............223....


A-63. Reactor conditions for Growth Run #652. .............. ...............224....


A-64. Reactor conditions for Growth Run #653. ............. ...............225....


A-65. Reactor conditions for Growth Run #654. .............. ...............226....


A-66. Reactor conditions for Growth Run #655. ............. ...............227....


A-67. Reactor conditions for Growth Run #656. .............. ...............228....


A-68. Reactor conditions for Growth Run #657. .............. ...............229....


A-69. Reactor conditions for Growth Run #658. ............. ...............230....


A-70. Reactor conditions for Growth Run #659 ................. ............_ .....231.........


A-71. Reactor conditions for Growth Run #660. .............. ...............232....


A-72. Reactor conditions for Growth Run #661 ...._.._.._ ................._._. .........23


A-73. Reactor conditions for Growth Run #662. .............. ...............234....


A-74. Reactor conditions for Growth Run #666. .............. ...............235....










LIST OF FIGURES


Figure page

1-1. Top view of the PMEE reactor ........... ..... ._ ...............25.

2-1. Spectral irradiance versus wavelength under AMO and AM1.5 conditions. .....................49

2-2. Photovoltaic system. ............. ...............49.....

2-3. Chalcopyrite structure of CulnSe2. ............. ...............50.....

2-4. CIGS/CGS monolithic tandem device structure ................. ...............51...............

3-1. UF growth recipes ................. ...............71................

3-2. Typical CIGS device structure ................. ...............72........... ...

4-1. Morphologies of films grown at lower growth temperatures by similar growth
recipes. ............. ...............94.....

4-2. Morphology of a film grown at a higher growth temperature. ............. .....................9

4-3. Morphologies of the Cu-rich domain region of CGS films grown by the same recipe
at different growth temperatures. .............. ...............95....

4-4. Morphologies of the Ga-rich matrix region of CGS films grown by the same recipe
at different growth temperatures.. ............. ...............95.....

4-5. Morphologies of the Cu-rich domain region of CGS films grown with different
growth recipes at 4910C ................. ...............95...............

4-6. Morphologies of the Ga-rich matrix region of CGS films grown with different
growth recipes at 4910C ................. ...............96...............

4-7. Morphologies of CGS films grown by the emulated 3-stage process at 4910C
(X3 0,000) ................. ...............96........... ....

4-8. Morphology of a Cu-rich film (#542) with large grains and a uniform surface. ...............96

4-9. Diffraction patterns of films grown at different temperatures with the same modified
three-stage process.. ............. ...............97.....

4-10. Diffraction patterns of films grown at different temperatures with the same modified
three-stage process featuring an initial GaSe layer. ....._._._ .... ... .__ ........_........98

4-11. Diffraction patterns of films grown at different rotational speeds ................. ................99

4-12. Diffracti on patterns of film s grown at different levels of overall Cu-ri chnes s................1 00











4-13. Effect of KCN-etch on the diffraction pattern of a Cu-rich fi1m.. ................ .................101

4-14. Diffraction patterns of films grown by the Constant Cu Rate Process. .................. .........102

4-15. Diffraction patterns of films grown with varying levels of peak Cu-richness. ................ 103

4-16. Diffraction patterns of films grown by the Emulated 3-Stage Process. ................... ........104

4-17. Diffraction pattern of a film grown by the Emulated 3-Stage Process that was never
Cu-rich. ............. ...............105....

4-18. Surface morphology of films grown by the Constant Cu Rate process (X100).. ........._...106

4-19. Surface morphology of a Cu-rich film grown by the Constant Cu Rate Process
(X 5000). .............. ...............107....

4-20. Surface morphology of a Cu-rich film grown by the Constant Cu Rate Process
(X 10,000)............... ...............10

4-21. Surface morphology of a Ga-rich film grown by the Constant Cu Rate Process. ...........109

4-22. Effect of KCN-etch on the surface morphology of the island region of a Cu-rich film..1 10

4-23. Effect of KCN-etch on the surface morphology of the field region of a Cu-rich film. ...11 1

4-24. Surface morphology of a Ga-rich film with rings around the islands..............._._...........112

4-25. Distinct grain structure of a Ga-rich film with rings around its islands. ................... .......1 13

4-26. Surface morphology of a film grown by the Emulated 3-Stage Process. ................... .....1 14

4-27. Surface morphology of a Cu-rich film grown by the Emulated 3-Stage Process. ...........1 15

4-28. Surface morphology of a Ga-rich film grown by the Emulated 3-Stage Process. ...........1 16

4-29. Diffraction pattern of a Ga-rich film grown by the Constant Cu Rate process. ..............1 17

4-30. Diffraction patterns of films grown by the Constant Cu Rate process. ................... ........1 18

4-31i. Diffraction patterns of films grown by 3-stage process ................. ................. .... 1 19

5-1. Dark and illuminated I-V curves for Device #523............... ...............132.

5-2. Spectral response curves comparing Device #523 and #452. ..............._ .............. ....132

5-3. Photo I-V curve for Device # 640............... ...............133..

5-4. Photo I-V curve for Device # 655............... ...............133..











6-1. Diffraction pattern of CIS film #582. ............. .....................144

6-2. Diffraction patterns of CIGS films grown to different thicknesses.. ............ ..................145

6-3. Diffraction patterns of CIGS films grown with different Cu/III ratios............................ 146

6-4. Diffraction patterns of CIGS films grown with different Ga/III ratios.............._.._.. ........147

6-5. Illuminated I-V curve for Device #582............... ...............148.

6-6. Comparison of illuminated I-V curves of Device #575 and #588. ............. ... ..........._..149

6-7. Comparison of the illuminated I-V curves of Device #588 and the calibration cell. ......149

7-1. Metal source crucible. ................. ...............157_.._ .....

7-2. Deposition flux Fs(r) (atoms/cm2-S) on the substrate at nine different melt levels. ........157

7-3. Positioning of the sources in the reactor. ...._ ......_____ .......___ ..........5

7-4. Cryoshroud ................. ...............158....._.. .....









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

COPPER GALLIUM DISELENIDE THIN FILM AB SORBER GROWTH
FOR SOLAR CELL DEVICE FABRICATION

By

Ryan Kaczynski

May 2007

Chair: Oscar Crisalle
Major: Chemical Engineering

A custom-built migration-enhanced epitaxy reactor originally optimized for CulnSe2 (CIS)

deposition was modified to grow gallium-containing compound semiconductor thin films, such

as CuGaSe2 (CGS) and Cunlnt-GaxSe2 (CIGS). The addition of gallium allows for the

manufacturing of solar cell absorber layers with wider band gaps.

Three distinct growth recipes under several growth temperatures and a wide range of

metal-composition ratios are used to deposit polycrystalline CGS thin films. The surface

morphology of gallium-rich fi1ms is typically very uniform, with long needle-like grains when

grown by the first recipe, a constant copper-rate process. In contrast, copper-rich fi1ms grown by

this same recipe or by a modified three-stage process have island structures with very large

grains embedded in a matrix region that possesses small grains. The surface morphology

becomes more uniform and the grains in the matrix region become larger when a higher growth

temperature is used. The third recipe, an emulated three-stage process, does not produce films

with an island-matrix structure, and the grains are uniformly large.

The highest conversion efficiency achieved for solar cells based on CGS is 5.3%, delivered

by a copper-rich absorber deposited at the highest sustainable growth temperature of 4910C.

This device has a large fill factor of 66 %, but the open-circuit voltage of 0.48 V is lower than









what is expected from a wide band-gap absorber. A set of CIGS solar cells was completely

fabricated and characterized in-house. This led to the most efficient device produced from an

absorber grown in our reactor, in the form of a 9 % CIS solar cell featuring a one-micron film

deposited at 4910C.

Finally, a dynamic reactor model was created to describe the deposition environment in our

epitaxial reactor. All relevant physical features are incorporated, including the cyclic motion of a

rotating platen and the spatial distribution of the flux produced by three metal effusion sources.

Reaction occurs under an excess of selenium, and operational variables such as rotational speed

and melt height can be simulated. The outputs are predicted film thickness and composition.

Further work is proposed to identify the values of adjustable sticking coefficients using

experimental data.









CHAPTER 1
INTTRODUCTION

This first chapter is intended to give the reader an overall description of the system under

study. This information will be followed by the statement of the obj ectives of the proj ect and

Einally by the proposed solution strategy to the problems. Not many details will be covered in

this chapter; the purpose is to give the reader an overall idea of the rationale behind the

remaining sections of the report.

The second chapter is an introduction to photovoltaics and thin film solar cells, especially

those based on copper chalcopyrites. Its purpose is to familiarize the reader with energy

production, solar cells and the important parameters that affect their efficiency. This chapter

may be skipped by readers who are already familiar with the field without a loss of continuity.

The third chapter is an in-depth description of the absorber growth and device fabrication

procedures. It is complimented by an explanation of the various techniques used to characterize

the respective films and devices. Our techniques are compared to those in the literature.

Chapter 4 is an account of Copper Gallium Diselenide absorber growth in our modified

molecular beam epitaxy reactor. The main motivation is the characterization of the films grown

under various processing conditions. This chapter is accompanied by Appendix A, which

includes the growth conditions for each absorber film described within the course of this work.

The fifth chapter is very similar in structure to the previous one, with the focal point shifting

to the device fabrication of CuGaSe2 Solar cells. The thin films grown in the PlVEE reactor that

are discussed in Chapter 4 are used as the absorber layers in solar cell devices. Most cells were

finished at the National Renewable Energy Laboratory (NREL), except for the final set which

was completely fabricated in-house at the University of Florida.









In Chapter 6, low gallium content Cu(In,Ga)Se2 absorbers are grown and completely

fabricated within our facilities. The as-grown absorbers and subsequent devices were

characterized to determine the effect of Ga composition on films grown by a simple single-stage

process at a substrate temperature below 5000C. Appendix A also contains the reactor

conditions pertaining to each of the growth runs.

In the seventh chapter, the focus is shifted to modeling of the reactor. A flux model had

already been developed and needed to be incorporated into overall dynamic reactor model. This

chapter is complemented by appendix B, which includes the details of model development.

The final two sections of the manuscript are conclusions and the list of references. The

conclusions chapter also contains a list of possible future directions this research can take.

System Description

This proj ect was initiated after the Boeing Company decided to terminate its photovoltaics

research program and donated some research equipment to the University of Florida. Billy

Stanbery, who was part of the Boeing Team, decided to enroll at the University of Florida to

pursue a PhD degree. This jump-started a comprehensive, multi-faceted and multidisciplinary

CIS solar cell research effort at the University of Florida.

PMEE Reactor

Physical Vapor Deposition (PVD) describes semiconductor thin film growth in a reactor

whose high vacuum conditions cause material to flow in the molecular regime. Molecular Beam

Epitaxy (MBE) describes this deposition process when epitaxial growth results. The main

attributes of MBE compared to other techniques are a low growth temperature that limits

diffusion, a slow growth rate that ensures two-dimensional growth, a simple growth mechanism,

and compatibility with in situ analysis. Because of its unprecedented control down to the atomic









scale, MBE has been employed for the growth of many novel devices that require "band gap

engmneening."

Migration Enhanced Epitaxy (MEE) is a variant of MBE based on sequential rather than

simultaneous exposure of the substrate to source fluxes. Rather than using shutters to control the

material deposition on the substrates, the substrates rotate on a donut-shaped platen that takes

them through the different deposition zones as well as fluxless relaxation steps in between. Each

substrate is sequentially exposed during a complete cycle to Cu+In+Ga, background vacuum

ambient, Se, and the background vacuum ambient again [1]. Chalcopyrite films have been

grown by MBE for nearly 30 years [2], but the rotating platen, which is the main concept of

MEE, makes our work unique compared to other research groups.

Our own reactor has been named with the acronym PMEE (plasma-assisted migration

enhanced epitaxy) because of the incorporation of a plasma cracker for selenium or sulfur

deposition. This reactor was originally designed to deposit CulnSe2 (CIS) absorber layers and

then was modified to support the growth of CuGaSe2 (CGS) and Cunlnt-GaxSe2 (CIGS) films as

well. The PMEE reactor can deposit Cu, In, Ga, Se, S, and Na, allowing for the deposition of a

wide-variety of Cu-chalcopyrite thin films. It can support device manufacturing based on

polycrystalline co-deposited CIS-based films, as well as an assortment of studies such as single

crystal growth and bilayer precursor design for RTP studies. This process is low throughput and

thus not economically feasible, but this research is concerned with investigating the film

properties, not large-scale production. The ultra-high vacuum (UHV) creates an extremely clean

condition and makes it possible to generate the molecular beam of each source so that the growth

system can be used to grow epitaxial CIS thin films of high crystalline quality. It can overcome,










to a certain extent, a disadvantage of MBE, which is low productivity by processing nine

samples in one batch.

There are some disadvantages of the system. Due to the rotational movement of the platen

and thus the substrates, direct in-situ measurement of the substrate temperature is virtually

impossible. The thermocouple is currently located in the gap between the platen and the heater

and reading sort of an average value of those two temperatures. The localized heater position

creates non-uniformity of the temperature distribution on the substrates. The growth rate is

significantly limited by the selenium flux delivery. Even with a high Se flux rate

([Se]/([Cu]+[In])>5), it is hard to obtain sufficient Se incorporation into a growing fi1m under a

high temperature condition since the Se deposition zone is confined, and the high vapor-pressure

material is easily re-evaporated from the surface. As a result, maximum flux rates of Cu and In

are limited, which makes it difficult to achieve high growth rate. A time-consuming and costly

problem is that the PMEE reactor has been custom designed and therefore requires extensive

customizations to the regular sources available from manufacturers.

Chamber

The reactor can be divided into four zones as is shown in Figure 1-1: load-lock, chalcogen,

heater, and metals. Materials are sequentially deposited as the substrate passes through their

respective zones rather than co-deposited. The total flux is highly enriched in the specie

evaporating from the nearer of the two sources as the substrate initially enters the metals

deposition zone from either side. Hence, substrate rotation direction can result in substantially

different compositions within the first metal layer. Counterclockwise rotation results in an initial

metal flux during each MEE cycle that is substantially Cu-enriched.

The high-vacuum chamber is divided vertically into two zones by the cryoshroud and is

maintained at a base pressure of around 10s torr by a series of diffusion and mechanical pumps.









Above the cryoshroud is the growth zone, where all the sources and substrates are located.

Liquid nitrogen, circulated in the cryoshroud, further reduces the pressure in the growth zone to a

base pressure around 10-9 torr. Pressure during deposition is in the range of 10-7 to 10-s torr

depending on the operating conditions. If the chamber is brought to atmospheric pressure, it

takes a few days to get the appropriate low pressure back.

Load-lock

A load-lock, attached to a port at the substrate platen level of the chamber, allows the

system to remain under vacuum for months during operation. The load-lock is independently

pumped with a small turbomolecular pump (TMP) and isolated by a gate valve from the loading

chamber. Chamber venting uses argon gas and the load-lock is equipped with a Venturi pump

and a liquid nitrogen sorption pump for rough-pumping down to the TMP's crossover pressure of

10-3 torr. Up to nine 2" diameter wafers or 2" x 2" square substrates can be loaded onto the

donut-shaped molybdenum platen via a two-prong fork, which is then rotated so that each

substrate travels through all four zones.

Chalcogen Zone

The chalcogen zone is where a thermal cracker for Selenium (Se) and a low-capacity

plasma cracker that can be charged with Selenium or Sulfur (S) are installed. Since Se does not

evaporate as a very reactive species, it must be further cracked to be incorporated into the film.

The thermal cracker breaks the large molecules into smaller, more reactive molecules by heating

them to very high temperatures in a double-oven reactor before deposition. Hence the

temperature in the evaporation zone of this double oven controls the flux of the material and the

temperature in the cracking zone controls the species distribution [3]. The plasma cracker

accelerates particles to make them more effectively reactive, which is an alternative to the high

temperature of the thermal cracker. No sensors are used to measure the flux since the Se is










deposited in excess; only the temperatures of the sources are measured. Excess Se deposits

everywhere so a chalcogen zone shield was incorporated to isolate this zone from the rest of the

reactor.

Heater Zone

After passing through the chalcogen zone, substrates are brought to the desired operation

temperature in the heater zone with radiative heating by a boron nitride-coated radiation heater.

Most of the substrate platen heating is provided here. In other zones, the substrates are slowly

cooled down since there is no direct heating there. Some extent of non-uniformity of the

temperature distribution on the platen is expected due to the complex design [4].

Metals Zone

Finally, the substrates enter the metal deposition zone where they are sequentially exposed

to Copper (Cu), Indium (In), and Gallium (Ga) fluxes. The Cu, In, and Ga sources are thermal

evaporation sources with conical shaped crucibles and free-evaporating surfaces. Deposition

uniformity is improved significantly with a conical instead of a cylindrical crucible [5]. The

effusion sources are identical in structure: 7.50 tapered angle, 30 cc capacity, and constructed

with Pyrolithic Boron Nitride (PBN). Cu and Ga have dual filament heating structures due to

their properties, whereas In has only one. The tip filament keeps the tip of the crucible hot so no

impurities can condense on the surface. Shielding prevents deposition on each individual

sub state except during that portion of each rotation cycle of the substrate platen when it is inside

the shield.

The platen then rotates back into the load-lock zone where the entire cycle restarts. The

dopant source is located in this area to introduce small quantities of impurities. It is charged with

a very small amount of NaF. The metals zone was expanded into the previously allocated load-

lock area so that the Gallium source could be added.









Control

Every material source is equipped with thermocouples for monitoring the temperature.

The source temperature is actually measured indirectly by putting the thermocouple in thermal

contact with the crucible. These are mainly c-type thermocouples, with the exception of the

evaporation zone in the two crackers which are fitted by k-type thermocouples due to the lower

temperatures involved. The substrate temperature is measured indirectly by means of a c-type

thermocouple suspended in between the rotating platen and the substrate heater. The rotating

nature of the platen prevents getting direct temperature readings on the substrate [6].

Rate control is provided by a Leybold-Heraeus Inficon Sentinel III with both EIES

(Electron Impact Emission Spectroscopy) sensors for monitoring and controlling the metals

deposition process and quartz crystal monitors (QCM) for calibration. Closed-loop feedback

control is conducted along with the in-situ rate measurement employed by the EIES sensors for

Cu and In sources. EIES sensors are calibrated by a QCM that is located right over the source

cells whenever source material is reloaded. A single QCM is used to monitor the Ga flux

because the reactor modifications were limited by space and the current setup. The dopant flux

is monitored by QCM because it doesn't need to be controlled tightly. No instrument is used to

measure the Se flux rate in-situ so closed-loop feedback control based on temperature has been

adopted.

An instrumentation and control interface for the PMEE reactor was designed to enable the

implementation of advanced control strategies envisioned for the local sources as well as the

supervisory control structure. A human-machine interface is programmed on a LABVIEW

platform so that real time control of the PMEE reactor can be administered through a central

computer [6]. Using microprocessor control, one can ensure run-to-run repeatability by

constantly monitoring and adjusting the various growth parameters.









Problem Statement

The purpose of this proj ect is to explore the key processing issues associated with growing

copper chalcopyrite fi1ms containing gallium in a migration enhanced epitaxy reactor. Copper

Gallium Diselenide is theoretically a good candidate as the top cell in a tandem solar device,

owing to its nearly ideal band gap of 1.7 eV and maximum theoretical efficiency of 26%.

Practical use in a tandem cell will require efficiencies greater than the current best cell efficiency

of approximately 10%. A low temperature process for the growth of CuGaSe2 absorber layers

should be developed to avoid the degradation of the junctions located underneath the top cell in a

monolithic tandem structure.

To grow high-quality CGS absorbers, several steps were required. First, a gallium source

needed to be retro-fit into a custom-built IVEE reactor used for the deposition of CulnSe2

absorber films. Then it needed to be shown that this reactor could grow polycrystalline CuGaSe2

thin films that produced working solar cell devices. Various growth recipes were investigated,

along with varying material compositions, growth temperatures, and post-deposition processes.

The structural and morphological properties of the films were characterized along with electrical

properties of the subsequently fabricated devices.

Another goal was to achieve complete in-house fabrication and characterization of CGS

devices. This was intended to decrease the feedback time needed in investigating the effect of

processing changes on electrical properties of the solar cell. This required a team effort of

several graduate students.

Finally, the flux models of the effusion sources needed to be incorporated into a dynamic

reactor model. This model will be the basis for a control feedback scheme that will correlate

film properties and hence, device properties with the input conditions of the reactor.












Chalcogen
Zone


Load-lock
Zone


Figure 1-1.


Top view of the PMEE reactor.









CHAPTER 2
SOLAR CELLS

Energy

Rapid industrialization combined with an expanding population is driving the world's

demand for energy, which is proj ected to triple by the end of the century (from 13 TW to 46 TW)

[7]. The fossil fuel reserves that currently furnish power to the globe will fall short of this

demand over the long term, and their continued use produces harmful side effects such as

pollution. Finding sufficient supplies of clean energy for the future may be civilization's most

difficult challenge. Alternative renewable fuels are currently not competitive with fossil fuels in

cost and production capacity, but solar cells have the potential to become a key part of the

solution to this problem.

Sunlight

Photovoltaic (PV) systems exploit an inexhaustible resource that is free to use and

available anywhere in the world. More energy from sunlight reaches the earth's surface in one

hour (4.3 x 1020 J) than is consumed by civilization in an entire year (4. 1 x 1020 J) [7]. If 0. 16%

of the land on Earth was covered with 10% efficient solar cells, 20 TW of power would be

provided, which is more than the world' s current consumption rate of fossil energy [7]. This

illustrates the impressive magnitude of the solar resource and the potential harbored by solar cell

technology.

The sun emits energy as a blackbody radiator at a temperature of approximately 6000 K

with a spectrum ranging from the ultraviolet (3.5 eV), through the visible, into the infrared (0.5

eV). The energy of the visible region ranges from 3.1 eV (violet) to 1.8 eV (red) with the peak

power of the sun occurring at approximately 2.25 eV. This distribution of photons in the

spectrum is one of the greatest limiting factors on solar cell performance. Under monochromatic









light, a typical PV cell might be able to convert 60% of the light to electricity, but under the

multicolored solar spectrum, the same cell would only be able to convert 10% of the light' s

energy to electricity [8]. Diffuse light is the portion of sunlight that has been refracted or

scattered in the atmosphere before it reaches the Earth's surface. When the sky is completely

clear, about 10% of the sunlight is diffuse. Most losses in the spectrum at lower energies are

caused by light being absorbed by molecules of water vapor while at energies higher than 3 eV,

almost all sunlight is absorbed by ozone [8].

The AM1 solar spectrum represents the sunlight on the Earth' s surface when the sun is at

its peak. At a solar zenith angle of 48.20, the equivalent of 1.5 of these noontime air masses

(AM1.5) is diminishing the intensity of the sunlight [8]. The AM1.5 condition has an incident

power of 84.4 mW/cm2 and is the most appropriate for calculating the conversion efficiency of a

solar cell in the terrestrial environment [9]. The irradiant power of the sun under AMO

conditions is 135.3 mW/cm2, which is the spectrum measured outside the earth's atmosphere.

AM1.5 is compared to AMO in Figure 2-1 [9]. The "peak watt" (WP) rating is the power (in

watts) produced by a solar cell module illuminated under the following standard conditions: 100

mW/cm2 intensity, 250C ambient temperature, and AM1.5. Because of day/night and time-of-

day variations and cloud cover, the average electrical power produced by a solar cell over a year

is about 20% of its WP rating [7].

The two most important variables controlling the amount of annual sunlight are latitude

and local cloud cover. Latitude is important to the amount of annual sunlight for two reasons:

the angle of the sun and the length of the days. The Northeast and Northwest United States are

the cloudiest regions, while the Southwest usually has the clearest skies. In Buffalo, NY, there is

about 60% and in Sacramento, CA, about 85% of the solar energy available in Albuquerque, NM









[10]. Almost 90% of the country gets between 6 and 8 kWh/m2 daily, which is enough for the

effective use of PV [8]. There is also a seasonal effect on the energy produced by PV modules

because in the summer the sun spends more time at high elevation angles (low air mass) than in

the winter (high air mass)

Costs

Clearly, solar energy can be exploited on the needed scale to meet the global energy

demand. Sunlight is readily available and its use does not harm the environment through

pollution or the climate through greenhouse gases. Yet, U.S. electricity production by solar cells

currently represents a tiny fraction (<0.02%) of the total electricity supply [7]. The wide-spread

use of PV has been hampered by the relatively high price of the solar cell module. The huge gap

between our present consumption of solar energy and its enormous undeveloped potential

defines a grand challenge in energy research.

Solar cells typically have a lifetime of at least 30 years and incur no fuel expenses, but they

do involve a capital cost. The cost for the electricity produced by the cell is estimated by

spreading the total capital cost over the entire lifetime of the cell while considering the total

electrical energy that will be produced during that time. Higher conversion efficiency thus

directly impacts the overall electricity cost because higher efficiency cells will produce more

electrical energy per unit cell area over the cell lifetime. The most useful cost calculation for PV

cell modules ($/WP) is determined by the ratio of module cost per unit area ($/m2) divided by the

maximum amount of electric power delivered per unit of area (module efficiency multiplied by

1000 W/m2, the peak isolation power) [7]. In addition to module costs, a PV system also has

costs related to the non-photoactive parts of the system, called balance of system (BOS) costs.

To compete with electricity produced from fossil fuels, solar cell costs must eventually approach

$0.40/WP [7].









Photovoltaic Systems


Efficiency = pwrot(2-1)
power mn

For a solar cell, this is the ratio of the electric power produced by the cell at any time

versus the power of the sunlight arriving at the cell. Efficiencies do not fluctuate much over the

life of a cell unless it is degrading. By definition, the higher a photovoltaic device's efficiency,

the more electricity it produces for a given exposed area. Besides device efficiency, the

definition of performance must also include uniformity, reproducibility, throughput, materials

utilization, and yield [11].

A single cell is only useful when powering wristwatches and calculators so many cells are

connected within a module. The module serves two purposes: it protects the solar cells from the

outside environment and it delivers a higher voltage than a single cell. Individual small-area

cells must be connected in series so some part of the module area is lost for the interconnection

of cells. Another important effect which reduces the performance of modules is the non-

uniformity of voltage and current density over a large area, which is mainly related to the

compositional variation of the absorber layer [12]. A critical issue that must be resolved is the

large gap in efficiency between small-area laboratory devices and large-area modules.

The basic components of a photovoltaic system are the modules, support structures, land,

and possibly sun trackers. There are two different kinds of modules: flat plates and

concentrators. Flat plates are large panels that can be assembled into even larger arrays. To

maximize energy production, a module can be mounted onto a two-axis solar tracker so that it

always points directly at the sun [10]. Concentrators use large, concentrating lenses to focus

sunlight onto small cells. These lenses replace large areas of expensive semiconductor material,

reducing the total module cost. A concentrator does not produce any energy when the weather is










cloudy because it cannot focus diffuse light. Hence, they are geographically limited to sunny

locations as opposed to flat plates, which can be useful in cloudy areas.

There are two major market sectors: grid-connected and stand-alone systems. The former

delivers power directly to the grid by converting the direct current of solar modules into

alternating current by an inverter. The latter supplies power to isolated sites and to small-scale

consumer products. Since solar energy is not always available, effective storage and distribution

are critical to matching supply with demand. Storage drives up the cost of solar cells. A

simplified photovoltaic system is shown Figure 2-2. Besides the terrestrial market, there is also

the space market, which has different material and cost requirements.

Future

The primary obj ective of worldwide solar cell research and development is to reduce the

cost of photovoltaics to a level that will be competitive with conventional ways of generating

power. The world market grew from less than 10 MWP/yr in 1980, to 80 MW in 1996 sold at

prices close to $10/WP [13], to finally exceeding 1000 MW in 2004 at less than $7/WP [14].

Costs need to be reduced even more for solar cells to become competitive with the likes of oil,

coal, and natural gas. Otherwise, future large-scale use of PV might depend more on

environmental concerns rather than economic competitiveness.

Behind the progress of photovoltaics as a technology is a loyalty to PV as an idea. There

are technical obstacles that must be overcome and their solutions depend on the funding of

fundamental research. As these issues are resolved, the costs will continue to fall. "Solar

electricity is part of America' s present and its future as is the R&D that enables it" Larry

Kazmerski, one of the pioneers of thin film solar cells [15].









Solar Cell History

Photovoltaics had its beginnings in the nineteenth century. French Scientist Alexandre-

Edmond Becquerel discovered the PV effect in 1839. He observed that a voltage and a current

were produced when two electrodes in a beaker full of fluid were exposed to sunlight. And in

1873, Willoughby Smith found that the element selenium conducted far more electricity when it

was illuminated than it did when it was dark [8].

Many consider the work accomplished during the 1950s at Bell Laboratories to be the true

origin of photovoltaics. Cal Fuller, Darryl Chapin, and Gordon Pearson made a silicon cell that

was able to convert 6% of sunlight into electricity [16], which was a large improvement over

selenium cells. Fuller and Chapin eventually reached 10% conversion efficiency, but compared

to traditional electricity in the 1950s, the cost of PV-produced power was a thousand times as

much [8].

From the mid 1950s to the early 1970s, PV research and development was directed

primarily toward space applications where it is the conventional power source. In space, payload

weight is critical and solar cells weigh very little compared to the power that they produce.

Nearly every communications satellite, military satellite, and scientific probe is powered by PV.

Satellite manufacturers can endure a high cost because it is a small fraction of their total cost, but

they cannot tolerate an unreliable power source that may jeopardize their entire investment.

Then in 1973, a greatly increased level of research and development on solar cells was initiated

following the oil embargo in that year, which led to the creation of the U. S. Department of

Energy, along with its PV program, a few years later.

Researchers at Bell Labs showed three decades ago that the I-III-VI2 Semiconductor

CulnSe2 (CIS) allows for efficient solar-to-electrical energy conversion. A single crystal CIS

cell of 12% efficiency was made at Bell Laboratories in 1975 [17] and the University of Maine









produced a polycrystalline CulnSe2 Cell Of nearly 6% efficiency in the following year [18].

Although Bell Labs and Maine initiated the study of CIS, the most important early research was

done by a small group at Boeing Aerospace Corporation in Seattle. Boeing's team, led by Reid

Mickelsen and Wen Chen, achieved over 10% efficiency with CIS cells in 1982 using elemental

co-evaporation of the source materials in vacuum [19].

Solar Cell Device Physics

Photovoltaics (PV) is the direct conversion of sunlight into electricity; solar cells absorb

sunlight and change it continuously into electricity. According to quantum theory, light can

behave either as waves or as particles. Discrete particle-like packets of light are called photons,

and each photon has a well-defined energy and wavelength.

Band Gap

The valence and conduction bands of an inorganic semiconductor are separated by a

forbidden energy range that the electrons cannot occupy [20]. This minimum threshold energy is

called the energy gap or band gap (E,), and it varies for different materials because each of them

has a different bond strength. Semiconductors with weak bonds have small energy band gaps.

Conduction is only possible if we can impart kinetic energy to an electron. When photons with

energies hv > E, impinge upon a pn junction solar cell, they are absorbed and the rate of

generation of electron-hole pairs as a function of distance x from the surface of the solar cell is

given by the following equation [9]:

gE(X) = a00(1-R)e-ax (2-2)

where a is the optical absorption coefficient, Go is the incident photon flux density per unit

bandwidth per second, and R is the reflection coefficient. Photons with energies below the band

gap pass through the absorber without being absorbed.









Electric Field

Most semiconductor devices incorporate both positive and negative regions, and it is the

space-charge region formed between them that leads to their useful electrical characteristics [20].

After a certain number of electrons and holes have flowed from one region to the other, an

electric field will be built up, preventing further net flow of the carriers. The greater the density

of free carriers initially on each side of the interface, the greater will be the electric field that

forms when they mix. This electric field is not tenuous; it does not come and go. The area of the

field is also called the depletion region because in that region there are no free carriers. They all

are either in bonds or swept away by the field. Under equilibrium conditions, electron-hole pairs

are continuously generated everywhere within the semiconductor and in the absence of an

applied voltage, the electron-hole pairs recombine and therefore no current flow results.

However, when a positive voltage is applied to the n-region of a diode with respect to the p-

region, the electron-hole pairs, once generated, will be separated and their probability of

recombination is diminished. The most important attribute of these p-n junctions is that they

rectify, i.e. they permit the passage of electric current in only one direction [20].

The electric field that drives the current is proportional to the material's band gap. A

semiconductor with a small band gap produces an insignificant voltage. As the band gap is

increased, more solar photons within the spectrum will lack the energy to produce electrons and

holes so a very large band gap would produce a high voltage with a tiny current. For a

semiconductor to be a sufficient absorber layer in a solar cell, it must have a band gap that allows

for both reasonable current and voltage.

Recombination

Radiative recombination, in which a hole reacts with an electron and produces a photon, is

exactly the reverse of absorption; it is the spontaneous transition of an electron from the









conduction band to an unoccupied state in the valence band [21]. In real solar cells,

recombination via impurities is the predominant recombination process. Recombination centers

located within the electric field can severely reduce the field's strength and thus the voltage.

These are called shunts [8]. Defects located at the interface can also be efficient recombination

centers because they introduce deep trap levels into the band gap [22].

Defects

In compound semiconductors, intrinsic point defects are introduced to compensate for

deviations from the stoichiometry [23]. The simplest point defect is the vacancy in which a

single atom is missing from the lattice. An interstitial is an extra atom occupying space between

the normal lattice sites. A component atom may also occur on a site intended for another. This

is called an antisite defect. Point defects influence the bulk properties of a semiconductor as

opposed to grain boundaries or interfaces that only affect the film locally [22].

A grain boundary is the complete fracturing of bonds along an entire surface. They occur

when a lattice takes shape in such a way that a defect spreads and makes it impossible for nearby

atoms to bond together. According to the grain boundary carrier-trapping theory, grain

boundaries work as trapping centers and therefore hinder the transport of charge carriers towards

the pn interface [24]. Cell performance would suffer considerably because these lost electrons

would not contribute to the cell current. It is very common for crystal defects to cause a

polycrystalline structure where the grain boundaries separate regions of different crystallographic

orientation. Many of the electrical and optical properties of these materials are determined by

the corresponding properties of the grain boundaries and these may differ considerably from a

single crystal [22].

When grain boundaries cannot be avoided, they need to be passivated to minimize their

effect. This consists of adding some extra material, usually oxygen, that can make the grain









boundary defects less harmful. Most added materials diffuse preferentially down grain

boundaries so it goes directly to the region where it can have the most effect. The added oxygen

might passivate defects in the grain boundary by grabbing loosely bound electrons, removing

many of them as undesirable recombination centers. Some materials have relatively harmless

grain boundaries like CulnSe2; they may be self-passivating or some growth step may passivate

them. They can be made inexpensively and yet behave almost as if they were single crystals.

Others have very harmful grain boundaries, like gallium arsenide (GaAs) and silicon (Si ) [8].

Thin Films

Crystalline silicon currently dominates the photovoltaic market despite a complicated

manufacturing process and a high production cost. Its advantages are a readily available raw

material, mature processing technology, and non-toxicity. Si-based products are also very

reliable and are capable of achieving high efficiencies [14]. The preeminence of the element Si,

in its amorphous or crystalline form, within the market is an overwhelming 99% [25].

A strategy for reducing costs is to use thin film materials that have a very high absorptivity

for solar photons. The leading thin film technologies, a-Si, CdTe, and CIS, offer the potential for

significant manufacturing advantages over crystalline Si. They have a lower consumption of

materials, independence from Si shortages, fewer processing steps, and a monolithic circuit

design so no assembly of individual solar cells into a Einal product is needed [26]. Less material

usage leads to lower material costs, thinner layers leads to faster processes and lower capital

costs, and the processing of large-area devices leads to reduced handling costs. They can also be

made into flexible and light-weight modules on alternative substrates, which provide multiple

advantages in processing [27]. The most serious threat to large-scale deployment of existing thin

fi1ms is materials availability of key elements such an In, Te, and Ge. Thus, Si-based technology

may always be relevant.









Direct vs Indirect Band Gap

Si has an indirect band gap so light absorption is much weaker than for thin films, which

are direct band gap semiconductors. The difference in absorption strength between direct and

indirect band gap semiconductors comes from the different processes by which they absorb

individual photons. Direct transitions are transitions in which the momentum of the electron-

hole pair does not change. Light of sufficient energy to free an electron from its fixed state is

absorbed by the electron, which is then freed. Light-absorption is more complicated in an

indirect band gap material. Promotion of an electron to the conduction band requires the

simultaneous interaction of a photon and a thermal vibration of the crystal lattice, called a

phonon. When a photon and a phonon of the proper energies are both absorbed, at the same

time, by a bound electron, a free electron-hole pair results. Almost all of the energy needed to

generate the electron-hole pair is carried by the photon; the phonon just acts to catalyze the

process.

Absorption Length

The absorption length of light in a semiconductor is determined by the likelihood that a

photon will be absorbed. The absorption length for crystalline Si is approximately 30 microns,

while it is about 0.3 microns for CIS. Higher energy photons have a greater probability of

interacting with a bound electron and being absorbed than lower energy photons, even though

both have more than the band gap energy. For instance, in CIS (Eg = 1.0 eV), the absorption

length of high-energy photons (>2.5 eV) is less than 0. 1 microns, but a beam of low-energy

photons (1.1 eV) might require 1 micron to be equally satisfied [8].

Diffusion Length

Free electrons on the p-side move around randomly before recombining. During this short

period of time, called the lifetime, they have a finite chance of encountering the electric field and









being sent to the n-type side of the device. This separation is a result of diffusion and the

average distance that minority carriers can move toward the built-in field before they return to

their fixed states is called the diffusion length. Materials with longer diffusion lengths are able

to produce more current. In solar cells, they can vary from less than a micron to over 100

microns in some single-crystal semiconductors. Large diffusion lengths are a necessity for

indirect band gap materials like crystalline silicon, but direct band gap materials absorb enough

light within their depletion region that their performance is not critical to diffusion length. If the

ratio of diffusion length to absorption length is greater than one, most carriers will be separated.

Crystal quality is the most important factor in determining a material's diffusion length because

diffusion is constrained by the propensity of free carriers to recombine.

CulnSe2-Based Solar Cells

CulnSe2-based solar cells are the most promising of the thin film solar cells and are the

basis for the investigations within this work. Cu-chalcopyrites, of the general composition

Cu(Inl-xGax)(SSexlv)2, Offer a wide range of band gaps from 1.0 eV for CulnSe2 to 2.4 eV for

CuGaS2. Recently, new world record total-area efficiencies of 15.0, 19.5, and 10.2% for

CdS/CIGS solar cells have been achieved for x=0 (CIS), x~0.28 (CIGS) and x=1 (CGS),

respectively [28]. Table 2-1 shows these record efficiencies alongside their theoretical output.

Material Properties

The direct optical band gap of single crystal CulnSe2 has a value near 1 eV at room

temperature [29]. The absorption coefficient of 105 cm-l for light greater than the band gap

means the thickness of the absorber can be reduced theoretically to less than 1 micron [30]. CIS-

based materials belong to the group of I-III-VI2 Semiconducting compounds. Their lattice

elements are tetrahedrally coordinated similar to diamond-like semiconductors [31]. The

chalcopyrite cell consists of two zinc blende cells with Cu and In occupying the same lattice sites









in the upper and lower cell, alternately as seen in Figure 2-3 [32]. In the chalcopyrite structure,

each I (Cu) or III (In, Ga) atom has four bonds to the VI atom (Se). In turn, each Se atom has

two bonds to the Cu and two to the In (or Ga). Because the strengths of the I-VI and III-VI

bonds are different, the ratio of the lattice constants c/a is not exactly 2; it varies from 2.01 for

CIS [33] to 1.96 in CGS [34].

Doping in chalcopyrites is mainly controlled by compositional variation and thereby

induced defects, which lead to strong compensation [35]. This makes the systematic study of

their electronic properties more difficult than in extrinsically dopable materials. The band gap

can be engineered, which offers a greater possibility of Einding the optimum photovoltaic

material with respect to cost, efficiency, and stability.

Four different phases have been found to be relevant: a-phase (CulnSe2), P-phase

(Culn3Ses), 6-phase (high temperature sphalerite phase), and Cu2-ySe [36]. The existence range

of single-phase CulnSe2 is Very small and does not include the stoichiometric composition of

25% Cu. The Cu content of absorbers for efficient thin-fi1m solar cells typically varies between

22 and 24% (at.) Cu. At the growth temperature of 500-5500C, this region lies within the single-

phase region of the a-phase.

Some interesting features of CIS-based solar cells include low open-circuit voltage to band

gap ratio compared to Si and III-V devices, insensitivity of the conversion efficiency to the

[Cu]/[III] ratio over a wide range, exceptional tolerance to grain boundaries, and loss of

performance for CIGS alloys with x greater than 0.3 [37]. Single crystal CIGS efficiencies also

lag behind those of polycrystalline CIGS. Optimal sodium incorporation is beneficial to device

performance, and excess sodium is detrimental. Na depth profiles typically exhibit some










qualitative features: enrichment at the CIS surface and a relatively lower concentration in the

bulk of the CIS with concentration increasing toward a maximum at the CIS/Mo interface [38].

Defects

For Chalcopyrite compounds, intrinsic defects are introduced to maintain the crystal

structure for non-stoichiometric composition. There are twelve intrinsic point defects that can be

formed in the ABX2 chalcopyrite lattice: three vacancies (VA, VB, Vx), three interstitials (Ai, Bi,

Xi), and six antisite defects. In addition to the antisite defects that can be formed by an exchange

of anions (X) and cations (A,B) as in binary compounds (Ax, Bx, XA, XB), One can also form

antisite defects on the same sublattice by an exchange of the cations (AB, BA) [22]. The Cu

vacancy, Vco, is considered to be the dominant acceptor in Cu-poor p-type material, while the Se

vacancy, Vse, is considered to be the dominant donor in n-type material [36]. Most of the off-

stoichiometry defects must be electronically inactive to allow for large deviations from

stoichiometry without the deterioration of the electronic quality of the film [39]. A critical issue

in regard to CIS-based solar cells is the control of these point defects, which are responsible for

recombination in the space charge region of the devices [40].

Gallium Addition

When indium is replaced by gallium, the band gap increases. This is an effect of the

smaller size of the Ga atom, when compared to In, and the subsequent formation energies

involved. With the addition of gallium, some of the material properties also change. These

include structural properties like lattice constants, film morphology, and adhesion, and chemical

changes such as defect levels, affinities, and carrier concentrations. The band gap of Cunlnt

xSe2GaxSe2 can be tuned from 1.0 to 1.7 eV by adjusting the Ga content (x) from 0 to 1.

Although the Ga/(Ga+In) ratio of roughly 0.65 would provide the optimal band gap by

theory, the actual Ga content in the current record device is 28%, which corresponds to a band









gap of approximately 1.2 eV [41]. This disparity implies that there are factors effecting

conversion efficiency other than band gap. The increase in efficiency in the range of 0 to 28%

Ga is due mainly to the increase of the band gap, and the potential on the grain boundary in this

gallium content range stays strong. However, at higher Ga content, the absence of the potential

on the grain boundary seems to be significant in comparison to the effect of the band-gap

widening [42].

CuGaSe2

Copper Gallium Diselenide, as a member of the I-III-VI2 COmpound semiconductors, has a

direct energy band gap of 1.7 eV [43], a very high absorption coefficient a = 3 x 104 Cm-1 at 1.7

eV [44], and an easily controllable electrical resistivity in a wide range of 10-1 to 105 ohm-cm

[45]. The band gap of CGS decreases with increasing Cu/Ga ratio [46]. The temperature

coefficient is lower for wide gap materials, which means that the efficiency loss at operating

temperature is less than for smaller band gap semiconductors [47]. To date, device efficiency of

greater than 9.5% is reported using less than 2 microns of CGS absorber film by NREL [48].

CuGaSe2 has been investigated for more than 25 years, but in comparison to the rapid

progress made for CulnSe2 and Cu(In,Ga)Se2, the efficiency of CGS-based solar cells is still

relatively low. CGS did not overcome a limitation of 6.2% until fairly recently when efficiencies

of up to 9.3% for thin films and 9.7% for single crystal devices were achieved [49]. These

reported values lie well below the theoretical limit for CGS of 9 = 26% [50]. The most critical

drawback of CGS solar cells is the low open-circuit voltage (Voc) compared to band gap. In the

case of CIS or CIGS, the Voc follows a relationship with the band gap according to Voc ~ E,/q-

500 mV, where q is the elementary charge. A Voc of 1.2 V should therefore be possible for

CGS-based devices, but Cu-rich absorbers have been limited to a Voc of 750 mV [51], while

high efficiency Ga-rich absorbers have peaked at 900 mV [48]. The improved device










performance of Ga-rich films results from decreased defect densities in the bulk and a decrease

of tunneling-enhanced recombination [52]. Cell performance may be fundamentally constrained

since it seems that the Fermi level might be limited to values less than 800 meV above the

valence band edge, which would always make CGS p-type [53].

Defects

Wide gap chalcopyrites, such as CGS seem to have extensive material and growth-related

problems that limit the device performance. The tetragonal structure is capable of sustaining a

large concentration of vacancy and antisite defects in CGS [54]. Devices based on slightly Cu-

poor CuGaSe2 absorbers have shown better performances than stoichiometric ones. In Cu-poor

material, the deviation from stoichiometry is not facilitated by the formation of a second phase,

as in the Cu-rich case, but the material develops a high density of defects. This leads to a high

degree of compensation, which in turn causes lateral potential fluctuations in the concentrations

of the charged defects [55]. This density of defects is higher at the increased Ga contents

because the lattice mismatch in that composition range is anticipated to be larger. Better lattice

matching between the surface and the bulk leads to better performance [56]. Optimal growth

conditions of CGS thin films are realized by a trade-off between growing Ga-rich films, reducing

the density of states in the band gap, and growing stoichiometric CGS to achieve optimal grain

size and crystallinity [55].

Recombination

The actual path of recombination, which limits open circuit voltage, is important; it can

take place at the interface or within the space charge region, or it can be tunneling supported

[57]. Increasing the Ga content in CIGS intensifies the contribution of tunneling, which is

observed as a larger characteristic tunneling energy, Eoo. This facilitates recombination and in

turn increases the recombination loss. For CGS, Rau et al. obtained significant values offoo and









found that the devices with the highest Voc are those with the lowest charge density and the

lowest tunneling currents [58]. At room temperature, the tunneling contribution to

recombination is insignificant for low gallium content CIGS [59]. Hence, there is no

fundamental difference between CIS and CGS with regard to recombination path, but

recombination losses in CGS are enhanced due to a higher contribution of tunneling [57].

Cu-rich CGS devices are controlled by high Eoo values due to tunneling enhanced interface

recombination. The efficiency gain achieved by the use of Ga-rich absorbers is mainly explained

by a reduced doping level and the decreased tunneling rate. All the beneficial device

modifications like air-annealing or the increase of CdS deposition temperature lead to a further

decrease of Eoo. The increased Cd diffusion into the absorber material during CBD explains the

reduction of tunneling [51i].

Type inversion

The efficiency of a CulnSe2 device is aided by the fact that its surfaces can be inverted to

become n-type even though the bulk of the sample is p-type. This is done via deposition of CdS,

which leads to band bending. The amount of band bending equals the shift in Fermi level with

respect to the valence band maximum from the p-type to the n-type region. A large amount of

band bending results in a Fermi level close to the conduction band minimum, which is needed to

get n-type conditions. The beneficial effects of this weakly n-type surface layer include the

reduction of the recombination rate and the enhancement of the carrier collection efficiency by

shifting the electrical junction away from the interface between CdS and the absorber [60].

The photovoltaic performance of CIGS devices deteriorates for x>0.3. Due to the lack of

type inversion for CIGS with x>0.3, the pn junction moves to the 'real' CdS/CIGS interface

causing higher recombination losses [61]. CGS has been reported p-type for all compositions,

for thin films as well as single crystals; this is true for all deviations from stoichiometry and









molecularity. The doping-pinning rule predicts both n- and p-type behavior for CIS and only p-

type behavior for CGS. The maximum Fermi level positions are basically the same for both

these materials [57]. Thus, it depends on the relative position of the band edges with respect to

those maximum Fermi level positions whether the material can be p-type or n-type or both. As

one applies to CGS the same process that converts CIS to n-type, the Fermi level does not rise

towards the conduction band minimum [62]. The larger band gap of CGS leads to a higher

difference between the Fermi level and the upper edge of the valence band at the interface [63].

In CGS, type inversion cannot be achieved under normal conditions, but it can be reached

due to doping via non-equilibrium effects [64]. Schon et al. have demonstrated that n-CGS can

be obtained using ion-implantation and Zn, Ge co-doping [65]. As-grown p-type CGS single

crystals are first doped by Ge-implantation and then heated in vacuum. Finally, annealing of the

implanted samples in Zn atmosphere results in n-type conduction of CGS [66].

Sulfide-based Chalcopyrites

CulnS2 has an optical band gap (E,) of approximately 1.5 eV [67], which is an excellent

match for the solar spectrum. Its high absorption coefficient, a = 104 Cm-1 (at h = 500 nm) is also

very good for solar cell devices [68]. Masse et al. calculated a maximum solar energy

conversion efficiency of 28% for a CulnS2 homojunction [69], but to date, the highest reported

efficiency for a solar cell is about 12% [70]. Although CulnS2 Solar cells are theoretically

expected to possess higher efficiencies than CIGS, sulfides have reached only about 60% of the

performance of selenides so far. Selenide-based chalcopyrite solar cells that are slightly copper

poor have shown the best efficiencies, but sulfide absorbers must be prepared Cu-rich to produce

a working device [57]. CuGaS2 has a band gap E, = 2.5 eV [69], but it has not been used in the

PV industry to date [71].










Deposition Processes

The progress of the solar industry depends not only on conversion efficiency, but also on

the development of techniques, which must be conducive to producing large-area devices at a

low cost. The absorber deposition method generally has a significant impact on the resulting

fi1m properties as well as on production cost. Common thin film deposition methods for CIS-

based solar cells are co-evaporation from elemental sources, selenization of metallic precursor

layers, evaporation from compound sources, chemical vapor deposition, closed-space vapor

transport, and low-temperature liquid phase methods like electrodeposition, spray pyrolysis, and

particle deposition techniques.

Siemens Solar Industries (SSI) was the first company in the world to produce CIS modules

using the selenization process. Low temperature (2000C) CIS precursor deposition is followed

by high temperature selenization [72]. At 5000C, a complete recrystallization of the precursor

film occurs [73]. The material quality of the absorber is determined by the structural features of

the precursor and the experimental conditions during selenization. Some disadvantages of this

process are that it involves complicated intermediate phases, interdiffusion, and reaction, which

can affect the controllability of the fi1m quality.

Usually high-quality I-III-VI2 absorber thin films are prepared on a laboratory scale by

physical vapor deposition (PVD) or molecular beam epitaxy (1VBE). These techniques require

high temperatures for the source metal evaporation [74]. Molecular beam epitaxy is used to

grow CGS epilayers [45] while two and three-stage co-evaporation are the benchmark methods

for depositing polycrystalline CIGS. Although these growth methods are relatively easy to

implement on a small R&D scale, scale-up to a commercial level proves to be challenging.

High-quality CIGS deposition requires a high substrate temperature (>5000C) which limits the

selection of substrate materials and decreases throughput due to heat-up and cool down periods.









Co-deposition of the elements requires precise control of the flux of each element and an

overpressure of chalcogens (Se or S) during deposition, which results in low material utilization

and high equipment maintenance costs. An alternative low temperature route to the formation

of CIS is the rapid thermal processing of stacked metal/Se layers [75].

Most PVD deposition techniques are quite wasteful of materials, but printing or

electrodeposition techniques are quite efficient in depositing materials. The co-electrodeposition

technique [76], where Cu-In-Ga-Se species are present in the same chemical bath, is a simple

process to prepare low-cost thin films. It is crucial to control the deposition parameters like pH,

chemical bath composition, deposition time, deposition temperature, and the applied potential,

owing to their influence on the film properties and quality.

Printing, spraying, or coating of inks involves the deposition of particulate precursor

materials onto substrates at low temperatures and the subsequent sintering under chalcogen

overpressure. The reaction kinetics of nanoparticle-derived CIGS precursor films, typically 0.5-

2.0 microns, is much different than those prepared by evaporation [77]. ISET's non-vacuum

process utilizes nanoparticles of mixed oxides of Cu, In, and Ga with a Eixed Cu/(In+Ga) ratio

that are synthesized into precursor inks [78].

Multij unctions

The maj or reason for losses in simple photovoltaic devices is the inefficient use of the solar

spectrum by cells that have only one built-in electric Hield. Some proportion of the sunlight is

not used because certain photons do not have enough energy to be absorbed and to free electrons.

For those photons that do have enough energy, there is no distinction between them; they are all

treated as if they have just enough energy to free an electron. The band gap at which these

spectrum-driven losses are smallest in single-junction cells is about 1.4 eV [8]. A range of band

gap values at which losses are still manageable extends from about 1.0 eV to 1.8 eV.









Theoretical Multijunctions

The single junction thermodynamic limit for solar cell conversion efficiency was

determined to be 32% by Shockley and Queisser [79], but the practical lab limit of

polycrystalline thin film solar cells is about 20% under 1-sun illumination. The major

assumption in the calculation of the theoretical limit is that electrons and holes created by the

absorption of photons with energies above the band gap lose their excess energy by phonon

emission.

One way to achieve efficiencies above the Shockley-Queisser limit is to use a series of

semiconductor pn junctions arranged in tandem configuration. The efficiency of solar cells can

be significantly increased by stacking several cells with different band gaps such that the gap

energy decreases from top to bottom. This multijunction cell uses more than one electric field to

separate electrons and holes. Light is incident on the top cell which has a high band gap.

Photons with energies greater than the band gap of the top cell are absorbed while those with

lower energies pass through to the next semiconductor where they are absorbed if their energy is

greater than the band gap of that cell. Thus, the solar spectrum is split so that photons are used

more efficiently; losses due to the mismatch between the energies of the photons and the cell's

band gap are reduced. Two cells in series connection have a maximum theoretical efficiency of

41.9% and with a larger number of cells, 50% efficiency can be exceeded [8]. The

thermodynamic limit for solar energy conversion is significantly higher still, 66% at 1-sun and

86% at full solar concentration (46,200 suns), for an infinite tandem [21]. The grand challenge is

to push solar cell efficiency towards its theoretical limit while maintaining low cost.

The efficiency benefit of a tandem solar cell to that of a single junction has been known for

quite some time, but it has only been practically observed in expensive crystalline III-V

materials. Multijunction cells under concentrated light have just recently exceeded 40%









efficiency (Spectrolab). Traditionally, they have been used to power satellites and other

spacecraft. The use of multijunction cells to generate clean energy for terrestrial applications has

been sought because, when combined with high concentration, multijunction cell modules have

the potential of producing the lowest $/watt amongst solar cell technologies [80].

Coutts et al. identified optimum band gaps for two-junction tandem thin film solar cells. A

current-matched, 28% efficient tandem is possible with a top cell absorber of 1.72 eV and a

bottom cell absorber of 1.14 eV [81]. These band gaps are ideally matched to the CIS-CGS

material system. Low Ga content CIGS has the band gap and performance to be the low gap

cell. The wide band gap top cell material of the tandem is critical; it is estimated that

approximately two-thirds of the tandem cell efficiency originates there [82]. A high band gap,

transparent top cell with efficiency greater than 17% is needed to form a tandem with an

efficiency of at least 25% [83].

Tandem Structure

In typical CIGS thin film solar cells, metallic Mo back contacts are used, which makes it

impossible for light to pass through this layer. However, a semitransparent solar cell is required

for the top cell of tandem devices. Nakada et al. report that the cell performances of CIGS

devices incorporating tin oxide (SnO2) and indium tin oxide (ITO) back contacts are similar to

those using molybdenum [84]. The superstrate configuration, where the glass substrate is not

only used as a supporting structure, but also as a window for illumination, has an advantage of

easy and reliable encapsulation. Since the diffusion of sodium from the soda-lime glass is

strongly inhibited by the front contact in this design, Na-doping is necessary. The addition of Na

from co-evaporated Na2Se has been reported to more than double the efficiency in superstrate

cells [85].









Monolithic vs Mechanical

There are pros and cons to monolithic or mechanical tandems. In a two terminal device, the

stacked cells are connected at a common boundary where the bottom contact of one is the top

contact of another. Current flows continuously between the cells under illumination. Using this

monolithic approach, only one thick transparent conducting oxide (TCO), one grid, and one anti-

reflective coating (ARC) would be needed. However, current-matching and thermal stability

issues arise. The lowest current will limit the entire device so band gaps must be chosen that

split the spectrum equally: half of the sunlight absorbed on the top and half transmitted to the

bottom cell and absorbed there.

Several difficult technical issues need to be addressed in order for high efficiency

monolithic tandem cells to be developed. The bottom, first deposited, cell must not be destroyed

by the processing conditions of the top cell. High-efficiency CIGS devices are vulnerable to

temperatures greater than 2000C where diffusion destroys the pn junction. Therefore, a

successful tandem device fabrication procedure will require a bottom cell that is not affected by

the processing conditions of the top cell or a top cell that can be grown at a much lower

processing temperature [86]. Some clever tandem structures are being investigated because of

the need to grow thin films at temperatures greater than 5000C to obtain high-quality absorbers

[87]. Another critical issue for a monolithically interconnected tandem cell is providing a

transparent interconnect between the top and bottom cells [86].

The mechanical stack may appear much simpler, but there are other issues involved. In a

four-terminal device, each cell has a top and bottom contact connected to an external circuit so

their output is taken off separately. Performance of each cell is independent so the spectrum

doesn't need to be split between them. More materials (ARCs, TCOs, and glass) are needed for

the overall structure, which increases the cost [82].












2100








7400









0.3 0.5


AMO


1 1.5 2 2.5
wavelength (ptm)


Figure 2-1.









Solar Rays


Spectral irradiance versus wavelength under AMO and AM1.5 conditions.


AC loads


Battery Storage


Figure 2-2.


Photovoltaic system.









Table 2-1. Efficiencies of copper chalcopyrites.
Material Band gap (eV) Theor. rl (%)
CulnSe2 1.0 [29] 25 [57]
Culn0.72~Ga0.2Se2 1.1 [28] 27.5 [57]
CulnS2 1.5 [67] 28.5 [71]
CuGaSe2 1.7 [43] 26 [49]


Achieved rl (%)
15.0 [28]
19.5 [28]
12.2 [70]
10.2 [28]


Cu


a


Figure 2-3.


Chalcopyrite structure of CulnSe2.









0.5-1.5 Cpm ITO 3.7 eV
0.03-0.05 pm CdS 2.4 eV

1.0-2.0 CLm CGS 1.7 eV

01.01 11m tunnel ing junctio~n 3.3 eVc~
0.01 mITO 3.7 e~V
0.03-0.05 um CdS 2.4 eV

]1.5-2.0 Cpm CIGS 1.1 eV

0.5-1.0 I-m Mo


Glass/SS~/Po,~llmer


Figure 2-4.


CIGS/CGS monolithic tandem device structure.









CHAPTER 3
ABSORBER GROWTH AND DEVICE FABRICATION

Growth Calibration

Fabricating and testing a working solar cell requires multiple steps including equipment

calibration, the deposition of multiple layers, and film and device characterization. When the

source materials are depleted, the reactor must be shut down and brought to atmospheric

conditions. Copper (Cu) is annealed in a hydrogen furnace to remove the oxide film present on

the Cu pellets received from the supplier. The oxides have a higher melting point than copper so

their presence in the source may cause sputtering and thus non-uniformity in the flux

distribution. The indium (In) and gallium (Ga) source materials are introduced into the vacuum

system in the same condition that they are received from the manufacturer. To add Cu or In to

the reactor, the respective source shutter must be disconnected while an optical port must be

removed to add Ga.

After the source material is replenished, the Sentinel III rate controller, using Electron

Impact Emission Spectroscopy (EIES) sensors, is calibrated by Quartz Crystal Monitors (QCM).

Both the Cu and In sources are equipped with an EIES optical sensor located adj acent to the

rotating platen at substrate level and a QCM sensor located directly above the source center at a

fixed distance above the substrates. EIES is a system of evaporant excitation by electrons that

uses the optical intensity of the subsequent de-excitation as a means of process control. These

sensors are used for online measurement while the QCMs provide an absolute value of the flux

for the calibration of the optical sensors and cannot be used online because they are located

directly above the substrates. The Ga source is equipped with a single sensor, which is a QCM

that is in an identical position to the EIES sensors used for the Cu and In sources. QCM rate

control of gallium was inadequate so a temperature control scheme was implemented. Once the










Sentinel's parameters are adjusted, fi1ms can be deposited based upon the deposition rate sensed

by the EIES sensors.

Before a growth run series is initiated, the Cu and/or In deposition rates must be calibrated

with a specific Ga source temperature. Assumed Cu-rich, Ga-rich, and near-stoichiometric thin

fi1ms, typically 0.25-0.5 microns, are grown and the composition is measured through ICP

analysis. It is assumed that the Ga deposition rate is Eixed throughout a run since the temperature

is manually controlled to a specific value. Cu and/or In rates can be adjusted based on the ICP

results of the previous run. For example, an average Cu rate is determined from the previous run

and is divided by the Cu/Ga ratio verified from ICP to give the Cu rate needed to produce

stoichiometric CGS. The Cu rate for the current run can be adjusted appropriately to give the

desired overall composition. ICP feedback results must be maintained throughout a growth

series because reactor conditions may change. This procedure is only as good as the

repeatability of reactor conditions between successive runs.

Standard Growth Procedure

System startup is a lengthy process during which stringent guidelines must be followed to

ensure proper operation of the reactor. A cryotrap is filled with liquid nitrogen so that the reactor

chamber reaches a certain crossover pressure to safely switch the pumping to the diffusion pump,

which is necessary to get to high vacuum. As the trap fills, samples can be loaded into and

unloaded out of the reactor through the load-lock. After switching the system into high vacuum,

the ionization gauges are degassed for a minute to remove any deposits from them. The platen is

then started to its desired rotating speed, which is typically 12 rotations per minute. Platen

rotation must be started before the substrate heater is engaged so as to not warp it. The PMEE

reactor Supervisory Control Panel is then opened on the attached system PC. Film pre-

deposition parameters are set for the metals such as soak power, rise time, and soak time.









Heating layers are also set to determine the order in which the heaters are turned on and how

much power should be supplied.

When the appropriate parameters are entered into the LABVIEW program, heating can

begin. The pyrolytic boron nitride (PBN) substrate heater is started first and brought up to the

growth temperature. The inputted temperature is actually the temperature in the gap between the

heater and the platen as a thermocouple cannot be directly placed on the rotating platen. As the

PBN heater' s power is increased, the Cu tip is turned on and the power is gradually increased

manually. Before turning on the selenium source heaters, the cryoshroud that surrounds the

metal sources is filled with liquid nitrogen. The cryoshroud helps keep the excess Se in its

designated reactor zone. First the cracker is heated and then the crucible. As the Se crucible

approaches its final temperature set point, the metals primary heaters are switched on. The

practice for source preparation in this IVBE system before initiating deposition is to hold the

sources at a particular soak power for a set period of time. The metal sources first go through a

period of rising temperature and then a soaking period so that the solid metal sources become

melts. Since the Ga source is manually temperature controlled, control is taken over manually

after an initial rise time and the power is adjusted to reach the Ga temperature set point before

deposition. The Ga shutter needs to be manually opened when the other metals shutters open

automatically after their soak period. If a certain metal source is not being deposited in the film,

its heaters are not turned on and its shutter remains closed throughout the run. Selenium is

depositing on the substrates as they pass through the chalcogen zone as the Se crucible

temperature approaches its final value prior to the start of metals deposition. Startup time

leading up to deposition is approximately two hours.










The deposition time begins once the metal shutters are opened and metal beam fluxes are

impinging upon the rotating substrates. Cu and In rates are controlled by adjusting the local set

point and corresponding offset. Desired element deposition rates are set in advance and layer

thickness is controlled by adjusting open shutter time. The average deposition rate over a certain

period is calculated and these parameters can be adjusted appropriately to achieve the desired

composition. The Se source is kept at a constant temperature during evaporation. The power of

the Ga source is manually adjusted to maintain temperature control since QCM rate control was

ineffective. The reactor conditions are closely monitored with the cryoshroud periodically being

filled throughout the run. Different growth strategies can be administered by closing certain

metal shutters during the growth run; selenium is supplied in excess while the Cu and In rates

can be adjusted. Dopant NaF can be added at a constant rate for a set period of time. When the

appropriate thickness is reached, the metals' shutters are closed and the heaters are shut down.

Se can be deposited in an annealing procedure under the designated growth temperature for a set

period of time as the metal heaters cool down. Otherwise, power to the Se crucible and the

substrate heater are decreased at the end of metals deposition.

When all heaters are cooled to below 2000C, which typically takes at least an hour and a

half, the system is taken out ofHi-Vac. Once the system cools completely, the grown films can

be removed from the reactor by way of the load-lock. Films to be used for device fabrication are

immediately vacuum sealed to isolate them from the atmospheric conditions prior to buffer

deposition. The films are also re-sealed after the buffer layer is added and prior to ZnO

sputtering. One sample can be cut up and used for absorber characterization. Sometimes, the

samples were exposed to a normal room temperature air ambient for over a month between









deposition and analysis. This was also the case for absorbers used for device fabrication prior to

the purchase of a vacuum sealing system.

Growth Schemes

High quality absorber layers that are well-controlled are essential to the fabrication of high

efficiency solar cells. Thermal evaporation processes have been mainly designed on a basis of

experience and intuition to grow polycrystalline thin film layers [88]. Therefore a fundamental

understanding of CIGS film deposition is necessary to design the best absorbers.

Single Stage

In the simplest single-step process, all rates as well as the substrate temperature are kept

constant during the whole process. A one-stage process typically produces low-quality material

when compared to the bilayer or three-stage processes. In CIGS growth, three-stage co-

evaporation leads to an absorber with a graded band gap, while single step co-deposition results

in a uniform band gap [89].

Bilayer

The first growth strategy used to synthesize highly-efficient CulnSe2 filmS was developed

at Boeing by Mickelesen and Chen. In the Boeing bilayer process, a two-phase film containing

CIS and Cu2-xSe is first deposited at low temperature and then reacted with a Cu-deficient flux of

co-evaporated Cu, In, and Se vapors at a higher temperature. The precursor deposition and re-

growth chemistry are shown in the following equations [90]:

Cu(v) + In(v )+ Se(v) -CulnSe2(S) + CU2-xSe(s) (3-1)

CulnSe2(S) + CU2-xSe(s) + Cu(v) + In(v )+ Se(v) -CulnSe2(S) (3 -2)

Copper-rich material tends to form larger grains. This is typically true above

approximately 5250C because of a liquid phase assisted re-growth process due to the melting of

Cu2-xSe in the presence of excess Se. Thus, by first depositing a layer containing excess copper,









larger CIS grains are formed. In-rich layers generally have smaller grains, but when grown on

top of Cu-rich layers they are inclined to conform to the same growth pattern [8].

Three-Stage

In the mid 1990s, NREL developed the three-stage process to grow high-quality CIGS

fi1ms [91]. Indium, gallium, and selenium are evaporated at 2600C to form a (In,Ga)2Se3

precursor. The temperature is then ramped up to 5500C within a Se flux. At this point, sufficient

Cu is co-deposited with Se to make the fi1m Cu-rich. Additional In, Ga, and Se are added to

bring the overall composition back to Cu-poor. The amount of In and Ga deposited in the third

stage is usually 10% of the total in the first and third stages combined. The film is finally cooled

down within a flux of Se at about 3500C. The three-stage process is based on the following

reaction chemistry (precursor deposition, re-growth, and titration) [90]:

In(v)>+ Se(v) In2Se3(S) (3-3)

In2Se3(S) + Cu(v )+ Se(v) -CulnSe2(S) + CU1-xSe(1) (3-4)

CulnSe2(S) + CU1-xSe(1) + In(v )+ Se(v) -CulnSe2(S) (3-5)

The intermediate Cu-rich growth stage has been shown to be beneficial to the morphology

and electronic quality of CIGS layers. The Cu2-xSe secondary phase has a higher emissivity in

the IR range than Cu-deficient CIGS, and the increased emission of heat radiation leads to a

lowering of the substrate temperature. Cu2-xSe segregations begin to appear once the film

reaches stoichiometric composition, i.e. [Cu]/[III] =1.00. This is reflected in a drop of the

substrate temperature, which is recorded by the thermocouple on the sample rear side. When the

substrate temperature is ramped up to 5500C after the completion of the first stage, the PID

temperature controller is cut off and a constant heating power is supplied [92]. The third stage is









terminated when the temperature reading reaches the value recorded before the film became

stoichiometric during the second stage.

Growth Strategies in the PMEE reactor

Three different growth recipes were investigated for the deposition of CuGaSe2 absorber

layers. Another growth strategy was employed in the reactor during the earliest investigations of

CGS growth, an initial Cu-rich layer followed by a Ga-rich layer similar to the Boeing bilayer

process, but it never resulted in quality devices. We refer to each strategy as follows:

Constant Copper Flux Process (Figure 3-1A)

Modified Three-Stage Process (Figure 3-1B)

Emulated Three-Stage Process (Figure 3-1C)

The Constant Cu Flux Process is illustrated in Figure 3-la. The selenium crucible is

maintained at a constant temperature, typically 2650C, so that selenium is provided in excess for

all film growth in our PMEE reactor. Since temperature control is used for gallium deposition,

we maintained a specific gallium temperature for each growth run. Thus, we can change the

overall composition of the absorber by manipulating the copper flux. Calibration runs were

performed to determine the relationship between the average copper deposition rate and the

Cu/Ga ratio. This strategy simply keeps the same Cu flux over the entire growth run so that the

absorber maintains either Cu-richness or Ga-richness throughout the deposition of the film.

Figure 3-1b shows the Modified Three-Stage Process. This growth recipe starts by

depositing GaSe for a set period of time, followed by a Cu-rich layer, and ending with a Ga-rich

layer. Since the Ga temperature is maintained for the complete growth run, the Cu flux must be

adjusted to achieve either a Cu-rich layer or a Ga-rich layer. The overall composition and peak









Cu-richness can be adjusted by varying the duration of growth and the Cu flux employed for

each step.

The Emulated Three-Stage Process is illustrated in Figure 3-10. In contrast to NREL's

approach, our Emulated Three-Stage Process does not use end-point detection, but is based on

the composition results of previous runs. The thermocouple for substrate temperature

measurement is placed in the gap between the heater and the platen for temperature measurement

since it could not be placed in contact with the substrate because of the rotating nature of the

platen. An additional possibility to control the composition is to monitor the emissivity of the

films, but problems with pyrometry were presented by selenium condensation on the optical

ports. In this recipe, we deposit GaSe for a certain amount of time and then deposit CuSe until

we reach the desired thickness. The greatest copper-richness is reached at this point, and then

GaSe is deposited until the overall composition becomes Ga rich. The gallium temperature

remains constant throughout the first and third stages, while the same Cu rate is maintained

during the second stage.

Absorber Characterization

Characterization of the absorber film is integral to the production of high quality devices.

Many research groups have implemented in-situ techniques to observe the growing film, but this

is not possible inside our reactor. Typically, one absorber is set aside strictly for characterization

purposes. The techniques described below are used extensively within this research.

ICP

Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) is most commonly

used for bulk analysis of liquid samples or solids dissolved in liquids [93]. The ICP operates on

the principle of atomic emission by atoms ionized in an argon plasma. Photons are emitted as

electrons return to the ground state of the ionized elements, which allows for the quantitative









identification of the species that are present. The strengths of ICP-OES include its speed, low

detection limits, and relatively small interference effects, but it is a destructive technique that

provides only elemental composition. Calibration curves must be made using a series of

standards to relate emission intensities to the concentration of each element of interest. The

Perkin-Elmer Plasma 3200 ICP system used in this study is located in the Particle Engineering

Research Center, University of Florida. This system is capable of analyzing materials with a

detection limit range of less than 1 part per million.

Known concentrations of Cu-In-Ga-Se dissolved in solution (0, 1, 5, and 10 ppm) are used

to create a calibration curve for ICP characterization. A small piece of the characterization

absorber, typically 2 cm x 1 cm, is dissolved in a 10 mL nitric acid solution. After the film

reacts for a few hours, the solution is diluted with 50 mL of deionized water. The overall

composition determined from this small sample may not be representative of the entire film if it

does not have uniform morphology.

SEM

Scanning Electron Microscopy (SEM) can be used to determine the grain size and shape of

absorber films [94]. The SEM is commonly used for image analysis by focusing a source

electron beam into a fine probe and rastering over the surface of the sample. Secondary electron

and backscattered images are obtained to provide the surface topographical information. SEM,

using the SEM JEOL JSM 6400, characterization measurements were done at the Major

Analytical Instrumentation Center, Department of Materials Science and Engineering, University

of Florida.

XRD

X-ray diffraction (XRD) is a powerful technique used to uniquely identify the crystalline

phases present in materials and to measure the structural properties of these phases [95].










Polycrystalline thin films can have a distribution of orientations, which influences the thin-film

properties. When sizes of crystal grains are less than about 100 nm, x-ray diffraction lines will

become broadened. Hence, grain size can be estimated by measuring the broadening of a

particular peak. XRD is noncontact and nondestructive, which makes it ideal for in-situ studies.

Characterization measurements, using the XRD Philips APD 3720, were done at the Maj or

Analytical Instrumentation Center, Department of Materials Science and Engineering, University

of Florida.

The most important use of thin-film XRD is phase identification. XRD provides positive

phase identification by comparing the measured d-spacings in the diffraction pattern with known

standards in the JCPDS Powder Diffraction File. Some thin films have a preferred orientation,

but the JCPDS file contains measurements for films with random orientations so there can be

some disagreement between the measured values and the standard. For films possessing several

phases, the proportion of each phase can be determined from the integrated intensities in the

diffraction pattern.

Device Fabrication

The most commonly used structure for CIS-based solar cells is the substrate configuration;

the absorber layer is evaporated on Mo-coated glass, and on top of this is a thin CdS buffer layer

and a transparent ZnO front contact. The universally accepted device design for fabricating high

efficiency, thin-film CIGS solar cells: MgF2/ZnO/CdS/CIGS/Mo/SLG is shown in Figure 3-2

[96].

Substrate and Back Contact

Soda-lime glass (SLG) is commonly used as the substrate for high-efficiency solar cells,

but it deforms at the temperatures used for highest device efficiencies, 550-6000C. Soda-lime

glass contains significant amounts (15.6 wt %) of sodium in the form ofNa20 [97], and it is









typically coated with molybdenum, which serves as the back contact. When the substrate

temperature approaches the softening point of the glass, Na ions diffuse from the glass through

the Mo back contact into the growing CIGS film. The extent of Na diffusion is related to the Mo

sputtering pressure. At low pressure, the amount of Na out-diffused from the SLG is low, while

at the highest pressure, the amount of Na out-diffused exceeds the optimal value required for

high-quality devices due the formation of microvoids and microcracks [97].

For good electronic device properties, the formation of an ohmic contact for the maj ority

carriers (holes) from the p-type CIGS and a low recombination rate for the minority carriers

(electrons) at the CIGS/back contact interface is essential. The back contact should be inert to

the highly corrosive environment during deposition and it must impede the diffusion of

impurities from the substrate into the absorber. Finally, a high optical reflectance is necessary to

minimize optical losses. Molybdenum, the historical back contact material for CIGS solar cells,

complies well with most requirements; it is inert during deposition, allows for the growth of

large grains, and forms an ohmic contact via an intermediate MoSe2 layer [98].

CIGS can also be deposited onto various substrates other than glass, even flexible ones.

Stainless Steel (SS) can be heated up over 5000C which is necessary to achieve high-quality

absorbers, but Na-doping is needed since SS doesn't contain sodium like soda-lime glass.

Unlike metal foils, polymer substrates are electrically insulating, which simplifies

monolithically-integrated module fabrication. However, polymer substrates have a limited

maximum operating temperature and can cause adhesion problems between the CIGS film and

the Mo back contact [99]. Stainless steel substrates have generated CIGS films with 17%

efficiency [100] while applying a low temperature, 4500C, CIGS deposition process and a









reliable method for controlled Na incorporation on polyimide substrates has yielded cells with

14% efficiency [101].

Post Absorber Deposition

Exposure of CIGS absorbers to the atmosphere for significant amounts of time prior to the

buffer layer deposition step leads to surface oxidation. Yamada et al. observed large oxidation

rates for polycrystalline films; the surface of the fi1m is likely to be oxidized to a depth of a

couple nm in a brief time after removal from the growth chamber [102]. Oxidation of the surface

can also lead to large changes in resistivity of up to three orders of magnitude in just a few days

[103]. This shows clearly that it would indeed make a difference if the application of the buffer

layer and the completion of the solar cells is done immediately after absorber deposition, a few

days, or even a few hours later. Nadenau reported a dramatic decrease in cell performance if the

CGS layers were exposed to air for one day prior to the CdS deposition [104].

CIGS grown under a large Cu-excess condition contains copper selenide phases at the

surface and along the grain boundaries. A cyanide-based chemical treatment is used to remove

any secondary phases while being inert to the CIGS phase [105]. The remaining material after

potassium cyanide (KCN) etching is expected to be the stoichiometric phase, but a rough surface

may remain [102]. This type of morphology may affect the device performance adversely

because it can lead to poor metallurgical contact at the interface [106].

Buffer Layer

The main purpose of the buffer layer in a solar cell device is to act as barrier to diffusion of

impurities from the transparent conducting oxide layer into the absorber. Other benefits may

include interface passivation and the establishment of an inverted region in the absorber [107].

The properties of a buffer layer often depend on the deposition technique used in its fabrication

and the ability to control the growth parameters for each technique.









Most high-efficiency CIGS device structures employ a high-resistivity CdS buffer layer

deposited by chemical bath deposition (CBD). Because the band gap of CdS is too low (2.4 eV)

to permit transmission of all useful light, a balancing act is employed to optimize the structure.

If the CdS layer is too thick, unacceptable absorption occurs, leading to the reduction in short

circuit current (Jsc). If the CdS layer is too thin, shunt paths are generated, which leads to a

decrease in the open-circuit voltage (Voc) [108]. The high efficiencies that have been achieved

by the CBD process are the result of a set of critical interactions that can produce n-type doping

or inversion, compositional grading, and interface passivation [109]. The direct diffusion of

cadmium into the CIGS layer has been observed and this may lead to the formation of a buried

pn-junction [110].

A CBD bath temperature at 800C instead of 600C, which is the common standard for

CIGS, is employed for CGS devices. The growth speed increase due to the elevated temperature

so the concentrations in the solution are modified. The quality of the buffer layer and the

interface with the absorber are improved by the 800C procedure [111]. Chemical bath deposition

also affects the defects in the bulk of the absorber material so tunneling recombination is reduced

in samples grown with the higher CdS deposition temperature.

There are a few problems associated with cadmium sulfide technology. The band gap of

the CdS layer is still low enough to limit the short wavelength part of the solar spectrum that can

reach the absorber, and this leads to a reduction in the current that can be collected. This current

reduction becomes proportionally more severe for higher band gap cells [112]. The substitution

of the heavy metal compound, CdS, is also desirable from an environmental and economical

point of view. A large-scale CBD-CdS buffer deposition process creates extra costs for the









necessary safety precautions needed for the handling and disposal of toxic material, especially

for such an inefficient and exceedingly wasteful process [113].

Alternative Buffers

An alternative buffer layer such as ZnS is attractive due to its wide optical band gap and

reduced ecological issues. By widening the E, beyond that of CdS, a higher short-wavelength

quantum efficiency is expected in CIGS solar cells, thereby increasing the short-circuit current

[114]. Using a ZnS CBD buffer, a champion cell of 18.6% was achieved by Hariskos et al.

[115].

Based on Anderson' s model of heterojunctions, the electron affinities of both layers should

match in order to obtain the maximum built-in potential and hence a high open-circuit voltage.

The electron affinity of CdS (4.5 eV) is larger than that of CGS (3.9 eV) so current loss can be

minimized by mixing ZnS with CdS to form a ternary compound ZnxCdl-xS in which the electron

affinity can be varied from 3.7 eV to 4.5 eV. Electron affinities are equal at about x = 0.78 and

any variation in the buffer layer composition from this optimum value results in a deterioration

of the cell performance [105]. Voc increases with increasing Zn concentration whereas the Jsc

decreases. Ramakrishnu et al. produced a CGS cell with moderate efficiency (rl ~ 5 %) with a

Zn composition fixed at x = 0.5 [105].

Due to good lattice matching and to ideal electronic band offsets, ZnSe is expected to

provide a perfect buffer layer for CGS [116]. For the CdS/CGS interface, the CdS conduction

band minimum is below that of CGS resulting in a "cliff" structure. Devices with this type of

band alignment show interface recombination dominated behavior, and hence suffer from a loss

in Voc. In order to avoid this effect, a material with a smaller valence band offset and a larger

gap is required. ZnSe has a band gap of 2.7 eV and the lattice constants of ZnSe and CGS are

closely matched, which should result in an almost strain-free interface [117]. Rusu et al.









produced ZnSe/CGS heterojunctions with very high voltages, but poor conversion efficiency due

to very low current (Voc ~0.96 V and Jsc ~2 mA/cm2) [118].

Window Layers

From the aspect of band alignment, the II-VI buffer layer could be omitted, but solar cells

prepared with a direct CIGS/ZnO heterocontact show only poor efficiencies [119]. Ramanathan

et al. observed that the direct sputtering of ZnO on CIGS typically yields only 2-5% devices

[120]. These cells are characterized by enhanced current losses probably due to tunneling or

recombination processes via trap levels associated with impurities that diffuse into the absorber

during transparent conducting oxide (TCO) deposition [107].

The basic properties for making high quality transparent conductors are high conductivity,

high optical transmission, minimal surface roughness, thermal stability to withstand the

processing temperature, chemical stability, and crystallinity. Typical TCOs used in solar cell

fabrication have band gaps in the range of 3.3-3.8 eV, carrier concentrations in the range of 1020_

1021 CA-12, a conductivity of 104 (Ohm-cm)- a sheet resistance of about 5-10 ohms/square, and an

optical transmission greater than 85% over the visible part of the spectrum [121]. Almost all of

the well-known TCOs that are used in solar cell devices, such as ZnO, In203, and SnO2 have n-

type conductivity [122].

In the fabrication of CIGS solar cells, it is customary to use a high/low resistivity grading

of the ZnO layer. An undoped layer of ZnO (high resistivity) is first deposited on CdS, followed

by the deposition of a doped layer. Ramanathan reports that solar cells made without the

undoped ZnO layer are identical to those made with the bilayer so the undoped ZnO layer may

be unnecessary even when CdS is very thin [89].









Metallization

To collect the current, contacts are placed across the entire surface of a PV cell. This is

normally done with a "grid" of metal strips. However, placing a large, opaque grid on top of the

cell shades the active parts of the cell from the sun so they are designed with many thin,

conductive "fingers" spreading to every part of the cell's surface. The Eingers of the grid must be

thick enough to conduct well with low resistance, but thin enough not to block much of the

incoming light. Ni/Al grids are deposited by e-beam evaporation using a mask. Cell areas are

then delineated by mechanical scribing to give individual cell areas of 0.429 cm2 and a Einal In

contact is soldered on after the fi1m is scratched away to reveal the Mo back contact far away

from the grid.

Anti-Reflective Coating

Bare solar cells can reflect about 30% of the sunlight. Since the power output is

proportional to the amount of sunlight that is absorbed, these losses are detrimental to the device

performance. Surface reflection loss can be reduced by adding anti-reflection coatings (ARC) to

the solar cell. An ARC is typically deposited onto CIGS devices by the E-beam evaporation of

MgF2 to a thickness of 800-1200 Angstroms. The gain in short-circuit current is typically 4-8%

with a corresponding enhancement in conversion efficiency [106].

Device Characterization

Current-Voltage

Current-voltage (I-V) analysis is a critical tool used to study solar cell performance. The

electrical parameters, including the conversion efficiency (rl), open-circuit voltage (Voc), short

circuit current density (Jsc), fill factor (FF), series resistance (Rs), shunt resistance (RSH), diode

ideality factor (n), and saturation current density (Jo), of a device can be determined from the

measured illuminated and dark I-V curves. The conversion efficiency is defined by









FF V, I,
4 = ocs (3-6)
P,

PIN is the total power of incident light. Considering the general expressions for Voc and Isc, the

key material parameters that determine the efficiency of the solar cell are the lifetime and

mobility of the minority charge carriers and the surface recombination velocities [22].

The power that a cell provides is a product of its operating current and voltage. Under

short-circuit conditions, current is maximum, but voltage is nearly zero so almost no power is

provided to the circuit. Under open-circuit conditions, voltage is highest, but no current flows so

power is again zero. Fill factor is the percentage of maximum power as compared to the product

of open-circuit voltage and short-circuit current. Some cells can have good Voc and good Jsc,

but a poor FF, which results in not much power and a low efficiency. The series resistance can

affect the shape of the photo I-V curve, mainly the FF [9].

To make a good measurement, two other parameters are controlled: total power in the light

and the temperature of the cell. Standard power is 100 mW/cm2, which is approximately the

power density of sunlight at the Earth's surface at noon on a cloudless day. The cell is held at a

Eixed temperature of 250C because cell voltage and thus power output varies with temperature.

To get an accurate efficiency, the precise cell area must be known because the amount of input

sunlight depends on the cell area. Total area is the total area of the top surface of the cell, while

active area is the surface area of the cell without counting metal contacts even if those are on top

of the active portion of the cell. The solar cell to be measured is exposed to simulated sunlight,

and as a resistive load is varied from open-circuit voltage through short-circuit conditions, the

cell's I-V characteristics are measured [8].









I-V Measurement Technique

The reference cell method, which basically uses a reference cell to adjust the illumination

level of the solar simulator, is employed in the performance measurement of CIGS (and CGS)

solar cells in this study. The solar simulator intensity is adjusted by changing the distance

between the tungsten-halogen lamp and the test plane so that the measured Jsc of the reference

cell is equal to its calibrated value at the standard measurement intensity of 100 mW/cm2. We

use a CIGS solar cell calibrated against a primary reference cell and the global reference

spectrum by NREL to set the illumination level of the solar simulator.

The open-circuit voltage of CIGS solar cells decreases with increasing temperature at 100

mW/cm2. The temperature of the test cell is maintained at 250C & loC by a temperature

controller that circulates cooling water through the assembly during the illuminated I-V

measurement. The temperature controller of the cooling system is set at 200C to keep the

reading of the thermocouple and hence the temperature of the test cell at 25 A loC. The semi-

automated I-V measurement system is controlled by a personal computer with the data

acquisition and data analysis software LabVIEW [123].

Quantum Efficiency

Quantum efficiency (QE) is defined as the number of electron-hole pairs generated per

absorbed photon and is a measure of the effectiveness of a cell in converting light of various

energies into electricity [9]. The cell is illuminated with monochromatic light while its electrical

output is being recorded. We know the number of photons in the monochromatic light and we

can measure the resulting electric current, which tells us how many electrons are being produced

by the cell. By slowly changing the monochromatic light to various energies, we can measure

the cell's response to the spectrum of solar photons. If the photons making up the

monochromatic light have less energy than the cell's band gap, they will pass through it without









producing any current (QE = 0). Just above the band gap of the cell, light will be very weakly

absorbed and unless the material's diffusion length is very large, the quantum efficiency will be

small. QE will begin to rise sharply as the energy of the incident photons is increased. In very

good cells, quantum efficiency of over 90% can be reached across most of the solar spectrum [8].

QE Measurement Technique

A spectral response measurement system employing a grating monochromator is used to

analyze the quantum efficiency of our solar cells. The monochromator, which is controlled via a

computer program written in LabVIEW, scans the spectral range from 400 to 1400 nm using 10

nm incremental steps. Two order sorting fi1ters are use to block the undesired harmonic terms

from the monochromator; one is applied for the range from 630 to 1000 nm and the other for

1000 to 1400 nm. The incident power density on the test plane is first measured by calibrated

silicon and germanium photodetectors, and it is saved on the hard disk of the computer. The

measured spectral response is calculated from the data stored in the computer previously and the

measured photocurrent of the test cell (Itest cel1(h)). Finally, the external quantum efficiency as a

function of the wavelength can be converted from the spectral response using the following

equation [123]

h-c-I ,, l(Al)
QE (l) = ts x 100% (3-7)
q Ai power densitydetector Ae test cell

where h, c, q, and h are Planck' s constant, the speed of light, the electronic charge, and the

photon wavelength, respectively.














Ga





Cu


Se





0 50 100 150 200 250 300
Time (minutes)

A


25


20



15





5


0


1200


1000


800


600


400


200


O


1200


1000


800


600


400


200


rr10


0 50 100 150
Time (minutes)

B


1200


1000


800


600


400


200


0


rr10


0100 200 300 400
Time (minutes)



Figure 3-1. UF growth recipes. A) Constant Cu-Flux. B) Modified 3-Stage. C) Emulated 3-
Stage.









S0.1 prn


Figure 3-2. Typical CIGS device structure.









CHAPTER 4
COPPER GALLIUM DISELENIDE AB SORBER GROWTH

Like other I-III-VI2 COmpounds, CuGaSe2 has a wide phase stability region. The existence

range of CGS extends down to a Cu/Ga ratio of approximately 0.7 [53]. Optical and electrical

properties are greatly affected by film composition because intrinsic point defects exist in the

material as its composition deviates from stoichiometry. It is not surprising that different

research groups have presented slightly different results depending on their specific preparation

method since the defects present are strongly dependent on the growth method and thermal

treatment [124].

Growth Matrix

During the last few years, CGS films were grown in order to optimize the performance of

devices. Eight sets of CGS films were grown on Mo coated soda-lime glass substrates in the

PMEE system under different growth conditions. The specific PMEE reactor conditions for each

growth run described within this document are available in Appendix A. Growth temperature

and recipe were adjusted for the films and the growth rate fluctuated from about 0.4 to 0.9 8Js.

The thickness estimated for each run was varied from 1.0 to 1.5 Cpm based upon the total Cu

thickness sensed by the Sentinel III. This estimation method has been a better predictor of

thickness for films containing indium rather than gallium. ICP analysis of film composition

gives the parts per million of each species in the solution. Knowing the area of the

characterization sample and the density of each species allows you to determine a thickness

estimate of the film. There is of course error involved in the exact measurement of the

characterization sample's area, but this procedure estimates the actual thickness range to be

approximately 0.6 to 1.3 pm. Each table related to the specific growth run series summarizes the

growth process, as-grown composition and if available, the composition of the film after a KCN









etch for each of the films. The column labeled "growth process" shows the intended

composition of each layer of the film.

The first set of films, samples 443 through 447 shown in Table 4-1, was deposited at

386oC. They were grown mostly gallium rich by following a procedure similar to Boeing's

bilayer process: Cu-rich deposition followed by a Ga-rich layer. Three different compositional

stages were incorporated: an initial Cu-rich stage, an intermediate stage that varied from less Cu-

rich, to near stoichiometric, to Ga-rich depending on the desired overall Cu/Ga ratio, and a final

Ga-rich layer. The layers were approximately the same thickness with each constituting one-

third of the total thickness. The growth rate for these films was approximately 0.5 a/s and the

film thickness was estimated to be about 1.2 pm.

Films with Cu-rich initial stages seem to give poor quality absorber films when grown in

the PMEE reactor. We believed this may have to do with poor adhesion of these films to the

molybdenum, like in Stacked Elemental Layer processing where only gallium as the first layer

led to good adhesion of the absorber layer to the Mo back contact [103]. Klenk et al. suggest

that Cu is not useful as the first material deposited onto the Mo as it caused severe adhesion

problems [125]. Films that start and finish with Ga sequences should adhere better to the

substrate and result in a more uniform morphology [126]. We intended to test this by changing

our growth procedure to either include a Ga-rich CGS initial layer or a thin GaSe layer.

The second set of films, samples 452-459 shown in Table 4-2, were also grown at 386oC.

The first part of this growth series consisted of films grown overall copper rich followed by

gallium rich samples. The intended growth procedure was a Modified Three-Stage Process. A

rather thin initial Ga-rich layer was deposited followed by a Cu-rich layer. Most of the films had

a thicker final Ga-rich layer deposited on them except for 452, which had only 2 stages, and 453,









whose Einal layer was composed of GaSe. This Ga-rich layer was typically half the total film

thickness. These fi1ms were grown at a similar rate to the previous set, but they were much

thinner, approximately 0.8 pm. Selenium annealing was performed for 30 minutes after metal

deposition was complete at the growth temperature for all the samples. The Se flux remained the

same as it was during metals deposition.

Copper selenide is a degenerate p-type solar cell found in CGS cells with overall Cu/Ga

ratios greater than one. If present, the Cu2-xSe phase will tend to short-circuit the device, not

allowing for a measure of device performance that is representative of the underlying absorber

material quality. It must be removed before deposition of the buffer layer [127]. A cyanide etch

should remove any Cu2-xSe phase at the surface and between the network of grains in the fi1m so

we dipped the CuGaSe2 film in a 10% KCN solution for fiye minutes. After the removal of the

Cu-Se surface phase, the composition of Cu-rich CGS films has been found to be near

stoichiometry [35]. It has also been shown to increase the photoluminescence intensity of Cu-

rich grown CIGS film up to five times by Keyes [128]. However, the removal of the secondary

phase did not result in the Cu-rich sample becoming more like the (In, Ga)-rich samples since the

dominant defects and recombination processes are inherent to the CIGS phase. This result

should be analogous to CuGaSe2 Samples.

The third set of films, samples 472 to 480 shown in Table 4-3, consisted of films that were

mostly gallium rich or near-stoichiometric. For many of the films, an initial layer of gallium

selenide was grown. This was done to promote adhesion of the film to the substrate. The initial

GaSe layer was followed by a Cu-rich CGS layer, and a thin GaSe or Ga-rich layer was

deposited on top. For two of these films, the Emulated Three-Stage growth recipe was used.

The final GaSe layer was very thin so it is assumed that since the final composition is slightly









Ga-rich for these two samples that the film may have never been Cu-rich during the growth

procedure. The gallium primary temperature was increased to 10070C, compared to 9750C that

was used for the previous growth series, to increase the growth rate. The growth rate for these

samples ranged from 0.8 to 0.9 sis, except for the emulated three-stage process, which had a

growth rate of about half that value. The film thickness was approximately 0.9 pm. The same

final Se vapor treatment was performed after deposition.

In the PlVEE system, the substrates are radiatively heated by a resistive heater located

above the rotating substrate platen. This platen carries the substrates through each of the four

zones in the system, the metals zone, the fluxless load-lock zone, the chalcogen zone, and the

heater zone. A thermocouple is placed in the gap between the heater and the rotating platen for

temperature measurement. This temperature is then controlled during growth. Previous work

had been done to determine the relationship between the gap thermocouple temperature, T,, and

the actual temperature, Ts, of the substrates, yielding the relationship [6]:

Ts = 0.5247 T, + 18.856 (4-1)

It had been previously believed that for the PlVEE system, using a T, greater than 7000C

may result in damage to the heater. However, the efficiencies of devices grown at this

temperature were relatively low. It was decided that a higher growth temperature was needed to

improve device efficiencies. After much investigation, it was concluded that the system could

likely be used safely at a higher temperature.

CGS solar cells with the highest performance were for a long time based on Cu-rich

composition, but the current record cells incorporate absorber films with an overall as-grown

composition that is gallium rich. The disadvantages of an overall as-grown Cu-rich composition

are a high doping density and a high concentration of electrically active traps [1 11]. Air









annealing mainly diminishes the density of deep traps in Cu-rich CGS [52]. It is well known that

the grain size of Cu-rich chalcopyrite fi1ms is larger as compared to Cu-poor films [129]. The

Cu-content of the fi1m determines the activation energy for grain boundary motion. Higher Cu-

contents lead to lower activation energies for grain boundary motion and therefore to the

formation of larger grains [88].

A Cu-rich growth period has been deemed to have beneficial effects on device

performance by some, but others have defined these benefits as limited to depositions at reduced

temperatures or times. Comparing different flux profiles, it was shown that at 4000C, Cu-rich

growth is necessary to achieve good performance in CIGS by Shafarman et al. At higher

substrate temperatures, device performance is insensitive to growth sequence allowing greater

process flexibility [130]. Two-stage, rather than three-stage growth was utilized there. The

simultaneous deposition of group I and III atoms during the two-stage process may provide more

time for the necessary reactions and lessen the benefit of the Cu-rich growth period since the

benefit of the Cu-rich growth period has been surmised to come from a fluxing of the CIGS

grains by excess liquid Cu2Se [131].

The lack of quality absorbers obtained by deposition in the PlVEE from overall Ga-rich

composition is a perplexing phenomenon. It is possible that the fi1ms were not grown Ga-rich

enough, but many were in the compositional existence range of high quality absorbers. It is

possible that the processing conditions arrived at after years of optimizing the performance of the

low-Ga absorber material are not optimal for use with higher Ga-containing films, especially

when the Cu/III ratio falls much below 1 [128].

Since previous CGS absorber layers grown by PlVEE with overall Cu-rich compositions

had shown higher efficiencies, subsequent film growth series were grown copper rich. Table 4-4









displays the fourth set, which consisted of seven CGS films. Samples 510-516 were grown at T,

= 7500C, which corresponds to a substrate temperature of 4120C. These samples used growth

recipes that were similar to the ones that resulted in our previous best cells at a slightly elevated

growth temperature. The growth rate was similar to the previous runs, about 0.4 to 0.5 8Js, and

the films were slightly thinner at 0.6-0.8 pm. Cu-rich samples became nearly stoichiometric

after etching them in a 10% KCN solution for 5 minutes to remove the unwanted copper selenide

secondary phase. Se annealing was also performed for these films.

High substrate temperatures may be even more important for CGS deposition than for CIS

or low gallium content CIGS growth. Purwins et al. found that the formation of CIS is finished

before CGS starts to form at an elevated temperature of approximately 3900C [132]. Most of our

film growth occurred at a substrate temperature near this lower limit of CuGaSe2 COnstitution.

Growth temperature seemed to be a limiting factor in producing high-quality films so a

much higher gap temperature of Tg = 900oC, corresponding to a growth temperature of 491oC,

was used for the fifth set of films, 521-525, shown in Table 4-5. These films had a similar

growth rate and thickness to the previous set. Film #523 was grown at a very low growth rate of

0.4 8Js to a thickness of only about 0.6 Gum. The final selenium vapor treatment for 30 minutes

occurred at the new elevated growth temperature of 4910C.

The growth temperature used while depositing CGS films in our PMEE reactor is lower

than ideal resulting in lower quality films. This may be a result of inadequate sodium

incorporation into the film. Na presence is due to diffusion from the soda-lime glass. During

absorber growth and potential annealing steps, Na diffuses through the Mo film into the

absorber, improving the doping concentration of the absorber. Ideally, a higher Na amount is

found closer to the Mo contact and the concentration gradually decreases in the bulk. Rusu et al









found a sodium concentration of I atomic percent at the surface [74]. A moderate level of Na

improves the efficiency of the cells by enhancing the p-type conductivity. It incorporates into

the lattice of the CIGS by reacting with Se and forming Na-Se compounds [97]. These

compounds slow the growth of CIGS and facilitate the Se incorporation into the film. Excessive

Na diffusion may limit the efficiency of the cells because of the introduction of additional deep

states.

Table 4-6 shows a sixth set of films, samples 535-542, which were mostly grown copper

rich at 491oC. Growth runs 535-540 used an elevated gallium primary crucible temperature of

10050C to increase the Ga flux. Previous attempts at growing quality absorbers by the Emulated

Three-Stage Process were unsuccessful so the procedure was attempted again for some of these

films while maintaining an overall Cu-rich composition. The initial GaSe layer was grown at

450oC and the final two stages were deposited at 491oC. Films #541 and #542 used the same

processing sequence as our best absorber to date, #523. Se annealing, as described above, was

only performed on absorbers #541 and #542. The thickness of the films ranged from 0.6 to 0.8

ym while employing a growth rate of approximately 0.4 a/s.

Three-stage co-evaporation imposes stringent limits on the parameter space if highly

efficient devices are to result. The growth kinetics, substrate temperature profile, and reaction

time will make the outcome of local equilibria unique to the growth process. The Ga and Se

delivered in the third stage reacts with the CuxSe to form additional CuGaSe2 until the CuxSe is

consumed. Cu must diffuse out of the CGS grains to react with new Ga and Se while some Ga

will diffuse into the bulk grains to bring them to more Cu-poor compositions. By varying the

temperature during this stage, the counterdiffusion process can be enhanced or inhibited such

that the thickness and/or composition of surface Cu-poor phases can be controlled [91]. The









evolution of the intrinsic defects depends on the dynamics of the reaction pathway, i.e. the

composition changes that occur when the film transitions from Cu-rich to Ga-rich [133]. The

degree of overall Cu-richness or lack of Cu-richness that the film has after the second stage may

have a large effect on the absorber quality.

The three growth strategies described in the preceding chapter were incorporated into a

seventh set of runs, 628-641, that produced CGS absorber layers. The second column of Table

4-7 identifies the growth process utilized. For example, Sample #628 was grown using the

Constant Cu Flux Process described previously. When the Modified Three-Stage Process was

utilized, the second column of the table indicates the target composition of each sublayer of the

resulting CGS film. For example, Sample #634 has a growth process denoted as

"O/1.2/1.62/1.2". This indicates that the absorber film in this sample is composed of four layers.

In the first layer, the ratio of the copper flux to the gallium flux is zero, indicating that the

effective metal flux reaching the substrate was composed only of gallium. In the second layer

the ratio of the copper flux to the gallium flux is 1.2, indicating that there was a 20% excess of

copper relative to gallium reaching the substrate and thus this sublayer was grown under copper-

rich conditions. In an analogous fashion, the ratio values of 1.62 and 1.2 describe the

relationship between the fluxes imposed when growing the third and fourth sublayers. Finally,

Sample #640 incorporates an Emulated Three-Stage Process designated as "GaSe/CuSe/GaSe".

This indicates that the growth was done with the intent of defining three sublayers, where the

bottom most and the top sublayers are grown under gallium and selenium fluxes, while the

middle sublayer is grown under copper and selenium fluxes.

The third column of Table 4-7 indicates the overall ratio of copper to gallium content in the

final CGS film, as measured via ICP, and whenever applicable, the fourth column gives the










copper to gallium ratio in the film after a 5 min etch procedure in a 10% KCN solution carried

out to eliminate surface CuSe material. For example, Sample #629 had a copper-to-gallium ratio

of 1.17 as determined by ICP, and hence was a copper-rich film. The last column shows that

after the KCN etch procedure the ratio of copper to gallium in Sample #629 was reduced to 1.00,

putting the film in a stoichiometric Cu:Ga composition.

In every growth run the gallium source primary temperature was maintained at 9700C for

each run, and the substrate temperature was estimated to be approximately 4400C. Although an

elevated substrate temperature of approximately 4900C gave higher quality CGS absorbers, the

heat being generated was also warping the rotating platen, which led to scraping and erratic

rotational movement of the substrates. A gap temperature of 8000C, corresponding to a substrate

temperature of 4400C, was deemed safe so this is the maximum growth temperature that is used

in the PMEE reactor. Each absorber layer was estimated to be grown to a thickness of 1.5

microns except for Sample #641, which was grown to a thickness of 2 microns. The actual

thickness is more likely to be in the range of 1.1 to 1.3 Cpm for those grown to an estimated

thickness of 1.5 Cpm by analysis of the component masses over a defined film surface area. A

thickness of 1 micron is sufficient for the absorption of photons up to 750 nm, however thicker

layers result in a better performance of the solar cell [134]. The film growth rate is estimated to

be about 0.8 AJs, except for those using the emulated three-stage process where the growth rate

is about half of this value.

Three samples were grown under the constant copper flux strategy: one was grown under

near a 1:1 (i.e., stoichiometric) Ga:Cu fluxes (Sample #628), one under Cu-rich conditions

(Sample #629), and one under Ga-rich conditions (Sample #630). Thicker versions of the

process that resulted in our best absorber, namely #523, were grown. Overall Ga-rich and near-









stoichiometric fi1ms were grown incorporating the modified three-stage process with varying

levels of peak copper richness. For this process, a thin initial GaSe layer was deposited followed

by Cu-rich layer and then a Ga-rich layer. The Einal two layers had similar thicknesses

incorporating half of the total film thickness.

Gallium rich samples in this growth series were slightly more selenium rich than KCN-

etched copper rich samples. Etched samples ranged from 0.490 to 0.494 selenium composition

while the overall gallium rich samples varied from 0.494 to 0.500. Epitaxial CGS films grown at

a substrate temperature of 5000C by Gu et al. showed a similar trend. The Se content value was

slightly higher than 50 at.% in the Ga-rich region and slightly lower than 50 at.% for the Cu-rich

region [45].

A final set of films, 647-662, were grown at 4400C by the Constant Copper Flux Process

over a range of Cu/Ga ratios from approximately 0.9 to 1.25. Films #648, grown by the

procedure that produced our best absorber to date, and #649, grown by the three-stage process,

were included for comparison. All of the films were grown at a rate of 0.7 to 0.9 a/s, except for

#649, which was grown at a rate around 0.45 is. Approximate film thickness varied from about

1.0 to 1.3 pm.

A growth series of bilayer precursors was started, represented by growth run #666 in

Appendix A, but the Ga source was damaged during or following GaSe deposition. It is likely

that the gallium crucible cracked and the metal leaked out and shorted the source. The intent was

to grow a low temperature CuSe/GaSe stack that was to be rapidly thermally processed (RTP).

Absorber Characterization

The effect of different growth conditions on the film morphology such as growth

temperature, overall Cu to Ga ratio and growth recipe were shtdied by using Scanning Electron

Microscopy (SEM). All CGS films having overall Cu to Ga ratio greater than one at some point









during the growth showed the morphology that had matrix and domain structure. In this

structure, the domain region showed highly Cu-rich composition and large grains while the

matrix region had small grains and stoichiometric or Ga rich composition. It means at the point

we had the overall Cu-rich composition for the film, there was a formation of liquid-like CuSe

secondary phase in the film and it made the grain size in the domain region larger than that in the

matrix region. As this kind ofinhomogeniety was observed in Cu-rich films with large grains

that lead to better efficiencies, the growth temperature was increased to get more uniform films.

Figure 4-2 shows that film #523 has better uniformity than the films grown by a similar

process at a lower growth temperature as seen in Figure 4-1. More uniform films are more likely

to produce high-quality CGS absorbers. Figure 4-3 and 4-4 shows the morphologies of films

grown with the similar Cu to Ga ratio profile but at different growth temperatures during the

growth. As seen in those figures, the grain size in the domain region appears to be largest at the

lowest growth temperature, while the grain size in the matrix region is the smallest. The grain

size in domain region was smaller at an intermediate growth temperature and got bigger at the

highest growth temperature. It appears that the grains in the matrix region got bigger as the

growth temperature increased. This means there might have been possible phase separation

between the Cu-rich domain region and Ga-rich matrix region at the lowest growth temperature,

and the film got more homogeneous as growth temperature increased. This improved bulk

crystal quality may be due to a more ideal incorporation of sodium into the film from the soda-

lime glass substrate at an elevated temperature [135].

Shafarman et al. showed that with Cu-rich growth of CIGS, the mean lateral grain area

decreases from 1.8 to 0.3 squared microns as substrate temperature is reduced from 550 to

4000C, but only at the highest substrate temperature does the grain size depend on growth recipe










[130]. Films deposited at 4000C have a greater average sodium concentration than those

deposited at higher substrate temperatures. Thus, improved device performance with increased

substrate temperature cannot be explained by greater availability of Na. Films with smaller grain

size or a greater density of grain boundaries may have greater average sodium concentration

since nearly all Na probably resides along those boundaries.

In Eigure 4-5 and 4-6, the morphologies of films grown at the highest growth temperature

were compared for three different growth processes. Film #521 was grown by using a reverse

Boeing process, which used Ga-rich and then Cu-rich conditions. Film #523 was grown by the

Modified Three-Stage Process to utilize the liquid-like CuSe secondary phase to get larger

grains. Finally, film #525 was made by a process similar to Boeing's. As shown in those

figures, absorbers #521 and #523 have similar morphologies, both for the matrix and the domain

regions. And the grain size in domain region is only slightly larger than that in the matrix region

for those two films. For film #525, we could see that the grain sizes in two regions appear to be

much different from each other and there might have been possible phase separation again.

In figure 4-7, the morphologies of Cu-rich and Ga-rich films grown by the Emulated 3-

Stage Process were compared. Even though both films showed good homogeneity, the grains of

Cu-rich film are much larger than that of Ga-rich film. The Ga-rich film (#536) has very small

grains (~100 nm) while the Cu-rich film has larger grain size (300 ~ 900 nm). Films #541 and

#542 have a more uniform morphology, as seen in Figure 4-8, than those grown at lower

substrate temperature. They also didn't show the domain (large grain size region) and matrix

structure so it is assumed that devices made from both absorbers #541 and #542 will show good

performance.









Preference for a certain orientation seems to be dependent upon the growth recipe that was

followed in the PMEE reactor. Films grown by similar growth recipes, but at different substrate

temperatures, exhibit nearly identical XRD patterns. Shafarman et al. claim that XRD

measurements did not show any significant difference in the film orientation for different

processes or substrate temperatures. All their films had nearly random orientation [130]. Figure

4-9 shows that films #511 and #522 both have a preference for the (1 12) orientation of CGS

while Figure 4-10 shows that #515 and #523 have comparatively less of a preference for this

orientation. Absorbers #515 and #523 were grown with an initial GaSe layer while #511 and

#522 had a Ga-rich initial layer and a final layer of GaSe. The films that had no initial Cu flux

during deposition had much more intense (220) peaks compared to the (204) peaks, whereas

films without this good adhesion layer seem to have slightly more intense (204) peaks than (220)

peaks. The full width half maximum of the (1 12) diffraction peak of film #523 is sufficiently

small indicating that the crystalline quality is fairly good. Some groups claim that there is a clear

correlation between higher (1 12) orientation and smoothness of the films [136], but that does not

seem to be the case for CGS films grown by PMEE at lower than ideal substrate temperatures.

Figure 4-11 shows two films with the same growth conditions, but grown at different

rotational speeds. The XRD patterns are very similar, but the film grown at the higher rotational

speed, #479, has slightly sharper peaks. A higher rotational speed may lead to larger grains. The

film grown at the lower speed, #476, also seems to have a stronger preference for (112)

orientation.

XRD patterns also show which secondary phases are present. Films #452 and #455 were

grown at the same low growth temperature, but film #455 has an as-grown Cu/Ga ratio of about

1.4 while #452 has a ratio of 1.1. Figure 4-12 shows that the Cu2-xSe peaks are much more









intense for the very Cu-rich film. After the 10 % KCN etch for Hyve minutes, these Cu2-xSe peaks

disappear as shown in Figure 4-13.

As shown earlier, the growth recipe can have an effect on the preferred film orientation.

Films grown by the Constant Cu Rate Process have similar XRD patterns no matter the

composition. Figure 4-14 shows that the peaks for the KCN-etched Cu-rich film, #629, are

nearly identical to the Ga-rich film, #630. The extent to which a film goes copper rich in the

Modified Three-Stage Process can also have an affect on the pattern. Films #635 and #636 have

nearly identical compositions and similar growth processes, but #636 becomes more Cu-rich

during deposition. Figure 4-15 shows a greater (220) peak intensity for the film that has a lower

copper peak composition.

The Emulated Three-Stage Process produced absorbers that favored the (204) orientation

of CGS rather than (1 12), which is the more prevalent configuration for the other two growth

recipes. Figure 4-16 shows that Cu-rich and Ga-rich films employing GaSe deposition followed

by CuSe deposition have (220) CGS peaks that are more intense than even the (112) peaks. The

preference for these orientations may be due to the fact that the Emulated Three-Stage Process

deposits copper and gallium in separate layers, while they are deposited concurrently in the other

growth strategies. Yet, the XRD pattern for film #478 shown in Figure 4-17, which was also

grown by a three stage process, but at a lower growth temperature does not show these same

characteristic peaks. Due to the very thin GaSe final layer and overall Ga-rich composition, film

#478 may not have ever been Cu-rich and hence may have had different growth kinetics.

The surface morphology of films #628 through #641, grown at 4400C by various growth

processes, was investigated by SEM. Figure 4-18 shows the Constant Cu Rate Process of a Cu-

rich (#629) and a Ga-rich film (#630) at 100X magnification. Both Samples were grown under a









constant Cu flux throughout the entire deposition run with no Cu:Ga profile grading in any

sublayers. The Ga-rich film is very uniform while the Cu-rich film has many island structures.

Figures 4-19 and 4-20 show the Cu-rich film's grain structure in the field region and the island

region at 5000X and 10,000X, respectively. The field region shows very small grains and the

island structures have large grains up to a micron in size. The Ga-rich film morphology that is

shown in Figure 4-21 exhibits long needle-like grains. The Cu-rich films have Cu2-xSe

secondary phase on their surface. Figure 4-22 shows the island region and Figure 4-23 shows

the field region of film #634 before and after the 10 % KCN etch. The gaps left by the etching of

the copper nodules are very apparent in the island region while the field region appears to be

unchanged.

Co-evaporation of Ga-rich samples in an in-line deposition process revealed that a bilayer

process yields large, columnar grains, whereas a single layer process leads to absorbers with very

small grains. The bilayer-like process had a Cu-rich growth regime at the beginning and the

single-layer like process had constant rates throughout [137]. Shafarman et al. showed that at

lower temperatures, the uniform flux process appears to give more columnar grains and a

smoother surface than CIGS films with a Cu-rich growth period. There is no apparent difference

between films grown with Cu-rich flux at either the beginning or middle of deposition [130].

The morphology of CGS films is strongly dependent on composition. Haug et al. observe

long and needle-like grains that are of small size for Cu/Ga~0.3. Grains of somewhat Ga-rich

CGS layers with Cu/Ga ratios between 0.9 and 0.7 are triangular, and layers deposited at higher

temperatures have an increased grain size. The Cu-rich layer consists of grains of irregular

shapes with a typical grain size of 1-3 microns (Cu/Ga~-1.1) [134].









Orsal et al. observed slightly different morphology [138]. For slightly Ga-rich, there are

grains in the background with thin and long shaped grains starting to grow on the surface. With

an even higher Ga-content, the morphology is again homogenous and constituted of platelet-

shaped grains that are tilted on the surface. Cu-rich films exhibit small and homogenous grains

on the bottom with large polyhedral and packed grains on the surface. While it is not the case for

Cu-rich or Ga-rich films, the morphology is very sensitive near stoichiometric composition and

seems to depend on growth temperature and kind of substrate. Triangular crystallites are more

evident at 4500C with a grain size of approximately 0.6 microns. At 4000C, grains are

polyhedral whereas the layer is composed of a melt of triangular and polyhedral grains at 5000C.

Columnar growth is observed at each growth temperature.

Films 635-637 and #639 show some very peculiar surface morphology. The growth recipe

involved an initial GaSe layer followed by a Cu-rich layer and a Ga-rich layer. The overall

composition was either Ga-rich or near stoichiometric. The surface has islands surrounded by

rings that show a grain transition of large grains to smaller grains to long thin grains as can be

seen in Figure 4-24. Figure 4-25 shows the long needle-like grains present in the field region

that are the same as those in films grown Ga-rich by the Constant Cu Rate Process and the large

grains in the island region that are of similar size and shape to the ones found in the island region

of Cu-rich films grown by the Constant Cu Rate Process. It is likely that these films may have

copper selenide phases in the island region although the overall composition may be Ga-rich.

The sample taken for ICP compositional characterization may not have been representative of

the entire film since it is very non-uniform. This could be the case for film #639 since the Cu/Ga

ratio changed to 0.95 after the KCN etch from 1.04. Typically the film becomes nearly

stoichiometric, usually very slightly Ga-rich, after the cyanide etch. The pieces of the










characterization absorber used for the as-grown and post-etch composition analysis may have

started with drastically different overall compositions because of the non-uniformity of the film.

Smooth surface morphology is characteristic of films deposited by the three-stage process

[139]. The Emulated Three-Stage Process produced very uniform films as can be seen in Figure

4-26. Figure 4-27 shows that the Cu-rich process that did not incorporate a final GaSe layer has

long tubular grains that have a larger axial diameter than the needle-like grains of the Ga-rich

films. A SEM picture on a 450 tilt in Figure 4-26 gives a clear view of these tubular grains. The

Ga-rich absorber film, #640 shown in Figure 4-28, seems to have somewhat triangular-shaped

grains that have not been produced by any other growth recipe used in the PMEE reactor.

Films 647-662 were mostly grown with a Constant Copper Rate Process to investigate the

difference in the absorber film properties based on composition. Figure 4-29 shows a slightly

different XRD pattern for the films grown the most Ga-rich like #647. The peak intensity for

(220) CGS is slightly greater than that of (204) CGS. Nearer stoichiometric Ga-rich films and

Cu-rich films, as can be seen in Figure 4-30, demonstrate the characteristics peaks of a constant

copper rate process that were seen in the previous growth series. Figure 4-31 also shows that the

Modified and Emulated Three-Stage Processes exhibit the same orientation as those in previous

growth series.

In all the diffraction patterns of the CGS films grown in the PMEE reactor, the (220) and

(204) peaks are clearly separated, showing that the films have the chalcopyrite type

crystallographic structure. This was true for the near-stoichiometric and Cu-rich CGS films

grown by MBE by Yamada et al., but they also observed sphalerite crystallite structure for films

with Ga-rich composition [140]. But this was for very Ga-rich (Cu/Ga~0.66) films, which we

never grew in these growth run series.









Conclusions

Growth temperature, growth recipe, and overall Cu/Ga ratio each had varying yet

substantial effects on the film morphology and orientation. Growth temperature seems to be the

most critical variable in achieving high-quality absorber films. The fact that Cu-rich absorbers

grown by PlVEE have been more successful than Ga-rich ones is likely due to the lower

deposition temperature. Ga-rich absorbers produce the highest efficiency CGS cells in the

literature, but they are also grown at an elevated temperature of at least 5500C. The intrinsic

defects produced at different processing conditions results in absorbers with distinct properties.

For the final growth series, we were able to maintain very consistent growth conditions

between runs in our reactor, which gave us great confidence in the compositional results for each

run, even though we lack in-situ measurement techniques. A packaging system allows us to

vacuum seal the absorbers after growth to lessen any degradation that may occur before the cell

has been processed. The biggest drawback in using the PlVEE reactor to grow polycrystalline

films is the limit that we must observe on the maximum substrate growth temperature. A more

effective technique may be to grow CGS bilayer precursors at a low substrate temperature and

then utilize an RTP system to rapidly raise the temperature for a brief period of time. Klenk et

al. performed a post-growth rapid thermal treatment at 5500C for 6 minutes on stacked elemental

layers that were deposited by evaporation at a low deposition temperature [103]. Our research

group has been successful in the past employing an analogous strategy to grow CIS films via the

RTP processing of a bilayer [141]; we anticipate that the RTP processing route is likely to

produce similar satisfactory results for the CGS material.










Table 4-1. First CGS growth series.
Film # Process Cu/Ga ratio
443 1.1/0.9/0.7 0.89
444 1.15/0.95/0.75 1.00
445 1.15/0.95/0.75 0.96
446 1.3/1.1/0.9 1.13
447 1.15/0.95/0.75 0.98
Note: Process refers to the intended Cu/Ga ratio of each graded layer.

Table 4-2. Second CGS growth series.


Film #
452
453
454
455
456
457
458
459


Process
0.8/1.25
0.8/1.4/0
0.85/1.4/1.1/0.87
0.85/1.4/1.1/0.87
0.85/1.4/1.05/0.81
O/0.85/1.5/1.1/0.75
0.85/1.4/1.2/0.8
0.85/1.2/1.05/0.75


Cu/Ga ratio
1.11
1.00
1.11
1.40
1.04
0.97
0.91
0.85



Cu/Ga ratio
0.93
1.00
0.97
1.01
0.97
0.98
1.01
0.95



Cu/Ga ratio
1.14
1.59
1.07
0.97
1.19
1.19
1.06


Table 4-3. Third CGS growth series.
Film # Process
472 0/1.3/0
74 0/1.4/0.96
475 0/1.3/0.90
476 0/1.3/0
477 0/1.3/0
478 GaSe/CuSe/GaSe
479 0/1.3/0
480 GaSe/CuSe/GaSe

Table 4-4. Fourth CGS growth series.
Film # Process
510 0.8/1.25
511 0.8/1.4/0
512 0/1.4/0.8
513 1.15/0.95/0.75
514 0.8/1.25
515 0/0.9/1.45/0.9
516 Constant


Cu/Ga after KCN-etch

0.98





0.99


Cu/Ga after KCN-etch
0.99
1.01
0.99
0.96
0.99
0.99
1.01










Table 4-5. Fifth CGS growth series.
Film # Process
521 0.8/1.25
522 0.8/1.4/0
523 0/0.9/1.45/0.9
524 0/1.4/0.8
525 1.15/0.95/0.75


Cu/Ga ratio Cu/Ga after KCN-etch


1.14
1.23
1.36
1.28
1.09


1.00

0.97


Table 4-6. Sixth CGS growth series.
Film # Process Cu/Ga ratio
535 GaSe/CuSe/GaSe 1.04
536 GaSe/CuSe/GaSe 0.98
537 GaSe/CuSe/GaSe 1.12
538 0/1.3 1.58
540 GaSe/CuSe/GaSe 1.54
541 /11611.80
542 /121611.67
Note: These samples were etched by KCN, but the composition of the etched
samples was not measured.


Table 4-7. Seventh CGS Growth Series.
Film # Process
628 Constant
629 Constant
630 Constant
634 0/1.2/1.62/1.2
635 0/1.3/0.7
636 0/1.6/0.7
637 0/1.3/0.87
638 GaSe/CuSe
639 0/1.6/0.8
640 GaSe/CuSe/GaSe
641 0/0.8/1.37/1.35
Note: Ga-rich samples were not KCN-ete


Cu/Ga ratio
0.97
1.17
0.89
1.37
0.89
0.90
0.98
1.26
1.04
0.91
1.49
ched.


Cu/Ga after KCN-etch

1.00

0.98




1.01
0.95

0.99


I











Cu/Ga ratio
0.92
1.41
0.95
0.98
0.99
1.23
0.95
1.17
0.98
1.17
1.12
1.04
1.00
1.23


Table 4-8. Eighth CGS growth series.
Film # Process
647 Constant
648 0/1.2/1.6/1.2
649 GaSe/CuSe/GaSe
652 Constant
653 Constant
654 Constant
655 Constant
656 Constant
657 Constant
658 Constant
659 Constant
660 Constant
661 Constant
662 Constant





A B
Figure 4-1. Morphologies of films grown at lower growth temperatures by similar growth
recipes (X1000). A) Tsub = 3860C. B) Tsub = 4120C.


Figure 4-2. Morphology of a film grown at a higher growth temperature (X3000). Tsub
4910C.




















Figure 4-3.


Morphologies of the Cu-rich domain region of CGS films grown by the same
recipe at different growth temperatures (X30,000). A) Tsub = 3860C. B) Tsub'
4120C. C) Tsub= 4910C.


Figure 4-4. Morphologies of the Ga-rich matrix region of CGS films grown by the same
recipe at different growth temperatures (X30,000). A) Tsub = 3860C. B) Tsub
4120C. C) Tsub= 4910C.


A B C
Morphologies of the Cu-rich domain region of CGS films grown with different
growth recipes at 4910C (X30,000). A) (.8/1.25). B) (0/.9/1.45/.9). C)
(1.15/.95/.75).


Figure 4-5.




















Figure 4-6.


Morphologies of the Ga-rich matrix region of CGS films grown with different
growth recipes at 4910C (X30,000). A) (.8/1.25). B) (0/.9/1.45/.9). C)
(1.15/.95/.75).


A B
Morphologies of CGS films grown by the emulated 3-stage process at 4910C
(X30,000). A) Ga-rich. B) Cu-rich.


Figure 4-7.


A B~
Figure 4-8. Morphology of a Cu-rich film (#542) with large grains and a uniform surface. A)
X3000. B) X30,000.











16000


14000-


12000-


10000-


~8000-
8 CGS(112)

6000-


4000-


2000 ~~CGS(220/204)CS3216



10 20 30 40 50 60 70
2 theta (degrees)

A
10000

Mo(110) ,52
9000-

8000-

7000-
CGS(112)
6000-

S5000-

4000-

3000-

2000 -1 IICGS(220/204)
CGS(312/116)
1000-


10 20 30 40 50 60 70
2 theta (degrees)

B

Figure 4-9. Diffraction patterns of films grown at different temperatures with the same
modified three-stage process. A) Tsub = 4120C. B) Tsub = 4910C.






































































Figure 4-10.


Diffraction patterns of films grown at different temperatures with the same
modified three-stage process featuring an initial GaSe layer. A) Tsub = 4120C. B)
Tsub = 4910C.


Mo(110)


10 20 30 40 50 60 7(
2 theta (degrees)

A


I


I __


14000


12000


10000


8000


6000


4000


2000


515KI
Mo(110)












CGS(220)
CGS(112)


CGS(204)
CGS(312/116)


0


523K


12000


10000


8000


6000


4000


CGS(112)


CGS(220)



CGS(204
CGS(312/116)


2 theta (degrees)











18000
CGS(112) 476
16000-


14000-


12000-


S10000-Mo10

a8000-


6000-


4000-


2000- 20204)CGS(312/116)


10 20 30 40 50 60 70
2 theta (degrees)

A
14000
CGS 112) 47

12000-

Mo(110)
10000-


8000-


6000-


4000-
CGS(220/204)

2000 -CGS(312/116)

200


10 20 30 40 50 60 70
2 theta (degrees)

B
Figure 4-11i. Diffraction patterns of films grown at different rotational speeds. A) 12 RPM. B)
20 RPM.











12000


10000-



8000-

CGS(112)

6000-



4000-
CGS(220)


2000 G (0)CGS(312)

Cu2-xSe I cU2-xSell CU2-xSe lCGS(116)

10 20 30 40 50 60 70
2 theta (degrees)

A
10000
Mo(110)45
9000-

8000-

7000-

6000 -1 CGS(112)

5000-

4000-

3000-

2000 -1 IICGS(220/204)

CGS(312/116)
1000 C2-xSe~l C~~ CU2-xSe


10 20 30 40 50 60 70
2 theta (degrees)


Figure 4-12. Diffraction patterns of films grown at different levels of overall Cu-richness. A)
Cu/Ga = 1.11. B) Cu/Ga = 1.40.