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Pulsed Laser Annealing and Rapid Thermal Annealing of Copper-Indium-Gallium-Diselenide-Based Thin-Film Solar Cells

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PULSED LASER ANNEALING AND RAPID THERMAL ANNEALING OF COPPER-INDIUM-GALLIUM-DISELENIDEBASED THIN-FILM SOLAR CELLS By XUEGE WANG 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 2005

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Copyright 2005 By Xuege Wang

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To my parents and my friends

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iv ACKNOWLEDGMENTS I would like to express my sincere gratitude to my supervisory committee chair (Professor Sheng S. Li), for his support and encouragement in the past 5 years. Without his patience and guidance, none of this work w ould have been possible. I would also like to thank Professors Timothy J. Anderson, Oscar D. Crisalle and Gijs Bosman for serving on my supervisory committee. I extend special thanks to Dr. James C. K eane for his help on device fabrication; Dr. Leon Chen, for supplying of CIGS samples; Professor Omar Manasreh, for his help on thermal annealing; Dr. Valentin Craciun, for his help on laser annealing; Dr. Chia-Hua Huang, for training me on device performance test s; and Dr. Lei Li Kerr, for her valuable discussions. I would like to extend my sincere appreciati on to my colleagues (Jiyon Song, Woo Kyoung Kim, Seokhyun Yoon and Wei Liu), in th e laboratories for their assistance. I could not have accomplished this work without their cooperation and help. Finally I am greatly indebted to my pa rents in China for their constant love, support, and encouragement. I dedi cate this dissertation to them.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES.............................................................................................................x ABSTRACT....................................................................................................................... xv CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW OF THIN-FILM SOLAR CELLS......................................7 Current Status of Photovoltaic Technologies...............................................................7 Thin-Film Solar Cells...................................................................................................8 Amorphous Silicon (a-Si:H) Thin-Film Solar Cells..............................................8 The CdTe Thin-Film Solar Cells...........................................................................9 The Cu(In,Ga)Se2 (CIGS) Thin-Film Solar Cells.................................................9 3 PULSED LASER ANNEALING (PLA) OF FILM PROPERTIES AND DEVICE PERFORMANCE FOR CIGS SOLAR CELLS.........................................................18 Introduction.................................................................................................................18 Experimental Details..................................................................................................19 Results of PLA Treated CIGS Films..........................................................................20 Results of First Set of PLA Treated CIGS Solar Cells...............................................21 PhotoJ-V Measurements of PLA CIGS Cells...................................................21 The Q-E Measurement of PLA CIGS Cells........................................................22 Results of Second Set of PLA Treated CIGS Solar Cells..........................................23 PhotoJ-V Results...............................................................................................23 DarkJ-V Results................................................................................................23 Quantum Efficiency Results................................................................................25 The C-V and DLTS Characterizations................................................................25 Additional PLA Effect Study......................................................................................26 Summary.....................................................................................................................27 4 RAPID THERMAL ANNEALING (RTA) OF FILM PROPERTIES AND DEVICE PERFORMANCE FOR CIGSBASED SOLAR CELLS.........................48

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vi Progressive RTA Treatment on CIGS Solar Cells.....................................................48 The RTA Effects of Separated CIGS Films...............................................................49 The XRD and SEM Results.................................................................................49 Hall-Effect Measurements...................................................................................50 The RTA Effects of Separate d CIGS Solar Cell Devices...........................................50 Experimental Details...........................................................................................50 PhotoJ-V Performance......................................................................................51 DarkJ-V Characteristics....................................................................................51 The S-R and Q-E Performance............................................................................52 Thermal Annealing on CIGS Solar Cells by High Temperature XRD System..53 Summary.....................................................................................................................53 5 FABRICATION PROCESS OF CIGS SOLAR CELLS............................................78 Substrate.....................................................................................................................7 8 Back Contact Layer....................................................................................................80 CIGS Absorber Layer.................................................................................................80 Chemical Bath Deposition ( CBD) of CdS Buffer Layer............................................82 Alternative CdFree Buffer Layer.............................................................................86 Transparent Conducting Oxide (TCO) Layer.............................................................87 Metal Contact..............................................................................................................90 6 CHARACTERIZATION AND MEASUREMENT SYSTEMS FOR CIGS SOLAR CELLS..........................................................................................................96 Hall-Effect Measurements..........................................................................................96 Introduction.........................................................................................................96 Measurement Procedure and Apparatus..............................................................97 Related Studies, Results and Discussion.............................................................98 The Cd-partial-electrolyte treatment on CIGS films....................................98 Hall-effect data of CIGS films supplied by NREL TFPPP team.................99 Photoand DarkJ-V Measurements.......................................................................100 Solar Cell Parameters........................................................................................100 PhotoJ-V Measurement System......................................................................103 Light source................................................................................................104 Temperature control...................................................................................104 Contacts and connections...........................................................................105 DarkJ-V Measurement....................................................................................106 Quantum Efficiency Measurement...........................................................................107 Quantum Efficiency Measurement Instrumentation.........................................107 Monochomator and Monochromatic Light Source...........................................108 Monochromatic Light Chopper.........................................................................109 Bias Light Source..............................................................................................110 Spectral Detector and Synchronous Detection Instrumentation........................110 Spectral Response Measurement Procedures....................................................111 Deep Level Transient Spect roscopy (DLTS) Measurement.....................................112

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vii Features and Principles......................................................................................112 Related Results and Discussions.......................................................................115 7 DEVICE MODELING AND SIMULA TION OF CIGS SOLAR CELLS..............128 Simulation Model of CIGS Solar Cell Devices........................................................128 Simulation Results of CIGS Solar Cells...................................................................130 Conclusions...............................................................................................................132 8 RADIATION EFFECTS ON CI GS FILM AND SOLAR CELLS..........................140 Introduction...............................................................................................................140 ELDRS Effects..................................................................................................141 Total Dose Effects.............................................................................................141 Displacement Damage.......................................................................................142 High-K Dielectrics.............................................................................................146 Experimental Details and Discussion.......................................................................146 9 SUMMARY AND FUTURE WORK......................................................................161 Summary...................................................................................................................161 Future Work..............................................................................................................163 LIST OF REFERENCES.................................................................................................164 BIOGRAPHICAL SKETCH...........................................................................................169

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viii LIST OF TABLES Table page 3-1. Effective minority carrier lifetime ( ) of CIGS films before and after PLA treatment as measured by DBOM............................................................................29 3-2. Hall-effect results for CIGS fi lms before and after PLA treatments........................30 3-3. Effective lifetimes and photoJ-V results of PLA CIGS/CdS samples and devices......................................................................................................................31 3-4. Photo-J-V performance of control cell and PLA-treated CIGS solar cells..............32 3-5. PhotoJ-V performance and darkJ-V parameters for the control cell and selected PLA-treated CIGS cells..............................................................................33 3-6. Results of the DLTS measurements on the control cell and PLA treated CIGS solar cells..................................................................................................................34 3-7. Annealing condition of new PLA samples...............................................................35 4-1. PhotoJ-V results of CIGS solar cells before and after progressive RTA...............54 4-2. CIGS film number and the annealing conditions.....................................................55 4-3. Hall-effect data of NREL CIGS sa mples before and after RTA treatments............56 4-4. Annealing conditions of RTA treated CIGS devices...............................................57 4-5. PhotoJ-V results of separated CIGS solar cells before and after RTA..................58 4-6. PhotoJ-V results of CIGS solar cells before and after RTA by using high temperature XRD system.........................................................................................59 5-1. Conversion efficiencies of CIGS cells grown by non-vacuum processes (by ISET) on different substrates....................................................................................91 6-1. Hall-effect results for the different Cd -partial-electrolyte treatment times on the CIGS samples.........................................................................................................116 6-2. Results from Hall-effect measurements for the CIGS samples..............................117

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ix 6-3. Hall measurement results for the CIS/CGS samples deposited on different substrates (GaAs/glass) by PMEE method.............................................................118 6-4. PhotoJ-V results for EPV CIGS device before and after DLTS scan..................119 7-1. Performance parameters of the CIGS (Eg=1.2 eV) solar cells versus the defect density in the CIGS absorber layer........................................................................133 7-2. PhotoJ-V data of cell#1 before and after progressive RTA treatment (From experiment and AMPS 1D simulation results).......................................................134 8-1. Dose rate and tota l dose of the tested CIGS samples and devices.........................150 8-2. Dose conditions and PhotoJ-V resu lts of CIGS devices before and after radiation..................................................................................................................151

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x LIST OF FIGURES Figure page 2-1. Best performance of va rious thin-film solar cells....................................................11 2-2. Sketch of the layer structure for (a) aSi p-i-n (superstrate), and (b) a-Si n-i-p (substrate) solar cells................................................................................................12 2-3. Schematic sketch of band di agram for a-Si p-i-n solar cell.....................................13 2-4. Device structure of CdS/CdTe thin-film solar cell...................................................14 2-5. Band gap versus lattice consta nt diagram of CIGS solar cell..................................14 2-6. Absorption coefficients of different solar cell materials..........................................15 2-7. PhotoJ-V result of 19.2% CIGS solar cell from NREL (under 1000W/m2, AM 1.5 global spectrum at 25oC)....................................................................................16 2-8. Device structure of a CdS/CIGS-based solar cell....................................................17 3-1. XRD spectra before and after PLA treatments for (a) a CIGS and (b) a CdS /CIGS sample...........................................................................................................36 3-2. Surface morphology of CIGS films (a) without and (b) with PLA treatments at an energy density of 55 mJ/cm2 (SEM images with magnification of 6000x).........37 3-3. Quantum efficiency of CIGS cells with and without PLA treatment......................38 3-4. Spectral response of CIGS cells with and without PLA treatment..........................38 3-5. PhotoJ-V parameters ve rsus different PLA condition...........................................39 3-6. DarkJ-V curves comparing the c ontrol cell to two PLA treated cells...................40 3-7. DarkJ-V curves (semi-log plot) of the control cell and two PLA treated cells......41 3-8. Quantum efficiency (Q-E) versus wave length comparing the control cell to the two PLA-treated cells...............................................................................................42 3-9. The DLTS scans of the controland PLACIGS cell..............................................43

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xi 3-10. Surface morphology of CdS/CIGS films (a ) before and after PLA treatment with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 4000x)..................................44 3-11. Surface morphology of CdS/CIGS films (a ) before and after PLA treatment with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 40000x)................................44 3-12. Theta-2 theta (symmetrical geometry) diffraction patterns of PLA samples and control sample..........................................................................................................45 3-13. Grazing incidence XRD analysis (G IXD) of PLA samples and the control sample (omega=3deg.).............................................................................................46 3-14. Grazing incidence XRD analysis (G IXD) of PLA samples and the control sample (omega=1deg.).............................................................................................47 4-1. Cycle time for Rapid Thermal Annealing (one run)................................................60 4-2. XRD results of RTA treated CIGS films and the control sample............................61 4-3. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 3000x).......................................................................62 4-4. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 10000x).....................................................................63 4-5. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 30000x).....................................................................64 4-6. DarkJ-V curves of tested CIGS solar cell (#D-R1) before and after RTA (300oC, 1-min) treatment..........................................................................................65 4-7. DarkJ-V curves of tested CIGS solar cell (#D-R2) before and after RTA (300oC, 2-min) treatment..........................................................................................66 4-8. DarkJ-V curves of tested CIGS solar cell (#D-R3) before and after RTA (350oC, 1-min) treatment..........................................................................................67 4-9. DarkJ-V curves of tested CIGS solar cell (#D-R4) before and after RTA (350oC, 2-min) treatment..........................................................................................68 4-10. DarkJ-V curves (semi-log plot) of te sted CIGS solar cell (#D-R1) before and after RTA (300oC, 1-min) treatment........................................................................69 4-11. DarkJ-V curves (semi-log plot) of te sted CIGS solar cell (#D-R2) before and after RTA (300oC, 2-min) treatment........................................................................70 4-12. DarkJ-V curves (semi-log plot) of te sted CIGS solar cell (#D-R3) before and

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xii after RTA (350oC, 1-min) treatment........................................................................71 4-13. DarkJ-V curves (semi-log plot) of te sted CIGS solar cell (#D-R4) before and after RTA (350oC, 2-min) treatment........................................................................72 4-14. Quantum efficiency (Q-E) versus wavele ngth of tested CIGS solar cell (#D-R1) before and after RTA (300oC, 1-min)......................................................................73 4-15. Quantum efficiency (Q-E) versus wavele ngth of tested CIGS solar cell (#D-R2) before and after RTA (300oC, 2-min)......................................................................74 4-16. Quantum efficiency (Q-E) versus wavele ngth of tested CIGS solar cell (#D-R3) before and after RTA (350oC, 1-min)......................................................................75 4-17. Quantum efficiency (Q-E) versus wavele ngth of tested CIGS solar cell (#D-R4) before and after RTA (350oC, 2-min)......................................................................76 4-18. PhotoJ-V curves of tested CIGS solar cells before and after (a) 100oC, 30 seconds and (b) 150oC, 30 seconds RTA treatments by using high temperature XRD system.............................................................................................................77 5-1. Layer structure of CIS/CIGS based solar cells.........................................................92 5-2. Schematic profile of the NREL ‘threestage physical evapora tion process for the fabrication of CIGS solar cells’................................................................................93 5-3. Simple CBD experimental set-up.............................................................................94 5-4. The E-Beam evaporation technique fo r depositing metal c ontact films on solar cells.......................................................................................................................... .95 6-1. Schematic representation of the Hall effect sample...............................................120 6-2. Typical four-point van der Pa uw and Hall effect measurements...........................120 6-3. The equivalent circuit diagram of a CIGS PN junction solar cell..........................121 6-4. The I-V characteristics under dark and illumination conditions of a PN junction solar cell.................................................................................................................121 6-5. Apparatus and block diagram of I-V measurement system for the CIS-based cells.........................................................................................................................1 22 6-6. The photoJ-V measurement and analysis for solar cells using the LabVIEW program. (The tested CIGS solar cell is fabricated by EPV.)................................122 6-7. The dark-J-V measurement and analysis for solar cells using the LabVIEW program. (The tested CIGS solar cell is fabricated by NREL.).............................123

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xiii 6-8. The block diagram of a spectral response measurement system for the CIS-based solar cells................................................................................................................124 6-9. The spectral response and quantum e fficiency measurements for solar cells using LabVIEW program. (The tested CIGS so lar cell is fabricated by UF.)........124 6-10. The DLTS scans for (a) EPV device at VR = -0.1V, VH = 0.3V, and W = 10ms. And (b) An Arrhenius plot obtained from the DLTS scans shown in (a)..............125 6-11. The DLTS scans for (a) EPV device at VR = 0.5V, VH = 0.7V, and W = 10ms. And (b) An Arrhenius plot obtained from the DLTS scans shown in (a)..............126 6-12. DarkJ-V curves of tested EPV CIGS device before and after DLTS measurement...........................................................................................................127 6-13. PhotoJ-V curves of tested EP V CIGS device before and after DLTS measurements.........................................................................................................127 7-1. The schematic energy band diagram of a typical ZnO/CdS/CIGS solar cell under equilibrium condition.............................................................................................135 7-2. Performance parameters of CIGS (Eg=1.2eV) solar cells versus the defect density of CIGS absorber layer..............................................................................136 7-3. PhotoJ-V curves of cell#1 before and after progressive RTA treatment (From AMPS 1D simulation results)................................................................................137 7-4. DarkJ-V curves of cell#1 before and after progressive RTA treatment (From AMPS 1D simulation results)................................................................................138 7-5. Quantum efficiency of cell#1 before and after progressive RTA treatment (From AMPS 1D simulation results)................................................................................139 8-1. Five basic effects of a defect energy level (Et) on the electrical performance of a device.....................................................................................................................152 8-2. Gamma absorption crosssection versus photon energy........................................153 8-3. Photon interaction proba bilities versus photon energy..........................................154 8-4. Photograph of irradiated CI GS films and the control film.....................................155 8-5. The XRD results of irradiated CIGS sa mples (#1-4) and the control sample (#5).156 8-6. PhotoJ-V results of CIGS device #1 (cell#1) before and after total dose of 2.12 Mrads (Si) radiation (Device from GSE)...............................................................157 8-7. PhotoJ-V results of CIGS device #1 (cell#2) before and after total dose of 2.12 Mrads (Si) radiation (Device from GSE)...............................................................158

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xiv 8-8. PhotoJ-V results of CIGS device #2 (cell#1) before and after total dose of 6.5 Mrads (Si) radiation (Device from SSI).................................................................159 8-9. PhotoJ-V results of CIGS device #2 (cell#2) before and after total dose of 6.5 Mrads (Si) radiation (Device from SSI).................................................................160

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xv 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 PULSED LASER ANNEALING AND RAPID THERMAL ANNEALING OF COPPER-INDIUM-GALLIUM-DISELENIDEBASED THIN-FILM SOLAR CELLS By Xuege Wang August 2005 Chair: Sheng S. Li Major Department: Electrica l and Computer Engineering Effects of Pulsed Laser Annealing (PLA ) treatment on the film properties and the performance of CIGS solar cells have been studied under various annealing conditions. This technique has been used for the first ti me to modify near-surface defects and related junction properties in Cu(In,Ga)Se2 (CIGS) solar cells. CIGS films deposited on Mo/glass substr ates were annealed using a 25 ns pulsed laser beam (248 nm wavelength) and a 250 ns pulsed laser beam (308 nm wavelength) with a larger beam size at se lected laser energy densities in the range of 20 to 110 mJ/cm2 and pulse number in the range of 5 to 20 pulses. The narrowing of X-Ray Diffraction (XRD) peak, new shoulders of Grazing Inci dence X-ray Diffraction (GIXD) and the increase of Scanning Electron Microscopy (SEM) surface feature size suggest near surface structure changes. The Dual-Beam Op tical Modulation (DBOM) and Halleffect measurements indicate PLA treatment increases the effective carrier lifetime and mobility as well as the sheet resistan ce. In addition, several annealed CdS/CIGS films processed

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xvi by PLA were fabricated into solar cells a nd characterized by Phot oand DarkJ-V and Quantum Efficiency (Q-E) measurements. Si gnificant improvement was observed in the overall cell performance, diode quality, a nd spectral response when compared to pre-annealed cells. DeepLevel Transient Spectroscopy (DLTS) results showed a 50% reduction of the density of shallow defect trap after low-power PLA treatments. The energy density of the laser beam and the pulse number were found to play key roles in modifying the optical and elec trical properties of the CIGS films and hence the cell performance. Results suggest that the optim al PLA energy density and pulse number are around 30 mJ/cm2 and 5 pulses, respectively. A comprehensive study of the effects of Rapid Thermal Annealing (RTA) on the film properties and the performance of CIGS solar cells has been carried out in this dissertation. CIGS samples and devices were characterized by using XRD, GIXD, SEM, Hall effect, Photoand DarkJ-V and Q-E measurements before and after RTA treatment under various ramp up and down rates, peak temperatures, holding times, and ambient conditions. Results show that progressive RT A treatments could significantly improve the overall uniformity and performance of la rge-area CIGS solar cells. Under low RTA temperatures, the surface composition and morphology remain unchanged. The simple RTA treatment on CIGS cells shows an in crease of quantum efficiency and some improvement of cell performance. The estima ted optimal annealing temperature should be between 200 and 300oC with a holding time of 1minute or less. Device simulation using one-dimensional (1-D) AMPS program has also been carried in this work for a typical CIGS cell with different defect densities. A well fitting between simulation results and progressive RT A results has been obtained. The results

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xvii show that the device performance and spect ral response can be positively improved by reducing the defect density of CIGS absorber layer. In addition, Gamma-Ray radiation tole rance study on several CIGS samples and solar cells were performed under different tota l dose by varying the sample displacements and exposure times. XRD and PhotoJ-V measur ements were taken before and after each radiation. The results show that the su rface morphology, crystallinity and the device performance of CIGS sola r cells change under certai n radiation dose condition.

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1 CHAPTER 1 INTRODUCTION Solar cells are now the most important a nd viable power sour ce for satellites and space vehicles. Solar cells have also been us ed successfully in large-scale terrestrial power generation applications. With world en ergy consumption growing each year, most of the energy comes from petroleum, natu ral gas, and fossil fuel, and nuclear power generation. These power systems are polluting, costly, or will be exhausted in the near future. Therefore, a renewable energy source such as solar energy is needed to replace these systems and protecting our natural envi ronment and future use. The sun has been radiating energy for over 500 million years, and is expecting to continue indefinitely. Photovoltaic (solar) en ergy conversion becomes an excelle nt candidate because it is clean, inexhaustible, uninte rruptible, and nonpolluting. Sola r cells use the internal photovoltaic (PV) effect in semiconductors, and are capable of providing electricity directly from the sun for a wide variety of applications with the advantage of long-duration power generation at lower maintenance cost. Recent interest has increased in research and development of low-cost, flat-panel solar cells, thin-film PV de vices, concentrator systems, and many innovative concepts. In 2004, worldwide PV module production has generated 750 MW electric power for terrestrial applications. The PV technology is growing at more than 20% annual growth rate, and is expecting to produ ce 5 to 10% of electricity used in United States in 10 to 20 years.

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2 As a simple definition, the basic functi on of a solar cell module is to absorb incident sun light and to crea te electron-hole pairs and collect ed at the external contacts of solar cells to produce elec tricity for a wide variety of applications. A semiconductor p-n junction structure is commonly used in solar cells. The useful spectral range to convert the incident sun light into electricity is usually de termined by the bandgap (Eg) of the semiconductor. Separation and transport of the photo-generated electron – hole pairs are accomplished by the built-in electric fiel d formed in the depletion region of the p-n junction. A transparent conductive oxide (TCO) or ohmic contact grid is normally used for current collection in the front, and a metal c ontact is used in the back side of the solar cell. Since the first silicon solar cell was repor ted by Chapin, Fuller, and Pearson in 1954, the solar cell has evolved from being a low-efficiency device to being a major power generation source for the spacecraf t and many terrestrial power generation systems. Currently, high efficiency singleand multi-junction solar cells based on III-V compound semiconductors such as GaAsand In Pmaterials have been developed for space applications, while crystalline silic on solar cells are still the dominant PV technology for terrestrial power generation appl ications. Commercial silicon solar cells available today are made from solar-grade si ngle crystalline silic on or polycrystalline silicon materials. Although high efficiencies are achieved with single crystalline silicon, the silicon technology is not expe cted to become a low-cost PV technology. One reason is that silicon is an indirect band gap semi conductor which requires a thick absorber layer (about 250-400 m thick) to absorb 90% of the useful sun light for electricity generation.

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3 In the past few years, much effort has b een devoted to developing various low-cost, high-efficiency, and high-stability solar cells for both terrestrial and space power generation and for applications in consumer el ectronics. Efficient solar cells can be made by thin-film technology. By using semiconducto r material with high absorption, a film thickness of a few micrometers is sufficient to effectively collect the sun light. A wide variety of absorber materials is av ailable for solar ce ll applications. Today, the most common thin-film solar cells are made from amorphous silicon materials. The commercial deposition proce ss of a-Si:H using plasma-enhanced chemical vapor deposition (PECVD) technique is comp atible with largearea deposition and low temperature processing. This allows the use of a wide variety of inexpensive substrate materials. The a-Si:H films can be easily doped by adding phosphorus or boron containing gases during deposit ion process for nand p-t ype doping. The optical band gap of a-Si is typically around Eg 1.7eV and can be tuned. For example, the band gap energy of a-Si:H can be incr eased by alloying with carbon or oxygen, and decreased with incorporation of germaniu m to form a-SixGe1-x (01x ) films. The energy band gap can also be fine tuned by changing the hydrogen content using different deposition parameters and methods. The a-Si:H thin film solar cells have reached an efficiency of 13.1%, while long term stability is a key issue for a-Si:H solar cells. Other thin film PV technologies based on CdTe (with a ma ximum efficiency of 16.5% [1]), and CdS/CuIn(Ga)Se2 (CIGS) material systems ha ve shown great promising for large scale terrestrial power systems. CIGS thin film solar cells with efficiency exceeding 19% AM1.5G have been demonstrated recently by NREL research team. Thin-film PV technology benefits from low material consumption and lo w price as compared to

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4 crystalline silicon solar cells Scaling the PV technology fr om single sola r cells to large-area PV modules is straightforward si nce many cells can be interconnected from material deposited on one substrate in the fo rm of stacked film layers. Compared to the crystalline material, thin-film solar cells can be manufactured with less energy input. This shortens the energy payback time (the time needed for photo-generated energy output to equal the energy consumed to produce the device). Specific advantages of the CuIn(Ga)Se2 a lloy are its wide compositional tolerance and a direct band gap material high optical absorption in th e visible spectrum. One major drawback for large scale production is the lim ited extraction rate of indium from mining. Cost and conversion efficiency are the two key factors that determine the compatibility of a solar cell. Thin-film solar ce lls give the best hope for obt aining PV devices with high efficiency and low cost. Copper indium disele nide (CuInSe2 or CIS) and copper indium gallium diselenide (CuInGaSe2 or CIGS) film s are the most promising materials of all thin-film solar cells for achieving these goa ls. Such material has certain exceptional characteristics particularly suitable for phot ovoltaic heterojunction applications. CIGS is a direct band gap material, which mi nimizes absorber layer thickness (1-2 m). Thin-films of CIS/CIGS are p-type conductivity, and a surf ace inversion layer can be formed when deposited with a CdS buffer layer. Therefore, an n-p heterojunction structure can be formed solar cell applic ation. CIGS material with 30% of gallium content has a band-gap energy of around 1.3 eV at 300 K, which is nearly ideal for a photovoltaic device operating in the sola r spectrum. There is no reported photon degradation of CIGS solar cells because of good thermal stabili ty. It can absorb the solar spectrum within a few micrometers (1-2 m) with high optical absorption coefficient.

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5 The CIGS thin-film solar cells have recentl y achieved efficiencies in excess of 19.5% AM1.5G. However, further optimization of the device performance could be substantially accelerated by a better understanding of th e processing effects on the interface between the CdS and CIGS layers and the tran sport properties of CIGS films. The pulsed laser annealing (PLA) and hal ogen lamp based rapid thermal annealing (RTA) on silicon-based devices and solar cell s have been extensively investigated in recent years [2-9]. Today, PLA technique is wide ly used to activate boron ion implants in silicon wafers to remove undesirable boron clus tering, defect evolution, or damage to the lattice created by implant and to recrystalyze th in Si amorphous films. The possibility of using this approach to modify the near surf ace defects in CIGS thin-film solar cells was motivated by the positive results reported for laser processing of silicon wafers. The basic idea is to promote atom mobility by local heating, on a nanometer-length scale; and thus confine the impact of processing to the near -junction region of the device. Results from characterizing the CIGS films and cells sugges t that interfacial recombination near the CdS/CIGS metallurgical junction is a majo r limitation to optimizing of the device performance. The PLA treatment promises to pr ovide defect annealing in the near-surface region, while preserving the beneficial composition gradients in CIGS films. Another powerful annealing technique using halogen-ba sed RTA process has been widely used in the semiconductor industry. It offers several advantages such as short cycle time, reduced thermal exposure and lotsize flexibility compared to conventional furnaces. Strong demand for thermal-budget reduction and cycle-time reduction had made RTA treatment a popular thermal-pr ocessing method in recent years. This technique has been successfully app lied to the fabrication of low-cost,

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6 low-thermal-budget silicon solar cells. Conversio n efficiency of more than 17% has been reported for RTA processing time of less than 3 minutes [10]. The main scope of this research is to investigate the effects of the above two techniques (PLA and RTA) on CIGS films pr operties and solar cell performance. Results show that both of these two techniques can positively improve the CIGS cell’s performance. In addition, device modeling a nd investigation of ra diation tolerance of CIGS-based solar cells are well discussed in this dissertation.

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7 CHAPTER 2 LITERATURE REVIEW OF THIN-FILM SOLAR CELLS Current Status of Photovoltaic Technologies The photovoltaic (PV) industry has grown at an annual rate of over 20% since 1990’s. Many PV manufacturers have invested in expanding their producti on facilities during the last few years. New PV companies have also been formed in Japan, USA, and Europe to increase the PV module produc tion. Remarkably, Japan manufactures produce almost half of today’s world PV m odules for terrestrial power generation use. The worldwide PV module produc tion has reached 750 MW in 2004. In the past few years, most of the PV systems are built using single-crystalline silicon wafers. The key issue and drawback in silicon based PV technology is the reduction of wafer costs, which is difficult to attain with single crystal silicon wafers. As a result, the PV industry has looked into developing low cost thin film PV technologies using alternative semiconductor ma terials to replace the silicon-wafer-based PV technology. Thin-film solar-cell producti on accounted for 13% of all PV production in 1999. Single-crystalline and multi-crystalline silicon solar cells are still accounted for most solar-cell production (~84%). Other crys talline products (such as ribbon growth or thin silicon films) contributed only about 3% of the PV modules. The main difficulties of thin-film technology are its bad image of long term stability from the first-generation amorphous silicon PV modules; its low conversion efficiency as compared to crystalline silicon solar cells; the toxicity of some of th e materials in fabrication and disposal; and its

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8 short lifetime. All these problems must be re solved if thin-film technology is to take a major share of the booming PV ma rket in the future [11-14]. Thin-Film Solar Cells The biggest advantage of thin-film t echnology is that it greatly reduces the manufacturing cost of solar cells by reduci ng material and deposition cost on large area low cost foreign substrates [15-19]. Typical thin-film solar-cell materials include a-Si:H alloys, CdTe, Cu(In, Ga)Se2 (CIGS), poly-Si, c-Si/poly-Si, and dye/TiO2. The most advanced and commonly used thin-film tec hnologies are a-Si-alloys, CdTe, and CIGS. Figure 2-1 shows the best performan ce of these thin-film solar cells. Amorphous Silicon (a-Si:H) Thin-Film Solar Cells Hydrogenated amorphous silicon (a-Si:H) is distinguished from the crystalline silicon (c-Si) by the l ack of long-range order (disorder) in the atomic structure and by its high bonded-hydrogen content (~10% in device quality a-Si:H). Although the overall properties of a-Si:H and c-Si materials are similar, the long range disorder in a-Si:H distorts bond lengths and bond angl es, which introduces large densities of broken-bond defects and micro voids. The di sorder relaxes the mo mentum conservation rules associated with crystalline materials, thus leading to high er optical-absorption coefficients than in c-Si for photons with energies (h ) greater than the bandgap energy (Eg). The optical band gap of a-Si:H is 1.7 eV and can be fine-tuned by changing the hydrogen content as a function of specific deposition parameters and methods. In addition, a-Si has a higher optical absorption coe fficient than c-Si in the visible range of the spectrum and hence the absorber laye r thickness of a-Si can be than 1 m. Typical a-Si:H-based solar cells have a p-i-n superstrate (Fig ure 2-2a) or n-i-p substrate (Figure 2-2b) structure, depe nding on the deposition sequence of doped and

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9 intrinsic layers [24]. For both structures light enters through the p-layer, which efficiently supports hole collection in the de vice. Heavily doped wide band gap a-SiC:H and a-SiO:H alloys or microcrystalline Si f ilms are applied as p-doped window layers to reduce absorption losses. Electrons and holes gene rated in the i-layer are driven to the nand player, respectively, by the intern al built-in electric field (Figure 2-3). The advantage of its unique material prope rties also makes thin-film a-Si:H an excellent candidate in sophi sticated multi-junction solar cell design. Achieved stable conversion efficiencies in ex cess of 13% [20-23] have b een reported for multi-junction a-Si:H thin-film solar cells. The CdTe Thin-Film Solar Cells With a direct band gap of Eg = 1.45 eV [25] and steep optical absorption edge, thin-film CdTe solar cells can absorb 90% of the incident sunlight in 1-2 m absorber layer thickness, and hence are considered as a promising thin-f ilm PV technology. The world record so far was achieved by NREL w ith16.5% conversion efficiency. Theoretical maximum efficiency of CdTe solar cells is over 27% [26, 27]. The depositing sequence can be changed fo r CdS/CdTe solar cells; the frontwall type and backwall type. The most common devi ce structure of CdTe solar cells is the backwall type (Figure 2-4 [25]). The CdTe layer usually is deposited by close-space vapor transport technique. Acting as a filter fo r sunlight, and with a cut off wavelength of 514 nm, n-type CdS becomes the ideal partner for p-type CdTe absorber to form n-p heterojunction solar cells. The Cu(In,Ga)Se2 (CIGS) Thin-Film Solar Cells The compounds of CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) with their chalcopyrite structure, are among the most promising materi als used in thin film solar cells [28, 29].

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10 The main advantages of CIS/CIGS-based so lar cells are the high conversion efficiency and low cost of materials. Although some problems still prevent the large-scale commercialization and use of CIGS cells fo r terrestrial power ge neration, significant progress has been made in CIS/CIGS-based PV technology, and single cell efficiency of 19% and module efficiency exceeding 10% AM 1.5G have been achieved in the last couple years. The CIGS absorber is direct band gap ma terial with bandgap tunable by adjusting the ratio of In to Ga, to maximize absorpti on of the solar spectrum. Its bandgap can be varied from 1.02 eV (CIS) to 1.68 eV (CGS) (F igure 2-5). Thin-film CIGS material also has a higher absorption coefficien t than any other thin-film ( > 105 cm-1) (Figure 2-6), which allows almost 99% of the incoming light to be absorbed with the first micron of the material according to the solar spectrum. Thus, the highest-efficiency (19.2% from NREL) CIGS solar cell (Figure 2-7) is fast approaching the efficiency of already commercialized multi-crystalline silicon cells (20.4%) and is higher than other thin-film PV technologies (Figure 2-1). The typical structure of CIGS solar cel ls is shown in Figure 2-8. The CIGS absorber layer is typically deposited on a Mo-coated soda-lime glass (SLG) substrate (Mo/SLG by using PVD), sputtering, PMEE, or RTP deposition technique. A 500 CdS buffer layer is deposited on t op of the CIGS absorber layer by CBD technique, followed by deposition of a TCO layer su ch as sputtered ZnO films. Finally, a 500 Nifilm and a 3 m Al metal contact layer are depo sited by using electron-beam evaporation technique. Chapter 5 gives detailed description of each layer and device fabrication processes.

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11 Figure 2-1. Best performance of various thin-film solar cells.

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12 (a) (b) Figure 2-2. Sketch of the layer st ructure for (a) a-Si p-i-n (superstrate), and (b) a-Si n-i-p (substrate) solar cells.

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13 (b) Figure 2-3. Schematic sketch of band diagram for a-Si p-i-n solar cell.

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14 Figure 2-4. Device structure of Cd S/CdTe thin-film solar cell. Figure 2-5. Band gap versus lattice cons tant diagram of CIGS solar cell.

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15 Figure 2-6. Absorption coefficients of different solar cell materials.

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16 Figure 2-7. PhotoJ-V result of 19.2% CI GS solar cell from NREL (under 1000 W/m2, AM 1.5 global spectrum at 25oC).

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17 Figure 2-8. Device structure of a CdS/CIGS-based solar cell.

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18 CHAPTER 3 PULSED LASER ANNEALING (PLA) OF FILM PROPERTIES AND DEVICE PERFORMANCE FOR CIGS SOLAR CELLS Pulsed Laser Annealing (PLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se2 (CIGS) solar cells. Several film surface and de vice performance characterizations were employed to investigate the effects of PLA on CIGS film and solar cells. In this chapter, the annealing effects, optimal conditions and characterization results are discussed in detail. Introduction The PLA technique is widely used to activ ate boron ion implants in silicon wafers and to remove undesirable boron clustering, defe ct evolution, and da mage to the lattice created by the implantation process. Benefici al effects of PLA are derived in part from selective absorption and limiting elevated temperature processing to the near-surface region [30, 31]. The possibility of using this approach to modify the near-surface defects and crystalline structure in Cu(In,Ga)Se2 (CIGS) thin-film solar cells was motivated by the positive results reported for PLA processing of silicon wafers. The basic idea is to promote atom mobility by local heating on the nanometer length scale and thus confine the impact of processing to the near-junction region of the device. The CIGS cells contain a thin (~ 50 nm) n+ CdS buffer layer deposited on a th ick (~ 1 to 2 m) p-type CIGS absorber. Although the exact location of th e electrical junction relative to the metallurgical one is not known, it does lie near the surface of the ab sorber. Furthermore, analyses of CIGS film characterization resu lts and cell performance data suggest that

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19 interfacial recombination near the CdS/CIGS metallurgical junction is a major limitation to the device performance. The possibility of repairing damage near this shallow junction, while preserving the composition gradients in the bulk CIGS films motivated exploration of PLA treatment. In this work most of our CIGS absorber films were annealed using a 248 nm pulsed laser beam and the propertie s of the films and subsequent device performances were compared to those for untre ated films. Furthermore, CIGS films were annealed using a 250 ns pulsed 308 nm laser beam with larger beam size at selected laser energy density in the range 30 to 110 mJ/cm2. Results of XRD, GIXD and SEM surface characterizations suggested near surface structure changes by PLA. Experimental Details The CIGS samples used in this study were provided by the National Renewable Energy Lab (NREL) and Shell Solar Inc. (SS I), and both sample sets were grown on Mo coated soda-lime glass substrates. For the Hall-effect measurements, however, the CIGS films were grown on the insula ting soda-lime glass (SLG) substrates. A thin (50 nm) CdS buffer layer was deposited on the CIGS samp les using Chemical Bath Deposition (CBD) at 75oC for 30 min. The PLA treatment was carried out using a pulsed 248 nm line derived from a KrF excimer laser system [32] In the main part of this study the laser pulse width was fixed at 25 ns and the energy density was varied in the range 20 to 60 mJ/cm2 and the number of pulse cycles in the range 5 to 20. To study the effect of PLA on the CIGS film properties, PLA treatments were performed on the CIGS films with and without CdS buffer layers. After PLA treatme nts, a ZnO window layer was deposited on the CdS/CIGS/Mo/SLG sample by RF sputtering. Subsequent metallization (Ni-Al front contact grids) was carried out by e-beam ev aporation through a sh adow-mask. Finally, finished devices were produced by cutting th e sample into separate cells with 0.429 cm2

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20 active area and attaching wires with Indium bumps on the Mo-coated glass substrates for back contacts. Detailed fabrication process is discussed in Chapter 5. The performance of these cells was then tested as described in the next section. Results of PLA Treated CIGS Films The non-destructive Dual Beam Optical Modulation (DBOM) characterization method was used to measure the effective car rier lifetimes in the CIGS absorbers [33, 34], and to evaluate the effect of PLA tr eatment on the performa nce of CIGS cells. Within the sensitivity of the DBOM technique the effective carrier lifetime was found to increase for CIGS films on Mo/SLG annealed using 5 cycles of laser pulse with an energy density in the range 30 to 60 mJ/cm2. The results (Table 3-1) show that a low energy PLA treatment can increase the effectiv e carrier lifetime of the annealed samples and improve the performance of CIGS cells. XRD patterns of CIGS and CdS/CIGS samp les before and after PLA treatments show substantial narrowing of the diffraction peaks that belong to CIGS (Figure 3-1). These results can be interprete d as an improvement of the layer’s crystallinity after PLA treatment, which is consistent with the obs erved increase of the grain size (SEM) and the effective carrier lifetime in the CIGS films. As illustrated by SEM micrographs (F igure 3-2), the surface morphology and apparent grain size increased upon laser annea ling. This result sugge sts that the energy density used was sufficient to cause atomic rearrangement in the near surface region, and thus the potential exists to modify the atomic defects in the near surface region. Based on the encouraging DBOM results from the initial PLA treated CIGS samples, a second set of experiments were performed in which the energy density and number of the incident laser pulse were va ried. Hall-effect measur ements were made on

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21 the CIGS samples without a Mo layer prior and after annealing to determine the effects of PLA treatment on the carrier concentration, carri er mobility, and sheet resistivity of CIGS films. Four CIGS films deposited on the glass substrate underwent pulsed PLA treatments at room temperature. Hall-effect measurem ents (using an MMR Hall and Van Der Pauw measurement system) were made for all samples before and after the PLA treatments. The results (Table 3-2) show a significant in crease in the values of Hall mobility and a decrease in film resistivity following PLA treatment. The carrier mobility in the PLA treated CIGS films were 3 to 4 times greater than the values befo re annealing. Although the hole concentration decreased slightly af ter annealing, film resi stivities decreased by 72 and 64% for CIGS films (samples H1 a nd H2) treated at an energy density of 20 mJ/cm2 with 10 and 20 laser pulses, respectivel y. At an energy density of 40 mJ/cm2, the film resistivity was reduced by more than 95% from that of the non-annealed samples (H3 and H4). Thus, both the energy density and the number of laser pulse play an important role in determining the resistivit y of PLA treated CIGS absorber layers. Results of First Set of PLA Treated CIGS Solar Cells PhotoJ-V Measurements of PLA CIGS Cells Four CIGS/CdS samples were annealed by a 50 mJ/cm2 laser beam with different pulse number. Two samples were followed by a 100 extra CdS buffer layer re-growth after PLA treatment on the CIGS samples initially coated with a 400 CBD CdS buffer layer, and one control sample without any treatment. These samples were then fabricated into cells fo r testing. The DBOM and photoJV results (Table 3-3) show an increase in the effective carrier lifetimes on the PLA treated samples. No explicit improvements, however, were found in the photoJ-V results of the annealed cells. The

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22 data also show slight decreasing in the f ill factor and conversion efficiency of cells annealed with 20 pulses PLA compared with the 10-pulse-annealed cells. Some high energy density (i.e., 80 mJ/cm2) PLA treatments were also used on other CIGS samples, and the results show a drastic reduction in the cell efficiency. These results suggest that an optimal PLA energy density should be less than 50 mJ/cm2, and no significant influence on the cell performance because of the additional CdS buffer layer re-growth was found in this study. The Q-E Measurement of PLA CIGS Cells Two CIGS films with a 500 CdS buffer laye r were annealed at an energy density of 50 mJ/cm2, and then fabricated into cells. To study the effect of pulsed PLA treatment, the spectral response and quantum efficiency (Q-E) were measured on these cells. The results (Figures 3-3, 3-4) indicate that for incident light with wa velengths greater than 650 nm, the Q-E and spectral response of the PLA cells are higher than those of the control cell, indicating that the PLA treatment increases the effective carrier lifetime and diffusion length in the absorber layer and hence increases the short-circuit current density in comparison to the control cell without PL A treatment. In the short wavelength regime ( < 0.65 m), however, the Q-E and spectral resp onse decrease after PLA treatment, which suggest damages near the interface re gion of CIGS/CdS films by the laser beam. As a result, the surface recombination veloci ty is increased and the Q-E and spectral response are lower in the shorter wavelength re gion. It is also noted that the values of Q-E and spectral response for sample annealed with 20 cycles lase r pulse were found to be lower than the sample with 10 cycles of annealing pulse with same energy density.

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23 Results of Second Set of PLA Treated CIGS Solar Cells PhotoJ-V Results Since the previous results suggest that an optimal PLA energy density should be less than 50 mJ/cm2, an additional set of CIGS/CdS samples treated by PLA with different energy densities (less than 50 mJ/cm2) and pulse number were fabricated into finished devices. Six CdS/CIGS samples trea ted by PLA with selected energy density and pulse number plus one control sample wit hout PLA treatment were fabricated into finished devices. There were five cells on each device and each cell was tested using a photoJ-V system at room temperature. The PLA conditions and averaged photoJ-V results of these devices are summarized in Tabl e 3-4 and Figure 3-5. Note that the data in Table 3-4 were averaged from the measured values of five cells on each device and the standard deviation computed on the 5cel l measurements. In these experiments, improvements were found in the photoJ-V results of the annealed devices under 30 mJ/cm2, using 5 or 10 pulses (CIGS-D1 and CIGS-D2), as compared to results from the control sample (Ctrl-D). The data also show degradation in cell pe rformance parameters such as open circuit voltage ( Voc) short-circuit current (Jsc), Fill Factor ( F.F. ), and cell efficiency (c) of the devices laser a nnealed with energy densities higher than 30 mJ/cm2. It was also found that for devices annealed at the same energy density, the fill factor and conversion efficiency of the 5cycle annealed devices are hi gher than those of 10-cycle annealed devices. These results suggest that an optimal PLA energy density and pulse number are approximately 30 mJ/cm2 and 5 pulses, respectively. DarkJ-V Results To better understand the specific difference s between the control cell and the two 30 mJ/cm2 PLA treated cells, darkJ-V and quantum efficiency (Q-E) measurements as

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24 well as theoretical calculations of device parameters were made for selected cells from each of the three devices (Ctrl-D, CIGS-D1 and CIGS-D2). The J-V characteristics for a p-n junction solar cell under uniform illumination condition [35, 36] are expressed by Equation 3-1. ph sh s sJ R J R V nkT J R V q J V J ) 1 ) ( (exp ) (0 (3-1) Where Jph, Jo, n Rs and Rsh are the photocurrent density, sa turation current density, diode ideality factor, series resistance, a nd shunt resistance, respectively. To study the effect of pulsed PLA treatm ent on the diode characteristics, photoand darkJ-V measurements were performed at room temperature for each the selected cell. The measured parameters and calculated values of Jo, n and Rs [37, 38] are summarized in Table 3-5 (note that Rsh is quite high in all tested CIGS cells). From the darkJ-V curves (Figure 3-6, 3-7), both dark current density and saturation current density, Jo, of the PLA treated cells ar e consistently smaller as compared to the control cell. These curves suggest that the recomb ination current through surface defects dominates the darkJ-V curv e under forward bias condition before PLA treatment (with diode ideality factor n >4). After the PLA treatment, a significant reduction in the dark current density was obs erved apparently because of reduction of surface defect density, and the recombination current was dominated by bulk defects in the junction space charge region at higher bias (n 2). These results indicate that defects in the surface region and inte rface of absorber and buffer layers can be effectively reduced by PLA treatment, thus decreasing the recombinati on of minority carriers via interface states and defects in the juncti on space-charge region of CdS/CIGS cells. Although cells treated by the PLA e xhibit higher series resistance ( Rs), the negative effect

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25 of Rs increase on the diode characteristic remains relatively small because of the extremely low value of Jo. Quantum Efficiency Results The quantum efficiency (Q-E) was measur ed on the control and PLA treated cells. The results (Figure 3-8) indicate that for inci dent light with wavele ngths greater than 480 nm, the Q-E of the PLA cells is higher than that of the control cell, indicating that the PLA treatment improves the effective carrier lifetime and diffusion length in the absorber layer and hence increases the short-circuit cu rrent density in comparison with the control cell. In the shorter wavelength regime ( < 0.48 m), no improvement of Q-E was observed between the control cell an d the PLA treated cells. It is also noted that values of the Q-E for cell annealed with 10 cycles of lase r pulse were found to be lower than that of the cell with 5 cycles of la ser pulse at same energy dens ity, again suggesting that the optimal laser pulse number is between 5 and 10 cycles. The C-V and DLTS Characterizations In order to investig ate the defect property change after PLA processing of CIGS cells, the Deeplevel transient spectro scopy (DLTS) and C-V measurements were employed on 2 finished CIGS devices with 30 mJ/cm2, 10 pulses and without PLA treatment. The DLTS measurement wa s performed with a reverse bias (VR) of 0.5 V, a trap-filling pulse amplitude of 0.7 V, and a saturation pulse width of 10 ms. The DLTS spectra were shown in Figure 3-9 and the DLTS measurement results were summarized in Table 3-6. As shown in Table 3-6. A minor ity carrier trap with an activation energy of 0.065 ~ 0.069 eV, which was quite close to the shallow donor energy level (~0.06 eV) known as the selenium vacancy (VSe), was detected, and its density was reduced by about 50% after PLA treatment.

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26 Additional PLA Effect Study Furthermore, CIGS films were annealed using a new 250 ns pulsed 308 nm laser beam with lager beam size in France at sel ected laser energy density in the range 30 to 110 mJ/cm2. XRD, GIXD and SEM surface character izations were made which suggest near surface structure changes. The sample number and annealing condition are shown in Table 3-7 below. To study the laser annealing effect on f ilm surface morphology by this new pulsed laser annealing system, 5 identical CdS/CIGS films were annealed at selected laser energy densities in the range 30 to 110 mJ/cm2, 5 pulses. As illustrated by SEM micrographs (Figure 3-10, 3-11), the surface morphology and apparent grain size increased upon laser annealing wi th energy density of 70 mJ/cm2 or higher. This result suggests that the selected energy densities we re sufficient to cause atomic rearrangement in the near surface region, and thus the potential exists to modify the atomic defects in the near surface region. From the XRD and GIXD plots (Figure 3-12, 3-13 and 3-14), new CdS peaks and shoulders (at 25, 28.5, and 48.3 degree) were observed as the energy density was increased, suggesting a signifi cant improvement of the crysta llinity of the buffer layer after the PLA treatment. For Figure 3-14, by using a smaller grazing incident angle ( =1o), the analysis is more sensitive to the sa mple surface. As one can see, the new CdS peaks becomes shaper compared to Figure 3-12 and 3-13, indicating a higher crystallinity for the topmost part of the buffer layer. Co mparing Figure 3-14 with 3-12 and 3-13, we found that the film surface changes more than bulk. Close look at the new peaks in all figures, we found that the peaks appear only wh en the energy density is equal or greater than 70 mJ/cm2, (sample #3, 4 and 5) which suggest 70 mJ/cm2 might be a critical

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27 threshold PLA condition. This energy density value could represent the melting threshold for the buffer layer. Summary The results of pulsed PLA treatment on the film properties and the performance of CIGS solar cells are very encouraging. Ex amination of the structural and electrical properties of films from 2 different sources clearly showed the annealing step modified the near surface region. Increased surface feature size evidenced in SEM photos and CIGS diffraction peak narrowing is consistent with increased crystallinity. Furthermore, DBOM measurements of the effective carrier lifetime indicated the lifetime could be increased by as much as a factor of 2.75. Hall measurements of CIGS samples deposited on SLG revealed PLA treatment increased the mobility and resistiv ity and decreased the net free hole concentration of each sample, c onsistent with the hypothesis of annealing out electrically active defects in the near su rface region. Based on a parametric study, the best PLA result was obtained with a pul sed laser energy density of 30 mJ/cm2 and 5 pulse cycles. J-V and Q-E measurements were also made to study the eff ect of PLA treatment at the best condition on the performance of CI GS cells. The results show that pulsed PLA treatment has a beneficial effect on the cell performance with the cell efficiency increasing from 7.69 to 12.22 and 13.41% after annealing 2 different samples prior to device processing. The energy density of the laser beam and the number of pulse cycle were found to play a key role in changing th e optical and electric al properties of the CIGS films and hence the cell performance. Based on these promising results, future efforts will focus on PLA study using a commercial type of Excimer laser system that offers a large laser beam size with uniform surface energy density and variable pulse widths and scan rates to access a wider range of

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28 PLA treatment conditions. In particular, it is hoped that a vari able pulse width and wavelength will allow control of the anneal depth. Given its application to other industrial materials, laser a nnealing has the pote ntial to be an effective method to improve solar cell performance in an industrial setting.

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29 Table 3-1. Effective minority carrier lifetime ( ) of CIGS films before and after PLA treatment as measured by DBOM Sample # before PLA (ns) after PLA (ns) PLA condition (mJ/cm2, pulses) CIGS-S1 1.77 4.87 30, 5 CIGS-S2 2.82 3.39 40, 5 CIGS-S3 4.11 5.43 50, 5 CIGS-S4 4.51 6.31 60, 5

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30 Table 3-2. Hall-effect results for CIGS films before and after PLA treatments Sample # Hole-concentration* (1016/cm-3) Hall-mobility* (cm2/V-s) Resistivity* ( -cm) PLA condition (mJ/cm2, pulses) CIGS-H1 0.53 / 0.445 8.89 / 37.6 133 / 37.3 20, 10 CIGS-H2 2.9 / 2.43 0.93 / 2.98 235 / 86.4 20, 20 CIGS-H3 4.3 / 1.8 1.54 / 6.1 94 / 2.67 40, 10 CIGS-H4 7.1 / 3.3 0.60 / 2.8 148 / 4.64 40, 20 Data taken before/after PLA treatments

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31 Table 3-3. Effective lifetimes and photoJ-V results of PLA CIGS/CdS samples and devices Sample # Lifetime (ns) Voc (V) Jsc (mA/cm2) F.F. (%) Eff. (%) PLA condition (mJ/cm2, pulses) CIGS/CdS #0 3.76 0.42 27.2 53.11 6.14 N/A CIGS/CdS #1 4.77 0.45 24.83 54.43 6.17 50, 10 CIGS/CdS #2 4.11 0.44 26.98 51.37 6.15 50, 20 CIGS/CdS #3 5.2 0.45 26.19 54.06 6.27 50, 10 CIGS/CdS #4 3.86 0.39 26.43 44.25 4.57 50, 20 * CdS re-growth was performed after PLA treatment.

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32 Table 3-4. Photo-J-V performance of contro l cell and PLA-treated CIGS solar cells Device # Voc (V) Jsc (mA/cm2) F.F. (%) c (%) PLA condition (mJ/cm2, pulses) CIGS-D1 0.576 31.60 66.50 12.07 30, 5 CIGS-D2 0.568 31.25 63.11 11.12 30, 10 CIGS-D3 0.497 26.79 55.29 7.368 40, 5 CIGS-D4 0.493 26.71 53.08 7.040 40, 10 CIGS-D5 0.436 25.56 51.80 5.773 50, 5 CIGS-D6 0.433 25.23 47.77 5.201 50, 10 Ctrl-D 0.543 30.05 55.74 9.064 N/A

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33 Table 3-5. PhotoJ-V performance and dark J-V parameters for the control cell and selected PLA-treated CIGS cells Cell # Ctrl-D-C3 CIGS-D1-C3 CIGS-D2-C3 PLA condition None 30 mJ/cm2, 5 pulses 30 mJ/cm2, 10 pulses Cell area (cm2) 0.429 0.429 0.429 Voc (V) 0.528 0.577 0.572 Jsc (mA/cm2) 29.78 34.24 32.00 F.F. (%) 48.86 67.88 66.78 Efficiency (%) 7.69 13.41 12.22 Vm (V) 0.365 0.458 0.453 Jm (mA/cm2) 21.37 28.99 26.88 N ~4.13 ~1.98 ~1.96 J0 (mA/cm2) ~3.2210-3 ~1.110-3 ~1.0610-3 Rs ( ) ~10.17 ~14.06 ~15.94

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34 Table 3-6. Results of the DLTS measuremen ts on the control cell and PLA treated CIGS solar cells Cell # Control cell PLA cell (30 mJ/cm2, 10 pulses) Trap type Minority (electron) Minority (electron) Trap activation energy, Ea (eV) Ec0.069 Ec0.065 Trap density, Nt (cm-3) 5.6 1013 2.8 1013

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35 Table 3-7. Annealing condition of new PLA samples Sample # Energy density (mJ/cm2) Pulse # Note 1 30 5 PLA sample 2 50 5 PLA sample 3 70 5 PLA sample 4 90 5 PLA sample 5 110 5 PLA sample Ctrl N/A N/A Control sample

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36 Figure 3-1. XRD spectra before and after PL A treatments for (a) a CIGS and (b) a CdS /CIGS sample.

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37 Figure 3-2. Surface morphology of CIGS films (a) without and (b) with PLA treatments at an energy density of 55 mJ/cm2 (SEM images with magnification of 6000x).

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38 Figure 3-3. Quantum efficiency of CIGS cells with and without PLA treatment. Figure 3-4. Spectral response of CIGS cells with and without PLA treatment.

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39 Figure 3-5. PhotoJ-V parameters versus different PLA condition.

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40 Figure 3-6. DarkJ-V curves comparing th e control cell to two PLA treated cells.

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41 Figure 3-7. DarkJ-V curves (semi-log plot ) of the control cell and two PLA treated cells.

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42 Figure 3-8. Quantum efficiency (Q-E) versus wavelength comparing the control cell to the two PLA-treated cells.

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43 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 010020030040 0 Temperature (K)Delta capacitance (pF)Control CIGS cellReverse bias = 0.5 V Pulse height = 0.7 V Pulse width = 10 ms y = -0.805x + 5.5049 R2 = 0.9812 -4 -3 -2 -1 0 89101112 1000/T, K-1ln(T2/En) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0100200300400 Temperature (K)Delta capacitance (pf) 0.05 ms 0.1 ms 0.2 ms 0.5 ms 1 msNLA CIGS cellReverse bias = 0.5 V Pulse height = 0.7 V Pulse width = 10 ms y = -0.755x + 4.9714 R2 = 0.9397 -4 -3 -2 -1 0 89101112 1000/T, K-1ln(T2/En)Initial dela y Figure 3-9. The DLTS scans of th e controland PLACIGS cell.

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44 Figure 3-10. Surface morphology of CdS/CIGS f ilms (a) before and after PLA treatment with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 4000x). Figure 3-11. Surface morphology of CdS/CIGS f ilms (a) before and after PLA treatment with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 40000x).

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45 Figure 3-12. Theta-2 theta (symmetrical ge ometry) diffraction patterns of PLA samples and control sample.

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46 Figure 3-13. Grazing incidence XRD analysis (GIXD) of PLA samples and the control sample (omega=3deg.)

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47 Figure 3-14. Grazing incidence XRD analysis (GIXD) of PLA samples and the control sample (omega=1deg.)

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48 CHAPTER 4 RAPID THERMAL ANNEALING (RTA) OF FILM PROPERTIES AND DEVICE PERFORMANCE FOR CIGSBASED SOLAR CELLS Investigation of the halogen-based Ra pid Thermal Annealing (RTA) process on CIGS films and solar cells are presented in this chapter. Annealing conditions, RTA procedures and experimental results are well discussed. Progressive RTA Treatment on CIGS Solar Cells The progressive RTA experiments were performed on one CIGS device, which contains 3 cells (f abricated by NREL), and the results show that progressive RTA treatment (with N2 ambient) improves cell performance and overall uniformity of large area CIGS solar cells. The RTA procedure is shown in Figure 4-1 and the basic sequence of the progressive RTA is given as follows: Prepare a CIGS device Test photoJ-V perfor mance before RTA Treat device by 100oC 30 seconds RTA Test photoJ-V pe rformance again Treat device by 200oC 30 seconds RTA Test photoJ-V pe rformance again…… Keep increasing the annealing temperatur e of each RTA until the cell performance dramatically drops or device is damaged. From the photoJ-V results (Table 4-1), significant improvement in cell performance was observed in all 3 cells after each progressive RTA treatment at temperatures of 100, 200, and 300oC for 30 seconds. A dramatic increase in the

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49 performance of cell #1 (increase from 9.52 to 15.77%) shows that progressive RTA could be used to enhance the structure uniformity and the performance of large area cells. Since we measured photoJ-V right af ter each annealing, the light soaking effect might be also a contribution of the performance improvement For CIGS material photovoltaic modules there are several references reporting the mate rial light induced beha vior and the possible reasons (e.g. [39], [40] and [ 41]). These references all observed the improvement in open circuit voltage (Voc) and fill factor (F.F.) b ecause of light soaking and the effect relaxes when kept in dark again. The same effect was observed when keeping the CIGS solar cells in dark with forward bias applied. We also tried 400oC for 30 seconds; unfortunately th e metal grids were severely damaged after the annealing. As a result, no further tests on J-V and Q-E characteristics were performed on these cells. The RTA Effects of Separated CIGS Films The XRD and SEM Results Five 1x1 inch CIGS samples (from SSI), which were grown on the Mocoated soda-lime glass (SLG) substrates, were us ed to investigate the RTA effects under different annealing conditions. XRD, SEM measurements were conducted before and after RTA treatments. The annealing condition (Table 4-2) and results are discussed in the following section. XRD measurements give information about the preferred film orientation and composition. SEM photos depict the surface morphology and grain size. Results (Figure 4-2 to 4-5) suggest that the overall fi lm composition and surface morphology do not change during the annealing. Only some slight ly blurry grain edges were observed in the

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50 SEM images for the 2 samples treated at 350oC (sample #F-R3 and F-R4) as compared to the 300oC RTA treated samples and control sample. Hall-Effect Measurements In order to determine the effects of RTA treatment on the carrier concentration, carrier mobility, and sheet resistance of CIGS films, resistivity and Halleffect measurements were carried out on two CI GS films directly deposited on the SGL substrates before and after the RTA treatment s. The annealing conditions and the results of resistivity and Halleffect measurem ents (using a MMR Halland Van Der Pauwmeasurement system) before and after the RT A treatments are summarized in Table 4-3. The Hall-effect measurements show a signi ficant increase of hole density and a decrease in film resistivity, Hall coefficient, and sheet resistance following the RTA treatment. For sample#H300-1, which was treated by 300oC RTA, the carrier mobility after RTA was found 2 times larger than the value before annealing, while for sample #H350-1, which was treated by 350oC RTA, the hole mobility was dropped by more than 50% after RTA treatment. The results revealed that 350oC RTA increased the carrier concentration by nearly two orders of magnitude which also led to the decrease of carrier mobility. The 300oC RTA treated sample showed beneficial results on the film resistivity, carrier mobility and carrier density. Therefor e, the peak annealing temperature plays an important role in determining the electrical properties of RTA treated CIGS absorber layers. The RTA Effects of Separated CIGS Solar Cell Devices Experimental Details Four 1x1 inch CIGS samples (from SSI), which were grown on the Mocoated soda-lime glass (SLG) substrates were used to investigate the RTA effects under different

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51 annealing conditions. A thin (50 nm) Cd S buffer layer was deposited on the CIGS samples using Chemical Bath Deposition (CBD) at 65oC for 13 minutes. A ZnO window layer was then deposited on the CdS/CI GS/Mo/SLG sample by RF sputtering. Subsequent metallization (Ni-Al front contact grids) was carried out by e-beam evaporation through a shadow-mask. Finally, finished devices were produced by cutting the sample into separate cells with 0.429 cm2 active area and attachi ng wires with Indium bumps on the Mo-coated glass substrates for back contacts. The performance of these cells was then tested by darkJ-V, photoJ-V and Q-E measurements before and after RTA treatment (with air ambient) under diffe rent annealing conditions. To exclude the light soaking effect, the phot oJ-V and Q-E re-measurement were conducted two weeks after RTA treatments. The annealing condition (Table 4-4) and results are discussed in the following section. PhotoJ-V Performance From the photoJ-V results (Table 4-5) for the cell D-R1, D-R2 and D-R3, we found that Voc, Jsc and conversion efficiency increa sed and F.F. decreased after RTA treatments. For cell D-R4 which was annealed at 350oC, 2 min, only Jsc increased after RTA and the overall performance dropped. By ta king a close look at the F.F. data, we found that for the cell treated by 2 min, the F.F. dropped more than that of the cell treated by 1 min RTA at the same temperature. This suggests that 1 min is enough for the RTA treatment and too long holding time can cause performance drops. DarkJ-V Characteristics To find the reason of Fill Factor loss during the RTA, the darkJ-V measurements were conducted of these the te sted devices before and after RTA treatments. From the measured darkJ-V curves (Figure 4-6 to 49), we found the overall dark current density

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52 decreased in forward bias region while it increased in reverse bias region after RTA treatment except cell D-R1 has increased da rkJ-V when applied voltage below 0.38 V after RTA treatment, and the shapes of all JV curves after RTA have a trend to become linear, which suggest the resist ivity decrease during the RTA and the Fill factor loss. One possible reason is the metal contact on the top of the devices diffused down through the whole device during the annealing and incr eased the conductivity, made the whole device become more like a conductor rather than a semiconductor. Some other tested cells even showed pure linear J-V curve and zero F.F, which is caused by metal spiking effects. Several recent studies already showed that the spiking effect is more likely to occur during the RTA for the E-beam evaporated Al metal grids than sputtered metal contacts. Another possible reason is because of the low melting point of In (156.61oC). Indium diffusion from the back contact might also cause the decrease of F.F. during the RTA treatment. The results suggest that RTA treatment is more suitable for devices with sputtered metal contacts or applying RTA treatments before metal grids and contacts deposition. From the semi-log plots of the darkJ-V cu rves (Figure 4-10 to 4-13), cell # D-R1 has deteriorated diode quality factor after the RTA treatment while other cells remain same diode qualities. Though the photoJ-V resu lts of cell # D-R1 is very promising after RTA, the relationship between RTA condition and the afterward darkJ-V result is still unclear for cell # D-R1. The S-R and Q-E Performance From the measured Q-E curves (Figure 4-14 to 4-17), improvements on Q-E were observed on entire interested wavelengths after RTA treatment s, which suggest that RTA has positive effect on all layers (absorber laye r, buffer layer, and window layer) of CIGS

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53 solar cells. These results are consisted with the photoJ-V results showed before; even for cell D-R4 which has over all decreased photoJ-V performance after RTA, the Q-E still showed that photo current density for th e interested wavelength region increase after RTA treatment which evidently proved the Jsc improvement of such cell. Thermal Annealing on CIGS Solar Cells by High Temperature XRD System Because of the limitation on the minion temperature of the RTA system, a high temperature XRD system was used to anne al 2 CIGS solar cells at temperature 100oC and 150oC, 30 seconds. The Photoand DarkJ-V m easurements were taken before and after annealing. DarkJ-V curves did not show much difference before and after low temperature RTA treatments. From the photoJ-V results (Table 4-6 and Figure 4-18), the overall performance for tested cells has been improved after low temperature RTA, furthermore. The cell treated by 150oC RTA has more significant improvement on all IV parameters while the cell treated by 100oC RTA only has improvement on F.F. and Combined with the previous results (progressive RTA results) shown before, these data suggest that the optimal RTA temperature fo r CIGS solar cells is in the range of 200oC to 300oC. Summary Progressive RTA treatments have shown significant im provement of the overall uniformity and performance of large area CIGS solar cells. Under low RTA temperature, the surface composition and morphology remain unchanged. Our study of the RTA effect on CIGS devices shows increa se in the values of Voc, Jsc, and conversion efficiency, but some decrease in fill factor (F.F.). The esti mated optimal annealing temperatures should be between 200 and 300oC with 1minute or less holding time.

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54 Table 4-1. PhotoJ-V results of CIGS so lar cells before and after progressive RTA Pre-annealing Cell #1 Cell #2 Cell #3 After 100oC RTA Cell #1 Cell #2 Cell #3 Voc (V) 0.628 0.652 0.655 Voc (V) 0.633 0.656 0.660 Jsc (mA/cm2) 31.66 32.97 33.50 Jsc (mA/cm2) 34.30 35.47 34.05 F.F. (%) 47.88 68.52 70.71 F.F. (%) 56.81 71.10 73.03 Eff. (%) 9.52 14.73 15.51 Eff. (%) 12.32 16.55 16.19 After 200oC RTA Cell #1 Cell #2 Cell #3 After 300oC RTA Cell #1 Cell #2 Cell #3 Voc (V) 0.652 0.657 0.650 Voc (V) 0.627 0.623 0.630 Jsc (mA/cm2) 34.85 35.11 32.34 Jsc (mA/cm2) 35.39 36.35 35.08 F.F. (%) 68.43 72.14 76.15 F.F. (%) 71.05 70.48 74.32 Eff. (%) 15.55 16.65 16.01 Eff. (%) 15.77 15.96 16.42

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55 Table 4-2. CIGS film number and the annealing conditions Sample # RTA temperature Holding time note F-R1 300oC 1 minute RTA sample F-R2 300oC 2 minutes RTA sample F-R3 350oC 1 minute RTA sample F-R4 350oC 2 minutes RTA sample F-Ctrl N/A N/A Control sample

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56 Table 4-3. Hall-effect data of NREL CIGS samples before and after RTA treatments Sample #H300-1 Sample #H350-1 RTA condition 300oC, 1-min 350oC, 1-min Ambient Ar Ar Resistivity (ohm-cm)* 70.58 / 4.21 55.76 / 2.17 Mobility (cm2/Vs)* 2.80 / 6.77 4.28 / 1.86 Hole density (cm-3)* 3.161016 / 2.191017 2.621016 / 1.551018 Hall coefficient cm3/Coul)* 197.77 / 28.48 238.53 / 4.03 Sheet resistance (ohm/cm2)* 470536.4 / 28044.4 371761.8 / 14435.8 Carrier type holes / holes holes / holes *Data taken before/a fter RTA treatments

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57 Table 4-4. Annealing conditions of RTA treated CIGS devices Sample # RTA Temperature Holding Time note D-R1 300oC 1 minute RTA device D-R2 300oC 2 minutes RTA device D-R3 350oC 1 minute RTA device D-R4 350oC 2 minutes RTA device

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58 Table 4-5. PhotoJ-V results of separated CIGS solar cells before and after RTA Cell # D-R1 D-R2 D-R3 D-R4 RTA condition 300oC, 1-min 300oC, 2-min 350oC, 1-min 350oC, 2-min Voc (V)* 0.455 / 0.471 0.471 / 0.487 0.465 / 0.471 0.465 / 0.422 Jsc (mA/cm2)* 27.49 / 31.43 26.55 / 30.51 28.75 / 33.94 27.67 / 30.41 F.F. (%)* 52.69 / 49.01 57.48 / 49.96 52.14 / 48.58 48.00 / 39.36 Eff. (%)* 6.591 / 7.259 7.196 / 7.422 6.975 / 7.759 6.177 / 5.051 Data taken before/after RTA treatments

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59 Table 4-6. PhotoJ-V results of CIGS sola r cells before and after RTA by using high temperature XRD system Device# RTA Voc (V)* Jsc (mA/cm2)* FF (%) Eff (%)* 1 100oC,30s 0.50/0.503 24.74 / 24.68 61.45 / 67.60 8.19 / 8.35 2 150oC,30s 0.43/0.509 25.56 / 28.85 45.91 / 51.9 5.03 / 7.62 Data taken before/after RTA treatments

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60 Figure 4-1. Cycle time for Rapid Thermal Annealing (one run).

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61 Figure 4-2. XRD results of RTA treate d CIGS films and the control sample.

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62 Figure 4-3. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 3000x).

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63 Figure 4-4. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 10000x).

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64 Figure 4-5. Surface morphology of RTA treated CIGS films and the control sample. (SEM images with magnification of 30000x).

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65 Figure 4-6. DarkJ-V curves of tested CI GS solar cell (#D-R1) before and after RTA (300oC, 1-min) treatment.

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66 Figure 4-7. DarkJ-V curves of tested CI GS solar cell (#D-R2) before and after RTA (300oC, 2-min) treatment.

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67 Figure 4-8. DarkJ-V curves of tested CI GS solar cell (#D-R3) before and after RTA (350oC, 1-min) treatment.

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68 Figure 4-9. DarkJ-V curves of tested CI GS solar cell (#D-R4) before and after RTA (350oC, 2-min) treatment.

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69 Figure 4-10. DarkJ-V curves (semi-log plot) of tested CIGS solar cell (#D-R1) before and after RTA (300oC, 1-min) treatment.

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70 Figure 4-11. DarkJ-V curves (semi-log plot) of tested CIGS solar cell (#D-R2) before and after RTA (300oC, 2-min) treatment.

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71 Figure 4-12. DarkJ-V curves (semi-log plot) of tested CIGS solar cell (#D-R3) before and after RTA (350oC, 1-min) treatment.

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72 Figure 4-13. DarkJ-V curves (semi-log plot) of tested CIGS solar cell (#D-R4) before and after RTA (350oC, 2-min) treatment.

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73 Figure 4-14. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R1) before and after RTA (300oC, 1-min).

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74 Figure 4-15. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R2) before and after RTA (300oC, 2-min).

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75 Figure 4-16. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R3) before and after RTA (350oC, 1-min).

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76 Figure 4-17. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R4) before and after RTA (350oC, 2-min).

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77 0 5 10 15 20 25 30 00.10.20.30.40.5Applied Voltage (v)Photocurrent Density (mA/cm^2) After 100C RTA Before RTA (a) 0 5 10 15 20 25 30 00.10.20.30.40.5Applied Voltage (v)Photocurrent Density (mA/cm^2) After 150 C RTA Before RTA (b) Figure 4-18. PhotoJ-V curves of tested CIGS solar cells before and after (a) 100oC, 30 seconds and (b) 150oC, 30 seconds RTA treatments by using high temperature XRD system.

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78 CHAPTER 5 FABRICATION PROCESS OF CIGS SOLAR CELLS In this chapter description of the CIGS solar cell device structure and typical fabrication process are given in detail. Based on the typical device structure and fabrication process of CIGS based thin-film solar cell (Figure 5-1), the first two steps comprise the deposition of a 1~2 m Mo back contact on top of a thin chemical barrier (SiO2) on a glass substrate. This can be acco mplished by two-step magnetron sputtering in an in-line PVD system. Next, a CIGS abso rber layer with thickness of 1.5~2 m is deposited by using PVD, sputtering, PMEE, co-evaporation or RTP technique. Followed by deposition of a 500 thin CdS buffer laye r (forming the hetero-junction with CIGS) using chemical bath deposition (CBD). An intrinsic ZnO of 500~1000 and a conducting ZnO:Al (0.3 to 0.5 m) TCO laye rs are then deposited by the sputtering system. Finally a 500 Ni and a 3 m Almeta l contacts are deposit ed by using E-Beam evaporation technique. For finished device, an Anti-reflection (AR) coating (e.g., MgF2) was used on the top of the cell to minimize the reflection loss. Substrate The most widely used substrate for the fabr ication of CIGS based solar cells is the soda-lime glass (SLG), which allowed cham pion efficiencies of up to 19.2% AM1.5G (Ramanathan et al., 2003). This low-cost SLG substrate material can be produced in large scale and with reproducible qua lity mainly for the window industry. The most commonly used back contact is the sputtered Mometal layer of approximately 1 m thickness. The following are the requirements for an excellent CIGS substrate.

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79 Vacuum compatibility. The substrate shoul d not degas during the various vacuum deposition steps, especially during CIGS absorber layer deposition, when the substrate must be heated. Thermal stability. For the deposition of high performance CIGS absorbers, the substrate temperature should reach 500-600oC during part of the deposition process. Substrate temperatures of less than 350oC usually lead to severely degraded absorber layer quality and poor cell performance. Therefore, substrates should withstand the temperature no less than 350oC. Proper thermal expansion. The coefficient of thermal expansion (CTE) of the substrate should be compatible with th at of CIGS; otherwise CIGS adhesion problems may be occurred. Also cracki ng of the Mo back contact can be encountered because of its low CTE. Chemical inactivity. The substrate shoul d not corrode, neither during processing nor during use. In particular, it should not react with Se during the CIGS deposition or decompose during aqueous solution de position of buffer layer (CdS). A good substrate should also not release any impurities that can diffuse into the absorber, except when the diffusion is desired. Sufficient humidity barrier. The substrate co uld protect the active solar cells layers during the long-term usage against environmental attack from the back such as the penetration of water vapor. Surface smoothness. A smooth substrate surface is essential. First, abrupt changes in the surface such as spik es or cavities may lead to shunts between the front and the back contact. Second, the deposition of impurity diffusion barriers or insulation layers may be easier and more successful on a smooth substrate. Cost, energy consumption, availability and weight. An excellent substrate should be cheap, little energy consumption, consis ts of available and abundant materials, and with light weight. Because of the above requirements and dema nds of CIGS substrates, the SLG is the preferred substrate material for the industr ial manufacturing of CIGS based solar cells since it fulfils most of these requirements Recent results from ISET show CIGS solar cells formed on SLG substrates with higher c onversion efficiency than those with other alternative substrates (Table 5-1). Additionally, the incorporation of sodium has a beneficial effect on the CIGS film quality. However, the main disadvantages of glass

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80 substrates are their high brittleness and nonflexibility, which limit the applications considerably. Back Contact Layer The molybdenum (Mo) layers with a thickness of around 1 m deposited by d.c.-magnetron sputtering process are used in the bottom contact layers for the CIGS-based solar cells. With the excellent properties of low contact resistance to the CIGS, relative stability at the processing temperatures (400-600 C), and resistance to reacting with Cu, In, and Ga, the Mo is the most widely used metal for the back contacts of CIGS-based solar cells. Followed the deposition of CIGS abso rber layer on the Mo-coated glass substrates, an interfacial MoSe2 layer between the CIGS absorbers and the Mo layers has been identified. This layer structure of MoSe2 was suggested to have a bandgap energy of about 1.4eV with a thickness of around 0.1 m. Besides that the wide-band-gap MoSe2 layer can be served as the back surface field in the CIGS-based solar cells to enhance the carrier collection, the MoSe2 layers are considered to improve the adhesion of CIGS films to the Mo contac ts to form good ohmic contacts. CIGS Absorber Layer The Cu(In,Ga)Se2 compound belongs to the semiconducting I-III-VI2 materials family that crystallize in th e tetragonal chalcopyrite struct ure. Chalcopyrite is another name for copper iron sulfide (CuFeS2), a common copper ore, which gave name to these materials. An interesting property of the semiconducting chalcopyrites in general and Cu(In,Ga)Se2 in particular is that bandgap energy, Eg, can be varied, for instance by varying the amount of Ga. The optimal bandgap for a solar cell with respect to the solar

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81 spectrum is around 1.4 eV [42]. The bandgap of CuInSe2 (CIS) is 1.04 eV and 1.68 eV for CuGaSe2 (CGS) while the bandgap of the alloys Cu(In,Ga)Se2 lies in between. A Ga/(In+Ga) ratio of 30% results in Eg of around 1.3 eV, which has been shown empirically to give the best device pe rformance. Another way to increase Eg is by replacing part of the Se for S to form Cu(In,Ga)(Se,S)2. It is expected that a wider Eg would yield a higher Voc. At a given efficiency higher voltage and lower current is preferred, since lower current results in smaller resistive losses with P = I2R. In a module with several interconnected cells an increas ed voltage is of interest. Since a higher voltage corresponds to a lower current for th e same power, a superior design of wider cells with smaller geometri cal losses is preferred. CIGS thin-films are grown by the sequential evaporation of metals in the presence of Se. The vacuum base pressure is in the 6-10-7-torr range. To help the reader better understand the growth of the CIGS thin-films, we describe first the integrated deposition scheme. Figure 5-2 shows a schematic profile of the elemental fluxes and substrate temperature used for the deposition of the CIGS films [43]. The process consists of three stages: (1) the formation of an (In,Ga)2Se layer on the soda-lime glass (SLG)/Mo substrate at a substrate temperature of 400oC; followed by (2) the deposition of Cu and Se at a substrate temperature of ~570oC, at which the compound formation and crystallization of Cu (In,Ga)2Se takes place such that the co mposition is slightly Cu-rich; and (3) additional In, Ga and Se is ad ded at the same substrate temperature of 570oC, so that the final composition expected is Cu-poor. The sample is cooled down to approximately 400oC in the presence of Se, after which the system is allowed to cool down naturally to room temperature. CIGS solar cells were usually completed by

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82 chemical-bath deposition (CBD) of approxima tely 500 CdS, followed by RF sputtering of 500 of intrinsic ZnO and 2000 of Al-dop ed ZnO. Ni/Al grid contacts were applied with approximately 5% coverage. The substrat e was SLG coated with Mo film sputtered using a DC magnetron cathode. Chemical Bath Deposition (CBD) of CdS Buffer Layer The technique of CBD involves the cont rolled precipitation fr om solution of a compound on a suitable substrate. The CBD technique offers several advantages over the CVD, MBE and spray pyrolysis methods. In the CBD process, the film thickness and deposition rate can be controlled by va rying the solution pH, temperature, and reagent concentration, and it is capable to coat large ar eas in a reproducible and low cost way. In addition, the homoge neity and stochiometry of the deposited films are partially maintained. The CBD process was first re ported by by J. E. Reynolds in 1884 for the deposition of PbS films. Sin ce then a wide range of chal cogenide (e.g. CdS, ZnS and MnS) and chalcopyrite materials (e.g. CuInS2 and CuInSe2) have been prepared by the CBD method. Recently, there has been considerable in terest for developing new polycrystalline thin-film semiconductors using various tech niques. Among them, the CBD process has found very attractive for being a low temper ature and low cost process. The CBD technique is a useful method for the deposit ion of thin-film semiconductor materials. Many of them form important components within polycrystalline solar cells. Such devices may offer advantages in low cost solar energy conversion. The CBD process has advantages over alternative methods of th in-film deposition: The technique is simple and requires relatively low capital expenditure ; films may be deposited at very low temperatures on a wide variet y of substrates. The CBD process may be easily adapted to

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83 large area processing at low fabrication cost and the thickness of the deposited layers may be controlled by varying the length of the deposition time. A major drawback of the CBD process is the inefficiency of the pr ocess in terms of the utilization of starting materials and their conversion to thin-films. The competing homogeneous reaction in the solution, which results in massive precipit ation in solution and deposition of materials on the CBD reactor walls, limits the extent of the heterogeneous reaction on the substrate surface. There are several methods for depositing thin-film CdS: vacuum evaporation, chemical vapor deposition, and spray pyrolys is. However, the most convenient method for low cost growth is the CBD process. The CBD technique is a simple solution growth process for creating polycrystalline CdS thin-films. This involves dipping a substrate into a reaction mixture for a time de pending on the film thickness required. It is well known that the structures and propertie s of CdS films are influenced by the recipe used in the growth. Composition, grain si ze, crystallinity, photosensitivity, defect density, and the covering ability have all related to the bath composition, temperature, and duration of the deposition. In the deposition of solid thin-films in a chemical bath by the CBD process the nucleation centers are regularly formed by the absorption of metal hydroxo species on the surface of the substrate. Th e initial layer of the thin -film is formed through the replacement of hydroxo group by the sulphide io ns, and subsequently the solid film is grown by the condensation of metal and sulfide ions onto the top of the initial layer. Two competing processes, the heterogeneous process of th e solid film deposited on the substrate and the homogeneous process of pr ecipitation in the reac tion bath are taking

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84 place simultaneously in the chemical bath dur ing the CBD process. In order to remove the possible precipitation, which may be at tached on the surface of the films, the substrates are rinsed with DI water after th e deposition and blown dr y with nitrogen gas. The CBD process set-up (Figure 53) is very simple and makes it very convenient for thin film CdS deposition. In our lab, we use NREL CBD Recipe to deposit the high quality CdS buffer layer. Materials used: NH4OH (28-30%) CdSO4(8/3)H2O (0.015M) for pure CdSO4 (0.015M=1.56345g/500ml water) NH2CSNH2 (1.5M) HCl (for the cleaning of the reaction vessel) DI H2O (DI: Deionized water) Prepare of the following solutions: {1} [CdSO4(8/3)H2O] 1.923 g 500 mL DI water {2} [NH2CSNH2] 57.09 g 500 mL DI water Stir the solution for 30 minutes, vacuum filter the solution using a clean 0.2 m tissue culture filter unit. Hardware: Two 1000 mL beakers Sample holder Hot plate with a magnetic stirrer Magnetic stirring rod Thermometer Filter unit

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85 Deposition Procedure: Set the temperature control system (circulator) at T=65 C Place the first 1000 mL beaker (referred as the “reactor” in the suite) Put the reactor on the magnetic stirrer Put a magnetic stirring rod into the beaker. Select a slow stirring speed. Place the substrates/samples into the sample holder. Place the sample holder with samples into the reactor. Place the thermometer in the react or to monitor temperature. Mix 366 mL DI H2O, 62.5 mL NH4OH, 50 mL of the solution {1} and 25 mL of the solution {2} into the second 1000 ml beaker. Add above mixture to the reactor. Wait for 13 minutes of deposition time. Remove the sample holder and immerse it into the glass beaker filled with DI water. Remove the substrates from the sample holder. Rinse the substrates with DI wa ter and blow dry with nitrogen. The bath is covered to help stabili ze the temperature and reduce the loss of ammonia. The solution, which is initially clear, becomes pale green, yellow, gold and then bright orange during the deposition. The s ubstrates become coated with a thin layer of smooth, shiny CdS film, which, in turn, is covered by a thick laye r of loosely adherent powder for longer duration deposition. Th is powder can be removed by ultrasonic cleaning. CdS films are formed through the reaction of adequately dissolved ammonia, cadmium and thiourea precursors. Under certai n conditions, these precursors form thin

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86 polycrystalline CdS films on any material su rface. The deposited films are dependent on temperature, relative concentrations of precursors, solution PH and stirring. Several studies reported that an ultra thin inverted n-type layer is created after the CdS buffer layer deposition. It has been observe d that this inverted layer is an n-type ordered vacancy compound (OVC) with energy bandgap around 1.3 eV, such as CuIn3Se5, and hence the buried homojunction is formed in the CIGS cells. Alternative CdFree Buffer Layer In the past few years, the CBD process has drawn considerable attention as an excellent method for the deposition of th e Cadmium Sulfide (CdS) buffer layer in efficient thin-film solar cells w ith either chalcopirytes (CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS)) or CdTe absorbers. Very good e fficiencies have been obtained using the CBD-CdS buffer layers. Among various buffer layer materials such as CdS, (Cd,Zn)S, ZnS, Zn(O,S,OH)x, ZnO, ZnSe, Inx(OH,S)y, In2S3, In(OH)3, SnO2, Sn(S,O)2, ZnSe, or ZrO2 deposited by CBD, ALE, MOCVD, or other deposition methods, the best performance CIGS solar cells with a total-ar ea conversion efficiency of 18.8% and other high-efficiency (> 17%) CIGS solar cells were obtained using the CdS buffer layer deposited by the CBD method. Using wide r band gap materials to replace the CdS (Eg=2.4eV) buffer layer could improve the quant um efficiency of the CIGS cells at shorter wavelengths, resulting in an increase of the shortcircuit current. The (Cd,Zn)S buffer layer has a band gap energy greater th an 2.4eV and can provi de a better lattice match to the CIGS absorber layer. Other alternative buffer layers have also been investigated as possible replacement for the CdS buffer layer to avoid the use of Cd because of its toxicity. Using ZnS (Eg=3.6eV) and In(OH)xSy (Eg=2.54eV) buffer layers for CIGS solar cells have achieved hi gh active-area conversion efficiencies of 16.9

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87 and 15.7%, respectively. Thus, both ZnS and In(OH)xSy films deposited by CBD process are promising candidates for the Cdfree buffer layers among the reported alternative materials for CIGS cells. Transparent Conducting Oxide (TCO) Layer A transparent conductive film is a material that is highly transparent in the range of visible light, and at the same time, elect rically conductive. The interest in these transparent conductors can be traced back to the early 20th century when reports of CdO films first appeared. Since then there has been a steadily growing interest in these materials with their unique properties. It is well known that non-st ochiometric and doped films of oxides based on Tin, Indium, Cadmium, Gallium, Copper and Zinc and their blends exhibit high transmittance and conductivit y. Products such as flat panel displays, solar cells, optoelectronic and electronic components and thermally insulating architectural glass have one thing in co mmon: they have to combine the opposing material properties transparency and elec trical conductivity. Tran sparent conductive films can be produced by multilayer coatings ba sed on thin metal films or by a homogeneous coating based on wide band gap semiconductors. Transparent Conductive Oxides are key compone nts in flat panel displays and solar cells. The main deposition technology used for large area deposition is the Physical Vapor Deposition (PVD) technology. Such “int elligent” films can be produced by using conductive oxides based on n-type TCOs and p-type TCOs in well defined layer stacks, which will lead to new applications in the field of transparent electronics and optoelectronics, like transi stors, diodes, active sources and detectors. For many manufacturing companies of TCO films it is th e aim to achieve stable film properties for large area coating processes with low film resistance and high transmittance within the

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88 visible spectrum range. A lot of efforts we re made to develop technologies to produce TCO coatings by reactive sputter technology. With adding oxygen or nitrogen gas into the coating chamber it is possible to produce oxide or nitride dielectric layers from metal targets. But all of these reactive sputtering co ating processes suffer from the fact that a chemical reaction takes place not only at the pr oduced layer, but at the metal target as well. Additionally a homogeneous gas flow and gas distribution in th e coating chamber is necessary to achieve high quality TCO coati ngs. For several applications the substrate geometry is limiting the gas distribution as we ll. This has led more and more to coating machines with highly sophisti cated in si-tu process measurements and gas flow control systems to stabilize the reactive sputter pr ocess. Mainly becaus e of quality and cost reasons more and more manufacturing co mpanies were using the conductive oxides which are easier to handle than pure metal targ ets. For the last three decades Indium Tin Oxide (ITO) has been the most popular n-type TCO material used for layer stacks for these applications. It is we ll known that ITO is an expens ive coating material in the thin-film market, and for quite some times a substitute that could provide cost savings while provide similar properties is still missing. This forces the use of ITO, and has kept many companies at the mercy of price fluctuations of the raw material. In the technological development of the various solar cells as described above, there are several aspects of cross-fertilizat ion and common problems. Examples are glass washing, glass bending and warp age at elevat ed temperatures, roll-to-roll processing, vacuum deposition; laser scribing, heat treatments, etc. While special attention was drawn to this during the interviews, no common answers were found, ex cept in the area of transparent conductive oxides. For all thin-film PV technol ogies, optimization of the

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89 transparent conductive oxides fo r the transparent contacts is of crucial importance. Improvement is needed, since current material s are far from optimal with respect to the light transmittance and conductiv ity that are desired for PV applications. Development and exploring characterizatio ns techniques are important for the understanding of the material properties and behaviors. All thin -film solar-cell technologies will benefit from these developments. For PV applications a thorough understanding of the optical and electrical properties of the TCO material is important in order to minimize the optical and electrical losses. However, traditionally indium tin oxide (ITO) has been the TCO of choice but it is expensive because of the low natural abunda nce of indium. With appropriate doping, transparent ZnO conducting films can be produc ed. From a purely financial viewpoint doped ZnO TCO films are preferable to ITO as the natural abundance of Zn is 1000 times higher than that of indium. Therefore, ZnO becomes a very promising candidate of TCO layer of thin-film solar cells especially for CIGS-based solar cells. As grown, nominally undoped, ZnO usually demonstrates n-type condu ctivity because of the presence of either Zn interstitials or O vacancies [44]. The literat ure is still unclear, how ever, as to which of these intrinsic defects are th e cause of the conductivity. Unfo rtunately the as-grown films are not suitable for device applications as the resistivity is too high and the re-oxidation of the Zn rich films at ambient temperatur es removes the source of the conductivity. To introduce stable n-type conduc tivity two dopants that are us ed most are Ga and Al [45–52], although doping with other elements such as B [53], In [54, 55] and Zr [47] has also been investigated. Al doped ZnO and Ga doped ZnO films can be used as electrodes for flat panel displays, low emissivity glass, as thin-film solar cells and as anode material

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90 for organic light emitting diodes [46]. In CIGS solar cell fabrication, magnetron sputtering technique is used to deposit an u ltra thin intrinsic ZnO layer followed by a 300 nm Al doped n-type ZnO conducting layers. Metal Contact Several runs of E-beam evaporati on experiments were made in our microelectronics lab. During the fabrication of CIGS cells, as a result, details of E-beam technique related to the deposition of Ni/Al c ontacts on solar cells were learned from this practice. Figure 5-4 shows the basic system of the E-Beam machine in the Microelectronics Lab of ECE department at UF An ultra thin Ni film was deposited on CIGS to prevent the diffusion from Al ohmic contact layer to ZnO layer because of the activity of Al. Since Ni is much harder than Al, it can protect th e ZnO layer from being damaged by the needle probe when characterizing the device.

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91 Table 5-1. Conversion efficiencies of CI GS cells grown by non-vacuum processes (by ISET) on different substrates. Substrate Conversion efficiency Air mass index Soda Lime Glass 13.6% AM 1.5G Molybdenum Foil 11.5% AM 1.5G Titanium Foil 9.5% AM 1.5G Polyimide Film 9.3% AM 0

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92 • E-beam vapor deposition • Optimized sputtered ZnO • Buffer layer-CdS, ZnS, or Inx (OH, S)yby CBD• CIS absorber layer-PMEE precursor film deposition -RTP growth• Sputtered Mo i-ZnO CdS CuIn1-xGaxSe2 Mo TCO (ZnO) Glass/SS/Polymer • E-beam vapor deposition • Optimized sputtered ZnO • Buffer layer-CdS, ZnS, or Inx (OH, S)yby CBD• CIS absorber layer-PMEE precursor film deposition -RTP growth• Sputtered Mo i-ZnO CdS CuIn1-xGaxSe2 Mo TCO (ZnO) Glass/SS/Polymer Figure 5-1. Layer structure of CIS/CIGS based solar cells.

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93 Figure 5-2. Schematic profile of the NREL ‘t hree-stage physical eva poration process for the fabrication of CIGS solar cells’.

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94 Figure 5-3. Simple CBD experimental set-up.

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95 Sample crucible E-Beam Mechanical Pump Diffsion Pump E-Beam deposition of Ni/Al metal contact Figure 5-4. The E-Beam evaporation techni que for depositing me tal contact films on solar cells.

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96 CHAPTER 6 CHARACTERIZATION AND MEASUREMENT SYSTEMS FOR CIGS SOLAR CELLS Characterization and measurements of CIGS based thin-film solar cells are important for determining and evaluating the quality of absorber layers, film surface morphology, film compositions, th e electrical and optical prop erties of absorber layers and the overall device performance. In this chapter, the fundament al device physics and cell structure, experimental setups, charac terization of CIGS ab sorber layers, and measurements and analysis of the device perf ormance parameters of CIGS solar cells are presented. Hall-Effect Measurements Introduction The Hall Effect was discovered more than 100 years ago when Hall observed a transverse voltage across a conductor subjected to a magnetic field. This technique can be used to determine the material type, carri er concentration and carrier mobility in a semiconductor. The basic principle of Hall e ffect in a semiconducto r is illustrated in Figure 6-1. The Hall coefficient, RH, is defined by Equation 6-1. X Z H HI B tV R (6-1) Since the magnetic field, BZ, will deflect either electrons or holes (nor p-type) to the bottom of the sample, the sign of VH changes depending on the material type, so does the Hall coefficient. All the qua ntities in Equation 6-1 for RH are measured

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97 experimentally, and the electr on and hole densities can be determined from the measured Hall coefficients, which are given by Equation 6-2. HqR n p 1 (6-2) Where the “+” sign applies for holes and the “–“ sign for electrons. The majority carrier mobility can be determined from the measured Hall coefficient and electrical conductivity, which is given by Equation 6-3. H H HR R (6-3) Where and is the electrical resistivity and co nductivity of the semiconductor, which can be determined by using fourpoint-resistivity measurements. Measurement Procedure and Apparatus CIGS absorbers deposited directly on SL G substrates are used in Hall effect measurements. Ohmic contacts are made by attaching 4 small indium dots placed at the corners of a square CIGS sample annealed at ~ 160 C in ambient. Contact quality is verified by the linear I-V characteristics (Figure 6-2). Standard Hall effect and Van der Pauw re sistance measuremen ts [56, 57] were employed to determine the hole concentration and mobility in CIGS films. The Hall voltage was measured using a constant magne tic field of 3000 G, at room temperature. The Hall effect and resistivity measurements on the CIGS samples were made using is a computer-controlled MMR Hall effect measurement system, which employs the four-point probe Van der Paul technique fo r determining the electrical conductivity and the Hall coefficient of the measured sample.

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98 Related Studies, Results and Discussion The objective of this research task is to develop and optimize the Cd-partial-electrolyte treatments of CIGS films -and to analyze their effects on the performance of CIGS-based solar cells. Hall effect and resistivity measurements were made on CIGS samples supplied by NREL, GSE, ISET, and EPV to determine the carrier densities and Hall mobilities in these samples. In addition, Hall data of CIS/CGS films deposited by UF PMEE growth system are also presented. The Cd-partial-electrolyte treatment on CIGS films Similar to the procedure of depositing the CdS buffer layer on CIGS films using CBD, the aqueous solution consisting of 2.4 10-4 M CdCl2, 7.43 10-4 M NH4Cl, 6.61 10-4M NH4OH at a bath temperature in the range of 80 to 85 C was applied for the Cd partial-electrolyte treated CIGS samples. In order to remove the possible precipitation, which may be deposited on the surface of the films, the CIGS samples were thoroughly rinsed with DI water after the de position and blown dry w ith nitrogen stream. Four 1 1 inch identical CIGS samples, which were cut from a 2 2 inch uniform slab, were applied by using Cd-p artial-electrolyte treatment with different soaking time. Hall measurements were made to characteri ze the effect of Cd-partial-electrolyte treatments on the carrier density and mobility of the samples. The results (Table 6-1) reveal that Cd-partial-electrolyte treatmen ts increase hole density and decrease Hall mobility with increasing Cd-treatment tim e. No correlation was found between the resistivity and the Cd-dip time. One possible ex planation of this effect is that during the Cd-partial-electrolyte treatment, Cd ions occupy the Cuvacancy sites and create donors with densities in the range of 1017-1018cm-3, thus an ultra-thin ntype surface inversion

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99 layer (5-10 nm) is formed [ 58]. The mixed conduction of el ectrons in such layer and holes in the p-CIGS absorber layer decr eases the measured Hall coefficient and consequently yields higher hole de nsity in the Cd-dip CIGS films. In addition, Hall effect and resistivit y measurements were made on the CIGS samples received from GSE, ISET, EPV and NREL, along with the CIS/CGS samples deposited by our group using PMEE method on different substrates (GaAs/glass). The experimental results are summarized in Table 6-2 and Table 6-3, respectively. Hall-effect data of CIGS f ilms supplied by NREL TFPPP team Four CIGS films (sample A, B, C and D) deposited on glass substrates were tested by our MMR Hall and Van Der Pauw measuremen t system at a fixed magnetic field of 3000 G and at 300 K. To measure the Hall volta ge laterally across the CIGS film, the film must be deposited on the insulating subs trate. Thus, the portions of samples A, B, and D used for Hall effect measurements we re deposited on bare soda-lime glass (SLG) substrate, rather than on the Mo-coated SL G substrate. Sample C was deposited on the stainless steel substrate, and th en peeled from the steel subs trate after coa ting with epoxy. Hall effect was not measured on samples E a nd F, as these films were not available on insulating substrates. Table 6-2 shows a summary of the Hall measurement results, including the hole concentration, Hall mobility, and resistivity data. Order of magnitude variations over the sample group are evident in each quantity. Possible complicati ons in interpretation of the Hall effect data include contribution from the mixed conduction types, differences in films grown without Mo versus under stan dard conditions, and changes to films during subsequent window layer processing.

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100 Photoand DarkJ-V Measurements J-V measurement is a very important dia gnostic tool for evaluating the electrical performance of solar cells. The electrical parameters in cluding the open-circuit voltage Voc, short-circuit current density Jsc, fill factor F.F., conversion efficiency series resistance Rs, shunt resistance Rsh, diode quality factor n, a nd reverse saturation current density Jo of a solar cell can be determined fr om the measured darkand photoJ-V curves. These parameters are very importa nt for assessing the performance of CIGS solar cells. Solar Cell Parameters In order to evaluate the performance of a solar cell it is essential to know the key device parameters of solar cells. The fo llowing derivation is ba sed on a typical PN junction solar cell theory from [59]. The equivalent circuit for a CIGS solar cell (Figure 6-3) is composed of the photocurrent component represente d by a constant current source, Iph=Isc, a dark current component, ID, a shunt resistance, Rsh, and a series resistance, Rs. If one neglects the effects of shunt resistance (assuming Rsh ), series resistance ( Rs ~ 0 ), and the recombination current ( Ir = 0 ) in the depletion region. Based on the assumption given above, the photoI-V characteristics of a p-n junction solar cell under illumination condition can be expressed by Equation 6-4. o1[exp(/)1]B phIIIqVkT (6-4) Where the total photocurrent density Iph generated in a p-n ju nction solar cell under one-sun condition can be obt ained by Equation 6-5. 2 1()phL J Jd (6-5)

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101 Where 1 and 2 are the cutoff wavelengths at the shortand long-wavelength limits of the solar spectrum, respectively. Fo r a typical p-n junction solar cell, 1 can be set at 0.3 m (from UV light), and 2 is determined by the cutoff wavelength or the band gap energy of the semiconductor (i.e., 2 = g = 1.24/Eg ( m)). The reverse saturation current Io1 because of the injection of electro ns and holes across the p-n junction is given by Equation 6-6 21p n ij pDnAoD D IqnA LNLN (6-6) Where Aj is the junction area; ni is the intrinsic carrier density; Dn and Dp denote the electronand holedi ffusion coefficients; Ln and Lp are the electronand holediffusion lengths, respectively. The short-circuit current ca n be obtained by setting V = 0 in Equation 6-4, which yields sc phII (6-7) Which shows that the shortcircuit current Isc is equal to the phot o-generated current, -Iph. The open-circuit voltage, Voc, can be obtained by setting I = 0 in Equation 6-4, and the result yields o o11n1Tsc cI VV I (6-8) Where VT = kBT/q is the thermal voltage. It is seen from Equation 6-8 that Voc depends on the ratio of the short-circuit cu rrent and the dark current, and Voc can be increased by keeping the ratio of Isc/Io1 as large as possible. This can be achieved by reducing the dark current, either by increasing the substrate doping density or in creasing the minority carrier lifetimes in the solar cell. Increasi ng the short-circuit current can also enhance Voc but it is not as drastic as re ducing the dark current in the solar cell. In practice, the Voc can

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102 be improved by incorporating a p-p+ back surface field (BSF) st ructure in the n-p junction solar cell. The BSF structure not only can de flect the minority carriers back into the junction but also reduces the back contact resistance of the cell. As a result, Voc, Jsc, fill factor ( FF ) and conversion efficiency can be impr oved with the BSF structure. Values of Vo c for a silicon p-n junction solar cell ma y vary between 0.5 V and 0.7 V depending on the cell structure, doping dens ities, and other device para meters used in the cell’s design and fabrication. If one includes the series resistance Rs and neglects the shunt resistance effect (i.e., Rsh ) in the I-V equation, then the output curr ent of the solar cell can be expressed as {exp[()/]1}DT s phVIRV I II (6-9) And the output power is given by 1||[ln]T Dph s II IPIVIVIR (6-10) The maximum output power can be calculated using the expression mmm P VI (6-11) Where o1(/) () (1/)m T msc m TVV III VV (6-12) Im is the current corresponding to the ma ximum power output, which is obtained by differentiating Equation 6-10 with respect to current I and setting/0 PI It is noted that Vm is obtained by solving the equation give n below using the iteration procedure. oexp(/)[1/]exp(/)mTmTcTVVVVVV (6-13) Another important solar cell parameter known as the fill factor (F.F.), which measures the squareness of the photoI–V curve shown in Figure 6-4, is defined by

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103 oo/ /1 F.F.1 1m T Tmmm csccocVV VVVIV VIVe e (6-14) Depending on the values of diode ideality f actor and shuntand seriesresistances, the fill factor for silicon p-n junction solar cells may vary between 0.75 and 0.85, while for a GaAs solar cell it may vary between 0.79 and 0.87. Finally, the conversion efficiency of a pn junction solar cell can be calculated by 100%out incP P (6-15) Where Pin is the input power from the sunlight, and Pout is the output power from the solar cell. The input power from the sunlig ht under one-sun AM0, AM1, AM1.5G, and AM2 conditions are given by 135.3, 92.5, 100, and 69.1 mW/cm2, respectively. PhotoJ-V Measurement System Our photoand darkJ-V and quantum effi ciency (Q.E.) measurement system are constructed by the previous Ph.D student, Dr. Chia-Hua Huang [60]. The photoJ-V performance of solar cells can be considered as accurate and useful only when the photoJ-V measurements are performed under the Standard Reporting Conditions (SRC) specifying the total radiation level, cell te mperature, and reference spectral radiance distribution. The typical SRC for terrestri al solar cells are a total irradiance of 100 mW/cm2, a reference spectrum of AM1.5 Global (ASTM Standard E892), and a cell junction temperature of 25C. The apparatus used for the construction of our J-V measurement system and measurement procedur e are based on the standard test method for electrical performance of photovoltaic ce lls using reference cells under simulated sunlight. The measurement system consists of a halogen lamp as the solar simulator, a temperature-controlled test chuck, a program mable power supply as the variable load,

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104 electronic instruments for measuring the terminal voltage and current of the test cells, and a computer program for the control of the measurement procedure and for the data acquisition. The J-V curve of the test cell is measured from the forward bias to the reverse bias using the voltage mode with a resolution of 1mV. Light source The ELH (tungsten-halogen bulb) lamp is utilized as a c-cl ass solar simulator in our photoJ-V measurement system. The referenc e cell method, which basically uses the reference cell to adjust the illumination level of the solar simulator, is employed in the photoJ-V measurement of CIGS solar cells. The solar simu lator intensity is adjusted by changing the distance between the solar simulator and the test plane so that the measured short-circuit current of the referen ce cell (from NREL) is equal to its calibrated value at the standard measurement intensity of 100 mW/cm2. Temperature control Since the open-circuit voltage of the CI GS solar cells decreases with increasing temperature, it is important to maintain the cell junction temperature at a standard value (25oC) during the photoJ-V measurement. In our system, the temperature of the test cell is maintained at 25 1 C by using a thermoelectric cooler assembly with a temperature controller during the photo-J-V m easurement. Because the CIGS solar cells are deposited on the 2 mm glass substrat es, a temperature difference of 3-5oC exists between the top and bottom surfaces of the solar cell under a light intensity of 100 mW/cm2. A thermocouple is used to measure the top surface temperatur e of the test cell during the measurement. Therefore the temperature controller of the cooling system is set at about ~20oC to keep the reading of the thermocouple and hence the temperature of the top surface of the test cell at 25 1oC.

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105 Contacts and connections The four-terminal contacts (also known as the Kelvin connections) are used for the connection between the test cell and the measurement system. Not only the wiring resistance but also the contact resistance betw een the probe tips and the contact pads of the solar cell can be neglected for the effici ency measurements by us ing the four-terminal contact method. In addition, a more accurate efficiency measurement for the solar cells can be achieved by using this method. Four micromanipulators with tungsten probe tips with radii in the range of 0.6 to 25 m are utilized to adjust th e position of the probe tips for contacting the small contact pads of the test cell. The voltage and current probes for both the top and bottom contacts should be pl aced as close as possi ble to avoid a high voltage drop between the two probes, and th e resistance between the voltage and current probes is hence minimized (less than 5 ). The photo and darkJ-V measurement system (Figure 6-5 [60] and 6-6), controlled by a personal computer with the da ta acquisition and data analysis software LabVIEW is composed of a programmable bipolar power supply served as the variable applied voltage source, an electrometer for measuring the terminal current, and a digital multimeter for measuring the terminal voltage of the test cell. The measuring range for current measurements by the electrometer is from 1 fA to 20 mA. Although the upper limit of the electrometer is only 20 mA, th e photo-generated current of the CIGS cell under test is typically smaller than 20 mA because of the small area (usually 0.429cm2) of the CIGS cells fabricated in a laboratory sc ale. With the sensitivity in the low-current measurements, the electrometer with a current resolution of 0.1 fA is perfectly ideal for the darkJ-V measurement.

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106 The PC based control program is writte n by the previous Ph.D. student, Dr. Chia-Hua Huang in the LabVIEW for data acquisition from the readings of electrometer, multimeter, and power supply. The program also controls the measurement procedure by sending commands to the programmable pow er supply via interf ace of GPIB for the completion of a full sweep of the J-V curve. For photoJ-V measurements, the illuminati on intensity of the solar simulator is first set by using the reference cell method de picted above. The test cell is exposed under this condition and biased at around the maximum power point for about 10 minutes (i.e., light soaking effect). The J-V curve is then swept from the forward bias to the reverse bias using the voltage mode w ith a resolution of 1 mV. Using the LabVIEW data acquisition software (Figure 6-6) the basic parameters such as VOC, JSC, fill factor F.F., output voltage at maximum power point VMAX, output current density at maximum power point JMAX, maximum power point PMAX, and conversion efficiency of test cells can be directly obtained from the measured photo-J-V curves. The open-circuit voltage VOC is determined from a linear fit to the J-V curve near zero output current, and similarly JSC is determined from a linear regression to the J-V points near zero output voltage. The maximum power point PMAX is obtained from an at least fifth-order polynomial fit to the data points of the output power versus voltage with the constraints in which the PMAX must be greater than 85% of th e measured maximum power and the VMAX must be greater than 85% of the measured VMAX. DarkJ-V Measurement The experimental procedure of DarkJ-V measurement is similar to the PhotoJ-V measurement except there is no incident light illuminating on the test device. From the

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107 measured dark JV curve and data (Figure 67), we can further extract and calculate many important parameters of the device such as reverse saturation current density ( J0), diode quality factor ( n ) and series resistance ( Rs). Quantum Efficiency Measurement Measurements of the spectral response in terms of the wavelength dependence of the photo-generated current density for a sola r cell as the character ization and diagnostic techniques are extremely important for quality control of each layer in cell fabrication, cell design, and understanding the diffusion mech anisms and separation of the individual photocurrent loss mechanisms. The external quantum efficiency, which is defined as the ratio of the generated elec tron-hole pairs per incident pho ton with certain wavelength, of the solar cell can be calculated from the measured absolute spectral response curve. A computer-controlled spectral response meas urement system using a monochromator for wavelength selection to measure the spec tral response (S-R) and quantum efficiency (Q-E) of the CIGS-based solar cells was de signed and constructed. The measurement system scanning the spectral range from 400 nm to 1400 nm with 10 nm as an increment step has the capability of applying white light bias and voltage bias to the test cell. During the measurement the entire area of th e test cell is covere d with a uniform and sufficient illumination-level monochromatic light. A computer program based on the software of LabVIEW was used to control the m easurement procedure and data acquisition. Quantum Efficiency Measurement Instrumentation Two types of measurement systems, i.e., the filter wheel and grating monochromator systems, are commonly used to measure the spectral response of the solar cells. The grating monochromator system has the basic feature of the flexibility to

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108 select wavelength, but has low light intens ity, poor beam uniformity, and small beam size. While the filter wheel system has the f eatures of higher light in tensity, better beam uniformity, and larger beam size, but has th e drawback of limited and fixed wavelengths in spectral response measurements. With th e small area of the CIGS-based solar cells fabricated at UF, we have constructed a spectral response measurement system using a grating monochromator to anal yze the spectra l response and quantum efficiency of our solar cells. The measurement system scanning the spectral range from 400 nm to 1400 nm with 10 nm as an increment step has the capability of applying white light bias and voltage bias to the test cell when the measurement is performed. Monochomator and Monochromatic Light Source As illustrated in Figure 6-8 [60], a 30Watt tungsten-halogen lamp is coupled with the monochromator as a light source to pr oduce a monochromatic light with a narrow bandwith of about 10 nm in the wavelength range from 350 nm to 2500 nm and with a resulting beam size of around 9mm 14mm on the test plane. With the consideration of possible spatial non-uniformity of the test cell to the spectral response, significant errors arising from the test cell and reference detector with different size or shape under a non-uniform monochromatic light [61], and the potential disadvantages, namely the low light intensity, uneven light di stribution, and small beam size, directly inheriting from a typical monochromator measurement system, th e geometric location and selection of the optical components including th e lenses, mirror, and optic al diffuser are specially arranged with caution so that the entire area of the cell on the test plane is covered with a uniform and adequate illumination-level m onochromatic light. The entrance slit width of the monochromator is opened to its ma ximum to increase the throughput of light

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109 intensity at the expense of the resulti ng image resolution from the output of the monochromator. The exit slit width of th e monochromator is opened to around 2.8 mm to keep the bandwidth of the monochromatic light at about or less than 10 nm for the wavelength from 400 nm to 1400 nm. The divergent monochromatic beam from the exit of the monochromator is collimated through the condenser lens, subsequently reflected onto the test plane via the high-re flection broadband flat mirror, and finally homogenized by a high-transmission (>85%) opti cal diffuser to make the monochromatic light more uniform without substantia lly sacrificing the light intensity. Since there is no real-time calibration and th e data of the incident power density on the test plane are stored before the photocurre nt measurement, the care must be taken for the stability of the light s ource used for the monochromator. A stable well-regulated power supply is served as the power source fo r the light source of the tungsten-halogen lamp. Two order sorting filters are used to block the undesired harmonic terms from the monochromator. One with th e cut-on wavelength of around 610 nm and the other with the cut-on wavelength of about 830 nm are a pplied for the ranges of wavelength from 630 nm to 1000 nm and from 1000 nm to 1400 nm, respectively. It is not necessary to use the order sorting filter for the range of wavelength from 400 nm to 630 nm because the silicon detector, which only responses to the wavelength above 360 nm, is used as the reference detector in the measurement system. Monochromatic Light Chopper An optical chopper used toge ther with a lock-in amplifier in the spectral response measurement system can discriminate the chopped ac signal from the undesirable noise and strong dc signal from the bias light, and hence increases the sign al-to-noise ratio in the spectral response measurement system However, errors can occur for the

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110 inadequate use of chopped light method when th e test cell and refere nce detector are of different size and/or shape [62]. These er rors can be minimized by locating the chopper blade in the narrowest location of the monoc hromatic beam pathway [62]. Therefore, we put the chopper right next to the exit of the monochromator in the measurement system to reduce the errors. In order to av oid the interference of the harmonics from the power lines of the bias light, a chopping frequenc y of 150 Hz is used in the measurement. Bias Light Source Besides the monochromatic lig ht a bias light is typical ly used in the spectral response measurement not only to approximate the standard operating conditions but also to compensate to effects, which might be attributed to trapping mechanisms of the test cell, about the non-linearity of the photo-gene rated current in the ce ll to the illumination level. An ELH lamp is used as the bias light source in the measurement system. The light intensity of the ELH lamp is adjusted during the spectral response measurement, which is one hundred times greater than that of the monochromatic light, to provide with sufficient illumination level such that the s hort-circuit current is within 70-100% of the ISC measured from the photoI-V measurement with respect to the SRC. A screen is placed between the chopper and lens to el iminate the undesired noise, whose magnitude can be comparable to the measured ac signa l because of the high illumination-level bias light, resulting form the stray light reflected from the components and the possible direct illumination from the bias light through the chopper onto the test cell. Spectral Detector and Synchron ous Detection Instrumentation The NIST traceable calibrated silicon and germanium photodetectors together with a lock-in amplifier are employed to meas ure the incident power density of the frequency-chopped monochromatic light beam on the test plane of the measurement

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111 system. In order to guarantee the measured incident power density is exactly same as the power density received on the solar cell surface, the surface heights of both detectors and the test cell on the testing stage are well adjusted to be equal. A zero-inductance four-terminal resistor of 2 is used as the current-voltage converter to convert the ac photocurrent generated from the photodetectors or the tested cell in to the photovoltage, which is then fed into the lock-in amplif ier. The monochromatic signal through the chopper becomes a trapezoidal waveform. Fo r absolute photocurr ent measurement of the detectors, the signal measured from the lock-in amplifier must be multiplied by a waveform correction factor (i.e. multiplicativ e constant) because the lock-in amplifier typically measure the amplitude of fundament al component of the trapezoidal waveform (root-mean-square signal), which is not exactly the same as the peak amplitude. Since the signals for the reference detectors and test cell are measured with similar electronic instruments, all multiplicative errors drop out and the absolute spectral response of the test cell can then be achieved. Spectral Response Measurement Procedures The monochromator, which is controlle d by a computer program written in LabVIEW software via the interface of GPIB, sc ans the spectral range from 400 nm to 1400 nm with 10 nm as an incremental step. The incident power density on the test plane is first measured by the photodetectors a nd the data are stored in the hard disk of the computer. The spectral response measurem ent is operated at the short-circuit mode by adjusting the variable load in the circuit loop to set the measurement at short-circuit condition with the terminal volta ge of the test cell within 5 mV. The light intensity of the light-bias lamp is adjusted such that the short-circuit current is within 70-100% of the

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112 ISC measured from the photoI-V measuremen t with respect to the standard reporting conditions (100 mW/cm2, 25oC, and reference spectrum). The ac photocurrent Itest cell( ) of the test cell is converted into photovoltage with a zero-inductance four-terminal precision resistance and is measured by using a lock-in amplifier. Subsequently the spectral response is calculated from the data stored in the computer previously and the measured photocurrent of the test cell. The external quantum effici ency as a function of wavelength can be converted from the spect ral response using the following expression: % 100 ) (cm Area ) W/cm ( density power q ) ( I c h ) ( QE2 cell test 2 detector cell test (6-16) Where h, c, q, and are the Plank constant, speed of light, electronic charge, and the photon wavelength, respectively. Figure 6-9 shows the spectral response and external quantum efficiency of a UF CIS solar cell taken by this measurement system. Deep Level Transient Spectroscopy (DLTS) Measurement Features and Principles The Deep-Level Transient Spectroscopy (D LTS) technique developed by Lang [63] provides a fast thermal scan of all the defect levels that are electrically active in the junction region of a diode. The main features of DLTS include: The saturated peak height is directly propor tional to the density of the defect level. The temperature at which the peak occurs is related to the ionization energy of the defect level. The sign of the peak indicates whether it is because of majority or minority carrier emission. The activation energy of the defect level can be determined from an Arrhenius plot of the emission rate versus inverse temp erature from several DLTS scans of the trap level with different rate windows.

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113 The carrier capture cross sect ion can be determined directly from the dependence of DLTS peak height on pulse width. Therefore, the DLTS technique allows a scan of all defect levels and provides information concerning defect density, energy level, and capture cross section. From the DLTS scans, one can obtain the useful information concerning defects in the depletion region of a p-n junction by observi ng the capacitance transient associated with the return to thermal equilibrium followi ng an applied non-equilibrium condition. Majority and minority carrier traps can be investigated by a mo mentary zero bias ( VH = VR, where VH is the applied pulse height and VR is the quiescent reverse bias) and a forward bias condition ( VH > VR), respectively. Before applying the pulse, the diode is under reverse bias condition with a wide depletion region and small capacitance. During the pulse, the depletion width decrea ses and the carrier tr aps in the depletion region are filled with the carriers. After the pulse, the depletion region returns to the equilibrium condition but the capacitance reach es its quiescent value slowly because of the trapped carriers. It is possible to characterize the hole-emission rate ( ep) as a function of inverse temperature ( T-1) in the p-type absorber layer by scanning the capacitance change over a wide range of temperatures under different rate windows in a CIGS n-p junction cell. The holeemission-rate is related to the hol ecapture cross-secti on and activation energy by kT E N v ea v th p pexp (6-17) Where th is the thermal velocity, p is the hole captur e cross-section, and Nv is the effective density of the valence band st ates [59]. From an Arrhenius plot ( i.e. ln(T2 )

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114 vs. T-1, where = 1/ep), the activation energy ( Ea) and the capture cross-section (p) of the holetrap level can be extracte d. If the capture cross-section for the hole trap is because of a thermally activated process, then it can be expressed by p = exp(Eo/ kT ), where is the hole capture crosssection when the temperature approaches infinity, and Eo is the capture cross-section activati on energy. The defect density ( NT) can be calculated using the expression A TN C C N02 (6-18) Where NA is the net holedensity in the absorber layer; C is the capacitance change because of the emission of hole carriers from th e trap level, which is proportional to the trap density, and Co is the junction capacitance measur ed at the DLTS peak temperature with a quiescent reve rse bias voltage ( VR). Equation 6-18 is valid for NT < NA. The net hole-density in a p-type CIGS abso rber can be determined from the C-V measurements. The DLTS spectra (Figure 6-10 (a)) meas ured for a CIGS device from EPV, obtained using a pulse height of VH = 0.3 V, a reverse bias of VR = -0.1 V and a saturation pulse width of W = 10 ms. This CIGS cell shows a deep holetrap at T 270 K. The activation energy calculated from the Arrhenius plot of Figure 6-10 (b) for this holetrap is Ea = Ev + 0.94 eV The average hole-density ( Na) obtained from the C-V measurement at 270 K was 3 1015 cm-3. Using the capacitance vs. temperature (C-T) data, Co was found equal to 301 pF, and from Equation 6-18 the hole trap density ( NT) was found to be 6.5 1013 cm-3 [64].

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115 Related Results and Discussions Because of the fast thermal scan of the test cell during DLTS measurement, it is very important to know whether there are so me effects on CIGS electrical performance during or after DLTS. Based on this idea, a CIGS solar cell (fabricated by NREL) was measured by photoand darkJ-V system befo re and after the DLTS scans. The DLTS scan (Figure 6-11 (a)) measured for this CI GS device, obtained using a pulse height of VH = 0.7 V, a reverse bias of VR = 0.5 V and a saturation pulse width of W = 10 ms. This CIGS cell shows a shallow trap at T 120 K. The activation energy calculated from the Arrhenius plot (Figure 6-11 (b)) for this holetrap is Ea = Ec 0.038 eV The average hole-density ( Na) obtained from the C-V measurement at 120 K is 7.53 1015 cm-3. From Equation 6-18 the trap density ( NT) was found to be 1.10 1014 cm-3. (DLTS data and calculation were made by W. K. Kim) Darkand photoJ-V measurements were taken before and after the DLTS scan. The results (Table 6-4, Figure 6-12 and 13) s how that during the thermal scan (between 80 and 300 K), the J-V curve almost remains unchanged, which suggest that the DLTS scan between 77 and 300 K does not affect th e J-V characteristics of the CIGS cell.

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116 Table 6-1. Hall-effect results for the different Cd-partial-electrolyte treatment times on the CIGS samples Samples Hole density (cm-3) Hall mobility (cm2/V-s) Resistivity ( -cm) No treatments 5.31015 8.89 133 Cd-dip (15 min) 2.91016 0.93 235 Cd-dip (15 min) 4.31016 1.54 94 Cd-dip (30 min) 7.11016 0.60 148 Note: The carrier conduction types for all CI GS samples tested are p-type assuming a uniform sample.

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117 Table 6-2. Results from Hall-effect measurements for the CIGS samples Sample # Hole-concentratio n (#/cm-3) Hall-mobilit y (cm2/V-s) Resistivity ( -cm) Film type A (NREL) 5.31015 8.9 133 P-type B (EPV) 2.61014 20.5 1166 P-type C (GSE) 1.51016 9.0 51 P-type D (ISET) 8.41015 0.4 2046 P-type E (SSI) N/A N/A N/A N/A F (IEC) N/A N/A N/A N/A

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118 Table 6-3. Hall measurement results for th e CIS/CGS samples deposited on different substrates (GaAs/glass) by PMEE method Samples Carrier density (cm-3) Hall mobility (cm2/V-s) Resistivity ( -cm) Type of carriers 355-CGS/GaAs 2.6771012 520.5991 4478.535 Electrons 354-CIS/GaAs 3.7951010 5732.827 28965.06 Electrons 353-CGS/GaAs 2.5001016 10.22044 24.42644 Holes 350-CIS/GaAs 1.1531017 19.54468 1.384847 Holes 355-CGS/glass 1.5461018 0.07958 50.68024 Holes 354-CIS/glass 3.8421011 298.8994 54362.13 Holes 353-CGS/glass 4.8291018 1.105717 1.168984 Holes 350-CIS/glass 5.0861017 4.137745 2.966456 Holes

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119 Table 6-4. PhotoJ-V results for EPV CI GS device before and after DLTS scan Voc (v) Jsc (mA/cm^2) F.F. (%) Eff. (%) Before DLTS 0.517 28.7 56.6246 8.4 After DLTS 0.51725 27.6 57.834 8.257

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120 Figure 6-1. Schematic representati on of the Hall effect sample. Figure 6-2. Typical fourpoint van der Pauw and Ha ll effect measurements.

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121 Figure 6-3. The equivalent circuit diag ram of a CIGS PN junction solar cell. Figure 6-4. The I-V characteristics under dark and illumination conditions of a PN junction solar cell.

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122 ELH lamp A VCooling systemCell Voltage Cell Current Variable LoadTemperature monitor CIS solar cell PC control (LabView) Temperature Controller Lab Jack ELH lamp A VCooling systemCell Voltage Cell Current Variable LoadTemperature monitor CIS solar cell PC control (LabView) Temperature Controller Lab Jack Figure 6-5. Apparatus and block diagram of I-V measurement system for the CIS-based cells. Figure 6-6. The photoJ-V measurement a nd analysis for solar cells using the LabVIEW program. (The tested CIGS solar cell is fabricated by EPV.)

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123 Figure 6-7. The dark-J-V measurement a nd analysis for solar cells using the LabVIEW program. (The tested CIGS solar cell is fabricated by NREL.)

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124 Cooling systemVariable Load Test cell A Lock-in Amplifier Optical diffuser MonochromatorChopperOrder sorting filter Bias Light Mirror Reference detector Voltmeter PC Control (LabView) Light source DC Power Supply Cooling systemVariable Load Test cell A Lock-in Amplifier Optical diffuser MonochromatorChopperOrder sorting filter Bias Light Mirror Reference detector Voltmeter PC Control (LabView) Light source DC Power Supply Figure 6-8. The block diagram of a spect ral response measurement system for the CIS-based solar cells. Figure 6-9. The spectral response and quantum efficiency measurements for solar cells using LabVIEW program. (The tested CIGS so lar cell is fabricated by UF.)

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125 50100150200250300350 -4.0 -3.0 -2.0 -1.0 0.0 1.0 100 ms 50 ms 20 ms 10 ms 5 ms 2 ms 1 ms 0.5 ms 0.2 ms 0.1 ms 0.05 ms 0.02 msC (pF) EPV CIGS cell VR=-0.1 V Pulse Height = 0.3 V Pulse Width = 10 ms Temperature (K)(a)50100150200250300350 -4.0 -3.0 -2.0 -1.0 0.0 1.0 100 ms 50 ms 20 ms 10 ms 5 ms 2 ms 1 ms 0.5 ms 0.2 ms 0.1 ms 0.05 ms 0.02 msC (pF) EPV CIGS cell VR=-0.1 V Pulse Height = 0.3 V Pulse Width = 10 ms Temperature (K)(a) 0.0000.0010.0020.0030.0040.005 -12.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0 Ea=EV+0.94 eVln(T2)1/T(K-1)(b)0.0000.0010.0020.0030.0040.005 -12.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0 Ea=EV+0.94 eVln(T2)1/T(K-1)(b) Figure 6-10. The DLTS scans for (a) EPV device at VR = -0.1V, VH = 0.3V, and W = 10ms. And (b) An Arrhenius plot obtaine d from the DLTS scans shown in (a).

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126 DLTS CIGS (NREL)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 050100150200250300350400450500Temperature (K)Delta capacitance (pf) 0.02 ms 0.05 ms 0.1 ms 0.2 ms 0.5 ms 1 msReverse bias = 0.5 V Pulse height = 0.7 V Pulse width = 10 ms y = -0.44x + 2.0018 R2 = 0.9744-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 6789101112131000/T, K-1ln(T2/En) Figure 6-11. The DLTS scans for (a) EPV device at VR = 0.5V, VH = 0.7V, and W = 10ms. And (b) An Arrhenius plot obtained from the DLTS scans shown in (a).

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127 0 5 10 15 20 25 30 00.10.20.30.40.50.6 V (V)J (mA/cm^2) Jd(before DLTS) Jd ( after DLTS ) Figure 6-12. DarkJ-V curves of tested EPV CIGS device before and after DLTS measurement. 0 5 10 15 20 25 30 00.10.20.30.40.5 V (V)J (mA/cm^2) Jph(before DLTS) J p h ( after DLTS ) Figure 6-13. PhotoJ-V curves of tested EPV CIGS device before and after DLTS measurements.

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128 CHAPTER 7 DEVICE MODELING AND SIMULATI ON OF CIGS SOLAR CELLS In chapter 4, we discussed the RTA effects on CIGS solar cells, and the best result we obtained was from the first set of CIGS samples with progressive RTA treatments. Unfortunately the sample, which contains three CIGS cells, was destroyed after we applied 400oC RTA treatment. Therefore the only e xperimental data we have on these cells are the values of Voc, Jsc, F.F. and measured before and after each progressive annealing up to 300oC. In order to extract other important paramete rs of these cells, the device modeling and simulation of these ce lls were carried out using the AMPS-1D (Analysis of Microelectronic and Photonic St ructures) device simulation program [65]. The main objective of this study is to obtai n the simulated defect density in the CIGS cells before and after each pr ogressive annealing, and to extract the darkJ-V and Q-E curves, which we could not perform on the damaged cells experimentally. Simulation Model of CIGS Solar Cell Devices It has been reported [66] that an In-ric h n-type surface layer, which was identified as an ordered vacancy compound (OVC) and te ntatively assigned as the stoichiometry CuIn2Se3.5 or CuIn3Se5, exists between CdS buffer layer and CIGS absorber layer. A junction model was proposed that consis ts of a chalcopyrite /defect chalcopyrite heterojunction between the p-type bulk CIGS absorber and n-type OVC layers. The electrical characteristics of low hole mobility ( p 10 cm2/V-s), high resistivity (105-106 cm), and low carrier densities (1011-1012 cm-3) were proposed for the defect

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129 chalcopyrite Cu(In1-xGax)3Se5 material s for 1>x>0 [67]. Furthermore, the conductivity type changes from nto p-type when the Ga conten t exceeds 30% [67]. The CIGS solar cell structure used in this simulation c onsists of a 155 nm ZnO (Eg = 3.3 eV ) TCO layer, a 51 nm CdS (2 .4 eV) buffer layer; an ultra thin high-recombination interface layer (1.5 eV) with a high dens ity of effective recombination centers is placed at the meta llurgical junction betw een the CIGS surface layer and CdS buffer layer; an inverted su rface layer (or OVC) with a thickness of 60nm, net carrier density n=3x1012 cm-3, band-gap energy, Eg=1.3 eV, is inserted between the CdS and CIGS layers, and an in terfacial layer; and finally th e CIGS absorber layer with band-gap energy Eg=1.2 eV (CuIn0.68Ga0.32S e). The total thickness of the absorber layer is assumed equal to 2 m for all simulation cases in this study. The computer simulation tool AMPS-1D is employed by sp ecifying the semiconductor parameters in each defined layer of the cell structure as input parameters in the simulation. In view of a limited knowledge of the semiconductor parame ters in each layer and uncertainties in the interface and junction properties arisi ng from possible interdiffusion and reaction during the cell processing, the division of the layers for the cell structure is limited to a simplified device structure as depicted a bove. The schematic energy band diagram under equilibrium condition for a typical CI GS/CdS/ZnO solar cell with a uniform band-gap is illustrated in Figure 7-1 [60]. Ba sed on the Hall-effect results depicted in the previous chapters, some device parameters such as carrier mobility, carrier concentration were specified by using the experimental data. To proceed with the simulation, other unknown material parame ters are reasonably employed according to some previous literatures.

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130 The objective of this study is to analyze th e trend in the performance of CIGS cells versus the defect density of the CIGS absorb er layer, and to fit the experimental photoJ-V data by adjusting the defect density of the CIGS absorber layer and varying other parameters in a small range, finally backward extract the parameters and cells performance which we could not obtain from the damaged device experimentally. Simulation Results of CIGS Solar Cells In this study, the defect density of CIGS absorber is varied from 1x1014 cm-3 to 1x1019 cm-3, all other parameters are unchanged. Th e results (Table 7-1 and Figure 7-2) reveal that the cell performance is nearly unchanged for defect density below 2x1016 cm-3. Above this value, Voc, Jsc and conversi on efficiency start dropping along with the increase of defect density, which suggests th at to gain an optimal device performance, the defect density should be less than 2x1016 cm-3. It is also found that the defect density does not affect the fill factor in the simulations. In chapter 4, we have discussed the eff ect of progressive RTA treatment on CIGS solar cells. Unfortunately the whole de vice was destroyed after we tried the 400oC RTA treatment. The only experimental data av ailable for simulation is the photoJ-V parameters. In this chapter, an excellent f it of the experimental J-V data was obtained by our simulation. In the progressive RTA study, the most significant performance improvement occurs on cell#1, and other cells on ly showed a slight increase in the photoJ-V parameters (Table 4-1). Therefore, the simulation was focused on fitting the progressive RTA data in cell #1. In the simulations, some parameters (such as carrier mobilities and carrier concentrations) of CIGS absorber layer are set and varied based on the Hall-effect data listed in Table 4-3. By mainly adjusting the defect density along with varying other parameters such as carrier co ncentration of ZnO TCO layer and CdS buffer

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131 layer in a small range, a good agreement betw een the experimental data and simulation results of cell#1 before and after progre ssive RTA was obtained (Table 7-2). The simulation results show that defect de nsity of CIGS layer decreased from 4x1017 to 1x1016 cm-3 after 3 consecutive progressive RT A treatments, which suggest that progressive RTA might be an effective treatment to reduce the defect density of CIGS solar cells. Figure 7-3 shows the photoJV curve of cell#1 before and after each progressive RTA treatme nt from the simulation results. To continue extract other important parameters, data of the dark curre nt and spectral respons e were obtained from the simulation. From the darkJ-V curves (Figure 7-4), in the low bias region (Va < 0.6 V) there is no difference of darkJ-V cu rves between data before and after each annealing. When the applied voltage is between 0.6 and 0.8 V, the darkJ-V curves after 200oC and 300oC were found to be slightly higher th an data before RTA, and the darkJ-V after 100oC was similar as the curve before RTA. In the high bias region with applied voltage greater than 0.88 V, the darkJ-V curves after each progressive RTA were found to be apparently lower than data before RTA. These re sults suggest that RTA under certain annealing conditions can possibly re duce the defect density and hence decrease the recombination current th rough the device. The recovere d Q-E curves (Figure 7-5) show explicitly higher photocurrent density after each progressive RTA than data before RTA. It is also noticeable that improveme nts in each progressive RTA treatment occur only for photon wavelengths longer than 650 nm which suggests that the RTA treatment has positive effects on the CIGS absorber laye r. It is not clear if RTA has any direct effect on the ZnO TCO layer and CdS and buffer layer. It should be noted that from the

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132 simulation results we did not see any phot ocurrent improvement in the shorter wavelength region. Conclusions In this chapter, simulation study of def ect density versus CIGS device performance was conducted. Results show that cell perf ormance start dropping with the increase of defect density from 2x1016 cm-3, and there is no strong relati on between defect density and F.F. in the simulation. A well fitting of experimental progressive RTA data was also obtained by simulation, the results show that progressive RTA treatment can reduce the defect density in CIGS absorber layer a nd hence improve the photoJ-V, darkJ-V performance and spectral response of the devi ce. Future effort will continue detailed study on effects of defect de nsity on CIGS cells performa nce, and will correlate the simulation data and the experimental data from our previous annealing study.

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133 Table 7-1. Performance parameters of the CIGS (Eg=1.2 eV) solar cells versus the defect density in the CIGS absorber layer. Defect density (cm-3) Jsc (mA/cm2)Eff. (%) F.F. Voc (V) 1 1014 30.434 14.699 0.734 0.658 2 1014 30.433 14.699 0.734 0.658 5 1014 30.431 14.698 0.734 0.658 1 1015 30.427 14.695 0.734 0.658 2 1015 30.420 14.691 0.734 0.658 5 1015 30.399 14.678 0.734 0.657 1 1016 30.366 14.657 0.734 0.657 2 1016 30.303 14.617 0.734 0.657 5 1016 30.148 14.518 0.734 0.656 1 1017 29.958 14.393 0.734 0.655 2 1017 29.718 14.226 0.733 0.653 5 1017 29.396 13.970 0.733 0.648 1 1018 29.205 13.755 0.733 0.643 2 1018 29.076 13.583 0.734 0.636 5 1018 28.977 13.314 0.736 0.624 1 1019 28.937 13.073 0.737 0.613

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134 Table 7-2. PhotoJ-V data of cell#1 before and after progressive RTA treatment (From experiment and AMPS 1D simulation results) Before 100oC 200oC 300oC Voc (V)* 0.628 / 0.6282 0.633 / 0.6331 0.652 / 0.6525 0.627 / 0.6318 Jsc (mA/cm2)* 31.66 / 31.64 34.30 / 34.30 34.85 / 34.796 35.39 / 35.392 F.F. (%)* 47.88 / 46.8 56.81 / 56.8 68.43 / 68.2 71.05 / 71.5 Eff. (%)* 9.520 / 9.647 12.32 / 12.276 15.55 / 15.409 15.77 / 15.763 Defect density ** 41017 cm-3 11017 cm-3 31016 cm-3 11016 cm-3 Data obtained from experiment / simulation ** Data obtained from simulation only

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135 ZnO Inverted Layer CdS CIGS E c E v E f Figure 7-1. The schematic energy band diagra m of a typical ZnO/CdS/CIGS solar cell under equilibrium condition.

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136 Figure 7-2. Performance parameters of CIGS (Eg=1.2eV) solar cells versus the defect density of CIGS absorber layer.

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137 0 5 10 15 20 25 30 35 40 00.10.20.30.40.50.60.7 Applied Voltage (V)Photo Current Density (mA/cm^2) Before RTA After 100 C RTA After 200 C RTA After 300 C RTA Figure 7-3. PhotoJ-V curves of cell#1 be fore and after progressive RTA treatment (From AMPS 1D simulation results).

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138 0 100 200 300 00.20.40.60.81Applied Voltage (V)Dark Current Density (mA/cm^2) Before RTA After 100 C RTA After 200 C RTA After 300 C RTA Figure 7-4. DarkJ-V curves of cell#1 before and after progressive RTA treatment (From AMPS 1D simulation results).

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139 0 0.2 0.4 0.6 0.8 1 0.40.50.60.70.80.911.11.2Wavelength (um)QE Before RTA After 100 C RTA After 200 C RTA After 300 C RTA Figure 7-5. Quantum efficiency of cell#1 be fore and after progressive RTA treatment (From AMPS 1D simulation results).

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140 CHAPTER 8 RADIATION EFFECTS ON CIGS FILM AND SOLAR CELLS One of the most promising applications of thin film solar cells is to work in the outer space. Therefore it is ve ry important to investigate the radiation tolerance of CIGS solar cell material. In this chapter, our re search focuses on advancing the understanding of radiation effects on CuInGaSe2 films and devices. Introduction Two main areas of interest focus on the displacement effects and enhanced low dose rate sensitivity (ELDRS) in devices with only a few atomic layers controlling the basic device physics. Displacement damage at ohmic contact interfaces may be more sensitive to degradation mechanisms caused by ionizing radiation than device junctions. Similarly, the fundamental m echanisms underlying ELDRS may be revealed and controlled at these reduced dimensions. The usually accepted view is that displacement damage is proportional to the non-ionizing energy loss (NIEL defined in units of keVcm2/g) or with higher energies by the displacement cross-secti on (D in MeVmb). It is unclear if the NIEL hypothesis (more general than non-impurity divacancies) can be used to relate damage because of different particles and energies in reduced scale photovoltaic devices UF has excellent capabilities for in-situ electrica l and elevated temperature i rradiations. This study on the radiation induced damage in CuInGaSe2 solar cells utilizes irradiations performed unbiased at room temperature. This met hod is an acceptable simplified experimental procedure for conducting such a study.

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141 ELDRS Effects The ELDRS effect is normally associat ed with linear devi ces having junction isolated bipolar transistors (and is not a part of this work). Detailed characterization of point defects, defect clusters, interstitials and electrical active traps offers a valuable insight leading to a better understanding of the basic degr adation mechanism. Future opportunities are envisioned w ith ultra-thin boundaries betw een materials of widely differing atomic number (copper interc onnects, high impedance bonds, complex metallized layers, etc.). These may be se nsitive to dose-enhan cement under electron or bremsstrahlung irradiation. Total Dose Effects Total dose effects in semiconductor devices depend on the creation of electron-hole pairs within dielectric layers (oxides, nitrid es etc.) and subsequent generation of traps at or near the interface with the semiconductor or of trapped charge in the dielectric. This can produce a variety of device effects such as flat-band and threshol d voltage shifts and surface leakage currents. There is an extensive literature on total dose effects. One of the most recent and detailed reviews has been given by Dre ssendorfer in his 1998 NSREC Short Course Notes [68]. Although the concept of total ionizing dose is a useful first order approximation for quantifying effects there are dependencies on a number of other parameters. Notably, the linear energy transfer (ener gy deposited per unit pa th length) of the radiation and the applied electric field both influence the rate of recombination of electron-hole pairs, while the dose-rate influences the relative importance of hole traps and interface states. These are extensively discussed in [68]. Strictly dose should be defined in relationship to

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142 the material affected. This is commonly th e silicon dioxide rather than silicon. Giving average doses in a device can lead to significant errors because of dose enhancement effects at boundaries. The basic mechanisms of total dose effects c ontinue to be a matter of research but it is now well established that hydrogen (i.e., prot on) plays an important role [69]. In recent years considerable effort has been directed to processing optimizati ons to 'harden' the dielectric materials (usually silicon dioxide). With the trend towards thinner oxides, attention has shifted to the removal of 'parasiti c' leakage paths in the thicker field oxides. This can often be achieved 'by design' so that provided a circuit is designed using cells from a radiation tolerant lib rary, performance can be guaranteed regardless of the particular foundry used. However the device s caling which results in thinner oxides also leads to microdosimetry effects where the to tal dose deposited by single energetic ions can cause permanent effects such as stuck bits in memories [70] or permanent damage to CMOS readout circuits for imagers [71]. In ultra-thin oxides new phenomena such as radiation induced leakage current (RILC) are becoming apparent because of me chanisms such as trap assisted tunneling. Also the trend to nitrided oxides (to suppre ss hot carrier effects) has implications for radiation tolerance [72]. Displacement Damage Energetic particles such as neutrons, protons, electrons, -particles and heavy ions can create damage in semiconductor materials by displacing atoms in the crystal lattice. Secondary electrons produced by high-ener gy photons will also produce displacement effects. The result is that st able defect states are created within the bandgap that can give

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143 rise to any of the five effects (Figure 8-1), depending on the temperature, carrier concentration and the location at which the defect resides. Generation of electron-hole pairs (leading to thermal dark current in detectors) Recombination of electronhole pairs (leading to reduc tion of minority carrier lifetime and effects in LEDs and laser diodes) Trapping of carriers, leading to loss in char ge transfer efficiency in CCDs (minority carrier trapping) or carrier rem oval (majority carrier trapping) Compensation of donors or acceptors, also leading to carrier removal in some devices (for example the resistance in a lightly doped collector in a bipolar transistor can increase) Tunneling of carriers, leading to increased current in reverse biased junctions particularly for small bandgap mate rials and high electric fields Displacement damage is proportional to the non-ionizing energy loss, NIEL (usually defined in units of keVcm2/g, though in high energy physics the displacement damage cross section (D) in MeVmb is usua lly used). The NIEL hypothesis can be used to relate damage because of different partic les and energies this greatly reduces the amount of testing needed (usually only one pa rticle and energy, e.g. 10MeV proton, is used). The NIEL scaling hypothesis leads to the concept of displacement damage equivalent dose (=NIEL x partic le fluence) [73]. This can be measured in keV/g or in (non-ionizing) rads. Displacement damage does not depend significantly on irradiation bias or temperature, hence irradiations can be perf ormed unbiased at room temperature this simplifies experimental procedures. These a ssumptions appear to be adequate for many cases. However when the exact nature of the defect is important then damage may not always scale with NIEL. Such an exception has been discussed by Dressendorfer [68] (in that case the difference between diffusion leng th damage in nand p-type silicon solar

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144 cells). Another case has recently emerged in the field of high energy physics where neutron and proton effects in oxygen doped silicon microstrip detectors have been found to differ. The reasons for this probably lie in the differences in damage clustering (and subsequent defect kinetics) between di fferent particle types and energies. It is now well established th at the amount of formation of defect clusters depends on the particle type. Electron irradiation gives primary knock-on atoms (PKAs) with low recoil energies and hence leads to almost ex clusive production of poi nt defects; whereas neutrons give a flat PKA spectrum and a mu ch greater proportion of cluster formation. For protons the situation is in between. In some cases the amount of clustering may not matter, only the total number of defects. Howe ver the clustering can be expected to affect the defect kinetics. Recent work by Watts suggests that when impurity related defects (e.g. the E-centre) are involved then NIEL sca ling may not always be strictly valid. The implication for space instrumentation is that tests at a single proton energy may not allow an accurate prediction in all cases. Fortunate ly there are still many cases where the NIEL hypothesis is valid, in particul ar where non-impurity related de fects, such as divacancies are involved. Recently Srour [74] has sugge sted a universal damage factor for the displacement-damage-induced dark current in silicon devices. This suggests a common defect such as the divacancy is involved, and it is seen th at NIEL scaling is effective. Other factors, which can affect the gene ration of defects, are the irradiation temperature and post-irradiation annealing. Us ually it is assumed that neither irradiation temperature nor bias has an important effect and that annealing at room temperature takes place in only the first few weeks after irradiation. However, this has not been studied in

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145 detail for situations (such as CCDs at low te mperature) where the details of the defect kinetics may be important. Although the largest bod y of displacement damage results applies to silicon devices, effects in other semiconductors are becoming increasingly important, particularly for photonics de vices involving materials such as GaAs, InP and SiGe. Prediction of the NIEL for advanced materials is therefore an important topic, especially as some devices (such as optocouplers, amphot erically doped and si ngle-heterojunction LEDs) are especially vulnerable in orbits subjected to intense proton fluxes. A simple technique for predicting NIEL using the SRIM code has been disc ussed recently [75]. However this does not include nuclear reactio ns, which are important at high energies. Note that discrepancies between the measur ed and predicted high energy NIEL for GaAs devices have been actively discussed in re cent years and the issue is not yet fully resolved. As for silicon devices, it is well known th at imaging devices, such as CCDs and CMOS active pixel sensors, s how dark current spikes beca use of displacement damage. These are individual pixels with higher th an average dark current. The dark current non-uniformity then depends not only on the average NIEL but also on its variance (as recently reviewed by Robbins [76]). In fact there are cases where a detailed Monte Carlo simulation of the damage cascades is necessary e.g. when interactions of highenergy protons within small depletion vol umes have to be considered. Displacement damage in linear devices (suc h as operational amplifiers) has recently been discovered to be a potentially im portant phenomenon, making prediction of displacement effects important.

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146 High-K Dielectrics Another area of new technology is the use of high-K dielectrics (i.e. materials with a high dielectric constant) and copper interconnections. Total ionizing dose can also influence mate rials properties. For example transparent materials, such as cover glasses, can beco me opaque via the populat ion of color centers, while plastic materials can become embritt led. Radiation-induced conductivity is an important phenomenon in mitigating charging of dielectric materials. In general, the important issues for total dose effects include the following parts. Dependence on bias during irradiation (irr adiation whilst the device is biased is usually the worst case). Annealing effects (trapped charge reduces after irradi ation, while interface traps tend to build-up). Dependence on dose rate (mainly because of annealing effects) Dependence on package and burn-in (especiall y for some types of plastic package) Variability from batch to batch a nd device to device (especially for commercial-off-the-shelf devices) In linear devices with junction isolated bipolar transistors there is a pronounced "enhanced low dose rate sensit ivity" (ELDRS) effect where the damage is greater at low dose rates. Of relevance to potential work under this project is the problem of dose-enhancement under electron or brem sstrahlung irradiation where there are boundaries between materials of widely di ffering atomic number. These can occur in packaging and shielding as well as on the die (e.g. copper interconnects, high-Z bump bonds, metallized layers, Au-Si die attachments). Enhancement factors can approach a factor two. Experimental Details and Discussion The gamma rays from cobalt-63 (1.17 & 1.33MeV) are above the pair production threshold energy. This is s hown in the gamma absorption cross-section graph (Figure 8-2). The predominant interactions of photon with the lattice are shown in the photon

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147 interaction probabilities (Fi gure 8-3). While the gamma rays are not capable of displacement damage directly, the hot electrons they create do have the ability to do this. The electron in most materials require s less than 30 eV to join in the conduction process if within a diffusion length of the sp ace charge region in pn junctions. Increased leakage currents, decreased gains and trapped charges in oxides are some of the common results of radiation damage. However in this st udy, our intent is to analyze the radiation tolerances of CIGS materials by charact erizing films surface morphology and devices performance before and after irradiation. By mounting four CIGS films/devices with difference distance from the radiation source, th e radiation dose is de termined by use of a radio chromatic film and measuring its cha nge in optical density. Total dose is extrapolated using time and information from a calibration exposure of ~ 1 Mrad (Si). In this study, 4 identical CI GS films and 4 CIGS devices were irradiated with 4 different dose rates. There is one extra CIGS sample (absorber only) served as the control sample for the surface comparison of the irradiated samples. Table 8-1 lists the dose rate and total dose of the tested samples and devi ces. PhotoJ-V measurem ents were taken for the 4 irradiated devices before and after th e radiation treatment. There is an explicit surface color change and fade of the meta l contact on each device after irradiation. Unfortunately all the irradiated devices show ed a linear J-V curve after radiation even new metal contacts were depos ited. For the irradiated CIGS samples (absorber layer only), the surface color also changed dramatically and the color changed area becomes larger as the total dose of sample received goes higher, as one can see from the photograph (Figure 8-4), the dark area increased along with th e increase of the total dose. XRD measurements were taken in order to analyze the change of the film composition

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148 and crystallinity by radiation. From the XRD results (Figure 8-5) of 4 irradiated CIGS samples and the control sample, there is no new peak observed among the irradiated samples in comparison to that of the control sample in the XRD plots, indicating that the film composition remains unchanged after radia tion. By close looking at each CIGS peak, we found a slight decrease of peak height s (XRD counts, from #5 down to #1) as we increase the total dose of radiation on the sa mples, indicating that the crystallinity of CIGS sample becomes poor once the radiation dose goes higher. Because of the strong J-V performance drop in the previous experiment, a new set of radiation experiment wa s carried out on 2 CIGS devi ces (from GSE and SSI) with much lower total dose. Two cells on each de vice were measured by photoJ-V system before and after irradiation. From the radi ation condition and PhotoJ-V results/curves (Table 8-2, Figure 8-6, to 8-9) of each cell before and after ra diation, for the 2 tested cells of GSE CIGS device (#1) with total received dose of 2.12 Mrads (Si), we found a explicit increase of Voc, F.F. and after radiation, wh ile a decrease of Jsc was also observed. For the 2 tested cells of SSI CIGS device (#2) irradiated with to tal received dose of 6.5 Mrads (Si), only Voc was improved after radiation, all other parameters: Jsc, F.F. and has strong decrease after radiation, indi cating that CIGS solar cells performance could possibly be improved by Gamma-ray radiation under low dose condition (2.12 Mrads) and 6.5 Mrads or higher radiation dose coul d probably damage the solar cell and hence cause the performance drop. These results suggest that th e estimated threshold of the total radiation dose in this radiation tolera nce study should be some point between 2.12 and 6.5 Mrads to remain the device performance un-dropped.

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149 Our future effort will focus on detailed study of effects of low dose Gamma-ray radiation on CIGS solar cells, as well as to find the accurate radiation dose threshold in our future radiation toleran ce study on CIGS materials.

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150 Table 8-1. Dose rate and total dose of the tested CIGS samples and devices Sample/device # Dose rate (Krads(Si)/hr) Total dose (Mrads(Si)) 1 82.0 61 2 65.8 49 3 61.7 46 4 44.2 33 5 (control sample) N/A N/A

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151 Table 8-2. Dose conditions and PhotoJ-V results of CIGS devices before and after radiation Device # #1 (cell#1) #1 (cell #2) #2 (cell#1) #2 (cell#2) Dose (Mrads) 2.12 2.12 6.5 6.5 Voc (V)* 0.501 / 0.538 0.505 / 0.538 0.495 / 0.566 0.498 / 0.562 Jsc (mA/cm2)* 28.89 / 27.83 29.12 / 25.89 34.51 / 27.60 33.36 / 20.25 F.F. (%)* 47.12 / 63.55 46.33 / 63.03 45.46 / 44.30 46.15 / 43.37 Eff. (%)* 6.823 / 9.639 6.818 / 8.778 7.767 / 6.918 7.671 / 6.215 Data taken before / after radiation

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152 Figure 8-1. Five basic effects of a defect energy level (Et) on the electrical performance of a device.

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153 Figure 8-2. Gamma absorption cros s-section versus photon energy.

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154 Figure 8-3. Photon interaction pr obabilities versus photon energy.

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155 Figure 8-4. Photograph of irradiated CIGS films and the control film.

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156 0 10000 20000 30000 40000 50000 60000 2025303540455055606570 2 theta (degree)CountsCIGS (112)Mo (110) CIGS (204)/(220) CIGS (312)/(116) Sample #1 Sample #2 Sample #3 Sample #4 Sample #5 Figure 8-5. The XRD results of irradiated CI GS samples (#1-4) and the control sample (#5).

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157 0 5 10 15 20 25 30 00.10.20.30.40.50.6 Voltage (V)Photocurren Density (mA/cm^2) before after Figure 8-6. PhotoJ-V results of CIGS device #1 (cell#1) be fore and after total dose of 2.12 Mrads (Si) radiation (Device from GSE).

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158 0 5 10 15 20 25 30 00.10.20.30.40.50.6 Voltage (V)Photocurren Density (mA/cm^2) before after Figure 8-7. PhotoJ-V results of CIGS device #1 (cell#2) be fore and after total dose of 2.12 Mrads (Si) radiation (Device from GSE).

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159 0 5 10 15 20 25 30 35 00.10.20.30.40.50.6 Voltage (V)Photocurren Density (mA/cm^2) before after Figure 8-8. PhotoJ-V results of CIGS device #2 (cell#1) be fore and after total dose of 6.5 Mrads (Si) radiation (Device from SSI).

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160 0 5 10 15 20 25 30 35 00.10.20.30.40.50.6 Voltage (V)Photocurren Density (mA/cm^2) before after Figure 8-9. PhotoJ-V results of CIGS device #2 (cell#2) be fore and after total dose of 6.5 Mrads (Si) radiation (Device from SSI).

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161 CHAPTER 9 SUMMARY AND FUTURE WORK Summary The effects of Pulsed Laser Annealing (PLA) treatment on the film properties and the performance of CIGS so lar cells have been studi ed under various annealing conditions. This technique has been used to modify near-surface defects and related junction properties in Cu(In,Ga)Se2 (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using tw o different laser system s at selected laser energy densities and pulse number. The na rrow of XRD peak, the new shoulders of GIXD and the increase of SEM surface featur e size suggest near surface structure changes in the CIGS films. The Dual-B eam Optical Modulation (DBOM) and Halleffect measurements reveal that PLA treatmen t increases the effectiv e carrier lifetime and mobility as well as the sheet resistance of the CIGS absorber. In addition, several annealed CdS/CIGS films processed by PLA were fabricated into solar cells and characterized by photoand darkJ-V and quantum efficiency (Q-E) measurements. The results show a signi ficant improvement in the overall cell performance, diode quality and spectral respon se when compared to pre-annealed cells. Deeplevel transient spectroscopy (DLTS) re sults show a reduction of the density of shallow defect trap after low power PLA tr eatments. The energy density of the laser beam and the pulse number were found to play a key role in modi fying the optical and electrical properties of the CIGS films and hence the cell performance. A near optimal PLA condition was obtained in this study.

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162 A comprehensive study of the effect of Rapid Thermal Annealing (RTA) on the film properties and the performance of CIGS so lar cells has also been carried out in this work. CIGS samples and devices have been characterized by XRD, GIXD, SEM, Halleffect, photoand darkJ-V and Q-E measur ements on CIGS samples before and after RTA treatments under various ramp up and down rates, peak temperature, holding time, and different ambient conditions. Device m odeling and simulation were performed to study the progressive RTA effect on defect de nsity and other important parameters of CIGS solar cells. The results show that pr ogressive RTA treatment could significantly improve the overall uniformity and performan ce of large area CIGS solar cells. Under low RTA temperature, the surface compos ition and morphology remain unchanged. The RTA effect on CIGS devices shows partia l improvements of cell’s performance and overall increase of quantum efficiency. Th e estimated optimal annealing temperatures should be between 200 and 300oC with 1minute or less holding time. Device simulation using AMPS-1D program has also been carried in this work for a typical CIGS cell with different defect densities. A well fitting between simulation results and progressive RTA results has been obtained. The results show that the device performance and spectral response can be positively improved by reducing the defect density of CIGS absorber layer. An explorative study of the Gamma-Ray ra diation tolerance of CIGS films and solar cells were conducted to investigat e the total dose eff ects with different displacements on the film property and de vice performance by varying the sample displacements and exposure times. XRD and PhotoJ-V results before and after each

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163 radiation show that Gamma-Ray radiation is able to affect the crystallinity of CIGS samples and the device performance under certain dose condition. Future Work Based on the promising results from this work, future efforts should focus on PLA study using different types of Excimer laser sy stems that offer various laser beam sizes, different energy densities and wavelengths w ith uniform surface energy density, variable pulse widths and scan rates to access a wi der range of PLA treatment conditions. In particular, it is suggested that a variable pul se width and wavelength laser can be used to allow the control of anneal depth. Given its application to other industrial materials, laser annealing has the potential to be an effective method to improve solar cell performance in an industrial setting. Comparison of RTA and conventional furnace annealing should also be studied in the near future. A detailed simulation of effects of defect density on CIGS cells performance a nd the correlation between the simulation data and the experimental PLA and RTA data will be studied. Effects of low dose Gamma-ray radiation on CIGS solar cells, as well as the accurate radiation dose threshold will be investigated in the future radiati on tolerance study on CIGS materials.

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168 66. D. Schmid, D, M. Ruckh, F. Grunwald, and H.W. Schock, Journal of Applied Physics, vol. 73, no. 6, pp. 2902-2909, 1992. 67. M.A. Contreras, H. Wiesner, D. Niles, K. Ramanathan, R. Matson, J. Tuttle, J. Keane, and R. Noufi, Conference Record of the 25th IEEE Photovoltaic Specialists Conference, pp. 809-812, 1996. 68. P.V. Dressendorfer, 1998 IEEE Nuclear and Space Radiation Effects Conference, Short Course Notes, Ch. III, Newport Beach, CA, USA, 1998. 69. S.T. Pantelides, S.N. Rashkeev, R. Bu czko, D.M. Fleetwood and R.D. Schrimpf, IEEE Transactions on Nuclear Science, vol 47, pp 2262-2268, 2000. 70. C. Poivey, T. carriere, J. Beaucour and T.R. Oldham, IEEE Transactions on Nuclear Science, vol 41, pp 2235-2239, 1994. 71. J.C. Pickel, IEEE Transactions on Nucl ear Science, vol 43, pp 912-917, 1996. 72. C. Claeys and E. Simoen, European Space Research and Technology Centre contract report P35284-IM-RP-0013, March 30 2000. 73. C.J. Dale, P.W. Marshall, G.P. Summ ers and E.A. Wolicki, Applied Physics Letters, vol 54, pp 451-453, 1989. 74. J.R. Srour, IEEE Transactions on Nuclea r Science, vol 47, pp 2451-2459, 2000. 75. S.R. Messenger, E.A. Burke, G.P. Summers, M.A. Xapsos, R.J. Walters, E.M. Jackson and B.D. Weaver, IEEE Transact ions on Nuclear Science, vol 46, pp 1595-1602, 1999. 76. M.S. Robbins, IEEE Transactions on Nuclear Science, vol 47, pp 2473-2479, 2000.

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169 BIOGRAPHICAL SKETCH Xuege Wang was born in Beijing, China. He received a Bachelor of Science degree from the department of Electrical Engineeri ng at the Beijing Univer sity of Aeronautics and Astronautics, China, in 1999. Since 2000, he has been a research assistant in the Optoelectronics Laboratory at the University of Florida, under Dr. Sheng S. Li’s guidance. In December 2002, he was awarded a Master of Science degree in ECE from the University of Florida. After his graduati on, he continued to work toward his Ph.D. degree. His research interests include char acterization and fabrication of CIGS-based thin-film solar cells for low-cost terrestrial power generation.


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Title: Pulsed Laser Annealing and Rapid Thermal Annealing of Copper-Indium-Gallium-Diselenide-Based Thin-Film Solar Cells
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Copyright Date: 2008

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PULSED LASER ANNEALING AND RAPID THERMAL ANNEALING OF
COPPER-INDIUM-GALLIUM-DISELENIDE-BASED THIN-FILM SOLAR CELLS
















By

XUEGE WANG


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


2005




























Copyright 2005

By

Xuege Wang



























To my parents and my friends















ACKNOWLEDGMENTS

I would like to express my sincere gratitude to my supervisory committee chair

(Professor Sheng S. Li), for his support and encouragement in the past 5 years. Without

his patience and guidance, none of this work would have been possible. I would also like

to thank Professors Timothy J. Anderson, Oscar D. Crisalle and Gijs Bosman for serving

on my supervisory committee.

I extend special thanks to Dr. James C. Keane for his help on device fabrication;

Dr. Leon Chen, for supplying of CIGS samples; Professor Omar Manasreh, for his help

on thermal annealing; Dr. Valentin Craciun, for his help on laser annealing; Dr. Chia-Hua

Huang, for training me on device performance tests; and Dr. Lei Li Kerr, for her valuable

discussions.

I would like to extend my sincere appreciation to my colleagues (Jiyon Song, Woo

Kyoung Kim, Seokhyun Yoon and Wei Liu), in the laboratories for their assistance. I

could not have accomplished this work without their cooperation and help.

Finally I am greatly indebted to my parents in China for their constant love,

support, and encouragement. I dedicate this dissertation to them.















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ......... ... ............. ....... .. .. ........ ...... .............. viii

LIST OF FIGURES ............................... ... ...... ... ................. .x

A B S T R A C T ............. .............................. ... ....................... ................ x v

CHAPTER

1 IN T R O D U C T IO N ............................................................................. .............. ...

2 LITERATURE REVIEW OF THIN-FILM SOLAR CELLS ............................. ..7

Current Status of Photovoltaic Technologies .......................................... ...............7
Thin-Film Solar C ells ................................................................................ .... .8
Amorphous Silicon (a-Si:H) Thin-Film Solar Cells.................... ................8
The C dTe Thin-Film Solar C ells............................................ ............... ... 9
The Cu(In,Ga)Se2 (CIGS) Thin-Film Solar Cells ...........................................9

3 PULSED LASER ANNEALING (PLA) OF FILM PROPERTIES AND DEVICE
PERFORMANCE FOR CIGS SOLAR CELLS ...................................................18

In tro d u ctio n ........................................................................................1 8
Experim ental D details ...................................... ..... ........ .. ............. 19
Results of PLA Treated CIGS Film s .................................... ............ ......... ...... 20
Results of First Set of PLA Treated CIGS Solar Cells.............................................21
Photo- J-V Measurements of PLA CIGS Cells ..........................................21
The Q-E Measurement of PLA CIGS Cells ................. ...............................22
Results of Second Set of PLA Treated CIGS Solar Cells ....................................23
Photo- J-V R results .................. ............................. .. .. ..... .... ...... .... 23
D ark- J-V R results ........................................ ................... .... .. ... 23
Quantum Efficiency Results........... ................... ................... .................... 25
The C-V and DLTS Characterizations ..................................... ............... 25
A additional PLA Effect Study........................................................... ............... 26
Summary .............. ............ .... ....... ....... .......................... 27

4 RAPID THERMAL ANNEALING (RTA) OF FILM PROPERTIES AND
DEVICE PERFORMANCE FOR CIGS- BASED SOLAR CELLS .........................48









Progressive RTA Treatment on CIGS Solar Cells ............................................. 48
The RTA Effects of Separated CIGS Films .................................... ............... 49
The X RD and SEM R esults........................................... .......................... 49
Hall-Effect Measurements...................................................................... 50
The RTA Effects of Separated CIGS Solar Cell Devices........................................50
E x p erim ental D details ........................................ ............................................50
Photo- J-V Perform ance .............................................. ............................ 51
D ark- J-V Characteristics ............................................................................. 51
The S-R and Q -E Perform ance....................................................... ..................52
Thermal Annealing on CIGS Solar Cells by High Temperature XRD System ..53
S u m m ary ...................................... .................................................. 5 3

5 FABRICATION PROCESS OF CIGS SOLAR CELLS............................................78

S u b stra te ............................................................................................................... 7 8
B ack C contact Layer .......................................... ... .... ........ ......... 80
C IG S A bsorber L ayer ................... .................. ...................... .... ...... .......... 80
Chemical Bath Deposition (CBD) of CdS Buffer Layer ............................................82
A alternative Cd- Free Buffer Layer ........................................ ......... ............... 86
Transparent Conducting Oxide (TCO) Layer.................................. ...............87
M etal C o n ta ct ...................................... ............................... ................ 9 0

6 CHARACTERIZATION AND MEASUREMENT SYSTEMS FOR CIGS
SOLAR CELLS ................................... ..... .. ...... ............... 96

H all-Effect M easurem ents .......................................................... ............... 96
Introduction ........................................................ ................. 96
Measurement Procedure and Apparatus............................................................97
Related Studies, Results and Discussion..........................................................98
The Cd-partial-electrolyte treatment on CIGS films.............................. 98
Hall-effect data of CIGS films supplied by NREL TFPPP team .................99
Photo- and D ark- J-V M easurem ents ............................................ .....................100
Solar Cell Param eters ........................................ ............................... 100
Photo- J-V Measurement System ........... ................................ ...............103
L eight sou rce ................................................................... ............... 104
Tem perature control ............................................................................104
Contacts and connections ................................ ................................... 105
D ark- J-V M easurem ent ........................................................ ............. 106
Quantum Efficiency Measurement.. ...................... ....................................... 107
Quantum Efficiency Measurement Instrumentation ......................................107
Monochomator and Monochromatic Light Source .......................................108
Monochromatic Light Chopper .............. .................. ............... 109
Bias Light Source ................................. .. ............... ........ ............ .. 110
Spectral Detector and Synchronous Detection Instrumentation .....................110
Spectral Response Measurement Procedures ............................... ..................111
Deep Level Transient Spectroscopy (DLTS) Measurement................................112









Features and Principles............................................................... ............... 112
Related Results and Discussions ................................................................ 115

7 DEVICE MODELING AND SIMULATION OF CIGS SOLAR CELLS .............128

Simulation Model of CIGS Solar Cell Devices ............... .................................. 128
Sim ulation Results of CIG S Solar Cells.................................................................130
C o n clu sio n s.................................................... ................ 13 2

8 RADIATION EFFECTS ON CIGS FILM AND SOLAR CELLS..........................140

In tro d u ctio n ............................ ..................................... ................ 14 0
ELD R S Effects ............. ...... ................ ............................ 141
Total D ose Effects ............ ...................... ........ ..... ....... .......... .... 141
Displacement Damage ............ ................................... 142
H igh-K D ielectrics ................................................ .... .... .. ............ 146
Experimental Details and Discussion ........... ................................ ...............146

9 SUMMARY AND FUTURE WORK ........................................... ............... 161

S u m m a ry ........................................................................................1 6 1
F u tu re W o rk ........................................................................................................ 1 6 3

L IST O F R E FE R E N C E S ....................................................................... .................... 164

BIOGRAPHICAL SKETCH ............................................................. ............... 169
















LIST OF TABLES


Table page

3-1. Effective minority carrier lifetime (') of CIGS films before and after PLA
treatm ent as m measured by D BOM ........................................ ........................ 29

3-2. Hall-effect results for CIGS films before and after PLA treatments........................30

3-3. Effective lifetimes and photo- J-V results of PLA CIGS/CdS samples and
devices ................................................................ ..... ..... ......... 31

3-4. Photo-J-V performance of control cell and PLA-treated CIGS solar cells .............32

3-5. Photo- J-V performance and dark- J-V parameters for the control cell and
selected PLA -treated CIG S cells........................................ .......................... 33

3-6. Results of the DLTS measurements on the control cell and PLA treated CIGS
so la r c ells .......................................................................... 3 4

3-7. Annealing condition of new PLA samples............................................................35

4-1. Photo- J-V results of CIGS solar cells before and after progressive RTA..............54

4-2. CIGS film number and the annealing conditions.............................................55

4-3. Hall-effect data ofNREL CIGS samples before and after RTA treatments ............56

4-4. Annealing conditions of RTA treated CIGS devices ............... ....... ............57

4-5. Photo- J-V results of separated CIGS solar cells before and after RTA ..................58

4-6. Photo- J-V results of CIGS solar cells before and after RTA by using high
tem perature X R D sy stem .............................................................. .....................59

5-1. Conversion efficiencies of CIGS cells grown by non-vacuum processes (by
ISE T) on different substrates......................................................... ............... 91

6-1. Hall-effect results for the different Cd-partial-electrolyte treatment times on the
C IG S sam p les ................................................................... ................ 1 16

6-2. Results from Hall-effect measurements for the CIGS samples............................117









6-3. Hall measurement results for the CIS/CGS samples deposited on different
substrates (GaAs/glass) by PMEE method........................................ ..............118

6-4. Photo- J-V results for EPV CIGS device before and after DLTS scan................19

7-1. Performance parameters of the CIGS (Eg=1.2 eV) solar cells versus the defect
density in the CIGS absorber layer. ............................................ ............... 133

7-2. Photo- J-V data of cell#1 before and after progressive RTA treatment (From
experiment and AMPS ID simulation results)...........................................134

8-1. Dose rate and total dose of the tested CIGS samples and devices .........................150

8-2. Dose conditions and Photo- J-V results of CIGS devices before and after
radiation ............................................................... ..... .... ......... 151















LIST OF FIGURES


Figure p

2-1. Best performance of various thin-film solar cells ............................................. 11

2-2. Sketch of the layer structure for (a) a-Si p-i-n (superstrate), and (b) a-Si n-i-p
(substrate) solar cells. ................................................................. .. 12

2-3. Schematic sketch of band diagram for a-Si p-i-n solar cell. ....................................13

2-4. Device structure of CdS/CdTe thin-film solar cell................................................ 14

2-5. Band gap versus lattice constant diagram of CIGS solar cell. .................................14

2-6. Absorption coefficients of different solar cell materials ........................................15

2-7. Photo- J-V result of 19.2% CIGS solar cell from NREL (under 1000W/m2, AM
1.5 global spectrum at 25 C). ...... ...................................................................... 16

2-8. Device structure of a CdS/CIGS-based solar cell. ................................................17

3-1. XRD spectra before and after PLA treatments for (a) a CIGS and (b) a CdS
/C IG S sam ple. ........................................................................36

3-2. Surface morphology of CIGS films (a) without and (b) with PLA treatments at
an energy density of 55 mJ/cm2 (SEM images with magnification of 6000x).........37

3-3. Quantum efficiency of CIGS cells with and without PLA treatment. .....................38

3-4. Spectral response of CIGS cells with and without PLA treatment ........................38

3-5. Photo- J-V parameters versus different PLA condition. ........................................39

3-6. Dark- J-V curves comparing the control cell to two PLA treated cells. .................40

3-7. Dark- J-V curves (semi-log plot) of the control cell and two PLA treated cells......41

3-8. Quantum efficiency (Q-E) versus wavelength comparing the control cell to the
tw o PLA -treated cells ............................................... ........ .. ............ 42

3-9. The DLTS scans of the control- and PLA- CIGS cell................... ........ ........43









3-10. Surface morphology of CdS/CIGS films (a) before and after PLA treatment with
energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2,
(f) 110 mJ/cm2. (SEM images with magnification of 4000x) ...............................44

3-11. Surface morphology of CdS/CIGS films (a) before and after PLA treatment with
energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90 mJ/cm2,
(f) 110 mJ/cm2. (SEM images with magnification of 40000x) .............................44

3-12. Theta-2 theta (symmetrical geometry) diffraction patterns of PLA samples and
control sam ple. ..................................................... ................. 45

3-13. Grazing incidence XRD analysis (GIXD) of PLA samples and the control
sam ple (om ega=3deg.) .......................... .................. ...... ............46

3-14. Grazing incidence XRD analysis (GIXD) of PLA samples and the control
sam ple (om ega= ldeg.) .................................................................. 47

4-1. Cycle time for Rapid Thermal Annealing (one run). .............................................60

4-2. XRD results of RTA treated CIGS films and the control sample. ...........................61

4-3. Surface morphology of RTA treated CIGS films and the control sample. (SEM
images with magnification of 3000x)............................................ ...............62

4-4. Surface morphology of RTA treated CIGS films and the control sample. (SEM
images with magnification of 10000x).....................................................63

4-5. Surface morphology of RTA treated CIGS films and the control sample. (SEM
images with magnification of 30000x)............................................................64

4-6. Dark- J-V curves of tested CIGS solar cell (#D-R1) before and after RTA
(300C 1-m in) treatm ent............................................................................ .... 65

4-7. Dark- J-V curves of tested CIGS solar cell (#D-R2) before and after RTA
(300C 2-m in) treatm ent.............................................................. ......... ............ 66

4-8. Dark- J-V curves of tested CIGS solar cell (#D-R3) before and after RTA
(350C 1-m in) treatm ent............................................................................ .... 67

4-9. Dark- J-V curves of tested CIGS solar cell (#D-R4) before and after RTA
(350C 2-m in) treatm ent ........................................ ........... ....................... 68

4-10. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R1) before and
after RTA (300C, 1-min) treatment. ............................................ ............... 69

4-11. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R2) before and
after RTA (300C, 2-min) treatment. ............................................ ............... 70

4-12. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R3) before and









after RTA (350C, 1-min) treatment. ............................................ ............... 71

4-13. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R4) before and
after RTA (350C, 2-min) treatment. ............................................ ............... 72

4-14. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R1)
before and after RTA (300C 1-min). ........................................ ............... 73

4-15. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R2)
before and after RTA (300C, 2-min). ........................................ ............... 74

4-16. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R3)
before and after RTA (350C, 1-min). ........................................ ............... 75

4-17. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell (#D-R4)
before and after RTA (350C, 2-min). ........................................ ............... 76

4-18. Photo- J-V curves of tested CIGS solar cells before and after (a) 100C, 30
seconds and (b) 150C, 30 seconds RTA treatments by using high temperature
X R D system .........................................................................77

5-1. Layer structure of CIS/CIGS based solar cells............................................. 92

5-2. Schematic profile of the NREL 'three-stage physical evaporation process for the
fabrication of CIGS solar cells'.............. ........... ................... ........ 93

5-3. Sim ple CBD experim ental set-up....................................... .......................... 94

5-4. The E-Beam evaporation technique for depositing metal contact films on solar
c e lls ..............................................................................9 5

6-1. Schematic representation of the Hall effect sample........ .......... .................120

6-2. Typical four-point van der Pauw and Hall effect measurements.........................120

6-3. The equivalent circuit diagram of a CIGS PNjunction solar cell........................121

6-4. The I-V characteristics under dark and illumination conditions of a PN junction
so lar cell. ........................................................................ 12 1

6-5. Apparatus and block diagram of I-V measurement system for the CIS-based
cells ...................................................... .... .................. 12 2

6-6. The photo- J-V measurement and analysis for solar cells using the LabVIEW
program. (The tested CIGS solar cell is fabricated by EPV.) .............................122

6-7. The dark-J-V measurement and analysis for solar cells using the LabVIEW
program. (The tested CIGS solar cell is fabricated by NREL.) ...........................123









6-8. The block diagram of a spectral response measurement system for the CIS-based
solar cells............................................................................................ 124

6-9. The spectral response and quantum efficiency measurements for solar cells
using LabVIEW program. (The tested CIGS solar cell is fabricated by UF.) ........124

6-10. The DLTS scans for (a) EPV device at VR = -0.1V, VH= 0.3V, and W= 10ms.
And (b) An Arrhenius plot obtained from the DLTS scans shown in (a). .............125

6-11. The DLTS scans for (a) EPV device at VR = 0.5V, VH= 0.7V, and W= 10ms.
And (b) An Arrhenius plot obtained from the DLTS scans shown in (a). .............126

6-12. Dark- J-V curves of tested EPV CIGS device before and after DLTS
m ea su rem en t ...................................... ......... ................. ................ 12 7

6-13. Photo- J-V curves of tested EPV CIGS device before and after DLTS
m easurem ents ............................................................ ...................... 127

7-1. The schematic energy band diagram of a typical ZnO/CdS/CIGS solar cell under
equilibrium condition. ...................... .. .................... ............................. 135

7-2. Performance parameters of CIGS (Eg=1.2eV) solar cells versus the defect
density of CIGS absorber layer. ................................. 136

7-3. Photo- J-V curves of cell#1 before and after progressive RTA treatment (From
AM PS ID sim ulation results). ........................................ ......................... 137

7-4. Dark- J-V curves of cell#1 before and after progressive RTA treatment (From
AM PS ID sim ulation results). ........................................ ......................... 138

7-5. Quantum efficiency of cell#1 before and after progressive RTA treatment (From
AM PS ID sim ulation results). ........................................ ......................... 139

8-1. Five basic effects of a defect energy level (Et) on the electrical performance of a
d ev ice .......................................................................... 15 2

8-2. Gamma absorption cross-section versus photon energy. .......................................153

8-3. Photon interaction probabilities versus photon energy. .......................................154

8-4. Photograph of irradiated CIGS films and the control film...................................155

8-5. The XRD results of irradiated CIGS samples (#1-4) and the control sample (#5).156

8-6. Photo- J-V results of CIGS device #1 (cell#l) before and after total dose of 2.12
M rads (Si) radiation (Device from GSE). ................................... ............... 157

8-7. Photo- J-V results of CIGS device #1 (cell#2) before and after total dose of 2.12
M rads (Si) radiation (Device from GSE). ................................... ............... 158









8-8. Photo- J-V results of CIGS device #2 (cell#l) before and after total dose of 6.5
Mrads (Si) radiation (Device from SSI).......................................................159

8-9. Photo- J-V results of CIGS device #2 (cell#2) before and after total dose of 6.5
M rads (Si) radiation (Device from SSI) ............. .............................................. 160















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

PULSED LASER ANNEALING AND RAPID THERMAL ANNEALING OF
COPPER-INDIUM-GALLIUM-DISELENIDE-BASED THIN-FILM SOLAR CELLS

By

Xuege Wang

August 2005

Chair: Sheng S. Li
Major Department: Electrical and Computer Engineering

Effects of Pulsed Laser Annealing (PLA) treatment on the film properties and the

performance of CIGS solar cells have been studied under various annealing conditions.

This technique has been used for the first time to modify near-surface defects and related

junction properties in Cu(In,Ga)Se2 (CIGS) solar cells.

CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed

laser beam (248 nm wavelength) and a 250 ns pulsed laser beam (308 nm wavelength)

with a larger beam size at selected laser energy densities in the range of 20 to 110 mJ/cm2

and pulse number in the range of 5 to 20 pulses. The narrowing of X-Ray Diffraction

(XRD) peak, new shoulders of Grazing Incidence X-ray Diffraction (GIXD) and the

increase of Scanning Electron Microscopy (SEM) surface feature size suggest near

surface structure changes. The Dual-Beam Optical Modulation (DBOM) and Hall- effect

measurements indicate PLA treatment increases the effective carrier lifetime and mobility

as well as the sheet resistance. In addition, several annealed CdS/CIGS films processed









by PLA were fabricated into solar cells and characterized by Photo- and Dark- J-V and

Quantum Efficiency (Q-E) measurements. Significant improvement was observed in the

overall cell performance, diode quality, and spectral response when compared to

pre-annealed cells. Deep- Level Transient Spectroscopy (DLTS) results showed a 50%

reduction of the density of shallow defect trap after low-power PLA treatments. The

energy density of the laser beam and the pulse number were found to play key roles in

modifying the optical and electrical properties of the CIGS films and hence the cell

performance. Results suggest that the optimal PLA energy density and pulse number are

around 30 mJ/cm2 and 5 pulses, respectively.

A comprehensive study of the effects of Rapid Thermal Annealing (RTA) on the

film properties and the performance of CIGS solar cells has been carried out in this

dissertation. CIGS samples and devices were characterized by using XRD, GIXD, SEM,

Hall effect, Photo- and Dark- J-V and Q-E measurements before and after RTA treatment

under various ramp up and down rates, peak temperatures, holding times, and ambient

conditions. Results show that progressive RTA treatments could significantly improve

the overall uniformity and performance of large-area CIGS solar cells. Under low RTA

temperatures, the surface composition and morphology remain unchanged. The simple

RTA treatment on CIGS cells shows an increase of quantum efficiency and some

improvement of cell performance. The estimated optimal annealing temperature should

be between 200 and 300C with a holding time of 1- minute or less.

Device simulation using one-dimensional (1-D) AMPS program has also been

carried in this work for a typical CIGS cell with different defect densities. A well fitting

between simulation results and progressive RTA results has been obtained. The results









show that the device performance and spectral response can be positively improved by

reducing the defect density of CIGS absorber layer.

In addition, Gamma-Ray radiation tolerance study on several CIGS samples and

solar cells were performed under different total dose by varying the sample displacements

and exposure times. XRD and Photo- J-V measurements were taken before and after each

radiation. The results show that the surface morphology, crystallinity and the device

performance of CIGS solar cells change under certain radiation dose condition.














CHAPTER 1
INTRODUCTION

Solar cells are now the most important and viable power source for satellites and

space vehicles. Solar cells have also been used successfully in large-scale terrestrial

power generation applications. With world energy consumption growing each year, most

of the energy comes from petroleum, natural gas, and fossil fuel, and nuclear power

generation. These power systems are polluting, costly, or will be exhausted in the near

future. Therefore, a renewable energy source such as solar energy is needed to replace

these systems and protecting our natural environment and future use. The sun has been

radiating energy for over 500 million years, and is expecting to continue indefinitely.

Photovoltaic (solar) energy conversion becomes an excellent candidate because it is

clean, inexhaustible, uninterruptible, and nonpolluting. Solar cells use the internal

photovoltaic (PV) effect in semiconductors, and are capable of providing electricity

directly from the sun for a wide variety of applications with the advantage of

long-duration power generation at lower maintenance cost.

Recent interest has increased in research and development of low-cost, flat-panel

solar cells, thin-film PV devices, concentrator systems, and many innovative concepts. In

2004, worldwide PV module production has generated 750 MW electric power for

terrestrial applications. The PV technology is growing at more than 20% annual growth

rate, and is expecting to produce 5 to 10% of electricity used in United States in 10 to 20

years.









As a simple definition, the basic function of a solar cell module is to absorb

incident sun light and to create electron-hole pairs and collected at the external contacts

of solar cells to produce electricity for a wide variety of applications. A semiconductor

p-n junction structure is commonly used in solar cells. The useful spectral range to

convert the incident sun light into electricity is usually determined by the bandgap (Eg) of

the semiconductor. Separation and transport of the photo-generated electron hole pairs

are accomplished by the built-in electric field formed in the depletion region of the p-n

junction. A transparent conductive oxide (TCO) or ohmic contact grid is normally used

for current collection in the front, and a metal contact is used in the back side of the solar

cell.

Since the first silicon solar cell was reported by Chapin, Fuller, and Pearson in

1954, the solar cell has evolved from being a low-efficiency device to being a major

power generation source for the spacecraft and many terrestrial power generation

systems. Currently, high efficiency single- and multi-junction solar cells based on III-V

compound semiconductors such as GaAs- and InP- materials have been developed for

space applications, while crystalline silicon solar cells are still the dominant PV

technology for terrestrial power generation applications. Commercial silicon solar cells

available today are made from solar-grade single crystalline silicon or polycrystalline

silicon materials. Although high efficiencies are achieved with single crystalline silicon,

the silicon technology is not expected to become a low-cost PV technology. One reason

is that silicon is an indirect band gap semiconductor which requires a thick absorber layer

(about 250-400 atm thick) to absorb 90% of the useful sun light for electricity generation.









In the past few years, much effort has been devoted to developing various low-cost,

high-efficiency, and high-stability solar cells for both terrestrial and space power

generation and for applications in consumer electronics. Efficient solar cells can be made

by thin-film technology. By using semiconductor material with high absorption, a film

thickness of a few micrometers is sufficient to effectively collect the sun light. A wide

variety of absorber materials is available for solar cell applications.

Today, the most common thin-film solar cells are made from amorphous silicon

materials. The commercial deposition process of a-Si:H using plasma-enhanced chemical

vapor deposition (PECVD) technique is compatible with large-area deposition and low

temperature processing. This allows the use of a wide variety of inexpensive substrate

materials. The a-Si:H films can be easily doped by adding phosphorus or boron

containing gases during deposition process for n- and p-type doping. The optical band

gap of a-Si is typically around Eg & 1.7eV and can be tuned. For example, the band gap

energy of a-Si:H can be increased by alloying with carbon or oxygen, and decreased with

incorporation of germanium to form a-SixGel-x (0 < x 1) films. The energy band gap

can also be fine tuned by changing the hydrogen content using different deposition

parameters and methods. The a-Si:H thin film solar cells have reached an efficiency of

13.1%, while long term stability is a key issue for a-Si:H solar cells. Other thin film PV

technologies based on CdTe (with a maximum efficiency of 16.5% [1]), and

CdS/Culn(Ga)Se2 (CIGS) material systems have shown great promising for large scale

terrestrial power systems. CIGS thin film solar cells with efficiency exceeding 19%

AM1.5G have been demonstrated recently by NREL research team. Thin-film PV

technology benefits from low material consumption and low price as compared to









crystalline silicon solar cells. Scaling the PV technology from single solar cells to

large-area PV modules is straightforward since many cells can be interconnected from

material deposited on one substrate in the form of stacked film layers. Compared to the

crystalline material, thin-film solar cells can be manufactured with less energy input. This

shortens the energy payback time (the time needed for photo-generated energy output to

equal the energy consumed to produce the device).

Specific advantages of the CuIn(Ga)Se2 alloy are its wide compositional tolerance

and a direct band gap material high optical absorption in the visible spectrum. One major

drawback for large scale production is the limited extraction rate of indium from mining.

Cost and conversion efficiency are the two key factors that determine the compatibility of

a solar cell. Thin-film solar cells give the best hope for obtaining PV devices with high

efficiency and low cost. Copper indium diselenide (CuInSe2 or CIS) and copper indium

gallium diselenide (CuInGaSe2 or CIGS) films are the most promising materials of all

thin-film solar cells for achieving these goals. Such material has certain exceptional

characteristics particularly suitable for photovoltaic heterojunction applications. CIGS is

a direct band gap material, which minimizes absorber layer thickness (1-2 [tm).

Thin-films of CIS/CIGS are p-type conductivity, and a surface inversion layer can be

formed when deposited with a CdS buffer layer. Therefore, an n-p heterojunction

structure can be formed solar cell application. CIGS material with 30% of gallium

content has a band-gap energy of around 1.3 eV at 300 K, which is nearly ideal for a

photovoltaic device operating in the solar spectrum. There is no reported photon

degradation of CIGS solar cells because of good thermal stability. It can absorb the solar

spectrum within a few micrometers (1-2 [tm) with high optical absorption coefficient.









The CIGS thin-film solar cells have recently achieved efficiencies in excess of 19.5%

AM1.5G. However, further optimization of the device performance could be substantially

accelerated by a better understanding of the processing effects on the interface between

the CdS and CIGS layers and the transport properties of CIGS films.

The pulsed laser annealing (PLA) and halogen lamp based rapid thermal annealing

(RTA) on silicon-based devices and solar cells have been extensively investigated in

recent years [2-9]. Today, PLA technique is widely used to activate boron ion implants in

silicon wafers to remove undesirable boron clustering, defect evolution, or damage to the

lattice created by implant and to recrystalyze thin Si amorphous films. The possibility of

using this approach to modify the near surface defects in CIGS thin-film solar cells was

motivated by the positive results reported for laser processing of silicon wafers. The basic

idea is to promote atom mobility by local heating, on a nanometer-length scale; and thus

confine the impact of processing to the near-junction region of the device. Results from

characterizing the CIGS films and cells suggest that interfacial recombination near the

CdS/CIGS metallurgical junction is a major limitation to optimizing of the device

performance. The PLA treatment promises to provide defect annealing in the near-surface

region, while preserving the beneficial composition gradients in CIGS films.

Another powerful annealing technique using halogen-based RTA process has been

widely used in the semiconductor industry. It offers several advantages such as short

cycle time, reduced thermal exposure and lot-size flexibility compared to conventional

furnaces. Strong demand for thermal-budget reduction and cycle-time reduction had

made RTA treatment a popular thermal-processing method in recent years. This

technique has been successfully applied to the fabrication of low-cost,






6


low-thermal-budget silicon solar cells. Conversion efficiency of more than 17% has been

reported for RTA processing time of less than 3 minutes [10].

The main scope of this research is to investigate the effects of the above two

techniques (PLA and RTA) on CIGS films properties and solar cell performance. Results

show that both of these two techniques can positively improve the CIGS cell's

performance. In addition, device modeling and investigation of radiation tolerance of

CIGS-based solar cells are well discussed in this dissertation.














CHAPTER 2
LITERATURE REVIEW OF THIN-FILM SOLAR CELLS

Current Status of Photovoltaic Technologies

The photovoltaic (PV) industry has grown at an annual rate of over 20% since

1990's. Many PV manufacturers have invested in expanding their production facilities

during the last few years. New PV companies have also been formed in Japan, USA, and

Europe to increase the PV module production. Remarkably, Japan manufactures

produce almost half of today's world PV modules for terrestrial power generation use.

The worldwide PV module production has reached 750 MW in 2004.

In the past few years, most of the PV systems are built using single-crystalline

silicon wafers. The key issue and drawback in silicon based PV technology is the

reduction of wafer costs, which is difficult to attain with single crystal silicon wafers.

As a result, the PV industry has looked into developing low cost thin film PV

technologies using alternative semiconductor materials to replace the silicon-wafer-based

PV technology. Thin-film solar-cell production accounted for 13% of all PV production

in 1999. Single-crystalline and multi-crystalline silicon solar cells are still accounted for

most solar-cell production (-84%). Other crystalline products (such as ribbon growth or

thin silicon films) contributed only about 3% of the PV modules. The main difficulties of

thin-film technology are its bad image of long term stability from the first-generation

amorphous silicon PV modules; its low conversion efficiency as compared to crystalline

silicon solar cells; the toxicity of some of the materials in fabrication and disposal; and its









short lifetime. All these problems must be resolved if thin-film technology is to take a

major share of the booming PV market in the future [11-14].

Thin-Film Solar Cells

The biggest advantage of thin-film technology is that it greatly reduces the

manufacturing cost of solar cells by reducing material and deposition cost on large area

low cost foreign substrates [15-19]. Typical thin-film solar-cell materials include a-Si:H

alloys, CdTe, Cu(In, Ga)Se2 (CIGS), poly-Si, [ac-Si/poly-Si, and dye/TiO2. The most

advanced and commonly used thin-film technologies are a-Si-alloys, CdTe, and CIGS.

Figure 2-1 shows the best performance of these thin-film solar cells.

Amorphous Silicon (a-Si:H) Thin-Film Solar Cells

Hydrogenated amorphous silicon (a-Si:H) is distinguished from the crystalline

silicon (c-Si) by the lack of long-range order (disorder) in the atomic structure and by its

high bonded-hydrogen content (-10% in device quality a-Si:H). Although the overall

properties of a-Si:H and c-Si materials are similar, the long range disorder in

a-Si:H distorts bond lengths and bond angles, which introduces large densities of

broken-bond defects and micro voids. The disorder relaxes the momentum conservation

rules associated with crystalline materials, thus leading to higher optical-absorption

coefficients than in c-Si for photons with energies (hu) greater than the bandgap energy

(Eg). The optical band gap of a-Si:H is 1.7 eV and can be fine-tuned by changing the

hydrogen content as a function of specific deposition parameters and methods. In

addition, a-Si has a higher optical absorption coefficient than c-Si in the visible range of

the spectrum and hence the absorber layer thickness of a-Si can be than 1 am.

Typical a-Si:H-based solar cells have a p-i-n superstrate (Figure 2-2a) or n-i-p

substrate (Figure 2-2b) structure, depending on the deposition sequence of doped and









intrinsic layers [24]. For both structures, light enters through the p-layer, which

efficiently supports hole collection in the device. Heavily doped wide band gap a-SiC:H

and a-SiO:H alloys or microcrystalline Si films are applied as p-doped window layers to

reduce absorption losses. Electrons and holes generated in the i-layer are driven to the n-

and p- layer, respectively, by the internal built-in electric field (Figure 2-3).

The advantage of its unique material properties also makes thin-film a-Si:H an

excellent candidate in sophisticated multi-junction solar cell design. Achieved stable

conversion efficiencies in excess of 13% [20-23] have been reported for multi-junction

a-Si:H thin-film solar cells.

The CdTe Thin-Film Solar Cells

With a direct band gap of Eg = 1.45 eV [25] and steep optical absorption edge,

thin-film CdTe solar cells can absorb 90% of the incident sunlight in 1-2 |tm absorber

layer thickness, and hence are considered as a promising thin-film PV technology. The

world record so far was achieved by NREL withl6.5% conversion efficiency. Theoretical

maximum efficiency of CdTe solar cells is over 27% [26, 27].

The depositing sequence can be changed for CdS/CdTe solar cells; the frontwall

type and backwall type. The most common device structure of CdTe solar cells is the

backwall type (Figure 2-4 [25]). The CdTe layer usually is deposited by close-space

vapor transport technique. Acting as a filter for sunlight, and with a cut off wavelength of

514 nm, n-type CdS becomes the ideal partner for p-type CdTe absorber to form n-p

heterojunction solar cells.

The Cu(In,Ga)Se2 (CIGS) Thin-Film Solar Cells

The compounds of CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) with their chalcopyrite

structure, are among the most promising materials used in thin film solar cells [28, 29].









The main advantages of CIS/CIGS-based solar cells are the high conversion efficiency

and low cost of materials. Although some problems still prevent the large-scale

commercialization and use of CIGS cells for terrestrial power generation, significant

progress has been made in CIS/CIGS-based PV technology, and single cell efficiency of

19% and module efficiency exceeding 10% AM1.5G have been achieved in the last

couple years.

The CIGS absorber is direct band gap material with bandgap tunable by adjusting

the ratio of In to Ga, to maximize absorption of the solar spectrum. Its bandgap can be

varied from 1.02 eV (CIS) to 1.68 eV (CGS) (Figure 2-5). Thin-film CIGS material also

has a higher absorption coefficient than any other thin-film (a > 105 cm-) (Figure 2-6),

which allows almost 99% of the incoming light to be absorbed with the first micron of

the material according to the solar spectrum. Thus, the highest-efficiency (19.2% from

NREL) CIGS solar cell (Figure 2-7) is fast approaching the efficiency of already

commercialized multi-crystalline silicon cells (20.4%) and is higher than other thin-film

PV technologies (Figure 2-1).

The typical structure of CIGS solar cells is shown in Figure 2-8. The CIGS

absorber layer is typically deposited on a Mo-coated soda-lime glass (SLG) substrate

(Mo/SLG by using PVD), sputtering, PMEE, or RTP deposition technique. A 500 A CdS

buffer layer is deposited on top of the CIGS absorber layer by CBD technique, followed

by deposition of a TCO layer such as sputtered ZnO films. Finally, a 500 A Ni- film and

a 3 tm Al metal contact layer are deposited by using electron-beam evaporation

technique. Chapter 5 gives detailed description of each layer and device fabrication

processes.













25


20


S15

a,
u 105


5


) K'


p p p I 1 I I I


P ^ ^ ^ s&fl.
Uslc


Figure 2-1. Best performance of various thin-film solar cells.









light


light


TCO (0,5-1.5 pm)

P

n
TCO (100 nm)
metal (110 nm)


I


TCO 80 n m)
p

n
TCO
m metal

substralte

(e. g. metal- or plastic roil. glass)

(b)
Figure 2-2. Sketch of the layer structure for (a) a-Si p-i-n (superstrate), and (b) a-Si n-i-p
(substrate) solar cells.



























TCO


I.'


Figure 2-3. Schematic sketch of band diagram for a-Si p-i-n solar cell.


of


~nn~
hu
















Contact pads


Area contact


p-CdTe (3-5pm)

n-CdS (0 1p i
TCO
Sglas substrate


Figure 2-4. Device structure of CdS/CdTe thin-film solar cell.


lam i-4- i. --- -


54


5.5


5 I
5,6


5,7


5.8


Lattice constant


Figure 2-5. Band gap versus lattice constant diagram of CIGS solar cell.


CuGaS2


CuGaSe
2


CulnS,


CulnSe
































M





n
1200



4 h

1u




400


1.0 1.S 2. 2.5
h v (eV)


Figure 2-6. Absorption coefficients of different solar cell materials.






16










,15 0 ,1 I m. 1 I | I m1 1 I t L 11 F I r I I


Voc = 0.689V
Jy = 35.71 mA/cmi
10 FF = 78.12% /
r = 19.20-o









0- -



0.0 02 0.4 0.6
Voltage (V)



Figure 2-7. Photo- J-V result of 19.2% CIGS solar cell from NREL (under 1000 W/m2,
AM 1.5 global spectrum at 250C).























ZnO (frovt contact)


/CdS
/,


Cu (In, Gai Se.
/ absorber)


r- .L c MQ back
/ .-- .-''"' ^ c ,tacT)

SiO (barrier)

SGlass
(substrate)


Figure 2-8. Device structure of a CdS/CIGS-based solar cell.














CHAPTER 3
PULSED LASER ANNEALING (PLA) OF FILM PROPERTIES AND DEVICE
PERFORMANCE FOR CIGS SOLAR CELLS

Pulsed Laser Annealing (PLA) has been used for the first time to modify

near-surface defects and related junction properties in Cu(In,Ga)Se2 (CIGS) solar cells.

Several film surface and device performance characterizations were employed to

investigate the effects of PLA on CIGS film and solar cells. In this chapter, the annealing

effects, optimal conditions and characterization results are discussed in detail.

Introduction

The PLA technique is widely used to activate boron ion implants in silicon wafers

and to remove undesirable boron clustering, defect evolution, and damage to the lattice

created by the implantation process. Beneficial effects of PLA are derived in part from

selective absorption and limiting elevated temperature processing to the near-surface

region [30, 31]. The possibility of using this approach to modify the near-surface defects

and crystalline structure in Cu(In,Ga)Se2 (CIGS) thin-film solar cells was motivated by

the positive results reported for PLA processing of silicon wafers. The basic idea is to

promote atom mobility by local heating on the nanometer length scale and thus confine

the impact of processing to the near-junction region of the device. The CIGS cells contain

a thin (- 50 nm) n+ CdS buffer layer deposited on a thick (- 1 to 2 atm) p-type CIGS

absorber. Although the exact location of the electrical junction relative to the

metallurgical one is not known, it does lie near the surface of the absorber. Furthermore,

analyses of CIGS film characterization results and cell performance data suggest that









interfacial recombination near the CdS/CIGS metallurgical junction is a major limitation

to the device performance. The possibility of repairing damage near this shallow junction,

while preserving the composition gradients in the bulk CIGS films motivated exploration

of PLA treatment. In this work most of our CIGS absorber films were annealed using a

248 nm pulsed laser beam and the properties of the films and subsequent device

performances were compared to those for untreated films. Furthermore, CIGS films were

annealed using a 250 ns pulsed 308 nm laser beam with larger beam size at selected laser

energy density in the range 30 to 110 mJ/cm2. Results of XRD, GIXD and SEM surface

characterizations suggested near surface structure changes by PLA.

Experimental Details

The CIGS samples used in this study were provided by the National Renewable

Energy Lab (NREL) and Shell Solar Inc. (SSI), and both sample sets were grown on Mo

coated soda-lime glass substrates. For the Hall-effect measurements, however, the CIGS

films were grown on the insulating soda-lime glass (SLG) substrates. A thin (50 nm) CdS

buffer layer was deposited on the CIGS samples using Chemical Bath Deposition (CBD)

at 75C for 30 min. The PLA treatment was carried out using a pulsed 248 nm line

derived from a KrF excimer laser system [32]. In the main part of this study the laser

pulse width was fixed at 25 ns and the energy density was varied in the range 20 to 60

mJ/cm2 and the number of pulse cycles in the range 5 to 20. To study the effect of PLA

on the CIGS film properties, PLA treatments were performed on the CIGS films with and

without CdS buffer layers. After PLA treatments, a ZnO window layer was deposited on

the CdS/CIGS/Mo/SLG sample by RF sputtering. Subsequent metallization (Ni-Al front

contact grids) was carried out by e-beam evaporation through a shadow-mask. Finally,

finished devices were produced by cutting the sample into separate cells with 0.429 cm2









active area and attaching wires with Indium bumps on the Mo-coated glass substrates for

back contacts. Detailed fabrication process is discussed in Chapter 5. The performance of

these cells was then tested as described in the next section.

Results of PLA Treated CIGS Films

The non-destructive Dual Beam Optical Modulation (DBOM) characterization

method was used to measure the effective carrier lifetimes in the CIGS absorbers [33,

34], and to evaluate the effect of PLA treatment on the performance of CIGS cells.

Within the sensitivity of the DBOM technique, the effective carrier lifetime was found to

increase for CIGS films on Mo/SLG annealed using 5 cycles of laser pulse with an

energy density in the range 30 to 60 mJ/cm2. The results (Table 3-1) show that a low

energy PLA treatment can increase the effective carrier lifetime of the annealed samples

and improve the performance of CIGS cells.

XRD patterns of CIGS and CdS/CIGS samples before and after PLA treatments

show substantial narrowing of the diffraction peaks that belong to CIGS (Figure 3-1).

These results can be interpreted as an improvement of the layer's crystallinity after PLA

treatment, which is consistent with the observed increase of the grain size (SEM) and the

effective carrier lifetime in the CIGS films.

As illustrated by SEM micrographs (Figure 3-2), the surface morphology and

apparent grain size increased upon laser annealing. This result suggests that the energy

density used was sufficient to cause atomic rearrangement in the near surface region, and

thus the potential exists to modify the atomic defects in the near surface region.

Based on the encouraging DBOM results from the initial PLA treated CIGS

samples, a second set of experiments were performed in which the energy density and

number of the incident laser pulse were varied. Hall-effect measurements were made on









the CIGS samples without a Mo layer prior and after annealing to determine the effects of

PLA treatment on the carrier concentration, carrier mobility, and sheet resistivity of CIGS

films.

Four CIGS films deposited on the glass substrate underwent pulsed PLA treatments

at room temperature. Hall-effect measurements (using an MMR Hall and Van Der Pauw

measurement system) were made for all samples before and after the PLA treatments.

The results (Table 3-2) show a significant increase in the values of Hall mobility and a

decrease in film resistivity following PLA treatment. The carrier mobility in the PLA

treated CIGS films were 3 to 4 times greater than the values before annealing. Although

the hole concentration decreased slightly after annealing, film resistivities decreased by

72 and 64% for CIGS films (samples H1 and H2) treated at an energy density of 20

mJ/cm2 with 10 and 20 laser pulses, respectively. At an energy density of 40 mJ/cm2, the

film resistivity was reduced by more than 95% from that of the non-annealed samples

(H3 and H4). Thus, both the energy density and the number of laser pulse play an

important role in determining the resistivity of PLA treated CIGS absorber layers.

Results of First Set of PLA Treated CIGS Solar Cells

Photo- J-V Measurements of PLA CIGS Cells

Four CIGS/CdS samples were annealed by a 50 mJ/cm2 laser beam with

different pulse number. Two samples were followed by a 100 A extra CdS buffer layer

re-growth after PLA treatment on the CIGS samples initially coated with a 400 A CBD

CdS buffer layer, and one control sample without any treatment. These samples were

then fabricated into cells for testing. The DBOM and photo- J-V results (Table 3-3) show

an increase in the effective carrier lifetimes on the PLA treated samples. No explicit

improvements, however, were found in the photo- J-V results of the annealed cells. The









data also show slight decreasing in the fill factor and conversion efficiency of cells

annealed with 20 pulses PLA compared with the 10-pulse-annealed cells. Some high

energy density (i.e., 80 mJ/cm2) PLA treatments were also used on other CIGS samples,

and the results show a drastic reduction in the cell efficiency. These results suggest that

an optimal PLA energy density should be less than 50 mJ/cm2, and no significant

influence on the cell performance because of the additional CdS buffer layer re-growth

was found in this study.

The Q-E Measurement of PLA CIGS Cells

Two CIGS films with a 500 A CdS buffer layer were annealed at an energy density

of 50 mJ/cm2, and then fabricated into cells. To study the effect of pulsed PLA treatment,

the spectral response and quantum efficiency (Q-E) were measured on these cells. The

results (Figures 3-3, 3-4) indicate that for incident light with wavelengths greater than

650 nm, the Q-E and spectral response of the PLA cells are higher than those of the

control cell, indicating that the PLA treatment increases the effective carrier lifetime and

diffusion length in the absorber layer and hence increases the short-circuit current density

in comparison to the control cell without PLA treatment. In the short wavelength regime

(k< 0.65 [tm), however, the Q-E and spectral response decrease after PLA treatment,

which suggest damages near the interface region of CIGS/CdS films by the laser beam.

As a result, the surface recombination velocity is increased and the Q-E and spectral

response are lower in the shorter wavelength region. It is also noted that the values of

Q-E and spectral response for sample annealed with 20 cycles laser pulse were found to

be lower than the sample with 10 cycles of annealing pulse with same energy density.









Results of Second Set of PLA Treated CIGS Solar Cells

Photo- J-V Results

Since the previous results suggest that an optimal PLA energy density should be

less than 50 mJ/cm2, an additional set of CIGS/CdS samples treated by PLA with

different energy densities (less than 50 mJ/cm2) and pulse number were fabricated into

finished devices. Six CdS/CIGS samples treated by PLA with selected energy density and

pulse number plus one control sample without PLA treatment were fabricated into

finished devices. There were five cells on each device and each cell was tested using a

photo- J-V system at room temperature. The PLA conditions and averaged photo- J-V

results of these devices are summarized in Table 3-4 and Figure 3-5. Note that the data in

Table 3-4 were averaged from the measured values of five cells on each device and the

standard deviation computed on the 5- cell measurements. In these experiments,

improvements were found in the photo- J-V results of the annealed devices under 30

mJ/cm2, using 5 or 10 pulses (CIGS-D1 and CIGS-D2), as compared to results from the

control sample (Ctrl-D). The data also show degradation in cell performance parameters

such as open circuit voltage (Voc), short-circuit current (Jc), Fill Factor (F.F.), and cell

efficiency (rc) of the devices laser annealed with energy densities higher than 30 mJ/cm2.

It was also found that for devices annealed at the same energy density, the fill factor and

conversion efficiency of the 5-cycle annealed devices are higher than those of 10-cycle

annealed devices. These results suggest that an optimal PLA energy density and pulse

number are approximately 30 mJ/cm2 and 5 pulses, respectively.

Dark- J-V Results

To better understand the specific differences between the control cell and the two

30 mJ/cm2 PLA treated cells, dark- J-V and quantum efficiency (Q-E) measurements as









well as theoretical calculations of device parameters were made for selected cells from

each of the three devices (Ctrl-D, CIGS-D1 and CIGS-D2).

The J-V characteristics for a p-n junction solar cell under uniform illumination

condition [35, 36] are expressed by Equation 3-1.

q(Vy RJ) V-RJ
J(V) J,(exp q 1) ) + J Jph (3-1)
nkT R,h

Where Jph, Jo, n, Rs and Rsh are the photocurrent density, saturation current density, diode

ideality factor, series resistance, and shunt resistance, respectively.

To study the effect of pulsed PLA treatment on the diode characteristics, photo-

and dark- J-V measurements were performed at room temperature for each the selected

cell. The measured parameters and calculated values of Jo, n, and R, [37, 38] are

summarized in Table 3-5 (note that Rsh is quite high in all tested CIGS cells).

From the dark- J-V curves (Figure 3-6, 3-7), both dark current density and

saturation current density, Jo, of the PLA treated cells are consistently smaller as

compared to the control cell. These curves suggest that the recombination current through

surface defects dominates the dark- J-V curve under forward bias condition before PLA

treatment (with diode ideality factor n >4). After the PLA treatment, a significant

reduction in the dark current density was observed apparently because of reduction of

surface defect density, and the recombination current was dominated by bulk defects in

the junction space charge region at higher bias (n z 2). These results indicate that defects

in the surface region and interface of absorber and buffer layers can be effectively

reduced by PLA treatment, thus decreasing the recombination of minority carriers via

interface states and defects in the junction space-charge region of CdS/CIGS cells.

Although cells treated by the PLA exhibit higher series resistance (R,), the negative effect









of R, increase on the diode characteristic remains relatively small because of the

extremely low value of Jo.

Quantum Efficiency Results

The quantum efficiency (Q-E) was measured on the control and PLA treated cells.

The results (Figure 3-8) indicate that for incident light with wavelengths greater than 480

nm, the Q-E of the PLA cells is higher than that of the control cell, indicating that the

PLA treatment improves the effective carrier lifetime and diffusion length in the absorber

layer and hence increases the short-circuit current density in comparison with the control

cell. In the shorter wavelength regime (k< 0.48 [tm), no improvement of Q-E was

observed between the control cell and the PLA treated cells. It is also noted that values of

the Q-E for cell annealed with 10 cycles of laser pulse were found to be lower than that of

the cell with 5 cycles of laser pulse at same energy density, again suggesting that the

optimal laser pulse number is between 5 and 10 cycles.

The C-V and DLTS Characterizations

In order to investigate the defect property change after PLA processing of CIGS

cells, the Deep- level transient spectroscopy (DLTS) and C-V measurements were

employed on 2 finished CIGS devices with 30 mJ/cm2, 10 pulses and without PLA

treatment. The DLTS measurement was performed with a reverse bias (VR) of 0.5 V, a

trap-filling pulse amplitude of 0.7 V, and a saturation pulse width of 10 ms. The DLTS

spectra were shown in Figure 3-9 and the DLTS measurement results were summarized

in Table 3-6. As shown in Table 3-6. A minority carrier trap with an activation energy of

0.065 0.069 eV, which was quite close to the shallow donor energy level (-0.06 eV)

known as the selenium vacancy (Vse), was detected, and its density was reduced by about

50% after PLA treatment.









Additional PLA Effect Study

Furthermore, CIGS films were annealed using a new 250 ns pulsed 308 nm laser

beam with lager beam size in France at selected laser energy density in the range 30 to

110 mJ/cm2. XRD, GIXD and SEM surface characterizations were made which suggest

near surface structure changes. The sample number and annealing condition are shown in

Table 3-7 below.

To study the laser annealing effect on film surface morphology by this new pulsed

laser annealing system, 5 identical CdS/CIGS films were annealed at selected laser

energy densities in the range 30 to 110 mJ/cm2, 5 pulses. As illustrated by SEM

micrographs (Figure 3-10, 3-11), the surface morphology and apparent grain size

increased upon laser annealing with energy density of 70 mJ/cm2 or higher. This result

suggests that the selected energy densities were sufficient to cause atomic rearrangement

in the near surface region, and thus the potential exists to modify the atomic defects in the

near surface region.

From the XRD and GIXD plots (Figure 3-12, 3-13 and 3-14), new CdS peaks and

shoulders (at 25, 28.5, and 48.3 degree) were observed as the energy density was

increased, suggesting a significant improvement of the crystallinity of the buffer layer

after the PLA treatment. For Figure 3-14, by using a smaller grazing incident angle

(Q=1), the analysis is more sensitive to the sample surface. As one can see, the new CdS

peaks becomes shaper compared to Figure 3-12 and 3-13, indicating a higher crystallinity

for the topmost part of the buffer layer. Comparing Figure 3-14 with 3-12 and 3-13, we

found that the film surface changes more than bulk. Close look at the new peaks in all

figures, we found that the peaks appear only when the energy density is equal or greater

than 70 mJ/cm2, (sample #3, 4 and 5) which suggest 70 mJ/cm2 might be a critical









threshold PLA condition. This energy density value could represent the melting threshold

for the buffer layer.

Summary

The results of pulsed PLA treatment on the film properties and the performance of

CIGS solar cells are very encouraging. Examination of the structural and electrical

properties of films from 2 different sources clearly showed the annealing step modified

the near surface region. Increased surface feature size evidenced in SEM photos and

CIGS diffraction peak narrowing is consistent with increased crystallinity. Furthermore,

DBOM measurements of the effective carrier lifetime indicated the lifetime could be

increased by as much as a factor of 2.75. Hall measurements of CIGS samples deposited

on SLG revealed PLA treatment increased the mobility and resistivity and decreased the

net free hole concentration of each sample, consistent with the hypothesis of annealing

out electrically active defects in the near surface region. Based on a parametric study, the

best PLA result was obtained with a pulsed laser energy density of 30 mJ/cm2 and 5 pulse

cycles. J-V and Q-E measurements were also made to study the effect of PLA treatment

at the best condition on the performance of CIGS cells. The results show that pulsed PLA

treatment has a beneficial effect on the cell performance with the cell efficiency

increasing from 7.69 to 12.22 and 13.41% after annealing 2 different samples prior to

device processing. The energy density of the laser beam and the number of pulse cycle

were found to play a key role in changing the optical and electrical properties of the

CIGS films and hence the cell performance.

Based on these promising results, future efforts will focus on PLA study using a

commercial type of Excimer laser system that offers a large laser beam size with uniform

surface energy density and variable pulse widths and scan rates to access a wider range of






28


PLA treatment conditions. In particular, it is hoped that a variable pulse width and

wavelength will allow control of the anneal depth. Given its application to other

industrial materials, laser annealing has the potential to be an effective method to

improve solar cell performance in an industrial setting.
















Table 3-1. Effective minority carrier lifetime (z) of CIGS films before and after PLA
treatment as measured by DBOM


T before PLA (ns)
1.77
2.82
4.11
4.51


T after PLA (ns)
4.87
3.39
5.43
6.31


PLA condition
(mJ/cm2, pulses)
30, 5
40, 5
50, 5
60, 5


Sample #
CIGS-S1
CIGS-S2
CIGS-S3
CIGS-S4















Table 3-2. Hall-effect results for CIGS films before and after PLA treatments


Hole-concentration Hall-mobility Resistivity PLA condition
Sample # (1016/cm-3) (cm2/V-s) (Q-cm) (mJ/cm2, pulses)
CIGS-H1 0.53/0.445 8.89/37.6 133/37.3 20, 10
CIGS-H2 2.9/2.43 0.93/2.98 235/86.4 20, 20
CIGS-H3 4.3/1.8 1.54/6.1 94/2.67 40, 10
CIGS-H4 7.1/3.3 0.60/2.8 148/4.64 40, 20
Data taken before/after PLA treatments















Table 3-3. Effective lifetimes and photo- J-V results of PLA CIGS/CdS samples and
devices


Lifetime Voc Jsc F.F.
Sample # (ns) (V) (mA/cm2) (%)
CIGS/CdS #0 3.76 0.42 27.2 53.11
CIGS/CdS #1 4.77 0.45 24.83 54.43
CIGS/CdS #2 4.11 0.44 26.98 51.37
CIGS/CdS #3 5.2 0.45 26.19 54.06
CIGS/CdS #4 3.86 0.39 26.43 44.25
* CdS re-growth was performed after PLA treatment.


Eff.
(%)
6.14
6.17
6.15
6.27
4.57


PLA condition
(mJ/cm2, pulses)
N/A
50, 10
50, 20
50, 10 *
50, 20 *
















Table 3-4. Photo-J-V performance of control cell and PLA-treated CIGS solar cells


Vo, (V) Js, (mA/cm2)


0.576
0.568
0.497
0.493
0.436
0.433
0.543


31.60
31.25
26.79
26.71
25.56
25.23
30.05


F.F. (%)
66.50
63.11
55.29
53.08
51.80
47.77
55.74


PLA condition
ri (%o) (mJ/cm2, pulses)
12.07 30, 5
11.12 30, 10
7.368 40, 5


7.040
5.773
5.201
9.064


40, 10
50, 5
50, 10
N/A


Device #

CIGS-D1
CIGS-D2
CIGS-D3
CIGS-D4
CIGS-D5
CIGS-D6
Ctrl-D
















Table 3-5. Photo- J-V performance and dark- J-V parameters for the control cell and
selected PLA-treated CIGS cells


Cell #
PLA condition
Cell area (cm2)
Voc (V)
Jsc (mA/cm2)
F.F. (%)
Efficiency (%)
Vm (V)
Jm (mA/cm2)
N
Jo (mA/cm2)
Rs (Q)


Ctrl-D-C3
None
0.429
0.528
29.78
48.86
7.69
0.365
21.37
-4.13
-3.22 x10 3
-10.17


CIGS-D1-C3
30 mJ/cm2, 5 pulses
0.429
0.577
34.24
67.88
13.41
0.458
28.99
-1.98
-1.1x10 3
-14.06


CIGS-D2-C3
30 mJ/cm2, 10 pulses
0.429
0.572
32.00
66.78
12.22
0.453
26.88
-1.96
-1.06x10-3
-15.94






34








Table 3-6. Results of the DLTS measurements on the control cell and PLA treated CIGS
solar cells


Control cell


Trap type
Trap activation energy, Ea (eV)
Trap density, Nt (cm-3)


Minority (electron)
Ec- 0.069
5.6 x 013


PLA cell (30 mJ/cm2, 10
pulses)
Minority (electron)
Ec- 0.065
2.8 x1013


Cell #
















Table 3-7. Annealing condition of new PLA samples


Energy density (mJ/cm2)
30
50
70
90
110
N/A


Pulse # Note
5 PLA sample
5 PLA sample
5 PLA sample
5 PLA sample
5 PLA sample
N/A Control sample


Sample #
1
2
3
4
5
Ctrl





























10 20 30 40 50 60 10 20 30 40 50 60
(a) 2 theta(degree) (1) 2 theta(degree)




Figure 3-1. XRD spectra before and after PLA treatments for (a) a CIGS and (b) a CdS
/CIGS sample.






























(a) m (b) 10m


Figure 3-2. Surface morphology of CIGS films (a) without and (b) with PLA treatments
at an energy density of 55 mJ/cm2 (SEM images with magnification of 6000x).
















0.8
0.7 .-


Q05 20 pulses 50mJ/cm'
0.4 10 pulses 5OmJlcrm
E

Non-NLA
00.2
0.1
10is
400 600 800 1000 1200 1400
Wavelength (nm)

Figure 3-3. Quantum efficiency of CIGS cells with and without PLA treatment.









20 pulses 60SOncm^2
OA 0 pulses SOmJJcm^2


S023


01


400 600 r.. 1000 1200 1400
Wavelength (nm)


Figure 3-4. Spectral response of CIGS cells with and without PLA treatment.


2
^2


!





























- I-
E


4-
-I,


NLA Condition (mJicm2)


NLA Condition (mJ/cm2) NLA Condition (mJ/cm2i


Figure 3-5. Photo- J-V parameters versus different PLA condition.







































0 0.1 0.2


0.3 0.4 0.5
Applied Voltage (V)


0.6 0.7


Figure 3-6. Dark- J-V curves comparing the control cell to two PLA treated cells.
















100



10



1



0.1



0.01



0.001


Figure 3-7. Dark- J-V curves (semi-log plot) of the control cell and two PLA treated
cells.


Temperature = 300 K
Cell's Area = 0.429 cm^2

n-4.1

-n2









S/ 00nn00 Control cell
30mJ/cm^2, 5 pulses NLA cell
30mJ/cm 2, 10 pulses NLA cell


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Applied Voltage (V)





































400 500 600 700 800 900 1000 1100 1200

Wavelength (nm)




Figure 3-8. Quantum efficiency (Q-E) versus wavelength comparing the control cell to
the two PLA-treated cells.


Temperature = 300 K
Ce's Area = 0.429 cm^2





0a00n




oDonoD Control cell
- 30mJ'cm^2, 5 pulses NLA cell
- 30mJ/cm^2, 10 pulses NLA cell








43














0.9 Control CIGS cell
Reverse bias = 0.5 V
0.8 Pulse height= 0.7 V
Pulse width = 10 ms
0.7 -
0
0.6 y 0.805x + 5.5049
S1 R2 0.9812
z 0.5 -
a\ lIn(T2/En)
C-
a 0.4- -2 -
o 0.4 -

0.3 \

0.2 \
0 8 9 10 11 12
S1000/T, K
0.1\\


0 100 200 300 40C
Temperature (K)





Initial
0.9- NLA CIGS cell delay
Reverse bias = 0.5 V
0.8 -Pulse height = 0.7 V
-0.05 ms
Pulse width = 10 ms
S0.7 0.1 ms
0.7
S0.6 0 y= 550x+49714 0.2 ms
1 R = 0 9397
S0.5-
Sn\ (T 2/ ) 0.5ms
0 0.4
S- -1 ms
0 0.3 ,\ \ ?
0.3
0.2 \ 8 9 10 11 12
1000/T, K
0.1


0 100 200 300 400
Temperature (K)



Figure 3-9. The DLTS scans of the control- and PLA- CIGS cell.































Figure 3-10. Surface morphology of CdS/CIGS films (a) before and after PLA treatment
with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90
mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 4000x).


Figure 3-11. Surface morphology of CdS/CIGS films (a) before and after PLA treatment
with energy densities at (b) 30 mJ/cm2, (c) 50 mJ/cm2, (d) 70 mJ/cm2, (e) 90
mJ/cm2, (f) 110 mJ/cm2. (SEM images with magnification of 40000x).


1MM
















counts/s

Mo

5000-



4000-
CIGS
CdS
3000-







#5
7 ........... #4

1000


crl

20 30 40 50 60
'2Theta



Figure 3-12. Theta-2 theta (symmetrical geometry) diffraction patterns of PLA samples
and control sample.


















counts/




4:000



ano CdS
CdS



2000









20 30 40 50 60 70
-3Theta



Figure 3-13. Grazing incidence XRD analysis (GIXD) of PLA samples and the control
sample (omega=3deg.)







47








counts/s


CIGS
3000-


2500-
CdS






S000-I CdS









20 30 40 02 6
*2Thet


sample (omega= deg.)














CHAPTER 4
RAPID THERMAL ANNEALING (RTA) OF FILM PROPERTIES AND DEVICE
PERFORMANCE FOR CIGS- BASED SOLAR CELLS

Investigation of the halogen-based Rapid Thermal Annealing (RTA) process on

CIGS films and solar cells are presented in this chapter. Annealing conditions, RTA

procedures and experimental results are well discussed.

Progressive RTA Treatment on CIGS Solar Cells

The progressive RTA experiments were performed on one CIGS device, which

contains 3 cells (fabricated by NREL), and the results show that progressive RTA

treatment (with N2 ambient) improves cell performance and overall uniformity of large

area CIGS solar cells. The RTA procedure is shown in Figure 4-1 and the basic sequence

of the progressive RTA is given as follows:

* Prepare a CIGS device

* Test photo- J-V performance before RTA

* Treat device by 100C 30 seconds RTA

* Test photo- J-V performance again

* Treat device by 200C 30 seconds RTA

* Test photo- J-V performance again......

* Keep increasing the annealing temperature of each RTA until the cell performance
dramatically drops or device is damaged.

From the photo- J-V results (Table 4-1), significant improvement in cell

performance was observed in all 3 cells after each progressive RTA treatment at

temperatures of 100, 200, and 300C for 30 seconds. A dramatic increase in the









performance of cell #1 (increase from 9.52 to 15.77%) shows that progressive RTA could

be used to enhance the structure uniformity and the performance of large area cells. Since

we measured photo- J-V right after each annealing, the light soaking effect might be also

a contribution of the performance improvement. For CIGS material photovoltaic modules

there are several references reporting the material light induced behavior and the possible

reasons (e.g. [39], [40] and [41]). These references all observed the improvement in open

circuit voltage (Voc) and fill factor (F.F.) because of light soaking and the effect relaxes

when kept in dark again. The same effect was observed when keeping the CIGS solar

cells in dark with forward bias applied.

We also tried 400C for 30 seconds; unfortunately the metal grids were severely

damaged after the annealing. As a result, no further tests on J-V and Q-E characteristics

were performed on these cells.

The RTA Effects of Separated CIGS Films

The XRD and SEM Results

Five lxl inch CIGS samples (from SSI), which were grown on the Mo- coated

soda-lime glass (SLG) substrates, were used to investigate the RTA effects under

different annealing conditions. XRD, SEM measurements were conducted before and

after RTA treatments. The annealing condition (Table 4-2) and results are discussed in

the following section.

XRD measurements give information about the preferred film orientation and

composition. SEM photos depict the surface morphology and grain size. Results (Figure

4-2 to 4-5) suggest that the overall film composition and surface morphology do not

change during the annealing. Only some slightly blurry grain edges were observed in the









SEM images for the 2 samples treated at 350C (sample #F-R3 and F-R4) as compared to

the 300C RTA treated samples and control sample.

Hall-Effect Measurements

In order to determine the effects of RTA treatment on the carrier concentration,

carrier mobility, and sheet resistance of CIGS films, resistivity and Hall- effect

measurements were carried out on two CIGS films directly deposited on the SGL

substrates before and after the RTA treatments. The annealing conditions and the results

of resistivity and Hall- effect measurements (using a MMR Hall- and Van Der Pauw-

measurement system) before and after the RTA treatments are summarized in Table 4-3.

The Hall-effect measurements show a significant increase of hole density and a

decrease in film resistivity, Hall coefficient, and sheet resistance following the RTA

treatment. For sample- #H300-1, which was treated by 300C RTA, the carrier mobility

after RTA was found 2 times larger than the value before annealing, while for sample

#H350-1, which was treated by 350C RTA, the hole mobility was dropped by more than

50% after RTA treatment. The results revealed that 350C RTA increased the carrier

concentration by nearly two orders of magnitude, which also led to the decrease of carrier

mobility. The 300C RTA treated sample showed beneficial results on the film resistivity,

carrier mobility and carrier density. Therefore, the peak annealing temperature plays an

important role in determining the electrical properties of RTA treated CIGS absorber

layers.

The RTA Effects of Separated CIGS Solar Cell Devices

Experimental Details

Four lxl inch CIGS samples (from SSI), which were grown on the Mo- coated

soda-lime glass (SLG) substrates were used to investigate the RTA effects under different









annealing conditions. A thin (50 nm) CdS buffer layer was deposited on the CIGS

samples using Chemical Bath Deposition (CBD) at 65C for 13 minutes. A ZnO window

layer was then deposited on the CdS/CIGS/Mo/SLG sample by RF sputtering.

Subsequent metallization (Ni-Al front contact grids) was carried out by e-beam

evaporation through a shadow-mask. Finally, finished devices were produced by cutting

the sample into separate cells with 0.429 cm2 active area and attaching wires with Indium

bumps on the Mo-coated glass substrates for back contacts. The performance of these

cells was then tested by dark- J-V, photo- J-V and Q-E measurements before and after

RTA treatment (with air ambient) under different annealing conditions. To exclude the

light soaking effect, the photo- J-V and Q-E re-measurement were conducted two weeks

after RTA treatments. The annealing condition (Table 4-4) and results are discussed in

the following section.

Photo- J-V Performance

From the photo- J-V results (Table 4-5), for the cell D-R1, D-R2 and D-R3, we

found that Vo,, Js, and conversion efficiency increased and F.F. decreased after RTA

treatments. For cell D-R4 which was annealed at 3500C, 2 min, only Jsc increased after

RTA and the overall performance dropped. By taking a close look at the F.F. data, we

found that for the cell treated by 2 min, the F.F. dropped more than that of the cell treated

by 1 min RTA at the same temperature. This suggests that 1 min is enough for the RTA

treatment and too long holding time can cause performance drops.

Dark- J-V Characteristics

To find the reason of Fill Factor loss during the RTA, the dark- J-V measurements

were conducted of these the tested devices before and after RTA treatments. From the

measured dark- J-V curves (Figure 4-6 to 4-9), we found the overall dark current density









decreased in forward bias region while it increased in reverse bias region after RTA

treatment except cell D-R1 has increased dark- J-V when applied voltage below 0.38 V

after RTA treatment, and the shapes of all J-V curves after RTA have a trend to become

linear, which suggest the resistivity decrease during the RTA and the Fill factor loss. One

possible reason is the metal contact on the top of the devices diffused down through the

whole device during the annealing and increased the conductivity, made the whole device

become more like a conductor rather than a semiconductor. Some other tested cells even

showed pure linear J-V curve and zero F.F, which is caused by metal spiking effects.

Several recent studies already showed that the spiking effect is more likely to occur

during the RTA for the E-beam evaporated Al metal grids than sputtered metal contacts.

Another possible reason is because of the low melting point of In (156.61C). Indium

diffusion from the back contact might also cause the decrease of F.F. during the RTA

treatment. The results suggest that RTA treatment is more suitable for devices with

sputtered metal contacts or applying RTA treatments before metal grids and contacts

deposition.

From the semi-log plots of the dark- J-V curves (Figure 4-10 to 4-13), cell # D-R1

has deteriorated diode quality factor after the RTA treatment while other cells remain

same diode qualities. Though the photo- J-V results of cell # D-R1 is very promising after

RTA, the relationship between RTA condition and the afterward dark- J-V result is still

unclear for cell # D-R1.

The S-R and Q-E Performance

From the measured Q-E curves (Figure 4-14 to 4-17), improvements on Q-E were

observed on entire interested wavelengths after RTA treatments, which suggest that RTA

has positive effect on all layers (absorber layer, buffer layer, and window layer) of CIGS









solar cells. These results are consisted with the photo- J-V results showed before; even

for cell D-R4 which has over all decreased photo- J-V performance after RTA, the Q-E

still showed that photo current density for the interested wavelength region increase after

RTA treatment which evidently proved the Js, improvement of such cell.

Thermal Annealing on CIGS Solar Cells by High Temperature XRD System

Because of the limitation on the minion temperature of the RTA system, a high

temperature XRD system was used to anneal 2 CIGS solar cells at temperature 100C and

150C, 30 seconds. The Photo- and Dark- J-V measurements were taken before and after

annealing. Dark- J-V curves did not show much difference before and after low

temperature RTA treatments. From the photo- J-V results (Table 4-6 and Figure 4-18),

the overall performance for tested cells has been improved after low temperature RTA,

furthermore. The cell treated by 150C RTA has more significant improvement on all IV

parameters while the cell treated by 100C RTA only has improvement on F.F. and l.

Combined with the previous results (progressive RTA results) shown before, these data

suggest that the optimal RTA temperature for CIGS solar cells is in the range of 200C to

3000C.

Summary

Progressive RTA treatments have shown significant improvement of the overall

uniformity and performance of large area CIGS solar cells. Under low RTA temperature,

the surface composition and morphology remain unchanged. Our study of the RTA effect

on CIGS devices shows increase in the values of Voo, Js, and conversion efficiency, but

some decrease in fill factor (F.F.). The estimated optimal annealing temperatures should

be between 200 and 3000C with 1- minute or less holding time.

















Table 4-1. Photo- J-V results of CIGS solar cells before and after progressive RTA


Pre-annealing
Voc (V)
Jsc (mA/cm2)
F.F. (%)
Eff (%)
After 200C
RTA
Voc (V)
Jsc (mA/cm2)
F.F. (%)
Eff (%)


Cell #1
0.628
31.66
47.88
9.52
Cell #1
0.652
34.85
68.43
15.55


Cell #2
0.652
32.97
68.52
14.73
Cell #2
0.657
35.11
72.14
16.65


Cell #3
0.655
33.50
70.71
15.51
Cell #3
0.650
32.34
76.15
16.01


After 100C
RTA
Voc (V)
Jsc (mA/cm2)
F.F. (%)
Eff. (%)
After 300C
RTA
Voc (V)
Jsc (mA/cm2)
F.F. (%)
Eff (%)


Cell #1
0.633
34.30
56.81
12.32
Cell #1
0.627
35.39
71.05
15.77


Cell #2
0.656
35.47
71.10
16.55
Cell #2
0.623
36.35
70.48
15.96


Cell #3
0.660
34.05
73.03
16.19
Cell #3
0.630
35.08
74.32
16.42






55








Table 4-2. CIGS film number and the annealing conditions


Sample # RTA temperature Holding time note
F-R1 3000C 1 minute RTA sample
F-R2 3000C 2 minutes RTA sample
F-R3 3500C 1 minute RTA sample
F-R4 3500C 2 minutes RTA sample
F-Ctrl N/A N/A Control sample
















Table 4-3. Hall-effect data of NREL CIGS samples before and after RTA treatments


Sample #H300-1


RTA condition 3000C, 1-mi,
Ambient Ar
Resistivity (ohm-cm)* 70.58 4.21
Mobility (cm2/Vs)* 2.80 / 6.77
Hole density (cm-3)* 3.16x10'6 /2
Hall coefficient (cm3/Coul)* 197.77 28.4
Sheet resistance (ohm/cm2)* 470536.4/2
Carrier type holes /holes
*Data taken before/after RTA treatments


Sample #H350-1
3500C, 1-min


.19x101
O8
8044. 4


55.76 2.17
4.28 1.86
2.62 x016/ 1.55x10]
238.53 4.03
371761.8 14435.8
holes /holes






57








Table 4-4. Annealing conditions of RTA treated CIGS devices


RTA Temperature
3000C
3000C
3500C
3500C


Holding Time note
1 minute RTA device
2 minutes RTA device
1 minute RTA device
2 minutes RTA device


Sample #
D-R1
D-R2
D-R3
D-R4















Table 4-5. Photo- J-V results of separated CIGS solar cells before and after RTA


Cell# D-R1 D-R2 D-R3 D-R4
RTA condition 3000C, 1-min 3000C, 2-min 3500C, 1-min 3500C, 2-min
Voc (V)* 0.455 /0.471 0.471/0.487 0.465 /0.471 0.465 /0.422
Jsc (mA/cm2)* 27.49/31.43 26.55/30.51 28.75/33.94 27.67/30.41
F.F. (%)* 52.69/49.01 57.48/49.96 52.14/48.58 48.00/39.36
Eff (%)* 6.591 / 7.259 7.196/7.422 6.975 / 7.759 6.177/5.051
* Data taken before/after RTA treatments






59








Table 4-6. Photo- J-V results of CIGS solar cells before and after RTA by using high
temperature XRD system


Device# RTA Voc (V)* Jsc (mA/cm2)*
1 1000C,30s 0.50/0.503 24.74/24.68
2 1500C,30s 0.43/0.509 25.56/28.85
* Data taken before/after RTA treatments


FF (%)
61.45 /67.60
45.91/51.9


Eff (%)*
8.19/8.35
5.03/7.62


































time


Figure 4-1. Cycle time for Rapid Thermal Annealing (one run).








































Figure 4-2. XRD results of RTA treated CIGS films and the control sample.



































Figure 4-3. Surface morphology of RTA treated CIGS films and the control sample.
(SEM images with magnification of 3000x).



































Figure 4-4. Surface morphology ofRTA treated CIGS films and the control sample.
(SEM images with magnification of 10000x).



































Figure 4-5. Surface morphology ofRTA treated CIGS films and the control sample.
(SEM images with magnification of 30000x).


















< 40




N 30

a

= 20
1-
I-
u

C 10


0.6


Applied voltage(V)




Figure 4-6. Dark- J-V curves of tested CIGS solar cell (#D-R1) before and after RTA
(3000C, 1-min) treatment.


Temperature = 300 K
Cell's Area = 0.43 cm^2


I

i

Before RTA

--After RTA
F_





















Temperature = 300 K
Cell's Area = 0.43 cm^2







/
1








Before
After
jB
l


Applied voltage (V)






Figure 4-7. Dark- J-V curves of tested CIGS solar cell (#D-R2) before and after RTA
(3000C, 2-min) treatment.















50


S40 Temperature = 300 K
Cell's Area = 0.43 cm^2
E /
30 -





10 0
S -- After ..-""

0
0 0.2 0.4 0.6
Applied voltage (V)




Figure 4-8. Dark- J-V curves of tested CIGS solar cell (#D-R3) before and after RTA
(350C, 1-min) treatment.
























Temperature = 300 K
Cell's Area = 0.43 cm^ 2


1
1
F

p
/


- -Before

-After


-----


0 0.1 0.2 0.3 0.4


0.5 0.6


Applied Voltage (V)






Figure 4-9. Dark- J-V curves of tested CIGS solar cell (#D-R4) before and after RTA
(350C, 2-min) treatment.


I I I

















100
er Temperature = 300 K
Cell's Area = 0.43 cm^2

10






1 -BeforeRTA

-3 ,I -- After RTA
0.1 /-
-
I


0.01
0 0.1 0.2 03 0.4 0.5 0.6
Applied Voltage(V)





Figure 4-10. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R1) before
and after RTA (300C, 1-min) treatment.







70








100
Temperature = 300 K
Cells Area = 0.43 cm^2 -

E -*
< 10




= 0.1








0 0.1 0.2 0.3 0.4 0.5 0.6

Applied voltage (V)




Figure 4-11. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R2) before
and after RTA (300'C, 2-min) treatment.














100
STemperature = 300 K
E Cell's Area = 0.43 cm^2 --


10




1 ---Before
After



0.1
0 0.2 0.4 0.6

Applied voltage (V)



Figure 4-12. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R3) before
and after RTA (350C, 1-min) treatment.






































0.01


0 0.1 0.2 0.3 0.4 0.5 0.6

Applied voltage (V)





Figure 4-13. Dark- J-V curves (semi-log plot) of tested CIGS solar cell (#D-R4) before
and after RTA (350C, 2-min) treatment.


Temperature = 300 K
Cell's Area = 0.43 cm^2 ,-











Before
-After


















0. 7
After RTA

0 6 Before RTA

0.5



0.43






0.1

0 -------------------I

400 600 800 1000 1200
wavelength (rm)

Figure 4-14. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell
(#D-R1) before and after RTA (300C, 1-min).














0. 9

0. 8

0. 7

0. 6

0. 5

0. 4

0.3 -

0. 2

0. 1

0 -
400


600 800 1000


1200


wavelength (m)




Figure 4-15. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell
(#D-R2) before and after RTA (300C, 2-min).

















0. 8




0.6 -




0.4




0.2




0 -
400


600 800 1000


1200


wave egth (rm)




Figure 4-16. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell
(#D-R3) before and after RTA (350C, 1-min).














0.6


0.5


0.4


at O.


0..2 '/
*0


0. 1


0

400


600 800 1000


1200


wavelength (nm)



Figure 4-17. Quantum efficiency (Q-E) versus wavelength of tested CIGS solar cell
(#D-R4) before and after RTA (350C, 2-min).






















-- After 100C RTA
- Before RTA


0 0.1 0.2 0.3 0.4


Applied Voltage (v)


- After 150 C RTA
-Before RTA


0.1 0.2


0.3
applied Voltage (v)


Figure 4-18. Photo- J-V curves of tested CIGS solar cells before and after (a) 100C, 30
seconds and (b) 150C, 30 seconds RTA treatments by using high temperature
XRD system.














CHAPTER 5
FABRICATION PROCESS OF CIGS SOLAR CELLS

In this chapter description of the CIGS solar cell device structure and typical

fabrication process are given in detail. Based on the typical device structure and

fabrication process of CIGS based thin-film solar cell (Figure 5-1), the first two steps

comprise the deposition of a 1-2 tm Mo back contact on top of a thin chemical barrier

(Si02) on a glass substrate. This can be accomplished by two-step magnetron sputtering

in an in-line PVD system. Next, a CIGS absorber layer with thickness of 1.5-2 [tm is

deposited by using PVD, sputtering, PMEE, co-evaporation or RTP technique. Followed

by deposition of a 500 A thin CdS buffer layer (forming the hetero-junction with CIGS)

using chemical bath deposition (CBD). An intrinsic ZnO of 500-1000 A and a

conducting ZnO:Al (0.3 to 0.5 tm) TCO layers are then deposited by the sputtering

system. Finally a 500 A Ni and a 3 tm Al- metal contacts are deposited by using E-Beam

evaporation technique. For finished device, an Anti-reflection (AR) coating (e.g., MgF2)

was used on the top of the cell to minimize the reflection loss.

Substrate

The most widely used substrate for the fabrication of CIGS based solar cells is the

soda-lime glass (SLG), which allowed champion efficiencies of up to 19.2% AM1.5G

(Ramanathan et al., 2003). This low-cost SLG substrate material can be produced in large

scale and with reproducible quality mainly for the window industry. The most commonly

used back contact is the sputtered Mo- metal layer of approximately 1 |tm thickness. The

following are the requirements for an excellent CIGS substrate.









* Vacuum compatibility. The substrate should not degas during the various vacuum
deposition steps, especially during CIGS absorber layer deposition, when the
substrate must be heated.

* Thermal stability. For the deposition of high performance CIGS absorbers, the
substrate temperature should reach 500-600C during part of the deposition
process. Substrate temperatures of less than 350C usually lead to severely
degraded absorber layer quality and poor cell performance. Therefore, substrates
should withstand the temperature no less than 350C.

* Proper thermal expansion. The coefficient of thermal expansion (CTE) of the
substrate should be compatible with that of CIGS; otherwise CIGS adhesion
problems may be occurred. Also cracking of the Mo back contact can be
encountered because of its low CTE.

* Chemical inactivity. The substrate should not corrode, neither during processing
nor during use. In particular, it should not react with Se during the CIGS deposition
or decompose during aqueous solution deposition of buffer layer (CdS). A good
substrate should also not release any impurities that can diffuse into the absorber,
except when the diffusion is desired.

* Sufficient humidity barrier. The substrate could protect the active solar cells layers
during the long-term usage against environmental attack from the back such as the
penetration of water vapor.

* Surface smoothness. A smooth substrate surface is essential. First, abrupt changes
in the surface such as spikes or cavities may lead to shunts between the front and
the back contact. Second, the deposition of impurity diffusion barriers or insulation
layers may be easier and more successful on a smooth substrate.

* Cost, energy consumption, availability and weight. An excellent substrate should
be cheap, little energy consumption, consists of available and abundant materials,
and with light weight.

Because of the above requirements and demands of CIGS substrates, the SLG is the

preferred substrate material for the industrial manufacturing of CIGS based solar cells

since it fulfils most of these requirements. Recent results from ISET show CIGS solar

cells formed on SLG substrates with higher conversion efficiency than those with other

alternative substrates (Table 5-1). Additionally, the incorporation of sodium has a

beneficial effect on the CIGS film quality. However, the main disadvantages of glass









substrates are their high brittleness and non-flexibility, which limit the applications

considerably.

Back Contact Layer

The molybdenum (Mo) layers with a thickness of around 1 |tm deposited by

d.c.-magnetron sputtering process are used in the bottom contact layers for the

CIGS-based solar cells. With the excellent properties of low contact resistance to the

CIGS, relative stability at the processing temperatures (400-600 C), and resistance to

reacting with Cu, In, and Ga, the Mo is the most widely used metal for the back contacts

of CIGS-based solar cells.

Followed the deposition of CIGS absorber layer on the Mo-coated glass

substrates, an interfacial MoSe2 layer between the CIGS absorbers and the Mo layers has

been identified. This layer structure of MoSe2 was suggested to have a bandgap energy

of about 1.4eV with a thickness of around 0.1 itm. Besides that the wide-band-gap

MoSe2 layer can be served as the back surface field in the CIGS-based solar cells to

enhance the carrier collection, the MoSe2 layers are considered to improve the adhesion

of CIGS films to the Mo contacts to form good ohmic contacts.

CIGS Absorber Layer

The Cu(In,Ga)Se2 compound belongs to the semiconducting I-III-VI2 materials

family that crystallize in the tetragonal chalcopyrite structure. Chalcopyrite is another

name for copper iron sulfide (CuFeS2), a common copper ore, which gave name to these

materials. An interesting property of the semiconducting chalcopyrites in general and

Cu(In,Ga)Se2 in particular is that bandgap energy, Eg, can be varied, for instance by

varying the amount of Ga. The optimal bandgap for a solar cell with respect to the solar









spectrum is around 1.4 eV [42]. The bandgap of CuInSe2 (CIS) is 1.04 eV and 1.68 eV

for CuGaSe2 (CGS) while the bandgap of the alloys Cu(In,Ga)Se2 lies in between. A

Ga/(In+Ga) ratio of 30% results in Eg of around 1.3 eV, which has been shown

empirically to give the best device performance. Another way to increase Eg is by

replacing part of the Se for S to form Cu(In,Ga)(Se,S)2. It is expected that a wider Eg

would yield a higher Vo. At a given efficiency higher voltage and lower current is

preferred, since lower current results in smaller resistive losses with P = I2R. In a module

with several interconnected cells an increased voltage is of interest. Since a higher

voltage corresponds to a lower current for the same power, a superior design of wider

cells with smaller geometrical losses is preferred.

CIGS thin-films are grown by the sequential evaporation of metals in the presence

of Se. The vacuum base pressure is in the 6-10-7-torr range. To help the reader better

understand the growth of the CIGS thin-films, we describe first the integrated deposition

scheme. Figure 5-2 shows a schematic profile of the elemental fluxes and substrate

temperature used for the deposition of the CIGS films [43]. The process consists of three

stages: (1) the formation of an (In,Ga)2Se layer on the soda-lime glass (SLG)/Mo

substrate at a substrate temperature of 400C; followed by (2) the deposition of Cu and Se

at a substrate temperature of 570C, at which the compound formation and

crystallization of Cu (In,Ga)2Se takes place such that the composition is slightly Cu-rich;

and (3) additional In, Ga and Se is added at the same substrate temperature of 570C, so

that the final composition expected is Cu-poor. The sample is cooled down to

approximately 400C in the presence of Se, after which the system is allowed to cool

down naturally to room temperature. CIGS solar cells were usually completed by









chemical-bath deposition (CBD) of approximately 500 A CdS, followed by RF sputtering

of 500 A of intrinsic ZnO and 2000 A of Al-doped ZnO. Ni/Al grid contacts were applied

with approximately 5% coverage. The substrate was SLG coated with Mo film sputtered

using a DC magnetron cathode.

Chemical Bath Deposition (CBD) of CdS Buffer Layer

The technique of CBD involves the controlled precipitation from solution of a

compound on a suitable substrate. The CBD technique offers several advantages over

the CVD, MBE and spray pyrolysis methods. In the CBD process, the film thickness

and deposition rate can be controlled by varying the solution pH, temperature, and

reagent concentration, and it is capable to coat large areas in a reproducible and low cost

way. In addition, the homogeneity and stochiometry of the deposited films are partially

maintained. The CBD process was first reported by by J. E. Reynolds in 1884 for the

deposition of PbS films. Since then a wide range of chalcogenide (e.g. CdS, ZnS and

MnS) and chalcopyrite materials (e.g. CuInS2 and CuInSe2) have been prepared by the

CBD method.

Recently, there has been considerable interest for developing new polycrystalline

thin-film semiconductors using various techniques. Among them, the CBD process has

found very attractive for being a low temperature and low cost process. The CBD

technique is a useful method for the deposition of thin-film semiconductor materials.

Many of them form important components within polycrystalline solar cells. Such

devices may offer advantages in low cost solar energy conversion. The CBD process

has advantages over alternative methods of thin-film deposition: The technique is simple

and requires relatively low capital expenditure; films may be deposited at very low

temperatures on a wide variety of substrates. The CBD process may be easily adapted to









large area processing at low fabrication cost, and the thickness of the deposited layers

may be controlled by varying the length of the deposition time. A major drawback of

the CBD process is the inefficiency of the process in terms of the utilization of starting

materials and their conversion to thin-films. The competing homogeneous reaction in

the solution, which results in massive precipitation in solution and deposition of materials

on the CBD reactor walls, limits the extent of the heterogeneous reaction on the substrate

surface.

There are several methods for depositing thin-film CdS: vacuum evaporation,

chemical vapor deposition, and spray pyrolysis. However, the most convenient method

for low cost growth is the CBD process. The CBD technique is a simple solution

growth process for creating polycrystalline CdS thin-films. This involves dipping a

substrate into a reaction mixture for a time depending on the film thickness required. It

is well known that the structures and properties of CdS films are influenced by the recipe

used in the growth. Composition, grain size, crystallinity, photosensitivity, defect

density, and the covering ability have all related to the bath composition, temperature,

and duration of the deposition.

In the deposition of solid thin-films in a chemical bath by the CBD process the

nucleation centers are regularly formed by the absorption of metal hydroxo species on the

surface of the substrate. The initial layer of the thin-film is formed through the

replacement of hydroxo group by the sulphide ions, and subsequently the solid film is

grown by the condensation of metal and sulfide ions onto the top of the initial layer.

Two competing processes, the heterogeneous process of the solid film deposited on the

substrate and the homogeneous process of precipitation in the reaction bath are taking