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Nanoscale Materials in Novel Solar Cell Designs

Permanent Link: http://ufdc.ufl.edu/UFE0021489/00001

Material Information

Title: Nanoscale Materials in Novel Solar Cell Designs
Physical Description: 1 online resource (113 p.)
Language: english
Creator: WADHWA,POOJA
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: CARBON -- ELECTROLYTE -- GATING -- NANOTUBES -- PHOTOVOLTAIC -- SCHOTTKY -- SILICON
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The accessibility of nanoscale materials developed over the past decade provides new opportunities in solar cell design. The work described here-in explores some of the possibilities. Three advances are reported along with some preliminary work in a related area: 1) Bulk heterojunction polymer solar cells, known since the mid-1990s, rely on naturally occurring phase segregation between the n-type and p-type charge transporters. Control over that phase segregation remains a major challenge. The advent of techniques to create silicon nanowires provides an opportunity to engineer the bulk heterojunction without relying on phase segregation. This is demonstrated in a simple n-type silicon nanowire/PEDOT:PSS polymer heterojunction solar cell. The bulk heterojunction nature of the cell imparts to these devices a power conversion efficiency (PCE) that is over four times greater than that of planar heterojunction devices made from these materials. 2) In a second solar cell design the intrinsic porosity of single wall carbon nanotube (SWNT) films is exploited to permit electronic modulation of the junction characteristics in a SWNT/n-Si Schottky junction solar cell. Electronic modulation occurs via electrolyte gating in which the porosity of the nanotube films permits the electrolyte direct access to the junction. The ungated, native device has a PCE of 8.5% (under AM1.5G illumination). Modulation of the gate voltage (which consumes negligible power in the steady state) of ?0.75 V yields a continuous, reversible modulation of the device PCE from ~4 - 11%. 3) In Schottky junction solar cells, the depletion layer responsible for charge separation is only of the order of a micron from the metal electrode. Accordingly, if large areas of the semiconductor surface are not in contact with the metal the current collection efficiency decreases. In the course of the studies described in 2 it was discovered that the free ions available in the electrolyte induce an inversion layer in the silicon that permits charge collection despite the nanotube-film/Si junction occupying only a fraction of the Si surface. By avoiding the parasitic absorption in the SWNTs over the electrolyte-Si only regions, the gated PCE could be increased to 12% which exceeds that of dye sensitized solar cells. Finally preliminary results of an alternative nanotube film preparation method is discussed.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by POOJA WADHWA.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Rinzler, Andrew G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0021489:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021489/00001

Material Information

Title: Nanoscale Materials in Novel Solar Cell Designs
Physical Description: 1 online resource (113 p.)
Language: english
Creator: WADHWA,POOJA
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: CARBON -- ELECTROLYTE -- GATING -- NANOTUBES -- PHOTOVOLTAIC -- SCHOTTKY -- SILICON
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The accessibility of nanoscale materials developed over the past decade provides new opportunities in solar cell design. The work described here-in explores some of the possibilities. Three advances are reported along with some preliminary work in a related area: 1) Bulk heterojunction polymer solar cells, known since the mid-1990s, rely on naturally occurring phase segregation between the n-type and p-type charge transporters. Control over that phase segregation remains a major challenge. The advent of techniques to create silicon nanowires provides an opportunity to engineer the bulk heterojunction without relying on phase segregation. This is demonstrated in a simple n-type silicon nanowire/PEDOT:PSS polymer heterojunction solar cell. The bulk heterojunction nature of the cell imparts to these devices a power conversion efficiency (PCE) that is over four times greater than that of planar heterojunction devices made from these materials. 2) In a second solar cell design the intrinsic porosity of single wall carbon nanotube (SWNT) films is exploited to permit electronic modulation of the junction characteristics in a SWNT/n-Si Schottky junction solar cell. Electronic modulation occurs via electrolyte gating in which the porosity of the nanotube films permits the electrolyte direct access to the junction. The ungated, native device has a PCE of 8.5% (under AM1.5G illumination). Modulation of the gate voltage (which consumes negligible power in the steady state) of ?0.75 V yields a continuous, reversible modulation of the device PCE from ~4 - 11%. 3) In Schottky junction solar cells, the depletion layer responsible for charge separation is only of the order of a micron from the metal electrode. Accordingly, if large areas of the semiconductor surface are not in contact with the metal the current collection efficiency decreases. In the course of the studies described in 2 it was discovered that the free ions available in the electrolyte induce an inversion layer in the silicon that permits charge collection despite the nanotube-film/Si junction occupying only a fraction of the Si surface. By avoiding the parasitic absorption in the SWNTs over the electrolyte-Si only regions, the gated PCE could be increased to 12% which exceeds that of dye sensitized solar cells. Finally preliminary results of an alternative nanotube film preparation method is discussed.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by POOJA WADHWA.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Rinzler, Andrew G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0021489:00001


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1 NANOSCALE MATERIALS IN NOVEL SOLAR CELL DESIGNS By POOJA WADHWA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOP HY UNIVERSITY OF FLORIDA 2011

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2 2011 Pooja Wadhwa

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3 To my mom and dad

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4 ACKNOWLEDGMENTS I would like to sincerely thank my academic advisor Prof. Andrew Rinzler for his indispensable guidance, enriching discussions and invaluable time withou t which this thesis would not have been completed. He has been a constant source of inspiration, guidance and support. I want to thank him for all the great opportunities he has given me to explore my potential. I would like to thank Prof. Reynolds for his insightful discussions and his group for their collaborative work. I would also like to thank Prof. Jing Guo and Jason Seol for their collaboration. I would like to thank my committee members, Prof. Hebard, Prof. Tanner, Prof. Hershfield and Prof. Hirschf eld for their guidance and encouragement. A special thanks to Prof. David Reitze for showing his confidence in me and being so wonderful to me. I would also like to thank Prof. Jiangeng Xue for being an inspirational teacher to me. I want to acknowledge all members of my lab for their help and support. I would also like to thank National Science Foundation and Nanoholdings for funding these research projects. I would like to thank Bill Lewis and David Hays from Nanoscale Research Facility for training me and allowing me to access the clean room facilities. I also want to thank Brent Nelson for solving all my computer issues and salvaging my laptop time and again. I want to thank Larry Phelps and Pete Axson from the Electronic shop and Marc Link, Bill Malph urs and Edward Storch from the Machine shop for doing such incredible work and helping me with the electronics, designing and machining for my projects I want to express my sincere gratitude to Darlene Latimer for her prompt help, support and friendship, making my life smooth sailing as a graduate student in UF. I

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5 also want to thank Pam Marlin for always answering my last minute questions. I want to thank Jay Horton for having insightful discussions with me on life, goals and career and for always greeting me with a smile. Most importantly, I want to thank my mother, Kiran Wadhwa, my father, Mohan Lal Wadhwa and my brother, Sachin Wadhwa for their unconditional love, immeasurable affection and constant support and for having faith in me and for believing i n me. I want to thank them for preparing me for my future. Words cannot express how much their love means to me and has motivated me to move forward in my life and to never give up. I am especially thankful and grateful to my uncle, Subhash Sangar and his family for their good wishes, prayers and love. I want to thank my friend Manoj Srivastava for being a great teacher of physics and of life to me. I also want to thank Basak Selcuk, Sinan Selcuk, Mansi Bahl and Ranie Bansal for their precious and beautifu l friendships, for their understanding and patience and for always being there for me. I would like to dedicate this work to my late grandmother, Rampyaari Kapoor, she would have been proud.

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6 TABLE OF CONTENTS page ACKNO WLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBREVIATIONS ................................ ................................ ........................... 13 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 17 1.1 Carbon Nanotubes ................................ ................................ ............................ 17 1.2 Nanotube Films ................................ ................................ ................................ 19 1.2.1 Nanotube Suspension ................................ ................................ ........... 19 1.2.2 Filtration Process ................................ ................................ ................... 20 2 SOLAR CELLS ................................ ................................ ................................ ....... 24 2.1 B ackground ................................ ................................ ................................ ....... 24 2.2 Theory ................................ ................................ ................................ ............... 24 2.2.1 Solar Energy ................................ ................................ ......................... 24 2.2.2 Working o f a Solar Cell ................................ ................................ .......... 25 2.2.2.1 Effect of series resistance ................................ ........................ 27 2.2.2.2 Performance characteristics of photovoltaics .......................... 28 2.3 Instrumentation ................................ ................................ ................................ 28 3 SILICON NANOWIRES IN A HYBRID SOLAR CELL ................................ ............. 36 3.1 Motiva tion ................................ ................................ ................................ ......... 36 3.2 Challenge ................................ ................................ ................................ .......... 37 3.3 Hybrid Solar Cell ................................ ................................ ............................... 37 3.3.1 Device D esign and Fabrication ................................ .............................. 38 3.3.2 Synthesis of SiNWs ................................ ................................ ............... 39 3.3.3 Results ................................ ................................ ................................ .. 40 3.3.4 Simulated Effect of Series Resistance ................................ .................. 40 3.3.5 Discussion ................................ ................................ ............................. 41 3.4 Interpenetrating Heterojunction Solar Cell ................................ ........................ 42 3.4.1 Concept ................................ ................................ ................................ 42

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7 3.4.2 Fabrication Details ................................ ................................ ................. 42 4 ELECTROLYTE GATED NAN OTUBE/SILICON SOLAR CELL ............................. 52 4.1 Concept ................................ ................................ ................................ ............ 52 4.2 Background ................................ ................................ ................................ ....... 52 4.3 Experimental Details ................................ ................................ ......................... 54 4.3.1 Device Architecture ................................ ................................ ............... 54 4.3.2 Device Fabrication ................................ ................................ ................. 54 4.4 Results ................................ ................................ ................................ .............. 56 4.4.1 Conventional Solar Cell ................................ ................................ ......... 56 4.4.2 Electrolyte Gated Solar Cell ................................ ................................ .. 56 4.4.3 Equivalent Circuit ................................ ................................ .................. 58 4.5 Discussion ................................ ................................ ................................ ........ 59 4.5.1 Effect on Built in Potential ................................ ................................ ..... 59 4.5.2 Nanotube Film Resistivity ................................ ................................ ...... 60 4.5.3 Energy Gap Feature ................................ ................................ .............. 61 4.5.3 .1 Schottky Mott model ................................ ................................ 61 4.5.3.2 Bardeen model ................................ ................................ ........ 62 4.5.3.3 Modern Schottky model ................................ ........................... 63 4.5.4 V OC Saturation ................................ ................................ ....................... 64 4.6 Concluding Remarks ................................ ................................ ......................... 65 4.7 Future Work ................................ ................................ ................................ ...... 66 5 ELECTROLYTE INDUCED INVERSION LAYER SCHOTTKY JUNCTION SOLAR CELL ................................ ................................ ................................ .......... 73 5.1 Background ................................ ................................ ................................ ....... 73 5.2 Device Desi gn ................................ ................................ ................................ ... 73 5.2.1 Device Architecture ................................ ................................ ............... 73 5.2.2 Device Fabrication ................................ ................................ ................. 74 5 .3 Results ................................ ................................ ................................ .............. 74 5.3.1 Conventional Grid Cell ................................ ................................ .......... 74 5.3.2 Electrolyte Gated Grid Cell ................................ ................................ .... 75 5.4 Inversion Layer ................................ ................................ ................................ 76 5.5 Electrostatic Simulations ................................ ................................ ................... 79 5.6 Quantitative Analysis ................................ ................................ ........................ 82 5.7 Discussion and Future Work ................................ ................................ ............. 84 6 CARBON NANOTUBE SPRAYED FILMS ................................ .............................. 93 6.1 Theory ................................ ................................ ................................ ............... 93 6.2 Ink preparation ................................ ................................ ................................ .. 95 6.3 SWNT Ink Based Films ................................ ................................ ..................... 96 6.4 Decomposing Py rene HPC ................................ ................................ ............... 96 6.5 Spray Casting ................................ ................................ ................................ ... 97 6.6 Pyrene HPC/SWNTs Ethanol Ink ................................ ................................ ..... 98

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8 6.7 Stability Measurement ................................ ................................ ...................... 99 6.8 Future Work ................................ ................................ ................................ .... 100 LIST OF REFERENCES ................................ ................................ ............................. 108 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 113

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9 LIST OF TABLES Table page 3 1 Gives the output parameters for different junctions under light. ......................... 44 4 1 Solar cell characteristics extracted from the gated J V curves. .......................... 67

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10 LIST OF FIGURES Figure page 1 1 The unrolled honeycomb lattice of a nanotube.. ................................ ................. 22 1 2 DOS of (10, 0) and (9, 0) nanotube. ................................ ................................ ... 23 2 1 Thirty years evolution in con version efficiencies of different photovoltaic technologies. ................................ ................................ ................................ ...... 30 2 2 Shows the air mass ratio. ................................ ................................ ................... 31 2 3 Solar spectrum as a functio n of wavelength at different air mass ratios.. ........... 32 2 4 A p n junction solar cell with resistive load. ................................ ........................ 33 2 5 Shows the effect of varying bias voltage on the net current and band bending of a p n junction.. ................................ ................................ ................................ 33 2 6 I V characteristics of a solar cell under illumination. ................................ ........... 34 2 7 Equivalent solar cell circuit. ................................ ................................ ................ 34 2 8 I V characteristics of a solar cell depicting the effect of series resistance. ......... 35 2 9 The optical set up used to simulate AM1.5G solar spectrum ............................. 35 3 1 Shows a heterogeneous blend of two organic materials. ................................ ... 44 3 2 Cross section of an organic inorganic hybrid device. ................................ ......... 44 3 3 Chemical structure of PEDOT:PSS.. ................................ ................................ .. 45 3 4 Top view. Sho ws a SEM images of the as prepared SiNWs with Ag. ................ 45 3 5 SEM images of SiNWs after Ag etching. ................................ ............................ 46 3 6 Cross sectional view of Si ................... 47 3 7 Cross sectional SEM image of infiltrated polymer between the nanowires. ....... 47 3 8 J V curve of a SiNWs and PEDOT:PSS junction solar cell w ithout CNTs ......... 48 3 9 Show the simulated J V plots under illuminatio n for different values of R s ....... 48 3 10 A schematic diagram of a hete rogeneous mixture of CNTs and SiNW s ............ 49 3 11 Schematic diagram of a solar cell with a mixed layer at the junction. ................. 49

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11 3 12 S how s the SEM images of pure SiNWs on a mixed cellulose membrane. ......... 50 3 13 S how s the SEM images of a mixed SiNWs/CNTs film on a MCE membr a ne. .... 51 4 1 Solar cell model J V curves following Prince. ................................ ..................... 67 4 2 Device illustration. ................................ ................................ .............................. 68 4 3 Photograph of the sample in the measurement fixture ................................ ...... 69 4 4 J Si cell in the dar k and under AM1.5G .............. 69 4 5 Gating effects. ................................ ................................ ................................ .... 70 4 6 Equivalent circuit of Figure 4 5B. ................................ ................................ ........ 71 4 7 Dark and light current J V curves at the indicated gate voltages. ....................... 71 4 8 Trend of the V OC with gate voltage ................................ ................................ .... 72 5 1 Schematic of the nanotube grid/silicon device. ................................ ................... 86 5 2 J V curves for a continuous SWNT film covering the Si window and for an etched f ilm .. ................................ ................................ ................................ ........ 87 5 3 Shows J V curves of the grid SWNT film before and after addition of the electrolyte. ................................ ................................ ................................ .......... 88 5 4 Time measurements of short circuit current of the grid solar cell. ....................... 89 5 5 Experimental J V curves under illumination at the specified gate voltages. ....... 90 5 6 Simulation geometry and parameters of a cross sectional slice of the devic e .. 90 5 7 Simulation results at V g = 0.75, 0, +0.75 V and V bias = 0, 0.3 V. ........................ 91 5 8 Simulation results at V g = 0.75, 0, +0.75 V and V bias = 0.4 V. ........................... 91 5 9 T he reflectance and transmittance measurements as a function of wavelength of the NES solar cell. ................................ ................................ ....... 92 6 1 Shows the st ructure of pyrene HPC. ................................ ................................ 101 6 2 A flowchart describing the steps of making SWNTs / polymer dispersant inks. .. 102 6 3 AFM image of a drop cast film of pyrene HPC/SWNTs water based ink. ......... 102 6 4 Optical micrographs of the pyrene HPC/SWNTs inks spray cast onto glass .. 103

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12 6 5 AFM image of pyrene HPC/SWNTs film spra yed from a water/ethanol .......... 103 6 6 Optical Microscope image of a sprayed SWNT film after removal of p HPC f rom ethanol based ink. ................................ ................................ .................... 104 6 7 AFM image of SWNT film s prayed from pyrene HPC/SWNT ethanol based ink after removal of p HPC. ................................ ................................ .............. 104 6 8 UV vis spectra of sprayed pyr ene HPC/SWNT film after etching p HPC. ........ 105 6 9 Optical micrograph image of sprayed SW NTs film after removal of p HPC. ..... 105 6 10 P hotograph of SWNT film sprayed from the p HPC/SWNT ethanol based ink 106 6 11 UV vis spectrum of the SWNT sprayed film after etching pyrene HPC. ........... 106 6 12 Log plot of stability measurement of sheet resistance of a sprayed SWNTs film after removal of p HPC. ................................ ................................ ............. 107

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13 LIST OF ABBREVIATION S AFM Atomic force microscope AM Air mass BOE Buffered oxide etch CM C Critical micelle concentration CNT Carbon nanotube DCMC Double critical micelle concentration DI De ionized DOS Density of states E GaIn Gallium Indium eutectic EMI BTI 1 Ethyl 3 methylimidazolium bis(trifluoromethylsulfonyl)imide HF Hydrofluoric acid HP C H ydroxypropyl cellulose IL Ionic liquid MCE Mixed cellulose ester MS Metal semiconductor NES Nanotube electrolye/semiconductor PC Propylene carbonate PCE Power conversion efficiency PEDOT:PSS Poly(3,4 ethylene dioxythiophene): poly(styrene sulfonate) PTF E Polytetrafluoroethylene PV Photovoltaic SEM Scanning electron microscope Si Silicon SiNW Silicon nanowire

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14 STC Standard test conditions SWNT Single wall carbon nanotube

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15 Abstract of Dissertation Presented to the Graduate School of the University of Flori da in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy NANOSCALE MATERIALS IN NOVEL SOLAR CELL DESIGNS By Pooja Wadhwa May 2011 Chair: Andrew G. Rinzler Major: Physics The accessibility of nanoscale materia ls developed over the past decade provides new opportunities in solar cell design. The work described here in explores some of the possibilities. Three advances are reported along with some preliminary work in a related area: 1) Bulk heterojunction polymer solar cells, known since the mid 1990s, rely on naturally occurring phase segregation between the n type and p type charge transporters. Control over that phase segregation remains a major challenge. The advent of techniques to create silicon nanowires pr ovides an opportunity to engineer the bulk heterojunction without relying on phase segregation. This is demonstrated in a simple n type silicon nanowire/PEDOT:PSS polymer heterojunction solar cell. The bulk heterojunction nature of the cell imparts to thes e devices a power conversion efficiency (PCE) that is over four times greater than that of planar heterojunction devices made from these materials. 2) In a second solar cell design the intrinsic porosity of single wall carbon nanotube (SWNT) films is explo ited to permit electronic modulation of the junction characteristics in a SWNT/n Si Schottky junction solar cell. Electronic modulation occurs via electrolyte gating in which the porosity of the nanotube films permits the electrolyte direct access to the j unction. The ungated, native device has a PCE of 8.5% (under AM1.5G illumination). Modulation of the gate voltage (which

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16 consumes negligible power in the steady state) of 0.75 V yields a continuous, reversible modulation of the device PCE from ~4 11%. 3 ) In Schottky junction solar cells, the depletion layer responsible for charge separation is only of the order of a micron from the metal electrode. Accordingly, if large areas of the semiconductor surface are not in contact with the metal the current coll ection efficiency decreases. In the course of the studies described in 2 it was discovered that the free ions available in the electrolyte induce an inversion layer in the silicon that permits charge collection despite the nanotube film/Si junction occupyi ng only a fraction of the Si surface. By avoiding the parasitic absorption in the SWNTs over the electrolyte Si only regions, the gated PCE could be increased to 12% which exceeds that of dye sensitized solar cells. Finally preliminary results of an altern ative nanotube film preparation method is discussed

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17 CHAPTER 1 INTRODUCTION 1.1 Carbon Nanotubes A single wall carbon nanotube is geometrically a graphene sheet rolled into a seamless cylinder with diameter of about 0.7 10.0 nm. Most of the single wall carbon nanotubes (SWNT or CNT) have diameters < 2 nm. Du e to their large aspect ratio (i.e. length/diameter which can be as large as 10 4 ), the nanotube are quasi one dimensional. The structure of a SWNT can be specified or indexed by its circumferential periodicity and its chiral vector ( C h ) in terms of a pair of integers ( n, m ) 1 The unwrapped graphene sheet in F igure 1 1 shows a unit cell of a SWNT, where a 1 and a 2 are the real space unit vectors of the hexagonal lattice that is related to the chiral vector as given by Equation 1 1. C h = n a 1 + m a 2 n m ) (n m | n ) (1 1) It has been shown that the electronic properties of the carbon nanotubes are very sensitive to their structure. 2 4 Although graphene is a zero gap semiconductor, theory had predicted that carbon nanotubes can be either metals or semiconductors with different size energy gaps, depending very sensitively on the diameter and helicity of the tubes, i.e., on the indices ( n, m ). 1 This has since been confirmed through a host of experiments including scanning tunneling spectros copy 4 as well as optical absorption and emission spectroscopies. The physics behind this sensitivity of the electronic properties of carbon nanotubes to their structure can be understood within a zone folding picture. The density of states (DOS) of SWNTs c an be derived from the energy dispersion relationship of the nanotubes. Due to the quasi one dimensional characteristic of SWNTs, the DOS is

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18 found to be limited in its capacity and can be easily filled or emptied upon injecting or depleting electrons from it. As shown in Figure 1 2, (10 0) and ( 9, 0 ) nanotube density of states show that the ( 10 0) nanotu be has a band gap at the Fermi level indicating a semiconducting nanotube, while the ( 9 0 ) nanotubes are metallic with low density of states at the Ferm i level. The sharp features in the DOS of the nanotubes are called von Hove singularities. In this picture which ignores excitonic effects, the only allowed electronic transitions are between symmetric valence and conduction band von Hove singularities, re sulting in absorption bands in the optical spectrum of the SWNTs. The band gap for the semiconducting nanotubes is inversely proportional to the diameter of the SWNT. 5 For a n (n, m) SWNT, if 2n+m is a multiple of 3, then the nanotube is metallic, otherwise its semiconducting. While the density of states of the nanotubes is low compared to typical metals the ir charge mobility is very high (near ballistic) 6,7 making the m good conductors. But, because of their low density of states, t he Fermi level of the nan otube s can be easily tuned which has motivated their application in photovoltaics in this dissertation Chapter 2 presents the theory and working of a basic solar cell and Chapter 3 introduces the application of nanotubes in a hybrid solar cell with silic on nanowires. Chapter 3 presents two different models to maximize the junction area in a solar cell using carbon nanotubes. Chapter 4 presents a nanotube/silicon photovoltaic where the Fermi level of the nanotubes is electrically modulated by electronic ga ting. Chapter 5 depicts and discusses in detail the effect of the electrolyte gating on a nanotube grid/silicon solar cell. A new type of solar cell has been introduced and its performance is shown to be

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19 increased via electrolytic gating. Finally, Chapter 6 discusses an alternative method to prepare uniform thin films of nanotubes by spray coating. To comprehend this alternate approach to nanotube film fabrication and understand the nanotube films used in Chapters 3 to 5 it is useful to discuss the backgrou nd and procedure of the conventional nanotube film preparation scheme, which is discussed in Section 1.2. 1.2 Nanotube Films 1.2.1 Nanotube Suspension The nanotubes are not soluble in any known solvents so the nanotubes when placed in solvents, rather than being homogeneously dispersed, forms large inhomogeneous clumps which are held together by van d er Waals force s To form homogeneous nanotube suspensions, they are suspend ed in an aqueous solution with the aid of surfactants. 8 For example 1 % (by weight ) o f Triton X 100 surfactant solution stably suspend s the nanotubes for extended periods of time. 8, 9 The general surfactant is a molecule with a hydrophobic side and a hydrophilic side To suspend particles which are hydrophobic (like carbon nanotubes), the surfactants assemble on the particles with their hydrophobic side facing them while presenting their hydrophilic side to the aqueous phase. The hydrophilic side of the surfactants meanwhile generates a hydration shell that then prevents the suspended parti cles from coming into direct contact with each other and flocking out of suspension. Because the hydrophobic interaction of surfactants with the substances they are to suspend is generally weak, surfactant molecules are continuously desorbed from the surfa ces. To replace these molecules and prevent flocculation the free surfactant concentration should be high however surfactants that are placed into water self assemble into micellular structures, often spherical, that face their hydrophobic sides toward eac h other. It is found that only

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20 once the surfactant concentration in the aqueous solution is raised above a certain concentration (specific to each surfactant) that the free surfactant concentration is maintained at a level sufficient to suspend particles. critical micelle concentration 100, above which there are free surfactant molecules available to suspend the nanotubes The nanotube suspension in the surfactant solution is meta stable an d with time the nanotubes in the solution begins to flock. The nanotubes can be re dispersed in the surfactant solution by supplying external energy via ultrasonication, which breaks the aggregates apart. Upon ultrasonication, the nanotubes solution can be used to make uniform films by the filtration method which is discussed in Section 1.2.2. 1.2.2 Filtration Process A nanotube film is made by the filtration process 10 by collecting the nanotubes on a filtration membrane with pores that are too small in dia meter for the nanotubes to permeate through A mixed cellulose ester (MCE) membrane m i s used for filtration (as it can later be easily dissolved away by acetone during the transferring of the nanotube film to a substrate). Th e filtration process is self regulat ory and form s a uniform thickness film with fine contr ol over the film thickness by the nanotube concentration in the suspension and the volume of the suspension filtered The nanotube surfactant solution is filtered through the MCE membrane and the film so formed on the membrane is then washed with sufficien t de ionized (DI) water to rinse off the major fraction of the surfactant. The nanotube film so formed is dried on the filtration membrane and is ready to be transferred to a substrate for use. The nanotube transfer step generally proceeds as follows. The nanotube film (backed by the membrane) is cut to the desired size and shape, wetted with DI water and pressed against the substrate

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21 using modest pressure in a clamp assembly. The assembly is dried in an 80C oven for an hour leaving the nanotube film and b acking membrane adhered to the substrate. The membrane is then dissolved away starting with an acetone vapor bath followed by transfer to multiple fresh acetone liquid baths to ensure removal of the major fraction of the mixed cellulose ester membrane material.

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22 Figure 1 1. The unrolled honeycomb lattice of a nanotube. It can be constructed by connecting sites O and A, and sites B and C a nanotube can be constructed. The vectors OA and OB define the chiral vector C h of the nanotube and a 1 and a 2 are the unit vectors. The rectangle OAC B defines the unit cell for the nanotube

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23 Figure 1 2. DOS of (10, 0) and (9, 0) nanotube. The (10 0) nanotube displays a gap at the Fermi level, indicating semiconducting behavior. Whereas the (9, 0) shows finite DOS at the Fermi level indicating a metallic nanotube. Reprinted in part with permission from [ Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992 60 2204 2206 ] Copyright [ 1992 ] American Institute of Physics

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24 CHAPTER 2 SOLAR CELLS 2.1 Background Photovoltaic cells convert light energy into electricity. The photovoltaic effect was first discovered by nineteen year old Edmund Becquerel, a French experimental physicist in 1839 while he was experimenting with an el ectrolytic cell made up of two metal electrodes. For over a century, there were on going efforts to understand the photovoltaic effect with significant contributions made by several imminent scientists including Albert Einstein and Milikan. In 1954, the Be ll Labs researchers Pearson, Chapin, and Fuller reported their discovery of 4.5% efficient silicon solar cells; this was raised to 6% a few months later by a work team including Mort Prince. The first commercial solar cell was announced by Hoffman Electron ics's Semiconductor Division at 2% efficiency; priced at $25/cell and at 14 mW power each, making the cost of energy from such cells $1500/W. Figure 2 1 shows the evolution of efficiencies of different solar cell technologies (from National Renewable Energ y Lab). 11 2.2 Theory 2.2.1 Solar Energy The total mass of the sun is now about 2 10 30 kg, with a nearly constant radiative energy output driven by nuclear fusion, and a stable life projected of over 10 billion (10 10 ) years. The sun light reaching the surfa ce of earth is attenuated by various layers of the atmosphere principally due to water vapor absorption in the infrared, ozone absorption in the ultraviolet and scattering by airborne dust and aerosols. The degree to which the atmosphere affects the sunlig ht received at the surface of earth is defined as ) between the sun and

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25 the zenith and it measures the atmospheric path length relative to the minimum path length when the sun is directly overhead as shown in Figure 2 2. atmosphere and AM1.5 spectrum ( = 48.2 ) corresponds to the spectrum at the surface of the earth and is the standard by which different solar cells are compared. The global clear sky spectrum corresponds to AM1.5G with a light intensity of 100 mW/cm 2 12 Figure 2 3 shows the spectral irradiance (power per unit area per unit wavelength) as a function of wavelength at different air mass ratios. 2.2.2 Worki ng of a Solar Cell A prototypical homojunction solar cell consists of a semiconducting material doped to be p type on one side and n type on the other side of their junction with electrical contacts on each end. When the two materials are brought in contac t, charge transfer takes place which develops a built in potential at equilibrium Once light shines on the device, charge carriers (electron hole pairs, also called excitons) are created and dissociated by the internal electric field generated by the buil t in potential in the depletion region which are collected by the electrodes generating the photocurrent ( I L ) 13 ( Figure 2 4 ). When the external load is zero this photo illuminated current is called the short circui t current ( I SC ). The voltage generated when the external impedance is infinite is called the open circuit voltage ( V OC ); providing two of the figures of merit for the device. When a junction between two materials is formed, thermalization of electrons acro ss the junction results in a built in potential. This thermal equilibrium is represented on an energy level diagram by a line up of the Fermi levels across the junction, while

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26 the built in potential appears as a bending of the semiconductor valence and con duction bands across the depletion region (Figure 2 5A). Photons absorbed within a semiconductor generate excitons provided the photon is absorbed within the region of the built in potential near the junction; the electron and the hole are driven by the po tential in opposite directions to provide power from the cell. Given the open circuit potential, V OC and the short circuit current, I SC the power of an ideal solar cell can be defined as the product of these. The power extracted from real cells however i s found to depend on the load. This occurs because of the charge accumulation on the terminals of the cell, which in turn shifts the energy levels of the device. An experimental means for mapping out the load impedance dependent power from a cell is to hav e the load be a power supply that provides a forward bias to the cell terminals (the forward bias here refers to the forward current ( I F ) direction for a diode, opposite to the direction of the photo current), thus mimicking the effect of the load impedanc e. By mapping out the current from the illuminated cell as a function of the forward bias voltage, the I V curve for the device can be generated. Figure 2 5 shows the graphical depiction of a solar cell demonstrating the effect of the voltage collected acr oss the load impedance on the depletion region, electric field and band bending at the junction of the device. When a photon generates an exciton in the depletion region, a photocurrent is produced in the device by virtue of exciton dissociation due to ban d bending arising from the initial difference in the Fermi levels ( qV bi where V bi is the built in potential) of the two materials. As mentioned above, when no load resistance is connected to the device, the generated photocurrent is called the short circu it current (Figure 2 5A). This is also shown on the I V curve of a solar cell in Figure 2 6A. In

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27 Figure 2 5B, as a forward bias voltage ( V F ) is applied to the device (mimicking the effect of a load), the net band bending is reduced by qV F thus reducing the net output current. As the forward biased voltage is increased further (Figure 2 5C), the band bending is further reduced, until beyond a point (V OC ) it changes direction. At this point, the electric field due to V F exceeds the electric field due to photo generated carriers, flipping its direction and reversing the direction of the current (Figure 2 6C). The net current in the circuit of Figure 2 5 is given by, 14 I = I F I L = I S [exp( q V / kT ) 1] I L (2 1) where, I L results from the excitation of excess carriers by solar radiation and I S is the diode saturation current. The output power is given by, P = IV = I S V [exp( q V / kT ) 1] I L V (2 2) Figure 2 6 shows current voltage characteristics of a solar cell under illumination. The condition for maximum pow er ( P M ) can be obtained when dP / dV = 0 and is depicted by the shaded rectangle in Figure 2 6. 2.2.2.1 Effect of series resistance For a practical solar cell, Figure 2 4 is modified to include series resistance ( R S ) to account for the ohmic losses within t he device. 15 The equivalent circuit is shown in Figure 2 7. Thus the output current reduces to, I = I S {exp[ q ( V I R S )/ k T ] 1} I L (2 3) The I V characteristics of a solar cell for varying values of R S are shown in Figure 2 8. 15 This data clearly shows that the series resistance plays a key role in determining the output characteristics of a solar cell. It has been observed 15,16 that the series resistance of a given solar cell depends on numerous factors including the bulk resistance of the organic and inorganic

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28 materials, the contact resistance between the electrodes and the junction materials, the bulk resistances of the electrodes, the junction depth and the doping concentrations of the semiconductors. It is essential for all practical purposes to kee p each of these contributions to the series resistance as low as possible. 2.2.2.2 Performance characteristics of p hotovoltaic s The power conversion efficiency (PCE) of a solar cell is a primary performance metric defined as the fraction of incident solar energy converted into electricity and is given by the ratio of the output electrical power to the incident optical power, which is given by PCE = P M /P inc 100% = I M V M /P inc 100% (2 4) where P inc is the incident power, I M and V M are the current and voltag e values at maximum power. The maximum PCE achieved thus far is 30% using inorganic semiconductors 17 and that using polymer based organic semiconductors still remain significantly lower at 5.7% efficiency. 18 The ratio, I M V M /I SC V OC is called fill factor ( FF ) and is a measure of the realizable power from a solar cell or in other words, it defines the deviation from an ideal solar cell as a result of losses. Therefore, the PCE in terms of FF can be re written as PCE = FF I SC V OC /P inc 100% (2 5) 2.3 Instrume ntation Standard test conditions (STC) under which the I V curve measurements of a solar cell are made, specify a cell temperature of 25 C, an illumination intensity of 100 m W /cm 2 and an AM1.5G spectrum. 19 The setup used to simulate the STC is shown in Fi gure 2 9. It involves a 150 W xenon lamp (Oriel 6255) in an Oriel 6136 housing

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29 powered by a model 8500 power supply. Light approximating th e solar spectral distribution wa s obtained using an Oriel 81094 AM1.5G filter. Light from the inhomogeneous source wa s focused into the acceptance aperture of a 150 mm l ong, fused silica homogenizing r od (Edmund Optics P65 837) by a 50 mm diameter fused silica lens with a 65 mm focal length. The output face of the h omogeniz ing rod wa s imaged in the horizontal focal plane of the sample by a 50 mm diameter, 100 mm focal length fused silica lens after rotation by 90 degrees with a broad band mirror (Newport 66225). The intensity at the sample plane wa s adjusted to 100 mW/cm 2 by translation of the 65 mm focal length lens, cut ting down on the fraction of the light entering the h omogenizing r od. The homogeneity of the light intensity over the ~1 cm 2 central region of the homogen ized beam at the sample plane wa s measured to be within 5%. Measurements were performed using a Keithl ey 2400 source meter controlled by L abTracer 2.0 software. Gating was done using a potentiometer controlled voltage divided 9 V battery (to provide a highly stable source of the voltages less than 1 V).

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30 Figure 2 1. Thirty years evolution in conversion efficiencies of different photovoltaic technologies. Reprinted in part with permission from Don Gwinner. Data compiled by Dr. Lawrence Kazmerski, National Renewable Energy Laboratory, Golden, Colorado. www.nrel.gov/pv/thin_film/docs/kaz_best_re search_cells.ppt

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31 Figure 2 2. Shows the air mass ratio which gives a measure of sunlight reaching the surface of earth.

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32 Figure 2 3. Solar spectrum as a function of wavelength at different air mass ratios. Global solar rad including the diffused light scattered to the earth from the atmosphere. Reproduced in part with permission from Standard ASTM G173 03, Standard Tables for References Solar Spectra l Irradiance at Air Mass 1.5, Amer. Society for Testing M atls., West Conshocken PA, USA. http://rredc.nrel.gov/solar/spectra/am1.5/

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33 Figure 2 4 A p n junction solar cell with resistive load. Figure 2 5. Shows the effect of varying bias voltage on th e net current and band bending of a p n junction. Parts A ), B ) and C ) show no bias voltage (no load resistance), a small forward bias voltage (small load resistance) and a large forward bias voltage (large load resistance) respectively.

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34 Figure 2 6 I V characteristics of a solar cell under illumination. Figure 2 7 Equivalent solar cell circuit.

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35 Figure 2 8. I V characteristics of a solar cell depicting th e effect of series resistance. Reprinted with permission from [Prince, M. J. Appl. Phys 1955 26 534 540]. Copyright [1955], American Institute of Physics Figure 2 9. The optical set up used to simulate AM1. 5G solar spectrum with standard test conditions. The red dashed line shows the path of light.

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36 C HAPTER 3 SILICON NANOWIRES IN A HYBRID SOLAR CELL 3.1 Motivation The key operational component of numerous electronic and opt o electronic devices arises at the interface between dissimilar materials possessing distinct workfunctions. This so called junction is critical to the operation of diodes, photodetectors, photovoltaics and light emitting diodes. Common manufacturing techn iques for solar cell junctions that for example evaporate (or implant) one material between the two materials. This is often less than ideal. For example, in a photov oltaic of inorganic materials, the width of the built in potential responsible for separation of the photo generated electron hole pairs is of the order of 300 nm for highly doped s ilicon. Relatively few photons can be absorbed within such a thin layer mea ning that only a small fraction of the incident light is converted directly to electrical power. Photons not absorbed in that layer can still contribute to the power if the absorption occurs within a diffusion length of the junction but that, depending on the material system, can be short. One known approach to improving this circumstance is to roughen or texture the surface at which the junction is formed to increase the effectiv e surface area of the junction 20 24 however, even texturing by microlithograph y techniques can only go so far in terms of the increased effective surface area. The advent of nanoscale materials presents new opportunities for enormous increases in the effective junction area 25 30 which by increasing the net amount of incident light absorbed within or near the junction can in principle increase the net amount of power produced by the device. For example, attempts have been made to form a blend of two distinct types of nanoparticles (one

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37 acts as an electron donor and the other as elect ron acceptor) to form a mixed heteroju nction 25,31,32 which greatly increases the net interfacial area between the two materials. There has also be en work done in realizing bulk 30,33,34 heterojunction which forms a network of interpenetrated donor and accep tor materials. The se two approach es will be exploited in this chapter with some added advantages which will be apparent as the chapter progresses. Though, this work concentrates on the potential benefits of nanoscale materials to so lar cell technology, it can be note d that other devices are also likely to benefit from such efforts (e.g. high powe r diodes, which also rely on junctions ). 3.2 Challenge An issue in the use of nanocrystalline materials for solar cell applications is the series resistance arising from the transport of the photo generated carriers across multiple nano particle boundaries before those carriers can reach their corresponding electrodes. 35,36 Clear percolating paths across the entire film do not exist as shown in Figure 3 1, leading to trapping of charges and recombination 16 To overcome this problem nanowires were employed, which by their large aspect ratios minimize the number of such impedance generating boundaries, providing facile pathways for extraction of the charge. 3.3 Hybrid S olar C ell This section discusses a hybrid solar cell consisting of an organic and an inorganic semiconduc tor to form a diode. This cell wa s designed to maximize the interface junction and to provide a low impedance pathway for dissociated excitons to reach the electrodes, thus increasing the overall efficiency of the cell.

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38 3.3.1 Device Design and F abrication The approach wa s to form a heterojunction between vertically oriented inorganic semiconducting nanowires and organic polymer layer infiltrated between them. The schematic device design is shown in Figure 3 2. S ilicon nanowires (SiNWs) were etched to be vertically oriented to be 5 m in length on an 8 8 mm 2 n Si sample. These SiNWs were infiltrated with a polymer to form a junction. The chemical potential of the polymer, on equilibrating with th e Fermi level of the Si, induced a built in potential in Si, where exciton dissociation o ccu red Electrons propagate d down the n SiNWs while holes were transported throu gh the polymer to the electrode The polymer used wa s poly(3,4 ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) (Figure 3 3). It was selected for its high electric al conductivity (70 S/cm, as measured on thin spin cast films), reasonable transparency and good thermal stability. Its water solubility moreover permit ed convenient room temperature solution casting of uniform thin films. Indeed it was found that spin coa ting of the PEDOT:PSS provide d the desired polymer layer infiltration between the SiNWs. An ad ditional hole extraction layer wa s used on top of the polymer, which helped the holes to reach the anode. This layer wa s a conductive, transparent, sin gle walled carbon nanotube (CNT) film which wa s transfer red onto the polymer and aided the hole extraction. It has previously been used as a transparent electrode in various solar cells benefiting from low sheet resistance and high transmittance 37 The sheet resistan ce for a 45 nm thick CNT s film used in this device wa Aluminum (Al) and gold (Au) wer e used as the bottom electrodes (cathode) with work functions of 4.08 eV and 5.1

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39 e V respectively. Palladium (Pd) wa s used as the top electrode (anode) with a work f unction of 5.12 eV. The silicon nanowires were formed on a flat n Si substrate by an etching process described in Section 3.3.2. A protective layer wa s created on the back of the Si to prevent its chemical etching and avoid having SiNWs etch ed on the bac k surface of the device making it difficult to form electrical c ontacts. This protective layer was p arylene C, which is an electrical insulator and is a chemically inert compound with no organic and inorganic solvents at room temperature. It has water abso rption of less than 0.1% (af ter 24 hrs of soaking in water). 38 All these physical properties make parylene well suited for this application. A layer of a parylene C (poly para xylene) wa s formed by pyrolysis of an unreacted dimer charge, di pa ra xylene int o a monomer (p xylene) which wa s deposited on the back surface and on a small portion of the top surface of the Si wafer in a chemi cal vapor deposition system. 39 3.3.2 Fabrication of SiNWs Vertically oriented SiNWs we re synthesized by etching n type single crystalline Si < 100 > with a resistivity of 0.002 0.0 ric acid/ferric nitrate ( HF/Fe(NO 3 ) 3 ) at 50 C preceded by an electroless deposition of silver from silver nitrate ( AgNO 3 ), which deposited in a dendritic morphology as shown in Figure 3 4. 40 48 SiNWs with diameters 50 200 nm. The etching time can be controlled to vary the length of the Si dopant type or its crystallographic orientation The silver etche s down into the wafer leaving the nanowires behind. E xcess Ag ends up around the bottom of the SiNWs and must be removed. 49 This was done in an aqueous nitric acid solution containing sodium nitrite ( NaNO 2 ) at 85C Representative SiNWs generated by this

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40 m ethod are shown in F ig ures 3 5 through 3 7 Fig ures 3 6 and 3 7 show cross sectional scanning electron microscope ( SEM ) image of the SiNWs after silver etching and after polymer infiltration respectively 3.3.3 Results M easurements in this early work were conducted in the lab using a 150 W halogen light source which is by no means representative of the solar energy spectrum, nevertheless the relative values are meaningful Table 3 1 lists four types of devices comparing SiNWs/PEDOT:PSS to planar Si/PEDOT:P SS devices with two distinct contacts to the PEDOT:PSS, with and without CNTs. Four to five of each device type were fabricated and measured. The values shown in Table 3 1 are specific representative devices. Surprisingly, the devices that included CNTs ha d substantially smaller short circuit currents. The absorbance of a 45 nm thick SWNT film is much too small to account for the difference so the conclusion is that a barrier develops between the nanotubes and the PEDOT:PSS which reduces the currents. Compa rison of the SiNWs to planar devices shows that in one case (with the CNTs) the J SC is 2.7 times greater and in the other (without the CNTs) 4.4 times greater for the SiNWs based devices. The J V curve for a SiNW/PEDOT:PSS device is shown in Figure 3 8 und er standard test conditions of AM1.5G. The V oc and J sc obtained from the plot are 194 mV and 2.4 mA/cm 2 respectively with a PCE of 0.104% and a FF of 0.22. 3.3.4 Simulated E ffect of Series R esistance The J V curve of Figure 3 8 suggests a high series res istance. Accordingly, simple The input parameters used in the diode Equation 2 3 from Chapter 2 a re photocurrent I L

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41 = 8 mA, the diode saturation current I S = 0.9 A and series resistance R S (measured from Figure 3 8). The J SC and V OC obtained from this simulated curve are 2.4 mA/cm 2 and 200 mV respectively as shown in Figure 3 9 These values are in close agreement with the measured values. Keeping the inpu t parameters the same, and only changing R S (Figure 3 9 ), the J SC will increase to 7.95 mA/cm 2 the V OC to 211 mV, the FF and PCE to 0.53 and 0.9% respectively. Hence lowering the series resistance in these devices would result in a major improvemen t. 3.3.5 Discussion The work function of the heavily doped n Si under consideration is 4.1 eV while that of PEDOT:PSS polymer is frequently quoted as 5.1 eV. 50 Within the Schottky Mott model this should yield a built in potential of the difference and the V OC should be close to this value. A far lower V OC was observed. This is generally attributed to interface states at the semiconductor surface and is likely the case here. Experience has shown that passivation of the surface with a thin oxide layer (less t han 25 A to avoid creating a tunneling barrier) can reduce this surface state density thus greatly improving performance. In the present case, polymer is introduced into the devices immediately after etching the wires in HF. Future work could explore intr oduction of a time delay before polymer introduction to permit the formation of a thin native oxide layer. As shown by the simulation, the devices clearly have a large series resistance that should be reduced. Two sources of series resistance are likely. Ohmic contacts to n type Si are notoriously difficult to make without introducing n++ regions. One source of series resistance in these devices is thus the Al/Au back contact. More recent work (Chapter 4) has identified Gallium/Indium (GaIn) eutectic 51 as forming much better Ohmic contacts and can be tried. The other likely source of series resistance in the

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42 devices built was the long path length for holes through the PEDOT:PSS polymer to the metal contact along only one edge of the device. More closely sp aced metal finger electrodes can improve this. 3.4 Interpenetrating Heterojunction Solar Cell 3.4.1 Concept Some preliminary work to fabricate a mixed nanaotube/semiconducting nanowire heterojunction solar cell was also performed. Silicon nanowires (of n Si) and carbon nanotubes were mixed together to form a heterogeneous blend. This layer was to be deposited onto a pure n Si wafer to collect the electrons while a pure CNT transparent film was to be used as the transparent hole collection electrode. The mi xed layer consists bundles. By the fabrication method employed the nanowire s and nanotubes in this device we re preferentially oriented to generally lie along a plane (Figure 3 1 0 ) as oppo sed to the vertically oriented SiNWs in the first approach. A schematic of the device design is shown in Figure 3 11 Fabrication challenges did not get the work to the point of testing devices however it may be useful to others to know what was done. 3.4. 2 Fabrication Details The mixed nanotube/nanowire layer was formed by dispersing the nanotubes and/or nanowires in DI water aided by Triton X 100 surfactant and following Section 1.2.2 from Chapter 1 to make a uniform film having the SiNWs/CNTs lying paral lel to the membrane. The film was transferred onto a pure n Si substrate. Figures 3 12 and 3 13 show the SEM images of pure SiNWs film and of a mixed SiNWs/CNTs film on a mixed cellulose ester membrane, respectively. A second layer of pure CNTs was attempt ed to

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43 transfer on the mixed layer but with no success. The first transfer step of the mixed layer onto Si had a high success rate but the rigidity of SiNWs (present in the mixed layer) preven ted the successful transfer of the second layer. Future work can be done to overcome this problem by filtering pure CNTs, followed by a mixed layer on the same filter membrane and perform one transfer step on Si substrate to make the device. Longer and mor e flexible SiNWs can also be fabricated using c hemical vapor deposition or laser ablation to successfully fabricate such a device.

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44 Table 3 1. Gives the output parameters for different junctions under light. Device V OC (mV) J SC (mA/cm 2 ) SiNWs/PEDOT:PSS /CNT 182 0.625 P lanar Si/PEDOT:PSS/CNT 160 0.234 SiNWs/PEDOT:PSS 200 3.125 P lanar Si/PEDOT:PSS 200 0.703 Figure 3 1 Shows a heterogeneous blend of two organic materials which increase the net surface area in contact but leads to charge trapping Figure 3 2. Cross section of an organic ino rganic hybrid device

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45 Figure 3 3 Chemical structure of PEDOT:PSS Figure 3 4. Top view. Shows a SEM images of the as prepared SiNWs with Ag dendrites.

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46 A. B. Figure 3 5. SEM images of SiNWs after Ag etching A) shows the top view and B) show s the side view

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47 Figure 3 6. Figure 3 7. Cross sectional SEM image of infiltrated polymer between the nanowires

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48 Figure 3 8. J V curv e of a SiNWs and PEDOT:PSS junction solar cell without CNTs under light and dark. Figure 3 9. Show the s imulated J V plots under illumination for different values of series resistance

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49 Figure 3 10. A schematic diagram of a heterogeneous mixture of C NTs (blue) and SiNWs (black) forming an interpenetrating network. Figure 3 11. Schematic diagram of a solar cell with a mixed layer at the junction

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50 A. B. Figure 3 12. A) and B) show the SEM images of pure SiNWs on a mixed cellulose membrane.

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51 A. B. Figure 3 13. A) and B) show the SEM images of a mixed SiNWs/CNTs film on a mixed cellulose membrane.

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52 CHAPTER 4 ELECTROLYTE GATED NA NOTUBE/SILICON SOLAR CELL 4.1 Concept In the Schottky Mott model of metal semiconductor (M S) junctions the built in potential that develops at the interface between a metal and a semiconductor is a function of the difference in the work functions (or equivalently the difference in Fermi levels) of the two materials. In Schottky junction solar cells (and related photodetectors) the built in potential provides the electromotive force for charge separation that powers the device. 14 Carbon nanotubes can be made into electrically conducting, transparent films. 10 When positioned on an appropriate semiconductor the nanotubes can act as the metal to establish an M S junction solar cell. 52 However, carbon nanotubes, and related sp 2 bond ed carbons, provide a unique metallic system possessing a low density of electronic states. Unlike normal metals this allows their Fermi level to be readily shifted via chemical charge transfer doping or electrical gating. This provides new opportunities i n contact barrier and built in potential control. 4.2 Background Lonergan first demonstrated electronic contact barrier modulation in a poly (pyrrole)/n indium phosphide Schottky diode. 53 More recently electronic control over contact barriers was exploite d in a novel, carbon nanotube enabled, vertical, field effect transistor and a related light emitting transistor. 54 Active control of the Fermi level offsets and thus the built in potential in a nanotube/semiconductor junction solar cell should also be pos sible. Such control is demonstrated in this chapter. Prince first modeled the now widely used current voltage relation for a solar cell treated as a voltage biased, photocurrent generating diode (including series and shunt

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53 resistances) 15 as described in C hapter 2. For typical values, the shunt resistance is found to have a negligible effect on the current density voltage (J V) characteristic while the series resistance (R S ) in contrast has profound effects. Figure 4 1 shows the J V curves within this model (parameters adapted from Prince) 15 for no series resistance, and series resistances of 20 2 and 40 2 (R S = V/ J). Power from the cell is generated in the 4 th quadrant where the area of the largest rectangular box (P M = [J V] M ) that can be fit betwe en the curve and the axes gives the maximum power density the cell can generate. The relative areas of the boxes associated with the zero and 20 2 curves illustrate the deleterious effect of series resistance on solar cell performance. For a diode in t he dark (not shown, but essentially the curves of Figure 4 1 shifted vertically so that they take off exponentially from the J = 0 axis) the forward bias current becomes appreciable when the applied forward bias voltage counteracts the built in potential, so that on a band diagram (inset Figure 4 1) the bands on the semiconductor side are raised and flattened sufficiently to permit forward tunneling and thermionic currents. When the junction is exposed to light the J V curve additionally includes a counter propagating photocurrent (inset Figure 4 1). The voltage at which the net current is zero now corresponds to the applied bias voltage at which the forward diode current equals the photocurrent flowing in the opposite direction. Since the forward current re quires a flattening of the bands, while the electromotive force for photo carrier separation is provided by the bent bands, the voltage at which these currents are equal, i.e. the open circuit voltage, V OC, (approaching the flat band condition) provides a sensitive measure of the built in potential. The value of the V OC in illuminated J V curve

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54 measurements can thus characterize the degree to which the built in potential has been modified, something that has previously required a change of material partners which is demonstrated here in a single device. 4.3 Experimental Details 4.3.1 Device Architecture A schematic of the single wall carbon nanotube (SWNT)/Si photovoltaic (PV) cell is shown in Figure 4 2. The cell substrate used was a moderately doped, n Si wafer (phosphorous, 4 20 layer deposited on the oxide frames a 24 mm 2 window was etched through to the bare Si. A thin, transparent SWNT film (~45 nm thick) was transferred to the wafer lying across the window, in contact with the metal. The flexibility of the nanotubes lets the film conform into the area of the window contacting the bare Si to form the SWNT/Si junction. The gold provided electrical contact to the SWNTs and constituted t he positive terminal of the device when it was illuminated. Contact to the n Si side of the cell was made via a gallium indium eutectic (E GaIn) painted directly onto the n Si on the opposite side of the wafer underlying the junction in contact with a stai nless steel electrode. 4.3.2 Device Fabrication Wafers were phosphorous doped n Si, 4 20 m thermal oxide These were diced into 2.5 2.5 cm 2 subst rates. The two contact pads were thermally evaporated onto the oxide surface using a 5 nm Cr adhesion layer and 80 nm of Au. The junction film contact pad surrounded a 2 4 mm 2 window of exposed oxide. The oxide within the window wa s etched to the bare Si with buffered oxide etch (BOE)

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55 using the gold pad as the etch mask. Subsequently, t he substrate backside underlying the window was etched followed by a de ionized ( DI ) water rinse and an N 2 blow dry. Two 6 8 mm 2 45 nm thick SWNT films were transferred from mixed cellulose ester membranes as described in Section 1.2.2 of Chapter 1, one film draped across the window in the junction pad, forming the SWNT/Si junction, and the other across the gate electrode pad. Prior to making the backside c ontact the substrate backside wa s exposed to several drops of BOE for 2 minutes, removing native ox ide formed during the SWNT film transfer steps. After a DI rinse and N 2 blow dry GaIn eutectic wa s painted onto the backside (underlying the junction) in an approximately 1 cm 2 patch using a stainless steel blunt tip nee dle. A similarly sized region was pa thi ck stainless steel sheet that i s larger than the substra te. The substrate wa s placed onto the steel sheet overlapping the painted regions, forming the backside contact between the substrate and the steel sheet and taped in place. An indium dot contact on the stainless steel coupled to a silver wire completed the backside electrical contact. Indium dots were used to coupl e the gold electrodes on the fro nt side to silver wires providing the junction film and gate electrode electrical co ntacts. A final HF etching step of the junction was introduced with the SWNT film in place, immediately prior to device testing to remove the oxide layer that developed at the interface during the SWNT film transfer step (the etchant and subsequent aqueous rinse permeating through the porous SWNT film before drying the device). Figure 4 3 shows an assembled device in the test fixture for AM1.5 G measurements before addition of ionic liquid.

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56 4.4 Results 4.4 .1 Conventional S olar C ell To this point the construc (additional structures in Figure 4 2 are discussed below). Within the Schottky Mott model thermodynamic equilibration of the Fermi level offset between the SWNTs and the n Si transfers carriers (electrons) from the Si to the SWNTs, producing the built in potential and corresponding depletion layer in the Si, adjacent to the junction. When exposed to light, photons transmitted through the transparent SWNT film and absorbed within the underlying Si depletion layer generate electron hole pairs that are driven in opposite directions by the field associated with the built in potential, with holes extracted on the SWNT film side and electrons on the n Si side. Figure 4 4 shows the J V characteristic for such a de vice in the dark, and under AM 1.5 illumination (instrumentation details are provided in Chapter 2). From the illuminated curve, the conventional parameters were extracted that characterized the cell performance: an open circuit voltage (V OC ) of 0.52 V, a short circuit current density (J SC ) of 22. 1 mA /cm 2 a fill factor (FF) of 0.75, and a power conversion efficiency (PCE) of 8.5%. This performance was somewhat better than similar such devices made by Jia et al. (who used thin films of double walled carbon nanotubes rather than the SWNTs used here and reported a maximum 7.4% PCE). 52 4.4.2 Electrolyte Gated Solar Cell To attain control over the Fermi level offsets at the SWNT/Si junction, the natural porosity of the SWNT film was exploited and an electrolyte gate was used to apply a gate field at the junction. This gate was electrically addressed by a second gold contact deposited as a strip on the oxide near the SWNT/Si junction and a second identical

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57 SWNT film was transferred to contact this strip (Figure 4 2). The first (junction) and second (gate electrode) SWNT films were not in electrical contact except through an ionic liquid: 1 Ethyl 3 methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI BTI), that was drawn between, and saturated both SWNT films (F igure 4 2B). A potential applied between the junction and gate electrode films modifies the electronic population of the SWNTs at the junction. The electrolyte boosts the capacitance of the nanotubes, providing substantial change in their electronic popula tions for relatively small applied voltages. A SWNT film was used as the gate electrode for its large surface area, which avoids limiting the charge accumulation in the junction film by a limited series capacitance of the gating electrode. By keeping the a pplied gate potentials well below the redox potential of the ionic liquid, the gate drew effectively no current in the steady state (once charge reorganization was complete). Importantly, this means that under steady operation the applied gate potential ne ither consumes nor supplies power. As mentioned in Section 4.3.2, the junction was exposed to a final etching step, leaving the silicon surface hydrogen terminated which was stable against oxidation for tens of minutes to hours in ambient atmosphere 55,56 giving plenty of time for measurement before the ionic liquid was added (despite the porous nature of the nanotube film). Since atmospheric water was complicit in silicon oxide formation the EMI BTI ionic liquid, which excludes water by its hydrophobicity actually affords protection against oxide formation at the junction. Figure 4 5A shows the dramatic, reversible, effect of steady state gate potentials on the J V characteristics of the AM1.5G illuminated device. Figure 4 5B represents a SWNT at the ju nction and a gate SWNT on the oxide, both in cross section (directed out

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58 of the page) and their charge state in response to gate potentials (negative gate shown). From the J V curves of Figure 4 5A positive voltages applied to the gate electrode (versus th characteristics and introduce a kink feature near the V OC Negative voltages applied to the gate electrode enhance the PV cell characteristics. Figure 4 5C shows a zoom of the zero current cro ssing (the V OC ) for the curves. Negligible gate current (< |30 nA|) was drawn at the extreme gate voltages (|0.75 V|). The solar cell characteristics extracted from each curve are listed in Table 4 1. At the largest negative gate potential applied ( 0.75 V ) the PV cell achieves ~ 11% power conversion efficiency, nearly 30% higher than its original value of 8.5%. 4.4.3 Equivalent Circuit Figure 4 6 is the equivalent circuit for the device shown in Figure 4 5B. In the equivalent circuit the cell power suppl y potential is explicitly labeled V C The measured currents that can flow in the cell loop, I C and the gate loop I G are also shown. The ionic liquid electrolyte is shown as a leaky capacitor allowing for current flow through the parallel resistor. Because the maximum gate voltage was kept below the voltage where appreciable redox reactions occur this is a very large resistor (with I G ~ 30 nA at the greatest gate voltages values used V G = |0.75 V| this resistance is 25 M ). From the equivalent circuit, Figure 4 clear that I C and V C are independent of I G and V G It might be argued that equivalent circuit does not capture the physical situation because the electrolyte also cont acts the bare silicon adjacent to the nanotubes. The argument against this is that what is relevant is which side of the depletion layer is contacted (the same side as the

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59 nanotubes). Nevertheless, to consider all possible objections even if the connection of the electrolyte were to the other side of the diode or indeed to both sides, this circumstance that the power dissipated in the gate loop cannot influence the power measured in the cell loop remains. Moreover, the power dissipated in the gate loop is a mere: I G V G = (30 nA) (0.75 V) = 22.5 nW. If this were somehow incorrect, appreciable dark currents would be expected when the cell is gated. Figure 4 7 shows the dark and light current J V curves for the device at gate voltages of zero volts and the ex treme gate voltages 0.75 V. As is clearly seen there is no dark current with gate voltage. 4.5 Discussion This gate voltage induced behavior can be explained on the basis of four mechanisms that are inferred to be relevant in this system. These are the ga te voltage induced modulation of 1) the built in potential 2) the nanotube film resistivity 3) an interface dipole at the SWNT/Si junction and 4) an electric field induced across the depletion layer in the n Si. Further work and detailed modeling will be needed to parse the quantitative contributions of these mechanisms to the gated J V curves of Figure 4 5A. Here the evidence is framed that these processes are necessary and sufficient to explain the data and observations. 4.5.1 Effect on Built in Potential The ch ange in the V OC (0.22 V over the gate voltage range of 0.75 V as seen in Figure 4 5C) is consistent with a change in the built in potential (V bi ) of the device as

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60 indicated qualitatively in the inset of Figure 4 5A. There, negative gate voltages withdraw electrons from the nanotubes at the junction, shifting their Fermi level further from the vacuum level relative to the n Si. The Fermi level equilibration in that case results in the larger V bi reflected in the correspondingly larger V OC Positive gate vo ltages have the opposite effect. 4.5.2 Nanotube Film Resistivity A simple shift of V bi due to a shift of the nanotube Fermi level should, however, manifest itself as a mere horizontal translation in the sort of curves displayed in Figure 4 1. There is cle arly more occurring in the data of Figure 4 5A than a simple such shift can explain. To make further sense of these results, first consider the region of high current, forward bias slopes in the first quadrant. These exhibit a decreasing slope as the gate voltage increases from 0.75 V to +0.75 V. This slope is inversely related to the series resistance indicating an increasing series resistance in the device with increasing gate voltage (compare with Figure 4 1 curves, bearing in mind that V OC simultaneous ly shifts in this device). Electrolyte gate induced changes of SWNT film resistivity are well known to occur, even in films far above the percolation threshold. 10 These changes occur as the Fermi levels of the semiconducting nanotubes in the bulk film mix ture of typically ~1/3 metallic and ~2/3 semiconducting nanotubes are pushed by the gate potential into (or out of) their band gap, effectively switching off (or on) the conductance in ~2/3 of the film constituents. Changes in the SWNT film resistivity and the series curves with changing gate voltage. Evaluation of the series resistance from the most steeply sloped, linear regions of the extreme gate voltage curves yie lds values of ~0.67 2 and ~1.67 2 at 0.75 V and +0.75 V gate voltage s respectively.

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61 4.5.3 Energy Gap Feature Neither a shift of V bi nor a modified series resistance however provides an explanation of the energy gap (kink) feature near V OC and its increasing prominence with increasing gate voltage. The shape of this feature suggests an energy barrier. Similarly kinked J V curves have sometimes been reported in organic solar cells where the observation is attributed to a transport barrier at one of the metal contacts. 57 In the device introduced in this chapter, electrical connection to the junction SWNT film is via a gold contact which is also exposed to the gating electrolyte. A separate experiment to test the possibility that the gated gold/SWNT co ntact was responsible for the barrier, observed only ohmic behavior independent of the gate voltage, ruling this out as the source of the kink feature. The possibility of a reversible, gate induced, electro oxidation of the Si surface (via oxygen dissolve d in the air exposed electrolyte) suggested itself, but the reproducibility of the curves in Figure 4 5A at intermediate gate voltages rules this out. That is, once the gate voltage exceeds the oxidation potential, electrochemical oxidation should proceed in one direction only (with only the rate depending on voltage). The well defined kink at +0.45 V implies being above the oxidation potential. To see the feature grow on going to +0.75 V and then recover the +0.45 V curve on going back (as is observed) is inconsistent with electro oxidation. 4.5.3.1 Schottky Mott model In discussing the gate effect on the Schottky junction, the zero th order model was assumed so far, where the magnitude of V bi depends only on the Fermi level offset between the metal and the semiconductor. Metal semiconductor Schottky junctions however rarely obey this Schottky Mott rule. The rational in favoring its validity in this

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62 case was based on two properties of the nanotubes: 1) their lack of chemical reactivity implies no chemical bon ds across the junction thus avoiding chemical bond induced interface states and 2) the compact nature of the carbon p z orbitals forming the nanotube bands and the large van der Waals bond distance (relative to chemical bonds) across the junction makes it improbable that the nanotubes would cause metal induced gap states within the Si bandgap. Without these principle mechanisms for generating in gap s tates it seemed plausible that the Fermi level would be unpinned and thus follow the Schottky Mott rule. The inability to explain the origin of the gate modulated kink feature along with other features of the data however compel a reconsideration of this a ssumption. 4.5.3.2 Bardeen model had no need of extrinsically induced surface states. 58 The simple termination of a bulk semiconductor at its surface already leads to surfa ce states. Treated as a continuum, these states have their own energy dependent density of states and the energy distribution of that density depends on the particular crystal face involved, surface atomic reconstruction, defects and impurities. Because th e bulk semiconductor must be in thermodynamic equilibrium with its surface the equilibrium spatial distribution of charge between the surface and the bulk can itself lead to an essentially intrinsic band bending and associated depletion layer. If the surfa ce states have a band of high density around the highest occupied surface state (in thermodynamic equilibrium with its bulk) then thermodynamic equilibration with the Fermi level of the contacting metal occurs via electrons exchanged with this high density band of surface states. In that

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63 case there is relatively little change in the band bending upon making the metal semiconductor contact and the Schottky barrier is independent of the workfunction of the metal used, effectively pinning the barrier. The Scho ttky Mott and the Bardeen models comprise the extreme limits of the behaviors typically observed wherein the Schottky barrier height does show dependence on the metal workfunction, with a generally weaker (and often much weaker) dependence than predicted b y the Schottky Mott rule. 4.5.3.3 Modern Schottky model To allow for the degree of dependence on the metal workfunction (incomplete pinning) generally seen, more modern Schottky barrier models incorporate the idea of an additional interface dipole between the metal and the semiconductor. In recent years this dipole has been associated with bond polarization across the chemical bonds between the metal and the semiconductor, 59 however earlier models recognized the existence of an interface dipole, independen t of chemical bonds (more relevant to the case of carbon nanotubes), as the charge transferred by energy equilibration between the semiconductor surface states on the one side and the corresponding image charge in the metal on the other. 60 Importantly, bec ause this interface dipole is assumed thin, it contributes as a tunneling barrier whose effect is folded into the Schottky barrier height. Work in recent years has indeed evidenced systematic changes in the Schottky barrier height in response to the dipole moments and orientations of polar molecules grafted to the semiconductor surface (or the metal) before forming the contact with the metal. 61 63 The modern view of Schottky contacts thus allows for modulation of the built in potential mediated by charge ex change with surface states, combined with an interface dipole that contributes to the Schottky barrier height.

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64 Within this context, the kink feature in Figure 4 5A can be explained. Concomitant with the gate induced modulation of the nanotube Fermi level t here is a gate modulated enhancement or suppression of the interface dipole at the nanotube Si junction which, in turn, feeds back to the band bending and V bi in the n Si. Because the electrolyte gate has direct access to the n Si surface immediately adjac ent to the nanotubes at the junction (through the nanotube film porosity) its effect on the interface dipole can be dramatic. In the case of more positive gate bias the enhanced dipole contributes to the Schottky barrier height manifesting itself as a redu ced forward current in the first quadrant of Figure 4 5A. In the fourth quadrant the additional tunneling barrier due to this enhanced interface dipole increases recombination losses, manifesting itself as the reduced current kink feature. Further contribu ting to this barrier is an electric field induced across the depletion layer due to the positive charges accumulated in the ionic liquid at the Si surface, in the regions adjacent to the nanotubes (generating an electric field in a direction opposing the f ield associated with the built in potential). Going toward negative gate bias reverses these trends. 4.5.4 V OC Saturation A further feature stands out in the data. The changes in the V OC saturate in this sample at 0.55 V with negative gate voltage and at most at 0.58 V in the numerous other devices tested as shown in Figure 4 8. Within the described picture this finds explanation as a region of high silicon surface state density that once reached prevents further change in V bi Reported measurements of th e surface state density as a function of energy for partly oxidized hydrogen terminated Si indeed show a steep rise in the density on approach of the valence band from the mid gap. 64 As mentioned previously,

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65 detailed modeling is needed to quantify the rela tive contributions of these effects to the behaviors seen, which will be addressed to in Chapter 5. 4.6 Concluding Remarks Finally, returning to the point made earlier that the steady state gate voltage draws no power. This means that the ~11% power conve rsion efficiency obtained (with room for further optimization in e.g. the nanotube film thickness) can be realized with negligible loss of power to drive the gate. Indeed, two relatively small ungated solar cells wired in series (to get the 0.75 V gate vo ltage) can be used to drive the gate of a much larger device, giving up in the power conversion efficiency only the fractional surface area dedicated to the small gating cells. Given the 30 nA current drawn by the gate for our 8 mm 2 area cell, two 1 cm 2 un gated cells could power the gate of over 6 m 2 of solar cell. Since the gating electrode need not be illuminated it need not occupy cell surface real estate and while it requires a large capacitance, this can be achieved compactly using activated carbon or a pseudo capacitive electrode of a type common in supercapacitors. Commercial devices would likely require sealing and while the fluid state of the ionic liquid electrolyte is convenient in research, sealing may be complicated by a fluid layer. Replacement with a solid state electrolyte could be useful in that case. the Si surface is left chemically unperturbed. If modification of the Si surface can change its surface state density to unpin the V OC from the observed saturation value, the resulting gate induced increase in V OC should allow for substantial further performance enhancement.

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66 4.7 Future Work Further experiments (e.g. gate modulated capacitance voltage measu rements) should be useful in gaining a more detailed understanding. Particularly interesting would be the results of similar electrolyte gated devices constructed with high quality single layer graphene as the transparent junction electrode. 65 66 Indeed gr aphene/n Si Schottky junction devices have recently been reported. 67 Since a continuous graphene layer can avoid direct contact of the electrolyte with the junction, and the graphene layer should screen the gate field from a direct influence on the interfa ce dipole, this could separate out its direct effect on the dipole from the gate induced modification of the (graphene) Fermi level. The electronically gated device platform reported here provides a model system that may provide new insights into some long standing questions in the physics of Schottky junctions.

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67 Table 4 1 Solar cell characteristics extracted from the gated J V curves of Figure 4 5A. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society Gate Bias (V) 0.75 0.45 0.15 0.0 + 0.15 + 0.45 + 0.75 V OC (V) 0.55 0.54 0.51 0.49 0.47 0.41 0.33 J SC (mA/cm 2 ) 25.0 25.3 25.2 25.0 25.0 24.9 24.8 FF 0.79 0.77 0.71 0.68 0.62 0.54 0.44 PCE(%) 10.9 10.5 9.2 8.4 7.4 5.5 3.6 Figure 4 1 Solar cell model J V curves Inset: Band bending at a M S junction and the flattening of the be nt bands with increasing forward bias (V F ). Reproduced i n part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society

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68 Figure 4 2 Device i llustration. A) 3 D view, hash lines are merely aids for 3 D visu alization (i onic liquid not shown ). B ) Cross sectional view, ionic liquid (IL) shown. The larger gold contact on the SiO 2 surface includes a 2 4 mm 2 rectangular window in which the oxide was etched to the bare n Si surface. A SWNT film contacts both the gold electrode and the n Si within the window forming the SWNT/n Si junction. E GaIn provides the back side contact between the n Si and a stainless steel sheet (not shown). The large gold con tact and the steel sheet comprise the two terminals of the solar cell. The second smaller gold contact and its associated SWNT film lie entirely on the oxide. These provide the gate electrode for the IL electrolyte which lies as a puddle across both SWNT films. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society.

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69 Figure 4 3. Photograph of the sample in the measurement fixture recorded from above (the sample sits horizon tally). IL is introduced within the region of the dotted oval line and once added, it saturates both SWNT films and bridges betwee n them Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society. Figure 4 4 J V plots : Si cell in the dark and under AM1.5 G illumination. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society.

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70 Figure 4 5 Gating effe cts. A ) J V plots of the illuminated SWNT /n Si cell under the indicated gate voltage a pplied to the gate electrode. B ) Charge state of the junction SWNTs with negative voltage applied to the gate electrode. C ) Zoom of the zero current crossings (V OC ) regio n in A) Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society.

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71 Figure 4 6. Equiv alent circuit of Figure 4 5B. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society. Figure 4 7. Dark and light current J V curves a t the indicated gate voltages. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society.

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72 Figure 4 8. Trend of the V OC with gate voltage exhibiting the saturation of the V OC as the gate is made increasingly negative. Points are data. Reproduced in part with permission from [ Wadhwa, P.; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G. Nano Lett. 2010 10 5001 5005 ] Copyright [ 2010 ] American Chemical Society.

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73 CHAPTER 5 ELECTROLYTE INDUCED INVERSION LAYER SCHO TTKY JUNCTION SOLAR CELL 5.1 Background Chapter 4 described the electronic modulation of the fundamental characteristics (open circuit voltage, fill factor, power conversion efficiency) of a nanotube/n Si Schottky junction solar cell. 68 The native device exhibited PCE of 8.5%. By exploiting an electrolyte gate, having access to the junction through the thin, porous SWNT film, a continuous, reversible modulation of the PCE from ~4% to nearly 11% was demonstr ated (the gate circuit drawing negligible power in the steady state). The presence of an electrolyte at the junction, presents the opportunity for a new type of solar cell that is described in this chapter. 5.2 Device Design 5.2.1 Device Architecture The d evice construction that demonstrates this new cell has much in common with the gated nanotube/n Si device discussed in Chapter 4, except that rather than use a continuous nanotube film to form the junction the film was etched in a grid pattern to cover onl y a fraction of the n Si surface. Figure 5 1A shows the schematic of such a device. A gold electrode with a 2 4 mm 2 rectangular window was evaporated onto a 1 m thick oxide layer on an n Si wafer. The window in the gold electrode is used as an etch mask t o etch the oxide within the window down to the bare n Si. A 45 nm thick, 6 8 mm 2 rectangular area SWNT film was deposited across the window contacting the gold electrode and forming the junction with the exposed n Si. As was done in Chapter 4, a second gol d electrode and second SWNT film that is to act as the gate electrode once the electrolyte was added, were deposited on the oxide layer near the junction. Figure

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74 5 1B shows a photograph of the grid pattern in the junction SWNT film formed by photolithograp hy and etching in an oxygen barrel asher. The grid lines shown are 100 m wide with a 300 m spacing between the vertical lines. 5.2.2 Device Fabrication To define the SWNT grid lines in the junction film photolithography was used. The substrate was spin coated with Shipley S1813 photoresist and a patterned chrome Si window. On developing the regions exposed the SWNT film underneath. Antech barrel plasma asher was used to t was completely stripped off the substrate using propylene glycol monomethyl ether acetate, acetone and methanol. This finally formed the required SWNTs in contact with n Procedure similar to that described in Chapter 4 was followed to make the back contact with GaIn eutectic stainless steel sheet On making the electrical contacts using indium dots and silver wires, a final BOE etching step of the grid/Si junction was introduc ed and the device was ready for measurement. 5.3 Results 5.3.1 Conventional Grid Cell Figure 5 2 shows the cell current density versus voltage (J V) characteristic in the dark and under illumination (AM1.5G, 100 mW/cm 2 ) for two distinct devices in the abs ence of electrolyte One device with a continuous SWNT film across the entire

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75 window and the other with the nanotube film etched into a grid pattern limiting the Si coverage with the nanotubes to 27% of the window area. The nanotube film/n Si contact forms semiconductor Schottky junction solar cell. Fermi level equilibration of the n Si with the nanotubes, transfers electrons from the n Si to the SWNTs generating a depletion layer and band bending in the Si. Photons absorbed in the Si generate free electrons and holes that upon diffusing to the depletion layer are separated by the built in potential in the vicinity of the nanotubes, enabling power generation from the device. For doping density of ~10 15 donors/cm 3 this depletion layer e xtends ~1 m into the Si from the contact with the nanotubes. Given the relatively small extent of this depletion layer the reduced junction area of the grid device yielded a reduced short circuit photocurrent. The reduction in the photocurrent did not scale in dire ct proportion to the reduced junction area because high quality single crystal silicon has long diffusion lengths, allowing photocarriers generated far from the junction to diffuse there and contribute to the photocurrent. Nevertheless, the photocurrent in the grid film is reduced by more than a factor of two over that of the continuous film, yielding a corresponding decrease in the full window area normalized power conversion efficiency. 5.3.2 Electrolyte Gated Grid Cell The situation became substantially more interesting on the simple addition of the EMI BTI ionic liquid electrolyte. Figure 5 3 compares the illuminated J V curves of the grid SWNT film before and after addition of the electrolyte for the gate electrically floating and with 0.75 V applied t o the gate electrode. The simple addition of the electrolyte, gate floating or not, more than recovers the short circuit photocurrent lost due to the reduced areal coverage of the n Si by the nanotubes. To explain this

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76 behavior, it is concluded that the el ectrolyte induces its own depletion (or inversion) layer in the Si across the large gaps between the nanotube grid lines. 5.4 Inversion Layer Electrolyte induced depletion layers are well known from photoelectrochemistry studies of electrolyte semiconduc tor interfaces. 69 By incorporating a counter electrode and a suitable regenerative redox couple in the supporting electrolyte such junctions form the basis of the liquid junction, regenerative solar cell, the best known example of which is the Gratzel cell 70 In such cells the counter electrode forms one terminal of the cell and the redox couple serves as a shuttle necessary to ferry charge between the semiconductor surface and the counter electrode, effectively completing the internal circuit of the cell. The SWNT grid electrolyte/n Si cell must be distinguished from photoelectrochemical cells in that there is no redox couple and the EMI BTI electrolyte used here has a very broad electrochemical window ranging from 2.6 V to +2.0 V (vs. Fc/Fc+ or 5.1 V rel ative to the vacuum level). 71 Hence this electrolyte does not participate in the charge transport. Instead photogenerated holes that make it to the electrolyte induced inversion layer in the Si are trapped by the electric field within the layer and are dif fused along it until they encounter a nanotube grid line where they are collected. Because the electric field, which accumulates holes at the surface also repels electrons, deleterious surface recombination is largely avoided. Consideration of how the depl etion layer develops in photoelectrochemical cells further demonstrates that the inversion layer appearing in the nanotube electrolyte/semiconductor (NES) solar cells must be of distinct origin. In the electrolyte of a photoelectrochemical cell the electro chemical (Nernst) potential of the incorporated

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77 redox couple sets the equilibrium distribution of the couple between its reduced and oxidized states. When the electrolyte comes into contact with the semiconductor the two exchange charge, simultaneously shi fting the electrochemical potential of the redox couple and the Fermi level of the semiconductor until they are in equilibrium (thus establishing the depletion layer in the semiconductor). But in NES devices, the large electrochemical window over which the EMI BTI electrolyte undergoes no redox precludes such charge exchange. Accordingly, the cause of the inversion layer required by the NES J V curves is different from that of photoelectrochemical cells. etal insulator semiconductor (MIS) cells first described by Godfrey and Green in the late 1970s. 72 In those devices narrow metal lines (Al or Mg) on the front surface of p type silicon collected electrons trapped by an inversion layer formed at the p Si s urface in the regions between the widely spaced metal lines. The inversion layer in those devices was induced by positive charge trapped in an SiO layer grown on the Si. In the present case the gate voltage is certainly capable of inducing charge (of eith er sign depending on the polarity of the gate) adjacent to the surface of the n Si but interestingly the high short circuit current (V bias = 0 V) seen in the grid film of Figure 5 3 occurs immediately on introduction of the ionic liquid. This implies that negative ions accumulate at the n Si surface upon simple introduction of the electrolyte. An experiment was conducted to explore whether the depletion layer in the n Si induced by the ions in the electrolyte between nanotube grid lines existed prior to th e light exposure or if it was created by ion migration in the electrolyte initiated by light exposure. The short circuit current generated by the cell was measured as a function of

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78 time immediately after exposing the device to the light (AM1.5G) using a fa st electromechanical shutter. If the depletion layer did not exist prior to the light exposure, the current should have built up relatively slowly as ions migrate to the surface inducing the depletion layer over time. Realizing that the high ionic conducti vity of the ionic liquid EMI BTI, which is 8 mS/cm may reorganize too quickly for the electronics to see the current rise (the fastest sampling rate of Keithley 2400 is ~2.4 ms, also about the speed of the electromechanical shutter used), a different elect rolyte system was used: lithium triflate (LiClO 4 ) in propylene carbonate (PC). In this electrolyte system, the ionic concentration could be reduced, thereby increasing the timescale for the build up of a depletion layer if it was somehow photo initiated Figure 5.4 shows the effect of the different ionic conductivities of: 0 (no electrolyte), 2.2 S/cm; 29 S/cm (different concentrations of LiClO 4 in PC) and 8000 S/cm (EMI BTI) on the short circuit current developed in the cell as a function of time (for multiple blocking and unblocking cycles of the light by the shutter). For the non electrolyte curve the short circuit current was limited by the depletion layer existing only in the immediate vicinity of the nanotube grids covering 25% of the Si window. Si nce the grid pattern limited this to only about a quarter of the n Si surface, the short circuit current density was only ~ 6 mA/cm 2 With the lowest ionic conductivity electrolyte ( 2.2 S/cm), the short circuit current density was already substantially greater than without the electrolyte showing that, as expected, relatively little electrolyte conductivity was required to begin to establish a depletion layer in the n Si. The rapid incre ase of the current beyond the 6 mA/cm 2 indicated that the major fraction of the depletion layer pre existed the light exposure on the electrolyte. The small time dependence seen in the rounded corners of the curves near the opening

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79 of the shutter (unblocki ng of light) indicated that there was some further enhancement of the depletion layer likely due to image charges in the electrolyte migrating to the interface in response to the photo generated hole build up, but this is a small effect that disappears as the electrolyte conductivity is increased. Increasing the electrolyte ionic concentration increases the current density, which in turn indicates that the depletion layer due to the electrolyte increases with electrolyte ionic concentration. Since however the still quite low ionic concentration of the 29 S/cm LiClO 4 in PC shows a saturation current density that is already 75% of that of the 8000 S/cm ionic liquid this dependence on the electrolyte conductivity clearly saturates at well below the highest conductivity. The p re existence of the electrolyte induced inversion layer for conducting electrolytes wa s also supported by electrostatic simulations discussed in Section 5.5 5.5 Electrostatic Simulations To determine if the charge separation could be explained by the native electrostatics, the system w as modeled using the solar simulation design package: Synopsys TCAD Sentaurus 73 This work was done in collaboration with Professor Jing Guo (UF Dept. of Electrical and Computer Engineering) and the simulations were performed in his group by Jason Seol. Th e electrolyte was simulated using a dielectric coating with a very large dielectric constant ( = 5000), i.e. the mobile free ions of the extreme polarizablity (the free charge of electrolytes precludes definition of a real, DC dielectric constant for them so that the AC dielectric constants available in the literature were not relevant). The value of = 5000 comes from the ratio of the characteristic dielectric layer dim ension (~100 m) relative to that of the characteristic Debye layer dimension in the electrolyte (< 20 nm). The simulation confirmed the formation of an

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80 inversion layer generating an electric field upon addition of the electrolyte. Figure 5 5 shows the exp erimental J V curves for the grid device where the current density remained normalized to the full window area. Figure 5 6 shows the simulation parameters and geometry (not to scale) for a cross sectional slice through a SWNT grid line of 100 m width, hav ing its long axis perpendicular to the page. The SWNT line was treated as a simple metal of constant work function CNT = 4 .9 eV (consistent with that of nitric acid purified nanotubes). The gate electrode was a gold line ( Au = 5.1 eV) situated on a 1 m thick SiO 2 dielectric ( = 3.9). Below the SWNT grid line lied the junction with the n Si ( Si = 4 .3 eV for the 1 10 15 cm 3 phosphorous doping density), with = 5000 ) that coated the entire structure. The Neumann boundary conditions used placed mirror planes at the left and right sides of the figure making the gold gate electrode line (including its reflection on the left side) 100 m wide (equal area to the SWNT grid line) and the spacing to the next SWNT grid line (including the reflection on the right side) 300 m. Figures 5 7A, B, C show the electric field developed in the depletion layer below the SWNT/n Si junction and in the adjacent n Si at a bias voltage V bias = 0 V for gate voltages: V g = 0.75, 0, +0.75 V (left column 5 7A, C, E, respectively). Figures 5 7B, D, F show this at a forward bias voltage V bias = 0.3 V for the same gate voltages: V g = 0.75, 0, +0.75 V (right column 5 7B, D, F, respectively). The sim ulation plots for the reverse bias case V b ias = 0.4 V at gate voltages V g = 0.75, 0, +0.75 V are shown in Figure 5 8. This simulation is concerned only with the inversion layer generated by the electrolyte. The other gate field dependent features of the J V curves discussed in

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81 Chapter 4 were not modeled here. Thus, the model did not for example include the gate induced shift in the SWNT Fermi level (nor the resistivity changes in the SWNT film, irrelevant to the electrostatics). Nevertheless, the model cap tured the existence of an inversion layer extending well beyond the direct depletion layer in the vicinity of the SWNT/n Si contact as required to explain the increased saturation currents upon addition of the electrolyte. This behavior can be understood as follows. When the nanotubes and the n Si are first placed in intimate contact the free energy of electrons in the n Si (work function: Si = 4 .3 eV) is reduced by their transfer to the carbon nanotubes (work function: CNT = 4 .9 eV). Such transfer stops when the Coulombic restoring forces due to the charge imbalance raise the local potential (the built in potential) to prevent further charge exchange, establishing equilibrium. In the presence of electrolyte ions, having freedom to migrate, the ions compe nsate the transferred charge to permit the exchange of substantially more charge before equilibrium is reached. Additional electrons are transferred to the nanotubes from the n Si regions between the nanotube grid lines compensated by positive electrolyte ions surrounding the nanotubes, while the holes left behind in the n Si inversion layer are compensated by negative electrolyte ions accumulated at the Si surface. The electrolyte here serves much as it does in an electrolytic capacitor to raise the capaci tance of the system with a self potential provided internally by the original Fermi level offset between the nanotubes and the n Si, or externally by the gate field.

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82 5.6 Quantitative Analysis Photons in transit to the Si surface that are absorbed in the S WNT film do not contribute to the power generation. This was confirmed by looking for photocurrent with a filter in the light path that only transmits light energies below the silicon bandgap (looking for photocurrent from absorption in the semiconducting nanotubes, having a bandgap of ~0.6 eV). In fact light absorbed in the nanotube films is not transmitted to the n Si thus degrading the PCE. Thinner nanotube films would transmit more power to the silicon, enhancing the PCE, but thinning the nanotube films increases their resistance and cell series resistance degrades the fill factor and thus the PCE. The ability to use a liquid junction and reduce the area of the Si that must be covered by the nanotube film suggested that a grid pattern of optimized spacin g could minimize the overall absorptive losses while minimally increasing the series resistance, yielding a net gain in the PCE. This turned out to be the case. The gated device discussed in Chapter 4 using a continuous SWNT film and 0.75 V applied to the gate achieved a best PCE of 10.9%. At this same gate voltage the grid film having 100 m wide SWNT lines with 300 m spaces between them has a PCE of 12%, an increase of 10% over the continuous film (Figure 5 3). The increased photon flux arriving at the electrolyte/n Si junction of the gridded cell should manifest itself as a larger short circuit current density, as was observed. The short circuit current density for the grid film was while that for the continuous film was The small increase in the J SC of the full film device before and after the addition of the electrolyte (22 mA/cm 2 to 25 mA/cm 2 ) was likely due a refractive index matching effect of the ionic liquid.

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83 The principle difference betwee n the short circuit current density in the case of a grid and a full film device (both electrolyte coated) should be in the absorptive losses due to the nanotubes. This was quantitatively corroborated as follows. The current collected for the grid device i s proportional to the photon flux reaching the silicon as given by: (5 1) Where A G is the area occupied by the nanotube grid lines as a fraction of the total window area, was the measured reflectance of an ionic liquid saturated nanotube film (45 nm thick) on n Si, where is the measured transmittance of an ionic liquid saturated nanotube film (45 nm thick) on glass (making the absorptive loss in the SWNT film) is the AM1.5G solar irradiance (mW/cm 2 /nm), is the reflectance of n Si coated with a thin layer of the ionic liquid and the integrals are over the region of the solar spectrum relevant to silicon (300 nm to 1107 nm). The current collected in the full film covered window device will similarly be prop ortional to the photon flux reaching the silicon in that case and be given by: (5 2) Reflectance and transmittance measurements were performed using a Perkin Elmer Lambda 900, dual beam spectrophotometer (plots shown in Figure 5 9). The solar irradiance was derived from the ASTM G173 03 t ables 74 and the integrals were performed numerically. The ratio of the currents in the two devices evaluated in this way yields which should yield the ratio of the grid to the film short circuit current

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84 densities of These are in good agreement given no adjustable parameters and errors to be expected from the neglect of the second transit through the nanotube film that appears in the reflectan ce measurement, and the neglect of the reflection from the front surface of the nanotube film/ionic liquid occurring in the transmittance measurement. Reassuringly, correction for these effects would raise the calculated value, further improving the agreem ent. 5.7 Discussion and Future Work The grating MIS cell mentioned above was intensely studied in the late 1970s and early 1980s. Although the PCE of the grating MIS cells could exceed 17% it was found that they degraded rather drastically with time. 75 The degradation was traced to electrons that accumulated from the environment at the SiO layer surface. Because this charge tended to neutralize the trapped positive charge in the SiO layer (responsible for generating the inversion layer that permitted the wi de electrode spacing) the magnitude of the inversion layer decreased, degrading the cell performance. This should not be a problem with electrolytes which are intrinsically neutral and induce the inversion layer via a spontaneous charge separation. As is s een in the data of Figure 5 3 simple addition of the electrolyte, even with the gate electrically floating, yields a short circuit photocurrent equal to the saturation photocurrent implying existence of the inversion layer even before any gate field is app lied. In the case of the SWNTs the appearance of the gap like feature on electrolyte addition reduces the fill factor so that the gate field is necessary to achieve the maximum power conversion efficiency. Such gap like feature is not anticipated in the ca se of conventional metal electrodes. In that case a grating MIS Schottky junction cell with the SiO layer replaced by the ionic liquid electrolyte may

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85 provide the solution to the previous degradation problem even without the need for active gating (while g ating incurs little energetic penalty it does add complexity). Finally we note that such cells should also benefit from a texturing of the Si to trap more of the light reflected from its surface. Recently arrays of nanoholes in a p n junction Si solar cell have been demonstrated to enhance the device performance. 76 The large spacing permitted between the grid lines in the electrolyte coated NES device indicates that by filling the nanoholes with electrolyte, the device could benefit from both the inversion layer and the additional light trapping. Substantial further improvements may be possible for such electrolyte induced, inversion layer cells.

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86 A. B. Figure 5 1 A) Schematic of the device. Not shown is the EMI BTI ionic liquid electrolyte that extend s across both the gate electrode SWNT film and the n Si junction. B) Photograph of a SWNT film across the exposed n Si within the gold electrode window in which the SWNT film was etched to form the grid pattern shown. The SWNT film grid lines are 100 m wi de with 300 m between them. The seeming break in the grid lines at the bottom edge of the window is an illusion. The lines run continuously up onto the gold electrode.

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87 Figure 5 2. J V curves in both the light (AM 1.5G, 100 mW /cm 2 ) and dark for a continuous SWNT film covering the n Si window and for an etched film (as in Figure 5 1B). The etched film in this case covers 27% of the n Si window.

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88 Figure 5 3. Shows illuminated J V curves of the grid SW NT film before and after addition of the electrolyte for the gate electrically floating and with 0.75 V applied to the gate electrode.

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89 Figure 5 4. Time measurements of short circuit current of the grid solar cell with 25% o f the silicon window covered with the nanotubes at different electrolyte concentrations (as indicated by the ionic conductivity).

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90 Figure 5 5. Experimen tal J V curves under illumination at the specified gate voltage s. Figure 5 6. Simulation geomet ry and parameters of a cross sectional slice of the device through the nanotube grid line.

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91 Figure 5 7. Simulation results at V g = 0.75, 0, +0.75 V and V b ias = 0, 0.3 V. Figure 5 8. Simulation results at V g = 0.75, 0, +0.75 V and V b ias = 0.4 V.

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92 Figure 5 9. Shows the reflectance and transmittance measurements as a function of wavelength taken for different components of the NES solar cell.

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93 CHAPTER 6 CARBON NANOTUBE SPRA YED FILMS 6.1 Theory This chapter discusses an alternate nanotube film preparation approach. The aim is to develop a process capable of producing larger area nanotube films than can be fabricated by the filtration method (disc ussed in Chapter 1) without sacrificing the homogeneity, low sheet resistance and high transmittance achieved by that method. The filtration process is limited by the size of the filtration apparatus and available membrane sizes. A conventional means of pr oducing thin solid films of particulates over large areas is spray coating. This requires that the particles be homogeneously layer leaving the solid particulates behi nd once the solvent evaporates. As discussed in Chapter 1, surfactants can be used to suspend carbon nanotubes in solvents to form a sprayable ink. Unfortunately, as also discussed in Chapter 1 nanotube suspension with surfactants requires at least the c ritical micelle concentration of the surfactant. That turns out to be a far larger concentration than the nanotubes and since the surfactants are non volatile they are left behind along with the nanotubes once the solvent evaporates. Because surfactants ar e not electrically conducting, they greatly impede the film electrical transport and attempts to remove the surfactant without disturbing the homogeneity of the film are problematic. Much effort has gone into improv ing the dispersibility of carbon nanotube s by surface functionalization involving the covalent attachment of chemical groups to CNTs 77,78 While these methods can be effective at dispersing the nanotubes the covalent attachment disrupt s the sp 2 structure and conjugation of the CNTs responsible

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94 fo r their high electrical conductivity 79 Hence these methods are not useful in the fabrication of conductive, transparent CNT films. Alternative to covalent attachment carbon nanotube dispersants that bind non covalently to the nanotubes have also been expl ored. The non covalently bound dispersants fall into three broad categories, surfactants 80 (already discussed), polymers 81 and others like DNA, proteins, starch etc. 82 Polymer dispersants increase the solubility of nanotubes in a broad range of solvents. This chapter describes the use of one such polymer to disperse the nanotubes in a desired solvent. This project is part of a collaborative effort between Rinzler group (UF Dept. of Physics) and the Polymer Chemistry group of Professor John Reynolds (UF Dep t. of Chemistry). The polymer system used was synthesized by the Reynolds group based on a system described in the literature to disperse mutiwalled carbon nanotubes well in water. 83 This system is a pyrene derivitized hydroxypropyl cellulose. Pyrene and its derivatives have been widely used to functionalize the CNTs because of their ability to interact with the CNTs via stacking. 84 Commercially available hydroxypropyl cellulose (HPC) is one of the most commonly used cellulose derivatives with good solubility in most common solvents. So, a pyrene derivitized HPC, shown in Figure 6 1, can be used as a dispersant to susp end carbon nanotubes. One advantage of this system is that unlike surfactants only the pyrene HPC associated with a nanotube need exist in the solvent, minimizing the dispersant amount that must be used. Another advantage is that the HPC backbone is readil y decomposed under relatively mild conditions so that once the film is formed, the major fraction of this material can be decomposed and washed away.

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95 6.2 Ink preparation Purified single wall carbon nanotube material suspended in 1% surfactant (Triton X 100 ) was used as the starting material. This solution was filtered through a porous filter membrane to collect the nanotubes on the membrane and without letting the nanotubes dry a Triton X 100 solution at critical micelle concentration (CMC) was used to rins e off the excess surfactant while leaving the nanotubes coated with their full complement of surfactant. Without letting the film dry, the nanotubes were scrapped off the membrane and re dispersed in an aqueous solution of Triton X 100 at a concentration o f 2 CMC. After letting the nanotubes soak in the solution for 15 hr, 20 minutes of bath ultrasonication was used to re suspend the SWNTs in 2 CMC solution. Pyrene HPC was dissolved in deionized water at a concentration of 0.33 mg/ml and filtered through 45 m polytetrafluoroethylene (PTFE) filter. Equal volumes of the SWNTs in 2 CMC aqueous solution and the pyrene HPC (p HPC) solution were mixed and left stirring on a magnetic stir plate for 4 days. The rationale behind this procedure is to present the SWNTs with Triton X 100 at precisely the CMC simultaneously with excess pyrene HPC. During the long term stirring step it was anticipated that the pyrene HPC, with its high affinity for the nanotubes would replace the surfactant (as is confirmed below). To elim inate the surfactant and excess pyrene HPC the solution was filtered with DI water washing collecting the SWNTs. Again without drying the filtered product was scraped into the desired solvent and re dispersed by bath ultrasonication. The fact that this mat erial could be stably suspended in pure solvents in which HPC is soluble is clear evidence of the success of this association process. Two control experiments following this protocol but using no HPC in one experiment and a second

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96 experiment using HPC that had not been pyrene derivitized both failed to permit suspension of the SWNTs. This procedure is summarized in the flowchart in Figure 6 2. 6.3 SWNT Ink Based Films The pyrene HPC dispersed SWNTs (p HPC/SWNTs) were stable in water for day s with only litt le flocculation visible over several weeks. This ink was then used to make f ilms by filtration, drop casting and spray casting methods from water and ethanol Figure 6 3 shows an atomic force microscope (AFM) image of a drop cast film on glass (from water) which was dried in air at 80 C. For a direct comparison to filter fabricated films a film of p HPC/SWNTs was made by the fitration method. The starting quantity of nanotubes (as a mass was known) assuming negligible losses in the pyrene HPC association pr ocess the nanotube concentration in the 0.6 ml used to make a film (of 15 mm diameter) was 0.012 mg/m l. This gave a film thickness measured by AFM of 80 nm. This quantity of nanotubes without pyrene HPC made by the standard filtration method would yield a film of 57 nm thickness. Consistent with the SWNTs coated with pyrene HPC which should occupy some additional volume. The sheet resistance of the film measured by the van Der Pauw method 85 to be 855 was much higher than a pure film of 57 nm which would typically be less than 80 Unsurprisingly the pyrene HPC coating interferes with the intimate tube tube contact necessary for the high conductance 6.4 Decomposing Pyrene HPC Because hydroxyproy l cellulose is readily decomposed in dilute acid a 10 mM solution of sulfuric acid was used to attempt degrading and removing the pyrene HPC from the filtered film. It was found that the films delaminated from the surface during this

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97 step. It was subsequen tly found that a 24 hr bake at 80 C of the pyrene HPC/SWNT films prior to attempting the HPC dissolution would avoid such delamination. After removal of p HPC from the nanotube film, the sheet resistance was measured to be 784 There was a decrease observed in the resistance of the film, but it was not comparable to the value of sheet resistance mentioned above for a filtered film which was not treated with p HPC. The reason for such high R s has not been understood. 6.5 Spr ay Casting More ink was prepared to make films by spray casting method. Pure water, pure ethanol, and a mixture of the two in different ratios were used as solvents to suspend SWNTs with pyrene HPC to form inks. Pyrene HPC/SWNT inks were sprayed using an I wata Eclipse HP BS air brush sprayer onto microscope glass slides with a 0.35 mm diameter needle and a 1/16 oz solution holder with pressure set to 30 psi. Pure ethanol based pyrene HPC/SWNT sprayed films were found to be more homogeneous in comparison to those sprayed from pure water and from water/ethanol mixtures as shown in the optical images in Figure 6 4. The pyrene HPC (without SWNTs) when sprayed from water, ethanol mixture solution formed dendrite type aggregates whereas; pure ethanol solution of t he pyrene HPC uniformly covered the glass surface. Similarly, in comparison to the water/ethanol mixture based pyrene HPC/SWNTs inks, pure ethanol based inks were more uniform and homogeneous. Figures 6 4A and 6 4B are optical micrographs of the sprayed fi lms from water/ethanol and from ethanol, respectively. The black specks are aggregated pyrene HPC. An AFM image (Figure 6 5) of a sprayed film from water, ethanol mixture shows the presence of SWNTs with

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98 pyrene HPC. E thanol wa s chosen to be the preferred s olvent over water for spraying pyrene HPC/SWNTs based inks. 6.6 Pyrene HPC/SWNTs Ethanol Ink Pyrene HPC/SWNTs ethanol ink was prepared with a concentration of 16 g/m l which was used to spray a 1 1 cm 2 film on a glass substrate. The thickness of the film was measured to be ~ 20 nm Pyrene HPC was removed from the film by the acid treatment as mentioned above. Figures 6 6 and 6 7 show the optical image and the AFM im age of the sprayed film. The optical image shows inhomogeneity in the film with varying concentration of nanotubes which was confirmed by varying intensities in different regions as observed by Raman spectroscopy. The s heet resistance of this thin inhomoge neous SWNT s film after removal of pyrene HPC wa s measured to be R s = 1070 The R s of a standard film prepared by the filtration method at this thickness is that this film gave reasonabl e conductivity compared to earlier samples. To increase the conductance of sprayed films, it was necessary to spray thicker films. It was challenging to do so because of persistent clogging of the air brush nozzle with pyrene HPC coated nanotubes. To avoid clogging, dilute p HPC/ SWNT ink s were prepared and used. T he air brush nozzle was cleaned with ethanol and t he sprayed film was dried in the air at 80 C for 2 min between successive sprayed layers This method was successful in spraying thick films. The SWNT film that was made w ith a low ink densit y of 1.6 g/ml as opposed to 16 g/ ml used for previous film, resulted in a successful spraying of a thick film. The t hickness of the SWNT film was deter mined from its UV vis spectra (Figure 6 8) to be 65 70 nm. The AFM imaging was done and it showed a high surface roughness of the film so the step height (thickness)

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99 measurement was not performed. Figures 6 9 and 6 10 show the optical image and the optical transparency of this thick film after removal of p HPC respectively. The sheet resistance of this sprayed pyren e HPC/SWNT film before and after removal of pyrene HPC wa s measured to be R s s respectively. This not only showed an increase in conductance of the SWNT film after the removal of p HPC, but also showed a substantial increase in conductance of the film sprayed using dilute inks when compared to the conducta nce of the film sprayed using higher concentration ink. The R s of a filtered film of the same thickness would typically be 40 indicating reasonable success with thicker SWNT sprayed films. 6.7 Stability Measurement To study the changes in the sheet r esistance of the sprayed film as a function of time, long term R s measurements were conducted over a period of four months. A pyrene HPC/SWNTs film was sprayed from ethanol o n a glass substrate with 40 nm g old contacts pre deposited on the four corners of the glass such that the film overlaps the Au contacts when sprayed. The contacts were deposited to avoid tearing of the film during frequent probe contacts for van Der Paw measurement. The sheet resistance of the sprayed pyrene HPC/ SWNT film was measured b efore and aft er the removal of p HPC and wa s found to be R s s The thickness of this film as determined from the UV vis spectrum as shown in Figure 6 11 is 60 65 nm. Figure 6 12 shows the change in R s of the SWNTs film over time where the film was stored in a petri dish in ambient air and the measurements were also conducted in air. The stability curve indicates that the sheet resistance of the sprayed nanotube film only doubles in a period of four months. The conductanc e and the stability of these sprayed

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100 films can be useful in various applications of nanotubes. One such application is in the electrochromic devices which can be explored in the future. 6.8 Future Work This chapter demonstrated an alternative method of mak ing SWNT films, spray casting. Pyrene derivitized HPC was bound to SWNTs to disperse them in ethanol to form inks which was used to successfully spray SWNT films. The sheet resistance measurements indicate that the sprayed SWNT films can be used in differe nt applications. These experiments also motivate future work to be done on synthesizing new polymer dispersants which can bind to the nanotubes to disperse them in desired solvents for spraying.

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101 Figure 6 1. Shows the structure of pyrene HPC. Adapted in part with permission from [ Yang, Q.; Shuai, L.; Zhou, J.; Lu, F.; Pan, X. J. Phys. Chem. B 2008 112 12934 12939 ] Copyright [ 2008 ] American Chemical Society.

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102 Figure 6 2. Shows a flowchart describing the steps involved in making SWNT s with polymer dispersant inks. Figure 6 3 AFM image of a single drop cast film of pyrene HPC/SWNTs water based ink. The scale is in microns.

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103 Figure 6 4. Optical micrographs of the pyrene HPC/SWNTs inks spray cast onto glass slides from: A) water/ ethanol mixture (2 : 3 volume ratio), B) pure ethanol. Figure 6 5. AFM image of pyrene HPC/SWNTs film sprayed from a water/ethanol mixture.

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104 Figure 6 6 Optical Microscope image of a sprayed SWNT film after removal of pyrene HPC from ethanol based ink. Figure 6 7. AFM image of SWNT film s pray ed from pyrene HPC/ SWNT e thanol based ink af ter removal of p HPC.

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105 Figure 6 8 UV vis spectra of the as sprayed pyrene HPC/SWNT film after etching pyrene HPC. Figure 6 9. Optical micrograph image of spra yed SWNTs film after removal of pyrene HPC.

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106 Figure 6 10 Photograph of SWNT film sprayed from the p HPC/SWNT ethanol based ink onto glass slide after removing p HPC shows the optical transparency of the nanotube film. Figure 6 11. UV vis spectrum of the SWNT sprayed film after etching pyrene HPC.

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107 Figure 6 12. Log plot of stability measurement of sheet resistance of a sprayed SWNT s film af ter removal of pyrene HPC.

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108 LIST OF REFERENCES 1. Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A. Annu. Rev. Mater. Res. 2004 34 247 278. 2. Mintmire, J. W.; Dunlap, B. I.; White C. T. Phys. Rev. Lett. 1992 68 631 634. 3. Hamada, N.; Sawada, S.; Oshiyama, A. Phys. Rev. Lett. 1992 68 1579 1581. 4. Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992 60 2204 2206. 5. Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical properties of carbon nanotubes London: World Scientific Publishing Company (Imperial College Press, 1998 ). 6. Javey A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Nature 2003 424 654 657. 7. Avouris, P.; Chen, Z.; Perebeinos, V. Nature Nanotechnology 2007 2 605 615. 8. Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003 3 1379 13 82. 9. Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodriguez, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Applied Physics A 1998 67 29 37. 10. Wu, Z. C.; C hen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004 305 1273 1276. 11. Data complied by Kazmerski, L. National Renewable Energy Laboratory, Golden, Colorado, www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt 12. Bird, R. E.; Hulstrom, R. L. J. of Solar Energy Eng. 1981 103 182 192. 13. Neamen, D. A. Semiconductor physics and devices ( McGraw Hill, 3rd edition). 14. Sze, S. M. Physics of Semiconductor Devices (Wiley, 1981 ). 15. Prince, M. J. Appl. Phys. 1955 26 534 540. 16. Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. App. Phys. Lett. 2004 84 3013 3015. 17. Green, M. A.; Emery, K.; King, D. L.; Igari, S.; Warta, W. Prog. Photovoltaics 2001 9 287 293.

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113 BIOGRAPHICAL SKETCH Pooja Wadhwa was born in the year 1981 in New Delhi, India. She did her undergraduate studies from University of Delhi in electronic science graduating with electrical and computer engineering from National University of Singapo re and graduated in 2004. She continued to live in Singapore and worked in Data Storage Institute as a research engineer and then she moved to United States of America in 2005 to purse her PhD in physics from University of Florida.