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Indium oxide as a high resistivity buffer layer for CdTe/CdS thin film solar cells

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Title:
Indium oxide as a high resistivity buffer layer for CdTe/CdS thin film solar cells
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Book
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English
Creator:
Balasubramanian, Umamaheswari
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University of South Florida
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Tampa, Fla.
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Subjects / Keywords:
optical properties
TCOs
front contact
Dissertations, Academic -- Electrical Engineering -- Masters -- USF   ( lcsh )
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government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
ABSTRACT: Transparent conductive oxides are an essential part of technologies that require both large-area electrical contact and optical access in the visible portion of the lightspectrum. SnO₂ doped with Fluorine (SnO₂: F) and In₂O₃ doped with tin (ITO) are the most popular choices of front contacts for CdTe solar cells. In this thesis, CdS/CdTe devices were fabricated with SnO₂: F (MOCVD) and ITO (sputtering) as front contacts without a high resistivity (resistivity relatively greater than front contact) buffer layer. The device characteristics of these devices were low but improved considerably after the inclusion of an intrinsic SnO₂ (SnO₂-i) deposited by MOCVD as buffer. Thus having emphasized and demonstrated the benefits of a buffer layer in these devices, the use of reactively sputtered SnO₂ (intrinsic), SnO₂ doped with Zinc (5% and 10% Zinc) and In₂O₃(intrinsic) as buffer layers in SnO₂:F/buffer/CdS/CdTe devices were explored. Experiments were also carried out on the photovoltaic active layers of SnO₂:F/SnO₂-i/CdS/CdTe Solar cells. Namely, the effect of window layer thickness was studied by making a series of devices in which the CdS thickness was progressively reduced and the effect of substrate temperature (Tsub) during the deposition of the absorber layer was also studied by increasing Tsub > 600 degree C during CdTe CSS. In order to determine the effectiveness of In₂O₃ as a buffer layer, a series of ITO/In₂O₃/CdS/CdTe cells were fabricated with varying thickness of In₂O₃ (250 to 2000 &#506) and also the CdS thickness was reduced in steps (~800 &#506 to~500 &#506) in these devices. ITO/ In₂O₃ device with efficiency greater than 14% (Voc: 820 mV, FF: 72% and Jsc: 24 mA/cm²) was fabricated for an In2O3 thickness of 250 &#506 and CdS thickness of ~ 600 &#506. However the best efficiency of 14.7% (Voc: 830 mV, FF: 77%, Jsc: 23 mA/cm²) was achieved for SnO₂:F/SnO₂-i/CdS/CdTe device. ITO films with resistivity as low as 1.9X10-4 ohm-cm, mobility 32 cm2V-1s-1 and average transmission ~ 90% in the visible region were obtained for carrier concentration in the order of 1.1XE21cm-3.
Thesis:
Thesis (M.S.E.E.)--University of South Florida, 2004.
Bibliography:
Includes bibliographical references.
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Mode of access: World Wide Web.
Statement of Responsibility:
by Umamaheswari Balasubramanian.
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Title from PDF of title page.
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Document formatted into pages; contains 100 pages.

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oclc - 55654263
notis - AJR1150
usfldc doi - E14-SFE0000297
usfldc handle - e14.297
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SFS0024992:00001


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ABSTRACT: Transparent conductive oxides are an essential part of technologies that require both large-area electrical contact and optical access in the visible portion of the lightspectrum. SnO doped with Fluorine (SnO: F) and InO doped with tin (ITO) are the most popular choices of front contacts for CdTe solar cells. In this thesis, CdS/CdTe devices were fabricated with SnO: F (MOCVD) and ITO (sputtering) as front contacts without a high resistivity (resistivity relatively greater than front contact) buffer layer. The device characteristics of these devices were low but improved considerably after the inclusion of an intrinsic SnO (SnO-i) deposited by MOCVD as buffer. Thus having emphasized and demonstrated the benefits of a buffer layer in these devices, the use of reactively sputtered SnO (intrinsic), SnO doped with Zinc (5% and 10% Zinc) and InO(intrinsic) as buffer layers in SnO:F/buffer/CdS/CdTe devices were explored. Experiments were also carried out on the photovoltaic active layers of SnO:F/SnO-i/CdS/CdTe Solar cells. Namely, the effect of window layer thickness was studied by making a series of devices in which the CdS thickness was progressively reduced and the effect of substrate temperature (Tsub) during the deposition of the absorber layer was also studied by increasing Tsub > 600 degree C during CdTe CSS. In order to determine the effectiveness of InO as a buffer layer, a series of ITO/InO/CdS/CdTe cells were fabricated with varying thickness of InO (250 to 2000 Ǻ) and also the CdS thickness was reduced in steps (~800 Ǻ to~500 Ǻ) in these devices. ITO/ InO device with efficiency greater than 14% (Voc: 820 mV, FF: 72% and Jsc: 24 mA/cm) was fabricated for an In2O3 thickness of 250 Ǻ and CdS thickness of ~ 600 Ǻ. However the best efficiency of 14.7% (Voc: 830 mV, FF: 77%, Jsc: 23 mA/cm) was achieved for SnO:F/SnO-i/CdS/CdTe device. ITO films with resistivity as low as 1.9X10-4 ohm-cm, mobility 32 cm2V-1s-1 and average transmission ~ 90% in the visible region were obtained for carrier concentration in the order of 1.1XE21cm-3.
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Indium Oxide as a High Resistivity Buffer Layer for CdTe/CdS Thin Film Solar Cells by Umamaheswari Balasubramanian A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Major Professor: Christos S. Ferekides, Ph.D. Don Morel, Ph.D. Robert Mamazza, Jr., Ph.D. Andrew M. Hoff, Ph.D. Date of Approval: March 24, 2004 Keywords: TCOs, front cont act, optical properties. Copyright 2004 Umamaheswari Balasubramanian

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DEDICATION This Thesis is dedicated to my family and my loving husband Rajender, without whose help and support, I would not have been able to complete my thesis.

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ACKNOWLEDGEMENTS I am indebted to my major professor Dr Chris Ferekides, who gave me this wonderful opportunity to do my masters under his meticul ous guidance. I am very grateful to Dr. Robert Mamazza, Jr., for always being there for me, right from getting me started in research and helping me through out the course of this program. I also like to thank my other committee members Dr. D on Morel and Dr. Andrew Hoff for their valuable time and consideration. I also thank my colleagues in the lab, for their timely help as and when required. I would also like to express my deep gratit ude and respect for my husband Rajender, who had been patient and understanding through ou t the course of this program, giving me moral support and literally st eering me through hard times. W ithout his constant love and support, I would not have been able to complete my thesis.

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i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES v ABSTRACT x CHAPTER 1 INTRODUCTION 1 1.1 Photovoltaic Effect and Origin of Solar Cells 2 1.2 Principle of Operation of Solar Cells 2 1.3 Basic Solar Cell Device Structure 4 1.4 Basic Solar Cell Physics 8 CHAPTER 2 TRANSPARENT CONDUCTIVE OXIDES 15 2.1 Transparent Conductive Oxides 15 2.1.1 Optical Properties 15 2.1.2 Electrical Properties 17 2.2 Electrical Conduc tion in Polycrystalline Thin Films 21 2.3 TCOs in general 22 CHAPTER 3 MATERIALS A ND LITERATURE REVIEW 25 3.1 Electronic Properties of SnO2 and ITO 25 3.2 The Effect of SnO2 and ITO as Front Contacts to CdS/CdTe Solar Cells 28 3.3 Properties of Cd2SnO4, Zn2SnO4 33 3.4 The Effect of Cd2SnO4 and Zn2SnO4 on CdS/CdTe Solar Cells 37 CHAPTER 4 EXPERIMENTAL METHODS 40 4.1 Material Deposition 40 4.1.1 Chemical Vapor Deposition Process 40

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ii 4.1.2 Sputtering 41 4.1.3 Chemical Bath Deposition Process 42 4.1.4 Closed Spaced Sublimation Process 43 4.1.5 Evaporation 43 4.1.6 Graphite Paste Application 44 4.2 Standard Device Structure 44 4.3 Material Characterization 45 4.4 Solar Cell Measurements 45 CHAPTER 5 RESULTS AND DISCUSSION 47 5.1 Properties of MOCVD SnO2 Thin Films 47 5.2 Properties of Sputtered In2O3 Thin Films 48 5.3 Properties of Sputtered ITO Thin Films 50 5.4 CdTe Solar Cells with SnO2: F and ITO as Front Contacts 53 5.4.1 Effect of High Resistivity (intrinsic) CVD SnO2 Layer 55 5.4.2 Effect of CdS Thickness on SnO2:F, ITO/SnO2-i CdS/CdTe Solar Cells 58 5.5 Effect of Substrate Temperature during CdTe CSS Deposition 61 5.6 SnO2:F CdS/CdTe Solar Cells with Sputtered SnO2 Doped with Zinc (SnO2:Zn) as Buffer Layer 64 5.7 CdS/CdTe Solar Cells with Indium Oxide as Buffer Layer 68 5.7.1. SnO2:F / In2O3 CdS/CdTe Solar Cells 68 5.7.2. ITO/ In2O3 CdS/CdTe Solar Cells 70 5.8 Effect of CdS Thickness on ITO/ In2O3 Devices 73 5.9 Window less Junctions 77 CHAPTER 6 CONCLUSIONS 81 REFERENCES 84

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iii LIST OF TABLES Table 1. The Summary of Different Types of Material s Used to Make Solar Cells. 7 Table 2. Photovoltaic Parameters of SnO2 and ITO CdS/CdTe Solar Cells 29 Table 3. Summary of the Parameters of the So lar Cell with Different TCOs. 38 Table 4. Jsc Losses in Different TCOs. 39 Table 5.The Parameters of Highest Efficiency CTO/ZTO/CdS/CdTe Solar Cell 39 Table 6. Properties of Reactively Sputtered In2O3 Thin Films. 49 Table 7. Properties of SnO2:F and ITO Films. 52 Table 8. Device Parameters of CdS/CdTe Solar Cells with SnO2:F,ITO as Front Contacts. 53 Table 9. Device Parameters of CdS/CdTe Solar Cells with SnO2:F,ITO as Front Contacts with Intrinsic SnO2 Buffer Layer. 57 Table 10. Device Parameters of SnO2, ITO Solar Cells with Different CdS Thickness. 59 Table 11. Device Parameters of SnO2:F / SnO2-i CdS/CdTe Solar cells with Different Substrate Temperat ure during CdTe CSS Deposition. 62 Table 12. Device Parameters of SnO2:F / SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 500 . 65 Table 13. Device Parameters of SnO2:F/ SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 2000 . 67 Table 14. Device Parameters of SnO2 :F cells with In2O3 as Buffer Layer. 69 Table 15. Device Parameters of ITO/ In2O3 Cells with ~800 of CdS. 71 Table 16. Device Parameters of ITO/ In2O3 Cells with ~700 of CdS. 73

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iv Table 17. Device Parameters of ITO/ In2O3 Cells with ~600 of CdS. 73 Table 18. Device Parameters of ITO/ In2O3 Cells with ~500 of CdS. 73 Table 19. Properties of Spinel TCOs. 78 Table 20. Device Parameters of Spinel TCO/CdTe Devices. 79 Table 21. SnO2: F/CdS/CdTe Devices with Different Buffer Layers 81 Table 22. SnO2: F/CdS/CdTe Devices with Thin Sputtered SnO2 Buffer Layer. 82 Table 23. ITO/CdS/CdTe Devices with Different Buffer Layers 82 Table 24. Summary of Best ITO/ In2O3 Devices with Thick CdS Layer. 83 Table 25. Summary of Best ITO/ In2O3 Devices with Thin CdS Layer. 83

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v LIST OF FIGURES Figure 1. Solar Power. 1 Figure 2.The Fermi Level Positions of P-type and N-type Semiconductors. 3 Figure 3. Bending of Valence and Co nduction Band due to Fermi Level Lining Up. 3 Figure 4. Origin of photocurrent. 4 Figure 5. Band Alignments in Homojunction (top) and Heterojunction (bottom). 5 Figure 6. Basic Structure of a Solar Cell. 6 Figure 7. The Current –Voltage (I-V) Characteristics of an P-N Junction Diode. 8 Figure 8. Equivalent Circuit of a "Real" Solar Cell Showing Both Shunt and Series Resistive Loss. 9 Figure 9. Solar Cell Under Short Circuit Condition. 11 Figure 10. Solar Cell Under Open Circuit Condition. 11 Figure 11. Spectral Distribution of Solar Radiation Intensity. 14 Figure 12. The Fill Factor. 14 Figure 13. Possible Photon Interactions W ith a TCO. 16 Figure 14. Photon-TCO Interaction Types by Charac teristic Wavelength Re gion. 17 Figure 15. Sample Dimensions for Definition of Sheet Resistance. 18 Figure 16. Schematic of the Four Point Probe Set Up. 19 Figure 17 .The Hall Effect Phenomenon. 21 Figure 18. Structure of Tin Oxide. 25 Figure 19. Structure of ITO. 27

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vi Figure 20. The J-V Characteristics of SnO2, ITO Cells 29 Figure 21. The Dark IV Characteristics of SnO2/CdS/In in the Forward and Reverse bias (a) Before Annea ling (b) After Annealing. 30 Figure 22. The Dark IV Characteristics of ITO/CdS/In in the Forward and Reverse bias (a) Before Annea ling (b) After Annealing. 31 Figure 23. The Dark IV Characteristics of ITO/CdS/In at Different Lengths of Annealing Times. 32 Figure 24. The Spinel Structure. 34 Figure 25. Transmission and Absorbance Spectra of CTO and SnO2 Thin Films. 35 Figure 26. Relationship Between Jsc and Transmission of SnO2 and CTO Films. 35 Figure 27. Transmittance, Reflectance, and Absorption for Wavelengths Between 300 and 2500 nm for a ZTO Film. 36 Figure 28. Shift in the Band Gap of ZTO Films with Increase in Carrier Concentration. 37 Figure 29. The Structure of CdS/CdTe So lar Cell to Find the Effect of Both CTO and ZTO Layers is as Follows. 37 Figure 30. Relative Quantum Efficiencies of CTO/CdS/CdTe Solar Cell and CTO/ZTO/CdS/CdTe Solar Cells. 38 Figure 31. Simplified Schematic of MOCVD Set-up. 40 Figure 32. Position of Sputtering Sources and Substrate Holder. 41 Figure 33. The Chemical Bath Deposition Set-up. 42 Figure 34. Simplified CSS Experimental Set-up. 43 Figure 35. Standard Device Structure. 44 Figure 36. XRD Pattern MOCVD SnO2:F Film at 460C. 47 Figure 37. AFM Photograph of the SnO2:F Surface. 48 Figure 38. XRD Pattern of In2O3 Thin Films. 49

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vii Figure 39. Transmission Spectra of In2O3 Thin Films. 49 Figure 40.The Transmission Spectra of SnO2:F and ITO. 50 Figure 41. XRD Pattern of ITO Film Deposited at 300C. 51 Figure 42. XRD Pattern of the ITO Film Deposited at 200C. 51 Figure 43. AFM Photograph of ITO Deposited at 200 C (left) and 300C (right) 52 Figure 44. Structure of CdS/CdTe Solar Cells with SnO2:F, ITO as Front Contacts. 53 Figure 45. Equivalent Circuit Diagram for a Solar Cell with a Barrier at the Back Contact (a), and a Solar Cell with a Barri er at the Front Contact (b). 54 Figure 46. Device Structure after the Inclusion of Buffer Layer. 55 Figure 47. I-V Characteristics of SnO2, ITO CdS/CdTe Solar Cells with and without Intrinsic SnO2 Layer. 56 Figure 48. Spectral Response of SnO2:F CdS/CdTe Cells with and without Intrinsic SnO2 Layer. 56 Figure 49. Spectral Response of ITO Cd S/CdTe Cells with and without Intrinsic SnO2 Layer. 57 Figure 50. J-V Characteristics (left) Spectral Response (right) of the Best SnO2/SnO2-i Devices with Different CdS Thickness. 59 Figure 51. J-V Characteristics (left) Spectral Response (right) of the Best ITO/SnO2-i Devices with Different CdS Thickness. 60 Figure 52. Effect of CdS Thickness on the Device Parameters of SnO2/SnO2-i CdS/CdTe Solar Cells. 61 Figure 53. Effect of CdS Thickness on the Device Parameters of ITO/ SnO2-i CdS/CdTe Solar Cells. 61 Figure 54. J-V Characteristics (left) and Spectral Res ponse (right) of SnO2:F / SnO2-i CdS/CdTe Solar Cells with Different Substrate Temperature during CdTe CSS Deposition. 62

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viii Figure 55. Effect of Different Substr ate Temperatures during CSS CdTe Deposition on Device Parameters. 63 Figure 56. Effect of High CdTe Substrate Temperature. 64 Figure 57. Device Structure of SnO2:F CdS/CdTe Solar Cells with Sputtered SnO2 Doped with Zinc (SnO2:Zn) as Buffer Layer. 65 Figure 58. IV Characteristics (Left) and Spectral Response (Right) of SnO2:F/ SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 500 66 Figure 59.Change in Device Parameters of SnO2:F/ SnO2:Zn CdS/CdTe Solar Cells for Di fferent Concentrations of Zinc (SnO2:Zn layer thickness of 500 ). 66 Figure 60. J-V Characteristics (Left) and Spectral Response (Right) SnO2:F/ SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 2000 67 Figure 61. Change in Device parameters of SnO2:F/ SnO2:Zn CdS/CdTe Solar Cells for Different Concentrations of Zinc (SnO2:Zn layer thickness of 2000 ). 68 Figure 62. Device Structure of SnO2:F/In2O3 CdS/CdTe Solar Cell. 69 Figure 63. I-V Characteristics (left) and Spectral Respons e (right) of SnO2:F Cells with In2O3 as Intrinsic Layer. 70 Figure 64. Device Structure of ITO/ In2O3 CdS/CdTe Solar Cells. 71 Figure 65. I-V Characteristics (Left) and Spectral Response (Right) ITO/ In2O3 Cells with 800 of CdS. 72 Figure 66. Change in Device Parameters of ITO/ In2O3 Cells with 800 of CdS with Change in Thickness of In2O3. 72 Figure 67. I-V Characteristics (Left) and Spectral Response (Right) ITO/ In2O3 Cells with 700 of CdS. 74 Figure 68. I-V Characteristics (Lef t) and Spectral Response (Right) ITO/ In2O3 Cells with 600 of CdS. 74

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ix Figure 69. I-V Characteristics (Left) and Spectral Response (Right) ITO/ In2O3 Cells with 500 of CdS. 75 Figure 70. Change in Device Parameters of ITO/ In2O3 Cells with 700 of CdS with Change in Thickness of In2O3. 75 Figure 71. Change in Device Parameters of ITO/ In2O3 Cells with 600 of CdS with Change in Thickness of In2O3. 76 Figure 72. Change in Device Parameters of ITO/ In2O3 Cells with 500 of CdS with Change in Thickness of In2O3. 76 Figure 73. Device Structure of TCO/CdTe Solar Cell. 77 Figure 74. Spectral Response of CTO/Cd Te Cells (left) and SnO2/ZTO/CdTe Devices (right). 80 Figure 75. Spectral Response of CIO/ CdTe Devices. 80

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x INDIUM OXIDE AS A HIGH RESISTIV ITY BUFFER LAYER IN CDTE/CDS THIN FILM SOLAR CELLS Umamaheswari Balasubramanian ABSTRACT Transparent conductive oxides are an essen tial part of technol ogies that require both large-area electrical cont act and optical access in the visible portion of the light spectrum. SnO2 doped with Fluorine (SnO2: F) and In2O3 doped with tin (ITO) are the most popular choices of front contacts for CdTe sola r cells. In this thesis, CdS/CdTe devices were fabricated with SnO2: F (MOCVD) and ITO (sputtering ) as front contacts without a high resistivity (resistivity relatively greater than front contact) buffer layer. The device characteristics of these devices were low but improved considerably after the inclusion of an intrinsic SnO2 (SnO2-i) deposited by MOCVD as buffer. Thus having emphasized and demonstrated the benefits of a buffer laye r in these devices, the use of reactively sputtered SnO2 (intrinsic), SnO2 doped with Zinc (5% and 10% Zinc) and In2O3(intrinsic) as buffer layers in SnO2:F/buffer/CdS/CdTe devices were explored. Experiments were also carried out on the photovoltaic active layers of SnO2:F/SnO2-i/CdS/CdTe Solar cells. Namely, the e ffect of window layer thickness was studied by making a series of devices in which the CdS thickness was progressively reduced and the effect of substrate temperature (Tsub) during the deposition of the absorber layer was also studied by increasing Tsub > 600C during CdTe CSS. In order to determine the effectiveness of In2O3 as a buffer layer, a series of ITO/In2O3/CdS/CdTe cells were fabricated with varying thickness of In2O3 (250 to 2000 ) and also the CdS thickness was reduced in steps (~800 to~500 ) in these devices.

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xi ITO/ In2O3 device with efficiency greater than 14% (Voc: 820 mV, FF: 72% and Jsc: 24 mA/cm2) was fabricated for an In2O3 thickness of 250 and CdS thickness of ~ 600 However the best efficiency of 14.7% (Voc: 830 mV, FF: 77%, Jsc: 23 mA/cm2) was achieved for SnO2:F/SnO2-i/CdS/CdTe device. ITO films with resis tivity as low as 1.9X10-4 -cm, mobility 32 cm2V-1s-1 and average transmission ~ 90% in the visible re gion were obtained for carrier concentration in the order of 1.1X1021cm-3.

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1 CHAPTER 1 INTRODUCTON Most of the energy that we use in the day to day life comes from fossil fuel and nuclear power stations. In energy terms, oil makes the single largest contribution to world energy supply, at forty percent, followed by coal at twenty six percent and natural gas at about twenty four percent. When these foss il fuels are being burnt, harmful emissions are being released into the environment. Th ese include carbon and sulphurdioxides which contribute to climate change and acid rain. Some of the effects due to the pollution caused by combustion of these fossil fuels are the green house effect and the depletion of the atmospheric ozone layer. Non-renewable, fossil fuels take millions of years to form, so they are finite and, ultimate ly, exhaustible energy resources. Figure 1. Solar Power [2]. On the other hand, there are serious doubts surrounding the safety of nuclear technology and how to dispose of the radioa ctive waste products. Therefore, we must develop and use alternative energy resources.

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2 Over the last twenty years there have been major steps in the development of renewable energy technology for both industr y and domestic use. As the amount of sunshine energy that reaches th e surface of the Earth every minute is greater than the total amount of energy that the world's human popula tion consumes in a year, the sun is our only long-term natural resource. The earth’s atmosphere and clouds absorb, reflect and scatter some of the radiation from the sun nonetheless enormous amounts of direct and diffused sunlight energy reaches the earth which can be converted to electricity. Photovoltaic devices that direct ly convert the sunlight into electricity are called solar cells. Solar cells are considered a major candi date in providing n early permanent power at low operating cost. The major drawback of solar cells at present is the high manufacturing cost involved. So research has to be done to improve device structure and reduce the manufacturing costs. Solar cells at present furn ish the most important longduration power supply for satellites and spac e vehicles. Solar cells have also been successfully employed in small to medi um scale terrestrial applications. 1.1 Photovoltaic Effect and Origin of Solar Cells The photovoltaic effect is the phenomenon by which solar cells convert sunlight into electricity. Sunlight co mprises packets of energy call ed photons. In 1839, a nineteen year old Henry Becquerel disc overed that electric current could be produced by shining light onto certain chemical solu tions. In 1877, this effect was ob served in a so lid material, Selenium for the first time. Practical solar cells have been made only since the mid 1950s after Einstein in 1905 and Schottky in 1930 provided a deeper insight into the understanding of the scientific princi ples involved in solid state physics. A silicon solar cell which converted 6% of sunlight incident upon it into electricity was developed by Chapin, Pearson and Fuller in 1954 and ce lls of this kind were used in space applications in 1958. No w solar cells with more than 30% of efficiency are being produced [2, 3]. 1.2 Principle of Operati on of Solar Cells Solar cells, in the simplest form are p-n junction devices which use sunlight to create electrical energy.

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3 The Fermi Level is a theoretical leve l derived from the Fermi-Dirac electron distribution in a solid that defines an electr on’s probability of occupying a specific energy level or state. The Fermi level also define s the magnitude of energy required to remove an electron from a semiconductor into fr ee space (or the vacuum level in a band diagram), which is the work function of the material. In intrinsic materials, the Fermi level is approximately centered between the conduction band and the valence band. For n-type materials, it is shifted up towards th e conduction band; for p-type materials, it is shifted downwards towa rds the valence band. Figure 2. The Fermi Level Positions of P-type and N-type Semiconductors. When a p-type semiconductor and ntype semiconductor are brought together, the Fermi levels get lined up, causing band bending in valence and conduction band. Electrons from the n-side combine with the holes on the p-si de resulting in a positive charge on the nside of the junction and a ne gative charge accumulation on th e p-side. This separation of charge creates a junction potential or built-in potential, and is denoted Vbi [1]. Figure 3. Bending of Valence and Conduction Band due to Fermi Level Lining Up. n-type material Conduction band p-type material Valence Band Fermi level p -ty p e n-type Fermileve l Depletion Region Built-in Potential (q V bi ) p-type n-type Before After

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4 In, Figure 3, the built-in potential ( qVbi) is defined as the difference in the relative energies of the conduction bands between the p-type and the n-type materials on either side of the junction. When a photon, with energy greater than the band gap of the semiconductor, passes through the solar cell, it may be absorbed by the material. This absorption takes the form of a band-to-band elec tronic transition, so an electron/hole pair is produced. If these carriers can diffuse to the depletion region befo re they recombine, then they are separated by th e electric field, cau sing one quantum of charge to flow through an external load. This is termed as photovoltaic effect. The current thus produced is termed as photo cu rrent (see figure 4). Figure 4. Origin of Photocurrent. 1.3 Basic Solar Cell Device Structure As described in the previous section, a so lar cell consists of a p-n junction. This junction could be made of p-t ype and n-type semiconductor ei ther of the same material (homojunction) or different ma terials (heterojunction). A ty pical heterojunction is made between a wide band gap materi al and a narrow band gap ma terial. There are inherent advantages and disadvantages in each of these types. n-type pt yp e E Photons Photo Current,I Depletion region qVbi E n-type p-type Photons

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5 Figure 5. Band Alignments in Homojunction (top) and Heterojunction (bottom). The obvious, advantage of a homojunction is the perfect band alignment. Band alignments are illustrated in figure 5, which shows both a homojunction (top) and a heterojunction (bottom) at equilibrium. Also, having the same material will also result in perfect lattice match at the junction, which leads to significantly fewer interfacial traps than that in a heterojunction. The existence of traps near the junction can reduce carrier life times. Mostly for heterojunctions, careful materi al selection (and or manipul ation) can approximate many of the advantages of the ho mojunction, with some focus on reduction of lattice mismatch effects and reduce the formation of interfacial defects and knowledge of work functions and electron affinities to reduce band related eff ects. It should also be noted that lattice match (or mismatch) and band related issues are not solely responsible for solar cell performance, the band gap and the absorption coefficient of the absorber, among other things, are also very important. In figure 5, the heterojunction shows a band alignment discontinuity that is prominent in the conduction band minimum ( Ec), provided this value is relatively small, the obstruction to the flow of minority carriers across the junction will be almost negligible. An impor tant advantage a heterojunction has over its counterpart is that the window layer may have a wider band gap than the absorber.

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6 This wider band gap will allow greater current generation from high energy photons. Further series resistance can also be lower for a heterojunction if the window layer is able to be heavily doped. These various considerations are device structure specific, and are not intended to cover all scenarios. The only modification to the p-n junction to turn it into a solar cell is the addition of appropriate contacts. One of the contacts would have to be semi-transparent to permit the light; or alternatively, it could also be a metal grid that only covered a small portion of the device. When the front contact used is a semitransparent oxide, the material used is classified as transparent conducting oxide (TCO) (as shall be discussed further in the subsequent chapters). The back contact need only be chemically and physically compatible with the adjacent material. With respect to figure 6, it can be seen that the n-type material is facing the light. So light passes through the n-type mate rial and reaches the p-type material. Figure 6. Basic Structure of a Solar Cell. So the p-type material is referred to as absorber and it should be capable of absorbing the photons of energy equal to or greater than it’s band gap. So in this arrangement, ideally speaking both the front co ntact and n-type material should be large band gap devices, to prevent optical losses, wh ich could have other wise been converted to electricity. And the n-type material should be as thin as possible to permit most of the light to enter the p-type material. Solar cells can be manufactured in a variety of ways and with a variety of materials. They can be categorized into three main types (as shown in Table 1).

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7 Table 1. The Summary of Different Types of Materials Used to Make Solar Cells. Crystalline silicon currently makes up over 86% of the photovoltaic market. The reason for this dominance is that the materi al, technology, and equipment come right out of the solid state integrated circuit industry. Th e development of c-Si cells is very energy intensive. It is therefore very expensive to process these cells and the technology is leaning toward the production of polycrystal line Si cells and other thin film based technologies. Gallium arsenide can be alloyed with indium (In), phosphorous (P), and aluminum (Al) to produce multijunction cell s with very high efficiencies. Amorphous silicon makes up most of the remaining 14% of the PV market. Cadmium telluride is a promising thin film technology. CdTe has a ba nd gap of 1.45 eV and is therefore best suited for the solar spectrum and the material has a very high absorption coefficient, so only a micro meter of the material is enough to absorb 90% of the incident radiation. With time it is thought to be the most promisi ng thin film to meet the cost goals needed for PV. The highest efficiency reported fo r CdTe solar cell is 16.4% by NREL [15] and the University of S.Florida is holding the s econd place with an efficiency of 15.8% [31]. Crystalline Other Thin Film

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8 CIS and its alloys is also a promisi ng thin film material with laboratory efficiencies of 18% and modul e efficiencies greater than 11%. This product is currently on the market and boasts 20+ years of research and development. Another type of thin film is thin film polycrystalline silicon. This is a new technology that is in the experimental stages. Among the “other” category include quantum dot solar cells in which a nanocyrstalline CdSe semiconductor is embedded in the conductive polymer/C60 composite. This has the potential for low-co st, large-area production. Dye-sensitized photochemical cells have a dye sensitizer that absorbs light and generates electron hole pairs in a nanocrystalline titanium dioxide semiconductor layer. Only certain wavelengths can be absorbed but because th e device is clear, research is being conducted to create a clear window that will absorb and co nvert UV light into energy [4]. 1.4 Basic Solar Cell Physics Figure 7. The Current –Voltage (I-V) Charac teristics of a P-N Junction Diode. The Current –Voltage (I-V) characteristic s of a p-n junction diode for different optical stimuli are shown in the figure above. If the p-n junction is operated in the f ourth quadrant, the product of a negative current and a positive voltage will yield a nega tive power. Physically, this corresponds to a source of power. Consequent ly, a p-n junction operated in the four th quadrant can be used as a source of power ; this is the principle behind the solar cell [5].

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9 The current and voltage relationship in a p-n junction diode can be written as 1 exp AkT qV I Io (1) Where I0 is the reverse saturation current and A is an ideality factor that is introduced to modify the theoretical expr ession for use with "Ideal" diodes. Under the presence of an optical stimulus photons are absorbed to create electron hole pairs. Pairs that are generated within a diffusion length of the depletion region will be swept by the built in potential across th e depletion region. Cons equently, when being excited by light, a current is produced due to the optical generation of carriers. Including this generation current ( IL) in equation (1): L oI AkT qV I I ] 1 exp [ (2) Figure 7 demonstrates how op tical excitation affects the I-V characteristic of the solar cell. It should be noted that a larger generation rate corre sponds to a larger generation current, corresponding to a larger downward shift in the I-V. Figure 8 shows an equivalent circuit that may be used to model the behavior of a solar cell. The current generated by the photons is represented by an independent source. The two resistors shown in Figure 8 model two types of the losses in a solar cell. Rseries is a series loss primarily due to resistive losses (cont act resistance and bulk resistance). Figure 8. Equivalent Circuit of a "Real" So lar Cell Showing Both Shunt and Series Resistive Loss [5]. The shunt resistance, Rshunt, is used to model leakage currents. Including the effects of series resistance Rseries and shunt resistance Rshunt in the current equation, the equation transforms as shown in equation 3.

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10 L shunt series series oI R IR V AkT IR V q I I ] 1 ) ( exp [ (3) Two quantities of interest for solar cells are the following: short circuit current ( Isc) and open circuit voltage ( Voc). Expressions for both Isc and Voc can be found from equation (2). The short circui t current is f ound by setting V = 0 in equation (2). This results in the following expression for Isc: L scI I (4) Similarly the open circuit volta ge can be found by letting I = 0 in equation (2) and solve for V : 1 lno L ocI I q AkT V (5) Under short circuit conditions, the phot o generated charges flow through the external circuit and no charge build up is produced, hence no voltage is developed. The photo generation current is dire ctly proportional to the intensity of the sunlight. Under direct sun conditions, the Isc is at its maximum. Under open circuit conditions there is a charge buildup on each side creating a diode current. The device reaches equilibrium when the diode current is equal and opposite to the photo generation current. In or der for the diode current, otherwise referred to as the internal or injection current, the in ternal potential barrier (voltage) is lowered. This further demonstrates the forward bias characteristics. Figure 9 below demonstrates the short circuit condition and Figur e 10 open circuit condition [4].

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11 Figure 9. Solar Cell Under Short Circuit Condition. Figure 10. Solar Cell Under Open Circuit Condition. Ideally the Rshunt of a solar cell needs to be infinite and the Rseries be zero but real devices have Rshunt in the order of se veral thousand ohms and Rseries in the order of 1-2

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12 A method to find the series resistance used by B.E.McCandless, et al., when Rshunt can be neglected, is as follows and is derived from plotting dV/dJ against 1/J ,where the intercept through the y-axis yi elds the series resistance a nd the slope provides the diode factor. Here J is the current density (Amperes/cm2), which can be substituted for I in the above equations when the area of the de vice is taken into account. Substituting J for all I in equation (3) and treating Rshunt (shunt resistance of the de vice) as negligible, we get AkT JR V q J J Jseries o L) ( exp 1 (7) Taking the natural log on both sides AkT JR V q J J Jseries o L) ( 1 ln (8) Solving for V in the above equation, 1 lno L seriesJ J J q AkT IR V (9) Differentiating V with respect to J we get L seriesJ J q AkT R dJ dV 1 (10) For a solar cell which is not under illumi nation (or in the da rk) in the above equation, we could simply remove JL(light generated current). J q AkT R dJ dVseries1 (11) The above equation is in the slope inter cept form:y=b+mx. So, from a plot of dV/dJ Vs 1/J, the yintercept provides Rseries and from the slope th e diode factor can be obtained. However, an approximation of series resi stance can be obtained from the slope of a J-V curve at high current values in the first quadrant; dV/dJ when J and similarly, the Rshunt can also be ob tained from the third quadrant, dV/dJ at large reverse bias. If the forward J-V characteristics of a devi ce are influenced by barriers, then Rseries cannot be calculated reliably by this method and if the reverse bias is a ffected by collection, then Rshunt cannot be found out reliably.

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13 Another characteristic propert y of solar cells of interest is the fill factor, FF, which is defined as: oc sc oc sc m mV I P V I V I FFmax (12) where Im ,Vm are the current and Voltage associated with maximum power generation. It is a measure of quality of the cell. The efficiency is a measure of the maximum power over the input power. % 100max inP P, where (13) Pin is the power due to the photons incident on the cell, given by: d c h N Ephotons (14) where Nphotons is the number of photons incident on the surface of the solar cell, h is Planck’s constant, c is the velocity of light and is the wavelength associated with the photons. The energy of the sun is created by nucle ar fusion reaction of hydrogen to helium. The gaseous surface of the sun radiates lik e a black body at 5900K.The radiation power density outside the atmosphere is known as th e solar constant and it has a value of 1.353 kW/m2. This radiation density at sea level is weakened by absorption and scattering in the atmosphere. In the infrared region there is absorption due to wate r and carbon dioxide in the air, in the visible region absorption is caused mainly by oxygen, in the ultraviolet spectral range there is interaction with the oz one in the air. AM 0 or AM 1.5 defined in the figure 11 are standards defining specific solar radiation conditions. AM (air mass) indicates the amount of air, the radiation passes through the atmosphere. AM 0 defines the intensity of extraterrestrial solar radiatio n (in the space), AM 1 de fines the intensity of radiation incident vertic ally at equator and AM 1.5 define s the intensity of sunlight with the sun is positioned at an angle of 41.8 above the horizon. For terrestrial photovoltaics the standard to determine the efficiency of solar cells is AM1.5 global.

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14 Figure 11. Spectral Distribution of Solar Radiation Intensity. Figure 12. The Fill Factor The efficiency can then be rewritten as: EA FFI Vsc ocor simplysc ocFFJ V (15) In figure12, fourth quadr ant of the IV-characteris tics of a solar cell under illumination is depicted with the points where Voltage and Current are maximum Vm, Im respectively [4]. The fill factor is the ratio of the area of maximum power rectangle to the area covered by Voc*Isc. Max power rectangle

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15 CHAPTER 2 TRANSPARENT CONDUCTIVE OXIDES In this chapter the properties of transp arent conductive oxides (TCO) and a brief discussion of research on TCO s is reviewed. 2.1 Transparent Conductive Oxides A TCO is a wide band gap semiconductor, and as the name implies has an inherent relative transparency in the visible region and it is highly conductive due to the presence of relatively higher co ncentration of free electrons in its conduction band. These arise either from defects in the material or from extrinsic dopants, the impurity levels of which lie near the conduction band edge. The high electron carrier concentration can cause absorption of radiation in both the visi ble and infra-red portions of the spectrum. A TCO is a compromise between electrical c onductivity and optical transmittance, a careful balance between the properties is required. 2.1.1 Optical Properties Photons incident on TCOs are absorbed, transmitted or reflected. A TCO absorbs some portion of the light incident upon it, corresponding to its band gap. TCOs with band gap less than 3.0eV tend to appear yellow-gr een, which corresponds to a wavelength of about 400nm. Such coloration is a function of an electron transition from the valence band to conduction band that is the band gap. These absorptions are a function of both the band gap and the thickness of the material. Th e relation can be described in terms of the absorption coefficient which is defined as 2 / 1) ( ) (d gE h C h (16) where hcorresponds to the energy of the photon d gEis the direct band gap of the material and C is essentially a constant. A plot of 2 as a function of hyields a straight line for a direct absorption, with an intercep t on the photon energy axis equal to the direct band gap of the material [32].

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16 at a particular wavelength can also be expressed in terms of thickness of the material t and absorption A at that particular wavelength as follows. ) ( 1 1 ln 1 ) ( A t (17) For a TCO to be suitable for applications in solar cells, it is desirable that it has large band gap, greater than 3.0 eV. Figure 13. Possible Photon Interactions with a TCO. The total transmission in a TCO is given as Total Specular DiffuseT T T % % % (18) where specular and diffuse transmission are defined in figure 13. Absorption is associated with the band ga p. Since TCOs are wide band gap semi conductors, the energy corresponding to this ab sorption is large; ty pically in the ultra violet region, or in the near UV region. Ge nerally transmission, both specular and diffuse occurs in the visible region. Below the band gap of the TCO, the radiation is typically transmitted with the only “losses” being prim arily due to reflection from the surface of the TCO. The portion of the solar spectrum of wa velengths from 300 to 1000nm is most rucial to the characterization of TCOs a nd also critical to the photovoltaic device performance.

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17 Figure 14. Photon-TCO Interaction Types by Characteristic Wavelength Region. Reflection of a photon, in a TCO can take place in two ways. One is that the photon can be reflected from the surface of the material; the second is that the photon can be reflected from the bulk of the material. Th e greatest level of re flection occurs in the near infra red region. This re flection is dominated by bulk refl ection that occurs as result of photon-electron interactions which induce scattering. Th e electrons and photons react due to the electro-magnetic nature of the phot on. The point at which this takes place is termed as the plasma frequency. The word pl asma is used because the electrons behave as a gas in highly doped TCOs (as they do in metals). The reflection in the infra red is highly dependent on carrier concentration. Heavily doped materials have increased amount of reflection in this region. 2.1.2 Electrical Properties Most of the semiconductor oxide film s have n-type conductivity. The high conductivity of these film s mainly results from stoichiome tric deviations. The TCOs are also heavily doped to make them more conductive [6]. A TCO must necessarily represent a compromise between electrical conductivity and optical transmittance, a careful balance between the properties is required. Since conductivity is given as = e N ,where is the mobility of the carriers in a TCO e is electronic charge and N is the carrier concentration .This gives one two ways to increase the co nductivity of a material.1.by increasing carrier concentration(N) and 2.by increasing the mobility ( ). Increasing the

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18 former also leads to an increase in free carrier absorption. Increasing the mobility does not however have any delirious effects and is therefore the focus of most research. Having emphasized the importance of the electrical properties of a TCO in the preceding paragraph, methods to find the resist ivity, carrier concentration and mobility of TCOs are discussed below. Electric current in a conductor is define d as how much charge passes through an arbitrary cross section of a conductor in a specific am ount of time. This can be represented mathematically as I = dt dq (19) Where I is the current in amperes, q is the charge in coulombs and t is time in seconds. The current density ,J can be expressed as the ratio of current to area. J = A I (20) If an electric field E is applied to the material, an electric current will flow whose density will be given by J= E (21) Where is called the electrical conductivity of the mate rial (discussed in the previous paragraph). The reciprocal of c onductivity is known as electrical resistivity For a rectangular shaped sample as shown in the figure 15, the resistance R is given as wt l R (22) Where l is the length, w is the width and t is the thickness of the sample. Figure 15. Sample Dimensions for De finition of Sheet Resistance. Thickness t Length l of the sample Width w

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19 When length and breadth are equal, that is l = w, the resistance of the sample becomes t R = Sheet resistance (23) The quantity sheet resistance is the resi stance of one square of the film and independent of the size of the square. The sh eet resistance is expressed as ohms/square. The most commonly used met hod to find the sheet resistan ce is a four point probe technique, first proposed by L.B.Valdes in 1954[6]. A typical schema tic set up is shown in Figure 16. Figure 16. Schematic of the Four Point Probe Set Up. When the probes are placed on a material of semi infinite volume, the resistivity is given by 3 2 2 1 2 11 1 1 1 2d d d d d d I V (24) When the probes are equispaced, that is d1=d2=d3=d, then I d V 2 (25) If the material is in the form of an infi nitely thin film resting on an insulating support, then

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20 2 lnt I V (26) /ln2 is the correction factor. So if the thickness of the film were known, the resistivity of the film can be calculated by calculating the sheet resistance of the film using a simple four probe set up, using the following equation I V Rsheet532 4 (27) where the correction factor /ln2 = 4.532 and subsequently multiplying sheet resistance with the thickness of the film, since t Rsheet (28) Under the influence of an electric field, the electrons begin to move in a specific direction and such directional motion is termed drift. The average velocity of this motion is known as the drift velocity Vd If N is the number density of electrons the current density is given by J = N e Vd. (29) where e is the electron charge. Combining equations (21) and (29) Vd ={ /Ne}E (30) In the above equation the proportiona lity factor is called Mobility of charge carriers. = /Ne (31) The charge carrier mobility is related to the effective mass of the charge carriers (m*) and the relaxation time ( ) according to = e /m* (32) In order to determine the conductivity type of the sample (whether the sample is n-type or p-type), Hall Effect measurement is performed. It also allows determination of density of charge carriers and their mobility values. When a current is passed through a slab of material in the presence of a transverse magnetic field a small potential difference, known as the hall voltage, is developed (between face 1 and face 2), in a direction perpendicular to both the current

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21 and the magnetic field. Face 1 is positive for p-type samples, whereas it is negative for ntype samples, mathematically it is given by VH = RH I(B/t) (33) Figure 17. The Hall Effect Phenomenon. where VH is the Hall voltage, B is the magnetic field and I is the current through the sample. RH is the Hall coefficient and is relate d to the carrier density according to the relation RH=rH(1/Ne) (34) where rH is the Hall scattering f actor. The value of it depends on the geometry of the scattering surface and the mechanisms by which th e carriers are scattered. In general, it dopes not vary significantly from unity. For n-type semiconductors, RH is negative, while for p-type it is positive. Hall effect measurements in conjunction with the measurement of conductivity, enable the calculation of the mobility of charge carriers [6]. From equations (31) and (34) Mobility = RH (35) 2.2 Electrical Conduction in Polycrystalline Thin Films In polycrystalline thin films, the conduction mechanism is dominated by the inherent inter crystalline boundaries (grain boundaries) rath er than intra crystalline characteristics. These boundaries generally cont ain fairly high densit ies of interface states which trap free carriers from the bulk of the grain and scatter free carriers by virtue of inherent disorders and the presence of trappe d charges. The interface states result in a space charge region in the grain boundaries. Du e to this space charge region, band bending occurs resulting in potential barriers to charge transport. The most commonly Face2 Face1 Direction of Current B

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22 used model to explain the transport phenomenon in poly crystalline films is the Petritz model. According to this model, the cu rrent density is given by the relation J = E kT e n egb o exp (36) Where o = (M/ nckT ), gb is the height of the potential barrier, nc is the number of crystallites per unit leng th along the film, and M is a factor that is barrier dependent. The grain boundary potential barrier gb is related to Ng and Ngb, the number of carriers in the grain and the grain boundary, respectively by gb g gbN N kT ln (37) The prefactor in the (36) is the conductivity of the ch arge carriers dominated by grain boundaries g Thus the grain boundary limited mobility can be written as = KT eb exp (38) An alternative equation for g, is kT e kT m elb gb exp ) 2 (2 1 (39) Where l’ is the average width of the grains. In order that conduction of any sort takes place a certain amount of energy needs to be given to the system, which is called conductivity activation energy E. gb ne E E where En is the carrier activation energy. 2.3 TCOs in General As discussed in the previous section electrical conductivity depends on the concentration (N) and mobility of relevant free carrier. In order to obtain films with high conductivity, high carrier concentration and mobility should be simultaneously realized. The electrical prope rties of the oxides depend crit ically upon the oxidation state of the metal component, stoichiometry of th e oxide and on the nature and quantity of impurities incorporated in the films, either intentionally or unintentionally.

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23 Stoichiometric oxides are insulators, i.e., ZnO, SnO2, In2O3, etc, due to their large band gaps. They can be made highly conducti ve by doping (incorporation of impurity). For example, in ZnO, Zinc atom contributes two electrons to the formation of the bond with oxygen. When ZnO is aluminum doped, aluminum replaces some of the Zinc. But when oxidized aluminum provides three elec trons, one more than Zinc, this electron would disrupt the electro static distribution of the lattice; th is electron is not “free” to appreciably move about the la ttice. It is unstable and ca n be easily elevated to the conduction band with the external application of energy. The most commonly used TCOs SnO2 and In2O3 are doped with Fluorine and Tin respectively. Fluorine doping gives superior performance compared with metallic dopants, in TCOs. A theoretic al understanding of this a dvantage of fluorine can be obtained by considering that the conduction band of oxide semiconductors is derived mainly from metal orbitals. If a metal dopant is used, it is electrically active when it substitutes for the primary metal (such as zi nc or tin). The conduction band thus receives a strong perturbation from each metal dopant, the scattering of conduction electrons is enhanced, and the mobility and conductivity are decreased. In contrast, when fluorine substitutes for oxygen, the electronic perturbatio n is largely confined to the filled valence band, and the scattering of conduction electro ns is minimized [7]. Doping essentially increases conductivity by increasing carrier concentration but very high doping would lead to a decrease in mobility due to variou s scattering mechanisms like ionized impurity scattering, neutral impurity scattering etc. The conducti on in heavily doped polycrystalline thin films is largely limited by dominant scattering mechanisms in addition to grain boundaries. Additionally, introduction of point defects or oxygen vacancies in the lattice of the oxides can increase th e conductivity of a TCO. In an oxide, oxygen vacancies produce donor sites. This is done by the re duction of the cation to which the oxygen atoms were bound, thus creating a dangling bo nd— an electron pair. These electrons, upon receipt of a specific amount of energy (the ac tivation energy fo r the defect) can enter the conduction band.

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24 The oxygen vacancies act as doubly ionized donors and contribute at a maximum two electrons to the electr ical conductivity as shown in the following equation: OO 0.5 O2(g) + VO (40) Where OO represents bonded oxygen; VO represents an oxygen vacancy with being the electron pair. TCOs can have oxygen vacancies introdu ced in two ways. The first is by synthesizing the material with a deficiency of O2 as one of the reactants or having a reducing ambient while synthesi zing. Alternatively, the compound can be subjected to a reducing post synthetic ambient, such as H2, often at elevated temperatures. Ternary TCOs like (Zn2SnO4, Cd2SnO4, and CdIn2O4) have intrinsic n-type conductivity due to a pr ocess of self doping by forcing one cation to assume the lattice site of the other cation type. For example in the invers e spinel structure of Cd2SnO4, half of the Cd occupies the tetrahedral sites and the other half occupies the octahedral sites, whereas Sn occupies only octahedral sites. The oxygen is fourfold coordinated. Some of the Sn atoms replace the cadmium atoms in the octahedral sites, termed as Sn on cadmium anti site. Due to the two extra electrons of Sn, the latti ce is disturbed leading to intrinsic n-type conductivity [8]. The Electronic properties of the materials will be discussed in the next chapter.

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25 CHAPTER 3 MATERIAL PROPERTIES A ND LITERATURE REVIEW 3.1 Electronic Properties of SnO2 and ITO Due to excellent optical a nd electrical properties, SnO2 is the most commonly used TCO for CdTe solar cells. SnO2 is an n-type, wide-band-gap semiconductor with the tetragonal rutile (TiO2) structure. The electr ical conductivity of SnO2 results primarily from the existence of oxygen vacancies, whic h act as donors. The Sn atoms are on a body centered tetragonal lattice and oxygen atoms in a hexagonal closed packed structure as shown in figure 18 [9,10]. Figure 18. Structure of Tin Oxide. Commercially available SnO2 thin films typically ar e deposited by atmospheric pressure chemical-vapor deposition (APCVD) using tin tetrachloride (TTC).Researchers have demonstrated higher-quality film results when TTC is replaced with tetramethyltin (TMT). The temperature window for SnO2 film growth is betw een 500 and 700C. The film is well crystallized at 500C. Intrinsic SnO2 films are randomly oriented, whereas Fdoped films exhibit a strong (200) preferred orientation.

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26 As the growth temperature increases, gr ain size increase, he nce surface roughness also increases. Without F-doping, the film resistivity is ~1 -cm. With F-doping, the film resistivity is ~5x10-4 -cm. Fluorine doping not only increases the carrier concentration, but also increases the electron mobility ( ) of the film. This observation is c ontrary to what is expected from ionized impurity scattering. For undoped SnO2 films, is ~1 cm2V-1s-1 and electron concentration is in the low-10 18 cm-3 range. Assuming O2 vacancies are dominant donors in undoped SnO2, and each oxygen vacancy contributes two electrons to SnO2, for a carrier concentration of low 1018 cm-3, the oxygen vacancy concentration should be about high-1017 cm-3. For F doped SnO2 films, the electron concentration increases to mid-1020 cm-3, which indicates the F ion concen tration should also be mid-1020 cm-3. Although the ion concentration increases significantly for F doping, the does not decrease, but increases from ~1 cm2V-1s-1 to 40 cm2V-1s-1. This observation indicates that the ion scattering is not a dominant scattering mechanism for SnO2: F film. Grain boundary contribution to the film resistivity is neglig ibly small for the films with thickness above 350 nm and carrier concentration above 2x1020cm-3. Generally, for SnO2 films that are ~1 m thick, the average transmission is >80% in the visible spectral range. F-doped films have higher absorption than th e undoped films. For undoped SnO2 films, the optical absorption (average between 500 and 900nm) is ~600 cm-1 compared to ~1200 cm-1 for F-doped film [11]. In2O3 is an n-type transparent conductive oxide crystallizes in the cubic bixbyte structure, which is similar to the fluorite structure, but one fourth of the anions are vacant allowing for small shifts of the ions. In2O3 has two non-equivalent six-fold coordinated cation sites. Figure 18 shows the two cation sites, which are referred to as equipoints "b" and "d". The b site cations have six eq uidistant oxygen anion ne ighbors, which lie approximately at the corners of a cube with two anion structural vacancies along one body diagonal. The d site cations are coordina ted to six oxygen anions at three different distances, which lie near the corners of a dist orted cube with two em pty anions along one face diagonal [19].

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27 As prepared, In2O3 generally lacks stoichiometry due to oxygen-array vacancies. At high O2 vacancies, a vacancy band forms and overlaps Ec at the bottom of the conduction band producing a degenerate semiconductor. The O2 vacancies act as doubly ionized donors and contribute at a maximum tw o electrons to the el ectrical conductivity [17]. Indium Tin Oxide is essentially formed by subsititutional doping of In2O3 with Sn which replaces the In3+ atoms from the cubic bixbyte structure of In2O3. Tin acts as a cationic dopant in the In2O3 lattice and substitutes the indium. In In2O3, since indium has a valence of three, the tin doping results in n doping of the lattice by providing an electron to the conduction band. The ITO films are extremely conductive and conductivities as low as 1.5 X10-4 -cm have been achieved for carrier concentrations of the order of 1021 cm-3. The average transmittance in the visible spectrum ranges from 85% to 90%. The band gap of ITO has a wi de range from 3.6 eV to 4.0eV. It is important to note that at high dopant concentrations, the obser ved carrier concentration of ITO films is lower than that expected assu ming that every soluble tin atom contributes one free electron. This implies that a portion of the tin remains electrically inactive. According to the Frank and Kostlin model, formation of the following two neutral defects compensates for the Sn donor. Two Sn4+ ions which are not on nearest neighbor positions loosely bound to an interstitial oxygen anion. This interstitia l defect dissociates on annealing under reducing conditions( Sn2Oi” 2Sn+2e+1/2O2(g)).The tin pair substituting two neighboring indium atoms st rongly binds additional oxygen, forming a neutral complex which consists of the two Sn4+ and the additional oxygen, that is, Sn2O4[17]. d site cation (24) b site cation ( 8 ) anion vacancy (16) lattice anion ( 48 ) Figure 19. Structure of ITO.

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28 The mobility of ITO films decreases as carrier concentration increases due scattering by neutral and ionized impurity sca ttering, with ionized impurity scattering dominating. 3.2 The Effect of SnO2 and ITO Front Contacts on CdS/CdTe Solar Cells Thin film CdS/CdTe solar cells fabricated on SnO2-coated glass have been found to have higher efficiencies than similar cells produced on indium–tin oxide (ITO) coated glass substrates. Further investigation implicated the CdS/ITO interface as the cause of the problem; current-voltage measurements re vealed that on heating in air the CdS/ITO junction became rectifying. This is consiste nt with recently reported findings which show that the work function of ITO is raised above that of CdS by oxidative treatments (such as heating in air) which would make the ITO/ CdS junction rectifying rather than ohmic. The cells used for study were fabricated in an identical fashion. The basic structure of all three cells was the same : ~200 nm CdS deposited by physical vapor deposition (PVD) onto the TCO (SnO2 and ITO), followed by ~5.5 m CdTe deposited by close spaced sublimation at source and substrate temperatures of 600 C and 500 C respectively. The cells were then treated with CdCl2 (known to improve the performance significantly by depositing ~150 nm CdCl2 by PVD and then heated in air at 400 C for 30 min in the case of a) SnO2 b) ITO30 and 15 min in the case of ITO15. After thorough rinsing in water to remove any remaining CdCl2, they were etched in 0.03% Br–methanol for 10 to 15 seconds before gold contacts were deposited. The figure 20 shows the photovoltaic out put characteristics of three cells measured under AM1.5 illumination at room temperature. The characteristics labeled ITO30 and ITO15 were recorded from cells fo rmed on indium–tin oxide coated glass; that labeled SnO2 was from a cell on tin oxide coated glass. Clearly the superiority of SnO2 as TCO is evident. The photovoltaic parameters (short circuit current density, Jsc, open circuit voltage, Voc, fill factor, FF, and conversion efficiency, series resistance, Rs, and shunt resistance, Rsh ) of the three cells are listed in table 2.

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29 Figure 20.The J-V Characteristics of SnO2, ITO Cells. Table 2. Photovoltaic Parameters of SnO2 and ITO CdS/CdTe Solar Cells. TCO Sheet R[ /sq] Voc [mV] Jsc [mA/cm2]FF[%] [%] Rs[ -cm2] Rsh[ -cm2] SnO2 17 684 27.3 54.4 10.1 6 196 ITO15 7 714 22.6 44.9 7.3 54.1 171 ITO30 7 623 20.2 36.7 4.6 34 82 The saturation of IV char acteristics of ITO cells for V > Voc suggests the presence of some kind of additional series diode component such a rectifying barrier at one of the contacts. Generally th is barrier is assumed to be at the back contact since the electron affinity of CdTe is approximately 4.5 eV and the distance between the conduction band and the fermi leve l, in the case of p-CdTe, can be estimated to be 1.381.45 eV. Thus the value of the work function of CdTe is about 5.9eV. Due to this high work function of CdTe (=5.9 eV), it is very difficu lt to avoid the formation of a Schottky contact. Typically CdTe contacts are mostly tunneling. But from the IV characteristics it can be inferred that any su ch barrier could be present at the CdS/TCO interface since all the cells have the same back contact and the fact that the characteristics

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30 of the cells become worse with incr ease in annealing times after the CdCl2 deposition, indicates that the effect may be associated with heating. To investigate the effects of heating on the TCO/CdS inte rface, a number of SnO2/CdS/In and ITO/CdS/In samples were prepared. Indium was used as the back contact as it is known to make a good ohmic contact to CdS. Room temp erature current-voltage (IV ) characteristics were measured in the dark for both asdeposited devices and after heating in air for 30 min at 400 C, i.e. mimicking the CdCl2 treatment. The results are shown in figure 21. (The saturation at approximately 3.8 mA was a consequence of the cu rrent limit on the instrumentation.). Figure 21. The Dark IV Characteristics of SnO2/CdS/In in the Forward and Reverse bias (a) Before Annealing (b) After Annealing. In the figure above the dark IV characteristics of SnO2/CdS/In in the forward and reverse bias, before annealing (a) and af ter annealing(b) indicate that the SnO2/CdS contact was ohmic and remained ohmic even after annealing with an exception of a small increase in series resistance from 0.62 to 1.3 cm2. This suggests that any

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31 interaction between the CdS and SnO2 was probably of minimal significance for the performance of the cell. Figure 22. The Dark IV Characteristic s of ITO/CdS/In in the Forward and Reverse bias (a) Before Annealing (b) After Annealing. In figure 22 the dark I-V characteristics of ITO/CdS/In in the forward and reverse bias before anneali ng (a) and after ann ealing (b) indicates that the ITO/CDS interface does not remain ohmic but becomes rectifying after annealing. Assuming that the contact between the In and CdS layer was ohmic, since that was common to both the SnO2/CdS/In and the ITO/CdS/In devices, then th e rectifying effect which appeared after heating of the samples would seem to be due to the contact between the ITO and CdS.

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32 Hence, annealing the CdS/ITO interface for 30 min at 400 C in air, as used in the CdCl2 treatment clearly resulted in some interaction, which impeded the flow of charge. Figure 23. The Dark IV Characteristics of ITO/CdS/In at Different Lengths of Annealing Times. To determine at what time of annealing does the interface becomes rectifying, the dark IVs of ITO/CdS/In at different ann ealing times of 10, 20 and 30 minutes were measured and the results obtained are shown in figure 23. It is clear from the figure above that the ITO/CdS interface started to become rectifying within 10 minut es heating time at 400 C, and had become fully rectifying after ~20 minutes. Since In is a donor impurity in CdS, cro ss diffusion of In from the ITO into the CdS would act to increa se its conductivity.

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33 Generally, this might be expected to im prove the injection of carriers across the ITO/CdS interface, reducing the contact resistance and rendering the contact even more ohmic, i.e. the reverse of what was obs erved. Another recent study of the oxidativereductive behavior of ITO surfaces us ing x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy has in dicated that the work fu nction is dependent on the surface oxygen concentration. In particular the UPS measured work function of asreceived ITO was found to be low (~ 4 eV) as a consequence of adventitious contamination, but increased substantially (5 eV) following various surface treatments. Importantly, oxidative treatments raised th e work function of the surface [21]. The annealing treatments carried out in the pres ent study were all undertaken in air and, assuming that oxygen was able to diffuse through the thin CdS layer, may be presumed to be oxidative. In that case, the UPS studies w ould imply an increase in the work function. However, since CdS is a n-type semiconductor, contact materials should have a low work function, ideally less than the electron affinity if they are to be injecting. The electron affinity of CdS is 4.5 eV, greater than as-rece ived ITO, but less than fully oxidized (i.e. heat treated) ITO. The implication is that the heat treatment changes the ITO from an injecting (ohmic) to a rectifying (Schott ky barrier) contact by ra ising the ITO work function through surface oxidation. The observati ons presented here strongly suggest that ITO is not well suited to us e in CdS/CdTe thin film solar cells. Although the sheet resistance of ITO coated glass is generally less than corresponding SnO2 coated glass, ITO is not as stable to subsequent processing steps as SnO2. Moreover, it would appear that in its ‘clean’ state, i.e. the more c ontrolled surface, the high work function of ITO makes it unsuitable in principle as an ohmic contact to n-CdS [20]. 3.3 Properties of Cd2SnO4 and Zn2SnO4 The spinel structure has the form of AB2O4, where A is a metal ion with a +2 valence and B is a metal ion with a +3 valence or +4 valency. This structure is viewed as a combination of the rock salt and zinc-b lend structures .The oxygen ions are in facecentered cubic close packing. A and B ions oc cupy tetrahedral and octahedral interstitial sites.

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34 Spinel structure can be divided into tw o types: normal and inverse spinel. In normal spinels, A2+ ions are on tetrahed ral sites and the B3+(or higher example B4+) ions are on octahedral sites. In inverse spinels, the A2+ ions and half the B3+ ions are on octahedral sites; the other half of the B3+ are on tetrahedral sties, B(AB)O4as shown in figure 24 [22,23]. Figure 24. The Spinel Structure. Cd2SnO4(CTO) ,Zn2SnO4 (ZTO) are TCOs with inve rse spinel structure with exceptional electrical and optical properties.Zn2SnO4 (ZTO)(inverse spinel), is superior to CTO in optical properties but has poor electrical properties. By optimizing the sputtering method and pot annealing conditions of CTO films, electr on mobilities as high as 65cm2V-1s-1 have been achieved for a carrier concentration of 2X1020cm-3. Even at a high carrier concentration of ~9x1020cm-3, mobilities as high as 55 cm2V-1s-1 have been observed. These mobilities are 2-3 times higher than those of commercial SnO2 films doped to similar levels.

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35 Free carriers in CTO films are thought to result from oxygen deficiencies in the film accommodated either as oxygen vacancies or cadmium interstitial s or a combination of both. This results in re sistivities as low as 1.3x10-4 -cm. Figure 25. Transmission and Absorbance Spectra of CTO and SnO2 Thin Films. Figure 26. Relationship Between Jsc and Transmission of SnO2 and CTO Films. This is almost seven times lower than conventional TTC based SnO2 films and about 2.5 times lower than TMT based SnO2 films with resistivities of ~3.3X10-4 -cm. Cd2SnO4 films have significantly better opti cal properties than conventional SnO2 films. The absorbance of the CTO film, in the visibl e range, is much smaller than that of the SnO2 film of the same sheet resistance. For example, at 600 nm, the absorbance of the CTO film is only 0.6%, compared to 12.2% of the SnO2 film. This appears to be due to the high electron mobility of CTO films.

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36 In CdTe cells replacing the 10000 SnO2 film with a 2500 CTO film yielded an increase in Jsc of more than 1.5 mA/cm2. Zn2SnO4 has better optical properties than CTO but low electrical conductivity and mobility due to its structure [12]. Polycrystalline ZTO thin films form in the inverse spinel crys tal structure. However, the spinel lattice is locally distorted enough to form two distinct octahedrally c oordinated Sn and Zn sites. The disorder in the Sn and Zn sites in the lattice significantly limits the mobility of the carriers, possibly by disrupting the edge-sharing nature of the octahedrally coordinated cations [14]. Single-phase, spinel zinc stannate (Zn2SnO4) thin films were grown by rf magnetron sputtering onto glass substrates. The as deposited films were amorphous but subsequent annealing at 600 C gave polycrystalline uniaxially oriented films with resistivities of 10-2-10-3 -cm, mobilities of 16–26 cm2/V s, and n-type carrier concentrations in the low 1019 cm3 range were achieved. X-ray diffraction peak intensity studies established the films to be in the inverse spinel configuration. A pronounced Burstein–Moss shift moved the optical band gap from 3.35 to as high as 3.89 eV. Figure27 shows transmittance, reflectance, and absorptance for wavelengths between 300 and 2500 nm for a ZTO film. The absorptan ce is less than 1.5% over the visible spectrum. The slight rise in the absorptance curve near 2300 nm is due to the onset of absorption by free electrons in the conduction band [13]. Figure 28 give s the shift in the band gap of ZTO films with incr ease in carrier concentration. Figure 27. Transmittance, Reflectance, a nd Absorption for Wavelengths Between 300 and 2500 nm for a ZTO Film.

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37 Figure 28. Shift in the Band Gap of ZTO Films with Increase in Carrier Concentration. 3.4 The Effect of Cd2SnO4 and Zn2SnO4 on CdS/CdTe Solar Cells The solar cell structure that was used to find the effect of both CTO and ZTO layers is as follows. Figure 29. The Structure of CdS/CdTe Solar Cell to Find the Effect of Both CTO and ZTO Layers is as Follows. In this study, it was found that the interdiffusion of the CdS and Zn2SnO4 ~ZTO layers can occur either at high temperature (550– 650 C) in Ar or at lower temperature (400– 420 C) in a CdCl2 atmosphere. By integrating a Zn2SnO4 film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both SnO2-based and Cd2SnO4-based CdTe cells. Glass Substrate Cd 2 SnO 4 or SnO 2 Zn 2 SnO 4 CdS CdTe Back Contac t Front Contac t

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38 Interdiffusion consumes the CdS film fr om both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn sour ce to alloy with the CdS film, which results in increases in the band gap of the window layer and in shortcircuit current density Jsc. Interdiffusion can also significantly improve device adhesion after CdCl2 treatment, thus providing much greater process latitude when optimizing the CdCl2 process step. The optimum CdCl2-treated CdTe device has high quantum efficiency at long wavelength, because of its good junction properties and we ll-passivated CdTe film. Figure 30. Relative Quantum Efficienci es of CTO/CdS/CdTe Solar Cell and CTO/ZTO/CdS/CdTe Solar Cells. The figure above gives the relative quantum efficiencies of CTO/CdS/CdTe solar cell and CTO/ZTO/CdS/CdTe solar cells. Th e table below gives a summary of the parameters of the solar cell with different TCOs [16]. Table 3. Summary of the Parameters of th e Solar Cell with Different TCOs [16]. Device structure Voc [mV] Jsc [mA/cm2] FF[%] [%] SnO2/CdS/CdTe 806.7 22.61 74.02 13.5 CTO/CdS/CdTe 805.2 23.53 73.77 14.0 SnO2/ZTO/CdS/CdTe 830.1 24.10 74.15 14.8 CTO/ZTO/CdS/CdTe 844.3 25.00 74.82 15.8

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39 The highest efficiency for CdTe solar cell was achieved by NREL by reduction of photocurrent loss in the TCO( by using CTO, reduced the loss in Jsc to 0.62 mA/cm2) ,reduction of photocurrent loss in CdS (by using ZTO from which interdiffusion of Zn occurs and CdS gets consumed from both ends ,reduced Jsc loss to 1 to 1.3 mA/cm2)and photocurrent loss in recombination cent ers in CdTe (by optimizing the CdCl2 treatment with the help of ZTO film, up to 0.5 mA/cm2).The table below gives the Jsc losses in different TCOs [15]. Table 4. Jsc Losses in Different TCOs. TCO Rs( /sq) Jsc loss due to TCO absorption SnO2 (TTC) 8-10 2.8 SnO2(TMT) 7-8 1.3 CTO 7-8 0.62 CTO/ZTO 7-8/~105-106 0.68 The parameters of the highest efficiency CTO/ZTO/CdS/CdTe solar cell achieved by NREL are as follows. Table 5.The Parameters of Highest E fficiency CTO/ZTO/CdS/CdTe Solar Cell. Cell# Voc [mV] Jsc [mA/cm2] FF[%] [%] Area (cm2) W547-A 847.5 25.86 74.45 16.4 1.131 W567-A 845.0 25.88 75.51 16.5 1.032

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40 CHAPTER 4 EXPERIMENTAL METHODS 4.1 Material Deposition 4.1.1 Chemical Vapor Deposition Process The substrates used were Corning 7059 borosilicate glass (1.25” X 1.45” X 0.032”). These substrates we re cleaned in a 10 % by volume HF solution followed by a thorough rinse in deionized water. They we re subsequently dried with compressed nitrogen. Primarily SnO2 was deposited on the substrates by chemical vapor deposition technique. The reactor consiste d of a quartz tube which was heated by RF coils to above 450C. The organometallic tin source used wa s TMT (Tetra Methyl Tin) and ultra high purity Helium was used as the carrier gas, and Oxygen served as the oxygen source. The Fluorine dopant used was the halocarbon 13B1. The SnO2 was deposited in a bilayer form for the standard cell, the first layer being Fluorine doped and the next layer being intrinsic. All the gases are introduced into the reactor via MFC (Mass Flow Controller).The oxidation reduction chemical reaction for this process is as follows. Sn(CH3)4 (Vap) + 14O2 (gas) SnO2 (solid) + 4CO2 (gas) + 6H2O (gas) (41) A simplified schematic of the deposit ion system is presented below. Figure 31. Simplified Sche matic of MOCVD Set-up.

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414.1.2 Sputtering SnO2 was also deposited by RF magnetron sputtering. SnO2 was reactively sputtered from a 99.99% pure Sn (metal) target in 25% O2 in an Argon environment. In2O3 was also reactively sputtered from 99.99% pure Indium target in 25% O2 in Argon. Indium Oxide doped with Tin (ITO) was sputtered from a 90% In2O3:10% SnO2, ITO target in pure Argon. Cadmium stannate was co-sputtere d from 99.999% pure CdO and SnO2 targets. Cadmium Indium Oxide was co-sputtered reactiv ely from a Cd and In metallic targets (99.999%) in 25% O2 with Argon. Zinc stannate was prepared by co-sputtering from ZnO and SnO2 (99.99%) targets. The vacuum chamber was a Consolidated Vacuum Corp. model; the sputtering guns were Kurt J. Lesker Torus TRS3FSA models; an d Advanced Energy RFX-600 power supplies supplied the RF power. The s puttering sources were positioned such that the center line of each source created an an gle of approximately forty degrees with respect to a plane parallel to the substrate holder. The substrate holder was rotated to ensure uniform film thickness. The total pressure in the chamber was main tained at 3.0 mT in all cases. Before each deposition, the chamber was pumped down to a background pressure of approximately in the low 10-6 Torr range. Figure 32. Position of Sputtering Sources and Substrate Holder.

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42 Annealing experiments were carried out in a simple quartz tube which has a vacuum outlet and gas inlet in which the sa mples were placed on graphite holders and heated by two halogen lamps(1000 W)-one on top and one at the bottom. The films were mostly annealed in He ambient, however O2 and H2 were also available to carry out oxidizing and reducing experiment s respectively if necessary. 4.1.3 Chemical Bath Deposition Process The CdS window layer used in this th esis is deposited by a process called Chemical Bath Deposition (CBD) .The Cadmium source was Cadmium acetate and Thiourea was used as the sulphur source. Ammonium hydroxide, a base and ammonium acetate, which served as a buffer, were also used, all combined in a solution of 600ml of H2O held in a double jacketed beaker with a capacity of 1000 ml. At specific time intervals measured volumes of the thiour ea and Cadmium acetate precursors were added to the solution, which was maintained at a constant temperature of 85C by circulating heated ethylene glycol thr ough the walls of the double jacket ed beaker.(The set up is illustrated in figure 33). A proposed mechanism for the reactions was given by J. Herrero et al [24]: Cd(CH3COO)2 Cd2+ + 2CH3COO(42) NH3 + HOH NH4 + + OH(43) Cd(NH3)4 2+ + 2OH[Cd(OH)2(NH3)2] + 2NH3 (44) [Cd(OH)2(NH3)2] + SC(NH2)2 [Cd(OH)2(NH3)2SC(NH2)2] (45) [Cd(OH)2(NH3)2SC(NH2)2] CdS(s) + CN3H5 + NH3 + 2HOH (46) Figure 33. The Chemical Bath Deposition Set-up.

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434.1.4 Close Spaced Sublimation Process The CdTe absorber layer is deposite d by Closed Spaced Sublimation process (CSS). In which the CdTe powder is sublimed on to a substrate with dimensions similar to that of the corning 7059 substr ate, the source such prepared is placed on a graphite holder. The substrate on which the CdTe la yer is to be deposited is placed on another graphite holder, on the top f acing the source. The source and the substrate are separated by spacers. The whole set-up, as described above was pl aced in a closed end quartz tube, that had a vacuum outlet and gas inle t (identical to that of th e annealing experimental set up).Both graphite holders (of the source and the substrate) were heated by two 2000 W halogen lamps, that caused the CdTe from th e source to sublime and get deposited on the substrate which is facing it, in such close proximity. Since the source and the substrate are in close proximity to each other, this process was termed as Close Spaced Sublimat ion. The experimental set up is shown in the figure below. Figure 34. Simplified CSS Experimental Set-up. 4.1.5 Evaporation The substrates after CdTe deposition were CdCl2 treated. 99.999% pure CdCl2 obtained from Fischer was pressed into circul ar pellets of 1cm in diameter, which were then placed on a metal boat, mounted between two copper electrodes.

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44 The samples were placed on top of a hollo w cylindrical glass holder, about 15 cm above the boat. The whole set up was placed inside a bell jar. The bell jar was pumped down to 10-4 Torr, and the boat containing the CdCl2 pellets was heated by passing electric current through th e copper electrodes. The CdCl2 was so evaporated on the substrates to about 8000 in thickness. Then the CdCl2 treated substrates were annealed in a He/O2 ambience at 390 C for 25 minutes after which the excess CdCl2 was rinsed away by using methanol. Then the substrates we re prepared for back contact application. 4.1.6 Graphite Paste Application The substrates from the previous step we re subjected to a bromine methanol etch for 7-10 seconds. This was done to create a tellurium rich surface. Then graphite paste doped with HgTe and Cu was painted on the etched surface w ith a paint brush. Then the substrates were left to dry for about 24 hours in a evacuated dessicator, after which they were annealed in He for about 20 minutes at 240C. Then the graphite paste was smoothened and silver paste was applied on it. After the cell ar eas were defined by scraping, Indium metal was applied as a me tallic grid on the front contact for better collection, using a soldering gun. 4.2 Standard Device Structure Figure 35. Standard Device Structure. e

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45 In this thesis, the performance of SnO2 and ITO as front contacts was compared, with and without a buffer layer. The use of SnO2 and In2O3 as buffer layers was also explored. SnO2 was deposited by MOCVD and sputtering and In2O3 and ITO were deposited only by sputtering. In all of th e above mentioned devices the CdS/CdTe junction and the back contact were kept consta nt and the front contact s were changed. In the section of window less junc tions of chapter 5, the junction between spinel TCOs and CdTe is studied without the CdS layer. The back contact however remained the same. All the spinel TCOs were deposited by sputtering. 4.3 Material Characterization The sheet resistance of the deposited TC O films was measured by using a simple four point probe set up. Then a portion of th e film was etched by HCl and Zinc and the thickness of the film was determined by measuring with a Tencor Alpha-step 200 profilometer, and the resistivity of th e film was subsequently calculated. The crystallinity and orientation of th e film was determined by powder X-Ray Diffraction pattern studies. The diffractometer was a Mini flex (VD01417) manufactured by Rigaku with CuK radiation ( = 1.540562 nm). The surface topography and the surface roughnes s of the films were studied using an Atomic Force Microscope (Nanoscope Dimension 3000). Hall mobilities, and carrier concentrations, were obtained using a Keithle y 920 Series Hall Test equipment set up. This method also provided material resistivit ies and sheet resistances to validate values obtained using the four-point-probe. For optical measurements, an Oriel Cornerstone m onochromator (model 74100) with an integrating sphere was utilized a nd transmission of the TCO films at different wavelengths was measured, using a LABVIE W program specifically written for this purpose. The optical band gaps of the TCOs we re also estimated from plotting square of absorption coefficient (at different wavele ngths) vs wavelength. 4.4 Solar Cell Measurements Dark and light J-V data was measured using a specifically written LABVIEW program from a Keithley 2410 1100 V source meter, keeping the cell under illumination from a solar simulator and covering the cell with a black cloth respectively.

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46 The open Circuit Voltage and Fill Factor were directly determined from the LABVIEW program. Since in this measurem ent the Jsc depended on the area of the devices calculated manu ally under a simple microscope, it was prone to e rror, hence the Jsc was calculated from QE of the devices measured using the same Oriel Cornerstone monochromator (model 74100), via another LA BVIEW program dedicated for operating the spectral response set up. The light s ource was a GE 400W/120V Quartz Line lamp (model # 43707). Values for series resistance and shunt resistance were determined by methods described in chapter one under basic device physics.

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47 CHAPTER 5 RESULTS AND DISCUSSION In this section the properties of fluorine doped SnO2 (SnO2:F) deposited by MOCVD process and indium oxide doped with tin (ITO) prepared by sputtering as described in chapter 4, are discussed. 5.1 Properties of MOCVD SnO2 Thin Films Tin Oxide films were deposited at a temperature of 460C by the MOCVD technique as described in ch apter 4. The thickness of th e films was approximately 6000 and the sheet resistance value was in the range of 7-10 /sq. The average transmission in the visible spectrum, for these films was above 90%. The films were polycrystalline with a preferred orientation of (110), as can be s een from the XRD pattern shown in figure 36. The root mean square roughness of the films was about 7.322 nm and the grain size was about 0.12 m as determined by AFM. Figure 37 shows an AFM photograph of the surface of SnO2 film deposited at 460C. These MOCVD SnO2 films have been in use for several years as the standard TCO for USF CdTe solar cells. Figure 36. XRD Pattern MOCVD SnO2:F Film at 460 C.

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48 Figure 37. AFM Photograph of the SnO2:F Surface. 5.2 Properties of Sputtered In2O3 Thin Films Indium Oxide was reactively sputte red from an In target in Ar/O2 mixture (25% O2). The total pressure was maintained at 3.0 mT. The film was deposited at various temperatures from room temperature (RT) to 400C. The films deposited at RT were amorphous. From the XRD patterns of the films (f igure 38 ), it could be seen that the films started to crystallize between 200 to 300C a nd at 400C they were well crystalline, as indicated by the intensity a nd FWHM, with increase in the deposition temperature, the crystallinity of the films improved. The average transmission of the films deposited at 200C was above 92% and the average transmission of the films deposited at higher temperatures averaged 90%. This is well within the measurement errors and th erefore no real effect of the deposition temperature was seen on the transmission of the films. The resistivities the films deposited at various temperatures and their average %T in the visible region is summarized in table 6. It can be seen that as the deposition temp erature increased the resistivities of the films decreased, this is due to the improveme nt in the crystallinity of the films.

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49Table 6. Properties of Reactively Sputtered In2O3 Thin Films. Deposition Temperature[C] Resistivity [ -cm] Avg %T in Visible region RT Too high Could not be measured 86.00 200 11.5 92.48 300 6.8 90.11 400 0.1 90.55 Figure 38. XRD Pattern of In2O3 Thin Films. Transmission of In2O3 Films0 10 20 30 40 50 60 70 80 90 100 350450550650750850950 Lambda, nmTransmissio n RT 200'C 300 400 Figure 39. Transmission Spectra of In2O3 Thin Films. 400 C 300 C 200 C

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50 From the transmission spectra and the XRD patterns, it could be seen that the amorphous films (deposited at RT and 200C) ha ve a similar transmission spectra and the polycrystalline films (deposited at 300C and 400C) group together and have a similar transmission spectra, this coul d be due to the differences in the refractive indices of amorphous and polycrystalline films. These diffe rences can affect the interference fringes. 5.3 Properties of Sputtered ITO Thin Films The ITO films were deposited at 200 and 300C from a 99.99% ITO target, in pure Ar at a pressure of 1.2 mT. Films were also deposited in different partial pressures of O2, but the resistivity of the resulting films increased with O2 partial pressure (most likely due to the reduction of oxygen vacancies), so the films were deposited in pure Ar with no O2. The sheet resistance of ITO films ranged from 9 to 11 /sq, for 2000 thick films deposited at 300C and for films deposited at a temperature of 200C the sheet resistance ranged from 13 to 15 /sq, for the same thickness. The average transmission of these films ranged from 88 to 90 % in the spectra l region of 400 to 900 nm. Figure 41 shows a comparison of ITO with standard SnO2. Total transmission of ITO and SnO2:F0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 lambda, nm%T SnO2:F at 460 C avg %T 400900nm 92% ITO at 300 C avg %T 400-900nm 89.9% Figure 40. The Transmission Spectra of SnO2:F and ITO.

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51 The film deposited at 200C displayed esse ntially random orientations as seen in figure 41, there were many crystalline orientat ions like <222>,(the pref erred orientation of In2O3),<211> etc and the films deposited at 300 C showed improvement in crystallinity with the preferred orientation of <400>, as indicated by the highest intensity of the<400> peak in figure 42. Figure 41. XRD Pattern of the ITO Film Deposited at 200C. Figure 42. XRD Pattern of ITO Film Deposited at 300C.

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52 Table 7. Properties of SnO2:F and ITO Films. Figure 43. AFM Photograph of ITO Deposite d at 200C (left) and 300C (right). The RMS roughness of the ITO was appr oximately 1.715 nm and 2.664nm for samples deposited at temperatures of 200 a nd 300C respectively. The roughness of ITO deposited at 300C was almost 3 times smaller than that of standard SnO2. Table 7 shows the results of hall measurements done on these films. The ITO film deposited at 300C has the lowest resistivity owing to high carrier concentration and mobility. The AFM images of the surface of ITO deposited at 200 and 300C are shown in figure 43. Sample Thickness[] Resistivity [ -cm] Carrier concentration [Cm-3] Mobility [cm2V-1s-1] Avg %T (400nm to 900nm) SnO2:F(460 C) 5000 4.16X10-4 3.95X1020 38 90-92 ITO(200C) 2000 3.08X10-4 7.14X10 20 28 88-89 ITO(300C) 2000 1.9X10-4 1.02X10 21 32 89-90

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535.4 CdTe Solar Cells with SnO2:F and ITO as Front Contacts The SnO2:F and ITO films whose properties we re described in the previous section were used as front contacts for CdTe solar cells. The CdS window layer was deposited by chemical bath deposition, and the absorber layer CdTe was deposited by Close space sublimation, graphite paste dope d with Cu and HgTe was applied as back contact, after CdCl2 treatment(every step as described in chapter 5).The cell structure of these devices is shown in figure 44. Figure 44. Structure of CdS/CdTe Solar Cells with SnO2:F, ITO as Front Contacts. Typical solar cell results of the devices are shown in table 8. Table 8. Device Parameters of CdS/CdTe Solar Cells with SnO2:F and ITO as Front Contacts [34]. Front contact Voc [mV] FF [%] Jsc [mA/cm2]Rseries [ -cm2] Rshunt [ -cm2] SnO2:F 740 63 22.44 0.68 1660 ITO 770 60 22.49 2.3 1250 The devices exhibited low ope n circuit voltage and FF. It could be inferred from the I-V characteristics of ITO device (see figur e 47) that a barrier to the current flow is present, due to the existence of a diode in the opposite direction to the main junction (see figure 45) either in the front or in the back contact.

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54 Figure 45. Equivalent Circuit Diagram for a So lar Cell with a Barrier at the Back Contact (a), and a Solar Cell with a Barrier at the Front Contact (b). Since all the devices have the same cont act and yet only those with ITO as front contacts showed this anamoly repeatedly, l eads to the conclusion that the barrier was formed at the ITO – CdS interface. A r ecent study on the influence of oxidative and reducive treatments on the surface of ITO film s lead to the conclusion that the work function of ITO films strongly depended on the surface oxygen concentration, and also that oxidative treatment lead to increase in the work function of ITO films[21]. The work function of as deposited ITO f ilm may be around 4.0 to 4.45 eV, lesser than the electron affinity of CdS (4.5 eV), assuming that the oxygen is able to diffuse through the CdS film during device fabrication (like CdTe deposition), it would increase the work function of ITO to around 5.0 eV. However, since CdS is a n-type semiconductor, contact materials should have a low work function, ideally less th an the CdS electron affinity if they are to be injecting. So the ITO junction becomes r ectifying due to the formation of a Schottky barrier after oxidative surface treatments. a) b)

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55 However, it is noted here that the work f unction of ITO film used for this thesis is not known. It is only assumed that the heat treatment changes the ITO from an injecting (ohmic) to a rectifying (S chottky barrier) contact by raising the ITO work function through surface oxidation as concluded by S.N.Alamri et al [20]. The addition of a high resistivity SnO2 layer seems to improve the device performance, most likely by affecting the IT O surface leading to a lower work function. 5.4.1 Effect of High Resist ivity (intrinsic) CVD SnO2 Layer A buffer layer of SnO2 was deposited on the SnO2:F and ITO substrates discussed in the previous section by MOCVD in the same chamber as SnO2:F, at 460C for a thickness of about 3000 . The only difference being that this film was not doped. Its resistivity was approximately 0.5 to 1 -cm. The structure of these devices is as follows Figure 46. Device Structure after the Inclusion of Buffer Layer.

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56 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current [A] ITO-without intrinsic layer SnO2-without intrinsic layer ITO with SnO2-i layer SnO2 with SnO2-i Figure 47. I-V Characteristics of SnO2 and ITO CdS/CdTe Solar Cells with and without Intrinsic SnO2 Layer. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength, (nm)Q.E. SnO2:F without SnO2-i SnO2:F with SnO2-i Figure 48. Spectral Response of SnO2:F CdS/CdTe Cells with and without Intrinsic SnO2 Layer.

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57 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength, (nm)Q.E. ITO ITO-with SnO2-i Figure 49. Spectral Response of ITO CdS/Cd Te Cells with and without Intrinsic SnO2 Layer. The introduction of this buffer layer improved the solar cell performance considerably. The Voc increased approximately 60 to 80 mV and the FF 7 to 8 %. No significant change in Jsc was observed because Jsc is primarily determined by CdS thickness. The shunt resistances of the devices improved by 2000 -cm2. The formation of barrier at the front contact in the case of ITO ,was elim inated ,as seen from the J-V data. Table 9 summarizes the performance of the devices after the inclusion of a SnO2 buffer layer. Table 9. Device Parameters of CdS/CdTe Solar Cells with SnO2:F and ITO as Front Contacts with Intrinsic SnO2 Buffer Layer [34]. Front contact Buffer Layer Voc [mV] FF [%] Jsc [mA/cm2] Rseries [ -cm2] Rshunt [ -cm2] SnO2:F SnO2-i 820 72 22.3 0.60 3300 ITO SnO2-i 820 69 22.1 0.53 3300 The SnO2:F/SnO2-i bi layer CdS/CdTe solar cell is the base line device to be used as a bench mark for this work

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585.4.2 Effect of CdS Thickness on SnO2:F and ITO/SnO2-i CdS/CdTe Solar Cells To date CdS/CdTe solar cell photocu rrents have reached 86% of their 30.6 mA/cm2 theoretical maximum for the 100 mW/cm2 terrestrial spectrum. A significant photocurrent loss about 5 mA/cm2 is due to window layer CdS band gap absorption below 520 nm depending on the th ickness of the CdS layer. The thickness of the CdS layer could be decreased to reduce the photocur rent loss considerably but there appears to be a process dependent “critical CdS thickness” between 400 to 1000 below which the junction parameters degrade significantly [25]. X-ray diffracti on and opto-electronic measurements of the interfacial region in hi gh efficiency cells show that the photovoltaic active layer consists of a n early homojunction of n-CdS1-yTey/p-CdTe1-xSx/p-CdTe. The CdS1-yTey and CdTe1-xSx alloys form via diffusion across the interface during CdTe deposition and post-deposition treatments a nd affect the photocur rent and junction behavior. The interfacial values of x and y co rrespond to the solubility limits in the CdTeCdS system at the device processing temperature. Formation of the CdS1-yTey alloy on the S-rich side of the junction reduces the band gap and increases absorption, reducing photocurrent below 520nm. Formation of the CdTe1-xSx alloy on the Te-rich side of the junction reduces the absorber layer band gap, due to the optical bowing parameter of the CdTe-CdS alloy system. The increase in long wavelength spectral re sponse increases the photocurrent by ~ 0.5 mA/cm2, which is nearly offset by small reduction in Voc ~ 25 mV. On the other hand, non-uniform consumption of CdS leads to paralle l junctions between CdTe1-xSx / CdS1-yTey and CdTe1-xSx / ITO (or SnO2), resulting in a net increase in Jo, which reduces Voc. Penetration of S-rich species into the CdTe grain boundaries can produce a three dimensional junction, which increases the actual junction area, also reducing Voc[18] The thickness of the CdS layer should be sufficient enough to facilitate the optimum formation of these alloys to maintain good junc tion properties. So, as the thickness of the CdS is varied, the short circuit current density Jsc decreases from thin to thick and Voc and FF deteriorate from thick to thin.

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59 The thickness of CdS was varied in these devices by reducing the time of deposition of the film. The factor [1QE] of the device at 450 nm is taken as a relative measure for the final thickness of CdS. The factor [1-QE @ 450nm] varies from low to high, as CdS thickness increases. Table 10. Device Parameters of SnO2 and ITO Solar Cells with Different CdS Thickness [34]. Jsc under 510 nm [mA/cm2] Voc [V] (average) FF[%] (average) Jsc [mA/cm2] (average) 1-QE@ 450nm SnO2 ITO SnO2 ITO SnO2 ITO SnO2 ITO 0.25 4.51 4.46 0.7850.790 60 60 24.9 24.1 0.36 4.10 3.76 0.8260.815 66 65 24.0 23.9 0.48 3.48 3.16 0.8300.833 68 67 23.4 22.5 0.55 2.82 2.98 0.8200.820 71 69 22.3 21.9 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current Density [A/cm2] 500 A of CdS 600 A of CdS 700 A of CdS 800 A of CdS 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 500 A of CdS 600 A of CdS 700 A of CdS 800 A of CdS Figure 50. J-V Characteristics (left) Spectral Response (right) of the Best SnO2/SnO2i Devices with Different CdS Thickness [34]. Table 10 summarizes the device parameters of SnO2 and ITO solar cells with varying CdS thickness, to be di scussed further in this section. Figure 50 shows the J-V and

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60 spectral response data for SnO2 devices with varying CdS thickness and figure 51 shows the J-V and spectral response data for IT O devices with different CdS thickness. -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current Density [A/cm2] 500 A of CdS 600 A of CdS 700 A of CdS 800 A of CdS 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 500 A of CdS 600 A of CdS 700 A of CdS 800 A of CdS Figure 51. J-V Characteristics (left) Spectral Response (right) of the Best ITO/SnO2i Devices with Different CdS Thickness [34]. As expected the Jsc under 510 nm decreased as the thickness of the CdS increased. The SnO2 and ITO devices showed similar trend, however the Jscs in ITO were slightly lower than that of SnO2. The Vocs and FF improved with in creasing CdS thickness, implying better junction quality (see figure 52). The Rseries decreased with increase in CdS thickness. Rshunt increased with CdS thickness, since thin CdS films may contain pinholes leading to direct contact of CdTe and SnO2. However, the range of Rshunt cannot explain the increase in FF(60 to 71%) from thin to thick Cd S ,therefore it is suggested that collection is the main limiting factor.

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61 45 50 55 60 65 70 75 0.250.360.480.55 Thickness of CdS(1-QE@450 nm)FF [%]15 16 17 18 19 20 21 22 23 24 25 785826830820 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.250.360.480.55 Thickness of CdS(1-QE@450 nm)Series Resistance [ -cm2] 800 1000 1200 1400 1600 1800 2000 2200 60666871 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 52. Effect of CdS Thickness on the Device Parameters of SnO2/SnO2-i CdS/CdTe Solar Cells [34]. 45 50 55 60 65 70 75 0.250.360.480.55 Thickness of CdS(1-QE@450nm)FF [%]15 16 17 18 19 20 21 22 23 24 25 790815833820 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.250.360.480.55 Thickness of CdS(1-QE@450 nm)Series Resistance [ -cm2] 800 1000 1200 1400 1600 1800 2000 2200 60656769 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 53. Effect of CdS Thickness on the Device Parameters of ITO/SnO2-i CdS/CdTe Solar Cells [34]. 5.5 Effect of Substrate Temperatu re during CdTe CSS Deposition A series of devices were fabricated to study the effect of CdTe deposition temperature, the substrate (SnO2/CdS) temperature was increased from 580C to 620C, while keeping the source temperature consta nt at 660C. Table 11 shows solar cell parameters for these cells. 0.25 0.36 0.48 0.55 1-QE @ 450 nm 0.25 0.36 0.48 0.55 1-QE @ 450 nm 0.25 0.36 0.48 0.55 1-QE @ 450 nm 0.25 0.36 0.48 0.55 1-QE @ 450 nm 0.25 0.36 0.48 0.55 1-QE @ 450 nm Jsc [mA/cm2] Jsc [mA/cm2]

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62 Table 11. Device Parameters of SnO2:F /SnO2-i CdS/CdTe Solar cells with Different Substrate Te mperature during CdTe CSS Deposition [34]. CdTe Temperature [C] Voc [mV] FF [%] Jsc [mA/cm2] Rseries [ -cm2] Rshunt [ -cm2] 580/660 830 77 23.00 0.32 3300 600/660 830 69 23.29 0.88 2500 610/660 760 69 23.82 0.53 1000 620/660 740 63 23.25 0.65 400 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current Density [A/cm2] 600/660 610/660 580/660 620/660 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 600/660 580/660 620/660 610/660 Figure 54. J-V Characteristics (left) and Spectral Response (right) of SnO2:F /SnO2-i CdS/CdTe Sola r Cells with Different Substrate Temperature during CdTe CSS De position [34].

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63 45 50 55 60 65 70 75 580/660600/660610/660620/660 CSS Sub/source TemperatureFF [%]15 16 17 18 19 20 21 22 23 24 25 830830760740 Voc [mV]Jsc [mA/cm2] FF Jsc 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 580/660600/660610/660620/660 CSS Sub/Source TemperatureSeries Resistance [ -cm2] 300 800 1300 1800 2300 2800 3300 3800 77696963 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 55. Effect of Different Substrate Temperatures during CSS CdTe Deposition on Device Parameters [34]. As seen in the figures 54 & 55, increasi ng the substrate temperature from 580 to 620C, degraded the device parameters. The Voc, FF and Rshunt of the devices decreased with increase in substrate temperature (Tsub). CSS employs high substrate temperatures during film growth, which can promote the formation of larger grains and higher Vocs yielding better device performance. However, as Tsub increases, the thickness of the CdS layer reduces as can be seen from the spectral response of these devices (see figure 54), due to increased consumption of CdS by CdTe. The devices fabricated at Tsub of 580 and 600C show the same effect on CdS thickne ss but in the devices with higher Tsub, due to the increased non uniform consumption of CdS, the CdTe may be penetrating further into CdS leading to direct contact of CdTe and SnO2(see figure 56), causing shunting there by resulting in lower Vocs. So it can be concluded that it is not advisable to attempt to reduce the final thickness of CdS by increasi ng the CdTe deposition temperature, as it results in poor junction parameters.

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64 Figure 56. Effect of High CdTe Substrate Temperature. 5.6 SnO2:F CdS/CdTe Solar Cells with Sputtered SnO2 Doped with Zinc (SnO2:Zn) as Buffer Layer Jordan and Albright developed a method to avoid shorting of CdTe and the SnO2 (as described in the previous section) due to flaws in the CdS layer, by providing a conductive layer which is formed by two SnO2 layers each having substantially dissimilar electrical conductivity, such that an electrically-conductive SnO2 layer, interconnects the plurality of photovoltaic cells (in the case of a panel), while the low conductivity layer prevents shorting of the cell. The elec tron density of th e low conductivity SnO2 layer may be adjusted to be within approximately th ree orders of magnitude of the presumed electron density of the p-type CdTe layer, such that a energy producing junction is formed in any area of flaws in the CdS layer by the CdTe and the SnO2 layer. In their work Cadmium was used to dope the low conductivity SnO2, but Zinc may also be used [33]. In this section, the effect of such a fr ont contact comprising a bilayer of dissimilar resistivities is studied by depositing a layer of SnO2 doped with Zinc on standard highly conducting SnO2:F and the results are discusse d. On the MOCVD deposited SnO2: F, a high resistance layer of SnO2 was reactively sputtered from a Sn target, with 25% O2 in pure Ar, the total pressure being 3.0 mT at 300C. The temperature of 300C was determined to be the optimum temperature. Zi nc was added, from a Zn target. The doping concentration of Zinc was controlled by th e amount of RF power supplied to the Zn sputtering source.

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65 The SnO2 was sputtered for two thicknesses of 500 and 2000 at two different Zinc concentrations of 5% and 10%.The device structure of these cells are shown in figure 57. Figure 57. Device Structure of SnO2:F CdS/CdTe Solar Cells with Sputtered SnO2 Doped with Zinc (SnO2:Zn) as Buffer Layer. Table 12. Device Parameters of SnO2:F /SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 500 . %Zn Voc [mV] FF [%] Jsc [mA/cm2] Rshunt [ -cm2] Rseries [ -cm2] Eff [%] 0 830.3 65 23.57 750 1.46 12.8 5 672.1 55 21.50 600 2.24 7.9 10 672.5 53 22.10 400 2.76 7.3 Table 11 summarizes the performan ce of devices with 500 of SnO2:Zn layer. The J-V and spectral response of the devices are shown in figure 58. As can be seen from figures 58 and 59, the device with no Zn had the highest Voc and Jsc. The FF and Rshunt decreased with increase in Zn concentration. The Rseries increased with increase in Zn concentration. The Jsc for the device with 10% zinc was slightly higher than that of the device with 5% Zinc.

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66 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current Density [A/cm2] 0% Zn 5% Zn 10% Zn 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength [nm]Q.E. 0% Zn 5% Zn 10% Zn Figure 58. J-V Characteristics (Left) a nd Spectral Response (Right) of SnO2:F/SnO2:Zn CdS/CdTe solar cells for SnO2:Zn Layer Thickness of 500 [34]. 45 50 55 60 65 70 75 0510 % ZnFF [%]15 16 17 18 19 20 21 22 23 24 25 830672673 Voc [mV]Jsc [mA/cm2] FF Jsc 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0510 Zn Content [%]Series Resistance [ -cm2] 300 400 500 600 700 800 65.654.7552.38 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 59. Change in Device Parameters of SnO2:F/SnO2:Zn CdS/CdTe Solar Cells for Different Concentrations of Zinc (SnO2:Zn layer thickness of 500 ) [34]. From the spectral response, it can be s een that QE @ 450 nm showed an increase (even though it is a small change) with incr easing Zn concentration. This might be attributed to the interdiffusion of Zn into th e CdS layer. The Zn in the CdS layer forms an alloy ZnxCd(1-x)S, whose band gap almost varies lin early with the atomic fraction of Zn(x). The band gap of CdS is around 2.4 eV and the band gap of ZnS is 3.8 eV.

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67 Consequently as Zn from the SnO2:Zn layer diffuses into and alloys with the CdS film ,the band gap increases. This shifts the short wave length cutoff to a smaller value, which leading to an increase in Jsc [16]. Cd could also be diffusing into the SnO2 layer, making it more resistive at the same time and formation of ZnS is also a possibility. It can also be seen from the QE plot that the devices wi th higher concentrations of Zn suffer from collection losses which may be due to lowering of electric field in the CdTe due to modification of interface properties. In the de vices with higher concentration of Zn the Voc drop is consistent with the light J-V of devices without a buffer layer, one possible explanation may be that the Zn in the SnO2 layer could be in the form ZnO, and as CdS reacts with ZnO, the buffer layer might get shorted but this possibility does not explain the increase in Rseries with increase in Zn concentration. Table 13. Device Parameters of SnO2:F/SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn Layer Thickness of 2000. %Zn Voc [mV] FF Jsc [mA/cm2] Rshunt [ -cm2] Rseries[ -cm2] Eff [%] 0 780 55 23.10 600 3.25 9.8 5 823 67 22.95 900 1.15 12.6 10 668 47 22.36 300 5.24 6.9 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current Density [A/cm2] 0% Zn 5% Zn 10% Zn 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength [nm]Q.E. 0% Zn 5% Zn 10% Zn Figure 60. J-V Characteristics (Left) and Spectral Response (Right) SnO2:F/SnO2:Zn CdS/CdTe Solar Cells for SnO2:Zn layer Thickness of 2000 [34].

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68 45 50 55 60 65 70 75 0510 % ZnFF [%]15 16 17 18 19 20 21 22 23 24 25 780823668 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0510 Zn Content [%]Series Resistance [ -cm2] 200 300 400 500 600 700 800 900 1000 54.6567.1246.76 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 61.Change in Device parameters of SnO2:F/SnO2:Zn CdS/CdTe solar cells for Different Concentrations of Zinc(SnO2:Zn layer thickness of 2000 ) [34]. Table 12 summarizes the performan ce of devices with 2000 of SnO2:Zn layer. Figure 59 shows the J-V and spec tral response data for these devices. The device with no zinc exhibited low Voc and FF, though higher th an the device with 10% Zn. This may be a bad device and the data pertaining to th is device can be discarded as an anamoly because devices with 1000 of sputtered SnO2 showed excellent device performance [27]. The device with 5% Zn displayed highest Voc and FF and did not suffer from collection losses in the red region. The device with 10% Zn showed a marked increase in the QE in the below 520 nm, due to the formation of the alloy ZnxCd(1-x)S as described in the previous paragraphs. This device also su ffered from pronounced collection loss in the red region. It can be seen that small amounts of Zn in thicker SnO2 layer seemed to increase device performance. It should be re membered that the device with no zinc could also be as good as this device with 5% device, if not better. 5.7 CdS/CdTe Solar Cells with Indium Oxide as Buffer Layer 5.7.1 SnO2:F /In2O3 CdS/CdTe Solar Cells To study the effect of In2O3 as buffer layer, 2000 of reactively sputtered In2O3 at a temperature of 300C was de posited on, MOCVD deposited SnO2:F at 460C ( standard SnO2:F) and the device was completed as give n in the device structure in figure 62. 500 A 2000 A

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69 Figure 62.Device Structure of SnO2:F/In2O3 CdS/CdTe Solar Cell. To determine the influence of the surface properties of In2O3, a series of devices were fabricated (a) SnO2:F/In2O3 (b) SnO2:F/In2O3 annealed at 600C (c) SnO2:F/In2O3/SnO2 annealed at 600C all these de vices were completed with CBD grown CdS, standard CSS CdTe and contact ed with doped graphite, conforming to the device structure given above. The performance of the devices are summarized in table 14. Table 14. Device Parameters of SnO2 :F cells with In2O3 as Buffer Layer [34]. Front contact Voc [mV] FF[%] Jsc [mA/cm2] Rshunt[ -cm2] Rseries [ -cm2] Eff [%] SnO2:F+In2O3 Not annealed 827 67 22.75 1100 0.74 12.6 SnO2:F+In2O3 Annealed at 600C in He 825 53 21.46 800 1.56 9.4 SnO2:F+In2O3 +SnO2 Annealed at 600C in He 841 63 22.1 1250 0.86 11.7

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70 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 -0.5-0.2500.250.50.751 Voltage[V]Current[A] SnO2/In2O3/SnO2annealed in He SnO2/In2O3-annealed in He SnO2/In2O3-not annealed at 600C 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%400500600700800900Wavelength, (nm)Q.E. SnO2/In2O3-annealed at 600C in He SnO2/In2O3/SnO2annealed at 600 C in He SnO2/In2O3-not annealed Figure 63. I-V Characteristics (Left) a nd Spectral Response (Right) of SnO2:F Cells with In2O3 as Intrinsic Layer. The device in which the SnO2:F/In2O3 bi layer was annealed at 600C in He ,showed evidence of a barrier formation an e ffect similar to the ITO devices without the high resistance layer and when the In2O3 layer was “encapsulated” by a layer of CVD deposited intrinsic layer and the tri-layer was annealed at 600C in He the device did not show any barrier formation, although the devi ce exhibited higher seri es resistance due to the addition of a 3000 layer of SnO2-i layer as expected, confirming that the barrier formation was due to the surface properties of In2O3 layer which appear to be sensitive to processing. The best device was the one with as deposited SnO2/In2O3 bi-layer, suggests that In2O3 can be used as buffer laye r in CdS/CdTe solar cells. 5.7.2. ITO/In2O3 CdS/CdTe Solar Cells In this section, results for ITO/In2O3 based devices are discussed. CdS/CdTe solar cell was fabricated with 2000 of ITO sputte red at 200C in Ar as front contact and 250 to 2000 of reactively sputtered In2O3 (at 300C) in Ar/O2 mixture (25% O2) from Indium target was deposited on it as the buffer layer. The device was then completed conforming to the device structure given below.

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71 Figure 64. Device Structure of ITO/In2O3 CdS/CdTe Solar Cells. Table 15.Device Parameters of ITO/In2O3 Cells with ~800 of CdS. Thickness of In2O3 [] Voc [mV] FF[%] Jsc [mA/cm2] Rshunt [ -cm2]Rseries[ -cm2] Eff [%] 250 830 0.66 24.14 2000 0.76 13.22% 500 840 0.64 23.49 1666 0.77 12.71% 1000 830 0.67 23.56 2000 0.53 13.17% 2000 830 0.62 23.00 2500 1.10 11.89% The solar cell characteristics for devices different thickness of In2O3 are listed in table 15.performance of the best devices is summarized in the following table. Figures 65 and 66 show the I-V and Spectra l response characteristics of these devices respectively. As the thickness of the In2O3 layer increases, the Vocs remain fairly constant about 830 mV. The series resistance of the devices is expected to increase with the increase in the thickness of th e high resistance In2O3 layer; In general, this is true but the device with 1000 of In2O3 does not fit the trend. The 0 devi ce shows the barrier formation as discussed earlier. The FFs also follow a sim ilar trend, decreasing with the increase of thickness of the buffer layer with the excep tion of the 1000 device having the highest fill factor.

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72 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current [A] 0A 500A 1000A 250A 2000A 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 0A 1000A 500A 250A 2000A Figure 65. I-V Characteristics (Left) a nd Spectral Response (Right) ITO/In2O3 Cells with 800 of CdS. 45 50 55 60 65 70 75 25050010002000 Thickness of In2O3FF [%]15 16 17 18 19 20 21 22 23 24 25 830840830830 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.2 0.4 0.6 0.8 1.0 1.2 25050010002000 Thickness Of In2O3 in ASeries Resistance [ -cm2] 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 6664.46762.3 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 66. Change in Device Parameters of ITO/In2O3 Cells with 800 of CdS with Change in Thickness of In2O3 [34]. The Rshunt of the devices, initially decrease d from 250 device to 500 device, but increased with increase in thickness then on. The highest value for shunt resistance was for the device with 2000 of In2O3. The Jscs are fairly similar as seen from the spectral response characteristics.

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735.8 Effect of the CdS Thickness on ITO/In2O3 Devices In order to determine the effectiveness of In2O3 as a buffer layer, several CdTe cells were fabricated with CdS of varying th ickness. Specifically, using the same range of In2O3 thickness, CdS was deposited for a th ickness of approximately 700,600,500 . Table 16.Device Parameters of ITO/ In2O3 Cells with ~700 of CdS. Thickness of In2O3[] Voc [mV] FF[%] Jsc [mA/cm2] Rshunt [ -cm2]Rseries [ -cm2] Eff [%] 250 830 67 23.64 2000 0.77 13.1 500 840 66.4 23.08 1666 0.78 12.8 1000 820 70.5 23.76 2500 0.55 13.6 2000 840 67 22.86 3500 1.14 12.9 Table 17. Device Parameters of ITO/ In2O3 Cells with ~ 600 of CdS. Thickness of In2O3 [] Voc [mV] FF[%] Jsc [mA/cm2] Rshunt [ -cm2]Rseries [ -cm2] Eff [%] 250 820 71.9 24.04 2000 0.76 14.2 500 810 70.9 23.66 2000 0.90 13.6 1000 800 66.4 22.58 800 0.44 12.0 2000 810 70.5 23.98 2000 0.47 13.7 Table 18. Device Parameters of ITO/ In2O3 Cells with ~ 500 of CdS. Thickness of In2O3 [] Voc [mV] FF [%] Jsc [mA/cm2] Rshunt [ -cm2] Rseries [ -cm2] Eff [%] 250 790 63.0 25.09 1600 0.89 12.4 500 820 64.0 24.81 1400 1.26 13.0 1000 810 60.0 24.31 1100 0.92 11.4 2000 710 62.8 23.92 1250 0.51 10.6

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74 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current [A] 0A 500A 1000A 250A 2000A 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900Wavelength, (nm)Q.E. 0A 250A 1000A 500A 2000A Figure 67. I-V Characteristics (Left) a nd Spectral Response (Right) ITO/In2O3 Cells with 700 of CdS. -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.50-0.250.000.250.500.751.00 Voltage [Volts]Current [A] 500 A 1000 A 250 A 2000 A 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 2000A 250A 500A 1000 A Figure 68. I-V Characteristics (Left) a nd Spectral Response (Right) ITO/In2O3 Cells with 600 of CdS.

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75 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03-0.50-0.250.000.250.500.751.00 Voltage [Volts]Current [A] 0A 500A 1000 250A 2000A 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. 250A 500A 2000 A 1000 A 0 A Figure 69. I-V Characteristics (Left) a nd Spectral Response (Right) ITO/In2O3 Cells with 500 of CdS. 45 50 55 60 65 70 75 25050010002000 Thickness of In2O3FF [%]15 16 17 18 19 20 21 22 23 24 25 830840820840 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.2 0.4 0.6 0.8 1.0 1.2 25050010002000 Thickness Of In2O3 in ASeries Resistance [ -cm2] 200 700 1200 1700 2200 2700 3200 3700 6766.470.567 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 70. Change in Device Parameters of ITO/In2O3 Cells with 700 of CdS with Change in Thickness of In2O3 [34]. Figures 67, 68, and 69 show the J-V char acteristics and spectral response of the ITO/In2O3 devices with 700,600 and 500 of CdS re spectively. Tables 16, 17 and 18and figures 70, 71, 72 summarize the device parameters of these devices.

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76 45 50 55 60 65 70 75 25050010002000 Thickness of In2O3FF [%]15 16 17 18 19 20 21 22 23 24 25 820810800810 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 25050010002000 Thickness Of In2O3 in ASeries Resistance [ -cm2] 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 71.970.966.470.5 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 71. Change in Device Parameters of ITO/In2O3 Cells with 600 of CdS with Change in Thickness of In2O3 [34]. 45 50 55 60 65 70 75 25050010002000 Thickness of In2O3FF [%]15 16 17 18 19 20 21 22 23 24 25 26 790820810710 Voc [mV]Jsc [mA/cm3] FF Jsc 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 25050010002000 Thickness Of In2O3 in ASeries Resistance [ -cm2] 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 63646062.8 FF [%]Shunt Resistance [ -cm2] Rseries Rshunt Figure 72. Change in Device Parameters of ITO/In2O3 Cells with 500 of CdS with Change in Thickness of In2O3 [34]. As seen from all the figures given above the following conclusions can be made. The parameters of the devices with 700 of CdS are very much similar to those of 800 . From the spectral response pl ots of all the devices, it can be seen that the blue region shows CdS thickness effect and the re d region is essentially identical.

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77 All the devices without the buffer layer, i.e., 0 of In2O3 show the barrier formation at the ITO/CdS interface (the 0 device of the 600 CdS set was a bad device and hence was excluded from the plots) Considering all the data, there does not seem to be any trend for R series, with In2O3 thickness or CdS thickness, even t hough it was suggested earlier that In2O3 thickness had an effect on Rseries. Rshunt is lower for thin CdS devices. The Vocs start to decrease for CdS thickness of 600 and beco me considerably lower for devices with 500 of CdS. The Jscs increase as CdS thickness decreases. This is typical as seen with base line devices in previous sections of this chapter. However, the thin CdS set results suggest that increasing the In2O3 thickness degrades solar cell performance. Almost all the QE data for devices with thicker In2O3 (1000 and 2000 ) display collection losses due to deeply penetrating photons. The highest efficiency of 14.2 % was achieved for the device with 250 of In2O3 and a CdS thickness of approximately 600. This suggests that In2O3 can be used as buffer layer in CdTe solar cells successfully. 5.9 Window less Junctions In this section, the performance of di fferent TCO-CdTe j unctions have been explored. Devices were made with different TCOs as front contact without the window layer CdS. So a direct junction was formed between n-type TCO and CdTe. The device structure of these devices is as follows Figure 73. Device Structure of TCO/CdTe Solar Cell. Cd2SnO4 (CTO) thin films were sputtered from oxide targets at room temperature in Ar ambient. The as deposited films were amorphous but subsequent annealing in He at 600C rendered the film poly crystalline in inverse spinel struct ure with preferred

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78 orientation of <222>. The properties of the 2000 film after annealing, discussed in this section are listed in table 19. Devices were made with three types of CTO: (1) as deposited CTO film, (2) annealed CTO film and (3) annealed CT O film with a buffer layer of CVD SnO2 of about 3000 in thickness, all the devices were then completed conforming to the device structure in figure 72. Since the CdS window layer is not present in these devices, the quality of junction formed between TCO an d CdTe turned out very poor and devices were suffering from very high series resist ance and extremely low shunt resistance. The performance of the CTO devi ces is listed in table 20. Table 19. Properties of Spinel TCOs. Material Annealing at 600 C in He Metallic ratio Film crystallinity Resistivity in -cm Average transmission in visible region No ~2 Zn/Sn Amorphous 4.3 X 10-3 90% ZTO at 450 C* Yes ~2 Zn/Sn Polycrystalline in inverse spinel 6.6 X 10-2 92% CTO at RT yes ~2 Cd/Sn Polycrystalline in inverse spinel 3.9X10-4 89% CdInO3 at 200 C yes ~1 Cd/In Polycrystalline 7.35X10-4 89% CdIn2O4 at RT yes ~0.5 Cd/In Polycrystalline in normal spinel 4.6X10-4 90% *taken from [27] The device with the as deposited CTO was inferior to the devices with annealed CTO because the as deposited film was amorphous and highly resistive. When the film was annealed, the film became highly conductive leading to higher Jscs and improved Voc.

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79 With the addition of a high resistiv ity buffer layer of CVD deposited SnO2, the Vocs improved further but the Jsc decreased. All the films showed very high shunting (due to high leakage currents due to poor junction fo rmation), and the shunt resistance improved with the incorporation of buffer layer. Table 20. Device Parameters of Spinel TCO/CdTe Devices. Front Contact Annealed in He at 600C (spinel TCO) Buffer layer Voc [mV] FF Jsc [mA/cm2] Rsh [ -cm2] Eff[%] No No 174 0.320 13.69 47 0.76 yes No 299 0.315 22.05 68 2.08 CTO Yes CVD SnO2 3000 454 0.300 17.33 101 2.33 No ZTO 330 0.356 17.3 149 2.05 SnO2:F Yes ZTO 463 0.353 22.06 140 3.61 CdInO3 yes 240 0.200 12.94 44 0.64 CdIn2O4 yes 310 0.385 14.67 135 3.86 In order to study the junction between Zn2SnO4 (ZTO) and CdTe, devices were made with SnO2: F as front contact with a thin layer of ZTO as buffer layer. The ZTO films were sputtered from oxide targets at 450 C in Ar. The as deposited film was amorphous and when subjected to annealing in He at 600C it became polycrystalline in the inverse spinel structure. The films were not deposited at room temperature like CTO because, the as deposited films at room temp erature did not become polycrystalline even after annealing at 600C in He. The film seemed to require more energy to crystallize. So it was deposited at the highest temperature pos sible at the sputtering chamber. The films became more resistive after annealing, but th e device with annealed ZTO layer was better than the device with the as de posited layer, in terms of Voc and Jsc. The device parameters

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80 of these devices are listed in table 20. To study the junction be tween Cadmium Indium Oxide (CIO) and CdTe, CIO was deposited from Cadmium and Indium target in 25% O2 with pure Ar. The properties of the film ar e listed in table 20. Though both the devices suffered from heavy shunting and series resistance effects the device with Cd to In ratio of 0.5 was more efficient than the device with Cd to In ratio of 1:1, with an efficiency of 3.8%, offering sufficient incentive to study and develop this TCO furthe r, for use in CdTe solar cells. All the devices were extrem ely poor with low shunt and high series resistances. All QE plots showed consiste nt collection losses at higher wavelengths. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900Wavelength, (nm)Q.E. Cd2SnO4-annealed without SnO2-i Cd2SnO4-annealed with SnO2-i 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 1 00% 40050060070080090 0 Wavelength, (nm)Q.E. SnO2:F+Zn2SnO4 not annealed SnO2:F+annealed Zn2SnO4 Figure 74. Spectral Response of CTO/CdTe Cells (left) and SnO2/ZTO/CdTe Devices (right). 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400500600700800900 Wavelength, (nm)Q.E. CdIn2O4-annealed at 600 C in He CdInO3-annealed at 600 C in He Figure75. Spectral Response of CIO/ CdTe Devices.

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81 CHAPTER 6 CONCLUSION In the previous chapter, SnO2:F and ITOthe two most popular choices as front contacts to CdTe solar cells were compared. It can be observed that the ITO film with only 2000 thickness (1/3 rd of the thickness of standard SnO2) lead to excellent device performance on par with the standard SnO2 devices. Devices made with both SnO2 and ITO without a high resistance buffer layer ha d low device parameters but ITO showed a barrier formation at the CdS/IT O interface, leading to large se ries resistance. So within reasonable limits, it can be conc luded that ITO by itself (without a buffer layer) is not an effective as front contacts for CdTe solar cells, but considering the facts that devices without buffer layers performed poorly in either case (SnO2 and ITO), and barrier formation was not observed when a high resi stivity buffer layer was included, this conclusion would not be a deterrent for the use of ITO as front contact in solar cell applications. Table 21. SnO2: F/CdS/CdTe Devices with Different Buffer Layers. TCO Buffer layer(2000) Voc [mV] FF Jsc [mA/cm2] Rshunt [ -cm2] Rseries [ -cm2] Eff [%] SnO2CVD(3000 ) 830 0.77 23.0 3300 0.32 14.7% SnO2Sputtered 780 0.55 23.1 600 3.25 9.84% SnO2 :Zn (5%)-sputtered 823 0.67 22.9 900 1.15 12.67% SnO2:F MOCVD at 460C In2O3sputtered 827 0.67 22.7 1100 0.74 12.60%

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82 The performance of SnO2: F/CdS/CdTe devices with different high resistance buffer layers were explored and the best de vices are summarized in table 21. All the devices had standard 800 CBD CdS, 5 m thick CSS CdTe and doped graphite back contact. As can be seen fr om table 21, the use of In2O3 as high resistivity layer resulted in decent device performance. Also it might seem from the table that the best performance was achieved for the device with MOCVD SnO2 as intrinsic layer, but a similar performance was achieved for devices w ith thinner layer of sputtered SnO2 (see table 22). Table 22. SnO2: F/CdS/CdTe Devices with Thin Sputtered SnO2 Buffer Layer. TCO Sputtered SnO2-i Voc [mV] FF[%] Jsc [mA/cm2] Rshunt [ -cm2] Rseries [ -cm2] Eff [%] 250 * 847.0 72.5 23.72 2200 0.63 14.60 SnO2:F 500 830.3 65.6 23.57 750 1.46 12.84 Taken from reference 27. The performance of the best ITO devices with MOCVD SnO2 and sputtered In2O3 are summarized in the table below. Since ITO devices with sputtered SnO2 were not studied, it can be expected within reasonable limits that ITO devices with thin sputtered SnO2 as buffer layer would also lead to exce llent device parameters comparable to the values listed in table 22. Table 23. ITO / CdS/CdTe Devices with Different Buffer Layers TCO Buffer layer Voc [mV] FF[%] Jsc [mA/cm2] Rshunt [ cm2] Rseries [ cm2] Eff [%] SnO2 MOCVD(3000) 820 69 22.1 3300 0.53 12.53% ITO sputtered In2O3 (2000 ) 830 62 23.0 2500 1.10 11.89% ITO devices with, thin sputtered In2O3 layer as the buffer layer also had decent performance. Table 24 summarizes the be st devices with thick CdS and table 25 summarizes the best devices with thin CdS. As seen from table 24, the devices with 250 and 1000 of In2O3 have almost similar good performance, for relatively thick CdS. But in the case of thinner CdS devices, devices with thicker In2O3 layer displayed lower device parameters. However, in either case (ITO/ In2O3 devices with thick or thin CdS),

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83 the use of thin In2O3, resulted in good device parameters consistently. The best efficiency was achieved for the devi ce with only 250 of In2O3 and 600 of CdS with an efficiency of 14.2 %, even when the thickne ss of CdS was further reduced the device performance was appreciable with an efficiency of 13% for a In2O3 thickness of 500 (see table 25). However as often is the case Jsc gains were offset by lower Vocs and FFs. Table 24. Summary of Best ITO/In2O3 Devices with Thick CdS Layer. TCO In2O3 thickness[] CdS thickness[] Voc [mV]FF[%] Jsc [mA/cm2] Rshunt [ cm2] Rseries [ cm2] Eff [%] 250 830 66 24.14 2000 0.76 13.2% 1000 800 830 67 23.56 2000 0.53 13.1% 250 830 67 23.64 2000 0.77 13.1% ITO sputtered 1000 700 820 71 23.76 2500 0.55 13.6% Table 25. Summary of Best ITO/In2O3 Devices with Thin CdS Layer. TCO In2O3 thickness[] CdS thickness[] Voc [mV] FF [%] Jsc [mA/cm2] Rshunt [ cm2] Rseries [ cm2] Eff [%] 250 600 820 72 24.04 2000 0.76 14.2% 250 790 63 25.09 1600 0.89 12.4% ITO sputtered 500 500 820 64 24.81 1400 1.26 13.0% Since the loss of Jsc due to the absorption of Cd S layer in the below 520 nm region of the visible spectrum is an important issue, when it comes to improving, the CdTe solar cell efficiency, this performance of ITO/In2O3 CdTe solar cells seems like a promising possibility. Co sputtering offered an opportunity to deposit and study the effects of ternary spinel TCOs, which is dealt with in the wi ndowless junctions section of the previous chapter, the junction between different tern ary spinel oxides and CdTe layers was explored. The SnO2:F/ZTO/CdTe device had an effici ency of 3.6% and so did the CdIn2O4/CdTe device. The Cd2SnO4/CdTe device had an efficiency of 2.1% and the device performance improved after the inclusion of MOCVD deposited SnO2 as intrinsic layer with an efficiency of 2.4%. The Ternary TCOs have to be studied further, especially Cd2SnO4 and Zn2SnO4 TCOs to achieve the highest efficiency for CdTe solar cells.

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84 REFERENCES [1]. http://matse1.mse.uiuc.edu/~tw/sc/prin.html,accessed August 2003 [2]. http://www.tva.gov/greenpowerswitch/solar.html, accessed September 2003 [3]. http://acre.murdoch.edu.au/refiles/pv/text.html, accessed September 2003 [4]. http://www.cse.ucsc.edu/classes/ee145/Spring03/EE145Lab7.pdf, accessed September 2003 [5]. http://www.ece.drexel.edu/courses/ECEE302/lab/Lab%20VI%20Solar%20cell.doc, accessed September 2003 [6]. H.L.Hartnagel, A.L.Dawar, A.K.Jain, C.Jagadish, “Semiconducti ng transparent thin films”, IOP Publishing ltd, 1995 [7]. http://www.mrs.org/membership/preview/aug2000bull/Gordon.pdf accessed September 2003 [8]. S. B. Zhanga and Su-Huai Wei “Selfdoping of cadmium sta nnate in the inverse spinel structure”, Applied P hysics Letters, 80 (2002) p.1376 [9]. http://www.cstl.nist.gov/div836/836.04/SensorProj/TinOxideSurf.html, accessed September 2003 [10]. http://cst-www.nrl.navy.mil/lattice/ struk/c4. html, accessed September 2003 [11]. X. Li, T. Gessert, C. DeHart, T. Barnes, H. Moutinho, Y. Yan, D. Young, M. Young, J. Perkins, and T. Coutts, “A Comparison of Composite Transparent Conducting Oxides Based on the Binary Compounds CdO and SnO2”, NCPV Program Review Meeting; Lake wood, Colorado, 14-17 October 2001 [12]. X. Wu, P. Sheldon, T.J. Coutts, D.H. Rose, and H. Moutinho, “Application of Cd2SnO4 Transparent Conducting Oxides in CdS/CdTe Thin-Film Devices”, 26th PVSC; Anaheim, California, September 30-October 3, 1997

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85 [13]. David L. Young, Helio Mo utinho, Yanfa Yan, and Timot hy J. Coutts, “ Growth and characterization of radio frequency magne tron sputter-deposited zinc stannate, Zn2SnO4, thin films”, Journal of App lied Physics, 92 (2002) p. 310 [14]. D. L. Young, D. L. Williamson, T. J. Coutts, “Structural characterization of zinc stannate thin films”, Journal of Applied Physics, 91 (2002) p.1464 [15]. X. Wu, R.G. Dhere, D.S. Albin, T.A. Gessert, C. DeHart, J.C. Keane, A. Duda, T.J. Coutts,S. Asher, D.H. Levi, H.R. Moutinho, Y. Yan, T. Moriarty, S. Johnston, K. Emery, and P. Sheldon, “ High-Efficiency CTO/ZTO/C dS/CdTe Polycrystall ine Thin-Film Solar Cells”, NCPV Program Review Meeti ng; Lakewood, Colorado,14-17 October 2001 [16]. X. Wu, S. Asher, D. H. Levi, D. E. King, Y. Yan, T. A. Gessert, and P. Sheldon, “Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline thin-film so lar cells”, Journal of App lied Physics, 89 (2001) p.4564 [17]. Radhouane Bel Hadj Tahar, Takayuki Ban, Yutaka Ohya, and Yasutaka Takahashi, “Tin doped indium oxide thin films: Electrica l properties”, Journal of Applied Physics, 83 (1998) p.2631 [18]. http://www.udel.edu/iec/RWB102.pdf, accessed September 2003 [19]. http://www.matsci.northwestern.edu/f aculty/links/tom/gabyresearch.htm, accessed September 2003 [20]. S N Alamri and A W Brinkman, “The e ffect of the transparent conductive oxide on the performance of thin film CdS/CdTe sola r cells”, Journal of Physics. D: Applied Physics, 33 (2000) p.L1 [21]. M. G. Mason, L. S. Hung, and C. W. Tang, S. T. Lee, K. W. Wong, and M. Wang, “Characterization of treated indium–tin–oxi de surfaces used in electroluminescent devices”, Journal of App lied Physics, 86 (1999) p.1688 [22].http://www.eng.ku.ac.th/~mat/MatDB/MatDB/source/Struc/ceramics/spinel/spinel.ht m, accessed September 2003 [23]. http://www.tf.unikiel.de/matwis/ama t/def_en/kap_2/basics/b2_1_6.html,accessed, September 2003 [24]. J.herrero, M.T.guierrez, C.Guillem, J.M.Cona, M.A.Martinez, A.M.Chaparro, and R.Bayon, “ Photovoltaic windows by chemical ba th deposition”, Journal of Thin Solid Films, 361 (2000) p.28 [25]. J.E.Granata and J.R.Sites, “Effect of CdS thickness on CdS/CdTe Quantum Efficiency”, 25th PVSC; Washington D.C., 13-17 May 1996

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86 [26]. X. Li, D. Albin, S. Asher et al, “The effect of substrate temperature on material properties and the device pe rformance of close-spaced sublimation deposited CdTe/CdS devices “, AIP Conference Proceedings, 353 (1996) p. 376 [27]. R.Mamazza, “Ternary Spinel Cd2SnO4, CdIn2O4, and Zn2SnO4 and Binary SnO2 and In2O3 Transparent Conducting Oxides as Fr ont Contact Materials for CdS/CdTe Photovoltaic Devices”, Electrical Engineer ing, University of South Florida, 2000 [28]. http://www.mrs.org/membership/previ ew/aug2000bull/Lewis.pdf, accessed September 2003 [29]. Keran Zhang, Furong Zhu, C.H.A.Huan, A.T.S.Wee, “Indium tin oxide films prepared by radio frequency magnetron s puttering method at a low processing temperature”, Journal of Thin Solid Films, 376 (2000) p.255 [30]. K. Durose, P.R. Edwards, D.P. Halliday, “Materials aspects of CdTe/CdS solar cells”, Journal of Crystal Growth, 197 (1999) p.733 [31]. C.Ferekides, and J.Britt, “CdTe sola r cells with efficiencies over 15%”, Solar energy Materials & Solar Cells, 35 (1994) p.255 [32]. Alan L. Fahrenbruch, Richard H.Bube “Fundamentals of solar cells”, Academic press, 1983 [33]. John F. Jordan, Albright, United Stat es Patent, patent number 5,279, 678, 1994 [34] The Jsc values from light J-V curves depend on the area measurements, which can vary 10 %, therefore wherever Jsc values are cited for comp arison purposes, they are based on values obtained from the spectral response of the devices; SR measurements are based on NREL calibrated standards and Jsc values are accurate within a maximum error of 3%