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High resistivity zinc stannate as a buffer layer in cds/cdte solar cells

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High resistivity zinc stannate as a buffer layer in cds/cdte solar cells
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Gayam, Sudhakar. R
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Subjects / Keywords:
Semiconductors
Thin films
Transparent conducting materials
Sputtering
Interdiffusion
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 )

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Summary:
ABSTRACT: The electrical conductivity of transparent conducting oxides is well exploited in front surface electrodes for solar cells where high transmission is also important. Fluorine doped tin oxide (SnO2: F) is the most popular choice of front contacts for CdTe solar cells. In this thesis, Cd2SnO4 and Zn2SnO4 thin films are investigated focusing on their electrical and optical properties and used them in solar cells. Processing for these materials is optimized for optimum solar cell performance. Cd2SnO4 thin films are deposited by co-sputtering of CdO and SnO2 targets in Ar ambient at room temperature. Then films are subjected to high temperature annealing in He ambient. The films crystallize in inverse spinel structure. The average transmission of a Cd2SnO4 thin film with a thickness of 2500angstrom obtained in this study is 92%. The lowest resistivity obtained in this work for a Cd2SnO4 film with a thickness of 2500angstrom is 5.4 X10-4 cm.The effect of stoichiometry on structure, optical and electrical properties of Cd2SnO4 is studied by varying the amount of CdO and SnO2 in the Cd2SnO4 film. Zinc stannate thin films are deposited by co-sputtering of ZnO and SnO2 targets in Ar ambient at both room temperature and elevated temperatures. As deposited and high temperature annealed Zn2SnO4 thin films are highly resistive. The average transmission of a Zn2SnO4 thin film with a thickness of 2000angstrom and annealed at 600ʻC in He has been 94%. Zn2SnO4 thin films are incorporated as a buffer layers into CdTe solar cells. SnO2: F is used as a front contact in CdTe solar cells in conjunction with high resistive Zn2SnO4 buffer layer.The best SnO2:F /zinc stannate cell device performance for room temperature deposited zinc stannate film resulted for the device with Zn/Sn =2.1. It has an efficiency of 12.43% with VOC = 810mV, FF = 66.6% and JSC = 23.1 mA.
Thesis:
Thesis (M.S.E.E.)--University of South Florida, 2005.
Bibliography:
Includes bibliographical references.
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by Sudhakar R. Gayam.
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Title from PDF of title page.
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Document formatted into pages; contains 93 pages.

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ABSTRACT: The electrical conductivity of transparent conducting oxides is well exploited in front surface electrodes for solar cells where high transmission is also important. Fluorine doped tin oxide (SnO2: F) is the most popular choice of front contacts for CdTe solar cells. In this thesis, Cd2SnO4 and Zn2SnO4 thin films are investigated focusing on their electrical and optical properties and used them in solar cells. Processing for these materials is optimized for optimum solar cell performance. Cd2SnO4 thin films are deposited by co-sputtering of CdO and SnO2 targets in Ar ambient at room temperature. Then films are subjected to high temperature annealing in He ambient. The films crystallize in inverse spinel structure. The average transmission of a Cd2SnO4 thin film with a thickness of 2500[angstrom] obtained in this study is 92%. The lowest resistivity obtained in this work for a Cd2SnO4 film with a thickness of 2500[angstrom] is 5.4 X10-4 cm.The effect of stoichiometry on structure, optical and electrical properties of Cd2SnO4 is studied by varying the amount of CdO and SnO2 in the Cd2SnO4 film. Zinc stannate thin films are deposited by co-sputtering of ZnO and SnO2 targets in Ar ambient at both room temperature and elevated temperatures. As deposited and high temperature annealed Zn2SnO4 thin films are highly resistive. The average transmission of a Zn2SnO4 thin film with a thickness of 2000[angstrom] and annealed at 600C in He has been 94%. Zn2SnO4 thin films are incorporated as a buffer layers into CdTe solar cells. SnO2: F is used as a front contact in CdTe solar cells in conjunction with high resistive Zn2SnO4 buffer layer.The best SnO2:F /zinc stannate cell device performance for room temperature deposited zinc stannate film resulted for the device with Zn/Sn =2.1. It has an efficiency of 12.43% with VOC = 810mV, FF = 66.6% and JSC = 23.1 mA.
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High Resistivity Zinc Stannate As A Bu ffer Layer In CdS/CdTe Solar Cells by Sudhakar R. Gayam 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 L. Morel, Ph.D. Yun L. Chiou, Ph.D. Date of Approval: March 23, 2005 Keywords: semiconductors, thin films, tr ansparent conducting materials, sputtering, interdiffusion Copyright 2005 Sudhakar R. Gayam

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ACKNOWLEDGEMENTS I would like to thank my Major Professor, Dr Chris Ferekides, for giving the opportunity to work towards my Masters Thesis. I am very grateful to him for having providing me the inspiration to learn and explore all the areas that fall under our research group. I would like to thank Dr. Don Morel for his f eedback during our group meetings. I would also like to thank Dr. Y.L.Chiou fo r agreeing to be in my committee. I would like to thank Dr. Robert Mamazza fo r helping me get started in our group. I would like to thank my colleagues Zhao, Sril atha, Lin, Mathesh, Swetha, Venkat, Vikram and Prashant for being very supportive and co -operative all through my research. Last but not least, I would like to thank my family who has been the pillar of support during each and every phase of my educational career.

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DEDICATION This thesis is dedicated to the cherished memory of my mother

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TABLE OF CONTENTS LIST OF TABLES iv LIST OF FIGURES vi ABSTRACT x CHAPTER 1. INTRODUCTION 1 1.1 Different forms of energy 1 1.2 Sunlight 4 1.3 Solar cells 5 CHAPTER 2. SOLAR CELL DEVICE PHYSICS 7 2.1 Semiconductor fundamentals 7 2.1.1 Hetrojunctions 8 2.2 Principle of operation of sola r cell and solar cell physics 10 CHAPTER 3. TRANSPAREN T CONDUCTING OXIDES AND LITERTURE REVIEW 16 3.1 Transparent conducting oxides 16 3.1.1 Optical materials 17 3.1.2 Electrical properties 19 3.1.3 Compromise between optical and elec trical properties 20 3.2 Transparent conducting oxidematerials 22 3.2.1 SnO 2 : F 23 3.2.2 Cd 2 SnO 4 26 3.2.3 Zn 2 SnO 4 28 3.2.3.1 Structural properties 28 3.2.3.2 Optical properties 30 3.2.3.3 Electrical properties 31 3.3 Zn 2 S nO 4 as a buffer layer in CdS/CdTe solar cells 32 i

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CHAPTER 4. EXPERIMENTAL METHODS 36 4.1 Processing 36 4.1.1 Substrate cleaning 36 4.1.2 Chemical vapor deposition of SnO 2 :F 36 4.1.3 Sputtering of Zn-Sn-O (ZTO) 37 4.1.4 Chemical bath deposition process (CBD) of CdS 38 4.1.5 Close spaced sublimation of CdTe 40 4.1.6 Cadmium chloride heat treatment 40 4.1.7 Contacting 41 4.2 Material & device characterization 41 4.2.1 Material characterization 41 4.2.2 Solar cell measurements 42 CHAPTER 5. RESULTS AND DISCUSSION 43 5.1 Zn 2 SnO 4 (Zinc stannate) films sputtered at room temperature 43 5.1.1 Structural and optical properties 43 5.2 Zn 2 SnO 4 (Zinc stannate) films sputtered at 400C 48 5.2.1 Structural properties 48 5.3 CdS/CdTe solar cells us ing zinc stannate film as a buffer layer 53 5.3.1 Zinc stannate films sputtered at room temperature 53 5.3.1.1 J-V characteristics 53 5.3.1.2 Spectral Response 54 5.3.2 Zinc stannate films sputtere d at room temperature with varying Zn/Sn ratios 55 5.3.2.1 J-V characteristics 56 5.3.2.2 Spectral response 57 5.3.2.3 Collection issues 57 5.3.3 Zinc stannate films sputtered at 400C 59 5.3.3.1 Zinc stannate films annealed at 600C for 5 minutes 60 5.3.3.1.1 J-V characteristics 60 5.3.3.1.2 Spectral response 61 5.3.3.1.3 Collection issues 62 5.3.3.2 Zinc stannate films annealed at 600C for 20 minutes 63 5.3.3.2.1 J-V characteristics 64 5.3.3.2.2 Spectral response 65 ii

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5.3.3.3 Zinc stannate films annealed at 600C for 30 minutes 65 5.3.3.3.1 J-V characteristics 66 5.3.3.3.2 Spectral response 67 5.3.3.3.3 Collection issues 67 5.4 Highly conductive transparent conducting oxides 69 5.4.1 Cadmium stannate 69 5.4.1.1 Structural and optical properties 69 5.4.1.2 Effect of stoichiometry on structural, optical and electrical properties of cadmium stannate film 72 CHAPTER 6. CONCLUSIONS 75 6.1 Investigation of materials 75 6.2 Comparison of devices made with zinc stannate as high resistive buffer layer 75 REFERENCES 77 iii

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LIST OF TABLES Table 1. Table comparing various energy sources 2 Table 2. J SC losses in different TCOs 34 Table 3. I-V data of CdTe solar cells with different device structures 35 Table 4. The parameters of the highest efficiency CdS/CdTe solar cell achieved by NREL 35 Table 5. List of materials present in the XRD patterns of a room temperature deposited Zn 2 SnO 4 film and annealed at various temperatures 44 Table 6. List of materials present in the XRD patterns of a room temperature deposited Zinc stannate film with different Zn/Sn ratios(Annealed at 600C) 46 Table 7. List of materials present in the XRD patterns of Zn 2 SnO 4 films deposited at 400C 49 Table 8. List of materials present in the XRD patterns of Zn 2 SnO 4 films deposited at 400C and annealed at 600C 50 Table 9. Summary of SnO 2 / Zn 2 SnO 4 devices (ZTO room temperature deposited and Zn/Sn=2.0) 53 Table10. Summary of SnO 2 / Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 56 Table 11. Summary of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 60 Table 12. Summary of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 20 min) 64 Table 13. Summary of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 66 iv

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Table 14. Summary of the best devices with ZTO deposited at room temperature 76 Table 15. Summary of the best devices with ZTO deposited at 400C 76 v

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LIST OF FIGURES Figure1. Spectral distribution of sunlight 4 Figure 2. Basic solar cell device structure 6 Figure 3. Fermi levels in ptype and n-type semiconductor 7 Figure 4. Band alignments in homojunction 8 Figure 5. Band alignments in hetrojunction 9 Figure 6. Principle of operati on of the solar cell 11 Figure 7. Typical I-V curve for an illuminated solar cell 13 Figure 8. Equivalent circuit of a solar ce ll with series and shunt resistances, R s & R sh 14 Figure 9. Spectral dependence of a semiconducti ng transparent material 18 Figure 10. Spectral dependence of refl ectance of TCOs with different carrier concentrations 20 Figure 11. Spectral dependence of absorptance of TCOs with different carrier concentrations 21 Figure 12. Spectral dependence of absorptance of TCOs with different mobilities 22 Figure 13. Structure of tin oxide 24 Figure 14. Optical transmittance, reflectance and absorptance of SnO 2 : F film 25 Figure 15. X-ray diffraction pattern for CVD deposited SnO 2 : F film 25 Figure 16. Optical transmission of Cd 2 SnO 4 films with a thickness of 2000A before and after H 2 annealing 27 Figure 17. Transmission and absorbance of a Cd 2 SnO 4 and SnO 2 film with a sh eet resistivity of ~10 / 28 vi

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Figure 18. XRD pattern for a Zn 2 SnO 4 film grown at 550C 29 Figure 19. AFM image of a Zn 2 SnO 4 film grown at 600C in Argon 30 Figure 20. Percent transmittance, reflectance and absorptance vs wavelength for a Zn 2 SnO 4 film 30 Figure 21. Percent transmission vs wavelength for Zn 2 SnO 4 film with different carrier concentrations 31 Figure 22. Relation between J SC and transmission of Cd 2 SnO 4 and SnO 2 films 33 Figure 23. Modified CdS/CdTe device structure 34 Figure24. Schematic diagram of the CVD reactor used for SnO 2 :F deposition 37 Figure 25. Schematic of sputter gun and substrate holder set-up 38 Figure 26. Chemical bath deposition set-up 39 Figure 27. Schematic diagram of close spaced sublimation set-up 40 Figure 28. Standard device structure 41 Figure 29. XRD of a room temperature deposited Zn 2 SnO 4 film annealed at high temperatures 44 Figure 30. XRD of a room temperature de posited zinc stannate film with different Zn/Sn ratios 46 Figure 31. Transmission spectra of room temperature deposited zinc stannate film and annealed at various temperatures 47 Figure 32. Transmission spectra of zinc stannate films with different Zn/Sn ratios(annealed at 600C) 47 Figure 33. XRD of a 400C deposited zinc stannate film 49 Figure 34. XRD of a 400C deposited zinc stannate film and annealed at 600C 50 Figure 35. Two dimensional phase images of as-deposited zinc stannate film at 400C and annealed at 600C for 20 minutes in He 51 Figure 36. AFM images of zinc stannate film before and after annealing 52 vii

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Figure 37. Light J-V (left) and dark J-V (right) charac teristics of SnO 2 / Zn 2 SnO 4 devices (ZTO room temper ature deposited and Zn/Sn=2.0) 54 Figure 38. Spectral response graph for SnO 2 / Zn 2 SnO 4 devices (ZTO deposited at room temperature and Zn/Sn=2.0) 55 Figure 39. Light J-V (left) and dark J-V (right) char acteristics of SnO 2 / Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 56 Figure 40. Spectral response of SnO 2 / Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 57 Figure 41. Monochromatic J-V curves and FF vs. wavelength plots for SnO 2 / Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 58 Figure 42. Light J-V (left) and dark J-V (right) charac teristics of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 61 Figure 43. Spectral response of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 61 Figure 44. Monochromatic J-V curves and fill factor vs. wavelength plots for SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 62 Figure 45. Light J-V (left) and dark J-V (right) charac teristics of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 20 min) 64 Figure 46. Spectral response of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 20 min) 65 Figure 47. Light J-V (left) and dark J-V (right) charac teristics of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 66 Figure 48. Spectral response of SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 67 viii

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Figure 49. Monochromatic J-V curves and fill factor vs. wavelength plots for SnO 2 / Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn = 2.0, annealed @ 600C for 30 min) 68 Figure 50. XRD of Cd 2 SnO 4 films annealed at four different temperatures 70 Figure 51. Transmission spectra of Cd 2 SnO 4 films annealed at four different temperatures 71 Figure 52. Dependence of resistiv ity on annealing temperature 71 Figure 53. XRD patterns of cadmium stanna te films for varying stoichiometry 72 Figure 54. Transmission spectra of cadmium stannate films for varying stoichiometry 73 Figure 55. Dependence of resistivity of cadmium stannate films for varying stoichiometry 73 ix

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HIGH RESISTIVITY ZINC STANNATE AS A BUFFER LAYER IN CdS/CdTe SOLAR CELLS Sudhakar R. Gayam ABSTRACT The electrical conductivity of transparen t conducting oxides is well exploited in front surface electrodes for solar cells where high transmission is also important. Fluorine doped tin oxide (SnO 2 : F) is the most popular choice of front contacts for CdTe solar cells. In this thesis, Cd 2 SnO 4 and Zn 2 SnO 4 thin films are investigated focusing on their electrical and optical propert ies and used them in sola r cells. Processing for these materials is optimized for optimum solar cell performance. Cd 2 SnO 4 thin films are deposited by co-sputtering of CdO and SnO 2 targets in Ar ambient at room temperature. Then films ar e subjected to high temperature annealing in He ambient. The films crystallize in inverse sp inel structure. The average transmission of a Cd 2 SnO 4 thin film with a thickn ess of 2500 obtained in this study is 92%. The lowest resistivity obtained in this work for a Cd 2 SnO 4 film with a thickne ss of 2500 is 5.4 X 10 -4 -cm. The effect of stoichiometry on structur e, optical and electr ical properties of Cd 2 SnO 4 is studied by varying th e amount of CdO and SnO 2 in the Cd 2 SnO 4 film. Zinc stannate thin films are deposit ed by co-sputtering of ZnO and SnO 2 targets in Ar ambient at both room temperature and el evated temperatures. As deposited and high temperature annealed Zn 2 SnO 4 thin films are highly resistive. The average transmission x

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of a Zn 2 SnO 4 thin film with a thickness of 2000 and annealed at 600C in He has been 94%. Zn 2 SnO 4 thin films are incorporated as a buffer layers into CdTe solar cells. SnO 2 : F is used as a front contact in CdTe solar cells in conjunction with high resistive Zn 2 SnO 4 buffer layer. The best SnO 2 :F /zinc stannate cell device performance for room temperature deposited zinc stannate film resulted for the device with Zn/Sn =2.1. It has an efficiency of 12.43% with V OC = 810mV, FF = 66.6% and J SC = 23.1 mA. The best SnO 2 :F /zinc stannate cell device performance for Zn 2 SnO 4 thin film deposited at 400C resulted for the device with 500 thick zinc stannate. It has an efficiency of 14.21% with V OC = 830mV, FF = 69.3% and J SC = 24.74 mA. xi

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CHAPTER 1 INTRODUCTION The energy demand is ever increasing with time but the availability of energy resources is depleting day-by-day. Most of the energy that we use today comes from fossil fuels like coal, petroleu m and natural gas. With the su dden rise in the energy needs, we have already consumed a major percentage of these fossil fuels, which are nonrenewable and exhaustible. Acute shortage will occur in the very near future. Moreover the combustion of fossil fuels is the primary concern. It causes air pollution which is not only dangerous to human health but also caus es an imbalance among the gases present in the earths atmosphere. Combustion increases the concentration of carbon dioxide which is the main reason for global warming. 1.1 Different forms of energy Growing concern over the availability of fossil fuels drives mankind in search of viable alternatives like nucl ear fission, nuclear fusion, wi nd, hydroelectric, solar etc. Nuclear power generation encounters problems w ith the disposal of radioactive waste and the maintenance of nuclear re actors. Electrical power ge neration by wind is unreliable and economically not feasible. Most non contr oversial way of generating electricity is hydroelectric power but it acc ounts for only a small percentage of the energy needed. T 1

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able 1 gives the survey of various energy s ources with the advantages and disadvantages associated with them [24]. Compared to va rious energy sources, solar energy has several advantages. The source of solar energy (Sun) is unlimited for all purposes. Solar cells cause no harm to the environment. Solar power generation is also vers atile; it can be done on a large scale as well as on a small scale. Solar cells are also of great use in space applications. In order for solar cells to be available for commercial use, they have to overcome two important challenges: cost and st orage. The cost of solar power must be competitive to the cost of power generated by the present day generation schemes. This can be accomplished by increasing the efficien cy of solar cells, reducing the production costs, and increasing the lifetime. Energy stor age is one of the impor tant considerations for large scale use of solar energy. Solar ener gy is available during the day but most of the power consumption occurs in the night. So there is a need for efficient storage technology for solar energy. Table 1. Table comparing various energy sources [24] Energy Source Type of Processes Advantages Disadvantages Petroleum Drilling, Oil shale Convenient and low pollution Limited supply, nonrenewable Natural Gas Drilling Convenient and low pollution Very limited supply, nonrenewable Coal Deep or strip mining, solvent refining, pyrolysis, gasification, magneto hydrodynamic usage Easy to handle, Provides source of hydro carbons in gaseous or liquid form as well as normal solid form Atmospheric pollution, nonrenewable 2

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Table 1 (Continued) Nuclear Fission Light water reactors Existing technology, compact Limited fuel supply, operating and transport hazards Solar Thermal Direct heating and cooling via absorption of solar radiation Pollution-free unlimited energy source Need for extensive architectural changes, storage required Solar electric Use of solar energy to operate a steam turbine Pollution-free unlimited energy source Research needed on collection, focusing and storage Solar Photovoltaic Photovoltaic effect in semiconductor junction devices Pollution-free unlimited energy source High cost of the cells, energy storage and operating life time Hydroelectric Full of water gravity used to generate electricity Renewable, inexpensive, can be used as storage process Limited to special locations Tidal energy Motion of water under gravitational pull of moon used to generate electricity Pollution-free renewable Limited number of exploitable sites Wind Force of the wind used to generate electricity Pollution-free renewable Limited to special locations, large scale effect on weather unknown Ocean Thermal Uses thermal gradients to drive heat engine and generate electricity Pollution-free renewabl e Special materials problems in resisting corrosion, costly transmission Biomass Conversion of solid organic matter into synthetic fuel Ready supply, could utilize wastes Uses arable land if deliberately planted, cost uncertain, sludge disposal 3

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1.2 Sunlight Sun is the only long term natural resource available to us. The spectral distribution of the radiation from the sun is modified by the absorption of radiation in the solar atmosphere and the earths atmosphere. Mo st of the intense radiation from the suns deep interior is absorbed by a layer of H ions near the suns surface. Sunlight is attenuated by at least 30% during its passage through the ea rths atmosphere. A typical spectral distribution of sunli ght reaching the earths surfa ce is shown in figure1. The degree of attenuation is highly variable. The most important parameter determining the total incident power under cl ear conditions is the length of the lightpath through the atmosphere. This is shortest when the sun is directly overhead. The ratio of any actual path length to this minimum value is known as the optical air mass (AM). When the sun is directly over head, the optical air mass is unity and the radiation is described as air mass one (AM1) radiation [27]. Figure 1. Spectral distribution of sunlight 4

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1.3 Solar cells The conventional solar cell is a pn junction with a single bandgap, E g Solar cells operate on the principle of the photovoltaic e ffect. When a solar cell is exposed to the solar spectrum, a photon with en ergy greater than or equal to the bandgap of the material contributes to the cell output. The wavelength of the light absorbed can be related to the energy by the following equation. E = hc/ (1) where h is Planks constant, c is the velocity of light and is the wavelength of light. The basic structure of a sola r cell device is shown in figur e 2. When a photon of certain energy is incident on the front surface of th e device, it gets transmitted and may get absorbed in the neutral n-region, depletion re gion, or the neutral p-region of the device depending upon the bandgap of the material an d the energy of the incident photon. This energy of the absorbed photon excites the electron from the valence band to the conduction band generating electr on hole pair. The electron hole pair generated within their corresponding diffusion lengths diffuse to the depletion edge and gets drifted away by the electric field present in the depletion region to the opposite side. For this device structure, front contacts should be optically transparent and electrically conducting so that most of the light gets absorbed near the depletion region. 5

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Photon Front contact n-type p-type Back contact Figure 2. Basic solar cell device structure The objective of this th esis is to explore different transparent conducting oxides which are used as front contacts in thin film solar cells. In CdTe solar cells, fluorine doped tin oxide thin films are used as highly conducting transparent conducting oxides and zinc stannate thin films are used as hi gh resistive buffer layers. Zinc stannate films deposited at room temperature and at 400 C are studied for the high performance of CdTe solar cell. 6

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CHAPTER 2 SOLAR CELL DEVICE PHYSICS 2.1 Semiconductor fundamentals Consider a p-type material and n-type material. The band settings of the p-type and n-type semiconductors ar e shown in figure 3 where E is conduction band energy, E C V is valence band energy and E is the fermi level of a semiconductor. F When a p-n junction is formed, electrons and holes move across the metallurgical junction so that the fermi level E F becomes constant at equilibrium throughout the device. This migration, or diffusion of electrons a nd holes create a charge imbalance on their corresponding n side and p side by leavi ng behind ionized donors and acceptors. This produces a space charge region with an exce ss of negative charge on the p side and EF n-type EV EC p-type EV EF EC Figure 3. Fermi levels in ptype and n-type semiconductor 7

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excess of positive charge on the n side. This charge imbalance sets up an electric field in this region. The diffusion of carriers across the junction produces diffusion current. This diffusion continues until the stre ngth of the electric field is sufficient e nough to drift back these carriers. At this point, the junction is said to be in equilibrium. When a same material is used to form the both sides of the p-n junction but with different type of dopants, then the junction is called homojunc tion as shown in figure 4. The amount of band bending gives the built-i n potential of the device. Figure 4. Band alignments in a homojunction [23] 2.1.1 Hetrojunctions A hetrojunction is formed between two semiconductors with different band gap energies E & E g1 g2, different work functions 1 & different electron affinities & 2 1 2. Figure 5 shows a hetrojunction where E g1 > E g2 8

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Eg1Eg2 EF EV EC12 1 2 qVb1 EV EC EVAC Figure 5. Band alignments in hetrojunction E is difference in energy of conduc tion bands of two semiconductors. C E V is difference in energy of valence band edges of two semiconductors When a junction is formed between these se miconductors, there is discontinuity in the band profile due to different band gap energies and electron affinities. = E C (2) 2 1 E V = E g2 E g1 + (3) 2 1 A negative E C or E V produces a spike in the conduction band or valence band which is undesirable for photovoltaic applications The spike impedes th e flow of minority carriers across the junction from p-type to ntype regions and the photocurrent is reduced. Such spikes can be avoided by a suitable comb ination of material pr operities like electron affinities and band gap energies. 9

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The built in potential is equal to the sum of partial built-in potentials, V b1 & V b2 or it is equal to the difference between the work functions. = (4) 2 1 For hetrojunctions, the change in cr ystal structure across the junction is an inherent structural problem. Due to this, an interface is formed between two semiconductors. Interfaces can be efficient r ecombination centers be cause they introduce deep trap levels in the band gap. They also provide sites for quantum mechanical tunneling process which is important for current loss mechanisms across junction [28]. The origin of interface states may be due to the degree of mismatch between the crystal lattices of the semiconductor. Therefore for a good hetrojunction solar cell, a small E C and good lattice match are necessary. 2.2 Principle of operation of so lar cell and solar cell physics Figure 6 shows a p-n juncti on under illumination. The nside of the junction is thin and heavily doped. By doping the n-regi on heavily, most of the depletion region extends into the p-region. Since the n-region is heavily doped and the lifetime of the holes, which are minority carriers on the n-side is short, use of a thin n-layer helps in avoiding recombination of holes before they reach the depletion region edge. Moreover, if the n-layer is thin, most of the light ge ts transmitted and absorbed in the depletion region which is very important for the collec tion of generated carriers. The front contact is a transparent conducting material. Light is illuminated through th e front contact into the device. When the device is illuminated, as the n-side is narrow, most of the photons get absorbed in the depletion region and neutral p-region generati ng electron hole pairs. 10

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The electron hole pair s generated in the depletion re gion drift under th e influence of electric field present in the depletion region. The electrons generated by the absorption of Neutral n-region E Neutral p-region ln W lp Medium Long Short Back Electrode Figure 6. Principle of operat ion of the so lar cell [25] Depletion region + + Drift Le + Di ffusi on LhFront Electrode Back Electrode photons within the diffusion length of electron s in the neutral p-region diffuse to the depletion region edge and get collected. The el ectrons generated at a distance greater than the diffusion length from the depletion region edge are lost to recombination. Similarly the holes generated by the short wavelength photons absorbed in the n-region diffuse to 11

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the depletion region edge and get collected. If the external terminals of the cell are shorted, current caused by this excess electron-hole transport acros s the junction will flow in an external circuit. The current du e to the flow of photogenerated carriers is called photocurrent, and it depends on the light intensity. The current obtained under this condition is called shor t-circuit current (J SC ). Under short conditions, there is no change in the potential drop across the cell since its te rminals are shorted. If the terminals of the device are open, the current in the external circuit is zero. Charge builds up on both sides of the junction due to excess electrons on n-side and excess holes on the p-side. Eventually, a field builds up in the junction whose di rection is opposite to the direction of electric field in the depletion region cau sed by the ionized donors and acceptors. This effect reduces the height of in ternal barrier which allows the majority carriers in the nand pregions to overcome the barrier and diffuse to the opposite side of the junction. The flow of majority carrier s counteracts the flow of photogenerated minority carriers. Eventually, a steady state is established in which there is no net current in the device. Now an open-circuit voltage (V OC ) develops between the terminals of the device with pside positive with respect to the n-side. The J and the V SC OC depend on the level of incident illumination, geometry of th e device and the material properties. If an external load is connected to the solar cell, a positive voltage appears across the junction as a result of the current passi ng through it. This voltage reduces the built-in potential of the p-n junction [25]. The total current through th e solar cell is given as: I = -I L + I [exp (qV/AkT)-1] (5) 0 12

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Where I L is photogenerated current I 0 -Reverse saturation current A-Diode quality factor The I-V characteristics of a typical solar ce ll measured both in the dark and under the illumination are shown in figure 7. Several para meters involved in the characterization of a solar cell are listed below: I -Short-circuit current SC V -Open-circuit voltage OC I m -Current corresponding to the maximum power point V m -Voltage corresponding to the maximum power point P -Maximum power generation (Product of V max m & I m ) FF-Fill Factor The current I L is the current due to illumination. P max is determined from the I-V product in the 4 th quadrant. Fill factor is defined as FF= V m I / V m OC I (6) SC Figure 7. Typical I-V curve fo r an illuminated solar cell 13

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The photovoltaic conversion efficiency ( ) of the solar cell is a measure of the amount of light energy converted to electrical energy. = P / P max in (7) P in is the incident power i nput to the solar cell. Figure 8 represents the equivalent circuit of th e solar cell with a series resistance R s and a shunt resistance R sh The currentvoltage relationship equation with series and shunt resistance effects included is given by [(exp q (V-IR I = I 0 s ) /AkT)-1] + (V-IR )/R I s sh L (8) Figure 8. Equivalent circuit of a solar ce ll with series and shunt resistances, R & R [26] s sh Generally, the R s can be approximated from the slope of a JV curve at higher current values in the first quadrant and R sh can be approximated from th e slope of a J-V curve in 14

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the third quadrant where J is the current density of the device. For an ideal solar cell, the R is zero and R s sh is infinity. But, in practice, due to various reasons, the series resistance has some finite value greater than zero and shunt resistance is reduced. Lower shunt resistances are mainly due to the formation of defects like pinhol es. Fill factor and V OC are greatly affected by low R Low R also induce leakage currents which affect the J sh sh SC of the device. Series resistance is mainly cause d due to the resistances at the contacts and in the bulk of the device material. Fill fact or and short circuit current are affected by higher series resistances. [26] 15

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CHAPTER 3 TRANSPARENT CONDUCTING OX IDES AND LITERATURE REVIEW 3.1 Transparent conducting oxides Transparent conducting oxides (TCOs), as the name implies are transparent as well as conductive. High transparency co mbined with useful electri cal conductivity is achieved by selecting a wide bandgap oxide that is rend ered degenerate through the introduction of native or substitutional dopants. Transparen t conducting oxides, because of their high transmission and required conductivity, have diverse applications. The ability of TCOs to reflect thermal infrared heat is used to make energy conserving windows also known as low emissive windows. The elec trical conductivity of TCOs is exploited in front surface electrodes for solar cells and flat-panel displays where high transmission of the front electrodes is also important. In aircraft and automobile wind ows, TCOs offer advantages of thermal management and also act as thin film resistive heater elements for demisting and deicing windows. Transparent conducting oxides are also used in electrochromic windows. Electrical induced reduction results in an electrically controllable change in color and light transmission. This effect can be used in the design of smart windows. Transparent conducting oxides can be formed into transparent electro magnetic shields, invisible security circuits on windows, and transparent radio antennas built into automobile windows. [2] 16

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3.1.1 Optical properties The optical properties of a TCO depend on it s material properties such as bandgap and the plasma edge frequency. They also depend on the thickness of the material. When light is incident on the surface of a TCO, it may be absorbed, transmitted and reflected depending upon the above mentioned mate rial properties. If the energy h of the incident photon is greater than or equal to the band gap of the material, then that photon is absorbed. This absorbed photon energy aids in the electron transition from valence band to the conduction band. The amount of light absorbed in a thin film semiconductor depends on the materials wavelength depend ent absorption coefficient as shown in equation 9. d 1/2 ( ) = C (h E ) (9) g d where is the absorption coefficient, h is the Planks constant, is the frequency, E g is the direct bandgap of the material and C is a constant. The plot of 2 against the photon energy (h ) yields a straight line w hose intercept with the energy axis gives the bandgap of the material. The amount of light absorbed can be related to the thickness of the material by the following expression = (1/t) ln (1/ (1-A)) (10) where t is the thickness and A is the absorptance at specific wavelength. [22] If the energy of the incident photon is less than the bandgap, then the photons are transmitted. The important optical characteristic of TCOs is that they have a transmission window between wavelengths of about 0.4 m and 1.5 m. At longer wavelengths, reflection occurs due to the plasma edge. After the absorption edge from the bandgap, a major portion of the light that is not transmitte d is reflected. This reflection is because of 17

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both the surface and bulk of the material. Reflec tion is greater in the near infrared region of the spectrum. This reflection is dominated by bulk reflection that occurs as a result of photonelectron interactions, which induce sc attering mechanisms. Electrons and photons interact due to the electroma gnetic nature of th e photon. The frequency at which this reflection takes place is called plasma frequency given by the following expression 2 p = ( ne / 0 m c 1/2 ) (10) where n is the carrier concentra tion, e is the electronic charge, 0 is the permittivity of the free space, is the high frequency permittivity and m c is the conductivity effective mass [5]. Spectral dependence of a semiconducting transp arent material is shown in figure 9. At frequencies higher than the plasma frequency, electrons cannot respond and the material behaves as a transparent dielectric. At frequencies below the plasma frequency, TCO reflects and absorbs incident ra diation. For most of the TCOs the plasma frequency falls in the near-infrared part of the spectrum and the visible region is in the higher and transparent frequency range [1]. Figure 9. Spectral dependence of a semiconducting transparent material 18

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3.1.2 Electrical properties Electrical conductivity mainly depends on the carrier concentration and mobility of the free carriers. Electrical conductivity is given by the following expression = qn (11) where n is the carrier concentration and is the carrier mobility. Conductivity can be increased either by increasing the carrier concentration or mobility. The increase in the free carrier concentration also increases th e free carrier absorption which effects the transmission of the TCO. On the othe r hand, mobility can be expressed as = q / m c (12) where is relaxation time and m c is the conductivity effectiv e mass. Electron mobility is also determined by the electr on scattering mechanisms that operate in the material. At low doping levels, scattering of electrons by phonons is present. However in practical applications, TCOs with high doping levels are used. Under these conditions, scattering by ionized dopant atoms becomes the im portant scattering phenomenon. In polycrystalline films, grain boundary scat tering is present. All these scattering mechanisms limit the mobility. One way of in creasing the mobility is by increasing the relaxation time. This can be done by producing high quality films with fewer defects and improved orientation. Another way of increa sing the mobility is by decreasing the effective mass [5]. As the increase in carrier concentrat ion increases the free carrier absorption, increasing the mobility is the most effective way of increasing conductivity without affecting the materials optical properties. 19

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3.1.3 Compromise between optical and electrical properties Figure 10 shows the variation of the modeled reflectance of a film with wavelength for different carrier concentrations. Free carrier mo bility is assumed to be constant in this case with carrier concentrat ion, although this may not be achieved in practice. Figure 10. Spectral dependence of reflectan ce of TCOs with different carrier concentrations [5] 19 The carrier concentration is varied between 5 X 10 cm -3 and 1 X 10 21 cm -3 Assumed high frequency permittivity and electron effective mass are 4 and 0.3m e respectively. Film thickness is 0.5 m. For the two higher carrier concentrations, the plasma wavelength changed from approximately 1.6 m to about 1.1 m following the relation mentioned in equation 10. A mean reflectance of about 15% is observed in the visible range of wavelengths due to interference fringes. Beyond the plasma wavelength, reflectance is about 90%. Films with low carri er concentration did not exhibit plasma frequency reflectance. 20

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Figure 11 shows the variation of absorptanc e with wavelength for the same parameters used in figure 10. As the carrier concentratio n is increased, the height of free carrier absorption band increased because there are more carriers available to absorb photons. Figure 11. Spectral dependence of absorpta nce of TCOs with different carrier concentrations [5] Figure 12 shows the modeled variation of absorptance with wavelength for a film with different mobilities. The carrier concen tration is kept constant at 5 X 10 20 cm -3 As the mobility is increased, the wavelength at which the peak of absorption occurred did not change but the height of the free carrier absorption band decreased. Thus by increasing the mobility, the conductivity of TCO can be increased without compromising its optical properties [5]. 21

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Figure 12. Spectral dependence of absorptance of TCOs with different mobilities [5] An effective TCO should have high electrical conduc tivity combined with low absorption of visible light. A figure of merit fo r TCOs can be expressed as follows = {R ln (T+R)} -1 (13) S where R S is the sheet resistance in T is the total visible transmission and R is the total visible reflectance. A larger value of indicates better performance of the TCO [4]. 3.2 Transparent conducting oxide materials Most of the research to develop highly tr ansparent and conductive thin films is focused on n-type semiconductors consisting of metal oxides. These TCOs ma y be classified as binary compound or multicomponent oxide ma terials. Binary co mpound TCO materials which are in practical use, include SnO : F or SnO : Sb, In 2 2 2 O : Sn (indium tin oxide 3 22

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films) and ZnO: Al or ZnO: Ga. Multico mponent oxides which are in practical use include Cd SnO CdIn O 2 4 2 4, Zn SnO and GaInO 2 4 3 Transparent conducting oxides used in this work are SnO : F, Cd SnO undoped SnO and Zn SnO 2 2 4 2 2 4 3.2.1 SnO : F 2 Doped tin oxide is a n-type wide band gap degenerate semiconductor. The properties of SnO 2 depend on the deviation from stoichiometry such as oxygen deficiencies and on the dopants used. Tin oxide crystallizes in the rutile stru cture. The Sn atoms are on a body centered tetragonal latt ice and oxygen atoms in a hexagonal closed packed structure. The rutile structure is shown in figure 13. Each Sn cation has a coordination number of six, forming an octahedral geometry with anionic oxygen present. If SnO 2 is perfectly stoichiometric, it would be an insulator or at most an ionic conductor. However, in practice, the material is never stoichiometric. It is anion deficient due to the formation of oxygen vacancies in the perfect crystal. Th ese vacancies are responsible for making electrons available for the conduction process. Ionized oxygen vacancies predominate in SnO according to the following defect reaction. 2 Sn x Sn + 2O x O Sn x + 2V .. Sn O + 4e + O 2 (g) (14) where Sn x Sn and O x O represent bonded tin and bonded oxygen respectively; V .. O represents oxygen vacancy [1]. Even a perfectly stoichiometric SnO crystal can be made 2 conducting by creating oxygen deficiencies by h eating the film in a slightly reducing atmosphere. The other method of making SnO 2 conducting is by chemical doping, for example, by fluorine or antimony. Either the doping of cation or anion sites by higher valency impurities in the oxide mate rials increases th e ntype conductivity. 23

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Figure 13. Structure of tin oxide [23] Fluorine doped tin oxide film with a thickne ss of 5000 has an average transmission of 83.3% as shown in figure14 [22]. The loss in transmission is mainly due to reflection. The average reflection in the visible region is 13.2% and the averag e absorption is 3.6%. SnO 2 : F films are deposited by chemical vapor deposition using TMT (Tetra Methyl Tin) and oxygen at 450C. SnO 2 : F films are polycrystalline in the asdeposited form. XRD spectra from figure 15 shows that the preferred orientat ion is along [110] direction. 24

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Figure 14. Optical transmittance, reflectance and absorptance of SnO : F film [22] 2 Figure 15. X-ray diffraction pattern for CVD deposited SnO : F [22] 2 25

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3.2.2 Cd 2 SnO 4 Cd SnO 2 4 is a ternary mixed metal oxide. The lo west energy crystal structure is the thermodynamically stable orthorhombic system However, sputtered films are of high quality and crystallize in both the spinel and inverse spinel structures [10]. Half of the cadmium cations occupy tetrahedral sites and the remaining half are distributed with tin cations on the octahedral sites (Cd[SnCd]O 4 ). Oxygen ions are in face centered cubic close packing. The conductivity in Cd SnO is mainly due to defects in its structure. 2 4 The possible defects in Cd SnO are Sn Cd 2 4 Cd, I V Cd and V Cd Sn O which represent Sn on Cd antisite, Cd interstials, Cd vacancies, Cd on Sn antisite and Oxygen vacancies respectively. Among these defects, Sn and Cd Cd I are shallow donors; V O is a deep donor; V is a shallow acceptor and Cd Cd Sn is a deep acceptor. Of all these defects, the formation of Sn Cd antisites is energetically favorable which is responsible for the n-type conductivity of unint entionally doped Cd SnO Cd SnO 2 4 2 4 can be made conductive through the creation of a Cd-rich growth environment. The Cdrich growth environment is to generate a Snrich environment in the la ttice as a result of the chemical potentials, and being co-dependant. The high conductivity of Cd SnO Sn Cd 2 4 can be attributed to the high mobility of carriers. This is proba bly because of the high degree of structural order of Cd 2 SnO 4 films due to which longer relaxatio n times are obtained. Mobilities as high as 80 cm 2 V -1 -1 s have been obtained. [9, 10, 11]. Figure 16 shows the optical transmission for Cd SnO films with a thickness of 2000 and annealed in He and H 2 4 2 Hydrogen, a reducing agent, increased th e materials carrier concentration. 26

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Figure 16. Optical transmission of Cd SnO 2 4 films with a thickness of 2000 before and after H 2 annealing [18] In the short wavelength range of 300-400 nm, a s light shift in the absorption edge exists as a result of H 2 annealing. This might be a Moss-Bu rstein shift due to the increased electron density. The slight increase in reflection in the infrared region might be a free carrier reflection due to the incr eased carrier concentration [18]. Cd SnO 2 4 films have better optical properties compared to conventi onal TCOs. This is due to the lower resistivities of Cd SnO 2 4 films, which allow thinner films to be used. Figure 17 compares the tran smission and absorption of Cd SnO and SnO 2 4 2 films with similar sheet resistivities. The absorbance of the Cd SnO 2 4 film in the visible range is much lesser than that of SnO film [15]. 2 27

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Figure 17. Transmission and absorbance of a Cd SnO and SnO 2 4 2 film with a sheet resistivity of ~10 / [15] 3.2.3 Zn SnO 2 4 3.2.3.1 Structural properties Zn SnO 2 4 is an inexpensive, optically transparent and electrically conducting oxide. Zn SnO 2 4 has superior optical transparency but lower electrical conductivity by several orders of magnitude than Cd SnO 2 4 films. Zinc stannate has two different oxides with different crystallographi c structures and zinc to tin ratios: ZnSnO and Zn SnO ZnSnO 3 2 4 3 crystallizes in the orthorhombic phase and Zn SnO 2 4 crystallizes in the inverse spinel structure with half of the Zn cations occ upying the tetrahedral s ites and the remaining half are distributed with tin cations in the oc tahedral site. This spinel lattice is locally distorted enough to form two distinct octahedr ally coordinated Sn and Zn sites. The two Sn and Zn octahedral sites in the lattice si gnificantly limit the mobility of the carriers, possibly by disrupting the edge-s haring nature of the octahedr al sites and thereby forming the open circuits in the conductive path ways [13]. 28

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Figure 18. XRD pattern for a Zn 2 SnO film grown at 550C [14] 4 Figure 18 shows the XRD pattern of a film deposited by RF sputtering in Ar at substrate temperatures of 550C. It shows a preferred orientation of (220). As-g rown films at room temperature are amorphous in nature. Subsequent annealing at higher temperatures makes these films polycrystalline. Figure 19 shows an AFM image of a sample grown at 600C in Ar. The image reveals grains of 100nm in diameter and a surface roughness of 4.3nm. This unusually smooth surface is one of th e qualities that is thought to make Zn SnO 2 4 ideal as a buffer layer in solar cells [14]. 29

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Figure 19. AFM image of a Zn SnO film grown at 600C in Argon [14] 2 4 3.2.3.2 Optical properties Figure 20 shows the transmittance, reflectance, and absorptance of a Zn SnO 2 4 film for wavelengths between 300 and 2500nm. Figure 20. Percent transmittance, reflectance and absorptance vs wavelength for a Zn SnO film [14] 2 4 30

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Optical data on Zn SnO 2 4 films reveal that the material is highly transparent due to its low absorptance in the visible region which is less than 1.5%. The slight rise in the absorption curve near 2300 nm is due to the onset of absorption by free electrons in the conduction band. The sharp increase in the absorptance ne ar 350nm is due to the direct band gap of the material. The direct bandgap absorptionedge wavelength highly depends on the carrier concentration in the films. Figure 21 shows the shift in the absorption edge wavelength with varying carrier concentration indicating a Mo ss-Burstein shift. The band gap shifted from 3.35eV to 3.89eV with in creasing carrier co ncentration [14] SnO Figure 21. Percent transmission vs wavelength for Zn 2 4 film with different carrier concentrations [14]. 3.2.3.3 Electrical properties Zn SnO 2 4 films are resistive compared to ot her conventional TCOs. Low mobilities (< 27 cm 2 19 /V-s) and carrier concentrations (< 4X10 cm -3 ) account for the low resistivities. The increase in carrier concentration in the films annealed in reducing atmosphere and decrease in carrier concentra tion in the films annealed in oxidizing atmosphere indicate that oxygen vacancies might be the possible mechanism for the generation of carriers. 31

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Low mobilities in Zn SnO 2 4 films can be attributed to the short relaxation times. High density of defects responsible for the poor quality of the film limits the relaxation time of the carriers. Relatively low effective mass values in Zn SnO 2 4 films compared to other TCO materials is the reason for the large Burstein-Moss shift observed in the optical properties of the film [17]. SnO as a buffer layer in CdS/CdTe solar cells 3.3 Zn 2 4 There is a significant photocurrent loss in CdS/CdTe sola r cells due to window-layer CdS bandgap absorption below 520nm. With d ecrease in CdS thickn ess, there is an increase in the device short-circuit current (J ). However, the open-circuit voltage (V SC OC ) and fill factor (FF) tend to decrease as the CdS is thinned [19]. In order to increase the device J without affecting V SC OC and fill factors, there is a need for consumption of CdS in the device during the processing itself. By integrating the Zn SnO 2 4 film into a CdS/CdTe solar cell as a buffer layer, inte rdiffusion consumes th e CdS film from both the Zn SnO 2 4 and CdTe sides during the device fabrication process and improves the quantum efficiency at shorter wavelengths. The Zn SnO 2 4 film acts as a Zn source to alloy with the CdS film, which resu lts in the increase in the ba ndgap of the window layer and in the short circuit current density. This interdiffussion can also significantly improve the device adhesion after CdCl 2 treatment, thus providing mu ch greater latitude when optimizing CdCl process step. The optimum CdCl 2 2 treated CdTe device has high quantum efficiency at longer wavelengths be cause of its good junction properties and well passivated CdTe film [20]. 32

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On the other hand, Cd 2 SnO 4, because of its superior electr ical and optical properties, can serve as a better replacement for conve ntional TCOs like SnO 2 CdS/CdTe solar cells with Cd 2 SnO as TCO yield improved s hort-circuit densities (J 4 SC ). Figure 22 shows the relation between J and transmission of Cd SnO and SnO SC 2 4 2 films [16]. Replacing the 10000 SnO film with a Cd SnO film in CdTe solar cell yielded an increase in J 2 2 4 SC of more than 1.5 mA/cm 2 [15]. and transmission of Cd SnO and SnO films [15] Figure 22. Relation between J SC 2 4 2 Table 2 shows the J SC loss due to glass/TCO absorption for three CdTe cells on different TCO superstrates. It can be seen that the Cd SnO based cell has the lowest J 2 4 SC loss (0.7 mA/cm 2 ) which is two to four times lower than SnO 2 based cells prepared by TMT and SnCl 4 precursors respectively. Figure 23 s hows the modified device structure of CdTe solar cell with the inclusion of Cd SnO and Zn SnO 2 4 2 4 33

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Table 2. J losses in different TCOs [16] SC Glass/TCO Absorption loss(mA/cm TCO R S ( /Sq) 2 ) SnO (SnCl ) 8-10 2.8 2 4 SnO (TMT) 6-8 1.3 2 Cd SnO 6-8 0.7 2 4 Figure 23. Modified CdS/CdTe device structure [16] Table 3 consists of I-V data of the Cd S/CdTe solar cells with different device structures. The use of a Zn 2 SnO 4 buffer layer for both SnO 2 and Cd 2 SnO 4 based solar cells showed improvement in all the device parameters of the solar cell thereby increasing the efficiency of the solar cell. Cd 2 SnO 4 based solar cells show their Glass Cd2SnO4 Zn2SnO4 CdS Back Contact CdTe FrontContact 34

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dominance over SnO 2 based solar cells because of its superior electrical and optical properties as discussed earlier in this chapte r. Table 4 summarizes the parameters of the highest efficiency CdS/CdTe so lar cell achieved by NREL [23]. Table 3. I-V data of CdTe solar cells w ith different device structures [20] Device Structure V OC (mV) J (mA/cm 2 ) FF(%) (%) SC SnO /CdS/CdTe 806.7 22.61 74.02 13.5 2 Cd SnO /CdS/CdTe 805.2 23.53 73.77 14.0 2 4 SnO /Zn SnO /CdS/CdTe 830.1 24.10 74.15 14.8 2 2 4 Cd SnO /Zn SnO /CdS/CdTe 844.3 25.00 74.82 15.8 2 4 2 4 Table 4. The parameters of the highest effici ency CdS/CdTe solar cell achieved by NREL [23] Cell # V OC (mV) J SC (mA/cm 2 ) FF(%) (%) Area(cm 2 ) W547-A 847.5 25.86 74.45 16.4 1.131 W567-A 845.0 25.88 75.51 16.5 1.032 35

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CHAPTER 4 EXPERIMENTAL METHODS This chapter discusses various processi ng and characterizatio n techniques used for the fabrication and characteriza tion of thin films and solar cells. 4.1 Processing 4.1.1 Substrate cleaning Corning 7059 borosilicate gl ass with dimensions 1.25 X 1.45 X 0.032 is used as a substrate. The substrates are cleaned in a 10% by volume HF solution; after a brief dip (7-10 secs), the substrates are rinsed t horoughly in deionized wa ter and then dried. 4.1.2 Chemical vapor deposition of SnO :F 2 Tin oxide is deposited by the chemical vapor deposition (CVD) technique. The CVD chamber used for the deposition of SnO 2 is shown in figure 24. It consists of a quartz tube within which a substrate holder heated by RF coils is placed. The deposition is done at 450C. The precursors used are Tetra Methyl Tin (TMT) (Sn source) and O 2 Tin oxide films are doped with Fluorine by introducing CFBr 3 a commercially available Freon. All the gases are introduced into the chamber using Mass Flow Controllers (MFC). The oxidation chemical reacti on for this process is as follows: Sn (CH ) 3 4 (Vap) + 8O 2 (gas) SnO 2 (solid) + 4CO 2 (gas) + 6H O (15) 2 (gas) 36

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Figure 24. Schematic diagram of the CVD reactor used for SnO :F deposition [23] 2 4.1.3 Sputtering of Zn-Sn-O (ZTO) ZTO films are prepared by RF magnetron co-sputtering of SnO 2 (99.999% purity) and ZnO (99.999% purity) targets. ZTO films are deposited at room temperature and at 450C in Ar ambient. Cadmium stannate (Cd SnO 2 4 ) films are prepared by RF magnetron co-sputtering of SnO 2 (99.999% purity) and CdO (99.999% purity) targets at room temperature in Ar ambient. By co-sputteri ng, one can control the individual composition of each material in the compound film controlling the deposition rates of the constituent oxides. A schematic of the sputtering chamber is shown in figure 25.The vacuum chamber used is a Consolidated Vacuum Corp. mode l, magnetron sputtering sources are Kurt J. Lesker Torus TRS3FSA models and power supplies are Advanced Energy RFX-600 models. The sputtering sources are positione d such that the center line of each gun created an angle of approximately forty degrees with respect to a plane parallel to the 37

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substrate holder. To ensure uniformity in f ilm thickness, the substrate holder is rotated during the deposition. Before each depositi on, the chamber is pumped down to a base pressure of 4 X 10 5 torr. The operating pressure for ZTO and cadmium stannate films is 2 X 10 3 3 torr and 3 X 10 torr respectively. After the sputter deposition, in order to impr ove the crystallinity, f ilms are subjected to high temperature annealing. This is carried out in an evacuated closed end quartz tube with a vacuum line outlet and a gas inlet. Tw o graphite substrate ho lders, one on which the substrates are placed and the other on top of the substrates are used. High temperatures are provided by two halogen la mps of 2KW each, one on the top and other at the bottom of the graphite holder s. Helium is used as the ambient. Figure 25. Schematic of sputter gun and substrate holder set-up [23] 4.1.4 Chemical bath deposition (CBD) of CdS Cadmium sulphide is a commonly used se miconductor for optoelectronic devices. CdS films are deposited by several techniques includ ing vacuum evaporation, spray pyrolysis, close spaced sublimation, electrodeposition, and precipitation from aqueous solutions. In 38

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this work, CdS films are deposited from an aqueous solution. The solution growth process is based on the reaction of a cadmium salt, a complexing agent, and a sulphur compound in a heated aqueous solution. Ca dmium acetate and Thiourea are used as sources of Cadmium and Sulphur respectively. Ammonium hydroxide, which is used as a complexing agent and ammonium acetate, which is used as buffer are combined in 600ml of water. The deposition set-up for CBD proce ss is shown in figure 26. The substrates are placed in a 1000ml double jacketed beaker. The solution is maintained at a constant temperature of 85C by circulating heated ethy lene glycol through the hollow walls of the beaker. At regular time intervals, measured volumes of Sulphur and Cadmium precursors are added to the solution. A proposed mechanis m was given by J.Herrero et al as follows. 2+ Cd(CH3COO) 2 Cd + 2CH COO 3 NH 3 + HOH NH 4+ + OH 2+ Cd(NH3) + 2OH [Cd(OH) (NH ) 4 2 3 2 ] + 2NH 3 [Cd(OH) (NH ) 2 3 2 ] + SC(NH ) [Cd(OH) (NH ) 2 2 2 3 2 SC(NH ) ] 2 2 [Cd(OH) (NH ) 2 3 2 SC(NH ) 2 2 ] CdS + CN H + NH + 2HOH (s) 3 5 3 Figure 26. Chemical bath deposition set-up [23] 39

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4.1.5 Close spaced sublimation of CdTe CdTe absorber film is deposited on CdS/ZTO/SnO 2 :F/glass film stack by the close spaced sublimation (CSS) technique. The deposition set-up for CSS is shown in figure 27. The substrates are annealed in H 2 at 400C before CdTe deposition. The deposition is done at source temperatures of 680C and substrate temperatures of 580C to 600C in a He/O 2 ambient. The spacing between the source and the substrate is 2 mm. Both source and substrate are heated using 2KW halogen lamps. Figure 27. Schematic diagram of close spaced sublimation set-up [23] 4.1.6 Cadmium chloride heat treatment After the CdTe deposition, a CdCl 2 heat treatment of the hetrojunction is essential to improve device performance. Cadmiu m chloride is evaporated from CdCl 2 pellets pressed from powder (99.999% purity) to a th ickness of 8000 at room temperature. Following the deposition, the samples are annealed in a He/O 2 ambient at 390C for 25 minutes. 40

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4.1.7 Contacting The final step in the fabrication process of the cell is contacting. Graphite paste doped with HgTe:Cu is applied on the CdTe film, and then the substrates are annealed at 250C in He ambient. The graphite contact is then coated with a thin conductive layer of silver. This completes the back contact of the solar cell. The CdTe around the cell is scraped exposing the TCO surface. Indium solder is applied to this exposed surface. This serves as a front metal contact, completing th e fabrication of the CdS/CdTe solar cell. The basic device structure is shown in the figure 28. 7059 Corning glass High conductive layer High resistive buffer layer CdS CdTe In Front Contact Graphite Ag paste Load e Figure 28. Standard device structure 4.2 Material & device characterization 4.2.1 Material characterization Film characterization is carried out by various techniques. The thickness of the film is measured using a Tencor Alpha-step profilometer. The sheet resistance of the film 41

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is measured using a simple four point probe set-up. The resistivity of the film is calculated using the thickness and sheet resistance values. Atomic composition is determined using a Tracor Northern (T N) 550 Energy Dispersive Spectrometer. Crystallinity and phase composition is de termined using powder XRD. The surface topography and roughness of the films are studied using an Atomic Force Microscopy (Nanoscope Dimension 3000). For optical measurements, an Oriel Cornerstone monochromator (model 74100) with an integrating sphere is utilized. 4.2.2 Solar cell measurements Light and dark I-V data is measured using a Keithley 2410 1100V source meter, keeping the cell under illumination from a sola r simulator and covering the cell with a black cloth respectively. Open-circuit voltage (V OC ), fill factor (FF) and short-circuit current density (J SC ) are determined using a LABVIEW program used for data collection and processes. Quantum efficiency of th e devices is measured using the Oriel Cornerstone monochromator (model 74100) with the support of another LABVIEW program used for operating the spectral respon se set-up. The light source used is a GE 400W/120V Quartz line lamp (model#43707). 42

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CHAPTER 5 RESULTS AND DISCUSSION 5.1 Zn SnO (Zinc stannate) films spu ttered at room temperature 2 4 5.1.1 Structural and optical properties Zn SnO films are deposited by co-sputtering of SnO 2 4 2 and ZnO targets in Ar ambient at room temperature and at 400C. Zn SnO films are deposited on SnO 2 4 2 :F /glass substrates. Figure 29 shows the X-ray diffraction (XRD) patterns of a room temperature deposited Zn SnO 2 4 film annealed at various temper atures. Table 5 shows the list of various materials appeared in the XRD pa tterns of room temperature deposited Zn SnO 2 4 films annealed at various temperatures. The peaks at approximate 2 values of 26.15, 52.01, 54.47 and 66.19 are asso ciated with the SnO 2 :F. XRD analysis of the asdeposited film shows the peaks of the SnO 2 :F layer. It also shows the emergence of ZnSnO 3 peaks. Subsequent annealing at higher temperatures in He ambient made these films polycrystalline. Th e films started to crystallize at 575C. This is evident with the onset of the (311) peak corres ponding to the sp inel phase (Zn SnO 2 4 ) of the zinc stannate structure. Higher temperature annealing s upplied the required ener gy for crys tallization of these films. The preferred orientation is along [311] direction. The orthorhombic phase 43

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0 200 400 600 800 1000 1200 1400 1600 20 30 40 50 60 70Intensity[cps ] As Deposited @ RT Annealed @550C Annealed @ 575C Annealed @ 600C Angle 2 SnO2(211) SnO2(220) ZnSnO3(006) SnO2(301) ZnSnO3(027) SnO2(110) SnO2(211) SnO2(220) ZnSnO3(006) SnO2(110) Zn2SnO4(311) SnO2(211) SnO2(220) ZnSnO3(006) SnO2(110) Zn2SnO4(311) SnO2(301) SnO2(110) SnO2(301) SnO2(211) Zn2SnO4(222) Zn2SnO4(222) SnO2(220) ZnSnO3(006) Zn2SnO4(440) Zn2SnO4(440) Zn2SnO4(400) Zn2SnO4(400) SnO2(301) SnO Figure 29. XRD of a room temperature deposited Zn 2 4 annealed at various temperatures Table 5. List of materials present in the XR D patterns of a room temperature deposited Zn SnO film and annealed at various temperatures 2 4 ZTO HT [C] Zn SnO ZnSnO ZnO SnO 2 4 3 2 As Deposited NO YES{(006)} NO NO 550C NO YES{(006),(027)} NO NO 575C YES{(311),(222),(400), (440)} YES{(006)} NO NO 600C YES{(311),(222),(400), (440)} YES{(006)} NO NO 44

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of zinc stannate (ZnSnO 3 ) is present in both the as-deposited and annealed films with no significant improvement in its inte nsity. No evidence of ZnO and SnO 2 related peaks in the Zn SnO is observed. 2 4 Figure 30 shows the XRD pattern analysis of Zn X SnO Y films for X = 1.5, 1.9, 2.0, 2.1 and 2.5 annealed at 600C. Table 6 shows the list of various materials appeared in the XRD patterns of room temperature deposited zinc stannate films with different Zn/Sn ratios. The peaks at approximate 2 values of 26.15, 52.01, 54.47 and 66.19 are associated with the SnO :F. For X=1.5, there is no evidence of spinel (Zn SnO 2 2 4 ) phase of zinc stannate in the film. However, an emerging orthorhombic phase (ZnSnO 3 ) of zinc stannate is observed. At X value close to 2 and above (Zn rich), Zinc stannate films became crystalline in the spinel phase (Zn SnO 2 4 ) with preferred orientation along [311] direction. ZnO and SnO 2 related peaks are not observed in the films at all Zn/Sn ratios characterized. 45

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 20 30 40 50 60 70 Angle[2 ]Intensity[cps ] Zn/Sn=1.5 Zn/Sn=1.9 Zn/Sn=2.0 Zn/Sn=2.1 Zn/Sn=2.5 SnO2(211) SnO2(220) SnO2(301) SnO2(110) SnO2(211) SnO2(220) SnO2(110) Zn2SnO4(311) SnO2(211) SnO2(220) ZnSnO3(006) SnO2(110) Zn2SnO4(311) Zn2SnO4(222) SnO2(220) SnO2(301) SnO2(110) Zn2SnO4(311) SnO2(301) Zn2SnO4(440) SnO2(211) Zn2SnO4(311) ZnSnO3(006) Zn2SnO4(222) Zn2SnO4(222) Zn2SnO4(222) Zn2SnO4(400) Zn2SnO4(400) Zn2SnO4(220) SnO2(211) SnO2(220) ZnSnO3(006) SnO2(301) SnO2(110) Zn2SnO4(440) ZnSnO3(006) Figure 30. XRD of a room temperature deposited zinc stannate film with different Zn/Sn ratios Table 6. List of materials present in the XRD patterns of room temperature deposited Zinc stannate films with different Zn/Sn ratios (Annealed at 600 C) SnO ZnSnO ZnO SnO Zn/Sn Zn 2 4 3 2 1.5 NO YES{(006)} NO NO 1.9 YES{(220),(311),(222),(400) ,(440)} YES{(006)} NO NO 2.0 YES{(311),(222),(400), (440)} YES{(006)} NO NO 2.1 YES{(311),(222)} NO NO NO 2.5 YES{(311),(222)} YES{(006)} NO NO 46

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Figures 31 and 32 show the transmission spectra of the room temperature deposited ZTO films and subsequently annealed at different temperatures. The films are subjected to post deposition annealing at 575C and 600C in helium ambient. It is 0 10 20 30 40 50 60 70 80 90 100 400450500550600650700750800850900 Wavelength [nm]T [%] As Deposited Annealed @ 575C Annealed @ 600C Figure 31. Transmission spectra of room temperature deposited Zn SnO 2 4 film and annealed at various temperatures 0 10 20 30 40 50 60 70 80 90 100 400450500550600650700750800850900 Wavelength [nm]T [%] Zn/Sn=1.0 Zn/Sn=1.5 Zn/Sn=2.0 Zn/Sn=2.5 Figure 32. Transmission spectra of ZTO films w ith different Zn/Sn ratios [Annealed at 600C] 47

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evident from the transmission spectra shown in figure 31 that the films annealed at higher temperatures show a slight improvement po ssibly due to the increase in crystallinity. Figure 32 shows that the ZTO films w ith Zn/Sn=2.0 and Zn/Sn=2.5 are highly transparent. This is because the ZTO film s with Zn/Sn ratios close to 2 became crystalline at higher temperatures while other films with Zn/Sn=1 and Zn/Sn=1.5 remained amorphous. 5.2 Zn SnO (Zinc stannate) films sputtered at 400C 2 4 5.2.1 Structural properties Figure 33 compares the XRD analysis of Zn SnO 2 4 films deposited at 400C with thicknesses 500, 750 and 1250. Table 7 shows th e list of various materials appeared in the XRD patterns of Zn 2 SnO 4 films deposited at 400C. St ructural data shows that randomly oriented polycrystalline films are produced. Both the spinel phase and orthorhombic phase of zinc stannate film are observed. Zinc oxide related peak is observed in 500, 750 films. The films are subsequently annealed at 600C for 20 minutes. Figure 34 compares the XRD analysis of Zn SnO 2 4 films deposited at 400C and annealed at 600C for 20 minutes. Table 8 lis ts various materials appeared in the XRD patterns of Zn SnO 2 4 films deposited at 400C and anneal ed at 600C. Upon annealing of the films, the phases of zinc stannate appeared for the as-deposited zinc stannate is more pronounced and zinc oxide related peak is obser ved in all the films. Zinc stannate films deposited at room temperature crystallized in the inverse spin el structure and have shown no evidence of zinc oxide related peaks wh ile the films deposited at 400C strongly displayed the presence of orthorhombic phase (ZnSnO ) and zinc oxide related peaks. 3 48

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0 200 400 600 800 1000 1200 20 30 40 50 60 70 Angle[2 ]Intensity[cps ] 500 As Deposited@400C 750 As Deposited@400C 1250 As Deposited@400C SnO2(211) SnO2(220) ZnSnO3(006) ZnO2(211) SnO2(301) ZnSnO3(027) SnO2(110) SnO2(211) SnO2(220) ZnSnO3(006) ZnO2(211) SnO2(110) Zn2SnO4(311) SnO2(211) SnO2(220) ZnSnO3(006) SnO2(110) Zn2SnO4(311) ZnSnO3(012) ZnSnO3(027) ZnSnO3(027) Zn2SnO4(311) SnO2(301) SnO2(301) Figure 33. XRD of a 400C deposited zinc stannate film Table 7. List of materials pres ent in the XRD patterns of Zn SnO 2 4 films deposited at 400C SnO ZnSnO ZnO SnO ZTO Zn 2 4 3 2 Thickness 500 Possible{(311)} YES{(006)} YES{(211)ZnO } NO 2 750 Possible{(311)} YES{( 012),(006),(027)} YES{(211)ZnO } NO 2 1250 Possible{(311)} YES{(006),(027)} NO NO 49

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0 200 400 600 800 1000 1200 20 30 40 50 60 70Intensity[cps ] 500 HT 600C-20min 750 HT 600C-20min 1250 HT 600C-20min Angle 2 SnO2(211) SnO2(220) ZnSnO3(006) ZnO2(211) SnO2(301) SnO2(110) SnO2(211) SnO2(220) ZnSnO3(006) ZnO2(211) SnO2(110) Zn2SnO4(311) SnO2(211) SnO2(220) ZnSnO3(006) ZnO2(211) SnO2(110) Zn2SnO4(311) Zn2SnO4(311) Figure 34. XRD of a 400C deposited zinc stannate film and annealed at 600C Table 8. List of materials pres ent in the XRD patterns of Zn SnO 2 4 films deposited at 400C and annealed at 600C SnO ZnSnO ZnO SnO ZTO Zn 2 4 3 2 Thickness 500 Possible YES(006) YES(211)ZnO NO 2 750 YES(311) YES(006) YES(211)ZnO NO 2 1250 Possible YES(006) YES(211)ZnO NO 2 50

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The surface of a 750 th ick zinc stannate film is anal yzed using AFM. The AFM images of the as-deposited films at 400C and ann ealed films are shown in figures 35 and 36 respectively. The scan size is 2m. AFM measurements calculate the surface roughness for the as deposited ZTO film in argon ambi ent as 4.2 nm. Upon annealing the film at 600C for 20 minutes in He made the su rface rougher. The surface roughness of the annealed film is 5.3nm. Figure 35. Two dimensional phase images of as-deposited ZTO film at 400C (above) and annealed at 600C for 20 minutes in He (below) 51

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Figure 36. AFM images of ZTO film before (above) and after annealing (below) 52

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5.3 CdS/CdTe solar cells using zinc stannate film as a buffer layer 5.3.1 Zinc stannate films spu ttered at room temperature As mentioned in the earlier chapter, the zinc stannate (ZTO) films are integrated as high resistive buffer layers in CdS/CdTe solar cells. ZTO film s with a Zn/Sn=2.0 and film thickness of 2000 is sputtered on SnO 2 :F layer. This bilayer is subjected to a heat treatment at high temperatures ranging from 550C to 600C in He ambient for 20 minutes. This thin film is further processed to make a CdS/CdTe solar cell using standard processes.CdCl 2 heat treatment is done at 420C for 15 minutes. The back contact used in this process is a Cu doped graphite paste. Table 9 shows the performance of a CdTe/CdS solar cell with 2000 Zn SnO buffer layer. 2 4 Table 9. Summary of SnO :F/ Zn SnO 2 2 4 devices (ZTO room temperature deposited and Zn/Sn=2.0) J SC ZTO HT [C] V OC [mV] FF [%] [mA/cm 2 ] Efficiency [%] R R series shunt [ -cm 2 ] [ -cm 2 ] 550 730 55.4 23.8 9.34 1.90 950 575 680 54.4 23.4 8.66 1.90 530 600 780 58.2 24.5 11.13 2.34 800 5.3.1.1 J-V characteristics The devices with SnO :F/ ZTO bilayer annealed at 600C exhibited higher V 2 OC FF and J than the devices annealed at 550C and 575C. Nevertheless the V SC OC and FF values for all the devices are lower than the typical CdTe/CdS solar cell values. Figure 37 shows the light J-V and dark J-V characteristics for SnO :F/ Zn SnO devices. ZTO films 2 2 4 53

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are deposited at room temperature and h eat treated at 550C, 575C and 600C. The observed trend may be due to the decrease in th e shunt resistance of the devices. The dark -0.025 -0.020 -0.015 -0.010 -0.005 0.000 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 550C 575C 600C 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.00.30.50.81.01.31.5 Voltage [Volts]Current Density [A/cm2] 550C 575C 600C Figure 37. Light J-V (left) and dark J-V (right) characteristics for SnO :F/ Zn 2 2 SnO 4 devices (ZTO room temperat ure deposited and Zn/Sn=2.0) currents of all the devices are higher but the device with ZTO film annealed at 600C has lower dark current compar ed to other two devices. 5.3.1.2 Spectral response It is evident from th e spectral response data of figure 38 that the devices displayed differences in the current generation. This difference is mainly pronounced for the wavelengths below 550nm. The device with anne aled bilayer at 600C has higher current generation below 550nm. This might be due to the high degree crystalline nature of the zinc stannate film annealed at high temperat ures. The thinner CdS for films annealed at higher temperatures suggest that the inte rdiffusion phenomenon between zinc stannate and CdS has consumed more CdS in this cas e compared to other temperature annealed 54

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films [20]. This result seem s counterintuitive as one might expect more interdiffusion with amorphous zinc stannate film. This th inner CdS has reduced the amount of light 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength [nm]Q.E. 550C 575C 600C Figure 38. Spectral response graph for SnO :F/ Zn SnO 2 2 4 devices (ZTO deposited at room temperature and Zn/Sn=2.0) absorbing in the CdS region there by allowing the light to be absorbed in the CdTe absorber region which led to the higher quant um efficiency in the blue region of the spectrum. 5.3.2 Zinc stannate films sputtered at room temperature with varying Zn/Sn ratios The effect of stoichiometry of the zinc stannate buffer layer on the performance of solar cells is studied by varying Zn/Sn ratio in the zinc stannate film. Solar cells are made using the zinc stannate films with Zn /Sn =1.5, 1.9, 2.0, 2.1 and 2.5. The zinc stannate film is sputtered on SnO 2 : F layer at room temperature in Ar ambient and later annealed in He at 600C for 20 minutes. Table 10 show s the performance of CdTe/CdS solar cells with 2000 ZTO buffer layer. The ZTO film is deposited at room temperature and heat treated at 600C. CdCl heat treatment is done at 420C for 15 minutes. 2 55

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Table 10. Summary of SnO 2 :F/ Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) J SC Zn/Sn V OC [mV] FF [%] [mA/cm 2 ] Efficiency [%] R R series shunt [ -cm 2 ] [ -cm 2 ] 1.5 710 54.6 24.4 9.45 2.32 500 1.9 770 44.3 24 8.17 2.34 730 2.0 780 58.2 24.5 11.13 2.30 800 2.1 810 66.6 23.1 12.43 2.47 800 2.5 790 66.7 23 12.12 1.39 900 5.3.2.1 J-V characteristics Figure 39 shows the light J-V and dark J-V characteristics for SnO 2 :F/Zinc stannate devices with various Zn/Sn ratios. The devices with Zn/Sn ratios of 2.0 and above have shown better performance in terms of V OC s and FFs. The shunt resistances of all the devices are low. The dark currents are higher fo r all the devices. Dark J-V characteristics -0.025 -0.02 -0.015 -0.01 -0.005 0 -0.20.050.30.550.8 Voltage[Volts]Current density[A/cm2] Zn/Sn=1.5 Zn/Sn=1.9 Zn/Sn=2.0 Zn/Sn=2.1 Zn/Sn=2.5 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.00.30.50.81.01.31.5 Voltage [Volts]Current Density [A/cm2] Zn/Sn=1.5 Zn/Sn=1.9 Zn/Sn=2.0 Zn/Sn=2.1 Zn/Sn=2.5 Figure 39. Light J-V (left) and dark J-V (right) characteristics of SnO 2 :F/ Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 56

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are inconclusive to determine the effect of Zn/Sn ratio on the solar cell performance. 5.3.2.2 Spectral response The spectral response data from figure 40 showed clear difference in the quantum efficiency of the devices below 550nm. This quantum efficiency data suggests that the CdS has been entirely consumed in the devi ces with Zn/Sn ratios less than 2.0 during the fabrication process. The device with Zn/S n=1.9 suffered from poor collection at longer wavelengths. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength [nm]Q.E. Zn/Sn=1.5 Zn/Sn=1.9 Zn/Sn=2.0 Zn/Sn=2.1 Zn/Sn=2.5 Figure 40. Spectral response of SnO 2 :F/ Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 5.3.2.3 Collection issues For all the devices, monochromatic J-V data is collected and anal yzed to develop an insight to the specific collection mechanism. The monochromatic J-V graphs are shown in figure 41. The conditions for each set of gra phs are inserted in the text box. The FF vs Wavelength graphs for each condi tion are also included in figure 41 adjacent to the monochromatic J-V. The dotted line indicate s the FF. The device wi th Zn/Sn=1.5 has low R shunt at all wavelengths. At 800nm, this de vice has poor collection. The device with Zn/Sn=1.9 has high monochromatic FFs than th e white light FF. This device suffers 57

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from high series resistance effect for all wave lengths. Higher series resistance effect is also obvious for the devices with Zn/Sn=2.0. -0.003 -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460nm 520nm 560nm 640nm 700nm 800n m Zn/Sn=1.5 25 30 35 40 45 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] Zn/Sn=1.5 -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460nm 520nm 560nm 640nm 700nm 800nm Zn/Sn=1.9 40 45 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] Zn/Sn=1.9 Figure 41. Monochromatic I-V curves and FF vs. wavelength plots for SnO 2 :F/ Zinc stannate devices (ZTO room temperature deposited and Zn/Sn=1.5, 1.9, 2.0, 2.1 and 2.5) 58

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-0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460nm 520nm 560nm 640nm 700nm 800nm Zn/Sn=2.0 40 45 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] Zn/Sn=2.0 -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460nm 520nm 560nm 640nm 700nm 800nm Zn/Sn=2.1 40 45 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] Zn/Sn=2.1 Figure 41. (Continued) 5.3.3 Zinc stannate films sputtered at 400C Zinc stannate films are deposited at 400C in Ar ambient for three different thicknesses, 500, 750 and 1250. The films are subsequen tly annealed at 600C for 5, 20 and 30 59

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minutes in He ambient. Fo r all the devices, the CdCl 2 heat treatment was carried out in same conditions. 5.3.3.1 Zinc stannate films annealed at 600C for 5 minutes The results for the cells processed with ZTO sputtered at 400C and annealed at 600C for 5 minutes are listed in table 11. Table 11. Summary of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) ZTO thickness [] V [mV] FF [%] J Efficiency [%] R R OC SC series shunt [mA/cm 2 ] [ -cm 2 ] [ -cm 2 ] 500 840 68.2 24.40 13.98 1.64 1700 750 830 68.2 24.84 14.04 1.75 1900 1250 820 69.1 24.39 13.83 1.47 2000 5.3.3.1.1 J-V characteristics Figure 39 shows the light J-V and dark J-V characteristics for SnO :F/ Zn SnO 2 2 4 devices with ZTO deposited at 400C and annealed at 600C for 5 minutes. All the devices showed improved performance in all aspects compared to the devices made with room temperature deposited ZTO films. The R shunt for all the devices with ZTO deposited at 400C is much higher than that of the devices with room temperature deposited ZTO. The XRD data of ZTO films deposited at 400 C showed the presence of zinc oxide (ZnO ) which may be the reason for improved R 2 shunt in these devices. The 500 and 750 zinc stannate films showed similar performance. 60

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-0.03 -0.02 -0.02 -0.01 -0.01 0.00 -0.20.10.30.60.8 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 5 min 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.00.30.50.81.01.31.5 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 5 min Figure 42. Light J-V (left) and dark J-V (right) characteristics of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn /Sn= 2.0, annealed @ 600C for 5 min) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength [nm]Q.E. ZTO thickness 750,600C-5min ZTO thickness 500,600C-5min ZTO thickness 1250,600C-5min Figure 43. Spectral response of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 5.3.3.1.2 Spectral response The spectral response da ta of all the devices as shown in figure 43 is similar at all wavelengths above 550nm.The differences in the spectral response below 550nm can be contributed to the amount of CdS consumed during the fabrication process. The CdS seems to have been thinned by the ZTO with little impact on the V of the device. The OC 61

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CdS is thinner for the device with 750 ZTO film. The thinner CdS allows for a greater current collection at lower wavelengths. Increas e in the quantum efficiency of all devices above 600nm is observed. 5.3.3.1.3 Collection issues For all the devices, the monochromatic J-V graphs are shown in the figure 44. The conditions for each set of gra phs are inserted in the text box. The FF vs Wavelength graphs for each condition are also included in figure 44 adjacent to the monochromatic J-V. The dotted line indicates the white light FF. The devices with larger zinc stannate thickness i.e. 750 and 1250 have higher cu rrent at 460nm. All the devices showed good collection at longer wavelengt hs ranging from 600nm to 800nm. -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460 nm 520nm 560nm 640nm 700nm 800nm 500A ,600C 5 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 500A ,600C 5 min Figure 44. Monochromatic I-V curves and fi ll factor vs. wavele ngth plots for SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 5 min) 62

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-0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm 2 460 nm 520nm 560nm 640nm 700nm 800nm 750A ,600C 5 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 750A ,600C 5 min -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460 nm 520nm 560nm 640nm 700nm 800nm 1250A ,600C 5 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 1250A ,600C 5 min Figure 44. (Continued) 5.3.3.2 Zinc stannate films anne aled at 600C for 20 minutes The results for the cells processed with ZTO sputtered at 400C and annealed at 600C for 20 minutes are listed in table 12. 63

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Table 12. Summary of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 20 min) [mV] FF [%] J ZTO thickness [] V Efficiency [%] R R OC SC series shunt [mA/cm 2 ] [ -cm 2 ] [ -cm 2 ] 500 830 70.8 23.8 13.99 1.44 1730 750 820 69.2 24.3 13.79 1.26 1700 1250 610 61.2 22.8 8.51 1.91 1000 5.3.3.2.1 J-V characteristics Figure 45 shows the light J-V and dark J-V characteristics for SnO :F/ Zn SnO 2 2 4 devices with ZTO deposited at 400C and annealed at 600C for 20 minutes. The devices with ZTO film thickness 500 and 750 showed similar performance. The presence of zinc oxide in the ZTO films improved the shunt resistance of the devices. The degraded performance of the device with 1250 thick ZTO must have been an anomaly in the device processing and cannot be attributed to the ZTO buffer layer. It suffered from high dark currents. -0.025 -0.020 -0.015 -0.010 -0.005 0.000 -0.20.10.30.60.8 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 20 min 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.00.30.50.81.01.31.5 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 20 min Figure 45. Light J-V (left) and dark J-V (right) characteristics of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/S n= 2.0, annealed @ 600C for 20 minutes) 64

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5.3.3.2.2 Spectral response The spectral response of the devices is s hown in figure 46.The device with 750 thick ZTO has higher quantum efficiency below 550nm. The device with 500 thick ZTO annealed at 600C for 20 minutes behaved sim ilarly with the one annealed at 600C for 5 minutes. But the longer wavelength quantum ef ficiency ranging from 600nm to 800nm is low for this set of devices compared to th e ones annealed at 600C for 5 minutes. This may be due to the poor collection in that region. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength [nm]Q.E. ZTO thickness 500, 600C-20min ZTO thickness 750, 600C-20min ZTO thickness 1250, 600C-20min Figure 46. Spectral response of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 20 min) 5.3.3.3 Zinc stannate films annealed at 600C for 30 minutes The results for the cells processed with ZTO sputtered at 400C and annealed at 600C for 30 minutes are listed in table 13. 65

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Table 13. Summary of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn = 2.0, annealed @ 600C for 30 min) [mV] FF [%] J ZTO Thickness [] V Efficiency[%] R R OC SC series shunt [mA/cm 2 ] [ -cm 2 ] [ -cm 2 ] 500 830 69.3 24.74 14.21 1.33 1900 750 820 70.1 22.8 13.11 1.25 1890 1250 660 61.6 23.1 9.40 2.25 1200 5.3.3.3.1 J-V characteristics Figure 47 shows the light J-V and dark J-V characteristics for SnO :F/ Zn SnO 2 2 4 devices with ZTO deposited at 400C and annealed at 600C for 30 minutes. The devices with zinc stannate film thickness 500 and 750 showed similar performance in terms of V OC and FFs. The presence of zinc oxide in the ZTO films improved the shunt resistance of the devices. The series resistance of the devi ce with 1250 thick zinc stannate film is also higher compared to ot her devices in this set. -0.025 -0.020 -0.015 -0.010 -0.005 0.000 -0.20.10.30.60.8 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 30 min 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.00.30.50.81.01.31.5 Voltage [Volts]Current Density [A/cm2] 500A 750A 1250A 600C 30 min Figure 47. Light J-V (left) and da rk J-V (right) curves of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 66

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5.3.3.3.2 Spectral response The spectral response data from figure 48 sh ows that the quantum efficiency of the device with 500 thick zinc stannate film has high qua ntum efficiency at all wavelengths. The quantum efficien cy of the devices with 750A and 1250A thick zinc stannate films has a drop at longer wavelengths ranging from 600nm to 800nm. This may be due to the poor collection in the devices. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 400450500550600650700750800850900 Wavelength [nm]Q.E. ZTO thickness 1250, 600C-30min ZTO thickness 500, 600C-30min ZTO thickness 750, 600C-30min Figure 48. Spectral response of SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 400C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 5.3.3.3.3 Collection issues The monochromatic J-V graphs are shown in the figure 49. The conditions for each set of graphs are inserted in the text box. The FF vs Wavelength graphs for each condition are also included in figure 49 adjacent to the monochromatic J-V. The dotted line indicates the white light FF. The device with 1250 thick zinc stannate film has higher monochromatic FFs than the white light FF This device suffers from high series resistance effect for all wavelengths. This led to the decrease in the fill factor of the device. 67

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-0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460 nm 520nm 560nm 640nm 700nm 800nm 500A ,600C 30 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 500A 600C 30 min -0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460 nm 520nm 560nm 640nm 700nm 800nm 750A ,600C 30 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 750A 600C 30 min Figure 49. Monochromatic J-V cu rves and fill factor vs. wavelength plots for SnO 2 :F/ Zinc stannate devices (ZTO sputtered @ 450C and Zn/Sn= 2.0, annealed @ 600C for 30 min) 68

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-0.002 -0.001 0.000 0.001 0.002 -0.200.050.300.550.80 Voltage [Volts]Current Density [A/cm2] 460 nm 520nm 560nm 640nm 700nm 800nm 1250A ,600C 30 min 50 55 60 65 70 75 80 400500600700800 Wavelength[nm]FF[%] 1250A 600C 30 min Figure 49. (Continued) 5.4 Highly conductive transparent conducting oxides 5.4.1 Cadmium stannate 5.4.1.1 Structural and optical properties Cadmium stannate films are de posited by co-sputtering of SnO 2 and CdO targets in Ar ambient at room temperature. As-deposited films are amorphous in nature. As-deposited films can be made polycrystalline by subject ing them to high temperature annealing in He ambient. Cd SnO 2 4 begins to crystallize at approxi mately 525C [22]. Figure 50 shows the XRD patterns of Cd SnO 2 4 films annealed at four different temperatures. The films turned crystalline with a preferred orientati on in the [222] direction. At all the annealing temperatures, films crystal lized in single phase of Cd SnO 2 4. There is no evidence of secondary phases of CdSnO SnO and CdO in the films. This shows that Cd 3 2 2 SnO films 4 69

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0 200 400 600 800 1000 1200 1400 1600 20 30 40 50 60 70 Angle[2 ]Intensity[cps ] 550C 575C 600C 625C (220) (311) (222) (422)(511) (440) (533) (220) (311) (222) (422)(511) (440)(533) (220) (311) (222) (422) (511) (440)(533) (220) (311) (222) (422) (511)(440)(533) Figure 50. XRD of Cd SnO films annealed at four different temperatures 2 4 are stable even at high temperatures which is an important parameter in view of the fact that films are subjected to high temperatur es in further processing steps. Optical transmission is determined for films of 2500A annealed in He ambient. Figure 51 shows the transmission spectra of Cd SnO 2 4 films annealed at different temperatures. It is evident from the transmission spectra that the Cd SnO 2 4 films annealed at higher temperatures are highly transparent at all wave lengths between 350 and 1000nm. This might be due to the increase in crystallinity of the film as the temperature increases. The average transmission for Cd SnO films, between wavelengths of 400 and 1000nm, annealed at 2 4 70

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550C, 575C, 600C and 625C is 88%, 91%, 92% and 93% respectively. The change in resistivity of the films with the annealing temperature is also studied. 0 10 20 30 40 50 60 70 80 90 100350450550650750850950Wavelength [nm]Transmission [% ] 550C 575C 600C 625C Figure 51. Transmission spectra of Cd SnO films annealed at four different temperatures 2 4 3.00E-04 3.50E-04 4.00E-04 4.50E-04 5.00E-04 5.50E-04 6.00E-04 6.50E-04 7.00E-04 7.50E-04 550 575 600 625 Annealing Temperatue[C] Resistivity[ -cm ] Figure 52. Dependence of resistiv ity on annealing temperature 71

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Figure 52 s hows an improvement in the resistivity of the film as the annealing temperature is increased from 550C to 625C. This might be attributed to the improvement in the crystallinity of the film at higher temperatures. As the crystallinity increases, the mobility of carriers also increases thereby increasing the conductivity of the film. 5.4.1.2 Effect of stoichiometry on structur al, optical and elect rical properties of cadmium stannate film The effect of stoichiometry on structure, optical and electrical properties of Cd SnO is studied by varying the amount of CdO and SnO in the Cd SnO 2 4 2 2 4 film. This is done by controlling the thickness of CdO and SnO 2 materials while sputtering. Figures 53, 54 and 55 show the XRD patterns, transm ission spectra and resistivity of the cadmium-rich Cd SnO film. 2 4 0 200 400 600 800 1000 20 30 40 50 60 70 Angle[2 ]Intensity[cps ] Cd/Sn=2.0 Cd/Sn=2.1 Cd/Sn=2.2 (220) (311) (222) (422) (511) (440)(533) (220) (311) (222) (422)(511) (440) (533) (222) (511) (533) (311) (422)(440) (220) Figure 53. XRD patterns of cadmium stanna te films for varying stoichiometry 72

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0 10 20 30 40 50 60 70 80 90 100 350450550650750850950 Wavelength [nm]Transmission [% ] Cd/Sn=2.2 Cd/Sn=2.0 Cd/Sn=2.1 Figure 54. Transmission spectra of cadmium st annate films for varying stoichiometry 3.00E-04 4.00E-04 5.00E-04 6.00E-04 7.00E-04 22 12 2 Cd/SnResistivity[ -cm ] Figure 55. Dependence of resistivity of cadmium stannate films for varying stoichiometry All the films are deposited at room temperature and annealed at 600C in He ambient. The XRD patterns of all the films are similar ex cept that the (311) peak of the film with Cd/Sn = 2.2 is starting to emerge. This is because the energy supplied by 600C annealing is not sufficient to totally crystall ize the cadmium stannate film with Cd/Sn = 73

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2.2. All the films show preferred orientation in [222] di rection. The average transmission for the films with Cd/Sn=2.0, 2.1 and 2.2 is 92.3%, 93% and 91.5% respectively. Low transmission of Cd SnO 2 4 film with Cd/Sn=2.2 is due to it s semi-crystalline nature and it can be improved by annealing it at much hi gher temperatures. The lowest resistivity obtained in this work is 5.4 X 10 -4 -cm for the film with Cd/Sn = 2.0. 74

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CHAPTER 6 CONCLUSION 6.1 Investigation of materials The structural and optical properties of the cadmium stannate and zinc stannate materials are studied. Zinc stannate films depos ited at room temperature are amorphous. Subsequent annealing at high temperatures ma de the films crystalline. This annealing also improved the transmission of the films to a great extent. The films deposited at 400C turned crystalline. Zinc stannate film s deposited at room temperature and elevated temperatures are highly resistive. The cadmium stannate films deposited at r oom temperature are amorphous. Annealing the films at high temperatures made them crysta lline. The electrical and optical properties of the films improved upon annealing. 6.2 Comparison of devices made with zinc stannate as high resistive buffer layer The best devices obtained are summarized in table 10 and table 11. The CdTe solar cells made with room temperature deposited zinc stannate films suffered from the effect of shunting which led to low open circuit voltages. Th e high series resistance in the devices led to decreased fill factors. The effect of stoichiometry of zinc stannate film on CdTe solar cell is studied. The Zn rich films of zinc stannate showed better performance compared to other stoichiometric zinc sta nnate films. The best devices in this study 75

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Table 14. Summary of the devices with ZTO deposited at room temperature come from the zinc stannate film deposited at elevated temperatures. The devices from these films showed high shunt resistance due to the existence of multiple phases of zinc stannate in the films. This helped in achieving better performance of the devices Table 15. Summary of the best devices with ZTO deposited at 400C ZTO Thickness[] V OC [mV] FF[%] J SC [mA/cm 2 ] R series [ -cm 2 ] R shunt [ -cm 2 ] Efficiency [%] 500 830 69.3 24.74 1.33 1900 14.21 750 830 68.2 24.84 1.75 1900 14.04 Zn/Sn V OC [mV] FF[%] J [mA/cm 2 ] R R Efficiency[%] SC series shunt [ -cm 2 ] [ -cm 2 ] 1.5 710 54.6 24.4 2.32 500 9.45 1.9 770 44.3 24 2.34 730 8.17 2.0 780 58.2 24.5 2.30 800 11.13 2.1 810 66.6 23.1 2.47 800 12.43 2.5 790 66.7 23 1.39 900 12.12 76

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REFERENCES 1. H.L.Hartnagel, A.L.Dawar, A.K.Jain, C. Jagadish, Semiconducting transparent thin films, IOP Publishing ltd, Philadelphia, 1995. 2. Brain G.Lewis, David C.Paine,Applications and processing of transparent conducting oxides, MRS bulletin, August 2000. 3. David S.Ginley, Clark Bri ght, Transparent conducting ox ides, MRS bulletin, August 2000. 4. Roy G.Gordon, Criteria for choosing tran sparent conductors, MRS bulletin, August 2000. 5. Timothy J.Coutts, David L.Young, Xiaonan Li Characteization of transparent conducting oxides, MRS bulletin, August 2000. 6. A.J. Freeman, K.R. Poeppelmeier, T.O. Mason, R.P.H. Chang, T.J Marks, Chemical and thin film strategies for new transp arent conducting oxides, MRS bulletin, August 2000. 7. D.R. Kammler, T.O.Mason, Phase relations hips, transparency and conductivity in the cadmium indate-cadmium stannate sy stem, Chem.Mater. 12 (2000) 1954. 8. T.Stapinski, E. Leja, T. Pisarkiewicz, Point defects and their influence on electrical properties of reactive sputtered Cd SnO 2 4 thin films, J.Phys.D: Appl. Phys. 17 (1984) 407. SnO 9. Y.Dou, R.G. Egdell, n-type doping in Cd 2 4 : A study by EELS and photoemission, Physics Review B. 53 (1996) 23. 10. Timothy J.Coutts, David L.Young, Xiaona n Li, Search for improved transparent conducting oxides: A fundamental investigation of CdO, Cd SnO and Zn SnO 2 4 2 4 , J.Vac.Sci.Technol.A. 18 (2000) 2646. 11. S. B. Zhanga and Su-Huai Wei, Self-doping of cadmium sta nnate in the inverse spinel structure, Applied Physics Letters, 80 (2002) 1376. 77

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25. S.O Kasap, Principles of Electronic materials and devices Second Edition, Mc Graw Hill, 2002. 26. S.M.Sze, Physics of semiconductor devices Second Edition, John Wiley & Sons, Newyork, 1981. 27. Martin A. Green, Solar Cells: Oper ating Principles, Technology and System Applications, Prentice-Hall, New Jersey, 1982. 28. H.J. Moller, Semiconductors for solar cells, Artech house, London 1993. 79