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Characterization of Cadmium Zinc Telluride Solar Cells University of South Florida Scholar Commons Graduate Theses and Dissertations Graduate School 11-12-2003 Characterization Of Cadmium Zinc Telluride Solar Cells Gowri Sivaraman University of South Florida Follow this and additional works at: https://scholarcommons.usf.edu/etd Part of the American Studies Commons Scholar Commons Citation Sivaraman, Gowri, "Characterization Of Cadmium Zinc Telluride Solar Cells" (2003). Graduate Theses and Dissertations. https://scholarcommons.usf.edu/etd/1478 This Thesis is brought to you for free and open access by the Graduate School at Scholar Commons. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. Characterization Of Cadmium Zinc Telluride Solar Cells by Gowri Sivaraman 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: November 12, 2003 Keywords: czt,thin films, tandem solar cells,sublimation,widegap semiconductors © Copyright 2003 , Gowri Sivaraman DEDICATION This thesis is dedicated to my family and friends for their everlasting love and support. ACKNOWLEDGMENTS I would like to thank my Major Professor, Dr. Christos S. Ferekides, for his guidance and support during the course of this work. His enthusiasm and profound knowledge in the subject has been a great source of inspiration. I would like to thank Dr. Don L. Morel and Dr. Yun L. Chiou for their guidance in the fulfillment of this thesis. I would like to express my special thanks to Su Yu whose invaluable guidance has been of great help to me. I would also like to thank all the fellow colleagues of the Thin Film Electronic Materials laboratory for their help and support. TABLE OF CONTENTS LIST OF TABLES iv LIST OF FIGURES v ABSTRACT viii CHAPTER 1 INTRODUCTION 1 CHAPTER 2 SEMICONDUCTORS AND SOLAR CELLS 4 2.1 p-n Junction 5 2.1.1 Heterojunction 8 2.2 Solar Cells 9 2.3 PhotoVoltaic Effect 10 2.4 Operation 10 2.5 Equivalent Circuit 12 2.6 Tandem Solar Cells 13 CHAPTER 3 OVERVIEW OF MATERIALS 15 3.1 Cadmium Telluride 16 3.2 Zinc Telluride 16 3.3 Cadmium Zinc Telluride 17 3.4 Cadmium Sulphide 19 3.5 Zinc Selenide 19 3.6 Cadmium Oxide 20 i CHAPTER 4 CZT SOLAR CELLS 22 4.1 CdS/CZT Structure 22 4.1.1 Cadmium Sulphide 23 4.1.2 Close Spaced Sublimation 24 4.1.3 Back Contacts 25 4.1.3.1 Zinc Telluride 25 4.1.3.2 Graphite Back Contact 26 4.2 Heterojunction Structures Investigated 26 CHAPTER 5 RESULTS AND DISCUSSION 27 5.1 CZT on ZnSe Films 27 5.1.1 Microstructures of CZT/ZnSe Films 29 5.1.2 SIMS Analysis 29 5.2 CZT/ZnSe Solar Cells 30 5.3. CZT/CdO Cells 33 5.4 CZT/CdS Films 35 5.4.1 Effect of Deposition Conditions 35 5.4.2 SIMS Analysis 38 5.5 Characterization of CZT/CdS Solar Cells 39 5.5.1 Effect of CdS Thickness 39 5.5.2 Effect of Annealing 41 5.5.3 Analysis of the Best Device 43 CHAPTER 6 SCAPS SIMULATION 46 6.1 Effect of Bulk Traps on Device Performance 50 ii 6.2 Effect of Interface States 51 CHAPTER 7 CONCLUSION 55 REFERENCES 57 iii LIST OF TABLES Table 1. Lattice Constants 18 Table 2. Properties of Relevant Materials 21 Table 3. VOC’s of CZT/ZnSe Devices 31 Table 4. Effect of Ambient on Composition 35 Table 5.Composition of a CZT Film with Eg ~1.6 eV 37 Table 6. CZT Devices in Various Annealing Ambient 42 Table 7. Comparison of As-Deposited & H2 Annealed Devices 43 Table 8. Tabulation of Results 45 Table 9. Input Values for SCAPS Simulation of CZT 47 iv LIST OF FIGURES Figure 1. Electric Field Distribution of a p-n Junction 6 Figure 2. Energy Band Diagram of p-n Junction 7 Figure 3. Energyband of Heterojunction at Equilibrium 8 Figure 4. The Solar Spectrum 9 Figure 5. Dark and Light I-V Curves of an Ideal Solar Cell under AM 1.5 Spectra 11 Figure 6. Equivalent Circuit of Solar Cell 12 Figure 7. Two Terminal Tandem Structure 13 Figure 8. Tandem Structure 14 Figure 9. Zinc Blende Structure 17 Figure 10. Structure of CdS/CZT Solar Cell 22 Figure 11.Experimental Setup for Chemical Bath Deposition 23 Figure 12. Experimental Setup of Close Spaced Sublimation 25 Figure 13. Structure of ZnSe Based Devices 26 Figure 14. Structure of CdO Based Devices 26 Figure 15. Transmission of a CZT Film 28 Figure 16. Energy Band Diagram of CZT Film 28 Figure 17. SEM of CZT Film on ZnSe 29 Figure 18. SIMS of CZT/ZnSe [24] 30 Figure 19. Light and Dark J-V of CZT/ZnSe Devices 32 v Figure 20. Spectral Responses of CZT/ZnSe Devices 32 Figure 21. Light and Dark J-V’s of CZT/CdO Devices 33 Figure 22. Spectral Response of CZT/CdO Devices 34 Figure 23. XRD of CZT 36 Figure 24. SEM Image of CZT 37 Figure 25.SIMS on ZnTe/CZT/CdS Structure 38 Figure 26. VOC Vs CdS Thickness 39 Figure 27. FF vs CdS Thickness 40 Figure 28. Spectral Response of CZT Devices 40 Figure 29. Effect of Annealing Ambient on VOC and FF 41 Figure 30. SR of CZT Devices for Various Annealing Ambient 42 Figure 31. SR of H2-anneal and As-deposited Devices 43 Figure 32. Spectral Response of the Best Device 44 Figure 33. Dark J-V of the Best Performance Device 44 Figure 34. Light J-V of the Best Device 45 Figure 35. Effect of Work Function on VOC- Simulated Results 48 Figure 36. Effect of Work Function on Fill Factor- Simulated Results 48 Figure 37. Light I-V Characteristics of Various Work Functions @ Na=1014 cm-3 49 Figure 38. Energy Band Diagram for Different Work Functions in Dark @ Zero Bias 49 Figure 39. VOC vs Trap Density for Different Capture Cross Section Ratios 50 Figure 40. Effect of NT on VOC, Jsc and FF’s 51 Figure 41. VOC vs Trap Density for Various Acceptor Interface Defects 52 Figure 42. Current Density vs Trap Density for Various Acceptor Interface Defects 52 vi Figure 43. Device Performance for Various Energy Levels 53 Figure 44. Simulated Results of QE for Acceptor Interface Defects 53 vii CHARACTERIZATION OF CADMIUM ZINC TELLURIDE SOLAR CELLS Gowri Sivaraman ABSTRACT Currently thin film solar cells have efficiencies in the range of 16-18%. Higher efficiencies of 20% or more can be achieved by two junction solar cells in which two p-n junctions are connected in series one on top of the other in a tandem structure. The ideal bandgaps for optimum efficiency in a tandem structure are about 1eV for the top cell and 1.7 eV for the bottom cell. Copper Indium Gallium di-Selenide (CIGS) with a bandgap of 1 eV is a suitable candidate for the bottom cell and Cadmium Zinc Telluride (CZT) with a tunable bandgap of 1.44-2.26 eV is a suitable candidate for the top cell. This work involves characterization of cadmium zinc telluride films and solar cells prepared by close spaced sublimation. CZT is deposited by co-sublimation of CdTe and ZnTe. The process has been investigated on various wide bandgap semiconductor materials including cadmium sulphide, cadmium oxide and zinc selenide. Different post deposition heat treatments were carried out to determine their effect on film and device properties. Characterization of the CZT devices was done using XRD, EDS, SIMS, J-V and spectral response measurements. CZT (Eg~1.7 eV) /CdS exhibited best performance when compared to the other window layers investigated. The best device exhibited Voc=640mV, FF=40% and Jsc=4.5 mA/cm2. The theoretical performance of CZT based solar cells were investigated using SCAPS. The effect of bulk and interface defects on the device parameters were studied. viii CHAPTER 1 INTRODUCTION Globalization came about as the industrialized countries began to exploit global fossil fuel and mineral reserves in pursuit of greater economic efficiency. There is an increasing demand for a cost effective renewable source of energy. Fossil fuels are assumed to offer low prices and greater potential. Coal, Oil and Gas are called "fossil fuels" because they have been formed from the fossilized remains of prehistoric plants and animals. They provide around 66% of the world's electrical power, and 95% of the world's total energy demands. But the basic disadvantage of fossil fuels is pollution. Burning any fossil fuel produces carbon dioxide, which contributes to the "greenhouse effect" adding to global warming. The fundamental economic reality of fossil fuels is that they are found in relatively small number of locations across the globe, yet consumed everywhere [7]. This has led to an increased interest in renewable sources of energy such as solar, hydroelectric and wind. Solar power is one of the world's fastest growing renewable energy sources, offering a potentially endless supply of power generation capable of meeting the electricity demands of the entire world. Sun is one of the most important non- conventional sources of energy. The radiative energy from the sun derives from a nuclear fusion reaction. The intensity of solar radiation in free space, at the average distance of earth from the sun is defined as the solar constant and has a value of 1353 W/m2[1].
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