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Solution Growth of Polycrystalline Silicon Thin Films on Glass Substrates for Low-Cost Photovoltaic Cell Application

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Solution Growth of Polycrystalline Silicon Thin Films on Glass Substrates for Low-Cost Photovoltaic Cell Application SOLUTION GROWTH OF POLYCRYSTALLINE SILICON THIN FILMS ON GLASS SUBSTRATES FOR LOW-COST PHOTOVOLTAIC CELL APPLICATION by ZHENGRONG SHI, B.Sc, M.Sc A thesis submitted to the University of New South Wales in fulfilment of the requirements for the degree of Doctor of Philosophy February, 1992 UNIVEFTTY Or N.S.W. 2 2 JUL 1393 LIBRARIES To My Wife: Wei ACKNOWLEDGEMENT I am indebted to Professor Martin A Green, my research supervisor, who has given me his invaluable academic guidance, financial assistance and encouragement during my thesis work. Special thanks are given to Dr. Trevor L Young who has helped me to set up the experimental facility and to improve the fluency of the text, and Benjamin Chan who has helped to characterize the silicon thin film and the silicon thin film solar cells, and also Michael Taouk who has helped with some of the thin film silicon solar cell fabrication. I also acknowledge the contribution of past and present members of the Centre for Photovoltaic Devices and Systems, particularly, Dr. Jurek Kurianski, Dr Stuart Wenham, Dr. Mark Gross, Dr. Soo Hong Lee, Steve Healy, Mike Willison, Ted Szpitalak, Mark Silver, John Willison, Shiqun Cai, Jenny Hansen, and the Electrical Engineering and Mechanical Engineering workshop staffs. This work was supported by the Energy Research and Development Corporation of Australia and the New South Wales Department of Minerals and Energy. The Centre for Photovoltaic Devices and Systems is Supported by the Commonwealth Special Research Centre Scheme. ABSTRACT The purpose of this thesis is to find an appropriate technique for depositing polycrystalline silicon thin film on glass substrates for low cost photovoltaic cell application. Solution growth has been chosen for such polycrystalline silicon deposition on glass substrates. Liquid phase epitaxial layers on both (111) and (100) oriented silicon wafers have been grown from A1 and Au based alloy solutions, in particular, Al/Ga, Al/Sn, Al/Zn, Au/Bi, Au/Pb, and Au/Sn. These alloys have higher silicon solubility below 600°C than solvents previously used. The crystal quality and electronic properties of the grown silicon thin films were characterized by a variety of methods such as microscopy, chemical etching and spreading resistance measurements. The aim of these experiments was to search for an appropriate solution for solution growth of silicon on dissimilar substrates. Graphoepitaxially grown (111) oriented silicon crystals have been demonstrated on patterned Si02 substrates. This technique can possibly be used to obtain oriented silicon thin films on glass substrates due to the similarity between Si02 and glass. Polycrystalline silicon thin films have been successfully deposited on glass substrates by various methods. The common feature of these methods was that a silicon rich surface, either amorphous or crystalline, had been created before silicon deposition. The silicon rich surface had two functions in silicon thin film growth on glass, namely, a) improving wettability between the solution and the glass; b) acting as a seeding layer for subsequent silicon thin film growth. Using the solution growth technique, thin film photovoltaic devices have been fabricated on various substrates, including single crystalline silicon, polycrystalline silicon, and glass. A computer modelling program, PC-ID, has been used to predict the performance of thin film silicon solar cells and to estimate the quality of silicon films by matching cell parameters with experimental cell output. Open circuit voltage exceeding 600 mV has been achieved from cells grown on single crystalline silicon substrates. CONTENT Chapter One: Introduction 1.1 Silicon Solar Cells 1 1.2 Cost of Silicon Solar Cells 3 1.3 Thin Film Silicon Solar Cells 6 1.4 Current Status of Polycrystalline Thin Film Silicon Solar Cells 11 1.5 Technologies for Silicon Deposition on Dissimilar Substrates 1.5.1 Vapour Deposition 12 1.5.2 Recrystallization 15 1.5.3 Graphoepitaxy 16 1.5.4 Solution Growth of Silicon 16 1.6 Solution Growth as a Method for Si Deposition on Dissimilar Substrates 18 1.7 Purpose of the Thesis 21 1.8 References 22 Chapter Two: Mechanisms of Crystallization from Solution 2.1 Introduction 34 2.2 Supersaturation 34 2.3 Wetting 37 2.4 Nucleation 2.4.1 Homogeneous Nucleation 39 2.4.2 Heterogeneous Nucleation 40 2.4.3 Nucleation by Cavitation 44 2.5 Crystal Growth 45 2.6 Liquid Phase Epitaxy (LPE) 48 2.6.1 Phase Equilibria 49 2.6.2 LPE Growth Techniques 50 2.6.3 Segregation 54 2.7 Computer Simulation of Silicon LPE 56 2.8 References 65 Chapter Three: Liquid Phase Epitaxy of Silicon 3.1 Introduction 71 3.2 Solvent Selection 71 3.3 Growth Facilities 73 3.4 Sample Preparation 75 3.5 Alloy Solution Preparation 76 3.6 Experimental Procedure 77 3.7 Characterization of Thin Films 3.7.1 Surface Morphologies of Thin Films 79 3.7.2 Doping Properties of Silicon Epitaxial Layers 87 3.7.3 Quality Analysis of Silicon Thin Films 1. Chemical Etching 89 2. Composition Analysis 94 3.8 Applicability of the Solutions for Silicon Growth on Dissimilar Substrates 97 3.9 References 98 Chapter Four: Graphoepitaxy of Silicon 4.1 Introduction 104 4.2 Background Review 104 4.3 Mechanisms of Graphoepitaxy 106 4.4 Preparation of Relief Patterns on SiC>2 Substrates 108 4.5 Experimental 112 4.6 Random Nucleation of Silicon on SiC>2 Substrates 113 4.7 Silicon Crystals on Patterned SiC>2 Substrates 115 4.8 Analysis and Discussion 116 4.9 References 123 Chapter Five: Polycrystalline Silicon Thin Films on Glass Substrates 5.1 Introduction 127 5.2 Substrate Selection 128 5.3 Glass Properties 133 5.4 Experimental 134 5.5 Deposition of Polycrystalline Silicon Thin Film on Glass Substrates 5.5.1 Silicon Particles Seeding 137 5.5.2 Bare Glass Substrates 140 5.5.3 Amorphous Silicon Coated Glass Substrates 146 5.5.4 Rheotaxy on Glass Substrates 150 5.5.5 Growth Kinetics and Properties of Silicon Thin Films 153 5.5.6 Surface Quality Improvement 162 5.6 References 171 Chapter Six: Thin Film Silicon Photovoltaic Devices 6.1 Introduction 176 6.2 PC-ID Modelling of Thin Film Silicon Solar Cells 177 6.3 Silicon Thin Film Solar Cells on Crystalline Silicon Substrates 6.3.1 P-N Junction Growth 185 6.3.2 Device Fabrication 186 6.3.3 I-V Characteristics of Solar Cells 186 6.3.4 EBIC Characterization of Thin Film Silicon Solar Cells 193 6.4 Polycrystalline Thin Film Silicon Solar Cells on Glass Substrates 197 6.5 References 200 Chapter Seven: Conclusion and Future Directions 7.1 Introduction 201 7.2 Summary and Discussion 201 7.3 Future Prospects 203 7.4 References 205 Appendix I: Numerical Modelling of Solution Growth 1. Numerical Method 206 2. Boundary Condition 207 3. Growth Rate 207 4. References 208 5. Program Listing 211 CHAPTER ONE INTRODUCTION 1.1 Silicon Solar Cells A solar cell is a semiconductor device that converts the energy of sunlight into electrical energy. Basically, a solar cell consists of an appropriately doped semiconductor having either p-type or n-type conductivity in which an internal electrical field is created by forming a region of the opposite conductivity type (p-n junction). The formation of a p-n junction can be achieved in several ways, such as by diffusion, ion-implantation, or epitaxial growth. Metals such as titanium and aluminium are deposited onto the front and back surface of the semiconductor to form electrical contacts. When sunlight shines on the cell's surface, the energy is absorbed by the semiconductor, thus creating electron-hole pairs in the bulk of the cell. These carriers are collected and separated by the internal electrical field and distributed to an external load. The first solar cell was discovered by Becquerel in 1839. The potential for large-scale use of photovoltaics as a power source was recognized upon the development of diffused-junction silicon solar cells by Chapin, Fuller and Pearson[l] at Bell Telephone Laboratories in the 1950's. Since then many reserachers have sought to improve the photovoltaic conversion efficiency of these cells. By the early of 1960's, an efficiency of 15% was achieved and cell design had reached a stage which was to remain relatively stable for a decade. 1 With the advent of the "energy crisis" in the 1970's, much more attention has been paid to the development of solar cells to meet the requirements of large scale terrestrial power supply systems. By the early 1980’s, there had been rapid improvements in silicon solar cell design. These have raised cell performance to a level much closer to the theoretical limit. Moreover, as this progress has been made, it has become increasingly clear that there remains considerable scope for further improvement. Silicon has been the most widely used photovoltaic material to date. It is the second most abundant element in the earth's crust and is the most extensively characterized and best understood semiconductor due to its dominant role in the IC (integrated circuits) industry. Silicon also has the advantage of low toxicity over other semiconductors such as GaAs and CdS. Thermally-grown silicon dioxide very effectively passivates interfaces and so reduces surface recombination losses. Silicon is an indirect band-gap semiconductor. Its band-gap (l.leV) is somewhat below the optimum value of 1.4eV for semiconductors matched to the solar spectrum. A theoretical conversion efficiency limit of about 29% has been predicted [2-5]. In practice, conversion efficiencies of silicon solar cells of up to 24% have been achieved to date [6]. This is very close to the highest efficiency of GaAs solar cells, which has been considered as the best photovoltaic material from the energy band-gap point of view.
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