Advanced Silicon Materials for Photovoltaic Applications

Today, the silicon feedstock for photovoltaic cells comes from processes which were originally developed for the microelectronic industry. It covers almost 90% of the photovoltaic market, with mass production volume at least one order of magnitude larger than those devoted to microelectronics.

However, it is hard to imagine that this kind of feedstock (extremely pure but heavily penalized by its high energy cost) could remain the only source of silicon for a photovoltaic market which is in continuous expansion, and which has a cumulative growth rate in excess of 30% in the last few years. Even though reports suggest that the silicon share will slowly decrease in the next twenty years, finding a way to manufacture a specific solar grade feedstock in large quantities, at a low cost while maintaining the quality needed, still remains a crucial issue. Thin film and quantum confinement-based silicon cells might be a complementary solution.

Advanced Silicon Materials for Photovoltaic Applications has been designed to describe the full potentialities of silicon as a multipurpose material and covers:

• Physical, chemical and structural properties of silicon

• Production routes including the promise of low cost feedstock for PV applications

• Defect engineering and the role of impurities and defects

• Characterization techniques, and advanced analytical techniques for metallic and non-metallic impurities

• Thin film silicon and thin film solar cells

• Innovative quantum effects, and 3rd generation solar cells

With contributions from internationally recognized authorities, this book gives a comprehensive analysis of the state-of-the-art of process technologies and material properties, essential for anyone interested in the application and development of photovoltaics.

Autorentext
Professor Sergio Pizzini is Chairman of NED SILICON SpA, a company which focuses on renewable energies. He is retired from the University of Milano-Bicocca, where he was a Professor of Physical Chemistry until 2008. He also held positions as Director of the Nanotechnology Science Doctorate and Director of the Doctorate School of the Faculty of Sciences at the University.
Professor Pizzini is currently a member of the Scientific Committee of the Solar Lab, a joint initiative of the University of Camerino, Department of Physics and of Renergies Italia, Spa. His scientific expertise spans from semiconductor physics and chemistry to surface defect science and silicon processes for photovoltaic uses. He is the author or co-author of four books as well as more than two hundred technical papers.

Inhalt
Preface xiii List of Contributors xvii

1. Silicon Science and Technology as the Background of the Current and Future Knowledge Society 1*Sergio S. Pizzini*

1.1 Introduction 1

1.2 Silicon Birth from a Thermonuclear Nucleosynthetic Process 2

1.3 Silicon Key Properties 2

1.3.1 Chemical and Structural Properties 2

1.3.2 Point Defects 7

1.3.3 Radiation Damage and Radiation Hardness 7

1.4 Advanced Silicon Applications 9

1.4.1 Silicon Radiation Detectors 9

1.4.2 Photovoltaic Cells for Space Vehicles and Satellite Applications 11

1.4.3 Advanced Components Based on the Dislocation Luminescence in Silicon 12

1.4.4 Silicon Nanostructures 14

References 15

2. Processes 21
Bruno Ceccaroli and Sergio S. Pizzini

2.1 Introduction 21

2.2 Gas-Phase Processes 23

2.2.1 Preparation and Synthesis of Volatile Silicon Compounds 23

2.2.1.1 Production and Utilization of SiHCl3 24

2.2.1.2 Production and Utilization of SiCl4 25

2.2.1.3 Production of SiH2Cl2 (and other Chlorosilanes) 26

2.2.1.4 Production and Applications of SiH4 27

2.2.1.5 Production of SiF4 29

2.2.1.6 Other Silicon Compounds 30

2.2.2 Purification of Volatile Silicon Compounds 30

2.2.3 Decomposition of Volatile Precursors to Elemental Silicon 30

2.2.3.1 Metal Reduction 30

2.2.3.2 Hydrogen Reduction 31

2.2.3.3 Thermal Decomposition of Volatile Silicon Precursors 32

2.2.4 Most Common Reactors 33

2.2.5 Recovery of By-Products 38

2.2.5.1 By-Products in the Case of Thermal Decomposition or Hydrogen Reduction of TCS 38

2.2.5.2 By-Products in Case of Thermal Decomposition of Silane 39

2.2.5.3 By-Products in the Case of Metal Reduction of Silicon Precursors 40

2.3 Production of MG and UMG Silicon and Further Refining Up to Solar Grade by Chemical and Physical Processes 40

2.3.1 MG Silicon Production 42

2.3.2 Metallurgical Refining Processes 47

2.3.3 MetalMetal Extraction Processes 52

2.3.4 Solid/Liquid Extraction Techniques 54

2.3.5 Final Purification by Directional Solidification 55

2.3.6 Solar-Grade Silicon Production from Pure Raw Materials or Via the Direct Route 58

2.4 Fluoride Processes 59

2.5 Silicon Production/Refining with High-Temperature Plasmochemical Processes 61

2.5.1 Silicon Production Via Plasma Processes 62

2.5.2 Silicon Refining Via Plasma Processes 63

2.6 Electrochemical Processes: Production of Silicon Without Carbon as Reductant 64

2.7 Conclusions 68

References 70

3. Role of Impurities in Solar Silicon 79
Gianluca Coletti, Daniel Macdonald and Deren Yang

3.1 Introduction 79

3.2 Sources and Refinements of Impurities 79

3.3 Segregation of Impurities During Silicon Growth 86

3.3.1 Equilibrium Segregation Coefficients 86

3.3.2 Effective Segregation Coefficient 87

3.3.3 Distribution of Impurities in Silicon Crystal Due to Segregation 90

3.4 Role of Metallic Impurities 92

3.4.1 Solubility and Diffusivity 92

3.4.2 Impact on Charge-Carrier Recombination 94

3.4.3 Modeling the Impact of Metallic Impurities on the Solar-Cell

Performance 96

3.5 Role of Dopants 101

3.5.1 Carrier Mobilities in Compensated Silicon 101

3.5.2 Recombination in Compensated Silicon 103

3.5.3 Dopant-Related Recombination Centers 105

3.5.4 Segregation Effects During Ingot Growth 106

3.5.5 Detecting Dopants in Compensated Silicon 107

3.6 Role of Light Elements 108

3.6.1 Oxygen 108

3.6.2 Carbon 109

3.6.3 Nitrogen 111

3.6.4 Germanium 113

3.7 Arriving at Solar-Grade Silicon Feedstock Definitions 114

References 118 &l...