Advanced Silicon Materials for Photovoltaic Applications

Preface xiii
List of Contributors xvii
1. Silicon Science and Technology as the Background of the Current and Future Knowledge Society 1Sergio 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
2.2.2 Purification of Volatile Silicon Compounds 30
2.2.3 Decomposition of Volatile Precursors to Elemental Silicon 30
2.2.4 Most Common Reactors 33
2.2.5 Recovery of By-Products 38
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 Metal-Metal 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
4. Gettering Processes and the Role of Extended Defects 127
Michael Seibt and Vitaly Kveder
4.1 Introduction 127
4.2 Properties of Transition-Metal Impurities in Silicon 130
4.2.1 Solubility of Transition Metal Impurities 131
4.2.2 Diffusion of Transition-Metal Impurities 136
4.3 Gettering Mechanisms and their Modeling 139
4.3.1 Segregation Gettering 140
4.3.2 Relaxation Gettering 142
4.3.3 Injection Gettering 142
4.3.4 Modeling of Gettering Kinetics 143
4.3.5 Aluminum Gettering 144
4.3.6 Phosphorus-Diffusion Gettering 146
4.3.7 Boron-Diffusion Gettering 149
4.4 Bulk Processes Affecting Gettering Efficiency and Kinetics 150
4.4.1 Metal-Silicide Precipitates 150
4.4.2 Dislocations 154
4.4.3 Grain Boundaries 167
4.4.4 Light-Element Impurities and Related Defects 169
4.5 Gettering Strategies and Defect Engineering 170
4.6 Conclusions 173
References 174
5. Advanced Characterization Techniques 189
Anna Cavallini, Daniela Cavalcoli and Laura Polenta
5.1 Introduction 189
5.2 Surface Photovoltage Spectroscopy 190
5.2.1 The Basic Principles 191
5.2.2 SPS Setup 193
5.2.3 Surface Photovoltage Spectroscopy of Hydrogenated Nanocrystalline Silicon (nc-Si:H) 194
5.3 Photocurrent Spectroscopy 196
5.3.1 Basic Principles 197
5.3.2 Spectral Photoconductivity Setup 199
5.3.3 Application of Spectral Photoconductivity to Silicon and Silicon Devices 201
5.4 Optical (Light) Beam Induced Current (OBIC or LBIC) 202
5.4.1 Basic Principles of Optical Beam Induced Current Method 202
5.4.2 Determination of the Electric Field and Depletion Region Extent in Particle Detectors by OBIC 204
5.5 Scanning Probe Microscopy for the Nanoscale Electrical Characterization of Semiconductors for PV Applications 207
5.6 Concluding Remarks 210
References 210
6. Advanced Analytical Techniques for Solar-Grade Feedstock 215
Richard S. Hockett
6.1 Introduction 215
6.2 Review of Analytical Techniques 216
6.3 GDMS Analysis of PV Si 222
6.4 SIMS Analysis of PV Si 223
6.5 Applications of SIMS and GDMS for PV Si Feedstock Studies 227
6.5.1 Impurity Segregation in Directional Solidified (DS) Silicon Blocks 227
6.5.2 Specification of [C], [O] and [N] in Solar-Grade Silicon Feedstock to be Used in DS Furnaces 229
6.5.3 SIMS Capability for Reduced-Cost Measurement of [C, O, B, P] 230
6.5.4 Problems in Conversion Between Resistivity and Dopant
Concentration in Highly Compensated Silicon 231
References 232
7. Thin-Film Deposition Processes 235
J.K. Rath
7.1 Introduction 235
7.2 Deposition Techniques of Thin-Film Silicon 235
7.2.1 Standard Radio-Frequency Plasma-Enhanced CVD 236
7.2.2 Very High Frequency Plasma-Enhanced CVD 236
7.2.3 Microwave Plasma-Enhanced CVD 237
7.2.4 ETP 237
7.2.5 Low-Energy Plasma-Enhanced PECVD 238
7.2.6 Hot-Wire CVD 238
7.3 In Situ Diagnosis of Growth Conditions 239
7.3.1 Electrical: Current-Voltage (I -V ) Probe 239
7.3.2 Optical Emission Spectroscopy (OES) 240
7.3.3 Infrared Spectroscopy 243
7.3.4 Ellipsometry 244
7.3.5 Ion Energy Probe 245
7.4 Challenges of Deposition at High Growth Rates and Low Substrate Temperatures 246
7.4.1 Growth-Process Models 246
7.4.2 Inhomogeneity of Growth 250
7.4.3 Growth at High Deposition Rates 251
7.4.4 Silane Dissociation Efficiency and Depletion Criteria for nc-Si Deposition 252
7.4.5 Low-Temperature (LT) Deposition 254
7.4.6 Structural Evolution at Low Temperature 257
7.4.7 Transient Plasma 260
7.5 Upscaling to Large-Area and Industrial Processing: Critical Analysis of Various Fabrication Processes 270
References 273
8. Modeling of Thin-Film Deposition Processes 287
Carlo Cavallotti
8.1 Introduction 287
8.2 Modeling the Plasma Discharge 290
8.3 Modeling of the Gas Phase and Surface Kinetics 295
8.3.1 Gas-Phase Kinetic Scheme 297
8.3.2 Surface Kinetic Scheme 301
8.3.3 On the Consistent Solution of the Plasma Discharge and Kinetic Models: Theory and Examples 303
8.4 Modeling of the Thin-Film Morphological Evolution 303
8.5 Status of the Field and Perspectives 308
References 309
9. Thin-Film Silicon Solar Cells 311
J.K. Rath
9.1 Introduction 311
9.2 Second-Generation Solar Cells: Advantages Compared to the First Generation 312
9.3 Drift-Type Thin-Film Silicon Solar Cells: Substrates and Configuration 314
9.4 Material Considerations for Thin-Film Silicon Solar Cells 316
9.4.1 Amorphous Silicon 316
9.4.2 Amorphous Silicon-Germanium 317
9.4.3 Nanocrystalline Silicon 317
9.4.4 Light Confinement 318
9.5 Present Status of Drift-Type Thin-Film Silicon Solar Cells 321
9.5.1 Recent R&D Results on Thin-Film Silicon Solar Cells 322
9.5.2 Industrial Scenario 322
9.6 Technological Issues 325
9.6.1 High Deposition Rate 325
9.6.2 Thin Cells 325
9.7 Third-Generation Thin-Film Silicon Cell 329
9.8 Solar Cells on Plastics 331
9.8.1 Transfer Method 331
9.8.2 Direct Deposition 332
9.9 Hybrid Cells 334
9.10 Industrial Scenario of Thin-Film Silicon-based Solar Cells 336
9.11 Challenges for Thin-Film Silicon Solar-Module Fabrication 338
References 341
10. Innovative Quantum Effects in Silicon for Photovoltaic Applications 355
Zhizhong Yuan, Aleksei Anopchenko and Lorenzo Pavesi
10.1 Basic Principles of 3rd-Generation Solar Cells 355
10.1.1 The Need for a New Generation of Solar Cells 355
10.1.2 Limitations in Early Generations 356
10.1.3 3rd-Generation Options 357
10.2 The Advantages of Using Silicon Nanocrystals 359
10.2.1 Fabrication and Advantages of Si-NC 359
10.2.2 Quantum Confinement Effect in Si-NCs 360
10.3 Applications of Si-NC in the 3rd-Generation Solar Cells 362
10.3.1 All-Silicon Tandem Solar Cells 362
10.3.2 Hot-Carrier Solar Cells 364
10.3.3 Intermediate-Band Solar Cells 366
10.3.4 Multiple-Carrier Generation 368
10.3.5 Downshifter Cell 372
10.4 Challenges and Solutions 375
10.4.1 Size Control 375
10.4.2 Carrier Transport 376
10.4.3 Absorption 378
10.4.4 Technological Constraints 380
10.5 Conclusions 381
References 382
Index 393