Physics of Thin-Film Photovoltaics

Preface xi

Part I General and Thin Film PV 1

I. Introduction to Thin Film PV 1

A. The Origin of PV. Junctions 1

B. Fundamental Material Requirements 3

C. Charge Transport. Definition of Thin Film PV 4

D. Distinctive Features of Thin Film PV 7

References 11

Part II One-Dimensional (1D) Diodes and PV 13

II. 1D Diode 13

A. Metal-Insulator-Metal Diode 13

B. Schottky, Reach-Through, and Field-Compensation Diodes 19

1. Schottky Diode 19

2. Reach Through Diodes 21

3. Field Compensation Diode 23

C. P-N Homo-Junctions 24

D. Heterojunctions 26

E. Other Relevant Types of Diodes 28

F. Field Reversal Diode: A Counterintuitive Case 29

G. Cat's Whisker Diode 30

III. 1D Solar Cell 32

A. 1D Solar Cell Base Model 32

B. Numerical Modeling of 1D PV 38

1. Governing Equations 39

2. Device Model Parameters 40

3. Some Modeling Results 42

IV. Photovoltaic Parameters 43

A. Second-Level Parameters 44

B. Practical Solar Cells and Third-Level Metrics 46

C. Indicative Facts 49

D. Phenomenological Interpretation. Ideal Diode with Other Circuitry Elements 52

V. Case Study 54

A. Field Reversal PV 54

1. Analytical Approach 55

2. Numerical Modeling of the Field Reversal Device Operations 60

B. Miraculous Back Contact 68

References 72

Part III Beyond 1D: Lateral Effects in Thin Film PV 79

VI. Examples of Multidimensional Numerical Modeling 79

VII. Introduction to Random Multidimensional Phenomena 81

VIII. Lateral Screening Length 84

A. Shunt Screening 84

B. Bias Screening 85

C. Quantitative Approach and Linear Screening Regime 88

IX. Schottky Barrier Nonuniformities 91

X. Semi-Shunts 93

XI. Random Diodes 96

A. Weak Diodes 96

B. Random Diode Arrays in Solar Cells 99

C. Random Diode Arrays in PV Modules and Fields 105

XII. Nonuniformity Observations 109

A. Cell Level Observations 109

B. Module Level Observations 118

XIII. Nonuniformity Treatment 121

References 125

Part IV Electronic Processes in Materials of Thin Film PV 131

XIV. Morphology, Fluctuations, and the Density of States 132

A. The Materials of Thin Film PV are Fundamentally Different 132

B. Noncrystalline Morphology 134

C. Long Range Fluctuations of Potential Energy 136

D. Random Potential in Very Thin Structures 139

E. Numerical Estimates and Implications 142

XV. Electronic Transport 144

A. Band Transport in Random Potential 144

B. Hopping Transport Through Thin Noncrystalline Films 147

1. Hopping Between Ideal Electrodes 149

2. Hopping Between Resistive Electrodes 151

3. Critical Area and Mesoscopic Fluctuations 153

XVI. Recombination in Quasi-Continuous Spectrum 155

XVII. Noncrystalline Junctions 161

XVIII. Piezo and Pyro-PV 164

A. The Nature of Piezo-PV 164

B. Piezo-PV Observations 169

C. The Significance of Piezo-PV 171

References 174

Part V Electro-Thermal Instabilities in Thin Film PV 181

XIX. The Two-Diode Model 182

A. Linear Stability Analysis 183

B. The Two-Diode Modeling: Numerical Estimates and Scaling 184

XX. Distributed Diode Model 186

A. Introduction 186

B. Linear Stability Analysis 187

XXI. Simplistic Numerical Modeling 188

XXII. Spontaneous Hot Spots 190

A. Introduction 190

B. Observations 191

C. Numerical Modeling 195

1. Electrical Model 195

2. Thermal Model 199

D. Modeling Results 200

E. Approximate Analytical Model 205

XXIII. Related Work 207

XXIV. Conclusions on the Electro-Thermal Instabilities in Thin Film PV 209

References 210

Part VI Degradation of Thin Film PV 213---

XXV. Thin Film vs Crystalline PV Degradation Processes 213

XXVI. Observations 215

A. Cell Degradation 216

B. Module Degradation 222

XXVII. Categories of Degradation 225

A. General Categories 225

B. Thin-Film PV Instabilities 227

1. Shunting Instability 227

2. Contact Delamination Instability 229

XXVIII. Accelerated Life Testing 231

A. Examples of Very Strong ALT: HALT 232

1. EBIC HALT 232

2. LBIC HALT 234

B. Actuarial Approach to ALT 235

C. Concluding Remarks on Degradation 236

References 237

Appendix. Some Methodological Aspects of Device Modeling 243

Appendix A: Model of Series Connection 243

Appendix B: The Diffusion Approximation 245

Appendix C: Long Range Potential 248

1. Point Changes 248

2. Columnar Charges 251

References 253

Index 255

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Photovoltaics; Solar cell; Thin film; Chalcogenide; CdTe; CdS; CIGS; TCO; Back contact; Front Contact: p-n junction; AMPS; Solar cell modeling; Piezo-photovoltaic; Degradation; Reliability; Thermal instability; Run away instability; Lateral nonuniformity; Nonuniformity screening; Efficiency; Open circuit voltage; Short circuit current; Fill factor; Nonideality factor; Series resistance; Shunt resistance; Recombination; Diffusion length; EBIC; OBIC; Mapping; Shunt; Shunting; Semi-shunt; Cat’ s whisker diode; Schottky diode; PV statistics