Physics of Thin-film Photovoltaics

Tackling one of the hottest topics in renewables, thin-film photovoltaics, the authors present the latest updates, technologies, and applications, offering the most up-to-date and thorough coverage available to the engineer, scientist, or student.

It appears rather paradoxical that thin-film photovoltaics (PVs) are made of materials that seem unacceptable from the classical PV perspective, and yet they often outperform classical PV. This exciting new volume solves that paradox by switching to a new physics paradigm.

Many concepts here fall beyond the classical PV scope. The differences lie in device thinness (microns instead of millimeters) and morphology (non-crystalline instead of crystalline). In such structures, the charge carriers can reach electrodes without recombination. On the other hand, thin disordered structures render a possibility of detrimental lateral nonuniformities (“recombination highways”), and their energy spectra give rise to new recombination modes. The mechanisms of thermal exchange and device degradation are correspondingly unique.

The overall objective of this book is to give a self-contained in-depth discussion of the physics of thin-film systems in a manner accessible to both researchers and students. It covers most aspects of the physics of thin-film PV, including device operations, material structure and parameters, thin-film junction formation, analytical and numerical modeling, concepts of large area effects and lateral non-uniformities, physics of shunting (both shunt growth and effects), and device degradation. Also, it reviews a variety of physical diagnostic techniques proven with thin-film PV. Whether for the veteran engineer or the student, this is a must-have for any library.

This outstanding new volume:

• Covers not only the state-of-the-art of thin-film photovoltaics, but also the basics, making this volume useful not just to the veteran engineer, but the new-hire or student as well
• Offers a comprehensive coverage of thin-film photovoltaics, including operations, modeling, non-uniformities, piezo-effects, and degradation
• Includes novel concepts and applications never presented in book format before
• Is an essential reference, not just for the engineer, scientist, and student, but the unassuming level of presentation also makes it accessible to readers with a limited physics background
• Is filled with workable examples and designs that are helpful for practical applications
• Is useful as a textbook for researchers, students, and faculty for understanding new ideas in this rapidly emerging field

Audience:

Industrial professionals in photovoltaics, such as engineers, managers, research and development staff, technicians, government and private research labs; also academic and research universities, such as physics, chemistry, and electrical engineering departments, and graduate and undergraduate students studying electronic devices, semiconductors, and energy disciplines

Victor G Karpov, PhD, is a professor in the Department of Physics and Astronomy at the University of Toledo in the USA, having received his Dr. of Sciences degree from the Institute for Nuclear Physics (Academy of Sciences of USSR) and PhD from Leningrad Polytechnical Institute. With almost 40 years of teaching and industry experience, he has published nearly 200 scholarly papers and has numerous grants and awards to his credit. 

Diana Shvydka, PhD, is a professor in the Department of Radiation Oncology at the University of Toledo, having also received her doctorate in Physics from the University of Toledo. With almost 20 years of teaching and industry experience, she has over 100 publications in scientific and technical journals and several patents.

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

COPYRIGHTED MATERIAL

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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