Handbook of Photovoltaic Science and Engineering

Handbook of Photovoltaic Science and Engineering

(Parte 1 de 7)

Handbook of

Photovoltaic Science and Engineering

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Handbook of

Photovoltaic Science and Engineering

Edited by

Antonio Luque Instituto de Energıa Solar, Universidad Politecnica de Madrid, Spain and

Steven Hegedus Institute of Energy Conversion, University of Delaware, USA

Copyright 2003 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England

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Library of Congress Cataloging-in-Publication Data

Handbook of photovoltaic science and engineering / edited by Antonio Luque and Steven Hegedus. p. cm.

Includes bibliographical references and index. ISBN 0-471-49196-9 (alk. paper) 1. Photovoltaic cells. 2. Photovoltaic power generation. I. Luque, A. (Antonio) I. Hegedus, Steven.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-471-49196-9

Typeset in 10/12 Times by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production.

Some images in the original version of this book are not available for inclusion in the eBook.

We dedicate this book to all those who have worked so hard for half a century to bring solar electricity to where it is today, and to our colleagues present and future who must work even harder in the next half century to make sure that it fulfills its potential as a widely available clean energy source.

The editors also owe much appreciation to the authors of the chapters contained in this book. Their long hours spent writing the best possible chapter covering their field of expertise, and then suffering through a storm of editorial criticisms, has hopefully made this a high-quality publication of lasting value.

Finally, we want to express our gratitude to our loved ones (Carmen, Ignacio, Sofıa, Victoria, Ines, and Debbie, Jordan, Ariel) for the many hours stolen from family life while working on this book.

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Contents

List of Contributors xi

1 Status, Trends, Challenges and the Bright Future of Solar Electricity from Photovoltaics 1 Steven S. Hegedus and Antonio Luque 1.1 The Big Picture 1 1.2 What Is Photovoltaics? 3 1.3 Six Myths of Photovoltaics 5 1.4 History of Photovoltaics 1 1.5 PV Costs, Markets and Forecasts 15 1.6 What Are the Goals of Today’s PV Research and Manufacturing? 19 1.7 Global Trends in Performance and Applications 20 1.8 Crystalline Silicon Progress and Challenges 23 1.9 Thin Film Progress and Challenges 27 1.10 Concentration PV Systems 31 1.1 Balance of Systems 32 1.12 Future of Emerging PV Technologies 37 1.13 Conclusions 39 References 41

2 Motivation for Photovoltaic Application and Development 45

Joachim Luther 2.1 Characteristics of Photovoltaic Energy Conversion 45 2.2 A Long-term Substitute for Today’s Conventional Electricity

Production – The Ecological Dimension of Photovoltaics 48 2.2.1 In Summary 54 2.3 A Technological Basis for Off-grid Electricity Supply – The

Development Dimension of Photovoltaics 54 2.3.1 In Summary 57 2.4 Power Supply for Industrial Systems and Products – The

Professional Low Power Dimension 57 2.5 Power for Spacecraft and Satellites – the Extraterrestrial Dimension of Photovoltaics 59 References 60 viii CONTENTS

3 The Physics of the Solar Cell 61

Jeffery L. Gray 3.1 Introduction 61 3.2 Fundamental Properties of Semiconductors 64 3.2.1 Crystal Structure 64 3.2.2 Energy Band Structure 65 3.2.3 Conduction-band and Valence-band Densities of State 6 3.2.4 Equilibrium Carrier Concentrations 67 3.2.5 Light Absorption 70 3.2.6 Recombination 74 3.2.7 Carrier Transport 78 3.2.8 Semiconductor Equations 81 3.2.9 Minority-carrier Diffusion Equation 82 3.3 PN-Junction Diode Electrostatics 83 3.4 Solar Cell Fundamentals 87 3.4.1 Solar Cell Boundary Conditions 87 3.4.2 Generation Rate 89 3.4.3 Solution of the Minority-carrier Diffusion Equation 89 3.4.4 Terminal Characteristics 89 3.4.5 Solar Cell I –V Characteristics 92 3.4.6 Properties of Efficient Solar Cells 95 3.4.7 Lifetime and Surface Recombination Effects 96 3.4.8 An Analogy for Understanding Solar Cell Operation: A

4 Theoretical Limits of Photovoltaic Conversion 113

CONTENTS ix

4.3.1 The Balance Equation of a PV Converter 120 4.3.2 The Monochromatic Cell 124 4.3.3 Thermodynamic Consistence of the Shockley–Queisser

Photovoltaic Cell 126 4.3.4 Entropy Production in the Whole Shockley–Queisser

Solar Cell 129 4.4 The Technical Efficiency Limit for Solar Converters 131 4.5 Very High Efficiency Concepts 132 4.5.1 Multijunction Solar Cells 132 4.5.2 Thermophotovoltaic Converters 135 4.5.3 Thermophotonic Converters 136 4.5.4 Higher-than-one Quantum Efficiency Solar Cells 140 4.5.5 Hot Electron Solar Cells 141 4.5.6 Intermediate Band Solar Cell 144 4.6 Conclusions 148 References 149

5 Solar Grade Silicon Feedstock 153

Bruno Ceccaroli and Otto Lohne 5.1 Introduction 153 5.2 Silicon 154 5.2.1 Physical Properties of Silicon Relevant to Photovoltaics 154 5.2.2 Chemical Properties Relevant to Photovoltaics 156 5.2.3 Health Factors 156 5.2.4 History and Applications of Silicon 157 5.3 Production of Metallurgical Grade Silicon 161 5.3.1 The Carbothermic Reduction of Silica 161 5.3.2 Refining 163 5.3.3 Casting and Crushing 166 5.3.4 Economics 167 5.4 Production of Semiconductor Grade Silicon (Polysilicon) 167 5.4.1 The Siemens Process 168 5.4.2 The Union Carbide Process 172 5.4.3 The Ethyl Corporation Process 173 5.4.4 Economics and Business 175 5.5 Current Silicon Feedstock to Solar Cells 175 5.6 Requirements of Silicon for Crystalline Solar Cells 179 5.6.1 Solidification 179 5.6.2 Effect of Crystal Imperfections 182 5.6.3 Effect of Various Impurities 186 5.7 Routes to Solar Grade Silicon 193 5.7.1 Crystallisation 193 5.7.2 Upgrading Purity of the Metallurgical Silicon Route 194 5.7.3 Simplification of the Polysilicon Process 198 5.7.4 Other Methods 201 5.8 Conclusions 201 References 202 x CONTENTS

6 Bulk Crystal Growth and Wafering for PV 205

W.K och, A.L .E ndros,D .F ranke,C . Haßler,J .P .K alejs and H. J. Moller 6.1 Introduction 205 6.2 Bulk Monocrystalline Material 206 6.2.1 Cz Growth of Single-crystal Silicon 207 6.2.2 Tri-crystalline Silicon 211 6.3 Bulk Multicrystalline Silicon 214 6.3.1 Ingot Fabrication 214 6.3.2 Doping 216 6.3.3 Crystal Defects 217 6.3.4 Impurities 219 6.4 Wafering 223 6.4.1 Multi-wire Wafering Technique 224 6.4.2 Microscopic Process of Wafering 226 6.4.3 Wafer Quality and Saw Damage 229 6.4.4 Cost and Size Considerations 230 6.5 Silicon Ribbon and Foil Production 230 6.5.1 Process Description 232 6.5.2 Productivity Comparisons 238 6.5.3 Manufacturing Technology 239 6.5.4 Ribbon Material Properties and Solar Cells 240 6.5.5 Ribbon/Foil Technology – Future Directions 243 6.6 Numerical Simulations of Crystal Growth Techniques 244 6.6.1 Simulation Tools 245 6.6.2 Thermal Modelling of Silicon Crystallisation Techniques 245 6.6.3 Simulation of Bulk Silicon Crystallisation 247 6.6.4 Simulation of Silicon Ribbon Growth 249 6.7 Conclusions 251 6.8 Acknowledgement 252 References 252

7 Crystalline Silicon Solar Cells and Modules 255

Ignacio Tobıas, Carlos del Canizo and Jesus Alonso 7.1 Introduction 255 7.2 Crystalline Silicon as a Photovoltaic Material 257 7.2.1 Bulk Properties 257 7.2.2 Surfaces 257 7.3 Crystalline Silicon Solar Cells 259 7.3.1 Cell Structure 259 7.3.2 Substrate 260 7.3.3 The Front Surface 263 7.3.4 The Back Surface 266 7.3.5 Size Effects 266 7.3.6 Cell Optics 268 7.3.7 Performance Comparison 270

CONTENTS xi

8 Thin-film Silicon Solar Cells 307

Bhushan Sopori 8.1 Introduction 307 8.2 A Review of Current Thin-film Si Cells 310 8.2.1 Single-crystal Films Using Single-crystal Si Substrates 317 8.2.2 Multicrystalline-Si Substrates 320 8.2.3 Non-Si Substrates 321 8.3 Design Concepts of TF-Si Solar Cells 324 8.3.1 Light-trapping in Thin Si Solar Cells 326 8.3.2 Description of PV Optics 327 8.3.3 Electronic Modeling 3 8.3.4 Methods of Making Thin-Si Films for Solar Cells 341 xii CONTENTS

8.3.5 Methods of Grain Enhancement of a-Si/µc-Si

Thin Films 343 8.3.6 Processing Considerations for TF-Si Solar Cell

9 High-Efficiency I-V Multijunction Solar Cells 359

J. M. Olson, D. J. Friedman and Sarah Kurtz 9.1 Introduction 359 9.2 Applications 363 9.2.1 Space Solar Cells 363 9.2.2 Terrestrial Energy Production 363 9.3 Physics of I-V Multijunction and Single-junction Solar Cells 363 9.3.1 Wavelength Dependence of Photon Conversion Efficiency 363 9.3.2 Theoretical Limits to Multijunction Efficiencies 364 9.3.3 Spectrum Splitting 364 9.4 Cell Configuration 365 9.4.1 Four-terminal 365 9.4.2 Three-terminal Voltage-matched Interconnections 366 9.4.3 Two-terminal Series-connected (Current Matched) 366 9.5 Computation of Series-Connected Device Performance 366 9.5.1 Overview 366

9.5.2 Top and Bottom Subcell QE and JSC 367 9.5.3 Multijunction J –V Curves 368

9.5.4 Efficiency versus Band Gap 370 9.5.5 Top-cell Thinning 372

9.5.6 Current-matching Effect on Fill Factor and VOC 373 9.5.7 Spectral Effects 374

CONTENTS xiii

10 Space Solar Cells and Arrays 413

Sheila Bailey and Ryne Raffaelle 10.1 The History of Space Solar Cells 413 10.1.1 Vanguard I to Deep Space I 413 10.2 The Challenge for Space Solar Cells 416 10.2.1 The Space Environment 417 10.2.2 Thermal Environment 420 10.2.3 Solar Cell Calibration and Measurement 424 10.3 Silicon Solar Cells 425 10.4 I-V Solar Cells 426 10.4.1 Thin-film Solar Cells 428 10.5 Space Solar Arrays 431 10.5.1 Body-mounted Arrays 432 10.5.2 Rigid Panel Planar Arrays 432 10.5.3 Flexible Fold-out Arrays 433 10.5.4 Thin-film or Flexible Roll-out Arrays 435 10.5.5 Concentrating Arrays 436 10.5.6 High-temperature/Intensity Arrays 438 10.5.7 Electrostatically Clean Arrays 439 10.5.8 Mars Solar Arrays 440 10.5.9 Power Management and Distribution (PMAD) 441 10.6 Future Cell and Array Possibilities 441 10.6.1 Low Intensity Low Temperature (LILT) Cells 441 10.6.2 Quantum Dot Solar Cells 442 10.6.3 Integrated Power Systems 442 10.6.4 High Specific Power Arrays 443 10.6.5 High-radiation Environment Solar Arrays 443 10.7 Power System Figures of Merit 4 References 446

1 Photovoltaic Concentrators 449

Richard M. Swanson 1.1 Introduction 449 1.1.1 The Concentrator Dilemma 450 1.2 Basic Types of Concentrators 452 1.2.1 Types of Optics 452 1.2.2 Concentration Ratio 455 xiv CONTENTS

1.2.3 Types of Tracking 456 1.2.4 Static Concentrators 456 1.3 Historical Overview 460 1.3.1 The Sandia National Laboratories Concentrator Program (1976 to 1993) 461 1.3.2 The Martin Marietta Point-focus Fresnel System 462 1.3.3 The Entech Linear-focus Fresnel System 463 1.3.4 Other Sandia Projects 465 1.3.5 The Concentrator Initiative 465 1.3.6 Early Demonstration Projects 466 1.3.7 The EPRI High-concentration Program 467 1.3.8 Other Concentrator Programs 471 1.3.9 History of Performance Improvements 472 1.4 Optics of Concentrators 474 1.4.1 Basics 474 1.4.2 Reflection and Refraction 478 1.4.3 The Parabolic Concentrator 479 1.4.4 The Compound Parabolic Concentrator 482 1.4.5 The V-trough Concentrator 483 1.4.6 Refractive Lenses 485 1.4.7 Secondary Optics 489 1.4.8 Static Concentrators 491 1.4.9 Innovative Concentrators 492 1.4.10 Issues in Concentrator Optics 494 1.5 Current Concentrator Activities 495 1.5.1 Amonix 496 1.5.2 Australian National University 496 1.5.3 BP Solar and the Polytechnical University of Madrid 496 1.5.4 Entech 497 1.5.5 Fraunhofer-Institut fur Solare Energiesysteme 497 1.5.6 Ioffe Physical-Technical Institute 498 1.5.7 National Renewable Energy Laboratory 498 1.5.8 Polytechnical University of Madrid 498 1.5.9 Solar Research Corporation 499 1.5.10 Spectrolab 499 1.5.1 SunPower Corporation 499 1.5.12 University of Reading 500 1.5.13 Tokyo A&T University 500 1.5.14 Zentrum fur Sonnenenergie und Wasserstoff Forschung

Baden Wurttenberg (ZSW) 500 References 500

12 Amorphous Silicon–based Solar Cells 505

Xunming Deng and Eric A. Schiff 12.1 Overview 505 12.1.1 Amorphous Silicon: The First Bipolar Amorphous Semiconductor 505

CONTENTS xv

12.1.2 Designs for Amorphous Silicon Solar Cells: A Guided Tour 508 12.1.3 Staebler–Wronski Effect 511 12.1.4 Synopsis of this Chapter 512 12.2 Atomic and Electronic Structure of Hydrogenated Amorphous

Silicon 513 12.2.1 Atomic Structure 513 12.2.2 Defects and Metastability 514 12.2.3 Electronic Density-of-states 515 12.2.4 Bandtails, Bandedges, and Band Gaps 516 12.2.5 Defects and Gap States 517 12.2.6 Doping 518 12.2.7 Alloying and Optical Properties 518 12.3 Depositing Amorphous Silicon 520 12.3.1 Survey of Deposition Techniques 520 12.3.2 RF Glow Discharge Deposition 521 12.3.3 Glow Discharge Deposition at Different Frequencies 523 12.3.4 Hot-wire Chemical Vapor Deposition 525 12.3.5 Other Deposition Methods 526 12.3.6 Hydrogen Dilution 526 12.3.7 Alloys and Doping 528 12.4 Understanding a-Si pin Cells 528 12.4.1 Electronic Structure of a pin Device 528 12.4.2 Photocarrier Drift in Absorber Layers 530 12.4.3 Absorber Layer Design of a pin Solar Cell 533 12.4.4 The Open-circuit Voltage 534 12.4.5 Optical Design of a-Si:H Solar Cells 537 12.4.6 Cells under Solar Illumination 540 12.4.7 Light-soaking Effects 541 12.5 Multiple-Junction Solar Cells 542 12.5.1 Advantages of Multiple-junction Solar Cells 542 12.5.2 Using Alloys for Cells with Different Band Gaps 544 12.5.3 a-Si/a-SiGe Tandem and a-Si/a-SiGe/a-SiGe Triple-junction

(Parte 1 de 7)

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