Handbook of Photovoltaic Science and Engineering

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3 Handbook of Photovoltaic Science and Engineering

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5 Handbook of Photovoltaic Science and Engineering Second Edition Edited by Antonio Luque Instituto de Energía Solar, Universidad Politécnica de Madrid, Spain and Steven Hegedus Institute of Energy Conversion, University of Delaware, USA A John Wiley and Sons, Ltd., Publication

6 This edition first published , John Wiley & Sons, Ltd First Edition published in 2003 Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloguing-in-Publication Data Handbook of photovoltaic science and engineering / edited by A Luque and S Hegedus. 2nd ed. p. cm. Includes bibliographical references and index. ISBN (cloth) 1. Photovoltaic cells Handbooks, manuals, etc. 2. Photovoltaic power generation Handbooks, manuals, etc. I. Luque, A. (Antonio) II. Hegedus, Steven. TK8322.H dc A catalogue record for this book is available from the British Library. Print ISBN: epdf ISBN: obook ISBN: epub ISBN: Set in 9/11 Times by Laserwords Private Limited, Chennai, India.

7 Contents About the Editors List of Contributors Preface to the 2nd Edition xxiii xxv xxxi 1 Achievements and Challenges of Solar Electricity from Photovoltaics 1 Steven Hegedus and Antonio Luque 1.1 The Big Picture What is Photovoltaics? Rating of PV Modules and Generators Collecting Sunlight: Tilt, Orientation, Tracking and Shading PV Module and System Costs and Forecasts Photovoltaics Today But First, Some PV History The PV Picture Today The Crucial Role of National Policies Grid Parity: The Ultimate Goal for PV The Great Challenge How Much Land Is Needed? Raw Materials Availability Is Photovoltaics a Clean Green Technology? Energy Payback Reliability Dispatchability: Providing Energy on Demand Trends in Technology Crystalline Silicon Progress and Challenges Thin Film Progress and Challenges Concentrator Photovoltaics Progress and Challenges Third-Generation Concepts Conclusions 35 References 36

8 vi 2 The Role of Policy in PV Industry Growth: Past, Present and Future 39 John Byrne and Lado Kurdgelashvili 2.1 Introduction Changing Climate in the Energy Industry PV Markets Policy Review of Selected Countries Review of US Policies Europe Asia Policy Impact on PV Market Development Future PV Market Growth Scenarios Diffusion Curves Experience Curves PV Diffusion in the US under Different Policy Scenarios Toward a Sustainable Future 74 References 75 3 The Physics of the Solar Cell 82 Jeffery L. Gray 3.1 Introduction Fundamental Properties of Semiconductors Crystal Structure Energy Band Structure Conduction-band and Valence-band Densities of State Equilibrium Carrier Concentrations Light Absorption Recombination Carrier Transport Semiconductor Equations Minority-carrier Diffusion Equation pn-junction Diode Electrostatics Summary Solar Cell Fundamentals Solar Cell Boundary Conditions Generation Rate Solution of the Minority-carrier Diffusion Equation Derivation of the Solar Cell I V Characteristic Interpreting the Solar Cell I V Characteristic Properties of Efficient Solar Cells Lifetime and Surface Recombination Effects Additional Topics Spectral Response Parasitic Resistance Effects Temperature Effects Concentrator Solar Cells High-level Injection p-i-n Solar Cells and Voltage-dependent Collection 125

9 vii Heterojunction Solar Cells Detailed Numerical Modeling Summary 128 References Theoretical Limits of Photovoltaic Conversion and New-generation Solar Cells 130 Antonio Luque and Antonio Martí 4.1 Introduction Thermodynamic Background Basic Relationships The Two Laws of Thermodynamics Local Entropy Production An Integral View Thermodynamic Functions of Radiation Thermodynamic Functions of Electrons Photovoltaic Converters The Balance Equation of a PV Converter The Monochromatic Cell Thermodynamic Consistency of the Shockley Queisser Photovoltaic Cell Entropy Production in the Whole Shockley Queisser Solar Cell The Technical Efficiency Limit for Solar Converters Very-high-efficiency Concepts Multijunction Solar Cells Thermophotovoltaic and Thermophotonic Converters Multi-exciton Generation Solar Cells Intermediate Band Solar Cell Hot Electron Solar Cells Conclusions 164 References Solar Grade Silicon Feedstock 169 Bruno Ceccaroli and Otto Lohne 5.1 Introduction Silicon Physical Properties of Silicon Relevant to Photovoltaics Chemical Properties Relevant to Photovoltaics Health, Safety and Environmental Factors History and Applications of Silicon Production of Silicon Metal/Metallurgical Grade Silicon The Carbothermic Reduction of Silica Ladle Refining Casting and Crushing Purity of Commercial Silicon Metal Economics Production of Polysilicon/Silicon of Electronic and Photovoltaic Grade The Siemens Process: Chlorosilanes and Hot Filament The Union Carbide and Komatsu Process: Monosilane and Hot Filament 187

10 viii The Ethyl Corporation Process: Silane and Fluidised Bed Reactor Economics and Business Current Silicon Feedstock to Solar Cells Requirements of Silicon for Crystalline Solar Cells Directional Solidification Effect of Crystal Imperfections Effect of Various Impurities Routes to Solar Grade Silicon Further Polysilicon Process Development and New Processes Involving Volatile Silicon Compounds Upgrading Purity of the Metallurgical Silicon Route Other Methods Crystallisation Conclusions 214 References Bulk Crystal Growth and Wafering for PV 218 Hugo Rodriguez, Ismael Guerrero, Wolfgang Koch, Arthur L. Endrös, Dieter Franke, Christian Häßler, Juris P. Kalejs and H. J. Möller 6.1 Introduction Bulk Monocrystalline Material Cz Growth of Single-crystal Silicon Bulk Multicrystalline Silicon Ingot Fabrication Doping Crystal Defects Impurities Wafering Multi-wire Wafering Technique Microscopic Process of Wafering Wafer Quality and Saw Damage Cost and Size Considerations New Sawing Technologies Silicon Ribbon and Foil Production Process Description Productivity Comparisons Manufacturing Technology Ribbon Material Properties and Solar Cells Ribbon/Foil Technology: Future Directions Numerical Simulations of Crystal Growth Techniques Simulation Tools Thermal Modelling of Silicon Crystallisation Techniques Simulation of Bulk Silicon Crystallisation Simulation of Silicon Ribbon Growth Conclusions 260 References 261

11 ix 7 Crystalline Silicon Solar Cells and Modules 265 Ignacio Tobías, Carlos del Cañizo and Jesús Alonso 7.1 Introduction Crystalline Silicon as a Photovoltaic Material Bulk Properties Surfaces Crystalline Silicon Solar Cells Cell Structure Substrate The Front Surface The Back Surface Size Effects Cell Optics Performance Comparison Manufacturing Process Process Flow Screen-printing Technology Throughput and Yield Variations to the Basic Process Thin Wafers Back Surface Passivation Improvements to the Front Emitter Rapid Thermal Processes Other Industrial Approaches Silicon Ribbons Heterojunction with Intrinsic Thin Layer All-rear-contact Technologies The Sliver Cell Crystalline Silicon Photovoltaic Modules Cell Matrix The Layers of the Module Lamination Post-lamination Steps Automation and Integration Special Modules Electrical and Optical Performance of Modules Electrical and Thermal Characteristics Fabrication Spread and Mismatch Losses Local Shading and Hot Spot Formation Optical Properties Field Performance of Modules Lifetime Qualification Conclusions 307 References 308

12 x 8 High-efficiency III V Multijunction Solar Cells 314 D. J. Friedman, J. M. Olson and Sarah Kurtz 8.1 Introduction Applications Space Solar Cells Terrestrial Electricity Generation Physics of III V Multijunction and Single-junction Solar Cells Wavelength Dependence of Photon Conversion Efficiency Theoretical Limits to Multijunction Efficiencies Spectrum Splitting Cell Configuration Four-terminal Three-terminal Two-terminal Series-connected (Current-matched) Computation of Series-connected Device Performance Overview Top and Bottom Subcell QE and J SC Multijunction J V Curves Current Matching and Top-cell Thinning Current-matching Effect on Fill Factor and V OC Efficiency vs Bandgap Spectral Effects AR Coating Effects Concentration Temperature Dependence Materials Issues Related to GaInP/GaAs/Ge Solar Cells Overview MOCVD GaInP Solar Cells GaAs Cells Ge Cells Tunnel-junction Interconnects Chemical Etchants Materials Availability Epilayer Characterization and Other Diagnostic Techniques Characterization of Epilayers Transmission-line Measurements I V Measurements of Multijunction Cells Evaluation of Morphological Defects Device Diagnosis Reliability and Degradation Future-generation Solar Cells Lattice-mismatched GaInP/GaInAs/Ge Cell Inverted Lattice-mismatched GaInP/GaInAs/GaInAs (1.83, 1.34, 0.89 ev) Cell Other Lattice-matched Approaches Mechanical Stacks 358

13 xi Growth on Other Substrates Spectrum Splitting Summary 359 References Space Solar Cells and Arrays 365 Sheila Bailey and Ryne Raffaelle 9.1 The History of Space Solar Cells Vanguard 1 to Deep Space The Challenge for Space Solar Cells The Space Environment Thermal Environment Solar Cell Calibration and Measurement Silicon Solar Cells III V Solar Cells Thin Film Solar Cells Space Solar Arrays Body-mounted Arrays Rigid Panel Planar Arrays Flexible Fold-out Arrays Thin Film or Flexible Roll-out Arrays Concentrating Arrays High-temperature/Intensity Arrays Electrostatically Clean Arrays Mars Solar Arrays Power Management and Distribution (PMAD) Future Cell and Array Possibilities Low-intensity Low-temperature (LILT) Cells Quantum Dot Solar Cells Integrated Power Systems High Specific Power Arrays High-radiation Environment Solar Arrays Power System Figures of Merit Summary 398 References Photovoltaic Concentrators 402 Gabriel Sala and Ignacio Antón 10.1 What is the Aim of Photovoltaic Concentration and What Does it Do? Objectives, Limitations and Opportunities Objectives and Strengths The Analysis of Costs of Photovoltaic Concentrators Typical Concentrators: an Attempt at Classification Types, Components and Operation of a PV Concentrator Classification of Concentrators Concentration Systems with Spectral Change 411

14 xii 10.4 Concentration Optics: Thermodynamic Limits What is Required in Concentrator Optics? A Typical Reflexive Concentrator Ideal Concentration Constructing an Ideal Concentrator Optics of Practical Concentrators Two-stage Optical Systems: Secondary Optics Factors of Merit for Concentrators in Relation to the Optics Optical Efficiency Distribution or Profile of the Light on the Receptor Angular Acceptance and Transfer Function Photovoltaic Concentration Modules and Assemblies Definitions Functions and Characteristics of Concentration Modules Electrical Connection of Cells in the Module Thermal Mechanical Effects Related to Cell Fixing Description and Manufacturing Issues of Concentration Modules Adoption of Secondary Optics Modules with Reflexive Elements (Mirrors) Description and Manufacturing Issues of Concentrators Based on Assemblies Tracking for Concentrator Systems Tracking Strategies for CPVs Practical Implementation of Tracking Systems Tracking Control System Pointing Strategies The Cost of Structure and Tracking Control Measurements of Cells, Modules and Photovoltaic Systems in Concentration Measurement of Concentration Cells Measurement of Concentrator Elements and Modules Absolute and Relative Measurements with Simulators Optical Mismatch in CPV Modules and Systems Testing CPV Modules and Systems Equipped with Multijunction Solar Cells Multijunction Cells Inside Module Optics The Production of PV Concentrators versus the Effective Available Radiation Accounting for Daylight Spectrum Variations Summary 449 References Crystalline Silicon Thin-Film Solar Cells via High-temperature and Intermediate-temperature Approaches 452 Armin G. Aberle and Per I. Widenborg 11.1 Introduction Motivation for Thin c-si Solar Cells Classification of c-si Thin-Film PV Technologies and Materials Silicon Deposition Methods 455

15 xiii Seeded versus Non-seeded Silicon Film Growth Modelling Impact of Diffusion Length in Absorber Region on Cell Efficiency Impact of Surface Recombination Impact of Light Trapping Crystalline Silicon Thin-Film Solar Cells on Native and High-T Foreign Supporting Materials Native Supporting Materials High-T Foreign Supporting Materials Crystalline Silicon Thin-Film Solar Cells on Intermediate-T Foreign Supporting Materials Solar Cells on Metal Solar Cells on Glass Conclusions 480 Acknowledgements 481 References Amorphous Silicon-based Solar Cells 487 Eric A. Schiff, Steven Hegedus and Xunming Deng 12.1 Overview Amorphous Silicon: The First Dopable Amorphous Semiconductor Designs for Amorphous Silicon Solar Cells: A Guided Tour Staebler Wronski Effect Synopsis Atomic and Electronic Structure of Hydrogenated Amorphous Silicon Atomic Structure Defects and Metastability Electronic Density-of-States Band Tails, Band Edges, and Bandgaps Defects and Gap States Doping Alloying and Optical Properties Briefing: Nanocrystalline Silicon Depositing Amorphous Silicon Survey of Deposition Techniques RF Plasma-Enhanced Chemical Vapor Deposition (RF-PECVD) at MHz PECVD at Different Frequencies Hot-wire Chemical Vapor Deposition Other Deposition Methods Hydrogen Dilution High-rate Deposition of Nanocrystalline Si (nc-si) Alloys and Doping Understanding a-si pin Cells Electronic Structure of a pin Device Voltage Depends Weakly on Absorber-layer Thickness What is the Useful Thickness for Power Generation? 513