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Principles of Solar Cells, LEDs and Diodes� e role of the PN junction

ADRIAN KITAI

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KITAIKITAI

Principles of Solar Cells, LED

s and Diodes

� e role of the PN junction

Principles of Solar Cells, LEDs and Diodes� e role of the PN junctionADRIAN KITAI, Departments of Engineering Physics and Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada

A textbook introducing the physical concepts required for a comprehensive understanding of p-n junction devices, light emitting diodes and solar cells.

Semiconductor devices have made a major impact on the way we work and live. Today semiconductor p-n junction diode devices are experiencing substantial growth: solar cells are used on an unprecedented scale in the renewable energy industry; and light emitting diodes (LEDs) are revolutionizing energy e� cient lighting. � ese two emerging industries based on p-n junctions make a signi� cant contribution to the reduction in fossil fuel consumption.

Principles of Solar Cells, LEDs and Diodes covers the two most important applications of semiconductor diodes - solar cells and LEDs - together with quantitative coverage of the physics of the p-n junction. � e reader will gain a thorough understanding of p-n junctions as the text begins with semiconductor and junction device fundamentals and extends to the practical implementation of semiconductors in both photovoltaic and LED devices. � e treatment of a range of important semiconductor materials and device structures is also presented in a readable manner.

Topics are divided into the following six chapters;

• Semiconductor Physics• Th e PN Junction Diode• Photon Emission and Absorption• Th e Solar Cell• Light Emitting Diodes• Organic Semiconductors, OLEDs and Solar Cells

Containing student problems at the end of each chapter and worked example problems throughout, this textbook is intended for senior level undergraduate students doing courses in electrical engineering, physics and materials science. Researchers working on solar cells and LED devices, and those in the electronics industry would also bene� t from the background information the book provides.

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Principles of Solar Cells,LEDs and Diodes

The role of the PN junction

ADRIAN KITAI

Departments of Engineering Physics and Materials Science andEngineering, McMaster University, Hamilton, Ontario, Canada

A John Wiley & Sons, Ltd., Publication

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This edition first published 2011© 2011 John Wiley & Sons, Ltd

Registered officeJohn 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 reusethe copyright material in this book please see our website at www.wiley.com.

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Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and productnames used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. Thepublisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurateand authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is notengaged in rendering professional services. If professional advice or other expert assistance is required, the services of acompetent professional should be sought.

The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contentsof this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for aparticular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services.The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipmentmodifications, changes in governmental regulations, and the constant flow of information relating to the use of experimentalreagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert orinstructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions orindication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work asa citation and/or a potential source of further information does not mean that the author or the publisher endorses the informationthe organization or Website may provide or recommendations it may make. Further, readers should be aware that InternetWebsites listed in this work may have changed or disappeared between when this work was written and when it is read. Nowarranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall beliable for any damages arising herefrom.

Library of Congress Cataloging-in-Publication Data

Kitai, Adrian, 1957–Principles of solar cells, LEDs, and diodes : the role of the PN junction / Adrian Kitai.

p. cm.Includes bibliographical references and index.ISBN 978-1-4443-1834-0 (hardback) – ISBN 978-1-4443-1833-3 (paper)

1. Diodes, Semiconductor. 2. Light emitting diodes. 3. Semiconductors–Junctions. 4. Solar cells. I. Title.TK7871.86.K48 2011621.3815′2–dc22

2011010956

A catalogue record for this book is available from the British Library.

HB ISBN: 9781444318340PB ISBN: 9781444318333ePDF ISBN: 9781119974550oBook ISBN: 9781119974543ePub ISBN: 9781119975236eMobi ISBN: 9781119975243

Set in 10/12pt Times by Aptara Inc., New Delhi, India.Printed in Malaysia by Vivar Malaysia

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http://www.wiley.com

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Dedicated to my wife Tomoko

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Contents

Introduction xiAcknowledgements xv

1 Semiconductor Physics 11.1 Introduction 21.2 The Band Theory of Solids 21.3 The Kronig–Penney Model 31.4 The Bragg Model 81.5 Effective Mass 81.6 Number of States in a Band 101.7 Band Filling 121.8 Fermi Energy and Holes 141.9 Carrier Concentration 151.10 Semiconductor Materials 251.11 Semiconductor Band Diagrams 261.12 Direct Gap and Indirect Gap Semiconductors 291.13 Extrinsic Semiconductors 351.14 Carrier Transport in Semiconductors 401.15 Equilibrium and Non-Equilibrium Dynamics 431.16 Carrier Diffusion and the Einstein Relation 451.17 Quasi-Fermi Energies 481.18 The Diffusion Equation 501.19 Traps and Carrier Lifetimes 531.20 Alloy Semiconductors 561.21 Summary 59

Suggestions for Further Reading 61Problems 63

2 The PN Junction Diode 692.1 Introduction 702.2 Diode Current 722.3 Contact Potential 752.4 The Depletion Approximation 782.5 The Diode Equation 852.6 Reverse Breakdown and the Zener Diode 97

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viii Contents

2.7 Tunnel Diodes 1002.8 Generation/Recombination Currents 1012.9 Ohmic Contacts, Schottky Barriers and Schottky Diodes 1042.10 Heterojunctions 1132.11 Alternating Current (AC) and Transient Behaviour 1152.12 Summary 117


3 Photon Emission and Absorption 1233.1 Introduction to Luminescence and Absorption 1243.2 Physics of Light Emission 1253.3 Simple Harmonic Radiator 1283.4 Quantum Description 1293.5 The Exciton 1323.6 Two-Electron Atoms 1353.7 Molecular Excitons 1413.8 Band-to-Band Transitions 1443.9 Photometric Units 1483.10 Summary 152


4 The Solar Cell 1594.1 Introduction 1604.2 Light Absorption 1624.3 Solar Radiation 1644.4 Solar Cell Design and Analysis 1644.5 Thin Solar Cells 1724.6 Solar Cell Generation as a Function of Depth 1764.7 Solar Cell Efficiency 1794.8 Silicon Solar Cell Technology: Wafer Preparation 1844.9 Silicon Solar Cell Technology: Solar Cell Finishing 1874.10 Silicon Solar Cell Technology: Advanced Production Methods 1914.11 Thin Film Solar Cells: Amorphous Silicon 1924.12 Telluride/Selenide/Sulphide Thin-Film Solar Cells 1994.13 High-Efficiency Multijunction Solar Cells 2004.14 Concentrating Solar Systems 2034.15 Summary 204


5 Light Emitting Diodes 2155.1 Introduction 2165.2 LED Operation and Device Structures 2175.3 Emission Spectrum 220

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Contents ix

5.4 Non-Radiative Recombination 2215.5 Optical Outcoupling 2235.6 GaAs LEDs 2255.7 GaAs1−xPx LEDs 2265.8 Double Heterojunction AlxGa1−xAs LEDs 2285.9 AlGaInP LEDs 2345.10 Ga1−xInxN LEDs 2365.11 LED Structures for Enhanced Outcoupling and Power Output 2445.12 Summary 247


6 Organic Semiconductors, OLEDs and Solar Cells 2536.1 Introduction to Organic Electronics 2546.2 Conjugated Systems 2556.3 Polymer OLEDs 2606.4 Small-Molecule OLEDs 2666.5 Anode Materials 2706.6 Cathode Materials 2706.7 Hole Injection Layer 2716.8 Electron Injection Layer 2726.9 Hole Transport Layer 2726.10 Electron Transport Layer 2756.11 Light Emitting Material Processes 2766.12 Host Materials 2786.13 Fluorescent Dopants 2796.14 Phosphorescent Dopants 2836.15 Organic Solar Cells 2836.16 Organic Solar Cell Materials 2896.17 Summary 292


Appendix 1: Physical Constants 301Appendix 2: Properties of Semiconductor Materials 303Appendix 3: The Boltzmann Distribution Function 305Index 309

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Introduction

Semiconductor devices have revolutionized the way we work and live. Transistors arethought of as one of the most important developments of the twentieth century and theyhave given rise to the computer age as well as to compact, reliable electronics found ineverything from televisions to cell phones.

An even more fundamental semiconductor device exists, however. It is the semiconductordiode or p-n junction diode. Diodes had been developed before the transistor and wereused for rectification whereby alternating current can be converted to direct current byemploying the unidirectional property of diodes: current normally only flows efficientlyin one direction through a diode, and current flow is blocked in the opposite direction.This property of diodes is exploited in power supplies as well as in many other circuitssuch as those found in radios and limiters. Since an understanding of diodes is required toexplain the principles of transistors, diodes are frequently presented as a stepping stone tothe transistor.

In the twenty-first century, however, two new major industries are undergoing very rapiddevelopments based directly on the p-n junction diode. Photovoltaic (PV) solar cells andlight emitting diodes (LEDs) are both p-n junctions that are designed and optimized toeither absorb or emit light. In both cases, an energy conversion process between photonsand electrons occurs within a p-n junction.

The consequences of this development constitute a revolution in two major industrialsectors:

1. Energy production has relied on hydrocarbons and nuclear power, and although thesewill continue to be important, the direct conversion of solar radiation into useful power isthe key to a long-term, sustainable energy supply. Ninety-seven percent of all renewableenergy on earth is in the form of solar radiation. The beginning of the twenty-first centuryhas seen the abrupt growth of a global solar photovoltaic industry in conjunction with theinvolvement of governments worldwide, and unprecedented growth in PV productionand deployment is now underway. The worldwide consumption of silicon semiconductormaterial for the entire microelectronics industry is being overtaken by its use for solarcells alone.

2. Electric lighting was achieved by the incandescent lamp in the early twentieth century.In the second half of the twentieth century a significant movement to fluorescent lampsand discharge lamps in which a gas is excited into a plasma that radiates energy moreefficiently than a tungsten filament occurred. The twenty-first century will give riseto the virtual displacement of both incandescent lamps and fluorescent lamps by LEDlamps. In preparation for this revolution, the world’s major lighting companies have

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xii Introduction

acquired LED research, development and production capabilities. LEDs have alreadyachieved lower power consumption and longer life in small, rugged packages than eitherincandescent or fluorescent lamps. They also eliminate heavy metals such as mercuryused in fluorescent lamps.

The purpose of this book, therefore, is to introduce the physical concepts requiredfor a thorough understanding of p-n junctions starting with semiconductor fundamentalsand extending this to the practical implementation of semiconductors in both PV andLED devices. The treatment of a range of important semiconductor materials and devicestructures is also presented.

The book is aimed at senior undergraduate levels (years three and four). An introduc-tory background in quantum mechanics is assumed, together with general knowledge ofjunior mathematics, physics and chemistry; however, no background in electronic mate-rials is required. As such this book is designed to be relevant to all engineering studentswith an interest in semiconductor devices and not specifically to electrical or engineeringphysics/engineering science students only. This is intentional since solar cells and LEDsinvolve a wide range of engineering disciplines and should not be regarded as belonging toonly one branch of engineering.

In Chapter 1, the physics of solid state electronic materials is covered in detail startingfrom the basic behaviour of electrons in crystals. The quantitative treatment of electrons andholes in energy bands is presented along with the important concepts of excess carriers thatbecome significant once semiconductor devices are either connected to sources of poweror illuminated by light. A series of semiconductor materials and their important propertiesis also reviewed. The behaviour of semiconductor surfaces and trapping concepts are alsointroduced since they play an important role in solar cell and LED device performance.

In Chapter 2, the basic physics and important models of a p-n junction device arepresented. The approach taken is to present the diode as a semiconductor device that can beunderstood from the band theory covered in Chapter 1. Various types of diode behaviour,including tunnelling, metal-semiconductor contacts and heterojunctions, are presented aswell as reverse breakdown behaviour.

Chapter 3 introduces the theory of photon emission and absorption, a topic that bookson semiconductor devices frequently pay less attention to. The standard description thata photon is created when an electron and a hole recombine, or a photon is absorbedwhen an electron and a hole are generated, is not adequate for a deeper understanding ofphoton emission and absorption processes. In this chapter the physics of photon creationis explained with a minimum of mathematical complexity, and these concepts are muchbetter understood by following radiation theory and describing the oscillating dipole bothclassically and using simple quantum mechanics. A section of Chapter 3 describes theexciton relevant to inorganic semiconductors as well as the molecular exciton for organicsemiconductors. In addition lineshapes predicted for direct-gap semiconductors are derived.Finally the subject of photometric units introduces the concepts of luminance and colourcoordinates that are essential to a discussion of organic and inorganic light emitting diodes.

Chapter 4 covers inorganic solar cells. The concepts regarding the p-n junction introducedin Chapter 2 are further developed to include illumination of the p-n junction and the simplestpossible modelling is used to illustrate the behaviour of a solar cell. Then a more realisticsolar cell structure and model are presented along with the attendant surface recombination

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Introduction xiii

and absorption issues that must be understood in practical solar cells. A series of solar celltechnologies are reviewed starting with bulk single and multicrystalline silicon solar celltechnology. Amorphous silicon materials and device concepts are presented. Solar cellsmade using other semiconductors such as CdTe are introduced followed by multijunctionsolar cells using layered, lattice-matched III-V semiconductor stacks.

Chapter 5 on inorganic LEDs considers the basic LED structure and its operating prin-ciples. The measured lineshape of III-V LEDs is compared with the predictions of Chapter3. LEDs must be engineered to maximize radiative recombination, and energy loss mech-anisms are discussed. The series of developments that marked the evolution of current,high-efficiency LED devices is presented starting from the semiconductors and growthtechniques of the 1960s, and following trends in succeeding decades that brought better ma-terials and semiconductor growth methods to the LED industry. The double heterojunctionis introduced and the resulting energy well is analysed on the basis of the maximum currentdensity that can be accommodated before it becomes saturated. LED optical outcoupling,which must also be maximized to achieve overall efficiency, is modelled and strategies tooptimize outcoupling are discussed. Finally the concept of spectral down-conversion usingphosphor materials and the white LED are introduced.

Chapter 6 introduces new concepts required for an understanding of organic semicon-ductors in general, in which conjugated molecular bonding gives rise to π bands andHOMO and LUMO levels in organic semiconductors. The organic LED is introduced bystarting with the simplest single active layer polymer-based LED followed by successivelymore complex small-molecule LED structures. The roles of the various layers, includingelectrodes and carrier injection and transport layers, are discussed and the relevant candi-date molecular materials are described. Concepts from Chapter 3, including the molecularexciton and singlet and triplet states are used to explain efficiency limitations in the lightgeneration layer of small-molecule OLEDs. In addition the opportunity to use phosphores-cent host-guest light emitting layers to improve device efficiency is explained. The organicsolar cell is introduced and the concepts of exciton generation and exciton dissociation aredescribed in the context of the heterojunction and the bulk heterojunction. The interest inthe use of fullerenes and other related nanostructured materials is explained for the bulkheterojunction.

All the chapters are followed by problem sets that are designed to facilitate familiaritywith the concepts and a better understanding of the topics introduced in the chapter. Inmany cases the problems are quantitative and require calculations; however, a number ofmore conceptual problems are presented and are designed to give the reader experience inusing the Internet or library resources to look up further information. These problems areof particular relevance in Chapters 4, 5 and 6, in which developments in solar cells andLEDs are best understood by referring to the recent literature once the basic concepts areunderstood.

Adrian Kitai

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