Solar Cell Materials E-Book

Arthur Willoughby

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Wiley Series in Materials for Electronic & Optoelectronic Applications
• Sprache: Englisch

Seitenzahl: 719

Veröffentlichungsjahr: 2014

Beschreibung

This book presents a comparison of solar cell materials, including both new materials based on organics, nanostructures and novel inorganics and developments in more traditional photovoltaic materials.

It surveys the materials and materials trends in the field including third generation solar cells (multiple energy level cells, thermal approaches and the modification of the solar spectrum) with an eye firmly on low costs, energy efficiency and the use of abundant non-toxic materials.

Leseprobe

Contents

Cover

Series

Title Page

Copyright

Dedication

Series Preface

List of Contributors

Chapter 1: Introduction

1.1 INTRODUCTION

1.2 THE SUN

1.3 BOOK OUTLINE

REFERENCES

Chapter 2: Fundamental Physical Limits to Photovoltaic Conversion

2.1 INTRODUCTION

2.2 THERMODYNAMIC LIMITS

2.3 LIMITATIONS OF CLASSICAL DEVICES

2.4 FUNDAMENTAL LIMITS OF SOME HIGH-EFFICIENCY CONCEPTS

2.5 CONCLUSION

NOTE

REFERENCES

Chapter 3: Physical Characterisation of Photovoltaic Materials

3.1 INTRODUCTION

3.2 CORRESPONDENCE BETWEEN PHOTOVOLTAIC MATERIALS CHARACTERISATION NEEDS AND PHYSICAL TECHNIQUES

3.3 X-RAY TECHNIQUES

3.4 ELECTRON MICROSCOPY METHODS

3.5 SPECTROSCOPY METHODS

3.6 CONCLUDING REMARKS AND PERSPECTIVES

ACKNOWLEDGEMENTS

REFERENCES

Chapter 4: Developments in Crystalline Silicon Solar Cells

4.1 INTRODUCTION

4.2 PRESENT MARKET OVERVIEW

4.3 SILICON WAFERS

4.4 CELL PROCESSING

4.5 CONCLUSION

ACKNOWLEDGEMENTS

REFERENCES

Chapter 5: Amorphous and Microcrystalline Silicon Solar Cells

5.1 INTRODUCTION

5.2 DEPOSITION METHODS

5.3 MATERIAL PROPERTIES

5.4 SINGLE-JUNCTION CELL

5.5 MULTIJUNCTION CELLS

5.6 MODULES AND PRODUCTION

ACKNOWLEDGMENTS

REFERENCES

Chapter 6: III-V Solar Cells

6.1 INTRODUCTION

6.2 HOMO- AND HETEROJUNCTION III-V SOLAR CELLS

6.3 MULTIJUNCTION SOLAR CELLS

6.4 APPLICATIONS

6.5 CONCLUSION

REFERENCES

Chapter 7: Chalcogenide Thin-Film Solar Cells

7.1 INTRODUCTION

7.2 CIGS

7.3 KESTERITES

ACKNOWLEDGEMENTS

REFERENCES

Chapter 8: Printed Organic Solar Cells

8.1 INTRODUCTION

8.2 MATERIALS AND MORPHOLOGY

8.3 INTERFACES IN ORGANIC PHOTOVOLTAICS

8.4 TANDEM TECHNOLOGY

8.5 ELECTRODE REQUIREMENTS FOR ORGANIC SOLAR CELLS

8.6 PRODUCTION OF ORGANIC SOLAR CELLS

8.7 SUMMARY AND OUTLOOK

REFERENCES

Chapter 9: Third-Generation Solar Cells

9.1 INTRODUCTION

9.2 MULTIPLE-ENERGY-LEVEL APPROACHES

9.3 MODIFICATION OF THE SOLAR SPECTRUM

9.4 THERMAL APPROACHES

9.5 OTHER APPROACHES

9.6 CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES

Concluding Remarks

Index

Wiley Series in Materials for Electronic and Optoelectronic Applications

www.wiley.com/go/meoa

Series Editors

Professor Arthur Willoughby, University of Southampton, Southampton, UK

Dr Peter Capper, SELEX Galileo Infrared Ltd, Southampton, UK

Professor Safa Kasap, University of Saskatchewan, Saskatoon, Canada

Published Titles

Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper

Properties of Group-IV, III–V and II–VI Semiconductors, S. Adachi

Charge Transport in Disordered Solids with Applications in Electronics, Edited by S. Baranovski

Optical Properties of Condensed Matter and Applications, Edited by J. Singh

Thin Film Solar Cells: Fabrication, Characterization and Applications, Edited by J. Poortmans and V. Arkhipov

Dielectric Films for Advanced Microelectronics, Edited by M. R. Baklanov, M. Green and K. Maex

Liquid Phase Epitaxy of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper and M. Mauk

Molecular Electronics: From Principles to Practice, M. Petty

CVD Diamond for Electronic Devices and Sensors, Edited by R. S. Sussmann

Properties of Semiconductor Alloys: Group-IV, III–V and II–VI Semiconductors, S. Adachi

Mercury Cadmium Telluride, Edited by P. Capper and J. Garland

Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, Edited by C. Litton, D. C. Reynolds and T. C. Collins

Lead-Free Solders: Materials Reliability for Electronics, Edited by K. N. Subramanian

Silicon Photonics: Fundamentals and Devices, M. Jamal Deen and P. K. Basu

Nanostructured and Subwavelength Waveguides: Fundamentals and Applications, M. Skorobogatiy

This edition first published 2014 © 2014 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Solar Cell Materials: Developing Technologies / [edited by] Gavin Conibeer, Arthur Willoughby. pages cm – (Wiley series in materials for electronic & optoelectronic applications) Includes bibliographical references and index. ISBN 978-0-470-06551-8 (hardback) 1. Photovoltaic cells–Materials. I. Conibeer, Gavin, editor of compilation. II. Willoughby, Arthur, editor of compilation. TK8322.P4584 2014 621.3815′42–dc23 2013037202

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

ISBN: 9780470065518 (13 digits)

This book is dedicated to our wives, Julie and Jenni, without whose support it would not have been possible.

Series Preface

WILEY SERIES IN MATERIALS FOR ELECTRONIC AND OPTOELECTRONIC APPLICATIONS

This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much-needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers and technologists, engaged in research, development and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices and circuits for the electronic, optoelectronic and communications industries.

The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics. The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure-property relationships have always been fundamentally and intrinsically important to materials science and engineering.

Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

ARTHUR WILLOUGHBY PETER CAPPER SAFA KASAP

List of Contributors

Tayebeh Ameri, Konarka Technologies Austria GmbH, Austria

Hamed Azimi, Konarka Technologies Austria GmbH, Austria

Daniel Bellet, Laboratorie des Matériaux et du Génie Physique, CNRS, France

Edith Bellet-Amalric, CEA-CNRS-UJF group ‘Nanophysique et Semiconducteurs’, CEA/INAC/SP2M, France

Christoph J. Brabec, Konarka Technologies Austria GmbH, Austria and Konarka Technologies Germany GmbH, Germany

Gavin Conibeer, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia

S. Delbos, Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

N. J. Ekins-Daukes, Department of Physics, Imperial College, London, UK

Martin A. Green, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia

J. F. Guillemoles, Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

Claudia Hoth, Konarka Technologies Germany GmbH, Germany

N. Naghavi, Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

M. Paire, Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

R. E. I. Schropp, Energy research Center of the Netherlands (ECN), Solar Energy, The Netherlands and Department of Applied Physics, Eindhoven University of Technology, The Netherlands

Andrea Seemann, Konarka Technologies Germany GmbH, Germany

Roland Steim, Konarka Technologies Germany GmbH, Germany

J. Vidal, Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

Arthur Willoughby, Faculty of Engineering and the Environment, University of Southampton, UK

1

Introduction

Gavin Conibeer1 and Arthur Willoughby2

1School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia

2Faculty of Engineering and the Environment, University of Southampton, UK

1.1 INTRODUCTION

The environmental challenges to the world are now well known and publicised, and all but a small minority of scientists accept that a reduction on dependence on fossil fuels is essential for addressing the problems of the greenhouse effect and global warming. Everyone is aware of the limited nature of fossil-fuel resources, and the increasing cost and difficulty, as well as the environmental damage, of extracting the last remnants of oil, gas and other carbonaceous products from the earth's crust.

Photovoltaics, the conversion of sunlight into useful electrical energy, is accepted as an important part of any strategy to reduce this dependence on fossil fuels. All of us are now familiar with the appearance of solar cell modules on the roofs of houses, on public buildings, and more extensive solar generators. Recently, the world's PV capacity passed 100 GW, according to new market figures from the European Photovoltaic Industry Association (14 February 2013), which makes a substantial contribution to reducing the world's carbon emissions.

It is the aim of this book to discuss the latest developments in photovoltaic materials which are driving this technology forward, to extract the maximum amount of electrical power from the sun, at minimal cost both financially and environmentally.

1.2 THE SUN

The starting point of all this discussion is the sun itself. In his book ‘Solar Electricity’ (Wiley 1994), Tomas Markvart shows the various energy losses to the solar radiation that occur when it passes through the earth's atmosphere (Figure 1.1).

Figure 1.1 Solar radiation in the atmosphere. (Reproduced with permission from Markvart, 2000. Copyright © 2000, John Wiley & Sons, Ltd.)

The atmosphere also affects the solar spectrum, as shown in Figure 1.2.

Figure 1.2 The solar spectrum. (Reproduced with permission from Markvart, 2000. Copyright © 2000, John Wiley & Sons, Ltd.)

A concept that characterises the effect of a clear atmosphere on sunlight, is the ‘air mass’, equal to the relative length of the direct beam path through the atmosphere. The extraterrestrial spectrum, denoted by AM0 (air mass 0) is important for satellite applications of solar cells. At its zenith, the radiation from the sun corresponds to AM1, while AM1.5 is a typical solar spectrum on the earth's surface on a clear day that, with total irradiance of 1 kW/m2, is used for the calibration of solar cells and modules. Also shown in Figure 1.2 are the principal absorption bands of the molecules in the air. AM1.5 is referred to frequently in a number of the chapters in this book, and readers should be aware of its meaning.

1.3 BOOK OUTLINE

The book starts with a clear exposition of the fundamental physical limits to photovoltaic conversion, by Jean-Francois Guillemoles. This covers the thermodynamic limits, the limitations of classical devices, and develops this theme for more advanced devices. The identification of device parameters used in other chapters can also be found in this chapter.

Material parameters, of course, also require a thorough understanding of characterisation tools, and the second chapter, by Daniel Bellet and Edith Bellet-Amalric, outlines the main material characterisation techniques of special interest in solar cell science. X-ray analysis, electron microscopy, ion-beam techniques and spectroscopy characterisation methods are discussed, including Raman, X-ray photoelectron and UV/Visible spectroscopy, which are rarely detailed in such a materials book.

The next chapter, by Martin A Green, concentrates on developments in crystalline silicon solar cells. Despite the fact that silicon is an indirect-bandgap semiconductor, and therefore is a much less efficient absorber of above-bandgap light than direct-gap semiconductors (such as GaAs), silicon is still the overwhelming choice for solar cell manufacture. As the second most abundant element in the earth's crust, with a well-established technology, the chapter explores recent developments that have produced low-cost devices with efficiencies approaching the maximum physically possible.

Amorphous and microcrystalline silicon solar cells, are next reviewed by Ruud E I Schropp. These thin-film technologies are finding many exploitable applications with their lower usage of absorber materials and use of foreign substrates.

Turning next to direct-bandgap semiconductors, Nicholas J Ekins-Daukes outlines recent developments in III-V solar cells. III-Vs give the highest efficiencies of any solar cell materials. But despite their large absorption coefficients for above-bandgap light, the materials are relatively expensive, and often difficult and rare to extract from the earth's crust. Their place in the technology is assessed, together with recent advances.

Chalcogenide thin-film solar cells are next reviewed by Miriam Paire, Sebastian Delbois, Julien Vidal, Nagar Naghavi and Jean-Francois Guillemoles. Cu(In Ga)Se2 or CIGS cells have made impressive progress in recent years with the highest efficiencies for thin-film cells, while Cu2ZnSn (S,Se)4 or CZTS or kesterite uses less-rare elements than CIGS, and so has significant potential for large-scale production.

The field of organic photovoltaics (OPV) has become of great interest since the efficiency achieved rapidly increased from around 1% in 1999, to more than 10% in 2012 (Green 2013). The chapter by Claudia Hoth, Andrea Seemann, Roland Steim, Tayebeh Amin, Hamed Azimi and Christoph Brabec reviews this novel technology, concentrating on the state-of-the-art in realising a photovoltaic product.

Lastly, one of us (Gavin Conibeer) looks to the future, by outlining third-generation strategies that aim to provide high conversion efficiency of photon energy at low manufacturing cost. Approaches covered include multiple energy level cells (such as tandem cells and multiple exciton generation), modification of the solar spectrum (such as by down- and upconversion), and thermal approaches (such as thermophotovoltaics and hot-carrier cells). The emphasis in all these approaches is on efficiency, spectral robustness, and low-cost processes using abundant nontoxic materials. The book ends with some concluding remarks by the editors, looking to the future in this rapidly developing field.

Finally, no book in this very extensive field can claim to be complete. To explore the field further, readers are recommended to consult ‘Thin Film Solar Cells’ by Jef Poortmans and Vladimir Arkipov (Wiley 2006), a companion volume in this Wiley Series on Materials for Electronic and Optoelectronic Applications, which includes such areas as dye-sensitised solar cells (DSSCs), in the chapter by Michael Gratzel. We hope that this book, with its emphasis on technological materials, will be of use to all who are interested in this field.

REFERENCES

Markvart, T., ‘Solar Electricity’ Wiley, Chichester 2000.

Poortmans, J., and Arkipov, V., ‘Thin Film Solar Cells’ Wiley, Chichester 2006.

Green, M.A., Emery, K, Hishikawa, Y., Warta, W., and Dunlop, E.D., Solar cell efficiency tables (version 41), Progress in Photovoltaics: Research and Applications, 21 (2013) p. 1–11.

2

Fundamental Physical Limits to Photovoltaic Conversion

J.F. Guillemoles

Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP), France

2.1 INTRODUCTION

Where to stop the quest for better devices? What does better mean? The conversion efficiency arises prominently in this respect.

More efficient devices, everything kept equal, would first translate into cheaper solar electricity. Are there limits to reducing the cost of PV electricity? In 2012, modules were sold 0.5–0.7 €/W and the cost of solar electricity is around 20 cts/kWh. In the longer term, development of photovoltaics (PV) has to be based on a major technological breakthrough regarding the use of processes and materials at very low cost, or/and on the engineering of devices offering far higher performance, harvesting most of the available solar energy. Two approaches are targeted at this issue today: the first aims at low-cost materials and low-cost processes to reduce the surface cost of PV devices, possibly sacrificing some of the device efficiency, and the second, aiming at the maximal possible efficiency, at the same cost as today's modules (see Figure 2.1). There is a major difference between these two approaches: the conversion concepts, the materials and the processes.

Figure 2.1 Relation of the cost per watt of solar energy to the surface cost of manufacturing solar devices (modules) and the device efficiency. The light gray, dark gray, and white oval regions represent the ranges found for crystalline silicon (first-), thin-film (second-), and third-generation solar technologies, respectively. The white zone marks the anticipated range for very high efficiency devices. For comparison, limiting efficiencies derived from thermodynamic constraints are also indicated as horizontal bands (low range: no concentration, high range: maximal concentration). Stars indicate industrial production costs as they could be estimated in 2010 from available data: filled stars for c-Si modules and hollow stars for thin-film technologies. (Adapted with permission from Green, 2003. Copyright © 2003, Springer.)

If we think in terms of the manufacturing costs of PV modules, the target aimed at requires that the system needs to produce 1 MWh (comprising about 0.2 m2 of high-end c-Si modules lasting 25 yr) cost less than €30 for parity with the base load or €120 for grid parity. For a very low-cost device, for instance based on polymers or organic–inorganic hybrids, with an expectation for conversion efficiencies on a par with those achieved by the amorphous Si line (on the grounds of similar structural disorder and a low carrier mobility) and shorter life durations, the budget is €7.5/m2 (5-year life duration with 5% efficiency, including power electronics and installation), closer to the cost of structural materials than of functional electronic materials. Finally, for profitable electricity production, we need to pay attention to the system costs. Thus, one sees that it might be extremely difficult to attempt to reduce production costs far beyond what is currently being obtained with inorganic thin-film systems.

This chapter will deal with the scientific issues behind the photovoltaic conversion process, keeping in mind what would make a difference to having this technology more widely used.

The first of these questions is of course the efficiency of the processes. Since the appearance of the first PV devices, the question of the conversion efficiency limits arose, and for a good reason: not only does it have high scientific and technological visibility, it is also one of the major factors in lowering the cost of generating solar electricity. Interestingly, this question of efficiency limit took quite a bit of time before being settled [Landsberg and Badescu, 1998].

The paper of Schockley and Queisser, devising an approach based on a detailed balance approach of photovoltaic conversion is still one of the most quoted papers on PV, yielding the limit of single-junction, standard PV devices.

This question has also been approached on a more general basis, using thermodynamics (Landsberg and Tonge, 1980, Parrott 1992, De Vos 1992) to give device-independent or even process-independent limits (Section 2.1). These limits are essentially related to the source (the sun) characteristics and to the conditions of use (e.g. ambient temperature). Perhaps more useful, and practical, limits have been proposed for defined processes.

In very general terms, photovoltaic conversion in its simpler form supposes several steps:

1. solar photon transferred to the active part of the system;

2. absorption of the photons and energy transfer to the electronic system;

3. selective extraction of electrons to contacts (2 at least);

4. channelling of e-free energy to useful load whose impedance is adjusted.

These steps are illustrated in Figure 2.2 and describe PV process as it is working in all working devices, with nonessential modifications for organic PV (in which electron and holes are coupled as excitons) and multijunction cells (where the incident spectrum on a cell is a part of the total solar spectrum).

Figure 2.2 Photovoltaic action proceeds in 3 steps. (a) photons have to be collected and coupled to the converting system; (b) the converting systems contains occupied (VB) and empty states (CB) separated by a gap between which light induces transitions are allowed; (c) upon photon absorption, two populations of charge carriers are created; (d).