Solar Fuels Materials, Physics, and Applications Solar Fuels Materials, Physics, and Applications Theodore Goodson III CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2017 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-1-4398-7491-2 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Goodson, Theodore G. Title: Solar fuels : materials, physics, and applications / Theodore Goodson III. Description: Boca Raton : CRC Press, [2017] | Includes bibliographical references and index. Identifiers: LCCN 2016051830| ISBN 9781439874912 (hardback : alk. paper) | ISBN 9781439874936 (ebook) Subjects: LCSH: Solar batteries. | Solar energy. | Renewable energy sources. Classification: LCC TK2960 .G66 2017 | DDC 621.31/244--dc23 LC record available at https://lccn.loc.gov/2016051830 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com To my wife Stephanie Lynn and my kids Jared (Theodore IV ), Elizabeth, and Sean Contents List of Figures ...........................................................................................................ix Preface .....................................................................................................................xiii Author ......................................................................................................................xv Chapter 1 Historical Background and Structure of This Book .............................1 In the Beginning ...................................................................................1 Goals for Future Solar Cells .................................................................2 Molecular Processes in Organic Solar Cells ........................................4 Excitons and Organic Solar Cells .........................................................4 Problem at the Interface .......................................................................8 Measurements of Solar Cell Surface Properties ...................................9 Spectroscopy of Organic Solar Cell Materials ...................................10 New Approaches with Perovskites .....................................................16 Summary and Outlook .......................................................................16 References ..........................................................................................17 Chapter 2 New Materials for Solar Energy Conversion ......................................21 Small Molecules for Organic Solar Cells ...........................................21 Organic Conjugated Polymer for Solar Cells .....................................29 References ..........................................................................................37 Chapter 3 Measurements of Organic Solar Devices and Materials ....................43 Standard Test Measurements of Solar Cell Efficiencies .....................43 Kelvin Probe Spectroscopy of Organic Solar Cell Materials .............49 Impedance Spectroscopic Measurements in Organic Solar Cells ......51 Measurement of Exciton Diffusion in Organic Solar Cells ................54 Femtosecond Transient Absorption Spectroscopy of Organic Solar Cell Materials ............................................................................57 Fluorescence Upconversion Spectroscopy and Anisotropy Measurements in Organic Solar Materials .........................................60 Two-Photon Excited Fluorescence Measurements in Organic Solar Cell Materials ............................................................................64 References ..........................................................................................67 Chapter 4 Organic Solar Cell Architectures Modernized ...................................75 Basic Constructions Revisited ............................................................75 Essential Diagrams and Ideas .............................................................77 Tandem Organic Cell Construction ....................................................82 New Electrodes for Organic Solar Cells ............................................85 vii viii Contents Universal Approach Toward Modification of Electrodes ...................85 Low-Cost ITO-Free Electrode Solar Systems ....................................86 New Geometries for Organic Solar Cell Construction .......................88 References ..........................................................................................89 Chapter 5 Interface Engineering in Organic Solar Cells .....................................95 Basic Mechanisms Involving Interface Layers in Organic Solar Cells ....................................................................................................95 Building Delocalized Charge Transfer States at the Interface............97 Energy Level Tuning at the Electrode/Active Layer Interface and the Integer Charge Transfer Model ..............................................99 Molecular Design of Interface Materials ..........................................101 Exciton Diffusion at the Interface in Organic Solar Cells ................104 Surface Control on Interfacial Dipoles in Organic Solar Cells ........107 References ........................................................................................108 Chapter 6 New Approaches: Testing the Limits in Organic Solar Cells ...........115 Introduction ......................................................................................115 Singlet Exciton Fission .....................................................................116 Singlet Exciton Fission in Organic Materials ...................................118 Perovskites and a New Hope ............................................................121 Perovskite Synthesis and Structure ..................................................122 Making Perovskite Films for Solar Applications .............................124 Mechanism of Charge Transport in Perovskite Solar Cells and Their Stability ...................................................................................127 Looking Forward with Perovskites and the Challenges Facing Future Commercialization ................................................................130 References ........................................................................................132 Chapter 7 A Look Forward with Organic Solar Fuels ......................................137 The Right Time for Organic Solar Technology ................................137 Are the Components of Organic Solar Cells Already Good Enough? ............................................................................................139 Organic Solar Cell Device Constructions Continue to Improve ......140 Challenge at the Interface Still Remains ..........................................141 Will New Approaches Continue to Rise? .........................................145 Future of Organic Solar Cells ...........................................................146 References ........................................................................................146 Index ......................................................................................................................151 List of Figures Figure 1.1 The gods of ancient Egypt—Aten and Ra. Ra in the solar bark. ........2 Figure 1.2 Power plant using renewable solar energy from the sun ....................3 Figure 1.3 House equipped for the use of solar energy. .......................................5 Figure 1.4 The evolution of crystalline and multicrystalline silicon solar cell efficiency .............................................................................7 Figure 1.5 Basic structure of dye-sensitized solar cell. ........................................9 Figure 1.6 The basic setup for fluorescence time-resolved measurements. .......11 Figure 1.7 Molecular structures of C60, carbon nanotube, and two-dimensional carbon network. ..............................................12 Figure 1.8 Silicon wafer fabrication and testing facility ....................................14 Figure 1.9 High-performance computing has played a major role in the discovery of new organic solar materials ....................................15 Figure 2.1 Fabrication of flexible solar cells. .....................................................22 Figure 2.2 Molecular structures of ZnPc-DOT dyads 1 and 2 and reference compound Zn(t-Bu) Pc. ....................................................23 4 Figure 2.3 (a) Device structure of the inverted BHJ-OSCs. (b) Scheme of energy levels of the materials involved in inverted polymer solar cells. .........................................................................................24 Figure 2.4 The molecular structure of the Tetracene and C and BCP 60 chromophores. ..................................................................................25 Figure 2.5 The molecular structure of rubrene and related chromophores. .......28 Figure 2.6 The molecular structure of MEH-PPV and BBL and the device structure. ...........................................................................................31 Figure 2.7 (a) The molecular structure of PPV and BBL and (b) the energy level diagram. ........................................................................33 Figure 2.8 The molecular structure and device construction of the ketolactam and azafulleroid systems. ...............................................34 Figure 2.9 The open circuit voltages of the fulleroids. ......................................34 Figure 2.10 The PTB polymers with exceptionally high organic solar cell efficiencies .................................................................................35 Figure 2.11 The correlation of the fill factor with pi-distance in the PTB polymers .............................................................................37 ix