OLED DispLays anD Lighting OLED DispLays anD Lighting Mitsuhiro Koden Yamagata University, Japan This edition first published 2017 © 2017 John Wiley & Sons, Ltd 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 www.wiley.com. 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 1988. 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. 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It is sold on the understanding that the publisher is not engaged in rendering professional services and neither the publisher nor the author shall be liable for damages arising herefrom. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloging‐in‐Publication Data Names: Koden, Mitsuhiro, author. Title: OLED displays and lighting / Mitsuhiro Koden. Description: Chichester, UK ; Hoboken, NJ : John Wiley & Sons, 2016. | Includes bibliographical references and index. Identifiers: LCCN 2016020346 (print) | LCCN 2016025002 (ebook) | ISBN 9781119040453 (cloth) | ISBN 9781119040507 (ePDF) | ISBN 9781119040484 (ePUB) | ISBN 9781119040507 (pdf) | ISBN 9781119040484 (epub) Subjects: LCSH: Light emitting diodes. | Organic semiconductors. Classification: LCC TK7871.89.L53 K63 2016 (print) | LCC TK7871.89.L53 (ebook) | DDC 621.3815/22–dc23 LC record available at https://lccn.loc.gov/2016020346 A catalogue record for this book is available from the British Library. Cover image: caracterdesign/gettyimages Set in 10/12pt Times by SPi Global, Pondicherry, India 10 9 8 7 6 5 4 3 2 1 Contents Preface ix 1 History of OLEDs 1 References 10 2 Fundamentals of OLEDs 12 2.1 Principle of the OLED 12 2.2 Fundamental Structure of the OLED 14 2.3 Features of the OLED 15 3 Light Emission Mechanism 17 3.1 Fluorescent OLEDs 17 3.2 Phosphorescent OLEDs 19 3.3 Thermally Activated Delayed Fluorescent OLEDs 21 3.4 Energy Diagram 21 3.5 Light Emission Efficiency 23 References 24 4 OLED Materials 25 4.1 Types of OLED Materials 26 4.2 Anode Materials 27 4.3 Evaporated Organic Materials (Small Molecular Materials) 29 4.3.1 Hole Injection Materials 29 4.3.2 Hole Transport Materials 32 4.3.3 Emitting Materials and Host Materials in Fluorescent Emission Layer 33 4.3.4 Emitting Materials and Host Materials in Phosphorescent Emission Layer 34 vi Contents 4.3.5 Emitting Materials and Host Materials in TADF Emission Layers 42 4.3.6 Electron Transport Materials 43 4.3.7 Electron Injection Materials and Cathodes 45 4.3.8 Charge‐Carrier and Exciton Blocking Materials 46 4.3.9 N‐Dope and P‐Dope Materials 49 4.4 Solution Materials 50 4.4.1 Polymer Materials 50 4.4.2 Dendrimers 61 4.4.3 Small Molecules 69 4.5 Molecular Orientation of Organic Materials 70 References 71 5 OLED Devices 75 5.1 Bottom Emission, Top Emission, and Transparent Types 75 5.2 Normal and Inverted Structures 79 5.3 White OLEDs 81 5.4 Full‐Color Technology 84 5.4.1 RGB‐Side‐by‐Side 87 5.4.2 White + CF 87 5.4.3 Blue Emission with Color Changing Medium (CCM) 88 5.5 Micro‐Cavity Structure 89 5.6 Multi‐Photon OLED 91 5.7 Encapsulation 94 5.7.1 Thin Film Encapsulation 99 5.7.2 Desiccant Technologies 100 References 100 6 OLED Fabrication Process 103 6.1 Vacuum Evaporation Process 103 6.1.1 Mask Deposition 104 6.1.2 Three Types of Evaporation Methods 104 6.1.3 Ultra‐High Vacuum 105 6.2 Wet Processes 107 6.3 Laser Processes 114 References 115 7 Performance of OLEDs 117 7.1 Characteristics of OLEDs 117 7.2 Lifetime 120 7.2.1 Storage Lifetime 121 7.2.2 Driving Lifetime 121 7.3 Temperature Measurement of OLED Devices 124 References 126 Contents vii 8 OLED Display 127 8.1 Features of OLED Displays 128 8.2 Types of OLED Displays 128 8.3 Passive‐Matrix OLED Display 130 8.4 Active‐Matrix OLED Display 132 8.4.1 TFT Circuit Technologies 133 8.4.2 TFT Device Technologies 137 8.4.3 Commercialized and Prototype AM‐OLED Displays 139 References 144 9 OLED Lighting 147 9.1 Appearance of OLED Lighting 147 9.2 Features of OLED Lighting 148 9.3 Fundamental Technologies of OLED Lighting 152 9.4 Light Extraction Enhancement Technologies 154 9.5 Performance of OLED Lighting 159 9.6 Color Tunable OLED Lighting 159 9.7 Application of OLED Lighting – Products and Prototypes 161 References 164 10 Flexible OLEDs 166 10.1 Early Studies of Flexible OLEDs 166 10.2 Flexible Substrates 167 10.2.1 Ultra‐Thin Glass 168 10.2.2 Stainless Steel Foil 171 10.2.3 Plastic Films 172 10.3 Flexible OLED Displays 174 10.3.1 Flexible OLED Displays on Ultra‐Thin Glass 176 10.3.2 Flexible OLED Displays on Stainless Steel Foil 176 10.3.3 Flexible OLED Displays on Plastic Film 177 10.4 Flexible OLED Lighting 181 10.4.1 Flexible OLED Lighting on Ultra‐Thin Glass 182 10.4.2 Flexible OLED Lighting on Stainless Steel Foil 184 10.4.3 Flexible OLED Lighting on Plastic Films 184 10.5 Toward the Flexible 186 References 186 11 New Technologies 189 11.1 Non‐ITO Transparent Electrodes 189 11.1.1 Conducting Polymer 190 11.1.2 Stacked Layer Using Ag 194 11.1.3 Silver Nanowire (AgNW) 195 11.1.4 Carbon Nanotube (CNT) 196 viii Contents 11.2 Organic TFT 197 11.3 Wet‐Processed TFT 198 11.4 Novel Wet‐Processed or Printed OLED 201 11.5 Roll‐to‐Roll Equipment Technologies 203 11.6 Quantum Dot 204 References 206 Index 209 Preface Since a bright organic light emitting diode (OLED) device was first reported by C. W. Tang and S. A. VanSlyke of Eastman Kodak in 1987, the high technological potential of OLEDs has been recognized in the display and lighting field. This technological potential of OLEDs has been proved and demonstrated by various scientific inventions, technological improvements, prototypes, and commercial products. Indeed, OLEDs have various attractive features such as colored or white self‐emission, planar and solid devices, fast response speed, thin and light weight, and applicability to flex- ible applications. Therefore, it should be seen that OLEDs are not only an interesting scientific field but also have great potential for major market applications. In the past 10 years, OLEDs have experienced serious and complicated business competi- tion from liquid crystal displays (LCDs) and light emitting diodes (LEDs) due to the rapid performance improvement and rapid cost reduction of LCDs and LEDs. However, at the time of writing (2016), new major business possibilities for OLED displays and lighting devices seem to be appearing, in particular, being induced by the huge potential of flexible OLEDs, although LCDs and LEDs are still major devices in displays and lighting, respectively. Rapid growth toward huge market size is forecast for OLED displays and OLED lighting by several market analysts. For about ten years I have been developing practical OLED technologies at Sharp Corporation, after developing LCD technologies there. Since 2013, I have developed practical flexible OLED technologies at Yamagata University, involving collaborations with a number of private companies. The purpose of this book is to give an overview of fundamental science and practical technologies of OLEDs, accompanied by a review of the developmental history. This book provides a breadth of knowledge on practical OLED devices, describing materials, devices, processes, driving techniques, and applications. In addition, this book covers flexible technologies, which must be key technologies for future OLED business. I trust that this book will contribute to not only university students but also researchers and engineers who work in the fields of development and production of OLED devices. 1 History of OLEDs Summary Active research and development of OLEDs (organic light emitting diodes) started in 1987, when Tang and VanSlyke of Eastman Kodak showed that a bright luminance was obtained in an OLED device with two thin organic layers sandwiched between anode and cathode. Since their report, OLEDs have been an attractive field from scientific and practical points of view because OLEDs have great potentials in practical applications such as displays and lighting. This chapter describes the history of the OLED. Key words history, Tang, Kodak, Friend, Forrest, Kido, Adachi Light emission by organic materials was first discovered in a cellulose film doped with acridine orange by Bernanose et al. in 1953 [1]. Ten years later, in 1963, Pope et al., reported that a single organic crystal of anthracene showed light emission induced by carrier injection in a high electric field [2]. Also, since it became known that a large number of organic materials showed high fluorescent quantum efficiency in the visible spectrum, including the blue region, organic materials have been considered as a candidate for practical light emitting devices. However, early studies did not give any indication of the huge potential of OLEDs because of issues such as very high electric field (e.g. some needing 100 V), very low luminance, and very low efficiency. Therefore OLED studies remained as scientific and theoretical fields, not indicating any great motivation towards practical applications. OLED Displays and Lighting, First Edition. Mitsuhiro Koden. © 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd. 2 OLED Displays and Lighting <10V N O O Al N N O Mg:Ag Electron transporting <100nm and emitting material Alq 3 Hole transporting <100nm material Diamine CH CH ITO 3 3 Glass substrate N N H3C CH3 B(>r1ig,0h0t 0emcdis/msi2o)n Figure 1.1 The structure and materials of an OLED device reported by Tang et al. [3] A major impact was made by C. W. Tang and S. A. VanSlyke of Eastman Kodak in 1987. They reported a bright emission obtained in an OLED device with two thin organic layers sandwiched between anode and cathode, as shown in Fig. 1.1 [3]. They introduced two innovative technologies, which used very low thicknesses (<150 nm) of organic layers and adoption of a bi‐layer structure. They reported that light emission was observed from as low as about 2.5 V and that high luminance (>1000 cd/m2) was obtained with a dc voltage of less than 10 V. Although the obtained external quantum efficiency (EQE) was still as low as about 1% and the power efficiency was still as little as 1.5 lm/W, the reported results were enough to draw huge attention from scientists and researchers. Indeed, their report started the age of the OLED not only in the academic field but also in industry. The history of OLEDs is summarized in Table 1.1. The device reported by Tang and VanSlyke in 1987 consists of a bottom emission structure and small molecular fluorescent monochrome organic material evaporated on glass substrates, but various other novel technologies have been studied and developed, aiming at a revolution in OLED technologies. In the academic fields, several novel disruptive technologies giving drastic changes in performance of OLEDs have been discovered or invented. These include polymer OLEDs [4], white OLEDs [5], phosphorescent OLEDs [7], multi‐photon OLEDs [10], TADF OLEDs [17]. In 1990, Burroughes et al. of the group led by Friend in the Cavendish Laboratories (Cambridge, UK) reported OLED devices with a light emitting polymer [4]. This invention opened the huge possibility of wet‐processed OLED technologies. The first scientific report of white‐emission OLED was published in 1994 by Kido et al. of Yamagata University [5]. This report generated active development, aimed at lighting applica- tions for OLEDs. The report also led to developments of the combination of white OLED emission with color filters, aimed at full‐color OLED displays.