Efficient Organic Light-Emitting Diodes (OLEDs) Yi-Lu Chang
Efficient Organic Light-Emitting Diodes (OLEDs)
Efficient Organic Light-Emitting Diodes (OLEDs) Yi-Lu Chang
Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore 038988 Email: editorial@panstanford.com Web: www.panstanford.com British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Efficient Organic Light-Emitting Diodes (OLEDs) Copyright 2015 by Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN 978-981-4613-80-4 (Hardcover) ISBN 978-981-4613-81-1 (ebook) Printed in the USA
Contents Preface vii 1. Introduction 1 1.1 Brief Overview of OLEDs 1 2. OLED Working Principles 7 2.1 Performance Evaluation 7 2.2 Emitter Classifications 9 2.3 Excitonics 13 3. Charge Carrier Injection and Transport 21 4. Efficient Device Architectures 25 4.1 Exciton and Carrier Confinement 25 4.2 Energy Barrier Minimization 26 4.3 Emissive Layer Expansion 29 5. Advanced Device Architectures: Exciton Harvesting 31 5.1 Exciton Harvesting via Phosphorescent Dopants 32 5.2 Exciton Harvesting via TADF 37 5.3 Exciton Harvesting via Exciplex-Forming Cohosts 43 6. p-type Intrinsic n-type (p-i-n) OLEDs 51 7. Top-Emission OLEDs 55 8. Efficient White OLEDs 63 8.1 Single-Emissive-Layer White OLEDs 63 8.2 Hybrid White OLEDs 65 8.3 Stacked White OLEDs 67 8.4 Multiple-Emissive-Layer White OLEDs 68 8.4.1 Separate Emissive Layers 68 8.4.2 Cascade Emissive Layers 70
vi Contents 8.4.3 Cascade Emissive Layers with Exciton Conversion 71 9. Optical Light Out-Coupling 75 9.1 Organic Waveguide Modes 77 9.2 Substrate Modes 80 9.3 Surface Plasmon Modes 81 10. Stability and Degradation 83 10.1 Efficiency Roll-Off 83 10.1.1 Triplet Triplet Annihilation 84 10.1.2 Triplet Polaron Annihilation 84 10.1.3 Charge Carrier Balance 85 10.2 Material Degradation 85 10.2.1 Cathode Oxidation 86 10.2.2 Anode Degradation 86 10.2.3 Electromigration 86 10.2.4 Molecular Aggregation 87 10.2.5 Molecular Fragmentation 87 11. Applications in Displays and Lighting 89 11.1 Displays 89 11.2 Solid-State Lighting 93 12. Conclusions and Outlook 97 References 101 Index 113
Contents vii Preface Organic light-emitting diodes (OLEDs) are already prevalent in our daily lives in the form of mobile phone and tablet displays. More exotic products such as curved and wearable displays and even largearea TV panels have emerged of late. At the same time, intensive work is being carried out globally on OLEDs for design-friendly and energy-efficient artificial lighting. Moving toward this direction, we are expected to experience in our society a dramatic change of the magnitude no less than how halogen bulbs and fluorescent tubes had revolutionized our world several decades ago. This book aims to provide a comprehensive understanding of this fast-growing subject. It is ideal for college students from multiple natural and applied science disciplines as well as industry professionals, especially from closely related fields such as inorganic light-emitting diodes (LEDs) and liquid crystal displays (LCDs). This book thoroughly explains the fundamental principles of key OLED concepts, buttressing them with simple mathematic formulations where necessary. It not only covers the most industrially applicable concepts such as top-emission OLEDs, white OLEDs, and tandem OLEDs, but also includes newer, advanced topics such as OLEDs based on thermally activated delayed fluorescence (TADF) and exciplex-forming co-hosts. The book stems from my five years of research experience at the University of Toronto and as vice president of research at OTI Lumionics. In particular, thinking back to when I was first exposed to the field of OLEDs, I realized that there still wasn t a suitable book on the market that covered the subject in sufficient depth and, at the same time, explained it at the level convenient for college students and industry professionals in related fields. I hope the readers will find this book fruitful and intriguing in the way it tackles complicated concepts, approaching them in a manner that is as simple as it is intuitive. Any feedback, inquiries, or reports of misprints from the readers of this first edition are greatly welcomed. Yi-Lu Chang April 25, 2015