Efficient Organic Light-Emitting Diodes (OLEDs)

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1 Efficient Organic Light-Emitting Diodes (OLEDs) Yi-Lu Chang

3 Efficient Organic Light-Emitting Diodes (OLEDs)

5 Efficient Organic Light-Emitting Diodes (OLEDs) Yi-Lu Chang

6 Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore editorial@panstanford.com Web: 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 (Hardcover) ISBN (ebook) Printed in the USA

7 Contents Preface vii 1. Introduction Brief Overview of OLEDs 1 2. OLED Working Principles Performance Evaluation Emitter Classifications Excitonics Charge Carrier Injection and Transport Efficient Device Architectures Exciton and Carrier Confinement Energy Barrier Minimization Emissive Layer Expansion Advanced Device Architectures: Exciton Harvesting Exciton Harvesting via Phosphorescent Dopants Exciton Harvesting via TADF Exciton Harvesting via Exciplex-Forming Cohosts p-type Intrinsic n-type (p-i-n) OLEDs Top-Emission OLEDs Efficient White OLEDs Single-Emissive-Layer White OLEDs Hybrid White OLEDs Stacked White OLEDs Multiple-Emissive-Layer White OLEDs Separate Emissive Layers Cascade Emissive Layers 70

8 vi Contents Cascade Emissive Layers with Exciton Conversion Optical Light Out-Coupling Organic Waveguide Modes Substrate Modes Surface Plasmon Modes Stability and Degradation Efficiency Roll-Off Triplet Triplet Annihilation Triplet Polaron Annihilation Charge Carrier Balance Material Degradation Cathode Oxidation Anode Degradation Electromigration Molecular Aggregation Molecular Fragmentation Applications in Displays and Lighting Displays Solid-State Lighting Conclusions and Outlook 97 References 101 Index 113

9 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

Organic Light Emitting Diodes (OLEDs) Physics 496/487 Matt Strassler

Organic Light Emitting Diodes (OLEDs) Physics 496/487 Matt Strassler Why OLEDs Lighting efficiency Incandescent bulbs are inefficient Fluorescent bulbs give off ugly light LEDs (ordinary light emitting