Introduction to Flat Panel Displays

Introduction to Flat Panel Displays

Wiley-SID Series in Display Technology Series Editor: Anthony C. Lowe Consultant Editor: Michael A. Kriss

Display Systems: Design and Applications Lindsay W. MacDonald and Anthony C. Lowe (Eds) Electronic Display Measurement: Concepts, Techniques, and Instrumentation Peter A. Keller Projection Displays Edward H. Stupp and Matthew S. Brennesholtz Liquid Crystal Displays: Addressing Schemes and Electro-Optical Effects Ernst Lueder Reflective Liquid Crystal Displays Shin-Tson Wu and Deng-Ke Yang Colour Engineering: Achieving Device Independent Colour Phil Green and Lindsay MacDonald (Eds) Display Interfaces: Fundamentals and Standards Robert L. Myers Digital Image Display: Algorithms and Implementation Gheorghe Berbecel Flexible Flat Panel Displays Gregory Crawford (Ed.) Polarization Engineering for LCD Projection Michael G. Robinson, Jianmin Chen, and Gary D. Sharp Fundamentals of Liquid Crystal Devices Deng-Ke Yang and Shin-Tson Wu Introduction to Microdisplays David Armitage, Ian Underwood, and Shin-Tson Wu Mobile Displays: Technology and Applications Achintya K. Bhowmik, Zili Li, and Philip Bos (Eds) Photoalignment of Liquid Crystalline Materials: Physics and Applications Vladimir G. Chigrinov, Vladimir M. Kozenkov and Hoi-Sing Kwok Projection Displays, Second Edition Matthew S. Brennesholtz and Edward H. Stupp Introduction to Flat Panel Displays Jiun-Haw Lee, David N. Liu and Shin-Tson Wu

Introduction to Flat Panel Displays By Jiun-Haw Lee National Taiwan University, Taiwan

David N. Liu Industrial Technology Research Institute, Taiwan

Shin-Tson Wu University of Central Florida, USA

This edition first published 2008 © 2008 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. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. 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 Lee, Jiun-Haw. Introduction to flat panel displays / by Jiun-Haw Lee, David N. Liu, and Shin-Tson Wu. p. cm. Includes bibliographical references and index. ISBN 978-0-470-51693-5 (cloth) 1. Flat panel displays. I. Liu, David N. II. Wu, Shin-Tson. III. Title. TK7882.I6L436 2008 621.3815 422—dc22 2008032204 A catalogue record for this book is available from the British Library. ISBN: 978-0-470-51693-5 Set in 9/11pt Times by Integra Software Services Pvt. Ltd, Pondicherry, India Printed in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire

Contents

Series Editor’s Foreword About the authors Preface Acknowledgements

xi xiii xv xvii

1 Introduction 1.1 Flat panel displays 1.2 Emissive and nonemissive displays 1.3 Display specifications 1.3.1 Physical parameters 1.3.2 Brightness and color 1.3.3 Contrast ratio 1.3.4 Spatial and temporal characteristics 1.3.5 Efficiency and power consumption 1.3.6 Flexible displays 1.4 Applications of flat panel displays 1.4.1 Liquid crystal displays 1.4.2 Light-emitting diodes 1.4.3 Plasma display panels 1.4.4 Organic light-emitting devices 1.4.5 Field emission displays References

1 1 3 3 3 5 5 5 6 6 6 7 7 8 8 9 9

2 Color science and engineering 2.1 Introduction 2.2 The eye 2.3 Colorimetry 2.3.1 Trichromatic space 2.3.2 CIE 1931 colorimetric observations 2.3.3 CIE 1976 uniform color system 2.3.4 Color saturation and color gamut 2.3.5 Light sources

11 11 12 15 15 16 19 21 22

vi

Contents

2.3.5.1 Sunlight and blackbody radiators 2.3.5.2 Backlights of transmissive displays 2.3.5.3 Color rendering index 2.3.6 Photometry 2.4 Production and reproduction of colors Homework problems References

22 23 24 25 27 28 28

3 Thin-film transistors 3.1 Introduction 3.2 Basic concepts of crystallized semiconductor materials 3.2.1 Band structure of crystallized semiconductors 3.2.2 Intrinsic and extrinsic semiconductors 3.3 Disordered semiconductors 3.3.1 Amorphous silicon 3.3.2 Polycrystalline silicon 3.4 Thin-film transistor characteristics 3.5 Passive matrix and active matrix driving schemes 3.6 Non-silicon-based thin-film transistors Homework problems References

31 31 31 32 36 38 39 41 43 47 53 55 56

4 Liquid crystal displays 4.1 Introduction 4.2 Transmissive thin-film transistor liquid crystal displays 4.3 Liquid crystal materials 4.3.1 Phase transition temperatures 4.3.2 Eutectic mixtures 4.3.3 Dielectric constants 4.3.4 Elastic constants 4.3.5 Rotational viscosity 4.3.6 Optical properties 4.3.7 Refractive indices 4.3.7.1 Wavelength effect 4.3.7.2 Temperature effect 4.4 Liquid crystal alignment 4.5 Homogeneous cell 4.5.1 Phase retardation effect 4.5.2 Voltage-dependent transmittance 4.6 Twisted nematic 4.6.1 Optical transmittance 4.6.2 Viewing angle 4.6.3 Film-compensated TN cells 4.7 In-plane switching 4.7.1 Device structure 4.7.2 Voltage-dependent transmittance 4.7.3 Viewing angle 4.7.4 Phase compensation films 4.8 Fringe field switching

57 57 58 60 60 61 62 65 65 66 67 67 68 70 71 72 73 73 74 75 76 78 78 79 79 80 81

Contents

vii

4.9 Vertical alignment 4.9.1 Voltage-dependent transmittance 4.9.2 Response time 4.9.3 Overdrive and undershoot voltage method 4.9.4 Multidomain vertical alignment 4.10 Optically compensated bend cell 4.10.1 Voltage-dependent transmittance 4.10.2 Compensation films for OCB 4.10.3 No-bias bend cell 4.11 Transflective liquid crystal displays 4.11.1 Introduction 4.11.2 Dual cell gap transflective LCDs 4.11.3 Single cell gap transflective LCDs 4.12 Future directions Homework problems References

83 83 83 85 86 88 88 89 91 91 91 93 95 101 101 103

5 Plasma display panels 5.1 Introduction 5.2 Physics of gas discharge 5.2.1 I–V characteristics 5.2.2 Penning reaction and Paschen curve 5.2.3 Priming mechanism 5.3 Plasma display panels 5.3.1 DC PDP 5.3.2 AC PDP 5.3.3 Panel processes 5.4 Front plate techniques 5.4.1 Substrate 5.4.2 Sustain electrode 5.4.3 Dielectric 5.4.4 Protection layer 5.5 Rear plate techniques 5.5.1 Substrate 5.5.2 Address electrode 5.5.3 Dielectric 5.5.4 Barrier rib 5.5.5 Phosphor 5.6 Assembly and aging techniques 5.6.1 Sealing layer formation and panel alignment 5.6.2 Sealing, gas purging and display gas filling 5.6.3 Aging 5.7 System techniques 5.7.1 Cell operation mechanism 5.7.2 Driving 5.7.3 Energy saving 5.7.4 PDP issues Homework problems References

109 109 109 110 111 112 112 112 113 115 117 118 118 119 119 120 121 121 121 122 124 126 126 127 128 128 129 130 130 132 132 132

viii

Contents

6 Light-emitting diodes 6.1 Introduction 6.2 Material systems 6.2.1 AlGaAs and AlGaInP material systems for red and yellow LEDs 6.2.2 GaN-based systems for green, blue and UV LEDs 6.2.3 White LEDs 6.3 Diode characteristics 6.3.1 The p-layer and n-layer 6.3.2 Depletion region 6.3.3 J–V characteristics 6.3.4 Heterojunction structures 6.3.5 Quantum well, quantum wire and quantum dot structures 6.4 Light-emitting characteristics 6.4.1 Recombination model 6.4.2 L–J characteristics 6.4.3 Spectral characteristics 6.5 Device fabrication 6.5.1 Epitaxy 6.5.2 Process flow and device structure design 6.5.3 Extraction efficiency improvement 6.5.4 Package 6.6 Applications 6.6.1 Traffic signals, electronic signage and huge displays 6.6.2 LCD backlight 6.6.3 General lighting Homework problems References

137 137 140 142 143 145 147 148 149 152 153 154 155 156 157 158 161 161 164 165 167 168 169 169 172 173 174

7 Organic light-emitting devices 7.1 Introduction 7.2 Energy states in organic materials 7.3 Photophysical processes 7.3.1 Franck–Condon principle 7.3.2 Fluorescence and phosphorescence 7.3.3 Jablonski diagram 7.3.4 Intermolecular processes 7.3.4.1 Energy transfer process 7.3.4.2 Excimer and exciplex formation 7.3.4.3 Quenching process 7.3.5 Quantum yield calculation 7.4 Carrier injection, transport and recombination 7.4.1 Richardson–Schottky thermionic emission 7.4.2 SCLC, TCLC and PF mobility 7.4.3 Charge recombination 7.4.4 Electromagnetic wave radiation 7.5 Structure, fabrication and characterization 7.5.1 Device structure 7.5.1.1 Two-layer OLED 7.5.1.2 Dopant in the matrix as the EML

177 177 178 179 180 182 183 184 184 185 187 187 189 190 192 193 193 195 196 197 198

Contents

ix

7.5.1.3 HIL, EIL and p–i–n structure 7.5.1.4 Top-emission and transparent OLEDs 7.5.2 Polymer OLEDs 7.5.3 Device fabrication 7.5.3.1 Thin-film formation 7.5.3.2 Encapsulation and passivation 7.5.3.3 Device structures for AM driving 7.5.4 Electrical and optical characteristics 7.5.5 Degradation mechanisms 7.6 Improvement of internal quantum efficiency 7.6.1 Phosphorescent OLEDs 7.6.2 Tandem structure 7.6.3 White OLEDs 7.7 Improvement of extraction efficiency Homework problems References

200 203 204 205 206 209 210 211 213 218 218 220 222 224 225 226

8 Field emission displays 8.1 Introduction 8.2 Physics of field emission 8.2.1 Work function and field enhancement 8.2.2 Vacuum mechanism 8.3 FED structure and display mechanism 8.4 Emitter 8.4.1 Spindt emitter 8.4.2 CNT emitter 8.4.3 Surface conduction emitter 8.5 Panel process 8.6 Field emission array plate techniques 8.7 Phosphor plate techniques 8.8 Assembly and aging techniques 8.8.1 Spacer 8.8.2 Sealing layer formation and panel alignment 8.8.3 Sealing 8.8.4 Evacuation and sealing off 8.8.5 Aging 8.9 System techniques Homework problems References

233 233 233 233 236 237 238 239 240 243 244 247 248 249 251 251 252 252 253 253 254 254

Index

259

Series Editor’s Foreword Article 2 of the bylaws of the Society for Information Display begins “1. The purpose of SID shall be: a) To encourage the scientific, literary and educational advancement of information display and its allied arts and sciences. . .’’. This book series was begun eleven years ago with the express object of extending that encouragement, which in the printed form amounted to publishing conference proceedings and a Journal of peer refereed papers, to the provision of a series of books which would satisfy the needs of scientists and engineers working in the wide and complex field of displays. More recently in 2006, we published “Fundamentals of Liquid Crystal Devices’’ by Deng-Ke Yang and Shin-Tson Wu (who – not coincidentally – is a co-author of this book). That book extended the readership because it was written primarily as a post graduate textbook. This latest volume in the series extends that educational scope still further by describing the operating principles and the methods of fabrication of technologies used or of potential use in flat panel displays, their methods of addressing, systems aspects and the underpinning science. Although general books on flat panel displays have been published in the past, this is the first comprehensive flat panel display textbook to have been written at this academic level. Its readership and its use will extend far beyond post graduate courses as it offers in a single volume material of great value to practising industrial engineers and scientists across the whole range of flat panel technologies. In my foreword, I usually provide a précis of the contents of a book, but the authors have done this so comprehensively that such an effort on my part would be superfluous. It merely remains for me to thank them for the great effort they have put into writing this book and wholeheartedly to commend it to our present and expanding readership. Anthony C Lowe Series Editor Braishfield, UK.

About the authors Jiun-Haw Lee Jiun-Haw Lee received BSEE, MSEE and PhD degrees in electrical engineering in 1994, 1995 and 2000, respectively, all from National Taiwan University, Taipei, Taiwan. From 2000 to 2003 he was with the RiTdisplay Corporation as the director. In 2003 he joined the faculty of National Taiwan University in the Graduate Institute of Photonics and Optoelectronics and the Department of Electrical Engineering, where he is currently an associate professor. His research interests include organic light-emitting devices, display technologies and solid-state lighting. Dr Lee is a member of the IEEE, OSA, MRS and SPIE. He received the Exploration Research Award of Pan Wen Yuan Foundation and Lam Research Award in both 2005 and 2006. He has published over 40 journal papers, 100 conference papers and 20 issued patents. David N. Liu David N. Liu has been the director of the Strategic Planning Division in the Display Technology Center (DTC) of the Industrial Technology Research Institute (ITRI) since 2006. He worked on IC and field emission displays at ERSO (Electronics Research and Service Organization)/ITRI and Bellcore (Bell Communication Research) from 1983 to 1996. He started his research and development work on plasma display panels at Acer Peripheral Inc. and AUO from 1996 to 2002. After his service at AUO, he was in charge of the flat panel display technology division in ERSO/ITRI until 2006. Dr Liu received his PhD degree in electrical engineering from New Jersey Institute of Technology in 1992. He has over 45 issued patents, 18 published papers and a contributed chapter of the Semiconductor Manufacturing Handbook (McGraw-Hill, 2005). He also successfully developed field emission displays, plasma display panels and flat panel displays followed by the receipt of many awards from ITRI, Photonics Industry and Technology DevelopmentAssociation,Administration Bureau of Science Base Industry Park and the Ministry of Economic Affairs (MOEA). He was also a recipient of the Outstanding Project Leader Award from MOEA in 2006. Shin-Tson Wu Shin-Tson Wu is a PREP professor at the College of Optics and Photonics, University of Central Florida (UCF). Prior to joining UCF in 2001, Dr Wu worked at Hughes Research Laboratories (Malibu, California) for 18 years. He received his PhD in physics from the University of Southern California (Los Angeles) and BS in physics from National Taiwan University (Taipei). Prof. Wu has co-authored four books: Fundamentals of Liquid Crystal Devices (Wiley, 2006), Introduction to Microdisplays (Wiley, 2006), Reflective Liquid Crystal Displays (Wiley, 2001) and Optics and Nonlinear Optics of Liquid Crystals (World Scientific, 1993), six book chapters, over 300 journal publications and 75 issued and pending patents.

xiv

About the authors

Prof. Wu is a fellow of the IEEE, OSA, SID and SPIE. He is a recipient of the SPIE G.G. Stokes award, SID Jan Rajchman Prize, SID Special Recognition Award, SID Distinguished Paper Award, Hughes team achievement award, Hughes Research Laboratories outstanding paper award, UCF Distinguished Researcher Award and UCF Research Incentive Award. He was the founding editor-in-chief of the IEEE/OSA Journal of Display Technology.

Preface Flat panel displays (FPDs) are everywhere in our daily lives: mobile phones, notebooks, monitors, TVs, traffic signals and electronic signage are a few examples. Several FPD technologies, such as liquid crystal displays (LCDs), plasma display panels (PDPs), light-emitting diodes (LEDs), organic light-emitting devices (OLEDs) and field emission displays (FEDs), have been developed. They coexist because each technology has its own unique properties and applications. However, due to the diversity of display materials and operating mechanisms, there has not been a textbook covering the fundamental physics of such a wide spectrum of display technologies. There are books dedicated to a specific display technology or book chapters covering different display technologies. This book is intended as a textbook for senior undergraduate and graduate students with a wide variety of backgrounds, such as electrical engineering, electronics, material science, applied physics and optical engineering. It can also be used as a reference book for engineers and scientists working in display industries. Parts of the material in this book and its organization follow the course ‘Introduction to display technologies’, which has been taught by Jiun-Haw Lee in the Graduate Institute of Photonics and Optoelectronics (GIPO) and Department of Electrical Engineering, National Taiwan University (NTU), Taipei, Taiwan, since 2003. This book introduces basic operation principles and underlying physics for thin-film transistors (TFTs) LCDs, PDPs, LEDs, OLEDs and FEDs in each chapter. The LCD is a nonemissive display. From the electrical viewpoint, each pixel is a light switch driven by a TFT. To reduce leakage current of the capacitor, the liquid crystal material should have a high resistivity. Moreover, to achieve a high contrast ratio, most direct-view TFT LCDs require two absorption-type sheet polarizers. These polarizers not only reduce the light efficiency but also limit the LCD’s viewing angle. Therefore, phase compensation films are required for wide-view LCDs. In contrast, the PDP is an emissive display. It can be considered as consisting of millions of miniature fluorescent lamps on a single panel. LEDs and OLEDs are electroluminescent devices with crystallized semiconductors and amorphous organic materials, respectively. Compared with liquid crystal materials which are also organic compounds, OLED materials should exhibit a low resistivity to reduce ohmic losses. A FED is a type of flat cathode ray tube, which has all the advantages of this mature technology. In this book, both basic physics and practical issues (such as material requirements, device configurations, fabrication methods and driving techniques) of different display technologies are addressed. Each display technology is at a different development stage; some are more mature than others. Generally speaking, they are still advancing so rapidly that it is difficult to keep up with the technological advancements. Thus, in this introductory book we have decided to emphasize the fundamental science and only highlight the key technological advancements of each technology. Another objective of this book is to provide background knowledge for readers from interdisciplinary fields to stimulate new ideas. Since display technologies cover very broad scientific spectra, any breakthrough from any aspect may result in substantial progress in this industry. Sometimes there is not only competition but also cooperation among different display technologies. For example, LCDs and LEDs are distinct technologies for different display applications. However, LEDs can be also used as

xvi

Preface

backlights for LCDs. As a result, the color gamut is widened, the dynamic contrast ratio is enhanced and power consumption is reduced. After reading this book, one may expect to have a whole picture of display technologies from scientific, technical and engineering viewpoints. There are different kinds of technologies suitable for different sizes (ranging from smaller than an inch to more than a hundred inches in diagonal measurement) and applications (such as outdoor, indoor and mobile displays). Furthermore, this book may serve as a stepping stone to more advanced research and development. The organization of this book is as follows. Chapter 1 introduces the classifications and specifications of display technologies, which are guidelines for developing a display and judging performance. Applications suitable for different technologies (LCD, PDP, LED, OLED and FED) are also illustrated. Displays are used to produce or reproduce color images. In Chapter 2 we introduce the stages of color formation from a scientific viewpoint. Then, the chromaticity diagram is used to quantitatively describe colors. Finally, one can use the background of color science to engineer the color performance of a display. Chapter 3 describes the TFTs based on semiconductor material, which are used to drive LCDs and OLEDs. Since this is an introductory textbook, some basic semiconductor physics are first introduced, which is also useful knowledge for Chapter 6. Material aspects of amorphous silicon and polycrystalline silicon are discussed. Then, device structures and their performances are introduced. Finally, driving techniques and circuits for LCDs and OLEDs are demonstrated. Emerging TFT technologies, such as organic and oxide TFTs, are briefly discussed. In Chapter 4 we begin with basic liquid crystal compound structures, mixture formulations and their physical properties, and then extend the discussion to device structures and display characteristics. Three major LCDs are introduced: transmissive, reflective and transflective. Most modern LCDs are of the transmissive type. However, these displays might be washed out by direct sunlight. In contrast, reflective displays work well under sunlight but are not readable in dark ambient. To retain the good images of a transmissive display while keeping good sunlight readability, transflective LCDs have been developed. Chapter 5 gives an overview of PDP fundamentals. We begin with a discussion of the physics of a gas discharge, covering the reactions of gas discharges and I–V characteristics. DC PDP and AC PDP panels as well as surface discharge and vertical discharge approaches are introduced. The panel process technologies and useful process approaches are also described. Finally, we discuss system techniques with cell operation and driving mechanism. Semiconductor LEDs are discussed in Chapter 6. We start from the material system because this determines the emission wavelength. Electrical properties of LEDs, typically p–n junctions, and corresponding optical characteristics are then discussed. The fabrication process is introduced, which highlights the practical electrical, optical and thermal issues. Finally, applications of LEDs for displays are described. Chapter 7 describes OLEDs, with fabrication processes and operation principles similar to LCDs and LEDs, respectively. The chapter starts from the material aspect. Opto-physical processes in an organic material are introduced. Electrical injection and transport in organic materials are then described. Device structures and fabrication are then discussed. One serious disadvantage of an OLED is its short lifetime; this issue is also addressed. In Chapter 8 an overview of FED fundamentals is provided. We begin by discussing the physics of field emission, covering the field enhancement and vacuum mechanism. FED structure, display mechanism and various emitters are introduced. The advantages and disadvantages of using low- and high-voltage phosphor are compared. The panel process technology and useful process approaches are also described. Finally, system techniques are discussed. Jiun-Haw Lee, Taiwan David N. Liu, Taiwan Shin-Tson Wu, Florida, USA

Acknowledgements Jiun-Haw Lee would like to thank his colleagues Profs. I-Chun Cheng, Chih-I Wu, Jian-Jang Huang, Yuh-Renn Wu, Hoang-Yan Lin and Ding-Wei Huang of GIPO, NTU, for many helpful discussions. Mr Jia-Xing Lin of ITRI is gratefully acknowledged for kindly providing useful information about TFT technologies. Dr Lee is also grateful to his students in NTU and Dr Zhibing Ge of the University of Central Florida, who helped to prepare drawings, references, homework problems and examples, together with providing valuable remarks and comments from a reader’s perspective. David N. Liu is grateful to his colleagues in ITRI and AUO for useful discussions, and Ted Knoy for his professional proofreading. In particular, he would like to express his gratitude to his wife Janice for her patience and support during the period of writing the book. Shin-Tson Wu is deeply indebted to his present and former group members at the University of Central Florida for their numerous technical contributions, and to Chi-Mei Optoelectronics for the funding support. He is grateful to his wife Cho-Yan for spiritual support during the writing of the book.

1 Introduction 1.1 Flat panel displays A display is an interface containing information which stimulates human vision. Information may be pictures, animation, movies and articles. One can say that the functions of a display are to produce or reproduce colors and images. Using ink to write, draw or print on paper is a traditional display, like a painting or a book. However, the content of such a traditional display is motionless and typically inerasable. In addition, a light source, synthetic or natural, is needed for reading a book or seeing a picture. There are lots of electronic displays that use an electronic signal to create images on a panel and stimulate the human eye. Typically, they can be classified as emissive and nonemissive. Emissive displays emit light from each pixel which constitutes an image on the panel. In contrast, nonemissive displays modulate light, by means of absorption, reflection, refraction and scattering, to display colors and images. For a nonemissive display, a light source is needed. Hence, these can be classified into transmissive and reflective displays. One of the most successful display technologies for home entertainment is the cathode ray tube (CRT), which is in widespread use in televisions (TVs). CRT is already a mature technology which has the advantages of self-emission, wide viewing angle, fast response, good color saturation, long lifetime and good image quality. However, a major disadvantage is its bulky size. The depth of a CRT is roughly equal to the length and width of the panel. For example, a monitor’s depth is about 40 cm for a 19-inch (38.6 cm × 30.0 cm) CRT with an aspect ratio of 4:3. Hence, it is not very portable. The bulky size and heavy weight limit its applications. In this book, we introduce various types of flat panel displays (FPDs). As the name implies, these displays have a relatively thin profile, i.e. several centimeters or less. For instance, the liquid crystal display (LCD) is presently the dominant FPD technology with diagonal sizes ranging from less than 1 inch (microdisplay) to over 100 inches. Such a display is usually driven by thin-film transistors (TFTs). A liquid crystal (LC) is a light modulator because it does not emit light. Hence, a backlight module is required for a transmissive LCD. In most LCDs, two crossed polarizers are employed in order to obtain a high contrast ratio. The use of two polarizers limits the maximum transmittance to about 35–40 %, unless a polarization conversion scheme is implemented. Moreover, the optical axes of two crossed polarizers are no longer perpendicular to each other when viewed at oblique angles. A LC is a birefringent medium which means its electro-optic effects are dependent on the incident light direction. Therefore, the viewing angle of a LCD is an important issue. Most wide-view LCDs require multiple optical phase compensation films; one for compensating the crossed polarizer and another for the birefringent LC. Film-compensated transmissive LCDs exhibit a high contrast ratio, high resolution, crisp image, good color saturation and wide viewing angle. However, the displayed images can be washed out under

Introduction to Flat Panel Displays c 2008 John Wiley & Sons, Ltd 

J.-H. Lee, D.N. Liu and S.-T. Wu

2

Introduction to Flat Panel Displays

direct sunlight. For example, if we use a notebook computer at outdoor ambient, the images may not be readable. This is because the reflected sunlight from the LCD surface is much brighter than that transmitted from the backlight so that the signal-to-noise ratio is low. A broadband antireflection coating will definitely help to improve the sunlight readability. Another way to improve sunlight readability is to use reflective LCDs.1 A reflective LCD uses ambient light to produce the displayed images. It does not carry a backlight; thus, its weight is reduced. A wristwatch is such an example. Most reflective LCDs have inferior performances compared to the transmissive ones in contrast ratio, color saturation and viewing angle. Moreover, at dark ambient a reflective LCD is not readable. As a result, its application is rather limited. To overcome the sunlight readability issue while maintaining high image quality, a hybrid display called a transflective liquid crystal display (TR-LCD) has been developed.2 In a TR-LCD, each pixel is divided into two subpixels: transmissive (T) and reflective (R). The area ratio between T and R can be adjusted depending on the application. For example, if the display is mostly used outdoors, then one can design to have 80 % reflective area and 20 % transmissive area. In contrast, if the display is mostly used indoors, then one can have 80 % transmissive area and 20 % reflective area. Within this TR-LCD family, there are still some varieties: double cell gap versus single cell gap, and double TFTs versus single TFT. These approaches are trying to solve the optical path length disparity between the T and R subpixels. In the transmissive mode the light from the backlight unit passes through the LC layer once, but in the reflective mode the ambient light traverses the LC medium twice. To balance the optical path length, we could make the cell gap of the T subpixels twice as thick as that of the R subpixels. This is the so-called dual cell gap approach. The single cell gap approach has a uniform cell gap throughout the T and R regions. To balance the different optical path lengths, several approaches have been developed, e.g. dual TFTs, dual fields (stronger field for T region and weaker field for R region) and dual alignments. Presently, the majority of TR-LCDs adopt the double cell gap approach for two reasons: (1) both T and R modes can achieve maximum light efficiency, and (2) the gamma curve matching between the voltage-dependent transmittance (VT) and reflectance (VR) is almost perfect. However, the double cell gap approach has two shortcomings: first, the T region has a slower response time than the R region because its cell gap is about twice as thick as that of the R region; second, the viewing angle is relatively narrow, especially when homogeneous cells are employed. To widen the viewing angle, a special rod-like LC polymeric compensation film has to be used. Chapter 4 gives detailed descriptions of various types of LCDs. A plasma display panel (PDP) is an emissive display which can be thought of as very many miniature fluorescent lamps on a panel. As an emissive display it typically has a better display performance, such as good color saturation and wide viewing angle. Due to the limitation of fabrication, the pixel size of a PDP cannot be too small. For a finite pixel size, the video content is increased by enlarging the panel size. PDPs are suitable for large-screen applications. In 2008, Panasonic demonstrated a 150-inch PDP TV with 4096 × 2160 pixels. This resolution is four times higher than that of the present full high-definition television (HDTV). Light-emitting diodes (LEDs) and organic light-emitting devices (OLEDs) are electroluminescent devices with semiconductor and organic materials, respectively. Electrons and holes recombine within the emissive materials, where the bandgap of the materials determines the emission wavelength. A field emission display (FED) uses sharp emitters to generate electrons. These electrons bombard the phosphors that are present to emit red (R), green (G) and blue (B) light. A FED is like a ‘flat’ CRT. Due to the mature technologies developed in CRTs, FEDs exhibit all the advantages of CRTs plus the smaller panel thickness. Compared to conventional displays (such as books, magazines and newspapers), electronic displays (such as TVs, mobile phones and monitors) are rigid because they are typically fabricated on glass substrates. Flexible FPDs are emerging. Several approaches have been developed, such as electrophoretic displays and polymer-stabilized cholesteric displays. Flexible displays are thin, robust and lightweight. In the remainder of this chapter, we first introduce FPD classifications in terms of emissive and nonemissive displays, where nonemissive displays include transmissive and reflective displays. Specifications

Introduction

3

of FPDs are then outlined. Finally, the FPD technologies described in the later chapters of this book are briefly introduced.

1.2 Emissive and nonemissive displays Both emissive and nonemissive FPDs have been developed. For emissive displays, each pixel emits light with different intensity and color which stimulate the human eye directly. CRTs, PDPs, LEDs, OLEDs and FEDs are emissive displays. An emitter is called Lambertian when the luminances from different viewing directions are the same. Most emissive displays are Lambertian emitters which results in a wide viewing angle performance. Also, due to the self-emissive characteristics, they can be used even under very low ambient light. When such displays are turned off, they are completely dark (ignoring the ambient reflection). Hence, display contrast ratios (see also Section 1.3.3) are high. Displays that do not emit light themselves are called nonemissive displays. A LCD is a nonemissive display in which the LC molecules in each pixel work as an independent light switch. The external voltage reorients the LC directors which causes phase retardation. As a result, the incident light from the backlight unit or ambient is modulated. Most high-contrast LCDs use two crossed polarizers. The applied voltage controls the transmittance of the light through the polarizers. If the light source is behind the display panel, the display is called a transmissive display. It is also possible to use ambient light as the light source. This resembles the concept of a conventional display, such as reading a book, which is called a reflective display. Since no backlight is needed in a reflective display, its power consumption is relatively low. In a very bright environment, images of emissive displays and transmissive LCDs can be washed out. In contrast, reflective displays exhibit an even higher luminance as the ambient light increases. However, they cannot be used in a dim environment. Hence, transflective LCDs have been developed, which are described in Chapter 4.

1.3 Display specifications In this section, we introduce some specifications which are generally used to describe and judge FPDs from the viewpoints of mechanical, electrical and optical characteristics. FPDs can be smaller than 1 inch for projection displays, 2–4 inches for mobile phones and personal digital assistants, 7–9 inches for car navigation systems, 8–18 inches for notebook computers, 10–25 inches for desktop computers and more than 100 inches for direct-view TVs. For different FPDs, their requirements for pixel resolutions also differ. Luminance and color are two important characteristics which directly affect the display performances. Dependences of these two parameters to viewing angles, uniformity, lifetime and response time should be addressed when describing the performances of an FPD. Contrast ratio is another important parameter, which changes with different ambient environments.

1.3.1 Physical parameters The basic physical parameters of an FPD include display size, aspect ratio, resolution and pixel format. The size of a display is typically described by diagonal length, in units of inches. For example, a 15-inch display means the diagonal of the viewable area of this display is 38.1 cm. There are three kinds of display format: landscape, equal and portrait, corresponding to the display width being larger than, equal to and smaller than its length. Most monitors and TVs use landscape format with a width-to-length ratio, which is called the ‘aspect ratio’, of 4:3, 16:9 or 16:10, typically. An FPD typically consists of a ‘dot matrix’ which can display images and characters. To increase resolution, one may use more dots in a display. Table 1.1 lists some standard resolutions of FPDs. For

4

Introduction to Flat Panel Displays

Table 1.1

Resolution of FPDs.

Abbreviation

Full name

Resolution

VGA SVGA XGA SXGA UXGA WXGA WSXGA WUXGA

Video graphics array Super video graphics array Extended graphics array Super extended graphics array Ultra extended graphics array Wide extended graphics array Wide super extended graphics array Wide ultra extended graphics array

640 × 480 800 × 600 1024 × 768 1280 × 1024 1600 × 1200 1366 × 768 1680 × 1050 1920 × 1200

example, VGA means the display is 640 dots in width and 480 dots in length. Higher resolution typically (but not necessarily) means better image quality. There are some resolutions listed in Table 1.1 starting with the letter ‘W’, which means wide screen with an aspect ratio larger than 4:3. Once the resolution, display size and aspect ratio are known, one may obtain the pitch of the pixels. For example, a 19-inch display with aspect ratio of 4:3 and resolution of UXGA has a pitch of 190.5 m. Note that not all of the pixel area contributes to the display. One can define the ‘fill factor’ or ‘aperture ratio’ as the ratio of the display area in a pixel over the whole pixel size, with its maximum value of 100 %. Besides, for a full-color display, at least three primary colors are needed to compose a color pixel. Hence, each color pixel is divided into three subpixels (RGB) sharing the area. For example, let us assume a color pixel has size of 240 m × 240 m; then the dimension of each subpixel is 80 m × 240 m. If the fill factor is 81 % which actually contributes to light emission or transmission, then the usable pixel area is reduced to 72 × 216 m2 . There are different layouts for RGB subpixels, as shown in Figure 1.1. For the stripe configuration, it is straightforward and easy for fabrication and driving circuit design. However, it has a poor color mixing performance for the same display area and resolution. For mosaic and delta configurations, their fabrication and/or driving circuit are more complicated but their image quality is better because of better color mixing capability. Also, displays with mosaic and delta configurations exhibit faster response times since the moving distance between the pixels is shorter. Actually, as the resolution gets high enough the subpixel arrangement becomes less critical. For medium and large displays, the stripe configuration is typically used. In contrast, for a small-size display which requires high resolution, e.g. video cameras, one may use the mosaic or delta configuration.

* R G B (a) Figure 1.1

(b)

(c)

Subpixel layout of an FPD: (a) stripe, (b) mosaic and (c) delta configurations.

Introduction

5

1.3.2 Brightness and color Luminance and color are two important optical characteristics of an FPD. A display with high luminance looks dazzling in a dark room. On the other hand, a display with insufficient brightness appears washed out under high ambient. Typically, the luminance of an FPD should be as bright as (or slightly brighter than) the real object. Under an indoor lighting environment, a monitor has a luminance of 200–300 cd m−2 (Section 2.3.6). For a large-screen TV, a higher luminance (500–1000 cd m−2 ) may be needed. An FPD is used to produce or reproduce colors; hence, how many colors of an FPD and how real the color is (color fidelity) between an FPD and a real object are two important characteristics of an FPD. Since the color of an FPD is mixed by (at least) three primary colors, i.e. RGB, more ‘pure’ (saturated) primaries results in a broader range of the possibly displayed colors, which is called ‘color gamut’ (Section 2.3.4). One can equally divide the stimuli to the eyes from dark to bright with 2, 4, 8 or more spacings, which is called ‘gray level’ or ‘gray scale’ (Section 2.3.3). For example, an FPD can display 16 million colors (28 × 28 × 28 ≈ 16.8 million) when each RGB subpixel is divided into 8 gray scales.

1.3.3 Contrast ratio The device contrast ratio (CR) of an FPD is defined as CR =

Lw , Lb

(1.1)

where L w and L b are the luminance at white and black states, respectively. Higher CR means higher on/off ratio and hence better image quality and higher color saturation. When CR is equal to or less than 1, the human eye cannot distinguish the on and off colors so that the information content of an FPD is lost or distorted. For most emissive displays, the off-state luminance is zero. Hence, the contrast ratio is infinity in a perfectly dark room. However, due to the surface reflection from the ambient, Equation (1.1) should be modified to Lw + Lar A-CR = , (1.2) Lb + Lar where A-CR is the ambient contrast ratio and L ar is the luminance from ambient reflection. A-CR is used to specify the ambient contrast ratio, to distinguish from the intrinsic ‘device’ contrast ratio as described in Equation (1.1). From Equation (1.2), as the ambient reflection increases, A-CR decreases sharply. To keep a good ambient contrast, one can: (1) increase the on-state luminance, and (2) reduce the reflectivity of the display surface. However, for a very strong ambient, e.g. in sunshine outdoors, luminance from the direct sun is four orders of magnitude higher than that of an FPD, which severely washes out the information content of the FPD. Sunlight readability is an important issue especially for mobile displays. In contrast, an adequate ambient light is required for conventional displays, such as books or newspapers. A similar situation applies to reflective displays, such as reflective LCDs.

1.3.4 Spatial and temporal characteristics Uniformity of an FPD means the luminance and color change over a display area. Human eyes are sensitive to luminance and color differences. For example, a 5 % luminance difference is noticeable between two adjacent pixels. For a gradual change, human eyes can tolerate up to 20 % luminance change over the whole display. Optical characteristics (luminance and colors) may also change at different viewing angles. For Lambertian emitters, such as CRTs, PDPs and FEDs, viewing angle performances are quite good. The emission profile of LEDs and OLEDs can be engineered by packaging and layer structure. However, the viewing angle of LCDs is one of the major issues because LC material is birefringent and crossed

6

Introduction to Flat Panel Displays

polarizers are no longer crossed when viewed at oblique angles. There are several ways to define the viewing angle of an FPD. For example, to find the viewing cone with: (1) a luminance threshold; (2) minimum contrast ratio, say 10:1; or (3) maximum value of color shift. For some cases that contrast ratio is smaller than 1; this is called ‘gray level inversion’. Response time is another important metric. If an FPD has a slow response time, one may see blurred images for fast moving objects. By switching the pixel from ‘off’ to ‘on’ and from ‘on’ to ‘off’, and calculating the time required from 10 to 90 % and 90 to 10 % luminance levels, one can obtain rise and fall time, respectively. One may also define the response time from one gray level to another, which is called the ‘gray-to-gray’ (GTG) response time. Most display scenes contain rich grayscales. Therefore, GTG response time is more meaningful. For LCDs, this GTG response time can be much longer than the black-to-white rise and fall time.3 A TFT is a holding type of active matrix. It is different from the CRT’s impulse type. Therefore, a motion picture response time4 is commonly used to define the response time of a TFT LCD. After long-term operation, the luminance of an FPD (especially an emissive display) decays. In an emissive display, if a fixed pattern is lit on for a long period of time before all the pixels are turned on for the full white screen, one can see nonuniformity of the fixed pattern with a lower brightness, which is called the ‘residual image’. As mentioned before, the human eye can detect less than 5 % nonuniformity between two adjacent pixels. Hence the lifetime of an FPD is crucial for static images. An alternative solution is to use moving pictures, rather than static images, for information display. Then the luminances of all pixels decay uniformly, since the average on time for all pixels is the same.

1.3.5 Efficiency and power consumption Power consumption is a key parameter, especially for mobile displays, as it affects battery life. For displays with wall-plug electrical input, lower power consumption implies lower heat generation, which means heat dissipation is less serious. Typically, one uses the unit lm W−1 to describe power efficiency of an FPD (Section 2.3.6). Lumen (lm) and watt (W) are units for describing light output and electrical input. A portable display with lower power consumption leads to a longer battery life. For notebooks and TVs, high optical efficiency also translates into less heat dissipation and a lower electricity bill. Thermal management in a small-chassis notebook is an important issue.

1.3.6 Flexible displays An FPD is usually fabricated on thin glass plates. Glass is a kind of rigid substrate. In contrast, conventional displays are printed on paper, which is flexible. An interesting research topic is to fabricate FPDs on flexible substrates, as a ‘paper-like’ display.5 Compared to the glass-based FPDs, flexible displays are thin and lightweight. Also, flexible displays can be fabricated by the roll-to-roll process, which is potentially of low cost. Substrate selection of flexible FPDs includes ultrathin glass, plastic and stainless steel. Bendable ultrathin glass substrate is achievable, but the cost is high. Plastic substrate is suitable for flexible displays, but the highest durable temperature is typically lower than 200 ◦ C. Stainless steel substrate is bendable, and durable for high temperature; however, it is opaque hence not suitable for transmissive displays. There are many technical bottlenecks for flexible FPDs, such as material selection, fabrication processes, device configurations, display package and measurement.

1.4 Applications of flat panel displays The following subsections briefly outline the applications of each technology. Detailed mechanisms are described in the related chapters.

Introduction

7

1.4.1 Liquid crystal displays Although LC materials were discovered more than a century ago,6,7 their useful electro-optic effects and stability were developed only in the late 1960s and 1970s. In the early stage, passive matrix LCDs were found useful in electronic calculators and wristwatches.8 With the advance of TFTs,9 color filters10 and low-voltage LC effects,11 active matrix LCDs have gradually penetrated into the market of notebook computers, desktop monitors and TVs. Today, LCDs have found widespread uses in everyday life, including (1) mobile applications, such as mobile phones, personal digital assistants, navigation systems, notebook personal computers; (2) office applications, such as desktop computers and video projectors; and (3) home applications, such as large-screen TVs.12 To satisfy these wide-spectrum applications, three types of LCDs have been developed: transmissive, reflective and transflective. Transmissive LCDs can be further separated into projection and direct-view. In a small-size, high-resolution LCD, the pixel size is around 40 m × 40 m. Here, the aperture ratio becomes particularly important because it affects the light throughput.13 To enlarge the aperture ratio, poly-silicon (p-Si) TFTs are commonly used because their electron mobility is about two orders of magnitude higher than that of amorphous silicon (a-Si). High mobility allows a smaller TFT to be used which, in turn, enlarges the aperture ratio. For the detailed structure of a TFT LCD, see Figure 4.1. For direct-view transmissive TFT LCDs, the pixel size (∼300 m × 300 m) is much larger than that of a microdisplay. Thus, a-Si is adequate although its electron mobility is relatively low. Amorphous silicon is easy to fabricate and has good uniformity. Thus, a-Si TFTs dominate the large-screen (>10 inches) LCD panel market. Similarly, reflective LCDs can also be divided into projection and direct-view displays. In projection displays using liquid-crystal-on-silicon (LCoS),14 the pixel size can be as small as ∼10 m × 10 m because of the high electron mobility of crystalline silicon (c-Si). In a LCoS device, the electronic driving circuits are hidden beneath the metallic reflector. Therefore, the aperture ratio can reach 90 % and the displayed picture is film-like. In contrast, most reflective direct-view LCDs use a-Si TFTs and a circular polarizer. Their sunlight readability is excellent, but they are not readable in dark ambient. Therefore, the application of reflective direct-view LCDs is rather limited. To maintain high-quality transmissive display and good sunlight readability, a hybrid TR-LCD has been developed. In a TR-LCD, each pixel is divided into two subpixels: one for transmissive and another for reflective display.15 In dark to normal ambient, the backlight is on and the TR-LCD works as a transmissive display. Under direct sunlight, the TR-LCD works in reflective mode. Therefore, its dynamic range is wide and its functionality does not depend on the ambient lighting conditions. TR-LCDs have been widely adopted in portable devices, such as mobile phones. For a detailed discussion of TR-LCDs, see Chapter 4.

1.4.2 Light-emitting diodes A LED is an electroluminescent device based on crystalline semiconductors.16 To convert electrical to optical power, one has to inject carriers into the LED through electrodes, and then they recombine to give light. The emission wavelength is mainly determined by the semiconductor materials, and can be fine tuned by device design. Since it is difficult to grow large-size single crystals, the wafer diameter of LEDs is limited to about 8 inches. After device processing, LEDs are diced from the wafer followed by the package process. The dimension of a single LED is typically several millimeters, which means the ‘pixel size’ of the LED is large. Hence, it is difficult to use a LED as a small display or it will have a very low resolution. An exception is to dice LED arrays from a wafer and use as a microdisplay with a size less than 1 to 2 inches. Due to their self-emissive characteristic, LEDs are commonly used for large displays, such as outdoor signages (single color, multicolor and full color), traffic signals and general lighting to replace light bulbs. Compared to conventional displays enabled by light bulbs, LED displays exhibit the advantages of lower

8

Introduction to Flat Panel Displays

power consumption, greater robustness, longer lifetime and lower driving voltage (so safer). There are also lots of outdoor screens with diagonals of over 100 inches which consist of millions of LED pixels. Rather than a display itself, a LED can also be used as the light source, such as the backlight module for a LCD, and general lighting. Compared to a conventional cold cathode fluorescent lamp (CCFL), which resembles a thin fluorescent tube, a LED exhibits a better color performance, longer lifetime and faster response. Another important driving force to the use of LEDs as LCD backlights is that the mercury in CCFLs is harmful to the environment. When using LEDs for general lighting applications, a broad spectrum is preferred to simulate natural light, such as sunlight, for obtaining a high color rendering of reflective objects (Section 2.3.5). This is quite different from the requirements for LED displays and LCD backlights, which usually need a narrow spectrum.

1.4.3 Plasma display panels The typical structure and operation principle for PDPs are similar to those of a fluorescent lamp. In the structure of a fluorescent lamp, two filament electrodes are formed in two ends of an inner glass tube. The wall of the inner glass tube is coated with phosphor. The cavity of the glass tube is filled with a gas mixture of argon and mercury. When a certain voltage is applied to the electrodes, plasma is generated from a gas discharge. Due to the energy level system of the plasma, ultraviolet (UV) radiation is generated with peak wavelength at λ = 254 nm. The phosphor of the fluorescent lamp is excited by the UV radiation which, in turn, emits light. PDPs use a similar operation mechanism to fluorescent lamps but the gases commonly used in PDPs are neon and xenon instead of the argon and mercury used in fluorescent lamps. Neon and xenon gases generate peak wavelengths at 147 and 173 nm which belong to the vacuum ultraviolet (VUV) region. VUV radiation can only propagate in a vacuum because it is strongly absorbed by air. Although the PDP structure is similar to a fluorescent lamp which is composed of two electrodes, phosphor and gases, an additional barrier rib structure is needed in PDPs to sustain the space between upper plate and lower plate.17 Because of the structure of the barrier rib, the unit cell size of PDPs cannot be made too small. In addition, PDP operation voltage is high because a typical plasma generation is needed. The high operation voltage demands a high voltage driver integrated circuit (IC) and results in a high cost of the electronics. However, PDPs exhibit a wider view angle, faster response time and wider temperature range than LCDs. In other words, PDPs remain good candidates for large-panel displays spanning from static pictures to motion pictures, from cold ambient to hot ambient and from personal use to public use. In addition to these performance advantages, PDPs can be fabricated with a low-cost and simple manufacturing process. For these reasons, many different PDP structures intended for a wide spectrum of applications have been developed.18−20

1.4.4 Organic light-emitting devices An OLED is also an electroluminescent device, like a LED, except its materials are organic thin films with amorphous structures.21 Amorphous organic material has a much lower mobility (typically five order of magnitude lower) than crystalline semiconductors, which results in a higher driving voltage of OLEDs. Also, the operation lifetime of OLEDs is one order of magnitude shorter than semiconductor LEDs. However, due to the amorphous characteristics, fabrication with large size (>40 inches) is possible. Since the conductivity of amorphous organic materials is very low, very thin organic films (100– 200 nm in total) are required to reduce the driving voltage to a reasonable value (i.e.