Flat Panel Displays - Advanced Organic Materials PDF
Preview Flat Panel Displays - Advanced Organic Materials
RSC MATERIALS MONOGRAPHS Flat Panel Displays Advanced Organic Materials S.M. Kelly Department of Chemistry, University of Hull, UK RSeC ROYAL SOCIETV OF CHEMISTRY ISBN 0-85404-567-8 A catalogue record for this book is available from the British Library. 0T he Royal Society of Chemistry 2000 All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review as permitted under the terms of the UK Copyright, Designs and Patents Act, 1988, this publication may not be reproduced,s tored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK,o r in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road Cambridge CB4 OWF, UK For further information see our web site at www.rsc.org Typeset by Paston PrePress Ltd, Beccles, Suffolk Printed by MPG Books Ltd, Bodmin, Cornwall Preface Liquid crystals have found wide commercial application over the last 25-30 years in electro-optical flat panel displays (FPDs) for consumer audio-visual and office equipment such as watches, clocks, stereos, calculators, portable telephones, personal organisers, note books and laptop computers. There are many other applications for liquid crystal displays (LCDs) such as information displays in technical instruments and in vehicles initially as clocks, then speedometers to a lesser extent and now increasingly as navigation and positional aids or entertainment consoles. They are also used in low-volume niche products such as spatial light modulators and generally as very fast light shutters. More importantly they have come to dominate the displays market in portable instruments due to their slim shape, low-weight, low voltage operation and low power consumption, see Table 1.1. LCDs are now starting to win market share from cathode ray tubes (CRTs) in the computer monitor market. The market share of LCDs of the total market for displays is expected to significantly increase over the next decade. There are a number of existing competing flat-panel display technologies, such as plasma display panels (PDPs), vacuum fluorescence displays (VFDs), inorganic semiconductor light- emitting diodes (LEDs), digital micromirror devices (DMDs) and field emission displays (FEDs). However, these have relatively small shares of the overall displays market, see Table 1.1. The most promising technology for FPDs being developed at the moment is represented by organic light-emitting diodes (OLEDs) using either low-molar-mass (LMM) materials (small molecules) or light-emitting polymers (LEPs). The first production lines for LEP technology have recently been commissioned and products are commercially available. However, in spite of these competing FPDs, especially OLEDs which are expected to exhibit significant growth, the value of LCDs is still expected to exceed that of CRTs in the near future. Manufacturing facilities for flat-panel displays are very capital intensive, e.g. a plant for TN (twisted nematic)-LCDs with active matrix addressing can cost upwards of $1 billion. As a consequence of a combination of factors, such as the capital already invested in LCD plants, which have to be depreciated, the steadily decreasing unit cost of LCDs and the expanding market requirement for them in existing products, it will take many years for competing technologies to gain a significant market share in the V vi Preface displays market in general. In particular, LCDs can be expected to maintain a dominant position in the market for portable displays. The successful development of LCD technology was dependent on parallel developments and progress in an unusual combination of scientific disciplines such as synthetic organic chemistry, physics, electronics and device engineering. These include improvements in batteries, polarisers, electrodes, CMOS drivers, spacers, alignment layers and nematic liquid crystals. These developments were made in response to a clear market requirement for a low-voltage, low-power- consuming flat-panel display screen for portable, battery-operated instruments in order to display graphic and digital information of ever more increasing volume, speed and complexity. In this monograph we will attempt to illustrate this development using the most important types of LCD currently in large- scale manufacture. We will not describe LCDs using smectic liquid crystals, although they may have the potential to become a major commercial product, since they have failed to make a commercial breakthrough after more than two decades of research and development. All of these types of LCDs can be modified to use nematic gels or polymer-dispersed liquid crystals. Therefore, these will not be dealt with here in any detail as the general principles of operation and electro-optical effects are essentially the same. OLEDs using electroluminescent small molecules have been in continuous development for over 30 years and intensive development for at least the last decade. Only now are OLEDs using low-molar-mass materials being manufac- tured on any significant scale. The development of OLEDs using conjugated organic polymers has been able to profit from the know-how and technology developed for low-molar-mass OLEDs. However, the time from discovery to manufacture has been much shorter, i.e.a bout 10 years. The first pilot plants for OLEDs using LEPs have just been commissioned at Philips and UNIAX and commercial products incorporating LEPs from Dow Chemicals, Clariant or Covion are to expected on the market in 2001 based on technology under licence, at least in part, from CDT. The development and successful commercialisation of LCDs and OLEDs as flat panel displays in consumer and industrial products and instruments is illustrative of the general dependence of consistent improvement in the performance of flat panel displays as a consequence of the invention, laboratory preparation, evaluation, optimisation, scale-up and then large-scale manufac- ture of organic compounds and their mixtures in high purity and at acceptable cost with the required spectrum of physical properties. This development in materials chemistry will be described in detail in the following chapters. The interdependence of technologies in electro-optics, .electronics and organic chemistry is illustrated by the imaginative use of liquid crystals in OLEDs as charge-carrier transport layers and as electroluminescent materials as the source of plane polarised light for hybrid OLED-LCDs with intrinsically higher brightness. The fundamental electro-optical principles of LCDs and OLEDs with their relative advantages and disadvantages for a diverse range of specific applica- tions are described in this monograph. These specifications then prescribe the Preface vii relative and absolute magnitude of a spectrum of physical properties for nematic liquid crystals and electroluminescent organic materials to fulfil in order for these types of flat panel displays to function as efficiently as possible and attain their maximum potential for a particular application. The nature of the nematic liquid crystalline state and the origin of electroluminescence in small organic molecules and organic conjugated polymers is described suffi- ciently to understand the correlation between molecular structure and those physical properties of relevance to LCDs and OLEDs, at least where such correlations are understood. This monograph concentrates primarily on the developments in the design and synthesis of these two different classes of organic materials specifically for use in the two most important types of FPDs using organic compounds. Therefore, the theoretical, especially mathematical, background to the phenomenon of liquid crystallinity and organic electrolumi- nescence are kept to a necessary minimum. Contents Chapter 1 Flat Panel Displays 1 1 Flat Panel Displays 1 Flat Panel Cathode Ray Tubes 2 Plasma Display Panels 3 Vacuum Fluorescence Displays 3 Field Emission Displays 4 Digital Micromirror Devices 4 Inorganic Semiconductor Light-Emitting Diodes 5 Organic Light-Emitting Diodes 5 Liquid Crystal Displays 6 2 Conclusions 7 3 References 8 Chapter 2 Liquid Crystals and Liquid Crystal Displays (LCDs) 9 1 Physical Properties of Nematic Liquid Crystals 20 2 Physical Properties of Liquid Crystals 20 Optical Anisotropy (Birefringence) 20 Elastic Constants 22 Viscosity 23 Dielectric Anisotropy 24 3 Liquid Crystal Displays 25 4 Cell Construction of LCDs 27 5 Addressing Methods for LCDs 30 Direct Addressing 30 Multiplex Addressing 30 Active Matrix Addressing 32 6 Organic Polymer Alignment Layers 33 7 Organic Compensation Films for LCDs 38 8 References 40 ix X Cont ents Chapter 3 Liquid Crystal Displays Using Nematic Liquid Crystals 45 1 Introduction 45 2 Dynamic Scattering Mode LCDs 45 Nematic Materials 48 3 Cholesteric-Nematic Phase Change (CNPC) LCDs 51 Chiral Nematic Materials 52 4 Electrically Controlled Birefringence (DAP/HN/ECB) LCDs 53 Nematic Materials of Negative Dielectric Anisotropy 56 5 Twisted Nematic LCDs 60 Nematic Materials of Positive Dielectric Anisotropy 66 Nematic Materials for Direct Addressing 66 Nematic Materials for Multiplex Addressing 74 Nematic Materials for Active Matrix Addressing 81 6 Super Twisted Nematic LCDs 85 Super Birefringent Effect LCDs 88 Electro-optical Performance of STN-LCDs 91 Temperature Dependence of Electro-optical Performance of STN-LCDs 92 Black-and-white STN-LCDs 93 Nematic Materials for STN-LCDs 93 Polar Nematic Materials for STN-LCDs 94 Apolar Nematic Materials for STN-LCDs 99 7 Guest-Jost LCDs 103 Negative Contrast Heilmeier and Zanoni GH-LCDs 110 White and Taylor GH-LCDs 112 Negative Contrast White and Taylor GH-LCDs 113 Positive Contrast White and Taylor GH-LCDs 114 Super Twisted Nematic (STN) GH-LCDs 115 Dichroic Dyes-Guests 117 Positive Contrast Dyes 117 Negative Contrast Dyes 121 Nematic Liquid Crystals-Hosts 122 Nematic Liquid Crystal Hosts of Positive Dielectric Anisot r opy 122 Nematic Liquid Crystal Hosts of Negative Dielectric Anisotropy 123 8 In-Plane Switching (IPS) LCDs 124 Nematic Materials 126 9 References 127 Chapter 4 Photoluminescence and Electroluminescence from Organic Materials 134 1 Introduction 134 2 Photoluminescence from Organic Materials 136 Cont ents xi 3 Electroluminescence from Organic Materials 138 4 References 14 4 Chapter 5 Organic Light-Emitting Diodes Using Low-Molar- Mass Materials (LMMMs) 147 1 Introduction 147 2 Monolayer Organic Light-Emitting Diodes Using LMMMs 150 3 Bilayer OLEDs Using LMMMs 151 4 Trilayer OLEDs Using LMMMs 154 5 Low-Molar-Mass Organic Materials for OLEDs 155 Non-Emissive Electron Transport Layers (ETLs) 156 Non-Emissive Hole Transport Layers (HTLs) 156 Liquid Crystals as Charge-Carrier Transport Layers 158 Columnar Liquid Crystals as HTL 160 Smectic Liquid Crystals as HTL and ETL 163 Liquid Crystals as Electroluminescent Materials 165 Green Electroluminescent, Low-Molar-Mass Organic Materials 166 Red Electroluminescent, Low-Molar-Mass Organic Materials 168 Blue Electroluminescent, Low-Molar-Mass Organic Materials 170 6 Performance of OLEDs Using LMMMs 173 Stability of OLEDs Using LMMMs 173 7 References 175 Chapter 6 Organic Light-Emitting Diodes Using Light- Emitting Polymers 179 1 Introduction 179 2 Monolayer OLEDs Using LEPs 179 Light-Emit ting Polymers (LEPs) 184 3 Bilayer OLEDs Using LEPs 196 Polymers as ETLs 199 4 Trilayer OLEDs Using LEPs 206 5 Polarised Light Emission from OLEDs 208 6 Performance of OLEDs Using LEPs 212 Stability of OLEDs Using LEPs 215 7 References 216 Conclusions and Outlook 222 Subject Index 229 CHAPTER 1 Flat Panel Displays 1 Flat Panel Displays The cathode ray tube (CRT) is still the dominant electro-optical display device today, although this is expected to change in the next few years. The CRT is still the benchmark display in terms of cost and performance. There are many areas of the market for electro-optic displays where one or more of the competing flat- panel display technologies offers a superior technological performance to a CRT, see Table 1.1.' Perhaps the most important are portable applications where the combination of physical properties, such as low power consumption, low operating voltage and light-weight of liquid crystal displays (LCDs) is clearly superior to that of CRTs. Most flat panel displays are emissive displays, i.e. they emit light without requiring absorbing polarisers like LCDs. Therefore, their brightness and viewing angle dependence are fundamentally superior to those of LCDs, which modulate the intensity of transmitted light from some independent internal or external light source. Therefore, they must be used with a back-light where insufficient ambient light is present. Light-emitting flat panel displays (FPDs) offer superior performance in poor ambient light conditions or in the dark whereas reflective FPDs are clearly superior in a bright light Table 1.1 Estimated world-wide market share offlat panel displays in the year 2000' Type offlat panel display (FPD) Number of units Liquid crystal displays (LCDs) with segmented characters 1 470 000 000 Super-twisted nematic liquid crystal displays (STN-LCDs) 45 000 000 Liquid crystal displays (AM-TFT-LCDs) with active matrix 48 000 000 thin film transistor addressing Organic electroluminescent displays (OLEDs) 300 000 Plasma display panels (PDPs) 630 000 Field emission displays (FEDs) 540 000 Inorganic semiconductor light-emitting diodes (LEDs) 181 000000 Vacuum fluorescent displays (VFDs) 166 000 000 Total 1 900 000 000 1 2 Chapter I environment. The former are not visible in the dark and the latter are washed out in bright light. A flat panel display may be several millimetres or several centimetres thick. There are many technologies capable of being used to create a flat panel display. The most important flat panel displays are described briefly below; the two most important are LCDs and OLEDs, which are the subject of this monograph. Both require organic materials in order to function. Therefore, these are described in much more detail. A high-information-content display must be capable of displaying an equiva- lent amount of information as a CRT of comparable size. The major segments of the displays market in general for CRTs are as television screens and static, i.e. non-portable computer monitors. Emissive displays are intrinsically brighter than commercial LCDs currently available, even those with a strong back-light. The use of crossed, absorbing polarisers limits the maximum intensity of incident light transmitted to 25%. Therefore, a large amount of research and development effort is being devoted to optimising internal reflectors, which replace one polariser, optical retarders and different types of LCDs, which use either one polariser or no polarisers. Advances in optimising the physical properties in organic materials such as nematic liquid crystals, electroluminescent small molecules and polymers are the topic of this monograph. Oligomers are intermediate compounds between low-molar-mass materials (small molecules) and polymers and serve as model compounds for studying polymers without the polydispersity of the latter. However, they are not used commercially, and probably will not be in the foreseeable future. Therefore, they will not form part of this monograph. Parallel developments in device peripherals such as organic polymer alignment layers, organic optical retarders and polarisers are also important. These are also described briefly. However, a satisfactory electro-optic performance of a particular display type is not always a sufficient criterium for commercialisa- tion. The properties of other electro-optic components, such as the cost of drivers can play a decisive role in deciding whether a particular display technology is manufactured at all, occupies a niche in the displays market or is manufactured in large volumes. However, these parameters often depend on the fundamental mode of operation of a particular display technology. These are described and compared briefly in this chapter for FPDs in general and in much more detail in Chapters 2-6 for LCDs and organic light-emitting diodes (OLEDs). Flat-Panel Cathode Ray Tubes2 The production of flat-panel cathode ray tubes (CRTs) is essentially a fabrica- tion issue. The basic principle of operation is the same as a standard CRT. Electrons are emitted from a hot cathode. These are guided by a magnetic field to the glass screen coated in a layer of phosphorescent material. Upon impact the energy of the electron is transferred to the phosphor and light is emitted. A regular pattern of red, green and blue phosphors creates a dense pattern of
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