Table Of ContentRSC
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
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0T he Royal Society of Chemistry 2000
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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
