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Flat Panel Displays
Advanced Organic Materials
By S.M. Kelly The Royal Society of Chemistry
Copyright © 2000 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-567-9
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 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 equivalent 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 commercialisation. 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 Tubes
The production of flat-panel cathode ray tubes (CRTs) is essentially a fabrication 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 pixels, which allows the generation of full colour. A gas plasma discharge may also be used as a source of electrons.
The high voltage requirement, i.e.< 200 V, and power consumption are the main restrictions to the utilisation of flat CRTs due to their incompatibility with battery operation over an extended period of time due to the high voltages and power consumption required. Other flat panel displays are more suitable and are usually preferred for portable, hand-held applications. The difficulty associated with manufacturing flat, rectangular large-area cathode ray tubes is an added problem preventing their use as screens for portable instruments. Such large CRTs would still be relatively heavy despite their relatively flat, thin construction due to the weight of the thick-walled glass vacuum tube required for mechanical stability.
Plasma Display Panels
Plasma display panels (PDPs) based on an emissive gas discharge phenomenon were invented over 30 years ago. Indeed large-area plasma panel displays have been commercially available since 1970. Monochrome PDPs use visible light emitted under the action of a small electric current flowing between the electrodes. Full colour displays use UV emission at 150 nm or 173 nm to address an alternating array of red, green and blue phosphorescent strips. Short response times and steep electro-optic transmission curves facilitate the fabrication of very large-area, high-information-content plasma display panels (> 60" diagonal). However, their high cost and substantial size and weight has restricted their acceptance for the consumer market. Moreover, flat-panel plasma displays require a large number of expensive, high-voltage, alternating current (AC) or direct current (DC) drivers. Furthermore, the high operating voltages and power consumption prohibit their use in portable, battery-operated applications. Therefore, PDPs have traditionally been used for non-portable, high-cost, low-volume display applications, which are far less cost-sensitive, such as industrial, commercial or military applications. LCDs with a very large area and high information content, e.g. for TVs with a 40" diagonal and above, are still very expensive and not competitive with PDPs. However, the unit-cost of large-area, high-information-content PDPs is also steadily decreasing. Consequently, the acceptance of PDPs as very large televisions and monitors in the consumer market is gradually increasing. Unfortunately the large pixel size (≈1 mm) gives rise to relatively low resolution and a grainy appearance for short viewing distances.
Vacuum Fluorescence Displays
Vacuum fluorescent displays (VFDs) are strongly related to flat-panel CRTs. Electrons are ejected from a cathode source, traverse a vacuum and then strike a pattern of triodes with individual anodes covered in red, green and blue phosphorescent material. However, the operating voltages, e.g. 12 V, and power consumption are much lower than those found for CRTs and PDPs. The fabrication costs of VFDs are also relatively low. They are rugged with long operating lifetimes. Therefore, small VFDs have been manufactured in large volume for several decades for a variety of applications, e.g. as part of car dashboards or orientation and navigation systems.
Once again the major problems associated with the commercialisation of large VPDs is their manufacture. These include increasing weight of the glass tubes, which are necessarily thick walled. Precise spatial matching of the cathode and anode matrices is also problematical at large display size. Multiplex addressing of larger displays results in unacceptably high operating voltages, e.g. 100 V, for battery-operated devices. VFDs with active matrix addressing use much lower operating voltages, but are correspondingly more expensive.
Field Emission Displays
Field emission displays (FEDs) utilise a very similar technology to the CRT tube, i.e. electron-impact induced light emission from a flat screen coated with alternating strips of red, green and blue organometallic phosphors. However, the main difference is that the electrons are not generated as a beam from a hot cathode, which is then directed by a magnetic field towards the screen, as in a CRT, but are emitted individually from a dense matrix of pointed pixel electrodes covering the active cathode area of the display. The narrow gap between the flat phosphor screen on top of the anode and the planar emission cathode layer and substrate is small, e.g. 2 mm. Therefore, considerably lower voltages are required for FEDs than for CRTs. However, the current density is significantly higher. This mode of operation allows light-weight flat panels to be constructed with a relatively low power consumption, wide viewing angle, high brightness, video-rate addressing and ruggedness. The contrast is generally relatively low (> 20:1). Flat-panel FEDs are available as monochrome and full-colour commercial products, although with a relatively small screen size (5" diagonal) for the moment. Larger prototypes have been demonstrated (12" diagonal). However, the most important factor holding back the wide-scale adoption of FEDs as a flat-panel display is the high operating voltage (> 20 V). This inhibits their use in portable device applications due to short battery lifetimes.
Digital Micromirror Devices
Digital light processing devices use micro-electromechanical systems referred to as a digital micromirror device (DMD). An array of rectangular polished aluminium mirrors, e.g. 640 x 480 pixels, each individual mirror situated above a CMOS memory chip, can be addressed by an applied voltage to reflect light through a microlense in the on-state or deflect light in the off-state. This is a bistable, black-on-white memory effect compatible with video-rate addressing with high contrast (> 100:1) and high brightness (≈ 300–400 lumens). The mirrors are fabricated in a series of lithographic steps on a single substrate. Grey scale can be realised using pulsed applied voltages with full colour achieved using colour filters. Therefore, DMDs are used as high-information-content front or rear projection devices, especially for home cinema and commercial cinema or stadia applications. However, they are essentially projection devices and the size and weight of the projector and light source are too large for portable applications.
Inorganic Semiconductor Light-Emitting Diodesg
Light-emitting diodes (LEDs) are flat panel displays which emit light under the action of an electric current passing through the emissive layer. Electroluminescence in inorganic semiconductors was discovered before the corresponding effect in organic materials was found. Consequently the first commercial alpha numeric display devices fabricated in the early 1960s used electroluminescence inorganic semiconductor materials, such as GaAs/P or ZnS/Mn on a glass substrate sandwiched between two dielectric layers. These separate the emissive material from the electrodes and limit the amount of current flowing through the display. Pulses of alternating current result in light emission. Monochrome semiconductor inorganic LEDs are manufactured on a large scale and are found in many electronic instruments.
High-information-content LEDs using inorganic semiconductors have been produced with .active matrix addressing using thin film transistors on a silicon substrate. However, the size of the displays is limited by the amount of power consumed by the large number of pixels due to the high capacitance at each individual pixel. The power consumption of a large-area LED, such as a notebook computer screen, would be considerable, e.g. 100 W. Other addressing problems, such as non-uniform grey scale due to the steep curve of brightness against voltage, also become disproportionately acute with increasing display size.
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Excerpted from Flat Panel Displays by S.M. Kelly. Copyright © 2000 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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