Liquid-Crystal and Electrophoretic Displays

The hrst reported OTFT display was a 16 x 16 pixel electrophoretic display demonstrated by researchers at Bell Laboratories and E Ink Corporation in 2001 [19]. Fignre 6.4.9 shows operation of the 5 in. display, which used pentacene TFTs on a flexible film of polyethylene terephthalate (PET). Later in 2001 AMLCDs nsing 

FIGURE 6.4.8 (See color insert following page 468.) Threshold voltage and mobility uniformity maps for a 1- x 1-cm 200-OTFT array using pentacene devices with a SiOj gate dielectric. 

FIGURE 6.4.10 A video-rate 64 x 64 PDLC OTFT display on a glass substrate using a solution-deposited polythienylenevinylene precursor film that was converted into a semiconductor. (From Huitema, H.E.A. et al., Nature, 414, 599, 2001. With permission.) 

2 Electro-Optic Behavior of Twisted-Nematic Liquid Crystals 

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Field-driven display elements such as liquid crystals and electrophoretic display materials (of which e-Ink is the leading example) are typically ma-trixed to create large panel sizes with a manageable number of external contacts. For a 1,000,000 pixel display, for example, a matrixed panel can use 2,000 contacts 1000 rows and 1000 columns. 

The demands that an AMOLED display places on TFT performance are more stringent than those placed by liquid-crystal or electrophoretic displays. Table 6.4.3 compares the TFT requirements of high-resolution displays of these types. The different TFT requirements arise from the fact that LCD and electrophoretic display pixels have a single TFT that is used as a switch, while an AMOLED pixel has one or more TFTs used as a switch, plus one or more used to drive current. A transistor used as a switch typically has stringent requirements for off-current because it must not leak significant charge when it is off, but the requirements for mobility, uniformity, and stability are modest. A transistor that is used to drive current has stringent requirements for mobility, uniformity, and stability, but off-current requirements are lenient. Because the AMOLED pixel has both types of... 

The method to manufacture cost-effective displays is to use proper processes for mass production or on a large scale. One exanple is a roll-to-roll process which needs a flexible substrate to be coated or printed display materials. Here, an important issue is to provide a monolayer of display materials. In order to supply this monolayer, a variety of methods have been tried for encapsulated cholesteric liquid crystals by Kent Display, microencapsulated electrophoretic display films by E-ink Coiporation, and microcup shapes by SiPiX. In the case of the first and second types, encapsulation processes are a significant step. 

For example, active-matrix backplanes for reflective displays using electrophoretic media is a potential application for large-area electronics. Electrophoretic media are usually charged colored particles suspended in an insulating fluid [35-37]. When the fluid is held in an electric field between two conductive plates, the particles will move between the plates creating a dark or a reflective state. Because the contrast in an electrophoretic display is because of reflection, no backlight is required and the display has relatively low-power consumption compared with a transmissive liquid-crystal display. 

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