Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1996-06-24
1998-08-04
Gross, Anita Pellman
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
349 92, 349 93, G02F 11333
Patent
active
057902178
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a liquid crystal display(LCD), and particularly to a novel LCD fabricated by dispersing ferroelectric liquid crystal(FLC) in a polymer matrix.
BACKGROUND ART
A Braun tube is generally adopted as the display for an image displaying apparatus such as a television. The Braun tube, however, has excessively large dimensions, and exhibits very high power consumption. Thus, the flat panel display(FDP) recently comes into wide use especially for the field of portable displays. Among these FDPs, the LCD is most widely used. The LCD is a typical non-emissive display utilizing electro-optic characteristics of a liquid crystal(LC), for example the dielectric anistropy.
Liquid crystals are divided into smectic, nematic and cholesteric phases according to their molecular structures. Among them, smectic and nematic LCs can be used for the purpose of display, but the smectic LC is not easily be aligned and the alignment is not stable under heat or shock. Most of practical LCDs, therefore, adopt nematic LCs.
As the nematic LCD, TN(Twisted Nematic) or STN(Super Twisted Nematic) LCDs have been used. TN or STN LCDs are constructed by aligning LC molecules to relatively twisted directions at upper and lower surfaces of the LC layer, and are already widely used. But, they cannot fulfill the rapid response speed and the viewability, such as a wide viewing angle, necessary for achieving the high resolution and a large screen. Accordingly, full-color or dynamic images cannot easily be achieved by these nematic LCDs. Moreover, they cannot be easily driven, as the optical response of simple matrix type nematic LCDs, depends upon the square of the applied voltage to result in little level difference between selected and non-selected pixels. As a result, the nematic LCD can only be applied to a portable television or a monitor of a notebook computer with unsatisfactory image quality. A TFT(Thin Film Transistor) LCD of an active matrix type has been developed, which enables a high speed action by independently switching each pixel by an array of driving elements. However, it has a highly complicated structure which results in high production cost and difficulty in fabricating a large screen.
All of the above described LCDs require the alignment of LC molecules in prescribed directions at boundary surfaces of the LC, and use separate polalizers to narrow the viewing angle and deteriorate the brightness. Thus, a Polymer-Dispersed (PD) LCD has been developed. It adopts the structure of dispersing submicron-sized nematic LC droplets in a polymer matrix. As the nematic LC has positive dielectric anistropy responding to the external electric field, it maintains an opaque state by scattering incident light when the electric field is not applied. When the electric field is applied, LC molecules of LC droplets are aligned in one direction to be a transparent state, and allow light to pass. As the PD LCD does not adopt an polarizer it results in a simple structure, a high brightness, and an wide viewing angle. Moreover, its polymer structure enables a flexible large LCD of a high strength at a low production cost. In the PD LCD, however, the driving voltage is applied to a LC through the polymer matrix, thereby revealing a high operating voltage, a large power consumption, and a low contrast. As a result, it can be utilized to control the lighting of a window, and not suitable for the image display such as a television.
All of above described LCDs utilize the dielectric anistropy of a LC, and the electro-optic characteristics thereof depend upon only the strength of applied voltage, regardless of its polarity. On the contrary, ferroelectric material is spontaneously polarized even if the electric field is not applied to exhibit a so-called bistable structure with two stable states. Recently, there has been reported that a smectic LC of a chiral-C phase exhibits ferroelectricty with a very high spontaneous polarization, and it has been adopted to a Ferroelectric LCD(FLCD).
In the FLCD, the dire
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Lee et al., "Fast Linear Electro-Optical Switching Properties of Polymer-Dispersed Ferroelectric Liquid Crystals", Applied Physics Letters, vol. 64, No. 6, Feb. 7, 1994, pp. 718-720.
Lee Kye Hun
Lee Sin Doo
Suh Seong Woo
Gross Anita Pellman
Orion Electric Co. Ltd.
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