Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Patent
1998-02-26
2000-07-04
Wu, Shean C.
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
25229961, 25229963, 25229901, C09K 1934, C09K 1930
Patent
active
060835737
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The present invention relates to a liquid-cry stalline medium, to the use thereof for electro-optical purposes, and to displays containing this medium.
Liquid crystals are used, in particular, as dielectrics in display devices, since the optical pro perties of such substances can be modified by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN (twisted nematic) cells, STN (supertwisted nematic) cells, SBE (superbirefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Further more, the liquid-crystal materials should have relatively low viscosity and give short addressing times, low threshold voltages and high contrast in the cells.
Furthermore, they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the abovementioned cells, at conventional operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used in the form of mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy, must satisfy different requirements depending on the cell type and area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, media of large positive dielectric anisotropy, broad nematic phases, relatively low bire fringence, very high resistivity, good UV and temperature stability and low vapor pressure are desired for matrix liquid-crystal displays having integrated non-linear elements for switching individual pixels (MLC displays).
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used for individual switching of individual pixels are active elements (i.e. transistors). This is then referred to as an "active matrix", and a differentiation can be made between two types: substrate.
Use of monocrystalline silicon as the substrate material limits the display size, since even modular assembly of the various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out worldwide on the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on the inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully color-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a way that each filter element is located opposite a switchable pixel.
The TFT displays usually operate as TN cells with crossed poarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display containing integrated nonlinear elements, i.e., in addition to the active matrix, also displays containing passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for
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patent: 5322638 (1994-06-01), Schadt et al.
patent: 5389289 (1995-02-01), Rieger et al.
patent: 5874022 (1999-02-01), Kudo et al.
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Poetsch Eike
Schuler Brigitte
Tarumi Kazuaki
Merck Patent Gesellschaft mit beschrankter Haftung
Wu Shean C.
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