Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2000-12-07

2003-02-04

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660

Reexamination Certificate

active

06514580

ABSTRACT:

The present invention relates to a liquid-crystalline medium, to the use thereof for electro-optical purposes and to displays containing this medium, in particular displays having active matrix addressing based on the ECB effect.
The principle of electrically controlled birefringence, the ECB effect or alternatively DAP effect (deformation of aligned phases), was described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon, “Deformation of nematic liquid crystals with vertical orientation in electrical fields”, Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Lett. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869)
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio between the elastic constants K
3
/K
1
, high values for the optical anisotropy &Dgr;n and values for the dielectric anisotropy &Dgr;∈ of from about −0.5 to about −5 in order to be suitable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment.
Technical use of this effect in electro-optical display elements requires LC phases which must satisfy a multiplicity of requirements. Particularly important here are chemical resistance to moisture, air and physical effects, such as heat, radiation in the infrared, visible and ultraviolet regions and direct and alternating electric fields.
Technically suitable LC phases are furthermore required to have a liquid-crystalline mesophase in a suitable temperature range and low viscosity.
None of the series of compounds having a liquid-crystalline mesophase which have been disclosed hitherto includes a single compound which meets all these requirements. In general, therefore, mixtures of from 2 to 25, preferably from 3 to 18, compounds are prepared in order to obtain substances which can be used as LC phases. However, optimum phases could not be prepared easily in this way, since no liquid-crystalline materials of significantly negative dielectric anisotropy were hitherto available.
Matrix liquid-crystal displays are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e. transistors). This is then referred to as an “active matrix”, and a distinction can be made between two types:
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
The use of single-crystal silicon as substrate material limits the display size, since even modular assembly of various part-displays results in problems at the joints.
In the case of more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A distinction is made between two technologies: TFTs comprising compound semiconductors, 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 its 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 expanded to fully colour-compatible displays, in which 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 polarizers in transmission and are backlit.
The term MLC displays here covers any matrix display containing integrated non-linear elements, i.e., besides 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 TVs) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in MLC displays due to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display drops, and the problem of after-image elimination can occur. Since the resistivity of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important for displays which must have acceptable resistance values over a long service life.
It is furthermore important that the resistivity increases as little as possible with increasing temperature and after heating and/or UV exposure. The low-temperature properties of the mixtures of the prior art are also particularly disadvantageous. It is required that crystallization and/or smectic phases do not occur, even at low temperatures, and that the temperature dependence of the viscosity is as low as possible. The MLC displays of the prior art thus do not satisfy today's requirements.
EP 0 474 062 discloses MLC displays based on the ECB effect. However, the LC mixtures described therein, which are based on 2,3-difluorophenyl derivatives containing an ester, ether or ethyl bridge, have low “voltage holding ratio” (HR) values after UV exposure.
There thus continues to be a great demand for MLC displays which have very high resistivity at the same time as a broad operating temperature range, short response times, even at low temperatures, and a low threshold voltage, and which do not have the disadvantages described above, or only do so to a reduced extent.
It is an object of the invention to provide MLC displays based on the ECB effect which do not have the abovementioned disadvantages, or only do so to a lesser extent, and preferably at the same time have very high resistivities and low threshold voltages.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that objects of the invention can be achieved by using media according to the invention in displays.
The invention thus includes a liquid-crystalline medium based on a mixture of polar compounds having negative dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I
in which R
1
is an alkoxy group having up to 12 carbon atoms.
The media according to the invention have very high HR values, low threshold voltages and in particular very good low-temperature stability and at the same time high clearing points.
The threshold voltage (V
o
) of the media according to the invention typically is in the range from 1.25 to 2.5V, preferably in the range from 1.5 to 2.2 V and most preferably 1.75 to 2.05 V. The clearing point of the inventive media is preferably in the range from 85° C. to 120° C., preferably from 70° C. to 110° C., and most preferably in the range from 80° C. to 101° C.
The HR after 5 minutes at 100° C. is in the range from 95% to 100% preferably 97% or greater most preferably 98% or greater. The media are stable against storage at −30° C. in test cells for preferably more than 500 h more preferably for more than 1000 h.
Some preferred embodiments are mentioned below:
a) A medium which additionally comprises one or more compound

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