Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2000-06-19

2002-10-15

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670, C568S583000, C568S664000, C568S669000, C568S670000

Reexamination Certificate

active

06465059

ABSTRACT:

The present invention relates to a liquid-crystalline medium and to its use for electro-optical purposes and for displays containing said medium.
Liquid crystals are used especially as dielectrics in display devices, as the optical properties of such substances can be affected by a voltage applied. Electro-optical devices on the basis of liquid crystals are very well known to those skilled in the art and can be based on various effects. Examples of such devices include cells with dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells (supertwisted nematic), SBE cells (superbirefringence effect) and OMI cells (optical mode interference) . 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 with respect to electrical fields and electromagnetic radiation. Additionally, the liquid crystal materials should have a low viscosity and in the cells give rise to short response times, low threshold voltages and high contrast.
Furthermore they should, at standard operating temperatures, i.e. in as wide a range as possible below and above room temperature, have a suitable mesophase, for example a nematic or cholesteric mesophase for the abovementioned cells. Since liquid crystals as a rule are used as mixtures of a number of components, it is important for the components to be readily miscible with one another. Other properties such as electrical conductivity, dielectric anisotropy and optical anisotropy must meet various requirements, depending on the cell type and field of application. For example, materials for cells having a twisted nematic structure should exhibit positive dielectric anisotropy and low electrical conductivity.
For example, matrix liquid crystal displays (matrix LCDs) comprising integrated nonlinear elements to switch individual pixels ideally require media having large positive dielectric anisotropy, broad-range nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability and low vapour pressure.
Such matrix liquid crystal displays are known. Suitable nonlinear elements for individually switching the separate pixels include active elements (i.e. transistors) . Such an arrangement is referred to as an “active matrix”, allowing a distinction between two types:
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as the substrate.
2. Thin-film transistors (TFT) on a glass sheet as the substrate.
The use of monocrystalline silicon as a substrate material limits the display size, since even modular assembly of separate subdisplays gives rise to problems at the joints.
In the 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 composed of compound semiconductors such as CdSe, or TFTs on the basis of polycrystalline or amorphous silicon. Work on the latter technology is being carried out worldwide with great intensity.
The TFT matrix is applied on the inside of the one glass sheet of the display, while the other glass sheet on its inside carries the transparent counter-electrode. Compared with the size of the pixel electrode, the TFT is very small and hardly interferes with the image. This technology can also be extended to full color capability pictorial representations, where a mosaic of red, green and blue filters is arranged in such a way that filter elements are located opposite switchable picture elements in a one-to-one arrangement.
The TFT displays usually function as TN cells comprising crossed polarizers in transmission and employ backlighting.
The term matrix LCDs in this context encompasses any matrix display comprising integrated nonlinear elements, i.e. in addition to the active matrix it also includes displays comprising passive elements such as varistors or diodes (MIM=metal-insulator-metal).
Matrix LCDs of this type are suitable, in particular, for TV applications (e.g. portable television) or for high information level displays for computer applications (laptop) and in motor vehicle or aircraft production. In addition to problems regarding angular dependence of contrast and switching times, matrix LCDs present difficulties owing to insufficiently high 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 et seq., Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 et seq., Paris]. As the resistance decreases, the contrast of a matrix LCD display deteriorates, and the problem of “afterimage elimination” can arise. As the resistivity of the liquid crystal mixture generally decreases over the lifetime of a matrix LCD, owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important to achieve acceptable service lives. Particularly with low-voltage mixtures it has hitherto been impossible to achieve very high resistivities. Moreover, it is important for the resistivity to exhibit as low an increase as possible with increasing temperature and after thermal exposure and/or exposure to UV. A further particularly disadvantageous feature are the low-temperature properties of the prior art mixtures. The requirement is for no crystallization and/or smectic phases to occur even at low temperatures and for the viscosity temperature dependence to be as small as possible. The prior art matrix LCDs therefore do not meet present-day requirements.
There is therefore still a great need for matrix LCDs having very high resistivity and at the same time a wide operating temperature range, short switching times even at low temperatures, and a low threshold voltage, which do not exhibit these drawbacks or exhibit them only to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desirable which permit the following advantages in these cells:
extended nematic phase domain (especially towards low temperatures)
switchability at extremely low temperatures (outdoor use, motor vehicles, avionics)
increased resistance to UV radiation (extended lifetime)
The media available from the prior art do not permit these advantages to be achieved while at the same time maintaining the other parameters.
For supertwisted cells (STN), media are desirable which permit higher multiplexability and/or lower threshold voltages and/or wider nematic phase domains (especially at low temperatures). For this purpose, a further expansion of the available parameter space (clearing point, transition smectic-nematic or melting point, viscosity, dielectric parameters, elastic parameters) is urgently required.
It is an object of the invention to provide media especially for such matrix LCDs, TN or STN displays which do not exhibit the abovementioned drawbacks or exhibit them only to a lesser extent, and preferably at the same time have very high resistivities and low threshold voltages.
We have found that this object can be achieved if displays employ media according to the invention.
SUMMARY OF THE INVENTION
The invention therefore relates to a liquid-crystalline medium on the basis of a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula IA
and one or more compounds of the formula IB
where
R, R* and R** each, independently of one another, are an alkyl or alkenyl radical having from 1 to 12 C atoms which is unsubstituted, singly substituted by CN or CF
3
or at least singly substituted by halogen, up to perhalogenated and, with the additional option of one or more CH
2
groups in these radicals, independently of one another in each case, being replaced by —O—, —S—,

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