Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2002-04-22

2004-07-20

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660

Reexamination Certificate

active

06764723

ABSTRACT:

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds having negative dielectric anisotropy, in particular for electro-optical displays having active matrix addressing based on the ECB effect.
The principle of electrically controlled birefringence, the ECB effect, or the 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).
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 An and values for the dielectric anisotropy &Dgr;∈ of −0.5 to −5 in order to be usable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have homeotropic edge alignment.
A newer variant of ECB displays are active matrix displays based on the VA (vertically aligned) effect, also known as VAN (vertically aligned nematic), or the VAC (vertically aligned cholesteric) effect. VA displays have been described, inter alia, in S. Yamauchi et al., SID Digest of Technical Papers, pp. 378ff (1989), and VAC displays have been described in K. A. Crabdall et al., Appl.Phys.Lett. 65, 4 (1994).
Like the ECB displays which were already known earlier, the more recent VA and VAC displays contain a layer of liquid crystalline medium with a negative dielecgric anisotropy &Dgr;∈ between two transparent electrodes. The molecules in the liquid crystal layer have a homeotropic or tilted homeotropic alignment in the switched-off state, i.e. are aligned substantially perpendicular to the electrode surfaces. Owing to the negative &Dgr;∈, realignment of the liquid crystal molecules parallel to the electrode surfaces takes place in the switched-on state.
In contrast to conventional ECB displays, in which the liquid crystal molecules in the switched-on state have a parallel alignment with uniform preferential direction over the entire liquid crystal cell, this uniform parallel alignment in VA and VAC displays is usually restricted only to small domains within the cell. Disclinations exist between these domains, which are also known as tilt domains. In another type of VA displays the domains are separated by polymer walls.
As a consequence, VA and VAC displays have a greater viewing-angle independence of the contrast and of the grey shades than conventional ECB displays. In addition, VA displays are often simpler to produce, since additional treatment of the electrode surface, like for example by rubbing, for uniform alignment of the molecules is no longer necessary.
In contrast to VAN displays, the liquid crystal media in VAC displays additionally comprise one or more chiral compounds, like for example chiral dopants, which in the switched-on state produce a helical twist of the liquid crystal molecules in the liquid crystal layer by an angle that is typically between 0 and 360°. The twist angle in the preferred case is about 90°.
In particular for these novel VA and VAC displays, special customized liquid crystal media are required. For example, it has been found that the liquid crystal media of negative dielectric anisotropy disclosed hitherto, as described for example in EP 0 474 062, have low values for the voltage holding ratio (HR) after UV exposure. They are therefore not very suitable for use in the displays described above.
The industrial application of the above described effects 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 infra-red, visible and ultra-violet regions and direct and alternating electrical fields. Furthermore, LC phases which can be used industrially need 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. Generally, therefore, mixtures of from two to 25, preferably from three to 18, compounds are prepared to give substances which can be used as LC phases. However, ideal phases cannot easily be produced in this way, since liquid-crystal materials having substantially negative dielectric anisotropy and adequate long-term stability were hitherto not available.
Matrix liquid-crystal displays (MLC displays) are known. Examples of nonlinear elements which can be used to individually switch the 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:
1. MOS (metal oxide semiconductor) transistors on silicon wafers as substrate,
2. Thin-film transistors (TFT) on a glass plate as substrate.
In the case of type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of monocrystalline silicon as substrate material restricts the display size, since even the modular assembly of 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 distinction is made between two technologies: TFTs comprising compound semi-conductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counter-electrode. 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, in which a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable pixel.
The TFT displays disclosed hitherto usually operate as TN cells with crossed polarizers in transmitted light and are illuminated from the back.
The term MLC display 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 in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, difficulties occur in MLC displays due to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORI-MACHI, 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]. As the resistance decreases, the contrast of an MLC display worsens. Since the resistivity of the liquid-crystal mixture generally decreases over the life of an MLC display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important for displays which must have acceptable resistance values over a long operating period.
The disadvantage of the MLC-TN displays disclosed hitherto is due to their comparatively low contrast,

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