The present invention relates to a MIM (Metal-Insulator-Metal Diode addressed Matrix) liquid crystal display, and to a nematic liquid-crystalline medium used in this display.
Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects. Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence 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 toward electrical fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have 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, for the above mentioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, the media desired for matrix liquid-crystal displays containing integrated nonlinear elements for switching individual image points (MLC displays) are those having high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability of the resistance and low vapor pressure.
MIM displays [J. G. Simmons, Phys. Rev. Vol. 155, No. 3, pp. 657-660; K. Niwa et al., SID 84 Digest, pp. 304-307, June 1984] of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependency of the contrast and the switching times, problems result in MIM displays due to inadequate specific resistance of the liquid-crystal mixtures. As the resistance decreases, the contrast of an MIM display worsens and the problem of xe2x80x9cafter image eliminationxe2x80x9d may occur. Since the specific resistance of the liquid-crystal mixture generally decreases over the life of an MIM display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important to give acceptable service lives. In particular, in the case of low-voltage mixtures, it was hitherto not possible to achieve very high specific resistances. It is furthermore important that the specific resistance increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation. The MIM displays of the prior art do not satisfy current demands.
A serious problem in fabricating the MIM diode addressed matrix liquid crystal display panel which has a large number of scanning electrodes is to reduce stray capacitances which are parallel to the MIM diodes. A higher addressing voltage is required if this capacitance is too large as compared with the capacitance of the pixel to be addressed with the MIM diode. Furthermore, the capacitance of the MIM diodes must be small relative to that of the pixels in order to hold the voltage across the pixels constant while the pixel is in the non-selected state. Otherwise, undesired voltages will be applied across the pixels of the non-selected scanning line, since many pixels are connected in the same electrode.
One possible solution of this problem is to use a special sandwich construction the so-called xe2x80x9clateral MIM diodexe2x80x9d [S. Morozumi et al., Japan Display ""83, pp. 404-407, 1983].
Another approach for the solution of this problem is the application of liquid-crystalline media with relatively high capacitances.
It was hitherto possible to prepare liquid-crystalline media with values for birefringence and phase range (e.g. clearing point:xe2x89xa770xc2x0) which are required for practical use, but only with relatively low capacitances of the liquid-crystalline media, if value is placed on values of about 98% for the holding ratio under extreme conditions (e.g. after exposure to UV).
Thus, there continues to be a great demand for MIM displays of very high specific resistance, high capacitances and at the same time a broad operating temperature range, short switching times and low threshold voltage which do not have these disadvantages or only do so to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures),
switchability at extremely low temperatures (outdoor use, automobiles, avionics),
increased stability to UV radiation (longer life)
The media available from the prior art do not make it possible to achieve these advantages whilst simultaneously retaining the other parameters.
The invention has the object of providing media, in particular for MIM displays of this type, which do not have the above mentioned disadvantages or only do so to a lesser extent, and preferably at the same time have very high specific resistances and low threshold voltages.
It has now been found that this object can be achieved if media according to the invention are used in displays.
The invention relates to a MIM-liquid crystal display comprising
a MIM diode array,
a pair of parallel substrates and a nematic liquid crystalline medium disposed between the pair of substrates, characterized in that the nematic liquid crystalline medium is based on
a) a component A comprising one or more compounds of the formula I 
xe2x80x83wherein
R1 is alkyl, alkoxy, alkenyloxy, oxaalkyl or alkenyl with 1 to 15 C atoms,
Q is CF2, OCF2, OCF2CF2 or a single bond,
X is F or Cl
Z1 and Z2 are each independently xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond, 
L1, L2,
L3 and L4 are each independently H or F, and
m is 0 or 1,
b) a component B comprising one or more compounds of the formula II 
xe2x80x83wherein
R2 has the meaning for R1,
R3 has the meaning of R1 or Xxe2x80x94Q,
Z3 and Z4 are each independently xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond, 
has the meaning given for 
and
n is 0 or 1.
The invention relates to a nematic liquid-crystal medium of the above given composition.
The liquid-crystal MIM displays according to the invention facilitate a significant broadening of the parameter latitude available.
The achievable combinations of clearing point, viscosity at low temperature, thermal and UV stability and dielectric anisotropy or threshold voltage are far superior to previous materials from the prior art.