STN liquid-crystal display

Details STN liquid-crystal display STN liquid-crystal display

2000-12-12

2003-03-11

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660, C252S299670, C349S186000, C349S101000

Reexamination Certificate

active

06531194

ABSTRACT:

The invention relates to supertwist liquid-crystal displays (SLCDs) or supertwisted nematic (STN) displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
SLCDs as defined in the preamble are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987), and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any relatively highly twisted display element with a value for the twist angle of between 160° and 360°, such as, for example, the display elements of Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electro-optical characteristic line and consequently better contrast values, and by significantly lower angle dependence of the contrast.
Of particular interest are SLCDs having very short response times, in particular at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimized using usually monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electro-optical characteristic line in SLCDs, the liquid-crystal mixtures should have relatively large values for K
3
/K
1
and relatively small values for &Dgr;&egr;/&egr;⊥.
In addition to optimization of the contrast and the response times, mixtures of this type are subject to further important requirements:
1. A broad d/p window
2. High long-term chemical stability
3. High electrical resistance
4. Low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex, but also for low- and medium-multiplex STNs (1/400). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs, in particular for high-resolution displays (XGAs), having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.
It is an object of the invention to provide SLCDs which do not have the abovementioned disadvantages, or only do so to a lesser extent, and at the same time have short response times, in particular at low temperatures, and very good steepnesses.
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 these objects can be achieved if nematic liquid-crystal mixtures are used which comprise compounds of the formula IA
in combination with compounds of the formula IB
and/or with compounds of the formula IC
in which
R
a
and R
c
are each, independently of one another, H or an alkyl group having 1 to 7 carbon atoms, and
R
b
and R
e
are each, independently of one another, an alkyl or alkoxy group having 1 to 10 carbon atoms or an alkenyl or alkenyloxy group having 2 to 10 carbon atoms,
R
d
is F, OCF
3
, OCHF
2
, alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl having 2 to 7 carbon atoms,
Y
1
, Y
2
, Y
3
, Y
4
are each, independently of one another, H or F,
q is 0 or 1 and
p is 0, 1 or 2.
The use of the compounds of the formulae IA and IB in the mixtures for SLCDs according to the invention produces
high steepness of the electro-optical characteristic line
low temperature dependence of the threshold voltage and
very short response times, in particular at low temperatures.
The compounds of the formula IA are covered by the broad generic claim in U.S. Pat. No. 5,744,058 as components of liquid-crystalline media. However, the specific combination of the compounds of the formula IA with the compounds IB and/or the compounds IC which produces, in particular, high steepness of the electro-optical characteristic line and low temperature dependence of the threshold voltage in connection with low rotation viscosities, is not described therein.
The combination of compounds of the formulae IA and IB and/or IC results, in particular, in a significant shortening of the response times of SLCD mixtures while simultaneously increasing the steepness and obtaining a low temperature dependence of the threshold voltage.
Furthermore, the mixtures according to the invention are distinguished by the following advantages:
they have a low viscosity,
they have low temperature dependence of the threshold voltage and the operating voltage, and
they result in long storage times in the display at low temperatures.
The invention therefore relates to a liquid-crystal display containing
two outer plates which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy which is present in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a pretilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of 22.5° to 600°, and
a nematic liquid-crystal mixture comprising of
a) 0-60% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 0-60% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between −1.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below −1.5; and
d) optionally an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
characterized in that the liquid-crystal mixture additionally comprises at least one compound of the formula IA
in which
R
a
is H or an alkyl group having 1 to 7 carbon atoms, and
R
b
is an alkyl or alkoxy group having 1 to 10 carbon atoms or an alkenyl or alkenyloxy group having 2 to 10 carbon atoms,
and at the same time at least one compound of the formula IB
in which
R
c
is H or an alkyl group having 1 to 7 carbon atoms, and
R
d
is F, OCF
3
, OCHF
2
, an alkyl or alkoxy group having 1 to 7 carbon atoms or an alkenyl group having 2 to 7 carbon atoms,
Y
1
and y
2
are each, independently of one another, H or F,
p is 0, 1 or 2, and
q is 0 or 1
and, in addition to the compounds of the formula IB or instead of the compounds of the formula IB, at least one compound of the formula IC
in which
R
e
is an alkyl or alkoxy group having 1 to 10 carbon atoms or an alkenyl or alkenyloxy group having 2 to 10 carbon atoms, and
Y
3
and Y
4
are each, independently of one another, H or F.
The invention also rela

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