STN liquid-crystal display

Details STN liquid-crystal display STN liquid-crystal display

2000-05-04

2003-05-27

Wu, Shean C. (Department: 2872)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06569503

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 are known, for example from EP 0 131 216 131; 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 electrooptical 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 also 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, further important requirements are made of mixtures of this type:
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.
The invention has an object of providing SLCDs which do not have the abovementioned disadvantages, or only do so to a lesser extent, and at the same time have very good 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.
Particularly, it has been found that advantages are 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
b
, independently of one another, are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl or alkenyloxy having 2 to 7 carbon atoms, where at least one of the radicals R
a
and R
b
is alkenyl or alkenylbxy having 2 to 7 carbon atoms,
R
c
is an alkenyl group having 2 to 7 carbon atoms,
R
d
and R
e
, independently of one another, are an alkyl or alkoxy group having 1 to 12 carbon atoms or an alkenyl group having 2 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH
2
groups may be replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another,
L is H or F,
Z is —CH
2
CH
2
— or a single bond, and
c is 0 or 1.
The use of the compounds of the formulae IA in combination with IB and/or IC 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 fast response times, in particular at low temperatures.
The compounds of the formulae IA in combination with IB and/or IC significantly shorten, in particular, the response times of SLCD mixtures while simultaneously increasing the steepness and retaining the low temperature dependence of the threshold voltage.
Furthermore, the mixtures according to the invention are distinguished by the following advantages:
they have low viscosity,
they have low temperature dependence of the threshold voltage and the operating voltage, and
they effect long storage times of the display at low temperatures.
The invention thus also includes liquid-crystal displays having
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 pre-tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 0 degrees 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 between 22.5° and 600°,
a nematic liquid-crystal mixture consisting of
a) 0-90% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 15-80% 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) if desired, 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 comprises at least one compound of the formula IA
in which
R
a
and R
b
are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl or alkenyloxy having 2 to 7 carbon atoms, where at least one of the radicals R
a
and R
b
is alkenyl or alkenyloxy having 2 to 7 carbon atoms, and
Z is —CH
2
CH
2
— or a single bond,
component B simultaneously comprises at least one compound of the formula IB
in which
R
c
is an alkenyl group having 2 to 7 carbon atoms,
R
d
is an alkyl or alkoxy group having 1 to 12 carbon atoms or an alkenyl group having 2 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH
2
groups may be replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another, and
c is 0 or 1,
and/or the liquid-crystal mixture simultaneously comprises at least one compound of the formula IC:
in which
R
e
is an alkyl or alkoxy group having 1 to 12 carbon atoms or an alkenyl group having 2 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH
2
groups may be replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another, and
L is H or F, and
q is 0 or 1.
The invention also r

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