TN and STN liquid-crystal displays

Details TN and STN liquid-crystal displays TN and STN liquid-crystal displays

2001-06-19

2003-04-29

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299670, C349S186000

Reexamination Certificate

active

06555186

ABSTRACT:

The invention relates to twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18,127 (1971). STN displays 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 STN here covers any relatively highly twisted display element having a twist angle with a value of between 160° and 360°, such as, for example, the display elements according to 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)).
STN displays are distinguished compared with standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, at moderate and relatively high multiplex rates, for example from 32 to 64 or higher, by better contrast values. On the other hand, the contrast in TN displays is generally higher owing to the better dark value and the angle dependence of the contrast is less than in STN displays with low multiplex rates of, for example, less than 32.
Of particular interest are TN and STN displays 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 mostly monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K
33
/K
11
and relatively small values for &Dgr;&egr;/&egr;
⊥
, where &Dgr;&egr; is the dielectric anisotropy and &egr;
195
is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimization of the contrast and response times, further important requirements are made of mixtures of this type:
1. 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 STN displays (with a multiplex rate in the region of about 1/400), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about 1/64 and 1/16 respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, a reduction in the operating voltage/threshold voltage in mixtures for TN and STN displays generally results in a significant increase in the response time. In addition, it has been found that a specific variation of the mixture components in order to improve the steepness and reduce the threshold voltage simultaneously has an adverse effect on the clearing point and the birefringence.
In addition, the known TN and STN displays often exhibit a strong temperature dependence of the operating voltage.
Thus, there continues to be a great demand for TN and STN displays, in particular for high-resolution displays (XGA), having high characteristic-line steepness and low threshold voltage at the same time as a large working-temperature range and favorable birefringence values, very short response times, low temperature dependence of the operating voltage and good angle dependence of the contrast which meet the above-mentioned requirements.
Among the objects of the invention is providing TN and STN displays which do not have the above-mentioned 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 and improved temperature dependence of the operating voltage owing to improved frequency dependence of the dielectric constants.
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 been found that objects of the invention may be achieved if use is made of nematic liquid-crystal mixtures which comprise one or more compounds of the formula I
in which
R
1
is an alkyl or alkenyl radical having 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF
3
or monosubstituted to per halo-substituted by halogen and in which, in addition, one or two CH
2
groups may be replaced by —O—, —S—,
 —CO—O—, —O—CO— or —O—CO—O— in such a way that 0 atoms are not linked directly to one another,
R
2
has one of the meanings indicated for R
1
or is X,
X is F, Cl, CN, OCN, NCS, SCN, or a halogenated alkyl, alkenyl, alkoxy or alkenyloxy radical having 1 to 6 carbon atoms, in particular OCF
3
or OCHF
2
,
L
1
and L
2
are each, independently of one another, H or F,
A is trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene or 1,3-dioxane-2,5-diyl, and
u is 1 or 1.
The use of the compounds of the formula I in the mixtures for TN and STN displays according to the invention results in
lower values for the threshold voltage and improved steepness values without a significant change in the clearing point and birefringence,
low temperature dependence of the operating voltage and threshold voltage, and
very fast response times, in particular at low temperatures.
The compounds of the formula I enable, in particular, a reduction in the threshold voltage and operating voltage of TN and STN displays without significantly impairing the response times or, in the favorable case, even with retention or shortening of the response times.
The mixtures according to the invention are furthermore preferably distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage, and
they effect long shelf lives in the LC display at low temperatures.
The invention 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 located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree 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) 15-80% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 20-85% 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 anisotr

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