TN and STN liquid crystal displays

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

2000-06-30

2003-02-04

WU, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06514579

ABSTRACT:

The invention relates to twisted nematic (acronym TN) and supertwisted nematic (acronym STN) liquid crystal displays having very short switching times and good steepness and angular dependence and to the novel nematic liquid crystal mixtures employed therein.
TN displays are known, e.g. from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, e.g. from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17
th
Freiburg Conference 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 herein comprises any relatively strongly twisted display element having a twist angle amounting to between 160° and 360°, for example the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3
rd
Intern. Display Conference, Kobe, Japan), the 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 in particular are distinguished, compared with standard TN displays, by considerably better slopes of the electro-optical characteristic curve and the attendant better contrast values and by a significantly reduced angular dependence of the contrast. Of particular interest are TN and STN displays having a very short switching time, especially at lower temperatures. To achieve short switching times it has hitherto been the practice to optimize the rotational viscosities of the liquid crystal mixtures, by employing usually monotropic additives having a relatively high vapour pressure. The switching times achieved were not, however, adequate for all applications.
To achieve a steep electro-optical characteristic curve in TN and STN displays, the liquid crystal mixtures should have relatively large values of K
3
/K
1
and relatively small values of &Dgr;∈/∈
1
.
Beyond optimized contrast and optimized switching times, such mixtures are subject to further important requirements:
1. wide d/p window
2. high chemical long-term stability
3. high electrical resistance
4. low frequency dependence and temperature dependence of the threshold voltage.
The parameter combinations achieved are still inadequate by a long shot, especially for high-multiplex, but also for low- and medium-multiplex STNs (1/400). One reason for this is that the effects of material parameters on the various requirements pull in opposite directions.
There is therefore still a great need for TN and STN displays, especially for high-resolution displays (XGA), having very short switching times in conjunction with a wide operating temperature range, a steep characteristic curve, good angular dependence of contrast and low threshold voltage, which comply with the above-specified requirements.
It is an object of the invention to provide TN and STN displays which do not have the above-specified draw-backs or which have them only to a lesser extent and at the same time have low threshold voltages and very good steepness.
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.
We have now found that this object can be achieved if nematic liquid crystal mixtures are employed which comprise compounds of formula IA
in conjunction with alkynyl-bonded compounds of formula IB,
where
R
1
and R
2
each, independently of one another, are an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH
2
groups being replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another,
R
3
is an alkenyl group having from 2 to 7 C atoms, and
L is H or F.
The use of the compounds of formulae IA and IB in the mixtures for TN and STN displays according to the invention results in
low viscosities,
wide nematic phase ranges,
a steeply sloping electro-optical characteristic curve, and
low threshold voltages.
The compounds of formulae IA and IB, in particular, cause a distinct increase in the steepness of TN and STN mixtures whilst at the same time reducing the threshold voltage.
The mixtures according to the invention are further distinguished by the following advantages:
they exhibit low temperature dependence of the threshold voltage and the operating voltage,
they have short switching times even at low temperatures,
they result in long persistence times in the display at low temperatures.
The invention therefore relates to a liquid crystal display comprising
two substrates which, together with an edging, form a cell,
contained in the cell, a nematic liquid crystal mixture having positive dielectric anisotropy,
electrode layers having alignment layers on the insides of the substrates,
a pre-tilt angle between the longitudinal axis of the molecules at the surface of the substrates and the substrates of from 0 degrees to 30 degrees, and
a twist angle of the liquid crystal mixture in the cell which, from alignment layer to alignment layer, amounts to between 22.5° and 600°,
a nematic liquid crystal mixture comprising
a) 20-90 wt % of a liquid-crystalline component A, consisting of one or more compounds having a dielectric anisotropy of more than +1.5;
b) 10-80 wt % 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 wt % of a liquid-crystalline component C, consisting of one or more compounds having a dielectric anisotropy of less than −1.5 and
d) optionally an optically active component D in such an amount that the ratio between layer thickness (spacing of the substrates) and natural pitch of the chiral nematic liquid crystal mixture is from about 0.2 to 1.3,
characterized in that component A of the mixture comprises at least one compound of formula IA,
 where
R
3
is an alkenyl group having from 2 to 7 C atoms,
 and the mixture comprises at least one compound of formula IB,
 where
R
1
and R
2
each, independently of one another, are an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH
2
groups being 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.
The invention also relates to corresponding liquid crystal mixtures for use in TN and STN displays, especially in medium- and low-multiplexed STN displays.
Particularly preferred compounds of formula IA are those of the following formulae
where R
3a
is H, CH
3
, C
2
H
5
or n-C
3
H
7
.
Particularly preferred compounds of formula IB are those where L is F.
The component A, in addition to the compounds of formula IA, preferably further comprises one or more compounds of formulae II and/or III,
where
R is an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH
2
groups being replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another,
 and
 each, independently of one another, are
 or
L
1
to L
6
each, independently of one another, are H or F,
Z
1
is —COO—, —CH
2
CH
2
— or a single bond,
Z
2
is —CH
2
CH
2
—, —COO—, —C≡C— or a single bond,
Q is —CF
2
—, —CHF—, —OCF
2
—, —OCHF— or a single bond,
Y is F or Cl,
a is 1 or 2, and
b is 0 or 1,
where, r

Affiliated with

Also associated with

Rating

TN and STN liquid crystal displays does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with TN and STN liquid crystal displays, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and TN and STN liquid crystal displays will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3127550