Supertwisted nematic liquid crystal displays

Details Supertwisted nematic liquid crystal displays Supertwisted nematic liquid crystal displays

1999-11-19

2002-08-27

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670, C349S179000, C349S186000

Reexamination Certificate

active

06440506

ABSTRACT:

The invention relates to supertwist nematic liquid-crystal displays (STN-LCDs) having short switching times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
STN-LCDs as in the heading are known, for example from EP 0,131,216 B1; DE 3,423,993 A1; EP 0,098,070 A2; M. Schadt and F. Leenhouts, 17th Freiburg conference on liquid crystals (Apr. 8-10, 1987); 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, L1784-L1786 (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-LCD here includes any relatively highly twisted display element having a twist angle with a value between 160° and 720°, 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), the STN-LCDs (DE OS 3,503,259), 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 OS 0,246,842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
Compared with standard TN displays, STN-LCDs of this type are distinguished by significantly better steepnesses of the electrooptical characteristic curve (in the following simply called ‘steepness’) and, associated therewith, better contrast values, and by a significantly lower angle dependence of the contrast. Of particular interest are STN-LCDs with extremely short switching times, in particular also at relatively low temperatures. In order to achieve short switching times, the rotational viscosities, in particular of the liquid crystal mixtures have hitherto been optimized using usually optimized combinations of liquid crystal components and optionally also monotropic additives of relatively high vapour pressure.
However, the switching times achieved were not adequate for all applications.
Shorter switching times can also be achieved by reducing the thickness of LC, layer of the STN-LCD and using liquid-crystal mixtures with a higher birefringence &Dgr;n.
All these approaches to shorter switching times, however, still end up with mixtures which were not adequate for every use.
Further demands for STN-LCD are a higher multiplexability (resulting in a smaller number of driving IC's), lower threshold voltages and a high steepness.
To achieve a high steepness in STN-LCDs, the liquid crystal mixtures should exhibit comparatively high values of the ratio of the elastic constants K
33
/K
11
and comparatively low values of &Dgr;&egr;/&egr;
⊥
, wherein &Dgr;&egr; is the dielectric anisotropy and &egr;
⊥
is the dielectric constant in vertical direction to the long molecular axis.
Apart of the optimization of the contrast and the switching times such liquid crystal mixtures should fulfill further important requirements such as:
1. A broad d/p window of cell gap d and pitch p, which is defined as the range between the lower and upper limit of the d/p values.
2. A high chemical stability.
3. A high electrical resistance.
4. A low dependence of the threshold voltage on temperature and frequency.
Optimal parameters, however, cannot be achieved simultaneously for all the properties mentioned above because of opposite influence of different material parameters such as dielectric and elastic properties. Thus, the parameter combinations achieved so far are still not sufficient, especially for high multiplex STN-LCDs (with a multiplex rate in the range of 1/400), but also for middle and low multiplex STN-LCD (multiplex rate in the range of 1/64 and 1/16, respectively).
Therefore, there continues to be a great demand for improved STN-LCDs having short switching times and, at the same time, a broad service-temperature range, a high steepness (i.e. low steepness values), good angle dependency of the contrast and low threshold voltage, which meet the abovementioned requirements.
The invention has the object of providing STN-LCDs which only have the above mentioned disadvantages to a small extent, or not at all, and at the same time have very useful overall properties, especially short switching times at low temperatures and a high steepness.
It has now been found that this object can be achieved if nematic liquid-crystal mixtures are used which comprise compounds having a 3,4,5-trifluorophenyl group together with compounds of formula I
wherein
R
3
is an alkenyl group with 2 to 7 C atoms,
R
4
is an optionally fluorinated alkyl, alkoxy, alkenyl or alkenyloxy group with 1 to 12 C atoms, wherein one or two non-adjacent CH
2
groups can be replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a manner that O atoms are not directly adjacent to one another, or, in case m=1, R
4
can also be Q—Y,
Q is CF
2
, OCF
2
, CFH, OCFH or a single bond,
Y is F or Cl,
L
1
and L
2
are each independently H or F, and
m is 0 or 1.
The use of compounds of having a 3,4,5-trifluorophenyl group together with compounds of formula I in the liquid crystal mixtures for STN-LCDs according to the invention especially leads to
a high steepness
a low threshold voltage with a low temperature dependence, and
short switching times especially at low temperatures.
Furthermore, the liquid crystal mixtures according to the present invention show the following advantageous properties
a low viscosity,
a low temperature dependence of the threshold voltage and the operating voltage,
improved stability of the mixture in the display at low temperatures.
The invention thus relates to an STN-LCD having
two plane-parallel 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 superposed 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 1 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°,
wherein the nematic liquid-crystal mixture essentially consists of
a) 30-90% by weight of a liquid-crystalline component A, comprising one or more compounds having a dielectric anisotropy of more than +1.5;
b) 10-45% by weight of a liquid-crystalline component B, comprising one or more compounds having a dielectric anisotropy from −1.5 to +1.5;
c) 0-10% by weight of a liquid-crystalline component D, comprising one or more compounds having a dielectric anisotropy of less than −1.5 and
d) optionally an optically active component C in such an amount that the ratio between the layer thickness (separation of the plane-parallel 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 component A having a 3,4,5-trifluorophenyl group, and at least one alkenyl compound of formula I
wherein
R
3
is an alkenyl group with 2 to 7 C atoms,
R
4
is an optionally fluorinated alkyl, alkoxy, alkenyl or alkenyloxy group with 1 to 12 C atoms, wherein one or two non-adjacent CH
2
groups can be replaced by —O—, —CH═CH—, —CO—, —OCO— or —COO— in such a manner that O atoms are not directly adjacent to one another, or, in case m=1, R
4
can also be Q—Y,
Q is CF
2
, OCF
2
, CFH, OCFH or a single bond,
Y is F or Cl,
L
1
and L
2
are each independently H or F, and
m is 0 or 1.
Another object of the invention is providing the inventive crystal mixtures as described above and below for use in STN-LCDs.
The formula I comprises the follow

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