Active matrix displays having high contrast values

Details Active matrix displays having high contrast values Active matrix displays having high contrast values

2000-07-26

2002-11-19

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299010, C252S299610, C252S299640, C252S299650, C252S299670, C349S172000

Reexamination Certificate

active

06482479

ABSTRACT:

BACKGROUND OF THE INVENTION
Replacement of the cathode ray tube with a flat panel screen requires a display technology which simultaneously makes it possible to achieve a high resolution, i.e. more than 1000 lines, a high brightness (>200 cd/m
2
), a high contrast (>100:1), a high frame rate (>60 Hz), an adequate color representation (>16 million), a large image format (>40 cm), a low power consumption and a wide viewing angle, at low production costs. At present, there is no technology which fully satisfies all these features simultaneously.
Many manufacturers have developed screens which are based on nematic liquid crystals and have been used in recent years in the field of notebook PCs, Personal Digital Assistants, desktop monitors etc. Use is made here of the technologies STN (supertwisted nematics), AM-TN (active matrix-twisted nematics) AM-IPS (active matrix-in plane switching) and AM-MVA (active matrix-multidomain vertically aligned), which are described in the relevant literature (see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein; SID Symposium 1997, ISSN-0097-966X, and the references cited therein). Furthermore, mention should be made of the technologies PDP (plasma display panel), PALC (plasma addressed liquid crystal), ELD (electroluminescent display), FED (field emission display) etc., which are also explained in the above-cited SID report.
Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (“twisted nematic”) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically suitable for areas of application such as computer displays and TV sets, as shown by a monitor marketed in Japan by Canon since May 1995.
The use of FLCs in electro-optical or fully optical components requires either compounds which form tilted or orthogonal smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping corn-pounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range to ensure that the display has a broad operating range. In particular, the contrast obtainable should be as high as possible over the entire operating range.
In so-called active matrix technology (AMLCD), a nonstructured substrate is usually combined with an active matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated into each pixel of the active matrix substrate. The nonlinear elements can also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the relevant literature (see, for example, T. Tsukuda, TTT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein).
Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in alignment and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of this process is the poor video capability, i.e. excessively slow response times, of nematic liquid crystals.
For this and other reasons, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, for example in WO 97/12355, Ferroelectrics 1996, 179, 141-152, or W. J. A. M. Hartmann (IEEE Trans. Electron. Devices 1989, 36 (9;Pt. 1), 1895-9, and dissertation, Eindhoven, The Netherlands, 1990).
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have suggested a monostable FLC geometry (Journal of the SID, 1/2, 1993, pages 163-169) in which the FLC material is aligned by means of relatively high voltages such that only a single stable position results from which a number of intermediate states are generated when an electric field is applied via a thin-film transistor. These intermediate states correspond to a number of different brightness values (gray shades) when the cell geometry is matched between crossed polarizers.
The disdavantage of the paper by Nito et al. is the occurrence of a streaky texture which limits contrast and brightness of this cell (see
FIG. 8
of the abovementioned citation). Furthermore, this method produces switching only in an angle range of up to a maximum of once the tilt angle, which is about 22° in the case of the material used by Nito et al. (cf. p. 165,
FIG. 6
) and thus produces a maximum transmission of only 50% of the transmission of two parallel polarizers. Terada et al. have suggested a monostable FLC configuration (Applied Physics Conference, Mar. 28, 1999, Tokyo, Japan; Abstract No. 28p-V-8). However, these displays are not yet suitable for practical use over a relatively large temperature range.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the present invention is to provide an active matrix liquid crystal display comprising a chiral smectic liquid crystal mixture where the liquid crystal mixture makes it possible to achieve a very high maximum transmission and a very high contrast and a constant threshold voltage over a broad temperature range.
In particular, a ferroelectric active matrix liquid crystal display comprising a ferrolelectric liquid crystal mixture is to be provided where the liquid crystal mixture assumes a monostable position, but without forming a streaky texture, is temperature-stable and makes it possible to achieve a very high maximum transmission and a very high contrast and a constant threshold voltage over a broad temperature range.
This object is achieved according to the invention by a chiral smectic active matrix display comprising a liquid crystal layer having the phase sequence I—N*—Sc*, a tilt angle which is virtually constant over a broad temperature range and a virtually constant deviation of the monostable position (i.e. the position in which the transmission of an FLC display arranged between two crossed polarizers is at a mininum) from the rubbing direction. The invention accordingly provides an active matrix display comprising a chiral smectic liquid crystal mixture where the liquid crystal mixture is characterized by the phase sequence I—N*—SmC*, a spontaneous polarization in the operating temperature range of <40 nC/cm
2
and a pitch of >10 &mgr;m at at least one temperature in the nematic or cholesteric phase and comprises at least one compound each from at least two of the substance classes (A), (B) and (C) and from 0.1 to 50% by weight, based on the liquid crystal mixture, of one or more compounds from substance class (D)
 R
20
—M
18
—(—A
14
—M
14
)
a
(—A
15
—M
15
)
b
—(M
16
—A
16
)
c
—(M
17
—A
17
)
d
—M
19
—R
21
  (D)
where:
R
1
, R
2
, R
3
, R
4
, R
5
, R
6
are each, independently of one another, hydrogen or a straight-chain or branched alkyl or alkenyl radical (with or without asymmetric carbon atoms) having 2 to 18 carbon atoms, where one or two nonterminal, nonadjacent —CH
2
— groups may be replaced by —O— and/or one —CH
2
— group may be replaced by —C≡C— or —Si(CH
3
)
2
— and one or more H atoms may be replaced by F with the provisos that heteroatoms cannot be adjacent and that in each case only one of R
1
, R
2
or R
3
, R
4
or

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