Active matrix liquid crystal device and smectic liquid...

Details Active matrix liquid crystal device and smectic liquid... Active matrix liquid crystal device and smectic liquid...

2002-10-16

2004-11-30

Wu, Shean C. (Department: 1756)

Compositions

Liquid crystal compositions

C428S001100

Reexamination Certificate

active

06824707

ABSTRACT:

The present invention relates to a novel smectic liquid crystal mixture and a novel active matrix liquid crystal device or display. More particularly, it relates to chiral smectic or more specifically ferroelectric liquid crystal mixtures, which enables the generation of gray levels and full color representation useful for computer monitors, TV, DVD, Video and other displays. In particular these displays are useful for high speed applications and for yielding a strong colour saturation by improved backlight techniques which rely upon a high speed liquid crystal. A further aspect of this invention are active matrix displays containing such a mixture, particularly in a monostable geometry.
Monostable chiral smectic liquid crystals have been proposed to be combined with active matrix technology to simultaneously allow the utilization of a very high ‘pixel speed’ and show a gray scale, contrary to bistable SSFLC displays.
Takatoh et. al. (6
th
International Conference on Ferroelectric Liquid Crystals, Jul. 20-24, 1997, Brest, France; M. Takatoh et al. 1998, SID Digest, 1171-1174) have demonstrated an AM display based upon chiral smectics using a very high P
s
material driven with an active matrix with polycrystalline Silicon-TFT. Nito et. al. (Nito et al., 1993, Journal of the SID, 1/2,163-169.) have suggested a monostable AM-FLC with much lower P
s
, however, with the disadvantage of a stripey FLC texture which is not suitable for high contrast displays without further improvements. Furue et. al. (Furue, H. et al., 1998, IDW '98, 209-212) suggested a polymer stabilized SSFLCD with a FELIX® mixture with a material having a moderate P
s
value.
Asao et. al. have presented a monostable FLC mode (Y. Asao et al., ILCC 2000, Sendai, and Jpn. J. Appl. Phys. 38, L534-L536, 1999 therein called “half-V-shape FLC” mode; see also T. Nonaka et. al., Liquid Crystals 26(11), 1599-1602, 1999, therein called “CDR” mode). Such displays provide, by virtue of their smaller Ps values, solutions for the gray scale problem and the resolution limitation caused by too large P
s
values in active matrix panels.
A remaining problem in the application of TFT-FLCD (e.g. monostable FLC) is the limited so-called “image sticking” effect (H. Pauwels et. al., Proceedings of the IDRC, Florida 2000, pp.72-75), caused by the activity of the manifold of charge carriers that are present in the pixel volume and which tend to generate a hysteresis effect, i.e. the type of gray level becomes dependent on the pixel switching history and the correct addressing of gray levels becomes therefore hardly reproducible in practice.
Thus it is seemingly an important factor to maintain a very low ion content and to achieve chemical stability against heat and light, both of which could cause additional ion formation. This demand has in practice lead to the exclusion of any material containing hetero atoms such as N, S, even O in liquid crystal mixtures for active matrix (i.e. TFT or MIM) applications {cp. e.g. Petrov et al., Liq. Cryst. 19(6), 729 (1995) [CAN 124:101494]; Petrov, Proc. SPIE-Int. Soc. Opt. Eng.(1995), 2408 [CAN 123:241500]; Dabrowski, Biul. Wojsk. Akad. Techn. 48(4), 5 (1999) [CAN 131:163227]; Kirsch, Angew. Chem., Int. Ed. 39(23),4216 (2000) and references cited in these papers}. Whereas, so far, this could successfully be done for nematics (cp. e.g. DE-A 1 962 9812, p. 12 to 16), there is hardly any smectic (S
c
) material without such hetero atoms. Thus the use of fast switching smectics for TFT application is strongly limited if not prohibited if the condition to exclude heteroatoms would be mandatory.
The objective of the present invention was therefore to provide a smectic liquid crystal mixture and an active matrix liquid crystal display with very high reproducibility of the gray levels irrespective of the containment of hetero atoms, in particular ferroelectric or antiferroelectric liquid crystal mixtures.
Contrary to the above-mentioned approach to keep liquid crystal materials free of ionic decontaminations, we have surprisingly found that certain dopants as additives to smectic liquid crystal mixtures are indeed effective in removing the image sticking in active matrix displays, while keeping a sufficiently high holding ratio. This is insofar unexpected, as the dopants, e.g. polyethylene glycoles, crown ethers, aza-crown ethers and the like usually contain several oxygen and/or nitrogen atoms and increase the conductivity of the material. Thus, in the traditional view, these dopants should cause a strong image sticking and a low holding ratio. Surprisingly, the opposite is observed.
Thus, the present invention provides a smectic liquid crystal mixture comprising a smectic liquid crystal base mixture and at least one dopant, which is either a complex ligand or an organic material in a concentration of 0.01 to 5.0 weight percent. The electrical conductivity of the instant liquid crystal mixtures is increased by at least 20 percent with respect to the base mixture
In a preferred embodiment of the instant invention the dopant is present in a concentration of 0.05 to 1.0 weight percent, which leads to a suppression or minimization of the image sticking effect up to 10% of the untreated value. The liquid crystal mixture according to the invention can be used in active liquid crystal displays.
In particular the present invention relates to a ferroelectric or antiferroelectric liquid crystal mixture, most particularly a monostable ferroelectric liquid crystal mixture, especially a monostable ferroelectric liquid crystal mixture having a phase sequence with decreasing temperature
isotropic-tilted smectic
or
isotropic-nematic or cholesteric-tilted smectic
or
isotropic-nematic or cholesteric-smectic A-tilted smectic,
comprising one or several dopants.
For dopants according to invention with —OH or —NH
2
— or —NH— or —SH groups, for instance those described in EP-B-385 688, U.S. Pat. No. 5,122,296 and U.S. Pat. No. 6,139,925, which are hereby incorporated by reference.
The complex ligands according to the invention are either podands, coronands or cryptands. Pertinent terminology and listing of pertinent compounds is hereby incorporated by reference to EP-B-385 688, EP-B-502 964 and EP-B-599 928.
The complex ligands can also be &agr;-cyclodextrin [10016-20-3], &bgr;-cyclodextrin [7585-39-9] or &ggr;-cyclodextrin [17465-86-0].
A preferred group of podands encompasses oligoethylendialkylether and polyethylenglycols, respectively, each preferably with a molar mass above 350 amu, both sub-groups optionally substituted by pendant groups.
A preferred group of coronands encompasses crown ethers (i.e. medio- and macro-cycles with oxygen donors), aza crown (i.e. medio- and macro-cycles with nitrogen and oxygen donors), each sub-group optionally substituted by pendant groups.
Especially preferred are coronands with one or two nitrogen donors, these nitrogen atoms being substituted by —C(═O)—R groups wherein R is an alkyl group (linear or branched) or a cyclic moiety; with this respect reference is made to EP-B-599 928 which is hereby incorporated by reference.
A prefered group of cryptands encompasses bicyclic species with nitrogen and oxygen donors (e.g. kryptofix®), bicyclic species exclusively with oxygen donors, and tricyclic species (e.g. soccerball cryptand) with either nitrogen and oxygen donors or exclusively oxygen donors; all sub-groups can be substituted by pendant groups.
In either group of preferred complex ligands their properties can be adjusted to the additional parameters of the display by modifying the pattern of the podand, coronand and cryptand, resp., through incorporation of hetero atoms (other than nitrogen and oxygen, e.g. sulfur or silicium) as donors, through incorporation of cyclic donor moieties (e.g. pyridine or quinoline end groups in case of podands; e.g. phenylen-diyl in case of benzo crowns) and through attaching pendant groups, respectively. Combinations of these different types of modifications are also possible

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