Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-03-26
2003-10-28
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299610, C252S299630, C252S299660
Reexamination Certificate
active
06638581
ABSTRACT:
The present invention relates to liquid-crystal displays, in particular to active matrix-addressed liquid-crystal displays (AMDs or AMLCDs), especially those which use an active matrix of thin-film transistors (TFTs) or varistors. In addition, the present application relates to liquid-crystal media for use in such displays. Such AMDs can use various active electronic switching elements. The most widespread are displays using three-pole switching elements. These are also preferred in the present invention. Examples of such three-pole switching elements are MOS (metal oxide silicon) transistors or the above-mentioned TFTs or varistors. In the case of TFTs, various semiconductor materials, predominantly silicon or cadmium selenide, are used. In particular, polycrystalline silicon or amorphous silicon is used. In contrast to the three-pole electronic switching elements, matrices of 2-pole switching elements, such as, for example, MIM (metal insulator metal) diodes, ring diodes or back-to-back diodes, can be employed in AMDs. However, these are, as also explained in greater detail below, usually not preferred owing to the inferior electro-optical properties achieved by the AMDs.
In liquid-crystal displays of this type, the liquid crystals are used as dielectrics whose optical properties change reversibly on application of an electric voltage. Electro-optical displays which use liquid crystals as media are known to the person skilled in the art. These liquid-crystal displays use various electro-optical effects. The most common conventional displays use the TN effect (twisted nematic, having a nematic structure which is twisted by about 90°), the STN effect (supertwisted nematic) or the SBE effect (supertwisted birefringence effect). In these and similar electro-optical effects, liquid-crystalline media of positive dielectric anisotropy (&Dgr;&egr;) are used.
Since the operating voltage should be kept as low as possible in displays in general, including in displays using these effects, use is made of liquid-crystal media of large dielectric anisotropy which are generally composed predominantly of dielectrically positive liquid-crystal compounds and contain at most smaller/lower proportions of dielectrically neutral compounds.
In contrast to the above-mentioned conventional displays using the above-mentioned electro-optical effects which require liquid-crystal media of positive dielectric anisotropy, there are other electro-optical effects which use liquid-crystal media of negative dielectric anisotropy, such as, for example, the ECB effect (electrically controlled birefringence) and its sub-forms DAP (deformation of aligned phases), VAN (vertically aligned nematics) and CSH (colour super homeotropics). The present invention relates to these effects.
The IPS effect (in-plane switching), which has been increasingly used recently, can use both dielectrically positive and dielectrically negative liquid-crystal media, similarly to guest/host displays, which, depending on the display mode used, can use dyes either in dielectrically positive or in dielectrically negative media. Here too, the present invention relates to those liquid-crystal displays mentioned in this paragraph which use dielectrically negative liquid-crystal media.
A further promising type of liquid-cyrstal display are axially symmetric microdomain (ASM) displays which are preferably addressed by means of plasma arrays (PA LCDs=plasma addressed liquid crystal displays). The present invention also relates to these displays.
The liquid-crystal media employed in the above-mentioned liquid-crystal displays and all liquid-crystal displays utilizing similar effects generally consist predominantly and usually even very substantially of liquid-crystal compounds having the corresponding dielectric anisotropy, i.e. compounds of positive dielectric anisotropy in the case of dielectrically positive media and compounds of negative dielectric anisotropy in the case of dielectrically negative media.
In the media used according to the present application, use can be made of at best significant amounts of dielectrically neutral liquid-crystal compounds, and usually of only very small amounts of dielectrically positive compounds or no dielectrically positive compounds at all, since in general the liquid-crystal displays must have the lowest possible addressing voltages. For this reason, liquid-crystal compounds having a dielectric anisotropy which is opposite in sign to the dielectric anisotropy of the medium are generally employed extremely sparingly or not at all.
The liquid-crystal media of the prior art have relatively poor low-temperature stabilities. Thus, the nematic phases frequently extend only down to −20° C. and in some cases even only down to −10° C. In addition, the threshold voltages (V
0
) are simultaneously relatively high, usually even greater than 2 V.
The majority of the liquid-crystal media of the prior art have relatively unfavourable values for &Dgr;n, frequently greater than 0.10. However, such large &Dgr;n values are not particularly advantageous for VAN displays, since VAN displays typically employ small optical retardation values. For example, use is made of a d·&Dgr;n of approximately 0.30 &mgr;m in the case of an untwisted director orientation, or of a d·&Dgr;n of approximately 0.40 &mgr;m in the case of a twist of 90°. Such large &Dgr;n values require very small layer thicknesses to be achieved, which are favourable for the response times observed, but result in low production yields.
&Dgr;n values in the range from 0.07 to 0.12 are often most favourable.
Furthermore, the response time of the displays of the prior art is frequently insufficiently long, in particular for video-capable displays. The viscosities of the liquid-crystal media are therefore to be improved, i.e. reduced. This applies in particular to the rotational viscosity and especially at low temperatures. A reduction in flow viscosity usually results in shorter filling times during production of the displays, in particular in the case of displays having a homeotropic edge alignment of the liquid crystals (e.g. in ECB and VAN displays).
There thus was and is a great demand for liquid-crystal media which do not have the disadvantages of the media of the prior art, or at least do so to a significantly reduced extent.
The liquid-crystal media according to the invention comprise
a) one or more dielectrically negative compound(s) of the formula I
in which
preferably
R
11
is alkoxy having 1 to 7 carbon atoms, preferably n-alkoxy and particularly preferably n-alkoxy having 2 to 5 carbon atoms, or alkenyloxy having 2 to 7 carbon atoms, preferably having 2 to 4 carbon atoms,
and, in the case where
can also be alkyl having 1 to 7 carbon atoms, preferably n-alkyl, particularly preferably n-alkyl having 1 to 5 carbon atoms, and
R
12
is alkyl having 1 to 7 carbon atoms, preferably n-alkyl, particularly preferably n-alkyl having 1 to 3 carbon atoms, alkoxy having 1 to 7 carbon atoms, preferably n-alkoxy, particularly preferably n-alkoxy having 2 to 5 carbon atoms, or alkenyloxy having 2 to 7 carbon atoms, preferably having 2 to 4 carbon atoms,
b) one or more dielectrically negative compound(s) of the formula II
in which
R
21
is alkyl having 1 to 7 carbon atoms, preferably n-alkyl and particularly preferably n-alkyl having 1 to 5 carbon atoms, alkoxy having 1 to 7 carbon atoms, preferably n-alkoxy and particularly preferably n-alkoxy having 2 to 5 carbon atoms, or alkenyloxy having 2 to 7 carbon atoms, preferably having 2 to 4 carbon atoms,
R
22
is alkyl having 1 to 7 carbon atoms, preferably n-alkyl, particularly preferably n-alkyl having 1 to 3 carbon atoms, alkoxy having 1 to 7 carbon atoms, preferably n-alkoxy, particularly preferably n-alkoxy having 2 to 5 carbon atoms, or alkenyloxy having 2 to 7 carbon atoms, preferably having 2 to 4 carbon atoms,
Z
21
and Z
22
are each, independently of one another, —CH
2
—CH
2
—, —CH═CH—, —C≡C—, —COO— or a single bond, preferably —CH
2
—CH
2
— or a single bond and
Bremer Matthias
Heckmeier Michael
Klement Dagmar
Merck Patent Gesellschaft mit
Millen White Zelano & Branigan P.C.
Wu Shean C.
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