Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-09-24
2003-11-04
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299650, C252S299660, C252S299610, C568S381000
Reexamination Certificate
active
06641871
ABSTRACT:
The present invention relates to mesogenic compounds, in particular liquid-crystalline compounds, and to liquid-crystalline media comprising these compounds. The present invention furthermore relates to liquid-crystal displays, in particular liquid-crystal displays addressed by means of an active matrix (AMDs or AM LCDs (active matrix addressed liquid crystal displays) and very particularly so-called plasma-addressed liquid-crystal displays (PA LCDs), in which the active electrically nonlinear medium is the plasma in the plasma channels.
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 of these are the TN effect (twisted nematic, with a homogeneous, virtually planar initial alignment of the liquid crystals and a nematic structure twisted by about 90°), the STN effect (supertwisted nematic) and the SBE effect (supertwisted birefringence effect). In these and similar electro-optical effects, liquid-crystalline media of positive dielectric anisotropy (&egr;B) are used.
Besides the said 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 chases), VAN (vertically aligned nematics) and CSH (color super homeotropics).
An electro-optical effect having excellent, low, viewing-angle dependence of the contrast uses axially symmetric micropixels (ASMs). In this effect, the liquid crystal in each pixel is surrounded cylindrically by a polymer material. This mode is particularly suitable for combination with addressing through plasma channels. Thus, in particular, large-area PA LCDs having good viewing-angle dependence of the contrast can be achieved.
The IPS effect (in plane switching), which has been increasingly employed recently, can use both dielectrically positive and dielectrically negative liquid-crystal media, similar to guest/host displays, which are able to employ dyes either in dielectrically positive or in dielectrically negative media, depending on the display mode used.
Since the operating voltage in displays should generally, i.e. including in displays based on these effects, be as low as possible, use is made of liquid-crystal media of large dielectric anisotropy, which generally predominantly and usually even very substantially consist of liquid-crystal compounds of the corresponding dielectric anisotropy, i.e. consist of compounds of positive dielectric anisotropy in the case of dielectrically positive media and of compounds of negative dielectric anisotropy in the case of dielectrically negative media. In the respective types of media (dielectrically positive or dielectrically negative), at most significant amounts of dielectrically neutral liquid-crystal compounds are typically employed. Liquid-crystal compounds having the sign of the dielectric anisotropy opposite to the dielectric anisotropy of the medium are generally employed in extremely small amounts, or not at all.
An exception here is formed by liquid-crystalline media for MIM (metal-insulator-metal) displays [J. G. Simmons, Phys. Rev. Vol 155 No. 3, pp. 657-660; K. Niwa et al., SID 84 Digest, pp. 304-307, June 1984], in which the liquid-crystal media are addressed by means of an active matrix of thin-film transistors. In this type of addressing, which utilizes the non-linear characteristic line of diode switching, it is not possible, in contrast to TFT displays, to charge a storage capacitor together with the electrodes of the liquid-crystal display elements (pixels). A reduction in the effect of voltage drop during the addressing cycle therefore requires the highest possible base value of the dielectric constant. In dielectrically positive media, as employed, for example, in MIM-TN displays, the dielectric constant perpendicular to the molecular axis (&egr;
⊥
) must therefore be as large as possible, since it determines the base capacitance of the pixel. To this end, as in WO 93/01253, EP 0 663 502 and DE 195 21 483, compounds of negative dielectric anisotropy are employed in addition to dielectrically positive compounds in the dielectrically positive liquid-crystal media.
A further exception is formed by STN displays in which, for example in accordance with DE 41 00 287, dielectrically positive liquid-crystal media comprising dielectrically negative liquid-crystal compounds are employed in order to increase the steepness of the electro-optical characteristic line.
The pixels of the liquid-crystal displays can be addressed directly, time-sequentially, i.e. in time multiplex mode, or by means of a matrix of active, electrically nonlinear elements.
The most common AMDs to date use discrete active electronic switching elements, such as, for example, three-pole switching elements, such as MOS (metal oxide silicon) transistors or thin-film transistors (TFTs) or varistors or 2-pole switching elements, such as, for example, MIM (metal insulator metal) diodes, ring diodes or back-to-back diodes. In TFTs, various semiconductor materials, predominantly silicone or alternatively cadmium selenide, are used. In particular, amorphous silicon or polycrystalline silicon is used.
In contrast to these conventional AMDs, the electrically nonlinear material in PA LCDs is not a solid semiconductor material, but instead a gaseous plasma. This plasma is located in discharge chambers which have the shape of channels. These channels run parallel to one another and thus define the rows (or columns) of the matrix. The signal electrodes, to which the addressing voltages for all pixels of a row (or column) are applied parallel to one another, run perpendicular to the plasma channels.
The plasma in PA LCDs is separated from the liquid-crystal layer by a thin insulating layer, known as the microsheet. As in conventional AMDs, the alignment layer is located on this microsheet in a conventional manner.
In contrast to conventional AMDs, however, voltage additionally drops over the microsheet in PA LCDs. This drop in voltage is generally significant, which means that the plasma voltage (V
plasma
) is divided as follows into the voltage drop over the liquid-crystal layer (V
LC
) and over the microsheet (V
sheet
):
V
plasma
=V
LC
+V
sheet
(I)
Taking into account the capacitance of the liquid-crystal layer (C
LC
) and of the microsheet (C
sheet
), which are determined by the respective layer thicknesses (d
LC
and d
sheet
respectively) and dielectric constants (&egr;
LC
and &egr;
sheet
respectively) since the active surfaces of the two layers correspond, the following is obtained:
V
plasma
/V
LC
=1+(&egr;
LC
/d
LC
)·(
d
sheet
/&egr;
sheet
) (II)
The effective dielectric constant of the liquid-crystal layer (&egr;E
LC,eff.
) is not constant, but instead depends on the applied voltage and the material properties of the liquid-crystal material, and on the initial alignment and the electrode configuration. It is always in the region of the two extreme values, namely the dielectric constants perpendicular to the director (to a first approximation the longitudinal molecular axis) (&egr;
&egr;
⊥
) and the dielectric constant parallel to the director (&egr;
||
).
In displays having an electric field perpendicular to the liquid-crystal layer and liquid-crystal media of negative dielectric anisotropy (&Dgr;&egr;<0) and a homeotropic edge alignment, which are preferred for the purposes of the present application, the liquid-crystal director in the fully switched-through state, i.e. on application of a correspondingly large voltage, is aligned parallel to the layer plane, and the effective dielectric cons
Heckmeier Michael
Reiffenrath Volker
Reuter Marcus
Merck Patent Gesellschaft mit
Millen White Zelano & Branigan P.C.
Wu Shean C.
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