Electro-optical liquid-crystal display

Details Electro-optical liquid-crystal display Electro-optical liquid-crystal display

2001-04-16

2004-10-05

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C428S001250, C252S299610, C252S299630, C252S299670, C349S134000, C349S136000, C349S179000, C349S180000

Reexamination Certificate

active

06800338

ABSTRACT:

The present invention relates to electro-optical liquid-crystalline displays, in particular STN displays having low addressing voltages.
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 (
t
wisted
n
ematic, having a homogeneous, virtually planar initial alignment of the liquid crystals and a nematic structure which is twisted by about 90°) and the STN effect (
s
uper
t
wisted
n
ematic) and the SBE effect (
s
uper
t
wisted
b
irefringence
e
ffect). In these and similar electro-optical effects, liquid-crystalline media of positive dielectric anisotropy (&Dgr;∈) are used.
In STN displays, which, in the present application, include all common and known types of display with relatively high twist, such as, for example, SBE (
s
upertwisted
b
irefringent
e
ffect), GH (guest/host), STN and OMI (optical mode interference) displays, as well as compensated STN displays, such as DSTN (
d
ouble layer STN) and film-compensated STN displays, the liquid-crystal director is twisted from one side of the liquid-crystal layer to the other by a given angle of from greater than 90°, typically of 180° or more, up to 600°, typically up to 270°. This is achieved on the one hand by corresponding alignment of the preferential directions of the liquid-crystal alignment of the two substrates to one another. The preferential direction of the alignment on the substrates is achieved by an anisotropic pretreatment, typically by rubbing a special, usually polymeric organic layer in one direction, or by vapour-deposition of SiO
x
at an angle. On the other hand, a chiral liquid-crystal medium is employed which consists of mesogenic chiral substances or, most widespread, consists of a non-chiral medium to which a chiral substance (a so-called dopant) is added. The latter alternative is usually preferred since, through variation of the concentration of the dopant, it enables the twist of the liquid-crystal layer to be set to virtually any desired values. It must be ensured here that the ratio of the layer thickness of the liquid-crystal layer (d) to the cholesteric pitch of the liquid-crystal (P) is sufficiently great in order to produce the desired twist. To this end, a twist value of more than 90° (or d/P=90°/360°=0.25) below the desired twist is generally set. This so-called geometrical limit is thus, for example, 0.5−0.25=0.25 for cells having a twist of 180° and 0.667−0.25=0.417 for a cell having a twist of 240°. The upper geometrical limit is in each case at a twist which is 180° higher, i.e. at a d/P value which is 0.5 higher. On application of an electric voltage, however, the cholesteric pitch increases, and thus the lower limit of the d/P ratio increases. Although the same effect can also occur at the upper limit, this is virtually impossible to utilize since an undesired electro-optical effect in the form of a refractive-index grid, the so-called striped transition, perpendicular to the director orientation in the centre of a liquid-crystal layer occurs on application of an electric voltage in the region of relatively high doping. This effect significantly reduces the upper limit of possible doping and is usually, in particular in the case of relatively high twist angles, much more pronounced than the increase in the lower limit on application of the voltage.
For fault-free operation of STN displays, however, a uniform transition of the liquid-crystal layer from the initial alignment to the final alignment, if possible over the entire display area, but at least over a pixel, is required. During this transition, the director of the liquid-crystal layer within each imaginary parallel sub-layer, irrespective of the location, moves toward one another in the same direction and at the same angle. This transition is also known as the Freedericks transition. However, this desired transition does not occur for all possible parameter combinations. Depending both on the properties of the liquid crystal and on the design of the display, an undesired transition known, owing to its optical appearance, as striped domain transition/distortion, occurs on application of an electric voltage. This transition takes precedence over the desired Freedericks transition if the liquid-crystal parameters, in particular the elastic constants and the dielectric anisotropy, are favorable for a steep electro-optical characteristic line. It is furthermore favored by a large d/P ratio and depends not least on the twist angle used and the surface tilt angle. The larger the twist angle, the greater the surface tilt angle has to be in order to enable stable operation of the display. Tilt angles of at least 2°, 3° or 4 to 5° are typically used at the twist angles of 180°, 220° and 240° that are generally frequently used.
Since the operating voltage in displays, i.e. also in displays with these effects, should generally be as low as possible, use is made of liquid-crystal media of high dielectric anisotropy, which 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. If need be, significant amounts of dielectrically neutral liquid-crystal compounds are typically employed. Liquid-crystal compounds with the sign of the dielectric anisotropy opposite to the dielectric anisotropy of the medium are generally employed extremely sparingly or not at all.
An exception is formed here by STN displays, which are also the subject-matter of the present application. In STN displays, dielectrically positive liquid-crystal media which comprise dielectrically negative liquid-crystal compounds can be employed, for example in accordance with DE 41 00 287, 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.
In STN displays, addressing in time multiplex mode is the most wide-spread. In this mode, the columns and rows of a matrix-form arrangement of liquid-crystal switching elements are addressed by means of an Alt and Pleschko addressing scheme. In this case, the liquid-crystal medium of the liquid-crystal display elements reacts to the root mean square (rms) of the addressing voltage. Particularly at relatively high multiplex ratios and in the case of very fast-switching liquid-crystal switching elements, however, this no longer applies. The addressing here can alternatively be carried out by “multiple line addressing”, by means of “active addressing” or the so-called “improved Alt-Pleshko addressing”.
The term “low multiplex drive” is commonly used for multiplex ratios of 1:32 or less, the term “mid multiplex drive” is commonly used for multiplex ratios in the range from about 1:48 to 1:100, and the term “high multiplex drive” is commonly used for multiplex ratios of about 1:128 or more (for example 1:240, 1:400 or 1:480).
The steepness of the electro-optical characteristic line of the liquid-crystal cell must be sufficiently large (the numerical value V
90
/V
10
must be correspondingly small) in order to be able to address the requisite number of rows. This also applies in the case of liquid-crystal displays having low addressing voltages. In these, however, the possible variations of the liquid-crystal medium are subject to relatively narrow limits. On the one hand, a large proportion of highly dielectrically positive compounds is required in order to achieve the requisite low threshold voltages. This results in a large proportion of the constituents of the medium being prespecified by substa

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