Electrode patterns for liquid crystal cells

Details Electrode patterns for liquid crystal cells Electrode patterns for liquid crystal cells

1999-11-12

2003-12-30

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S141000

Reexamination Certificate

active

06671019

ABSTRACT:

BACKGROUND OF THE INVENTION
Nematic, smectic C, chiral nematic, chiral Smectic C and other liquid crystal devices are routinely used in display and other electro-optic applications. For many display applications luminance, grey scale and grey scale as a function of angle are important characteristics. Promising devices which potentially exhibit favorable luminance, grey scale and grey scale as a function of angle properties include vertically or homeotropically aligned cholesteric (VAC) devices, such as described in U.S. Pat. No. 5,466,358, and in-plane switching cells such as described in U.S. Pat. No. 5,477,358. For good switching rates and grey scale behavior as a function of viewing angle these devices require rapid, multi-domain control of the liquid crystal director.
The use of in-plane electric fields for the multi-domain control of the alignment of a liquid crystal has been described, for example, in U.S. Pat. No. 5,309,264. Refinements have been described in, for example, U.S. Pat. No. 5,777,711. None of these devices exhibits adequate and sufficiently rapid domain control when the liquid crystal is homeotropically aligned. Some of these patterns also require relatively precise positioning of the top and bottom electrodes relative to each other, which is difficult to accomplish and typically results in an appreciable loss of manufacturing throughput. Another approach involving in-plane electric fields involves two electrodes on at least one of the surfaces of a liquid crystal device as seen in U.S. Pat. No. 5,745,207. This results in large in-plane electric fields, but presents practical difficulties and requires numerous steps in connection with the manufacture of such devices.
SUMMARY OF THE INVENTION
This invention makes possible exceptionally good control over the alignment of liquid crystals in displays or other electro-optic devices. Control of the alignment of the liquid crystal is achieved by use of appropriately patterned electrodes. In particular, small scale patterns are created in the electrodes by non-conducting gaps having dimensions and shapes selected in accordance with the invention. These small scale patterns are contained largely or entirely within a single domain of the liquid crystal, often covering much of the area of the electrode. As a result, the instant invention allows for good control of the liquid crystal directors, which results in fast switching and good control over the number, size and location of liquid crystal domains. Moreover, the advantages of the invention can generally be achieved by relatively simple and inexpensive manufacturing techniques.
Accordingly, in one aspect of the invention there is provided a liquid crystal cell comprising first and second substrates spaced apart by a distance t and a liquid crystal material disposed therebetween. The first and second electrodes are disposed on said first and second substrates, respectively, and connected to a power supply. At least one of the first and second electrodes has at least one pixel defined by dimensions in a plane parallel to the plane of the substrate and further including at least one non-conducting gap therein, the non-conducting gap being a small scale gap having at least one dimension in the plane of the electrode that does not exceed about 2.5 times the distance t. When in a field-on condition in cooperation with the other of said first or second electrode, the pixel contains an electric field effective to produce at least one liquid crystal domain having dimensions in a plane parallel to the plane of the substrates within which the liquid crystal molecules have an azimuthal orientation with predominantly the same sign and direction, and wherein at least a portion of the small scale gap is disposed within the domain at least about 1 times, and preferably at least 2.0 times, the distance t from the boundaries of the domain. Preferably, the cell includes a plurality of said small scale gaps within said at least one pixel.
In one embodiment, when in said field-on condition, the liquid crystal material exhibits a plurality of domains within the pixel, each said domain being adjacent at least one said small scale gap, or in another embodiment, each said domain is adjacent a plurality of said small scale gaps. In yet another aspect of the invention, both said first and second electrodes include at least one pixel containing at least one said small scale gap or, in another embodiment, a plurality of said small scale gaps. In another embodiment where both the first and second electrodes include at least one pixel, in a field-on condition, the liquid crystal material exhibits a plurality of domains within each pixel, each domain being adjacent at least one small scale gap or a plurality of said small scale gaps. In many embodiments of the invention, the pixels on the first and second substrates are substantially adjacent and coextensive.
In still other embodiments, at least a portion of a boundary of the domain is substantially linear, and the boundary is substantially adjacent and colinear with at least one of a non-conducting gap or portion of a non-conducting gap, or a difference in a location of edges of the electrodes on opposing substrates at an edge of the pixel, and a plurality of said small scale gaps is disposed at an angle thereto, or extend at an angle therefrom. In many embodiments, the small scale gaps are substantially rectangular and parallel to one another. In other embodiments, all the domain boundaries are substantially linear and disposed substantially adjacent and colinear with a non-conducting gap or a portion of a non-conducting gap, or with a difference in a location of edges of the electrodes on opposing substrates at an edge of the pixel, with a plurality of said small scale gaps disposed at an angle thereto or extending at an angle therefrom. Preferably, the small scale gaps are substantially rectangular and parallel. In some preferred embodiments, both said first and second substrates include at least one such pixel such that the pixels are substantially adjacent and coextensive. In other embodiments, when each pixel includes a plurality of said small scale gaps, the small scale gaps are substantially rectangular and parallel to one another, and the substantially parallel gaps on opposing said substrates are rotated relative to each other by an angle of from about 10° more to about 30° less than an angle by which the azimuthal orientation of said liquid crystal rotates as a consequence of its natural pitch on passing through the cell. Preferably, the liquid crystal material is selected from a nematic liquid crystal or a chiral nematic liquid crystal having negative dielectric anisotropy, and at least one said substrate is treated to align the liquid crystal.
It is another aspect of the invention wherein at least about 60% of the area within at least one pixel is within about 1.5 times the distance t, and preferably 0.7 times the distance t from an edge of a conducting portion of the electrode. In many preferred embodiments at least about 80%, and more preferably still 90%, of the area of the pixels will be so designed. In one such embodiment, when in a field-on condition, the liquid crystal material exhibits a plurality of domains within the pixel, each domain being adjacent a plurality of said small scale gaps wherein at least about 60% of the area within said at least one pixel is within about 1.5 times the distance t from an edge of a conducting portion of said electrode.
Preferably, the small scale gaps form a pattern within each liquid crystal domain which transforms according to a two-dimensional space group selected from the group consisting of Pg, Cm or Pl.
It is yet another aspect of the invention to provide a liquid crystal cell wherein the pixel includes a plurality of small scale non-conducting gaps having a length and a width in the plane of said electrode wherein said width does not exceed about 2.5 times the distance t and wherein at least about 60% of the area within said at least one pixel is within about 1.5 times the distance

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