Liquid crystal display device

Details Liquid crystal display device Liquid crystal display device

2001-02-06

2004-03-30

Kim, Robert H. (Department: 2871)

Liquid crystal cells, elements and systems

With specified nonchemical characteristic of liquid crystal...

Within nematic phase

C349S169000

Reexamination Certificate

active

06714276

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a surface mode LCD such as a pi-cell device or a splay-bend device (SBD).
2. Description of the Related Art
The term “surface mode LCD” as used herein means an LCD in which the optical change caused by varying the electric field across the liquid crystal layer occurs primarily in layers in the vicinity of a substrate of the liquid crystal. Examples of surface mode LCDs are the pi-cell and the splay-bend device, although other types of surface mode LCDs are known. Surface mode LCDs are disclosed in “Sov. J. Quantum Electronics”, 1973, Vol 3, p78-79.
The pi-cell (otherwise known as an “optically compensated birefringent device” or OCB) is described in “Mol. Cryst. Liq. Cryst.”, 1984, Vol 113, p329-339, and in U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG.
1
. The device comprises transparent substrates
1
,
1
′ on which are disposed alignment layers
2
,
2
′. A layer of nematic liquid crystal
3
is disposed between the substrates
1
,
1
′.
The alignment layers
2
,
2
′ create parallel alignment of the liquid crystal molecules in the liquid crystal layer
3
at its boundaries with the alignment layers
2
,
2
′. This can be achieved by using parallel-rubbed polyamide alignment layers.
Addressing electrodes (not shown) are provided on the substrates
1
,
1
′, so that an electric field can be applied to selected areas of the liquid crystal layer. The liquid crystal layer
3
is placed between linear polarizers
4
,
4
′, whose transmission axes are crossed with one another and are at 45° to the optic axis of the liquid crystal layer.
A retarder
5
with its optic axis perpendicular to the optic axis of the liquid crystal layer, may optionally be provided to compensate for the retardation of the liquid crystal layer. The retarder lowers the required range for the operating voltage by allowing zero retardation of the LCD to be achieved at a finite voltage across the liquid crystal layer.
FIG. 1
shows a transmissive LCD. A pi-call can also be embodied as a reflective device by providing a reflector below the liquid crystal layer, possibly by making the addressing electrode on the lower substrate a reflective electrode. The lower polarizer
4
′ is not required with a reflective pi-cell.
The principle of operation of the pi-cell device is illustrated in
FIGS. 2A
to
2
D.
When no electric field is applied across the liquid crystal layer, the liquid crystal is in an H-state (homogenous state or splay state), in which the liquid crystal molecules in the center of the liquid crystal layer are substantially parallel to the substrates. This is shown in FIG.
2
A. The short lines in the figures represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a band state). In this state, the liquid crystal molecules in the center of the liquid crystal layer are substantially perpendicular to the substrates.
FIG. 2C
shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and
FIG. 2D
shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V-state and the second, higher voltage V-state.
If the electric field across the liquid crystal layer should be reduced below the threshold value, the liquid crystal layer will relax to the H-state of
FIG. 2A
; in order to re-commence operation of the device, it is necessary to put the liquid crystal layer back into the V-state. This generally requires a large applied voltage, owing to the low pre-tilt of the liquid crystal molecules. The pre-tilt is usually below 45° and typically between 2 and 10° so as to provide sufficient optical modulation and fast switching between the two V-states (for instance of the order of a millisecond or less).
One problem with known OCB devices is the difficulty of nucleating and stabilising the V-state, which is topologically distinct from the H-state. One prior art technique is described in UK Patent Application 9521043.1/2 306 228. In this prior art technique, the V-state is nucleated under the application of a high voltage, and is stabilized by the polymerization of a network whilst a high voltage is applied. This prior art technique is, however, unsuitable for use in active matrix devices, since it is difficult to apply voltages having the required magnitude in a TFT panel. A further disadvantage is that the in-situ polymerization can lead to ionic contamination of the liquid crystal layer, and result in image sticking.
The SBD device, which is also a surface mode device, is described in UK Patent Application No. 9712378.0/2 326 245. The structure of an SBD device is generally similar to that of a pi-cell, except that the alignment layers in an SBD device have a high pre-tilt whereas the alignment layers in a pi cell have a low pre-tilt. An SBD device uses a liquid crystal material with a negative di-electric anisotropy, whereas a pi-cell uses a liquid crystal material having a positive di-electric anisotropy.
The principle of operation of an SBD is analogous to that of a pi-cell. When no voltage is applied across the liquid crystal layer of an SBD, the stable liquid crystal state is a V-state. When an electric field greater than a threshold value is applied across the liquid crystal layer, an H-state becomes stable. The SBD is operated by switching the liquid crystal between a first H-state which occurs at a low applied voltage across the liquid crystal layer and a second H-state which occurs when a higher voltage is applied across the liquid crystal layer. If the electric field across the liquid crystal layer is reduced below the threshold value, the liquid crystal Will relax into the V-state and it will be necessary to put the liquid crystal back into the H-state before operation can be re-commenced.
The high pre-tilt alignment layers required for an SBD can be produced, for example, by the photo-polymerization of a mixture of reactive mesogens.
SID 97 Digest, page 739, discloses a method of promoting nucleation of the V-state in a pi-cell. Voltages of the order of 20 V are applied across the liquid crystal layer to switch the liquid crystal from the H-state to the V-state. However, it is difficult to provide voltages of this magnitude in a TFT (thin film transistor) substrate.
Japanese published Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi-cell panel, and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls/pillars acting as nucleation sites for growth of the V-state into the existing H-state. This prior art has a number of disadvantages. Firstly, it requires additional process steps during fabrication of the panel, since it is necessary to align the liquid crystal molecules under the influence of an applied electric field. These additional process steps complicate the fabrication of the panel. Secondly, some spacer balls/pillars can cause the H-state to form in the desired V-state, thus destabilising the operating state of the panel.
Miwa et al disclose, in IDW 97-Digest page 85, a method of maintaining the stability of a V-state in a pi-cell. A resetting period is provided within each frame, and the high voltage V-state is addressed in this period. This prevents the liquid crystal layer relaxing to the H-state when low driving voltages are applied. This does not, however, address the initial nucleation of the V-state from the H-state.
U.S. Pat. No. 4,566,758 discloses a surface mode nematic liquid crysta

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