Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-03-01
2003-12-30
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S034000, C349S191000, C345S211000, C345S212000, C345S214000
Reexamination Certificate
active
06671009
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an OCB-mode liquid crystal display device with fast response and broad viewing angle for displaying TV images, personal computer or multimedia images, a manufacturing method for the same, and a driving method for a liquid crystal display device.
BACKGROUND ART
Conventional liquid crystal display devices employ, as one example of liquid crystal display modes, twisted nematic (TN) mode liquid crystal display elements using a nematic liquid crystal with positive dielectric anisotropy, but these have the shortcomings of a slow response and narrow viewing angles. There are also display modes with slow response and broad viewing angles, using a ferroelectric liquid crystal (FLC) or anti-ferroelectric liquid crystal, but these have shortcomings with regard to burn-in, shock resistance, and temperature dependence. There is also the in-plane switching (IPS) mode which has extremely broad viewing angles, in which the liquid crystal molecules are driven within the display plane by a transversal electric field, but the response times are slow, and numerical aperture and luminance are low. When trying to display full-color moving images on large screens, a liquid crystal mode with broad viewing angle, high luminance and fast display properties is necessary, but at present, a liquid crystal display mode that perfectly satisfies all these requirements in practice does not exist.
Among the conventional liquid crystal display devices that aimed for at least a broad viewing angle and high luminance are liquid crystal display devices in which TN mode liquid crystal regions are partitioned into two domains to widen the viewing angle vertically (see SID 92 DIGEST p.798-801). That is to say, using a nematic liquid crystal with positive dielectric anisotropy in the display pixels of the liquid crystal display device, two TN mode liquid crystal regions with different alignment orientation of the liquid crystal molecules are formed, and the viewing angle is enlarged by this TN-mode with two alignment domains.
FIG. 48
is a diagram showing the configuration of such a conventional liquid crystal display device. In
FIG. 48
, numerals
701
and
702
denote glass substrates, numerals
703
and
704
denote electrodes, and numerals
705
,
705
′,
706
, and
706
′ denote alignment films. In the alignment region A, the nematic liquid crystal molecules
707
and
707
′ with positive dielectric anisotropy are slightly tilted away from the upper and lower boundaries to the opposing substrates, forming a larger and a smaller pretilt angle, whereas in the other alignment region B, the size of the pretilt angles with respect to the upper and lower boundaries of the opposing substrates is opposite to that in the alignment region A. Both the larger and the smaller pretilt angles are several degrees each, and are set to different angles. An example of a conventional manufacturing method for forming alignment regions with different pretilt angles at the upper and lower substrates is spreading photoresist on an alignment film, masking the photoresist photolithographically, and rubbing the desired alignment film surface in a predetermined direction, and repeating this procedure a certain number of times. As shown in
FIG. 1
, with this configuration, the liquid crystal molecules in the central portions of the liquid crystal layer in the alignment regions A and B are provided with opposite orientations, and since the liquid crystal molecules of the alignment regions rise in different directions when a voltage is applied, the refractive index anisotropy with respect to incoming light evens out for each pixel, and the viewing angle can be enlarged. With this conventional TN-mode with two alignment domains, the viewing angle can be made wider than with regular TN-mode, and the vertical viewing angle becomes about ±35° at a contrast of 10.
However, the response time is substantially the same as in TN-mode, namely about 50 ms. Thus, in this conventional TN-mode with two alignment domains, viewing angle and response are insufficient.
As for liquid crystal display modes utilizing the so-called homeotropic alignment mode, in which the liquid crystal molecules are aligned approximately vertically at the boundaries to the alignment films, there are liquid crystal display devices with broad viewing angle and fast response that are provided with film phase-difference plates and subjected to alignment partitioning, but again the response time between black and white display is about 25 ms, and in particular the response time for gray scales is slow at 50-80 ms, which is longer than the 1/30 s that are held to be the visual speed of the human eye, so that moving images appear blurred.
On the other hand, a bend alignment type liquid crystal display device (OCB-mode liquid crystal display device) has been proposed, which utilizes changes of the refractive index due to changes in the angle with which the liquid crystal molecules rise when the liquid crystal molecules between the substrates are in bend alignment. The speed with which the orientation of bend aligned liquid crystal molecules changes in the ON state and the OFF state is much faster than the speed of orientation changes between ON and OFF states in TN liquid crystal display devices, so that a liquid crystal display device with fast response time can be obtained. Moreover, in this bend alignment type liquid crystal display device, optical phase differences can be compensated automatically, because all the liquid crystal molecules are bend aligned between the upper and lower substrates, and the liquid crystal display device has potential as a liquid crystal display device with low voltage and broad viewing angle, because phase differences are compensated by the film phase difference plates.
Incidentally, these liquid crystal display devices are manufactured such that the liquid crystal molecules between the substrates are in splay alignment when no voltage is applied. In order to change the refractive index using bend alignment, the entire display portion has to be transitioned uniformly from splay alignment to bend alignment before use of the liquid crystal display device. When applying a voltage between the opposing display electrodes, the transition seeds for the transition from splay alignment to bend alignment do not appear in uniform distribution, but around the distributed spacers, at alignment irregularities at the boundary to the alignment films, or at damaged portions. Furthermore, the transition seeds do not necessarily appear always at the same locations, which may easily lead to display defects, in which the transition sometimes takes place and sometimes does not take place. Consequently, it is very important that at least all pixel portions of the entire display portion are transitioned uniformly from splay alignment to bend alignment before use.
However, conventionally, when applying a simple ac voltage, the transition sometimes does not take place, and when it does take place, the transition time is very long.
DISCLOSURE OF THE INVENTION
It is an object of the present invention bend alignment type liquid crystal display device with fast response, suitable for display of moving images and broad viewing angle, in which the transition into bend alignment takes place reliably, and the transition is concluded in short time so that there are no display defects, as well as a manufacturing method for such a liquid crystal display device, and a driving method for a liquid crystal display device.
To achieve this object, according to a first aspect of the invention, a method for driving a liquid crystal display device, for an alignment transition from splay alignment to bend alignment in a liquid crystal display device which includes a pair of substrates and a liquid crystal layer disposed between the substrates; wherein, when no voltage is applied, pretilt angles of the liquid crystal at an upper and at a lower boundary of the liquid crystal layer have opposite signs, and the liquid crystal layer
Adachi Katsumi
Hatta Shin-ichiro
Hattori Katsuji
Ishihara Shoichi
Kubota Hirofumi
Kim Robert H.
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Schechter Andrew
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