Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-05-04
2002-12-10
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000, C349S186000
Reexamination Certificate
active
06493055
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a matrix driving type transverse electric field type liquid crystal display device of a novel display mode. More particularly, the present invention relates to a matrix driving type transverse electric field type liquid crystal display device of a novel display mode using a homeotropically-oriented nematic liquid crystal material which, as a whole liquid crystal layer, generates spontaneous polarization in the presence of an applied transverse electric field; and to a liquid crystal material which may provide such a liquid crystal display device.
2. Description of the Related Art
A transverse electric field type liquid crystal display device has been conventionally known. The transverse electric field type liquid crystal display device has a liquid crystal layer between a pair of substrates, the liquid crystal layer containing a nematic liquid crystal material oriented parallel to the substrate surface, whereby the device is driven by an applied transverse electric field and by utilizing dielectric anisotropy of the nematic liquid crystal material (e.g., Japanese Laid-Open Patent Publication No. 6-160878).
The transverse electric field type liquid crystal display device has problems in that the aperture ratio is low; a high contrast display is not easily achieved; and the response speed is low. Referring to
FIGS. 12A
to
12
D, the configuration of a conventional transverse electric field type liquid crystal display device and the problems associated therewith will be described.
The liquid crystal display device includes a liquid crystal panel. The liquid crystal panel includes a pair of substrates
203
and
203
, transverse electric field electrodes
201
and
202
both of which are provided on one of the substrates, alignment films
204
and
204
each provided on one of the substrates on the liquid crystal layer side thereof, and a liquid crystal layer
210
as a display medium. The liquid crystal display device further includes polarizers
206
and
206
provided external to the liquid crystal panel. In the conventional transverse electric field type liquid crystal display device, in the absence of an applied voltage, liquid crystal molecules
205
contained in the liquid crystal layer
210
are not twisted between the pair of substrates
203
and
203
but are oriented generally parallel to the substrate
203
, as shown in
FIGS. 12A and 12C
. Each of the substrates is provided with the polarizer
206
in such a manner that the direction of a polarization axis
209
of one polarizer
206
is identical to the direction of a molecular axis
208
of the liquid crystal molecules
205
while the direction of a polarization axis
209
of the other polarizer
206
is orthogonal to the direction of the molecular axis
208
of the liquid crystal molecules
205
. For example, in the liquid crystal display device of the above-identified publication, the optical axis of linearly-polarized light which has passed through a polarizer provided on a lower substrate (hereinafter, referred to as the “lower polarizer”), i.e., the transmission axis of the lower polarizer, is identical to the molecular axis of the liquid crystal molecules. Therefore, there is no birefringence generated by the liquid crystal layer. As a result, the linearly-polarized light coming from the lower side of the liquid crystal panel reaches another polarizer provided on an upper substrate (hereinafter, referred to as the “upper polarizer”) without becoming elliptically-polarized light or changing the direction of its optical axis, whereby the linearly-polarized light is blocked by the upper polarizer.
On the other hand, as shown in
FIGS. 12B and 12D
, when an electric field E is applied in a direction
207
which is at a certain angle with respect to the molecular axis direction
208
of the liquid crystal molecules
205
and is generally parallel to the substrate surface, due to the dielectric anisotropy of the liquid crystal molecules
205
, the liquid crystal molecules
205
rotate in a plane parallel to the substrate surface so that the minor axis thereof is orthogonal to the line of electric force. As a result, the optical axis of the linearly-polarized light which has passed through the lower polarizer is shifted with respect to that of the liquid crystal molecules, whereby the light coming from the lower side of the liquid crystal panel passes through the upper polarizer.
The aperture ratio of the conventional transverse electric field type liquid crystal display device is low because the liquid crystal molecules are driven based upon the dielectric anisotropy. In order to maximize the transmission in the conventional liquid crystal display device, the liquid crystal molecules therein must be rotated by 45°. The field strength required for rotating the liquid crystal molecules may vary depending upon the dielectric anisotropy and the elastic constant of the liquid crystal molecules, and the like, but is about 1 V/&mgr;m for a commonly-employed liquid crystal material. When a liquid crystal display device having an ordinary pixel size is produced using an ordinary liquid crystal material, the short side of a pixel is about 80 &mgr;m long. Accordingly, a driving voltage of about 80 V is required to be applied between the transverse electric field electrodes
201
and
202
. However, such a driving voltage, as high as about 80 V, is not practical for an ordinary matrix driving type liquid crystal display device. Therefore, in the conventional liquid crystal display device, an additional electrode (not shown) needs to be provided between the electrodes
201
and
202
in
FIGS. 12A
to
12
D in order to reduce the interval between two electrodes and thus the driving voltage required therebetween. As a result, the additional electrode creates an additional light-blocking portion, thereby lowering the aperture ratio of the liquid crystal display device.
A high contrast display is not easily achieved in the conventional transverse electric field type liquid crystal display device due to the configuration thereof. As described above, in order to block light in the absence of an applied voltage, the direction of the polarization axis of one polarizer (e.g., the transmission axis of the lower polarizer) needs to be identical to the molecular axis direction of the liquid crystal molecules while the direction of the polarization axis of the other polarizer (e.g., the transmission axis of the upper polarizer) needs to be orthogonal to the molecular axis direction of the liquid crystal molecules. For example, if the polarization axis of the lower polarizer is not identical to the molecular axis of the liquid crystal molecules, linearly-polarized light which has passed through the lower polarizer becomes elliptically-polarized light due to the birefringence of the liquid crystal layer, and thus passes through the upper polarizer. Therefore, in order to achieve a high contrast display, it is necessary for the direction of the alignment treatment (e.g., rubbing treatment) for the upper substrate to be precisely identical to that for the lower substrate, for the direction of the alignment treatment to be precisely identical to the polarization axis direction of one of the polarizers, and for the direction of the alignment treatment to be precisely orthogonal to the polarization axis direction of the other polarizer. However, when actually producing a liquid crystal display device, it is very difficult to precisely arrange these components as described above. Accordingly, it is very difficult to achieve a high contrast display with the conventional transverse electric field type liquid crystal display device. Moreover, the productivity in the manufacture of such a liquid crystal display device is very low due to the precise arrangement of the components being required.
The response speed is low in the conventional liquid crystal display device for the following reason. The response speed can be generally classified into two factors, i.e.,
Kozaki Shuichi
Miyachi Koichi
Shimoshikiryo Fumikazu
Chung David
Nixon & Vanderhye P.C.
Parker Kenneth
Sharp Kabushiki Kaisha
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