U.S. patent application number 09/986889 was filed with the patent office on 2002-05-23 for liquid crystal display device and associated fabrication method.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Hattori, Katsuji, Ishihara, Shoichi, Yamazoe, Hiroshi.
Application Number | 20020060766 09/986889 |
Document ID | / |
Family ID | 27548303 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020060766 |
Kind Code |
A1 |
Hattori, Katsuji ; et
al. |
May 23, 2002 |
Liquid crystal display device and associated fabrication method
Abstract
In a liquid crystal display device which displays images by
changing light transmission through formation of a bend alignment
state of the liquid crystal, a large pretilt angle domain is formed
on at least either the surface of the pixel electrode or the
surface of the counter electrode and the large pretilt angle domain
is so conditioned as to cause a larger pretilt angle of the liquid
crystal molecules than the surrounding region does. This permits
quick, reliable transition from a splay alignment state to the bend
alignment state. Such transition is also facilitated by adding a
chiral agent to a liquid crystal. A combination of the large
pretilt angle domain and a chiral agent further facilitates
occurrence of the transition. Another liquid crystal display device
is designed such that the twist angle of the molecules of the
liquid crystal ranges from 160.degree. to 200.degree. and that the
fast response achieved by formation of an alignment state similar
to the bend alignment state can be achieved without formation of
the bend alignment state, by applying a driving voltage higher than
the voltage, which causes the extremum of transmission in the
driving voltage-transmission characteristic of the device, between
the pixel and counter electrodes. In another liquid crystal display
device, the pixels corresponding to the pixel electrode are divided
into at least two domains which cause bend director fields having
different orientations, thereby improving viewing angles. In still
another liquid crystal display device, the twist angle of the
molecules of the liquid crystal is in the range of from 160.degree.
to 200.degree. or from 250.degree. to 290.degree. and the liquid
crystal layer contains a dye or pigment. The device is driven with
voltages in a certain high range. With this arrangement, the
difficulty in causing the transition can be overcome. Another
liquid crystal display device has pretilt angles varying according
to the colors of pixels. This arrangement prevents hue shifts
caused by changes in the transmission of the liquid crystal, the
changes being due to the different wavelengths of transmitted
light.
Inventors: |
Hattori, Katsuji;
(Takarazuka-shi, JP) ; Ishihara, Shoichi;
(Katano-shi, JP) ; Yamazoe, Hiroshi; (Katano-shi,
JP) |
Correspondence
Address: |
PARKHURST & WENDEL, L.L.P.
Suite 210
1421 Prince Street
Alexandria
VA
22314-2805
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
|
Family ID: |
27548303 |
Appl. No.: |
09/986889 |
Filed: |
November 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09986889 |
Nov 13, 2001 |
|
|
|
08922021 |
Sep 2, 1997 |
|
|
|
Current U.S.
Class: |
349/136 |
Current CPC
Class: |
G02F 1/1395 20130101;
G02F 1/133707 20130101 |
Class at
Publication: |
349/136 |
International
Class: |
G02F 001/141 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 1996 |
JP |
8-234020 |
Sep 27, 1996 |
JP |
8-256103 |
Jan 24, 1997 |
JP |
9-010887 |
Apr 21, 1997 |
JP |
9-102960 |
Apr 22, 1997 |
JP |
9-104346 |
Jul 23, 1997 |
JP |
9-196684 |
Claims
What is claimed is:
1. A liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode and (3) a liquid crystal
enclosed between the pixel and counter electrodes, wherein the
respective opposed surfaces of the pixel and counter electrodes are
conditioned such that liquid crystal molecules contacting or in the
vicinity of said surfaces have specified pretilt angles, wherein
images are displayed by changing light transmission through
formation of a bend alignment state of the liquid crystal, and
wherein a large pretilt angle domain is formed on at least either
one of said surfaces of the pixel and counter electrodes, the large
pretilt angle domain causing a larger pretilt angle of liquid
crystal molecules than a region surrounding the large pretilt angle
domain does.
2. A liquid crystal display device according to claim 1, wherein
the pretilt angle of the liquid crystal molecules caused by the
large pretilt angle domain is 10.degree. or more larger than that
caused by the surrounding region.
3. A liquid crystal display device according to claim 1, wherein
the pretilt angle of the liquid crystal molecules caused by the
large pretilt angle domain is 15.degree. or more.
4. A liquid crystal display device according to claim 3, wherein
the pretilt angle of the liquid crystal molecules caused by the
large pretilt angle domain is 70.degree. or more.
5. A liquid crystal display device according to claim 1, wherein a
plurality of said pixel electrodes are provided and at least one
large pretilt angle domain is formed on each pixel electrode.
6. A liquid crystal display device according to claim 1, wherein
said large pretilt angle domain is formed by a surface alignment
agent which causes a larger pretilt angle of the liquid crystal
molecules than the surrounding region does.
7. A liquid crystal display device according to claim 1, wherein
said large pretilt angle domain is formed by a projection which
causes a larger pretilt angle of the liquid crystal molecules than
the surrounding region does.
8. A liquid crystal display device according to claim 1, wherein
said liquid crystal contains a chiral agent.
9. A method for fabricating a liquid crystal display device which
comprises (1) a pixel electrode, (2) a counter electrode and (3) a
liquid crystal enclosed between the pixel and counter electrodes
and wherein images are displayed by changing light transmission
through formation of a bend alignment state of the liquid crystal,
said method comprising the steps of: (a) forming a film made from a
mixture of a first surface alignment agent and a second surface
alignment agent on at least one of the surfaces of said pixel and
counter electrodes, the first surface alignment agent causing a
first pretilt angle of liquid crystal molecules in the vicinity of
said pixel electrode or counter electrode, the second surface
alignment agent causing a second pretilt angle larger than the
first pretilt angle, and (b) causing the phase separation of said
first and second surface alignment agents contained in said
film.
10. A method for fabricating a liquid crystal display device which
comprises (1) a pixel electrode, (2) a counter electrode and (3) a
liquid crystal enclosed between the pixel and counter electrodes
and wherein images are displayed by changing light transmission
through formation of a bend alignment state of the liquid crystal,
said method comprising the steps of: (a) forming an alignment film
made from a first surface alignment agent on at least one of the
surfaces of said pixel and counter electrodes, the first surface
alignment agent causing a first pretilt angle of liquid crystal
molecules in the vicinity of said pixel electrode or counter
electrode, and (b) partially forming an alignment film made from a
second surface alignment agent on the alignment film made from the
first surface alignment agent, the second surface alignment agent
causing a second pretilt angle larger than the first pretilt
angle.
11. A liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode, (3) a liquid crystal enclosed
between the pixel and counter electrodes, and (4) a phase
compensating layer, wherein images are displayed by changing light
transmission through formation of a bend alignment state of the
liquid crystal, and wherein said liquid crystal contains a chiral
agent.
12. A liquid crystal display device according to claim 11, wherein
said chiral agent produces a chiral pitch in said liquid crystal,
said chiral pitch ranging from 5 .mu.m to 100 .mu.m.
13. A liquid crystal display device according to claim 11, wherein
said chiral agent produces a chiral pitch in said liquid crystal,
said chiral pitch ranging from 7 .mu.m to 40 .mu.m.
14. A liquid crystal display device comprising (1) a first
substrate having a pixel electrode formed thereon, (2) a second
substrate having a counter electrode formed thereon and positioned
opposite the first substrate, (3) a liquid crystal enclosed between
the first and second substrates, (4) a first polarizer and a second
polarizer disposed so as to sandwich the first and second
substrates, the polarizing axes of the first and second polarizers
crossing at right angles, and (5) a driver circuit for applying
driving voltage between the pixel electrode and the counter
electrode, wherein the liquid crystal molecules of said liquid
crystal have a twist angle ranging from 160.degree. to 200.degree.,
and wherein said driver circuit applies driving voltage between the
pixel and counter electrodes, the driving voltage being higher than
the highest one of voltages that cause the maximal value of light
transmission in the driving voltage-transmission characteristic of
the liquid crystal display device.
15. A liquid crystal display device comprising (1) a first
substrate having a pixel electrode formed thereon, (2) a second
substrate having a counter electrode formed thereon and positioned
opposite the first substrate, (3) a liquid crystal enclosed between
the first and second substrates, (4) a first polarizer and a second
polarizer disposed so as to sandwich the first and second
substrates, the polarizing axes of the first and second polarizers
being parallel to each other, and (5) a driver circuit for applying
driving voltage between the pixel electrode and the counter
electrode, wherein the liquid crystal molecules of said liquid
crystal have a twist angle ranging from 160.degree. to 200.degree.,
and wherein said driver circuit applies driving voltage between the
pixel and counter electrodes, the driving voltage being higher than
the highest one of voltages that cause the minimal value of light
transmission in the driving voltage-transmission characteristic of
the liquid crystal display device.
16. A liquid crystal display device according to claim 14 or 15,
further comprising a phase compensating layer at least either
between the first substrate and the first polarizer or between the
second substrate and the second polarizer.
17. A liquid crystal display device according to claim 16, wherein
said phase compensating layer is a biaxial phase compensating
film.
18. A liquid crystal display device according to claim 16, wherein
said phase compensating layer is a laminated film composed of a
biaxial phase compensating film and a uniaxial phase compensating
film.
19. A liquid crystal display device according to claim 14 or 15,
wherein the chiral pitch of said liquid crystal is not less than
the thickness of the liquid crystal and not more than three times
the thickness of the liquid crystal.
20. A liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode and (3) a liquid crystal
enclosed between the pixel and counter electrodes, wherein images
are displayed by changing light transmission through formation of a
bend alignment state of the liquid crystal, and wherein pixels
corresponding to the pixel electrode are divided into at least two
domains which cause bend director fields having different
orientations in the liquid crystal.
21. A liquid crystal display device according to claim 20, wherein
the orientations of the bend director fields caused by the two
domains cross at right angles.
22. A liquid crystal display device according to claim 20, wherein
the pixel electrode and the counter electrode are respectively
provided with an alignment film at their surfaces facing the liquid
crystal, and wherein each alignment film is divided into said
domains which are conditioned so as to cause differently oriented
director fields in the liquid crystal.
23. A liquid crystal display device according to claim 22, wherein
the pairs of opposed domains, one domain of each pair being formed
on the alignment film of the pixel electrode and the other domain
of each pair being formed on the alignment film of the counter
electrode, are conditioned such that each pair of opposed domains
form a pair of director fields, the director fields of each pair
respectively including liquid crystal molecules aligned at pretilt
angles in a plane perpendicular to a displaying plane such that the
director fields of each pair are symmetric with respect to a plane
that is parallel to the displaying plane and located in the middle
of the distance between the pixel electrode and the counter
electrode.
24. A liquid crystal display device according to claim 20, further
comprising a phase compensating layer formed on at least either the
outer surface of the pixel electrode or the outer surface of the
counter electrode, for optically compensating the director
alignment of the liquid crystal molecules.
25. A liquid crystal display device according to claim 20, wherein
said liquid crystal contains a chiral agent.
26. A method for fabricating a liquid crystal display device,
wherein images are displayed by changing light transmission through
formation of a bend alignment state of a liquid crystal enclosed
between a pixel electrode and a counter electrode, and wherein
alignment films are respectively formed on the surfaces of the
pixel electrode and the counter electrode, the surfaces facing the
liquid crystal, the alignment film on the pixel electrode and a
portion of the alignment film on the counter electrode which
portion corresponds to the pixel electrode being respectively
divided into at least two domains having different conditioning
directions by directing ultraviolet lights to two regions
corresponding to the two domains to be formed, the ultraviolet
lights being directed in different directions or having different
polarizing directions.
27. A liquid crystal display device comprising (1) a twisted liquid
crystal cell having a liquid crystal layer sandwiched between a
pair of substrates, the liquid crystal layer having liquid crystal
molecules twisted between said pair of substrates and (2) a
polarizing plate disposed on either the light incoming side or
light outgoing side of the liquid crystal cell, wherein said
polarizing plate is disposed such that its polarizing axis is
substantially parallel to the longitudinal axis of the liquid
crystal molecules on the interface of one of said pair of
substrates, said substrate being on the light incoming side or
light outgoing side, wherein the twist angle of the liquid crystal
molecules in said liquid crystal layer is in the range of from
160.degree. to 200.degree. and said liquid crystal layer contains a
dye or pigment, which has a voltage-brightness characteristic
according to which when the voltage applied to said liquid crystal
cell exceeds the Freedericksz threshold voltage of the liquid
crystal, brightness first rises gently with a first gradient and
then rises with a second gradient sharper than the first gradient,
and which performs image displaying with driving voltages at least
higher than the voltage corresponding to the turning point where
brightness changes from the first gradient to the second
gradient.
28. A liquid crystal display device comprising (1) a twisted liquid
crystal cell having a liquid crystal layer sandwiched between a
pair of substrates, the liquid crystal layer having liquid crystal
molecules twisted between said pair of substrates and (2) a
polarizing plate disposed on either the light incoming side or
light outgoing side of the liquid crystal cell, wherein said
polarizing plate is disposed such that its polarizing axis is
substantially parallel to the longitudinal axis of the liquid
crystal molecules on the interface of one of said pair of
substrates, said substrate being on the light incoming side or
light outgoing side, wherein the twist angle of the liquid crystal
molecules in said liquid crystal layer is in the range of from
250.degree. to 290.degree. and said liquid crystal layer contains a
dye or pigment, which has a voltage-brightness characteristic
according to which when the voltage applied to said liquid crystal
cell exceeds the Freedericksz threshold voltage of the liquid
crystal, brightness first rises gently with a first gradient and
then rises with a second gradient sharper than the first gradient,
and which performs image displaying with driving voltages at least
higher than the voltage corresponding to the turning point where
brightness changes from the first gradient to the second
gradient.
29. A liquid crystal display device comprising (1) a twisted liquid
crystal cell having a liquid crystal layer sandwiched between a
pair of substrates, the liquid crystal layer having liquid crystal
molecules twisted between said pair of substrates and (2) a
polarizing plate disposed on either the light incoming side or
light outgoing side of the liquid crystal cell, wherein said
polarizing plate is disposed such that its polarizing axis is
substantially parallel to the longitudinal axis of the liquid
crystal molecules on the interface of one of said pair of
substrates, said substrate being on the light incoming side or
light outgoing side, wherein the twist angle of the liquid crystal
molecules in said liquid crystal layer is in the range of from
160.degree. to 200.degree. and said liquid crystal layer contains a
dye or pigment, which performs image displaying with driving
voltages which permit the average tilt angle of liquid crystal
directors relative to the plane of the substrates to be 10.degree.
or more.
30. A liquid crystal display device comprising (1) a twisted liquid
crystal cell having a liquid crystal layer sandwiched between a
pair of substrates, the liquid crystal layer having liquid crystal
molecules twisted between said pair of substrates and (2) a
polarizing plate disposed on either the light incoming side or
light outgoing side of the liquid crystal cell, wherein said
polarizing plate is disposed such that its polarizing axis is
substantially parallel to the longitudinal axis of the liquid
crystal molecules on the interface of one of said pair of
substrates, said substrate being on the light incoming side or
light outgoing side, wherein the twist angle of the liquid crystal
molecules in said liquid crystal layer is in the range of from
250.degree. to 290.degree. and said liquid crystal layer contains a
dye or pigment, which performs image displaying with driving
voltages which permit the average tilt angle of liquid crystal
directors relative to the plane of the substrates to be 20.degree.
or more.
31. A liquid crystal display device according to any one of claims
27 to 30, wherein said dye or pigment is black in color and image
displaying is performed with driving voltages equal to and less
than the Freedericksz threshold voltage of the liquid crystal only
when black images are displayed.
32. A liquid crystal display device comprising (1) a liquid crystal
cell having a liquid crystal layer sandwiched between a pair of
substrates which are rubbed in the same direction and (2) a
polarizing plate disposed on either the light incoming side or
light outgoing side of the liquid crystal cell, wherein said
polarizing plate is disposed with its polarizing axis being
substantially parallel to the rubbing direction of said substrates,
wherein said liquid crystal layer contains a dye or pigment, and
wherein said liquid crystal cell is a bend director alignment cell
in which a bend alignment state is formed at the time of
application of voltage.
33. A liquid crystal display device according to claim 32, wherein
the bend alignment state formed at the time of application of
voltage includes twist at the center of the liquid crystal
cell.
34. A liquid crystal display device according to claims 33, wherein
said liquid crystal contains a chiral agent.
35. A liquid crystal display device comprising (1) a plurality of
pixel electrodes constituting a plurality of pixels, (2) a counter
electrode, (3) a liquid crystal enclosed between the pixel
electrodes and the counter electrode, and (4) a color filter having
regions respectively corresponding to said pixels, each region
transmitting any one of a plurality of colors, wherein at least
either the surfaces of the pixel electrodes or the surface of the
counter electrode is conditioned such that liquid crystal molecules
in the vicinity of the surfaces or surface are aligned so as to
form specified pretilt angles, wherein images are displayed by
changing light transmission through formation of a bend alignment
state of said liquid crystal, and wherein said specified pretilt
angles vary according to the colors of the pixels.
36. A liquid crystal display device according to claim 35, wherein
said specified pretilt angles are determined such that when the
same voltage is applied between the pixel electrodes for different
colors and the counter electrode, the same transmission can be
obtained for the pixels of different colors.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The invention relates to liquid crystal display devices well
suited for use in computer displays, television receivers and other
industrial products and to methods for fabricating them. More
particularly, the invention pertains to light-transmissive type and
light-reflective type liquid crystal display devices capable of
providing rapid response and a wide range of viewing angles and to
fabrication methods thereof.
[0003] (2) Description of the Related Art
[0004] There have been practically used twisted-nematic (TN) liquid
crystal display devices incorporating a nematic liquid crystal. The
TN mode, however, has the drawback of poor response. Another
disadvantage of the TN mode is that viewing angles, that is, angles
through which the viewer can see images properly are narrow.
Concretely, when diagonally viewing images in a TN liquid crystal
display device, brightness and contrast decrease and gray scale
inversion occurs. For this reason, such TN mode is unacceptable for
liquid crystal display systems which operate at high speed to
provide animatic images or require good angular viewability when
viewed in diagonal directions. Another known type of liquid crystal
display devices is the Polymer Dispersed Liquid Crystal (PDLC) mode
that utilizes the effect of light dispersion. This mode
advantageously provides high brightness, because it does not
require use of a polarizing plate. However, the response speed of
the PDLC mode is as low as that of TN liquid crystal display
devices. Additionally, the PDLC mode provides a wide range of
viewing angles but the viewing angles of the PDLC mode cannot be
controlled in principle by a phase compensating layer like the TN
mode. There have been developed other types of liquid crystal
display devices: Ferroelectric Liquid Crystal (FLC) and
Anti-Ferroelectric Liquid Crystal (AFLC). These modes suffer from
the critical problems of poor shock resistance and temperature
characteristics and therefore have not been put to practical
use.
[0005] In attempt to overcome the foregoing problems, Optically
Compensated Bend (OCB) liquid crystal display devices have been
proposed, which exhibit extremely rapid response and a relatively
wide range of viewing angles. One example of such devices is
disclosed in Japanese Patent Laid-Open Publication No. 7-84254
(1995). One embodiment of the OCB liquid crystal display devices
according to this publication is designed as shown in FIG. 1 to
have a liquid crystal cell 11 in which a liquid crystal 12 is
enclosed between a pair of transparent substrates 13, 14 and in
which a pixel electrode 15, a counter electrode 16 and alignment
films 17, 18 are formed on the transparent substrates 13, 14. The
surfaces of the alignment films 17, 18 are conditioned so as to
form a bend alignment state in which liquid crystal molecules 12a,
12b proximate to or contacting the alignment films 17, 18 are
symmetrically tilted as shown in FIG. 1. More concretely, the
surfaces of the alignment films 17, 18 are rubbed in the same
direction to form a pretilt angle ranging from several degrees to
10 degrees. The bend alignment state may include twist in the
proximity of the centers of the transparent substrates 13, 14
(i.e., liquid crystal molecules in the proximity of the centers are
twisted so that they do not lie in the plane where X and Z axes
lie) depending on design conditions. Provided on both sides of the
liquid crystal cell 11 are polarizing plates 19, 20. Sandwiched
between the transparent substrate 14 and the polarizing plate 20 is
a phase compensating layer 21 for optically compensating the
director alignment of the liquid crystal 12. In the above-described
bend alignment state, the liquid crystal molecules change rapidly
with a change in the driving voltage applied between the pixel
electrode 15 and the counter electrode 16, and consequently, fast
response can be achieved. Such fast response due to the rapid
molecular change can be obtained even when changing applied voltage
between two levels corresponding two halftones which have a slight
difference in brightness. The symmetry of the bend alignment state
increases the angular viewability in the plane where X and Z axes
lie so that e.g., a viewing angle of about .+-.50.degree. can be
achieved, whereas the phase compensating layer 21 increases angular
viewability in the plane where Y and Z axes lie so that e.g., a
viewing angle of about .+-.40.degree. can be achieved. Note that,
in FIG. 1, X and Y axes designate the transverse direction and
vertical direction, respectively, of the display screen. The phase
compensating layer 21 also contributes to a reduction in driving
voltage.
[0006] The OCB liquid crystal display device presents a difficult
problem. That is, it requires formation of the bend alignment state
prior to image displaying, which is unfavorable for the following
reason. When no voltage is applied between the pixel electrode 15
and the counter electrode 16, the bend alignment state is not
formed but a splay alignment state P with the liquid crystal
molecules arranged fanwise is created as shown in FIG. 2, even if
the above surface treatment is applied to the alignment films 17,
18. Therefore, at the time such as when a power supply is turned
on, the splay alignment state P should be changed to the bend
alignment state Q by application of high electric energy. The
transition from the splay alignment state P to the bend alignment
state Q can be caused at relatively high speeds by applying a
comparatively high voltage ranging from e.g., 10V to 30V between
the pixel electrode 15 and the counter electrode 16. However, it
takes more than tens of minutes to cause the transition when
applying a voltage (several volts) which is low enough to avoid
excessive load on the driving ICs. In the worst case, such
transition does not occur after an elapse of more than one hour.
This hinders practical use of the OCB liquid crystal display
device.
[0007] As an attempt to solve the above problem, Japanese Patent
Laid-Open Publication No. 9-96790 (1997) proposes a technique in
which the twisted alignment of the liquid crystal molecules as seen
in the TN mode is combined with the rising alignment (in which the
liquid crystal molecules are aligned in a direction normal to the
substrates) similar to that of the OCB mode. This technique is
intended to solve the above problem by eliminating the need for
formation of the bend alignment state and to achieve higher
response speed than the TN mode by forming a director alignment
similar to the bend alignment state. In reality, however, fast
response can not be necessarily achieved even if a director
alignment similar to the bend alignment state is formed.
[0008] Although the above prior art OCB liquid crystal display
device succeeds in providing wide viewing angles to a certain
extent, it still has difficulty in largely increasing the viewing
angle within the plane where Y and Z axes lie (see FIG. 1) by the
phase compensating layer 21 alone and therefore the viewing angle
characteristics vary significantly according to viewing directions.
Accordingly, the OCB liquid crystal display device leaves much to
be desired in the viewing angle uniformity. As mentioned earlier,
the viewing angle within the plane where X and Z axes lie (FIG. 1)
can be improved by the symmetry of the bend alignment state. In
order to further increase the viewing angles not only in this
direction but also in other directions, it is conceivable to use a
biaxial phase compensating layer as the phase compensating layer
21. However, the fabrication of such a phase compensating layer
requires accurate control of index of refraction in triaxial
directions, so that where the OCB liquid crystal display device is
applied to a large screen display system, it is extremely difficult
to form such a compensating layer that posses uniform properties
throughout the display screen.
[0009] In many cases, the polarizing plates 19, 20 are placed as
shown in FIG. 3 such that their polarization axes respectively form
an angle of 45.degree. or a specified angle relative to the
conditioning direction of the alignment films 17, 18. In this case,
light incident on the liquid crystal cell 11 passes through the
liquid crystal 12 in the birefringence mode. Such propagation tends
to cause the viewing angle dependence of the hues of a display
image (i.e., hues and color stability may vary according to viewing
angles). Hue shifts would be caused not only by certain viewing
angles but also by the following factors even when images are
viewed squarely (i.e., in a direction perpendicular to the
substrates). FIG. 4 shows the transmission rates of blue, green and
red light where different voltages are applied between the pixel
and counter electrodes of a liquid crystal display device. The
liquid crystal display device used herein is produced under the
following conditions:
[0010] Alignment film: Polyimide director alignment film PSI-A2204
produced by Chisso Corporation.
[0011] Liquid crystal: MT-5540 produced by Chisso Corporation.
[0012] Phase compensating layer: Biaxial oriented film produced by
Nitto Denko Corporation.
[0013] Gap distance of a liquid crystal cell: about 5 .mu.m
[0014] Pretilt angle: 5.degree. to 6.degree.
[0015] Other conditions:
[0016] (1) The upper and lower substrates are bonded such that the
rubbing directions of the alignment films are parallel to each
other.
[0017] (2) Wavelengths at the centers of the spectra of blue, green
and red light are approximately 450 nm, 540 nm and 630 nm,
respectively.
[0018] As shown in FIG. 4, the light transmission of the liquid
crystal varies according to the wavelength of transmitted light.
More concretely, when a voltage of 2V is applied between the pixel
and counter electrodes, the transmission rates of blue, green, red
light are 0.08, 0.045 and 0.025, respectively. Accordingly, entire
images on the screen become bluish. Although it is conceivable that
hue shifts can be prevented by adjusting the voltage applied
between the pixel and counter electrodes according to the color of
light, such adjustment leads to an increased scale of the circuit
and a higher production cost.
SUMMARY OF THE INVENTION
[0019] According to the first aspect of the invention, there is
provided a liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode and (3) a liquid crystal
enclosed between the pixel and counter electrodes,
[0020] wherein the respective opposed surfaces of the pixel and
counter electrodes are conditioned such that liquid crystal
molecules contacting or in the vicinity of the surfaces have
specified pretilt angles,
[0021] wherein images are displayed by changing light transmission
through formation of a bend alignment state of the liquid crystal,
and
[0022] wherein a large pretilt angle domain is formed on at least
either one of the surfaces of the pixel and counter electrodes, the
large pretilt angle domain causing a larger pretilt angle of liquid
crystal molecules than a region surrounding the large pretilt angle
domain does.
[0023] One of the objects of the invention is to quickly and
reliably carry out the transition from the splay alignment state to
the bend alignment state in a liquid crystal display device which
displays images by changing light transmission through formation of
the bend alignment state of the liquid crystal.
[0024] To accomplish this object, a liquid crystal display device
according to the invention includes a large pretilt angle domain
which is formed on at least either the surface of the pixel
electrode or the surface of the counter electrode and which is
conditioned such that the pretilt angle of liquid crystal molecules
caused by the large pretilt angle domain is larger than the pretilt
angle of molecules caused by the region surrounding the large
pretilt angle domain. The liquid crystal molecules proximate to or
contacting the large pretilt angle domain are comparatively raised,
and therefore become a core for the transition from the splay
alignment state to the bend alignment state when voltage is applied
between the pixel electrode and the counter electrode. With this
core, the transition region grows and expands, which enables the
transition to occur reliably throughout the liquid crystal in a
short time. In addition, such transition does not consume large
amounts of electric energy so that the driver circuit is not
subjected to excessive load.
[0025] To achieve the inventive effect, that is, the rapid,
reliable transition, we tried to clarify the mechanism of the
transition of the director alignment state. After a rigorous study,
we found that just after application of voltage, the transition was
more likely to occur in the vicinity of spacers which were disposed
irregularly between the transparent substrates in order to keep a
constant gap between the transparent substrates. The reason for
this is that the alignment of the liquid crystal molecules
proximate to the spacers tends to be irregular under the influence
of the configuration of the spacers and other physical properties
of their surfaces so that some molecules near the spacers have
larger tilt angles than the tilt angle of the molecules far from
the spacers. Such molecules triggers an occurrence of the
transition from the splay alignment state to the bend alignment
state in the neighborhood of the spacers. However, such transition
is accidental and therefore does not occur in the neighborhood of
every spacer. Moreover, the spacers may shift and are not
necessarily positioned on all the pixels. Liquid crystal display
devices usually have a multitude of pixels, and if parts of the
pixels do not have such transition, sound images cannot be
displayed. To solve this problem and achieve the high-speed,
reliable transition, we have come to the idea of the provision of
the large pretilt angle domain. Such a large pretilt angle domain
may be formed, for example, by partially applying an alignment film
material, which imparts a large pretilt angle to liquid crystal
molecules, to the surface of an electrode, through phase separation
or printing. Alternatively, it may be formed by providing a small
projection on an electrode.
[0026] According to the second aspect of the invention, there is
provided a liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode, (3) a liquid crystal enclosed
between the pixel and counter electrodes, and (4) a phase
compensating layer, wherein images are displayed by changing light
transmission through formation of a bend alignment state of the
liquid crystal and wherein the liquid crystal contains a chiral
agent.
[0027] The above-described transition can be easily induced by
adding a chiral agent to the liquid crystal. A combination of the
large pretilt angle domain and the chiral additive causes the
transition more easily.
[0028] According to the third aspect of the invention, there is
provided a liquid crystal display device comprising (1) a first
substrate having a pixel electrode formed thereon, (2) a second
substrate having a counter electrode formed thereon and positioned
opposite the first substrate, (3) a liquid crystal enclosed between
the first and second substrates, (4) a first polarizer and a second
polarizer disposed so as to sandwich the first and second
substrates, the polarizing axes of the first and second polarizers
crossing at right angles, and (5) a driver circuit for applying
driving voltage between the pixel electrode and the counter
electrode,
[0029] wherein the liquid crystal molecules of the liquid crystal
have a twist angle ranging from 160.degree. to 200.degree., and
[0030] wherein the driver circuit applies driving voltage between
the pixel and counter electrodes, the driving voltage being higher
than the highest one of voltages that cause the maximal value of
light transmission in the driving voltage-transmission
characteristic of the liquid crystal display device.
[0031] According to the above liquid crystal display device of the
invention, the twist angle of the molecules of the liquid crystal
is in the range of from 160.degree. to 200.degree., and a voltage
higher than the voltage that causes the extremum (maximal or
minimal value) of light transmission in the driving
voltage-transmission characteristic of the liquid crystal display
device is applied between the pixel electrode and the counter
electrode. With this arrangement, response as fast as that achieved
by a device which forms the bend alignment state can be achieved
without forming an alignment state similar to the bend alignment
state. Concretely, since the liquid crystal molecules are kept in a
twisted condition in the above device, there is no need to make a
discrete phase transition such as the transition from the splay
alignment state to the bend alignment state. Additionally, the
liquid crystal molecules can be brought into an alignment state
similar to the bend alignment state by application of the
above-specified voltage. By virtue of this, images can be
displayed, for instance, just after turning on the power supply of
the liquid crystal display device and excellent response can be
ensured.
[0032] According to the forth aspect of the invention, there is
provided a liquid crystal display device comprising (1) a pixel
electrode, (2) a counter electrode and (3) a liquid crystal
enclosed between the pixel and counter electrodes,
[0033] wherein images are displayed by changing light transmission
through formation of a bend alignment state of the liquid crystal,
and
[0034] wherein pixels corresponding to the pixel electrode are
divided into at least two domains which cause bend director fields
having different orientations in the liquid crystal.
[0035] To improve viewing angle characteristics, thereby achieving
good viewability in various directions in a liquid crystal display
device which displays images by changing light transmission through
formation of the bend alignment state of liquid crystal molecules,
pixels corresponding to the pixel electrode are divided into at
least two domains which cause bend director fields having different
orientations in the liquid crystal. Such domain division can be
accomplished by rubbing a plurality of regions on the alignment
films in different directions or alternatively, by directing
ultraviolet rays having different polarizing or illuminating
directions onto the regions. With this arrangement, the self
compensating ability of viewing angles inherent in the bend
director alignment is exerted in a plurality of different
directions so that good viewability in various directions can be
ensured. Further, a phase compensator may be used in conjunction
with the above arrangement to improve the viewing angle
characteristics.
[0036] According to the fifth aspect of the invention, there is
provided a liquid crystal display device comprising (1) a twisted
liquid crystal cell having a liquid crystal layer sandwiched
between a pair of substrates, the liquid crystal layer having
liquid crystal molecules twisted between said pair of substrates
and (2) a polarizing plate disposed on either the light incoming
side or light outgoing side of the liquid crystal cell,
[0037] wherein said polarizing plate is disposed such that its
polarizing axis is substantially parallel to the longitudinal axis
of the liquid crystal molecules on the interface of one of said
pair of substrates, said substrate being on the light incoming side
or light outgoing side,
[0038] wherein the twist angle of the liquid crystal molecules in
said liquid crystal layer is in the range of from 160.degree. to
200.degree. and said liquid crystal layer contains a dye or
pigment,
[0039] which has a voltage-brightness characteristic according to
which when the voltage applied to said liquid crystal cell exceeds
the Freedericksz threshold voltage of the liquid crystal,
brightness first rises gently with a first gradient and then rises
with a second gradient sharper than the first gradient, and
[0040] which performs image displaying with applied voltages at
least higher than the voltage corresponding to the turning point
where brightness changes from the first gradient to the to second
gradient.
[0041] To solve the problems of (i) the difficulty in causing the
transition from the splay alignment state to the bend alignment
state, (ii) the difficulty in fabricating a phase compensator of
excellent properties for improving the viewing angles and (iii) the
viewing angle dependence of hues which results in hue variation and
color instability according to viewing angles, the twist angle of
the liquid crystal molecules should be in the range of from
160.degree. to 200.degree. or from 250.degree. to 290.degree., the
liquid crystal layer should contain a dye or pigment, and image
displaying is performed with driving voltage falling within a
specified high range. According to the above arrangement, since the
liquid crystal layer contains a dye or pigment, this liquid crystal
display device utilizes the guest-host effect. Therefore, the above
liquid crystal display device can overcome the viewing angle
dependence of hues that is one of the outstanding problems imposed
by the conventional OCB liquid crystal display devices
incorporating the birefringence mode. In addition, the above
display device is not the birefringence mode, there is no need to
include a phase compensating layer. Use of the twisted liquid
crystal cells enables it to display images without the transition
from the splay alignment state to the bend alignment state. Image
displaying with driving voltage in a specified high range permits
fast response and a satisfactorily high contrast.
[0042] According to the sixth aspect of the invention, there is
provided a liquid crystal display device comprising (1) a plurality
of pixel electrodes constituting a plurality of pixels, (2) a
counter electrode, (3) a liquid crystal enclosed between the pixel
electrodes and the counter electrode, and (4) a color filter having
regions respectively corresponding to said pixels, each region
transmitting any one of a plurality of colors,
[0043] wherein at least either the surfaces of the pixel electrodes
or the surface of the counter electrode is conditioned such that
liquid crystal molecules in the vicinity of the surfaces or surface
are aligned so as to form specified pretilt angles,
[0044] wherein images are displayed by changing light transmission
through formation of a bend alignment state of said liquid crystal,
and
[0045] wherein said specified pretilt angles vary according to the
colors of the pixels.
[0046] In the above liquid crystal display device, hue shifts
caused by the dependence of the transmission of the liquid crystal
on the wavelength of transmitted light can be overcome by setting
pretilt angles according to the colors of the pixels. Specifically,
different pretilt angles are formed for the three primary colors so
that the same voltage-transmission characteristic can be attained
for the three primary colors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a longitudinal sectional view showing the
structure of a prior art liquid crystal display device.
[0048] FIG. 2 is a longitudinal sectional view showing the
structure of another prior art liquid crystal display device.
[0049] FIG. 3 is a diagram of the orientations of optical elements
of a liquid crystal display device.
[0050] FIG. 4 shows the transmission-applied voltage characteristic
of another prior art liquid crystal display device.
[0051] FIG. 5 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to first,
second, third embodiments of the invention.
[0052] FIG. 6 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to a forth
embodiment of the invention.
[0053] FIG. 7 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to a fifth
embodiment of the invention.
[0054] FIG. 8 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to a seventh
embodiment of the invention.
[0055] FIG. 9 is a diagram of the orientations of optical elements
of the liquid crystal display device according to the seventh
embodiment.
[0056] FIG. 10 shows the voltage-transmission characteristic of the
liquid crystal display device according to the seventh
embodiment.
[0057] FIG. 11 shows the voltage-transmission characteristic of the
liquid crystal display device according to an eighth embodiment of
the invention.
[0058] FIG. 12 is a diagram of the orientations of optical elements
of the liquid crystal display device according to a ninth
embodiment of the invention.
[0059] FIG. 13 shows the voltage-transmission characteristic of the
liquid crystal display device according to the ninth
embodiment.
[0060] FIG. 14 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to a tenth
embodiment of the invention.
[0061] FIG. 15 is a partially sectional top view showing the
structure of the liquid crystal display device according to the
tenth embodiment.
[0062] FIG. 16 is a longitudinal sectional view showing the
structure of a liquid crystal display device according to an
eleventh embodiment of the invention.
[0063] FIG. 17 shows a surface alignment technique according to a
twelfth embodiment of the invention.
[0064] FIG. 18 is a sectional view of a liquid crystal display
device C according to a thirteenth embodiment of the invention.
[0065] FIG. 19 shows the orientations of optical elements of the
liquid crystal display device C according to the thirteenth
embodiment.
[0066] FIG. 20 shows the voltage-transmission characteristic of the
liquid crystal display device C according to the thirteenth
embodiment.
[0067] FIG. 21 is a partially enlarged view corresponding to FIG.
20.
[0068] FIG. 22 shows the voltage-transmission characteristic of a
liquid crystal display device E4 according to a fifteenth
embodiment of the invention.
[0069] FIG. 23 is a partially enlarged view corresponding to FIG.
22.
[0070] FIG. 24 shows the tilt angle of directors in the liquid
crystal display device C, the tilt angle being obtained from
simulation.
[0071] FIG. 25 shows the orientation of directors in the liquid
crystal display device C, the orientation being obtained from
simulation.
[0072] FIG. 26 shows the relationship between the tilt angle of
directors and applied voltage in the liquid crystal display device
C.
[0073] FIG. 27 shows the tilt angle of directors in the liquid
crystal display device E4, the tilt angle being obtained from
simulation.
[0074] FIG. 28 shows the orientation of directors in the liquid
crystal display device E4, the orientation being obtained from
simulation.
[0075] FIG. 29 shows the relationship between the tilt angle of
directors and applied voltage in the liquid crystal display device
E4.
[0076] FIG. 30 shows the orientations of optical elements of a
liquid crystal display device F according to an eighteenth
embodiment of the invention.
[0077] FIG. 31 shows the voltage-brightness characteristic of the
liquid crystal display device F according to the eighteenth
embodiment.
[0078] FIG. 32 shows the viewing angle characteristics of the
liquid crystal display device F according to the eighteenth
embodiment.
[0079] FIG. 33 is a sectional view of a liquid crystal display
device G according to a nineteenth embodiment of the invention.
[0080] FIG. 34 shows the transmission-applied voltage
characteristic of a liquid crystal display device according to a
twentieth embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0081] [Embodiment 1]
[0082] Now there will be explained one example of OCB liquid
crystal display devices, in which the transition from the splay
alignment state to the bend alignment state quickly occurs.
Referring to FIG. 5, a liquid crystal cell 38 that constitutes a
liquid crystal display device has transparent substrates 33, 36
made of glass, between which a nematic liquid crystal 37
("ZLI-4792" produced by Merck KGaA) having positive dielectric
anisotropy is enclosed. The transparent substrate 33 has
transparent pixel electrodes 31 and an alignment film 32 formed
thereon, whereas the transparent substrate 36 has a counter
electrode 34 and an alignment film 35 formed thereon. Spherical
spacers 51 each having a diameter of about 6 .mu.m are interposed
between the transparent substrates 33 and 36 whereby the gap
distance between the substrates 33, 36 can be kept constant.
Disposed on both sides of the liquid crystal cell 38 are polarizing
plates 39, 40. Between the transparent substrate 36 and the
polarizing plate 40 is a phase compensator 43. Each transparent
pixel electrode 31 is, for instance, in the form of a 100
.mu.m.times.300 .mu.m rectangle. While there are shown only three
pixels in FIG. 5, a plurality of such pixel electrodes are provided
in an actual display device to display bit map images. Large
pretilt angle domains 32h, 35h are formed on the alignment films
32, 35, respectively, and more specifically, at least one domain
32h or 35h is formed for each pixel. The alignment films 32, 35 are
conditioned in the same direction. With this arrangement, when no
voltage is applied to the liquid crystal cell 38, liquid crystal
molecules are arranged in a splay alignment state, and when a
specified voltage is applied, they are arranged in a bend alignment
state. The process of the surface treatment applied to the
alignment films 32, 35 is as follows.
[0083] (1) For forming a small pretilt angle, a polyimide surface
alignment agent of the polyamic acid type, which is capable of
forming a pretilt angle of about 5.degree. and commercially
available from Nissan Chemical Industries Ltd. under the number
SE-7492, is used. For forming a large pretilt angle, a polyimide
surface alignment agent of the prepolymerized type, which is
capable of forming a pretilt angle of about 15.degree. and
commercially available from Japan Synthetic Rubber Co., Ltd. under
the number JALS-246 is used. 100 parts of the former agent and 10
parts of the latter agent are mixed. The mixture is applied to the
transparent pixel electrodes 31 and the counter electrode 34, and
then dried and sintered to form the director alignment films 32,
35. During the drying process, the two surface alignment agents
undergo phase separation so that the large pretilt angle domains
32h, 35h are formed.
[0084] (2) The entire surfaces of the alignment films 32, 35 are
treated by rubbing, using, for example, a rubbing cloth made of
rayon, so that the above large and small pretilt angles are
attained.
[0085] A voltage of 8V was applied by a driver circuit 41 for 10
seconds to the liquid crystal cell 38 having the large pretilt
angle domains 32h, 35h which were formed on the alignment films 32,
35 as described earlier. The transition from the splay alignment
state to the bend alignment state or to the twisted bend alignment
state (this state is also hereinafter referred to as "bend
alignment state") was seen in all the pixels irrespective of the
presence or absence of the spacer 51 in their neighborhood. After
application of voltage had been repeated in the same way, good
repeatability was found in the occurrence of the transition. The
reason for such smooth transition is that a core of the transition
is first created in the large pretilt angle domains 32h, 35h and
then, the transition region grows and expands from this core.
[0086] [Embodiment 2]
[0087] In lieu of the polyimide surface alignment agent capable of
forming a pretilt angle of about 15.degree. used in Embodiment 1, a
polyimide surface alignment agent of the prepolymerized type
capable of forming a pretilt angle of about 70.degree. produced by
Japan Synthetic Rubber Co., Ltd. under the number JALS-204 is used
in Embodiment 2, for forming a high pretilt angle. The liquid
crystal cell 38 having the large pretilt angle domains 32h, 35h
formed on the alignment films 32, 35 was prepared and 5V was
applied for 2 seconds to the liquid crystal cell 38. After that,
the transition occurred without fail. When a small amount of
surface alignment agent that forms a substantially homeotropic
structure of around 90.degree. was added, the transition readily
occurred with low driving voltage.
[0088] [Embodiment 3]
[0089] Another method for forming the large pretilt angle domains
32h, 35h similar to those in Embodiment 2 will he explained.
[0090] (1) First, a polyimide surface alignment agent of the
prepolymerized type capable of forming a pretilt angle of about
5.degree. produced by Japan Synthetic Rubber Co., Ltd. under the
number JALS-212 is applied to the transparent pixel electrodes 31
and the counter electrode 34. Then, the product is dried and
sintered to form the alignment films 32, 35.
[0091] (2) A polyimide surface alignment agent of the
prepolymerized type capable of forming a pretilt angle of
70.degree. and commercially available from Japan Synthetic Rubber
Co., Ltd. under the number JALS-204 is printed on the alignment
films 32, 35 at the positions corresponding to the transparent
pixel electrodes 31 such that printed areas each having a diameter
of about 10 .mu.m are arranged at a pitch of 100 .mu.m. Then, the
product is dried and sintered to form the large pretilt angle
domains 32h, 35h.
[0092] (3) Surface treatment is applied in the same way as
described in the process (2) of Embodiment 1.
[0093] After a voltage of 5V had been applied for one second to the
liquid crystal cell 38 having the large pretilt angle regions 32h,
35h thus formed on the alignment films 32, 35, the transition
occurred without fail.
[0094] In Embodiments 1 to 3 described above, the mixing ratio of
the surface alignment agent for producing a large pretilt angle to
the surface alignment agent for producing a small pretilt angle and
the diameters of the large pretilt angle domains 32h, 35h, are not
limited to the above figures but may be determined in accordance
with the voltage and time required for the transition and with the
liquid crystal material used. However, it should be noted that at
least one large pretilt angle domain must be formed for every
pixel. Transition is more likely to occur with the greater large
pretilt angle and with the bigger difference between the large
pretilt angle and the small pretilt angle. For this reason, the
large pretilt angle may be in the range of from 15.degree. to
90.degree. and more preferably from 70.degree. to 90.degree., while
the difference between the larger pretilt angle and the small
pretilt angle may be 10.degree. or more. These ranges are, of
course, not limitative of the large and small pretilt angles, which
may be determined according to the above conditions.
[0095] [Embodiment 4]
[0096] There will be explained another liquid crystal display
device in which the transition from the splay alignment state to
the bend alignment state smoothly occurs. In the following
embodiments, elements having functions similar to those of
Embodiment 1 are designated by the same reference numerals given to
the elements of Embodiment 1 and the description of them is
omitted. As seen from FIG. 6, a square-pole projection 52 is formed
on each of the transparent pixel electrodes 31. The height and one
side of the cross-section of each square-pole projection 52 are 4
.mu.m. These projections 52 are formed from an acrylic
photosensitive polymer. It should be noted that the height of the
projections 52 is exaggeratedly illustrated in FIG. 6. The
projections 52 can be easily formed for example through exposure
and development, using corresponding masks. An alignment film 32 is
formed over the surface of each transparent pixel electrode 31 and
over the surface of each projection 52. This alignment film 32 is
formed by application, drying, sintering and surface treatment by
use of a low pretilt angle forming surface alignment agent, like
the preparation of ordinary alignment films.
[0097] A voltage of 3V was applied for one second to the liquid
crystal cell 38 having the above projections 52 and it was found
that the transition occurred on all the transparent pixel
electrodes 31 without fail. The occurrence of the transition on all
the pixel electrodes 31 is conceivably due to the fact that the
liquid crystal molecules in the vicinity of each projection 52 are
aligned in a virtually upright fashion along the surface of the
projection 52, forming a core and a transition area grows and
expands from this core.
[0098] It should be noted the projection 52 is not limited to the
shape, size, material and manufacturing method mentioned above, but
at least one projection 52 should be formed on each transparent
pixel electrode 31. For instance, the projections 52 may be
cylindrical, conical, spherical, pyramidal or prismatic, and are
lower than the spacers 51 in height. To eliminate the need for the
spacers 51, the projections 52 may be equal to the spacers 51 in
diameter.
[0099] [Embodiment 5]
[0100] This embodiment provides one example of the liquid crystal
display devices, in which a chiral agent is added to the liquid
crystal 37 to cause the smooth transition from the splay alignment
state to the bend alignment state.
[0101] Eleven liquid crystal display devices A1 to A11 (see FIG. 7)
were prepared, which were designed similarly to the display device
of Embodiment 1 except for the following points.
[0102] (a) The large pretilt angle domains 32h, 35h are not formed
on the alignment films 32, 35.
[0103] (b) A polyimide surface alignment agent of the
prepolymerized type capable of forming a pretilt angle of about
5.degree. and produced by Japan Synthetic Rubber Co., Ltd. under
the number JALS-212 is used for forming the alignment films 32,
35.
[0104] (c) The spacers 51 are disposed at positions where the
transparent pixel electrodes 31 do not lie.
[0105] (d) Cholesteryl nonanoate serving as a left-handed chiral
agent is added to a nematic liquid crystal having positive
dielectric anisotropy so that the chiral pitches of the liquid
crystal 37 of the devices A1 to A11 are as indicated in the
following Table 1.
1 TABLE 1 chiral time required for liquid pitch of observation
result uniform transition crystal liquid of transition at to bend
alignment display crystal the time of voltage state device (.mu.m)
application (second A1 5 transition locally 1 occurs and expands A2
7 uniform transition 0 occurs across surface with transmissivity
variation A3 10 the same as above 0 A4 20 the same as above 0 A5 40
the same as above 0 A6 60 transition locally 1 occurs and quickly
expands A7 80 the same as above 1 A8 100 transition locally 3
occurs and gradually expands A9 120 the same as above 60 A10 140
the same as above 120 A11 .infin. the same as above 600
[0106] A rectangular wave voltage (frequency=30 Hz, maximum voltage
3V) was applied to the liquid crystal display devices A to All
respectively. The transition between the alignment states and the
time required for causing uniform transition to the bend alignment
state over the entire surface of each pixel were observed. The
result of the observation is demonstrated in Table 1.
[0107] As seen from Table 1, in the liquid crystal display devices
A2 to A5 of chiral pitches from 7 .mu.m to 40 .mu.m (i.e., 7
.mu.m.ltoreq.chiral pitch.ltoreq.40 .mu.m), transmission changed
instantly across the surface of each pixel and no alignment defects
were found. That is, the transition from the splay alignment state
to the bend alignment state is thought to have taken place
smoothly.
[0108] In the liquid crystal display devices A1 (chiral pitch=5
.mu.m), A6 to A8 (60 .mu.m.ltoreq.chiral pitch.ltoreq.100 .mu.m),
the transition locally occurred and expanded in a considerably
short time. In other words, the transition occurred across the
surface of each pixel with a comparatively small amount of electric
energy.
[0109] In the liquid crystal display devices A9 to A11 (120
.mu.m.ltoreq.chiral pitch.ltoreq..infin.) the transition first
occurred locally, and voltage had been applied for a long time (1
to 10 minutes) before the transition occurred across the surface of
each pixel. This means that a large amount of electric energy was
need to cause the transition throughout the pixel surface.
[0110] It will be understood from the result that the electric
energy required for the transition from the splay alignment state
to the bend alignment state can be reduced and such transition can
be carried out without fail, by adding a chiral agent to the liquid
crystal 37 to impart a twist component to the liquid crystal 37 and
to achieve a chiral pitch of 5 to 100 .mu.m, and, more preferably,
7 to 40 .mu.m.
[0111] [Embodiment 6]
[0112] This embodiment provides a liquid crystal display device in
which a chiral agent is added to the liquid crystal 37 employed in
Embodiment 2. More specifically, cholesteryl nonanoate serving as a
left-handed chiral agent is added to the liquid crystal 37 to
attain a chiral pitch of 50 .mu.m in this embodiment. Like
Embodiment 2, the liquid crystal cell 38 having the large pretilt
angle domains 32h, 35h is designed to contain the above liquid
crystal 37 between the transparent substrates 33, 36. The spacers
51 are disposed at positions where the transparent pixel electrodes
31 do not lie.
[0113] A rectangular wave voltage (frequency=30 Hz, maximum
voltage=3V) was applied to the above liquid crystal display device
having the liquid crystal cell 38. Then, the transition to the bend
alignment state and the time required for attaining the uniform
transition across the surface of each pixel were observed. The
transition firstly occurred in the large pretilt angle domains 32h,
35h and then expanded in a considerably short time, say, in one
second over the entire surface of each pixel. This means that the
uniform transition across the pixel surface could be achieved by a
relatively small amount of electric energy. It is understood from
the observation that a core for the transition was first created in
the large pretilt angle domains 32h, 35h and the transition was
promoted by the addition of the chiral agent to the liquid crystal
37 so that the splay alignment state was changed to the bend
alignment state very quickly.
[0114] It should be noted that the same inventive effect can be
obtained by adding a chiral agent to the liquid crystal 37 of the
liquid crystal display device of Embodiment 1 or Embodiment 3. The
pretilt angle may be selected from a wide range as described in
Embodiment 3. While the spacers 51 are disposed at positions where
the transparent pixel electrodes 31 do not lie for the sake of
observation in Embodiment 5 and Embodiment 6, they may be placed at
position where the transparent pixel electrodes 31 are
disposed.
[0115] [Embodiment 7]
[0116] This embodiment relates to a liquid crystal display device
which does not need the transition between the alignment states
required by the OCB mode. That is, the liquid crystal display
device of this embodiment is not an OCB liquid crystal display
device, but capable of providing response as fast as the OCB mode
by virtue of the alignment condition similar to that of the OCB
mode and its mechanism. As seen from FIG. 8, the physical structure
of the liquid crystal display device of this embodiment is similar
to that of Embodiment 5 (see FIG. 7), but differs from the latter
in the directors of the alignment films 32, 35 as shown in FIG. 9.
Specifically, the twist angle .omega. of the liquid crystal
molecules of this embodiment is 180.degree..
[0117] The liquid crystal display device of Embodiment 7 is
fabricated in the following procedure.
[0118] (1) Two transparent substrates 33, 36 made of glass and
having the transparent pixel electrodes 31 and the counter
electrode 34 respectively are coated with a polyamic acid type
polyimide surface alignment agent RN-474 produced by Nissan
Chemical Industries Ltd. by spin coating. Then, the coating
material is cured at 180.degree. within a thermostatic chamber over
one hour, thereby preparing the alignment films 32, 35.
[0119] (2) The alignment films 32, 35 are rubbed with a rayon
rubbing cloth in the direction indicated in FIG. 9 so as to produce
a twist angle .omega. of 180.degree..
[0120] (3) The spacers 51 produced by Sekisui Fine Chemical Co.,
Ltd. are interposed between the transparent substrates 33, 36 so as
to create a gap distance of 6 .mu.m between the substrates 33, 36.
These substrates 33, 36 are then bonded by use of Structbond 352A
(sealing resin) produced by Mitsui Toatsu Chemical Co., Ltd.,
thereby forming the liquid crystal cell 38.
[0121] (4) Cholesteryl nonanoate serving as a left-handed chiral
agent is added to the positive nematic liquid crystal material
ZLI-2411 (NI point=65.degree., .DELTA.n=0.140) produced by Merck
KGaA to produce the liquid crystal 37.
[0122] (5) The liquid crystal 37 is injected into the gap between
the transparent substrates 33, 36 placed in an evacuated chamber
and then sealed.
[0123] (6) The polarizing plates 39, 40 and the phase compensator
43 (=bi-axial phase-different film) having a retardation of 50 nm
are bonded to the liquid crystal cell 38 such that they are
oriented as shown in FIG. 9, thereby fabricating the liquid crystal
display device.
[0124] FIG. 10 shows the result of a measurement of the
voltage-transmission characteristic of the liquid crystal display
device fabricated in the above procedure. In the measurement, a
rectangular wave having a frequency of 30 Hz was applied to the
liquid crystal display device with a known method. It is understood
from FIG. 10 that the change in the alignment of the liquid crystal
molecules is continual and an alignment state similar to the bend
alignment state can be obtained smoothly and reliably. When
displaying images with applied voltages of 1.8V to 6V, the contrast
ratio is 230:1.
[0125] Table 2 shows the sum of response times when the applied
voltage is changed from V1 to V2 and when the applied voltage is
changed from V2 to V1. It is understood from Table 2 that fast
response can be obtained when changing applied voltage between two
levels corresponding two halftones which have a slight difference
in brightness.
2TABLE 2 V1 .fwdarw. V2 .fwdarw. V1 V1 (V) V2 (V) sum of response
times (msec) 1.8 2.4 31 2.4 3.0 29 3.0 3.6 26 3.6 4.2 25 4.2 4.8 26
4.8 5.4 23 5.4 6.0 21
[0126] The operational condition of this liquid crystal display
device is as follows. When no voltage is applied to the device, the
alignment of the liquid crystal molecules is the same as that of
the STN (Super Twisted Nematic) mode because the alignment films
32, 35 are conditioned so as to produce a twist angle .omega. of
180.degree.. When a voltage of 1.8V (with which transmission
becomes maximal as shown in FIG. 10) or more is applied to the
device, the alignment of the liquid crystal molecules becomes
similar to that of the OCB mode. Therefore, fast response as
indicated above can be obtained. Even when the above voltage is
applied to the device, the liquid crystal molecules are kept in the
twisted condition so that discrete phase transition such as the
transition from the splay alignment state to the bend alignment
state as seen in the OCB mode will never occur. This permits image
displaying just after application of voltage.
[0127] According to the liquid crystal display device of Embodiment
7, as the liquid crystal molecules are in the twisted condition as
described above, the polarizing plates 39, 40 may be disposed with
their polarizing axes being parallel to each other (i.e., parallel
nicol) instead of the cross nicol arrangement where the polarizing
axes cross at right angles as shown in FIG. 9. In this case,
normally black display is carried out. Specifically, the brightness
of display images decreases as the applied voltage decreases. It
should be noted that the phase difference of the phase compensator
43 needs to be selected according to the arrangement of the
polarizing axes, because the appropriate value of phase difference
when the polarizing axes cross at right angles differs from that
when the polarizing axes are parallel to each other.
[0128] [Embodiment 8]
[0129] The liquid crystal display device of Embodiment 8 has the
same structure as that of Embodiment 7 but differs from the latter
principally in the twist angle .omega. of the liquid crystal
molecules.
[0130] Seven liquid crystal display devices B1 to B7 were
fabricated, which have the same structure as the liquid crystal
display device of Embodiment 7 except for the following points.
[0131] (a) As the liquid crystal 37, a positive nematic liquid
crystal ZLI-2293 (NI point=85.degree., .DELTA.=0.140) commercially
available from Merck KGaA is used. As a left-handed chiral agent,
cholesterol nonanoate is used to produce a chiral pitch of 10
.mu.m.
[0132] (b) The thickness of the liquid crystal layer is 5
.mu.m.
[0133] (c) The phase compensator 43 employed in Embodiment 7 is not
used.
[0134] (d) The twist angle .omega. of each device is as shown in
Table 3.
3TABLE 3 liquid crystal twist angle of response time display device
liquid crystal (msec) B1 150 41 B2 160 28 B3 170 27 B4 180 23 B5
190 27 B6 200 29 B7 210 40
[0135] FIG. 11 shows the voltage-transmission characteristic of
each of the liquid crystal display devices B1 to B7 when measured
at room temperature. Table 3 demonstrates the sum of response times
when the applied voltage is changed from V1 to V2 and when the
applied voltage is changed from V2 to V1 in the case of each
device, the values of V1 and V2 for each device being shown in
Table 4. For example, in the case of the device B1, the sum is
obtained by adding the response time when the applied voltage is
changed from 3.1V to 4.1V to the response time when the applied
voltage is changed from 4.1V to 3.1V.
4TABLE 4 liquid crystal display device V1 (V) V2 (V) B7 2.3 3.3 B6
2.4 3.4 B5 2.5 3.5 B4 2.6 3.6 B3 2.8 3.8 B2 3.0 4.0 B1 3.1 4.1
[0136] As seen from Table 3, in each case, fast response can be
obtained by applying a voltage higher than the voltage that causes
the maximal value of transmission, with the twist angle .omega.
being in the range of from 160.degree. to 200.degree.. When the
twist angle .omega. falls in the above range, the movement of the
liquid crystal molecules is little disturbed by the backflow caused
by application of voltage, so that response as fast as that of the
OCB mode can be achieved.
[0137] Although the phase compensator 43 is not provided in this
embodiment, the phase compensator 43 suited for each liquid crystal
display device may be employed to obtain higher contrast display
image. The brightness characteristics when viewing each device
squarely can be adjusted by optimizing the phase difference And of
the liquid crystal cell 38 when no voltage is applied. While the
chiral pitch of the liquid crystal 37 is twice the thickness of the
layer of the liquid crystal 37 in this embodiment, it may range
from one to three times, because if the chiral pitch is less than
the thickness of the layer 37, the twist angle .omega. becomes
larger than the desired angle by 180.degree. and if the chiral
pitch is more than three times the thickness of the layer 37, the
condition of the director alignment becomes instable.
[0138] [Embodiment 9]
[0139] This embodiment provides an OCB liquid crystal display
device where the transition from the splay alignment state to the
bend alignment state is caused continuously and reversibly, by
setting the twist angle .omega. to 10.degree.. The liquid crystal
display device of Embodiment 9 is an OCB liquid crystal display
device similar to that of Embodiment 5, but differs from the latter
in that the twist angle .omega. of the liquid crystal molecules is
10.degree. as indicated in FIG. 12.
[0140] The liquid crystal display device of Embodiment 9 is
fabricated in the same procedure as Embodiment 7 but different from
the latter in the following points.
[0141] (a) A prepolymerized type polyimide surface alignment agent
AL-5062 produced by Japan Synthetic Rubber Co., Ltd. is used as the
alignment films 32, 35.
[0142] (b) The alignment films 32, 35 are rubbed in the direction
shown in FIG. 12 to produce a twist angle .omega. of
10.degree..
[0143] (c) The transparent substrates 33, 36 are bonded with a gap
distance of 7 .mu.m.
[0144] (d) A positive nematic liquid crystal material LIXON-5052
(NI point=104.degree., .DELTA.n=0.102) produced by Chisso
Corporation which does not contain a chiral agent is used as the
liquid crystal 37.
[0145] (e) The phase compensator 43, which has a phase difference
of 45 nm when observed in a normal direction and is composed of a
uniaxial film and a biaxial film bonded to each other, is bonded as
shown in FIG. 12.
[0146] A rectangular wave having a frequency of 30 Hz was applied
by a known method to the liquid crystal display device fabricated
under the above conditions and then the voltage-transmission
characteristic of the device was measured. FIG. 13 shows the result
of the measurement. The liquid crystal molecules were in the splay
alignment state with no voltage applied to the device, but they
were brought into the bend alignment state when the applied voltage
was in the vicinity of about 2.3V. It was confirmed from the
observation that the change in the alignment of the liquid crystal
molecules at that time was continual and reversible and the
transition to the bend alignment state was smoothly performed
without fail. When image displaying was performed with applied
voltages from 2.3V to 10V, the contrast ratio was 315:1. The sum of
the response times when changing the applied voltage from 2.3V to
2.8V and when changing vice versa was 22 msec. Thus, fast response
could be obtained when changing applied voltage between two levels
corresponding two halftones which have a slight difference in
brightness. Additionally, faster response was observed when driving
the device with a large driving voltage amplitude.
[0147] As described above, the liquid crystal display device of
this embodiment is an OCB liquid crystal display device in which
twisting power is given to the alignment of the liquid crystal.
With this arrangement, the transition from the splay alignment
state to the bend alignment state can be carried with excellent
reliability and repeatability so that it finds a wide range of
applications.
[0148] [Embodiment 10]
[0149] This embodiment provides a bend mode liquid crystal display
device which has improved viewing angles in various directions
without use of the phase compensator 43. As seen from FIG. 14, the
mechanical structure of this liquid crystal display device is
similar to the structure of the device of Embodiment 5 (see FIG. 7)
except for the following points: (a) For fabricating the alignment
films 32, 35, a different material is used (described later). (b)
The alignment films 32, 35 are respectively divided into two
domains. (c) There is not provided the phase compensator 43. (d)
The gap distance between the transparent substrates 33, 36 is 8
.mu.m. (e) The liquid crystal 37 does not contain a chiral
agent.
[0150] Next, the division of the alignment films 32, 35 will be
described in detail. As shown in FIG. 15, the alignment films 32,
35 formed on the transparent substrates 33, 36 are divided into two
domains 32a, 32b and domains 35a, 35b respectively, at the regions
corresponding to the transparent pixel electrodes 31. The alignment
films 32, 35 are conditioned so as to form a bend director
alignment in which the liquid crystal molecules contacting the
opposed pairs of domains 32a, 35a lie in the plane including X and
Z axes whereas the liquid crystal molecules contacting the opposed
pairs of domains 32b, 35b lie in the plane including Y and Z axes.
More specifically, the domains 32a, 35a are conditioned such that
the director field proximate to them has a pretilt angle of about
5.degree. with respect to X axis, while the domains 32b, 35b are
conditioned such that the director field proximate to them has a
pretilt angle of about 5.degree. with respect to Y axis.
[0151] The above alignment films 32, 35 are formed and conditioned
in the following way.
[0152] (1) A prepolymerized-type, polyimide surface alignment agent
(e.g., AL-0656 produced by Japan Synthetic Rubber Co., Ltd.) is
applied to the transparent pixel electrodes 31 and the counter
electrode 34, dried and sintered, thereby forming the alignment
films 32, 35.
[0153] (2) The entire surfaces of the alignment films 32, 35 are
rubbed with a rubbing cloth made of rayon so that the liquid
crystal molecules on the surface of the alignment films 32, 35 form
a pretilt angle of about 5.degree. with respect to Y axis.
[0154] (3) Masking is carried out utilizing the photolithographic
technique such that only the domains 32a, 35a of the alignment
films 32, 35 are exposed.
[0155] (4) Rubbing is done with a rayon rubbing cloth similarly to
the step (2) such that only the domains 32a, 35a form a pretilt
angle of about 5.degree. with respect to X axis.
[0156] In the liquid crystal cell 38 thus formed, a rectangular
wave voltage (amplitude=3V, frequency=30 Hz) was applied between
the transparent pixel electrodes 31 and the counter electrode 34 by
the driver circuit 41. Then, the condition of the director
alignment of the liquid crystal 37 was observed with a polarization
microscope. It was found that, there are formed, in the liquid
crystal 37 contacting the alignment film 32 formed on the
transparent pixel electrodes 31, (i) the bend director field
oriented in the direction of X axis and proximate to the domain 32a
and (ii) the bend director field oriented in the direction of Y
axis and proximate to the domain 32b, these differently oriented
director fields being separated by a disclination line 42.
[0157] The plates 39, 40 were disposed on both sides of the liquid
crystal cell 38 respectively. A specified image signal voltage was
applied between the transparent pixel electrodes 31 and the counter
electrode 34, and viewing angles in various planes perpendicular to
the displaying plane were checked utilizing back light or
reflection light. The same large viewing angle characteristics
(e.g., about .+-.55.degree.) were obtained in the plane including X
and Z axes and in the plane including Y and Z axes. The
substantially similar viewing angle characteristics were found in
other planes than the above planes. This means that the liquid
crystal display device of Embodiment 10 is capable of displaying
images which are highly bright, well-contrasted and free from gray
scale inversion, when viewed from various directions.
[0158] [Embodiment 11]
[0159] In addition to the optical elements employed in Embodiment
10, there may be provided the negative-type, phase compensator 43
for optical compensation between the transparent substrate 36 and
the polarizing plate 40 as shown in FIG. 16. The use of the phase
compensator 43 permits a further improvement in viewing angles (for
example, about .+-.60.degree.) and a reduction in the driving
voltage. The phase compensator 43 may be provided between the
transparent substrate 33 and the polarizing plate 39 instead of
providing it between the transparent substrate 36 and the
polarizing plate 40, or alternatively provided at both
positions.
[0160] [Embodiment 12]
[0161] The surface treatment for the alignment films 32, 35 may be
carried out in the following way.
[0162] (1) Like the step (1) of Embodiment 10, a surface alignment
agent (e.g., PI-610 produced by Nissan Chemical Industries Ltd.) is
applied to the transparent pixel electrodes 31 and the counter
electrode 34, dried and then sintered to form the alignment films
32, 35.
[0163] (2) As shown in FIG. 17, ultraviolet light (wavelength 365
nm, energy density=4.5 mW/cm.sup.2, polarizing direction=the
direction of Y-axis) is directed in the direction of arrow A (i.e.,
at about 45.degree. with respect to X axis in the plane including X
and Z axes) onto the position (corresponding to the domain 32a to
be formed) of the alignment film 32 for 10 minutes so that the
liquid crystal molecules near the surface of the alignment film 32
in this position are aligned at a pretilt angle of about 5.degree.
relative to X axis.
[0164] (3) Similarly to the above step (2), ultraviolet light
(polarizing direction=the direction of X axis) is directed in the
direction of arrow B onto the position (corresponding to the domain
32b to be formed) of the alignment film 32 so that the liquid
crystal molecules near the surface of the alignment film 32 in this
position are aligned at a pretilt angle of about 5.degree. relative
to Y axis.
[0165] (4) Similarly to the steps (2) and (3), ultraviolet light is
directed in the directions of arrow C and arrow D onto the
positions (corresponding to the domains 35a, 35b to be formed) of
the alignment film 35, respectively so that the liquid crystal
molecules near the surface of the alignment film 35 in these
positions are aligned at pretilt angles, symmetrically to the
director fields of the domains 32a, 32b of the alignment film 32
respectively.
[0166] The director fields may be formed in the plane including X
and Z axes as well as in the plane including Y and Z axes like
Embodiment 10, by the above-described radiation of ultraviolet
lights having different polarizing directions and different
radiating directions. In addition, this technique provides the
advantages that it can facilitate uniform surface treatment and
that it avoids a possible decrease in yield which would be caused
by damage to the alignment films due to photolithography. In
consequence, highly improved stability of director alignment can be
achieved.
[0167] The radiating conditions, radiating directions and
polarizing directions of ultraviolet light are not limited to those
described above but may be varied according to the materials of the
liquid crystal 37 and the alignment films 32 35. Further, the
surface treatment may comprise not only the above-described
radiation of ultraviolet light but also rubbing carried out prior
to and/or after the radiation step. It should be noted that the
surface treatment of this embodiment may be applied to other
embodiments.
[0168] While Embodiments 10 to 12 have been described with cases
where the alignment films are respectively divided into two domains
which causes two director fields in planes crossing at right
angles, other ways of division may be possible. For example, the
films are respectively divided into a plurality of domains to form
a plurality of director fields so that viewing angles in various
directions can be improved. The domains do not necessarily have the
same size but may be varied in size according to the viewing angle
characteristics. The transition from the initial director alignment
state to the bend alignment state at the time of a start of voltage
application may be speeded up by adding an appropriate amount of
chiral agent (e.g., cholesteryl nonanoate) to the liquid crystal
37, so that the speed of response can be increased. In this case,
although the bend director alignment of the liquid crystal 37
includes twist, the same effect on viewing angles can be
achieved.
[0169] [Embodiment 13]
[0170] This embodiment provides a liquid crystal display device
which does not require, unlike the OCB mode, use of a phase
compensator nor the arrangement in which the polarizing plate is
disposed with its polarizing axis being oriented in a direction
different from the conditioning direction of the alignment films.
The liquid crystal display device of Embodiment 13 is similar to
the OCB mode in terms of the alignment condition, but has the same
principle as the Guest-host mode (hereinafter referred to as "GH"
mode) has in terms of reproducing light levels.
[0171] FIG. 18 illustrates a cross section of a liquid crystal
display device C according to Embodiment 13 of the invention. The
liquid crystal display device C is a light-transmissive type liquid
crystal display device made up of a liquid crystal cell 110 and a
polarizing plate 109 disposed on the light incoming side of the
liquid crystal cell 110, the liquid crystal cell 110 comprising a
pair of glass substrates 101, 108 between which a liquid crystal
layer 105 is sandwiched. The inner surfaces of the glass substrates
101, 108 are respectively provided with transparent electrodes 102,
107. Disposed on the inner surfaces of the transparent electrodes
102, 107 are alignment films 103, 106. The polarizing plate 109 is
arranged such that its polarizing axis is substantially parallel to
the direction of the longitudinal axis of the liquid crystal
molecules proximate to the interface of the glass substrate 108
which is positioned on the light incoming side.
[0172] The liquid crystal cell 110 is a twisted liquid cell in
which the liquid crystal molecules of the liquid crystal layer 105
are twisted between the glass substrates 101, 108. In this
embodiment, the twist angle .omega. (see FIG. 19) of the liquid
crystal layer 105 is 180.degree.. The liquid crystal layer 105
contains a black dye in addition to a liquid crystal material. The
black dye is a dichromatic dye such as an azoxy dye or
anthraquinone dye and is of the so-called posi-type which exerts a
significant absorbing effect on a light polarized in a direction
parallel to the longitudinal axis of liquid crystal molecules and a
small absorbing effect on a light polarized in a direction parallel
to the lateral axis of liquid crystal molecules. The liquid crystal
of the liquid crystal layer 105 is preformed so as to have a chiral
pitch of 12 .mu.m by adding a chiral agent. The liquid crystal
display device C is designed to keep a gap distance of 6 .mu.m
between the substrates by use of spacers 104.
[0173] The liquid crystal display device C having the above
structure is manufactured by the following fabrication method.
[0174] (1) A polyamic acid type polyimide surface alignment agent
RN-474 produced by Nissan Chemical Industries Ltd. is applied by
spin coating to the two glass substrates 101, 108 having the
transparent electrodes 102, 107. The agent is cured at 180.degree.
C. over one hour in a thermostatic chamber.
[0175] (2) The coated substrates are rubbed in the direction shown
in FIG. 19 using a rayon rubbing cloth. Note that, in FIG. 19,
reference numeral 121 designates the rubbing direction of the
substrate 101 on the light outgoing side, reference numeral 122 the
rubbing direction of the substrate 108 on the light incoming side,
and reference numeral 123 the direction of the polarizing axis of
the polarizing plate 109. Since the twist angle .omega. is
180.degree. in Embodiment 13, the rubbing direction 121 of the
glass substrate 101 is the same as the rubbing direction 122 of the
glass substrate 108.
[0176] (3) The glass substrates 101, 108 are bonded such as to
produce a gap distance of 6 .mu.m therebetween using the spacers
104 produced by Sekisui Fine Chemical Co., Ltd. and Structbond 352A
(the commercial name of a sealing resin produced by Mitsui Toatsu
Chemical Co., Ltd.), whereby the vacant liquid crystal cell 110 is
prepared.
[0177] (4) 100 parts by weight of a positive nematic liquid crystal
material ZLI-2411 commercially available from Merck KGaA (Nematic
isotropic transition point (NI point)=65.degree., anisotropy of
refractive index (.DELTA.n)=0.140) is mixed with 1 part by weight
of a black dye S-466 produced by Mitsubishi Chemical Corporation.
Note that the liquid crystal ZLI-2411 contains cholesteryl
nonanoate as a left-handed chiral agent and has a chiral pitch of
12 .mu.m. The liquid crystal thus prepared is injected into the
vacant, liquid crystal cell 110 placed in an evacuated chamber.
[0178] (5) The polarizing plate 109 is bonded to the liquid crystal
cell 110 such that the rubbing direction 122 of the glass substrate
108 is coincident with the direction of the polarizing axis 123 of
the polarizing plate 109, as shown in FIG. 19.
[0179] The voltage-brightness characteristic of the liquid crystal
display device C was measured while a rectangular wave voltage of
30 Hz being applied to it. The result of the measurement is shown
in FIGS. 20 and 21. It should be noted that FIG. 21 is a partially
enlarged diagram corresponding to FIG. 20. As clearly seen from
FIG. 20, the voltage-brightness characteristic of the liquid
crystal display device C is outlined as follows: the brightness
level is substantially zero when no voltage is applied and is
maintained at approximately zero from the time voltage application
starts to the time the applied voltage reaches a Freedericksz
threshold voltage V.sub.th. After that, the brightness level
increases as the applied voltage increases.
[0180] The above voltage-brightness characteristic is attributed to
the following fact. When the applied voltage is equal to or less
than the Freedericksz threshold V.sub.th, the liquid crystal
molecules are parallel to the substrates and the molecules of the
black dye are constrained by the liquid crystal molecules, so that
the longitudinal axis of the dye molecules is parallel to the
substrates. Therefore, an incident light 125 which has passed
through the polarizing plate 109 is mostly absorbed by the black
dye so that the brightness level becomes substantially zero. In the
range where the applied voltage is equal to or more than the
Freedericksz threshold voltage V.sub.th, the liquid crystal
molecules comparatively close to the center of the liquid crystal
cell rise vertically relative to the substrates. As the applied
voltage increases, the liquid crystal molecules closer to the
substrates rise substantially vertically. Under the influence of
the movement of the liquid crystal molecules, the dye molecules
also vertically rise toward the substrates. This causes a decrease
in the light absorption effect of the black dye, so that the level
of brightness increases.
[0181] According to the voltage-brightness characteristic of the
liquid crystal display device C, the level of brightness gently
increases with a first gradient just after the voltage applied to
the liquid crystal cell 110 exceeds the Freedericksz threshold
voltage V.sub.th, and then it further increases with a second
gradient sharper than the first one after the applied voltage
exceeds about 2.5V. This is obvious from FIGS. 24 and 25 to be
described later. In the first voltage range from the point the
applied voltage exceeds the Freedericksz threshold voltage V.sub.th
to the point the applied voltage reaches 2.5V, a big change is not
seen in the tilt angle and orientation of the liquid crystal
molecules. After the applied voltage exceeds 2.5V, the tilt angle
and orientation vary significantly. Therefore, the molecules of the
black dye affected by the movement of the liquid crystal molecules
have little fluctuation in the first voltage range and fluctuate
considerably after the applied voltage exceeds 2.5V. As a result,
the light absorbability of the black dye declines to a large extent
in the second stage compared to the prior stage, resulting in a
sharp increase in brightness.
[0182] The main feature of the liquid crystal display device of
this embodiment resides in that image displaying is performed with
voltages in the high voltage range which are higher than the
turning point, i.e., 2.5V at which brightness changes abruptly in
the voltage-brightness characteristic curve. It is confirmed by the
following test result that the liquid crystal display device C
achieves fast response and a high contrast ratio in gray scale
displaying.
[0183] We first measured the brightness of display images in the
liquid crystal display device C, with the applied voltage ranging
from 2.5V to 11.0V and calculated the contrast ratio. As a result,
it was confirmed that a contrast ratio of 136:1 was obtained which
was good enough for gray scale displaying.
[0184] Then, the voltage applied to the liquid crystal display
device C was changed from 2.5V to 3.7V, 4.9V, 6.1V, 7.3V, 8.5V,
9.7V and 10.9V sequentially, and the rise time and fall time of
each change were measured to obtain the sum of these times. The
respective sums for the voltage changes were 43 msec, 39 msec, 37
msec, 35 msec, 35 msec, 30 msec and 30 msec. The response time of
an ordinary liquid crystal display device is known to be as
follows: the sum of the rise time and fall time is about 150 msec.
when voltage is changed between 2.5V and 3.7V, and is 30 to 40
msec. when voltage is changed between 9.7V and 10.9V. As obvious
from the test result, the liquid crystal display device C has
excellent response characteristics.
[0185] It is well understood from the foregoing description that
the liquid crystal display device C can perform gray scale
displaying with fast response, when the driving voltage ranges from
2.5V to 10.9V. While this embodiment has been described with a case
where images are displayed in 8 tones, the invention is not limited
to this and enables high-speed image displaying likewise in cases
where display images have more than 8 tones. This is also easily
assumed from the above test result.
[0186] As described earlier, the liquid crystal display device C of
Embodiment 13 has a liquid crystal cell having a twist angle
.omega. of 180.degree. in which a guest-host (GH type) liquid
crystal material is injected, and the device C differs from STN
liquid crystal display devices in the range of driving voltage and
in the way of light propagation. Fast response can be ensured in
gray scale displaying like the OCB mode, by employing the above
range of driving voltage. Since light transmission is controlled by
controlling the light absorption by the black dye, there is no need
to provide an optical compensating layer and black hue never
fluctuates visually in this embodiment. Accordingly, the liquid
crystal display device of this embodiment is, in principle, free
from visual color changes while ensuring response as fast as that
of the conventional OCB liquid crystal display devices, so that it
finds a wide range of applications.
[0187] Although a black dye is used in this embodiment,
dyes/pigments of other colors may be used according to
applications. In cases where a black dye is used, image displaying
may be performed with voltages equal to and less than the
Freedericksz threshold voltage V.sub.th only when black color
images are displayed, in order to further decrease the brightness
level of black images.
[0188] For reference, the arrangement of the polarizing plate will
be explained. While the polarizing axis of the polarizing plate is
substantially parallel to the longitudinal axis of the liquid
crystal molecules in the vicinity of the interfaces of the
substrates in this embodiment, it is conceivable that the
polarizing axis may be arranged at a certain angle such as
20.degree. or 45.degree. relative to the longitudinal axis.
However, such non-parallel arrangement where the polarizing axis
and the longitudinal axis of the molecules are not parallel to each
other does not obtain a satisfactory black level, which results in
poor image quality. The reason for this is as follows. In the high
voltage range, the voltage-brightness characteristic in the case of
the parallel arrangement where the polarizing axis and the
longitudinal axis of the molecules are parallel to each other is,
in principle, identical to the voltage-brightness characteristic in
the case of the non-parallel arrangement. However, in the low
voltage range, the voltage-brightness characteristic in the case of
the parallel arrangement differs from that in the case of the
non-parallel arrangement. More precisely, where the polarizing axis
and the longitudinal axis of the molecules are not parallel, the
molecules of the dye are not parallel to the polarizing plate when
no voltage is applied and therefore, the absorbed light is small in
amount compared to the case of the parallel arrangement, so that
brightness remains at a certain level. Even when the applied
voltage slightly exceeds the Freedericksz threshold voltage
V.sub.th, brightness is maintained at a level substantially similar
to the level at the time of no voltage application. When the
applied voltage increases further, the tilt angle and orientation
of the liquid crystal molecules have a particular relationship with
the orientation of the polarizing plate and as a result, brightness
drops drastically. When the applied voltage increases still
further, brightness increases in conjunction therewith. Even when
brightness is at the lowest level, it is not zero but a level which
is not low enough to display black color, and therefore, the liquid
crystal display device having the non-parallel arrangement fails in
ensuring a satisfactory black level, leading to poor image
quality.
[0189] A test conducted by us has, however, proved that where the
polarizing plate is placed with its polarizing axis being
substantially perpendicular to the longitudinal axis of the liquid
crystal molecules, the lowest level of brightness is not zero but
acceptable for displaying black color. In consideration of this
fact, the polarizing axis of the polarizing plate may be arranged
substantially perpendicular to the longitudinal axis of the liquid
crystal molecules and with such perpendicular arrangement, image
displaying may be done with voltages higher than the voltage at
which brightness is at the lowest level.
[0190] [Embodiment 14]
[0191] Embodiment 14 has the same structure as Embodiment 13 except
that while the twist angle .omega. is 180.degree. in Embodiment 13,
the twist angle .omega. of Embodiment 14 is in the range of from
160.degree. to 200.degree.. With the structure of Embodiment 14,
the same inventive effect as that of Embodiment 13 can be obtained.
Details will be explained below.
[0192] Seven liquid crystal display devices D1 to D7 were
fabricated by the same method as that of the liquid crystal display
device C of Embodiment 13 except for the following points.
[0193] (a) As a liquid crystal material, a positive nematic liquid
crystal ZLI-2293 (NI point=85.degree., .DELTA.n=0.140) produced by
Merck KGaA and containing 1 wt% of a black dye S-466 (produced by
Mitsubishi Chemical Corporation) is used.
[0194] (b) The thickness of the liquid crystal layer is 5 .mu.m and
the chiral pitch is 10 .mu.m.
[0195] (c) The twist angle .omega. of the liquid crystal of each
device differs from that of Embodiment 13. As seen from Table 5,
the twist angles .omega. of the liquid crystal display devices D1
to D7 are 150.degree., 160.degree., 170.degree., 180.degree.,
190.degree., 200.degree., and 210.degree., respectively.
5 TABLE 5 liquid crystal twist angle of display device liquid
crystal D1 150 D2 160 D3 170 D4 180 D5 190 D6 200 D7 210
[0196] The following test was conducted to measure the response of
each of the liquid crystal display devices D1 to D7. Concretely,
the range of driving voltage (V1-V2) for each device was determined
as shown in Table 6. For evaluating the response of each device,
the response times when the applied voltage was changed from V1 to
V2 and when it was changed from V2 to V1 were respectively
measured, and then the sum of these response times was obtained.
Table 7 shows the test result. It should be noted that V1 is the
applied voltage when the gradient of the voltage-brightness
characteristic abruptly changes in each of the liquid crystal
display devices D1 to D7.
6TABLE 6 liquid crystal display device V1 (V) V2 (V) D1 2.2 3.1 D2
2.3 3.2 D3 2.4 3.3 D4 2.5 3.4 D5 2.6 3.5 D6 2.7 3.6 D7 2.8 3.7
[0197]
7 TABLE 7 liquid crystal response time display device (msec) D1 52
D2 35 D3 33 D4 31 D5 31 D6 35 D7 57
[0198] As seen from Table 7, while the liquid crystal display
devices D1, D7 exhibit poor response as their response times are
more than 50 msec., the display devices D2, D3, D4, D5, D6 exhibit
rapid response as their respective response times are less than 40
msec. It is understood from the result that the twist angle, which
permits rapid response, ranges from 160.degree. to 200.degree..
[0199] The reason why rapid response can be obtained when the twist
angle falls in the range of from 160.degree. to 200.degree. is as
follows. It is widely known that, in a liquid crystal display
device having a twisted liquid crystal cell and a polarizing plate,
the response is dependent of the angle between the twisted liquid
crystal molecules and the polarizing plate and becomes fast when
this angle falls in a certain range. Under the condition that the
polarizing plate is disposed with its polarizing axis being
parallel to the liquid crystal molecules in the interface of the
substrate 108 on the light incoming side, the twist angle for
obtaining fast response falls in the range of from 160.degree. to
200.degree.. Accordingly, if the twist angle ranges from
160.degree. to 200.degree., the degree to which the movement of the
liquid crystal molecules is prevented by the backflow caused by
actuation can be restricted as much as possible so that response as
fast as that of the OCB mode can be achieved.
[0200] Regarding the liquid crystal display devices D2, D4, D5, D6
of this embodiment, the viewing angle dependence of hues were
checked at various brightness levels. These devices were found to
be virtually free from hue shifts so that their usefulness was
proved.
[0201] Although the chiral pitch of the liquid crystal material is
set to be twice the thickness of the liquid crystal layer in this
embodiment, the preferable range of the chiral pitch is one to
three times the thickness of the liquid crystal layer. The reason
for this is that if the chiral pitch is smaller than the thickness
of the liquid crystal layer, the twist angle of the liquid crystal
layer becomes larger than the desired value by 180.degree. and if
the chiral pitch is more than three times the thickness of the
liquid crystal layer, the condition of the director alignment tends
to be instable.
[0202] [Embodiment 15]
[0203] The liquid crystal display device of Embodiment 15 has the
same structure as that of the liquid crystal display device D of
Embodiment 14 except that the twist angle .omega. of the liquid
crystal of Embodiment 14 ranges from 160.degree. to 200.degree.,
whereas the twist angle .omega. of Embodiment 15 ranges from
250.degree. to 290.degree.. FIGS. 22 and 23 show the
voltage-brightness characteristic of a liquid crystal display
device E4 having a twist angle .omega. of 270.degree., which is a
typical example of Embodiment 15. Note that FIG. 23 is a partially
enlarged view corresponding to FIG. 22.
[0204] It is obvious from FIGS. 22, 23 that the voltage-brightness
characteristic of the liquid crystal display device E of this
embodiment is essentially identical to that of the liquid crystal
display device C having a twist angle .omega. of 180.degree.. One
of the features of Embodiment 15 resides in that image displaying
is performed, similarly to Embodiments 13, 14, with driving
voltages higher than the point (=3.6V on the curves shown in FIGS.
22, 23) at which the gradient of the voltage-brightness
characteristic curve abruptly changes. It has been experimentally
verified by the test described below that fast response and a high
contrast ratio in gray scale displaying can be achieved in this
embodiment. The test will be concretely described.
[0205] Seven liquid crystal display devices E1 to E7 were
fabricated by the fabrication method that was similar to that of
the liquid crystal display device C of Embodiment 13 except for the
following points. In Embodiment 15, a positive nematic liquid
crystal ZLI-2293 (NI point=85.degree., .DELTA.n=0.140) produced by
Merck KGaA containing 1 wt % of a black dye S-466 produced by
Mitsubishi Chemical Corporation is used as the liquid crystal
material. The thickness of the liquid crystal layer is 20 .mu.m,
and the chiral pitch is 24 .mu.m. Twist angles .omega. different
from that of embodiment 13 are adapted. Specifically, the twist
angles of the liquid crystals in the liquid crystal display device
E1 to E7 are, as shown in Table 8, 240.degree., 250.degree.,
260.degree., 270.degree., 280.degree., 290.degree., and
300.degree., respectively.
8 TABLE 8 liquid crystal twist angle of display device liquid
crystal E1 240 E2 250 E3 260 E4 270 E5 280 E6 290 E7 300
[0206] The range of applied voltage (V1-V2) for each of the display
devices E1 to E7 is determined as shown in Table 9. Table 10 shows
the sum of the response times when the applied voltage is changed
from V1 to V2 and when it is changed vice versa-in each device. It
should be noted V1 is the applied voltage when the gradient of the
voltage-brightness characteristic abruptly changes in each of the
liquid crystal display devices E1 to E7.
9TABLE 9 liquid crystal display device V1 (V) V2 (V) E1 3.2 3.9 E2
3.3 4.0 E3 3.4 4.1 E4 3.5 4.2 E5 3.6 4.3 E6 3.7 4.4 E7 3.8 4.5
[0207]
10TABLE 10 liqiud crystal response time display device (msec)
contrast ratio E1 69 70:1 E2 50 120:1 E3 43 170:1 E4 37 196:1 E5 44
180:1 E6 48 135:1 E7 62 85:1
[0208] The twist angle and the thickness of the liquid crystal
layer of Embodiment 15 are large. Therefore, as seen from Table 10,
Embodiment 15 is somewhat poor in response characteristics compared
to Embodiment 14, but acceptable for practical use. Further, when
image displaying is performed with voltages higher than those shown
in Table 9, substantially similar response characteristics can be
obtained in operation for changing applied voltage between two
levels corresponding two halftones which have a slight difference
in brightness.
[0209] For the liquid crystal display device B4, the contrast ratio
was defined as the ratio of the brightness when 11.0V was applied
to the brightness when 3.0V was applied. The value of this contrast
ratio was found to be 196. For other liquid crystal display devices
E1 to E3 and E5 to E7, the contrast ratio was likewise defined and
their respective values were obtained. Table 10 demonstrates the
contrast ratio of each device. As seen from Table 10, a contrast
and response characteristics good enough for practical use can be
obtained with a twist angle ranging from 250.degree. to
290.degree.. The response when the twist angle is in the range of
from 250.degree. to 290.degree. is better than those when it is
240.degree. and when it is 300.degree. for the same reason that the
twist angle ranging from 160.degree. to 200.degree. achieves good
response. A high contrast can be obtained when the twist angle is
in the range of from 250.degree. to 290.degree. for the following
reason. Where the twist angle exceeds 290.degree., the twist angle
is so large that the light propagation within the liquid crystal
layer cannot follow the twist, which entails a loss of light and,
in consequence, a poor contrast.
[0210] The viewing angle dependence of hues at various brightness
levels was observed in the liquid crystal display devices E2 to E6
and virtually no hue fluctuation was observed. This proves the
usefulness of these display devices E2 to E6.
[0211] While the chiral pitch of the liquid crystal material in
this embodiment is 1.2 times the thickness of the liquid crystal
layer, the preferable range may be one to twice the thickness of
the liquid crystal layer. The reason for this is that if the chiral
pitch is smaller than the thickness of the liquid crystal layer,
the twist angle of the liquid crystal layer becomes 180.degree.
larger than the desired value and if the chiral pitch is more than
twice the thickness of the liquid crystal layer, the twist angle of
the liquid crystal layer becomes 180.degree. smaller than the
desired value.
[0212] [Embodiment 16]
[0213] While Embodiments 13 to 15 determine the range of driving
voltage for the liquid crystal display device from the
voltage-brightness characteristic, this range is determined from
the average tilt angle of the liquid crystal molecules in
Embodiment 16. Brightness usually varies according to the variation
of the voltage applied to the liquid crystal display device and
this fact is attributable to changes in the tilt angle of dye
molecules following changes in the tilt angle of the liquid crystal
molecules. For this reason, the range of driving voltage may be
determined not only from the voltage-brightness characteristic but
also from the average tilt angle of the liquid crystal molecules.
This embodiment provides one example in which the range of driving
voltage for the liquid crystal display device is determined from
the average tilt angle of the liquid crystal molecules.
[0214] This embodiment will be concretely explained. The director
distribution of the liquid crystal display device C of Embodiment
13 was calculated. The applied voltage was varied by 1V from 0V to
10V. FIGS. 24 to 25 show the result of the test. Note that FIG. 24
shows the tilt angle of the liquid crystal molecules in relation to
the substrate plane, whereas FIG. 25 shows the orientation of
director alignment. In FIG. 24, line X0 represents a case where a
voltage of 0V was applied. Similarly, lines X1, X2, X3, X4, X5, X6,
X7, X8, X9 and X10 represent cases where the applied voltage was
1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V and 10V, respectively. Referring
to FIG. 25, line Y0 represents a case where the applied voltage was
0V, and likewise, lines Y1, Y2, Y3, Y4, Y5, Y6, Y7, Y8, Y9 and Y10
represent cases where the applied voltage was 1V, 2V, 3V, 4V, 5V,
6V, 7V, 8V, 9V and 10V, respectively.
[0215] It is understood from FIGS. 24, 25 that the tilt angle of
the liquid crystal molecules and the orientation of director
alignment change slightly when the applied voltage was up to 2V,
and change greatly when the applied voltage was equal to or more
than 3V. It is conceivable that due to the changes in the tilt
angle and the director alignment orientation, which correspond to
changes in the applied voltage, the gradient of brightness levels
largely changes in the vicinity of 2.5V in the voltage-brightness
characteristic of the liquid crystal display device C (see FIG.
21). Accordingly, the same range of applied voltage determined by
the voltage-brightness characteristic can be obtained through
determination using the average tilt angle of the liquid crystal
molecules. We calculated the average tilt angle of the liquid
crystal molecules for each value of applied voltage. FIG. 26 shows
the result. It is understood from FIGS. 20, 26 that the average
tilt angle of the liquid crystal molecules corresponding to an
applied voltage of 2.5V is 10.degree.. Thus, in a liquid crystal
display device having a twist angle ranging from 160.degree. to
200.degree., image displaying is possibly carried out when the
average tilt angle of the liquid crystal molecules is 10.degree. or
more. When the average tilt angle is less than 10.degree., neither
satisfactory brightness nor a practicable contrast ratio can be
obtained.
[0216] [Embodiment 17]
[0217] While Embodiment 16 determines the range of driving voltage
from the average tilt angle in the liquid crystal display device
whose twist angle .omega. ranges from 160.degree. to 200.degree.,
Embodiment 17 carries out the driving voltage range determination
with the average tilt angle in the liquid crystal display device
whose twist angle .omega. ranges from 250.degree. to 290.degree..
It was experimentally proven that Embodiment 17 had the same
inventive effect of Embodiment 16.
[0218] This embodiment will be concretely explained. The director
configuration of the liquid crystal display device E4 prepared
according to Embodiment 15 was obtained through calculation, while
the applied voltage being changed by 1V from 0 to 10V. FIGS. 27 and
28 show the result of the calculation for each voltage value. FIG.
27 shows the tilt angle of the liquid crystal molecules relative to
the substrate plane, whereas FIG. 28 shows the orientation of
director alignment. FIG. 29 shows the average tilt angle of the
liquid crystal molecules corresponding to each applied voltage
value. It is understood from FIGS. 22, 29 that the average tilt
angle of the liquid crystal molecules corresponding to an applied
voltage of 3.6V is 20.degree.. Accordingly, in the case of the
liquid crystal display device whose twist angle is 250.degree. to
290.degree., image displaying is possible when the average tilt
angle of the liquid crystal molecules is 20.degree. or more. When
the average tilt angle is less than 20.degree., satisfactory black
color displaying cannot be performed, and a practicable contrast
ratio cannot be obtained.
[0219] [Embodiment 18]
[0220] While Embodiments 13 to 17 use a twisted liquid crystal
cell, Embodiment 18 is characterized by a splay liquid crystal cell
having a twist angle .omega. of 0.degree.. The liquid crystal
display device F of Embodiment 18 has the same structure as the
liquid crystal display device C of Embodiment 13, except that the
liquid crystal display device F has a liquid crystal cell formed by
adding a black dye in the conventional OCB mode liquid crystal cell
and that the polarizing plate is disposed with its polarizing axis
being substantially parallel to the rubbing direction of the
substrates. Another difference is that the liquid crystal display
device F does not incorporate the birefringence mode employed in
the conventional OCB liquid crystal display devices but utilizes
the Guest-host mode. The liquid crystal display device F of
Embodiment 18 is fabricated in the following method.
[0221] (1) A prepolymerized type polyimide surface alignment agent
AL-5062 produced by Japan Synthetic Rubber Co., Ltd. is applied by
spin coating to the two glass substrates 101, 108 having the
transparent electrodes 102, 107, and then cured at 180.degree. over
one hour within a thermostatic chamber.
[0222] (2) Then, the surfaces of the coated substrates are rubbed
in the direction shown in FIG. 30, using a rayon rubbing cloth.
Referring to FIG. 30, reference numeral 121 represents the rubbing
direction of the substrate 101 positioned on the light outgoing
side, reference numeral 122 the rubbing direction of the substrate
108 positioned on the light incoming side, and reference numeral
123 the direction of the polarizing axis of the polarizing plate
109. In Embodiment 18, the rubbing direction 121 of the substrate
101 is the same as the rubbing direction 122 of the glass substrate
108 in order to produce a twist angle .omega. of 0.degree..
[0223] (3) The substrates 101, 108 are bonded so as to have a gap
distance of 14 .mu.m therebetween by use of the spacers 104
produced by Sekisui Fine Chemical Co., Ltd. and Structbond 352A
(sealing resin) produced by Mitsui Toatsu Chemical Co., Ltd.,
whereby the vacant liquid crystal cell 110 is formed.
[0224] (4) 100 parts by weight of a positive nematic liquid crystal
LIXON-5052 (NI point=104.degree., .DELTA.n=0.102) produced by
Chisso Corporation and containing no chiral agent and 1 part by
weight of a black dye S-466 produced by Mitsubishi Chemical
Corporation are injected in the vacant liquid crystal cell 110
placed in an evacuated chamber.
[0225] (5) The polarizing plate 109 is bonded to the liquid crystal
cell 110 such that the rubbing directions 121, 122 of the
substrates coincide with the direction 123 of the polarizing axis
of the polarizing plate as shown in FIG. 30, thereby fabricating
the liquid crystal display device F.
[0226] The voltage-brightness characteristic of the liquid crystal
display device F thus fabricated was measured by a known method
while a rectangular wave voltage of 30 Hz being applied to it. The
result of the measurement is shown in FIG. 31. When no voltage was
applied, the director alignment of the liquid crystal layer was in
the splay alignment state, but when the applied voltage was in the
vicinity of about 2.3V, the director alignment was brought into the
bend alignment state. Referring to FIG. 31, when image displaying
was done with a driving voltage of 1.8V to 12V, the contrast ratio
was 80:1. The sum of the rise time and fall time when the voltage
was changed from 2.3V to 2.8V was 30 msec.
[0227] FIG. 32 shows the range of viewing angles when the
brightness ratio (i.e., contrast ratio) is more than 5:1 with
driving voltages of 10V and 1.8V. As seen from FIG. 32, the liquid
crystal display device F of this embodiment has good viewing angle
characteristics, providing a viewing angle of more than 120.degree.
in a vertical direction and a viewing angle of 160.degree. in a
lateral direction. Therefore, the liquid crystal display device F
proved itself very valuable in practical use. When checking the
viewing angle dependence of the displaying characteristics during
actuation of the liquid crystal display device F with driving
voltages ranging from 2V to 8V, gray scale inversion was not
recognized.
[0228] As has been described above, Embodiment 18 uses a splay
liquid crystal cell in which the liquid crystal layer can be
brought into the bend alignment state by voltage application and
uses a dye contained in the liquid crystal layer, so that it
presents several advantages. First, it ensures fast response equal
to that of the OCB mode as well as good viewing angle
characteristics. Second, it overcomes the viewing angle dependence
of the hues of display images that has been one of the outstanding
problems suffered by the conventional OCB liquid crystal display
devices employing the birefringence mode. In addition, since the
device F is not the birefringence mode, there is no need to include
a phase compensator layer.
[0229] Although voltages equal to and lower than the Freedericksz
threshold voltage are applied only when displaying black-color
images in this embodiment, black-color displaying may be done with
voltages higher than 2.3V (see FIG. 31) if there is not strong
requirement for a high contrast.
[0230] [Embodiment 19]
[0231] FIG. 33 shows a cross section of a liquid crystal display
device according to Embodiment 19 of the invention. The liquid
crystal display device G of this embodiment is a light
reflective-type liquid crystal display device having a reflector
140. In FIG. 33, elements having the same functions as those of the
elements of the liquid crystal display device F shown in FIG. 18
are designated by the same reference numerals given to the elements
of the device F. Essentially, the liquid crystal display device G
is fabricated by incorporating the reflector 140 in the structure
of the device F of Embodiment 18. However, the device G differs
from the device F of Embodiment 18 in that the liquid crystal layer
105 contains a chiral agent. Use of a chiral agent permits the
smooth transition from the initial state of the liquid crystal
molecules to a twisted, bend alignment state and increases response
speed. In this case, the director alignment of the liquid crystal
is in the bend alignment state having twist that exists at the
center of the liquid crystal, but the inventive effect of
Embodiment 18 in terms of viewing angles can be achieved by
Embodiment 19.
[0232] The fabrication method of the liquid crystal display device
G having the above features is as follows.
[0233] (1) A prepolymerized-type polyimide surface alignment agent
AL-5062 produced by Japan Synthetic Rubber Co., Ltd. is applied by
spin coating to the two glass substrates 101, 108 having the
transparent electrodes 102, 107, and then cured at 180.degree. over
one hour within a thermostatic chamber.
[0234] (2) Then, the surfaces of the coated, glass substrates 101,
108 are rubbed in the same direction, using a rayon rubbing cloth
to produce a twist angle .omega. of 0.degree.. The glass substrates
101, 108 are bonded so as to have a gap distance of 10 .mu.m
therebetween by use of the spacers 104 produced by Sekisui Fine
Chemical Co., Ltd. and Structbond 352A that is a sealing resin
produced by Mitsui Toatsu Chemicals Co. Ltd., whereby the vacant
liquid crystal cell 110 is formed.
[0235] (3) 100 parts by weight of a positive nematic liquid crystal
LIXON-5052 (NI point=104.degree., .DELTA.n=0.102) produced by
Chisso Corporation and having a chiral pitch of 20.mu.m and 1 part
by weight of a black dye S-466 produced by Mitsubishi Chemical
Corporation are injected in the vacant liquid crystal cell 110
placed in an evacuated chamber.
[0236] (4) The polarizing plate 109 is bonded to the liquid crystal
cell 110 such that the rubbing direction of the substrates
coincides with the direction of the polarizing axis of the
polarizing plate, and the reflector 140 is bonded to the liquid
crystal cell 110, thereby fabricating the liquid crystal display
device G.
[0237] The voltage-brightness characteristic of the liquid crystal
display device G thus fabricated was measured by a known method
while a rectangular wave voltage of 30 Hz being applied to it. The
contrast ratio obtained when the display device G was viewed
squarely was 30:1.
[0238] When displaying images in 8 tones in the liquid crystal
display device G, the response between every two tones was 30 msec
or less, and the viewing angle dependence of hues was not observed.
To obtain the range of viewing angles with which a contrast ratio
of 5:1 or more can be obtained, a measurement was conducted like
Embodiment 18. It was found that the display device G had a wide
range of viewing angles, providing a viewing angle of 100.degree.
in a vertical direction and a viewing angle of 115.degree. in a
lateral direction. The usefulness of the display device C was thus
confirmed. It should be noted that while the polarizing plate 109
is disposed on the light incoming side of the liquid crystal cell
110 in Embodiments 13 to 19, it nay be disposed on the light
outgoing side.
[0239] [Embodiment 20]
[0240] This embodiment provides a liquid crystal display device
incorporating the OCB mode or a similar mode, that is designed to
compensate the different transmission characteristics of the three
primary colors. Such compensation is accomplished by employing
different pretilt angles for the three primary colors, instead of
adjusting applied voltage for every primary color.
[0241] Concretely, the pretilt angle is so varied as to hold the
relationship described by: the pretilt angle for blue<the
pretilt angle for green<the pretilt angle for red. That is, the
pretilt angle corresponding to blue is the smallest among three. If
the pretilt angle is made too small, the energy necessary for the
transition from the splay alignment state to the bend alignment
state increases, so that the transition becomes difficult to carry
out. Therefore, it is necessary to set the pretilt angle for blue
in a range that causes the transition with ease. Red has the
largest pretilt angle and if the pretilt angle is made too large,
it will impair displaying with the appropriate bend director
alignment. Therefore, the pretilt angle for red should be no more
than around 30.degree.. There must be a preferable range for the
pretilt angle for each primary color, blue, green and red, to
satisfy the above conditions.
[0242] There will be explained on a surface treatment technique for
producing the director alignment having different pretilt angles
for the three primary colors.
[0243] (1) First, a polyamic acid type polyimide alignment film
PSI-A2204 produced by Chisso Corporation is applied using a spinner
to the entire surfaces of the electrodes formed on the substrates
and then, cured.
[0244] (2) For forming a pretilt angle for red, application of a
negative resist OMR-83 produced by Tokyo Ohka Kogyo Co., Ltd.
exposure by use of a photo mask and development are sequentially
carried out, such that only the region corresponding to red pixels
is exposed. In this condition, a homeotropic agent (produced by
Merck KGaA) is diluted, applied to and chemically combined to the
surface of the red region. By such application of the homeotropic
agent, the pretilt angle of only the region to which the agent has
been applied can be made larger than those of other regions, when
the cell is filled with a liquid crystal in the later step.
[0245] (3) After removing the resist, the surfaces of the
electrodes are entirely rubbed by the ordinary method.
[0246] (4) For forming a pretilt angle for blue, only the blue
pixel-corresponding region is exposed to radiation of an
ultraviolet ray of 360 nm, using a photo mask. The radiation of the
ultraviolet ray causes decomposition of the alignment film so that
when the cell is filled with a liquid crystal later, the pretilt
angle of only this region can be made smaller.
[0247] When the above surface treatment is carried out, various
pretilt angles can be obtained by adjusting the dilution rate of
the homeotropic agent, the radiation energy of ultraviolet light
and others. In an actual liquid crystal display device formed by
the foregoing technique, the pretilt angle of the blue
pixel-corresponding region on the upper and lower substrates is
about 2.degree. and the pretilt angle of the red
pixel-corresponding region on both substrates is about 19.degree..
The pretilt angle of the green-pixel corresponding region on both
substrates to which no special treatment has been applied is about
5.degree. to 6.degree. like the case of the prior art liquid
crystal display devices.
[0248] FIG. 34 shows the transmission-applied voltage
characteristic of the liquid crystal display device of Embodiment
20. As understood from FIG. 34, the virtually same transmission can
be obtained for each of the primary colors, blue, green, red,
irrespective of applied voltage. With this arrangement, voltage
adjustment for the three primary colors is no longer necessary, and
image displaying with appropriate hues is enabled without loosing a
balance even if the same voltage is applied to the regions of
different colors. Although Embodiment 20 does not use a phase
compensator, it may be included in Embodiment 20 in which case, the
same inventive effect can be obtained.
[0249] While Embodiment 19 has been described with a reflective
liquid crystal display device, Embodiment 19 is applicable to
transmissive liquid crystal display devices having no reflector.
Also, other embodiments described earlier are applicable to both
reflective and transmissive liquid crystal display devices. In the
case of a reflective liquid crystal display device, the substrates
may be made of silicon or reflective materials such as metals
including aluminum, or alternatively a reflective metal film may be
applied to either the pixel electrodes or the counter
electrode.
[0250] All of the above-described embodiments may be applied to
passive matrix-type liquid crystal-display devices and also to
active matrix-type liquid crystal display devices incorporating an
active element such as a TFT (Thin Film Transistor) or MIM (Metal
Insulated Metal) formed on either substrate. The active matrix-type
enables display images of better quality. The invention is
applicable to various types of liquid crystal display devices such
as normally-white liquid crystal display devices and normally-black
liquid crystal display devices which display white and black images
respectively, when no voltage is applied.
[0251] It should be noticed that the materials of the elements
constituting each device are not limited to those described above.
For example, plastic substrates may be used as the transparent
substrates and other surface alignment agents than polyimide
surface alignment agents may be used While left-handed cholesteryl
nonanoate is used as the chiral agent in the foregoing embodiments,
other types of chiral agents including the left-handed and
right-handed may be used. As a matter of course, the pretilt angle
and the gap distance between the transparent substrates are not
limited to the above values, but may be varied according to the
material of the liquid crystal and other optical design conditions.
Although it is preferable that the pretilt angles of the alignment
films on both sides of the liquid crystal cell be equal to each
other in view of the symmetry of viewing angles, they may differ
from each other in order to facilitate a change in the alignment
state of the liquid crystal molecules. Further, in Embodiment 1 and
other embodiments, a phase. compensator is provided on only one
side of the liquid crystal cell but both sides may be respectively
provided with a phase compensator.
* * * * *