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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and a
method for producing the same. More specifically, the present invention
relates to a liquid crystal display device having a display medium with a
structure in which liquid crystal regions are partitioned by a polymeric
material and a method for producing the same.
2. Description of the Related Art
As display devices utilizing an electro-optic effect, liquid crystal
display devices using nematic liquid crystals have conventionally been
used. Examples of such liquid crystal display devices include a twisted
nematic (TN) liquid crystal display device and a super-twisted nematic
(STN) liquid crystal display device. Liquid crystal display devices using
ferroelectric liquid crystal have also been proposed. These liquid crystal
display devices include a pair of glass substrates, nematic liquid crystal
or smectic liquid crystal provided between the substrates, and two
polarizing plates sandwiching the substrates.
Furthermore, as the display devices utilizing an electro-optic effect,
liquid crystal display devices utilizing a light scattering phenomenon of
liquid crystal, instead of using the polarizing plates, have been known.
Such liquid crystal display devices use a dynamic scattering (DS) mode and
a phase change (PC) mode.
In recent years, liquid crystal display devices requiring no alignment
treatment have been proposed. Such a liquid crystal display device
electrically regulates a transparent state and an opaque state by using
the birefringence of a liquid crystal. More specifically, in such a liquid
crystal display device, the refractive index of liquid crystal molecules
with respect to ordinary light is matched with the refractive index of a
supporting medium which supports the liquid crystal. Thus, liquid crystal
molecules are oriented under the application of a voltage and hence a
transparent state is displayed; whereas the orientation of the liquid
crystal molecules is disturbed under the application of no voltage and
hence a light scattering state is displayed. Japanese National Publication
No. 61-502128 discloses a specific method: liquid crystal and
photopolymerizable or thermosetting resin are mixed and resin is cured to
deposit liquid crystal, whereby liquid crystal droplets are formed in the
resin.
Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 disclose a
liquid crystal display device using polarizing plates with improved
viewing angle characteristics, i.e., a polymer dispersed liquid crystal
device sandwiched with polarizing plates disposed so that the respective
polarizing directions cross at right angles (hereinafter, referred to as
crossed polarizing plates). These conventional liquid crystal display
devices have improved viewing angle characteristics. However, the use of
scattering of light for the elimination of polarization makes the
brightness of the device 1/2 that of a TN liquid crystal display device.
Thus, these conventional devices have not found a wide range of use.
Furthermore, Japanese Laid-Open Patent Publication No. 5-27242 discloses a
method for improving viewing angle characteristics by disturbing the
orientation of liquid crystal molecules with walls or projections of a
polymer to form random domains. However, according to this method, liquid
crystal domains are formed at random, a polymeric material enters a pixel
portion, and a plurality of disclination lines formed at random between
the liquid crystal domains are not eliminated even under the application
of a voltage. For these reasons, the conventional liquid crystal display
devices have the disadvantage of low contrast, light transmittance under
the application of no voltage is low, that is, the black level is not
satisfactory under the application of a voltage.
Accordingly, the conventional liquid crystal display devices using
polarizing plates have poor viewing angle characteristics and hence are
unsuitable for use as a liquid crystal display device for a wide viewing
angle. For example, a TN liquid crystal display device undergoes alignment
treatment so that liquid crystal molecules rise in the same direction
under the application of a voltage. That is to say, the TN liquid crystal
display device has a structure in which liquid crystal molecules have an
initial orientation of 90.degree. twist and rise in one direction at a
certain angle, i.e., a pretilt angle. This makes the liquid crystal
molecules tilt in the same direction in the case where a gray scale
display is conducted to allow the liquid crystal molecules to rise, as
shown in FIGS. 51(a) to 51(c). Because of this, as shown in FIG. 51(b),
when the liquid crystal molecules are viewed from directions A and B,
apparent refractive indices become different. This also makes the
difference in contrast between directions A and B large, and in some
cases, results in an abnormal display such as a change in hue and the
reversal of black and white colors.
As described above, the conventional liquid crystal display devices have
the disadvantage of poor viewing angle characteristics.
Another method for producing a liquid crystal display device using
polarizing plates has been proposed. According to this method, first, a
mixture containing liquid crystal and a photopolymerizable material is
provided between a pair of substrates. Then, light is irradiated to the
mixture to a predetermined pattern through a photomask. At this time, the
liquid crystal is phase-separated from the polymeric material in a regular
manner. As shown in FIGS. 52(a) to 52(c), when a voltage is applied to the
device thus produced, liquid crystal molecules interact with the polymer
and consequently, the liquid crystal molecules rise along walls in each
direction. Because of this, apparent refractive indices become nearly the
same in directions A and B in FIG. 52(b), improving viewing angle
characteristics.
For improving the viewing angle characteristics most effectively, liquid
crystal molecules in each pixel should be oriented so as to be symmetric
with respect to an axis. However, the axisymmetric orientation requires
walls, pillars, or the like of a polymer in the middle of the pixels. This
leads to problems during practical use, such as the reduction of liquid
crystal regions and decreased light transmittance under the application of
no voltage. Furthermore, in this case, disclination lines between the
liquid crystal domains cannot be controlled, which makes it impossible to
eliminate the disclination lines even under the application of voltage. As
a result, the display quality is degraded. Alternatively, the decrease in
contrast due to difficulties in eliminating disclination lines degrades
the display quality.
SUMMARY OF THE INVENTION
The liquid crystal display device of this invention, comprises: two
substrates respectively defining a plurality of pixels, each pixel being a
display unit, at least one of the substrates being transparent; and a
display medium layer formed between the two substrates, having a
supporting medium made of a polymeric material and a liquid crystal, the
liquid crystal being respectively filled in a plurality of liquid crystal
regions being partitioned by supporting walls made of the polymeric
material in the supporting medium and each having a size corresponding to
a size of each of the plurality of pixel regions, wherein molecules of the
liquid crystal filled in the plurality of liquid crystal regions is
axisymmetrically oriented in an imaginary plane parallel with a surface of
the substrates, and at least one liquid crystal domain is positioned in
each of the plurality of the liquid crystal regions.
In one embodiment of the present invention, one liquid crystal domain is
positioned in each of the plurality of liquid crystal regions.
In another embodiment of the present invention, a plurality of liquid
crystal domains are positioned in each of the plurality of liquid crystal
regions, liquid crystal molecules in each domain are axisymmetrically
oriented, and the supporting walls made of the polymeric material are
present outside of each domain.
In still another embodiment of the present invention, a thin film made of a
material selected from the group consisting of an organic material and an
inorganic material is provided on surfaces of the two substrates.
In still another embodiment of the present invention, the two substrates
are sandwiched between polarizing plates.
In still another embodiment of the present invention, a product .DELTA.
n.multidot.d of anisotropy of refractive index .DELTA. n of the liquid
crystal and a cell gap d between the two substrates is in the range of 300
nm to 650 nm.
In still another embodiment of the present invention, a twist angle of the
liquid crystal between the two substrates when the liquid crystal is
injected therebetween is in the range of 45.degree. to 150.degree..
In still another embodiment of the present invention, the liquid crystal
has a viscosity .mu. of 50 mPa.s or less at 20.degree. C. and dielectric
constant anisotropy .DELTA..epsilon. of +3(1 kHz) or more.
In still another embodiment of the present invention, the liquid crystal
satisfies a condition under which a voltage V.sub.10 is 2 volts or less in
a voltage-light transmittance characteristic at 25.degree. C., in a TN
cell when light transmittance of the liquid crystal changes from an
initial state to 90%.
In still another embodiment of the present invention, a product .DELTA.
n.multidot.d of anisotropy of refractive index .DELTA. n of the liquid
crystal and a cell gap d between the two substrates is in the range of
1000 nm to 1400 nm, and a twist angle of the liquid crystal present in a
cell is in the range of 45.degree. to 150.degree..
In still another embodiment of the present invention, a product .DELTA.
n.multidot.d of anisotropy of refractive index .DELTA. n of the liquid
crystal and a cell gap d between the two substrates is in the range of 550
nm to 800 nm, and a twist angle of the liquid crystal present in a cell is
in the range of 240.degree. to 300.degree..
In still another embodiment of the present invention, the supporting walls
reach each of the two substrates.
In still another embodiment of the present invention, a center axis of
orientation of the liquid crystal regions present in the pixels is
orthogonal to at least one of the substrates.
In still another embodiment of the present invention, disclination lines
are formed at the periphery of the liquid crystal regions under an
application of a voltage.
In still another embodiment of the present invention, the liquid crystal
molecules in the liquid crystal regions are axisymmetrically oriented so
as to be in parallel with the surface of the substrates, a center axis of
an orientation of the liquid crystal regions is aligned in a vertical
direction to the substrates, and the polymer material in the supporting
walls is symmetrically oriented with respect to the center axis, whereby
disclination lines are not formed in the liquid crystal regions under the
application of a voltage.
In still another embodiment of the present invention, the liquid crystal
molecules in the liquid crystal regions are axisymmetrically oriented so
as to be in parallel with the surface of the substrates, a center axis of
orientation of the liquid crystal regions is aligned in a vertical
direction to the substrates, and the polymer material in the supporting
walls is oriented in one direction, whereby disclination lines are not
formed in the liquid crystal regions under the application of a voltage.
In still another embodiment of the present invention, a polymer present
between the substrates and the liquid crystal in the liquid crystal
regions has a pretilt angle axisymmetric with respect to a center axis of
orientation of the liquid crystal regions, whereby disclination lines are
not formed in the liquid crystal regions under the application of a
voltage.
In still another embodiment of the present invention, a black mask is
provided on one of the substrates so as to correspond to a center portion
of the domains in which the liquid crystal molecules are radially
oriented.
According to another aspect of the present invention, the method for
producing a liquid crystal display device of this invention comprises the
steps of: (1) providing a mixture containing a liquid crystalline compound
and a photopolymerizable compound between two substrates, at least one of
which is transparent; and (2) irradiating light having a predetermined
irradiation intensity distribution to the mixture between the two
substrates, allowing a phase separation of the mixture involved in
polymerization thereof to be effected, and uniformly distributing
supporting walls made of a resin and a liquid crystal.
In one embodiment of the present invention, a photopolymerization initiator
is added to the mixture.
In another embodiment of the present invention, in step (2), a uniform
distribution of the supporting walls and the liquid crystal is determined
so as to correspond to an arrangement pitch of a plurality of pixels
defined by the two substrates.
In still another embodiment of the present invention, step (2) includes
alignment treatment for allowing molecules of the liquid crystal
partitioned by the supporting walls to be axisymmetrically oriented in an
imaginary plane parallel with a surface of the substrates.
In still another embodiment of the present invention, a light-shielding
chip corresponding to a center portion of an axisymmetric orientation of
the liquid crystal molecules is formed on either of the two substrates.
In still another embodiment of the present invention, the mixture is
irradiated with light having uniform irradiation intensity distribution
under a condition that a UV-rays component in a short wavelength region of
300 nm or less is shielded.
In still another embodiment of the present invention, the UV-rays component
in a short wavelength region is shielded by using a UV-rays cut filter.
In still another embodiment of the present invention, the UV-rays component
in a short wavelength region is shielded by using an inorganic and organic
material which makes transmittance of light with a wavelength of 300 nm
not more than 10% and transmittance of light with a wavelength of 350 nm
at least 40%, assuming that light transmittance with respect to air is
100%.
In still another embodiment of the present invention, while being
substantially controlled, at least one of an electric field and a magnetic
field is applied to the mixture during light irradiation.
In still another embodiment of the present invention, the electric field is
applied by using an electrode for a display.
In still another embodiment of the present invention, light having the
predetermined irradiation intensity distribution is formed by using a
photomask.
Alternatively, the method for producing a liquid crystal display device of
this invention comprises the steps of: injecting a mixture containing a
liquid crystalline compound, a photopolymerizable compound, and a liquid
crystalline photopolymerizable compound between electrode substrates in a
cell, at least one of the substrates being transparent; and irradiating
the mixture with light having a uniform irradiation intensity distribution
while at least one of an electric field and a magnetic field is applied to
the mixture, thereby allowing phase separation involved in polymerization
to be effected.
In one embodiment of the present invention, a temperature of the cell
during light irradiation is set to be at least a temperature at which the
liquid crystalline compound to be used exhibits an isotropic phase, and
then the cell is cooled.
In another embodiment of the present invention, the photopolymerizable
compound includes a fluorinated compound.
The liquid crystal display device of the present invention has a structure
in which a display medium layer is sandwiched between two substrates. For
producing such a liquid crystal display device, in the first step, a
mixture containing a liquid crystalline compound, a photopolymerizable
compound, and a photopolymerization initiator is provided between the two
substrates. In the second step, the mixture is irradiated with light
having a predetermined irradiation intensity distribution, thereby
allowing phase separation involved in polymerization of the mixture to be
effected. In this way, the display medium layer having a structure in
which supporting walls made of the polymer (resin) and liquid crystal are
uniformly distributed is obtained.
Molecules in the liquid crystal filled in a plurality of liquid crystal
regions in the display medium layer are axisymmetrically oriented in an
imaginary plane parallel with a surface of the substrates. Furthermore, at
least one liquid crystal domain is positioned in each of the plurality of
liquid crystal regions.
When the angle and direction, in which the liquid crystal display device of
the present invention are observed from outside, are changed, the
dependence of display contrast on a viewing angle can be eliminated
because of the axisymmetric orientation of the liquid crystal molecules.
Also, the axisymmetric orientation of the liquid crystal molecules
prevents the disclination lines from being formed in the liquid crystal
region, and thus the display quality is remarkably improved.
According to the present invention, the product .DELTA. n.multidot.d of
anisotropy of refractive index .DELTA. n of the liquid crystal material
and a cell gap d (distance between substrates sandwiching a display
medium) is set to be in the range of 300 to 650 nm, and the twist angle of
liquid crystal between the substrates is set to be in the range of
45.degree. to 150.degree. when the liquid crystal is injected
therebetween. Because of this, the light transmittance of the display
device can be optimized, and the light transmittance of the liquid crystal
display device can be remarkably improved.
Thus, the invention described herein makes possible the advantages of (1)
providing a liquid crystal display device with markedly improved viewing
angle characteristics and display quality; and (2) a simplified method for
producing a liquid crystal display device.
These and other advantages of the present invention will become apparent to
those skilled in the art upon reading and understanding the following
detailed description with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a liquid crystal display device in
Example 1 according to the present invention.
FIG. 2 is a plan view of a photomask used for producing the liquid crystal
display device shown in FIG. 1.
FIG. 3 is a plan view of one pixel of the liquid crystal display device
shown in FIG. 1, observed with a polarizing microscope.
FIG. 4 is a state of disclination lines generated under the application of
a voltage in one pixel of the liquid crystal display device shown in FIG.
1.
FIG. 5 is a plan view of a liquid crystal display device in a modified
example according to the present invention, observed with a polarizing
microscope.
FIG. 6 is a plan view showing an orientation state of liquid crystal
molecules in an example according to the present invention.
FIG. 7 is a plan view showing another orientation state of liquid crystal
molecules in an example according to the present invention.
FIG. 8 is a plan view of one pixel of the liquid crystal display device.
FIG. 9 shows graphs illustrating the viewing angle characteristics of the
liquid crystal display device according to the present invention.
FIG. 10 shows graphs illustrating the viewing angle characteristics of a
conventional TN liquid crystal display device.
FIG. 11 is a plan view of a counter substrate having a color filter used in
Example 2 according to the present invention.
FIG. 12 is a plan view of a TFT substrate having a black mask used in
Example 2 according to the present invention.
FIG. 13 is a graph showing the relationship between the light transmittance
and the .DELTA. n.multidot.d characteristic of the liquid crystal display
device in Example 2 according to the present invention.
FIG. 14 is a graph showing the dependence of light transmittance with
respect to light with 3 wavelengths on the .DELTA. n.multidot.d
characteristic in the liquid crystal display device in Example 2 according
to the present invention.
FIG. 15 is a graph showing the dependence of the light transmittance on a
twist angle of the liquid crystal display device in Example 2 according to
the present invention.
FIG. 16 is a plan view of a photomask used in Example 2 according to the
present invention.
FIG. 17 is a plan view of pixel electrode regions of the liquid crystal
display device in Example 2 according to the present invention.
FIG. 18 is a plan view of a photomask used in Comparative Example 2.
FIG. 19 is a plan view of pixel electrode regions of the liquid crystal
display device produced in Comparative Example 2.
FIG. 20 is a plan view of a photomask used in Construction Example 10.
FIG. 21 is a graph showing the dependence of light transmittance on the
.DELTA. n.multidot.d characteristic of the liquid crystal display device
in Example 4 according to the present invention.
FIG. 22 is a graph showing the dependence of light transmittance on the
.DELTA. n.multidot.d characteristic of the liquid crystal display device
in Example 4 according to the present invention.
FIG. 23 is a graph showing the dependence of light transmittance on the
twist angle of the liquid crystal display device in Example 4 according to
the present invention.
FIG. 24 is a schematic view of a counter substrate having a color filter
used in Example 5 according to the present invention.
FIG. 25 is a schematic view of a substrate having a black mask used in
Example 5.
FIGS. 26(a) and 26(b) are graphs showing a spectral transmission
characteristic of a UV-rays cut filter used in Example 5 according to the
present invention.
FIG. 27(a) is a block diagram of a measurement device for measuring a
charge holding ratio; and FIGS. 27(b) to 27(d) are diagrams showing
signals.
FIG. 28 is a graph showing an optical characteristic of a plastic substrate
used in Construction Example 24.
FIG. 29(a) is a diagram of a pixel portion of the liquid crystal display
device in Example 6 according to the present invention; and FIG. 29(b) is
a diagram illustrating the structure of the pixel portion.
FIGS. 30(a) to 30(d) are schematic views showing an orientation state in
each part of the liquid crystal region of the liquid crystal display
device in Example 6 according to the present invention.
FIGS. 31(a) to 31(d) are schematic views showing an orientation state in
each part of the liquid crystal region of the liquid crystal display
device in Example 6 according to the present invention.
FIGS. 32(a) and 32(b) show the liquid crystal region of the liquid crystal
display device in Example 6 according to the present invention, observed
with a polarizing microscope.
FIGS. 33(a) to 33(c) are diagrams illustrating the effect of an external
field during a polymerization step of a method according to the present
invention.
FIG. 34 is a plan view of a photomask used in Construction Example 25.
FIG. 35 is a diagram of a schlieren texture of a liquid crystal display
device produced in Construction Example 25.
FIG. 36 shows graphs showing electro-optic characteristics of the liquid
crystal display device produced in Construction Example 25.
FIG. 37 is a view illustrating the principle of suppressing disclination
lines.
FIGS. 38(a) and 38(b) are views showing a state where disclination lines
are formed.
FIG. 39 is a plan view of a photomask used in Construction Example 27.
FIGS. 40(a) and 40(b) are diagrams showing observation results of a liquid
crystal display device produced in Construction Example 27.
FIGS. 41(a) to 41(c) are diagrams used for expecting the orientation of
liquid crystal molecules of a liquid crystal display device.
FIGS. 42(a) and 42(b) are diagrams showing observation results of a liquid
crystal display device produced in Construction Example 28.
FIGS. 43(a) to 43(c) are diagrams showing observation results of the liquid
crystal display device produced in Construction Example 28.
FIGS. 44(a) and 44(b) are diagrams used for expecting the orientation of
liquid crystal molecules of the liquid crystal display device produced in
Construction Example 28.
FIG. 45 is a plan view of a pixel portion of a liquid crystal display
device produced in Construction Example 29.
FIG. 46 is a diagram illustrating the size of each light-shielding portion
of a photomask in Example 8 according to the present invention.
FIG. 47 is a plan view of the photomask used in Example 8 according to the
present invention.
FIG. 48 is a plan view of the pixel portions of the liquid crystal display
device produced in Example 8 according to the present invention.
FIGS. 49(a) and 49(b) are diagrams respectively showing an example of a
photomask usable in the present invention and a liquid crystal region to
be obtained.
FIG. 50 is a diagram showing an example of a photomask usable in the
present invention and a liquid crystal region to be obtained.
FIGS. 51(a) to 51(c) are cross-sectional views illustrating the behavior of
liquid crystal molecules of a conventional liquid crystal display device.
FIGS. 52(a) to 52(c) are cross-sectional views illustrating the behavior of
liquid crystal molecules of the liquid crystal display device according to
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the present invention will be described by way of illustrative
examples with reference to the accompanying drawings. It is noted that the
present invention is not limited to the following examples in terms of
size, material, and structure.
EXAMPLE 1
FIG. 1 is a cross-sectional view of a liquid crystal display device 1 in
Example 1 according to the present invention. The liquid crystal display
device 1 includes a pair of glass substrates 2 and 3 having a thickness of
1.1 mm each. On the glass substrate 2, a plurality of segmented electrodes
4 and a plurality of switching devices 5 are provided in a matrix. The
segmented electrodes 4 are made of a mixture containing indium oxide and
tin oxide (ITO), and have a thickness of 50 nm. The switching devices work
for applying a signal voltage to the segmented electrodes 4, or for
interrupting the signal voltage. As the switching devices, for example, a
thin film transistor (TFT) is used. An organic thin film 6 covers the
segmented electrodes 4 and the switching devices 5. It is noted that the
organic thin film 6 can be omitted. Accordingly, a TFT substrate 12 is
constructed.
On the other glass substrate 3, a counter electrode 7 made of ITO is
provided. In addition, a black mask 8 having light-shielding portions is
positioned on the counter electrode 7 so that the light-shielding portions
correspond to portions between the segmented electrodes 4 on the glass
substrate 2. A smoothing film 9 covers the black mask 8. A color filter 10
having red (R), green (G), and blue (B) primitives, in an appropriate
color pixel arrangement, is incorporated on the smoothing film 9 so that
each color corresponds to each segmented electrode 4. The color filter 10
is covered with an organic thin film 11. It is noted that the organic thin
film 11 can be omitted. Accordingly, a counter substrate 13 is
constructed.
A display medium layer 14 is sandwiched between the TFT substrate 12 and
the counter substrate 13. The display medium layer 14 includes resin walls
16 (i.e., polymer walls) and liquid crystal regions 17. The resin walls 16
are formed in regions of the display medium layer 14 excluding regions
where the segmented electrodes 4 are formed, and the liquid crystal
regions 17 are formed between the respective resin walls 16 and in regions
of the display medium layer 14 where the segmented electrodes 4 are
formed. Disclination lines 18 are formed on interfaces between the resin
walls 16 and the liquid crystal regions 17. In the liquid crystal display
device 1, liquid crystal molecules in the liquid crystal regions 17 are
oriented symmetrically with respect to an axis, and at least one liquid
crystal domain is formed in each liquid crystal region 17.
A process for producing the liquid crystal d | | |
