U.S. patent application number 09/749156 was filed with the patent office on 2001-06-28 for liquid crystal display device having spontaneous polarization and evaluation method therefor.
Invention is credited to Ogura, Jun.
Application Number | 20010005259 09/749156 |
Document ID | / |
Family ID | 18505035 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005259 |
Kind Code |
A1 |
Ogura, Jun |
June 28, 2001 |
Liquid crystal display device having spontaneous polarization and
evaluation method therefor
Abstract
A liquid crystal display device comprises a pair of substrates
on whose opposing surfaces electrodes are formed; and a liquid
crystal provided between the substrates and having spontaneous
polarization and a physical property such that when a positive or
negative saturation voltage whose absolute value is sufficiently
large is applied between the electrodes, liquid crystal molecules
are aligned in a first direction or a second direction, and when an
arbitrary voltage lying between the positive saturation voltage and
the negative saturation voltage is applied between the electrodes,
a director is aligned in an arbitrary direction which corresponds
to the applied voltage and which lies in a cone angle formed by the
first direction and the second direction. The liquid crystal has a
physical property which satisfies the following equation:
.epsilon.(.theta..sup.2/P.sub.s.sup.2).ltoreq.8 where .epsilon.
[F/m] is a permittivity of the liquid crystal located between the
substrates, .theta. [.smallcircle.] is a tilt angle defined by 1/2
of the cone angle of the liquid crystal, and P.sub.s [nC/cm.sup.2]
is spontaneous polarization of the liquid crystal molecules of the
liquid crystal. The constructed liquid crystal display device can
provide gradation display and has less display burning.
Inventors: |
Ogura, Jun; (Tokyo,
JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN,
LANGER & CHICK, P.C.
25th Floor
767 Third Avenue
New York
NY
10017
US
|
Family ID: |
18505035 |
Appl. No.: |
09/749156 |
Filed: |
December 27, 2000 |
Current U.S.
Class: |
349/172 ;
349/174; 349/184 |
Current CPC
Class: |
G02F 1/141 20130101 |
Class at
Publication: |
349/172 ;
349/184; 349/174 |
International
Class: |
C09K 019/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
JP |
375137 / 1999 |
Claims
What is claimed is:
1. A liquid crystal display device comprising: a pair of substrates
arranged opposite to each other; electrodes arranged on opposing
surfaces of said pair of substrates and facing each other; and a
liquid crystal provided between said substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between said electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to said one
polarity and a sufficiently large level is applied between said
substrates, said liquid crystal molecules are aligned in a second
direction, and when a voltage lying between said voltage of said
one polarity for aligning said liquid crystal molecules in said
first direction and said voltage of said other polarity for
aligning said liquid crystal molecules in said second direction is
applied between said electrodes, a director is aligned in an
arbitrary direction corresponding to said applied voltage and in a
cone angle formed by said first direction and said second
direction, said physical property satisfying a following
equation:.epsilon.(.theta..sup.2/P.sub.s.- sup.2).ltoreq.8where
.epsilon. [F/m] is a permittivity of said liquid crystal located
between said substrates, .theta. [.smallcircle.] is a tilt angle
defined by 1/2of said cone angle of said liquid crystal, and
P.sub.s [nC/cm.sup.2] is spontaneous polarization of said liquid
crystal molecules of said liquid crystal.
2. The liquid crystal display device according to claim 1, wherein
said liquid crystal has such a physical property as to satisfy
.epsilon. (.theta..sup.2/P.sub.s.sup.2).ltoreq.5.
3. The liquid crystal display device according to claim 1, wherein
said liquid crystal is one of a ferroelectric liquid crystal and an
antiferroelectric liquid crystal.
4. A liquid crystal display device comprising: a pair of substrates
arranged opposite to each other; electrodes arranged on opposing
surfaces of said pair of substrates and facing each other; and a
liquid crystal provided between said substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between said electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to said one
polarity and a sufficiently large level is applied between said
substrates, said liquid crystal molecules are aligned in a second
direction, and when a voltage lying between said voltage of said
one polarity for aligning said liquid crystal molecules in said
first direction and said voltage of said other polarity for
aligning said liquid crystal molecules in said second direction is
applied between said electrodes, a director is aligned in an
arbitrary direction corresponding to said applied voltage and in a
cone angle formed by said first direction and said second
direction, said physical property satisfying a following
equation:C(.theta..sup.2/P.sub.s.sup.2).l- toreq.0.8where C
[F/cm.sup.2] is a capacitance of said liquid crystal display device
per unit area, .theta. [.smallcircle.] is a tilt angle defined by
1/2of said cone angle of said liquid crystal, and P.sub.s
[nC/cm.sup.2] is spontaneous polarization of said liquid crystal
molecules of said liquid crystal.
5. A method of evaluating display burning of a liquid crystal
display device comprising a pair of substrates arranged opposite to
each other, electrodes arranged on opposing surfaces of said pair
of substrates and facing each other, and a liquid crystal provided
between said substrates and having spontaneous polarization and a
physical property such that when a voltage of one polarity and a
sufficiently large level is applied between said electrodes, liquid
crystal molecules are aligned in a first direction, when a voltage
of a polarity opposite to said one polarity and a sufficiently
large level is applied between said electrodes, said liquid crystal
molecules are aligned in a second direction, and when a voltage
lying between said voltage of said one polarity for aligning said
liquid crystal molecules in said first direction and said voltage
of said other polarity for aligning said liquid crystal molecules
in said second direction is applied between said electrodes, a
director is aligned in an arbitrary direction corresponding to said
applied voltage and in a cone angle formed by said first direction
and said second direction, said method comprising: a property
evaluation step of acquiring a physical property including a
permittivity of said liquid crystal located between said
substrates, a tilt angle defined by 1/2of said cone angle of said
liquid crystal and spontaneous polarization of said liquid crystal
molecules of said liquid crystal; and a determination step of
determining a degree of display burning based on a normalized
permittivity .epsilon..sub.s defined by a following
equation:.epsilon..sub.s=.epsilon.-
(.theta..sup.2/P.sub.s.sup.2)where .epsilon. [F/m] is said
permittivity of said liquid crystal located between said
substrates, .theta. [.smallcircle.] is said tilt angle defined by
1/2of said cone angle of said liquid crystal, and P.sub.s
[nC/cm.sup.2] is said spontaneous polarization of said liquid
crystal molecules of said liquid crystal.
6. The method according to claim 5, wherein said determination step
determines whether or not said permittivity .epsilon., said tilt
angle .theta. and said spontaneous polarization P.sub.s obtained in
said property evaluation step satisfy a following
equation:.epsilon.(.theta..s- up.2/P.sub.s.sup.2).ltoreq.8.
7. The method according to claim 5, wherein said determination step
determines whether or not said permittivity .epsilon., said tilt
angle .theta. and said spontaneous polarization P.sub.s obtained in
said property evaluation step satisfy a following
equation:.epsilon.(.theta..s- up.2/P.sub.s.sup.2).ltoreq.5.
8. The method according to claim 5, wherein said liquid crystal is
one of a ferroelectric liquid crystal and an antiferroelectric
liquid crystal.
9. A method of evaluating display burning of a liquid crystal
display device comprising a pair of substrates arranged opposite to
each other, electrodes arranged on opposing surfaces of said pair
of substrates and facing each other, and a liquid crystal provided
between said substrates and having spontaneous polarization and a
physical property such that when a voltage of one polarity and a
sufficiently large level is applied between said electrodes, liquid
crystal molecules are aligned in a first direction, when a voltage
of a polarity opposite to said one polarity and a sufficiently
large level is applied between said electrodes, said liquid crystal
molecules are aligned in a second direction, and when a voltage
lying between said voltage of said one polarity for aligning said
liquid crystal molecules in said first direction and said voltage
of said other polarity for aligning said liquid crystal molecules
in said second direction is applied between said electrodes, a
director is aligned in an arbitrary direction corresponding to said
applied voltage and in a cone angle formed by said first direction
and said second direction, said method comprising: a property
evaluation step of acquiring a physical property including a tilt
angle defined by 1/2of said cone angle of said liquid crystal and
spontaneous polarization of said liquid crystal molecules of said
liquid crystal; a computation step of acquiring a capacitance of
said liquid crystal display device per unit area; and a
determination step of determining whether or not said acquired tilt
angle, spontaneous polarization and capacitance satisfy a following
equation:C(.theta..sup.2/P.sub.s.sup.2).ltoreq.0.8where C
[F/cm.sup.2] is said capacitance of said liquid crystal display
device per unit area, .theta. [.smallcircle.] is said tilt angle
defined by 1/2of said cone angle of said liquid crystal, and
P.sub.s [nC/cm.sup.2] is said spontaneous polarization of said
liquid crystal molecules of said liquid crystal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
device which uses a liquid crystal having spontaneous polarization,
and, more particularly, a liquid crystal display device with less
display burning. The invention also relates to a method of
evaluating and/or predicting the degree of display burning of a
liquid crystal display device which uses a liquid crystal having
spontaneous polarization.
[0003] 2. Description of the Related Art
[0004] Development is being actively made on liquid crystal display
devices that use a ferroelectric liquid crystal and an
antiferroelectric liquid crystal which are excellent in the fast
response characteristic and the angle of visibility in place of
liquid crystal display devices that use a conventionally popular
nematic liquid crystal. The liquid crystal display device that uses
an antiferroelectric liquid crystal is advantageous over the liquid
crystal display device that uses a ferroelectric liquid crystal in
easier alignment control of the liquid crystal molecules and higher
shock absorption. Therefore, active studies have been made on the
former liquid crystal display device.
[0005] Liquid crystal display devices which use a ferroelectric
liquid crystal or an antiferroelectric liquid crystal have such a
difficulty that their transmittances cannot be controlled
arbitrarily, thus making it hard to provide gradation display.
Recently, antiferroelectric liquid crystal display devices which
use an antiferroelectric liquid crystal and can accomplish
gradation display have been proposed as disclosed in U.S. Pat No.
563,175, U.S. Pat. No. 5,895,108 and Japanese Unexamined Patent
Publication (KOKAI) No. 64056/1995. As the antiferroelectric liquid
crystals disclosed in those publications have a wide voltage range
where an antiferroelectric-ferroelectric phase transition precursor
phenomenon occurs with respect to the applied voltage, they have a
number of intermediate optical states in that range. A liquid
crystal display device which is capable of achieving gradation
display can be provided by using such a liquid crystal and
controlling the applied voltage in such a range where the liquid
crystal molecules take intermediate optical states.
[0006] In the liquid crystal display devices that use a
ferroelectric liquid crystal or an antiferroelectric liquid
crystal, when a positive or negative voltage is applied to the
liquid crystal for a long period of time, the applied voltage
causes an ionic impurity present in the liquid crystal to gather in
the vicinity of the electrodes due to the spontaneous polarization
of the liquid crystal. The charges that are originated from the
ionic impurity interact with the spontaneous polarization of the
liquid crystal molecules, so that the alignment state of the liquid
crystal molecules is fixed. As a result, even if application of the
voltage is stopped, the liquid crystal display devices would have
so-called display burning which dimly shows the previously
displayed image.
[0007] At the stage of designing ferroelectric liquid crystal
display devices or antiferroelectric liquid crystal display
devices, it has been difficult to predict the aforementioned
display burning of the liquid crystal display devices. There is no
way but to actually design and make protocols of liquid crystal
display devices and take real measurements on the degree of display
burning empirically or the like. This takes a considerable time and
cost in designing and manufacturing display devices.
SUMMARY OF THE INVENTION
[0008] Accordingly, it is an object of the present invention to
provide an antiferroelectric liquid crystal display device with
suppressed display burning.
[0009] It is another object of the invention to provide a
display-quality evaluation method for a liquid crystal display
device, which can evaluate the degree of display burning without
actually measuring the degree of display burning.
[0010] To achieve the first object, according to the first aspect
of the invention, there is provided a liquid crystal display device
comprising:
[0011] a pair of substrates arranged opposite to each other;
[0012] electrodes arranged on opposing surfaces of the pair of
substrates and facing each other; and
[0013] a liquid crystal provided between the substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between the electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to the one
polarity and a sufficiently large level is applied between the
substrates, the liquid crystal molecules are aligned in a second
direction, and when a voltage lying between the voltage of the one
polarity for aligning the liquid crystal molecules in the first
direction and the voltage of the other polarity for aligning the
liquid crystal molecules in the second direction is applied between
the electrodes, a director is aligned in an arbitrary direction
corresponding to the applied voltage and in a cone angle formed by
the first direction and the second direction, the physical property
satisfying a following equation:
.epsilon.(.theta..sup.2/P.sub.s.sup.2).ltoreq.8 (1)
[0014] where .epsilon. [F/m] is a permittivity of the liquid
crystal located between the substrates, .theta. [.smallcircle.] is
a tilt angle defined by 1/2of the cone angle of the liquid crystal,
and P.sub.s [nC/cm.sup.2] is spontaneous polarization of the liquid
crystal molecules of the liquid crystal.
[0015] The liquid crystal display device according to the first
aspect of the invention is constructed in such a manner that the
value of a normalized permittivity .epsilon..sub.s [=.epsilon.
(.theta..sup.2/P.sub.s.sup.2)] which is acquired by normalizing the
permittivity .epsilon. with the square of the tilt angle .theta.
and the square of the spontaneous polarization P.sub.s meets the
equation 1. Although the liquid crystal display device uses the
liquid crystal having spontaneous polarization that is likely to
cause display burning as compared with the conventional TN liquid
crystal or the like, therefore, the occurrence of display burning
can be restrained. It is therefore possible to provide high-quality
gradation display which is less influenced by display burning while
making good use of the merits of the liquid crystal having
spontaneous polarization, such as an excellent response
characteristic.
[0016] It is preferable that the normalized permittivity
.epsilon..sub.s be 5 or lower. Further, the equation 1 can be
applied to both a ferroelectric liquid crystal and an
antiferroelectric liquid crystal, and a liquid crystal display
device which uses either type of liquid crystal can effectively
suppress display burning.
[0017] To achieve the first object, according to the second aspect
of the invention, there is provided a liquid crystal display device
comprising:
[0018] a pair of substrates arranged opposite to each other;
[0019] electrodes arranged on opposing surfaces of the pair of
substrates and facing each other; and
[0020] a liquid crystal provided between the substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between the electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to the one
polarity and a sufficiently large level is applied between the
substrates, the liquid crystal molecules are aligned in a second
direction, and when a voltage lying between the voltage of the one
polarity for aligning the liquid crystal molecules in the first
direction and the voltage of the other polarity for aligning the
liquid crystal molecules in the second direction is applied between
the electrodes, a director is aligned in an arbitrary direction
corresponding to the applied voltage and in a cone angle formed by
the first direction and the second direction, the physical property
satisfying a following equation:
C(.theta..sup.2/P.sub.s.sup.2).ltoreq.0.8 (2)
[0021] where C [F/cm.sup.2] is a capacitance of the liquid crystal
display device per unit area, .theta. [.smallcircle.] is a tilt
angle defined by 1/2of the cone angle of the liquid crystal, and
P.sub.s [nC/cm.sup.2] is spontaneous polarization of the liquid
crystal molecules of the liquid crystal.
[0022] This liquid crystal display device is constructed in such a
manner that as apparent from the equation 2, the value of a
normalized capacitance [C(.theta..sup.2/P.sub.s.sup.2)] which is
acquired by normalizing the capacitance C with the square of the
tilt angle .theta. and the square of the spontaneous polarization
P.sub.s becomes equal to or smaller than 0.8. Although the liquid
crystal display device uses the liquid crystal having spontaneous
polarization, therefore, display burning is unlikely to occur. This
makes it possible to provide high-quality gradation display which
is less influenced by display burning while making good use of the
merits of the liquid crystal having spontaneous polarization, such
as an excellent response characteristic.
[0023] To achieve the second object, according to the third aspect
of the invention, there is provided a method of evaluating display
burning of a liquid crystal display device comprising a pair of
substrates arranged opposite to each other, electrodes arranged on
opposing surfaces of the pair of substrates and facing each other,
and a liquid crystal provided between the substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between the electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to the one
polarity and a sufficiently large level is applied between the
electrodes, the liquid crystal molecules are aligned in a second
direction, and when a voltage lying between the voltage of the one
polarity for aligning the liquid crystal molecules in the first
direction and the voltage of the other polarity for aligning the
liquid crystal molecules in the second direction is applied between
the electrodes, a director is aligned in an arbitrary direction
corresponding to the applied voltage and in a cone angle formed by
the first direction and the second direction. The method comprises
a property evaluation step of acquiring a physical property
including a permittivity of the liquid crystal located between the
substrates, a tilt angle defined by 1/2of the cone angle of the
liquid crystal and spontaneous polarization of the liquid crystal
molecules of the liquid crystal; and a determination step of
determining a degree of display burning based on a normalized
permittivity .epsilon..sub.s defined by a following equation 3:
.epsilon..sub.s=.epsilon.(.theta..sup.2/P.sub.s.sup.2) (3)
[0024] where .epsilon. [F/m] is the permittivity of the liquid
crystal located between the substrates, .theta. [.smallcircle.] is
the tilt angle defined by 1/2of the cone angle of the liquid
crystal, and P.sub.s [nC/cm.sup.2] is the spontaneous polarization
of the liquid crystal molecules of the liquid crystal.
[0025] A liquid crystal display device which is to be evaluated by
the present invention comprises a liquid crystal having spontaneous
polarization, such as a ferroelectric liquid crystal or an
antiferroelectric liquid crystal, is likely to suffer display
burning and has a gradation display capability. As apparent from
the equation 3, the possible display burning of such a liquid
crystal display device can be evaluated objectively by using the
value of the normalized permittivity
.epsilon..sub.s[=.epsilon.(.theta..sup.2/P.sub.s.sup.2)] as an
index, so that the degree of display burning of the liquid crystal
display device can be predicted without actually measuring the
display burning of the liquid crystal display device. This
contributes to reducing the time and cost of producing the liquid
crystal display device.
[0026] The display burning of a liquid crystal display device is
allowable within the range where the normalized permittivity
.epsilon..sub.s is equal to or smaller than 8 or more preferably is
equal to or smaller than 5 as indicated by the equation 1.
Comparing the normalized permittivity .epsilon..sub.s with those
values can therefore make it possible to determine whether the
display quality is acceptable or not. The equation 1 is applicable
to both a ferroelectric liquid crystal and an antiferroelectric
liquid crystal.
[0027] To achieve the second object, according to the fourth aspect
of the invention, there is provided a method of evaluating display
burning of a liquid crystal display device comprising a pair of
substrates arranged opposite to each other, electrodes arranged on
opposing surfaces of the pair of substrates and facing each other,
and a liquid crystal provided between the substrates and having
spontaneous polarization and a physical property such that when a
voltage of one polarity and a sufficiently large level is applied
between the electrodes, liquid crystal molecules are aligned in a
first direction, when a voltage of a polarity opposite to the one
polarity and a sufficiently large level is applied between the
electrodes, the liquid crystal molecules are aligned in a second
direction, and when a voltage lying between the voltage of the one
polarity for aligning the liquid crystal molecules in the first
direction and the voltage of the other polarity for aligning the
liquid crystal molecules in the second direction is applied between
the electrodes, a director is aligned in an arbitrary direction
corresponding to the applied voltage and in a cone angle formed by
the first direction and the second direction. The method comprises
a property evaluation step of acquiring a physical property
including a tilt angle defined by 1/2of the cone angle of the
liquid crystal and spontaneous polarization of the liquid crystal
molecules of the liquid crystal; a computation step of acquiring a
capacitance of the liquid crystal display device per unit area; and
a determination step of determining whether or not the acquired
tilt angle, spontaneous polarization and capacitance satisfy the
following equation 2:
C(.theta..sup.2/P.sub.s.sup.2).ltoreq.0.8 (2)
[0028] where C [F/cm.sup.2] is the capacitance of the liquid
crystal display device per unit area, .theta. [.smallcircle.] is
the tilt angle defined by 1/2of the cone angle of the liquid
crystal, and P.sub.s [nC/cm.sup.2] is the spontaneous polarization
of the liquid crystal molecules of the liquid crystal.
[0029] This method can permit the display burning of such a liquid
crystal display device to be evaluated objectively by using the
value of the normalized permittivity .epsilon..sub.s
[=.epsilon.(.theta..sup.2/P.sub.s- .sup.2)] as an index. The use of
the normalized capacitance can allow the degree of display burning
of the liquid crystal display device to be predicted without
actually measuring the display burning of the liquid crystal
display device. This contributes to reducing the time and cost of
producing the liquid crystal display device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a cross-sectional diagram illustrating the
structure of a liquid crystal display device according to one
embodiment of the invention;
[0031] FIG. 2 is a diagram showing the relationship between the
transmittance axis of a sheet polarizer of the liquid crystal
display device shown in FIG. 1 and the alignment direction of the
liquid crystal molecules of an antiferroelectric liquid
crystal;
[0032] FIGS. 3A and 3B are diagrams for explaining a change .DELTA.
in the inclination angle or tilt angle of the liquid crystal
molecules which may be a factor for display burning; and
[0033] FIG. 4 is a diagram showing the relationship between a
change .DELTA. in the tilt angle of the liquid crystal molecules
and the normalized permittivity .epsilon..sub.s of the liquid
crystal display device according to this embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Liquid crystal display devices will be described below as
embodiments of the present invention with reference to the
accompanying drawings.
[0035] FIG. 1 shows the cross-sectional structure of an
antiferroelectric liquid crystal display device (hereinafter
referred to as "liquid crystal display device") 100 according to
one embodiment of the invention. In the following description of
the embodiment, the liquid crystal display device 100 will be
described as a display device which uses thin film transistors
(TFTs) as active elements and provides an active matrix type
monochromatic gradation display.
[0036] The liquid crystal display device 100 has a pair of
transparent substrates (e.g., glass substrates) 101 and 102.
Transparent pixel electrodes 103 and TFTs 104 connected to the
pixel electrodes 103 are arranged on one substrate (hereinafter
referred to as "lower substrate") 101 in a matrix form.
[0037] Formed on the other substrate (hereinafter referred to as
"upper substrate") 102 is a transparent opposing electrode 107
facing the pixel electrodes 103 formed on the lower substrate 101.
The single opposing electrode 107 has an area over the entire
display area.
[0038] Alignment films 108 and 109 are respectively formed on the
electrode-forming surfaces of the lower substrate 101 and the upper
substrate 102. The alignment films 108 and 109 are formed of an
organic polymer compound, such as polyimide. The opposing surfaces
of the alignment films 108 and 109 has undergone an alignment
process, such as rubbing, so that the alignment films 108 and 109
have alignment restricting force which causes crystal molecules in
the vicinity of the surfaces to be aligned in the direction of the
alignment process (a third direction 111c in FIG. 2 to be discussed
later).
[0039] The lower substrate 101 and upper substrate 102 are adhered
together at the peripheral edge portions by a frame-shaped seal
member 110. An antiferroelectric liquid crystal 111 is sealed in
the area that is surrounded by the seal member 110 between the
substrates 101 and 102. The layer of the antiferroelectric liquid
crystal 111 is kept at a predetermined thickness by the seal member
110 and a transparent gap member 112.
[0040] In the embodiment, the layer thickness of and the material
for the antiferroelectric liquid crystal 111 are selected in such a
way that the value of a normalized permittivity .epsilon..sub.s
which is acquired by normalizing the permittivity .epsilon. of the
antiferroelectric liquid crystal 111 with the square of the tilt
angle .theta. [.smallcircle.] (1/2of the cone angle of the
antiferroelectric liquid crystal 111 between the transparent
substrates 101 and 102 by the interaction of the applied electric
field and spontaneous polarization) and the square of the
spontaneous polarization P.sub.s [nC/cm.sup.2] becomes equal to or
lower than 8 [(.smallcircle.cm).sup.2/(CVm.times.10.sup.9], more
preferably, equal to or lower than 5, i.e., the normalized
permittivity .epsilon..sub.s meets the following equation 1. The
layer thickness is so set as to approximately coincide the
thickness that provides the optimal retardation (optical path
difference).
.epsilon..sub.s=.epsilon.(.theta..sup.2/P.sub.s.sup.2).ltoreq.8
(1)
[0041] A pair of sheet polarizers 113 and 114 are so arranged as to
sandwich the transparent substrates 101 and 102. The sheet
polarizer 113 is arranged so that its optical axis extends in the
direction of approximately 22.5 .smallcircle. with respect to the
direction of the alignment process. The sheet polarizer 114 is
arranged so that its optical axis is perpendicular or parallel to
the optical axis of the sheet polarizer 113.
[0042] Referring now to FIG. 2, a description will be given of the
positional relationship between the alignment direction of the
molecules of the antiferroelectric liquid crystal 111 and the
optical axes of the sheet polarizers 113 and 114 in the
embodiment.
[0043] The director (the average direction of the long axis of the
liquid crystal molecules) of the antiferroelectric liquid crystal
111 is set to a first direction 111a when a positive, first
saturation voltage large enough to cause a phase change of the
liquid crystal molecules is applied between the pixel electrodes
103 and the opposing electrode 107 and is set to a second direction
111b (which is shifted from the first direction 111a by twice the
tilt angle .theta.) when a second saturation voltage which has the
opposite polarity to that of the first saturation voltage and whose
absolute value is large enough to cause a phase change of the
liquid crystal molecules is applied to the antiferroelectric liquid
crystal 111. When no voltage is applied, the director of the
antiferroelectric liquid crystal is set to the aforementioned third
direction 111c which nearly matches with the direction of the
alignment process.
[0044] When an arbitrary voltage lying between the first saturation
voltage and the second saturation voltage is applied to the
antiferroelectric liquid crystal 111, the director of the
antiferroelectric liquid crystal is set to an intermediate
direction between the first direction 111a and the second direction
111b in accordance with the applied voltage. The application of a
voltage lying between the first saturation voltage and the second
saturation voltage can set the director of the antiferroelectric
liquid crystal 111 to any direction in the range of the cone angle
2 .theta. shown in FIG. 2. Therefore, controlling the voltage that
is applied and maintained between the pixel electrodes 103 and the
opposing electrode 107 can allow the liquid crystal display device
100 to demonstrate a gradation display capability.
[0045] In the embodiment, the optical axis of the sheet polarizer
113, e.g., a transmittance axis 113a, perpendicularly intersects
the direction 111c of the alignment process (approximately normal
to the chiral smectic layer) at an angle of 22.5 .smallcircle. as
shown in FIG. 2. The optical axis of the sheet polarizer 114, e.g.,
a transmittance axis 114a, perpendicularly intersects the
transmittance axis 113a of the sheet polarizer 113.
[0046] As the transmittance axes 113a and 114a of the sheet
polarizers 113 and 114 are set as shown in FIG. 2 in this
structure, the liquid crystal display device 100 has the lowest
transmittance when the director of the antiferroelectric liquid
crystal 111 is set to the direction 113a (the direction of
.theta.=22.5 .smallcircle.). The liquid crystal display device 100
has the highest transmittance when the director of the
antiferroelectric liquid crystal 111 is set to a direction 113d
(the direction of 45 .smallcircle. with respect to the direction
113a). When the director of the antiferroelectric liquid crystal
111 is set to a direction other than those directions, light passes
in accordance with the alignment state so that the liquid crystal
display device 100 provides a brightness according to the average
alignment state of the liquid crystal molecules. As an arbitrary
voltage lying between the first saturation voltage and the second
saturation voltage is applied to the antiferroelectric liquid
crystal 111, therefore, the liquid crystal display device 100 can
display a gradation level according to the applied voltage.
[0047] As described above, the antiferroelectric liquid crystal 111
is constructed in such a way that the normalized permittivity
.epsilon..sub.s acquired by normalizing the permittivity .epsilon.
of the antiferroelectric liquid crystal 111 with the square of the
tilt angle .theta. and the square of the spontaneous polarization
P.sub.s [nC/cm.sup.2] becomes equal to or lower than 8
[(.smallcircle.cm).sup.2/(- CVm).times.10.sup.9] as shown in the
equation 1, more preferably, equal to or lower than 5. With this
particular structure, even when an arbitrary voltage is applied
between the pixel electrodes 103 and the opposing electrode 107 for
a long period of time, the director of the antiferroelectric liquid
crystal 111 can easily return to the direction 111c that nearly
matches with the original alignment direction with no electric
field applied, once the application of the voltage is stopped.
[0048] Therefore, the liquid crystal display device 100 does not
easily have display burning and can keep displaying high-quality
images. Unlike the ordinary antiferroelectric liquid crystal
display device whose gradation display becomes unclear if the
influence of the voltage that has been applied to the liquid
crystal remains, the liquid crystal display device 100 has the
improved gradation display capability which is free of the
conventional problem and can display high-quality images.
[0049] A description will now be given of the conditions for the
equation 1 that can allow the liquid crystal display device 100 of
the embodiment to suppress display burning.
[0050] A plurality of liquid crystal display cells (hereinafter
referred to as "LCD cells") each having a structure common to the
liquid crystal display device 100 shown in FIG. 1, except for the
material for and the layer thickness of the antiferroelectric
liquid crystal used in the liquid crystal display device 100 are
prepared and their characteristics are set in the following
manner.
[0051] First, with no electric field applied, the director of the
antiferroelectric liquid crystal 111 is at the center of the cone
angle 2 .theta. as shown in FIG. 3A. Next, a DC (Direct Current)
voltage is applied between the pixel electrodes 103 and the
opposing electrode 107 of each LCD cell in such a way that the tilt
angle .theta. of the director of the antiferroelectric liquid
crystal 111 becomes equal to 22.5 .smallcircle. and each LCD cell
is left in this condition for one hour. The application of this
voltage sets the director of the antiferroelectric liquid crystal
111 in the direction 111a. Then, the pixel electrodes 103 are
short-circuited with the opposing electrode 107, and each LCD cell
is further left in this condition for one hour. As a result, the
LCD cells become a field-less state so that the liquid crystal
molecules aligned in the direction 111a tend to return to the
direction 111c of the alignment process.
[0052] As the same voltage is continuously applied to the LCD cells
for one hour, however, charges of the opposite polarity to that of
the spontaneous polarization which are originated from the ionic
impurity contained in the liquid crystal are stored in the vicinity
of the lower substrate 101 and upper substrate 102. Even if the LCD
cells comes to the field-less state, from the macroscopic point of
view, the interaction of the charges whose polarity is opposite to
that of the spontaneous polarization prevents the liquid crystal
molecules from freely moving so that the liquid crystal molecules
do not completely return to the original state as shown in FIG. 3B.
That is, the antiferroelectric liquid crystal 111 has a director in
the direction lying between the direction 111c of the alignment
process and .theta.=22.5 .smallcircle..
[0053] For each LCD cell, the return angle, .theta.r, (with
.theta.=22.5 .smallcircle. as a reference position) of the liquid
crystal molecules of the antiferroelectric liquid crystal 111
having the aforementioned director is measured by a polarization
microscope as shown in FIG. 3B, and a change .DELTA. in the tilt
angle of the director of the antiferroelectric liquid crystal 111
between the time when the voltage is applied and the time when the
voltage application is stopped is acquired from an equation 4
below.
.DELTA.=.vertline.100.times.(1-.theta.r/22.5).vertline. (4)
[0054] In the equation 4, when .theta.r=22.5 .smallcircle., i.e.,
when the director of the antiferroelectric liquid crystal 111
returns to the original alignment direction under no electric field
applied (nearly in the direction 111c of the alignment process),
the change .DELTA. becomes 0. When .theta.r=0 .smallcircle., i.e.,
when the director does not return at all from the state in the
field-less condition, the change .DELTA. becomes 100. The larger
the change .DELTA. is, therefore, the greater the degree of display
burning is, and the smaller the change .DELTA. is, therefore, the
smaller the degree of display burning is.
[0055] To normalize the acquired change .DELTA. with the
permittivity .epsilon. of the liquid crystal, the permittivity of
the sealed liquid crystal should be obtained. Because it is
difficult to directly acquire the permittivity .epsilon. of the
antiferroelectric liquid crystal 111 sealed in the liquid crystal
display device 100, the capacitance, C.sub.c, of each LCD cell per
unit area is obtained first.
[0056] First, an AC (Alternate Current) voltage V of a low
frequency equal to or lower than 10 kHz is applied between the
pixel electrodes 103 and the opposing electrode 107 in the LCD
cell, the charges Q stored in the LCD cell are measured, and the
capacitance C.sub.c per unit area is acquired from a following
equation 5 where S is the effective area of the pixel electrodes
103.
C.sub.c=Q/V .multidot.1/S (5)
[0057] The previously measured thickness of the alignment film and
the influence thereof are eliminated from the capacitance C.sub.c,
thus yielding a capacitance C.sub.LC of the liquid crystal layer
alone. Next, the permittivity .epsilon. of the liquid crystal is
acquired from the known thickness of the liquid crystal layer.
[0058] Further, the spontaneous polarization P.sub.s [nC/cm.sup.2]
is measured from the polarization inversion current that flows when
the direction of the spontaneous polarization of the liquid crystal
is inverted.
[0059] The tilt angle .theta. that is expressed by 1/2of the cone
angle 2 .theta. of the liquid crystal is measured by the
polarization microscope.
[0060] For each LCD cell, the normalized permittivity
.epsilon..sub.s which is the permittivity .epsilon. of the liquid
crystal of each LCD cell normalized with the tilt angle .theta. and
the spontaneous polarization P.sub.s is computed from an equation 6
below using the measured values.
.epsilon..sub.s=.epsilon..times.(.theta..sup.2/P.sub.s.sup.2)
(6)
[0061] For multiple LCD cells with different conditions, such as
the material for the liquid crystal and the thickness of the
alignment film, the change .DELTA. and the normalized permittivity
.epsilon..sub.s are acquired. The change .DELTA. and the normalized
permittivity .epsilon..sub.s have a positive exponential
correlation with each other as shown in FIG. 4.
[0062] The following is apparent from the correlation shown in FIG.
4. In the range over which the normalized permittivity
.epsilon..sub.s changes from the infinity to 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9], as the normalized
permittivity .epsilon..sub.s falls, the change .DELTA. or the
degree of display burning decreases sharply. With the normalized
permittivity .epsilon..sub.s being equal to or smaller than 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9], the change .DELTA.
or the degree of display burning is stable and hardly changes.
[0063] That is, with the normalized permittivity .epsilon. being
equal to or smaller than 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9], the change .DELTA.
or the degree of display burning is equal to or less than 10% which
is low enough to be practically allowable. More preferably, the
normalized permittivity .epsilon..sub.s should be equal to or
smaller than 5 to make the degree of display burning equal to or
less than 5%. It is empirically confirmed that the allowable range
of display burning is where the normalized permittivity
.epsilon..sub.s satisfies the equation 1, so that the change
.DELTA. becomes equal to or less than 10%.
[0064] From the above-described results of the experiment, it is
understood that the practically allowable range for the normalized
permittivity .epsilon..sub.s of the antiferroelectric liquid
crystal 111 used in the liquid crystal display device is equal to
or smaller than 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9].
[0065] To achieve gradation display with suppressed display
burning, therefore, the antiferroelectric liquid crystal 111 whose
normalized permittivity .epsilon..sub.s is adjusted to be equal to
or smaller than 8 [(.smallcircle.cm).sup.2/(CVm).times.10.sup.9]
should be used as in the liquid crystal display device 100 that has
been described in the foregoing description of the embodiment. It
is to be noted however that for the liquid crystal display device
100 to provide the adequate display, the proper retardation should
be secured. As in the ordinary liquid crystal display devices,
therefore, the material for the liquid crystal and the layer
thickness thereof should be adjusted to make the normalized
permittivity .epsilon..sub.s equal to or smaller than 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9] while maintaining
the proper retardation.
[0066] It is also apparent from the above-described experiment, the
degree of display burning can be predicted and evaluated by
referring to FIG. 4, without actually making a sample of the liquid
crystal display device 100, by acquiring the normalized
permittivity .epsilon..sub.s of the liquid crystal to be used and
using the normalized permittivity .epsilon..sub.s as an index for
the display burning. Specifically, it is predictable that if the
normalized permittivity .epsilon..sub.s becomes greater than 8
[(.smallcircle.cm).sup.2/(CVm).times.10.sup.9], the liquid crystal
display device 100 will have display burning prominently, while
with the normalized permittivity .epsilon..sub.s being equal to or
smaller than 8 [(.smallcircle.cm).sup.2/(CVm).times.10.sup.9], more
preferably, equal to or smaller than 5, the liquid crystal display
device 100 will not suffer display burning much.
[0067] Although the permittivity of the antiferroelectric liquid
crystal 111 is normalized in this embodiment, the capacitance of
the LCD cell per unit area (the capacitance per unit area between
the pixel electrodes 103 and the opposing electrode 107 facing each
other) may be normalized with the square of the tilt angle .theta.
and the square of the spontaneous polarization P.sub.s. In this
case, to set the change .DELTA. equal to or less than 10%, the
value of the normalized capacitance should be equal to or smaller
than 0.8 [(.smallcircle.cm).sup.2/(CV).times.10.sup.9].
[0068] As in the case where the normalized permittivity
.epsilon..sub.s is used as an index for display burning, the degree
of display burning can be predicted and evaluated by acquiring the
value of the capacitance per unit area, normalized with the square
of the tilt angle .theta. and the square of the spontaneous
polarization P.sub.s and using the obtained capacitance.
[0069] In the foregoing description of the embodiment, the liquid
crystal display device 100 has been described as using the
antiferroelectric liquid crystal 111. However, the conditions for
the normalized permittivity .epsilon..sub.s or the normalized
capacitance under which the value of the change .DELTA. in the tilt
angle of the director of the antiferroelectric liquid crystal
between the time when the voltage is applied and the time when the
voltage application is stopped becomes equal to or less than 10%,
more preferably, equal to or less than 5%, is applicable to general
liquid crystal display devices that use a liquid crystal and liquid
crystal composition which have spontaneous polarization, such as
ferroelectric liquid crystal, as well as those using an
antiferroelectric liquid crystal.
[0070] Although the liquid crystal display device 100 of the
embodiment provides monochromatic gradation display, the invention
can be adapted to a liquid crystal display device having, for
example, color filters or the like to ensure color display.
[0071] Further, although the liquid crystal display device 100 of
this embodiment has been described as a transmittive type, the
invention can be adapted to a reflection type liquid crystal
display device. In this case, the substrates or electrodes may be
formed of opaque materials, such as metal, and a sheet polarizer
may be provided only on one side of the liquid crystal display
device (on the viewer's side).
* * * * *