JPH06174920A - Optical compensation sheet and liquid crystal display element formed by using the same - Google Patents

Abstract

PURPOSE:To provide the liquid crystal display element of a high grade display which improves the visual angle characteristic of a TN type liquid crystal display element when used for this element and has excellent visibility. CONSTITUTION:This optical compensation sheet is a crystallooptically biaxial optically anisotropic element varying in all of the three refractive indices nx, ny, nz (Kz denotes the refractive index most approximate to a thickness direction) in the main axis direction. The angle formed by the direction of nz and the direction perpendicular to the sheet is defined as thetaz and the angle formed by the optical axis nearer the direction perpendicular to the sheet of the two optical axes and the direction perpendicular to the sheet as thetaopt. The thetaz is then 0 deg. to 40 deg. and the thetaopt is 0 deg. to 20 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical compensation sheet used for improving the viewing angle characteristics of display contrast and display color. Further, by using the optical compensation sheet, the viewing angle characteristics of display contrast and display color are improved. The present invention relates to an improved liquid crystal display device.

[0002]

2. Description of the Related Art CRTs, which are the mainstream of display devices for OA equipment such as Japanese word processors and desktop personal computers
Have been converted into liquid crystal display elements which have the great advantages of thinness, light weight, and low power consumption. Most of the liquid crystal display elements (hereinafter, referred to as LCDs) that are currently popular use twisted nematic liquid crystals. The display method using such a liquid crystal can be roughly classified into a birefringence mode and an optical rotation mode.

An LCD using a birefringence mode has a twisted angle of 90 ° or more in the alignment of liquid crystal molecules and has steep electro-optical characteristics. Therefore, a simple matrix without active elements (thin film transistor or diode). A large-capacity display can be obtained by time-division driving even with a striped electrode structure. However, the response speed is slow (hundreds of milliseconds) and gray scale display is difficult, and a liquid crystal display element (TFT-L) using an active element is used.
The display performance of CDs, MIM-LCDs, etc.) is exceeded.

For the TFT-LCD and MIM-LCD, there is used a display system (TN type liquid crystal display element) of optical rotation mode in which the alignment state of liquid crystal molecules is twisted by 90 °. This display method is the most effective method as compared with other LCDs because it has a high response speed (several tens of milliseconds), can easily obtain a monochrome display, and has a high display contrast. However, since the twisted nematic liquid crystal is used, there is a problem in the viewing angle characteristics that the display color and the display contrast change depending on the viewing direction due to the principle of the display system.
The display performance of is not reached.

Japanese Unexamined Patent Publication Nos. 4-229828 and 4-2.
As disclosed in Japanese Patent No. 58923, a method has been proposed in which a viewing angle is increased by disposing a retardation film between a pair of polarizing plates and a TN liquid crystal cell.

However, even with these methods, the viewing angle of the LCD is still insufficient, and further improvement is desired. In particular, when considered as a vehicle-mounted type or as a substitute for a CRT, the current viewing angle cannot cope with the situation.

It is generally known that liquid crystal molecules have different refractive indexes in the major axis direction and the minor axis direction of the liquid crystal molecules. When polarized light enters liquid crystal molecules exhibiting such anisotropy of refractive index, the polarized state of the polarized light changes depending on the angle of the liquid crystal molecules. The molecular arrangement of the liquid crystal cell of the twisted nematic liquid crystal has a structure in which the arrangement of the liquid crystal molecules is twisted in the thickness direction of the liquid crystal cell. The light is sequentially polarized and propagates depending on the orientation of the liquid crystal molecules. Therefore, the polarization state of the light propagating in the liquid crystal cell is different between the case where the light is vertically incident on the liquid crystal cell and the case where the light is obliquely incident, and as a result, the display pattern cannot be seen at all depending on the viewing direction. Appears as a phenomenon, which is not preferable for practical use.

[0008]

SUMMARY OF THE INVENTION The present invention provides an optical compensation sheet having improved viewing angle characteristics of display contrast and display color. Furthermore, the present invention provides a liquid crystal display device having improved display contrast and viewing angle characteristics of display color.

The above-mentioned objects have been achieved by the following means. (1) Three refractive indices nx, ny, nz (n
z is the refractive index closest to the thickness direction) are all optically anisotropic biaxial optically anisotropic elements, and the angle formed by the direction nz and the direction perpendicular to the sheet surface is θz. If one of the two optical axes closer to the direction perpendicular to the seat surface forms an angle with the direction perpendicular to the seat surface as θopt, then θ
An optical compensation sheet, wherein z is 0 ° or more and 40 ° or less, and θopt is 0 ° or more and 20 ° or less. (2) The maximum value of the refractive index in the in-plane direction is defined as nx ′, the refractive index in the thickness direction is defined as nz ′, and the thickness is defined as (nz′−n
When the retardation in the thickness direction is defined by x ′) × d, the retardation in the thickness direction is −50 to −9.
It is 00 nm, The optical compensation sheet as described in (1) above. (3) T with a twist angle of approximately 90 ° between the two electrode substrates
A liquid crystal display device comprising a liquid crystal cell sandwiching an N-type liquid crystal, two polarizing elements arranged on both sides of the liquid crystal cell, and at least one optical compensation sheet disposed between the liquid crystal cell and the polarizing element, A liquid crystal display device, wherein the optical compensation sheet is the optical compensation sheet described in (1) or (2) above.

The operation of the present invention will be described below with reference to the drawings, taking a TN type liquid crystal display device as an example. 1, 2, and 3
Shows the polarization state of the light propagating in the liquid crystal cell when a voltage higher than the threshold voltage is applied to the liquid crystal cell, and shows the bright state when no voltage is applied. Figure 2
FIG. 6 is a diagram showing a polarization state of light when light is vertically incident on a liquid crystal cell. When the natural light 0 is vertically incident on the polarizing plate 1 having the polarization axis 1.1, the light transmitted through the polarizing plate 1 is
Linearly polarized light becomes 1.3.

In the figure, 3.3 is a schematic representation of one liquid crystal molecule model showing the alignment state of the liquid crystal molecules when a sufficient voltage is applied to the TN liquid crystal cell. When the molecular long axis of the liquid crystal molecules 3.3 in the liquid crystal cell is parallel to the light traveling path 1.4, there is no difference in the refractive index on the incident surface (in the plane perpendicular to the light traveling path). There is no phase difference between the ordinary light and the extraordinary light propagating through it, and the linearly polarized light 1.3 propagates as it is when it passes through the liquid crystal cell. When the polarization axis 2.1 of the polarizing plate 2 is set to be perpendicular to the polarization axis 1.1 of the polarizing plate 1, the light 3.1 that has passed through the liquid crystal cell cannot pass through the polarizing plate and is in a dark state.

FIG. 3 is a diagram showing a polarization state of light when the light obliquely enters the liquid crystal cell. Natural light of incident light 0
Is incident obliquely, the polarized light transmitted through the polarizing plate 1 is 1.3.
Becomes almost linearly polarized light. (In the actual case, it becomes elliptically polarized due to the characteristics of the polarizing plate). In this case, the refractive index anisotropy of the liquid crystal causes a difference in the refractive index on the incident surface of the liquid crystal cell, and the light 3.1 transmitted through the liquid crystal cell is elliptically polarized and transmitted through the polarizing plate 2. The transmission of light in such an oblique incidence undesirably lowers the contrast.

The present invention is intended to prevent such a decrease in contrast due to oblique incidence and improve the viewing angle characteristics. FIG. 1 shows an example of the configuration according to the present invention. The optical compensation sheet 7 of the present invention is provided between the polarizing plate 2 and the liquid crystal cell 3.
Are arranged. The optical compensation sheet 7 is a birefringent body that polarizes more as the angle of incidence of light with respect to the optical axis increases. As in the case of FIG. 3, the elliptically polarized light 3.1 that is obliquely incident on the liquid crystal display element having such a structure and transmitted through the liquid crystal cell 3 is caused by the phase delaying action when transmitting through the optical compensation sheet 7. The elliptically polarized light was modulated to the original linearly polarized light, and a good liquid crystal display device having the same transmittance even at various oblique incidences and having no viewing angle dependency was realized.

The optical compensatory sheet used in the present invention is a crystallographically biaxial optically anisotropic element. Crystal optics 2
Axial means that the three refractive indices nx, ny, and
It means that nz are all different. Here, the refractive index in the direction of the principal axis close to the direction perpendicular to the sheet is nz. Usually, nx and ny of an optical compensation sheet obtained by uniaxially stretching a polymer material such as polycarbonate are in the plane, and nz is in the direction perpendicular to the sheet. The principal axes nx and ny are not necessarily in-plane, and nz may also be inclined from the direction perpendicular to the sheet. In the present invention, the angle formed by the direction of nz and the direction perpendicular to the sheet is defined as θz, and θz
Is preferably 0 ° or more and 40 ° or less. Furthermore,
It is preferably 5 ° or more and 30 ° or less.

When the crystal is optically biaxial, the optical axis is 2
Exist. The optical axis as referred to in the present invention means the direction in which the retardation becomes 0 when viewed from this direction. In the present invention, the angle between the direction closer to the sheet and the direction perpendicular to the sheet of these two optical axes is defined as θ opt, and θ opt is 0 ° or more 2
It is preferably in the range of 0 ° or less. In this case, the retardation when viewed from the front is close to zero. Specifically, the retardation from the front is preferably 200 nm or less, and more preferably 100 nm or less.

Most of TN-LCDs adopt a normal-white mode. In this mode, the transmittance of light from the black display portion remarkably increases as the viewing angle is increased, resulting in a sharp decrease in contrast. The black display is the state when a voltage is applied, but at this time, the TN cell can be regarded as a positive uniaxial optical anisotropic body in which the optical axis is slightly tilted from the direction normal to the surface of the cell. Also, in the case of intermediate gradation, the optical axis seems to further tilt from the normal direction of the LC cell.

When the optical compensatory sheet of the present invention is used, when the liquid crystal cell is viewed from the front, the retardation of the optical compensatory sheet is small, so that a high contrast similar to the conventional one can be obtained. When viewed from an oblique direction, the birefringence of the optical compensation sheet is exhibited, the birefringence of the liquid crystal in an oblique orientation is compensated, and the viewing angle characteristics are significantly improved.

When the maximum value of the refractive index in the in-plane direction is nx ', the refractive index in the thickness direction is nz', and the thickness is d, the retardation in the thickness direction is defined as (nz'-nx ') * d. To do. Retardation in the thickness direction is -50 to -900
When the thickness is nm, the effect of improving the viewing angle characteristic appears remarkably. Therefore, the retardation in the thickness direction is -50 to -9.
It is preferably in the range of 00 nm, and more preferably -50 to
It is preferably in the range of -700 nm.

The optical compensation sheet of the present invention is preferably provided as a polymer film or plate, and the optical transmittance of the optical compensation sheet is preferably 80% or more, more preferably 90% or more.

The polymeric material used in the optical compensation sheet of the present invention is not particularly limited, but it may be polycarbonate, polyarylate, polysulfone, polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, Polyvinyl alcohol, polyamide, polyimide, polyolefin, polyvinyl chloride, cellulosic polymer, polyacrylonitrile, polystyrene, also
Binary type, ternary type, various polymers, graft copolymers, blends and the like are preferably used. Further, a sheet in which a low molecular liquid crystal having positive or negative intrinsic birefringence is dispersed in a polymer matrix may be used.

A detailed description will be given below with reference to embodiments.

EXAMPLE A styrene-acrylonitrile copolymer pellet having a molecular weight of 150,000 was melted, extruded from a nozzle having an inner diameter of 100 mm and stretched by 30%, and a styrene-acrylonitrile copolymer rod rod having an outer diameter of 87 mm. Got Slice the rod rod in the plane perpendicular to the central axis of the rod rod, that is, the stretching axis,
A circular plate-shaped material having a thickness of about 3 mm was obtained. Using a lapping machine manufactured by U.S. Buhler, the plate-like material is 50 μm SiC powder, 30 μm SiC powder, 10 μm SiC powder, 3 μm diamond powder, 0.05 μm
Sequentially polishing with an alumina powder of No. 2 to obtain a mirror-like optical compensation sheet of a styrene-acrylonitrile plate having a thickness of 2 mm. The sample thus obtained is referred to as sample A.

A rod rod obtained in the same manner as in Sample A was sliced at a plane intersecting with the central axis, that is, the stretching axis at an angle of 40 ° to obtain an elliptical plate-like material having a thickness of about 3 mm. The plate-like material was polished in the same manner as in Sample A to obtain a mirror-like styrene-acrylonitrile plate having a thickness of 2 mm. The styrene-acrylonitrile plate was stretched by changing the stretching ratio to obtain optical compensation sheet samples B to F shown in Table 1. The case where the optical compensation sheet was not used was set as Comparative Example 1, the samples of Table 1 were used, and the optical compensation sheet was arranged in the TN type liquid crystal cell as shown in FIG. 1-4 and Comparative Examples 2-3.

[0022]

[Table 1]

Evaluation of Birefringence The prepared sample is an ellipsometer AE manufactured by Shimadzu Corporation.
The maximum value nx 'of the refractive index in the in-plane direction and the refractive index nz' in the thickness direction were measured using P-100. The thickness of the sample was d, and (nz'-nx ') xd was the retardation in the thickness direction. In addition, the sample is tilted, and the ordinary light refractive index no and the extraordinary light refractive index ne are measured.
The direction of = ne was taken as the optical axis. The angle formed by one of the two optical axes closer to the direction perpendicular to the sheet surface and the direction perpendicular to the sheet surface was defined as θopt. The direction right in the middle of the two optical axes was defined as the direction with the refractive index nz, and the angle between this direction and the direction perpendicular to the sheet surface was defined as θz.

The refractive index in the in-plane direction perpendicular to the nz direction was measured by AEP-100, and the maximum value was nx,
The minimum value was set to ny. It was confirmed that among the samples shown in Table 1, Samples B to F, which are biaxial in crystal optics, have different refractive indices nx, ny, and nz in the principal axis direction.

Evaluation of viewing angle characteristics of liquid crystal cell A rectangular wave voltage of 30 Hz was applied to the TN type liquid crystal cell, and the relationship between the transmittance and the voltage was measured using LCD-5000 manufactured by Otsuka Electronics. The result is shown in FIG. Here, the light transmittance in the state where no voltage is applied is set to 100%. LCD
Using -5000, the light transmittance at a voltage of 0 V and 5 V was measured, and the viewing angle characteristics of 0 V / 5 V contrast were evaluated. The viewing angle in the vertical and horizontal directions in which the contrast of each liquid crystal panel was 10 or more was evaluated. The results are shown in Table 2.

[0026]

[Table 2]

From Table 2, it can be seen that Examples 1 to 4 of the present invention have excellent viewing angle characteristics.

[0028]

According to the present invention, it is possible to provide a liquid crystal display device of high quality display in which the viewing angle characteristics of the TN type liquid crystal display device are improved and the visibility is excellent. Needless to say, even if the present invention is applied to an active matrix liquid crystal display element using a three-terminal or two-terminal element such as TFT or MIM, excellent effects can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram illustrating one example of a configuration of a liquid crystal display element of the present invention.

FIG. 2 is a diagram illustrating a configuration of a conventional TN type liquid crystal display element and a diagram for explaining a light transmission state when light is perpendicularly incident on a display surface.

FIG. 3 is a diagram illustrating a configuration of a conventional TN type liquid crystal display element and a light transmission state when light obliquely enters a display surface.

FIG. 4 is a diagram showing applied voltage characteristics of transmitted light of a liquid crystal display element in an example of the present invention.

[Explanation of symbols]

 1, 2: polarizing plate 1.1, 1.2: polarization axis 3: TN type liquid crystal cell 7: optical compensation sheet

Claims (3)

[Claims] 1. The three refractive indices nx, ny, n in the principal axis direction
A crystal-optically biaxial optically anisotropic element in which z (nz represents a refractive index closest to the thickness direction) is all different,
When the angle formed by the direction of nz and the direction perpendicular to the seat surface is θz, and the angle between the direction closer to the seat surface of the two optical axes and the direction perpendicular to the seat surface is θopt, θz is 0 ° or more and 40 ° or less, and θop
An optical compensation sheet, wherein t is 0 ° or more and 20 ° or less. 2. The maximum value of the refractive index in the in-plane direction is defined as nx ', the refractive index in the thickness direction is defined as nz', and the thickness is defined as (nz '
When the retardation in the thickness direction is defined by −nx ′) × d, the retardation in the thickness direction is −50 to
The optical compensation sheet according to claim 1, which has a thickness of -900 nm. 3. The twist angle between the two electrode substrates is approximately 90.
Liquid crystal cell sandwiching a TN type liquid crystal of 2 °, two polarizing elements disposed on both sides of the liquid crystal cell, and a liquid crystal display element in which at least one optical compensation sheet is disposed between the liquid crystal cell and the polarizing element. 3. The liquid crystal display device according to claim 1, wherein the optical compensation sheet is the optical compensation sheet according to claim 1.