JPH0713150A - Liquid crystal display element using optical anisotropic element - Google Patents

Abstract

PURPOSE:To provide a liquid crystal display element with improved view angle characteristics for the display contrast, gradation, display and display colors without decreasing the front-view contrast. CONSTITUTION:This liquid crystal display element consists of a liquid crystal cell comprising a liquid crystal interposed between two electrode substrates, two polarizers on both sides of the cell, and one optical anisotropic element between the two polarizers. This optical anisotropic element consists of a laminated body of thin films showing optically negative uniaxial property or biaxial but near uniaxial property and has a twisted structure in which directions of optic axes on both sides of the element are different from each other. The optic axis on the surface of the optical anisotropic element near the liquid crystal cell and the optic axis of the liquid crystal on the liquid crystal cell substrate near the optical anisotropic element when the cell shows black display make <=3 deg. angle. The optic axis on the surface of the optical anisotropic element far from the liquid crystal cell and the optic axis of the liquid crystal on the liquid crystal cell and substrate far from the optical anisotropic element when the cell, shows black make <=3 deg. angle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device having improved display contrast, gradation characteristics and display color viewing angle characteristics.

[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, a rotation mode display system (TN type liquid crystal display element) in which the alignment state of liquid crystal molecules is twisted by about 90 ° is used. This display method is the most effective method as compared with other LCDs because it has a high response speed (several + milliseconds), can easily obtain a black and white 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 are changed depending on the viewing direction due to the principle of the display method.
The display performance of T is exceeded.

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.

The retardation film proposed in the above patent publication has a retardation of almost zero in the direction perpendicular to the surface of the liquid crystal cell, has no optical action from the front, and is tilted. When the liquid crystal cell has a retardation, the retardation is manifested in the liquid crystal cell. However, even with these methods, the viewing angle of 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.

Further, Japanese Patent Laid-Open Nos. 4-366808 and 4-366809 propose a method for improving the viewing angle by using a liquid crystal cell containing a chiral nematic liquid crystal having an inclined optical axis as a retardation film. However, there is a problem that it is a two-layer liquid crystal system and the cost is high and it is difficult to reduce the weight. Further, JP-A-5-80323 proposes a method of using a retardation film whose optical axis is tilted with respect to a liquid crystal cell. However, since a uniaxial polycarbonate is sliced and used, There is a problem that it is difficult to obtain an area retardation film at low cost.

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. Twisted nematic liquid crystal has a structure in which the alignment of liquid crystal molecules is twisted in the thickness direction of the liquid crystal cell, but the light transmitted through the liquid crystal cell depends on the orientation of each liquid crystal molecule in the twisted alignment. Sequentially polarized and propagated. 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.

[0009]

SUMMARY OF THE INVENTION The present invention provides a liquid crystal display device having improved display contrast, gradation characteristics and viewing angle characteristics of display color without lowering the front contrast. is there.

[0010]

The above-mentioned objects have been achieved by the following means. (1) A liquid crystal cell in which a liquid crystal is sandwiched between two electrode substrates, two polarizing elements arranged on both sides of the liquid crystal cell, and one optical anisotropic element between the two polarizing elements. In the arranged liquid crystal display element, the optically anisotropic element is composed of a laminated body of thin films exhibiting optically negative uniaxiality or biaxiality close to uniaxiality. An optical axis having a different twisted structure and formed by the optical axis of the surface of the optical anisotropic element close to the liquid crystal cell and the optical axis of the liquid crystal cell surface of the liquid crystal cell substrate close to the optical anisotropic element at the time of black display are 30 °. The angle formed by the optical axis of the surface of the optically anisotropic element far from the liquid crystal cell and the optical axis of the liquid crystal cell surface of the liquid crystal cell substrate far from the optically anisotropic element during black display is 30 ° or less. Liquid crystal display device characterized by. (2) The liquid crystal display element according to (1), wherein the optically anisotropic element has a retardation in light of 632.8 nm of -50 nm to -1000 nm. (3) TN in which the liquid crystal cell has a twist angle of about 90 °
A liquid crystal display element according to (1), characterized in that a liquid crystal is sandwiched between the liquid crystal display elements. (4) The liquid crystal display device according to (3), wherein the optical anisotropic element has a retardation in the light of 632.8 nm of −50 nm to −600 nm.

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 and 2 show the polarization state of light propagating in the liquid crystal cell when a voltage higher than the threshold voltage is applied to the liquid crystal cell, and show a bright state when no voltage is applied. . FIG. 1 is a diagram showing a polarization state of light when light is vertically incident on a liquid crystal cell. When the natural light L0 is vertically incident on the polarizing plate A having the polarization axis PA, the light transmitted through the polarizing plate A is linearly polarized light L.
It becomes 1.

In the figure, LC indicates the state of alignment of the liquid crystal molecules when a sufficient voltage is applied to the TN type liquid crystal cell, by using one liquid crystal molecule model. When the molecular long axis of the liquid crystal molecule LC in the liquid crystal cell is parallel to the light path, there is no difference in the refractive index on the incident surface (in the plane perpendicular to the light path), and therefore the ordinary light propagating in the liquid crystal cell When the linearly polarized light L1 passes through the liquid crystal cell, the linearly polarized light L1 propagates as it is. The polarizing axis PB of the polarizing plate B is the polarizing axis P of the polarizing plate A.
When set to be perpendicular to A, the light L2 that has passed through the liquid crystal cell cannot pass through the polarizing plate and is in a dark state.

FIG. 2 is a diagram showing a polarization state of light when the light obliquely enters the liquid crystal cell. Natural light of incident light L
When 0 is obliquely incident, the polarized light L transmitted through the polarizing plate A
1 becomes almost linearly polarized light. (In the actual case, it will be elliptically polarized due to the characteristics of the polarizing plate). In this case, a difference in refractive index occurs on the incident surface of the liquid crystal cell due to the anisotropy of the refractive index of the liquid crystal,
The light L2 transmitted through the liquid crystal cell is elliptically polarized and transmitted through the polarizing plate B. The transmission of light in such oblique incidence is
This is not preferable because it causes a decrease in contrast.

The present invention is intended to prevent such a decrease in contrast due to oblique incidence, improve viewing angle characteristics, and at the same time improve front contrast.

It is presumed as follows that the viewing angle characteristic of the liquid crystal display device was significantly improved by the present invention. Most 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 type liquid crystal cell can be regarded as a positive uniaxial optical anisotropic body in which the optical axis is slightly inclined from the direction normal to the cell surface. This slight tilt of the optical axis not only causes birefringence even in front of the cell, but also causes significant asymmetry of the viewing angle in the vertical direction of the cell, that is, the main viewing angle direction, which significantly impairs the viewing angle in either one or both directions. become. This is in agreement with the actual phenomenon.

When the optical axis of the liquid crystal cell is tilted from the direction normal to the surface of the liquid crystal cell, it is expected that the optical anisotropic body having the optical axis in the normal direction will be insufficiently compensated. If the liquid crystal cell can be regarded as a positive optical anisotropic body, it is suitable to use a negative uniaxial optical anisotropic body as shown in FIG. 3 in order to compensate for it. Also,
Since the liquid crystal in the liquid crystal cell is twisted and aligned, it is preferable that the optical anisotropic element used in the present invention also has a twisted structure. For these reasons, it is presumed that the viewing angle characteristic is significantly improved by the optical anisotropic body made of a negative uniaxial laminate in which the optical axis of the present invention is inclined from the normal direction.

The optically anisotropic element of the present invention is a laminate of optically anisotropic uniaxial or biaxial optically anisotropic bodies close to uniaxial. The term “optically negative uniaxial” means that there is only one optical axis in which birefringence does not occur and the refractive index in the optical axis direction is smaller than the refractive index in the direction orthogonal to the optical axis. . Biaxiality means that there are two optical axes. Biaxiality near uniaxiality in the present invention means biaxiality in which an angle formed by two optical axes is 40 ° or less.

FIG. 4 schematically shows the refractive index of one layer of the thin film of the optically anisotropic element used in the present invention. As shown in FIG. 4, the main refractive indices of one layer of the thin film of the optically anisotropic element with respect to light having a wavelength of 632.8 nm are na, nb, and nc. In the case of negative uniaxiality, na <nb = nc. na
Is the optical axis.

The optically anisotropic element used in the present invention comprises a laminate of thin films having different optical axis directions. The laminated body here is a laminated body of two or more thin films having the same optical axis direction. In the present invention, the laminated body also includes a cholesteric liquid crystal having a twisted structure and the like, which can be regarded as a laminated body of an infinite number of thin films whose optical axis directions are continuously changed.

The optical anisotropic element used in the present invention has a twisted structure in which the directions of the optical axes on both surfaces are different. In the present invention, when the optical anisotropic element is mounted on a liquid crystal display element, the direction of the optical axis of the thin layer closest to the liquid crystal cell of the optical anisotropic element and the liquid crystal cell closer to the optical anisotropic element. The angle formed by the alignment direction of the liquid crystal when displaying black on the substrate surface is 30 ° or less. Further, in the present invention, the direction of the optical axis of the thin layer farthest from the liquid crystal cell of the optically anisotropic element and the orientation direction of the liquid crystal at the time of black display on the liquid crystal cell substrate surface farther from the optically anisotropic element. Is less than 30 °. Since the liquid crystal in the liquid crystal cell has a twisted structure, the optically anisotropic element used in the present invention preferably has a continuous twisted structure corresponding to the twisted structure of the liquid crystal. Therefore, the twist angle of the optically anisotropic element used in the present invention is preferably substantially equal to the twist angle of the liquid crystal in the liquid crystal cell. For example, the STN type liquid crystal cell having a twist angle of 240 ° is preferably about 240 °, and the TN type liquid crystal cell having a twist angle of 90 ° is preferably about 90 °. By controlling the orientation direction of the optically anisotropic element in this way, the display contrast, gradation characteristics, and display color viewing angle characteristics of the liquid crystal display element due to the birefringent viewing angle characteristics of the liquid crystal cell are almost completely compensated. It

The retardation in the present invention means (na−nb) × d, where d is the thickness of the optically anisotropic element.
Is. 632.8 nm of the optical anisotropic element in the present invention
Retardation in light of -50 nm to -100
It is preferably 0 nm. When a TN type liquid crystal cell is used, it is preferably -50 nm to -600 nm.

The optical anisotropic element used in the present invention is provided as a polymer film, a plate-like material, or a coating film on another support. The light transmittance of the optically anisotropic element is 80.
% Or more is preferable, and 90% or more is more preferable.

The polymer material used for the optical anisotropic element used in the present invention is not particularly limited, but polycarbonate, polyarylate, polysulfone, polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone. , Polyvinyl alcohol, polyamide, polyimide, polyolefin, polyvinyl chloride, cellulosic polymer, polyacrylonitrile, polystyrene, polymethylmethacrylate, product name Zeonex 280, which is a polyolefin material manufactured by Nippon Zeon, and also a binary system, Ternary polymers, 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 photoisomerizable substance may be used as the optically anisotropic substance used in the present invention. The photoisomerizable substance is one that causes stereoisomerization or structural isomerization by light, and is preferably one that causes reverse isomerization by light or heat of another wavelength. Examples of these compounds include azobenzene compounds, benzaldoxime compounds, azomethine compounds, stilbene compounds, spiropyran compounds, spirooxazine compounds, fulgide compounds, diaryl ethene compounds, cinnamic acid compounds, Examples thereof include retinal compounds and hemithioindigo compounds. These photoisomerizable substances may be monomers or polymers, and in the case of a polymer, the photoisomerizable group may be in the main chain or the side chain.

As the optically anisotropic substance used in the present invention, a liquid crystal compound or a compound capable of forming a liquid crystal may be used.

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

Example A photoisomerizable substance having a styrene-equivalent weight average molecular weight of about 20,000 (the following compound, m = 50%, n = 50%) was dissolved in methylene chloride to give a 2% by weight solution. The solution is coated on a glass plate with a doctor blade to a thickness of 2
A film of μm was obtained. The film was irradiated with linearly polarized light while heating to orient the photoisomerizable substance. Eight layers of this film were laminated with the angle shifted by 10 ° to obtain an optical anisotropic element of the present invention.

[0027]

[Chemical 1]

The main refractive index of the obtained optical anisotropic element with light of 632.8 nm is na = 1.630, nb = nc =
It was 1.610. Therefore, the retardation value at 632.8 nm is 320 nm. Also, n
b was parallel to the film surface, the angle between na and the film normal direction was 25 °, and the twist angle of the optical axis was 80 °.

A birefringence of 0.110 is provided between a pair of polarizing elements.
3. The nematic liquid crystal has a twist angle of 90 ° and a thickness of 4.
A viewing angle of contrast between a liquid crystal display element (samples B and C) in which the above-mentioned optical anisotropic element is mounted and a liquid crystal display element (sample A) in which the above-mentioned optical anisotropic element is not mounted between the liquid crystal cell sandwiched with a gap size of 5 μm and the polarizing element are provided. The characteristics were evaluated.
When the optical anisotropic element was mounted, the mounting angle of the optical anisotropic element was changed as shown in Table 1. Otsuka Electronics LCD-
At 5,000, the viewing angle in all directions based on the 2.0 V / 3.7 V (white / black) contrast 10 standard was measured. The results are shown in FIGS. 5 to 7 as isocontrast curves.

[0030]

[Table 1]

It can be seen from FIGS. 5 to 7 that the sample B of the present invention has excellent viewing angle characteristics.

[0032]

EFFECTS OF THE INVENTION 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 liquid crystal display device are improved and which is excellent in visibility. In addition, the present invention is a TFT
It goes without saying that excellent effects can be obtained even when applied to an active matrix liquid crystal display element using a 3-terminal or 2-terminal element such as a MIM.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a configuration of a conventional TN type liquid crystal display element and a diagram illustrating a light transmission state when light is perpendicularly incident on a display surface.

FIG. 2 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. 3 is a schematic diagram showing the principle of improving the viewing angle characteristics according to the present invention.

FIG. 4 is a diagram illustrating a main refractive index of one layer of a thin film of an optically anisotropic element used in the present invention.

FIG. 5 is a diagram illustrating a viewing angle in all directions with reference to contrast 10 of sample A.

FIG. 6 is a diagram illustrating a viewing angle in all directions of Sample B with a contrast of 10 as a reference.

FIG. 7 is a diagram illustrating a viewing angle in all directions of sample C with a contrast of 10 as a reference.

[Explanation of symbols]

A, B ... Polarizing plate, PA, PB ... Polarization axis, L0 ... Incident light,
L1 ... Linearly polarized light passing through the polarizing plate A, L2 ... Polarized light passing through a TN type liquid crystal cell (mainly elliptically polarized light), LC ... Explaining liquid crystal in the TN type liquid crystal cell

Claims (4)

[Claims] 1. A liquid crystal cell in which a liquid crystal is sandwiched between two electrode substrates, and two polarizing elements arranged on both sides of the liquid crystal cell.
In a liquid crystal display element in which one optical anisotropic element is arranged between the two polarizing elements, a laminate of thin films in which the optical anisotropic element exhibits optically negative uniaxiality or biaxiality close to uniaxiality The optical anisotropic element has a twisted structure in which the directions of the optical axes on both sides are different, and the optical axis of the surface of the optical anisotropic element close to the liquid crystal cell and the liquid crystal cell substrate close to the optical anisotropic element The angle formed by the optical axis of the liquid crystal when the surface is displayed in black is 30 ° or less,
The angle formed by the optical axis of the surface of the optically anisotropic element far from the liquid crystal cell and the optical axis of the liquid crystal cell surface of the liquid crystal cell substrate far from the optically anisotropic element during black display is 30 ° or less. Liquid crystal display device. 2. The retardation of the optically anisotropic element at a light of 632.8 nm is -50 nm to -1000 nm.
The liquid crystal display element according to claim 1, wherein 3. A liquid crystal display element according to claim 1, wherein the liquid crystal cell sandwiches a TN type liquid crystal having a twist angle of about 90 °. 4. The liquid crystal display element according to claim 3, wherein the optical anisotropic element has a retardation at a light of 632.8 nm of −50 nm to −600 nm.