JPH03276124A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To make color compensation by a double refractive layer and to improve visual angle characteristic by providing the double refractive layer having a distribution of the molecular major axis direction in a thickness direction on one surface of a polarizer. CONSTITUTION:The double refractive layer 7 is provided between the substrate 1 and polarizer 2 of an STN cell 1 and the molecular major axis direction thereof is provided with the distribution in the thickness direction; in addition, the molecular major axis direction on both surfaces of the refractive layer 7 is inclined with the plane parallel with the substrate 1. Further, the inclination direction thereof is made symmetrical with the plane parallel with the substrate 1. Such double refractive layer 7 is formed by using a liquid crystal cell which is formed by orienting the liquid crystal molecules in a prescribed direction or a high-polymer film which is thermally and mechanically deformed. The color compensation is executed and the dependency on the visual angle is lessened if such double refractive layer 7 is used. The improvement in the visual angle characteristic is thus made.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a liquid crystal display device, and particularly relates to a liquid crystal display device in which the liquid crystal having positive dielectric anisotropy has a structure twisted by 90 degrees or more in the thickness direction of the liquid crystal layer. The present invention relates to an STN type liquid crystal display element having cells.

[Prior art and problems to be solved by the invention] The display mode of liquid crystals that has been mainly used in the past is called the twisted nematic (TN) type, which has a structure in which liquid crystal molecules are twisted approximately 90 degrees between a pair of substrates. It takes advantage of the rotation of the plane of polarization by liquid crystals and the disappearance of this effect by applying voltage. This display mode was sufficient for low time-division driving of watches, calculators, etc., but when high time-division driving was used to increase display capacity, it had the drawbacks of reduced contrast and narrow viewing angles. there were. This means that in high time division driving, the ratio of the voltage applied to the selected point and the non-selected point is 1.
In order to obtain a display element with high contrast and a wide viewing angle, it is necessary to apply a voltage V that partially changes the relative transmittance of the element.
□. The ratio of voltage Vso changing by 50% to (V, ./
It's V. ) is required to be as small as possible.

In the case of twisted nematic type, this γ value is about 1°13. In order to reduce this γ value, a method has been proposed in which the twist angle of the liquid crystal molecules is increased and the polarization axis is shifted from the liquid crystal alignment direction, and this method is called SBE mode or STN mode. According to such a system, the γ value can be reduced to 1.1 or less, and high time-division driving of about 1/400 duty is possible.

However, this method uses coloring due to birefringence and its change due to voltage, so it is difficult in principle to display black and white, and the transmitted light or reflected light of the liquid crystal cell is colored. It will be displayed on the background.

A method of using a birefringent medium as a color compensating plate to eliminate coloration of STN type liquid crystal display elements is known (Japanese Patent Laid-Open No. 6
4-51.9). However, the STN type liquid crystal display element which is color-compensated using a birefringent layer has a problem in that its brightness and color change depending on the viewing angle direction.

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display element that eliminates the above-mentioned drawbacks of conventional liquid crystal display elements and can easily provide multicolor display.

[Means and effects for solving the problems] In order to achieve the above object, according to the present invention, a liquid crystal layer made of a liquid crystal composition having positive dielectric anisotropy is formed between a pair of substrates provided with electrodes. and a liquid crystal cell configured to be oriented substantially horizontally with respect to the substrate when no voltage is applied and have a twisted structure of 120° or more and 360° or less in the thickness direction of the liquid crystal layer, and the liquid crystal layer is sandwiched between the liquid crystal cell and the liquid crystal cell. At least one of the pair of polarizers is provided with a birefringent layer in which the long axis direction of the molecules is distributed in the thickness direction, in particular, the long axis directions of the molecules on both sides of the birefringent layer are in a plane parallel to the substrate. Provided is a liquid crystal display element characterized in that a birefringent layer is provided which is inclined with respect to the substrate and distributed such that the direction of inclination is symmetrical with respect to a plane parallel to the substrate.

Next, the configuration of the present invention will be explained in detail with reference to one drawing. 1st
The figure is a cross-sectional view showing a configuration example of a liquid crystal display element of the present invention. In this figure, the cell 10 is a 5-N cell, and the substrate 1
．． 11 are alignment films 3 and 13 each subjected to alignment treatment.
and transparent electrodes 4.14, which are arranged spaced apart and facing each other,
A liquid crystal 6 is sealed in between, and is isolated from the outside air by a sealant 5. This liquid crystal cell is connected to the first polarizer 2 and the second polarizer 2.
A birefringent layer 7 is disposed between the substrate 1 and the polarizer 2, sandwiched between the polarizers 12, and constitutes a liquid crystal display element.

This liquid crystal display element can also be used as a reflective type by disposing a reflective plate on the outside of one polarizer. Further, as another example of the structure of the liquid crystal display element of the present invention, the substrate itself can also serve as a birefringent layer. In this case, the layer structure of the device is the same as that of the conventional STN liquid crystal display device, except that at least one of the substrates 1.11 consists of two laminated birefringent layers. As a further embodiment of the invention, the birefringent layer can be incorporated as a component of the polarizing plate itself. In a commonly used polarizer that utilizes the dichroism of iodine or pigments, a stretched film has polarizing ability by adsorbing iodine or pigments, and then the stretched film is sandwiched between two other films to protect it. However, a birefringent layer can be disposed between the protective film on the liquid crystal layer side and the stretched film, and the protective film on the liquid crystal layer side can also be composed of a birefringent layer. can. As mentioned above, the birefringent layer used in the present invention is
It may be placed anywhere between the liquid crystal layer and the polarizer.

When a birefringent layer is provided between a substrate and a polarizer as in the configuration example of the liquid crystal display element of the present invention, a transparent glass, plastic, or the like is used as the substrate. When a plastic substrate is used, it is easy to reduce the thickness of the substrate to 0.2+am or less, and therefore the display element can be configured to be extremely thin and lightweight. Also,
Since the substrate is thin, a display element with a wide viewing angle can be obtained without displaying double images.

The alignment treatment on each substrate of the liquid crystal display element of the present invention is performed so that the liquid crystal molecules are aligned substantially horizontally when no voltage is applied,
Liquid crystal molecules are preferentially aligned along this alignment treatment direction. In this case, the term "substantially horizontal" with respect to the orientation of liquid crystal molecules means that the tilt angle of the liquid crystal molecules with respect to the substrate is approximately in the range of 0° to 30°. This orientation control can be performed by conventionally known oblique vapor deposition or by forming an inorganic or organic film on the substrate and then rubbing it with cotton cloth or the like. As the alignment film 3.13 used in the present invention, a polymer film made of polyamide, polyimide, etc., subjected to rubbing treatment, or an obliquely vapor-deposited film of Sin, MgO5MgF, etc. is used.

As shown in Figure 2, a normal birefringent film has a maximum refractive index direction a in a plane parallel to the substrate, and a direction a (refractive index na
), a direction b (refractive index nb) perpendicular to a in the same plane, and a direction C (refractive index n.) perpendicular to the substrate.

On the other hand, in the birefringent layer 7 of the present invention, as shown in FIG. It is sloping. Furthermore, the direction of inclination is symmetrical with respect to a plane parallel to the substrate. If the distance in the thickness direction with respect to the substrate side surface of the birefringent layer 7 is 2, then the molecular long axis direction of the birefringent layer 7 is 2.
is a function a (z), and the direction parallel to the substrate in the plane perpendicular to the long axis direction of the molecule is b (z)1. Let c(z) be a direction perpendicular to the (Z) direction and the b(z) direction. Also, a
The refractive indices in the (z), b (z), and c (z) directions are constant regardless of 2, and are assumed to be na□, nb7, and nct, respectively. Let α(z) be the angle that the a(z) direction makes with a plane parallel to the substrate. Further, when the thickness of the birefringent layer 7 is d, the inclination directions of the long axes of molecules on both sides of the birefringent layer 7 are symmetrical with respect to a plane parallel to the substrate, so that α(d) knee α(0). As such a birefringent layer, both sides of a liquid crystal cell and a polymer film, in which the liquid crystal molecules are aligned as shown in FIG. 3, are thermally treated.

Examples include those that are mechanically deformed.

FIG. 4 shows the definition of angles related to the present invention. Do and D2 are directions in which the liquid crystal molecules on the first substrate 1 and the second substrate 11 are projected onto the lower substrate 11.

The arrow indicates the direction slightly upward relative to the substrate 11 when projecting the liquid crystal molecules. The liquid crystal layer is Dl
The liquid crystal molecules have a twisted structure by ω between and D2. Note that as the liquid crystal for the cell 10, a nematic liquid crystal having positive dielectric anisotropy to which a chiral substance is added and adjusted to an appropriate pitch is preferably used. In this case, if ω1 is small, the steepness will deteriorate and the time division characteristics will deteriorate. On the other hand, if ω1 is too large, a scattering structure is generated when a voltage is applied, which deteriorates display quality, which is not preferable. From this, ω1 must be greater than or equal to 120'' and less than 360°. In Figure 2, when the cell is viewed from the substrate 1 side, the twist direction is clockwise from substrate 11 to substrate 1. However, the direction of the orientation process,
The rotation can also be made counterclockwise by selecting cholesteric liquid crystal or chiral nematic liquid crystal. The X direction is the direction in which a (z) is projected onto a plane parallel to the substrate, and is δ with respect to the liquid crystal molecule alignment direction D1 on the adjacent substrate 1.
It has an angle of Further, the transmission axis P of the polarizer 2 adjacent to the birefringent layer 7 is arranged at an angle of β1 with respect to the X direction. Further, the transmission axis P2 of the polarizer 12 forms an angle of β2 with the liquid crystal molecule alignment direction D2 on the substrate 11, with the angle being positive in the direction of twist of the liquid crystal.

The color produced when a birefringent layer is sandwiched between polarizers is determined by the thickness of the birefringent layer and the direction of the polarizing plate.

Also, the color changes depending on the viewing angle direction.

The thickness of the birefringent layer depends on the viewing angle dependence of the tarpsion. If the viewing angle dependence of the birefringent layer tarpaulin is small, the viewing angle dependence of color will also be small. therefore,
The smaller the viewing angle dependence of the retardation of the birefringent layer used for color compensation of the STN liquid crystal display element, the smaller the viewing angle dependence of the color of the liquid crystal display element.

FIG. 5 shows an example of the viewing angle dependence of a normal birefringent film and a birefringent layer tarpsion constituting the present invention. Retardation is normalized by tarpaulsion when viewed from the vertical direction of the birefringent layer. Further, the viewing angle direction θ indicates an angle from a direction perpendicular to the birefringent layer within a plane perpendicular to the birefringent layer and including the maximum refractive index direction. The refractive index of a normal birefringent film is nB=1.6. nb=:
1.5, nc"1.45, the refractive index and inclination angle of the birefringent layer of the present invention are nat"=1.6, nb,=1.5,
nc7=1.45, α(0)=30″.

As shown in FIG. 5, the birefringent layer constituting the present invention has smaller viewing angle dependence of retardation than a normal birefringent film. Therefore, the liquid crystal display element of the present invention has less viewing angle dependence of color than a liquid crystal display element constructed using a normal birefringent film, and has improved viewing angle characteristics.

〔Example〕

Examples of the present invention will be described below, but the present invention is not limited thereto.

(Example 1) It has a transparent electrode, the twist angle ω of the liquid crystal between the upper and lower glass substrates is 200°, and the liquid crystal layer turbulence (the product of the birefringence of the liquid crystal molecules and the thickness of the liquid crystal layer) is 0.921Ja
An STN cell was fabricated. The liquid crystal is a nematic liquid crystal ZLI2293 (manufactured by Merck & Co., Ltd.) with positive dielectric anisotropy and a chiral nematic liquid crystal 5811 (manufactured by Merck & Co., Ltd.).
was used. The alignment treatment was performed by rubbing the polyimide film.

Next, the upper and lower substrates are aligned by diagonal vapor deposition of SiO, so that the long axis direction of the liquid crystal molecules on both substrates is about 3
0@, and a liquid crystal cell (cell 1) was prepared so that the long axis of the liquid crystal molecules was oriented in the thickness direction as shown in Figure 3.
This was the birefringent layer of the present invention.

Then, the cell 1 was installed between the upper substrate and the upper polarizing plate so that the X direction was perpendicular to the alignment direction of liquid crystal molecules on the upper substrate of the STN cell (δ: 90°). The thickness of the liquid crystal layer in the direction perpendicular to the substrate of cell 1 was 0.834 mm, and the liquid crystal used was ZLI2293.

On the other hand, at the bottom of the STN cell, a neutral gray polarizing plate was arranged so that its transmission axis formed an angle of 45° (β2: 45°) with respect to the alignment direction of liquid crystal molecules on the lower substrate. Furthermore, a backlight using cold cathode tubes was installed below this polarizing plate. Moreover, a neutral gray polarizing plate was placed above the cell 1 so that its transmission axis formed an angle of -45° with respect to the X direction of the cell 1 (β1=−45°).

It was confirmed that the liquid crystal display element configured as described above is black when no voltage is applied, becomes white when voltage is applied, has small color change depending on the viewing angle direction, and has excellent viewing angle characteristics.

(Example 2) It has a transparent electrode, the twist angle ω1 of the liquid crystal between the upper and lower glass substrates is 240°, and the liquid crystal layer thickness is 1.
．． An STN cell with 0 voice was created. The liquid crystal.

A mixture of nematic liquid crystal ZLI2293 and chiral nematic liquid crystal 5811 was used. Orientation treatment.

This was done by rubbing a polyimide film.

Next, one side of the cast polymer film is stretched with a hot roll, and then the other side is stretched in the same direction with a hot roll, so that the long axis direction of the molecules on both sides of the polymer film is diagonal to the film surface.

The birefringent layer of the present invention was prepared by oriented symmetrically with respect to the film plane. Inclination angle α(0) of the birefringent layer
The stretching conditions and film thickness were adjusted so that the angle of the birefringent layer in the direction perpendicular to the polymer film was approximately 25° and the thickness of the birefringent layer was 0°94°. This polymer film was placed on top of the cell so that δ=90°.

On the other hand, a neutral gray polarizing plate with a reflector was placed at the bottom of the STN cell so that β2:45°. Moreover, a neutral gray polarizing plate was placed on top of the polymer film so that β1=knee 45°.

The liquid crystal display element configured as described above is also white when no voltage is applied and becomes black when voltage is applied, and as in Example 1, the color change depending on the viewing angle direction is small and has excellent viewing angle characteristics. was confirmed.

〔Effect of the invention〕

According to the present invention, with the above structure, it is possible to improve the viewing angle characteristics of an STN type liquid crystal display element that performs color compensation using a birefringent layer, and to obtain a monochrome display liquid crystal display element with excellent display quality. I can do it.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a configuration example of a liquid crystal display element of the present invention. FIG. 2 is a diagram showing the direction of the refractive index of a normal birefringent film. FIG. 3 is an explanatory diagram of a birefringent layer constituting the liquid crystal display element of the present invention. FIG. 4 is an explanatory diagram of the angular relationship between the orientation direction of liquid crystal molecules, the direction of polarizing plates, etc. of the liquid crystal display element of the present invention. FIG. 5 is a diagram showing an example of the viewing angle dependence of a birefringent film and a birefringent layer tarpsion constituting the present invention.

Claims (2)

[Claims] (1) A liquid crystal layer made of a liquid crystal composition having positive dielectric anisotropy is oriented approximately horizontally to the substrates between a pair of substrates provided with electrodes when no voltage is applied, and in the thickness direction of the liquid crystal layer. 120
At least one of the liquid crystal cells configured to have a twisted structure of not less than 360 degrees and a pair of polarizers sandwiching the liquid crystal layer has a structure in which the long axis direction of the molecules is in the thickness direction. A liquid crystal display element characterized by having a birefringent layer with a distribution. (2) Birefringence is distributed such that the long axis directions of the molecules on both sides of the birefringent layer are inclined with respect to the plane parallel to the substrate, and the direction of the inclination is symmetrical with respect to the plane parallel to the substrate. 2. The liquid crystal display element according to claim 1, further comprising a layer.