JPH055863A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To obtain the liquid crystal display device of high-quality display which has the visual angle characteristic improved and is superior in visual recognizability. CONSTITUTION:A display device consists of the liquid crystal display element and a driving means 3f which applies a voltage to a driving liquid crystal cell 3, and this liquid crystal display element consists of the driving liquid crystal cell 3, a compensating liquid crystal layer (cell 2) which is arranged adjacently to the driving liquid crystal cell and is twisted along a spiral axis approximately parallel with the substrate normal of the driving liquid crystal cell, and two polarizing plates between which the driving liquid crystal cell 3 and the compensating liquid crystal layer are interposed. In this liquid crystal display element, the compensating liquid crystal layer has >=360 deg. twisting angle, and the thickness of the liquid crystal layer of the cell 3 is approximately equal to that of the compensating liquid crystal layer. The refractive index to extraordinary rays of the cell 3 at the time of applying a voltage, which brings about a display picture to obtain the dark state, to the cell 3 by the driving means is approximately equal to that to ordinary rays of the compensating crystal layer, and the refractive index to ordinary rays the cell 3 is approximately equal to that to extraordinary rays to the compensating liquid crystal layer.

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, and more particularly to a liquid crystal display device in which the viewing angle dependence of a contrast ratio is controlled.

[0002]

2. Description of the Related Art Liquid crystal display devices, which are thin, lightweight, and have low power consumption, are actively used as display devices for personal OA equipment such as Japanese word processors and desktop personal computers. Most of liquid crystal display elements (hereinafter abbreviated as LCD) use twisted nematic liquid crystal, and the display system can be roughly classified into two systems of a birefringence mode and an optical rotation mode.

The LCD of the birefringence mode display system generally has a molecular arrangement twisted by 90 ° or more (called the ST system) and has steep electro-optical characteristics, so that a switching element (thin film transistor or diode) is provided for each pixel. Even without a simple structure, a large-capacity display can be easily obtained by time-division driving even with a simple matrix electrode structure. However, in this ST method, since the birefringence effect is used, the display colors are so-called yellow mode display of yellow and dark blue or so-called blue mode display of white and blue due to light interference,
Black and white display and color display were impossible.

As a means for eliminating such coloring of the display, it is possible to realize black and white display by disposing a second liquid crystal cell which is twisted in the opposite direction between a polarizing plate and a liquid crystal cell, as disclosed in Japanese Patent Publication No. 63-53528. Is disclosed in.

The principle of black and white is that the light of the ordinary light component and the extraordinary light component, which are elliptically polarized in the liquid crystal cell for display in which the liquid crystal molecules are arranged in a twisted pattern, are mutually exchanged by the second liquid crystal cell which is the optical compensator. To convert the elliptically polarized light into linearly polarized light. As a result, coloring due to light interference is eliminated, and black and white display can be realized. Here, in order to convert the elliptically polarized light into the linearly polarized light as described above, the optical compensator has a display liquid crystal cell and a retardation value that are substantially the same, and twist directions are opposite to each other. The arrangement is such that the orientations of the liquid crystal molecules closest to each other are orthogonal to each other. Moreover, in such an LCD, the display color is colored at an oblique angle deviated from the normal to the display surface, and black and white display cannot be obtained.

As an LCD of a birefringence mode display system other than the ST system, a system using a liquid crystal cell in which liquid crystals having negative dielectric anisotropy are arranged substantially perpendicular to a substrate (ECB
Method). This method is intended to perform display by changing the retardation value of the liquid crystal cell by tilting the liquid crystal molecules with respect to the substrate normal line by applying a voltage to the liquid crystal cell. The retardation value greatly changes depending on the angle at which the liquid crystal cell is viewed, and as a result, a phenomenon such that the displayed image becomes invisible or appears inverted may occur.

On the other hand, the LCD in the optical rotation mode has a molecular arrangement twisted by 90 ° (TN system), has a fast response speed of several tens of milliseconds, and shows a high contrast ratio and a good gradation display property. It is applied to a clock, a calculator, and a liquid crystal television (TFT-LCD or MIM-LCD) for color display in which a switching element is provided for each pixel and combined with a color filter.

Regarding color display, a CN liquid crystal cell exhibiting a certain hue utilizing selective scattering when no voltage is applied is arranged between a polarizing plate and a liquid crystal cell, and a combination of the presence or absence of voltage application to the CN liquid crystal cell and the liquid crystal cell is used. It is possible to switch to a two-color display mode of a specific hue and its complementary color, or a monochrome display mode Mol.Cryst.Liq.Cryst., 1977, VOL.39, PP.127-13
Reported in 8.

However, a liquid crystal display device utilizing these birefringence mode, optical rotation mode and selective scattering has a viewing angle dependency that a display color and a contrast ratio change depending on a viewing angle and an azimuth, and a cold cathode ray tube (CRT). The display performance of is completely exceeded.

[0010]

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 light of a certain polarization state enters a liquid crystal molecule exhibiting such anisotropy of refractive index, the light changes its polarization state depending on the angle of the liquid crystal molecule. Therefore, the polarization state of the light propagating in the liquid crystal cell is different depending on whether the light enters the liquid crystal cell perpendicularly or obliquely, and as a result, the display state depends on the azimuth and angle when the liquid crystal display element is viewed. The pattern appears in reverse, the display pattern disappears at all, or the display becomes colored, which is not preferable in practice.

The present invention solves the above inconvenience.

[0012]

According to the present invention, a driving liquid crystal cell and an array twisted by a spiral axis which is arranged adjacent to the driving liquid crystal cell and is substantially parallel to a substrate normal line of the driving liquid crystal cell are provided. A liquid crystal display device comprising a compensating liquid crystal layer and two polarizing plates sandwiching the driving liquid crystal cell and the compensating liquid crystal layer;
In a liquid crystal display device comprising a driving means for applying a voltage to the driving liquid crystal cell, the liquid crystal display element has a compensating liquid crystal layer having a twist angle of 360 ° or more, Refraction to extraordinary light of the driving liquid crystal cell when a voltage for generating a display image for obtaining a dark state is applied to the driving liquid crystal cell by the driving means and the thickness of the compensating liquid crystal layer is substantially equal. And the refractive index of the compensating liquid crystal layer for ordinary light are substantially equal to each other, and the refractive index of the driving liquid crystal cell for ordinary light and the refractive index of the compensating liquid crystal layer for extraordinary light are substantially equal to each other. Is what you get.

As the driving liquid crystal cell, it is possible to use a driving liquid crystal cell which is arranged substantially perpendicular to the substrates when no voltage is applied between the two substrates, or various cells arranged in a twisted arrangement.

[0014]

In general, there are roughly two types of arrangements of polarizing plates used in liquid crystal display devices. Light is not transmitted when a voltage is not applied to the liquid crystal cell, and a light transmission state is obtained when a voltage is applied ( There are a normally closed system and a system in which light is transmitted when a voltage is not applied to the liquid crystal cell and light is blocked when a voltage is applied (normally open). As an example, FIG. 3 shows the contrast ratio dependence of the conventional TN type normally open and normally closed when the display surface is tilted from 0 ° to 60 ° in the left and right directions. It is indicated by (10.0) in the case of normally open and (11.0) in the case of normally closed. Comparing these, it can be seen that normally closed has less viewing angle dependence of contrast ratio than normally open. The contrast ratio is a value obtained by dividing the brightness in a state where light is transmitted (bright state) by the brightness in a state where light is blocked (dark state), and the contrast ratio greatly affects the brightness in a dark state. Then, when the viewing angle dependence of the brightness in the left-right direction in the dark state of both the normally open type and the normally closed type is measured, the characteristics as shown in FIG. 4 are obtained. The case of normally open is shown in (10.1), and the case of normally closed is shown in (11.1). As is clear from the figure, normally closed has less viewing angle dependency in the dark state than normally open, and as a result, normally closed has better viewing angle characteristics of contrast ratio than normally open.

Considering the difference between the normally open and normally closed dark states, a voltage is applied to the liquid crystal cell in order to block light in the case of normally open, and in the case of normally closed. A voltage is applied to the liquid crystal cell to transmit light. FIG. 5 shows the molecular alignment state when a voltage value capable of obtaining a dark state is applied to the liquid crystal cell in the normally open state. Here, 7 and 8 in the figure are tilts, respectively.
The tilt angle 7 is an angle and a twist (twist) angle.
In the coordinate system shown in, when the substrate surface of the liquid crystal cell is the xy plane, the tilt angle of the major axis (5.1L) of the liquid crystal molecule (5.1) with respect to the xy plane is shown. The twist angle 8 is the liquid crystal molecule (5.1). It is the angle between the axis projected from the z axis to the xy plane and the x axis.

When a voltage is applied, the liquid crystal molecules tilt near 90 ° near the center of the liquid crystal cell, but near the upper and lower substrate surfaces, the liquid crystal molecules do not tilt much due to the influence of the alignment regulating force of the substrate surfaces. .. Further, the twist angle has an S-shaped distribution.

When the tilt angle of the liquid crystal molecules is constant with respect to the cell thickness direction and the twist angle is linearly twisted, that is, when compared with the dark state molecular arrangement state in the normally closed state, the molecular arrangement state when voltage is applied. Are viewed differently depending on the viewing angle and orientation of the liquid crystal cell, and as a result, the difference in the appearance of the liquid crystal molecule alignment state is reflected in the viewing angle characteristics as the difference in the polarization state propagating in the liquid crystal cell. Therefore, the reason why the normally closed characteristic is better is that the difference in the appearance of the molecular arrangement state in the dark state depending on the viewing direction angle is smaller than that in the normally open state. Therefore, if the device can be devised so that the same molecular arrangement state can be seen from any direction by some method, the viewing angle characteristics in the dark state in the normally open state can be improved.

The largest change in the appearance of the liquid crystal molecules depending on the viewing angle in the liquid crystal cell is where the liquid crystal molecules near the center of the liquid crystal cell are greatly inclined. Therefore, the viewing angle characteristics can be improved by reducing the change in the appearance of this portion.

The alignment state of the liquid crystal can be simply shown by a three-dimensional index ellipsoid. FIG. 7 shows the state in which the liquid crystal molecules stand vertically with the index ellipsoid 6.
The birefringence phenomenon is a phenomenon related to the difference in refractive index within the two-dimensional plane when the refractive index ellipsoid 6 is viewed from a certain direction, and therefore when viewed from the z direction (that is, when the liquid crystal cell is viewed from the front directly). ) The refractive index body (6.4) in the two-dimensional plane becomes a circle, and the refractive index body (6.5) when viewed from the refractive index difference and the visual axis (6.1)
Is different from. In the case of normally open (crossed nicols), the dark state is obtained because the refractive index difference when viewed from the z direction is 0, but the two-dimensional surface when viewed from the visual axis (6.1) is an ellipse, and As a result, there is a difference in refractive index, so that the dark state is not obtained. An ellipse in a two-dimensional plane that can be seen from the visual axis (6.1) when the angle (6.3) at which the refractive index ellipsoid 6 is viewed is increased.
(6.5) becomes larger in the length direction of n61, and larger transmitted light is observed when viewed from the direction of the visual axis (6.1).

Therefore, in order to optically compensate such a refractive index ellipsoid, the refractive index in the lengthwise direction of n62 increases as the angle (6.3) for viewing the refractive index ellipse increases. If an index ellipsoid (that is, an optically anisotropic body with negative optical property) is placed on the visual axis (6.1), an ellipse in the two-dimensional plane will be obtained.
(6.5) becomes a circle, and even if observed from various directions, the apparent refractive index becomes almost the same and the viewing angle characteristics are improved.

A cholesteric liquid crystal cell is an example of one which exhibits optically negative optical anisotropy. A cholesteric liquid crystal cell has a helical twisted arrangement of liquid crystal molecules.General liquid crystals have a positive optical anisotropy, whereas a cholesteric liquid crystal cell has a helical twisted arrangement that results in an optically negative optical. Shows anisotropy. Since the viewing angle characteristic of the contrast ratio largely depends on the change of the viewing angle of the luminance in the dark state, in order to obtain a better optical compensation effect using such a cholesteric liquid crystal cell, the thickness of the liquid crystal layer of the driving liquid crystal cell is required. And the thickness of the compensating liquid crystal layer is substantially equal to each other, and the length n1 of the major axis and the length of the minor axis of the refractive index ellipsoid when the voltage causing the dark state is applied to the driving liquid crystal cell as described above. The relationship between the length n2 and the major axis length n1c, the minor axis length n2c, and the minor axis length of the index ellipsoid 9 of the liquid crystal of the cholesteric liquid crystal cell shown in FIG. 8 is n1 = n1c n2 = n2c. When, the best compensation effect is obtained.

Although the TN liquid crystal cell has been described above as an example, not only the TN type but also the ST type and the LCD of the display type having a small twist angle of 90 ° or less, and further the ECB type liquid crystal display element in which the vertical arrangement is performed. Also has the same effect.

Needless to say, the above-mentioned cholesteric liquid crystal cell can obtain the same function by using a polymer liquid crystal layer having twistability. In this case, for example, at least one of the substrates of the driving liquid crystal cell is used. Further, it is obtained by applying such a polymer liquid crystal layer, and it is easy to manufacture and a more desirable liquid crystal display device can be obtained. In this case, it is possible to use, for example, a high molecular copolymer liquid crystal having a polysiloxane main chain and having biphenylbenzoate and cholesteryl groups in the side chains at an appropriate ratio.

[0024]

EXAMPLES Examples of the liquid crystal display device of the present invention will be described in detail below.

(Embodiment 1) FIGS. 1 and 2 show a cell structure in this embodiment. The liquid crystal display element consists of two polarizing plates 1,
4 and the compensating liquid crystal cell 2 and the driving liquid crystal cell 3 are sandwiched between them. The polarizing plate 1 is a transparent substrate 1a
The polarizing film 1b is attached to the inside of the polarizing plate 4, and the polarizing plate 4 is also formed by attaching the polarizing film 4b to the transparent substrate 4a. The absorption axes (1.1) and (4.1) of these polarizing plates 1 and 4 are arranged so as to be orthogonal to each other.

The compensating liquid crystal cell 2 comprises these polarizing plates 1, 4
The liquid crystal cell structure has a liquid crystal layer 2c interposed between the transparent substrates 2a and 2b.

The driving liquid crystal cell 3 is arranged between the compensating liquid crystal cell 2 and the polarizing plate 4. The upper substrate 3a and the lower substrate 3b form a space between the transparent electrodes 3c and 3d, respectively, and are connected to the driving power supply 3f. The twisted nematic liquid crystal layer 3e is introduced between the substrates 3a and 3b at a twist angle of 90 °, and the state changes according to the applied voltage from the driving power source 3f.

The optical axis of the liquid crystal of the driving liquid crystal cell 3 is twisted counterclockwise from the lower substrate 3b to the upper substrate 3a (left twist). (3.1) and (3.2) are the rubbing axes of the upper and lower substrates, respectively, which are orthogonal to each other. The thickness of the liquid crystal layer of the driving liquid crystal cell 3 is 6.0 μm.

The compensating liquid crystal cell 2 has a twist angle of 3690.
(2.1) and (2.2) are the rubbing axes of the upper and lower substrates 2a and 2b, respectively, which are orthogonal to each other. The refractive index n2c in the thickness direction of the liquid crystal layer of this compensating liquid crystal cell is 1.489, and the refractive index n1c in the cell in-plane direction is 1.524 (retardation value = −
0.210 μm), and the refractive index n2 of the driving liquid crystal cell in the dark state (when 5 V is applied between the driving power source 3f and the liquid crystal cell electrode) is the same as the refractive index n2 in the cell plane direction and the refractive index n1 in the thickness direction. The values were used (n1 = n1c, n2 = n2c).

The absorption axis (1.1) of the polarizing plate 1 and the rubbing axes (2.2), (3.2) of the lower substrate are parallel to each other, and the absorption axis (4.1) of the polarizing plate 4
And the rubbing axes (2.1) and (3.1) of the upper substrate are parallel.

An example of the viewing angle dependence of the y-axis azimuth in the dark state (when 5 V is applied between the driving power source 3f and the liquid crystal cell electrode) of the liquid crystal display element of this structure is shown in FIG.
FIG. 9 is a diagram showing the transmittance of the liquid crystal cell in the dark state when the measurement point is tilted from 0 ° to 60 ° in the y and -y directions from the z-axis of FIG. 2 of the liquid crystal cell. In the figure, (10.1) shows the viewing angle characteristics in the present embodiment, and (10.2) shows the viewing angle characteristics in the conventional example. Ideally, it is desirable that the transmittance is small and the change is constant regardless of the angle at which the liquid crystal cell is viewed. Compared with the conventional example, the change of the inclination angle of the transmittance is smaller in this example, and the display panel has a diagonal angle of 1
A 0-inch size TFT-LCD is created with this configuration.
When gradation display was performed, a high-contrast LCD capable of distinguishing between 16 gradations was realized even if the viewpoint was changed.
Measurement of the viewing angle characteristics of contrast ratio, 60 ° U
In contrast, a contrast ratio of 15: 1 or more was obtained, and even if the incident angle was 60 ° or more, a good display without reversal of display image or change of display color was obtained.

(Comparative Example 1) In Example 1, the viewing angle characteristics of the liquid crystal display device in the case where the compensating liquid crystal cell 2 was not arranged between the driving liquid crystal cell 3 and the upper polarizing plate 1 were measured. The measurement result is shown in (10.2) of FIG. The dark state changes depending on the viewing angle, and the maximum contrast ratio in the 60 ° cone is 5:
Only 1 was obtained, and when the incident angle was 60 ° or more, the display image was inverted or completely invisible depending on the viewing direction.

Comparative Example 2 In Example 1, the liquid crystal layer of the compensating liquid crystal cell has a refractive index n2c in the thickness direction of 1.34.
0, a refractive index n1c in the in-plane direction was 1.371, and a compensating liquid crystal cell having a layer thickness of 6.8 μm was used (n1 ≠).
n1c, n2 ≠ n2c). The characteristics of the liquid crystal display device having this configuration were measured in the same manner as in Example 1, and the results are shown in FIG.
It is shown in 3). The characteristics are better than those of Comparative Example 1, but the 60 ° coating
When the contrast ratio was measured with a microscope, only 7: 1 was obtained.

Example 2 In Example 1, a polymer liquid crystal film was used as the compensating liquid crystal cell 2, and Example 1 was used.
Arranged in the same way. When the electro-optical characteristics were measured, the same characteristics as in Example 1 were obtained, and 10 inch TFT-
When an LCD was made with this configuration and displayed with 16 gradations, a high contrast display LCD capable of distinguishing between 16 gradations even when the viewpoint was changed was realized. When the viewing angle characteristics were measured, a contrast ratio of 20: 1 or more was obtained with a 60 ° cone , and a good display without inversion of display images or change of display color was obtained even when the incident angle was 60 ° or more.

(Embodiment 3) In Embodiment 1, the substrate 3
A liquid crystal having homeotropic alignment is introduced between a and 3b, and the state changes according to the applied voltage from the driving power supply 3f. The refractive indices ne and no of the liquid crystal are 1.578 and
It is 1.468, and the thickness of the liquid crystal layer is 7 μm. The compensating liquid crystal cell 2 has a twist angle of 3690 ° and a thickness of 7.0.
μm, the refractive index n2c in the thickness direction of the compensating liquid crystal cell
Is 1.468, and the refractive index n1c in the cell in-plane direction is 1.57.
8, the refractive index n2 of the driving liquid crystal cell in the dark state (when no voltage is applied from the driving power source 3f to the liquid crystal cell electrode) is the same as the refractive index n1 in the cell plane direction and the refractive index n1 in the thickness direction. (N1 = n1c, n2 = n2c). 640 × 4 with this configuration
Create an ECB type LCD with 80 dots, 1/240 duty
When the simple multiplex drive was used, the transmittance in the dark state was almost constant even when the viewing angle changed to 50 °. When the viewing angle characteristics were measured, a contrast ratio of 21: 1 or more was obtained with a 60 ° cone , and a good display without inversion of display images or change of display color was obtained even when the incident angle was 60 ° or more.

[0036]

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. Needless to say, even if the present invention is applied to an active matrix liquid crystal display element using a 3-terminal or 2-terminal element such as TFT or MIM, excellent effects can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a liquid crystal display element of Example 1 of the present invention.

FIG. 2 is an exploded perspective view showing the configuration of the liquid crystal display element according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a viewing angle characteristic of a contrast ratio of a normally open and normally closed type in a left-right direction of a conventional TN type liquid crystal display device.

FIG. 4 is a diagram for explaining a viewing angle characteristic of luminance in a normally open and normally closed dark state of a conventional TN type liquid crystal display device.

FIG. 5 is a view showing a molecular arrangement in the thickness direction of a liquid crystal cell when a voltage is applied to the liquid crystal cell.

6 is a diagram showing a coordinate system of tilt angles and twist angles of the liquid crystal molecules of FIG.

FIG. 7 is a diagram showing a three-dimensional refractive index ellipsoid with liquid crystal molecules standing.

FIG. 8 is a diagram illustrating a refractive index ellipsoid for optically compensating the refractive index ellipsoid of FIG.

FIG. 9 is a diagram for explaining viewing angle characteristics in a dark state according to an example of the present invention and a conventional example.

[Explanation of symbols]

 1, 4 ... Polarizing plate 2 ... Compensation liquid crystal cell 3 ... Driving liquid crystal cell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hitoshi Hato 8 Shinsita-cho, Isogo-ku, Yokohama

Claims (1)

Claim: What is claimed is: 1. A driving liquid crystal cell and a compensation liquid crystal cell, which is arranged adjacent to the driving liquid crystal cell and has a twisted arrangement in a spiral axis substantially parallel to a substrate normal line of the driving liquid crystal cell. A liquid crystal display device comprising a liquid crystal display element comprising a liquid crystal layer, two polarizing plates sandwiching the driving liquid crystal cell and a compensating liquid crystal layer, and a driving means for applying a voltage to the driving liquid crystal cell,
In the liquid crystal display device, the twist angle of the compensating liquid crystal layer is 3
The driving liquid crystal has a voltage of 60 ° or more, a thickness of the liquid crystal layer of the driving liquid crystal cell and a thickness of the compensating liquid crystal layer are substantially equal to each other, and a voltage for generating a display image for obtaining a dark state by the driving means. The refractive index of the driving liquid crystal cell for extraordinary light when applied to the cell and the refractive index of the compensating liquid crystal layer for ordinary light are substantially equal to each other. A liquid crystal display device characterized by having substantially the same refractive index with respect to.