JPH0580323A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To uniform the intensity of light transmitted through a polarizing plate irrelevantly to the direction of the light and to improve the visual angle characteristics of a half-tone display by providing a phase difference film which has an optical axis slanting to a sheet surface on a projection light side. CONSTITUTION:A liquid crystal director 42 in a half-tone display state slants to a substrate surface halfway between the horizontal state and vertical state and its tilt angle is theta. Light beams (e) and (f) transmitted through a liquid crystal layer 6 are made incident on the phase difference film 2 whose optical axis 37 is at an angle beta to the film surface. In this case, the angle beta is set in the opposite direction from the tilt angle theta. Light having a plus incidence angle has a large angle to the optical axis 37 of the film 2 and, therefore, has a large phase difference across the film 2. Light having a minus incidence angle, on the other hand, has a small angle to the optical axis 37 and, therefore, has a small phase difference. Consequently, phase differences compensate each other in opposite directions, so the phase difference entering a projection side polarizing plate 1 does not vary greatly with variation in incidence 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, and more particularly to a liquid crystal display device in which display characteristics in gray scale display are small in variation depending on a viewing angle.

[0002]

2. Description of the Related Art Liquid crystals
Tal display (LCD) has a liquid crystal substance sandwiched between two transparent electrodes facing each other in parallel, and controls the alignment state of liquid crystal molecules by changing the applied voltage between the electrodes.
It operates based on the principle of realizing screen display by controlling transmission or reflection of light depending on the liquid crystal alignment state.

In such a liquid crystal display device, for example, the orientation direction of liquid crystal molecules (hereinafter referred to as liquid crystal director) is parallel to the electrode plane in a voltage-off state, and the angle between the liquid crystal directors on both sides of the electrode is large. Twisted nematic (TN) mode with 90 ° twist, 180-2
70 degree twisted super twisted nematic (ST
N) mode is typical.

Two polarizing plates are provided on the outside of a transparent substrate (usually a glass plate) carrying a transparent electrode, and the angle formed by the mutual polarization axes is usually 0 ° or 90 °. The incident light passes through the inside of the liquid crystal after being polarized by the polarizing plate on the incident light side, but the alignment state of the liquid crystal changes according to the applied voltage, and the polarization state of the passing light depends on the alignment state. Changes. In gradation display, the applied voltage continuously takes an arbitrary value between the threshold voltage (Vth) and the saturation voltage (Vsat), and the liquid crystal molecules continuously change their molecular orientation depending on the voltage. Change. In gradation display, half-tone density display is used, but at this time, the director of the liquid crystal molecules forms an oblique angle (tilt angle: θ °) with respect to the electrode plane. Therefore, a light ray incident on the substrate at an incident angle of 0 ° advances at an angle (90-θ) ° with the liquid crystal molecules in the liquid crystal layer. This state is shown in FIG.

The liquid crystal molecules 41 have a uniaxial refractive index anisotropy with the director direction 42 as the optical axis. Therefore, the polarized light incident depending on the angle between the orientation vector and the light is an ordinary light component (refraction). The index no) and the extraordinary light component (refractive index ne) proceed separately.

At the stage of passing through the liquid crystal layer, a phase difference (retardation) δ is generated between the ordinary light component and the extraordinary light component according to the difference between the refractive indices no and ne, and this phase difference δ
Based on, the vibration vector of light is in an elliptically polarized state in which it rotates in an elliptical shape. When the emitted light in the elliptically polarized state thus generated enters the polarizing plate on the emission side, only the component in the changed axis direction in the elliptically polarized light passes through the polarizing plate. Changes. When the applied voltage between the electrodes is changed, the orientation vector of the liquid crystal molecules is changed, which changes the elliptically polarized state of the light after passing through the liquid crystal layer, and thus the outgoing light intensity after passing through the outgoing side polarizing plate is changed. In this way, the transmitted light intensity can be changed according to the applied voltage.

Unlike the gradation display method in which the applied voltage is continuously changed, the applied voltages are Vth and Vs.
In the non-gradation display, the light transmittance is set to only two levels by using only the binary value of at.

In the gradation display of the liquid crystal display device, it is possible to express light and shade as compared with the non-gradation display, and by combining the three primary colors by gradation display, the color expression becomes rich (full color display). In addition, it greatly improves the expressibility of the liquid crystal display device.

[0009]

When gradation display is performed by the LCD, there is a problem that the brightness changes depending on the position of the eyes of the LCD even if the same applied voltage is applied. The principle will be described with reference to FIG.

The polarized light f incident on the liquid crystal layer at the incident angle α ° advances at an angle (90-α-θ) ° with the liquid crystal molecules having the tilt angle θ °. Thus, the elliptically polarized state of the light passing through the liquid crystal layer differs depending on the incident angle α. It is the angle (90-α-
The refractive index ne of extraordinary light differs depending on the difference in θ) °,
And the distance d / cos α through the liquid crystal layer depending on the angle α
This is because the elliptically polarized state of the light after passing is different for two reasons that (d represents the distance between opposing electrodes) is different, and therefore the transmittance of the outgoing light side polarizing plate is different. Therefore, when gradation is displayed
The brightness of the CD changes, which has the following adverse effects on the display quality:

(1) The brightness of a part desired to be displayed brightly and the brightness desired to be displayed dark may be reversed.

This occurs because the viewer's eyes see differently for each part of the LCD display screen. That is, a portion displayed brightly may appear dark depending on the vision seen from a certain position, and another portion displayed dark at the same time may appear bright depending on the vision seen from the same position.

(2) In the case of color display The hue changes depending on the viewing position.

The reason is as follows. In color display, the hues are the three primary colors (red (R), green (G), blue (B))
It is realized by the combination of the transmitted light from each of the color filters. However, since the elliptically polarized state of the light passing through the liquid crystal layer differs depending on the viewing angle, the balance of the transmitted light of the R, G, and B color filters changes, and as a result, the hue appears different from the original display.

[0015]

According to the present invention, in a liquid crystal display device in which a liquid crystal is sandwiched between two transparent electrode substrates and the light transmittance is changed by the voltage value between the transparent electrodes,
A liquid crystal display device is obtained which has a film having an optical axis inclined to the surface of the transparent electrode substrate on at least one surface of the transparent electrode substrate.

Further, according to the present invention, there can be obtained a liquid crystal display device in which the film whose optical axis is inclined with respect to the substrate surface is made of polycarbonate resin.

Furthermore, according to the present invention, a liquid crystal display device can be obtained in which the angle between the retardation film and the surface of the transparent electrode substrate is 45 °.

[0018]

What works is a gradation display type liquid crystal display device in which a retardation film having an optical axis whose angle is inclined with respect to a film surface is provided between a substrate and a polarizing plate on the side of outgoing light of an LCD. It will be explained with reference to FIG. 4 whether or not the above-mentioned problems in gradation display are alleviated for the reason.

FIG. 4 is a model cross-sectional view when transmitted light is incident on the liquid crystal display device according to the present invention.

Liquid crystal director 42 in the gradation display state
Has an inclined state between the horizontal and vertical directions with respect to the substrate surface, and its tilt angle is θ °. When the incident angle α ° is plus, the angle formed by the light ray e and the liquid crystal director 42 is relatively small, so that the ordinary component of the light is relatively large, and the refractive index ne of the extraordinary light and the refraction of the ordinary light are relatively large. The difference from the index no is also small, and therefore the phase difference δ is small, so that the elliptic polarization degree is small. Conversely, when the incident angle α is negative, the light ray e
The angle formed by the orientation director 42 and the orientation director 42 is increased, so that the phase difference δ is increased and the elliptic polarization degree is increased.

The lights e and f that have passed through the liquid crystal layer in this way are then incident on a film whose optical axis forms an angle β ° with respect to the film surface, that is, the retardation film 2.
In this case, the angle β ° is set in the opposite direction to the tilt angle θ °. Light having a positive incident angle has a large angle with the optical axis of the film 2, and therefore the phase difference before and after passing through the film 2 is large. On the contrary, the light having a negative incident angle forms a small angle with the optical axis of the film, and therefore the phase difference before and after passing through the film 2 is small.

Therefore, the sum of the retardation at the stage of passing through the liquid crystal layer and the retardation at the stage of passing through the retardation film 2, that is, the retardation entering the polarizing plate on the exit side is such that the retardations at the respective stages are reversed. Since they complement each other, they do not change significantly due to changes in the incident angle. Therefore, the visual change in the light intensity after passing through the polarizing plate is less than that in the case where the retardation film is not used, and the visual reversal of the brightness and the change in the hue are reduced.

[0023]

The present invention will be described below with reference to the drawings.

FIG. 1A is a perspective view of a liquid crystal display device showing a first embodiment of the present invention, and FIG. 1B is a sectional view taken along a sectional line AA in FIG. 1A.

The liquid crystal display device shown in FIG. 1 was manufactured in the following manner and its characteristics were evaluated.

Each electrode of two glass plates 3 and 9 having electrodes 4 and 7 made of a square indium tin oxide (ITO) thin film of 1 cm in length and width and 1400 angstroms having external connection terminals on the surface. Thickness 8 on
Polyimide thin films 5 and 8 of 00 angstrom were formed. Then, this polyimide thin film is rubbed in one direction by a rayon felt roller, and then the glass thin film surfaces 5 and 8 are set inside so that the upper rubbing direction is 90 ° to the lower rubbing direction. The plates 3 and 9 were opposed to each other. Next, the glass plates 3 and 9 were bonded together while adjusting the spacers so that the electrodes 4 and 7 face each other while maintaining a distance of 5 μm. The liquid crystal 6 was filled between the electrodes 4 and 7 to produce a liquid crystal cell.

Further, a polycarbonate resin having a uniaxial optical axis was sliced into a thin piece having a thickness of about 0.5 mm on a plane forming an angle of 45 ° with respect to the optical axis and used as a retardation film 2.

The retardation film 2 was attached to the upper surface of the liquid crystal cell so that the projection line direction of the optical axis to the film surface forms an angle of 135 ° to the right from the rubbing direction of the upper alignment film. Next, a polarizing plate was attached to both upper and lower surfaces of the cell so that the rubbing direction and the polarization axis direction coincided with each other, to obtain a liquid crystal display element.

FIG. 2A is a plan view showing a second embodiment of the present invention, and FIG. 2B is a sectional view taken along a sectional line BB in FIG. 2A.

The liquid crystal display device shown in FIG. 2 was manufactured as follows and its characteristics were evaluated.

On the surface of each of the glass substrate 9 on which an active matrix having a thin film transistor (TFT) 25 having the structure shown in FIG. 3 as a pixel constituent unit is formed, and the glass substrate 3 on which a color filter 21 is formed. Then, polyimide thin films 5 and 8 having a thickness of 800 angstrom were formed. Next, this polyimide thin film 5,8
After rubbing in one direction with a rayon felt roller, the TFT substrate 9 and the color filter substrate 3 with the polyimide thin film surface inside so that the upper rubbing direction forms a right angle of 90 ° with the lower rubbing direction. And faced each other. Next, the two substrates were bonded together while adjusting with a spacer so that the distance between the transparent electrodes was 5 μm.

Liquid crystal 6 was filled between the electrodes to prepare a liquid crystal cell.

Further, a polycarbonate resin having a uniaxial optical axis was sliced into a thin piece having a thickness of about 0.5 mm on a plane forming an angle of 45 ° with respect to the optical axis and used as a retardation film 2.

The retardation film 2 was attached to the upper surface of the liquid crystal cell so that the projection line direction of the optical axis to the film surface formed an angle of 135 ° to the right from the rubbing direction of the upper alignment film. Then, polarizing plates were attached to the upper and lower surfaces of the cell so that the rubbing direction and the polarization axis direction were coincident with each other to obtain a liquid crystal display device.

[0035]

In order to evaluate the gradation display of the liquid crystal display device of Examples 1 and 2, the drive voltage is adjusted so that the light transmittance is 0% at the vertical position of 0 ° and the horizontal position of 0 °. After being in the halftone state, the difference in visual contrast was compared to that without the retardation film (Table 1).

[0036]

[Table 1]

In Table 1, Comparative Example 1 is obtained by removing the retardation film of Example 1, and Comparative Example 2 is obtained by removing the retardation film of Example 2. The value is obtained by dividing the brightness at the maximum brightness by the brightness at the maximum darkness, and the larger the value, the clearer the contrast.

It can be seen from Table 1 that the change in the visual contrast is smaller when the retardation film is attached than when the retardation film is not attached.

Therefore, according to the liquid crystal display device of the present invention, there is little visual change in contrast, and it is possible to effectively adapt to gradation display.

[Brief description of drawings]

1A is a perspective view showing a first embodiment of the present invention, and FIG. 1B is a sectional view taken along a sectional line AA in FIG. 1A.

2A is a plan view showing a second embodiment of the present invention, and FIG. 2B is a sectional view taken along a sectional line BB in FIG. 2A.

3A is a plan view showing a TFT shape of a liquid crystal display device according to a second embodiment of the present invention, and FIG. 3B is a plan view of FIG.
It is sectional drawing in the sectional line CC of A. FIG.

FIG. 4 is a model cross-sectional view showing a liquid crystal display device according to the present invention in a gradation display state.

FIG. 5 is a model cross-sectional view showing a conventional liquid crystal display device in a gradation display state.

[Explanation of symbols]

1, 10 Polarizer 2 Phase difference film 3 Glass 4 Upper electrode 5, 8 Alignment film 6 Liquid crystal 7 Lower electrode 9 Glass 11 Lower glass plate edge 12 Upper and lower glass plate overlap 13 Upper glass plate edge 14 Lower electrode Terminal 15 lower lead portion 16 upper and lower electrode overlapping portion 17 upper lead portion 18 upper electrode terminal 19 pixel 20 black matrix 21 color filter 22 protective layer 23 transparent electrode 24 wiring 25 TFT 26 pixel electrode 27 passivation film 28 gate 29 channel 30 Drain side electrode 31 Source side electrode 32 ITO layer 33 Chrome layer 34 n ⁺ Silicon layer 35 Amorphous silicon 36 Gate insulating film 37 Optical axis 41 Liquid crystal molecule 42 Liquid crystal director 51 Optical axis of retardation film AA, BB, C -C section line a, b, c, d, e, f Excessive light

Claims (3)

[Claims] 1. A liquid crystal is sandwiched between two transparent electrode substrates,
In a liquid crystal display device that changes the light transmittance depending on the voltage value between the transparent electrodes, at least one surface of the transparent electrode substrate has a retardation film whose optical axis is inclined with respect to the transparent electrode substrate surface. Characteristic liquid crystal display device. 2. The liquid crystal display device according to claim 1, wherein the retardation film is made of a polycarbonate resin. 3. The liquid crystal display device according to claim 1, wherein an angle between the retardation film and the surface of the transparent electrode substrate is 45 °.