JPH0720471A - Reflection type liquid crystal display device - Google Patents

Abstract

PURPOSE:To increase contrast with a low driving voltage and to improve the visibility by rubbing vertical orientation films and controlling liquid crystal directors at the time of voltage application to parallel with the rubbing direction. CONSTITUTION:When a voltage is applied, the directors are directed in parallel to substrates 3 and 4 by the anisotropy of the dielectric constant that liquid crystal 7 itself has. The vertical orientation films 5 and 6 are previously rubbed and the directors of the liquid crystal 7 are directed in the rubbing direction. Dichroic dye 11 also faces in the same direction according to the motion of molecules of the liquid crystal 7. Consequently, light 13 which has a polarization plane in parallel to the director direction of the dichroic dye 11 is absorbed as to incident light, but light 14 having a vertical polarizing plate is passed. The passed light is rotated and polarized by a wavelength plate 12, and reflected by a reflecting plate 10 and then further rotated and polarized by the wavelength plate 12. Consequently, the light is rotated by 90 deg. and enters a liquid crystal layer 7 again, so it is absorbed on its return way. At this time, the light is not reflected to make a black display.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a personal computer, a word processor,
The present invention relates to a liquid crystal display device used for a display device such as a small television.

[0002]

2. Description of the Related Art A conventional liquid crystal display device is shown in FIG. IT
Transparent pixel electrodes 1, 2 such as O (Indium-Tin-Oxygen)
Alignment films 5 and 6 are formed on a light-transmitting substrate 3 and 4 such as glass on which a polyimide resin is formed to a thickness of about 1000 Å and rubbed in one direction. A liquid crystal 7 is sandwiched between the substrates 3 and 4, and the liquid crystal molecules are aligned in the rubbing direction. Polarizing plates 8 and 9 are provided on the outer walls of the substrates 3 and 4, respectively.
The polarizing axes of the polarizing plates 8 and 9 are aligned with the rubbing directions of the alignment films 5 and 6 of the substrates 3 and 4 to which the polarizing plates 8 and 9 are attached. Further, a reflection plate 10 is arranged further outside the polarizing plate 9.

Such a liquid crystal display device has a pixel electrode 1,
When no potential difference is generated between the two, the liquid crystal 7 is dominated by the rubbing directions of the alignment films 5 and 6 and is aligned in a fixed direction. Since the rubbing directions do not match between the transparent substrates 3 and 4, the liquid crystal molecules gradually rotate between the substrates 3 and 4. At this time, light incident from the substrate 3 side only enters the liquid crystal layer 7 that matches the polarization axis of the polarizing plate 8, and the plane of polarization of the incident light is rotated according to the rotation of the molecules of the liquid crystal 7. The rotated light has a polarization axis in the same direction as the other polarizing plate 9 and is transmitted. Further, the light is reflected by the reflecting plate 10, and the light passes through the polarizing plates 8 and 9 and emerges. At this time, the liquid crystal display device displays white.

On the other hand, when a potential difference is generated between the pixel electrodes 1 and 2, the molecules of the liquid crystal 7 rise due to their own dielectric anisotropy. Therefore, the light incident on the liquid crystal layer 7 from the substrate 3 side is not rotated because the liquid crystal molecules do not rotate, and is absorbed because it does not coincide with the polarization axis of the polarizing plate 9. Therefore, no light is emitted from the liquid crystal display device. At this time, the liquid crystal display device displays black. Black and white display is performed by such a method.

[0005]

However, in this conventional liquid crystal display device, since the polarizing plates 8 and 9 are used,
Approximately 50% of the incident light is the polarizing plate 8 regardless of white or black display.
Absorbed by Therefore, the light utilization rate is low. Therefore, sufficient brightness cannot be obtained even when displaying white,
Display with excellent visibility is impossible.

In order to improve the brightness of white display, it is usually not a reflection type, but a backlight is provided and a sufficient amount of light is made incident from the transparent substrate 4 side. The brightness at the time of display was improved. However,
When a backlight is used, its power consumption is so large that it has a problem that the original characteristic of the liquid crystal display device, which is excellent in portability, is impaired.

Since the alignment state when no voltage is applied is parallel to the transparent substrates 3 and 4, the alignment regulating force from the substrates 3 and 4 is large, and liquid crystal molecules are applied even when a voltage is applied. There is also a problem that the motor does not move easily and the driving voltage must be very high.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is characterized in that it has two transparent substrates provided with electrodes. , A liquid crystal material having a negative dielectric anisotropy mixed with a dichroic dye is sandwiched in a vertically aligned state, and a reflector is arranged on one side of this translucent substrate. A quarter
In the reflection type liquid crystal display device in which the wave plate is arranged, a rubbing treatment is applied to the vertical alignment film to control the liquid crystal director when voltage is applied so as to be parallel to the rubbing direction.

[0009]

By controlling the liquid crystal director to be parallel to the rubbing direction when a voltage is applied in this way, a difference occurs in the light transmittance depending on the polarization direction of the incident light when a voltage is applied,
Polarized light that could not be absorbed on the outward path in the reflection type liquid crystal display device can be sufficiently absorbed on the return path by rotating the light 90 degrees using a wave plate, and a polarizing plate is not necessary, and the contrast is high. The display device has good visibility. In addition, since the alignment state when no voltage is applied is vertical to the translucent substrate, the alignment regulating force from the substrate is weak and low voltage driving is possible.

[0010]

Embodiments of the reflective liquid crystal display device according to the present invention will be described in detail below with reference to the accompanying drawings. 1A and 1B are diagrams showing a configuration of a reflective liquid crystal display device according to the present invention, wherein FIG. 1A shows a white display state, and FIG. 1B shows a black display state. Translucent substrates 3 and 4 provided with pixel electrodes 1 and 2
Vertical alignment films 5 and 6 made of a polyimide resin or the like that has been rubbed in a predetermined direction are formed on the substrate 3, and a dichroic dye 11 such as an anthraquinone dye or an azo dye is provided between the substrates 3 and 4 to have a negative dielectric anisotropy. A guest-host type liquid crystal material mixed with the liquid crystal 7 having optical properties is enclosed, and the retardation is set to a range of 120 to 160 nm in order to rotate light.
A half-wave plate 12 and a reflection plate 10 are provided.

The reason why the liquid crystal having a negative dielectric anisotropy is used in the present invention is as follows. When a liquid crystal having a positive dielectric constant anisotropy is used, the director of the liquid crystal is substantially parallel to the substrates 3 and 4 when no voltage is applied. The reason for aligning in parallel is that the alignment regulating force is strong. Therefore, liquid crystal molecules in the vicinity of the alignment film cannot be switched unless a high voltage is applied. In particular, in a guest-host type liquid crystal display device, the viscosity of the liquid crystal increases due to the dichroic dye, and the saturation voltage also increases. On the other hand, in the case of liquid crystal having a negative dielectric anisotropy, the director of liquid crystal molecules is made perpendicular to the substrate in the state where no voltage is applied. The reason for being perpendicular to the substrate is that the alignment regulating force is weak. When a voltage is applied, the liquid crystal switches from the central portion of the liquid crystal layer 7, but since the alignment regulating force is weak, it is possible to switch at a low voltage near the alignment film.

In the state where no voltage is applied between the upper and lower pixel electrodes 1 and 2 (FIG. 1-a), the liquid crystal director is oriented vertically to the substrates 3 and 4 due to the vertical alignment regulating force of the vertical alignment films 5 and 6. There is. The dichroic dye 11 mixed in the liquid crystal 7 also has an elongated molecular structure, and its director also moves according to the movement of the liquid crystal molecules. Therefore, when no voltage is applied, the director of the dichroic dye 11 is also oriented in the direction perpendicular to the substrates 3 and 4, and thus does not absorb the light incident on the liquid crystal 7. Therefore, the incident light passes through the liquid crystal 7 layer regardless of the polarization plane, the polarization plane is rotated by the wave plate 12 and is reflected by the reflection plate 10, and then the polarization plane is rotated again by the wave plate 12. Then, in the return path, the light has a polarization plane rotated by 90 degrees with respect to the outward path, and passes through the liquid crystal layer 7. At this time, the display is white.

When a voltage is applied, due to the anisotropy of the dielectric constant of the liquid crystal 7 itself, its director direction is oriented parallel to the substrates 3 and 4 (FIG. 1-b). Since the vertical alignment films 5 and 6 have already been rubbed, the director of the liquid crystal 7 faces the rubbing direction. The dichroic dye 11 also faces the same direction according to the movement of the seven molecules of the liquid crystal. Therefore, of the incident light, the light 13 having the polarization plane parallel to the director direction of the dichroic dye 11 is absorbed, and the light 14 having the perpendicular polarization plane passes through. The light passing therethrough is rotated by the wave plate 12, reflected by the reflection plate 10, and further rotated by the wave plate. As a result, the light is rotated by 90 degrees and enters the liquid crystal layer 7 again, and is absorbed in the return path. At this time, light is not reflected and a black display is obtained.

FIG. 2 shows an experimental method for confirming whether or not the liquid crystal director is oriented in the rubbing direction when a voltage is applied. The polarizing plate 8 is shown with the wave plate 12 and the reflecting plate 10 shown in FIG. 1 removed. This is confirmed by the light transmittance of polarized light used. The polarization axis of the polarizing plate 8 is aligned so as to be parallel to the rubbing directions of the transparent substrates 5 and 6. Light is incident from the light source unit 15, and the transmitted light intensity is measured by the light receiving unit 16. The change in light transmittance when a voltage was applied between the pixel electrodes was measured. Next, the polarization axis of the polarizing plate was rotated by 90 degrees, and the voltage between the pixel electrodes and the change in light transmittance were measured in the direction perpendicular to the rubbing direction.

As a result of the experiment, as shown in FIG. 3, the voltage-transmittance characteristic of the light having the polarization plane in the direction parallel to the rubbing direction is shown by the mark [◇] when the director of the liquid crystal 7 is the substrate 3, Four
The absorption increases as it becomes parallel to. On the contrary, light having a plane of polarization perpendicular to the rubbing direction does not change in absorption even if a voltage is applied as indicated by the [+] mark.

[0016]

As described above, according to the reflection type liquid crystal display device of the present invention, by rubbing the vertical alignment film, the reflectance of light when no voltage is applied is high and the driving voltage is low. It is possible to provide a liquid crystal display cell having high contrast and excellent visibility.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a reflective liquid crystal display device according to the present invention, in which (a) shows white display and (b) shows black display.

FIG. 2 is a diagram showing an experiment for confirming a liquid crystal director when a voltage is applied.

FIG. 3 is a diagram showing a liquid crystal director and voltage-transmittance characteristics when a voltage is applied.

FIG. 4 is a cross-sectional view of a conventional reflective liquid crystal display device.

[Explanation of symbols]

1, 2 ... Pixel electrodes, 3, 4 ... Substrate, 5, 6, ...
・ Alignment film, 7 ・ ・ ・ Liquid crystal, 8, 9 ・ ・ ・ Polarizing plate, 10 ・
..Reflector, 11 ... Dichroic dye, 12 ... Quarter wave plate, 13 ... Incident light having polarization plane parallel to rubbing direction, 14 ... Perpendicular to rubbing direction Incident light having a polarization plane, 15 ... Light source, 16 ... Light receiving section

Claims (1)

[Claims] 1. Two translucent substrates provided with electrodes,
A liquid crystal material having a negative dielectric anisotropy is sandwiched, a wavelength plate and a reflection plate are provided outside one of the translucent substrates, and a vertical alignment film is formed on the inner surface of the translucent substrate. Type liquid crystal display device, the liquid crystal is vertically aligned when no voltage is applied by rubbing the vertical alignment film, and the rubbing direction and the liquid crystal director are controlled to be parallel when voltage is applied. Reflective liquid crystal display device.