JPH05232478A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To improve display characteristics, such as contrast by controlling the disturbance in liquid crystal orientation around spacers. CONSTITUTION:The spacers 5 for holding a cell gap and a liquid crystal material are clamped between two sheets of parallel plates having transparent electrodes 2 on the inside surfaces and liquid crystal oriented films 3 on these electrodes 2. The spacers 5 are subjected to an orientation treatment. Spherical or bar-shaped particles subjected to a perpendicular orientation treatment on the surface are used as the spacers 5 in such a case. The tilt angles of the liquid crystal molecules in the parts in contact with the spacers 5 are nearly parallel with the electrode surfaces as the spacer 5 surfaces are nearly perpendicular to the electrode surfaces of the cell in the central part of the cell when the voltage is not impressed to the element. On the other hand, the liquid crystal molecules near the oriented films 3 are originally liable to be risen perpendicularly by the influence of the spacer 5 surfaces and are easily perpendicularly risen by the influence of an electric field when the voltage is impressed to the element.

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 having a display contrast improved by treating a spacer surface in the liquid crystal display device.

[0002]

2. Description of the Related Art A liquid crystal display device (LSD) has a liquid crystal material sandwiched between two transparent electrodes facing each other in parallel, and the arrangement state of liquid crystal molecules is controlled by varying an 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 arrangement state.
In such a liquid crystal display element, for example, the liquid crystal molecular director is parallel to the electrode plane in the voltage-off state, and the angle between the liquid crystal directors on both sides of the electrode is 90 degrees.
° Twisted Twisted Nematic (TN) mode,
A super twisted nematic (STN) mode twisted by 180 to 270 ° is typical. Two polarizing plates are installed on the outside of a transparent substrate (usually a glass plate) carrying a transparent electrode, and the angle formed by their 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 arrangement state of the liquid crystal changes according to the applied voltage. Changes the polarization state of. In this way, the outgoing light that has passed through the liquid crystal layer has a different transmittance in the polarizing plate on the outgoing light side depending on the polarization state, and therefore the light transmittance can be controlled by the applied voltage, and image display is realized. It This principle is the TN or STN mode.

By performing such switching of light for each subdivided screen constituent element (pixel), it is possible to display characters, figures, and the like. Each pixel has three color components, that is, red (R: Red) and green (G: Gre).
en) and blue (B: Blue) are switched to realize color display. In a color display LCD, it is common to perform R, G, B switching by forming a structure in which R, G, B color filters are formed on one side of opposite electrodes for each pixel. is there. A transparent electrode for driving liquid crystal is formed on the uppermost layer of the color filter. Indium tin oxide (ITO) is most often used as the material of the transparent electrode.

In color display, black and white (monochrome)
Since higher image quality is required in terms of color tone and the like as compared with display, it is necessary to keep the voltage applied between the electrodes of each pixel constant during the scan period (frame period) of the screen. For this purpose, an LCD having a kind of memory function for holding the voltage written in each pixel is generally used for color display, and such an LCD is activated. It is called a matrix LCD. As a means for realizing a memory function of an active matrix LCD (AM-LCD), a thin film transistor (TFT) or metal / insulator / metal (MI) is provided for each pixel.
It is usual to install M) and make them carry a switching function.

The alignment of the liquid crystal molecules in the liquid crystal display device is realized by the alignment film layer formed on the transparent electrode on the inner surface of the glass plate of the device. Although the alignment film alone defines the angle (pretilt angle) of the liquid crystal molecules with respect to the electrode plane, it cannot define the direction (azimuth) within the plane of the liquid crystal molecules.
Defining the azimuth angle is usually realized by rubbing the surface of the alignment film in the desired azimuth direction. The operation of rubbing this surface is called "rubbing".

In the liquid crystal display device, spherical or rod-shaped solid particles called "spacers" are sandwiched between the upper and lower substrates in order to maintain a constant cell gap between the upper and lower substrates holding the liquid crystal. It is desirable that the spacers are uniformly distributed in the display screen. For example, in the case of the active matrix type, about 2 to 10 spacers per pixel are the current state of the market. By providing a spacer between both substrates, the uniformity of the thickness of the liquid crystal in the liquid crystal display element is secured,
Therefore, the uniformity of brightness and hue in the display screen is ensured.

[0008]

Liquid crystal molecules have the property of being aligned parallel to the alignment film on the surface of the alignment film. Therefore, on the surface of the alignment film after rubbing, the liquid crystal molecules are parallel to the surface of the alignment film and oriented in a certain direction. In this case, the liquid crystal molecules change stepwise from the parallel state to the vertical state according to the magnitude of the applied voltage, and accordingly the light transmittance in the pixel changes stepwise.

However, when the spacer is present in the pixel, the liquid crystal molecules around the spacer behave differently from other parts in the pixel. This is because the alignment state of liquid crystal molecules around the spacer is disturbed by the influence of the surface of the spacer. In the liquid crystal molecular layer around the spacer in which the alignment state is disturbed, the light transmittance is different as compared with the other normal parts, and therefore the originally intended pixel brightness and brightness are different. The deviation of the light transmittance from the ideal state due to the alignment disorder of the liquid crystal molecules adversely affects the display characteristics. For example, it is ideal that the light transmittance when the display is in the black state is practically zero, but some light leakage occurs in a pixel having alignment disorder. For this reason, the contrast, which is the ratio of the brightness in the white display and the brightness in the black state, decreases. In recent years, the characteristics required for liquid crystal display elements have become strict, and such a reduction in contrast becomes a defect in display quality.

[0010]

According to the present invention, a spacer for holding a cell gap and a liquid crystal substance are sandwiched between two parallel plates each having a transparent electrode on the inner surface and a liquid crystal alignment film on the electrode. In the liquid crystal display element, a liquid crystal display element in which the spacers are aligned is obtained.

Further, according to the present invention, there can be obtained a liquid crystal display element in which the above-mentioned spacer is a spherical or rod-shaped particle whose surface is vertically aligned.

The liquid crystal display device of the present invention is characterized in that spherical or rod-shaped particles whose surfaces are vertically aligned are used as spacers.

The alignment state around the spacer due to the vertical alignment treatment of the surface of the spacer is as shown in FIG. As shown here, when no voltage is applied, the spacer surface of the liquid crystal molecule that is in contact with the spacer surface is almost perpendicular to the cell electrode surface in the cell center portion, so that its tilt angle is approximately parallel to the electrode surface. In addition, in the part close to the cell electrode, the spacer surface affects the liquid crystal molecules in the direction perpendicular to the alignment film surface, while the alignment film surface affects the surrounding liquid crystal molecules horizontally. Therefore, the influence of both is canceled out, and the liquid crystal molecules are aligned with the tilt angle almost parallel to the surface of the alignment film like other ordinary molecules.

On the other hand, when a voltage is applied, the liquid crystal molecules near the alignment film originally tended to stand vertically due to the effect of the spacer surface, and thus easily stand vertically due to the effect of the electric field. The liquid crystal molecules in the central portion of the cell around the spacer have a tilt angle close to vertical due to the influence of the electric field except for a part of the spacer surface which is close to vertical.

On the other hand, the movement of the liquid crystal molecules in the liquid crystal display device using the conventional spacer in which the spacer surface is not subjected to the vertical alignment treatment is as shown in FIG. Here, since the alignment state of the liquid crystal molecules with respect to the spacer surface is random, the light transmittance of the pixel when the voltage is not applied and when the voltage is applied is different from that of the normal portion.

[0016]

EXAMPLES The specific contents of the present invention will be described in the following examples. Example 1 The surface of a resin spherical spacer having an average diameter of 5 μm was treated with dodecyltrimethylammonium chloride, so that the surface was rehydrated to have vertical alignment.

Using a glass substrate having an ITO (indium tin oxide) electrode having an area of 1 cm ^{2 on} the surface,
A liquid crystal display device using this modified spacer was manufactured.
Luminance of the device is in 0.22cd / m ² at the black display is 45 cd / m ² at the white display, the contrast was 205. Comparative Example 1 A liquid crystal display device was produced in the same manner as in Example 1 except that the spacer surface was not subjected to vertical alignment treatment.

The brightness of this element is 0.67 cd / in black display.
m ² , the white display was 44 cd / m ² , and the contrast was 66. Example 2 The surface of a glass rod-shaped spacer having an average diameter of 5 μm was treated with dodecyltriethyloxysilane, so that the surface was re-humidified to have vertical orientation.

Using a glass substrate having an ITO (indium tin oxide) electrode with an area of 1 cm ^{2 on} the surface,
A liquid crystal display device using this modified spacer was manufactured.
Luminance of the device is 0.35cd / m ² at the black display is 49cd / m ² at the white display, the contrast was 140. Comparative Example 2 A liquid crystal display element was produced in the same manner as in Example 1 except that the spacer surface was not subjected to vertical alignment treatment. Luminance of the device is 0.85cd / m ² at the black display, 50 cd / m ² in white display
And the contrast was 59. Example 3 The surface of a spherical spacer made of resin having an average diameter of 5 μm was treated with dodecyltrimethylammonium chloride to rehydrate the surface into vertical alignment.

Pixel display area 300 μm × 100 μm
The TFT (thin film transistor) active matrix substrate and the color filter substrate corresponding thereto were used to fabricate a liquid crystal display device to which the modified spacer was applied. The brightness of this device is 0.20 cd / m ² in black display and 4 in white display.
It was 3 cd / m ² and the contrast was 215. Comparative Example 3 A liquid crystal display device was produced in the same manner as in Example 1 except that the spacer surface was not subjected to vertical alignment treatment.

The brightness of this device is 0.48 cd / in black display.
m ² , the white display was 42 cd / m ² , and the contrast was 88.

[0022]

As shown in the embodiments, the liquid crystal display element of the present invention has the effect of improving display characteristics such as contrast by controlling the liquid crystal alignment disorder around the spacers.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a liquid crystal alignment state around a spacer of a liquid crystal display device according to the present invention, FIG. 1A shows a state in which no voltage is applied, and FIG. 1B shows a state in which voltage is applied.

FIG. 2 is a cross-sectional view showing a liquid crystal alignment state around a spacer of a conventional liquid crystal display device, and FIG.
B indicates the time when a voltage is applied.

[Explanation of symbols]

 1 glass 2 transparent electrode 3 alignment film 4 liquid crystal molecule alignment direction 5 spacer 6 alignment film 7 transparent electrode 8 glass

Claims (2)

[Claims] 1. A liquid crystal display device having a spacer for holding a cell gap and a liquid crystal substance sandwiched between two parallel plates having a transparent electrode on the inner surface thereof and an alignment film for liquid crystal on the electrode, wherein the spacer is aligned. A liquid crystal display device characterized by being processed. 2. The liquid crystal display device according to claim 1, wherein the spacer is a spherical or rod-shaped particle whose surface is vertically aligned.