KR19990016875A - Reflective color liquid crystal display using chiral nematic hybrid alignment structure - Google Patents

Abstract

The present invention relates to a reflective color LCD using a chiral nematic hybrid alignment structure, wherein a reflective color STN LCD having a hybrid alignment structure is formed, and the vertical alignment layer is rubbed in the same direction as the horizontal alignment to give direction to the vertical alignment. In addition, chiral additives are mixed to have a suitable rotational force in the liquid crystal to form a hybrid orientation LCD having a constant rotational direction, and green, blue, red, batting, and black can be displayed depending on the magnitude of the applied voltage, so that there is no backlight. It is easy to colorize the type LCD, which is advantageous for the show type of the LCD, the power consumption is reduced, and the process yield and the high magnetic behavior can be improved.

Description

Reflective color liquid crystal display using chiral nematic hybrid alignment structure

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflective color liquid crystal display (hereinafter referred to as LCD) using a chiral nematic hybrid alignment structure. In a liquid crystal display of a special hybrid alignment structure, the vertical alignment layer is rubbed in the same direction as the horizontal alignment. Directionality is given to the vertical alignment and the chiral additive is added to form an LCD having a constant orientation with appropriate rotational force in the liquid crystal, and thus green, blue, red, white and black can be displayed depending on the magnitude of the applied voltage. Reflective color liquid crystal using chiral nematic hybrid alignment structure that can easily form colorless reflective LCD, which is advantageous for miniaturization of LCD, power consumption is reduced, and process yield and device operation reliability can be improved It relates to a display device.

LCD, which is a kind of flat panel display device, uses an electro-optic effect in which the optical anisotropy is changed by applying an electric field to a liquid crystal that combines liquidity and optical properties of a crystal. Compared with the low power consumption, easy to thin and thin, large size and high definition are widely used.

In general, as shown in FIG. 1, the LCD panel has a failure turn 17 between a transparent electrode 13 pattern and two transparent substrates 11 coated with an alignment layer 15 capable of orienting liquid crystals. The liquid crystal 19 sealed by this is injected, and the polarizing plate 21 is affixed on the outer surface of both board | substrates.

The LCD panel, as described above, does not emit light by itself and displays the screen by using external light. The LCD panel is divided into a transmissive type and a reflective type according to the way the light is transmitted. It is used in the form of a module together with a light emitting device, for example, an electro luminescence (hereinafter referred to as EL) device or a light emitting diode (hereinafter referred to as LED) panel. The reflecting plate 23 which can reflect the reflected light again is attached.

Recently, LCD has been improved in quality and color needs to be attached to realize color. Existing color LCD has three colors of red (R), blue (B) and green (G). Use a color filter. Since each color constitutes one pixel, the number of pixels is three times larger than that of a black and white device, and the three colors are combined with each other to realize a desired color.

The color expression method using the color filter as described above is mainly used in the transmission type, there is a disadvantage that the brightness is sharply reduced when used in the reflection type.

The reason is that when the light transmitted from the outside passes through the color filter, all the colors other than the color are absorbed by the filter depending on which color the pixel passes through. For example, when light passes through a green pixel, all other colors other than green are absorbed by the filter, reducing the intensity of light to about one third. In the case of reflective LCDs, light is incident from the front of the LCD and the liquid crystal layer After passing through the color filter, the light is passed through the liquid crystal layer and the color filter by the reflector again, in which case the light passes through the color filter layer twice. For example, when the light passing through the green pixel is reflected when it is incident, the light is absorbed when passing through the blue pixel. Therefore, when the color filter is used in the reflective LCD, it is reduced to about 1/9 of the intensity of the incident light, which is very dark. In fact, when the color filter is applied to the reflective LCD, the display becomes very dark and display information is not distinguished. Therefore, only the transmissive LCD is used.

In the conventional super twisted nematic (hereinafter referred to as STN) LCD, there is a method of displaying color using birefringence of liquid crystal instead of a color filter without using a color filter. That is, as shown in FIG. 2, the principle of displaying the color differently by changing the applied voltage of d · ㅿ n, which is the product of the refractive index anisotropy n and the cell gap d of the liquid crystal, is displayed. Here, if the angle of the polarizing plate is properly controlled, the intensity of light transmitted or reflected by the change of d · ㅿ n is changed according to the wavelength. If the birefringence value is increased or decreased, the phenomenon that the wavelength of transmitted light periodically increases or decreases occurs, and the background color can be determined by appropriately adjusting d · ㅿ n. This phenomenon has been used as a basic feature in the existing STN LCD. Conventional monochromatic STN LCDs do not have such a high birefringence value, while the birefringence value is not large to use it for color implementation, while the birefringence value is used for the implementation of color. The effective birefringence of the liquid crystal exhibits a phenomenon such that the voltage is substantially reduced when a voltage is applied, because the orientation of the liquid crystal changes horizontally and vertically. For example, when all of the liquid crystals are in a vertically aligned state, birefringence is in a state in which there is no presence.

When the refractive index anisotropy of the liquid crystal is n and the cell gap is d, the birefringence value of the LCD is d · n. When the angle formed by the liquid crystal molecules from the horizontal is θ, the effective birefringence value is expressed by the following equation (1).

In this case, as the voltage increases, θ also increases, so the birefringence value of the LCD becomes smaller.

Reflective color LCD using birefringence according to the prior art as described above can realize only 3 or 4 colors, but since there is no color filter, there is no loss of light and the structure is similar to the conventional monochromatic LCD. It is simple and realizes all three or four colors in one pixel, so there is no need to triple the number of pixels as in the case of using a color filter.However, since all colors are spherical in one pixel, There is a sequence of colors implemented by increments. For example, as shown in the color coordinate of FIG. 2, the reflective color LCD using the conventional LCD is green depending on the voltage, as shown in FIG. blue Red White or white green blue The order is red. Therefore, the response time is defined as the time taken to move from the first color appearing to the newest appearing, which requires a relatively long time since the largest birefringence difference is required. Therefore, taking a mass-produced product as an example, the reflective color LCD has a problem that the response time is about 2 to 3 times longer than the monochromatic LCD is difficult to implement a moving image.

Therefore, the present invention is to solve the above problems, the object of the present invention is birefringent, but in the conventional STN LCD liquid crystal has a horizontal alignment structure in which the chiral additive is mixed with the liquid crystal to increase the torsion angle On the other hand, in the present invention, one side board has a horizontal alignment structure, and the other side board uses a hybrid alignment structure having a vertical alignment structure, thereby improving responsiveness, reducing driving voltage, and increasing the number of colors that can be displayed. The present invention provides a reflective color LCD using a chiral nematic hybrid alignment structure capable of improving process yield and device operation reliability.

1 is a cross-sectional view of a reflective STN LCD according to the prior art.

Fig. 2 is a graph of transmittance according to the change of d · ㅿ n of the reflective color LCD.

3 is a color coordinate of a reflective color STN LCD according to the prior art;

4 is a color coordinate of a reflective color STN LCD according to the present invention;

5 is a brightness graph according to an applied voltage of a reflective color STN LCD according to the present invention;

6 is a sectional view of a reflective color STN LCD according to the present invention;

* Explanation of symbols for the main parts of the drawings

11 substrate, 13 transparent electrode, 15 alignment film, 17 failed turn, 19 liquid crystal, 21 polarizing plate, 23 reflecting plate

The characteristics of the reflective color LCD using the chiral nematic hybrid alignment structure according to the present invention for achieving the above object,

In a reflective LCD having a hybrid alignment structure in which one side board is vertically aligned and the other board is horizontally aligned,

A chiral additive is added so that the ratio (d / p) of the cell gap d and the pitch p of the liquid crystal of the LCD is 0.4 to 1.

Hereinafter, a reflective color LCD using a chiral nematic hybrid alignment structure according to the present invention will be described in detail with reference to the accompanying drawings.

First, an important characteristic of the reflective color LCD according to the present invention is first, to align the liquid crystal in the horizontal and vertical direction on each of the two substrates, second, to give a direction through the rubbing process to the vertical alignment structure, third, to the liquid crystal Add chiral additives.

LCDs using hybrid alignment structures have been proposed in the related art, but such conventional hybrid alignment structures have little threshold voltage and thus are disadvantageous in multiple driving and the like, and are not particularly applicable to general devices due to poor viewing angle characteristics. In order to eliminate the problem, chiral additives were mixed in the liquid crystal fabric to cause distortion in the hybrid alignment structure.

At this time, it is difficult to find the torsion angle having the optimal electro-optic properties by appropriately adjusting the concentration of the chiral additive. The optical properties are most sensitive to d · ㅿ n and should have about 1200 to 3000 nm and the substrate surface orientation characteristics. It is also sensitive to surface fixation energy.

In order to horizontally align the liquid crystal substrate, rubbing, which is a process of constantly rubbing a polymer alignment film such as polyamide in one direction, is used. For the vertical alignment, an alignment material such as lecitin is used. Although not necessary, in the present invention, the surface of the vertical alignment layer was rubbed in one direction.

This is because when the nematic liquid crystal containing chiral material is injected into the hybrid alignment LCD, the liquid crystal makeup has a constant torsion angle. The torsion is stabilized because one substrate is in a horizontal orientation, while the other substrate is in a vertical orientation. This does not exist, so the direction of torsion cannot be accurately defined.

The nematic liquid crystal added with chiral additives shows a domain in which two or more alignment states are co-conditioned in a hybrid alignment LCD in which the vertical alignment substrate is not rubbed. The different alignment states have a constant area. The area is changed by the applied voltage, shock, or heat.

According to the experimental results of the present inventors, it can be seen that the two areas are due to the difference in the torsion steel, the torsion angle of the two areas differ by as much as 180 ℃.

The causes of the torsional angle difference are as follows.

That is, if the pitch of the chiral nematic liquid crystal formed by the chiral additive is p and the cell gap is d, the bit angle of the liquid crystal in the LCD is determined by d / p, and the twist angle of the liquid crystal in the LCD is When θ is expressed, the relation as in Equation 2 is established.

However, in the horizontal alignment substrate, since the liquid crystal has a property to be parallel to the rubbing direction, it does not have such a torsion angle. When the rubbing direction is set to 0 °, the torsion angle must be a multiple of 180 °. It should have a value of 180 °. Therefore, according to the size of d / p, the torsion angle is given in the range of equation 3, preferably has a value of 0.4≤d / p≤1.

In particular, in a hybrid alignment LCD, two twists always coexist according to d / p since one substrate has a vertical alignment structure.

In order to remove such a region, it is necessary to give a direction to the vertical alignment structure so that the vertical alignment layer is rubbed in one direction. The rubbed vertical alignment layer has a vertical alignment phase irrespective of the rubbing direction when no external force is required, and when the force is applied from the outside, the director of the liquid crystal molecules is deformed and the directional effect due to rubbing is exhibited.

In addition, the rotational force due to the chiral additive gives the liquid crystal molecules a force to rotate in one direction, thereby generating an effect due to rubbing in the vertical alignment structure. In fact, according to the experimental results of the present inventors, the above areas were removed when the vertical alignment layer was rubbed in a chiral hybrid alignment LCD.

The liquid crystal used in the LCD of the present invention has a positive dielectric anisotropy? Ε.

In the case of εε0, when the voltage is applied, the hybrid alignment structure is shifted to the almost vertical alignment structure, and d ㅿ n is effectively reduced to show characteristics similar to the conventional products. The hybrid alignment structure is transferred to the horizontal alignment structure, so that d ㅿ n is increased.

In one embodiment of the present invention, a chiral hybrid alignment LCD was manufactured using liquid crystal RDP-61408 and chiral additive S-811 manufactured by LODIC of Japan. The refractive index anisotropy of the liquid crystal was n = 0.2082, the LCD cell gap was set to 10 µm, and chiral additive S-811 was mixed with the liquid crystal such that d / p = 0.5.

In addition, one substrate was rubbed by applying polyamide SL-1054 manufactured by Japan Synthetic Rubber for horizontal orientation, and the other substrate was rubbed by applying polyamide JALS-203 manufactured by Japan Synthetic Rubber for vertical alignment. The two substrates were rubbed with rubbing directions parallel to each other.

In addition, the polarizers were attached to the front and rear of the LCD so that the polarizers were perpendicular to each other. The direction of one polarizer was 45 ° to the rubbing direction, and a reflective plate was attached to the rear polarizer.

Then, the results of measuring the color characteristics of the LCD according to the present invention is shown in Figure 4, Figure 4 is a color coordinate of the LCD according to the present invention, in this state by applying a voltage to the LCD as the voltage increases to the color LCD Represents green, blue, red, white, and black. White and black are not distinguished in color coordinates and are evaluated by the brightness of the LCD.

As a result of measuring the brightness of the LCD, as shown in Fig. 4, the voltage becomes the maximum brightness at about 2.4V, and the color at this time corresponds to white, and the brightness decreases rapidly as the voltage is further increased. On the color coordinate, the color converges by passing 2.4V, and the color becomes black at this time.

Therefore, in the present embodiment, the reflective color LCD using the chiral hybrid alignment structure can display five colors, but as shown in FIG. 3 in which the color coordinate characteristics of the conventional reflective color STN LCD are shown, the conventional reflective type Color LCD The nematic LCD can display only four colors, and therefore, a characteristic capable of displaying both white and black is one of the important advantages of the present invention.

As described above, the reflective color LCD which has a chiral nematic hybrid alignment structure according to the present invention has a hybrid alignment structure, but rubs the vertical alignment layer in the same direction as the horizontal alignment to give direction to the vertical alignment. Reflective LCD without backlight, by mixing chiral additives to have rotational force in liquid crystal to form hybrid orientation LCD with constant rotation direction, and green, blue, red, white and black depending on applied voltage It is advantageous to miniaturize the LCD by easily forming the color of the color, to reduce the power consumption, and to improve the process yield and the reliability of device operation.

Claims (3)

In a reflective LCD having a hybrid alignment structure in which one side substrate is vertically aligned and the other substrate is horizontally aligned, an alignment layer of a vertically aligned substrate that is rubbed in the same direction as the horizontal alignment direction, and a cell gap d of the liquid crystal of the LCD. A chiral additive is added so that the ratio (d / p) of the pitch (p) of a liquid crystal is 0.4-1, The reflective color LCD using the chiral nematic hybrid orientation structure characterized by the above-mentioned. The reflective color LCD using a chiral nematic hybrid alignment structure according to claim 1, wherein the dielectric anisotropy of the liquid crystal has a positive value. The reflective color LCD according to claim 1, wherein the product of the refractive index anisotropy n of the liquid crystal and the cell gap has a value of d · n n = 1200 to 3000 nm.