KR100816337B1 - Liquid crystal display - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, in order to improve visibility, a compensation film having an optical axis coinciding with the rubbing direction is provided between the retardation plate and the upper substrate. In the liquid crystal display device according to the present invention, the first substrate and the second substrate correspond to each other, a first alignment layer having a first rubbing direction is formed on an inner surface of the first substrate, and a second rubbing on the inner surface of the second substrate. A second alignment film having a direction is formed. A liquid crystal layer is interposed between the first alignment film and the second alignment film, and the first and second retardation plates are attached to the outside of the first substrate and the second substrate, respectively. The first and second polarizing plates are disposed outside the first and second retardation plates, respectively, and have optical axes corresponding to the first rubbing direction and the second rubbing direction, respectively, between the second substrate and the second retardation plate. First and second retardation compensation films are located.

Black-white contrast ratio, black luminance, retardation compensation film

Description

Liquid crystal display {LIQUID CRYSTAL DISPLAY}

1 and 2 illustrate a liquid crystal voltage-reflectance curve and a liquid crystal voltage-transmittance curve, respectively, according to a change in the twist angle of a liquid crystal in a reflection mode and a transmission mode of a twisted nematic liquid crystal display (TN) type. Will,

3 shows the polarization axis of the vertically staggered polarizing plate, the rubbing direction of the alignment layer, and the twist angle of the liquid crystal,

4 is a structural diagram of a liquid crystal display according to an exemplary embodiment of the present invention;

5 illustrates a rubbing direction of an alignment layer, a polarization axis of a polarizing plate, and an axial direction of a phase difference compensation film in a liquid crystal display according to an exemplary embodiment of the present invention.

6 schematically illustrates a light transmission model in an on voltage state in a liquid crystal display according to an exemplary embodiment of the present invention.

The present invention relates to a liquid crystal display device.

The liquid crystal display is one of the flat panel displays widely used at present. The liquid crystal display is composed of two substrates on which two electrodes facing each other are formed and a liquid crystal layer interposed therebetween. The liquid crystal molecules of the liquid crystal layer display an image in a manner of controlling the amount of light transmitted through the liquid crystal layer. Here, two opposite electrodes may be formed on one of two substrates.

The liquid crystal display device may be of a transmissive mode type that transmits light to display an image, a reflective mode type that reflects light to display an image, or a transflective type suitable for the transmission mode and the reflection mode.

By the way, in the transflective or reflective liquid crystal display device, since the optical path in the reflection mode is twice the optical path in the transmission mode and thus the light loss is large, when the high quality is displayed, the luminance is insufficient, that is, the reflectance and There is a problem that visibility is lowered due to a decrease in transmittance.

An object of the present invention is to provide a liquid crystal display device having improved visibility.

In order to solve this technical problem, in the present invention, a compensation film having an optical axis coinciding with the rubbing direction is provided between the retardation plate and the upper substrate.

Specifically, in the liquid crystal display device according to the present invention, the first substrate and the second substrate correspond to each other, a first alignment layer having a first rubbing direction is formed on an inner surface of the first substrate, and an inner surface of the second substrate. A second alignment film having a second rubbing direction is formed. A liquid crystal layer is interposed between the first alignment film and the second alignment film, and the first and second retardation plates are attached to the outside of the first substrate and the second substrate, respectively. The first and second polarizing plates are disposed outside the first and second retardation plates, respectively, and have optical axes corresponding to the first rubbing direction and the second rubbing direction, respectively, between the second substrate and the second retardation plate. First and second retardation compensation films are located. At this time, the first rubbing direction and the second rubbing direction may form an angle smaller than 90 degrees.

Here, the first substrate may have a reflective electrode and a transmissive electrode. At this time, it is preferable that the first and second retardation compensation films have a phase retardation value of 2 times or more and 4 times or less of the phase retardation value of the liquid crystal occurring near the first and second substrates. In addition, at least one of the first rubbing direction and the second rubbing direction may be parallel to or perpendicular to the polarization axis of the first polarizing plate, and the first and second retardation compensation films may have negative birefringence characteristics.

Hereinafter, with reference to the accompanying drawings will be described the present invention.

1 and 2 illustrate a liquid crystal voltage-reflectance curve and a liquid crystal voltage-transmittance curve, respectively, according to a change in the twist angle of a liquid crystal in a reflection mode and a transmission mode of a twisted nematic liquid crystal display (TN) type. will be.

1 and 2, it can be seen that in both the reflection mode and the transmission mode, when the twist angle of the liquid crystal is smaller than 90 degrees, the reflectance and the transmittance are greatly increased. On the other hand, although not shown in the figure, there is an advantage that the response speed also increases when the twist angle of the liquid crystal is reduced.

However, when the torsion angle of the liquid crystal is smaller than 90 degrees, it can be seen that the luminance of the black state also increases as the liquid crystal voltage increases, that is, as a voltage for displaying the black state is applied. This means that the ratio of the black-insulation film is reduced.

When a voltage is applied to the liquid crystal display of the TN mode, the liquid crystal molecules inside the cell are arranged perpendicular to the substrate, but the liquid crystal molecules near the substrate adhere to the surface of the alignment layer and thus maintain the existing horizontal arrangement despite the application of the electric field. Therefore, there is a portion where the liquid crystal director gradually changes between the vicinity of the alignment film surface and the bulk liquid crystal. That is, in the bulk liquid crystal, the liquid crystal molecules stand vertically, even if a sufficient voltage is applied, the liquid crystal molecules near the substrate are inclined with respect to the substrate while being twisted toward the rubbing direction of the alignment film due to the alignment regulating force of the alignment film formed on the substrate. Therefore, phase retardation of light occurs in the vicinity of the substrate.

In the liquid crystal display device when the twist angle of the liquid crystal is 90 degrees, since the director of the liquid crystal molecules near the substrate is perpendicular to or parallel to the polarization axis of the polarizing plate, there is almost no light leakage phenomenon caused by the phase retardation of the light. not.

However, when the torsion angle of the liquid crystal is not 90 degrees, even if the polarization axis of the polarizing plate coincides with the rubbing direction of one substrate, as shown in FIG. Can not. Therefore, light leakage phenomenon inevitably occurs due to the phase retardation of light in the vicinity of the substrate. This is because the luminance of the black state rapidly increases when the twist angle of the liquid crystal is changed.

For this reason, reducing the twist angle of the liquid crystal can increase the reflectance and transmittance, but also increases the luminance of the black state. Therefore, it is difficult to realize the improvement of the visibility of a liquid crystal display device only by adjusting the twist angle of a liquid crystal.

In the present invention, a phase difference compensation film is used to reduce the torsion angle of the liquid crystal to realize the improvement of reflectance and transmittance, and to simultaneously suppress the increase of the luminance in the black state to realize the improvement of the black-white contrast ratio.

The method of suppressing the increase in the luminance of the black state prevents the alignment of the liquid crystal on the surface of the alignment film to achieve isotropy of the liquid crystal medium, or compensate for the phase delay of light generated by the alignment of the liquid crystal in the vicinity of the substrate. will be. In the present invention, a phase difference compensation film is used to compensate for the phase difference generated in the liquid crystal layer. The most common method is to attach a retardation compensation film having a phase retardation of a polarity opposite to the polarity corresponding to the phase retardation of light in the vicinity of the substrate. That is, the phase retardation of the light generated by the alignment fixation of the liquid crystal molecules near the substrate is compensated in the opposite direction by using the phase difference compensation film.

The retardation compensation film has the same configuration as the compensation film using the polymer liquid crystal or the liquid crystal layer of the liquid crystal display device, but does not drive. The twist angle and direction of the retardation compensation film can be realized by rubbing, and can be defined by the alignment film.

Since the twisted nematic liquid crystal has positive birefringence, it is preferable to compensate for this by using a phase difference compensation film having negative birefringence. That is, the retardation of the light caused by the liquid crystal molecules is compensated for by using a phase difference compensation film in which the liquid crystals are arranged in a direction opposite to the twist direction of the liquid crystal molecules near the substrate of the liquid crystal display device.

4 and 5 are views for explaining a liquid crystal display according to a first exemplary embodiment of the present invention. FIG. 4 is a structural diagram of a liquid crystal display, and FIG. 5 is a rubbing direction of an alignment layer, a polarization axis, and a phase difference of a polarizing plate. Correlation with the compensation film is shown.

In the liquid crystal display according to the exemplary embodiment of the present invention, there is a pair of lower and upper substrates 101 and 102 having the alignment layers 11 and 12 coated on an inner surface thereof, and a liquid crystal layer between the two substrates 101 and 102. (10) is interposed. This liquid crystal layer 10 has a torsion angle smaller than 90 degrees.

A phase difference plate, for example, a lower quarter wave plate 301 that provides a phase difference of −45 degrees, is attached to the outside of the lower substrate 101, and is outside the lower quarter wave plate 301. The lower polarizer 401 is provided.

The upper quarter wave plate 302 is provided outside the upper substrate 102 to provide a 45 degree phase difference, but there are two sheets between the upper quarter wave plate 302 and the upper substrate 102. Retardation compensation films 201 and 202 are attached. These retardation compensation films 201 and 202 have a birefringent polarity opposite to that of the birefringent polarity of the liquid crystal layer 10. Each of the retardation compensation films 201 and 202 is configured to have an optical axis that matches the rubbing direction of the alignment films 11 and 12 formed inside the upper substrate 102 and the lower substrate 101.

When the electric field is applied to the liquid crystal display device having such a structure, the phase retardation of light caused by the liquid crystal molecules near the surfaces of the alignment films 11 and 12 can be suppressed by the two phase difference compensation films 201 and 202.

As shown in Fig. 5, in the reflection mode, the phase delay actually acting on the optical system is twice the sum of the phase delay in the vicinity of the upper substrate and the phase delay in the vicinity of the lower substrate. Therefore, when the thickness of the retardation compensation film is set to be twice the phase retardation value occurring in such a liquid crystal cell, it is possible to compensate for the phase retardation occurring in the liquid crystal cell in the reflection mode. In this case, the system including the phase difference compensation film and the liquid crystal cell in the black state acts as an isotropic medium with respect to the incident light. Therefore, after the reflected light passes through the upper quarter wave plate, the light becomes completely linearly polarized (the polarization direction is changed by 90 degrees) and is completely blocked by the upper polarizing plate 402, thereby greatly reducing the luminance in the black state.

 On the other hand, since the light paths are different in the transmission mode and the reflection mode, it may not be possible to achieve black luminance minimization simultaneously in both modes. When the retardation compensation film is suitably set in the reflection mode, the compensation of the phase delay is excessive in the transmission mode. When the retardation compensation film is set properly in the transmission mode, the compensation of the phase delay is insufficient in the reflection mode.

Therefore, assuming that the phase retardation value of light with respect to the liquid crystal on the substrate surface is a, the phase retardation value of the retardation compensation film is set to 4a in the reflection mode, and the phase retardation value of the retardation compensation film is set to 2a in the transmission mode. Is optimal. As mentioned, the light passes twice in the manner of incidence and reflection near the surfaces of the two substrates in the reflection mode, but once in the transmission mode passes in the vicinity of the surfaces of the two substrates.

FIG. 6 schematically illustrates a light transmission model in an ON voltage state in a reflection mode and a transmission mode in a conceptual structure of a liquid crystal display according to an exemplary embodiment of the present invention.

The basic structure of the liquid crystal display device has been described with reference to FIG. 4.

In the liquid crystal display panel 100, the reflective electrode 5 is formed in the "reflection mode", and the transparent electrode 6 is formed in the "transmission mode".

Here, the liquid crystal layer 10 is set so that the twist angle of the liquid crystal is 90 degrees or less in order to improve the transmittance and the reflectance, which is rubbing of the alignment layers 11 and 12 formed on the inner surfaces of the two substrates 101 and 102. This can be achieved by adjusting the direction.

Here, the lower quarter wave plate 301 is designed to provide a phase difference of -45 degrees to the light, and the upper quarter wave plate 302 is designed to provide a phase difference of 45 degrees to the light, and the lower polarizing plate 401 is provided. And the upper polarizer 402 are disposed to have polarization axes that cross vertically.

The backlight 500, which is a light source, is installed outside the lower polarizer 401.

Next, a model of the on voltage state in the liquid crystal display according to the exemplary embodiment of the present invention will be described.

First, the light transmission model in the ON state in the "transmission mode" will be described.

Unpolarized light from the backlight device 500 passes through the lower polarizer 401 to become first linear polarized light having the same axis as that of the lower polarizer 401.                     

The first linearly polarized light becomes left linear circular polarized light while passing through the lower quarter wave plate 301 providing a phase difference of -45 degrees.

The left linear circular polarization passes through the system including the liquid crystal display panel 100 and the phase difference compensation films 201 and 202 serving as an isotropic medium. At this time, when the phase delay compensation by the phase difference compensation films 201 and 202 is not completely performed, polarized light having a shape close to a perfect circular polarized light can be obtained.

This left linear circular polarization passes through the upper quarter wave plate 302 providing a phase difference of 45 degrees, and becomes linear polarization or elliptical polarization close to linear. For convenience of description, this light is referred to as second linearly polarized light. Since the second linearly polarized light has the same optical axis as the first linearly polarized light, the second linearly polarized light is absorbed by the upper polarizing plate 402 having a polarizing axis perpendicular to the polarizing axis of the lower polarizing plate 401, thereby showing a black display. In this case, since the second linearly polarized light is a completely linearly polarized light or an elliptical polarized light that is almost close to the linearly polarized light, the second linearly polarized light is all blocked or almost blocked by the upper polarizer 402. Therefore, the brightness of the black state is greatly reduced.

Next, the light transmission model in the ON state in the "reflection mode" will be described.

Natural light that enters through the liquid crystal display panel 100 from the outside passes through the upper polarizer 402 and becomes third linearly polarized light having the same optical axis as that of the upper polarizer 402. This third linearly polarized light becomes preferential circularly polarized light while passing through the upper quarter wave plate 302 which provides a 45 degree phase difference.

This preferential circularly polarized light enters and reflects a system including the liquid crystal display panel 100 and the phase difference compensation films 201 and 202 which act as an isotropic medium to become left linear circularly polarized light. At this time, when the phase delay compensation by the retardation compensation films 201 and 202 is not completely performed, polarized light having a shape almost close to circular polarized light can be obtained.

This preferential circularly polarized light becomes linearly polarized light or near elliptically polarized light while passing through the upper quarter wave plate 302 providing a phase difference of 45 degrees. For convenience of description, such light is referred to as fourth linearly polarized light. Since the fourth linearly polarized light has an optical axis perpendicular to the third linearly polarized light, the fourth linearly polarized light is absorbed by the upper polarizing plate 402 having a polarization axis parallel to the optical axis of the third linearly polarized light, thereby showing a black display. In this case, since the fourth linearly polarized light is a completely linearly polarized light or an elliptical polarized light almost close to the linearly polarized light, all of the fourth linearly polarized light is blocked or almost blocked by the upper polarizer 402. Therefore, the brightness of the black state is greatly reduced.

The present invention as described above can be applied to all types of liquid crystal display devices having a reflection mode or a transmission mode.

In the present invention, the reflectance and the transmittance can be improved by reducing the twist angle of the liquid crystal, and the increase in the luminance of the black state can be suppressed by using the phase difference compensation film. Therefore, the contrast ratio of black-white can be improved and the visibility of a liquid crystal display device can be improved.

Claims (6)

A first substrate; A second substrate corresponding to the first substrate; A first alignment layer formed on an inner surface of the first substrate and having a first rubbing direction; A second alignment layer formed on an inner surface of the second substrate and having a second rubbing direction; A liquid crystal layer interposed between the first alignment layer and the second alignment layer; First and second retardation plates attached to the outside of the first substrate and the second substrate, respectively; First and second polarizing plates disposed outside the first and second retardation plates, respectively; And First and second retardation compensation films positioned between the second substrate and the second retardation plate and having optical axes corresponding to the first rubbing direction and the second rubbing direction, respectively; Liquid crystal display comprising a. In claim 1, And the first rubbing direction and the second rubbing direction form an angle greater than 0 degrees and less than 90 degrees. In claim 1, The first substrate includes at least one of a reflective electrode and a transmissive electrode. The method of claim 1 or 2, The sum of the phase retardation value of the first retardation compensating film and the phase retardation value of the second retardation compensating film is two to four times less than the phase retardation value of the liquid crystal occurring near the first substrate or the second substrate. . In claim 1, And at least one of the first rubbing direction and the second rubbing direction is parallel or perpendicular to the polarization axis of the first polarizing plate. In claim 1, The first and second retardation compensation films have negative birefringence characteristics.

Images