KR20010045641A - Antiferroelectric liquid crystal display for reflexive type - Google Patents

Abstract

PURPOSE: A reflective antiferroelectric LCD(liquid crystal display) is to use a polarizing plate and a phase delaying plate to provide a high iso-contrast ratio and luminance, a wide viewing angle and a fast responding characteristic. CONSTITUTION: A metal electrode(20) is formed on a strip of a first substrate(10). A transparent electrode(40) is formed on a strip of a second substrate(30) orthogonal to the metal electrode. An alignment film(50) is coated on the second substrate. Antiferroelectric liquid crystal is injected between the first and second substrates. A phase delaying plate(70) is formed on the second substrate on which the electrode is not formed. A polarizing plate(80) is formed on the phase delaying plate. Each molecule of the liquid crystal has an inclined angle between 20 to 60. A multiplying value of a thickness of the antiferroelectric liquid crystal and an optical anisotropy is between one eights of a wavelength and a half of the wavelength.

Description

Reflective antiferroelectric liquid crystal display {Antiferroelectric liquid crystal display for reflexive type}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflective antiferroelectric liquid crystal display, and more particularly, to an antiferroelectric liquid crystal display having high contrast ratio, high luminance, wide viewing angle, and high response by using one polarizing plate and a phase delay plate.

The liquid crystal used in the conventional reflective liquid crystal display is mainly nematic liquid crystal, and there are various modes related thereto. However, the reflective liquid crystal display has a disadvantage of slow response speed and narrow viewing angle. Recently, an antiferroelectric liquid crystal display (AFLCD) has been disclosed, which has the potential to be a large area liquid crystal display. This is a transmissive type that has a wide viewing angle and a fast response speed but a relatively high driving power, and requires a backlight to be installed on the rear of the liquid crystal display.

Unlike the conventional transmissive liquid crystal display, in which the backlight is bonded to the rear, the reflective color liquid crystal display, which has been in the spotlight recently, uses ambient light instead of the backlight by attaching a mirror to the rear, and thus there is no power consumption due to the backlight. This light weight makes it suitable for portable personal digital assistant displays.

The reflective color liquid crystal display needs to have high brightness, high resolution, high contrast ratio, excellent gradation display characteristics, and fast response time with video speed.

However, since the early reflection type liquid crystal display uses two polarizing plates, the luminance and contrast ratio are low. In addition, due to the structure in which the mirror is bonded to the rear surface of the display device, a gap is generated between the liquid crystal layer and the reflecting plate, which causes aberration of incident light and reflected light passing through different pixels. In the case of a color liquid crystal display, the resolution and color purity have a fatal effect.

Therefore, the development of a new reflective color liquid crystal display device using a polarizer and consisting of an inner metal reflector, a scattering thin film, a phase delay thin film, and the like, has recently been reported.

However, although the reported reflection type liquid crystal display has improved luminance and contrast ratio to some extent, light transmission characteristics are largely dependent on wavelength, applied voltage, and viewing angle, and its limitations are revealed due to narrow viewing angle and slow response speed.

Projection type antiferroelectric liquid crystal panel under recent development is characterized by having wider viewing angle and faster response speed than conventional nematic liquid crystal. The response time of anti-ferroelectric liquid crystals is short, from tens to hundreds of microseconds, whereas the response speed of thin film transistor (TFT) -LCDs is tens of microseconds, and the response speed of super-twisted nematic (STN) -LCDs is also tens of microseconds. It is almost impossible to display a fast video at a response speed of several tens of Hz. Anti-ferroelectric liquid crystals have not only fast response speed but also tristab (ility) having memory characteristics.

Using this feature, a simple matrix drive is possible. The production cost is lower than TFT-LCD because TFT is not used. The characteristics of the simple matrix antiferroelectric liquid crystal panel provide higher contrast ratio and image quality than the STN-LCD panel using the simple matrix method. In most cases, the contrast ratio is between 30: 1 and 50: 1, compared to 10: 1 and 20: 1 for STN-LCDs. This is possible by using the antiferroelectric memory characteristic. When a voltage equal to or higher than the threshold voltage is applied to the liquid crystal layer, the device is turned on and maintained in the on state due to memory characteristics even when the voltage is considerably lowered. Conventional TN-LCD and STN-LCD panels have a problem that the state changes when the voltage drops. For example, if it is black in the off state and white in the on state, when the voltage decreases, the white color gradually approaches black over time. Because of this characteristic, the contrast ratio is relatively low, but has a gray scale display function.

Semiferroelectric liquid crystals have a disadvantage in that gray scale display is difficult to achieve due to memory characteristics. However, recently, a method has been found to maintain an intermediate state in which a stable ferroelectric phase and an antiferroelectric phase coexist.

In this way, the memory effect and gradation display advantages can be used simultaneously.

One of the best advantages of the antiferroelectric liquid crystal is its wide viewing angle of about 160 degrees, which is surprising when compared to the 40 and 100 degrees of conventional nematic liquid crystals. This is because the liquid crystal molecules in which the molecular tilt direction is alternately arranged between the glass substrates are rotated to create an on / off state.

The anti-ferroelectric liquid crystal display has a transmissive type that requires a backlight to be installed on the back of the liquid crystal display despite the excellent characteristics of overcoming the disadvantages of the conventional liquid crystal display. Therefore, high power consumption and high weight are not suitable for use in portable liquid crystal display devices.

An object of the present invention is to use a semi-ferroelectric liquid crystal to solve the problems of the prior art, and by using a single polarizing plate and a phase delay plate, a reflection type antiferroelectric liquid crystal having high contrast ratio, high brightness, wide viewing angle and fast response characteristics. It is to provide a display device.

1 is a cross-sectional view of a reflective antiferroelectric liquid crystal display device according to the present invention;

2 is a diagram showing the simulation of the intensity of the reflected light according to the thickness of the anti-ferroelectric liquid crystal cell and the phase difference of the phase delay plate, where a) is an antiferroelectric phase having alternating molecular tilt angles of 24.9 degrees and -24.9 degrees, and b) 24.9. C) is a ferroelectric phase having a molecular tilt angle of -24.9 degrees.

3 is a graph showing the intensity of reflected light according to an applied voltage.

4 is a diagram showing the intensity of reflected light according to an applied voltage on a time axis;

5 shows the contrast ratio of a reflective antiferroelectric liquid crystal cell;

** Description of symbols for the main parts of the drawing **

10. Glass substrate 20. Metal electrode

30. Second substrate 40. Transparent electrode

50. Alignment layer 60. AFLC

70. Phase delay plate 80. Polarizer plate

In order to achieve the above object, the apparatus of the present invention includes a first substrate on which a metal electrode on a strip provided as a reflecting plate is formed, and a second substrate on which a transparent electrode on a strip arranged orthogonally to the metal electrode is formed, and an alignment film is coated. And a semi-dielectric liquid crystal injected between the first and second substrates on which the surfaces on which the electrodes are formed face each other, a phase delay plate disposed on a surface on which the electrodes of the second substrate are not formed, and a polarizing plate formed on the phase delay plates. Characterized in that provided.

Here, the molecular tilt angle of the liquid crystal is preferably 45 degrees, and in this embodiment is 24.9 degrees. The thickness of the antiferroelectric liquid crystal is preferably a product having a quarter wave plate with optical anisotropy (birefringence), which is about 2 m in this embodiment. The optical axis of the phase delay plate is rotated by [pi] / 4 with respect to the optical axis of the polarizing plate.

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

1 shows the configuration of a reflective antiferroelectric liquid crystal display device according to the present invention. In the liquid crystal display of the present invention, a first glass substrate 10 having a metal electrode 20 formed on a strip provided as a reflecting plate, and a transparent electrode 40 formed on a strip disposed perpendicular to the metal electrode 20 are formed. And a second substrate 30 coated with the alignment layer 50.

That is, the metal electrodes 20 are deposited in a band shape on the entire surface of the first glass substrate 10. Here, the metal electrode 20 serves as a reflection plate at the same time as the electrode, it is located directly below the liquid crystal can prevent aberration. In addition, in order to prevent a decrease in contrast ratio and resolution due to low reflection of light incident on a region where the metal electrode 20 is not formed, it is preferable to encode a material having high reflectance. A first glass substrate having a uniform reflective film formed thereon and a transparent electrode formed thereon may be used.

A transparent electrode 40 such as indium thin oxide (ITO) is arranged on the second substrate 30 so as to be orthogonal to the metal electrode 20 of the first substrate 10. Then, a polyimide such as AL1051 manufactured by Japanese Synthetic Rubber Co., Ltd. is coated thereon and rubbed in a predetermined direction to form a horizontal alignment layer 50.

A spacer is inserted between the two sheets of first and second substrates 10 and 30 configured as described above to maintain the gap between the two substrates at about 2 μm, and then the antiferroelectric liquid crystal 60 is injected therebetween and sealed. Here, AFLC uses CS40001 etc. by Chisso Petrochimcal. The polarization Ps of this AFLC was -79.8 nC / cm 2, and the molecular tilt angle was 24.9 degrees.

The phase delay plate 70 is disposed on the front surface of the AFLC cell configured as described above, that is, the front surface of the second substrate 30, and the polarizer plate 80 is disposed on the phase delay plate 70.

The optical axis of the polarizing plate 80 is parallel to the optical axis of the AFLC 60. The optical axis of the phase delay plate 70 is rotated by π / 4 with respect to the optical axis of the polarizing plate 80. The phase delay plate 70 has an image delay of λ / 4 of incident light, for example, 136 nm in the case of 544 nm incident light.

Incident light transmitted through the polarizing plate is linearly polarized. As this light penetrates the phase delay plate, the light is changed into a circularly polarized state by the optical characteristic. In the antiferroelectric state of AFLC, even though light passes through the liquid crystal layer, optical changes are not made. Light passing through the liquid crystal layer is reflected by the metal electrode to change only the direction of circularly polarized light. That is, the left circularly polarized light is changed to the preferential circularly polarized light, and the preferential circularly polarized light is changed to the left circularly polarized light.

Light reflected from the metal electrode passes through the liquid crystal layer and the phase delay plate again. When the circularly polarized light passes through the phase delay plate, the light is changed back to linearly polarized light, but the polarization direction is perpendicular to the polarization direction of the light incident and transmitted through the polarizer. As the linearly polarized light passes through the phase delay plate twice, the polarization direction is rotated by 90 degrees. Therefore, the reflected light does not penetrate through the polarizing plate on the front surface, and thus becomes dark.

On the contrary, in the state where the liquid crystal layer is transferred to the ferroelectric phase by the voltage, the liquid crystal layer has an effective birefringence of a certain size. When light passing through the phase delay plate is incident on the liquid crystal layer, the optical effect is different depending on the magnitude of the phase difference of the liquid crystal layer.

For example, if the phase difference of the liquid crystal layer is lambda / 4 similar to the phase delay plate, the phase delays of the liquid crystal layer and the phase delay plate cancel each other out. Therefore, the polarization state of the light does not change at all, is reflected in the linearly polarized state, and the reflected light passes through the polarizing plate through the liquid crystal layer and the phase delay plate.

2 is a diagram showing the simulation of the intensity of the reflected light according to the thickness of the anti-ferroelectric liquid crystal cell and the phase difference of the phase delay plate, where a) is an antiferroelectric phase having alternating molecular tilt angles of 24.9 degrees and -24.9 degrees, and b) 24.9. C) represents a ferroelectric phase having a molecular tilt angle of -24.9 degrees.

Figure 2 simulates the intensity of the reflected light of the reflective AFLC cell of the present invention by the 2 x 2 Jones matrix method. Here, the inclination angle of the AFLC is 24.9 degrees and the birefringence is 0.088.

In the case of the antiferroelectric phase of Fig. 2 (a), antireflection (0) and total reflection (1) appear as horizontal lines at 135.9 nm, which is 1/4 wavelength, and 271.8 nm, which is half wavelength, for incident light of 543.5 nm.

In the ferroelectric phases of FIGS. 2 (b) and 2 (c), it can be seen that the intensity of the reflected light is 1 at the intersection point of 2.3 μm in thickness and 135.9 nm having a quarter wavelength. Therefore, since the reflectivity is 0 for the antiferroelectric phase and the reflectivity is 1 for the ferroelectric phase at a thickness of 2.3 μm and a quarter wavelength of 135.9 nm, a high contrast ratio can be obtained.

3 shows the intensity of the reflected light according to the applied voltage.

In FIG. 3, the solid line represents the simulation result, and the points represent the experimental result. It can be seen that the intensity of the reflected light is slightly increased by the propagation effect below the threshold value of 9V / μm, but rapidly increases above the threshold value. The bright state is at 10V / μm. Thus, bistable switching between darkness and light can be achieved through the transition from the antiferroelectric phase to the ferroelectric phase induced by the electric field. When using antiferroelectric liquid crystals having a large propagation effect, limited gray scale display can be achieved.

4 shows the intensity of the reflected light according to the applied voltage on the time axis. That is, in the present invention, when the electric field is applied, it has a response characteristic of 0.34 데 to change from a dark state to a bright state, and has a response characteristic of 4.72 ㎳ to change from a bright state to a dark state. Therefore, the video speed has the fastest response speed possible.

5 shows the iso-contrast ratio of reflective AFLC cells. As shown in FIG. 5, the symmetrical and wide viewing angle characteristics are shown. And the contrast ratio shows 5: 1 or more under white light.

As described above, the present invention implements a reflective antiferroelectric liquid crystal display device using a single polarizing plate and a phase delay plate to enable a high contrast ratio, a wide viewing angle, and a fast response speed in a portable device. A small display device can be provided.

Claims (7)

A first substrate on which a metal electrode on a strip provided as a reflecting plate is formed; A second substrate on which a transparent electrode on a strip disposed orthogonal to the metal electrode is formed, and an alignment layer is coated; A semi-electric liquid crystal injected between the first and second substrates with the electrodes formed to face each other; A phase delay plate disposed on a surface where the electrode of the second substrate is not formed; And A reflective antiferroelectric liquid crystal display device comprising a polarizing plate formed on the phase delay plate. The reflective antiferroelectric liquid crystal display device according to claim 1, wherein the molecular tilt angle of the liquid crystal is between 20 and 60 degrees. The reflective antiferroelectric liquid crystal display according to claim 1, wherein the product of the thickness of the antiferroelectric liquid crystal and the optical anisotropy (birefringence) is between 1/8 wavelength and 1/2 wavelength of incident light. The reflective antiferroelectric liquid crystal display device according to claim 1, wherein the phase delay of the phase delay plate is a positive integer of a value between 1/8 wavelength and 1/2 wavelength of incident light. The reflective antiferroelectric liquid crystal display according to claim 1, wherein the optical axis of the phase delay plate is rotated by [pi] / 4 with respect to the optical axis of the polarizing plate. The reflective antiferroelectric liquid crystal display device according to claim 1, further comprising a first substrate having a uniform reflective film formed thereon and a transparent electrode formed thereon. The reflective antiferroelectric liquid crystal display device according to claim 1, wherein an antireflective film is attached on the polarizing plate.