WO2002029481A1

WO2002029481A1 - Smectic liquid crystal display in a transverse electrode configuration

Info

Publication number: WO2002029481A1
Application number: PCT/KR2001/001645
Authority: WO
Inventors: Sin Doo Lee; Jin Yun Kim
Original assignee: Neo Tek Research Co., Ltd.
Priority date: 2000-10-04
Filing date: 2001-09-28
Publication date: 2002-04-11
Also published as: KR100383542B1; KR20020027049A; AU2001292425A1

Abstract

The present invention provides the LCD device which has the horizontal electrode arrangement structure having an improved viewing angle and high-response quality, comprising the 1st panel coated by an alignment layer on the surface where at least 2 horizontal electrodes are formed; the 2nd panel coated by an alignment layer; and the smetic liquid crystal which has positive or negative dielectrical anisotropy, injected between the 1st panel and the 2nd panel facing each other.

Description

SMECTIC LIQUID CRYSTAL DISPLAY IN A TRANSVERSE ELECTRODE CONFIGURATION

TECHNICAL FIELD The present invention relates to a smectic liquid crystal display and, more particularly, to a liquid crystal display with the horizontal electrode array structure having the wide and symmetric viewing angle and high speed response features.

BACKGROUND ART The liquid crystal display ("LCD") of the related art adopting the twisted nematic ("TN") mode has the fatal drawback of having a narrow viewing angle and slow response speed as more LCDs of higher definition and larger screen are developed. As means to remedy the problem of the narrow viewing angle, various methods are being introduced. Among such methods, the optical compensation method is frequently used. Observing that the TN-LCD is operated asymmetrically, the optical compensation method compensates the changes of the double refraction according to the bearing by using the uniaxial optical compensation film. This method has problems in that color divergence is caused by the wavelength-dependent refraction divergence of the liquid crystal and the production cost is high. On the other hand, there is the in-plane switching ("IPS") method which is to drive an LCD by forming electrodes on one side of the liquid crystal substrate. According to this method, if the liquid crystal is horizontally aligned and if the light axis of the liquid crystal and one polarizing axis are made coincident, when the electric field is not supplied, the dark state is obtained and as the electric field is supplied, the bright state is obtained. According to this method, a wide viewing angle may be achieved because the average light axis change occurs in a surface that is parallel to a sample surface. However, the response time is long and the aperture ratio is low. Moreover, the driving voltage is high.

Additionally, there is a method of improving the viewing angle by using the multi-domain alignment method. The axially symmetric aligned microcell ("ASM") method obtains circularly symmetric viewing angle by mixing liquid crystal and polymer and then using phase separation. In this method, it is possible to maintain constant sample gaps on a large area without a rubbing process. Thus, this method has a great potentiality for diverse applications. Another method, a-TN (amorphous twisted nematic) method, is a technique to improve the viewing angle characteristic by forming very small domains in a unit pixel where the small domains have arbitrary directions and sizes, so as to increase the number of pixel divisions. This method has an advantage in that the manufacturing process is very simple. However, the control of sizes of the small domains is practically impossible. Thus, it is difficult to apply this method to LCDs having large areas.

The MD-TN (multi-domain twisted nematic) method obtains the symmetrical viewing angle characteristic dependent upon the bearing, by dividing a pixel into multi-domains and by causing the direction of the distortion of the nematic molecules to be different in each domain. However, this method also has a shortcoming because the manufacturing process is complex since a rubbing process is required for each domain and thus the production yield is lowered.

The MD-VA (multi-domain vertical alignment) method is to divide a pixel into many different domains and to align them. In this method, the initial aligning direction is maintained to be vertical in each domain of a unit pixel. Thus, the light leakage is very low in the OFF state and the contrast rate is very high. However, this method has a problem in that each domain must go through the rubbing process in a direction different from the direction of another domain. Also, there is a method of causing each domain on the surface to have different plunge angle without the rubbing process. However, in this method, alignment materials such as SiOx must be uniformly coated. Thus, the manufacturing process becomes complicated.

On the other hand, so as to resolve the low-speed response of the LCD, various arts utilizing the ferroelectric liquid crystal have recently been introduced. The liquid crystal used in a LCD may be classified, depending upon the molecule array structure, as nemitic liquid crystal having the direction orders only or smectic liquid crystal which has the position orders as well. Further, depending on whether there is spontaneous polarization or not, the liquid crystal may be classified as paraelectric liquid crystal or ferroelectric liquid crystal. The interaction between the paraelectric nematic liquid crystal and the electric field is non-polarized. Thus, it is explained as the liquid crystal molecules' dielectric anisotropy. In a twisted LCD using such nematic liquid crystal, the response speed of the molecules by the dielectric anisotropy is tens of milliseconds (ms). Thus, it is difficult to apply such nematic liquid crystal for an LCD requiring high-speed response. In comparison with the nematic liquid crystal, the ferroelectric liquid crystal has the high-speed response time of tens of microseconds (μs) because the response is induced by the polarized interaction between the spontaneous polarization and the electric field. Thus, ferroelectric liquid crystal is suitable for the implementation of high-speed motion images. However, the surface stabilized ferroelectric liquid crystal (SSFLC) of the related art operates as the bistable mode. Therefore, it is impossible to implement the continuous gray-scale display. Furthermore, because the layer structure with position orders is formed, it is very difficult to embody the uniform alignment of a large area. In case of the antiferroelectric LCD which has been introduced recently, it shows high-speed response feature and a limited multi gray-scale display function in comparison with the nematic liquid crystal. However, the antiferroelectric liquid crystal also has the serious problems such as the uniform alignment in a large area and the screen tremor. On the other hand, a horizontal configuration deformed helix ferroelectric aligned liquid crystal (DHF) display has been introduced. In the DHF, electric field is applied to the horizontal alignment structure of the ferroelectric liquid crystal having a helix pitch that is considered very short in view of the light wavelength. Thus, in this method, the continuous gray-scale display is made possible by the variations of the helix structure. This method achieves the high-speed response feature. However, so as to have the uniform alignment on a large area, additional alignment processes such as shearing or electric field processing, are required. Moreover, the contrast rate is decreased due to the formation of band structures and there also is the problem of screen tremor. As explained above, the common serious problem in manufacturing ferroelectric or antiferroelectric LCDs with layer structure is the uniform alignment in large areas. Furthermore, each type of devices has other problems of difficulty in displaying gray-scales or low screen quality. More specifically, ferroelectric or antiferroelectric liquid ciystal generally has the helix pitch and band structure defects appear due to the strong polarized interaction between the spontaneous polarization and the electric field. Therefore, it is very difficult to implement the uniform single alignment structure. As a result, the horizontally aligned deformed helix ferroelectric liquid crystal display of the related art has the low contrast rate and it is impossible to have high transmittance.

Most recently, a vertical configuration deformed helix ferroelectric liquid crystal (NC-DHF) display has been introduced. In the NC-DHF, if a strong electric field is applied, the layer array is deformed or broken. Moreover, because there is no voltage threshold that is electric-optically clear, it is difficult to apply the VC- DHF to a manually operated liquid crystal display. Therefore, the active driving using the TFT is required.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a horizontal alignment configuration by forming two or more electrodes upon one side of the substrate and by injecting the smectic liquid crystal, so as to resolve the problems of the prior art. According to the present invention, there exists no interference between smectic layers differently from the case with nematic liquid crystal. Thus, high-speed response characteristic may be obtained. According to the present invention, there is a threshold voltage which does not exist in a deformed helix ferroelectric liquid crystal display. Moreover, the continuous gray-scale display which is impossible in a surface stabilized ferroelectric liquid crystal display is made possible as in a TN mode according to the present invention. Additionally, because the present invention utilizes techniques that are similar to the IPS of the related art, wide and symmetric viewing angle may be obtained.

In order to achieve the above objects, the present invention provides a liquid crystal display including the first substrate and the second substrate facing each other and liquid crystal injected between the first substrate and the second substrate. The present invention's liquid crystal display comprises: the first substrate having the alignment film coated over two or more electrodes formed on the internal surface of the first substrate; the second substrate having the alignment film coated over the internal surface of the second substrate; and the smectic liquid crystal injected between the first substrate and the second substrate. According to the present invention, one or more electrodes may be formed upon the second substrate and over the formed electrodes, the horizontal or vertical alignment film may be coated.

The polarizers may be attached to the external surfaces of the first substrate and the second substrate respectively. The polarizer attached to the first substrate and the polarizer attached to the second substrate are located perpendicularly from the other polarizer or in parallel to the other polarizer, or form an angle between 0° and 90° . The back light source is used for the polarizer.

Additionally, a front light source may be used by forming a reflection plate on the internal surface or the external surface of one substrate of the two substrates. Furthermore, an additional light compensation film may be inserted between the external surfaces and the polarizers of the first substrate and the second substrate.

The smectic liquid crystal injected between the first substrate and the second substrate is the positive or negative dielectric anisotropic liquid crystal.

Additionally, according to the present invention, by adjusting the distance between multiple electrodes formed on the first substrate, the light transmittance may be controlled.

The rotation angle (bearing) of liquid crystal molecules of the smectic liquid crystal moving from the first substrate to the second substrate ranges from 0° to 180° and the molecular tilt angle of the smectic liquid crystal ranges from 0° to 90°.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure la illustrates a sectional view of the horizontal alignment according to the present invention. Figure lb illustrates a projected diagram of the horizontal alignment according to the present invention.

Figure 2 illustrates a partial top view of the horizontal array structure.

Figure 3 a is a graph showing the level of the transmission light in case of the positive dielectric anisotropy. Figure 3b is a graph showing the level of the transmission light in case of the negative dielectric anisotropy.

Figure 4a illustrates the driving state as the electric field is not applied where there is no plunge angle.

Figure 4b illustrates the driving state as the electric field is applied where there is no plunge angle.

Figure 4c illustrates the driving state as the electric field is not applied where there is a plunge angle.

Figure 4d illustrates the driving state as the electric field is applied where there is a plunge angle. Figure 5 a illustrates a microscopic view in the case where the perpendicular polarizer is located and the electric field is not applied.

Figure 5b illustrates a microscopic view in the case where the perpendicular polarizer is located and the electric field is applied. Figure 6a illustrates the voltage waveform applied to the sample.

Figure 6b illustrates the response characteristic in case of the positive dielectric anisotropy.

Figure 6c illustrates the voltage waveform applied to the sample.

Figure 6d illustrates the response characteristic in case of the negative dielectric anisotropy.

Figure 7a illustrates the viewing angle in case of the positive dielectric anisotropy.

Figure 7b illustrates the viewing angle in case of the negative dielectric anisotropy. ** Descriptions of reference numerals for important parts of the drawings**

10: First substrate 11: Second substrate

20, 21 : Alignment film 30: Smectic liquid crystal

40: Electrode 50: Liquid crystal direction

60: Smectic liquid crystal layer 70, 71 : Polarizer 80: Rubbing direction 90: Width of the sample

100: Applied electric field 110: The direction of the liquid crystal's movement by the electric field

BEST MODE FOR CARRYING OUT THE INVENTION Reference will now be made in detail to the present invention as illustrated in the accompanying drawings.

Figure la and Figure lb illustrate a preferred embodiment of the present invention.

As shown in Figure' la and Figure lb, the smectic liquid crystal display using the horizontal electrode array structure according to the present invention comprises: the first substrate 10 having the alignment film 20 coated over two or more electrodes 40 formed on its internal surface; the second substrate 11 having the alignment film 21 coated over its internal surface; and the smectic liquid crystal 30 injected between the first substrate 10 and the second substrate 11. Figure 2 is a partial top view of the electrodes arranged horizontally on the first substrate.

The first and second substrates 10, 11 may be glass substrates or plastic substrates. The alignment film 20 is composed of a horizontal alignment high polymer film for the liquid crystal alignment. In the present invention, such horizontal alignment material was spin-coated on the substrate and heat-processed. The smectic liquid crystal 30 is composed of smectic liquid crystal having the positive or negative dielectric anisotropy.

As shown in Figure la and Figure lb, polarizers 70, 71 are attached to the external surfaces of the first and second substrates 10, 11. The polarizers 70, 71 are perpendicularly arranged so that the polarizing axes of the polarizer and the analyzer form the angle +90^° or -90°. Alternatively, the polarizers 70, 71 may be arranged parallel to each other so that the polarizing axes of the polarizer and the analyzer may be parallel. Additionally, by arranging the polarizers 70, 71 in such manner as to form an angle between 0° and 90°, the polarizing axes of the polarizer and the analyzer may be made to form the angle between 0° and 90°. In the smectic liquid crystal display using the horizontal electrode array structure according to a preferred embodiment of the present invention, the sample gap of 2μm is maintained by using a glass spacer. The smectic liquid crystal injected in the sample is IS5512(Δε >0) or IS5511(Δε <0) (Merck Co.), and has the positive or negative dielectric anisotropy.

The transmittance characteristic of the smectic liquid crystal display manufactured according to the present invention is as illustrated in Figure 3a and Figure 3b. Figure 3 a illustrates the transmittance upon voltage characteristic of the smectic liquid crystal having the positive dielectric anisotropy and Figure 3b illustrates the transmittance upon voltage characteristic of the smectic liquid crystal having the negative dielectric anisotropy. The high driving threshold voltage illustrated in the figures is because of the voltage decrease caused by the low dielectric anisotropy value and the wide electrode distance. As smectic liquid crystal's dielectric anisotropy value becomes higher and the electrode distance becomes shorter, the threshold voltage becomes lower.

Figures 4a to 4d illustrate the changes in the liquid crystal direction in the smectic liquid crystal display manufactured according to the preferred embodiment of the present invention. Figure 4a illustrates the driving state when the electric field is not applied to the smectic liquid crystal where the liquid crystal molecule's plunge angle is almost nil because the plunge angle (α) is 22.5° or less. Figure 4b illustrates the driving state when the electric field is applied to the smectic liquid crystal where the liquid crystal molecule's plunge angle is almost nil because the plunge angle (α) is 22.5° or less. In such cases, the liquid crystal direction moves as the bearing (Φ) on the smectic cone rotates 180°. Figure 4c illustrates the driving state when the electric field is not applied to the smectic liquid crystal where there is a substantial liquid crystal molecule's plunge angle because the plunge angle (α) is 22.5° or greater. Figure 4d illustrates the driving state when the electric field is applied to the smectic liquid crystal where there is a substantial liquid crystal molecule's plunge angle because the plunge angle (α) is 22.5° or greater.

In Figures 4a to 4d, '50' represents the liquid crystal's direction, '60' represents the smectic liquid crystal layer, '80' represents the rubbing direction, '90' represents the depth of the sample, '100' represents the applied electric field, and '110' represents the direction to which the liquid crystal's direction rotates according to the electric field, γ is the angle between axis z and the rubbing direction 80, which is greater than 0°. This causes the liquid crystal molecules to have the directional characteristic when the electric field is applied. In this case, the magnitude of the bearing within which the liquid crystal direction moves upon the smectic cone is 90° ≤ Φ < 180°. The liquid crystal molecules' plunge angle (θ_p) may be expressed as follows: 0° ≤ θ_p ≤ 2α - 45°. As shown in Figure 4a and Figure 4c, if the electric field is not applied, the liquid crystal within the sample is aligned toward the direction inclined from the electrodes. If the electric field is applied, the liquid crystal moves along the surface of the cone of the smectic liquid crystal as shown in Figure 4b and Figure 4d.

Figure 5a and Figure 5b are the views observed under the perpendicular polarizers. Figure 5a illustrates the case where the electric field is not applied. It displays the dark state. Figure 5b illustrates the case where the electric field is applied. It shows the circular symmetry. Figure 6a and Figure 6c illustrate the applied voltage waveforms. Figure 6b and Figure 6d illustrate the responses to the voltage waveforms shown in Figure 6a and Figure 6c. As shown in Figure 6b and Figure 6d, the response in the present invention may be characterized as much faster than it is in the IPS method using the nematic liquid crystal. Figure 6a illustrates the case of the positive dielectric anisotropy, where the ascending time is 3.20ms and the descending time is 13.80ms. Thus, the total time is 17.00ms. Figure 6b illustrates the case of the negative dielectric anisotropy, where the ascending time is 3.20ms and the descending time is 5.60ms. Thus, the total time is 8.80ms. This is at least three times faster than the speed of the TN method. Figure 7a and Figure 7b illustrate the viewing angle characteristics of the smectic liquid crystals having the positive and negative dielectric anisotropy respectively. Because changes in the average light axis of the liquid crystal molecule occur on a layer that is parallel to the sample surface, wide viewing angle is obtained according to the preferred embodiment of the present invention.

INDUSTRIAL APPLICABILITY

As explained above, the present invention implements a horizontal alignment smectic liquid crystal display having a great viewing angle symmetry and the high-speed response characteristic by forming two or more electrodes on one side of a substrate and coating the two substrate surfaces with horizontal alignment material. Because the present invention adopts the electrode structure that is similar to that in the in-plain switching method of the prior art, no new electrode-related technology is required to be developed in the present invention. The present invention provides the technology suitable for a liquid crystal display having the wide and symmetric viewing angle as in the IPS method and the continuous gray-scale display and high-speed response characteristic as in the TN mode.

Claims

WHAT IS CLAIMED IS:

1. A smectic liquid crystal display using the horizontal electrode array structure, wherein the liquid crystal is injected between the first substrate and the second substrate facing each other, comprising: the first substrate having the alignment film coated over two or more electrodes formed on the internal surface of the first substrate; the second substrate having the alignment film coated over the internal surface of the second substrate; and the smectic liquid crystal injected between the first substrate and the second substrate.

2. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1 , wherein one or more electrodes are formed upon the second substrate.

3. The smectic liquid crystal display using the horizontal electrode array structure according to claim 2, wherein the horizontal alignment film is coated on the said electrodes.

4. The smectic liquid crystal display using the horizontal electrode array structure according to claim 2, wherein the vertical alignment film is coated on the said electrodes.

5. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1, wherein the polarizers are attached to the external surfaces of the first substrate and the second substrate respectively and the back light source is used.

6. The smectic liquid crystal display using the horizontal electrode array structure according to claim 5, wherein the polarizer on the first substrate and the polarizer on the second substrate are located perpendicularly.

7. The smectic liquid crystal display using the horizontal electrode array structure according to claim 5, wherein the polarizer on the first substrate and the polarizer on the second substrate are in parallel.

8. The smectic liquid crystal display using the horizontal electrode array structure according to claim 5, wherein the polarizer on the first substrate and the polarizer on the second substrate form an angle between 0° and 90°.

9. The smectic liquid crystal display using the horizontal electrode array structure according to claim 5, wherein a light compensation film is additionally inserted between the external surface and the polarizer of the first substrate or the second substrate.

10. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1 , wherein a reflection plate is formed on the internal surface or the external surface of either the first substrate or the second substrate and a polarizer is attached to the external surface of the other substrate.

11. The smectic liquid crystal display using the horizontal electrode array structure according to claim 10, wherein a light compensation film is additionally inserted between the said one substrate and the polarizer.

12. The smectic liquid crystal display using the horizontal electrode array structure according to a claim among claims 1 to 11, wherein the said smectic liquid crystal is the positive dielectric anisotropic liquid crystal.

13. The smectic liquid crystal display using the horizontal electrode array structure according to a claim among claims 1 to 11, wherein the said smectic liquid crystal is the negative dielectric anisotropic liquid crystal.

14. The smectic liquid crystal display using the horizontal electrode array structure according claim 1, wherein the light transmittance is controlled by adjusting the distance between the said electrodes.

15. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1 , wherein the rotation angle (bearing) of liquid crystal molecules of the smectic liquid crystal moving from the first substrate to the second substrate ranges from 0° to 180°.

16. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1 , wherein the molecular tilt angle of the smectic liquid crystal ranges from 0° to 45°

17. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1 , wherein the molecular tilt angle of the smectic liquid crystal ranges from 45° to 60°.

18. The smectic liquid crystal display using the horizontal electrode array structure according to claim 1, wherein the molecular tilt angle of the smectic liquid crystal ranges from 60° to 90°.