JPH09325339A - Liquid crystal display element and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain good sharpens, a high contrast ratio and high transmittance in a bright state by providing a liquid crystal layer with a spiral structure which gradually changes the major axis direction of liquid crystal molecules between substrates when voltage is impressed on this layer. SOLUTION: Nematic liquid crystals 10 which are negative in dielectric constant anisotropy and are added with a chiral agent are injected between a pair of substrates 20 and 21 of a perpendicular orientation type ECB mode liquid crystal display element. At the time of not impressing the voltage, double refraction does not exist in the intra-surface direction of the substrates 20, 21 and since the liquid crystal molecules 10 are perpendicularly oriented, the good black display is obtd. When the voltage is impressed on this cell, the liquid crystal molecules 10 fall and retardation increases, thus attaining the state of allowing the transmission of light (bright state). At this time, the liquid crystal molecules 10 attain the twisted structure (spiral arrangement) to gradually change the major axis direction as shown by dotted lines between the upper and lower substrates 20 and 21 on account of the chiral agent. The sharpness which is the ratio of the change in the transmittance to the change in the voltage is made to have a high value by such chirality.

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 excellent display quality such as contrast characteristics and a manufacturing method thereof.

[0002]

2. Description of the Related Art A vertical alignment type ECB (E
Electrically Controlled Bi
In a refraction mode LCD (liquid crystal display device), liquid crystal molecules 10 are formed on the upper and lower substrates 20 when no voltage is applied,
It is known that since it is oriented perpendicularly to 21, the combination with the polarizing plates 23 and 24 in the crossed Nicols arrangement provides a high light shielding rate when no voltage is applied and a high contrast display can be obtained.

In this ECB mode, when a voltage is applied, as shown in FIG.
As shown in (1), the liquid crystal molecules begin to fall from the liquid crystal molecules at the center between the upper and lower substrates of the cell, and at the same time, the retardation of the liquid crystal layer changes to gradually increase the transmittance.

[0004]

However, in this ECB mode, in general, the rate of change in transmittance with respect to voltage change,
That is, the steepness is sharper than that of STN-LCD (super twist nematic liquid crystal display element). The sharpness is defined as the ratio of the applied voltage corresponding to a predetermined (close to dark state) transmittance to the applied voltage corresponding to another predetermined bright state. It shows that the ratio is large and the transmittance in the bright state is high.

[0005] Poor sharpness means that when a display is performed by simple matrix driving with a large duty ratio, the contrast ratio is small and the transmittance in the bright state is low. It is known that the steepness can be increased by increasing the thickness of the liquid crystal cell. However, another problem arises in that the response speed decreases when the cell thickness is increased.

An object of the present invention is to provide a vertical alignment type ECB mode liquid crystal display device having a good sharpness, a high contrast ratio and a high transmittance in a bright state, and a manufacturing method thereof.

[0007]

A liquid crystal display element of the present invention comprises a pair of substrates having electrodes formed on the surface thereof and facing each other at a predetermined interval, and a liquid crystal layer disposed between the pair of substrates. In the vertical alignment type ECB mode liquid crystal display device, in which liquid crystal molecules are aligned vertically to the substrate surface in the state where no voltage is applied to the liquid crystal layer, the liquid crystal layer has a negative dielectric anisotropy. Including nematic liquid crystal material with
Further, the liquid crystal material is characterized by having a spiral structure in which liquid crystal molecules gradually change their major axis directions between the substrates when a voltage is applied to the liquid crystal layer.

The method of manufacturing a liquid crystal display device of the present invention comprises the steps of preparing a pair of substrates that have been subjected to an alignment treatment so that liquid crystal molecules are vertically aligned, and disposing the pair of substrates facing each other at a predetermined interval. And a step of injecting a nematic liquid crystal material having a negative dielectric anisotropy with a chiral agent added between the substrates.

By imparting chiral properties to the liquid crystal molecules of the nematic liquid crystal material, sharpness is improved without increasing the cell thickness. It is considered that when the liquid crystal material has a chiral property, the tilting direction of the liquid crystal molecules with respect to the voltage increase becomes large.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A nematic liquid crystal having a negative dielectric anisotropy and a chiral agent is injected between a pair of substrates of a vertical alignment type ECB mode liquid crystal display element. When no voltage is applied, all liquid crystal molecules are aligned substantially perpendicular to the substrate surface, as in the conventional vertically aligned ECB mode liquid crystal display element shown in FIG. When this cell is sandwiched by polarizing plates with a crossed Nicols arrangement and the display surface is observed, there is no birefringence in the in-plane direction of the substrate when no voltage is applied, and the liquid crystal molecules are vertically aligned, which is favorable. Black display (low transmittance)
Is obtained.

When a voltage is applied to this cell, as shown in FIG. 1, the liquid crystal molecules are tilted and the retardation is increased so that light is transmitted (bright state). At this time, the liquid crystal molecules take a twisted structure (spiral arrangement) in which the major axis direction is gradually changed between the upper and lower substrates due to the chiral agent as shown by a dotted line. Due to this chirality, the sharpness, which is the ratio of the change in transmittance with respect to the change in voltage, can have a higher value than the conventional one in which no chiral agent is added. This phenomenon is also confirmed by computer simulation.

The empty cell is a vertical alignment type EC according to the conventional technique.
It can be obtained with the same manufacturing method and the same structure as the B-mode liquid crystal display element. However, by adding a chiral agent to the liquid crystal material to be injected, a liquid crystal display element having a high contrast ratio and a high transmittance can be obtained even when driven at the same duty ratio as the conventional one.

The process and the display performance when the liquid crystal display device of the embodiment of the present invention is actually manufactured will be described below.

A vertical alignment polyimide alignment film (manufactured by Nissan Chemical Industries, Ltd.) was applied by flexographic printing on a glass substrate on which an ITO (indium tin oxide) display transparent electrode was formed,
It was baked at 200 ° C for 1 hour. The thickness of the alignment film after firing was 600 angstroms.

The two glass substrates subjected to the orientation treatment in the above steps were stacked so that the cell thickness d was 4.5 μm to form an empty cell.

A chiral agent was added to the empty cell obtained in the above step by using a vacuum injection method, and nematic liquid crystal (manufactured by Merck Ltd.) having a negative dielectric anisotropy was injected. At that time, a device was produced in which the ratio of the value of the natural twist pitch p of the nematic liquid crystal material to the value of the cell thickness d was set to several values in the range of 0 to 0.9.

The completed liquid crystal cell was sandwiched between two polarizing plates (manufactured by Nitto Denko) arranged in a crossed Nicols state, and electro-optical characteristics at 1/240 duty driving were measured. As shown in Table 1 and FIG. Results were obtained. At that time, a vertical alignment type ECB mode cell (conventional 1 and conventional 2 in Table 1) according to a conventional technique was similarly prepared and measured for comparison.

The transmittance values in Table 1 are values when the contrast is constant. The contrast value is a value under the condition that the transmittance is constant at 10%. The slits in Conventional 1 and Example 2 in Table 1 are the ones in which the present invention is applied to the invention disclosed in the Example column of JP-A-3-259121 previously filed by the applicant of the present application. A slit-shaped opening is provided in one of the electrodes of the substrate, and when electric voltage is applied, diagonal electric fields with different directions are generated between the edges of the slit and the electrodes of the other substrate. Thus, the liquid crystal molecules are tilted in different directions to improve the viewing angle characteristics. Details will be described later.

[0019]

[Table 1]

From the measurement results shown in Table 1 and FIG. 2, it is found that the liquid crystal display device according to the embodiment of the present invention rapidly decreases the sharpness value when d / p is 0.5 or more.
That is, d / p is about 0.5 from the viewpoint of sharpness.
The above is preferable. When the d / p value becomes 0.8 or more depending on the material, a hysteresis phenomenon occurs. Therefore, it is actually desirable that the range is 0.5 ≦ d / p ≦ 0.8. As a more preferable condition, 0.6 ≦ d / p ≦ 0.7 can be recommended.

By the way, the present invention can be applied to the invention disclosed in the section of the specification example of Japanese Patent Application Laid-Open No. 3-259121 of the same applicant as the present application, and the same effect can be obtained. . According to the invention described in this publication, an elongated portion (slit) is formed in one of the electrode sides at a crossing portion (display pixel portion) of the electrode in a direction along an electrode edge opposed to the electrode side, so that the inside of a pixel is formed. A wide viewing angle is realized by tilting liquid crystal molecules in two or more directions. An embodiment of the invention described in this publication will be briefly described with reference to FIG.

FIG. 3A is an external view showing the configuration of the dot matrix type liquid crystal display element of the invention described in the above publication. A pair of glass substrates 11 and 12 are arranged so as to face each other so as to define a space having a predetermined gap in which liquid crystal is accommodated.
A plurality of common electrodes 16 are arranged in parallel on the lower glass substrate 11. On the surface of the upper glass substrate 12, plural thin segment electrodes 17 are arranged in parallel in the direction orthogonal to the common electrode 16. A pair of substrates 1
On the outer sides of 1 and 12, polarizing plates 13 and 14 having a relationship of mutually orthogonal Nicols are arranged. The common electrode 16 has an elongated opening (slit) 18 formed along the longitudinal direction of the segment electrode 17.

In FIG. 3, in the AA 'cross section, the direction of the electric field due to the electrode structure is as shown in FIG. 3B, and it can be seen that the tilting direction of the liquid crystal molecules indicated by the arrow can be controlled in two different directions. .

Furthermore, an invention in which the shape and arrangement of the electrodes are improved so that the transmittance and the aperture ratio are improved as compared with the invention disclosed in the above-mentioned publication is a Japanese Patent Application No. 8-40 filed by the same applicant as the present application.
No. 163. The present invention can be applied to the invention disclosed in the specification of Japanese Patent Application No. 8-40163 to obtain the same effect.

FIG. 4 shows an electrode structure of the invention in Japanese Patent Application No. 8-40163. FIG. 4A is a plan view showing the structure of the electrode intersection. A scanning electrode 1 shown by a solid line on a substrate (not shown)
Are formed, a plurality of signal electrodes 2 indicated by broken lines are formed on another substrate, and the scanning electrodes 1 and the signal electrodes 2 are arranged so as to be orthogonal to each other. The scan electrode 1 is formed with an elongated cutout 3, and the electrode member is removed from the cutout 3. The notches 3 are arranged at the arrangement pitch of the signal electrodes 2 in the direction along the edges 4 of the signal electrodes 2, and are formed so that the opposite edges 5, 6 of the scanning electrodes 1 are staggered.

4 (B) and 4 (C) show the electric field direction at the electrode edge in the side cross section taken along the lines AA 'and BB' in FIG. 4A by arrows. One pixel
The direction of the electric field at the edge of the signal electrode 2 is parallel at any position of the AA 'and BB' cross sections, and the cross section A- shows the upper part and the lower part in one pixel.
The electric field inclination directions are opposite in A ′ and BB.

Therefore, the liquid crystal molecules in one pixel are inclined by 180 ° in the opposite directions up and down (upward and downward in the plane of the drawing) with respect to the central portion of the scanning electrode 1 when a voltage is applied. The state of the liquid crystal molecules at this time is shown in FIG.
It is shown in (D). In FIG. 4D, the black-painted portions of the liquid crystal molecules 10 indicate the tilt directions, and it can be seen that the liquid crystal molecules 10 are directed in opposite directions in the upper and lower regions by the electric field E. It is also apparent from FIGS. 4B to 4D that the inclination relationship of the liquid crystal molecules is reversed between the left and right pixels adjacent to each other.

In the liquid crystal cell of FIG. 4, polarizing plates in a crossed Nicol arrangement are arranged outside a pair of substrates, and their polarization axes are at an angle of 45 ° with respect to the longitudinal direction of the signal electrode 2 (right arrow in FIG. 4D). ), The transmittance is set to be the most efficient for the liquid crystal array shown in FIG.

Although the present invention has been described with reference to the embodiments, the present invention is not limited to these. For example, the electrodes provided with the notches may be either scanning electrodes or signal electrodes. It will be apparent to those skilled in the art that various changes, improvements, combinations, and the like can be made.

[0030]

According to the present invention, in the vertical alignment type ECB mode liquid crystal display element, the sharpness characteristic is improved by giving chirality to the liquid crystal material and giving a twisted structure to the liquid crystal molecules when a voltage is applied. It is possible to improve the contrast ratio and the transmittance under the same driving conditions.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an alignment state of liquid crystal molecules when a voltage is applied to a vertical alignment type ECB mode liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a sharpness characteristic diagram measured by variously changing d / p values of a liquid crystal cell according to an example of the present invention.

FIG. 3 is a diagram showing an electrode structure and a diagonal electric field of a liquid crystal display device to which the present invention can be applied.

FIG. 4 is a diagram showing an electrode structure of a liquid crystal display device to which the present invention can be applied and an arrangement state of liquid crystal molecules.

FIG. 5 is a cross-sectional view showing an arrangement state of liquid crystal molecules in a vertical alignment type ECB mode liquid crystal display element when no voltage is applied.

FIG. 6 is a cross-sectional view showing an arrangement state of liquid crystal molecules when a voltage is applied to a conventional vertical alignment type ECB mode liquid crystal display device.

[Explanation of symbols]

 1, 2, 16, 17 Electrode 18 Slit 3 Notch 10 Liquid crystal molecule 20, 21 Substrate 23, 24 Polarizing plate

Claims (9)

[Claims] 1. A pair of substrates having electrodes formed on the surface thereof and arranged to face each other at a predetermined interval, and a liquid crystal layer arranged between the pair of substrates, wherein a voltage is applied to the liquid crystal layer. In a vertical alignment type ECB mode liquid crystal display device in which liquid crystal molecules are vertically aligned with respect to a substrate surface in a non-existing state, the liquid crystal layer includes a nematic liquid crystal material having a negative dielectric constant anisotropy. Is a liquid crystal display device having a spiral structure in which liquid crystal molecules gradually change their major axis directions between the substrates when a voltage is applied to the liquid crystal layer. 2. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined interval d is set to 0.5 ≦ d / p ≦ 0.8. Item 3. A liquid crystal display device according to item 1. 3. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined distance d is set to 0.6 ≦ d / p ≦ 0.7. Item 3. A liquid crystal display device according to item 1. 4. An opening is provided in one of the electrodes of the pair of substrates, and the liquid crystal molecules are moved by different oblique electric fields in different directions generated between the edge of the opening and the electrode of the other substrate when a voltage is applied. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is tilted in directions different from each other. 5. A notch is provided in one of the electrodes of the pair of substrates, and the liquid crystal molecules are separated from each other by different oblique electric fields in different directions generated between the edge of the notch and the electrode of the other substrate when a voltage is applied. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is tilted in different directions. 6. The nematic liquid crystal material is added with a chiral agent.
The liquid crystal display device according to the above. 7. A step of preparing a pair of substrates that have been subjected to an alignment treatment so that liquid crystal molecules are vertically aligned, and a pair of substrates that are opposed to each other at a predetermined interval and a chiral agent is added between the substrates. Alignment type EC having a step of injecting a nematic liquid crystal material having different dielectric anisotropy
A method for manufacturing a B-mode liquid crystal display element. 8. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined interval d is set to 0.5 ≦ d / p ≦ 0.8. Item 8. A method for manufacturing a liquid crystal display element according to item 7. 9. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined interval d is set to 0.6 ≦ d / p ≦ 0.7. Item 8. A method for manufacturing a liquid crystal display element according to item 7.