JPH10206860A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a normally closed type liquid crystal display device which suppresses a light leak accompanying a drop in threshold voltage in a spacer peripheral area to obtain a high contrast ratio and has, specially, excellent display characteristics by making the dielectric constant of spacers higher than that of a liquid crystal material. SOLUTION: The liquid crystal display device is constituted by sandwiching a liquid crystal layer 54 and spacers 50 between a 1st substrate 51 and a 2nd substrate 52, providing the 2nd substrate 52 with electrodes 41 and 42 in matrix for applying an electric field 55 to the liquid crystal layer 54 almost at right angles to the 1st substrate 51, and thus laminating them across the spacers 50 so that the thickness of the liquid crystal layer 54 is made constant. The dielectric constant of the spacers 50 is >=4/3 as large as that of the liquid crystal material. Then when a voltage is applied between the liquid crystal layer 54 between the electrodes 41 formed on the 1st substrate 51 and the 2nd electrodes 52 formed on the 2nd substrate 52, the electric field 55 becomes close in the spacers 50 and rough nearby the spacer surfaces. The electric field is made weak nearby the spacer surfaces and the threshold value appears to rise.

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 capable of reducing light leakage due to spacers for controlling the thickness of a liquid crystal layer, which is conspicuous in dark display and dark display, and obtaining good image quality. The present invention relates to a liquid crystal display device having normally closed characteristics.

[0002]

2. Description of the Related Art In a liquid crystal display device, when the gap of a liquid crystal panel varies, it greatly affects display quality such as contrast reduction, brightness unevenness, and color unevenness. In order to control the gap of the liquid crystal panel, a spacer is provided in the panel surface. The higher the scattering density of the spacer, the more the variation in the gap of the liquid crystal panel can be suppressed. However, if a spacer is present in the display area of the liquid crystal panel, the effect of light leakage from the spacer and light leakage from the liquid crystal layer around the spacer increases, and display quality such as contrast deteriorates. In particular, in the case of a normally closed type liquid crystal display device, light leakage of the liquid crystal layer around the spacer is conspicuous in dark display, so that the influence on the contrast is very large. For this reason, the spacer itself is colored black or the like to prevent light from passing through the spacer (Japanese Patent Laid-Open No.
63-89890, JP-A-5-257150, JP-A-7-5474,
JP-A-8-40707) has been proposed. As a method focusing on the periphery of the spacer, a method in which the liquid crystal layer at the interface of the spacer is vertically aligned by surface treatment to reduce alignment disturbance near the spacer (Japanese Patent Laid-Open No. 4-177324),
A method in which the surface of the spacer is roughened to randomly align liquid crystal molecules (JP-A-8-146433).
A method using a spacer composed of a polymerizable monomer and a polymer having a molecular weight of 0 or more (Japanese Patent Application Laid-Open No. 6-67182), and a method of making the dielectric constant of the liquid crystal and the spacer equal (Japanese Patent Application Laid-Open No. 5-19266).
Has been proposed.

[0003]

However, even if the light in the spacer is impermeable, the effect of light leakage of the liquid crystal layer around the spacer is large and cannot be ignored.
In the method of controlling the orientation of the liquid crystal layer on the surface of the spacer, if the orientation order changes when a shock such as voltage application or vibration is applied, light leakage around the spacer may increase. At this time, if the spacers having a large light leakage amount are locally concentrated, not only the contrast is reduced but also the luminance is uneven. Light leakage around the spacer is caused by a decrease in threshold voltage in this region. The factors may be the influence on the electric field due to the difference in the dielectric constant between the spacer and the liquid crystal material, and the disorder of the alignment order of the liquid crystal layer due to the spacer.
Therefore, not only the dielectric constant of the spacer affects the light leakage, but it is not possible to suppress the decrease in the threshold voltage only by making the dielectric constants of the liquid crystal and the spacer equal and making the applied electric field equal. difficult.

SUMMARY OF THE INVENTION It is an object of the present invention to suppress light leakage around a spacer due to a decrease in threshold voltage in a dark display and in the vicinity of a dark display. Liquid crystal display device having the same.

[0005]

SUMMARY OF THE INVENTION A pair of substrates formed at a predetermined interval with a plurality of spacers interposed therebetween and provided with electrodes on their respective opposing surfaces, and a predetermined thickness sandwiched between the pair of substrates. In a liquid crystal display device having a liquid crystal layer composed of a liquid crystal layer, the spacer is a spacer having a dielectric constant that is at least 4/3 times the vertical component of the dielectric constant of the liquid crystal material of the liquid crystal layer.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The outline of the present invention will be described below.

In the case of an STN (Super Twisted Nematic) type liquid crystal display device, the dark state is displayed not when no voltage is applied but when a certain voltage is applied. At this time, the light leakage area around the spacer is larger than when no voltage is applied. This indicates that the light leakage area around the spacer is a change in transmittance due to voltage application. Further, the light leakage region has a lower threshold voltage than other regions. Therefore, by suppressing the electric field applied to the region where the threshold voltage is lowered more than in other regions, the apparent threshold voltage is increased and the light leakage caused by the liquid crystal molecules rising faster than the surroundings is suppressed. I do.

[0008] In particular, ST having normally closed characteristics
In an N-type liquid crystal display device, the electric field applied to the liquid crystal layer in the immediate vicinity of the spacer surface is made lower than the electric field determined by the voltage between the electrodes formed on the substrate, so that the apparent appearance of the liquid crystal layer in the vicinity of the spacer surface is reduced. , The light leakage in this region can be reduced, and the contrast can be improved.

There are two main factors that cause the threshold voltage to decrease in the region around the spacer. First, if the dielectric constant of the spacer is smaller than the dielectric constant of the liquid crystal, the electric field applied between the electrodes is weakened in the spacer and strengthened on and around the spacer, and the alignment of the liquid crystal molecules is controlled by the spacer. It is disturbed to an orientation that reduces the voltage. If the former is the only factor, the dielectric constant of the spacer and the liquid crystal are made equal, and if the latter is the only factor, the liquid crystal molecules are processed to be vertically aligned on the spacer surface. However, in order to solve both of them, it is important to suppress a decrease in threshold voltage.
Therefore, light leakage can be significantly reduced by weakening the electric field applied to the liquid crystal layer around the spacer and rather increasing the threshold voltage. At this time, the brightness of the area around the spacer is different from the brightness of the other areas, but the adverse effect on the contrast is when light leaks while the surroundings are dark, and the threshold voltage is increased. The difference in transmittance from the surroundings when it is high is almost negligible in practical use.

The electric field at the interface between the spacer and the liquid crystal is expressed by the following equation.

E ′ = E0 · 3ε1 / (2ε1 + ε2) (1) where E ′ is the electric field at the interface between the spacer and the liquid crystal,
0 is the electric field determined by the voltage between the electrodes, ε1 is the dielectric constant of the liquid crystal,
ε2 represents the dielectric constant of the spacer. The dielectric constant of the liquid crystal has anisotropy of a vertical component (ε （) and a horizontal component (ε‖). If the liquid crystal molecules are perfectly horizontal to the substrate, ε
⊥ is measured, and if vertical, ε で あ れ ば is measured. Here, since it is near the threshold voltage, it is assumed that ε1 ≒ ε⊥.

On the other hand, as a result of intensive studies on the threshold voltage of the light leakage region around the spacer, it has been found that the rate of decrease in the threshold voltage is generally within 10%. Therefore, the electric field applied to the liquid crystal around the spacer is 1
It is considered that the threshold voltage drop can be substantially suppressed by reducing 0%. From the above equation, to make E ′ ≦ 0.9E0, ε
It is understood that it is necessary to satisfy the condition of 2 ≧ 4/3 · ε1.

When the liquid crystal molecules rise due to pretilt or disturbance of the alignment around the spacer, the component of ε‖ is added, and the dielectric constant becomes larger than ε⊥. Therefore, it is more preferable that E ′ ≦ 0.8E0. ε2 ≧ 3/2 · ε1 or average permittivity (εave = (2ε⊥ + ε‖)
/ 3) and ε2 ≧ εave is more reliable and effective.

Since the above relationship is determined by the ratio of the dielectric constant of the liquid crystal material used to the spacer, a cross-linkable polymer such as a cross-linkable polymer can be used as long as it satisfies this condition and has such a hardness that the gap can be controlled. There are no restrictions on the spacer material, such as organic substances and inorganic substances such as molten crystal type metal oxides.

The same effect can be obtained by covering the surface of the spacer base material with a thickness with a material having a high dielectric constant.

For example, as a liquid crystal material (ε⊥ = 3.
6, εave = 5.6), (ε⊥ = 4.1, εave
= 7.5), (ε⊥ = 4.6, εave = 8.3)
) And the liquid crystal material having the material characteristics of (ε⊥ = 4.0, εave = 6.8), the spacer materials are CaF ₂ (ε = 6.8) and MgF ₂ (ε = 6.
5), a molten crystalline metal oxide such as Al ₂ O ₃ (ε = 9.3) and MgO (ε = 9.7). Since these spacers have light impermeability, an effect of preventing light leakage inside the spacers can be expected. The spacer base material is made of divinylbenzene, polysulfone, polystyrene, or the like, and the surface thereof is made of polyvinyl fluoride (ε = 6.
8-8.5), cyanoethyl pullulan (ε = 18.5)
And a spacer coated with a high dielectric constant material. At this time, the coating layer is required to have a small thickness because the effect is small in a thin film.

For example, if a liquid crystal material is used as a liquid crystal compound having a cyclohexyl group, a cyclopentyl group, a cyclobutyl group, a cyclopropyl group, a bicyclopentyl group, a bicyclooctyl group and the like, and ε⊥ is reduced to about 2.7, Spacers such as fusible silica (ε = 3.7) can be used. As the spacer of the crosslinkable polymer, for example, the dielectric constant of the spacer containing divinylbenzene as a main component is about 3, that of polystyrene is about 2.6, and that of polysulfone is about 3.2. Must be used.

Next, specific embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the state of a liquid crystal layer near beads and the state of an electric field inside the beads of the liquid crystal display device of the present invention. In FIG. 1, the liquid crystal display device has a liquid crystal layer 54 and a spacer 50 sandwiched between a first substrate 51 and a second substrate 52, and the second substrate 52 is substantially perpendicular to the first substrate 51.
4, a matrix electrode 41 for applying an electric field 55 to
The liquid crystal layer 54 is laminated via a spacer 50 so as to make the thickness of the liquid crystal layer 54 substantially constant.

Here, the dielectric constant of the spacer has a value that is at least 4/3 times the dielectric constant of the liquid crystal material. When an electric field is applied to the liquid crystal layer between the electrode formed on the first substrate and the electrode formed on the second substrate, the electric field 55 becomes dense in the spacer and coarsely near the spacer surface. Become. Accordingly, the electric field is weakened in the vicinity of the spacer surface, so that the threshold voltage apparently increases. At this time,
In particular, in an STN type liquid crystal display device having normally closed characteristics, an increase in threshold voltage suppresses an increase in light transmittance, so that light leakage around the spacer is suppressed.

Hereinafter, the features of the present invention will be described in more detail.

FIG. 2 is a schematic sectional view showing the structure of the liquid crystal display device of the present invention. The upper and lower substrates 51 and 52 include XY electrodes 41 and 42 and alignment films 31 and 32, respectively.
Is sandwiched. The substrate 51 includes a color filter 61, a black matrix 62, and a flattening film 63. The XY electrodes 41 and 42 on the upper and lower substrates 51 and 52 are connected to drive circuits 91 and 92, and the phase plate 2 is provided outside the upper and lower substrates.
1 and 22 are placed. At this time, the phase plate 21 is located between the phase plate 22 and the substrate 52 or between the phase plate 22 and the polarizing plate 1.
2 may be arranged. Polarizing plates 11 and 12 are placed outside them, and a light source 8
1, a light guide 82 and a reflector 83. Although the prism sheet is omitted here, the prism sheet may be placed on the light guide 82 in order to improve the brightness in the front direction. The upper and lower substrates 51 and 52 are made of glass, and the XY electrodes 41 and 42 are made of ITO. The alignment films 31 and 32 are made of a polyimide polymer and are subjected to an alignment treatment by a rubbing method. The liquid crystal layer 54 has a vertical component ε⊥ of the dielectric constant.
Is 5.20, the horizontal component ε‖ is 20.0, and Δn is 0.2.
This mixture was prepared by adding a chiral agent to a nematic liquid crystal composition of 144 (589 nm, 20 ° C.), and the twist angle was 240 degrees. The above-mentioned chiral agent is added in an amount necessary for twisting 240 degrees (here, 0.85% by weight). Melted crystal type spherical alumina particles (dielectric constant 9.
3) Classified, a sphere diameter of 6.25 μm and a standard deviation of 0.4
The liquid crystal layer having a thickness of 8 μm was sprayed on the substrate, and the liquid crystal layer thickness was 6.2 μm.
m was obtained. d · Δn is 0.893 μm. The azimuth of the polarization axis of the lower polarizer is 80 degrees, the azimuth of the polarization axis of the upper polarizer is 10 degrees, the slow axis angle of the lower phase plate is 110 degrees, and the slow axis angle of the upper phase plate is 70 degrees. The retardation of the phase plate was set to 400 nm (wavelength: 555 nm) for both upper and lower sides. In this liquid crystal panel, light leakage near the spacer during dark state display was suppressed, and a liquid crystal display device having a good image quality with a contrast ratio of 43: 1 was obtained.

As a comparative example, a liquid crystal display device was obtained in the same manner as described above using a spacer (dielectric constant: 2.9) which was a polystyrene-divinylbenzene copolymer. At this time, light leakage near the spacer during the dark state display was large, and the contrast ratio was 20: 1.

The liquid crystal layer 54 has a vertical component ε⊥ of the dielectric constant.
Is 3.7, the horizontal component ε‖ is 10.6, and Δn is 0.13.
8 (589 nm, 20 ° C.) is a mixture obtained by adding a chiral agent to a nematic liquid crystal composition having a twist angle of 220 degrees. The molten crystal type spherical alumina particles (dielectric constant: 9.3) were subjected to classification treatment, and those having a sphere diameter of 6.25 μm and a standard deviation of 0.48 μm were scattered on a substrate to give a liquid crystal layer thickness of 6.25 μm.
2 μm was obtained. d · Δn is 0.893 μm. The azimuth of the polarization axis of the lower polarizer is 80 degrees, the azimuth of the polarization axis of the upper polarizer is 10 degrees, and the slow axis angle of the lower phase plate is 110.
Degree, the slow axis angle of the upper phase plate was set to 70 degrees, and the retardation of the phase plate was set to 400 nm (wavelength: 555 nm) for both upper and lower sides. In this liquid crystal panel, light leakage near the spacer during dark state display is suppressed, and the contrast ratio is 40: 1.
That is, a liquid crystal display device having good image quality was obtained.

The liquid crystal layer 54 has a vertical component ε⊥ of the dielectric constant.
Is 5.20, the horizontal component ε‖ is 20.0, and Δn is 0.2.
This mixture was prepared by adding a chiral agent to a nematic liquid crystal composition of 144 (589 nm, 20 ° C.), and the twist angle was 240 degrees. The amount of the chiral agent required for twisting 240 degrees (0.85% by weight here)
Is added. Particles having a sphere diameter of 5.9 μm made of a divinylbenzene-polystyrene copolymer were mixed with a 50% solution of cyanoethyl pullulan (solvent was 9: 1 by weight acetone-DM).
F), and after taking out, drying at 120 ° C. for 4 hours, a spacer having a sphere diameter of 6.3 μm was sprayed on the substrate,
A liquid crystal layer thickness of 6.2 μm was obtained. d · Δn is 0.893 μm
It is. The azimuth of the polarization axis of the lower polarizer is 80 degrees, the azimuth of the polarization axis of the upper polarizer is 10 degrees, the slow axis angle of the lower phase plate is 110 degrees, and the slow axis angle of the upper phase plate is 70 degrees. And the retardation of the phase plate are both 400 nm (wavelength 555
nm). In this liquid crystal panel, light leakage near the spacer during dark state display was suppressed, and a liquid crystal display device having a good image quality and a contrast ratio of 35: 1 was obtained. In the present invention, a method of immersing the spacer base material in a high-concentration solution of a high dielectric constant polymer was used.For example, by polymerizing a high dielectric constant polymerizable monomer in the presence of the spacer base material. , A method of forming a high dielectric constant resin layer on the spacer surface, or a method of introducing a vinyl group on the surface of the spacer base material, and starting with the polymerizable vinyl monomer having a high dielectric constant polymerizable monomer in the side chain. A method of introducing a resin layer having a high dielectric constant through chemical bonding with the surface of the spacer base material, such as polymerization, may be used.

[0026]

The dielectric constant of the spacer is made higher than that of the liquid crystal material so that the electric field applied to the liquid crystal layer near the spacer surface is reduced by at least 10%. Accordingly, light leakage due to a decrease in threshold voltage in the region around the spacer can be suppressed, and a liquid crystal display device having a high contrast ratio can be provided. A normally closed liquid crystal display device having particularly good display characteristics can be provided. .

[Brief description of the drawings]

FIG. 1 is a diagram showing a liquid crystal layer near beads and a state of an electric field inside the beads of the liquid crystal display device of the present invention.

FIG. 2 is a schematic cross-sectional view illustrating a configuration of a liquid crystal display device of the present invention.

[Explanation of symbols]

11, 12 ... polarizing plate, 21, 22 ... phase plate, 31, 32
... Orientation film, 41, 42 ... Electrode, 50 ... Spacer, 51,
52: substrate, 54: liquid crystal layer, 55: electric field, 61: color filter, 62: black matrix, 63: flattening film, 81: light source, 82: light guide, 83: reflector, 91,
92 ... Drive circuit.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor, Kazuwa Araya 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor, Naoki Kikuchi 3300, Hayano, Mobara City, Chiba Hitachi, Ltd. Electronic Device Division

Claims (5)

[Claims] 1. A pair of substrates arranged at a predetermined interval via a plurality of spacers and provided with electrodes on respective opposing surfaces, and a liquid crystal layer having a predetermined thickness sandwiched between the pair of substrates. 2. The liquid crystal display device according to claim 1, wherein the plurality of spacers have a dielectric constant that is at least 4/3 times as large as a vertical component of a dielectric constant of a liquid crystal material of the liquid crystal layer. 2. The liquid crystal display device according to claim 1, wherein the plurality of spacers have a dielectric constant larger than an average dielectric constant of a liquid crystal material of the liquid crystal layer. 3. A pair of substrates arranged at a predetermined interval via a plurality of spacers, each having an electrode on its opposing surface, and a liquid crystal layer having a predetermined thickness sandwiched between the pair of substrates. Wherein at least a part of the plurality of spacers has a layer having a dielectric constant larger than an average dielectric constant of the liquid crystal on at least a part of the surface thereof. . 4. The spacer according to claim 3, wherein at least a part of the surface of the plurality of spacers has a layer having a dielectric constant larger than the average dielectric constant of the liquid crystal, A liquid crystal display device comprising a polymer. 5. The liquid crystal layer according to claim 1, wherein the liquid crystal layer is formed of a twisted nematic liquid crystal having a twist angle of 200 to 270 degrees, and an alignment film is formed on a pair of substrates facing each other. A liquid crystal display, wherein the pair of polarizing plates are formed so as to sandwich the pair of substrates, and the absorption axis of the polarizing plate and the alignment axis of the alignment film have normally closed characteristics. apparatus.