JPH10186391A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain the compatibility between a wide visual field angle and display uniformity inconspicuous in display abnormality, by forming an oriented layer on at least one surface of a liquid crystal layer, specifying the specific resistance of this liquid crystal layer, and forming a part of electrode structure in direct contact with the oriented film. SOLUTION: Two sheets of transparent glass substrates having polished surfaces are used as substrates SUB1, 2. A thin-film transistors is formed on the one substrate of these substrates SUB1, 2. A silicon nitride film which is an insulating film is formed thereon. A pixel electrode, which is a slender wire-shaped or belt-like electrode and a common electrode are formed as the electrode structure for driving liquid crystal on the insulating film. Further, an oriented film is applied thereon. In such a case, the oriented film is formed on at least one surface of the liquid crystal layer and the specific resistance of the liquid crystal layer is >=10<13> Ω.cm. The electrode structure is partly formed in direct contact therewith.

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 system in which a parallel electric field is applied to a substrate. It relates to a display device.

[0002]

2. Description of the Related Art In a conventional liquid crystal display device, electrodes for driving liquid crystal are formed on the surface of two substrates, and electrodes which are opposed to each other are used. This is because a display method typified by a twisted nematic display method, which operates by applying an electric field in a direction perpendicular to the substrate to the liquid crystal, is employed. In this case, a transparent electrode such as ITO (Indium Tin Oxide) is used for the electrode.

On the other hand, as a method of making the direction of an electric field applied to the liquid crystal substantially parallel to the substrate (hereinafter referred to as a lateral electric field method), a method using a comb-shaped electrode provided on one substrate. Has been proposed in Japanese Patent Publication No. 63-21907 and U.S. Pat. No. 4,345,249.

In this case, the electrodes need not be transparent,
An opaque metal electrode having high conductivity is used. Further, in the horizontal electric field method using an active element, defined to the specific resistance of the liquid crystal is less than ¹ × ¹⁰ 1 ³ Ω · cm is, JP-A 7-33341, JP-A No. 7-3064
No. 17 publication.

[0005]

In a lateral electric field system in which the liquid crystal is aligned in the same direction on the upper and lower substrates, a normally black system in which black display is performed when no voltage is applied and white display is performed when a voltage is applied is generally used. Has been adopted.

In this system, black unevenness occurs locally after a long operation, for example, in white display, as compared to the normally white system in which white display is performed when no voltage is applied, which is adopted in the conventional twisted nematic system. There is a problem that display abnormalities become remarkable.

In addition, in the homogeneous alignment of liquid crystal, it is remarkable that slight alignment abnormality causes display failure. Furthermore, in the case of a method of driving liquid crystal using an active element, an insulating film for protecting electrodes and the active element is formed.

However, in the conventional in-plane switching mode liquid crystal display device, if the insulating film is provided so as to cover the electrodes for driving the liquid crystal, there is a disadvantage that the optical voltage holding ratio is reduced. . That is, although a large voltage holding ratio is not electrically lost even in a low-resistance liquid crystal which is an original feature of the horizontal electric field method, there is a problem that the voltage holding ratio is optically small. Therefore, there is a problem that the optical voltage holding ratio is reduced due to local contamination, and the display is recognized as abnormal display.

Further, the direct current is easily accumulate in the insulating film, the specific resistance of the liquid crystal using at most 10 ^{1 2} Ω · cm about the low resistivity liquid crystal material, remaining on the insulating film by ions in the liquid crystal DC If the components are not compensated, an afterimage will occur.
Therefore, in the conventional lateral electric field method in which the insulating film covers the electrodes for driving the liquid crystal, there is a restriction that a liquid crystal material having a low specific resistance must be used. This leads to high contamination of the liquid crystal, which is one of the causes of deteriorating the display quality. In general, a liquid crystal having a small specific resistance value is liable to be contaminated, and thus the resistance value does not tend to increase.

The present invention relates to a lateral electric field type active matrix type liquid crystal display device which solves these problems and achieves both a wide viewing angle and display uniformity in which display abnormalities are not noticeable.

[0011]

Means for Solving the Problems In order to solve the above problems and achieve the above object, the present invention uses the following means.

Means 1 include a liquid crystal layer composed of a liquid crystal having a structure capable of being twisted, two substrates sandwiching the liquid crystal layer, and the liquid crystal layer having a surface for displaying an image,
In an electrode structure for generating an electric field having a component substantially parallel to the surface, and in a liquid crystal display device that displays an image switched by being controlled by the electric field, an alignment layer is formed on at least one surface of the liquid crystal layer. the specific resistance of the liquid crystal layer ^{10 1 3} Omega
Cm or more, a part of the electrode structure is formed directly in contact with the alignment film.

According to a second aspect of the present invention, in the first aspect, the electrode structure includes a pixel electrode and a counter electrode, and is formed between a protective film covering an active element and the alignment film.

According to a third aspect, in the first aspect, the composition of the liquid crystal is mainly composed of a fluorine-based liquid crystal having a fluoro group.

Means 4 is characterized in that, in any one of means 1 to 3, the liquid crystal composition contains 1% or less of a liquid crystal compound having a cyano group at a terminal.

Means 5 is characterized in that in any one of means 1 to 3, the residual solvent in the alignment film is 100 ppm or less.

The means 6 is characterized in that in any one of the means 1 to 3, the imidation ratio of the alignment film is 65% or more.

The means 7 is characterized in that, in the means 1, the at least one conductive layer which does not affect the driving of the liquid crystal is provided on one of the substrates on which the electrode structure is not formed.

The operation of the means 1 to 3 is as follows.

It is known that the specific resistance of the liquid crystal has a great correlation with the liability of contamination of the liquid crystal due to external factors. In general, a low-resistance liquid crystal, whose resistance value does not increase even after purification, is contaminated by external factors such as moisture, and its resistance value decreases.

In the conventional in-plane switching type liquid crystal display device, the liquid crystal may be locally contaminated by foreign matter or the like on the display surface, and the locally contaminated liquid crystal may cause abnormal display unevenness such as black unevenness. It is one of the causes.

It has been found that the insulating film covering the electrodes for driving the liquid crystal is a cause of lowering the optical voltage holding ratio.
The insulating film also tends to cause a DC voltage to remain, and another object is to use a liquid crystal having a small resistance to compensate for the residual DC component.

Therefore, a DC voltage component hardly remains on the insulating film except for the insulating film on the electrode. In this case, a liquid crystal having high resistance can be used. Then, it has been found that the display abnormality that occurs locally can be greatly improved by this. Further, by using a high-resistance liquid crystal, the liquid crystal can be repeatedly sealed, which is effective in cost reduction.

The operation of the means 4 is as follows.

The high-resistance liquid crystal is easily achieved by a mixed system of only a fluorine-based liquid crystal having a terminal fluoro group. However, in general prone to high resistance of about 10 ^{1 4} Ω · cm is a fluorine-based liquid crystal.

In this case, since the influence of static electricity occurs,
This can be improved by mixing a liquid crystal compound containing 1% or less of a cyano group.

The operation of the means 5 or 6 is as follows.

As described above, the liquid crystal having a low resistance has a function of canceling the DC remaining in the pixel, and the problem of the afterimage based on the residual DC voltage is not so large. However, when such a high-resistance liquid crystal is used, an afterimage based on the residual DC voltage becomes apparent. Therefore, in this case, by using an alignment film in which the residual DC voltage is quickly relaxed, or in which the residual DC voltage is small, the after-image is not remarkable by the high-resistance liquid crystal, and the after-image and uneven display are abnormal. Reduction can be compatible.

The operation of the means 7 is as follows.

When a high-resistance liquid crystal is used, in the lateral electric field method of this embodiment, since one of the substrates has no electrode, the non-uniformity due to static electricity becomes more remarkable. Therefore, by providing at least one conductive or low-resistance insulating thin film that does not affect the driving of the liquid crystal on the one substrate side on which the electrodes are not formed, the static electricity caused by using the high-resistance liquid crystal is provided. Problem is solved.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described by way of examples.

[Embodiment 1] FIG. 1 is a conceptual diagram of the whole active matrix display device according to the present invention. In addition,
FIG. 10 shows a conceptual diagram of an active element and a drive circuit unit.

As the substrates SUB1 and SUB2, two transparent glass substrates having a thickness of 1.1 mm and a polished surface are used. A thin film transistor was formed on one of these substrates, and a silicon nitride film PSV as an insulating film was formed thereon.

As an electrode structure for driving the liquid crystal, a pixel electrode PX and a common electrode CT, which are elongated linear or band-like electrodes, are formed on the insulating film PSV, and an alignment film ORI1 is further coated thereon. did.

The pixel electrode PX and the common electrode CT which are in direct contact with the alignment film or the liquid crystal are formed of an ITO which becomes a transparent electrode in consideration of a chemical reaction of the alignment film with a residual solvent and the like and corrosion of metal.
Material was used.

In this embodiment, polyimide is used as the alignment film ORI1 on the side of the substrate SUB1 on which the active element is formed, and a rubbing process is performed on the surface to align the liquid crystal. On the other substrate SUB2 side, a color filter with a black matrix was formed, and an alignment film ORI2 of polyimide was applied on the outermost surface, and the same rubbing treatment was performed.

By maintaining the firing temperature of the alignment film at a peak temperature in the range of 200 to 240 ° C. for about 5 to 10 minutes, the residual solvent of the alignment film is 100 ppm or less and the imidization ratio is 65%. This is set so as to prevent display unevenness defects such as black unevenness from occurring even when used under severe conditions for a long time. The thickness of the alignment film is about 0.05 to
The film was formed to have a uniform thickness within a range of 0.15 μm.

FIG. 3 shows the initial alignment direction RDR and the transmission axis direction MAX of the polarizing plate with respect to the applied electric field EDR of the liquid crystal molecules used in this embodiment.

The rubbing directions on the upper and lower interfaces are almost parallel to each other, and the angle between the rubbing direction and the direction of the applied electric field is 75 degrees (φLC1
= ΦLC2 = 75 °). Further, the panel is sandwiched between two polarizing plates (G1220DU manufactured by Nitto Denko Corporation), and the polarization transmission axis of one of the polarizing plates is set to φP1 = 75 °, and the other is set orthogonal to it, that is, φP2 = -15 °. . Therefore, in the present embodiment, a dark (black) state at a low voltage (VOFF) and a high voltage (VO)
Normally closed characteristics that take a bright (white) state in N) are adopted.

The dielectric anisotropy Δε between these substrates is positive and the value is 7.3, and the refractive index anisotropy Δn is 0.074.
(589 nm, 20 ° C.). The gap d was formed by dispersing and holding spherical polymer beads between the substrates, and was 4.0 μm in a liquid crystal sealed state. Therefore Δ
n · d is 0.296 μm. Specific resistance of the liquid crystal used at this time is ^{8 × 10 1 3 Ω · cm} , and a mixed system of specific fluorine-based liquid crystal or the like having high resistance.

That is, the liquid crystal composition is disclosed in
JP-A-33341, wherein at least one fluoro group is introduced as a terminal group into the liquid crystal,
It contains a liquid crystal compound represented by the general formula (I).

[0042]

Embedded image

Here, at least one of X _{1 to} X ₃
One is a fluoro group, and the rest are a fluoro group, a cyano group,
Represents a trifluoromethyl group, a trifluoromethoxy group, a nitrile group or a hydrogen atom, R represents an alkyl group or an alkoxy group having 1 to 10 carbon atoms which may be substituted, and ring A represents a cyclohexane ring, a benzene ring, Represents a dioxane ring, a pyrimidine ring or a [2,2,2] -bicyclooctane ring, Z represents a single bond, an ester bond, an ether bond or methylene, methyleneoxy or ethylene, and n is an integer of 1 or 2 It is.

And / or contains a liquid crystal compound represented by the general formula (II).

[0045]

Embedded image

Here, at least one of X _{1 and} X ₂ is a fluoro group, and the rest represents a fluoro group, a cyano group, a trifluoromethyl group, a trifluoromethoxy group, a nitrile group or a hydrogen atom. Represents an alkyl group or an alkoxy group having 1 to 10 carbon atoms which may be substituted, and ring A represents a cyclohexane ring, a benzene ring, a dioxane ring, a pyrimidine ring, or [2, 2,
2] -bicyclooctane ring, Z represents a single bond, an ester bond, an ether bond or methylene, methyleneoxy or ethylene, and n is an integer of 1 or 2.

FIG. 2 shows the principle of operation of liquid crystal molecules in a bright state and a dark state.

In the initial alignment state, the liquid crystal molecules are homogeneously aligned substantially parallel to the substrate surface, and the liquid crystal molecules are rotated in the plane by an electric field substantially parallel to the substrate surface, thereby performing display in a birefringence mode. Was.

The light transmittance T / T ₀ at this time is expressed by the following equation.

T / T ₀ = sin ² (2αeff) · sin ² (πdeff · Δn / λ) (1) where αeff is the angle between the effective optical axis of the liquid crystal layer and the polarization transmission axis. In this example, the effective value of the rotation angle α of the liquid crystal molecules in the thickness direction of the liquid crystal layer is an apparent value that can be treated as an average value when uniform rotation is assumed.

Also, deff is the effective thickness of the birefringent liquid crystal layer, Δn is the refractive index anisotropy, and λ is the wavelength of light.

In the equation (1), at the time of the applied electric field EDR, the value of αeff increases according to the intensity, and becomes maximum at 45 degrees.

Further, in the simulation of this example, the retardation Δn · deff of the liquid crystal layer was selected to be half of the light wavelength λ to realize the zero-order mode of birefringence, and the dielectric anisotropy Δ
ε is set to be positive.

FIG. 4 is a plan view of an essential part showing one pixel of the liquid crystal display section and its periphery in the active matrix type color liquid crystal display device according to the embodiment of the present invention.

FIG. 5 is a sectional view taken along section line 6-6 in FIG.

FIG. 6 is a sectional view of the thin film transistor element TFT taken along the line 7-7 in FIG.

FIG. 7 is a sectional view of the storage capacitor Cstg taken along section line 8-8 in FIG.

The thin film transistor TFT comprises a source electrode SD1, a drain electrode SD2, a scanning signal line gate electrode GT, and amorphous silicon AS.

The scanning signal line GL and the counter voltage signal line CL are formed by patterning the same metal layer.

Further, the video signal line DL and the source electrode SD
1. Both drain electrodes SD2 were formed by patterning the same metal layer.

Further, after forming the insulating film PSV, the common electrode CT, which is a portion for driving the liquid crystal, is electrically connected to the above-described counter voltage signal line CL through the through hole TH2, and the pixel electrode PX is also connected to the through hole near the transistor portion. T
H1 electrically connects to the source electrode SD1. The common electrode CT and the pixel electrode PX were formed using a transparent ITO material.

The capacitance element Cstg is connected to the source electrode SD1 electrically connected to the pixel electrode PX by the through hole TH1.
And the counter voltage signal line CL with the insulating protective film GI interposed therebetween.

The pixel electrode PX is shown in the front view of FIG.
It is arranged between three common electrodes CT.

The pixel pitch is 100 μm in the horizontal direction and 300 μm in the vertical direction.

The wiring widths are scanning signal lines GL, video signal lines DL, and counter voltage signal lines C, which are wirings extending over a plurality of pixels.
L was made as large as possible to avoid line defects. That is,
The wiring width was about 10 μm, 8 μm, and 8 μm, respectively.

On the other hand, the widths of the pixel electrode and the common electrode, which are independently formed in pixel units, and the portions of the video signal lines extending in the longitudinal direction are slightly reduced to about 5 μm and 6 μm, respectively.

By reducing the width of these electrodes, the possibility of disconnection due to the entry of foreign matter or the like increases, but in this case, one pixel is partially missing but does not lead to a line defect.

The video signal line DL and the common electrode PX are provided with a gap of 2 μm via the insulating film PSV.

The number of pixels is 640 × 3 × by 640 × 3 (R, G, B) video signal lines and 480 scanning signal lines.
The number was 480.

FIG. 8 shows the structure of a color filter substrate with a black matrix (BM).

FIG. 8 shows a front view as seen from a direction perpendicular to the substrate surface, and a cross-sectional view along AA 'and BB' of the front view.

A material in which carbon and an organic pigment were mixed was used for the BMs 22a and 22b.

Thereafter, a pigment is dispersed in the photosensitive resin, and R,
The color matrix 23 was formed by coating, patterning exposure, and developing each of G and B. Then, an epoxy polymer was applied and formed as an overcoat film OC of the color filter.

The active matrix type liquid crystal display device obtained in this manner had a wide viewing angle in which no gradation inversion occurred while maintaining a contrast of 10 or more at 60 degrees, up, down, left and right, and good display quality.

[Embodiment 2] This embodiment has the same configuration as that of Embodiment 1 except for the following.

FIG. 9 shows the structure of the color filter substrate with BM.

FIG. 9 shows a front view as seen from a direction perpendicular to the substrate surface and side sectional views taken along AA 'and BB' of the front view.

First, an extremely thin film 26 of tin oxide of about 10 ° was formed on the substrate. After applying a carbon-containing resin thereon, patterning exposure and development were performed to form a black matrix BM22.

Thereafter, a pigment is dispersed in the photosensitive resin,
The color matrix 23 was formed by coating, patterning exposure, and developing each of G and B.

The thus obtained active matrix type liquid crystal display device has a wide viewing angle at which no grayscale inversion occurs while maintaining a contrast of 10 or more in the vertical and horizontal directions of 60 ° or more, and has good display quality. Even when the display screen was rubbed, no large disturbance of the alignment of the liquid crystal due to static electricity occurred.

[Embodiment 3] This embodiment is the same as Embodiment 1 except for the following.

The liquid crystal used had a positive dielectric anisotropy Δε of 10.1 and a refractive index anisotropy Δn of 0.084.
(589 nm, 20 ° C.). On the other hand, the specific resistance is 3 × 1
0 ^{1 4} Ω · cm at which was for a specific resistance value was a liquid crystal compound is mixed 10ppm is 5 × 10 ¹ containing cyano group
³ Ω · cm.

That is, the mixed liquid crystal is a liquid crystal compound represented by the general formula (I), and at least one of X _{1 to} X ₃ is a cyano group, and the rest is a fluoro group, a cyano group,
Represents a trifluoromethyl group, a trifluoromethoxy group, a nitrile group or a hydrogen atom, R represents an alkyl group or an alkoxy group having 1 to 10 carbon atoms which may be substituted, and ring A represents a cyclohexane ring, a benzene ring, Represents a dioxane ring, a pyrimidine ring or a [2,2,2] -bicyclooctane ring, Z represents a single bond, an ester bond, an ether bond or methylene, methyleneoxy or ethylene, and n is an integer of 1 or 2 It is.

And / or a liquid crystal compound represented by the general formula (II), wherein at least one of X _{1 and} X ₂ is a cyano group, and the rest is a fluoro group, a cyano group, a trifluoromethyl group, Trifluoromethoxy group,
R represents a substituted or unsubstituted alkyl or alkoxy group having 1 to 10 carbon atoms, and ring A represents a cyclohexane ring, a benzene ring, a dioxane ring, a pyrimidine ring, or [2 , 2,2] −
Represents a bicyclooctane ring, Z represents a single bond, an ester bond, an ether bond or methylene, methyleneoxy or ethylene, and n is an integer of 1 or 2.

The active matrix type liquid crystal display device thus obtained maintains a contrast of 10 or more at 60 degrees or more in the vertical and horizontal directions and has a wide viewing angle at which gradation inversion does not occur. Did not cause a large disturbance of the alignment of the liquid crystal.

In the present embodiment, the electrode structure is composed of the pixel electrode and the common electrode, and both electrodes are formed between the protective film PSV and the alignment film. There is no limitation, and it goes without saying that the invention can be applied even if at least one electrode is in direct contact with the alignment film, and the electrode is formed on an insulating film such as a substrate.

[0087]

According to the present invention, the horizontal electric field method in contact with the electrode directly alignment film for driving the liquid crystal, wide by specific resistance of the liquid crystal is more than ¹ × 10 1 ³ Ω · cm It is possible to obtain an active matrix type liquid crystal display device which has both viewing angle and high image quality with little display abnormality.

[Brief description of the drawings]

FIG. 1 is a simple conceptual diagram of an active matrix type liquid crystal display device of the present invention.

FIG. 2 is a diagram showing an operation of a liquid crystal in a horizontal electric field mode of the present invention.

FIG. 3 is a diagram showing definitions of a direction of an applied electric field, an initial alignment (rubbing) direction, and an axial direction of a polarizing plate in a horizontal electric field method of the present invention.

FIG. 4 is a plan view of a principal part showing one pixel of a liquid crystal display unit and its periphery in an active matrix type color liquid crystal display device according to an embodiment of the present invention.

FIG. 5 is a sectional view taken along section line 6-6 in FIG. 4;

FIG. 6 is a cross-sectional view of the thin film transistor element TFT taken along section line 7-7 in FIG.

FIG. 7 shows a storage capacitance Cstg along a section line 8-8 in FIG. 4;
FIG.

8A and 8B are a plan view and a cross-sectional view illustrating a configuration (1) of a color filter substrate.

FIG. 9 is a plan view and a cross-sectional view illustrating a configuration (2) of a color filter substrate.

FIG. 10 shows a TF in the liquid crystal display device according to the embodiment of the present invention.
It is a figure showing a T circuit system.

[Explanation of symbols]

CT: common electrode (common electrode), GI: gate insulating film,
DL: video signal line, SD2: drain electrode, PX: pixel electrode, SD1: source electrode, ORI: alignment film, LC: liquid crystal molecule, SUB: substrate, POL: polarizing plate, EDR: electric field, RDR: rubbing direction, MAX ... Polarizing plate transmission axis direction, GL ... Scan signal line, AS ... Amorphous silicon,
TFT: thin film transistor, Cstg: storage capacity, 17:
Control circuit, 18 ... scanning signal line driving circuit, 19
... Video signal line driving circuit, 20 ... Common electrode driving circuit,
Reference numeral 21: active matrix type liquid crystal display element, 22: black matrix, 23: color filter, OC: overcoat film, PSV: insulating film, 26: extremely thin transparent conductive film

Claims (7)

[Claims] 1. A liquid crystal layer comprising a liquid crystal having a structure capable of being twisted, two substrates sandwiching the liquid crystal layer, the liquid crystal layer having a surface for displaying an image, and being substantially parallel to the surface. An electrode structure that generates an electric field having various components, and a liquid crystal display device that displays an image that is switched by being controlled by the electric field. An alignment layer is formed on at least one surface of the liquid crystal layer. in resistivity 10 ^{1 3} Ω · cm or more, a portion of the electrode structure is characterized in that it is formed in direct contact with the alignment film. 2. The liquid crystal display device according to claim 1, wherein the electrode structure comprises a pixel electrode and a counter electrode, and is formed between a protective film covering an active element and the alignment film. 3. The liquid crystal display device according to claim 1, wherein the liquid crystal composition is mainly composed of a fluorine-based liquid crystal having a fluoro group. 4. The liquid crystal display device according to claim 1, wherein the liquid crystal composition contains 1% or less of a liquid crystal compound having a terminal cyano group. 5. The liquid crystal display device according to claim 1, wherein the residual solvent in said alignment film is 100 ppm or less. 6. The liquid crystal display device according to claim 1, wherein the imidation ratio of the alignment film is 65% or more. 7. The liquid crystal display device according to claim 1, wherein at least one of the substrates on which the electrode structure is not formed has at least one conductive layer that does not affect the driving of the liquid crystal.