JP3780211B2 - Liquid crystal display - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device.
[0002]
[Prior art]
In the conventional liquid crystal display device, the electrodes for driving the liquid crystal layer are the transparent electrodes facing each other formed on two substrates. This is because a display method represented by a twisted nematic display method that operates by setting the direction of the electric field applied to the liquid crystal to a direction substantially perpendicular to the substrate surface is employed. On the other hand, as a method of making the direction of the electric field applied to the liquid crystal substantially parallel to the substrate surface, methods using comb electrode pairs are disclosed in, for example, Japanese Patent Publication No. 63-21907, USP4345249, WO91 / 10936, Japanese Patent Application Laid-Open No. 6-222397. And JP-A-6-160878. In this case, the electrode does not need to be transparent, and a highly conductive and opaque metal electrode is used. In these known techniques, a display method (hereinafter referred to as a transverse electric field method) in which the direction of the electric field applied to the liquid crystal is substantially parallel to the substrate surface has an extremely wide viewing angle as compared with a conventional liquid crystal display device. .
[0003]
[Problems to be solved by the invention]
In the above-described lateral electric field type liquid crystal display device, display is performed by applying an electric field in a direction substantially parallel to the substrate to the liquid crystal and rotating the liquid crystal within the substrate surface. Therefore, even if the viewing angle direction is changed, the apparent Δn · d of the liquid crystal layer does not change so much, and an extremely wide viewing angle can be obtained as compared with the conventional vertical electric field (TN) method. However, it has been found that even in the horizontal electric field mode liquid crystal display device, there exists a viewing angle range in which gradation inversion occurs. The angle is related to the orientation of the liquid crystal molecules in white display, and gradation inversion occurs at the angle at which the major axis of the liquid crystal molecules faces. An object of the present invention is to solve this problem without increasing the number of processes such as the number of rubbing, and to prevent gradation inversion.
[0004]
[Means for Solving the Problems]
(The inventors have intensively studied to solve the above problems and achieve the above object, and as a result, have found that the above object can be achieved by the following means.)
A scanning signal electrode, a video signal electrode, a pixel electrode, a reference electrode, and an active element are formed on the substrate, and a liquid crystal alignment film is formed on the electrode group directly or via an insulating layer on the substrate. The substrate is disposed opposite to the substrate on which the alignment film is formed, and the liquid crystal layer is held by the two substrates. (The electrode group is configured to apply an electric field substantially parallel to the substrate to the liquid crystal layer. The initial alignment direction of the liquid crystal molecules is one direction, and a plurality of driving directions of the liquid crystal molecules are provided in one pixel.
[0005]
FIG. 1 shows a schematic view of an example of the liquid crystal display device of the present invention. When the pixel electrode 2 and the common electrode 3 are bent as shown in the figure, the electric field direction 7 has two directions in the pixel. The rotation directions of the liquid crystal molecules arranged along the initial alignment direction 6 of the liquid crystal molecules are different depending on the two electric field directions, and are directed in two vertical directions like the liquid crystal molecules 8 when the electric field is applied. As described above, the direction in which gradation inversion is likely to occur is the major axis direction of liquid crystal molecules, but if there are two liquid crystal molecule orientations in one pixel, the optical characteristics of the two orientations are averaged. It is considered that tone reversal has disappeared because of the above characteristics.
[0006]
In order to manufacture such a liquid crystal display device, several electrode structures are conceivable. However, the liquid crystal display device can be easily manufactured by using a pixel electrode and a common electrode having a bent structure as shown in FIG. It can also be achieved by making the pixel electrode and the common electrode non-parallel. The angle of the bent portion is not particularly limited, but is preferably 120 ° or more and 180 ° or less because the bending of the pixel is not visible to the naked eye. If the angle formed between the electrode and the rubbing direction is small, there is a problem that the voltage-transmittance characteristic of the liquid crystal element becomes too steep and multi-tone display cannot be performed. This problem can be solved by providing two or more inter-electrode distances in the pixel. In the voltage-transmittance characteristics of the horizontal electric field method, the threshold voltage can be changed by the distance between the electrodes. Therefore, when there are two or more types of inter-electrode distances, the voltage-transmittance characteristics of one pixel are the average of the voltage-transmittance characteristics at each inter-electrode distance, and the voltage-transmittance characteristics become gentle and multi-gradation display is performed. Is possible. In addition, when the pixel electrode and the common electrode are bent as shown in FIG. 1, an area not related to the display composed of the image signal electrode and the common electrode at both ends of the pixel becomes large, and the aperture ratio becomes low. This problem can be solved by making the image signal electrode or the scanning signal electrode have a similar bent structure.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1]
FIG. 2 is a diagram showing the structure of various electrodes in the unit pixel of the present invention. The scanning signal electrode 4 was formed on a polished glass substrate, and the surface of the scanning signal electrode was covered with an alumina film which was an anodic oxide film of Al. A SiN (gate SiN) film and an amorphous Si (a-Si) film, which are gate insulating films, are formed so as to cover the scanning signal electrodes, and an n-type a-Si film and a pixel electrode 2 are formed on the a-Si film. And the image signal electrode 1 was formed. Further, a common electrode 3 was formed in the same layer as the pixel electrode and the image signal electrode. As shown in FIG. 2, the structure of the pixel electrode and the image signal electrode is such that both the bent common electrode and the scanning signal electrode are parallel to each other, and the transistor element and the metal electrode group are formed on one substrate. Been formed. The angles of the bent portions of the pixel electrode and the common electrode are the same, and are set to 170 degrees. Further, the distance between the pixel electrode and the common electrode was all the same in the pixel, and was set to 30 μm. As a result, an electric field is applied between one of the substrate-like pixel electrodes and the common electrode, and the direction thereof is substantially parallel to the substrate surface. The substrate-like electrodes are all made of aluminum but are not particularly limited as long as they are made of metal with low electrical resistance, and may be chromium, copper, or the like. The number of pixels is 640 (X3) X480, and the pixel pitch is 100 μm in the horizontal direction (that is, between the common electrodes) and 300 μm in the vertical direction (that is, between the scanning signal electrodes). In addition, striped R, G, B color filters were provided on the substrate opposite to the substrate having the transistor elements. A transparent resin for flattening the surface was laminated on the color filter. Epoxy resin was used as the material for the transparent resin. Further, a polyimide alignment film was applied on the transparent resin. A driving LSI is connected to the panel as shown in FIG. 3, and a scanning signal supply circuit 9 and an image signal supply circuit 10 are connected to the TFT substrate, and a scanning signal voltage, a video signal voltage, and a timing signal are received from the image information signal source 11. Supplied and active matrix driven.
[0008]
On the other hand, the rubbing directions on the upper and lower substrates were substantially parallel to each other and parallel to the image signal electrodes. The gap was held by dispersing spherical polymer beads between the substrates to a dispersion density of 100 / mm ² , and 4.0 μm in a liquid crystal sealed state. The panel was sandwiched between two polarizing plates (manufactured by Nitto Denko Corporation, G1220DU), and the polarizing transmission axis of one polarizing plate was almost parallel to the rubbing direction, and the other was orthogonal to it. Thereby, normally closed characteristics were obtained. Between the substrates, a liquid crystal having a positive dielectric anisotropy Δε composed mainly of a compound having three fluoro groups at its terminals was held. As the alignment film, a polyimide alignment film made of 2,2-bis [4- (p-aminophenoxy) phenylpropane] and pyromellitic acid dihydrate was used. This alignment film is not limited to this material, and various polyimide films can be used. The viewing angle characteristics of the panel thus produced were evaluated within an elevation angle of ± 60 degrees using an LCD viewing angle characteristic inspection device (C5718, manufactured by Hamamatsu Photonics Co., Ltd.). The gradation was set to 8 gradations, and the viewing angle dependence of the luminance at each gradation voltage was measured. As a result, gradation inversion did not occur at all angles in the manufactured panel.
[0009]
[Example 2]
FIG. 4 is a diagram showing the structure of various electrodes in a unit pixel according to the second embodiment of the present invention. A liquid crystal display device was produced in the same manner as in Example 1 except that the shapes of the pixel electrode and the common electrode were changed as shown in FIG. 4 and the distance between the pixel electrode and the common electrode was 15 μm. When viewing angle characteristics were measured in the same manner as in Example 1, gradation inversion did not occur at all angles.
[0010]
[Example 3]
A liquid crystal display device was produced in the same manner as in Example 1 except that the angle of the bent portions of the pixel electrode and the common electrode was 178 degrees. When viewing angle characteristics were measured in the same manner as in Example 1, gradation inversion did not occur at all angles. Further, the voltage-transmittance characteristics were measured, and the voltage at which the transmittance became maximum and the voltage at which the transmittance became 1% of the maximum transmittance were calculated, and were found to be 2.5 V and 1.5 V, respectively. The difference was 1.0V, which was very small.
[0011]
[Example 4]
FIG. 5 is a diagram showing the structure of various electrodes in a unit pixel according to the third embodiment of the present invention. Example 2 except that the shapes of the image signal electrode, the scanning signal electrode, the pixel electrode, and the common electrode are changed as shown in FIG. 5 and the angle of the bent portion of the image signal electrode, the pixel electrode, and the common electrode is 170 degrees. Similarly, a liquid crystal display device was produced. As shown in FIG. 5, since the image signal electrode is also bent like the pixel electrode and the common electrode, the aperture ratio is about 1.13 times that of the second embodiment. When viewing angle characteristics were measured in the same manner as in Example 1, gradation inversion did not occur at all angles.
[0012]
[Example 5]
FIG. 6 is a diagram showing the structure of various electrodes in a unit pixel according to the fourth embodiment of the present invention. A liquid crystal display device was produced in the same manner as in Example 3 except that there were two distances between the pixel electrode and the common electrode in the pixel, which were 20 μm and 10 μm. When viewing angle characteristics were measured in the same manner as in Example 1, gradation inversion did not occur at all angles. In addition, the voltage-transmittance characteristics were measured, and the voltage at which the transmittance was maximum and the voltage at which the transmittance was 1% of the maximum transmittance were calculated. As a result, they were 3.4 V and 1.0 V, respectively. The difference was sufficiently large as 2.4 V, which was a voltage difference capable of multi-gradation display.
[0013]
[Comparative Example 1]
FIG. 7 is a diagram showing the structure of various electrodes in the unit pixel of the first comparative example of the present invention. The pixel electrode and the common electrode had a linear structure, and a liquid crystal display device was produced in the same manner as in Example 2 except that the rubbing angle was 15 degrees with respect to the image signal electrode. When viewing angle characteristics were measured in the same manner as in Example 1, gradation inversion occurred at an angle of 45 degrees and an elevation angle of 45 degrees or more with respect to all the image signal electrodes.
[0014]
[Example 6]
FIG. 8 is a diagram showing the structure of various electrodes in a unit pixel according to the fifth embodiment of the present invention. The liquid crystal display device is the same as in Example 2 except that the electrode structure is such that the driving directions of two liquid crystal molecules are generated on the left and right as shown in the figure, and the initial alignment direction of the liquid crystal molecules is parallel to the scanning signal electrodes. Produced. As a result of measuring the viewing angle characteristics in the same manner as in Example 1, gradation inversion did not occur at all angles.
[0015]
[Example 7]
FIG. 9 is a diagram showing the structure of various electrodes in a unit pixel according to the sixth embodiment of the present invention. A liquid crystal display device was manufactured in the same manner as in Example 2 except that the pixel electrode and the common electrode were non-parallel as shown in the figure and the angle formed by the pixel electrode and the common electrode was 5 degrees. As a result of measuring the viewing angle characteristics in the same manner as in Example 1, gradation inversion did not occur at all angles.
[0016]
【The invention's effect】
According to the present invention, a horizontal electric field type liquid crystal display device with a wide viewing angle and a liquid crystal display device that is completely free of gradation inversion can be provided without increasing the number of processes such as the number of rubbing.
[0017]
In addition, a liquid crystal display device having the above characteristics and capable of multi-gradation display can be provided. In addition, a high aperture ratio liquid crystal display device having the above-described characteristics can be provided.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an example of a liquid crystal display device of the present invention.
FIG. 2 is a plan view of a unit pixel of the present invention.
FIG. 3 is a circuit diagram of a system configuration in the liquid crystal display device of the present invention.
FIG. 4 is a plan view of a unit pixel according to the present invention.
FIG. 5 is a plan view of a unit pixel of the present invention.
FIG. 6 is a plan view of a unit pixel according to the present invention.
FIG. 7 is a plan view of a unit pixel of a liquid crystal display device of a comparative example.
FIG. 8 is a plan view of a unit pixel according to the present invention.
FIG. 9 is a plan view of a unit pixel of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Image signal electrode, 2 ... Pixel electrode, 3 ... Common electrode, 4 ... Scan signal electrode, 5 ... Transistor element, 6 ... Initial orientation direction of liquid crystal, 7 ... Electric field direction, 8 ... Liquid crystal molecule at the time of voltage application.

Claims (5)

The scanning signal electrode, the image signal electrode, the pixel electrode, the common electrode, and the active element are arranged to form a plurality of display pixels on the first substrate,
An alignment film is formed on the first substrate on the scanning signal electrode, the image signal electrode, the pixel electrode, the common electrode and the active element directly or through an insulating film,
A counter substrate is disposed opposite the first substrate;
A liquid crystal layer is sandwiched between the first substrate and the counter substrate,
The scanning signal electrode, the image signal electrode and the common electrode are connected to an external connection means; and
A liquid crystal display device comprising polarizing means for changing the optical characteristics of the liquid crystal layer,
The liquid crystal display device characterized by having the pixel electrode, the common electrode and the image signal electrode is bent in the same manner in the first display pixel portion in one display pixel. The scanning signal electrode, the image signal electrode, the pixel electrode, the common electrode, and the active element are arranged to form a plurality of display pixels on the first substrate,
An alignment film is formed on the first substrate on the scanning signal electrode, the image signal electrode, the pixel electrode, the common electrode and the active element directly or through an insulating film,
A counter substrate is disposed opposite the first substrate;
A liquid crystal layer is sandwiched between the first substrate and the counter substrate,
The scanning signal electrode, the image signal electrode and the common electrode are connected to an external connection means; and
A liquid crystal display device comprising polarizing means for changing the optical characteristics of the liquid crystal layer,
A liquid crystal display device, wherein the pixel electrode, the common electrode, and the scanning signal electrode in one display pixel have a bent portion in the same display pixel.   2. The liquid crystal display device according to claim 1, wherein the bent portions of the image signal electrode, the pixel electrode, and the common electrode are bent at an angle of not less than 120 degrees and not more than 180 degrees.   3. The liquid crystal display device according to claim 2, wherein the bent portions of the image signal electrode, the pixel electrode, and the common electrode are bent at an angle of not less than 120 degrees and not more than 180 degrees.   The liquid crystal display device according to claim 1, wherein the liquid crystal layer has a positive dielectric anisotropy.

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