JPH1130784A - Liquid crystal display - Google Patents

Abstract

(57) [Problem] To provide a multi-domain horizontal electric field type liquid crystal display device having a high aperture ratio. The liquid crystal layer includes a pair of substrates and a liquid crystal layer sandwiched between the substrates. At least one of the substrates has a plurality of electrodes for applying an electric field substantially parallel to the substrates to the liquid crystal layer, In a liquid crystal display device having a protective film that protects at least one, and an alignment film formed on the protective film or the electrode, the electrode has a structure that is bent on a substrate surface, and the number of bends is reduced. A liquid crystal display device having 3 to 11 pixels in one pixel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that drives a liquid crystal by applying an electric field in a direction substantially parallel to a substrate surface.

[0002]

2. Description of the Related Art In a conventional liquid crystal display device, electrodes for driving a liquid crystal layer are formed on two substrates, respectively, and use transparent electrodes which are opposed to each other. This is represented by a twisted nematic display system which operates by making the direction of an electric field applied to the liquid crystal substantially perpendicular to the substrate surface.

On the other hand, a system using a pair of comb-teeth electrodes as a system for making the direction of an electric field applied to the liquid crystal substantially parallel to the substrate surface is disclosed in, for example, Japanese Patent Publication No. 63-21907, US Pat.
45,249, WO91 / 10936, JP-A-6-106
No. 2,223,972 and JP-A-6-160878.

In this case, the electrode is not necessarily required to be transparent, and an opaque metal electrode having high conductivity is used. Regarding a display method in which the direction of the electric field applied to the liquid crystal is substantially parallel to the substrate surface (hereinafter referred to as a horizontal electric field method), a multi-domain is formed using bent electrodes, For the method of eliminating the color tone change and the gradation inversion when viewed, see S.M. Aratani et al.
Jpn. J. Appl. Phys. 36 (1A / B) L27-2
9 (1997).

[0005] However, there is no mention of a method for obtaining a high aperture ratio when using such a bent electrode.

[0006]

When a pixel is simply formed using the bent electrode as described above, the pixel itself has a bent structure. For this reason, when a straight line is displayed, there is a problem that minute bending of a pixel can be seen and display quality deteriorates.

Therefore, it has been considered to form a multi-domain by using a pixel having a rectangular shape and a bent electrode alone. When such a design is actually performed, a bent common electrode and a pixel electrode are arranged between the video signal electrodes on the straight lines at the left and right ends of the pixel.
In this case, there is a problem that the area between the video signal electrode which must be shielded from light and the adjacent common electrode (or pixel electrode) is increased, so that the aperture ratio is greatly reduced.

An object of the present invention is to solve the above problems and to provide a multi-domain horizontal electric field type liquid crystal display device having a high aperture ratio.

[0009]

Means for Solving the Problems When a rectangular pixel in which only the electrode is bent is used, the following three factors can be cited as factors that lower the aperture ratio.

[0010] A region sandwiched between the video signal electrode and an electrode adjacent thereto becomes wider.

The width of the electrode adjacent to the video signal electrode is wide.

[0012] Disclination (a region where the liquid crystal does not move) that occurs at a bent portion of the electrode causes a portion through which light does not pass.

The gist of the present invention that solves the above is as follows.

(1) A pair of substrates, and a liquid crystal layer sandwiched between the substrates. At least one of the substrates has a plurality of electrodes for applying an electric field substantially parallel to the substrates to the liquid crystal layer.
In a liquid crystal display device having a protective film that protects at least one of the electrodes and an alignment film formed on the protective film or the electrode, the electrode has a structure that is bent on a substrate surface, and is bent. Wherein the number of the pixels is 3 to 11 in one pixel.

(2) The liquid crystal display device, wherein the video signal electrode formed linearly and the bent electrode closest to the video signal electrode are formed so that only the video signal electrode side is linear. .

(3) The liquid crystal display device described above, wherein the bent electrode is provided with two or more portions having different angles at the bent portion, and the electrode is gradually bent.

In order to reduce the area, it is effective to increase the number of bent electrodes. Increasing the number of bends when the bending angles of the electrodes are the same reduces the area between the electrodes and the linear video signal electrodes.

In practice, when the number of bends is three or more, the above-mentioned area is sharply reduced and a high aperture ratio is obtained.
However, conversely, when the number of bends increases, the area through which light does not pass due to the disclination increases, and the aperture ratio decreases.

The lower limit of the aperture ratio is determined by ISO9243, and is required to be 30% or more. The number of bends is preferably 3 to 11 in order to satisfy the above standard.

When the video signal electrode side of the bent electrode closest to the video signal electrode is formed linearly (FIG. 1A), a region where light leaks from the video signal electrode 8 (FIG. 1A).
(b)) can be further reduced. If this area is reduced, the size of the black matrix that shields that part can be reduced, and the aperture ratio can be substantially increased.

The bending of the electrodes at both ends in one pixel is performed in _two or more steps (θ ₁ and θ _{2 in} FIG. 6). In particular, the width of the electrode adjacent to the video signal electrode 8 can be reduced. Thereby, the aperture ratio can be increased.

Further, in order to increase the luminance, not only the aperture ratio is increased, but also the overall light transmittance is increased as described below to improve the luminance.

(4) It has blue, green, and red pixels, and the peak transmittance wavelength of the blue pixels is 450 to 500 n.
m, the peak transmittance wavelength in the green pixel is 540 to 56
0 nm, the peak transmittance wavelength in the red pixel is 540-400.
The thickness of the liquid crystal layer is formed to be 600 nm, the electrode of each pixel has a bent structure on the substrate surface, and the number of bents is 3 to 11 in one pixel, and A liquid crystal display device, wherein a bending angle of the electrode is formed to be smaller than a bending angle of the electrode of the green pixel.

Since the transverse electric field method uses a so-called birefringent mode, its transmittance T can be generally expressed by the following equation [1].

[0025]

T = T ₀ · sin ² 2φ · sin ² [(π · d · Δn) / λ] (1) where T ₀ is a coefficient, and is a transmission of a polarizing plate mainly used for a liquid crystal panel. Is the angle between the effective optical axis of the liquid crystal layer and the polarization transmission axis, d is the thickness of the liquid crystal layer, Δn
Represents the refractive index anisotropy of the liquid crystal, and λ represents the wavelength of light.

As can be seen from equation (1), d · Δn / λ
Becomes the integral multiple of 1/2, the transmittance becomes the largest. Normally, this value is set to be approximately １ ／ to obtain a sufficient transmittance. However, in a liquid crystal display device using blue, green, and red pixels that are usually used, the wavelength of light that finally passes through each pixel is different. Therefore,
The value d that satisfies d · Δn / λ ≒ 1/2 differs for each pixel. Therefore, in order to maximize the light transmittance of each pixel of blue, green, and red, it is necessary to change the thickness d of the liquid crystal layer of each pixel. However, in the case of the in-plane switching method, when the thickness d of the liquid crystal layer is changed, the driving voltage changes.

The threshold voltage Ec of the horizontal electric field type liquid crystal display device is expressed by the following equation [2].

[0028]

Ec = π / d · √ [K ₂ / (ε ₀ · Δε)] (2) where d is the thickness of the liquid crystal layer, K ₂ is the elastic constant of the twist of the liquid crystal, and ε ₀ is The dielectric constant in vacuum, Δε, indicates the anisotropy of the dielectric constant of the liquid crystal.

As can be seen from the above equation (2), as the thickness d of the liquid crystal layer decreases, the threshold voltage increases, and the so-called drive voltage increases. Therefore, when the thickness d of the liquid crystal layer is changed to maximize the transmittance of each color pixel, there arises a problem that the drive voltage of each pixel changes.

In the case of the multi-domain horizontal electric field system using the bent electrode, the driving voltage can be changed by changing the bending angle, so that this problem can be solved. That is, the drive voltage of each pixel can be made equal by reducing the bending angle in a pixel having a small thickness of the liquid crystal layer and increasing the bending angle in a pixel having a large thickness of the liquid crystal layer.

[0031]

[Embodiment 1] FIG. 1 is a schematic view of a unit pixel of a liquid crystal display device on which an electrode of the present invention is formed. FIG. 2 is a schematic sectional view of the liquid crystal display device of FIG.

A common electrode 2 made of Al on a glass substrate 1
In addition, a scanning signal electrode 3 is formed, and the surface thereof is covered with an alumina film 4. Further, a gate insulating film 5 made of SiN is formed on those electrodes, and an amorphous Si (a-Si) film 6, an n-type a-Si film 7, and a video signal made of Al / Cr are further formed thereon. A TFT (Thin Film Transistor) including the electrode 8 and the pixel electrode 9 is formed.

Further, a protective film 10 made of SiN is formed thereon, and an alignment film 11 is further formed thereon.

The common electrode 2 is arranged in parallel with the scanning signal electrode 3. The pixel opening 25 is divided into four by the common electrode 2 and the pixel electrode 9. The pixel electrode 9 partially overlaps the common electrode 2 to form a storage capacitor. The pixel pitch is 100 μm in the horizontal direction and 300 μm in the vertical direction, and the size of the opening is 77 μm × 235.
μm.

The width of each electrode in the opening 25 is 6 μm. The size of the opening 25 was determined so that light leakage between the common electrode 2 and the video signal electrode 8 would not be seen from the front even if the alignment position of the upper and lower substrates was shifted by 5.5 μm left and right.

The feature of this embodiment is that the common electrode 2 and the pixel electrode 9 are bent as shown in FIG.
The number of bends is 5. The side of the common electrode 2 adjacent to the video signal electrode 8 is formed in a straight line. The angle between the rubbing direction 21 for aligning the liquid crystal at the interface and the electrodes 2 and 9, that is, the bending angle θ is the same for all the electrodes. In the present embodiment, the angle θ is 15 degrees.

If the number of bends (the number of bends in the opening) is three or more, a considerable aperture ratio can be obtained even if the pixel is divided into four. When the number of bends is five as shown in FIG. 1, the aperture ratio is 32.5%.

It can be easily inferred from FIG. 1 that if the number of bends is one at the bend angle θ, the common electrode 2 adjacent to the video signal electrode 8 becomes very large and the aperture ratio becomes extremely small. The aperture ratio in this case is 20.5%.
By setting the number of bends to three or more, the aperture ratio can be improved.

It is desirable that the number of bends be an odd number. The formation of the multi-domain by bending the electrode as shown in FIG. 1 is intended to reduce the change in color tone in the in-plane switching method. By bending the electrode, the same number of domains each having an anti-symmetric liquid crystal direction are formed, and each color tone change is compensated for, thereby reducing the color tone change. If the number of bends is even, the number of domains whose liquid crystal directions are antisymmetric is not the same, so that the effect of reducing the change in color tone due to multi-domain formation is reduced. Therefore, it is desirable that the number of bends be odd.

It is desirable that the actual bending angle θ is small from the viewpoint of the aperture ratio, but if it is too small, disclination occurs. When the bending angle θ is small, the response speed is sharply reduced. Therefore, the range is preferably 2 to 30 degrees, and more preferably 5 to 30 degrees.

[Embodiment 2] FIG. 3 is a schematic view of a unit pixel of a liquid crystal display device according to another embodiment in which the electrodes of the present invention are formed. This embodiment is the same as the first embodiment except that the side of the common electrode 2 adjacent to the video signal electrode 8 is not a straight line but is bent like other electrodes.

The size of the opening 25 is the same as in the first embodiment.
It becomes 56 μm × 235 μm. The aperture ratio in this case is 2
5.4%, which is smaller than that of the first embodiment. In this case, when the side of the common electrode 2 adjacent to the video signal electrode 8 is formed in a straight line as shown in FIG. 1, the aperture ratio can be increased.

FIG. 4 is a diagram showing the relationship between the number of bends and the aperture ratio. The pixel size and the like are the same as in the first embodiment,
The size of the opening was 77 μm × 235 μm, and the width of each electrode in the opening was 6 μm.

As described above, the video signal electrode 8 and the common electrode 2
Is smaller in inverse proportion to the number of bends. Therefore, when the number of bends is from 1 to 5, the relative transmittance sharply increases as the number of bends increases. However, the decrease in transmittance due to disclination occurring at the bent portion increases in proportion to the number of bends. Therefore, when the number of bends exceeds 6, as shown in FIG. Above this, the aperture ratio becomes 30% or less. Therefore, the number of bends is 3
~ 11 is desirable.

[Embodiment 3] FIG. 5 is a schematic view of a unit pixel of a liquid crystal display device of another embodiment in which the electrodes of the present invention are formed. The feature of the present embodiment is that the connection portion of the common electrodes 2 is not at the lower end of the pixel as shown in FIGS.

In this case, the disclination generated in the bent portion of the central electrode can be shielded by the common electrode. Therefore, a higher aperture ratio can be obtained as compared with the case where the connection portion between the common electrodes 2 is at the lower end of the pixel.

When the pixel size is the same as that of the first embodiment, the aperture ratio in this embodiment is 33.5%, which can be improved by about 1% compared to the first embodiment.

[Embodiment 4] FIG. 6 is a schematic view of a unit pixel of a liquid crystal display device according to another embodiment in which the electrodes of the present invention are formed. The feature of this embodiment is that the bend portion of the bend portion of the common electrode 2 adjacent to the video signal electrode 8 has two bend angles (θ ₁ and θ ₂ ).
That is the provision.

By doing so, the bending angle becomes θ ₂
The width of the electrode can be reduced as compared with the case of only the above, whereby the aperture ratio can be increased.

In this embodiment, θ ₂ is 15 degrees and θ ₁ is 3 degrees. Further, the ratio of the length of theta ₁ portion and theta ₂ of the portion of the electrode was 1: 1. Others are the same as the first embodiment. In this case, the aperture ratio is 37.2%, which can be significantly improved as compared with the case of the first embodiment.

[Embodiment 5] FIG. 7 is a schematic view of a unit pixel of a liquid crystal display device according to another embodiment in which the electrodes of the present invention are formed. FIG. 8 is a schematic sectional view of the liquid crystal display device. The feature of this embodiment is that the pixel electrode 9 is made of ITO from the middle, and the pixel electrode 9 and the ITO pixel electrode 22 on the protective film 10 are arranged.

When a horizontal electric field is applied between the electrodes, a slight horizontal electric field is also generated on the pixel electrode 8. Therefore,
The liquid crystal on the electrode is also driven by the electric field, similarly to the opening. Since the pixel electrode 9 is made of ITO in the middle, light transmission due to driving rotation of the liquid crystal on the electrode can be used, so that the overall light transmittance can be increased and high luminance can be realized.

Embodiment 6 FIG. 9 is a schematic front view and a schematic sectional view of a substrate of a liquid crystal display device having electrodes according to the present invention. The feature of this embodiment is that the thickness of the liquid crystal layer of the pixel B is smaller than the thickness of the pixels G and R, and the bending angle θ of the electrode of the pixel B is smaller than the bending angles of the pixels G and B. Actually, as described above, in order to maximize the transmittance of each pixel according to the wavelength of light transmitted through each pixel, it is necessary to control the thickness of the liquid crystal layer in this way. come.

In this embodiment, the retardation of the pixel B is 240 nm, and the retardation of the pixels G and R is 280.
nm. As can be seen from the equation (2), the drive voltage of the pixel B becomes higher than the drive voltages of the pixels G and R because the thickness of the liquid crystal layer is reduced. Therefore, as shown in FIG. 9, the bending angle θ of the electrode of the pixel B is reduced, and the driving voltage is reduced. In this way, the driving voltage of each pixel can be made the same.

The bending angle θ is the angle (rubbing angle) between the electrode and the rubbing direction when a linear electrode is used.
Corresponding to When the rubbing angle is reduced, the threshold voltage in the lateral electric field method and the voltage indicating the maximum transmittance are reduced. Therefore, when the rubbing angle is reduced, the driving voltage is reduced.

By utilizing this phenomenon, the light transmittance of each pixel can be maximized and the drive voltage of each pixel can be made equal as in this embodiment. In this embodiment, the bend angle of the pixel B is set to 8 degrees, and the bend angles of the pixels G and R are each set to 15 degrees.
Degree. As a result, the drive voltages of all the pixels could be made substantially the same. As described above, since the transmittance of each pixel can be maximized while the drive voltage of each pixel remains the same, the luminance of the liquid crystal display device can be increased.

[Embodiment 7] FIG. 10 is an overall configuration diagram of a liquid crystal display device of the present invention.

A vertical scanning circuit 17, a video signal driving circuit 18, and a common electrode driving circuit 19 are connected on the TFT substrate of the liquid crystal display element of the first embodiment, and a power supply circuit and a controller 20 are connected.
Supplies a scanning signal voltage, a video signal voltage, and a timing signal, and performs active matrix driving.

In the liquid crystal display device of Example 1, the rubbing directions of the upper and lower substrates are substantially parallel. Further, a polarizing plate having a transmission axis parallel to the rubbing direction 21 of FIG. 1 and a polarizing plate having a transmission axis perpendicular to the rubbing direction 21 are used by being attached to the outside of the substrate. With this arrangement, normally closed characteristics can be obtained.

With the above-described configuration, it is possible to provide a liquid crystal display device of a horizontal electric field type which has high luminance and does not have color tone change and gradation inversion in an oblique direction.

[0061]

According to the present invention, it is possible to provide a liquid crystal display device having a high luminance, a remarkably wide viewing angle, and free from oblique color tone change and gradation inversion.

[Brief description of the drawings]

FIG. 1 is a schematic view of a unit pixel of a liquid crystal display device according to a first embodiment on which electrodes of the present invention are formed.

FIG. 2 is a schematic sectional view of the liquid crystal display device of FIG.

FIG. 3 is a schematic diagram of a unit pixel of a liquid crystal display device according to a second embodiment on which electrodes of the present invention are formed.

FIG. 4 is a diagram showing the relationship between the number of bends and the aperture ratio of the electrode of the present invention.

FIG. 5 is a schematic view of a unit pixel of a liquid crystal display device according to a third embodiment on which electrodes of the present invention are formed.

FIG. 6 is a schematic view of a unit pixel of a liquid crystal display device according to a fourth embodiment on which electrodes of the present invention are formed.

FIG. 7 is a schematic diagram of a unit pixel of a liquid crystal display device according to a fifth embodiment on which electrodes of the present invention are formed.

8 is a schematic sectional view of the liquid crystal display device of FIG.

FIG. 9 is a schematic front view and a schematic cross-sectional view of a liquid crystal display device having an electrode according to a sixth embodiment of the present invention.

FIG. 10 is an overall configuration diagram of a liquid crystal display device using the liquid crystal display element of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Glass substrate, 2 ... Common electrode, 3 ... Scan signal electrode, 4
... Alumina film, 5 ... Gate insulating film, 6 ... Amorphous Si (a
-Si) film, 7 ... n-type a-Si film, 8 ... video signal electrode,
9: pixel electrode, 10: protective film, 11: alignment film, 12: liquid crystal layer, 13: counter glass substrate, 14: black matrix, 15: color filter, 16: protective film for color filter, 17: vertical scanning circuit, 18: video signal drive circuit, 19: common electrode drive circuit, 20: power supply circuit and controller, 21: rubbing direction, 22: ITO pixel electrode, 23: display area, 24: electrode, 25: opening.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masuyuki 3300 Hayano, Mobara-shi, Chiba Electronic Device Division, Hitachi, Ltd. (72) Katsumi Kondo 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture (72) Inventor Kazuyuki Funabata 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd.

Claims (5)

[Claims] 1. A liquid crystal display device comprising: a pair of substrates; a liquid crystal layer sandwiched between the substrates; at least one of the substrates includes a plurality of electrodes for applying an electric field substantially parallel to the substrates to the liquid crystal layer; A protective film that protects at least one of the above, and a liquid crystal display device having an alignment film formed on the protective film or the electrode, wherein the electrode has a structure bent on a substrate surface, and
A liquid crystal display device wherein the number of bends is 3 to 11 in one pixel. 2. The video signal electrode according to claim 1, wherein the video signal electrode formed linearly and the bent electrode closest to the video signal electrode are formed so that only the video signal electrode side is linear. Liquid crystal display. 3. The liquid crystal display device according to claim 1, wherein the bent electrode is provided with two or more portions having different angles at the bent portion, and the electrode is bent stepwise. 4. A liquid crystal display device comprising: a pair of substrates; and a liquid crystal layer sandwiched between the substrates. At least one of the substrates includes a plurality of electrodes for applying an electric field substantially parallel to the substrates to the liquid crystal layer; A liquid crystal display device having a protective film for protecting at least one of the above, and an alignment film formed on the protective film or the electrode, comprising blue, green, and red pixels, and having a peak transmittance wavelength in the blue pixel. Is 450-500 nm, the peak transmittance wavelength of the green pixel is 540-560 nm, the peak transmittance wavelength of the red pixel is 540-600 nm, and the thickness of the liquid crystal layer is formed. And the number of bends is 3 to 11 in one pixel, and the bend angle of the electrode in the blue pixel is smaller than the bend angle of the electrode in the green pixel. The liquid crystal display device characterized by being earthenware pots formed. 5. The retardation represented by the product of the thickness of the liquid crystal layer and the refractive index anisotropy of the liquid crystal is 450 pixels for blue pixels.
１／500 nm, about 、, green pixel: 540-56
Approximately 1/2 of 0 nm, 540 to 600 nm for red pixels
The liquid crystal display device according to claim 4, wherein the liquid crystal display device is configured to be approximately の.