JPH07209648A - Liquid crystal display panel - Google Patents

Abstract

PURPOSE:To improve the visual angle dependency of the liquid crystal display panel in an intermediate display state. CONSTITUTION:The liquid crystal display panel has a couple of substrates 1 and 2 which are arranged opposite each other across a specific gap and a nematic liquid crystal layer 3 held in the gap. A counter electrode 5 and pixel electrodes 4 are formed on the internal surfaces of both the substrates 1 and 2 and face each other to prescribe pixel areas in a matrix. Orientation films 15 and 14 are formed on the internal surfaces of the substrates 1 and 2 and come into contact with the nematic liquid crystal layer 3 to hold its liquid crystal molecules 16 in a twist array state. The oriented film 14 or 15 on at least one substrate side is irradiated partially with ultraviolet rays in individual pixel areas to control the twist array state of the liquid crystal molecules 16 on an orientation division basis. The orientation layers 14 and 15 are formed of polyimide oriented films which are rubbed and selectively irradiated with the ultraviolet rays to vary the pretilt angle of the liquid crystal molecules 16.

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 panel. More specifically, it relates to a technique for improving viewing angle characteristics in halftone display. More specifically, it relates to an orientation control technique for improving the viewing angle.

[0002]

2. Description of the Related Art Conventionally, nematic liquid crystals in a twisted alignment have been generally used in liquid crystal display panels. FIG. 7 is a schematic diagram showing the operation principle of a twist nematic mode liquid crystal display panel. The left side shows the state where no voltage is applied, and the right side shows the state where voltage is applied. Upper polarizing plate 10
The polarization axis A of 1 and the polarization axis B of the lower polarizing plate 102 are orthogonal to each other. Further, the alignment direction of the upper alignment film 103 and the alignment direction of the lower alignment film 104 are also orthogonal to each other. Therefore, the nematic liquid crystal molecules 105 are twist-aligned with a 90 ° twist. When no voltage is applied, the upper polarizing plate 1
The linearly polarized light component of the incident light passing through 01 is rotated by 90 ° by the twist-aligned liquid crystal molecules 105 and is transmitted through the lower polarizing plate 102. Therefore, white display is obtained when no voltage is applied. On the other hand, when a voltage is applied, the liquid crystal molecules 105
Stands up and the optical rotation is lost. Therefore, the linearly polarized component of the incident light is blocked by the lower polarizing plate 102, and black display is obtained. Such a display system is called a normally white mode. By properly selecting the level of the applied voltage, a halftone display (gray display) between white display and black display can be obtained. In the halftone display, the nematic liquid crystal molecules 105 are between the completely laid state and the completely raised state, and their molecular axes are tilted with respect to the substrate surface.
The halftone display or gradation display is necessary for reproducing, for example, a television image or a video image.

[0003]

The liquid crystal display panel of twisted nematic mode has a problem that the viewing angle characteristic is poor. That is, the transmittance of the liquid crystal display panel has a viewing angle dependency, and the display density changes depending on the viewing direction, resulting in poor visibility. This viewing angle dependency becomes particularly noticeable in halftone display,
Image quality is significantly impaired. In some cases, the display state is reversed depending on the viewing direction, and the image quality is significantly degraded. As described above, in the halftone display, the liquid crystal molecules are in a tilted state, and the transmittance changes extremely depending on the relationship between the direction of the molecular axis and the viewing direction, and in the worst case, the image is inverted. Will happen.

In order to improve such viewing angle dependence, for example, Japanese Patent Laid-Open No. 64-88520 discloses an alignment division technique. The orientation direction of liquid crystal molecules is divided into a plurality of regions within one pixel, and the orientation directions are made different in each region. As a result, even if the position of the eyes is tilted from the normal line of the liquid crystal panel, the image is inverted in black and white at a certain portion, and white spots occur at a certain portion, the whole pixel is averaged and the viewing angle is changed over the entire screen. It is possible to obtain a good image in which black-and-white inversion and white spots do not occur regardless of this. However, in actuality, it is extremely difficult in terms of manufacturing technology to make the orientation direction different for each region within each minute pixel.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, it is an object of the present invention to improve the viewing angle dependence of halftone display in a twisted nematic mode liquid crystal display panel by simple alignment control. And The following measures have been taken in order to achieve this object. That is, the liquid crystal display panel according to the present invention has, as a basic structure, a pair of substrates opposed to each other with a predetermined gap, and a nematic liquid crystal layer held in the gap. Predetermined electrodes are formed on the inner surfaces of both substrates and face each other to define a matrix of pixel regions. An alignment layer is formed on the inner surface of each substrate and is in contact with the nematic liquid crystal layer to maintain it in a twist alignment state. As a feature of the present invention, at least one of the alignment layers on the substrate side is partially subjected to ultraviolet irradiation treatment in each pixel region, and the twist alignment state of the nematic liquid crystal layer is controlled by alignment division. . For example, the alignment layer is made of a rubbing-treated polyimide alignment film, and is subjected to ultraviolet irradiation treatment to change the pretilt angle of liquid crystal molecules contained in the nematic liquid crystal layer. In this case, the alignment film always has a constant ultraviolet irradiation area even if the ultraviolet irradiation site changes in each pixel region. Liquid crystal display panels having such a structure include active matrix type and simple matrix type. In the case of the active matrix type, subdivided pixel electrodes are formed on one substrate, and continuous counter electrodes are formed on the other substrate. A matrix-shaped pixel region is defined in a portion where both electrodes overlap in a plane.

[0006]

According to the present invention, the alignment film is partially or selectively subjected to ultraviolet irradiation treatment in each pixel region.
As a result, the twist alignment state of the nematic liquid crystal layer is controlled by alignment division. For example, if the polyimide alignment film that has been subjected to the rubbing treatment is subjected to an ultraviolet irradiation treatment, the polyimide molecular chains on the surface are broken and the surface becomes microscopically rough. For this reason, the pretilt angle of the liquid crystal molecules becomes high and the spectroscopic threshold voltage thereof becomes low at the site where the ultraviolet rays are selectively irradiated. On the other hand, the pretilt angle of the liquid crystal molecule remains low near 0 ° in the portion not irradiated with the ultraviolet light, and the optical threshold voltage becomes high. In this way, in the state where the optical threshold voltage is changed in the orientation division manner,
When a voltage of a halftone level is applied, liquid crystal molecules are laid down in one part and liquid crystal molecules are erected in the other part, and a desired halftone display is obtained as a composite of the transmittances of both parts. Although each pixel region exhibits a halftone as a whole, the liquid crystal molecules are actually a mixture of a standing state and a lying state, and the tilted state with the worst viewing angle characteristic is not included. Therefore, it becomes possible to significantly suppress the viewing angle dependency in the halftone display as compared with the related art.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic partial cross-sectional view showing an embodiment of a liquid crystal display panel according to the present invention, showing an example of an active matrix type liquid crystal display panel. However, it goes without saying that the present invention is not limited to the active matrix type and can be applied to the simple matrix type. As shown in the figure, the liquid crystal display panel is composed of a pair of upper and lower substrates 1 and 2 that are opposed to each other with a predetermined gap, and a nematic liquid crystal layer 3 held in the gap. Predetermined electrodes are formed on the inner surfaces of the substrates 1 and 2 and face each other to define a matrix of pixel regions. In this example, the pixel electrodes 4 subdivided into a matrix are formed on the inner surface of the lower substrate 2, and the upper substrate 1
A continuous counter electrode 5 is formed on the inner surface of the. A matrix-shaped pixel region is defined in a portion where the pixel electrode 4 and the counter electrode 5 overlap in a plane. In addition, on the lower substrate 2, thin film transistors 6 for switching driving are also integrally formed corresponding to the individual pixel electrodes 4. The thin film transistor 6 is configured by using polysilicon 7 patterned in a predetermined shape as an element region, and a gate electrode G is patterned on the polysilicon 7 via an insulating film 8. In addition, the wiring electrode 10 is connected to the thin film transistor 6 via the first interlayer insulating film 9.
Is electrically connected to the source region S of. This wiring electrode 10
Is covered with a second interlayer insulating film 11, on which the above-mentioned pixel electrode 4 is patterned. The pixel electrode 4 is electrically connected to the drain region D of the thin film transistor 6 through a contact hole opened in the second interlayer insulating film 11 and the first interlayer insulating film 9. On the other hand, the upper substrate 1
A black mask 12 is provided on the inner surface of the so as to match the thin film transistor 6. This black mask 1
An insulating film 13 is interposed between 2 and the counter electrode 5.

An alignment layer 14 is formed on the inner surface of the lower substrate 2. An alignment layer 15 is similarly formed on the inner surface of the upper substrate 1. These alignment layers 15 and 14 are in contact with the nematic liquid crystal layer 3 from above and below to maintain the liquid crystal molecules 16 in a twisted alignment state. However, in the drawing, the twist alignment of the liquid crystal molecules 16 is simplified for easy understanding. Actually, as shown in FIG.
6 is an arrangement in which the upper substrate 1 and the lower substrate 2 are twisted by 90 °. In this example, the alignment layers 14 and 15 are made of a rubbing-treated polyimide alignment film.

As a feature of the present invention, at least one of the alignment layers 14 and 15 on the substrate side is subjected to ultraviolet irradiation treatment partially (selectively) in each pixel region, and the twist alignment of the liquid crystal molecules 16 is performed. The state is controlled by orientation division. In the illustrated example, the left half of the pixel area is selectively irradiated with ultraviolet rays, and the pretilt angle of the liquid crystal molecules 16 included in the nematic liquid crystal layer 3 is highly changed. On the other hand, the pretilt angle of the liquid crystal molecules 16 is kept low in the right half of the portion that has not been irradiated with the ultraviolet rays.

In this embodiment, the upper and lower alignment layers 14 and 15 were irradiated with ultraviolet rays using a high pressure mercury lamp. When the pretilt angle of the liquid crystal molecule 16 was actually measured,
It was 3 to 4 ° at the ultraviolet irradiation site. On the other hand, it was 0 to 1 ° at the site not irradiated with ultraviolet rays. In this way, it becomes possible to create regions having different optical threshold voltages in the same pixel region by selective irradiation with ultraviolet rays. As a result, when a voltage of a halftone level is applied to the pixel region, the liquid crystal molecules 16 rise at a relatively low voltage in the ultraviolet ray irradiation region, and the liquid crystal molecules rise after that in the ultraviolet ray non-irradiation region. Therefore, the liquid crystal molecules are less likely to be inclined even in the intermediate display in the entire pixel region. For example, when the liquid crystal molecule is in an intermediate position in either the ultraviolet ray irradiation site or the ultraviolet ray non-irradiation site, the liquid crystal molecule in the other site is in a completely standing or sleeping state.
That is, it is possible to improve the viewing angle characteristics of the liquid crystal display panel by reducing the tilted state of the liquid crystal molecules, which has the most remarkable viewing angle dependency.

FIG. 2 is a plan view schematically showing individual pixel regions arranged in a matrix. Each pixel region 21 is divided into an upper portion 21U and a lower portion 21L. In this example, UV irradiation is performed using a mask that is aligned with each pixel region 21, and UV irradiation is applied only to the lower portion 21L shown by hatching.

FIG. 3 is a schematic plan view showing an example of improvement of the ultraviolet selective irradiation system shown in FIG. In this example, the individual pixel regions 21 arranged in a matrix are irradiated with ultraviolet rays through a stripe-shaped mask arranged in the row direction. The opening width W of the striped mask is set to half the width P of each pixel region. In this example, it is not necessary to accurately align the stripe-shaped mask and the matrix-shaped pattern of the pixel region 21.
Even if the pattern of the stripe-shaped mask and the pattern of the pixel region are shifted in the vertical direction, the area of the irradiation site in each pixel region shown by hatching is kept constant. However, the position of the irradiation site changes. Specifically, the total area of the irradiation areas divided vertically by sandwiching the non-irradiation area in each pixel region 21 is always kept constant, but the area of each of the irradiation areas of the upper and lower parts varies complementarily. Become.

FIG. 4 is a graph showing the relationship between the transmittance and the applied voltage of the liquid crystal display panel according to the present invention. The normally white mode is adopted, and the transmittance becomes approximately 100% when no voltage is applied, and the transmittance becomes close to 0% when a sufficiently high voltage is applied. Curve A in the graph represents the transmittance characteristic of the portion not irradiated with ultraviolet rays. Curve B represents the transmittance characteristic at the ultraviolet irradiation site. As described above, since the pretilt angle of the liquid crystal molecules is kept low in the non-irradiated portion, the optical threshold voltage is high. On the other hand, at the irradiated area, the surface state of the alignment layer becomes rough, so the pretilt angle of liquid crystal molecules changes to a higher level
The spectroscopic threshold voltage shifts to the lower direction. In addition,
A curve C represents a transmittance characteristic obtained by combining the curves A and B. Now, to obtain a halftone display with a transmittance of 50%,
It is assumed that a predetermined voltage level Vm is applied. At the non-irradiated portion, the liquid crystal molecules are in a substantially lying state, and at the irradiated portion, the liquid crystal molecules are substantially upright. As a combination of both parts, the desired halftone display is obtained.

FIG. 5 schematically shows the alignment state of liquid crystal molecules in the halftone display shown in the graph of FIG. In each pixel region 51, the liquid crystal molecules 53 are in a lying state in the right half of the ultraviolet ray non-irradiated portion 52. On the other hand, the liquid crystal molecules 53 are almost upright at the ultraviolet irradiation site 54 on the left half. As described above, the liquid crystal molecules 53 included in the pixel region are in a mixture of a substantially raised state and a substantially lying state, even though the halftone display is performed in the pixel region 51 as a whole. Therefore, almost no liquid crystal molecules in an intermediate tilted state in which the viewing angle dependence is most prominent are included.

FIG. 6 schematically shows a halftone display state in a conventional liquid crystal display panel. As shown in the drawing, all the liquid crystal molecules 53 in the individual pixel regions 51 are in an intermediate tilted state, and the viewing angle dependency is remarkably exhibited, and image reversal or the like occurs, and the display quality is significantly impaired.

[0016]

As described above, according to the present invention, the alignment layer on the side of at least one substrate is partially subjected to the ultraviolet irradiation treatment in each pixel region, and the nematic liquid crystal layer is twist-aligned. The state is controlled by orientation division.
Since parts with different optical threshold voltages can be created in the same pixel, halftone display can be realized as a mixture of the liquid crystal molecules in the lying state and the rising state, and the viewing angle dependency can be significantly improved compared to the past. The effect of becoming is obtained. The change and inversion of the display density depending on the viewing angle are improved, and a display with good display quality can be realized.

[Brief description of drawings]

FIG. 1 is a schematic partial sectional view showing an embodiment of a liquid crystal display panel according to the present invention.

FIG. 2 is a schematic diagram showing an example of alignment-divisional ultraviolet irradiation processing.

FIG. 3 is a schematic view showing another example of an alignment-division type ultraviolet treatment system.

FIG. 4 is a graph showing transmittance characteristics of the liquid crystal display panel according to the present invention.

FIG. 5 is a schematic view showing a molecular alignment state of a liquid crystal display panel according to the present invention.

FIG. 6 is a schematic view showing a molecular alignment state in a conventional liquid crystal display panel.

FIG. 7 is an explanatory diagram showing an operation principle of a conventional twist nematic mode liquid crystal display panel.

[Explanation of symbols]

 1 Upper Substrate 2 Lower Substrate 3 Nematic Liquid Crystal Layer 4 Pixel Electrode 5 Counter Electrode 6 Thin Film Transistor 14 Alignment Layer 15 Alignment Layer 16 Liquid Crystal Molecules

Claims (4)

[Claims] 1. A pair of substrates, which are arranged to face each other with a predetermined gap, and a nematic liquid crystal layer held in the gap, and predetermined electrodes are formed on the inner surfaces of both substrates to face each other. To define a matrix of pixel areas, and an alignment layer is formed on the inner surface of each substrate to contact the nematic liquid crystal layer and maintain it in a twisted alignment state. The liquid crystal display panel is characterized in that the side alignment layer is partially subjected to ultraviolet irradiation treatment in each pixel region, and the twist alignment state of the nematic liquid crystal layer is controlled by alignment division. 2. The alignment layer is made of a rubbing-treated polyimide alignment film, and changes the pre-tilt angle of liquid crystal molecules contained in the nematic liquid crystal layer when subjected to ultraviolet irradiation treatment. 1. The liquid crystal display panel according to 1. 3. The liquid crystal display panel according to claim 1, wherein the alignment film has a constant ultraviolet irradiation area even if the ultraviolet irradiation site changes in each pixel region. 4. A subdivided pixel electrode is formed on one substrate, and a continuous counter electrode is formed on the other substrate, and a matrix-shaped portion is formed in a portion where both electrodes overlap in a plane. The liquid crystal display panel according to claim 1, wherein a pixel region is defined.