Liquid crystal display device

The liquid crystal display device includes a pixel structure provided with a large wall formed along a long side of a pixel with a rectangular plane, a small wall formed at a center of the pixel and extending in the same direction as the large wall, a wall electrode formed on a wall surface of the large wall, a plane electrode formed between the small and large walls, in which the wall electrode and the plane electrode form a pixel electrode, and a common electrode formed on a surface of the small wall. The large wall has a part with an increased thickness at an end part of the pixel. The wall electrode is bent toward the center of the pixel. This structure prevents decrease of reverse twist of liquid crystal at an end part of the pixel as well as generation of domain, thus improving transmittance of the screen.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent Application JP 2012-086872 filed on Apr. 10, 2012, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a display device, and more particularly, the present invention relates to an IPS type liquid crystal display device with excellent viewing angle property, which is capable of realizing a high-definition screen.

2. Related Art

The liquid crystal display panel, used for the liquid crystal display device includes a TFT substrate having pixels with pixel electrodes and thin film transistors (TFT) arranged in a matrix, a counter substrate that faces the TFT substrate and has color filters at positions corresponding to the pixel electrodes of the TFT substrate, and a liquid crystal interposed between the TFT substrate and the counter substrate. The image is generated by controlling the light transmittance of the liquid crystal molecule for the respective pixels.

Since the liquid crystal display device has a light-weight and flat structure, it has been widely employed for the use in various fields. The compact liquid crystal display device has been extensively applied to the mobile phone, DSC (Digital Still Camera) and the like. The liquid crystal display device has a problem concerning the viewing angle property as the phenomenon that brightness, chromaticity and the like on the screen viewed from front are different from those on the screen obliquely viewed. The IPS (In Plane Switching) type that activates the liquid crystal molecules by the horizontal electric field exhibits excellent viewing angle property.

Japanese Unexamined Patent Application Publication No. Hei 6-214244 discloses the structure as one of IPS types having an electrode formed on a part of the wall of the columnar spacer that defines the space between the TFT substrate and the counter substrate, which applies the voltage between the electrode on the wall and the electrode formed on the TFT substrate so as to efficiently generate the transverse electric field as shown inFIG. 4.

SUMMARY OF THE INVENTION

The high definition display device with fine resolution equal to or higher than WVGA (800×480) has been employed for high-end products such as the medium/small liquid crystal display devices. For the high definition display, the transmittance of the panel is a key factor. The screen, with far higher definition, has been demanded to evoke importance of transmittance of the panel. As the definition becomes higher, the voltage applied to the pixel electrode becomes more influential on the neighboring pixel. This may cause contamination between those pixels, resulting in deteriorated image quality.

To cope with the demand for the higher definition screen, the wall is formed in the pixel, on which the pixel electrode or the common electrode is formed, that is, the wall electrode IPS has been under development. The wall, electrode IPS allows improvement in efficiency of the display mode. The improved display mode efficiency increases the rate of the line of electric force in the transverse direction, thus enabling improvement in efficiency of the IPS type. The wall-like electrode makes it possible to apply the electric field to the entire pixel. Therefore, the voltage applied to the pixel electrode may be reduced.

Compared to the generally employed IPS type, the wall electrode IPS is capable of reducing intensity of the voltage applied to the liquid crystal layer. On the contrary, as the field intensity applied to the liquid crystal layer is low, the electric field in the reversely twisted direction generated in the pixel will cause the forward twisted liquid crystal to compete against the reversely twisted liquid crystal. This may generate the region within the pixel where the liquid crystal is immobilized, that is, domain is generated. As a result, the transmittance distribution in the pixel becomes inhomogeneous, thus causing the problem of significantly deteriorating the transmittance.

The present invention provides the liquid crystal display device with wall electrode IPS, which is configured to prevent generation of the domain within the pixel, and further prevent deterioration in the transmittance of the pixel.

In order to attain the object, the present invention is configured as described below.

(1) A liquid crystal display device includes a pixel structure provided with a large wall formed along a long side of a pixel with a rectangular plane, a small wall, formed at a center of the pixel and extending in the same direction as the large wall, a wall electrode formed on a wall, surface of the large wall, a plane electrode formed between the small wall and the large wall, in which the wall electrode and the plane electrode form a pixel electrode, and a common electrode formed on a surface of the small wall. The large wall has a part with an increased thickness at an end part of the pixel. The wall electrode is bent in a direction toward the center of the pixel.
(2) A liquid crystal display device includes a pixel structure provided with a large wall formed along a long side of a pixel with a rectangular plane, a small wall formed at a center of the pixel and extending in the same direction as the large wall, a wall electrode formed on a wall surface of the large wall, a plane electrode formed between the small wall and the large wall, in which the wall electrode and the plane electrode form a pixel electrode, and a common electrode formed on a surface of the small wall. The large wall has a part with an increased thickness at an end part of the pixel. The wall electrode is bent in a direction toward the center of the pixel. The small wall is bent in a direction away from the bent wall electrode at the end part of the pixel.
(3) A liquid crystal display device includes a pixel structure provided with a large wall formed along a long side of a pixel with a rectangular plane, a small, wall formed at a center of the pixel and extending in the same direction as the large wall, a wall electrode formed on a wall surface of the large wall, a plane electrode formed between the small wall and the large wall, in which the wall electrode and the plane electrode form a pixel electrode, and a common electrode formed on a surface of the small wall. The large wall, has a part with an increased thickness at an end part of the pixel. The wall electrode is bent in a direction toward the center of the pixel. The small wall has a length longer than that of the large wall. A part of the small wall corresponding to the length longer than that of the large wall is bent in a direction away from the bent wall electrode.

According to the present invention, the IPS type liquid crystal display device with the wall electrode is capable of preventing generation, of the domain within the pixel. This makes it possible to provide the liquid crystal display device with high definition screen as well as the property of high transmittance, therefore, high brightness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described referring to the following embodiments.

First Embodiment

FIG. 1shows a structure of a pixel according to the present invention as a plan view.FIG. 2is a cross-section taken on line A-A′ ofFIG. 1.FIG. 3is a cross section taken on line B-B′ ofFIG. 1. AsFIG. 1shows, the pixel according to the present invention has different cross section structures between the area around the center of the pixel (taken on line A-A′) and the area around the end part of the pixel (taken on line B-B′).

Referring toFIG. 1, a small wall60transversely extends along the center of the pixel. Large walls50transversely extend at the boundary of the pixel while interposing the small wall60. A common electrode14is formed on the upper surface of the small wall60. A wall electrode161is formed at the inner side of the large wall50, and a plane electrode162is formed between the small wall60and the large wall50. A pixel electrode16is formed of the wall electrode161and the plane electrode162. A through hole40for connecting a video signal line12and the pixel electrode is formed at the right side of the pixel. The wall electrode161formed on the large wall50is bent at both end parts of the pixel. The bent structure serves to prevent generation of the domain in the effective region of the pixel, that is, light transmission region, which is the characteristic of the present invention.

Definition, of a twisting direction of a liquid crystal molecule31is illustrated at the upper left side ofFIG. 1. Referring toFIG. 1, the initial orientation of the liquid crystal, molecule31is slightly angled clockwise with, respect to the horizontal direction. The twist in the same direction as the angle in the initial orientation denotes a forward twist, and the reversely directed, twist to the angle in the initial, orientation denotes a reverse twist.

FIG. 2is a cross section taken on line A-A′ at the center of the pixel, illustrating a basic structure of the wall electrode IPS. Referring toFIG. 2, a TFT layer11is formed on a TFT substrate10made of glass. The TFT layer11contains a gate electrode, a gate insulating film, a semiconductor layer and the like. The video signal line12is formed on the TFT layer11. A first interlayer insulating film13formed, of SiN is applied onto the video signal line12, on which the common electrode14is formed. The large wall50is formed on the video signal line12coated with the first interlayer insulating film13. The small wall60is formed at the center of the pixel. Both the large wall50and the small wall60are formed of an acrylic resin material, for example.

The common electrode14is formed to coat the large wall50and the small wall60. The common electrode14that covers the large wall50and the small wall60is coated with a second interlayer insulating film15made of SiN. The pixel electrode16is applied onto the second interlayer insulating film15. The pixel electrode16includes the wall electrode161and the plane electrode162.

A planarizing film17as an organic passivation film is formed to coat the plane electrode162of the pixel electrode16. The planarizing film17serves to make the layer thickness of a liquid crystal layer30uniform within the pixel, and to cause the line of electric force between the common, electrode14on the small wall60and the plane electrode162of the pixel electrode16to be a transverse component in the liquid crystal layer30. In other words, the line of electric force generated from the plane electrode162functions as the longitudinal electric field within the planarizing film17, while functioning as the transverse electric field within the liquid crystal layer30. This makes it possible to improve the display mode efficiency.

Referring toFIG. 2, a counter substrate20is provided, while having the liquid crystal layer30interposed. The counter substrate20made of glass includes a black matrix21, a color filter22, and an over coat film23that coats the color filter22. Alignment films formed at the sides of the TFT substrate10and the counter substrate20are not shown inFIG. 2.

Referring toFIG. 2, the large wall50serves as a columnar spacer that defines the distance between, the TFT substrate10and the counter substrate20. The large wall50forms a boundary with the neighboring pixel so that the electric field of the pixel electrode16is not applied to the neighboring pixel. The small wall60has its height equal to or higher than 1 μm.

FIG. 3is a cross section taken on line B-B′ of FIG.1, illustrating the characteristic of the present invention. Referring toFIG. 1, the part of the large wall50as indicated by B-B′ has a larger width, and correspondingly, the width of the plane electrode162is reduced.FIG. 1shows the difference L1in the thickness of the large wall50between the end part and the area around the center. The drawing also shows a thickness W+L1as a total of the width of the large wall50and the thickness of the wall electrode161at the end part of the pixel as well as a dimension D that is half the minor axis of the pixel.

The aforementioned relationship is indicated by the cross section ofFIG. 3. Referring toFIG. 3, the width of the plane electrode162to the left of the small wall60is smaller than that of the plane electrode162to the right, and correspondingly, the width of the liquid crystal layer30is reduced. In this way, the end part where the width of the liquid crystal layer30is reduced allows suppression of generation of the domain within the effective region of the pixel to be explained later. The rest of the structure shown inFIG. 3is the same as described referring toFIG. 2.

FIGS. 4A and 4Bare plan views showing an effect of the present invention in comparison with the related art.FIG. 4Ais a plan view showing a problem of the related art. Referring toFIG. 4A, a dotted line denotes an equipotential line generated between the wall electrode161formed on the large wall50and the common electrode14formed on the small wall60. The liquid crystal molecule31is likely to be oriented in the direction perpendicular to the equipotential line. However, the angle with respect to the equipotential line varies depending on the relationship with direction of the initially oriented liquid crystal molecule31.

As for the related art, at the end part of the pixel, the liquid crystal molecule31may have the twist in the direction along its twisting direction at the area around the center (hereinafter referred to as forward twist direction), and the twist in the reverse direction to the forward twist direction. The part of the liquid crystal molecule31, in which the forward and reverse twists occur may be the region where so called a domain100is generated. As the light cannot transmit through this part, the transmittance of the pixel is decreased to reduce brightness of the screen. Referring toFIGS. 4A and 4B, the liquid crystal molecules31that are forward twisted are shown in white, and the liquid crystal molecules31that are reversely twisted are shown in black.

FIG. 4Bis a plan view showing the pixel structure according to the present invention. Referring toFIG. 4B, the large wall50has the larger thickness at the end part of the pixel. This causes the wall electrode161to be bent toward the center. As the wall electrode161, that is, the pixel electrode16has bent portion at the end part, the resultant equipotential line has the larger component for twisting the liquid crystal molecules31in the forward direction. As a result, in the part of the pixel for transmitting light, all the liquid crystal molecules31are oriented in the forward direction, thus preventing generation of the domain.

Meanwhile, the equipotential line that reversely twists the liquid crystal molecule31is generated in the part of the pixel where the large wall50is not formed, that is, the part that does not allow transmission of the light from the backlight. In this part, the domain100is generated. Meanwhile, the part where the domain100is generated as shown inFIG. 4Boriginally does not; allow transmission of the light. This hardly influences the transmittance of the liquid crystal display panel.

In other words, when the wall electrode161is bent at the end part of the pixel, the distance between the wall electrode161and the common electrode14at the center is reduced. The electric field in the resultant region has the highest intensity in the pixel. As for the aforementioned structure, the electric field at the outer side of the pixel is directed to reversely twist the liquid crystal molecule, and the electric field at the inner side of the pixel is directed to strongly twist the liquid crystal molecule31forward. The liquid crystal reversely twisted at the outer side of the pixel is dammed by the forward twisted electric field at the inner side of the wall electrode161. In other words, the domain100at the end part of the pixel is generated only in the narrow area between the wall electrode161and the common electrode14at the center, which makes it possible to hide the dark line in the display region from view.

In this way, the present invention is configured to have the wall electrode161formed on the large wall50bent toward the center at the area around the end part of the pixel. The domain is not generated in the region of the pixel that allows light transmission, thus making it possible to suppress deterioration in the transmittance without, generating the domain.

FIG. 5is a graph representing the relationship between configuration of the end part of the pixel, which has the bent wall electrode161in association with increase in the width of the large wall50as shown inFIG. 1, and the transmittance of the pixel. The graph shows that the transmittance is kept high so long as no domain is generated. As shown inFIG. 1, W denotes the width of the large wall50including the thickness of the wall electrode161at the center of the pixel, L1denotes the bending length of the wall electrode toward the center of the pixel in association with increase in the width of the large wall, and D denotes the dimension ½ of the minor axis of the pixel.

In the aforementioned case, the value derived from the formula
(D−W)/X(1)
is defined so as to examine with respect to the bending length L1of the wall electrode161, which may prevent generation of the domain efficiently. Referring toFIG. 5, x-axis is represented by X, and y-axis denotes the relative transmittance. AsFIG. 5shows, the relative transmittance may be improved by 5% or higher in the range of X from 2 to 4. If X=2 in the formula (Id, the value of L1is equal to (D−W)/2, that is, the value L1as the bending length of the wall electrode161at the end part of the pixel becomes approximately ½ of the width of the plane electrode at the area around the center of the pixel. If X=4, the value of L1becomes approximately ¼ of the width of the plane electrode at the area around the center of the pixel. When the bending length L1of the wall electrode161is approximately ¼ to ½ of the width of the plane electrode at the area around the center of the pixel, generation of the domain may foe suppressed efficiently.

FIG. 6is a plan view showing that the structure described referring toFIG. 1is applied to the multi-domain pixel. The multi-domain denotes the structure having the common electrode14formed on the small wall60and the pixel electrode16formed of the wall electrode161and the plane electrode162bent at predetermined angles at the area around the center of the pixel in order to improve uniformity of the viewing angle. The end part of the pixel shown inFIG. 6shows the large wall50with larger thickness and the bent wall electrode161as described referring toFIG. 1. This makes it possible to efficiently prevent generation of the domain at the end part of the pixel as described referring toFIG. 4.

Second Embodiment

FIG. 7is a plan view of the pixel according to a second embodiment of the present invention.FIG. 8is a cross section taken on line C-C′ ofFIG. 7. The cross section taken on line A-A′ ofFIG. 7is the same as the one used for explanation referring toFIG. 2. Referring toFIG. 7, like the first embodiment, the width of the large wall50is increased at the end part of the pixel, and the wall electrode161is bent toward the center of the pixel. The characteristic of the embodiment is that the small wall60is bent in the direction where the wall electrode161is bent in association with increase in the width of the large wall50as shown inFIG. 7. In other words, the small wall60is bent at the area around where the pixel electrode161is bent in the direction away from the bent pixel electrode161.

FIG. 8is a cross section taken on line C-C′ ofFIG. 7representing a structure of the end part of the pixel. AsFIG. 8shows, one of the large walls50has the larger width. The small wall60is positioned to the right rather than the center of the pixel in the minor axis direction. The rest of the structure is the same as described referring toFIG. 2or3.

FIG. 9is a plan view of the pixel representing an operation of the embodiment. Referring toFIG. 9, in the pixel, the narrowest part between the wall electrode161and the common electrode14, which have the bent structures at the end parts of the pixel is the distance defined by the small wall60and the top end of the wall electrode161in the direction perpendicularly extending from the wall surface of the small wall60. Intensity of the electric field in this part becomes the highest. Meanwhile, at the top end of the wall electrode161at the outer side of the pixel, the common electrode14that coats the small wall60is diagonally bent, and the distance between the wall electrode161and the common electrode14is widened compared to the position at the top end of the bent wall electrode161at the inner side of the pixel, resulting in a weakened electric field. In this case, the liquid crystal molecule at the top end of the bent wall electrode161at the outer side is reversely twisted, and the one at the top end at the inner side is twisted forward. At the top end of the wall electrode161at the inner side, the liquid crystal is moved in the forward twisted direction by the electric field in the forward twisted direction. Meanwhile, at the top end of the wall electrode161at the outer side, the liquid crystal is moved in the reverse twisted direction by electric field in the reverse twisted direction. Accordingly, the domain is generated between the liquid crystal twisted forward and the liquid crystal twisted reversely. However, as the electric field in the forward twisted direction has higher intensity than the electric field in the reverse twisted direction, the domain is forced outside the pixel. Therefore, the display mode efficiency may be improved as the whole pixel without generating the domain within the display region of the pixel.

FIG. 10is a graph representing the relationship between configuration of the end part of the pixel, which has the bent wall electrode161in association with increase in the width of the large wall50as shown inFIG. 7and further has the small wall60that is bent in the direction away from the bent wall electrode161, and the transmittance of the pixel. The graph shows that the transmittance is kept high as long as no domain is generated. As shown inFIG. 7, W denotes the width of the large wall including the thickness of the wall electrode161at the region around the center of the pixel, L1denotes the bending length of the wall electrode toward the center of the pixel in association with increase in the width of the large wall, and D denotes the dimension half the minor axis of the pixel. The above conditions are the same as those defined in the first embodiment.

In the aforementioned case, like the first embodiment, the value derived from the formula
(D−W)/X(1)
is defined so as to examine with respect to the bending length L1of the wall electrode161, which may prevent generation of the domain efficiently. Referring toFIG. 10, x-axis is represented by X, and y-axis denotes the relative transmittance. AsFIG. 10shows, the relative transmittance may be improved by 5% or higher in the range of X from 1 to 4.5. If X is in the range from 2.5 to 4, the relative transmittance may further be improved by 10% or higher. If X is equal to 3, the relative transmittance may be improved by 11% or higher.

If X=1 in the formula (I), the value of L1is substantially equal, to the width of the plane electrode162at the area around the center of the pixel. If X=4.5, the value of L1is approximately 1/4.5 of width of the plane electrode162at the area around the center of the pixel. In other words, generation of the domain may be effectively suppressed when the bending length L1of the wall electrode161is approximately in the range from 1/4.5 to 1 of the width of the plane electrode161at the area around the center.

FIG. 11is a plan view showing that the structure described referring toFIG. 7is applied to the multi-domain pixel. As described referring toFIG. 1, at the end part of the pixel as shown inFIG. 1I, the thickness of the large wall50is increased and the wall electrode161is bent. At the end part of the pixel, the small wall60is bent in the direction away from the bent wall electrode161. The structure as shown inFIG. 11allows improvement in the transmittance in the same manner as shown in the graph ofFIG. 10.

Third Embodiment

FIG. 12is a plan view of a pixel according to a third embodiment of the present invention. The structure shown inFIG. 12is configured by extending the bent structure of the common electrode14formed on the small, wall60as shown inFIG. 7according to the second embodiment to the area around the through hole40at the end part of the pixel. Extending the common electrode14formed on the small wall60to the area around the through hole40allows the liquid crystal around the through hole40to serve as the display region, thus improving the transmittance.

AsFIG. 12shows, the structure has the bent small wall60, which allows the liquid crystal molecule31to be twisted forward in the liquid crystal layer around the through hole40, suppressing generation of the domain. That is, the transmission region may be widened while suppressing generation of the domain, resulting in improved transmittance. A region A enclosed by the dashed line as shown inFIG. 12serves as the generally employed display region. In this embodiment, in addition to the aforementioned region, a region B enclosed by the dashed line at the area around the through hole40is allowed to contribute to the display, which makes it possible to increase the transmittance correspondingly.

FIG. 13is a plan view showing that the structure described referring toFIG. 12is applied to the multi-domain pixel. Like the description referring toFIG. 1, at the end part of the pixel shown inFIG. 13, the thickness of the large wall50is increased, and the wall electrode161is bent. At the end part of the pixel, the small wall60is bent in the direction away from the bent wall electrode161, and extends to the area around the through hole40. This makes it possible to aim at improving the transmittance of the structure as shown inFIG. 13in the same manner as described referring toFIG. 12.

Fourth Embodiment

FIG. 14is a cross section of the pixel structure according to the embodiment.FIG. 14corresponds to the cross section taken on line B-B′ ofFIG. 1. The difference in the structure from the first embodiment as shown inFIG. 3is that the part of the large wall50corresponding to the thickness increased by L1is as high as the small wall60. Therefore, the thickness of the large wall50at the end part of the pixel is the same as that of the large wall50at the area around the center of the pixel.

Even if the wall that bends the pixel electrode161at the end part of the pixel is as high as the small wall60, the equipotential line as shown, inFIG. 4Bis generated, which makes it possible to prevent generation of the domain in the display region. Additionally the wall that bends the pixel electrode16at the end part of the pixel in the embodiment shown inFIG. 14is as high as the small wall60, which allows the liquid crystal layer formed thereon to serve as the display region. This makes it possible to improve the transmittance correspondingly.

Fifth Embodiment

This embodiment is an example derived from combining the structures as described in the first to the fourth embodiments.FIG. 15shows a structure derived from combining those of the first embodiment shown inFIG. 1and the second embodiment shown inFIG. 7. The structure of the second, embodiment shown inFIG. 7has the small wall60bent at both end parts of the pixel. Meanwhile, the structure shown inFIG. 15has the small wall60bent only at one end part of the pixel where the through hole40is not formed. The embodiment shown inFIG. 15has the intermediate feature between those shownFIG. 1andFIG. 7.

FIG. 16shows another example derived, from combining the structures as described in the first embodiment shown inFIG. 1and the second embodiment shown inFIG. 7. The structure of the second embodiment shown inFIG. 7has the small wall60bent at both end parts of the pixel. Meanwhile, the structure shown inFIG. 16has the small wall60bent only at one end part of the pixel where the through hole40is formed. Therefore, the embodiment shown inFIG. 16also has the intermediate feature between those shown inFIG. 1andFIG. 7.

FIG. 17shows an example derived from combining the structures of the first embodiment shown inFIG. 1and the third embodiment shown inFIG. 12. The structure of the third embodiment shown inFIG. 12has the small wall60bent at both end parts of the pixel up to the area around the through hole40. The structure shown inFIG. 17has the small wall60bent only at one end part of the pixel where the through hole40is formed up to the area therearound. The structure of the embodiment shown inFIG. 17allows the liquid crystal layer30to be used as the display region at the area around the through hole40. This makes it possible to improve the transmittance by the amount corresponding to the region B enclosed by the dashed line inFIG. 12.

FIG. 18shows an example derived from combining the first embodiment, shown inFIG. 6and the second embodiment shown inFIG. 11. The structure shown inFIG. 18is formed by applying the structures of the first embodiment and the second embodiment to the pixel with the multi-domain structure, that is, applying the first embodiment to the end part of the pixel at the side where the through hole40is formed, and the second embodiment to the end part of the pixel at the side where the through hole40is not formed. Accordingly, the embodiment shown inFIG. 18has the intermediate feature between those of the first embodiment shown inFIG. 6and the second embodiment shown inFIG. 11.

FIG. 19shows an example derived from, combining the first embodiment shown inFIG. 6and the second embodiment shown inFIG. 11. The structure shown inFIG. 19is formed by applying the structures of the first embodiment and the second embodiment to the pixel with the multi-domain structure, that is, applying the second embodiment to the end part of the pixel at the side where the through hole40is formed, and the first embodiment to the end part of the pixel at the side where the through hole40is not formed. Accordingly, the embodiment shown inFIG. 19has the intermediate feature between those of the first embodiment shown inFIG. 6and the second embodiment shown, inFIG. 11.