Liquid-crystal display apparatus with large viewing angle and high optical transmittance

The TFT substrate (10) of this liquid crystal display device (100) includes: a TFT (11) which is provided for each pixel; an upper electrode (12) which is electrically connected to the TFT's drain electrode (11d); a lower electrode (13) which is arranged under the upper electrode; and a dielectric layer (14) which is arranged between the upper and lower electrodes. Its counter substrate (20) includes a counter electrode (21) which faces the upper electrode. The upper electrode has first and second regions (R1, R2) which have mutually different electrode structures, and a third region (R3) which electrically connects the first and second regions to the drain electrode. The third region of the upper electrode includes a symmetrical connecting portion (12c) that is a conductive film pattern, of which the shape is substantially symmetrical with respect to a virtual line (L1) that splits each pixel into two adjacent regions in a row direction.

TECHNICAL FIELD

BACKGROUND ART

Recently, the display performances of liquid crystal display devices have been improved to the point that more and more manufacturers use them in TV receivers, for example. The viewing angle characteristic of liquid crystal display devices has been improved to a certain degree but is not satisfactorily in some respects. Among other things, there is still a high demand for improvement of the viewing angle characteristic of a liquid crystal display device which uses a vertical alignment liquid crystal layer. A liquid crystal display device with a vertical alignment liquid crystal layer is sometimes called a “VA (Vertical Alignment) mode liquid crystal display device”.

A VA mode liquid crystal display device which is currently used for a display device with a big screen such as a TV set adopts an alignment division structure in which multiple liquid crystal domains are formed in a single pixel to improve the viewing angle characteristic. An MVA (Multi-domain Vertical Alignment) mode is often adopted as a method of forming such an alignment division structure. The MVA mode is disclosed in Patent Document No. 1, for example.

Specifically, according to the MVA mode, an alignment control structure is provided on each of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so as to contact with the liquid crystal layer, thereby forming multiple liquid crystal domains with mutually different alignment directions (i.e., tilt directions), the number of which is typically four, in each pixel. As the alignment control structure, a slit (as a hole) or a rib (as a projection structure) may be provided for an electrode, thereby creating an alignment controlling force from both sides of the liquid crystal layer.

If such a slit or rib is adopted, however, the alignment controlling force will be applied onto liquid crystal molecules non-uniformly within a pixel, because the slit or rib has a linear structure unlike the situation where the pretilt directions are defined by an alignment film in a conventional TN (twisted nematic) mode LCD. As a result, the response speed may have a distribution unintentionally.

In order to improve the responsivity of the MVA mode, so-called “PSA (Polymer Sustained Alignment) technology” has been developed recently. The PSA technology is disclosed in Patent Documents Nos. 2 and 3. According to the PSA technology, to give a pretilt to liquid crystal molecules, a polymer layer, which is called an “alignment sustaining layer”, is used. The alignment sustaining layer is formed by polymerizing a photo-polymerizable monomer, which has been added in advance to the liquid crystal material, with a voltage applied to the liquid crystal layer after a liquid crystal cell is completed. By adjusting the distribution and intensity of an electric field to be applied to polymerize the monomer, the pretilt azimuth (i.e., azimuth angle within a substrate plane) and pretilt angle (i.e., an elevation angle with respect to the substrate plane) of the liquid crystal molecules can be controlled.

Meanwhile, Patent Document No. 3 discloses a configuration in which a pixel electrode with a fine-line striped pattern (which is sometimes called a “fishbone type pixel electrode”) is used in combination with the PSA technology. According to such a configuration, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules will be aligned parallel to the longitudinal direction of the striped pattern, which is in sharp contrast to the conventional MVA mode disclosed in Patent Document No. in which the liquid crystal molecules are aligned perpendicularly to the linear alignment control structure such as slits or ribs. The lines and spaces of the fine-line striped pattern may have a narrower width than the conventional MVA mode alignment control structure. That is why the fishbone type pixel electrode is applicable more easily to small pixels than the conventional MVA mode alignment control structure is.

According to these modified VA mode technologies (including the PSA technology and the fishbone type pixel electrode), an excellent viewing angle characteristic is realized. Recently, however, since there is a growing demand for further improvement of the viewing angle characteristic of VA mode liquid crystal display devices, a so-called “pixel division driving technique” has been incorporated into actual products one after another (see Patent Documents Nos. 4 and 5, for example).

According to the pixel division driving technique, the phenomenon that the γ (gamma) characteristic when the screen is viewed straight on is different from the γ (gamma) characteristic when the screen is viewed obliquely, i.e., the viewing angle dependence of the γ characteristic, can be significantly reduced. In this case, the γ characteristic is the grayscale dependence of the display luminance.

Also, according to the pixel division driving technique, a single pixel is comprised of a plurality of subpixels which can apply mutually different voltages to the liquid crystal layer (i.e., which can exhibit mutually different luminances), and a predetermined luminance corresponding to the display signal voltage to be input to a pixel is realized by the entire pixel. That is to say, the pixel division driving technique is a technique for reducing the viewing angle dependence of a pixel's γ characteristic by synthesizing together mutually different γ characteristics of a plurality of subpixels.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

If the pixel division driving technique is adopted, however, different effective voltages need to be applied to the liquid crystal layer from one subpixel to another, and therefore, the pixel electrode provided for each pixel includes a plurality of subpixel electrodes associated with its subpixels, and switching elements (such as TFTs) are provided for the respective subpixel electrodes. That is to say, since at least two switching elements are provided for each pixel, the aperture ratio and optical transmittance of each pixel both decrease. And such a decrease in aperture ratio and optical transmittance is particularly noticeable in a high-definition liquid crystal display device, of which each pixel has a small area.

The present inventors perfected our invention in order to overcome these problems by providing a liquid crystal display device which has an excellent viewing angle characteristic and each pixel of which has a sufficiently high optical transmittance.

Solution to Problem

A liquid crystal display device according to an embodiment of the present invention includes: an active-matrix substrate; a counter substrate which faces the active-matrix substrate; and a liquid crystal layer which is interposed between the active-matrix substrate and the counter substrate. The liquid crystal display device includes a plurality of pixels which are arranged in columns and rows to form a matrix pattern. The active-matrix substrate includes: a thin-film transistor which is provided for each of the plurality of pixels and which includes a gate electrode, a source electrode, and a drain electrode; an upper electrode which is electrically connected to the drain electrode of the thin-film transistor; a lower electrode which is arranged under the upper electrode; and a dielectric layer which is arranged between the upper and lower electrodes. The counter substrate includes a counter electrode which faces the upper electrode. The upper electrode has first and second regions which have mutually different electrode structures, and a third region which electrically connects the first and second regions to the drain electrode. The third region of the upper electrode includes a symmetrical connecting portion that is a conductive film pattern, of which the shape is substantially symmetrical with respect to a virtual line that splits each pixel into two adjacent regions in a row direction.

In one embodiment, the active-matrix substrate further includes an interlayer insulating layer which is provided to cover the thin-film transistor. A contact hole is formed in the interlayer insulating layer and the dielectric layer so that the drain electrode is partially exposed in the contact hole and that the third region of the upper electrode is electrically connected to the drain electrode inside the contact hole. And the center of the contact hole is located off the virtual line.

In one embodiment, the active-matrix substrate further includes: a scan line which is extended substantially parallel to the row direction and which is electrically connected to the gate electrode of the thin-film transistor; and a signal line which is extended substantially parallel to the column direction and which is electrically connected to the source electrode of the thin-film transistor.

In one embodiment, an electric field to be generated in a region of the liquid crystal layer over the first region of the upper electrode and an electric field to be generated in another region of the liquid crystal layer over the second region of the upper electrode when a voltage is applied to the liquid crystal layer have mutually different directions and/or intensities.

In one embodiment, if a particular one of the plurality of pixels displays a predetermined half-scale tone, the luminance in a region of the particularly pixel corresponding to the first region of the upper electrode is lower than the luminance in another region of the particular pixel corresponding to the second region of the upper electrode.

In one embodiment, a plurality of slits are formed in the first region of the upper electrode, but no slits are formed in the second region of the upper electrode.

In one embodiment, when a voltage is applied to the liquid crystal layer, liquid crystal molecules are aligned substantially parallel to the slits in a region of the liquid crystal layer over the first region of the upper electrode, and liquid crystal molecules are aligned radially in another region of the liquid crystal layer over the second region of the upper electrode.

In one embodiment, the third region of the upper electrode is continuous with the first region.

In one embodiment, the third region of the upper electrode is continuous with the second region.

In one embodiment, the liquid crystal layer is a vertical alignment liquid crystal layer.

In one embodiment, at least one of the active-matrix substrate and the counter substrate includes a vertical alignment film and an alignment sustaining layer which is arranged between the vertical alignment film and the liquid crystal layer and which defines the pretilt azimuth of liquid crystal molecules when no voltage is applied to the liquid crystal layer.

In one embodiment, mutually different potentials are applied to the upper and lower electrodes.

In one embodiment, the upper and lower electrodes are each made of a transparent conductive material.

In one embodiment, when viewed along a normal to a display screen, the upper electrode overlaps at least partially with the lower electrode with the dielectric layer interposed between them, and the upper electrode, the dielectric layer and the lower electrode together form a storage capacitor.

In one embodiment, the liquid crystal display device further includes a pair of polarizers which face each other with the liquid crystal layer interposed between them.

In one embodiment, the thin-film transistor includes an oxide semiconductor layer.

In one embodiment, the oxide semiconductor layer includes an In—Ga—Zn—O based semiconductor.

Advantageous Effects of Invention

Embodiments of the present invention provide a liquid crystal display device which has an excellent viewing angle characteristic and each pixel of which has a sufficiently high optical transmittance.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is in no way limited to the embodiments to be described below.

FIGS. 1 and 2illustrate a liquid crystal display device100as a first embodiment of the present invention. This liquid crystal display device100includes a plurality of pixels which are arranged in columns and rows to form a matrix pattern.FIG. 1is a plan view schematically illustrating a single pixel of the liquid crystal display device100, andFIG. 2is a cross-sectional view as viewed on the plane2A-2A′ shown inFIG. 1.

The liquid crystal display device100includes an active-matrix substrate (which will be referred to herein as a “TFT substrate”)10, a counter substrate20which faces the TFT substrate10, and a liquid crystal layer30which is interposed between the TFT substrate10and the counter substrate20.

The TFT substrate10includes thin-film transistors (TFTs)11, which are provided for respective pixels. Each TFT11includes a semiconductor layer11a, a gate electrode11g, a source electrode11sand a drain electrode11d. In this embodiment, only one TFT11is provided for each pixel.

The TFT substrate10further includes an upper electrode (first electrode)12which is electrically connected to the drain electrode11dof the TFT11, a lower electrode (second electrode)13which is arranged under the upper electrode12, and a dielectric layer14which is arranged between the upper and lower electrodes12and13. Mutually different potentials are applied to the upper and lower electrodes12and13. The upper and lower electrodes12and13are each made of a transparent conductive material (such as ITO). When viewed along a normal to the display screen, the upper electrode12overlaps at least partially with the lower electrode13with the dielectric layer14interposed between them. And the upper electrode12, the dielectric layer14and the lower electrode13together form a storage capacitor.

The TFT substrate10further includes a scan line (gate bus line)15which is extended substantially parallel to the row direction, and a signal line (source bus line)16which is extended substantially parallel to the column direction. The scan line15is electrically connected to the gate electrode11gof the TFT11and supplies a scan signal to the TFT11. The signal line16is electrically connected to the source electrode11sof the TFT11and supplies a display signal to the TFT11.

These components of the TFT substrate10are supported by a transparent insulating substrate (such as a glass substrate)10a. On the surface of the insulating substrate10a, the gate electrode11gand scan line15of the TFT11are arranged to face the liquid crystal layer30and are covered with a gate insulating layer17.

On the gate insulating layer17, arranged is a semiconductor layer11awhich functions as the channel, source and drain regions of the TFT11. The semiconductor layer11amay be made of any of various known semiconductor materials, examples of which include amorphous silicon, polysilicon and continuous grain silicon (CGS).

Also, the semiconductor layer11amay be an oxide semiconductor layer, which may include an In—Ga—Zn—O based semiconductor, for example. In this case, the In—Ga—Zn—O based semiconductor is a ternary oxide of In (indium), Ga (gallium) and Zn (zinc). The ratios (i.e., mole fractions) of In, Ga and Zn are not particularly limited. For example, In:Ga:Zn=2:2:1, In:Ga:Zn=1:1:1 or In:Ga:Zn=1:1:2 may be satisfied. The In—Ga—Zn—O based semiconductor may be either amorphous or crystalline. If the In—Ga—Zn—O based semiconductor is a crystalline one, a crystalline In—Ga—Zn—O based semiconductor of which the c axis is substantially perpendicular to the layer plane is suitably used. The crystal structure of such an In—Ga—Zn—C based semiconductor is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2012-134475, the entire disclosure of which is hereby incorporated by reference. A TFT including such an In—Ga—Zn—O based semiconductor layer has high mobility (which is more than 20 times as high as that of an a-Si TFT) and low leakage current (which is less than one hundredth of that of an a-Si TFT).

The oxide semiconductor layer does not have to be an In—Ga—Zn—O based semiconductor layer, but may also include a Zn—O based (ZnO) semiconductor, an In—Zn—O based (IZO) semiconductor, a Zn—Ti—O based (ZTO) semiconductor, a Cd—Ge—O based semiconductor, a Cd—Pb—O based semiconductor, an In—Sn—Zn—O based semiconductor (such as In2O3—SnO2—ZnO) or an In—Ga—Sn—O based semiconductor, for example.

The source and drain electrodes11sand11dare arranged to contact respectively with the source and drain regions of the semiconductor layer11a. The signal line16is also arranged on the gate insulating layer17.

An interlayer insulating layer18is provided to cover the TFT11and the signal line16. In this embodiment, the interlayer insulating layer18has a multilayer structure consisting of an inorganic insulating film18aand an organic insulating film18bstacked on the inorganic insulating film18a. The organic insulating film18bmay be made of a photosensitive resin, for example. Naturally, the interlayer insulating layer18may also have a single-layer structure.

The lower electrode13is arranged on the interlayer insulating layer18and is covered with the dielectric layer14. The upper electrode12is arranged on the dielectric layer14. A contact hole CH is formed in the interlayer insulating layer18and the dielectric layer14to partially expose the drain electrode11d. Inside the contact hole CH, the upper electrode12(more specifically, its third region R3(to be described later)) is electrically connected to the drain electrode11d.

The counter substrate20includes a counter electrode (third electrode)21which faces the upper electrode12. Typically, the counter electrode21is a common electrode which is shared in common by every pixel. The counter electrode21is made of a transparent conductive material (such as ITO). Although not shown, the counter substrate20typically further includes color filters and an opaque layer (black matrix). The scan lines15, signal lines16, TFTs11and contact holes CH of the TFT substrate10are shielded from light by the opaque layer. The components (including the counter electrode21) of the counter electrode21are supported by a transparent insulating substrate (such as a glass substrate)20a.

The liquid crystal layer30is a vertical alignment liquid crystal layer. That is to say, the liquid crystal molecules31included in the liquid crystal layer30are aligned substantially perpendicularly to (typically so as to define an angle of 85 degrees or more with respect to) the surface of the substrate as shown inFIG. 2when no voltage is applied to the liquid crystal layer30. Although not shown, at least one (typically both) of the TFT substrate10and counter substrate20includes a vertical alignment film which is provided in contact with the liquid crystal layer30.

Optionally, an alignment sustaining layer may be formed by the PSA technology as disclosed in Patent Documents Nos. 2 and 3. The alignment sustaining layer is provided between the vertical alignment film and the liquid crystal layer30, and defines the pretilt azimuth of the liquid crystal molecules31when no voltage is applied to the liquid crystal layer30. The alignment sustaining layer is made of a photopolymerized material, which may be obtained by photo-polymerizing a photopolymerizable compound added to the liquid crystal material (which is typically a photopolymerizable monomer) by irradiating the compound with an ultraviolet ray, for example, with a voltage applied thereto.

The liquid crystal display device100further includes a pair of polarizers40aand40bwhich are arranged to face each other with the liquid crystal layer30interposed between them. In the exemplary configuration shown inFIG. 2, these polarizers40aand40bare arranged outside of the pair of substrates10and20. These polarizers40aand40bare arranged so as to conduct a display operation in the normally black mode in combination with the vertical alignment liquid crystal layer30. For example, if the polarizers40aand40bare linear polarizers, the polarizers40aand40bare arranged so that their polarization axes (i.e., either their transmission axes or absorption axes) cross each other substantially at right angles (i.e., as crossed Nicols). Naturally, the polarizers40aand40bmay also be circular polarizers.

Hereinafter, the structures of the upper and lower electrodes12and13that the TFT substrate10has will be described more specifically with reference toFIGS. 3, 4 and 5.FIG. 3is a plan view illustrating only the upper electrode12of the liquid crystal display device100of this embodiment.FIGS. 4 and 5are cross-sectional views as respectively viewed on the planes4A-4A′ and5A-5A′ shown inFIG. 1.

The upper electrode12has first and second regions R1and R2which have mutually different electrode structures, and also has a third region R3to electrically connect the first and second regions R1and R2to the drain electrode11d.

In this embodiment, a plurality of slits12sare formed (by removing portions of a conductive film) in the first region R1of the upper electrode12as shown inFIGS. 3 and 4. The first region R1includes a plurality of branch portions12bwhich run substantially parallel to each other, and each of those slits12sis located between two adjacent ones of the branch portions12b. In this manner, the first region R1has a comb tooth electrode structure. In the following description, the first region R1will be sometimes referred to herein as a “slit cut region”.

On the other hand, in the second region R2of the upper electrode12, there are no slits as shown inFIGS. 3 and 5. That is to say, the second region R2has a planar (i.e., solid) electrode structure. In the following description, the second region R2will be sometimes referred to herein as a “slit uncut region” or a “solid region”.

Although there are two first regions R1with the second region R2interposed between them at the center of the pixel in the exemplary configuration shown inFIG. 3, the numbers and arrangements of the first and second regions R1and R2are not limited to the ones illustrated inFIG. 3, as will be described later.

In the exemplary configuration shown inFIG. 3, the third region R3of the upper electrode12is continuous with the first region R1. That is to say, the third region R3is physically connected to the first region R1. A more specific structure of the third region R3will be described in detail later.

The lower electrode13has been formed so that almost all of the pixel (but its portions over the TFT11and near the contact hole CH) is covered with the conductive film. That is to say, the lower electrode13is substantially a solid electrode. Naturally, the lower electrode13does not have to have such a structure. To generate a lateral electric field (to be described later), however, the lower electrode13suitably overlaps with at least some of the slits12sof the upper electrode12.

The upper electrode12is electrically connected to the drain electrode11dof the TFT11, and therefore, a potential corresponding to the display signal supplied through the signal line16can be applied to the upper electrode12. On the other hand, a different potential from the one applied to the upper electrode12is applied to the lower electrode13. Typically, the same potential as the one applied to the counter electrode21is applied to the lower electrode13.

Since the first region (slit cut region) R1and second region (slit uncut region, or solid region) R2of the upper electrode12have mutually different electrode structures, an electric field to be generated in a region of the liquid crystal layer30over the first region R1and an electric field to be generated in another region of the liquid crystal layer30over the second region R2when a voltage is applied to the liquid crystal layer30have mutually different directions and/or intensities. Specifically, a vertical electric field, of which the intensity corresponds to the potential difference between the upper electrode12and the counter electrode21, is generated in a region of the liquid crystal layer30over the second region R2. On the other hand, not only the vertical electric field, of which the intensity corresponds to the potential difference between the upper electrode12and the counter electrode21, but also a lateral electric field, of which the intensity corresponds to the potential difference between the upper and lower electrodes12and13, are generated in a region of the liquid crystal layer30over the first region R1.

As can be seen, since not only a vertical electric field but also a lateral electric field are generated in the region of the liquid crystal layer30over the first region R1, the liquid crystal molecules31tilt to a smaller degree in that region of the liquid crystal layer30over the first region R1than in the region of the liquid crystal layer30over the second region R2. That is why when a certain pixel displays a predetermined half scale tone, a region of that pixel corresponding to the first region R1has a lower luminance than another region of the same pixel corresponding to the second region R2. That is to say, two kinds of regions with mutually different luminances are formed within the same pixel. The voltage-transmittance (V-T) characteristic of that region corresponding to the first region R1(i.e., a relatively dark region) will shift to a higher voltage range compared to the voltage-transmittance (V-T) characteristic of that region corresponding to the second region R2(i.e., a relatively bright region).

FIG. 6schematically illustrates the alignment state of liquid crystal molecules31when a voltage is applied to the liquid crystal layer30. As shown inFIG. 6, in the region of the liquid crystal layer30over the first region R1of the upper electrode12, the liquid crystal molecules31are aligned substantially parallel to the slits12s(i.e., parallel to the branch portions12b, too). Nevertheless, their alignment azimuth in that region of the liquid crystal layer30over the first region R1in the upper part of the pixel is different by approximately 180 degrees from their alignment azimuth in that region of the liquid crystal layer30over the first region R1in the lower part of the pixel. On the other hand, in the region of the liquid crystal layer30over the second region R2of the upper electrode12, the liquid crystal molecules31are aligned radially (i.e., in every direction).

As described above, in the liquid crystal display device100of this embodiment, when a certain pixel displays a predetermined half-scale tone, two kinds of regions with mutually different luminances are created in the same pixel (i.e., two kinds of regions with mutually different voltage-transmittance (V-T) characteristics are created in the same pixel). As a result, the viewing angle dependence of the r characteristic can be reduced. In addition, since there is no need to provide multiple TFTs11for each pixel, the liquid crystal display device100of this embodiment can increase the aperture ratio and optical transmittance of each pixel sufficiently. Consequently, the liquid crystal display device100of this embodiment has an excellent viewing angle characteristic, and each pixel has sufficiently high optical transmittance.

From the standpoint of reducing the viewing angle dependence of the γ characteristic sufficiently, the ratio of the area of the first region R1(i.e., the combined area of the two first regions R1in this example) to that of the second region R1suitably falls within the range of 5 to 1 through 1 to 2, and more suitably falls within the range of 3 to 1 through 1 to 1.

Also, in the first region R1, the branch portions12bsuitably have a width w1of 2.0 μm to 8.0 μm, and the slits12ssuitably have a width w2of 2.0 μm to 8.0 μm. The ratio (w1/w2) of the width w1of the branch portions12bto the width w2of the slits12ssuitably falls within the range of 0.4 to 3.0, and more suitably falls within the range of 0.5 to 1.5.

The applicant of the present application proposed a liquid crystal display device which includes an upper electrode and a lower electrode on its TFT substrate and includes a counter electrode on its counter substrate and in which the upper electrode has two kinds of regions with mutually different electrode structures (that are called “slit regions” and “planar regions”) in PCT International Application Publication No. 2012/090773, the entire disclosure of which is hereby incorporated by reference.

In that application, however, the structure of the rest of the upper electrode which is used to electrically connect those slit regions and planar regions to a TFT (that is a region in the vicinity of the TFT or the contact hole and that corresponds to the region R3of the upper electrode12in the liquid crystal display device100of this embodiment) is not studied particularly specifically. For example, in that application, the regions of the upper electrode in the vicinity of the contact hole and over the TFT are supposed to have a “planar electrode structure” and a conductive film has been formed to cover those regions of the upper electrode entirely.

However, if such a region of the upper electrode located over the TFT had such a structure, the characteristic of the TFT could change. For that reason, the conductive film that functions as the upper electrode is suitably removed from over the TFT. In that case, if the contact hole is located at the center in the horizontal direction, there will be no problem, in particular. In a high-definition liquid crystal panel, however, sometimes the pixel size may be too small to arrange the contact hole at the center in the horizontal direction. As a result, the conductive film that defines a region of the upper electrode to electrically connect the slit regions and planar regions to the TFT will have a horizontally asymmetrical shape, thus debasing the display quality.

In contrast, since the third region R3of the upper electrode12has the structure to be described later in the liquid crystal display device100of this embodiment, the decline in display quality described above can be minimized. Hereinafter, the structure of the third region R3of the upper electrode12of the liquid crystal display device100of this embodiment will be described in comparison with the upper electrode12′ of a liquid crystal display device1000illustrated as a comparative example inFIGS. 7 and 8.FIG. 7is a plan view schematically illustrating a single pixel of a liquid crystal display device1000as a comparative example.FIG. 8is a plan view illustrating only the upper electrode12′ of the liquid crystal display device1000of the comparative example.

In both of the liquid crystal display device100of this embodiment and the liquid crystal display device1000of the comparative example, supposing a virtual line L1which splits each pixel into two regions which are adjacent to each other in the row direction (i.e., which splits each pixel into two horizontally) is drawn, the center of the contact hole CH is not located on that virtual line L1(which will be referred to herein as a “horizontally splitting line”) as shown inFIGS. 1 and 7.

The first and second regions R1and R2of the upper electrode12′ of the liquid crystal display device1000as the comparative example have the same electrode structures as the first and second regions R1and R2of the upper electrode of the liquid crystal display device100of this embodiment.

The third region R3of the upper electrode12of the liquid crystal display device100of this embodiment includes a symmetrical connecting portion12cwhich has a shape that is substantially symmetrical with respect to the horizontally splitting line L1as shown inFIG. 3. InFIG. 3, only the symmetrical connecting portion12cis shadowed to make it recognizable easily. In the configuration shown inFIG. 3, the symmetrical connecting portion12cis connected to (i.e., continuous with) the leftmost and rightmost branch portions12bof the first region R1.

On the other hand, the third region R3′ of the upper electrode12′ in the liquid crystal display device1000as the comparative example does not have such a conductive film pattern, of which the shape is substantially symmetrical with respect to the horizontally splitting line L1, and the conductive film in the third region R′ has quite an asymmetrical shape horizontally and is continuous with only the rightmost branch portion12bof the first region R1as shown inFIG. 8. Since the upper electrode12′ gets depressed significantly over the contact hole CH (as can be seen easily fromFIG. 2), the application of a voltage initiates a change in the orientations of the liquid crystal molecules31from those located over the contact hole CH in the third region R3to begin with, separately from the change in the orientations in the first and second regions R1and R2. In this case, if the conductive film in the third region R3′ had quite an asymmetrical shape horizontally, then the change in the orientations to start from the liquid crystal molecules31over the contact hole CH would propagate differently between the right and left portions of the pixel, and the orientations of the liquid crystal molecules31would also be significantly asymmetrical horizontally, thus causing an adverse effect on the display operation. On top of that, if the area of the conductive film accounted for a significant percentage of the third region R3′, the horizontally asymmetrical alignment controlling force produced from the third region R3′ would be too great to achieve an intended alignment state in some cases.

In contrast, if the third region R3of the upper electrode12has a symmetrical connecting portion12c(as a part of the conductive film pattern) which is substantially symmetrical with respect to the horizontally splitting line L1as in the liquid crystal display device100of this embodiment, then approximately equal alignment controlling forces will be applied to the liquid crystal molecules31in the right and left halves of each pixel in the third region R3. As a result, the alignment in the liquid crystal layer30will be closer to a horizontally symmetrical one and get stabilized.

FIG. 9(a)shows the results of simulations which were carried out on the liquid crystal display device1000of the comparative example to see how the alignment state of the liquid crystal molecules31and the optical transmittance changed with a voltage applied. On the other hand,FIG. 9(b)shows the results of simulations which were carried out on the liquid crystal display device100of this embodiment to see how the alignment state of the liquid crystal molecules31and the optical transmittance changed with a voltage applied. The simulations were carried out on the supposition that the pixel had a horizontal length (i.e., a length as measured in the X direction) of 28.25 μm and a vertical length (i.e., a length as measured in the Y direction) of 84.75 μm and a voltage of 5.0 V was applied to the liquid crystal layer30.

As can be seen fromFIG. 9(a), in the liquid crystal display device1000of the comparative example, the orientations of the liquid crystal molecules31began to change from the ones in the vicinity of the contact hole CH and the change in the orientations propagated onto the lower right portion of the conductive film (i.e., a portion of the conductive film in the third region R3′ of the upper electrode12′). As a result, the liquid crystal molecules31could not get aligned as intended in the lower left region of the pixel, and that region looked dark.

On the other hand, in the liquid crystal display device100of this embodiment, the change in the orientations propagated smoothly from the lower left and lower right regions of the pixel as can be seen fromFIG. 9(b). As a result, the liquid crystal molecules31in the lower left region of pixel could also be aligned as intended and that region looked bright.

As can be seen, since the third region R3of the upper electrode12includes the symmetrical connecting portion12cin the liquid crystal display device100of this embodiment, horizontally symmetrical alignments are realized and the display quality can be improved.

Hereinafter, a liquid crystal display device200as a second embodiment of the present invention will be described with reference toFIGS. 10 and 11.FIG. 10is a plan view schematically illustrating a single pixel of the liquid crystal display device200.FIG. 11is a plan view illustrating only the upper electrode12A of the liquid crystal display device200. The following description of the liquid crystal display device200of the second embodiment will be focused on only differences from the liquid crystal display device100of the first embodiment.

Just like the upper electrode12of the liquid crystal display device100of the first embodiment, the upper electrode12A of the liquid crystal display device200of this embodiment also has first, second and third regions R1, R2and R3.

The first and second regions R1and R2of the upper electrode12A in this liquid crystal display device200have the same electrode structures as the first and second regions R1and R2of the upper electrode12in the liquid crystal display device100of the first embodiment.

On the other hand, the third region R3of the upper electrode12A in this liquid crystal display device200has a somewhat different electrode structure from the third region R3of the upper electrode12in the liquid crystal display device100of the first embodiment. Specifically, in the liquid crystal display device100of the first embodiment, the symmetrical connecting portion12cof the third region R3of the upper electrode12is continuous with the leftmost and rightmost branch portions12bof the first region R1as shown inFIG. 3. Meanwhile, in the liquid crystal display device200of this embodiment, the symmetrical connecting portion12cof the third region R3of the upper electrode12A is continuous with two central branch portions12bof the first region R1as shown inFIG. 11.

In the liquid crystal display device200of this embodiment, the third region R3of the upper electrode12A also includes the symmetrical connecting portion12cthat is a conductive film pattern, of which the shape is substantially symmetrical with respect to the horizontally splitting line L1. That is why approximately equal alignment controlling forces will be applied to the liquid crystal molecules31in the right and left halves of each pixel in the third region R3. As a result, the alignment in the liquid crystal layer30will be closer to a horizontally symmetrical one and get stabilized. Consequently, the display quality improves.

In the liquid crystal display device200of this embodiment, the symmetrical connecting portion12cof the third region R3is continuous with the central branch portions12bof the first region R1. That is why the area of a portion of the upper electrode12which is located close to the signal lines16decreases, and therefore, the parasitic capacitance to be produced between the signal lines16and the upper electrode12can be reduced, which is also beneficial. On the other hand, in the liquid crystal display device100of the first embodiment, the symmetrical connecting portion12cof the third region R3is continuous with the leftmost and rightmost branch portions12bof the first region R1. Consequently, the liquid crystal molecules31will have the same alignment direction over the same branch portion12b, and it is possible to prevent the liquid crystal molecules31from having opposite alignment directions within a region corresponding to a single branch portion12and causing a dark line there. On the other hand, in this liquid crystal display device200, the alignment directions of the liquid crystal molecules31will invert where the central branch portions12bare connected to the symmetrical connecting portion12c(as indicated by the dashed circle inFIG. 12), and the alignment boundary will be sensed as a dark line as schematically illustrated inFIG. 12. And if such a dark line appears in the area contributing to the display operation within the pixel, the transmittance will decrease.

Next, another liquid crystal display device300according to this embodiment will be described with reference toFIGS. 13 and 14.FIG. 13is a plan view schematically illustrating a single pixel of the liquid crystal display device300.FIG. 14is a plan view illustrating only the upper electrode12B of the liquid crystal display device300. The following description of the liquid crystal display device300of the second embodiment will be focused on only differences from the liquid crystal display device100of the first embodiment.

In the liquid crystal display device100of the first embodiment, the upper electrode12is connected to the TFT11which is located under the upper electrode12as shown inFIG. 1. On the other hand, in the liquid crystal display device300of this embodiment, the upper electrode12B is connected to the TFT11which is located over the upper electrode12B as shown inFIG. 13. That is to say, although each pixel is scanned by the scan line15which is located under the pixel in the liquid crystal display device100of the first embodiment, each pixel is scanned by the scan line which is located over the pixel in the liquid crystal display device300of this embodiment.

In the liquid crystal display device300of this embodiment, the third region R3of the upper electrode12B also includes the symmetrical connecting portion12cthat is a conductive film pattern, of which the shape is substantially symmetrical with respect to the horizontally splitting line L1as shown inFIG. 14. That is why approximately equal alignment controlling forces will be applied to the liquid crystal molecules31in the right and left halves of each pixel in the third region R3. As a result, the alignment in the liquid crystal layer30will be closer to a horizontally symmetrical one and get stabilized. Consequently, the display quality improves.

Hereinafter, a liquid crystal display device400as a third embodiment of the present invention will be described with reference toFIGS. 15 and 16.FIG. 15is a plan view schematically illustrating a single pixel of the liquid crystal display device400.FIG. 16is a plan view illustrating only the upper electrode12C of the liquid crystal display device400. The following description of the liquid crystal display device400of the third embodiment will be focused on only differences from the liquid crystal display device100of the first embodiment.

Just like the upper electrode12of the liquid crystal display device100of the first embodiment, the upper electrode12C of the liquid crystal display device400of this embodiment also has first, second and third regions R1, R2and R3.

However, although the upper electrode12has two first regions R1in the liquid crystal display device100of the first embodiment, the upper electrode12C has only one first region R1in the liquid crystal display device400of this embodiment. Specifically, the first region R1is located in an upper part of the pixel, and the second region R2is located under the first region R1. Thus, the third region R3of the upper electrode13C is continuous with (i.e., connected to) the second region R2.

In the liquid crystal display device400of this embodiment, the third region R3of the upper electrode12C also includes the symmetrical connecting portion12cthat is a conductive film pattern, of which the shape is substantially symmetrical with respect to the horizontally splitting line L1. That is why approximately equal alignment controlling forces will be applied to the liquid crystal molecules31in the right and left halves of each pixel in the third region R3. As a result, the alignment in the liquid crystal layer30will be closer to a horizontally symmetrical one and get stabilized. Consequently, the display quality improves.

Next, another liquid crystal display device500according to this embodiment will be described with reference toFIGS. 17 and 18.FIG. 17is a plan view schematically illustrating a single pixel of the liquid crystal display device500.FIG. 18is a plan view illustrating only the upper electrode12D of the liquid crystal display device500.

The upper electrode12D of the liquid crystal display device500also has only one first region R1. Specifically, the second region R2is located in an upper part of the pixel, and the first region R1is located under the second region R2. Thus, the third region R3of the upper electrode13D is continuous with (i.e., connected to) the first region R1.

In this liquid crystal display device500, the third region R3of the upper electrode12D also includes the symmetrical connecting portion12cthat is a conductive film pattern, of which the shape is substantially symmetrical with respect to the horizontally splitting line L1. That is why approximately equal alignment controlling forces will be applied to the liquid crystal molecules31in the right and left halves of each pixel in the third region R3. As a result, the alignment in the liquid crystal layer30will be closer to a horizontally symmetrical one and get stabilized. Consequently, the display quality improves.

In the liquid crystal display devices400and500of this embodiment, the upper electrode12C,12D of each pixel has only one first region R1, and therefore, the liquid crystal molecules31are aligned in only one direction in a dark region of the pixel. On the other hand, in the liquid crystal display devices100,200and300of the first and second embodiments, the upper electrode12,12A,12B of each pixel has two first regions R1, and the liquid crystal molecules31are aligned in two directions in a dark region of the pixel. That is why from the standpoint of improving the viewing angle characteristic in the dark region sufficiently, the liquid crystal display devices100,200,300of the first and second embodiments are preferred.

Hereinafter, a liquid crystal display device600as a fourth embodiment of the present invention will be described with reference toFIGS. 19 and 20.FIG. 19is a plan view schematically illustrating a single pixel of the liquid crystal display device600.FIG. 20is a plan view illustrating only the upper electrode12E of the liquid crystal display device600. The following description of the liquid crystal display device600of the fourth embodiment will be focused on only differences from the liquid crystal display device100of the first embodiment.

In the liquid crystal display device600of this embodiment, the upper electrode12E has four first regions R1, which is a major difference from the liquid crystal display device100of the first embodiment. Specifically, the upper electrode12E includes not only the pair of first regions R1which are located over and under the second region R2but also another pair of first regions R1which are located on the left- and right-hand sides of the second region R2. In the latter pair of first regions R1, the slits12sand the branch portions12brun substantially perpendicularly to the direction in which the slits12sand branch portions12brun in the former pair of first regions R1. That is why the liquid crystal molecules31over the latter pair of first regions R1are aligned substantially perpendicularly to the liquid crystal molecules31over the former pair of first regions R1. Also, the alignment direction in the region of the liquid crystal layer30over the first region R1on the left-hand side of the second region R2is different by approximately 180 degrees from the alignment direction in the region of the liquid crystal layer30over the first region R1on the right-hand side of the second region R2. As a result, in the liquid crystal display device600of this embodiment, the liquid crystal molecules31are aligned in four directions in the dark region of the pixel. Consequently, the viewing angle characteristic can be further improved.

Next, another liquid crystal display device700according to this embodiment will be described with reference toFIGS. 21 and 22.FIG. 21is a plan view schematically illustrating a single pixel of the liquid crystal display device700.FIG. 22is a plan view illustrating only the upper electrode12F of the liquid crystal display device700.

In this liquid crystal display device700, the upper electrode12F also has four first regions R1. And in the dark region of the pixel, the liquid crystal molecules31are also aligned in four different directions. However, although the slits12sand branch portions12brun either substantially parallel or substantially perpendicularly to the row and column directions in the four first regions R1that the upper electrode12E of the liquid crystal display device600has, the slits12sand branch portions12brun in a direction that defines an angle of approximately 45 degrees with respect to the row and column directions in the four first regions R1that the upper electrode12F of this liquid crystal display device700has.

In addition, in the upper electrode12F of this liquid crystal display device700, the four first regions R1have substantially equal areas. Consequently, the viewing angle characteristic can be improved even more effectively.

INDUSTRIAL APPLICABILITY

Embodiments of the present invention provide a liquid crystal display device which has an excellent viewing angle characteristic and each pixel of which has a sufficiently high optical transmittance. A liquid crystal display device as an embodiment of the present invention can conduct a display operation with high quality even when its definition is high and its pixel size is small.

REFERENCE SIGNS LIST