Patent Grant
7990503
1. Field of the Invention
The present invention relates to a liquid crystal display device used as, for example, a display unit of an electronic apparatus.
2. Description of the Related Art
In recent years, liquid crystal display devices have come to be used for TV sets, monitor devices of personal computers, etc. In these purposes, liquid crystal display devices are required to have a superior viewing angle characteristic that the display screen is viewable from all directions.
However, VA (vertically aligned)-mode liquid crystal display devices, for example, have a problem that a transmittance vs. application voltage characteristic (T-V characteristic) obtained when the display screen is viewed from the direction perpendicular to it (i.e., front direction) is different from that obtained when the display screen is viewed from a oblique direction. More specifically, the color of an image on the display screen looks more whitish when the display screen is viewed from a oblique direction than when it is viewed from the front direction.
Liquid crystal display devices of the TN (twisted nematic) mode, which is a more conventional drive mode, have the same problem. JP-A-2-12 (Reference 1), U.S. Pat. No. 4,840,460 (Reference 2), and Japanese Patent No. 3,076,938 (Reference 3) disclose techniques for solving the above problem of TN-mode liquid crystal display devices. The pixel structure of a basic liquid crystal display device using these known techniques will be described below briefly. A pixel region is formed by, for example, two sub-pixels A and B in which separate pixel electrodes are formed, respectively. The pixel electrode of the one sub-pixel A is connected directly to the source electrode of a thin-film transistor (TFT), but the pixel electrode of the other sub-pixel B is not connected directly to the source electrode. The pixel electrode of the sub-pixel B has a region that coextends with part of a control capacitance electrode extending from the source electrode to a storage capacitor electrode with an insulating film interposed in between, and is connected indirectly to the source electrode via a control capacitance Cc formed in this region.
In the liquid crystal display device having the above pixel structure, a voltage applied to a portion of a liquid crystal layer in the sub-pixel A is different from a voltage applied to a portion of the liquid crystal layer in the sub-pixel B. As a result, distortion in the T-V characteristic is distributed in each pixel. Therefore, the phenomenon that an image looks whitish when viewed from a oblique direction is suppressed and the viewing angle characteristic is improved. This technique will be referred to below as “capacitive coupling HT (halftone/gray scale) technique.”
Whereas References 1-3 describe the above techniques for TN-mode liquid crystal display devices, a more remarkable effect is obtained when the capacitive coupling HT technique is applied to liquid crystal display devices of the VA-mode which has become the mainstream mode in recent years in place of the TN mode.
The pixel region has sub-pixels A and B. The sub-pixel A has a trapezoidal shape, for example, and is disposed at the center (in the vertical direction) closer to the left sideline in the pixel region. The sub-pixel B is disposed so as to occupy portions of the pixel region excluding the sub-pixel A, that is, a top portion, a bottom portion, and a right-hand end portion that is located at the center (in the vertical direction). Each of the sub-pixels A and B is approximately line-symmetrical with respect to the storage capacitor bus line 118. A pixel electrode 116 is formed in the sub-pixel A, and a pixel electrode 117 which is separated from the pixel electrode 116 is formed in the sub-pixel B. The pixel electrode 116 is electrically connected to the storage capacitor electrode 119 and the source electrode 122 of the TFT 120 via a contact hole 124. On the other hand, the pixel electrode 117 is in an electrically floating state. The pixel electrode 117 has a region that coextends with part of the control capacitance electrode 125 with a protective film 131 (not shown in
A linear slit (electrode-omitted portion) 144 is formed between the pixel electrodes 116 and 117 so as to extend obliquely with respect to the end lines of the pixel region. The slit 144 not only separates the pixel electrodes 116 and 117 but also functions as an alignment restriction structure for restricting the alignment of a liquid crystal 106 (not shown in
A counter substrate, which is opposed to the TFT substrate via the liquid crystal layer, has a common electrode 141 (not shown in
Now assume that the TFT 120 has been turned on, whereby a voltage is applied to the pixel electrode 116, that is, a voltage Vpx1 develops across the portion of the liquid crystal layer corresponding to the sub-pixel A. Since the voltage Vpx1 is divided according to the capacitance ratio of the liquid crystal capacitance Clc2 and the control capacitance Cc, a voltage that is applied to the pixel electrode 117 is different from the voltage applied to the pixel electrode 116. A voltage Vpx2 that develops across the portion of the liquid crystal layer corresponding to the sub-pixel B is given by
Vpx2={Cc/(Clc2+Cc)}×Vpx1.
In this manner, in the liquid crystal display device having the pixel structure of
However, the liquid crystal display device shown in
As shown in
Incidentally, prescribed capacitances are formed between the drain bus line 114 and the pixel electrodes 116 and 117. Particularly in the configuration in which no thick overcoat layer is formed between the drain bus line 114 and the pixel electrodes 116 and 117, the values of the capacitances formed tend to vary depending on the distances between the drain bus line 114 and the pixel electrodes 116 and 117 as measured parallel with the substrate surface. Therefore, if relative patterning deviations occur in the drain bus line 114 and the pixel electrodes 116 and 117 due to, for example, shot unevenness in divisional exposures, display unevenness that the display characteristics vary from one divisional exposure region to another will be found visually in a manufactured liquid crystal display device. It is therefore necessary to set the end portions of the pixel electrodes 116 and 117 as distant from the drain bus line 114 as possible so that differences in the display characteristics will be less likely found visually even if patterning deviations occur. However, if the end portions of the pixel electrodes 116 and 117 are set away from the drain bus line 114, the region where the pixel electrodes 116 and 117 are formed is made narrow, resulting in a problem that the pixel aperture ratio and the luminance decrease.
Further, a prescribed bonding registration error occurs when the TFT substrate and the counter substrate are bonded to each other. Therefore, it is necessary that the aperture of the BM 145 formed on the counter substrate side be set smaller than the region on the TFT substrate side where the pixel electrodes 116 and 117 are formed. This results in a problem that the pixel aperture ratio and the luminance decrease further.
An object of the present invention is therefore to provide a liquid crystal display device which is high in luminance and superior in display quality.
The above object is attained by a liquid crystal display device comprising a pair of substrates opposed to each other; a liquid crystal sealed between the pair of substrates; plural pixel regions each having a first pixel electrode formed on one of the pair of substrates and a second pixel electrode formed on the one substrate and separated from the first pixel electrode; a transistor disposed for each of the pixel regions and having a source electrode which is electrically connected to the first pixel electrode; a control capacitance portion which has a control capacitance electrode electrically connected to the source electrode and opposed to at least part of the second pixel electrode via an insulating film, and which thereby establishes capacitive coupling between the source electrode and the second pixel electrode; linear alignment restriction structures formed on the other substrate, for restricting alignment of the liquid crystal; and singular point control apertures formed in the first and/or second pixel electrode so as to occupy parts of overlap regions of the alignment restriction structures and the first and/or second pixel electrode as viewed perpendicularly to surfaces of the substrates, for controlling positions of singular points of alignment vectors of the liquid crystal.
The invention can realize a liquid crystal display device which is high in luminance and superior in display quality.
A liquid crystal display device according to a first embodiment of the present invention will be described below with reference to
A gate bus line driving circuit 80 incorporating a driver IC for driving the plural gate bus lines and a drain bus line driving circuit 82 incorporating a driver IC for driving the plural drain bus lines are connected to the TFT substrate 2. The driving circuits 80 and 82 output scanning signals and data signals to prescribed gate bus lines and drain bus lines on the basis of prescribed signals that are output from a control circuit 84. A polarizing plate 87 is disposed on that surface of the TFT substrate 2 which is opposite to its TFT elements formation surface and a polarizing plate 86 is disposed on that surface of the counter substrate 4 which is opposite to its common electrode formation surface, the polarizing plates 86 and 87 being in a crossed-Nicols arrangement. A backlight unit 88 is disposed on that surface of the polarizing plate 87 which is opposite to the TFT substrate 2.
A storage capacitor bus line 18 is formed so as to traverse a pixel region defined by the gate bus lines 12 and the drain bus lines 14 and to extend parallel with the gate bus lines 12. A storage capacitor electrode 19 is formed above the storage capacitor bus line 18 with the insulating film 30 interposed in between (the storage capacitor electrodes 19 are provided for the respective pixels). The storage capacitor electrode 19 is electrically connected to the source electrode 22 of the TFT 20 via a control capacitance electrode 25. A storage capacitor Cs is formed between the storage capacitor bus line 18 and the storage capacitor electrode 19.
The pixel region has sub-pixels A and B. The sub-pixel A has a trapezoidal shape, for example, and is disposed at the center (in the vertical direction) closer to the left sideline in the pixel region. The sub-pixel B is disposed so as to occupy portions of the pixel region excluding the sub-pixel A, that is, a top portion, a bottom portion, and a right-hand end portion that is located at the center (in the vertical direction). In the pixel, each of the sub-pixels A and B is approximately line-symmetrical with respect to the storage capacitor bus line 18. A pixel electrode 16 is formed in the sub-pixel A, and a pixel electrode 17 which is separated from the pixel electrode 16 is formed in the sub-pixel B. For example, both of the pixel electrodes 16 and 17 are transparent conductive films and are formed in the same layer. The pixel electrode 16 is electrically connected to the storage capacitor electrode 19 and the source electrode 22 of the TFT 20 via a contact hole 24 which is formed through the protective film 31 right above the storage capacitor electrode 19. On the other hand, the pixel electrode 17 is in an electrically floating state. The pixel electrode 17 has, in the top part of the pixel region (as viewed in
The pixel electrodes 16 and 17 are separated from each other by slits 44 and 47 which generally assume a “>” shape and surround the three sides of the trapezoidal pixel electrode 16. The slits 44 extend obliquely with respect to the end lines of the pixel region and the slit 47 extends parallel with the right-hand end line of the pixel region. The slits 44 also function as alignment restriction structures for restricting the alignment of a liquid crystal 6. The slits 44 and 47 should be thick enough to prevent short-circuiting of the pixel electrodes 16 and 17 (e.g., 10 μm or more).
The counter substrate 4, which is opposed to the TFT substrate 2 via the liquid crystal 6, has a common electrode (counter electrode) 41 formed on a glass substrate 11. A liquid crystal capacitance Clc1 is formed between the pixel electrode 16 of the sub-pixel A and the common electrode 41, and a liquid crystal capacitance Clc2 is formed between the pixel electrode 17 of the sub-pixel B and the common electrode 41. Bank-shaped linear projections (resin structures) 42 which function as alignment restriction structures are formed on the common electrode 41 so as to extend parallel with the slits 44 and obliquely with respect to the end lines of the pixel region. The slits 44 and the linear projections 42 extend in a direction that forms an angle of about 45° with the polarization axes of the polarizing plates 86 and 87 which are disposed outside and sandwich the TFT substrate 2 and the counter substrate 4. The linear projections 42 are made of a positive resist material such as a novolac resin. For example, the linear projections 42 are 1.0 μm in height and 10 μm in width. The linear projections 42 are located approximately at the centers of the sub-pixels A and B so as to divide each of the sub-pixels A and B approximately equally into regions in which the liquid crystal 6 have different alignment directions. In the pixel, the linear projections 42 are disposed so as to be approximately line-symmetrical with respect to the storage capacitor bus line 18, whereby in the pixel (more specifically, in the sub-pixels A and B) the liquid crystal 6 are aligned uniformly in four orthogonal directions. The control capacitance electrode 25 which connects the source electrode 22 to the storage capacitor electrode 19 overlaps with part of the linear projections 42 when viewed perpendicularly to the substrate surfaces. The liquid crystal 6 tends to suffer alignment failures in those regions of the pixel electrodes 16 and 17 which are relatively distant from the linear projections 42. To solve this problem, the pixel electrodes 16 and 17 are formed with, in those regions, minute slits 46 which extend approximately perpendicularly to the linear projections 42. Liquid crystal molecules 8 are aligned parallel with the extension directions of the minute slits 46, whereby the liquid crystal 6 is prevented from suffering alignment failures.
Auxiliary projections 43 are formed in regions that are close to the crossing positions of the linear projections 42 and the end lines of the pixel electrodes 16 and 17 and that form obtuse angles with the extension directions of the linear projections 42 when viewed perpendicularly to the substrate surfaces. The auxiliary projections 43 are formed, for example, in the same layer as the linear projections 42 and extend approximately parallel with the drain bus line 14. The auxiliary projections 43 are provided to cancel out influences of electric fields developing near the end lines of the pixel electrodes 16 and 17, and are disposed so as to overlap with the end portions of the pixel electrodes 16 and 17 when viewed perpendicularly to the substrate surfaces. A light shield film (BM) 45 for shielding an end portion of the pixel region from light is formed in the counter substrate 4.
Now assume that the TFT 20 has been turned on, whereby a voltage is applied to the pixel electrode 16, that is, a voltage Vpx1 develops across the portion of the liquid crystal layer corresponding to the sub-pixel A. Since the voltage Vpx1 is divided according to the capacitance ratio of the liquid crystal capacitance Clc2 and the control capacitance Cc, a voltage that is applied to the pixel electrode 17 is different from the voltage applied to the pixel electrode 16. A voltage Vpx2 that develops across the portion of the liquid crystal layer corresponding to the sub-pixel B is given by
Vpx2={Cc/(Clc2+Cc)}×Vpx1.
Since 0<Cc/(Clc2+Cc)<1, a relationship |Vpx1|>|Vpx2| holds except in a case of Vpx1=Vpx2=0. In this manner, in the liquid crystal display device according to this embodiment, the voltages Vpx1 and Vpx2 developing across the portions of the liquid crystal layer corresponding to the sub-pixels A and B can be made different from each other in each pixel. As a result, distortion in the T-V characteristic is distributed in each pixel and the phenomenon that an image looks whitish when viewed from a oblique direction is suppressed. The viewing angle characteristic is thus improved.
In this embodiment, singular point control apertures 50, 51, and 52 for controlling the positions of singular points of alignment vectors of the liquid crystal 6 are formed at portions of the overlap regions of the linear projections 42 and the pixel electrodes 16 and 17 as viewed perpendicularly to the substrate surfaces. The apertures 50 and 51 are formed by removing the pixel electrode 17 partially, and the apertures 52 are formed by removing the pixel electrode 16 partially. For example, the apertures 50, 51, and 52 are about 5 μm in width and about 15 μm in length. For example, the longitudinal directions of the apertures 50, 51, and 52 are approximately perpendicular to the extension directions of the linear projections 42. At least one aperture 50 or 51 is formed in the sub-pixel B and at least one aperture 52 is formed in the sub-pixel A.
The aperture 50 in the top portion of the sub-pixel B is disposed approximately at the center of the overlap region of the associated linear projection 42 and the pixel electrode 17 as viewed perpendicularly to the substrate surfaces (see
Each aperture 52 in the sub-pixel A is disposed approximately at the center of the overlap region, as viewed perpendicularly to the substrate surfaces, of the pixel electrode 16 and the associated linear projection 42 extending obliquely with respect to the end lines of the pixel region (see
Liquid crystal molecules 8 in a region α where each auxiliary projection 43 is formed are inclined toward the inside of the pixel electrode 16 due to influences of the auxiliary projection 43 and the edge of the pixel electrode 16. Therefore, a singular point (s=+1; indicated by mark “•”) is formed between the top-left end (in
Like the aperture 50, the aperture 51 in the bottom portion of the sub-pixel B is disposed approximately at the center in the overlap region of the associated linear projection 42 and the pixel electrode 17 as viewed perpendicularly to the substrate surfaces. Since no part of the control capacitance electrode 25 is formed in this region, a singular point (s=−1) is formed firmly in the region of formation of the aperture 51 like the singular point formed in the region of formation of the aperture 52 (see
The singular points in the regions of formation of the apertures 51, 52, and 53 do not move, that is, they are formed at the same positions stably. Therefore, no afterimage is found visually and the display quality of the liquid crystal display device is increased accordingly.
In this embodiment, light shield plates 54 of 4 μm in width, for example, are formed close to the ends (left-hand ends in
Since the light shield plates 54 are formed on the TFT substrate 2 side, no consideration need to be given to bonding misregistration etc. in this connection. Therefore, the light shield plates 54 can be disposed outside of the BM 45 when viewed perpendicularly to the substrate surfaces and the end line of the BM 45 can be shifted outward by a width d1 in a region corresponding to the light shield plates 54. When viewed perpendicularly to the substrate surfaces, in the regions corresponding to the light shield plates 54, the interval between the end line of the BM 45 and the drain bus line 14 is shorter than in the other regions.
The light shield plates 54 may overlap with the pixel electrode 16. Further, the light shield plates 54 reduce influences of the capacitance between the drain bus line 14 and the pixel electrode 16; a display failure due to crosstalk does not occur even if the pixel electrode 16 is disposed close to the drain bus line 14. In this embodiment, the width of the overlaps between the light shield plates 54 and the pixel electrode 16 is about 2 μm. If the light shield plates 54 overlapped with the pixel electrode 17 which is connected to the source electrode 22 by capacitive coupling, a new capacitance would occur between the light shield plates 54 and the pixel electrode 17. Therefore, the light shield plates 54 do not extend to come close to the ends, opposed to the drain bus line 14, of the pixel electrode 17. As a result of the above designs, the ends, opposed to the drain bus line 14, of the pixel electrode 16 are closer to the drain bus line 14 by a length d2 than the ends, opposed to the drain bus line 14, of the pixel electrode 17 are. In this manner, in this embodiment, the light shield plates 54 increases the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
As described above, according to this embodiment, not only can singular points be formed reliably in the regions where the linear projections 42 are formed but also their positions can be fixed substantially. Since the positions of the singular points do not vary due to, for example, a difference in the manner of a transition between gradation levels, no afterimage is found visually. Even when a local pressure is applied externally to the substrate surface by a finger push, for example, the singular points do not move and hence the degree of resulting display unevenness is suppressed. As such, this embodiment can increase the display quality of the liquid crystal display device.
Further, in this embodiment, the light shield plates 54 which are kept at the same potential as the common electrode 41 are disposed close to the ends, opposed to the drain bus line 14, of the pixel electrode 16 which is electrically connected to the source electrode 22. This prevents light leakage near the ends of the pixel electrode 16 as well as a display failure due to crosstalk. In addition, the light shield plates 54 increase the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
Next, a liquid crystal display device according to a second embodiment of the invention will be described with reference to
According to this embodiment, singular points can be formed reliably and their positions can almost be fixed, whereby the display quality of the liquid crystal display device is increased as in the case of the first embodiment. Further, this embodiment is the same as the first embodiment in that the light shield plates 54 prevent light leakage and a display failure due to crosstalk and increase the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
Next, a liquid crystal display device according to a third embodiment of the invention will be described with reference to
According to this embodiment, singular points can be formed reliably and their positions can almost be fixed, whereby the display quality of the liquid crystal display device is increased as in the case of the first embodiment. Further, this embodiment is the same as the first embodiment in that the light shield plates 54 prevent light leakage and a display failure due to crosstalk and increase the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
Next, a liquid crystal display device according to a fourth embodiment of the invention will be described with reference to
According to this embodiment, singular points can be formed reliably and their positions can almost be fixed, whereby the display quality of the liquid crystal display device is increased as in the case of the first embodiment. Further, this embodiment is the same as the first embodiment in that the light shield plates 54 prevent light leakage and a display failure due to crosstalk and increase the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
Next, a liquid crystal display device according to a fifth embodiment of the invention will be described with reference to
According to this embodiment, singular points can be formed more reliably and their positions can almost be fixed, whereby the display quality of the liquid crystal display device is increased more than in the first embodiment. Further, this embodiment is the same as the first embodiment in that the light shield plates 54 prevent light leakage and a display failure due to crosstalk and increase the aperture ratio of each pixel and hence the luminance of the liquid crystal display device.
Next, a liquid crystal display device according to a sixth embodiment of the invention will be described with reference to
The invention is not limited to the above embodiments and various modifications are possible.
For example, although the above embodiments are directed to the transmission-type liquid crystal display device, the application field of the invention is not limited to it and includes liquid crystal display devices of other kinds such as the reflection type and the semi-transmission type.
Further, although in the above embodiments employ the linear projections 42 as the counter-substrate-4-side alignment restriction structures, the invention is not limited to such a case. Slits (common-electrode-omitted portions) formed by removing linear portions of the common electrode 41 may be used as alignment restriction structures.
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