As of now, liquid crystal display devices under development which have wide viewing angle characteristics includes liquid crystal display devices utilizing the IPS (In-Plane-Switching) mode which is a transverse electric field mode or the FFS (Fringe Field Switching) mode, and liquid crystal display devices utilizing the VA (Vertical Alignment) mode. The VA mode is better for mass production than the transverse electric field mode and is therefore used in a wide range of TV applications and mobile applications.
The VA mode liquid crystal display devices are generally classified into MVA (Multidomain Vertical Alignment) mode liquid crystal display devices, in which one pixel includes a plurality of domains of different liquid crystal alignment directions, and CPA (Continuous Pinwheel Alignment) mode liquid crystal display devices in which the liquid crystal alignment direction continuously varies around a rivet or the like formed on an electrode at the center of a pixel.
In the MVA mode liquid crystal display devices, the alignment control means which extend in two mutually-orthogonal directions are provided to form four liquid crystal domains in one pixel, in which the azimuthal angles of the directors representing the liquid crystal domains are 45° relative to the polarization axes (transmission axes) of a pair of polarizing plates in a crossed nicols arrangement. Assuming that the direction of the polarization axis of one of the polarizing plates is azimuthal angle 0° and that the counterclockwise direction is the positive direction, the azimuthal angles of the directors of the four liquid crystal domains are 45°, 135°, 225°, and 315°. Selection of these azimuthal angles of the directors is most preferable in respect of transmittance because linearly-polarized light in the direction of 45° relative to the polarization axis is not absorbed by the polarizing plates. Such a structure which includes four domains in one pixel is referred to as “four-division alignment structure” or simply “4D structure”.
When slits (or ribs) are used as the alignment control means in the MVA mode liquid crystal display devices, the width of the slits need to be about 10 μm or more in order to obtain a sufficient alignment control force. If the slit width is narrower than this, sufficient alignment control force cannot be obtained. To form four domains, it is necessary to form in a counter electrode slits extending in directions different by 90° when seen in a direction normal to the substrate (“<”-shaped slit) and to form in a pixel electrode slits which are separated by a certain space from the counter electrode slits and which extend parallel to the counter electrode slits. Specifically, both the counter electrode and the pixel electrode in one pixel need to have a plurality of slits extending in the direction of 45°-225° and the direction of 135°-315° and having the width of about 10 μm.
However, when the above-described slits are employed, the ratio of the area of the slits to the pixel area increases, and accordingly, part of the pixel area which fails to contribute to display increases, so that the transmittance (brightness) significantly decreases. In the case of a small-size liquid crystal display device of finer definition, e.g., 2.4-inch VGA for use in mobile phones, the pixel pitch (row direction×vertical direction) is, for example, 25.5 μm×76.5 μm. In such a small pixel, the above-described slits cannot be formed.
In the CPA mode liquid crystal display devices, a rivet is formed of a resin or the like in the counter electrode at the pixel center, such that the rivet and a diagonal electric field produced at an edge of the pixel electrode serve to regulate the alignment of the liquid crystal. Provided in the respective gaps between the two polarizing plates and the liquid crystal layer are ¼-wave plates (quarter wave plates). By utilizing omniazimuthal, radial slope alignment domains and circular polarization, high transmittance (brightness) can be achieved.
The CPA mode which utilizes the ¼-wave plates achieves high transmittance but disadvantageously provides a low contrast ratio and a narrow viewing angle as compared with the MVA mode. Specifically, when the ¼-wave plates are used, the display (especially, the display at lower gray levels (lower brightness)) appears brighter, i.e., so-called “whitish dots” are conspicuous, when observed in a diagonal viewing angle than when observed in front of the display surface (when observed in a direction normal to the display surface (viewing angle 0°)).
To solve the above problems of the liquid crystal display device in the MVA mode and the CPA mode, liquid crystal display devices as disclosed in Patent Document 1, Patent Document 2, and Patent Document 3 have been proposed. In the liquid crystal display devices of these patent documents, the four-division alignment structure is realized by forming in the pixel electrodes a large number of narrow slits extending in the direction of 45°-225° and in the direction of 135°-315° (referred to as “fishbone pixel electrode”) such that the liquid crystal is aligned parallel to the slits. In liquid crystal display devices which use such fishbone pixel electrodes, large slits or rivets are not formed in pixels, and linearly-polarized light is used without using ¼-wave plates. Therefore, display can be realized with high transmittance, high contrast ratio, and wide viewing angle.
Note that the liquid crystal display devices of these patent documents include alignment sustaining layers on surfaces of the upper and lower substrates on the liquid crystal layer side for making the liquid crystal have an appropriate pretilt angle during absence of voltage application to the liquid crystal. These alignment sustaining layers are formed by polymerizing monomers contained in the liquid crystal layer during application of a voltage to the liquid crystal.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-107730
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-149647
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-330638
The pixel electrodes of the liquid crystal display devices described in the aforementioned patent documents have a plurality of linear electrode portions (also referred to as “line portions”) extending in the direction of 45°-22520 and the direction of 135°-315°. Abnormal alignment of the liquid crystal, or such a phenomenon that the liquid crystal alignment direction becomes unstable, can occur depending on the shape or size of the linear electrode portions or the shape or size of the plurality of slits extending parallel to the linear electrode portions (also referred to as “linear space portions”), resulting in the problems of variation in transmittance across the display surface, display unevenness, and abnormal gray scale.
The inventor of the present application conducted researches and found that, to meet severe display characteristic requirements for liquid crystal display devices in the future, it is necessary to increase the transmittance in each pixel and suppress the variation in transmittance among pixels in a display screen to be about 10% or lower. However, the shape and size of the linear electrode portions and slits which are optimum for achieving such high transmittance characteristics has not been researched. Also, the fishbone-type electrodes have not been researched as to what setting of the relationship between the width of the linear electrode portions and the width of the slits can achieve the best transmittance characteristics.
The present invention was conceived with the view of solving the above problems. One of the objects of the present invention is to provide a liquid crystal display device including fishbone-type electrodes of high display quality which has high transmittance and which has a reduced transmittance variation smaller than about 10% across the display surface.
A liquid crystal display device of the present invention is a vertical alignment type liquid crystal display device which has a plurality of pixels, including: a first electrode which includes, in each of the plurality of pixels, a plurality of first branch portions extending in a first direction and a plurality of second branch portions extending in a second direction that is different from the first direction; a second electrode disposed so as to oppose the first electrode; and a liquid crystal layer interposed between the first electrode and the second electrode, wherein a width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 8.0 μm.
In one embodiment, any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 1.4 μm and narrower than 3.2 μm.
In one embodiment, when any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 1.4 μm and narrower than 2.1 μm, the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 3.5 μm.
In one embodiment, the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 5.0 μm.
In one embodiment, when any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 1.4 μm and narrower than 2.1 μm, the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 3.2 μm.
In one embodiment, the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 5.0 μm while any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 1.8 μm and narrower than 3.2 μm, or the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range not less than 1.4 μm and not more than 3.2 μm while any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 1.4 μm and equal to or narrower than 1.8 μm.
In one embodiment, the width of each of the plurality of first branch portions and the plurality of second branch portions is in a range more than 2.1 μm and less than 2.8 μm while any adjacent two of the plurality of first branch portions and any adjacent two of the plurality of second branch portions are separated by a space wider than 2.1 μm and narrower than 2.8 μm.
In one embodiment, the first electrode includes a trunk portion, the plurality of first branch portions extend from the trunk portion in the first direction, and the plurality of second branch portions extend from the trunk portion in the second direction.
In one embodiment, the liquid crystal display device further includes a pair of polarizing plates which have transmission axes orthogonal to each other, wherein the first direction and the second direction are orthogonal to each other, and directions of the transmission axes of the pair of polarizing plates and the first direction are different by 45°, 135°, 225°, or 315°.
In one embodiment, each of the plurality of pixels includes a first region and a second region which are separated by a line parallel to or perpendicular to the directions of the transmission axes of the pair of polarizing plates, each of the first region and the second region includes the plurality of first branch portions and the plurality of second branch portions, each of the plurality of first branch portions in the first region has a first width, and each of the plurality of first branch portions in the second region has a second width that is different from the first width.
In one embodiment, each of the plurality of second branch portions in the first region has the first width, and each of the plurality of second branch portions in the second region has the second width.
In one embodiment, any adjacent two of the plurality of first branch portions in the first region are separated by a first space, and any adjacent two of the plurality of first branch portions in the second region are separated by a second space that is different from the first space.
In one embodiment, any adjacent two of the plurality of second branch portions in the first region are separated by the first space, and any adjacent two of the plurality of second branch portions in the second region are separated by the second space.
A liquid crystal display device of the present invention includes fishbone-type electrodes which have branch portions of the above-specified width or fishbone-type electrodes in which the space between adjacent branch portions has the above-specified value. Therefore, the abnormal alignment and alignment shift of the liquid crystal are suppressed so that the alignment direction of the liquid crystal in each pixel can be stabilized to be in a correct direction. Thus, high quality display with no roughness is possible in which the variation in transmittance across the display surface is smaller than about 10%.
In a liquid crystal display device of the present invention, one pixel includes two regions between which the widths of branch portions of the fishbone-type electrode or the spaces between adjacent branch portions are different. Therefore, the variation in brightness is suppressed, and high quality display with excellent gray scale and viewing angle characteristics is achieved.