(a) Field of the Invention
The present invention relates to an active matrix liquid crystal display (LCD) device having a wide viewing angle.
(b) Description of the Related Art
Conventional LCD devices include a static drive LCD device wherein an electric field is applied to the liquid crystal (LC) by a constant voltage signal. The static drive LCD device has a drawback in that a large number of signal lines are required in the case of a large capacity display panel.
Thus, a large capacity LCD panel generally uses a multiplex drive scheme, wherein the signal voltages are supplied to the LC in a time sharing scheme. Among the LCD devices using the multiplex drive scheme, an active matrix LCD device, wherein electric charge supplied to the electrodes in a frame period is maintained to a next frame period, exhibits a high image quality. On the other hand, LCD devices are categorized also into two types including one wherein the electric field is applied to the LC perpendicular to the glass substrates sandwiching therebetween the LC, and the other wherein the electric field is applied parallel to the substrate. The latter is called "In-plane Switching Scheme" and especially suited to a large size monitor due to its wide viewing angle.
FIG. 1 shows the structure of electrodes in a pixel element of a conventional active matrix LCD device, such as described in JP-B-63-21907. The LCD device includes a plurality of pixel elements arranged in a matrix, a plurality of scanning lines 108 each connected to an external drive circuit and gates of a corresponding row of the pixel elements, a plurality of signal lines 102 each for supplying a corresponding column of the pixel elements, a common electrode 103 disposed for all of the pixel elements and having a comb-shape electrode portion in each pixel area. Each pixel element includes a thin film transistor (TFT) 109 and a pixel electrode 104 having a comb shape corresponding to the comb shape of the electrode portion of the common electrode.
Referring to FIG. 2 taken along line II--II in FIG. 1, the conventional LCD device has a LC panel 300 including a TFT panel 100 and a counter panel 200. The TFT panel 100 includes, from the front side thereof, a TFT glass substrate 101 having a first polarizing plate 110 on the front side thereof, the common electrode 103, an insulator film 105, and the pixel electrode 104 and the signal line 102. The teeth of the comb-shape pixel electrode 104 and teeth of the comb-shape common electrode 103 are arranged alternatively with each other to oppose each other in the direction parallel to the LC panel 300. These electrodes 103 and 104 are protected by a protective insulator film 106, on which a first orientation film 107 is formed by coating. The first orientation film 107 is subjected to a rubbing operation in a first direction.
The counter panel 200 has, from the rear side thereof, a glass counter substrate 201 having a second polarizing plane 205 on the rear side thereof, a matrix shield film 203 for shielding light, a color film 204 for displaying multi-color image, a planarization film 202, and a second orientation film 207. The second orientation film 207 is subjected to rubbing operation in the second direction opposite to the first direction.
LC layer 301 is disposed between the TFT panel 100 and the counter panel 200, wherein the LC molecules are oriented in the first direction adjacent to the TFT panel 100 by the first orientation film 107 and in the second direction adjacent to the counter panel 200 by the second orientation film 207. The first polarizing plate 110 bonded onto the front side of the TFT substrate 101 has a light transmission axis perpendicular to the first direction, whereas the second polarizing plate 205 bonded onto the rear side of the counter substrate 201 has a light transmission axis perpendicular to the light transmission axis of the first polarizing plate 110.
In operation, the TFT 109 is turned on/off by the corresponding scanning line 108 formed as the common layer with the pixel electrode 103. When the TFT 109 is turned on, electric charge flows from the signal line 102 into the pixel electrode 104, and after the TFT 109 is turned off in the subsequent period, the pixel electrode 104 stores the electric charge. The common electrode 103 is maintained at a constant potential, thereby generating a transverse electric filed due to the potential difference between the same and the pixel electrode 104 in the direction parallel to the LC panel 300.
The transverse electric field rotates the crystal axis of the LC molecules due to the interaction between the electric field and the dielectric anisotropy of the LC molecules, as shown in FIG. 3, wherein the rotation is exemplarily shown in the case of a positive dielectric anisotropy of the LC molecules. If the dielectric anisotropy is negative, the rotation is opposite to the direction shown in FIG. 3. The rotation of the LC molecules generates retardation change, wherein the transmission (or permeability) changes at the locations where the light shield layer 203, pixel electrode 104, common electrode 103, scanning line 108 and TFT 109 are not disposed.
FIG. 4 shows the principle of the operation of the LCD panel, wherein the LC molecule exemplarily has a positive dielectric anisotropy. The direction of the initial orientation of the LC molecules 302 is determined depending on the rubbing direction of the first orientation film 107 on the TFT panel 100, and thus aligned perpendicular to the polarizing axis of the first polarizing plate 110 of the TFT panel 100. The incident light is polarized by the first polarizing plate 110, and thus is substantially completely shielded by the second polarizing plate 205 because the polarized light is not subjected to the retardation of the liquid crystal layer 301. In this state, the LCD panel exhibits black.
When the transverse electric field is applied to the LC molecule due to the potential difference between the common electrode 103 and the pixel electrode 104, the LC molecule 302 rotates due to the interaction between the dielectric anisotropy of the LC molecule and the transverse electric field. The incident light is subjected to the retardation of the LC due to the dielectric anisotropy, and generally assumes an elliptical polarization just before passing the second polarizing plate 205. The component of the elliptically polarized light aligned with the polarization axis passes the LCD panel 300, and the time average of the light intensity is sensed by human eyes.
The shape of the elliptically polarized light is changed depending on the angle .psi. defined by the mean orientation and the initial orientation of the LC molecules 302, wherein the normalized transmission T/T.sub.0 is determined by the following approximate expression: EQU T/T.sub.0 =sin.sup.2 (2.psi.)sin.sup.2 (.DELTA.n.times.d.times..pi./.lambda.) (1)
wherein .DELTA.n,d and .lambda. are anisotropy in the refractive index, cell gap and wavelength of the transmitted light, respectively. In equation (1), the minimum transmission is obtained by .psi.=0.degree., whereas the maximum transmission is obtained by .psi.=45.degree..
In the active matrix LCD device as described above, a color tint phenomenon is generally observed due to the refractive index anisotropy of the LC modules, wherein the polarized light exhibits a blue or yellow tint when the LC panel is observed with a relatively large viewing angle. As schematically illustrated in FIG. 5, the view angle along the major axes of the LC molecules involves the blue tint, whereas the view angle along the minor axes of the LC molecules involves the yellow tint. FIG. 6 shows x-y chromaticity change in the case of an intermediate gray scale level, wherein LCD device is observed with a viewing angle .theta.=60.degree., with the azimuth .phi. being between 0 and 360.degree.. The definition for the viewing angle .theta. and the azimuth .phi. are shown in FIG. 7, wherein the gothic arrow indicates the chromaticity coordinates as viewed from the front. Referring to FIG. 6 showing x-y coordinate chromaticity change, it will be noted that the viewing direction which is diagonal to the front view involves a large color shift toward blue or yellow.
The mechanism of generation of the tint phenomenon is detailed below. Table 1 shows theoretical formulae of the effective refractive index anisotropy .DELTA.n' and the effective thickness d' of the liquid crystal cell when the LC molecules 302 are observed from the direction along the major axes and the minor axes thereof, with a viewing angle of .theta..
TABLE 1 .DELTA.n' d' major axis ##EQU1## ##EQU2## minor axis .DELTA.n ##EQU3##
Typical values substituted for respective parameters in the formulae shown in Table 1 provide viewing angle dependency of retardation (.DELTA.n'.times.d'), as shown in FIG. 8. Specifically, the relationship between the retardation .DELTA.n.sub.0.times.d.sub.0 and the wavelength .lambda. of the maximum-transmitted light is expressed by: EQU .DELTA.n.times.d/.lambda.=1/2 (2)
That is, the retardation and the wavelength of the maximum-transmitted light are proportional to each other. From FIG. 8 and equation (2), the blue tint and the yellow tint are observed when the viewing direction is aligned with the major axes and the minor axes, respectively, of the LC molecules.
Referring to FIG. 9, there are shown diagrams of orientations of the LC molecules as viewed in the cell thickness direction in the cases of displaying black, intermediate and white levels on the LC panel. The constraint of the orientation by the substrate surfaces allows the LC molecules to align more uniformly in one direction in the vicinity of black level compared to the vicinity of white level (or less black level), as shown in FIG. 9. In this respect, the color tint is larger when the display indicates more black level. However, the color is in fact more visible in the vicinity of the intermediate level because the color tint is not clearly observed in the black level.
To solve the problem color tint, JP-A-9-105908 proposes disposition of diagonal edges of the common electrode 103 and the pixel electrode 104, as shown in FIG. 10, thereby rotating the LC molecules in both the rotational directions. The proposed technique reduces the color tint remarkably; however, it involves reduction of the ratio of the opening area to the total area in each pixel area due to the diagonal edges. In addition, the interface between the areas of the opposite rotational directions generates a reverse-twist disclination, which involves afterimage observed by human eyes.