Embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein:
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
Elements having same functions are given same reference numerals in all the drawings to describe these embodiments and no repeated description will be made.
The present invention relates a display device including a display panel in which TFT elements are disposed in pixel regions included in a display area. Among such display panels are liquid crystal display panels.
One of such liquid crystal display panels is a display panel in which a liquid crystal material 3 is sealed in between a pair of substrates 1, 2 as shown in
Of the pair of substrates 1, 2, the substrate 1 that is larger in size when seen from an observer is generally called a “TFT substrate.” Although not shown in
If the liquid crystal display panel adopts a drive mode called “vertical electric field mode” such as twisted nematic (TN) mode or vertical alignment (VA) mode, a counter electrode (also called “common electrode”) opposite to a pixel electrode on the TFT substrate 1 is provided adjacent to the counter substrate 2. If the liquid crystal display panel adopts a drive mode called “horizontal electric field mode” such as in-plane switching (IPS) mode, the counter electrode is provided adjacent to the TFT substrate 1.
Now a structure example of one pixel in the display area DA of the liquid crystal display panel will briefly be described with reference to
In a liquid crystal display panel to which the present invention is to be applied, one pixel in the display area DA may basically have any structure. As a liquid crystal display panel that the present invention is desirably applied, a liquid crystal display panel adopting IPS mode in which one pixel has the structure shown in
For an IPS liquid crystal display panel, pixel electrodes and counter electrodes (common electrode) are provided adjacent to the TFT substrate 1. The TFT substrate 1, for example, has a plurality of scan signal lines GL extend in the x direction and provided on a surface of a glass substrate SUB, as shown in
Provided in each pixel region on a surface of the glass substrate SUB is a flat counter electrode CT in each pixel region. The counter electrodes CT arranged in the x direction are electrically coupled to each other via common signal lines CL running in parallel to the scan signal lines GL. Provided on sides opposite to the sides of the scan signal lines GL on which the common signal lines CL are provided are common coupling pads CP, which are electrically coupled to the counter electrodes CT.
Provided on the first insulating layers PAS1 in addition to the video signal lines DL are semiconductor layers, drain electrodes SD1, and source electrodes SD2. Such semiconductor layers, for example, are made of amorphous silicon (a-Si) and include a semiconductor layer that serves as a channel layer SC for a TFT element disposed in each pixel region and a semiconductor layer (not shown) that prevents a short circuit between the scan signal lines GL and the video signal lines DL in regions in which those lines three-dimensionally intersects one another. For the semiconductor layer serving as a channel SC for a TFT element, both the drain electrode SD1 coupled to the video signal line DL, and the source electrode SD2 are coupled to the semiconductor layer.
Pixel electrodes PX are provided above the surface (layer) on which the video signal lines DL and the like are formed, with a second insulating layer PAS2 between the pixel electrodes PX and the surface. The pixel electrodes PX are independent electrodes provided in each pixel region, and are each electrically coupled to a source electrode SD2 via an opening (through hole) TH1 provided in the second insulating layer PAS2. When the counter electrode CT and pixel electrode PX are stacked with the first and second insulating layers PAS1 and PAS2 therebetween shown in
Provided on the second insulating layer PAS2 in addition to the pixel electrodes PX is, for example, bridge wiring BR for electrically coupling two counter electrodes CT disposed vertically with the scan signal line GL therebetween. Here the bridge wiring BR is coupled to the common signal line CL and common coupling pad CP disposed with the scan signal line GL therebetween, via through holes TH2, TH3.
Provided on the second insulating layer PAS2 so as to cover the pixel electrode PX and bridge wiring BR is an orientation film 5. Although not shown, the counter substrate 2 is disposed so as to be opposed to the surface of the TFT substrate 1 above which the orientation film 5 is provided.
The present invention is applied, for example, to a liquid crystal display device including the TFT substrate 1, one pixel region of which has the structure shown in
It is also assumed that a liquid crystal display panel to which the present invention is to be applied adopts the line-by-line inversion drive method. Specifically, it is assumed that one video signal line is given video signals having an identical polarity in one frame period and each two adjacent video signal lines are given video signals having mutually inverse polarities so as to be driven inversely. As a result, the pixel electrodes in the pixel regions each show, for example, the polarity relative to the common electrode as shown in
However, this type of display device has a problem that a stripe (transverse stripe) having light parts and dark parts appears on each of the lines of pixels arranged in the direction of extension of the scan signal lines. Such transverse stripes are more remarkable in the proximity of the region where a scan is completed, for example, in lower regions of the screen when the screen is scanned from top to bottom.
In a related art liquid crystal display panel (TFT substrate 1), a pixel electrode is formed such that the respective intervals between the pixel electrode and two adjacent video signal lines are identical so as to make the respective parasitic capacitances generated between the pixel electrode and the video signal lines identical.
However, if the relative positional relations between the pixel electrode and the video signal lines are changed due to mask misalignment or the like in the process of forming the pixel electrode, the respective parasitic capacitances generated between the pixel electrode and video signal lines differ from each other as shown in FIG. 8.
For example, the respective potentials of three pixels arranged between the video signal lines DLn and DLn+1 shown in
On the other hand, as the polarity of the signals given to the video signal line DLn that generates a larger parasitic capacitance with the pixel electrode is changed from negative to positive, the respective potentials of the pixel electrodes are increased by ΔV. In this case, the luminance of the pixels whose drain electrode of the TFT element is coupled to the video signal line DLn becomes less than that set up in the video signals, while the luminance of the pixel whose drain electrode of the TFT element is coupled to the video signal line DLn+1 becomes more than that set up in the video signals.
As described above, when there is a discrepancy between the respective parasitic capacitances between the pixel electrode and adjacent two video signal lines, the pixel coupled to the video signal line that generates a larger parasitic capacitance with the pixel becomes darker, while the pixel coupled to the video signal line that generates a smaller parasitic capacitance with the pixel becomes brighter. Such variations in luminance cause transverse stripes.
In the following embodiments of the present invention, methods for reducing transverse strips that appear in a liquid crystal display panel having the structure shown in
In the first embodiment, description will be made on a method for reducing the parasitic capacitance generated between the pixel electrode and video signal line to reduce transverse strips that appear when a liquid crystal display device in which TFT elements are disposed in a staggered shape is driven using the line-by-line inversion method.
When a liquid crystal display panel adopts the horizontal electric field drive mode and one pixel of the TFT substrate has the structure shown in
The relation between a gap “a” between the video signal line DL and the pixel electrode PX and the parasitic capacitance Csig is, for example, the relation shown in
In general liquid crystal display devices, the pixel electrode PX and counter electrode CT are designed so as to be as large as possible in order to increase the aperture ratio of each pixel. Therefore, it is very difficult to reduce the gap “b” between the video signal line DL and counter electrode CT to reduce the parasitic capacitance Csig. In other words, in order to reduce the parasitic capacitance Csig, it is preferable to reduce the width of the pixel electrode PX to increase the gap “a” between the video signal line DL and pixel electrode PX.
This allows the parasitic capacitance Csig generated between the video signal line DL and pixel electrode PX to be reduced. Therefore, it is possible to reduce the variation ΔV in the potential of the pixel electrode made at the timing when the polarity of the video signal line is inverted. As a result, transverse stripes that appear in the display area can be reduced.
In the first embodiment, the width of the pixel electrode PX is reduced to increase the gap “a” between the video signal line DL and pixel electrode PX. This allows the parasitic capacitance Csig between the video signal line DL and pixel electrode PX to be reduced, thereby reducing transverse strips. However, reducing the width of the pixel electrode PX will result in a reduction in the aperture ratio of the pixel.
In the second embodiment, a method for changing the drive method to reduce transverse stripes without reducing the aperture ratio of the pixel will be described.
In
Specifically,
In the third embodiment, description will be made on an example in which a drive method different from that according to the second embodiment is adopted as a method for reducing transverse stripes without reducing the aperture ratio of the pixel.
When displaying video pictures or images in a liquid crystal display device, each pixel is generally displayed in identical luminance (gray level) in one frame period. However, in recent years, there have been proposed, for example, method for displaying video pictures or images in different luminance in each of two fields obtained by dividing one frame period.
Among such methods for displaying video pictures or images in different luminance in each of two fields obtained by dividing one frame period is a method for dividing one frame period into a light field and a dark field as shown in
If such a drive method is adopted as a method for driving a liquid crystal display device according to the third embodiment of the present invention, the potential of the pixel electrode is changed, for example, as shown in
The third embodiment has another advantage in that a video picture is displayed in low luminance in the dark field display period, thereby making a variation in luminance less identifiable as well as transverse stripes less remarkable.
In the fourth embodiment, an example in which a drive method different from the second and third embodiments is adopted as a method for reducing transverse stripes without the aperture ratio of the pixel.
In the fourth embodiment, transverse stripes are reduced by inputting, to the video signal lines DL, “two-line inversion” video signals, that is, video signals such that the polarity is inverted for each two pixels arranged in the direction of extension of the video signal lines.
When two-line inversion is adopted, the potentials of pixel electrodes coupled to five scan signal lines GLm, GLm+1, GLm+2, GLm+3, and GLm+4 arranged between two adjacent video signal lines DLn and DLn+1 are changed, for example, as shown in
While the present invention has heretofore been described in detail based on the embodiments, the invention is not limited to these embodiments. Various modifications can be made to these embodiments without departing from the scope and spirit of the invention.
For example, in these embodiments, the liquid crystal display device adopting horizontal electric field drive mode in which one pixel has the structure shown in
Number | Date | Country | Kind |
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2006-151462 | May 2006 | JP | national |