JPH0815723A - Active matrix liquid crystal display - Google Patents

Abstract

PURPOSE:To obtain an active matrix liquid crystal display which is improved in the visual field angle characteristics of a liquid crystal panel by dividing the counter electrodes on pixel electrodes to plural regions and impressing driving waveforms of different voltages to the divided counter electrodes, thereby decreasing or eliminating gradation inversion regions. CONSTITUTION:This active matrix liquid crystal display has the pixel electrodes 4 consisting of transparent conductive films for driving liquid crystals, thin-film transistors 3 for transmitting image signals to the pixel electrodes 4 and the counter electrodes 6 facing the pixel electrodes 4 for driving the liquid crystals. The counter electrodes 6 on the pixel electrodes 4 are divided to plural regions A, B and the driving waveforms of the different voltages are impressed to the respective divided counter electrodes 6A, 6B. The voltage-transmittance characteristics of the liquid crystal panel are superposed with the voltage- transmittance characteristics of the liquid crystal panel divided to the shapes of the respective counter electrodes 6A, 6B by supplying the driving waveforms of the different voltages to the counter electrodes 6A, 6B dividing the pixel electrodes 4. The gradation inversion regions are thereby averaged and are decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a thin film transistor (Thi
The present invention relates to an active matrix liquid crystal display using an n film transistor).

[0002]

2. Description of the Related Art Thin film transistor (hereinafter referred to as TFT)
A display of an active matrix type display substrate using the OLED has been actively researched because it can obtain higher image quality than a display device using a simple matrix type display substrate.

FIG. 9 is a perspective view schematically showing a liquid crystal panel portion of a conventional active matrix liquid crystal display. In FIG. 9, 1 is a scanning line, 2 is a data line, and 3 is T.
FT, 4 are pixel electrodes for driving liquid crystal, 5 is a glass substrate, 6
Is a counter electrode made of a transparent conductive film, 7 is a counter substrate, 8 is a gate electrode connected to the scanning line 1, and 9 is a source electrode (or a drain electrode) connected to the data line 2. Source electrode), 10 is the pixel electrode 4
A drain electrode (relative to the source electrode 9) electrically connected to.

Normally, in a transmissive liquid crystal display device, it is necessary to transmit the light from the back light source.
The pixel electrode 4 and the counter electrode 6 must be transparent conductive films. The elements of the scanning line 1, the data line 2, the TFT 3, and the pixel electrode 4 are formed by repeating thin film formation, selective etching, and the like on the glass substrate 5 on the side where they are formed.

When color display is performed on the liquid crystal panel, color display can be performed by forming a color filter corresponding to each pixel on the glass substrate 5 on the counter substrate 7. In such a liquid crystal panel, when the TFT 3 is driven according to an image signal and the voltage applied to the liquid crystal layer is changed, the transmittance of the liquid crystal panel is changed accordingly, and an image can be displayed.

Next, a method of driving the liquid crystal panel of the active matrix liquid crystal display will be described. FIG. 10 shows an equivalent circuit of a liquid crystal panel of an active matrix liquid crystal display, where A is scanning lines A1, A2, ..., Am, B is data lines B1, B2, ..., Bn, and Q is TFTs Q11, Q1.
2, ..., Q1m, Q21, Q22, ..., Q2m, Qn1, Qn
2, ..., Qnm, T is counter electrode, Φ is drive pulse Φ1, Φ
2, ..., Φm.

The scanning line A is connected to the source electrode of the TFT Q at the intersection with the data line B. The image signal is applied to the data line B. Furthermore, TF is applied to scan line A.
The gate electrode of TQ is connected. The drain electrode of the TFT Q communicates with the counter electrode T through the driving liquid crystal. The driving pulse Φ is sequentially applied to the gate electrode from the scanning line A to turn on the TFT Q, and at this time, the image signal from the data line B is written to each pixel electrode through the source electrode. This state is maintained until the drive pulse Φ is applied to the gate electrode of the TFT Q in the next field. The image is displayed in this manner.

FIG. 11A is a voltage-transmittance characteristic of a liquid crystal panel.
(Normally white mode), the horizontal axis shows the applied voltage,
The vertical axis represents the transmittance of the liquid crystal panel. FIG. 11 (b) is an explanatory diagram of the case where the panel is rotated in the φ direction from 0 ° to 360 ° while maintaining the view angle θ. The change in each transmittance is
The downward viewing angle θ with respect to the liquid crystal panel is taken.
As the angle of view θ increases, as shown in Fig. 11 (a),
The slope of the curve showing the voltage-transmittance characteristic becomes gentle, and when the angle of view θ becomes a certain slope or more, contrary to the normal transmittance change, as shown by the broken line in FIG. A region where the transmittance increases when the applied voltage becomes high occurs at a low transmittance. This area is called a gradation inversion area 11 and causes deterioration of the viewing angle characteristics of the liquid crystal panel.

Next, the cause of the generation of the gradation inversion area 11 will be described with reference to FIG. FIG. 12 shows a state where liquid crystal molecules of a conventional liquid crystal panel are aligned, and 12 is a liquid crystal molecule. The display mode of an active matrix type liquid crystal panel is generally TN (twisted nematic) mode. In this display mode, the alignment direction of the liquid crystal molecules 12 between the glass substrate 5 and the counter substrate 7 is twisted by 90 °. When no voltage is applied, the liquid crystal molecules 12 are the glass substrate 5
And parallel to the counter substrate 7, and this state is shown in FIG.
Shown in (a). In this case, since the liquid crystal molecules 12 are almost parallel to the glass substrate 5 with respect to the viewing angle direction, the light is in a white state in which all the light is transmitted.

Next, when a voltage is applied to the liquid crystal panel from this state, the liquid crystal molecules 12 are aligned from parallel to perpendicular to the substrate. FIG. 12 (b) shows a state in which the liquid crystal molecules 12 are aligned when a voltage is applied. When such a display is made, when viewed with a certain inclination (perspective angle θ) with respect to the normal direction of the liquid crystal panel, the liquid crystal panel is oriented vertically as the liquid crystal is vertically aligned above a certain applied voltage. This is because the tilt of the liquid crystal molecules 12 with respect to that direction, particularly in the normally white mode, causes a large amount of leakage light due to the tilt of the liquid crystal molecules 12.

FIG. 13 is a radar chart showing a gradation inversion angle of a liquid crystal panel of a conventional active matrix liquid crystal display. The lines on the concentric circles have the same view angle θ with respect to the normal direction. What angle is when the panel is rotated in the φ direction from 0 ° to 360 ° while keeping the view angle θ as shown in Fig. 11 (b)? It shows whether or not gradation inversion occurs. According to this, when the panel is viewed from the bottom side at φ = 0 ° (right below the panel), gradation inversion occurs at the inclination of θ = 26 °. This gradation inversion causes a large deterioration in the display quality of the liquid crystal panel.

[0012]

However, the liquid crystal panel having such a structure has a certain inclination with respect to the normal direction.
When viewed with the (angle of view θ), as the liquid crystal is vertically aligned above a certain applied voltage, the leakage light due to the tilt of the liquid crystal molecules becomes larger depending on the viewing direction of the liquid crystal panel, and grayscale inversion occurs. Occur. This area is called a gradation inversion area, and there is a problem that the viewing angle characteristics of the liquid crystal panel are deteriorated.

The present invention solves the above-mentioned problems of the prior art, and an object thereof is to provide an active matrix liquid crystal display in which the viewing angle characteristics of a liquid crystal panel are improved by reducing or eliminating the gradation inversion region. To do.

[0014]

To achieve this object, the present invention provides a pixel electrode made of a transparent conductive film for driving a liquid crystal, and a thin film transistor for transmitting an image signal to the pixel electrode. In an active matrix liquid crystal display having a counter electrode facing the pixel electrode for driving the liquid crystal, the counter electrode on the pixel electrode is divided into a plurality of regions, and the divided counter electrodes have different voltages. It is characterized in that a driving waveform is applied.

Further, the counter electrode is divided into two regions.

Further, the counter electrodes divided into a plurality of regions are coupled by electric capacitance, and a drive waveform obtained by dividing the voltage by the ratio of the capacitance of the counter electrode and the capacitance for coupling the counter electrode is applied. It is configured to do.

[0017]

According to the above construction, by supplying driving waveforms having different voltages to the counter electrodes which divide one pixel electrode, the voltage-transmittance characteristic of the liquid crystal panel is divided into the shapes of the respective counter electrodes. The voltage-transmittance characteristic of is overlapped, and the gradation inversion region becomes smaller by being averaged.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment will be described in detail below with reference to the drawings.

FIG. 1 shows an active matrix liquid crystal display which is a first embodiment of the present invention.
FIG. 2 is a plan view of the divided counter electrodes A and B, and FIG. 2 is a schematic perspective view of the liquid crystal panel. The opposed electrodes 6A and 6B divided into the respective parts are connected at their ends, and a voltage is supplied from the external terminals A and B. FIG. 3 shows these equivalent circuits, FIG. 4A shows a waveform Vg (driving pulse) applied to the gate electrode 8 from the scanning line 1, and FIG. 4B shows a waveform Vsc applied to the source electrode 9 from the data line 2. (Image signal), FIG. 4 (c) shows a waveform Vtc1 (counter signal) applied to the counter electrode 6A in the area A, and FIG. 4 (d) shows a waveform Vtc2 (counter signal) applied to the counter electrode 6B in the area B. . Moreover, the same reference numerals are given to those having the same operation and effect described in FIG. 9 of the conventional example, and the detailed description thereof will be omitted.

Here, the respective portions of the counter electrodes 6A and 6B obtained by dividing the counter electrode 6 into two areas are designated as areas A and B, and the driving conditions of the liquid crystal panel are set as follows. As the image signal, the data line 2 has its polarity inverted every horizontal scanning (hereinafter, referred to as 1H) time, and the magnitude of its amplitude is changed from 0V to 5V to display white (Vs
The brightness is changed from (W)) to black display (Vs (B)) (FIG. 4 (b)). The counter signal is inverted at the counter electrode 6A in the area A at the same period (1H) as the data line 2 and the amplitude thereof is set to 3V (FIG. 4 (c)). In the region B, the counter electrode 6B has the same inversion cycle and phase as in the region A, and a signal of 5 V in amplitude is applied (FIG. 4 (d)).

FIG. 5 shows the voltage-transmittance characteristics when driven under the above conditions. The characteristic 13 of the liquid crystal portion in the area A is shown by the broken line in FIG. 5, and the characteristic 14 of the liquid crystal portion in the area B is shown at one point in FIG. Represented by a chain line. In each of the voltage-transmittance characteristics, the horizontal axis represents the counter electrode 6
The transmittance change is shifted by an amount corresponding to the difference between the amplitude components of the voltages applied to A and 6B. The reason why the voltage-transmittance characteristics are different is that the potential of the counter electrode 6 at the end of voltage application in each region is different, and thus the voltage applied to the pixel at the time of holding until the next voltage application is different. is there.

The voltage-transmittance characteristic of the liquid crystal panel in which the pixels are divided according to the shape of the counter electrode 6 in this way is determined by the superposition of both characteristics, and the characteristic 15 obtained by superimposing the areas A and B at this time is obtained. This is shown by the solid line in FIG. The gradation inversion region 11 that has been generated when the angle of view θ is large is reduced by averaging. Further, the opposing electrodes 6 of both
By changing and optimizing the voltage amplitude of
It is possible to eliminate 11.

FIG. 6 shows a radar chart of the gradation inversion angle when the device is manufactured as described above and operated under driving conditions. As shown in FIG. 6, when observed from directly below the panel, it is understood that gradation inversion does not occur. From the above, the display quality of the liquid crystal panel could be significantly improved.

Although the first embodiment shows the case where the pixel is divided in the vertical direction, the gradation inversion region 11 minimizes the division shape, the area ratio, etc. in the vertical direction or the horizontal direction. It is optional to adjust so that In principle, the more the counter electrode 6 is divided, the finer the voltage applied to each region can be set, and thus the finer the voltage-transmittance characteristic of the liquid crystal panel can be set. In general, it has been clarified from the results of experiments that the sufficient effect can be obtained for this purpose by dividing into two regions.

Next, a second embodiment will be described. In the first embodiment, the voltages applied to the respective areas A and B are supplied from different power sources, but in the second embodiment, the area A of the counter electrode 6A and the area B of the counter electrode 6B are supplied. And the part of are connected capacitively by an external capacitor.

FIG. 7 is a schematic perspective view of the second embodiment.
The equivalent circuit is shown in FIG. Further, Ca is the counter electrode 6A.
Let Cb be the load capacitance of the region A portion of C, and Cb be the load capacitance of the region B portion of the counter electrode 6B. When a voltage is applied to the external terminal A from the outside, the voltage applied from the outside is directly applied to the area A as shown in FIG. 7, but the voltage applied to the area B is the voltage applied from the outside to the external capacitor C1. The voltage is divided by the ratio between the value and the load capacitance Cb of the area B of the counter electrode 6B. Therefore, by properly selecting the value of the external capacitance, it is possible to reduce the number of power sources connected to the counter electrodes 6A and 6B and obtain the same effect as that described in the first embodiment.

In the second embodiment, the amplitude of the voltage applied to the area A is set to 5 V, and the value of the external capacitor C1 connected to the area A and the area B is set to 0.12 μF. The total load capacity of the counter electrode 6 of the liquid crystal panel is 0.4.
The load capacitance Ca in the area A of the counter electrode 6A is half, which is 0.2 μF. If the value of the external capacitor is determined as described above, the amplitude of the voltage of the signal applied to the area B is (0.12 /
0.2) × 5V = 3V, and the same effect as that of the first embodiment can be obtained. This makes it possible to reduce the number of power sources, reduce the cost of the liquid crystal panel, and improve the viewing angle.

Although the effect described in the above embodiment is described in the case where the image signal and the counter signal are inverted in the 1H period, this effect is not limited to the drive in which the signal is inverted in the 1H. The same effect can be obtained even in a driving method in which the amplitude of the opposite field periodically changes.

[0029]

As described above, according to the present invention,
One display pixel electrode is divided into a plurality of areas, and different voltages are applied to the counter electrodes to give different optical characteristics, thereby reducing or eliminating the gradation inversion region, thereby improving the viewing angle characteristics of the liquid crystal panel. .

Further, the divided counter electrode is structured to be capacitively connected by an external capacitor, and is connected to the counter electrode by dividing the applied voltage by the ratio of the values of the external capacitor and the capacitance of the counter electrode. As a result, the number of power sources to be used is reduced, the cost is reduced, and the viewing angle is improved.

[Brief description of drawings]

FIG. 1 shows two pixel electrodes in the first embodiment of the present invention.
FIG. 6 is a plan view showing a liquid crystal panel of an active matrix liquid crystal display having divided counter electrodes.

FIG. 2 is a perspective view schematically showing the liquid crystal panel of the first embodiment.

FIG. 3 is a diagram showing an equivalent circuit of the liquid crystal panel of the first embodiment.

FIG. 4 is a waveform (a) applied from a scanning line to a gate electrode by a signal applied to each electrode of the liquid crystal panel of the first embodiment, a waveform (b) applied from a data line to a source electrode, and a region A facing each other. It is a figure which shows the waveform (c) applied to an electrode, and the waveform (d) applied to the counter electrode of the area | region B.

FIG. 5 is a diagram showing voltage-transmittance characteristics obtained by overlapping the areas A and B of the counter electrode of the first embodiment and both of them.

FIG. 6 is a radar chart showing a grayscale inversion angle of the first embodiment.

FIG. 7 is a perspective view schematically showing a liquid crystal panel of a second embodiment.

FIG. 8 is a diagram showing an equivalent circuit of the liquid crystal panel of the second embodiment.

FIG. 9 is a perspective view schematically showing a liquid crystal panel of a conventional active matrix liquid crystal display.

FIG. 10 is a diagram showing an equivalent circuit of a conventional liquid crystal panel.

FIG. 11 is a diagram (a) showing a voltage-transmittance characteristic of a conventional liquid crystal panel, and an explanatory diagram (b) in the case where the panel is rotated in the φ direction from 0 ° to 360 ° while keeping the view angle θ.

FIG. 12 is a diagram showing a state in which liquid crystal molecules of a conventional liquid crystal panel in a voltage non-applied state (a) and a voltage applied state (b) are aligned.

FIG. 13 is a radar chart showing a grayscale inversion angle of a conventional liquid crystal panel.

[Explanation of symbols]

1 ... Scan line, 2 ... Data line, 3 ... TFT (thin film transistor), 4 ... Pixel electrode, 5 ... Glass substrate,
6, 6A, 6B ... Counter electrode, 7 ... Counter substrate, 8 ... Gate electrode, 9 ... Source electrode, 10 ... Drain electrode,
11 ... gradation reversal region, 12 ... liquid crystal molecule, 13 ... characteristic of liquid crystal portion of region A, 14 ... characteristic of liquid crystal portion of region B, 15 ... characteristic of overlapping region A and region B.

Claims (3)

[Claims] 1. A pixel electrode made of a transparent conductive film for driving a liquid crystal, a thin film transistor for transmitting an image signal to the pixel electrode, and a counter electrode facing the pixel electrode for driving the liquid crystal. In the active matrix liquid crystal display having, the counter electrode on the pixel electrode is divided into a plurality of regions, and driving waveforms having different voltages are applied to the divided counter electrodes. 2. The active matrix liquid crystal display according to claim 1, wherein the counter electrode is divided into two regions. 3. A counter waveform divided into a plurality of regions is coupled by an electric capacitance, and a drive waveform obtained by dividing a voltage by a ratio of a capacitance of the counter electrode and a capacitance of coupling the counter electrode is applied. The active matrix liquid crystal display according to claim 1 or 2, characterized in that: