KR20090036557A - Liquid crystal display - Google Patents

Abstract

A liquid crystal display device is provided to arrange two pixel electrodes and a TFT(Thin Film Transistor) on one pixel area and combine capacities of the two pixel electrodes, thereby improving visibility in a forward direction. The first signal line and the second signal line are formed on the first insulating substrate in the first direction. The third signal line is formed on the first insulating substrate in the second direction. The third signal line is insulated from the first and second signal lines. The first thin film transistor is connected to the first signal line and the third signal line. The second thin film transistor is connected to the second signal line and the third signal line. The first pixel electrode(190a) is connected to the first thin film transistor. The second pixel electrode(190b) is connected to the second thin film transistor. A reference electrode is formed on the second insulating substrate. A liquid crystal material layer is injected between the first substrate and the second substrate. The first pixel electrode and the second pixel electrode are capacitively coupled.

Description

Liquid crystal display {LIQUID CRYSTAL DISPLAY}

The present invention relates to a liquid crystal display device.

In general, a liquid crystal display device injects a liquid crystal material between an upper substrate on which a common electrode, a color filter, and the like are formed, and a lower substrate on which a thin film transistor and a pixel electrode are formed. By applying a different voltage to form an electric field to change the arrangement of the liquid crystal molecules, and through this to adjust the transmittance of light to represent the image.

However, it is an important disadvantage that the liquid crystal display device has a narrow viewing angle. In order to overcome these disadvantages, various methods for widening the viewing angle have been developed. Among them, liquid crystal molecules are oriented vertically with respect to the upper and lower substrates, and a method of forming a constant opening pattern or forming protrusions on the pixel electrode and the common electrode opposite thereto is performed. This is becoming potent.

As a method of forming the opening pattern, an opening pattern is formed in each of the pixel electrode and the common electrode, and the viewing angle is widened by adjusting the direction in which the liquid crystal molecules lie down using a fringe field formed by the opening patterns. .

The method of forming the protrusions is a method of controlling the lying direction of the liquid crystal molecules by using the electric field distorted by the protrusions by forming protrusions on the pixel electrode and the common electrode formed on the upper and lower substrates, respectively.

In another method, an opening pattern is formed on the pixel electrode formed on the lower substrate, and a protrusion is formed on the common electrode formed on the upper substrate, so that the liquid crystal lies down using the fringe field formed by the opening pattern and the protrusion. There is a way to form a domain by controlling.

In such a multi-domain liquid crystal display, the gray scale inversion reference viewing angle defined as a contrast ratio reference viewing angle based on a contrast ratio of 1:10 or a limit angle of luminance inversion between gray scales is excellent, more than 80 ° in all directions. However, the gamma curve of the front side and the gamma curve of the side do not coincide with each other, resulting in inferior visibility in the left and right sides compared to the TN (twisted nematic) mode liquid crystal display. For example, in the patterned vertically aligned (PVA) mode in which openings are formed by domain dividing means, the screen looks brighter toward the side and the color tends to shift toward white. Occasionally, the picture appears clumped and disappears. However, as liquid crystal display devices are used for multimedia in recent years, visibility has become increasingly important as pictures and moving pictures are viewed.

The technical problem to be achieved by the present invention is to implement a multi-domain liquid crystal display device having excellent visibility.

A liquid crystal display according to the present invention for achieving the above object is formed on the first insulating substrate, the first signal line and the second signal line, which are formed in the first direction on the first insulating substrate, on the first insulating substrate in the second direction. It is, and the first and the second thin film transistor which is cross-insulated and second signal line and third signal line, connected to the first signal line and the third first thin film transistor connected to the signal line, second signal line and third signal line, A first pixel electrode connected to the first thin film transistor, a second pixel electrode connected to the second thin film transistor, a second insulating substrate facing the first insulating substrate, a reference electrode formed on the second insulating substrate, and A liquid crystal material layer injected between the first substrate and the second substrate, and formed on at least one of the first insulating substrate and the second insulating substrate, and forming the first pixel electrode and the second pixel; Domain dividing means for dividing the small electrode into a plurality of small domains, the lower side of the first pixel electrode and the upper side of the second pixel electrode facing each other, and the lower side and the upper side of the two pixel electrodes facing each other; The first signal electrode and the second pixel electrode are engaged with the first or second signal line at a predetermined angle larger than 0 ° and smaller than 90 °, and the first pixel electrode and the second pixel electrode form a capacitive coupling with each other.

In the present invention, two pixel electrodes and a thin film transistor are disposed in one pixel region, and the two pixel electrodes are capacitively coupled to improve visibility in all directions. Further, since the domain is divided so that the average director of the liquid crystal is directed toward the 45 ° direction with respect to the gate line or the data line, a polarizing plate in which the polarization axis is parallel to the gate line or the data line can be used. Therefore, the manufacturing cost of the polarizing plate can be reduced.

Next, a liquid crystal display according to an exemplary embodiment of the present invention will be described with reference to the drawings.

1 is a layout view of a liquid crystal display according to an exemplary embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along line II-II 'of FIG. 1.

The liquid crystal display according to the exemplary embodiment of the present invention includes a thin film transistor substrate and a color filter substrate facing each other, and a liquid crystal layer injected therebetween. The thin film transistor and the pixel electrode having the cutout are repeatedly arranged in a matrix shape on the thin film transistor substrate, and the reference electrode having the color filter and the cutout is formed on the color filter substrate. The liquid crystal molecules of the liquid crystal layer are aligned so as to be perpendicular to the two substrates without an electric field applied thereto. Hereinafter, each of them will be described in more detail.

First, the thin film transistor substrate will be described.

A gate line 121 extending in a horizontal direction is formed on a transparent insulating substrate 110 such as glass, and a portion of the gate line 121 forms a gate electrode 123.

In addition, the storage electrode line 131 is formed on the insulating substrate 110. The storage electrode line 131 extends in the horizontal direction as a whole but is partially curved.

A gate insulating layer 140 is formed on the gate lines 121 and 123 and the storage electrode line 131, and an amorphous silicon layer 151 is formed on the gate insulating layer 140 on the gate electrode 123. The amorphous silicon layer 151 overlaps the gate electrode 123 to form a channel portion of the thin film transistor. On the amorphous silicon layer 151, drain resistive contact layers 165a and 165b made of amorphous silicon doped with N-type impurities such as phosphorus (P) at a high concentration are formed. Although not shown in the cross-sectional view, source resistive contact layers 163 are formed to face drain resistive contact layers 165a and 165b.

The data lines 171, 173, 175a, and 175b and the coupling electrode 174 are formed on the ohmic contacts 163, 165a, and 165b and the gate insulating layer 140. The data lines 171, 173, 175a, and 175b may include a data line 171 extending in the vertical direction, a source electrode 173 that is a part thereof, and first and second drain electrodes 175a and 175b separated from the data line 171. Include. The source electrode 173 faces the first and second drain electrodes 175a and 175b on the gate electrode 123, and one end of each of the first and second drain electrodes 175a and 175b has a gate line ( It extends inwardly of the first and second pixel regions positioned at both sides with respect to 121. As described below, the coupling electrode 174 electromagnetically couples the first pixel electrode 190a and the second pixel electrode 190b which are separated from each other. Here, the ohmic contacts 163, 165a, and 165b are formed only at portions where the amorphous silicon layer 151 and the data lines 171, 173, 175a, and 175b overlap.

The passivation layer 180 is formed on the data lines 171, 173, 175a, and 175b. In this case, the passivation layer 180 may include the first and second contact holes 181 and 182 exposing one end of the first and second drain electrodes 171, 173, 175a and 175b, respectively. It has the 3rd contact hole 183 which exposes one end.

First and second pixel electrodes connected to the first drain electrode 175a and the second drain electrode 175b through the first contact hole 181 and the second contact hole 182 on the passivation layer 180, respectively. 190a and 190b are formed. Here, the second pixel electrode 190b is connected to the coupling electrode 174 and the third contact hole 183, and the first pixel electrode 190a overlaps the coupling electrode 174 to be electromagnetically coupled ( Capacitively coupled). As a result, the first pixel electrode 190a and the second pixel electrode 190b form capacitive coupling through the coupling electrode 174. In addition, the first and second pixel electrodes 190a and 190b overlap the storage electrode line 131 to form a storage capacitor. The first and second pixel electrodes 190a and 190b are made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). Meanwhile, the first pixel electrode 190a has first to third lower cutouts 191, 192, and 193. The third lower cutout 193 is positioned at the top and bottom centers of the first pixel electrode 190a and is formed to dig from left to right, and the first and second lower cutouts 191 and 192 are respectively formed in a third shape. The upper and lower portions of the first pixel electrode 190a divided up and down by the lower cutout 193 are formed in diagonal directions. The first lower cutout 191 and the second lower cutout 192 are symmetrical with respect to the third lower cutout 193. Sides adjacent to the first pixel electrode 190a and the second pixel electrode 190b are bent in a V shape. At this time, the sides of the first pixel electrode 190a are convex, and the sides of the second pixel electrode 190b are concave, V.

Next, the color filter substrate will be described.

The black matrix 220 is formed on a transparent insulating substrate 210 such as glass, and the red, green, and blue color filters 230 are repeatedly formed in each pixel region defined by the black matrix 220. . An overcoat layer 250 is formed on the color filter 230, and a reference electrode 270 made of a transparent conductive material such as ITO is formed on the overcoat layer 250. The reference electrode 270 has first to fourth upper cutouts 271, 272, 273, and 274. The first to third upper cutouts 271, 272, and 273 are disposed at positions overlapping with the first pixel electrode 190a, and together with the first to third lower cutouts 191, 192, and 193, the first pixel. The region of the electrode 190a is divided into a plurality of small domains, and the fourth upper cutout 274 divides the region of the second pixel electrode 190b up and down into four small domains.

At this time, each of the small domains divided by the lower cutouts 191, 192, and 193 and the upper cutouts 271, 272, 273, and 274 substantially forms a quadrangular shape, and two long sides thereof are gate lines 121. ) And the data line 171.

Lower and upper polarizers 12 and 22 are attached to the outer sides of the two substrates 110 and 210. At this time, the polarization axes of these polarizing plates 12 and 22 are parallel to the gate line 121 or the data line 171, and are arranged to be orthogonal to each other.

In the liquid crystal display having the structure, the first pixel electrode 190a and the second pixel electrode 190b are switched by different thin film transistors, and thus may receive different voltages. In the case of sequentially driving the pixel rows from top to bottom, the voltage applied to the first pixel electrode 190a is maintained to be lower than the voltage applied to the second pixel electrode 190b. Therefore, the voltage applied to the first pixel electrode 190a according to each gray level is set to be lower than the voltage applied to the second pixel electrode 190b (voltage shift) by a certain amount, thereby reducing the distortion of the gamma curve. do. In this case, the visibility is greatly changed by the area ratio of the first pixel electrode 190a and the second pixel electrode 190b and the magnitude of the voltage shift. Therefore, care must be taken in determining these values.

Next, the reason why the voltage of the first pixel electrode 190a is lower than the voltage of the second pixel electrode 190b will be described with reference to FIG. 3.

Between two pixel electrodes P (n) -a, P (n) -b] voltages {V [P (n) -a], V [P (n) -b)] disposed in one pixel region To derive the relationship.

In FIG. 3, Clca is a liquid crystal capacitor formed between a pixel electrode and a reference electrode, Csta is a storage capacitor formed between a sustain electrode line and a pixel electrode, Clcb is a liquid crystal capacitor formed between a b pixel electrode and a reference electrode, and Cstb is The storage capacitor formed between the storage electrode line and the b pixel electrode, Cpp, represents the coupling capacitance formed between the a pixel electrode and the b pixel electrode.

3, the first and second thin film transistors are connected to the same gate line and the data line, and the first pixel electrode and the second pixel electrode are connected to the first and second thin film transistors, respectively. The first pixel electrode and the second pixel electrode having the storage electrode line 131 interposed therebetween form a capacitive coupling Cpp.

Based on one data line 171, when the n-th gate line 121 is turned on, two thin film transistor (TFT) channels are turned on and thereby the first and second pixel electrodes P (n). ) -a, P (n) -b] is applied. However, P (n) -b is capacitively coupled to P (n + 1) -a so that P (n) -b is affected when P (n + 1) -a is on. Therefore, the voltages of P (n) -a and P (n) -b are given by

V [P (n) -a] = Vd (n)

In Equations 1 and 2, Vd (n) denotes a voltage applied to a data line to drive a P (n) pixel, and Vd (n + 1) denotes a data applied to drive P (n + 1). Means line voltage. In addition, V'd (n + 1) means a voltage applied to the P (n + 1) pixel of the previous frame.

As shown in Equations 1 and 2, the voltage applied to the P (n) -b pixel and the voltage applied to the P (n) -a are different from each other. In particular, when dot inversion driving or line inversion driving is performed, and the next pixel row displays the same gray scale as the previous pixel row (actually, most of the pixels have a lot of time in this case), Vd (n) =-Vd Since (n + 1) and Vd (n) =-V'd (n) (assuming that the reference electrode voltage is the ground voltage), Equation 2 can be arranged as follows.

*

According to Equation 3, a voltage lower than P (n) -a is applied to P (n) -b. Therefore, when the P (n + 1) pixel displays the same gray level as the P (n) pixel, the voltage applied to P (n + 1) -a (corresponding to the second pixel electrode) is P (n) -b ( The voltage applied to the first pixel electrode).

4 is a graph showing visibility distortion amount according to voltage shift and domain ratio.

In FIG. 4, the vertical axis indicates the amount of visibility distortion, and the horizontal axis indicates the area ratio of the first pixel electrode 190a and the second pixel electrode 190b and the magnitude of the voltage shift.

Visibility of 0.1 ~ 0.2 means very good visibility at the level of cathode ray tube (CRT), 0.2 ~ 0.25 indicates very good visibility, 0.25 ~ 0.3 is good visibility, and 0.3 ~ 0.35 is good. It is enough. Less than 0.35 is inferior in visibility and the display quality is not good.

Judging from FIG. 4, excellent visibility is seen when the area ratio of the first pixel electrode and the second pixel electrode is 50:50 to 80:20, and the voltage shift amount is excellent when 0.4 to 1.0V near Vth. It shows visibility. In other words, it is preferable to design the first pixel electrode larger than the second pixel electrode. However, when the first pixel electrode is 80% or more than the second pixel electrode due to a problem such as a kick-back voltage, various problems such as flicker may occur, which is not preferable. In addition, the visibility is improved when the Vth voltage of the first pixel electrode is 0.4V to 1.0V lower than the Vth voltage of the second pixel electrode. The voltage difference between the first pixel electrode and the second pixel electrode may become larger when the gray level is high.

Next, the reason why the visibility is improved in the liquid crystal display according to the present invention will be described with reference to FIGS. 5 and 6.

FIG. 5 is a graph showing gamma curve distortion at the side of a conventional liquid crystal display, and FIG. 6 is a graph showing the reduction of gamma curve distortion at the side according to the present invention.

Referring to FIG. 5, it can be seen that when only one pixel electrode is formed as in the related art, the gamma curve of the side surface is greatly distorted upward compared to the front gamma curve. However, as in the present invention, when the pixel electrode is divided into two sub-electrodes (the first pixel electrode and the second pixel electrode), and these two sub-electrodes are capacitively coupled, they are applied to one sub-electrode (the first pixel electrode). The voltage is shifted. At this time, when the image signal voltage is set so that the voltage applied to the first pixel electrode is lower than the normal gray voltage, the voltage of the first pixel electrode is kept below the threshold voltage Vth in a predetermined range of low grayscale, so that the first The pixel electrode portion remains black and only the second pixel electrode portion contributes to the increase in luminance. However, since the area of the second pixel electrode portion is small, the luminance increase is also small (the 'only second pixel electrode on' portion of FIG. 6). When the gray level is higher than the predetermined gray level, the voltage of the first pixel electrode also rises above the threshold voltage, so that the first pixel electrode also contributes to the increase in brightness, thereby increasing the brightness ('first pixel electrode and second pixel electrode on' of FIG. 6). part). Therefore, as shown in Fig. 6, the distortion of the gamma curve is reduced.

FIG. 7 is a gamma curve obtained by measuring a conventionally patterned vertically aligned (PVA) mode liquid crystal display, and FIG. 8 is a gamma curve measured by a liquid crystal display according to an exemplary embodiment of the present invention.

7 and 8 illustrate that the gamma curve distortion of the liquid crystal display according to the exemplary embodiment of the present invention is smaller than that of the conventional liquid crystal display in all directions.

1 is a layout view of a liquid crystal display according to an exemplary embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line II-II 'of FIG. 1,

3 is a circuit diagram of a liquid crystal display according to an exemplary embodiment of the present invention.

4 is a graph showing the amount of visibility distortion according to voltage shift and domain ratio,

5 is a graph illustrating gamma curve distortion in a side of a conventional liquid crystal display device;

6 is a graph showing that the gamma curve distortion at the side is reduced according to the present invention,

7 is a gamma curve measured by a conventional patterned vertically aligned (PVA) mode liquid crystal display device.

8 is a gamma curve measured by the liquid crystal display according to the exemplary embodiment of the present invention.

110: insulated substrate 121: gate line

123: gate electrode 131: sustain electrode line

140: gate insulating film 171: data line

173: source electrodes 175a and 175b: drain electrodes

174: bonding electrode 180: protective film

190a, 190b: pixel electrode

191, 192, 193: lower incision 271, 272, 273, 274: upper incision

Claims (1)

First insulating substrate, First and second signal lines formed on the first insulating substrate in a first direction; A third signal line formed on the first insulating substrate in a second direction and insulated from and intersecting the first and second signal lines; A first thin film transistor connected to the first signal line and the third signal line, A second thin film transistor connected to the second signal line and the third signal line, A first pixel electrode connected to the first thin film transistor, A second pixel electrode connected to the second thin film transistor, A second insulating substrate facing the first insulating substrate, A reference electrode formed on the second insulating substrate, A liquid crystal material layer injected between the first substrate and the second substrate, and Domain dividing means formed on at least one of the first insulating substrate and the second insulating substrate and dividing the first pixel electrode and the second pixel electrode into a plurality of small domains; Including, The lower side of the first pixel electrode and the upper side of the second pixel electrode face each other, and the lower side and the upper side of the two pixel electrodes facing each other are larger than 0 ° and smaller than 90 ° with the first or second signal line, respectively. The liquid crystal display of claim 1, wherein the first pixel electrode and the second pixel electrode are capacitively coupled to each other at a predetermined angle.

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