KR100599962B1 - Fringe field switching mode lcd device - Google Patents

Abstract

The present invention discloses a fringe field driving liquid crystal display device which can improve response rate and viewing angle characteristics while improving transmittance and aperture ratio. The present invention has a unit pixel space defined, the upper and lower substrates facing each other at a predetermined distance; A liquid crystal layer interposed between the upper and lower substrates and including several liquid crystal molecules; A counter electrode formed in each unit pixel space of the lower substrate and receiving a common signal; A pixel electrode formed in a unit pixel space of a lower substrate so as to overlap with the counter electrode, and forming fringe fields in two directions symmetrical with the counter electrode; A first polarizer disposed on the outer surface of the lower substrate a second polarizer disposed on the outer surface of the upper substrate; A first polarizer disposed on an outer surface of the upper substrate and having a predetermined polarization axis; A second polarizer disposed on an outer surface of the lower substrate and having an absorption axis orthogonal to the polarization axis; A first vertical alignment layer disposed on an inner surface of the lower substrate; And a second vertical alignment layer disposed on an inner surface of the upper substrate, wherein the second vertical alignment layer has a rubbing axis that forms 10 to 90 ° with the fringe fields in two symmetrical directions.

Description

Fringe field driving liquid crystal display device {FRINGE FIELD SWITCHING MODE LCD DEVICE}

1 is a plan view of a typical fringe field driving liquid crystal display device.

2 is a plan view of a fringe field driving liquid crystal display for explaining an embodiment of the present invention.

3 is a view illustrating a polarization axis and a rubbing axis applied to the exemplary embodiment of FIG. 2.

4 is a plan view of a fringe field driving liquid crystal display for explaining another embodiment of the present invention.

5 is a view illustrating a polarization axis and a rubbing axis applied to the embodiment of FIG. 4.

(Explanation of symbols for the main parts of the drawing)

10-bottom substrate 11-gate bus line

13-data bus line 15-counter electrode

16-common electrode line 17-pixel electrode
P1-flat axis of first polarizer P2-flat axis of second polarizer
x-gate bus line y-data bus line
r, r '-rubbing axis of second vertical alignment layer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly, to a fringe field drive liquid crystal display device capable of securing high transmittance and high aperture ratio while securing response characteristics and viewing angles.

In general, a fringe field driving liquid crystal display is proposed to improve a low aperture ratio and transmittance of a general IPS mode liquid crystal display, and has been filed in Korean Patent Application No. 98-9243.

In the fringe field driving liquid crystal display, the counter electrode and the pixel electrode are formed of a transparent conductor, and the gap between the counter electrode and the pixel electrode is formed to be narrower than the gap between the upper and lower substrates, and the fringe field is formed on the counter electrode and the pixel electrode. field).

Such a fringe field driving liquid crystal display is shown in FIG. 1.

Referring to the drawing, the gate bus line 2 and the data bus line 4 are crossed and arranged on the lower substrate 1 to define a unit pixel. The thin film transistor TFT is disposed near the intersection point of the gate bus line 2 and the data bus line 4.

The counter electrode 5 is a transparent conductor and is formed for each unit pixel and has a rectangular plate shape. The counter electrode 5 is in contact with the common electrode line 7 to receive a common signal continuously. The common electrode line 7 is in parallel with the gate bus line 2 and is in contact with a predetermined portion of the counter electrode 5 and parallel with the data bus line 4 from the first portion 7a. And a second portion 7b extending between the counter electrode 5 and the data bus line 4, respectively. At this time, the second portion 7b is insulated from the data bus line 4. In addition, the first portion 7a of the common electrode line 7 is disposed between the gate bus lines 2.

The pixel electrode 9 is formed in each unit pixel space so as to overlap with the counter electrode 5. The pixel electrode 9 includes a comb portion 9a formed parallel to the data bus line 4 at equal intervals, and a bar 9b connecting the comb portions 9a. At this time, the bar (9b) is provided with a pair to connect between both ends of the comb portion (9a). In addition, any one of the pair of bars 9b is partially contacted with the drain electrode of the thin film transistor TFT. The pixel electrode 9 is formed of a transparent conductive layer similarly to the counter electrode 5. The counter electrode 5 and the pixel electrode 9 are insulated with the gate insulating film interposed therebetween.

On the other hand, although not shown in the figure, the upper substrate facing the lower substrate 1 is opposed and replaced with a width larger than the distance between the pixel electrode 9 and the counter electrode 5.

In addition, the second portion 7b of the common electrode line 7 is disposed between the data bus line 4 and the pixel electrode 9, more specifically, the comb portion 9b of the pixel electrode 9, thereby shielding light. Play a role.

On the inner surface of the lower substrate 1 and the resultant surface of the upper substrate (not shown), a horizontal alignment film (not shown) is formed, respectively, wherein the horizontal alignment film has a predetermined angle with respect to the gate bus line 2. It has a rubbing angle.

A fringe field drive liquid crystal display device having such a configuration operates as follows.

When an electric field is formed between the counter electrode 5 and the pixel electrode 9, the distance between the counter electrode 5 and the pixel electrode 9, that is, the distance between the upper and lower substrates is larger than the thickness of the gate insulating film, so that the two electrodes are larger. Between (5, 9) a fringe field is formed comprising vertical components. This fringe field extends over the counter electrode 5 and the pixel electrode 9 so that all of the liquid crystal molecules on the electrode are operated. As a result, a high opening ratio and a high transmittance can be realized.

However, although the fringe field drive liquid crystal display described above is very excellent in terms of aperture ratio and transmittance, the horizontal alignment film is used as the alignment film, so the response speed is rather slow. Accordingly, there is a difficulty in using it as a moving picture screen.

Accordingly, in order to supplement the above problems of the fringe field driving liquid crystal display, another technique of applying a vertical alignment layer to the fringe field driving mode has been proposed. Then, the response speed of the fringe field driving liquid crystal display is improved by the influence of the vertical alignment layer, and two domains are formed for each space where the electric field is formed, thereby obtaining a viewing angle symmetrically in the left and right directions.

However, in the method using the vertical alignment layer, the response speed can be improved, but since the direction is not imparted to the vertical alignment layer, the direction in which the liquid crystal molecules move by the electric field is not constant, so that the transmittance is partially different. . Furthermore, in the fringe field driving liquid crystal display device to which the vertical alignment layer is applied, a double domain is formed in the left and right directions of the screen, so that a viewing angle is reduced in a specific part of the screen, particularly in the vertical direction.

Accordingly, an object of the present invention is to provide a fringe field driving liquid crystal display device capable of improving the transmittance and aperture ratio, as well as improving response speed and viewing angle characteristics.

In order to achieve the above object of the present invention, according to an embodiment of the present invention, the unit pixel space is limited, the upper and lower substrates facing each other at a predetermined distance; A liquid crystal layer interposed between the upper and lower substrates and including several liquid crystal molecules; A counter electrode formed in each unit pixel space of the lower substrate and receiving a common signal; A pixel electrode formed in a unit pixel space of a lower substrate so as to overlap with the counter electrode, and forming fringe fields in two directions symmetrical with the counter electrode; A first polarizer disposed on the outer surface of the lower substrate a second polarizer disposed on the outer surface of the upper substrate; A first polarizer disposed on an outer surface of the upper substrate and having a predetermined polarization axis; A second polarizer disposed on an outer surface of the lower substrate and having an absorption axis orthogonal to the polarization axis; A first vertical alignment layer disposed on an inner surface of the lower substrate; And a second vertical alignment layer disposed on an inner surface of the upper substrate, wherein the second vertical alignment layer has a rubbing axis that forms 10 to 90 ° with the fringe fields in two symmetrical directions.

More preferably, the second vertical alignment layer has a rubbing axis of 20 to 60 ° with the fringe fields in the two symmetrical directions.

In addition, the dielectric anisotropy of the liquid crystal molecules may be negative or positive, the absolute value of the dielectric anisotropy is preferably 3 to 15, the phase retardation value of the liquid crystal layer is preferably 0.1 to 0.8㎛.

A phase compensating plate having a phase opposite to a phase delay value of the liquid crystal layer is further provided between the upper substrate and the second polarizing plate. Here, the phase delay value of the phase compensation plate is the same as the phase delay value of the liquid crystal layer. The polarization axis of the second polarizing plate may be parallel to the rubbing axis of the second vertical alignment layer.

According to the present invention, vertical alignment films are formed on the inner side of the substrate while forming fringe fields in two directions symmetrically in the unit pixels. In addition, the vertical alignment layer toward the upper substrate has a fringe field in two directions and a rubbing axis in a direction capable of forming 10 to 90 °.

Accordingly, when a voltage is applied to the pixel electrode, the liquid crystal molecules are deviated in a constant direction while forming a quadruple domain, and thus have high, even transmittance and excellent viewing angle characteristics in all regions of the unit pixel.

(Example)

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 2 is a plan view of a fringe field driving liquid crystal display for explaining an embodiment of the present invention, and FIG. 3 is a view showing a polarization axis and a rubbing axis applied to the embodiment of FIG. 2. 4 is a plan view of a fringe field driving liquid crystal display for explaining another embodiment of the present invention, and FIG. 5 is a view illustrating a polarization axis and a rubbing axis applied to the embodiment of FIG. 4.

First, an embodiment of the present invention will be described with reference to FIGS. 2 and 3.

Referring to FIG. 2, a plurality of gate bus lines 11 extend in the x-axis direction on the lower substrate 10, and the plurality of data bus lines 13 may extend in the y-axis direction substantially perpendicular to the x-axis direction. It extends to define the unit pixel of the matrix form.

A gate insulating film (not shown) is interposed between the gate bus line 11 and the data bus line 13 to insulate the two lines. That is, the gate bus line 11 is disposed on the surface of the lower substrate 10, the gate insulating film is coated on the upper portion thereof, and the data bus line 13 is disposed on the gate insulating film.

The counter electrode 15 is disposed in the unit pixel surrounded by the gate bus line 11 and the data bus line 13. The counter electrode 15 has a plate shape in which unit pixels are reduced. In addition, the counter electrode 15 is formed of a transparent conductive layer, for example, an ITO layer. In this case, the counter electrode 15 is in contact with the common electrode line 16, which is disposed in parallel with the gate bus line 11 and extends between the counter electrode 15 and the data bus line 15, as in the related art. To receive a common signal. The common electrode line 16 is formed of an opaque metal film.

The pixel electrode 17 is also formed in the unit pixel and becomes a transparent conductive layer, for example, an ITO layer. The pixel electrode 17 is formed in one-half region of the counter electrode 15 and is electrically connected to several vertical branches 17a parallel to the data bus lines and the vertical branches 17a, and the counter electrode 15 It includes several horizontal branches 17d formed in the remaining region of the substrate and parallel to the gate bus line. In addition, the pixel electrode 17 may include a first branch 17b connecting one end of the vertical branch 12a and a second branch 17c and first branch 17b connecting the other ends of the vertical branch 12a. And a third branch 17e connecting one end of the second branch 17b and the horizontal branch 17d. Here, the vertical and horizontal branches 17a and 17d are spaced at equal intervals and have a width such that all of the liquid crystal molecules on the electrode can be operated by a field generated between the exposed counter electrodes 15. . The first and second branches 17b and 17c are parallel to the horizontal branches 17d, and the third branch 17e is parallel to the vertical branches 17a. In addition, the first and second branches 17b and 17c may be larger than the width of the horizontal branch 17d.

Here, a gate insulating film is interposed between the counter electrode 15 and the pixel electrode 17 to insulate the two.

In the vicinity of the intersection of the gate bus line 11 and the data bus line 13, a thin film transistor TFT for switching the signal of the data bus line 13 to the pixel electrode 17 is included. The thin film transistor TFT is in contact with the horizontal branch 17d of the pixel electrode 17.

The upper substrate (not shown) is disposed above the lower substrate 10 at a predetermined distance. In this case, the distance between the upper and lower substrates is called a cell gap, and the cell gap is larger than the distance between the horizontal or vertical branches 17d and 17a of the pixel electrode and the counter electrode 15 (in this embodiment, the thickness of the gate insulating film). A color filter (not shown) is formed on one surface of the upper substrate.

A liquid crystal layer (not shown) including several liquid crystal molecules is interposed between the lower substrate 10 and the upper substrate. Here, the liquid crystal molecules may have negative or positive dielectric anisotropy, and have negative dielectric anisotropy of about -15 to -3 in the case of negative and about 3 to 15 in the case of positive. In addition, in order to obtain an optimized image quality, the product of the phase retardation value, i.e., the refractive index anisotropy (dΔn) of the cell gap and the liquid crystal molecules, is about 0.1 to 0.8 mu m.

First and second vertical alignment layers (not shown) are formed between the lower substrate 10 and the liquid crystal layer (not shown) and between the upper substrate (not shown) and the liquid crystal layer (not shown). At this time, the first vertical alignment layer formed on the lower substrate 10 does not rub. On the other hand, the second vertical alignment film formed on the upper substrate is rubbing substantially in the direction of about 10 to 80 degrees, more preferably 20 to 60 degrees with the electric field. 3, r represents the rubbing axis of the second vertical alignment layer.

Thereafter, a first polarizing plate (not shown) having a predetermined polarization axis P1 is attached to the outer surface of the lower substrate 10, and a polarization axis perpendicular to the polarization axis P1 of the first polarizing plate is attached to the outer surface of the upper substrate. A second polarizing plate (not shown) having a P2 is attached. In this case, the second polarizing plate is attached such that the polarization axis P2 is parallel to the rubbing axis r of the second vertical alignment layer, and thus, the x direction of the drawing, that is, about 10 to 80 °, more preferably 20 To 60 °. In addition, in order to realize complete dark, a phase compensation plate (not shown) having the same phase delay value as that of the liquid crystal layer and an opposite phase may be interposed between the upper substrate and the second polarizing plate.

Hereinafter, the operation of the liquid crystal display according to the present embodiment will be described.

First, if the gate bus line 11 is not selected, no image signal is applied to the pixel electrode 17, and no electric field is formed between the counter electrode 15 and the pixel electrode 17. FIG. Then, the linearly polarized light passing through the first polarizing plate does not change the traveling direction while passing through the liquid crystal layer. That is, since the long axis of the molecules in the liquid crystal (not shown) is arranged to be perpendicular to the substrate, the straight light does not change the traveling direction. Therefore, light cannot pass through the polarizing plate 45, and the screen becomes dark. At this time, since a phase compensation plate is interposed, perfect dark can be achieved.

On the other hand, when a scan signal is applied to the gate bus line 11 and an image signal is applied to the data bus line 13, a thin film transistor formed near the intersection of the gate bus line 11 and the data bus line 13 ( The TFT is turned on so that an image signal is transmitted to the pixel electrode 17. At this time, since a common signal having a predetermined voltage difference is continuously applied to the counter electrode 15, a field is formed between the counter electrode 15 and the pixel electrode 17. At this time, since the branch of the pixel electrode is very fine, and the distance between the counter electrode and the pixel electrode is less than the cell gap, a fringe field having a sufficient electric field is formed on the electrode.

Here, the part where the fringe field is formed is between the exposed counter electrode 15 and the vertical branch 17a and the exposed counter electrode 15 and the horizontal branch 17d, and the general field is formed in the form of a normal line of the electrode. The fringe field in the horizontal direction parallel to the gate bus line is formed at the portion where the vertical branch 17a is formed, and the fringe field in the vertical direction parallel to the data bus bus line is formed at the portion where the horizontal branch 17d is formed. do. Accordingly, the liquid crystal molecules arranged along the rubbing direction while the major axis is vertically aligned with respect to the substrate surface are twisted in the horizontal and vertical electric field directions. Thus, two domains are formed in one unit pixel in two symmetrical directions to form two domains, and a vertical alignment layer is used to form two domains in one direction. Therefore, four domains are formed in the unit pixel. Therefore, the viewing angle characteristic is greatly improved. In addition, as the second vertical alignment layer is rubbed in a predetermined direction, the liquid crystal molecules may be twisted in two directions while having a constant orientation, thereby obtaining a high and uniform transmittance. That is, conventionally, since no rubbing process is performed on the vertical alignment film, it is distorted in a random direction, so that the transmittance is partially different. However, in the present invention, since the liquid crystal molecules are distorted in two directions in a directional state, the liquid crystal molecules have a uniform transmittance in all parts of the unit pixel. In addition, since the vertical alignment film is used, the response characteristic is improved compared with the case of using the horizontal alignment film.

4 and 5, another embodiment of the present invention will be described.

In this case, the other embodiment of the present invention has the same configuration as the above embodiment except for the pixel electrode, the second vertical alignment layer, the first polarizing plate, and the second polarizing plate, and thus, the same parts are excluded without overlapping descriptions. Give a sign.

That is, as shown in FIG. 4, the pixel electrode 27 of this embodiment is formed of an ITO layer and overlaps with the counter electrode 15, similarly to the above embodiment. The pixel electrode 27 includes a first bar 27a formed at one side edge of the counter electrode in parallel with the data bus line, and a gate bus line (B) to divide the space of the counter electrode 15 from the first bar 27a. A second bar 27b extending parallel to 11). A plurality of third and fourth bars 27c and 27d branched from the first and second bars 27a and 27b and having an oblique shape are formed on the upper and lower portions of the second bar 27b. At this time, a substantial electric field is formed between the third and fourth bars 27c and 27d in diagonal lines, and between the third and second bars 27c and between the counter electrodes 15 between the fourth bars 27d.

On the other hand, the second vertical alignment layer is rubbing to have substantially an electric field and a predetermined angle, preferably, 10 to 90 degrees, more preferably 20 to 60 degrees. Here, the rubbing axis of the second vertical alignment film is represented by r ', and is formed in parallel with the data bus line 13 in this embodiment, for example. Accordingly, as shown in FIG. 5, the second polarizing plate is disposed such that the rubbing axis r ′ of the second vertical alignment layer and the polarizing axis P2 are parallel to each other, and the first polarizing plate has the flat polarization axis P1 as the second polarizing plate. Is attached so as to be perpendicular to the polarization axis P2.

Such a liquid crystal display device is completely dark as in the above embodiment before the electric field is applied.

On the other hand, when a voltage is applied to the pixel electrode 27, the diagonal fringe fields of two symmetrical directions are formed by the third and fourth bars 27c and 27d having a diagonal shape in the unit pixel. Accordingly, the liquid crystal molecules arranged along the rubbing direction while the major axis is vertically aligned with respect to the substrate surface are twisted in the form of diagonal fringe fields in two symmetrical directions. Thus, two domains are formed in one unit pixel in two symmetrical directions to form two domains, and a vertical alignment layer is used. Thus, two domains are formed in one electric field in one direction. Therefore, four domains are formed in the unit pixel, and the viewing angle characteristic is greatly improved. In addition, as the second vertical alignment layer is rubbed in a predetermined direction, the liquid crystal molecules may be twisted in two directions while having a constant orientation, thereby obtaining a high and uniform transmittance.

In the present exemplary embodiment, the counter electrode is formed in the form of a plate. However, the counter electrode is not limited thereto and may be formed in the form of a comb teeth so as to form a fringe field in two directions symmetric with the pixel electrode.

As described in detail above, according to the present invention, vertical alignment films are formed on the inner side of the substrate while forming fringe fields in two directions symmetrically in the unit pixels. In addition, the vertical alignment layer toward the upper substrate has a fringe field in two directions and a rubbing axis in a direction capable of forming 10 to 90 °.

Accordingly, when a voltage is applied to the pixel electrode, the liquid crystal molecules are deviated in a constant direction while forming a quadruple domain, and thus have high, even transmittance and excellent viewing angle characteristics in all regions of the unit pixel.

In addition, since the vertical alignment film is used, the response speed is greatly improved.

In addition, since the phase compensation plate is interposed, complete dark can be realized, and the contrast ratio is also greatly improved.

In addition, one rubbing process can be reduced compared to a fringe field driving liquid crystal display using a conventional horizontal alignment layer.

In addition, this invention can be implemented in various changes in the range which does not deviate from the summary.

Claims (7)

A unit pixel space is defined and includes upper and lower substrates facing each other at a predetermined distance; A liquid crystal layer interposed between the upper and lower substrates and including several liquid crystal molecules; A counter electrode formed in each unit pixel space of the lower substrate and receiving a common signal; A pixel electrode formed in a unit pixel space of the lower substrate and forming fringe fields in two directions symmetrical with the counter electrode; A first polarizer disposed on an outer surface of the lower substrate and having a predetermined polarization axis; A second polarizer disposed on an outer surface of the upper substrate and having a polarization axis orthogonal to the polarization axis of the first polarizer; A first vertical alignment layer disposed on an inner surface of the lower substrate; And A second vertical alignment layer disposed on an inner surface of the upper substrate; And the second vertical alignment layer has a rubbing axis that is between 10 and 90 ° with the fringe fields in the two symmetrical directions. The fringe field driving liquid crystal display device of claim 1, wherein the second vertical alignment layer has a rubbing axis formed between 20 and 60 ° with the fringe fields in two symmetrical directions. The fringe field driving liquid crystal display of claim 1, wherein the dielectric anisotropy of the liquid crystal molecules may be negative or positive, and an absolute value of the dielectric anisotropy is 3 to 15. The fringe field drive liquid crystal display device according to claim 1, wherein the phase retardation value of the liquid crystal layer is 0.1 to 0.8 mu m. 5. The fringe field driving liquid crystal display device according to claim 4, further comprising a phase compensating plate having a phase opposite to a phase delay value of the liquid crystal layer between the upper substrate and the second polarizing plate. 6. The fringe field driving liquid crystal display device according to claim 5, wherein the phase delay value of the phase compensation plate is the same as the phase delay value of the liquid crystal layer. The fringe field driving liquid crystal display device according to claim 1, wherein the polarization axis of the second polarizing plate is parallel to the rubbing axis of the second vertical alignment layer.

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