KR20020043941A - Fringe field switching mode lcd - Google Patents

Abstract

PURPOSE: A fringe field switching mode liquid crystal display is provided to shorten response time while increasing transmissivity and reducing driving voltage. CONSTITUTION: A fringe field switching mode liquid crystal display includes upper and lower substrates(40,20) facing each other, having the distance of 4 micrometers between the two substrates, and a liquid crystal layer(30) interposed between the upper and lower substrates. The liquid crystal layer contains liquid crystal molecules(30a) having positive dielectric anisotropy. The liquid crystal display further includes a transparent counter electrode(22) formed on the inner side of the lower substrate, a transparent pixel electrode(26) formed on the inner side of the lower substrate. The pixel electrode forms a fringe field together with the counter electrode to operate most of the liquid crystal molecules. The liquid crystal display also has the first horizontal alignment film(28) interposed between the lower substrate and the liquid crystal layer, and the second horizontal alignment film(44) formed between the upper substrate and the liquid crystal layer. The horizontal alignment films have a predetermined rubbing axis.

Description

Fringe field drive mode liquid crystal display device {FRINGE FIELD SWITCHING MODE LCD}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fringe field drive mode liquid crystal display, and more particularly, to a fringe field drive mode liquid crystal display capable of ensuring maximum transmittance while using a liquid crystal layer having a positive dielectric anisotropy.

In general, IPS (in-plane switching) LCD has been proposed by Hitachi, Japan, to compensate for the low viewing angle characteristics of twisted nematic (TN) mode LCD. This IPS mode liquid crystal display has the following configuration. The upper and lower substrates face each other at a predetermined distance, and a liquid crystal layer including several liquid crystal molecules is interposed between the upper and lower substrates. The opaque pixel electrode and the counter electrode for driving the liquid crystal molecules are both formed on the lower substrate, for example, and the pixel electrode and the counter electrode are spaced at a larger interval than the gap between the upper and lower substrates so that a parallel electric field is formed. In addition, the pixel electrode and the counter electrode have a relatively large width in order to secure a constant electric field intensity. In addition, a horizontal alignment film is interposed between the upper and lower substrates and the liquid crystal layer, respectively.

The IPS mode liquid crystal display device has an advantage that the viewing angle is excellent because the liquid crystal molecules are arranged in a state lying on the substrate surface, but the transmittance is very low.

In order to improve the low transmittance characteristics of the IPS mode liquid crystal display, a fringe field switching mode LCD (FFS-LCD) has been proposed by the present applicants. It was filed in US 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. Be sure to

This general fringe field driving liquid crystal display is shown in FIG. 1.

Referring to FIG. 1, the lower substrate 1 and the upper substrate 10 face each other with a predetermined distance d (hereinafter, referred to as a cell gap). Here, the distance between the lower substrate 1 and the upper substrate 10 is referred to as a cell gap d hereinafter. The liquid crystal layer 15 is interposed between the lower substrate 1 and the upper substrate 10. In this case, the liquid crystal layer 15 may include predetermined liquid crystal molecules, and the liquid crystal molecules may have negative or positive dielectric anisotropy. In addition, the retardation represented by the product of the cell gap d and the refractive index anisotropy Δn is set to 0.25 to 0.35 탆 so that the liquid crystal display can obtain the maximum transmittance.

Although not shown in the figure on the lower substrate 1, the gate bus lines and the data bus lines are arranged in an intersecting manner to define unit pixels, and a thin film transistor for active driving near the intersection point of the gate bus lines and the data bus lines (not shown). Is placed). The counter electrode 3 is formed in the unit pixel of the lower substrate 1. The counter electrode 3 is formed of a transparent indium tin oxide (ITO) layer, and is formed in the form of a comb teeth or a plate. The gate insulating film 4 is formed on the lower substrate 1 on which the counter electrode 3 is formed. The pixel electrode 5 is formed in the shape of a comb on the gate insulating film 4 so as to overlap the counter electrode 3. At this time, the interval l between the counter electrode 3 and the pixel electrode 5 is formed to be narrower than the cell gap d. In addition, a horizontal alignment layer 6 is formed on the resultant surface of the lower substrate 1 to control the initial arrangement of liquid crystal molecules. At this time, the horizontal alignment film 6 has a rubbing axis in a predetermined direction and has a predetermined pretilt angle.

Meanwhile, the color filter 12 is formed on the opposite surface of the upper substrate 10 corresponding to the lower substrate 1. Also on the surface of the color filter 12, a horizontal alignment film 14 for controlling the initial arrangement of the liquid crystal molecules is formed. At this time, the horizontal alignment layer 14 of the upper substrate also has a pretilt angle of a predetermined angle, and has a rubbing axis that forms an angle of 180 ° with the rubbing axis of the horizontal alignment layer 6 of the lower substrate.

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

When a voltage difference is generated between the counter electrode 3 and the pixel electrode 5, the fringe field is formed because the distance between the counter electrode 3 and the pixel electrode 5 is smaller than the cell gap d. At this time, the width of the counter electrode 3 opened through the space between the pixel electrode 5 and the pixel electrode 5 is sufficiently fine, so that the fringe field extends over both the counter electrode 3 and the pixel electrode 5. Thus, the liquid crystal molecules in the unit pixel are mostly operated. Accordingly, the transmittance and the aperture ratio are improved.

However, although both liquid crystals having positive dielectric anisotropy or liquid crystals having positive dielectric anisotropy can be used as the liquid crystal layer of FFS-LCD, liquid crystals having negative dielectric anisotropy used in IPS-LCD having similar structure are mainly used as liquid crystal layer. .

In this case, when the liquid crystal having a negative dielectric anisotropy is used as the liquid crystal layer of the FFS-LCD, as shown in FIG. 2, the transmittance is inversely proportional to the cell gap d. That is, as shown in FIG. 2, as the cell gap d increases to 3 μm, 3.9 μm, and 4.6 μm, the transmittance gradually increases.

Meanwhile, FIG. 3 is a graph illustrating a driving voltage for retardation dΔn expressed as a product of a cell gap d and a refractive index anisotropy Δn of a typical FFS-LCD, where the refractive index anisotropy Δn is 0.08. And a liquid crystal having a negative dielectric anisotropy was used. As a result, as shown in FIG. 3, it can be seen that as the retardation or the cell gap increases, the driving voltage decreases. This means that when the cell gap is increased, the portion of the liquid crystal layer which is affected by the surface effect due to the alignment is increased, so that a large effect can be obtained even at a small voltage.

However, the response time is proportional to the square of the cell gap d ² , and when the cell gap is increased, the response time is increased.

As a result of the above experiment, in the case of using a liquid crystal having a negative dielectric anisotropy as the liquid crystal layer of the FFS-LCD, increasing the cell gap increases the transmittance and lowers the driving voltage, but has a problem in that the response time is increased.

Accordingly, it is an object of the present invention to provide an FFS-LCD capable of reducing response time while increasing transmittance and lowering driving voltage.

1 is a cross-sectional view of a typical fringe field driving mode liquid crystal display device.

2 is a graph illustrating transmittance with respect to a cell gap of a typical fringe field driving mode liquid crystal display.

3 is a graph showing driving voltages for retardation of a typical fringe field driving mode liquid crystal display;

4A and 4B are cross-sectional views of an FFS-LCD in accordance with the present invention.

Figure 5 is a graph of transmittance according to the cell gap in the FFS-LCD according to the present invention.

Figure 6 is a graph showing the driving voltage according to the cell gap in the FFS-LCD according to the present invention.

7 is a graph of transmittance according to retardation in the FFS-LCD according to the present invention.

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

20-lower substrate 22-counter electrode

24-gate insulating film 26-pixel electrode

28-first horizontal alignment layer 30-liquid crystal layer

30a-liquid crystal molecule 40-upper substrate

42-color filter 44-second horizontal alignment layer

45a-polarizer 45b-decomposer

In order to achieve the above object of the present invention, according to an embodiment of the present invention, the upper and lower substrates facing a predetermined distance; A liquid crystal layer interposed between the upper and lower substrates and including liquid crystal molecules having positive dielectric anisotropy; A transparent counter electrode formed inside the lower substrate; A transparent pixel electrode formed inside the lower substrate and forming a fringe field together with the counter electrode to operate most of the liquid crystal molecules; An FFS-LCD interposed between the upper and lower substrates and the liquid crystal layer, respectively, and including a horizontal alignment layer having a predetermined rubbing axis, wherein the distance between the upper and lower substrates is 4 μm or less.

Here, the distance between the upper and lower substrates is 3 to 3.8㎛, the refractive index anisotropy of the liquid crystal layer is preferably 0.08 to 1.5.

(Example)

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

4A and 4B are cross-sectional views of an FFS-LCD according to the present invention, and FIG. 5 is a graph of transmittance according to cell gap in an FFS-LCD according to the present invention. 6 is a graph showing the driving voltage according to the cell gap in the FFS-LCD according to the present invention, Figure 7 is a graph of the transmittance according to the retardation in the FFS-LCD according to the present invention.

Referring to FIG. 4, the lower substrate 20 and the upper substrate 40 face each other with a predetermined cell gap d. The liquid crystal layer 30 including several liquid crystal molecules 30a is interposed between the lower substrate 20 and the upper substrate 40. At this time, a material having a low amount of dielectric constant anisotropy is used as the liquid crystal layer 30. In addition, the refractive index anisotropy (Δn) of the liquid crystal layer 30 is about 0.08 to 1.5, preferably about 0.085 to 0.09, and the dielectric anisotropy is about 3 to 15.

Although not shown in the figure above the lower substrate 20, the gate bus lines and the data bus lines are intersected and arranged to define unit pixels, and the thin film transistors for active driving may be formed near the intersection points of the gate bus lines and the data bus lines. Not shown) is disposed. The counter electrode 22 is formed in the unit pixel of the lower substrate 20. The counter electrode 22 is formed of an ITO layer, which is a transparent conductive layer, and may be formed in the form of a comb teeth or a plate. In this embodiment, for example, it was formed in the form of a plate. The gate insulating film 24 is formed on the lower substrate 20 on which the counter electrode 22 is formed. The pixel electrode 26 is formed in the shape of a comb to overlap the counter electrode 22 on the gate insulating layer 24. At this time, the interval l between the counter electrode 22 and the pixel electrode 26 is formed to be narrower than the cell gap d, so that a fringe field is formed between the counter electrode 22 and the pixel electrode 26. In addition, a first horizontal alignment layer 28 is formed on the resultant surface of the lower substrate 20 to control the initial arrangement of the liquid crystal molecules. At this time, the first horizontal alignment layer 28 is 45 to 90 °, more preferably 60 to 85 °, with the substrate projection line of the fringe field to be formed between the counter electrode 22 and the pixel electrode 26 so as to obtain the maximum transmittance. It has a rubbing axis to form a.

Meanwhile, the color filter 42 is formed on an opposite surface of the upper substrate 40 corresponding to the lower substrate 20. The second horizontal alignment layer 44 is formed on the surface of the color filter 42. The second horizontal alignment film 44 has a rubbing axis that forms a predetermined angle, for example, an angle of 180 ° with the rubbing axis of the first horizontal alignment film 28.

In addition, a polarizer 45a for linearly polarizing light incident from a backlight (not shown) is attached to an outer portion of the lower substrate 20, and an outer portion of the upper substrate 40 passes through the liquid crystal layer 30. Attached is a decomposer 45b that selectively absorbs and passes light. Here, the polarizer 45a has a polarization axis (not shown) that coincides with the rubbing axis of the first horizontal alignment film 28, and the decomposer 45b has an absorption axis (not shown) orthogonal to the polarization axis.

5 is a graph of transmittance according to cell gap in the FFS-LCD according to the present invention. According to the figure, when a material having a positive dielectric anisotropy is used as the liquid crystal layer 30, the smaller the cell gap d, that is, the cell gap d is about 4 μm or less, the transmittance gradually increases. Can be.

6 is a graph showing a driving voltage according to a cell gap in the FFS-LCD according to the present invention. According to this drawing, when a material having positive dielectric anisotropy is used as the liquid crystal layer 30, it can be seen that as the cell gap d decreases, the driving voltage decreases. That is, according to the drawing, in the cell gap of 3 to 3.7 mu m, a low driving voltage of 3.8 to 4.5V is shown.

7 is a graph of transmittance according to retardation in the FFS-LCD according to the present invention. According to the figure, the retardation (dΔn) shows excellent transmittance at 0.35 μm or more.

As a result, in the FFS-LCD using a material having a positive dielectric anisotropy as the liquid crystal layer as in the present invention, when the cell gap d is set to 4 µm or less, preferably 3 to 3.8 µm, which is a relatively small thickness, It is possible to drive at a low driving voltage while securing a high transmittance, and the upper and lower substrates are bonded together at a lower thickness than before, so that the driving can be performed at a high response speed.

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

First, if no electric field is formed between the counter electrode 22 and the pixel electrode 26, the liquid crystal molecules 30a having positive dielectric anisotropy coincide with the rubbing axes of the first and second horizontal alignment layers 28 and 44. Is arranged to. Accordingly, the light passing through the polarizer 45a from the backlight does not change the traveling direction while passing through the liquid crystal layer 30 arranged so that the rubbing axis and the long axis coincide. Accordingly, the light passing through the liquid crystal layer 30 is absorbed by the decomposer 45b having an absorption axis perpendicular to the polarization axis, so that the screen becomes dark.

On the other hand, when a voltage difference is generated between the counter electrode 22 and the pixel electrode 26, a fringe field f is formed between the counter electrode 22 and the pixel electrode 26 as shown in FIG. 4B. Then, the liquid crystal molecules 30a having positive dielectric anisotropy are arranged such that their long axes coincide with the fringe field f. As a result, the light passing through the polarizer 45a from the backlight changes its polarization state while passing through the liquid crystal layer 30 and thus passes through the absorption axis of the decomposer 45b. Thus, the screen becomes white.

As described in detail above, according to the present invention, the cell gap d is set to 4 µm or less while using the amount of dielectric anisotropy as the liquid crystal layer in the FFS-LCD. By setting the cell gap in this manner, it is possible to achieve a high transmittance while achieving a low driving voltage and a fast response speed.

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

Claims (9)

Upper and lower substrates facing each other at a predetermined distance; A liquid crystal layer interposed between the upper and lower substrates and including liquid crystal molecules having positive dielectric anisotropy; A transparent counter electrode formed inside the lower substrate; A transparent pixel electrode formed inside the lower substrate and forming a fringe field together with the counter electrode to operate most of the liquid crystal molecules; An FFS-LCD comprising a horizontal alignment layer interposed between the upper and lower substrates and the liquid crystal layer, each having a predetermined rubbing axis. The distance between the upper and lower substrates is FFS-LCD, characterized in that 4㎛ or less. The FFS-LCD of claim 1, wherein a distance between the upper and lower substrates is 3 to 3.8 µm. The FFS-LCD according to claim 1 or 2, wherein the refractive index anisotropy of the liquid crystal layer is 0.08 to 1.5. The FFS-LCD of claim 1, wherein the dielectric anisotropy of the liquid crystal molecules is 3 to 15. The FFS-LCD of claim 1, wherein the rubbing axis of one of the horizontal alignment layers is about 45 to 90 degrees with the substrate projection line of the fringe field formed between the counter electrode and the pixel electrode. 6. The FFS-LCD of claim 5, wherein the rubbing axis of the alignment film on the lower substrate side and the rubbing axis of the alignment film on the upper substrate side are about 180 degrees. The FFS-LCD of claim 1, wherein the counter electrode is formed in a plate shape, and the pixel electrode is formed in a comb teeth shape. The FFS-LCD of claim 1, wherein both the counter electrode and the pixel electrode are formed in the shape of a comb. The FFS-LCD according to any one of claims 2, 7, and 8, wherein the distance between the counter electrode and the pixel electrode is smaller than the distance between the upper and lower substrates.