KR20110039926A - Optically-isotropic liquid crystal display - Google Patents

Abstract

PURPOSE: An optically-isotropic (Liquid Crystal Display) device is provided to reduce driving voltage thereof and have high transmittance rate. CONSTITUTION: An optically-isotropic (Liquid Crystal Display) device comprises: a lower substrate(100) which includes a pixel electrode and a common electrode which are distanced from each other; an upper substrate(200) which faces the lower substrate; a liquid crystal layer(300) which is placed between the upper and lower substrates and is switched into an anisotropic state when an electric field is applied during the absence of the electric field in an isotropic state; and a polarizing plate(400) which crosses the upper and lower substrates.

Description

Optically isotropic liquid crystal display device {Optically-isotropic Liquid Crystal Display}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optically isotropic liquid crystal display device, and more particularly, to an optically isotropic liquid crystal display device including an electrode shape having a new structure having one or more inclined portions.

The liquid crystal display device displays an image by adjusting the light transmittance of the liquid crystal using an electric field. Such liquid crystal displays are classified into a vertical electric field type and a horizontal electric field type according to the direction of the electric field for driving the liquid crystal. The vertical field type liquid crystal display device drives the liquid crystal in VA (Vertical Alignment ) mode by the vertical electric field formed on the upper substrate.

In contrast, a horizontal field type liquid crystal display device generally includes an upper substrate 20 having a color filter (not shown), a lower substrate 10 having a thin film transistor, and two substrates 10 and 20, as shown in FIG. 1. The liquid crystal is filled in the polarizing plate 30 intersected at 90 degrees to the outside and the liquid crystal space provided between the two substrates. The liquid crystal display drives the liquid crystal in In Plane Switch (hereinafter referred to as IPS) mode by a horizontal electric field applied to the common electrode 12 and the pixel electrode 11. In the case of the IPS mode, the liquid crystal molecules do not rotate in the upper layer, so that the light efficiency is decreased, the driving voltage is high, and there is a problem of the color problem due to the viewing angle.

Meanwhile, liquid crystals used in the liquid crystal display panel include nematic liquid crystals, smectic liquid crystals and cholesteric liquid crystals, and nematic liquid crystals are mainly used. The nematic liquid crystal of the nematic liquid crystal is adjusted according to the electric field formed between the pixel electrode and the common electrode, the liquid crystal layer adjusts the light transmittance according to the inclination angle of the nematic liquid crystal. Accordingly, the luminance of the liquid crystal display panel is determined by the thickness of the liquid crystal layer, that is, the cell gap of the liquid crystal display panel and the anisotropic refractive index of the liquid crystal.

For this reason, a uniform cell gap of the liquid crystal display panel is required, and the viewing angle is lowered due to the anisotropic refractive index of the liquid crystal. In order to overcome such a decrease in viewing angle and cell gap dependency, a liquid crystal display panel having a blue phase liquid crystal has been proposed, and an example thereof is disclosed in US Pat. No. 4,767.149. In addition, such a blue phase liquid crystal does not use an alignment layer and has an advantage of having a faster response speed than conventional liquid crystals.

2A to 2E show the structure and optical state of a blue phase liquid crystal. As shown, the optically isotropic blue phase liquid crystal is a liquid crystal phase present in the temperature range between the cholesteric nematic liquid crystal phase and the isotropic liquid crystal phase and has a lattice structure (FIGS. 2A and 2B) and a cylinder (FIG. 2C) inside the lattice. ) In the form of a liquid crystal twisted by a chiral dopant inside the cylinder, and optically isotropically before the voltage is applied (FIG. 2D). However, when voltage is applied, refractive anisotropy is generated by the electric field formed outside (FIG. 2E), thereby realizing a bright state.

Δn = λ KE ²

Δn = induced refractive index anisotropy

λ = wavelength of light source used

K = Kerr constant

E = electric field

However, since the temperature range where blue phase liquid crystals exist is very narrow, around 1 degree, it is unsuitable for use as a display, and recently, as a technology for expanding the existence range at room temperature by forming a polymer network using ultraviolet rays in blue phase liquid crystals has been developed. As a result, a technique has been attempted to utilize a liquid crystal mixture having a nanostructure, including an isotropic blue phase liquid crystal, as a display. However, in the case of using the blue phase liquid crystal, despite the fast response speed of microseconds, the liquid crystal trapped in the polymer network has to be driven. Therefore, when using the conventional IPS, there is a problem in that high driving voltage and light transmittance are reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and to provide a horizontal field type liquid crystal display device having a novel structure having a low transmission voltage and a high transmittance at the same time.

One aspect of the present invention for achieving the above object is a lower substrate comprising a pixel electrode and a common electrode spaced apart from each other; An upper substrate disposed opposite the lower substrate; A liquid crystal layer disposed between the upper substrate and the lower substrate and changed into an anisotropic state when an electric field is applied in an isotropic state in the absence of an electric field; And a polarizing plate intersecting the upper substrate and the lower substrate, wherein the pixel electrode and the common electrode include at least one inclined portion.

According to the present invention, the inclination electric field is formed on the top surface of the electrode as the shape of the driving electrode includes one or more inclined portions, which are not conventional rectangular structures, and the horizontal electric field of the inclined electric field generates induction retardation in the blue phase liquid crystal to transmittance. Can increase. As a result, the liquid crystal display device according to the present invention can reach the maximum transmittance faster than the conventional structure, thereby significantly reducing the driving voltage.

Accordingly, the present invention can provide a liquid crystal display device capable of significantly lowering power consumption even though a moving image can be realized at a microsecond response speed by applying an optically isotropic blue phase liquid crystal.

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

3 is a cross-sectional view of a liquid crystal display device having an electrode including at least one inclined portion according to the present invention. Referring to FIG. 3, the liquid crystal display of the present invention includes a lower substrate 100, a pixel electrode 110 and a common electrode 120 formed on the lower substrate, an upper substrate 200, a liquid crystal layer 300, and a lower substrate. A polarizer 400 is disposed below the substrate and intersects 90 degrees on the upper substrate.

The pixel electrode 110 and the common electrode 120 are alternately disposed on the lower substrate 100. The pixel electrode 110 and the common electrode 120 may include a bottom portion a contacting a lower substrate, a top portion b spaced apart from the bottom portion by a predetermined height h, and a side portion connecting the bottom portion and the top portion. (c), wherein the side portion comprises one or more inclined portions (s).

4A to 4D are cross-sectional views of the pixel electrode 110 and the common electrode 120 having the inclined portion according to the present invention.

As used herein, the side portion (c) means both sides of the electrode, and the conventional electrode side is generally formed perpendicular to the substrate. In addition, the side portion c has a left portion c1 and a right portion c2 and may have the same or different lengths. In the present invention, the side portion c uses the concept encompassing them.

In the electrode of the present invention, the side portion c may include one or more inclined portions, and the upper limit of the number of inclined portions may be mathematically infinite, but preferably 1 to 10, most preferably one on each side. Two may be formed.

The inclined portion, which is a term used in the present invention, is used as a concept to encompass non-vertical shapes among the sides. For example, non-vertical shapes in the sides may include straight lines, curves, ellipses, parabolas, hemispheres, shoulders, sines, inverse ellipses, and the like having slopes.

As used herein, the inclination angle to the tangential angle represent the inclination of the inclined portion.

The inclination angle represents an angle formed with the bottom part (extension line) when the vertical section of the inclination part is a straight line. The tangential angle represents the angle when the tangential angle is tangential to the starting point of the curved portion when the vertical section of the inclined portion is curved.

4A and 4D, all of the side portions are inclined portions, and B side portions include both vertical portions and inclined portions. Referring to FIGS. 4A to 4, two inclined portions s1 and s2 have an inclination angle θ1 and θ2 at the electrode end surface. The inclination angle to the tangential angle θ1 and θ2 may be the same or different.

Referring to FIG. 4C, the side of the electrode includes a parabolic inclined portion, which is a point where the two inclined portions (total parabolic shape) intersect with each other (the parabolic maximum point) at the top portion (the parabolic shape). b).

The inclination angle of the inclined portion is in the range of more than 0 ° and less than 90 °. When the inclination angle is 90 °, the rectangular structure is exhibited as in the prior art, and thus the effect of the present invention cannot be exhibited. The inclination angle is preferably 15 ° to 85 °, more preferably 30 ° to 70 °.

On the other hand, when the side is curved, the tangential angle may be 0 ° to 90 °. An example where the tangential angle is 90 ° may be a semicircle, and an example that becomes 0 ° may be a parabola.

The top portion b may have a width less than the width W of more than zero bottom portion. Referring again to 4a to 4d, the top portion b may be a linear flat portion having a length, such as the upper surface of the trapezoid, and preferably, the length of the flat portion is shorter than the width W of the bottom portion. It is more preferable that the length of (b) is shorter, that is, the top portion has a sharp structure with protrusions (protrusions), and the top (b) portion is indicated by a dot that is an intersection point where both inclined portions meet. Most preferred. Here, the meaning that the width of the top portion is greater than zero indicates that there is no flat linear portion as in the dot, and thus the length thereof cannot be measured.

Figure 5 shows a preferred example of the shape of the pixel electrode and the common electrode that can be used in the present invention. Referring to FIG. 5, the pixel electrode and the common electrode may have an inclined portion, and the vertical cross-section may have a trapezoid, a triangle, an ellipse, an inverted ellipse, a sinusoidal curve, an inverted curve, a semicircle, and an S shape. Preferably, the cross section is preferably a symmetrical structure.

The top portion b of the trapezoidal electrode has a width smaller than the bottom portion width W, and triangles, inverted ellipses, and semicircles are represented as dot structures such as vertices and maximum points. .

According to the present invention, the inclination electric field is formed on the upper surface of the electrode as the shape of the driving electrode includes one or more inclined portions instead of the rectangular structure, and the horizontal electric field of the inclined electric field generates induction retardation in the blue phase liquid crystal to transmittance. Can increase. As a result, the liquid crystal display device according to the present invention can reach the maximum transmittance faster than the conventional structure, thereby significantly reducing the driving voltage.

On the other hand, in the electrode according to the present invention, its cross-sectional area is smaller than the product of the width W of the bottom portion and the height.

It is preferable to form the electrode height larger than the height (thickness) in the conventional rectangular structure in this invention. That is, it may be a structure that can protrude higher from the substrate.

The height of the pixel electrode and the common electrode is 0.04 to 30 µm, preferably 1 to 10 µm, and more preferably 1 to 3 µm.

As the height of the electrode increases, a higher horizontal electric field is generated, thereby obtaining a high transmittance and lowering the driving voltage. In terms of the aperture ratio, if the width of the electrode is constant, the aperture ratio is constant regardless of the height, but if a high transmittance occurs on the upper surface of the electrode, the aperture ratio may increase.

The electrode shape of the present invention is provided with an inclined portion and at the same time has a height (h) higher than that of a conventional electrode, thereby obtaining a higher transmittance and aperture ratio.

The length W of the bottom portion may be 1 to 10 μm, preferably 1.5 to 10 μm.

The distance L between the pixel electrode and the common electrode may be 1 to 40 μm, preferably 1.5 to 20 μm.

The value of W / L may be 1 / 40-2, preferably 1 / 4-1.

When the value of W / L is kept constant, it is preferable to reduce the value of L. For example, it is advantageous to reduce the value of L in the order of 10, 8, 6, 4, 2 while fixing the value of W / L to 1/2, since the strength of the horizontal electric field can be increased. In general, when W decreases, the electric line density decreases due to the decrease of the electrode thickness, whereas when L decreases, the distance between the electrode and the electrode decreases, thereby increasing the electric field strength. If both variables are taken into account, the decrease in L has a greater effect on the electric field strength, resulting in an increase in the overall electric field density.

The liquid crystal layer 300 that may be used in one embodiment of the present invention is optically isotropic (preferably isotropic in view of macroscopic) when no electric field is applied, such as a material having a Pockels effect or a Kerr effect, It may be a material that exhibits optical anisotropy by applying an electric field, and has an optical anisotropy when an electric field is not applied, and may be optically isotropic (preferably isotropic in view of macroscopicity) due to the loss of the optical anisotropy by applying an electric field. It may be. In addition, the liquid crystal layer may exhibit optical anisotropy when no electric field is applied, and the degree of optical anisotropy may be changed by applying an electric field. Preferably, when no electric field is applied, the liquid crystal layer is optically isotropic (preferably isotropic when viewed macroscopically) and is optically modulated by applying an electric field (in particular, birefringence is increased by applying an electric field).

There is no particular limitation on the liquid crystal layer 300. As an example, the liquid crystal layer 300 may include a nanostructured liquid crystal composite having an optically isotropic cholesteric blue phase liquid crystal. 5CB (pn pentyl-p'-cyanobiphenyl), JC-1041 (chiso) can be used as a liquid crystal, and chiral dopant is ZLI-4572 (Merck), ISO- (60BA) 2, CB-15 may be used, and RM275, EHA, etc. may be used as a single molecule used to stabilize a blue phase by forming a single molecule into a polymer through UV curing, and trimethylopropane triacrylate may be used as a photoinitiator. Can be. Their structure is as follows.

Liquid crystal 5CB (p-n pentyl-p'-cyanobiphenyl)

LCD JC-1041

Chiral dopant ZLI-4572

Chiral dopant ISO- (60BA) 2

Chiral dopant CB-15

Monomer RM-257

Monomer EHA (2-Ethylhexyl Acrylate)

Photo Initiator (Trimethylopropane triacrylate)

In addition, the liquid crystal layer 300 may include a smear blue (BPSm) phase. Like the cholesteric blue phase, the smectic blue phase has a high symmetry structure (for example, "Eric Grelet et al.," Structural Investigations on Smectic Blue Phases ", PHYSICAL REVIEW LETTERS, The American Physical Society, 4 2001 23, vol.86, No.17, p3791-3794 "), because it has an order (order structure, orientation order) below the visible light wavelength, it is almost a transparent material, and the degree of orientation order changes by application of an electric field. Optical anisotropy is expressed (the degree of optical anisotropy changes). That is, the smear blue phase is optically substantially isotropic, and since the liquid crystal molecules are directed toward the electric field by applying an electric field, the lattice is distorted to express optical anisotropy.

The lower substrate 100, the upper substrate 200, the electrodes 110 and 120, and the polarizer 400 may use conventionally known ones, but the present invention is not particularly limited thereto.

For example, the electrodes 110 and 120 may use a metal electrode material such as aluminum in addition to a transparent electrode material such as ITO.

A glass substrate may be used as the substrates 100 and 200, but is not limited thereto. It is preferable that at least one of the substrates 100 and 200 is a transparent substrate, for example, various substrates known in the art may be used. have. Moreover, a film type board | substrate may be sufficient and it may be what has flexibility.

The manufacturing method of the electrode formed on the lower substrate 100 may be a known method. For example, a polymer resin is coated on a substrate in a desired shape such as pyramid or triangle shape, and a metal material is deposited on the upper surface. Subsequently, after the photoresist (PR) is applied to the upper surface of the deposited metal, a portion of the electrode other than the electrode may be selectively UV exposed, developed, and etched to prepare a desired electrode shape.

As a method of coating the polymer resin in a desired shape, a method of coating the polymer resin on a substrate and then applying PR to use a mask having a desired pattern for UV exposure, development, and etching.

In addition, the polymer resin may be inserted into the master on which the desired pattern is formed in an intaglio, attached to the substrate, and then only the master may be removed to coat a polymer having a desired shape on the substrate.

FIG. 6 is a plan view illustrating a unit pixel 10 of an IPS liquid crystal display device including an electrode having an inclined portion shown in FIG. 3. Referring to FIG. 6, after forming a common electrode 1 having an inclined portion on a transparent glass substrate or a plastic substrate, forming a gate line 9, a TFT 8 layer, and a cutting film layer, the pixel electrode 2 is formed. It is formed in a direction parallel to the common electrode. At this time, the data bus line 7 and the gate bus line 9 do not limit the height of the electrode.

7A and 7B illustrate a principle of driving a liquid crystal mixture having a nanostructure including a blue phase liquid crystal by an electric field. As shown in FIG. 7A, the liquid crystal mixture between the common electrode 1 and the pixel electrode 2 is optically isotropic when the voltage is not applied (see FIG. 7A). Induced refractive index is generated in the liquid crystal mixture in the direction of the electric field due to the strong electric field formed between the pixel electrode 2 and the pixel electrode 2, thereby achieving an optically anisotropic state (see FIG. 7B) and implementing a bright state.

Comparative example  One

Based on the conventional structure shown in FIG. 1, the voltage dependence of the transmittance of the BPLC cell is examined and shown in FIG. 9, and the transmittance of the BPLC cell is shown according to voltage and distance.

Both the common electrode and the pixel electrode are located on the lower substrate having a thickness of 0.04 μm, the electrode width W and the pattern space L between the electrodes are 4 μm and 8 μm, respectively, and the LC cell gap spacing is 6 μm.

For the simulation, K, Δn , Δ were set to 5.5 × 10 ⁻¹⁰ m / V ² , 0.150, and 4.5, respectively. In order to calculate the electro-optic properties of the BPLC cells, induced birefringence by Kerr effect was calculated using "TechWiz LCD (Sanayisystem, Korea) as a commercially available software.

In Comparative Example 1, one transistor (hereinafter, 1TR (transistor), amplification of the pixel electrode voltage is controlled by one transistor to one pixel, while the common electrode voltage is fixed) and 2TR (two at one pixel) TR was used to control the voltages of the pixel electrode and the common electrode, respectively).

Referring to Fig. 8, because of the higher electric field generated at 2TR, the 2TR driving method shows a V-T characteristic that is twice as fast as 1TR.

Referring to FIG. 9A, it can be seen that although the portion between the pixel electrode and the common electrode (that is, the pattern space L) shows high transmittance, there is almost no transmittance on the electrodes. In addition, even in the case of the pattern space L, it can be seen that there is a problem that the variation in transmittance is very large at a high voltage.

Referring to b of FIG. 9, it can be seen that a strong horizontal electric field is concentrated at an edge (edge), whereas in the pattern space L, a weak horizontal electric field is widely distributed.

Referring to FIG. 9C, it can be seen that the opening ratio of the conventional structure is only 71.7%.

Example  One

The change in the voltage-transmittance of the BPLC cell is shown in FIG. 11 using a device having an electrode shape in which the vertical section of the inclined portion is straight. It implemented on the same conditions as the comparative example 1 except having adopted 2TR, changing the inclination angle of the inclination part to 15 degrees-85 degrees, and making the height of the electrode 2 nm at maximum.

Example  2

The change in the voltage-transmittance of the BPLC cell is shown in FIG. 11 using an element including an electrode shape having a curved vertical section of the inclined portion. 2TR was employed, and the electrode shape was ellipse, sine, triangle, S, inverse ellipse shape, and the electrode was made under the same conditions as in Comparative Example 1 except that the height of the electrode was 2 μm.

Example  3

2TR was used and the electrode shape was an inverted ellipse (R-Ellipse), and it carried out on the same conditions as the comparative example 1 except having applied the voltage at 98V. 12 shows the transmittance of the BPLC cell according to Example 3 according to the voltage and distance.

In comparison with FIG. 8 and FIG. 10, the maximum transmittance of 0.26 is shown in the case of a triangular shape of 15 °, and in Comparative Example 1, the maximum transmittance is smaller than 0.25 in the vicinity of 112 to 113. In addition, in the case of a triangular shape of 45 degrees, the transmittance was 0.27 around 90V, and in the case of a 60 ° trapezoidal shape, the transmittance was about 0.26 in the vicinity of 80V. In the case of 45 ° triangle and 60 ° trapezoidal shape, the effect is remarkably excellent.

When comparing b of FIG. 8 and FIG. 11, the ellipse shape exhibits a maximum transmittance of 0.25 at less than 80V, and the R.Ellipse shape shows a maximum transmittance of 0.32 at 98V. It can be seen that Example 2 shows a lower transmit voltage than Comparative Example 1 and shows excellent transmittance.

Fig. 11C shows the aperture ratio in the inverted ellipse (R.Ellipse) shape. The opening ratio of Comparative Example 1 is only 71.7%, whereas in Example 2, it is 98.3%, thereby showing a remarkable difference in transmittance.

Comparing FIG. 12 (Example 3) and FIG. 9 (Comparative Example 1), Example 3 has a higher transmittance in the upper part of the electrode than in Comparative Example 1, and also maintains high transmittance even in the case of pattern space L. It can be seen that the variation is small.

Although the present invention has been described in detail with reference to preferred embodiments of the present invention, the present invention is not limited to the above-described embodiments, and many modifications are made by those skilled in the art to which the present invention pertains within the technical spirit of the present invention. This possibility will be self-evident.

1 is a cross-sectional view of a conventional horizontal field type liquid crystal display device.

2A to 2E are schematic diagrams showing the structure and optical state of a blue phase liquid crystal.

3 is a cross-sectional view of a liquid crystal display device having an electrode including one or more inclined portions according to one embodiment of the present invention.

4 is a cross-sectional view of the pixel electrode 110 and the common electrode 120 having an inclination part according to an exemplary embodiment of the present invention.

5 shows examples of shapes of the pixel electrode and the common electrode that can be used in the present invention.

FIG. 6 is a plan view illustrating a unit pixel 10 of an IPS liquid crystal display device including an electrode having an inclined portion shown in FIG. 3.

7A and 7B illustrate a principle of driving a liquid crystal mixture having a nanostructure including a blue phase liquid crystal by an electric field.

8 graphically shows the change in voltage-transmittance of a BPLC cell based on the electrode structure of Comparative Example 1. FIG.

9 shows the transmittance of the BPLC cell based on the voltage and distance based on the electrode structure of Comparative Example 1.

FIG. 10 shows the change in voltage-transmittance of the BPLC cell based on the electrode structure of Example 1. FIG.

FIG. 11 shows the change in the voltage-transmittance of the BPLC cell based on the electrode structure of Example 2. FIG.

12 shows the transmittance of the BPLC cell according to Example 3 according to the voltage and distance.

Brief description of the main parts of the drawing

100; Lower substrate 200; Upper substrate 300; Liquid crystal layer

400 polarizing plate 110 pixel electrode 120 common electrode

Claims (12)

A lower substrate including a pixel electrode and a common electrode spaced apart from each other; An upper substrate disposed opposite the lower substrate; A liquid crystal layer disposed between the upper substrate and the lower substrate and changed into an anisotropic state when an electric field is applied in an isotropic state in the absence of an electric field; And a polarizing plate intersecting the upper substrate and the lower substrate, wherein the pixel electrode and the common electrode include at least one inclined portion. The display device of claim 1, wherein the pixel electrode and the common electrode have a bottom portion contacting a lower substrate, a top portion spaced apart from the bottom portion by a predetermined height, and a side portion connecting the bottom portion and the top portion. Liquid crystal display device comprising an inclined portion. The liquid crystal display device according to claim 1, wherein the inclination angle is in a range of 5 ° to 85 ° when the vertical section of the inclined portion is a straight line. The liquid crystal display device according to claim 1, wherein the tangential angle is 0 ° to 90 ° when the vertical section of the inclined portion is curved. 3. The liquid crystal display device according to claim 2, wherein the top portion is less than the width W of the bottom portion greater than zero. The liquid crystal display of claim 2, wherein the top portion of the pixel electrode and the common electrode are intersection points where the inclined portions meet. The liquid crystal display device according to claim 1, wherein the vertical cross sections of the pixel electrode and the common electrode are trapezoidal, triangular, elliptical, inverted elliptic, sinusoidal, inverted curve, semicircle, and S-shape. The liquid crystal display device according to claim 2, wherein the pixel electrode and the common electrode have a height of 1 to 10 µm. The liquid crystal display device according to claim 2, wherein the bottom portion length (W) of the pixel electrode and the common electrode is 1 to 10 mu m, and the interval L therebetween is 1 to 40 mu m. The liquid crystal display of claim 1, wherein the liquid crystal layer comprises a nanostructured liquid crystal composite having an optically isotropic blue liquid crystal. The liquid crystal display of claim 1, wherein a voltage is applied to the liquid crystal display by a thin film transistor connected to at least one of the pixel electrode and the common electrode to form a horizontal electric field between the electrodes. A display apparatus comprising the liquid crystal display device according to any one of claims 1 to 11.

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