KR20020078831A - Liquid crystal display - Google Patents

Abstract

PURPOSE: A liquid crystal display is provided to improve the viewing angle by compensating the difference of retardation values according to the path by using compensating films fixed with discotic liquid crystal. CONSTITUTION: A liquid crystal display includes first and second insulating substrates(10,11) facing to each other, first and second compensating films(30,31) attached to outer surface of the first and second substrates, first and second hybrid discotic liquid crystal(50) fixed to the first and second compensating films, first and second polarizing plates(40,41) attached to the first and second compensating films, and nematic liquid crystal(20) injected between the first and second substrates, wherein a retardation value in surfaces of the first and second compensating films is 30nm or higher.

Description

Liquid crystal display

The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device for securing a wide viewing angle.

The liquid crystal display is one of the most widely used flat panel display devices. The liquid crystal display includes two glass substrates on which electrodes are formed and a liquid crystal layer interposed therebetween. A display device for controlling the amount of light transmitted by rearranging them.

One substrate of such a liquid crystal display device generally includes a thin film transistor for switching a voltage applied to an electrode. In addition to the thin film transistor, the substrate includes a wiring including a gate line and a data line and a gate line receiving a signal from the outside. And gate pads and data pads respectively transferred to the data lines. A pixel electrode electrically connected to the thin film transistor is formed in the pixel region defined by the intersection of the gate line and the data line.

On the other substrate of the liquid crystal display, a color filter and a black matrix are formed, and a common electrode is formed on the entire surface thereon.

Polarizers are attached to the outside of the substrate of the liquid crystal display, and a compensation film for improving the viewing angle is attached between the substrates and the polarizers. However, such a compensation film is limited in improving the viewing angle because there is no product variety and there is no optimized condition.

The technical problem to be achieved by the present invention is to maximize the viewing angle.

1 is a cross-sectional view schematically illustrating a liquid crystal display device according to an exemplary embodiment of the present invention.

2 is a view showing the contrast ratio by color,

3A to 3C are views showing viewing angles according to retardation values of liquid crystals injected into a liquid crystal display device;

4 is a view showing the polar angle of the discotic liquid crystal attached to the compensation film,

5A to 5C are views showing a viewing angle according to a retardation value of a TAC film used as a substrate of a compensation film,

6a to 6c are views showing a viewing angle according to the thickness of the discotic liquid crystal attached to the compensation film;

7A to 7C are views illustrating viewing angles according to pretilt angles of liquid crystals injected into the liquid crystal display device.

8A to 8G are views showing viewing angles according to polar angles of a discotic liquid crystal attached to a compensation film;

9A and 9B are views illustrating a viewing angle according to a retardation value of a biaxial film used as a compensation film.

In order to achieve this problem, the present invention uses a biaxial film as a substrate of the compensation film.

According to the present invention, first and second compensation films are attached to the outside of the first and second insulating substrates facing each other, and the first and second discotic liquid crystals are attached to the first and second compensation films, respectively. It is. First and second polarizing plates are attached to the outside of the first and second compensation films, respectively, and a nematic liquid crystal is injected between the first and second substrates. Here, a biaxial film is used as the compensation film, and it is preferable that the values of R _e and R _th of the biaxial film have a range of 10 nm to 60 nm and 40 nm to 100 nm, respectively.

At this time, the retardation value (R _e , R _th ) of the biaxial film is preferably (20 nm, 60 nm).

On the other hand, the thicknesses of the first and second discotic liquid crystal layers are 1.5 µm to 3.0 µm, preferably 2.0 µm. The polar angle θ ₁ formed by the optical axes of the first and second discotic liquid crystals close to the first and second compensation films and a vertical line perpendicular to the first and second compensation films is 5 to 10 degrees, and the first and second compensation films The polar angle θ ₂ between the optical axes of the first and second discotic liquid crystals and the vertical line perpendicular to the first and second compensation films far away from is 55 to 90 degrees, and the ratio of elastic modulus (K ₃₃ / K ₁₁ ) is 0 to It has a range of 0.1, the polar angle θ ₁ is preferably 7.5 degrees, θ ₂ is preferably 60.8 degrees, the ratio of the elastic modulus (K ₃₃ / K ₁₁ ) is preferably 0.038, the retardation value of the nematic liquid crystal It is 0.35 micrometer-0.39 micrometer, Preferably it is 0.363 micrometer, and the pretilt angle of a nematic liquid crystal is 5 degrees.

In addition, the slow axis of a biaxial film is orthogonal to the orientation direction of a discotic liquid crystal, and it is preferable that the average optical axis direction of a discotic liquid crystal matches the orientation direction and the pretilt direction of a nematic liquid crystal.

In the present invention, by using a biaxial film as a compensation film, the viewing angle can be improved by appropriately adjusting the conditions of the retardation value and the liquid crystal.

Next, a liquid crystal display according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the same.

First, the structure of a liquid crystal display according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 and 2.

1 illustrates a schematic structure of a liquid crystal display according to an exemplary embodiment of the present invention.

As shown in FIG. 1, electrodes (not shown) are formed inside the two insulating substrates 10 and 11, and a nematic liquid crystal 20 is injected between the substrates 10 and 11. Compensation films 30 and 31 are attached to the outer sides of the two substrates 10 and 11, respectively, and polarizing plates 40 and 41 are attached to the outer sides of the compensation films 30 and 31, respectively. At this time, the transmission axes (or absorption axes) of the polarizing plates 40 and 41 are perpendicular to the rubbing direction of the nematic liquid crystal 20, and the transmission axes (or absorption axes) of the two polarizing plates 40 and 41 are perpendicular to each other. . The compensation films 30 and 31 are formed by coating and curing a hybrid discotic liquid crystal (50) on a film such as triacetyl cellulose (TAC). To improve.

The operation of the liquid crystal display of the twisted nematic (TN) mode will be briefly described. The nematic liquid crystals 20 adjacent to the two substrates 10, 11 are oriented parallel to the substrates 10, 11, and the nematic liquid crystals 20 not adjacent to the two substrates 10, 11 have one substrate. In a continuous 90 degree twisted orientation towards the other substrate. When a voltage is applied to the electrodes of the two substrates 10 and 11, an electric field is formed between the substrates 10 and 11 and the liquid crystal is oriented in a direction parallel to the electric field.

In the liquid crystal display, the change in the viewing angle when the conditions of the compensation films 30 and 31 and the liquid crystals 20 and 50 were changed was examined.

The drawing obtained through the simulation shows the contrast ratio with respect to the viewing angle on the screen as a circle, the color is displayed differently according to the contrast ratio in the circle, and the number outside the circle represents the angle. The contrast ratio according to color is shown in FIG. 2, and the viewing angle indicates that the contrast ratio is 10 or more.

First, the change in the viewing angle according to the retardation (Δn · d) value of the nematic liquid crystal injected between the substrates will be described with reference to FIGS. 3A to 3C.

3A to 3C are views showing viewing angles when the retardation values of nematic liquid crystals are changed to 0.363 μm, 0.378 μm, and 0.401 μm, respectively. In these three cases, all conditions except for the retardation value of a nematic liquid crystal are the same. A uniaxial film was used as a substrate of the compensation film, and the retardation values R _e and R _th were (0 nm and 40 nm), and the thickness of the discotic liquid crystal layer was 2.0 μm. In addition, the polar angle of the discotic liquid crystal is 7.6 degrees for the discotic liquid crystal adjacent to the compensation film substrate and 52.6 degrees for the discotic liquid crystal farther from the compensation film substrate. The elastic modulus ratio K ₃₃ / K ₁₁ of the discotic liquid crystal is 1.15, and the pretilt angle of the nematic liquid crystal is 5 degrees. Here, K ₃₃ is the bending elastic modulus of the liquid crystal, K ₁₁ is the spreading elastic modulus, the orientation of the discotic liquid crystal is determined by the interaction of these two coefficients. R _e is d · (n _x −n _y ), R _th is d · (n _x −n _z ), and in the case of a uniaxial film, n _x = n _y . Meanwhile, the polar angle is an angle formed by the optical axis of the discotic liquid crystal and a vertical line perpendicular to the compensation film substrate. Since the optical axis of the discotic liquid crystal passes through the center of the liquid crystal, the polar angle θ may be defined as shown in FIG. 4. The polar angle of the discotic liquid crystal adjacent to the film was defined as θ ₁ , and the polar angle of the discotic liquid crystal farther from the compensation film was defined as θ ₂ .

3A and 3B, the retardation value of the nematic liquid crystal is 0.35 μm to 0.39 μm, preferably 0.36 μm to 0.38 μm, and the upper and left viewing angles are best. As shown below, the left and right viewing angles become worse.

Next, the change in the viewing angle according to the change in the retardation values R _e and R _th of the TAC film used as the compensation film substrate will be described with reference to FIGS. 5A to 5C.

5A to 5C are views illustrating a viewing angle in a liquid crystal display device in which retardation values R _e and R _th of the compensation film substrate are different from each other. Each of (R _e , R _th ) in FIGS. 5A-5C is (0 nm, 0 nm), (0 nm, 40 nm), (0 nm, 80 nm), and n _x and n _y have the same value. It is a uniaxial film which has, and all conditions except the R _th value of a compensation film substrate are the same. The retardation value of the nematic liquid crystal is 0.363 μm, the thickness of the discotic liquid crystal layer is 2.0 μm, and in the case of the discotic liquid crystal whose polar angle is adjacent to the compensation film substrate, it is 7.6 degrees. In the case of 52.6 degrees, K ₃₃ / K ₁₁ of the discotic liquid crystal is 1.15, and the pretilt angle of the nematic liquid crystal is 5 degrees.

5A to 5C, the best viewing angles were obtained at 40 nm when R _th was changed to 0 nm, 40 nm and 80 nm, respectively. From these results, it means that there is an optimal R _th value according to the discotic liquid crystal.

Next, the change in the viewing angle according to the change in the thickness of the discotic liquid crystal layer will be described with reference to FIGS. 6A to 6C.

6A to 6C are views showing viewing angles when the thickness of the discotic liquid crystal layer is changed to 1.6 μm, 2.0 μm, and 2.4 μm, respectively. In all three cases, all conditions except for the thickness of the discotic liquid crystal layer are the same. A uniaxial film was used as a substrate of the compensation film and (R _e , R _th ) was (0 nm, 40 nm). The retardation value of a nematic liquid crystal is 0.363 micrometers. The polar angle is 7.6 degrees for the discotic liquid crystal adjacent to the compensation film substrate, 52.6 degrees for the discotic liquid crystal farther from the compensation film substrate, K ₃₃ / K ₁₁ of the discotic liquid crystal is 1.15, and the diameter of the nematic liquid crystal The square is 5 degrees.

As shown in FIGS. 6A to 6C, the left and right viewing angles have better characteristics as the thickness of the discotic liquid crystal layer is thicker, but the upper and lower viewing angles have the best characteristics when the upper and lower viewing angles are 2.0 μm. Therefore, in order to improve both the up, down, left, and right viewing angles, it is necessary to optimize the distribution of the polar angles of the discotic liquid crystal.

Next, the change of the viewing angle according to the change of the pretilt angle of the nematic liquid crystal will be described with reference to FIGS. 7A to 7C.

7A to 7C are views showing the viewing angle when the pretilt angles of the nematic liquid crystals are changed to 2 degrees, 5 degrees, and 8 degrees, respectively, by varying the rubbing method of the alignment layer. In all three cases, all conditions except for the pretilt angle of the nematic liquid crystal are the same. A uniaxial film was used as the substrate of the compensation film, and the retardation values R _e and R _th were (0 nm, 40 nm). The retardation value of a nematic liquid crystal is 0.363 micrometers. The thickness of the discotic liquid crystal layer is 2.0 μm, the polar angle is 7.6 degrees for the discotic liquid crystal adjacent to the compensation film substrate, 52.6 degrees for the discotic liquid crystal farther from the compensation film substrate, and K ₃₃ / K ₁₁ is 1.15.

As shown in FIGS. 7A to 7C, when the pretilt angle of the nematic liquid crystal is changed, there is almost no change in the left and right viewing angles, but when the pretilt angle is large, the contrast ratio moves downward when the pretilt angle is large. Therefore, it is difficult to improve the characteristics of the vertical and horizontal viewing angles only by changing the pretilt angle of the nematic liquid crystal.

Next, a change in the viewing angle according to the change in the polar angle of the discotic liquid crystal will be described with reference to FIGS. 8A to 8G.

8A to 8G are views showing viewing angles according to distribution of polar angles of liquid crystals. The polar angle distribution is respectively the polar angle of the discotic liquid crystal adjacent to the compensation film θ ₁ , the polar angle of the discotic liquid crystal farthest from the compensation film is θ ₂ , and the K ₃₃ / K ₁₁ value of the discotic liquid crystal (θ ₁ , θ ₂ , K ₃₃ / K ₁₁ ), and FIGS. 8A to 8G (7.0 degrees, 52.2 degrees, 5.00), (7.6 degrees, 52.6 degrees, 1.15), (7.5 degrees, 52.6 degrees, 0.28), (7.5 Degrees, 52.9 degrees, 0.086), (7.5 degrees, 53.4 degrees, 0.00), (7.5 degrees, 60.8 degrees, 0.038), (5.0 degrees, 58.3 degrees, 0.053), and all other conditions are the same. A uniaxial film was used as the substrate of the compensation film, and the retardation values R _e and R _th were (0 nm and 40 nm), and the thickness of the discotic liquid crystal layer was 2.0 μm. The retardation value of the nematic liquid crystal is 0.363 mu m, and the pretilt angle of the nematic liquid crystal is 5 degrees.

Here, the formula for calculating the thickness profile of the polar angle of the discotic liquid crystal is shown in [Equation 1].

Where θ is the polar angle and z is the thickness according to the polar angle of the discotic liquid crystal layer, to be.

As shown in FIGS. 8A to 8G, it can be seen that the top, bottom, left, and right viewing angles are improved in the order of FIGS. 8A, 8B, 8C, 8D, 8E, 8G, and 8F. This means that the greater the polar part of the liquid crystal in this order, the greater the standing portion of the liquid crystal relative to the compensation film substrate, the better the viewing angle. On the other hand, the upper portion of the upper and lower viewing angles changes with respect to the distribution of the liquid crystal layer, but the lower portion has little change.

Next, the change in the viewing angle when the biaxial film is used instead of the TAC film as the substrate of the compensation film will be described with reference to FIGS. 9A and 9B.

9A to 9C are views illustrating a viewing angle when a biaxial film is used as a compensation film substrate under the condition of FIG. 8F showing the best viewing angle in the viewing angle characteristic according to the polar angle distribution of the discotic liquid crystal described above. At this time, the values of (R _e , R _th ) of the biaxial film in FIGS. 9A and 9B are (20 nm, 60 nm) and (40 nm, 80 nm), respectively, and the other conditions are the same as those in FIG. 8F.

In the liquid crystal display of FIG. 8F, there was little change in the viewing angle when the retardation value or the pretilt angle of the liquid crystal was changed to achieve symmetry without deteriorating the viewing angle.

On the other hand, as a result of changing the values of R _e and R _th using the biaxial film of the substrate of the compensation film, the best viewing angle characteristic is shown in FIG. 9A in which (R _e , R _th ) is (20 nm, 60 nm). As shown in FIG. 9B, when the biaxiality is too large (40 nm and 80 nm), a phenomenon in which the viewing angles of the upper and lower sides are reversed occurs in contrast to FIG. 8F.

As a result of simulating the viewing angle by changing various conditions as described above, the liquid crystal display of FIG. 9A, that is, the retardation value of the nematic liquid crystal is 0.363 μm, the pretilt angle of the nematic liquid crystal is 5 degrees, and the compensation film substrate is a biaxial film. The retardation values R _e and R _th are (20 nm and 60 nm), the thickness of the discotic liquid crystal layer adhered to the compensation film substrate is 2.0 μm, and the polar angles of the discotic liquid crystals (θ ₁ , θ ₂₎ ) Shows the best viewing angle characteristics when (7.5 degrees, 60.8 degrees) and elastic modulus ratio (K ₃₃ / K ₁₁ ) is 0.038.

In this case, it is preferable that the draw axis of the biaxial film is orthogonal to the alignment direction of the discotic liquid crystal, and the average optical axis direction of the discotic liquid crystal coincides with the alignment direction and the pretilt direction of the nematic liquid crystal.

On the other hand, the thickness of the discotic liquid crystal layer fixed to R _e of 10 nm to 60 nm, R _th of 40 nm to 100 nm, and the compensation film substrate of the biaxial film serving as the compensation film substrate is 1.5 μm to 3.0 μm, and discotic. Even in the case of a liquid crystal display device in which the polar angle θ ₁ of the liquid crystal is 5 degrees to 10 degrees, θ ₂ is 55 degrees to 90 degrees, and the ratio of elastic modulus (K ₃₃ / K ₁₁ ) falls within the range of 0 to 0.1 Can be secured.

As described above, in the present invention, the retardation value of the nematic liquid crystal is 0.363 μm, the pretilt angle of the nematic liquid crystal is 5 degrees, and the compensation film substrate is used as the biaxial film, and the retardation values R _e and R _th are (20). Nm, 60 nm), the thickness of the discotic liquid crystal layer adhered to the compensation film substrate is 2.0 µm, the polar angles (θ ₁ , θ ₂ ) of the discotic liquid crystal (7.5 degrees, 60.8 degrees), and the ratio of the elastic modulus (K ₃₃ / K ₁₁ ) shows the best viewing angle characteristic when 0.038.

Claims (11)

First and second insulating substrates facing each other, First and second compensation films attached to an outer side of the first and second substrates, respectively, First and second discotic liquid crystals fixed to the first and second compensation films, respectively, First and second polarizers attached to an outer side of the first and second compensation films, respectively, Nematic liquid crystal injected between the first substrate and the second substrate Including; A biaxial film is used as the compensation film, and the values of R _e and R _th of the biaxial film have a range of 10 nm to 60 nm and 40 nm to 100 nm, respectively. In claim 1, The retardation value (R _e , R _th ) of the biaxial film is preferably (20 nm, 60 nm). In claim 1, The first and second discotic liquid crystal layer has a thickness of 1.5㎛ to 3.0㎛ respectively. In claim 3, The liquid crystal display device wherein the thickness of the first and second discotic liquid crystal layers is 2.0 μm. In claim 1, The polar angle θ ₁ formed between the optical axes of the first and second discotic liquid crystals close to the first and second compensation films and a vertical line perpendicular to the first and second compensation films is 5 to 10 degrees, and the first and second A polar angle θ ₂ between the optical axes of the first and second discotic liquid crystals far from the second compensation film and a vertical line perpendicular to the first and second compensation films is 55 to 90 degrees, and a ratio of elastic modulus (K ₃₃ / K ₁₁ ) having a range of 0 to 0.1. In claim 5, A polar angle θ ₁ of the first and second discotic liquid crystals is 7.5 degrees, θ ₂ is 60.8 degrees, and the ratio of elastic modulus (K ₃₃ / K ₁₁ ) is 0.038. In claim 1, A liquid crystal display device wherein the retardation value of the nematic liquid crystal is 0.35 µm to 0.39 µm. In claim 7, A liquid crystal display device wherein the retardation value of the nematic liquid crystal is 0.363 μm. In claim 1, A liquid crystal display device having a pretilt angle of 5 degrees for the nematic liquid crystal. In claim 1, The slow axis of the biaxial film is a liquid crystal display device orthogonal to the alignment direction of the discotic liquid crystal. In claim 1, And an average optical axis direction of the discotic liquid crystal coincides with an alignment direction and a pretilt direction of the nematic liquid crystal.