KR19990059998A - Liquid Crystal Display Using Compensation Film - Google Patents

Abstract

A compensation film having a hybrid arrangement, such as a FUJIFILM, is attached to the outer side of the twisted nematic liquid crystal display cell and a uniaxial compensation a plate film is attached to one or both sides of the cell. At this time, the oblique direction of the disk-shaped molecules constituting the hybrid type compensation film is the same as the buffing direction of the adjacent liquid crystal cell surface, and the slow axis of the refractive index of the plate compensation film is the transmission axis of the adjacent polarizing plate. To match or perpendicular to. In addition to the FUJI film, a combination of a plurality of uniaxial compensation films capable of exhibiting such an effect may be used, and a biaxial compensation film may be used instead of a plate compensation film.

Description

Liquid Crystal Display Using Compensation Film

The present invention relates to a wide viewing angle liquid crystal display device using a compensation film.

In general, a liquid crystal display device has a structure in which a liquid crystal is injected between two substrates, and the amount of light transmitted is controlled by adjusting the intensity of the electric field applied thereto.

Twisted-nematic (TN) type liquid crystal display device has a long axis of liquid crystal molecules filled between two substrates in a state in which no voltage is applied, and the liquid crystal molecules are twisted in a spiral with a constant pitch. It has a twisted structure in which the direction of its major axis changes continuously, and when voltage is applied, the liquid crystal molecules rise perpendicular to the substrate.

The liquid crystal material has birefringence in which the refractive indices in the major and minor directions of the molecules are different from each other. The birefringence causes a difference in the refractive index felt by light depending on the position of the liquid crystal display. Therefore, a difference occurs in the rate at which the polarization state is changed while the linearly polarized light passes through the liquid crystal, and thus the amount and color characteristics of the light when viewed from the front side are different from those seen from the front. As a result, a liquid crystal display having a twisted nematic structure may cause changes in contrast ratio, color shift, gray inversion, and the like depending on the viewing angle.

In order to solve this problem, a method of compensating for a phase difference generated in the liquid crystal layer using a compensation film having an opposite phase difference has been developed. Among them, Fuji Film's WV film shows the best compensation result, which uses a discotic molecular compound with a negative c plate compensating effect. Hybrid arrays are sequentially arranged to have an increasingly larger angle with respect to the normal of the display device substrate.

1 is a cross-sectional view showing the structure of a Fuji film. Normally, the FUJI film is divided into two parts. The first part (I) is a tri-acetyl cellulose (TAC) substrate film, which is a c plate uniaxial compensation film having a constant retardation value, and the remaining part (II) has an inclination angle of 4 It is a film having a thickness of about 2μm consisting of disk-shaped molecules that sequentially change from 68 degrees.

The viewing angle characteristic of the liquid crystal display device experimented using the FUJIFILM is shown in FIG.

In this experiment, five uniaxial compensation films, each having a different optical axis, were used instead of the Fuji film to simulate a compensation film with a hybrid arrangement.

3 shows the direction of the optical axis of the uniaxial compensation film used at this time. The optical axis θ, φ of the first uniaxial compensation film 21, 31 from the outside of the liquid crystal cell 1 is (90 ^o , 0 ^o ), and (θ, φ of the second film 22, 32. ) Is (7.5 ^o , 225 ^o ), (θ, φ) of the third film (23, 33) is (22.5 ^o , 225 ^o ), and (θ, φ) of the fourth film (24, 34) is ( 37.5 ^o , 225 ^o ), and the (θ, φ) of the fifth film 25, 35 is (52.5 ^o , 225 ^o ). Each film is 0.5 μm thick and n _o and n _e are 1.57 and 1.52, respectively. Here, θ and φ are said to have x-axis and y-axis in the plane parallel to the substrate of the liquid crystal display device, respectively. The angle between the axis and the z axis is shown.

As shown in FIG. 2, the use of the FUJIFILM film is excellent in compensating effect and there are many improvements in terms of viewing angle, but the contrast ratio is still low when viewed at an angle of 45 degrees to the transmission axis of the polarizer (90 degrees and 180 degrees). Can be. This is because an inherent light leakage phenomenon occurs when two polarizers whose transmission axes are perpendicular to each other are used.

An object of the present invention is to widen the viewing angle of a liquid crystal display device.

1 is a cross-sectional view showing the structure of a Fuji compensation film according to the prior art,

2 illustrates viewing angle characteristics of a liquid crystal display according to the related art.

3 shows the compensation film used in the simulation of the Fuji compensation film,

4 is an exploded perspective view of a liquid crystal display according to an exemplary embodiment of the present invention;

5 illustrates viewing angle characteristics of a liquid crystal display according to an exemplary embodiment of the present invention.

6 and 7 are exploded perspective views of a liquid crystal display according to another exemplary embodiment of the present invention.

In the liquid crystal display device according to the present invention, a compensation film having a hybrid array is attached to both sides of a liquid crystal cell into which a liquid crystal material having positive dielectric anisotropy is injected, and a hybrid array or between a compensation film having a hybrid array and a liquid crystal cell is formed. A plate uniaxial compensation film or biaxial compensation film is attached to one or both sides of the outer side of the compensation film having, and a polarizing plate is attached to the outside of the compensation film, respectively.

In this case, the transmission axis of the polarizing plate is perpendicular to the buffing direction of the adjacent liquid crystal cell surface, and a direction in which the refractive index of the plate compensation film or the biaxial compensation film is greatest is parallel to the transmission axis of the adjacent polarizing plate or The compensation film having a hybrid array is attached so as to be perpendicular to the same direction as the buffing direction of adjacent surfaces when the oblique direction of the disk-shaped molecules constituting the compensation film is projected onto the substrate.

a The sum of the retardation values (nx-ny) * d of the plate compensation film and the biaxial compensation film is preferably 5 to 150 nm. Here, the z-axis is in a direction perpendicular to the surface of the liquid crystal cell, and in the plane parallel to the surface of the liquid crystal cell and having the largest refractive index in a plate compensation film or biaxial compensation film, the x axis is the same as the x axis. When the direction in the plane and perpendicular to the x-axis is the y-axis, nx, ny, and nz respectively represent refractive indices in the x, y and z-axis directions of the compensation film, and d is the thickness of the compensation film.

An embodiment of the liquid crystal display according to the present invention will now be described in detail with reference to the accompanying drawings.

4 is an exploded perspective view of a liquid crystal display according to an exemplary embodiment of the present invention. The liquid crystal display shown in FIG. 4 illustrates a case of the o mode in which the transmission axis of the polarizer is perpendicular to the buffing direction of the adjacent alignment layer.

As shown in FIG. 4, compensation films 21 and 22 having a hybrid arrangement of a FUJIFILM type are attached to both sides 11 and 12 of the liquid crystal cell 10 into which the liquid crystal material is injected. Alignment films (not shown) are formed in each of both surfaces 11 and 12 of the liquid crystal cell, and the alignment films are buffed in one direction in a direction crossing each other. Arrows shown on both sides 11 and 12 of the liquid crystal cell 10 indicate this buffing direction. Compensation films 21 and 22 having a hybrid arrangement are each composed of two parts, one of which is 211 and 221 which is a hybrid arrangement in which the inclination angles of the disk-shaped molecules constituting the compensation film are sequentially changed. 212 and 222 are c plate uniaxial compensation films with a TAC substrate with a delay value of -40 nm. c plate compensation film refers to a compensation film having a relationship of nx = ny> nz. The direction of the compensation films 21 and 22 is attached to be the same as the buffing direction of the adjacent surfaces 11 and 12 when the oblique direction of the disk-shaped molecules constituting the compensation films 21 and 22 is projected onto the substrate. have.

The a plate uniaxial compensation film 32 is attached to the outer side of the FUJIFILM type compensation film 22 attached to the upper surface 12 of the liquid crystal cell 10. The x axis of the plate uniaxial compensation film 32 is attached so as to be perpendicular to the inclination direction of the compensation film 22 of the hybrid arrangement.

The polarizing plates 41 and 42 are attached to the outside of the FUJIFILM type compensation film 21 and the outside of the a plate uniaxial compensation film 32 attached to the lower surface 11 of the liquid crystal cell 10, respectively. . As described above, the transmission axes of the polarizing plates 41 and 42 are attached to be perpendicular to the buffing direction of the adjacent surfaces 11 and 12 of the liquid crystal cell 10.

5 illustrates a viewing angle characteristic of the liquid crystal display according to the exemplary embodiment of the present invention.

The spacing of the liquid crystal cells used in the simulation is 4.3 μm and the parameters of the other liquid crystal cells are shown in Table 1. Five uniaxial compensation films with different optical axes and retardation values were used to simulate the compensation film with the hybrid arrangement, and the optical axis (θ, φ) of the uniaxial compensation film used at this time. Are (90 ^o , 0 ^o ), (7.5 ^o , 225 ^o ), (22.5 ^o , 225 ^o ), (37.5 ^o , 225 ^o ), (52.5 ^o , 225 ^o ) from the outside. Each film is 0.5 μm thick and n _o and n _e are 1.57 and 1.52, respectively.

And the retardation value (nx-ny) * d of the a plate compensation film used at this time is 80 nm. The retardation value of a plate compensating film should be optimized in each case, but 5 to 150 nm is most preferred.

Table 1 Parameters of TN Cells Used in Optical Simulation

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Elastic constant (pN) K ₁ 11.7

K ₂ 5.1

K ₃ 16.1

Relative permittivity ε _∥ 10.8

ε _⊥ 3.4

Refractive Index n _e 1.5404 + 0.008591

n _o 1.46 + 0.005948

Cell Spacing / Pitch d / p 0.033

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As shown in Figure 5, it can be seen that the viewing angle is excellent at all angles. In particular, it can be seen that the gray scale display performance when viewed in a direction forming an angle of 45 degrees with the transmission axis of the polarizing plate is much improved compared to the liquid crystal display device compensated by only the FUJIFILM film.

In the embodiment of the present invention, a plate compensation film was used, but a biaxial compensation film of nx> ny> nz may be used instead.

The liquid crystal display according to the present invention may have various other structures as shown in FIGS. 6 and 7.

In the liquid crystal display shown in FIG. 6, the a-plate compensation film 32 is not between the hybrid array compensation film 22 and the polarizing plate 42, but the upper surface 12 of the liquid crystal cell 10 and the hybrid array compensation film. It is attached between 22.

The a plate compensating film may also be attached to the lower portion of the liquid crystal cell.

In the liquid crystal display shown in FIG. 7, two arrays of a-plate compensation films 31 and 32 are attached to both sides 11 and 12 of the liquid crystal cell 10, and the polarizing plates 41 and the hybrid array compensation films 21 and 22. , 42). In this case, the retardation values (nx-ny) * d of the two a plate compensating films 31 and 32 need not be equal to each other, and the sum of the retardation values (nx-ny) * d has a value between 5 and 150 nm. It is preferable.

The compensation film applied in the embodiment of the present invention is not necessarily a Fuji film, and any other compensation film capable of exhibiting such an effect may be used. For example, a compensation film having a structure as disclosed in SID'97, Digest, p. 687 may be used. The delay value and the like of the compensation film are not optimized values, and it is clear that better results can be obtained by optimizing the delay value and the arrangement direction of the molecules constituting the compensation film.

As described above, a liquid crystal display having a wide viewing angle can be obtained by attaching a compensation film having a hybrid arrangement such as a FUJI film and the like and a plate compensation film or a biaxial compensation film together.

Claims (14)

A liquid crystal cell injecting a liquid crystal material having positive dielectric anisotropy and having a first surface and a second surface opposite the first surface, First and second hybrid type compensation films attached to the first and second surfaces of the liquid crystal cell, A pair of polarizers attached to outer surfaces of the first and second hybrid type compensation films, A first a plate compensating film attached to an inner or an outer surface of the first or second hybrid type compensating film, And a first surface and a second surface of the liquid crystal cell are buffed in different directions. In claim 1, Liquid crystals coinciding with the buffing directions of the first and second surfaces, respectively, when the oblique directions of the optical axes of the disk-shaped molecules constituting the first and second hybrid type compensation films are projected onto the first and second surfaces, respectively. Display device. In claim 2, The transmission axes of the pair of polarizing plates are perpendicular to each other and perpendicular to the buffing directions of the adjacent first or second surfaces. In claim 3, And a direction in which the refractive index of the first a plate compensating film is greatest is parallel or perpendicular to a transmission axis of an adjacent polarizing plate. In claim 4, Further a second a plate compensation film attached to the inner or outer side of the first or second hybrid type compensation film attached to the opposite side to which the first a plate compensation film of the liquid crystal cell is attached Liquid crystal display device comprising. In claim 5, And a direction in which the refractive index of the second a plate compensating film is greatest is parallel to or perpendicular to a transmission axis of an adjacent polarizing plate. In any one of claims 1 to 6, Is in a plane parallel to the first surface of the liquid crystal cell and has a direction of the largest refractive index in the first or second a plate compensating film as the x-axis, is in the same plane as the x-axis and is perpendicular to the x-axis When the direction is y-axis, the refractive indexes in the x-axis and y-axis directions of the first or second a plate compensating film are nx and ny, respectively, and the cell spacing of the liquid crystal cell is d, When only the first a plate compensating film is attached, the retardation value (nx-ny) * d of the first a plate compensating film is 5 to 150 nm. When the first and second a plate compensation films are attached, the sum of the delay values (nx-ny) * d of the first and second a plate compensation films is 5 to 150 nm. A liquid crystal cell injecting a liquid crystal material having positive dielectric anisotropy and having a first surface and a second surface opposite the first surface, First and second hybrid type compensation films attached to the first and second surfaces of the liquid crystal cell, A pair of polarizers attached to outer surfaces of the first and second hybrid type compensation films, A first biaxial compensation film attached to an inner or outer surface of the first or second hybrid type compensation film, And a first surface and a second surface of the liquid crystal cell are buffed in different directions. In claim 8, Liquid crystals coinciding with the buffing directions of the first and second surfaces, respectively, when the oblique directions of the optical axes of the disk-shaped molecules constituting the first and second hybrid type compensation films are projected onto the first and second surfaces, respectively. Display device. In claim 9, The transmission axes of the pair of polarizing plates are perpendicular to each other and perpendicular to the buffing directions of the adjacent first or second surfaces. In claim 10, And a direction in which the refractive index of the first biaxial compensation film is greatest is parallel to or perpendicular to a transmission axis of an adjacent polarizing plate. In claim 11, A second biaxial compensation film attached to an inner side or an outer side of the first or second hybrid type compensation film attached to the opposite side to which the first biaxial compensation film of the liquid crystal cell is attached; Liquid crystal display device comprising. In claim 12, And a direction in which the refractive index of the second biaxial compensation film is greatest is parallel or perpendicular to a transmission axis of an adjacent polarizing plate. The method according to any one of claims 8 to 13, In the plane parallel to the first surface of the liquid crystal cell and the direction of the largest refractive index in the first or second biaxial compensation film as the x-axis, in the same plane as the x-axis and perpendicular to the x-axis When the direction is y-axis, the refractive indexes in the x-axis and y-axis directions of the first or second biaxial compensation film are nx and ny, respectively, and the cell spacing of the liquid crystal cell is d, When only the first biaxial compensation film is attached, the retardation value (nx-ny) * d of the first biaxial compensation film is 5 to 150 nm, When the first and second biaxial compensation films are attached, a sum of retardation values (nx-ny) * d of the first and second biaxial compensation films is 5 to 150 nm.