JP2695671B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a super twist type liquid crystal display device using a retardation plate as an optical compensator.

<Prior Art> In general, a super twist type liquid crystal display device is colored yellow green or blue, but by using an optical compensator, color correction is performed and a bright and clear white / black display is obtained. Therefore, the display quality is improved, and the display can be used as a display of OA equipment such as a word processor and a computer.

As a super twist type liquid crystal display device which has been subjected to color compensation, there is a two-layer type super twist type liquid crystal display device, and coloring generated in the first layer (driving cell) is performed in the second layer (optical compensation cell). The color is corrected and achromatic. This structure requires two liquid crystal cells as compared with a single-layer super twist type liquid crystal display device, and thus has a problem that the thickness of the display device is increased and the weight is increased.

In order to solve this problem, a thin and lightweight super twist type liquid crystal display device has been developed by using a retardation plate made of a uniaxially stretched polymer film as an optical compensator. However, since the retardation film is made by stretching a polymer film, the optical properties are different between the stretching direction of the film and the direction perpendicular to the stretching direction, and the retardation plate is different from the two-layer type super twist type liquid crystal display device. The phase difference plate type super twist type liquid crystal display device has a problem that a color change due to a viewing angle or an elevation angle is large, that is, a viewing angle is narrow.

<Problems to be Solved by the Invention> A retardation plate composed of a uniaxially stretched polymer film uses optical anisotropy as an optical compensator. However, the refractive index (birefringence) differs between the stretching direction of the polymer film and the direction perpendicular thereto. The retardation (Δn · d), which is given by the product of the refractive index Δn and the thickness d of the film, is a physical quantity that gives the phase of light. This retardation differs between the stretching direction of the polymer film and the direction perpendicular thereto. In a retardation plate corresponding to a regular crystal, the retardation tends to decrease in a stretching direction and increase in a direction perpendicular to the retardation. When combined with a liquid crystal display cell, the difference between the retardation of the retardation plate and the retardation of the liquid crystal display cell becomes large, especially in the stretching direction of the retardation plate, so that the transmitted light causes a phase difference and is colored. I do. That is, color compensation is not performed, and the display contrast is reduced, so that the viewing angle is narrowed.

The present invention solves such a problem,
It is an object of the present invention to provide a liquid crystal display device which is thin, lightweight, can provide clear white / black display, and has a wide viewing angle.

<Means for Solving the Problems> The present invention relates to a super twist type liquid crystal display device using a retardation plate composed of a uniaxially stretched polymer film as an optical compensator, wherein at least one surface of a liquid crystal display cell has retardation. Having two laminated retardation plates to be added,
The intersection angle between the slow axis of the first-layer retarder and the slow axis of the second-layer retarder is not less than 20 degrees, and the retardation value of the first-layer retarder depends on the elevation angle. The second layer is laminated such that the slow axis direction of the second layer is parallel to the direction in which the liquid crystal display cell is minimized, and the retardation plate of the retardation plate adjacent to the liquid crystal display cell and the position adjacent to the liquid crystal display cell. The crossing angle between the slow axis of the phase difference plate and the rubbing axis of the adjacent substrate of the liquid crystal display cell is 70 degrees to 90 degrees, and each phase difference plate is made of a uniaxially stretched polycarbonate polymer film.

Even if the laminated retardation plate is only on one side of the liquid crystal display cell,
You may arrange | position on the front surface and back surface of a liquid crystal display cell.

Further, in the case of a combination of lamination and single layer, one surface of the liquid crystal display cell is a laminated retardation plate, and the other surface is a slow axis of the retardation plate adjacent to the liquid crystal display cell and a retardation plate of the adjacent substrate of the liquid crystal display cell. A single-layer retardation plate having a crossing angle of 70 to 90 degrees with the rubbing axis is provided, and the single-layer retardation plate is made of a uniaxially stretched polymer film of polycarbonate.

<Function> The reason why the uniaxially stretched polymer film is used as a retardation plate is due to its optical anisotropy. That is, the relative phase difference between the light (the ordinary ray and the extraordinary ray) transmitted through the liquid crystal panel is canceled out by transmitting the phase difference plate, or the phases are made uniform to achromatic.

This color compensation utilizes the property that the refractive index in the stretching direction of the polymer film and the refractive index in the direction perpendicular thereto are different. On the other hand, the viewing angle characteristics of the retardation plate must consider a three-dimensional refractive index. Now, the refractive index in the three-dimensional direction of the retarder is
n _MD (drawing direction), n _TD (direction orthogonal to drawing direction), n
_{Assuming that ZD} (thickness direction), the refractive index and the retardation as viewed from the stretching direction and the direction orthogonal thereto are given by the following equation, where φ is the elevation angle from the normal direction of the retardation plate.

(1) When viewed from the stretching direction Refractive index Δn _MD = ｛n _MD ² n _ZD ² / (n _MD ² sin ² φ + n _ZD ² cos ² φ)｝ ^1/2 −n _TD phase difference R _MD = Δ _MD · d / cosφ (2) When viewed from the direction perpendicular to the stretching direction Refractive index Δn _TD = n _MD − ｛n _TD ² n _ZD ² / (n _TD ² sin ² φ + n _ZD ² cos ² φ)｝ ^1/2 Phase difference R _TD = Δn _TD · d / cosφ When the refractive indices in the three-dimensional directions are measured and substituted into the above equation, FIG. 3 is obtained. FIG. 3 shows the relationship between the elevation angle of the retardation plate and the change in retardation obtained from the theoretical formula.
From this result, it is understood that the retardation tends to decrease in the stretching direction of the retardation plate, and to increase in the direction perpendicular to the stretching direction (in the case of a uniaxial positive sign).

Incidentally, uniaxially stretched polymer films of polycarbonate (PC) and polyvinyl alcohol (PVA) are known as retardation films, and the results of measuring the retardation of the retardation film with respect to the elevation angle by the method of Senarmont for these films are shown in FIG. (A) <Polycarbonate> and FIG. 4 (B) <Polyvinyl alcohol>. However, the normalized value is 1.0 as the retardation value when viewed from directly above. The orientation is measured at intervals of 15 °, with the stretching direction being 0 ° and the direction orthogonal to the stretching direction being 90 °. This result is consistent with the tendency obtained from the above theoretical formula. When the rate of change of the retardation with respect to the elevation angle is obtained from FIGS. 4 (A) and 4 (B), FIGS. 5 (A) and 5 (B) are obtained.

On the other hand, FIG. 6 shows the result of determining the rate of change of the retardation with respect to the elevation angle of the liquid crystal display cell. When such a retardation plate and a liquid crystal display cell are combined, in the stretching direction of the retardation plate, the difference between the retardation of the liquid crystal display cell and the retardation of the retardation plate is large, and the transmitted light (ordinary light,
The extraordinary ray) causes a phase difference and is colored. As a result, the display angle is reduced because the display contrast is reduced. Therefore, to widen the viewing angle, an optical compensator having a small retardation change rate may be obtained by laminating retardation plates.

FIGS. 5 (A) and 5 (B) show the changes in azimuth and elevation angle retardation with respect to the respective stretching directions of polycarbonate and polyvinyl alcohol retardation plates. (FIG. 5 (A)) shows that the retardation change depending on the azimuth is slight at the elevation angles of 15 ° and 30 °. However, at 45 ° and 60 °, there is a clear change in retardation depending on the direction. In particular, at an elevation angle of 60 °, the rate of change is about twice in the stretching direction and the direction perpendicular to the stretching direction. On the other hand, the polyvinyl alcohol phase difference plate (FIG. 5 (B))
At 45 °, little change in retardation is seen depending on the azimuth, but at 45 ° and 60 °, the change is more intense than at the polycarbonate retardation plate.
In the ° direction, the rate of change is 8 times.

Therefore, when the elevation angle is increased, the retardation of the polycarbonate phase difference plate is smaller than the rate of change of the polyvinyl alcohol phase difference plate. Therefore, in order to suppress a change in retardation with respect to the viewing angle (elevation angle), the former polycarbonate phase difference plate needs to be formed. Appropriate.

At this time, it is necessary to consider the vibration plane of the ordinary ray and the extraordinary ray passing through the phase difference plate and the oscillation plane of the ordinary ray and the extraordinary ray passing through the liquid crystal display cell. The oscillation plane of the ordinary ray is the fast axis,
The plane of vibration of the extraordinary ray is called the slow axis, and when two phase difference plates are superimposed, the superposition method of making the fast axes (or slow axes) parallel to each other is added, and the fast axes (slow) of each other The direction in which the axes are perpendicular to each other is called subtraction. As a result of many studies, we have found that when the retardation plates are additively laminated, the change in retardation is reduced, and as a result, the viewing angle is increased. Further, when two layers of retardation plates are additionally laminated, the first layer and the second layer
It was found that the viewing angle would not be effectively widened unless the crossing angle of the slow axis of the phase difference plate of the layer was 20 degrees or more.

In particular, when stacking two retardation plates, it is effective to use the direction in which the elevation angle dependence shown in FIG. 5A is minimized. At this time, the liquid crystal may be laminated so that the slow axis direction of the second-layer retardation plate is parallel to the direction in which the retardation change with respect to the elevation angle of the first-layer retardation plate is minimized. It has been found that when combined with a display cell, the change in retardation is reduced and the viewing angle is increased.

Placing the slow axes of the retarders of the second layer in parallel in the direction in which the retardation changes of the retarders of the first layer are minimized means that the retardation changes of the retarders of each other are minimized. Therefore, in the laminated retardation film, a decrease in the retardation value in the stretching direction is suppressed, and the retardation change is made uniform, so that when combined with a liquid crystal display cell. In addition, the retardation difference in the stretching direction of the retardation plate is reduced, and the viewing angle is widened.

The most appropriate crossing angle between the slow axes of the first and second phase difference plates was in the range of 20 ° to 40 °.

Also, when performing lamination, the retardation plate adjacent to the liquid crystal display cell is not color-compensated unless it is arranged subtractively with respect to the liquid crystal cell, so the slow axis of the retardation plate adjacent to the liquid crystal display cell is: It has been found that optimal color compensation is obtained when the rubbing axis of the substrate adjacent to the liquid crystal display cell is at 70 to 90 degrees.

The above-described lamination arrangement condition is the same when the retardation plate is disposed on the front and back surfaces of the liquid crystal display cell, or when one side is a laminated retardation plate and the other surface is a single-layer retardation plate. The effect is obtained. In addition, when the phase difference plates provided on the front surface and the rear surface are disposed in a symmetric relationship with respect to the liquid crystal display cell, a liquid crystal display device having a wide viewing angle can be obtained.

<Example> An example of the present invention will be described with reference to FIGS.

FIG. 1 is an explanatory view showing a structure of an embodiment of the present invention described below, wherein 1 is an upper polarizing plate, 2 is an upper laminated retardation plate, 3 is an STN liquid crystal cell, 4 is a lower retardation plate, 5 is a lower polarizing plate. The upper polarizer 1 has a single transmittance of 42% and a degree of polarization.
Using a 99.99% neutral gray type polarizing plate,
The upper laminated retardation plate 2 is made of a polycarbonate uniaxially stretched polymer film having a thickness of 50 μm and has a retardation value of 380 to 580 nm when laminated. The STN liquid crystal cell 3 has an LC material to which a left-handed chiral dopant is added. And twist angle 240 degrees, dΔn (d is the liquid crystal layer thickness, Δn
Is a value of refractive index anisotropy) = 0.83 to 0.92 μm. The lower retardation plate 4 may not be provided at all, may be a single-layer retardation plate, or may be provided with two retardation plates. When a laminated or single-layer retardation plate is provided, the retardation plate is made of a uniaxially stretched polymer film of polycarbonate and has the same retardation value as the upper retardation plate 2. As the lower polarizing plate 5, the same one as the upper polarizing plate 1 was used.

With reference to FIG. 2, a description will be given of an arrangement positional relationship in stacking the respective constituent members in this case.

2, P1 is the liquid crystal molecule orientation axis of the upper substrate constituting the STN liquid crystal cell 3, P2 is the liquid crystal molecule orientation axis of the lower substrate, P3 is the absorption axis of the upper polarizing plate 1, and P4 is the arrow. The absorption axis of the lower polarizer 5, P5 is the slow axis (stretching direction) of the retarder adjacent to the STN liquid crystal cell of the upper laminated retarder 2, and P6 is adjacent to the STN liquid crystal cell of the lower retarder 4. The retardation axis (stretching direction) of the retardation plate, θ ₁ is the angle between the liquid crystal molecule orientation axis P1 of the upper substrate and the slow axis of the retardation plate adjacent to the STN liquid crystal cell of the upper laminated retardation plate P5, theta ₂ is the angle between the slow axis of the retardation plate adjacent to the STN liquid crystal cell of the liquid crystal molecular orientation axis P2 and the lower phase difference plate P6 of the lower substrate, alpha liquid crystal molecular orientation axis P2 and the lower side of the lower substrate The angle β formed between the absorption axis P4 of the polarizing plate 5 and β represents the angle formed between the liquid crystal molecule alignment axis P1 of the upper substrate and the absorption axis P3 of the upper polarizing plate 1.

In order to stack the retardation plates, color compensation at the time of stacking must be ensured. Therefore, first, optimization of the arrangement of the phase difference plates was examined without stacking the phase difference plates.

The upper retardation plate 2 has a retardation value of 580 nm, and the liquid crystal cell 3 has a dΔn of 0.83 μm and a twist angle of 240 degrees. θ ₁ = 50 °, 60 °, 70 °, 80
°, 90 °, 100 °, and 110 ° were set, and color compensation was examined. still,
At this time, α and β were arbitrarily moved and adjusted so as to obtain white / black display. As a result, it was found that color compensation was obtained when θ ₁ = 70 ° to 90 °.

Next, upper and lower retardation plates 2 and 4 having a retardation value of 400 nm were used, and dΔn of the liquid crystal cell 3 was 0.92 μm.
m, having a twist angle of 240 degrees. θ ₁ = 50 °, 60
°, 70 °, 80 °, 90 °, 100 °, 110 °, θ 2 = 130 °, 120 °, 1
The color compensation was set at 10 °, 100 °, 90 °, 80 °, and 70 °. As a result, color compensation was obtained when θ ₁ = 70 ° to 90 ° (θ ₂ = 110 ° to 90 °). I found out.

At this time, it has been found that the viewing angle becomes wider as the color compensation is obtained. Therefore, when laminating the retardation plates, the retardation plate adjacent to the liquid crystal cell should be 70 ° ≦ θ ₁ ≦ 90 ° (90 ° ≦ θ
₍₂ ≦ 110 °).

Example 1 In the lamination of a retardation plate, a second retardation plate will be described below with the retardation plate adjacent to the liquid crystal cell as the first layer. Each retardation plate is made of a polycarbonate uniaxially stretched polymer film, and two of them are laminated. The first and second retarders have a retardation of 400 nm, and the liquid crystal cell 3 has a dΔn of 0.92 μm and a twist angle of 240 °. In the first embodiment, the lower retardation plate 4 is not provided.

Here, θ ₁ is set to 70 °, and the intersection angles of the slow axes (stretching directions) of the retardation plates of the first layer and the second layer are 0 °, 10 °, and 20 °.
Setting the viewing angle and color compensation at °, 30 °, 40 °, and 50 °, good results were obtained when the crossing angle was between 20 ° and 40 °, and especially when the crossing angle was 25 ° Met. At this time, α = 40 ° and β = 50 °. At this time, 29
° and 14 ° in the opposite viewing angle direction, and the viewing angle in the viewing angle direction was increased by about 7 ° compared to the case where the layers were not stacked.

Example 2 The number of layers of the upper laminated phase difference plate 2 was set to two, the first and second layer phase difference plates having a retardation of 200 nm were used, and the dΔn of the liquid crystal cell 3 was 0.90 μm. Use a twist angle of 240 degrees. The lower retardation plate 4 is not laminated, but is made into one sheet, and one having a retardation of 400 nm is used. Each of the laminated and single-layer retardation plates is made of a uniaxially stretched polymer film of polycarbonate. θ
₁ = 90 °, θ ₂ = 90 °, and the intersection angles of the slow axes (stretching directions) of the retardation plates of the first layer and the second layer are 0 °, 10 °, 20 °.
Setting the angle to 30 °, 30 °, 40 °, and 50 °, and examining the viewing angle and color compensation, good results were obtained when the crossing angle was between 20 ° and 40 °, especially when the crossing angle was 30 °. Met. At this time, α = 130 ° and β = 20 °. At this time, about 5 ° in the viewing angle direction and about 3 ° in the opposite viewing angle direction as compared with the case where the layers are not stacked.
The viewing angle has widened.

Example 3 When the number of layers of the upper laminated phase difference plate 2 is two, the retardation plates of the first and second layers have a retardation of 200 nm or the number of laminated lower phase difference plates 4 is two. The retardation plates of the first and second layers have a retardation of 200 nm
Use those. Each of the laminated upper and lower retardation plates is made of a uniaxially stretched polymer film of polycarbonate. The liquid crystal cell 3 has a dΔn of 0.90 μm and a twist angle of 240 degrees. θ ₁ = 90 °, θ ₂ = 9
0 °, the upper laminated retarder 2 and the lower laminated retarder 4 are both 0 °, 10 °, and 20 ° at the intersection of the slow axis (stretching direction) of the first and second layers. , 30 °, 40 °, and 50 °, and the results of examining the viewing angle and color compensation showed that good results were obtained when the crossing angle was between 20 ° and 40 °, and that the optimum was obtained when the crossing angle was 30 °. there were. At this time, α = 80 ° and β = 5 °. At this time, the viewing angle was increased by about 2 ° in the viewing angle direction and by about 7 ° in the opposite viewing angle direction as compared with the case where the layers were not stacked.

Comparative Example 1 In Example 1, the material of each retardation plate of the upper laminated retardation plate 2 was changed from polycarbonate to polyvinyl alcohol. Other configurations, conditions, and the like are exactly the same as those described in the first embodiment. As a result of the viewing angle measurement when the crossing angle was set to 25 °, in Comparative Example 1, the viewing angle was 20 ° in the viewing angle direction and 12 ° in the anti-viewing angle direction, which was smaller in the viewing angle direction than when the polycarbonate retardation plate of Example 1 was used. The viewing angle was 9 ° and the viewing angle was 2 ° in the opposite viewing direction.

Comparative Example 2 In Example 2, similarly, the material of each of the phase difference plates of the upper laminated phase difference plate 2 and one lower phase difference plate 4 was changed from polycarbonate to polyvinyl alcohol. Other configurations, conditions, and the like are exactly the same as those described in the first embodiment. However, since a retardation plate having a retardation of 200 nm was not present in polyvinyl alcohol, polycarbonate was used as a material of each retardation plate of the upper laminated retardation plate. Intersection angle
The angle was set to 30 °, and as a result of the visual angle measurement, in Comparative Example 2, it was 35 ° in the visual angle direction and 14 ° in the opposite visual angle direction. The 9 ° viewing angle in the anti-viewing angle direction was inferior to the case where the polycarbonate retardation plate in Example 2 was used.

As is clear from the results of Comparative Examples 1 and 2, the liquid crystal display device using the polycarbonate retarder has a wider viewing angle than the liquid crystal display device using the polyvinyl alcohol retarder. This is because, as shown in FIGS. 5A and 5B, the polycarbonate retardation plate has an elevation angle of 15 °, 30 °.
This is because no change in retardation due to the azimuth is observed at 45 °, and the retardation change rate at elevation angles of 45 ° and 60 ° is 1/4 of the change rate of the polyvinyl alcohol phase difference plate.

 A comparison table between Examples 1 and 2 and Comparative Examples 1 and 2 is shown below.

FIG. 7 shows the results of measuring the viewing angle characteristics of the optimal combination example in the first embodiment among the above-described embodiments. FIG. 8 shows the results of measuring the viewing angle characteristics of the optimal combination examples in the second and third embodiments.

<Effects of the Invention> As described above, according to the present invention, the change in retardation with respect to the viewing angle (elevation angle) of the phase difference plate can be reduced. A high-quality liquid crystal display device that has good characteristics and has little dependence on the viewing angle can be obtained.

[Brief description of the drawings]

FIG. 1 is a structural explanatory drawing of a liquid crystal display device for explaining an embodiment of the present invention, FIG. 2 is a plan view showing a positional relationship of the embodiment, and FIG. 3 is an elevation angle of a retardation plate obtained from a theoretical formula. FIG. 4 (A), which shows the relationship of the retardation change to
(B) is a diagram showing the relationship between the retardation change and the actually measured elevation angle of each of the polycarbonate and the polyvinyl alcohol phase difference plate, and FIGS. 5 (A) and (B) are omnidirectional directions of the polycarbonate and the polyvinyl alcohol phase difference plate, respectively. FIG. 6 is a diagram showing the relationship of the retardation change rates in all directions of the liquid crystal panel, and FIG. 7 is a graph showing the viewing angle characteristics of the optimum combination example in the case of Example 1. FIG. 8 is a diagram showing the results, and FIG. 8 is a diagram showing the results of actually measuring the viewing angle characteristics of the optimal combination examples in the second and third embodiments. 1: Upper polarizer, 2: Upper laminated retarder, 3: STN liquid crystal cell, 4: Lower laminated retarder, 5: Lower polarizer.

Claims (3)

(57) [Claims] In a super twist type liquid crystal display device using a retardation plate made of a uniaxially stretched polymer film as an optical compensator, at least one surface of a liquid crystal display cell is provided with two laminated layers in which retardation is added. Has a retardation plate,
The intersection angle between the slow axis of the first-layer retarder and the slow axis of the second-layer retarder is not less than 20 degrees, and the retardation value of the first-layer retarder depends on the elevation angle. The second layer is laminated such that the slow axis direction of the second layer is parallel to the direction in which the property is minimized, and the slow axis of the retardation plate adjacent to the liquid crystal display cell and the substrate adjacent to the liquid crystal display cell 70 crossing angle with the rubbing axis
A liquid crystal display device, wherein each of the retardation plates is made of a uniaxially stretched polymer film of polycarbonate. 2. The liquid crystal display device according to claim 1, wherein the laminated retardation plates are disposed on the front and back surfaces of the liquid crystal display cell. 3. A liquid crystal display device according to claim 1, wherein a retardation plate laminated on one surface of the liquid crystal display cell is arranged, and a retardation plate of the retardation plate adjacent to the liquid crystal display cell is disposed on the other surface. A single-layer retardation plate whose crossing angle between the axis and the rubbing axis of the adjacent substrate of the liquid crystal display cell is 70 degrees to 90 degrees is arranged, and the single-layer retardation plate is formed from a uniaxially stretched polymer film of polycarbonate. A liquid crystal display device comprising: