JPH06167691A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide a supertwisted nematic type color liquid crystal display device having good visual angle characteristics and stable display colors. CONSTITUTION:Three-dimensional refractive index control phase difference plates 9, 10 consisting of uniaxially stretched high-polymer films are installed between polarizing plates 8, 11 and liquid crystal display element 15 of this liquid crystal display device 14. The coefft. K of the phase difference plates 9, 10 is selected within a -0.75<=K<=0 range and the change in the retardation of the angle of elevation and azimuth directions generated by the liquid crystal display element 15 is adequately compensated, by which the visual angle characteristic of the liquid crystal display device 14 is improved and the stable color display is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an STN (super twisted nematic) type liquid crystal display device.

[0002]

2. Description of the Related Art Generally, an STN type liquid crystal display device is colored in yellow green or blue. However, by using an optical compensator, color correction is performed and a bright and clear black and white display can be obtained. Therefore, color display is possible, display quality is improved, and it can be used as a display means of an office automation (OA) device such as a word processor or a computer.

As an STN type liquid crystal display device to which color compensation is applied, there is a two-layer type STN type liquid crystal display device, in which coloring generated in the first layer (driving panel) is applied to the second layer (optical compensation panel). The color is corrected with to make it achromatic. This structure is
Compared with the single layer STN type liquid crystal display device, the liquid crystal panel has 2
Since the number of sheets is required, there is a problem in that the display device becomes thick and the weight increases. In order to solve this problem, a thin and lightweight STN type liquid crystal display device has been developed by using a positive retardation plate having a coefficient K = 1 made of a uniaxially stretched polymer film as an optical compensation plate. .

[0004]

However, since the retardation plate is generally made by stretching a polymer film, the optical properties are different between the slow axis direction and the fast axis direction of the film, and the two-layer type is used. The STN type liquid crystal display device of the retardation film type has a problem in that the color change due to the azimuth angle or the elevation angle is large, that is, the viewing angle is narrower than the STN type liquid crystal display device.

The uniaxially stretched polymer film is used as a retardation plate because it has optical anisotropy. That is, in the slow axis direction and the fast axis direction of the polymer film,
The refractive index is different. This is called birefringence. The retardation (Δn · d), which is given by the product of the refractive index anisotropy Δn and the film thickness d, is a physical quantity that gives the phase difference of light that occurs when passing through the film. The change is different in the slow axis direction and the fast axis direction. For example, a retardation plate made of polyvinyl alcohol has a property that the retardation in the slow axis direction decreases and increases in the fast axis direction as the elevation angle increases. As a result, when combined with a liquid crystal display panel, even if the optical compensation relationship is perfect in the normal direction, the difference between the retardation of the retardation plate and the retardation of the liquid crystal display panel increases as the elevation angle increases, resulting in optical compensation. The relationship breaks down. That is, there is a problem in that the viewing angle becomes narrow because the color changes and the display contrast decreases.

An object of the present invention is to provide a liquid crystal display device having improved display angle characteristics and high display quality.

[0007]

According to the present invention, there is provided a liquid crystal display device having a super twisted nematic liquid crystal layer interposed between a pair of transparent substrates so that the major axis direction of molecules is twisted in a spiral shape by about 240 °. In a liquid crystal display device which is arranged between a pair of polarizing plates, a retardation plate made of a uniaxially stretched polymer film is arranged between the pair of polarizing plates and the liquid crystal display element. The retardation plate, when the rate of retardation change depending on the elevation angle of the uniaxially stretched polymer film is expressed by a coefficient K using a three-dimensional refractive index,
The liquid crystal display device is characterized in that the coefficient K is selected to take a value in the range of -0.75≤K≤0.

[0008]

According to the present invention, a liquid crystal display element having a pair of transparent plates is provided with a super twisted nematic liquid crystal layer which is oriented between the pair of transparent substrates so that the major axis of the molecule is twisted by about 240 °. In the liquid crystal display device arranged between the pair of polarizing plates, the retardation plate made of a uniaxially stretched polymer film is disposed between the liquid crystal display element, and each of the retardation plates is The rate of retardation change depending on the elevation angle of the uniaxially stretched polymer film is set to 3
When the coefficient K is expressed by using the dimensional refractive index, the coefficient K is −
It is selected to take a value in the range of 0.75 ≦ K ≦ 0.

In order to expand the viewing angle in the STN type liquid crystal display device using the retardation plate, if the retardation of the liquid crystal display panel and the retardation plate to be arranged are the same even if the elevation angle becomes large, the optical compensation relation is obtained. A wide viewing angle can be obtained without deteriorating. That is, it is necessary to match the retardation change of the retardation plate to be arranged with the elevation angle retardation change of the liquid crystal panel. In the prior art, a retardation plate having a retardation change due to an elevation angle different from that of a positive retardation plate of K = 1 has two different retardation plates of K = 1 and K = −1. The slow axes of the plates must be aligned. The K = −1 retardation plate is a retardation plate having a property that retardation change due to elevation angle is opposite to that of K = 1, that is, a retardation plate having negative optical anisotropy, and is made of, for example, polystyrene. By using a three-dimensional refractive index control retardation plate that achieves this with a single film and finding and arranging the optimum relational coefficient K of the three-dimensional refractive index,
It is possible to obtain a wide-viewing-angle color liquid crystal display device having a small number of films, a thin shape, a light weight, and a clear display.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a liquid crystal display device 1 according to an embodiment of the present invention.
4 is an exploded cross-sectional view showing the overall configuration of FIG. Glass substrate 1
The color filter 12 is formed over the entire surface. The transparent common electrode 2 is formed thereon, and the organic alignment film 3 is further formed thereon. The transparent segment electrode 6 is formed on the glass substrate 7, and the organic alignment film 5 is further formed thereon. Such a glass substrate 1,
The liquid crystal layer 4 is sealed between the glass substrate 7 and the glass substrate 7 with the sealant 13 to form the liquid crystal display element 15. The organic alignment films 3 and 5 are each subjected to rubbing alignment treatment, and the liquid crystal layer 4 is installed so that the major axis direction of the liquid crystal molecules is twisted by 240 °. On the outer surface of the glass substrates 1 and 7 of the liquid crystal display element 15 formed as described above, the phase difference plate 9 and
10 is installed, and the polarizing plates 8 and 11 are installed on the outer side thereof.

The liquid crystal layer 4 is a nematic liquid crystal having a positive dielectric anisotropy, such as phenylcyclohexane (PC).
1. C) cholesteric nonanate (CN) as a chiral dopant for controlling the twist direction in (H) type liquid crystal.
A mixed liquid crystal added with 10% is used. The refractive index anisotropy Δn of the mixed liquid crystal is 0.142. In addition, the glass substrate 1,
In order to keep the interval between 7 constant, a plastic spacer (not shown) is sandwiched between these substrates. With this spacer, the thickness of the liquid crystal layer 4 is 6.0 μm.
set to m.

The polarizing plates 8 and 11 are neutral gray type polarizing plates having a single transmittance of 42% and a polarization degree of 99.9%, and the retardation plates 9 and 10 are made of polycarbonate and have a retardation value. Are both 428 nm. The three-dimensional refractive index control retardation plate is an NZ plate manufactured by Nitto Denko Corporation (trade name NRZ, characteristic value −0.30 ≦ K ≦ −
0.10) was used. The color filter 12 is composed of, for example, three color filters formed in a fine band shape, for example, a red filter R, a green filter G and a blue filter B, and a black light shielding layer BL provided in a gap between the filters. And formed by a dyeing method, a printing method, an electrodeposition method, a pigment dispersion method, or the like. Further, the color filter 12 may be formed on the segment electrode 6.

FIG. 2 is a plan view of the color filter 12 shown in FIG. 1, FIG. 3 is a sectional view of the color filter 12, and FIG. 4 is an enlarged plan view of the vicinity of the color filter 12. 2 to 4, the red filter R, the green filter G, and the blue filter B are formed in strips, for example, and the red filter R, the green filter G, and the blue filter B are arranged in parallel in this order. As shown in FIG. 4, the color filter 12 is formed with a width L1 = 70 μm for each color filter and has a filter interval L2.
= 25 μm. The filter interval L2 is determined by the manufacturing accuracy of a color filter such as a photo process, and is generally 15 μm to 40 μm. The filter array is, for example, a red filter R, a green filter G, and a blue filter B in the width direction of the filter indicated by arrow E in FIG.
The sequence of is repeated. Further, a light shielding layer BL is formed between the filters R, G, B. For the light-shielding layer BL, for example, a photosensitive pigment dispersion method in which carbon black is dispersed in photopolymer is used.

FIG. 5 is a diagram showing the installation conditions of each member of the embodiment shown in FIG. In FIG. 5, the liquid crystal display device 14
The direction of the viewing angle when installed horizontally is 6 o'clock to 12 o'clock. Arrows P3 and P5 are the directions of the liquid crystal molecule alignment axis, which is the rubbing axis direction of the alignment films 3 and 5, and are twisted by 240 ° counterclockwise with respect to each other. The rubbing axis direction P3 of the alignment film 3 is 120 ° clockwise from the 12 o'clock direction,
The rubbing axis P5 of the alignment film 5 is set so as to form an angle of 120 ° counterclockwise from the 12:00 direction. Polarizer 8
Is 80 clockwise from the 12 o'clock direction in the absorption axis direction P8.
The retardation plate 9 is installed so that the slow axis direction P9 forms an angle of 20 ° clockwise from the 12 o'clock direction. The phase difference plate 10 is in the slow axis direction P1.
It is installed so that 0 makes an angle of 10 ° clockwise from the 12 o'clock direction. Further, the polarizing plate 11 has an absorption axis direction P1.
1 is installed so as to make an angle of 20 ° counterclockwise from the 12 o'clock direction.

The uniaxially stretched polymer film is used as a retardation plate because of its optical anisotropy. That is, the property that the refractive index in the slow axis direction and the refractive index in the fast axis direction are different is utilized. Is the relative phase difference between the light (ordinary ray and extraordinary ray) passing through the liquid crystal display panel canceled by the product of the refractive index anisotropy Δn and the film thickness d, that is, the retardation when passing through the retardation plate? , Or all wavelengths are aligned in phase. However, this is a case where the liquid crystal display device is viewed from the normal direction of the display surface, and when viewed from an oblique direction, that is, when the viewing angle direction is viewed at an angle to the normal direction and the azimuth direction, the retardation plate The three-dimensional index of refraction must be taken into account. Now, let the refractive indices of the retardation plate in the three-dimensional directions be n _x (slow axis direction), n _y (fast axis direction),
Let n _z (thickness direction), and the relationship of the refractive index in this three-dimensional direction be a coefficient K, which is expressed as in the following equations (1) to (4).

[0016] When the phase difference plate having positive optical anisotropy, a _{(n x + n y) /} 2> n z (1), K = 1- (n z -n y) / (n x - _nz ) (2) (where n _x ≠ n _z ).

In the case of a retardation plate having negative optical anisotropy, (n _x + n _y ) / 2 <n _z (3) and K = (n _x −n _z ) / (n _z −n _y). ) -1 (4) (where n _z ≠ n _y ).

In particular, when (n _x + n _y ) / 2 = n _z (5), K = 0 (6).

With respect to the refractive index and the retardation when viewed from the slow axis direction and the fast axis direction, when the elevation angle from the normal direction of the retardation plate is φ, the following equations (7) to ( Given in 10).

[0020] (1) slow refractive index when viewed from the axis direction anisotropy ^{_{Δn x = {n x 2 n}} z 2 / (n x 2 sin 2 φ + n z 2 cos 2 φ)} 1/2 -n y (7) Phase difference R _x = Δn _x · d / cos φ (8) (2) When viewed from the fast axis direction Refractive index anisotropy Δn _y = n _x − {n _y ² n _z ² / (n _{y ^{2 sin 2 φ + n z 2}} cos 2 φ)} 1/2 (9) the phase difference _{R y = Δn y · d /} cosφ (10) FIG 6 is a graph showing the retardation change due to the elevation angle of the different phase difference plate with coefficient K Is. FIG. 6 shows the three-dimensional refractive indexes of the positive retardation plate with K = 1, the negative retardation plate with K = −1, and the retardation plate with K = 0. It is obtained by substituting it in the formula. FIG. 6A shows K = 1, FIG. 6B shows K = −1, and FIG. 6C shows the retardation change depending on the elevation angle of the retardation plate of K = 0. From these results, the retardation change due to the elevation angle is different between the positive retardation plate and the negative retardation plate, in the case of the positive retardation plate,
In the slow axis direction, the retardation decreases as the elevation angle increases, and in the fast axis direction, the retardation increases as the elevation angle increases. In the case of a negative retardation plate, the retardation increases in the slow axis direction as the elevation angle increases, and the retardation decreases in the fast axis direction as the elevation angle increases. Thus, it can be seen that the positive retardation plate and the negative retardation plate have opposite tendencies in retardation change depending on the elevation angle. Further, as shown in FIG. 6C, in the case of the retardation plate with K = 0, the retardation shows a constant value in both the slow axis direction and the fast axis direction regardless of the elevation angle change.

FIG. 7 is a graph showing the results of actually measuring the retardation change depending on the elevation angle for a positive retardation plate of K = 1 made of polycarbonate, which is a typical material for the retardation plate. The measurement of the retardation change depending on the elevation angle was performed by using the method of Senarmont. That is,
Linearly polarized light is incident on the retardation plate at an azimuth angle of 45 ° with respect to the fast axis on a plane including the fast axis and the slow axis of the retardation plate. Further, the light passing through the phase difference plate is arranged so that the fast axis direction thereof coincides with the fast axis direction of the phase difference plate.
Observe through an analyzer through a / 4 wave plate. Assuming that the retardation of the retardation plate is Δ and the extinction is obtained at the azimuth angle θ by adjusting the azimuth of the polarizer, the retardation Δ of the retardation plate is obtained from the following formula (11).

Δ = π / 2 + θ (11) As shown in FIG. 7, the actually measured change in retardation according to the elevation angle of the retardation plate matches the tendency derived based on the above-described theoretical formula shown in FIG. To do. That is, in the positive retardation plate with K = 1, the retardation decreases as the elevation angle increases in the slow axis direction, and the retardation increases as the elevation angle increases in the fast axis direction.

FIG. 8 is a graph showing the change in retardation of the positive retardation film shown in FIG. 7 depending on the elevation angle, and the change in the azimuth direction for each elevation angle. As shown in FIG. 8, when the elevation angle is 15 °, the variation in retardation in the azimuth direction is small because the displacement of the elevation angle itself from the normal direction is small, and the variation in retardation in both the fast axis direction and the slow axis direction is small. Is almost a perfect circle. When the elevation angle becomes 30 °, the change in retardation in the fast axis direction increases, while the change in retardation in the slow axis direction decreases, so the graph becomes an ellipse having the fast axis as the major axis.
When the elevation angle becomes 45 °, the retardation change in the fast axis direction becomes larger and the retardation change in the slow axis direction becomes smaller. Therefore, the graph shows that the ellipse whose major axis is the fast axis direction is the slow axis direction. You will have a constriction.
When the elevation angle is 60 °, the tendency when the elevation angle is 45 ° becomes more remarkable, and the constriction in the slow axis direction becomes larger in the graph.

FIG. 9 is a graph showing the retardation change in the azimuth direction of the liquid crystal display element 15 shown in FIG. 1 for each elevation angle. For the liquid crystal display element 15, the measurement was performed with the direction of the viewing angle set to the 12 o'clock direction. As shown in FIG.
The change in retardation of the liquid crystal display element 15 in the azimuth direction is small in all azimuths at elevation angles of 15 ° and 30 °, and the graph is represented as a nearly perfect circle. When the elevation angle becomes 45 °, the change in retardation of the liquid crystal display element 15 becomes large in the 6 o'clock direction, and the graph is 1 when the 3 o'clock-9 o'clock direction is the boundary.
The 2 o'clock direction is almost semi-circular, and the 6 o'clock direction is semi-elliptical with the major axis being the 12 o'clock-6 o'clock direction. When the elevation angle becomes 60 °, the change in retardation of the liquid crystal display element 15 becomes large both in the 12 o'clock direction and the 6 o'clock direction. It looks like a combination of two ellipses.

As described above, the change in retardation of the liquid crystal display element 15 in the azimuth direction changes in the viewing angle direction of 12 o'clock and 6 o'clock, and hardly changes in the direction of 3 o'clock-9 o'clock. Further, it can be seen that the degree of change is larger in the case of the 6 o'clock direction retardation change than in the case of the 12 o'clock direction retardation change. Such a change in retardation is due to the liquid crystal molecules having a twisted structure in the liquid crystal layer 4 of the liquid crystal display element 15.

A retardation plate as shown in FIGS. 7 and 8,
In the liquid crystal display device in which the liquid crystal display element 15 as shown in FIG. 9 is combined, the retardation change tendency between the retardation plate and the liquid crystal display element 15 is different, so the optical compensation relationship is broken,
Light leakage and color change occur. This reduces the display contrast and narrows the viewing angle. Therefore, in order to expand the viewing angle, it is necessary to reduce the retardation change due to the elevation angle of the retardation plate.

FIG. 10 is a graph showing the measurement result of the retardation change depending on the elevation angle of the three-dimensional refractive index control retardation plate having the coefficient K = -0.57. Coefficient K = -0.
In the three-dimensional refractive index control retardation plate No. 57, the retardation change between the slow axis direction and the fast axis direction is opposite to that of the K = 1 retardation plate, and the elevation angle becomes large in the fast axis direction. The retardation decreases in accordance with the above, and the retardation increases as the elevation angle increases in the slow axis direction. Also, it can be seen that the degree of change is also different. Therefore, if a negative three-dimensional refractive index control retardation plate as shown in FIG. 10 is installed with the coefficient K selected so as to cancel the change in retardation depending on the elevation angle of the liquid crystal display element 15, a wide viewing angle with little change in color tone can be obtained. The color liquid crystal display device of

Table 1 shows the coefficient K of the three-dimensional refractive index control retardation plate made of a uniaxially stretched polymer film.
The viewing angle characteristic of the liquid crystal display device 14 using this is shown. The viewing angle characteristics of the liquid crystal display device 14 are the same as those of the phase difference plates 9 and 1 shown in FIG.
The value was set to 0, and the retardation plates having different coefficients K were used for the measurement, and the retardation plate having the conventional coefficient K = 1 was also measured. Regarding the viewing angle characteristics, the viewing angle range is shown by the angle such that the contrast of the measurement points on the liquid crystal display device 14 in the 12 o'clock to 6 o'clock direction and the 9 o'clock to 3 o'clock direction is such that contrast ≧ 4. Regarding the color tone characteristics of the liquid crystal display device 14, those which are superior to the conventional liquid crystal display device in the 12 o'clock to 6 o'clock direction and 9 o'clock to 3 o'clock direction are ○, and those which are similar to the conventional liquid crystal display device. Shown with Δ. The color tone characteristics are visually evaluated by comparison with conventional techniques.

[0029]

[Table 1]

As shown in Table 1, the coefficient K is -0.75≤
In the liquid crystal display device 14 including the retardation films 9 and 10 in the range of K ≦ 0, the viewing angle characteristics in the 9 o'clock to 3 o'clock direction are improved, and the conventional retardation film having the coefficient K = 1 is provided. A wide viewing angle of about 70% or more can be obtained as compared with a liquid crystal display device. Further, with respect to the change in color tone, the degree of change is small as compared with the conventional liquid crystal display device including the retardation plate having the coefficient K = 1. Therefore, in the liquid crystal display device 14, color display with stable color tone and a wide viewing angle is performed. It turns out that is obtained.

As described above, according to this embodiment, the three-dimensional refractive index control retardation plates are combined to form the retardation plates 9 and 1.
By adjusting 0 and the retardation change depending on the elevation angle of the liquid crystal display element 15, it is possible to suppress the color tone change due to the elevation angle, the deterioration of the contrast and the inversion of the color display, which are possessed by the conventional color liquid crystal display device having the retardation plate, and the wide field of view is suppressed. A color display of the corners can be obtained.

[0032]

As described above, according to the present invention, a retardation plate made of a uniaxially stretched polymer film is arranged between a pair of polarizing plates and a liquid crystal display element, and each of the retardation plates comprises: When the rate of retardation change depending on the elevation angle of a uniaxially stretched polymer film is expressed by a coefficient K using a three-dimensional refractive index,
The coefficient K is selected so as to take a value in the range of −0.75 ≦ K ≦ 0. As a result, it is possible to more appropriately compensate for the retardation change caused by the elevation angle change and the azimuth angle change caused by the super twisted nematic liquid crystal layer of the liquid crystal display element which is twisted and aligned by 240 °. Therefore,
A high-contrast display can be obtained in a wide viewing angle range, and a stable color display with a small change in display color depending on the viewing angle can be obtained.

[Brief description of drawings]

FIG. 1 is an exploded cross-sectional view showing an overall configuration of a liquid crystal display device 14 according to an embodiment of the present invention.

FIG. 2 is a plan view of a color filter 12 of the embodiment shown in FIG.

FIG. 3 is a sectional view of a color filter 12 of the embodiment shown in FIG.

FIG. 4 is an enlarged plan view of the color filter 12 of the embodiment shown in FIG.

FIG. 5 is a diagram showing installation conditions of each member of the embodiment shown in FIG.

FIG. 6 is a graph showing retardation changes according to elevation angles of retardation plates having different coefficients K.

FIG. 7 is a graph showing a measurement result of retardation change depending on an elevation angle for a positive retardation film having a coefficient K = 1.

FIG. 8 is a graph showing measurement results of changes in the azimuth direction for each elevation angle for a positive retardation film having a coefficient K = 1.

9 is a graph showing measurement results of retardation change in the azimuth direction for each elevation angle of the liquid crystal display element 15 shown in FIG.

FIG. 10 is a graph showing a result of measuring a retardation change according to an elevation angle of a three-dimensional refractive index control retardation plate having a coefficient K = −0.57.

[Explanation of symbols]

 1,7 Glass substrate 2 Common electrode 3,5 Alignment film 4 Liquid crystal layer 6 Segment electrode 8,11 Polarizing plate 9,10 Retardation plate 12 Color filter 13 Sealing material

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Asao Kiguchi 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture

Claims (1)

[Claims] 1. A liquid crystal display device, in which a super twisted nematic liquid crystal layer is interposed between a pair of transparent substrates, in which the long axis direction of molecules is twisted in a spiral shape by about 240 °, is arranged between a pair of polarizing plates. In the liquid crystal display device configured as described above, a retardation plate made of a uniaxially stretched polymer film is arranged between the pair of polarizing plates and the liquid crystal display element, and each of the retardation plates has a uniaxially stretched height. When the rate of retardation change depending on the elevation angle of the molecular film is expressed by a coefficient K using a three-dimensional refractive index, the coefficient K is selected so as to take a value in the range of −0.75 ≦ K ≦ 0. Display device.