JP2000275661A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal display device, in which transmittance during white display is increased, irregular brightness of the panel face is decreased, and homogeneous display with high brightness can be obtd. SOLUTION: This liquid crystal display device has an interdigital electrode group, formed on at least one of a pair of transparent substrates 3, 4 to apply voltage parallel mainly to the substrates 3, 4, and a liquid crystal layer 1 which can be twisted and is held between the pair of substrates 3, 4. The thickness d of the liquid crystal layer 1 and a double refraction Δn of the liquid crystal of the liquid crystal layer 1 satisfy the relations d=0.55/(2×Δn×r) and 0.75<=r<=0.85.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device using a thin film transistor or the like, which has a high transmittance, a high contrast and no display unevenness.

[0002]

2. Description of the Related Art Active matrix type liquid crystal display devices are widely used as display devices for personal computers, word processors and other information devices because of their thin and lightweight features and high image quality. . Currently mainstream display mode
In addition to the twisted nematic (TN) method,
In-plane switching (IPS) systems with a very wide viewing angle have begun to spread. In the IPS method, torsional deformation is induced in the liquid crystal layer by an electric field parallel to the substrate surface, and the transmittance is controlled by the resulting birefringence effect of the liquid crystal layer. IP
For details of the S system, see, for example, Oh-e, et al., Liquid Crystals 22 (4), pp.
379-390 (1997). IPS
The feature of the system is that the viewing angle is wide, 80 ° up / down / left / right
A liquid crystal display device having a contrast of 10 or more at the above viewing angles can be realized. Therefore, the display mode is expected to be widely used as a large monitor or a television in the future.

[0003]

The IPS system is a display mode utilizing a birefringence effect generated by applying a voltage, and is a normally black mode in which a black display is obtained when no voltage is applied. Since the display mode is a birefringence effect display mode, the transmittance of the liquid crystal layer is determined by the thickness d of the liquid crystal layer and the birefringence Δ
n, depending on the wavelength λ of the incident light. Therefore, the transmission spectrum of the liquid crystal display device, that is, the transmittance and the chromaticity change depending on the setting of the thickness d of the liquid crystal layer and the birefringence Δn. Since the birefringence Δn of the liquid crystal layer changes (decreases) by applying a voltage, the optimum combination of the thickness d of the liquid crystal layer and the birefringence Δn has not been clarified until now.

If the combination of the thickness d of the liquid crystal layer and the birefringence Δn is not optimal, a large transmittance cannot be obtained, and a high-brightness backlight must be used to realize a high-brightness liquid crystal display device. I couldn't. Further, in the IPS type liquid crystal display device, since an opaque metal film such as aluminum or chromium is used as a comb-shaped electrode, the transmittance is reduced by the electrode area. From this point as well, it is desirable that the transmittance of the liquid crystal layer be as large as possible.

Japanese Patent Application Laid-Open No. Hei 9-15650 discloses a technique that defines the thickness d and the birefringence Δn of the liquid crystal layer. In the liquid crystal display device described in this publication, the thickness d and the birefringence Δn of the liquid crystal layer are set to be 3.0 μm <d <4.5 μm and 0.07 <Δn.
<0.09 is defined independently. If defined independently, the transmission will not be at a maximum.

SUMMARY OF THE INVENTION The present invention provides a liquid crystal display device having high luminance and capable of uniform display by increasing the transmittance during white display and reducing the luminance unevenness on the panel surface.

[0007]

The invention according to claim 1 is
A pair of transparent substrates, a comb-shaped electrode group formed on at least one of the pair of substrates and mainly applying a voltage parallel to the substrates, and a liquid crystal layer sandwiched between the pair of substrates and capable of twisting deformation. In the liquid crystal display device, when the thickness d of the liquid crystal layer is Δn, the birefringence of the liquid crystal in the liquid crystal layer is d = 0.55 / (2 × Δn × r), 0.75 ≦ r ≦ 0.
85.

According to a second aspect of the present invention, in the liquid crystal display device according to the first aspect, the liquid crystal molecules in the liquid crystal layer are parallel to the substrate and have a twist angle of 10 ° or less when no voltage is applied. They are oriented in a certain way.

The invention according to claim 3 is the invention according to claim 1 or 2
In the liquid crystal display device described above, the angle between the alignment direction of the liquid crystal molecules in the liquid crystal layer when no voltage is applied and the long side of the comb-shaped electrode is in the range of 10 to 20 °.

[0010]

FIG. 2 is a perspective view schematically showing the configuration of an IPS type liquid crystal display device according to the present invention.
In the figure, 1 is a liquid crystal layer, 2 is a liquid crystal molecule, 3 and 4 are transparent substrates, and usually a glass substrate is used. In the case of a color liquid crystal display device, among the pair of glass substrates, 3 is an array substrate on which a comb electrode group is formed, and 4 is a color filter substrate. Reference numerals 5 and 6 denote polarizing plates, which are glass substrates 3 and
4 is attached. One of the polarizing plates 5 and 6 has a transmission axis parallel to the alignment direction of the liquid crystal on the glass substrate to be bonded, and the other has a transmission axis relative to the alignment direction of the liquid crystal on the glass substrate to be bonded. It is vertical. When the twist angle of the liquid crystal molecules in the liquid crystal layer is 0 ° when no voltage is applied, that is, when the liquid crystal layer is in a uniaxial alignment state, the pair of polarizing plates is crossed Nicols. Reference numeral 7 denotes a backlight.

FIGS. 3 and 4 are cross-sectional views schematically showing an IPS mode liquid crystal display device according to the present invention. In the figure, 8 and 9 are comb-shaped electrodes, of which 8 is a signal electrode and 9 is a common electrode. Reference numeral 10 denotes a thin film transistor (TFT) as a switching element, 11 denotes an alignment film, and 12 denotes a spacer for controlling the thickness of the liquid crystal layer 1. 3 and 4 show a case where p-type liquid crystal is used.

FIG. 3 shows a pixel portion to which no voltage is applied, and FIG. 4 shows a pixel portion to which a voltage is applied. In a pixel to which a voltage is applied, torsional deformation is induced in the liquid crystal molecules 2. As a result, when a driving voltage is applied, the birefringence Δn _eff becomes Δn _eff when the physical property value of the liquid crystal material is Δn.
n _eff = r · Δn, (r <1). The relationship between the incident light wavelength λ and the transmittance T when the driving voltage is applied is represented by T∝sin ² (π · Δn _eff · d / λ). Here, d is the thickness of the liquid crystal layer 1.

[0013] The present invention, birefringence [Delta] n _d of the liquid crystal layer, the configuration of the physical properties [Delta] n and a liquid crystal display device of a liquid crystal material, i.e., regardless of the electrode width and the electrode spacing of the comb electrodes, the thickness of the liquid crystal layer, the drive voltage applied This is based on the finding that it sometimes changes to a value of 75% to 85% of the physical property value Δn (r = 0.75 to 0.85). That is, the transmittance of the transmission spectrum when the driving voltage is applied to the liquid crystal display device is obtained by calculating the thickness d of the liquid crystal layer 1 and d =
0.55 / (2 × Δn × r), 0.75 ≦ r ≦ 0.85
By setting to the range defined by, the range becomes the range shown by the diagonal line of the curve shown in FIG. 1, and as can be seen from FIG.
When a driving voltage is applied, the range of green light (high visibility) that is most perceived by human eyes (high visibility) (500 nm to 600 n)
In m), the transmittance can be made high, and the transmittance can hardly change due to the change in the thickness d.

[0014]

EXAMPLES Specific examples will be described below. Embodiment 1 FIG. The birefringence Δn is 0.068 (5
89 nm, 20 ° C.) and a dielectric anisotropy Δε of +12.0 were used. The smaller the birefringence Δn, the greater the thickness of the liquid crystal layer. Therefore, in order to shorten the liquid crystal injection time and reduce the unevenness in the thickness of the liquid crystal layer, it is desirable that the birefringence Δn is small. The larger the dielectric anisotropy Δε of the liquid crystal material, the lower the drive voltage.

The thickness d of the liquid crystal layer is 4.8 μm (= 0.5 μm).
5 / (2 × 0.068 × 0.85)) or more and 5.4 μm
(= 0.55 / (2 × 0.068 × 0.75)) or less.

Spacer 1 used for controlling thickness d of liquid crystal layer
For 2, polymer beads were used. In the alignment film 11,
A polyimide film was used. The alignment film was subjected to an alignment process by rubbing. For the rubbing, a rayon cloth is used, and the array substrate side has a length of 15
Performed at an angle of °. The color filter side substrate was rubbed in a direction antiparallel to the rubbing direction on the array substrate side. The transmission spectrum at the time of applying the driving voltage was within the range of the oblique line in FIG. 1, and a high transmittance was obtained. Further, changes in transmittance and chromaticity due to unevenness in the thickness d of the liquid crystal layer could not be visually recognized.

Embodiment 2 FIG. The liquid crystal material has a birefringence Δn of 0.091 (589 nm, 20 ° C.) and a dielectric anisotropy Δε.
Used was +14.0. The thickness d of the liquid crystal layer is
3.6 μm (= 0.55 / (2 × 0.091 × 0.8
5)) or more and 4.0 μm (= 0.55 / (2 × 0.09)
1 × 0.75)). A liquid crystal display device was manufactured in the same manner as in Example 1, except for the liquid crystal material and the thickness d of the liquid crystal layer. The transmission spectrum when the driving voltage was applied was within the range of the oblique line in FIG. 1, and a high transmittance was obtained. Further, changes in transmittance and chromaticity due to unevenness in the thickness d of the liquid crystal layer could not be visually recognized.

Embodiment 3 FIG. The rubbing direction was parallel to the long side direction of the comb-shaped electrode on the array substrate side. The substrate on the color filter side was rubbed antiparallel at an angle of 10 ° with respect to the rubbing direction on the array substrate side. A liquid crystal display device was manufactured in the same manner as in Example 1, except for the rubbing direction.
In this case, rubbing was performed in parallel with the long side direction of the comb-shaped electrode, so that rubbing defects near the comb-shaped electrode could be reduced. As a result, light leakage when no voltage was applied, that is, when displaying black, was reduced. As in the case of the first embodiment, the transmission spectrum when the driving voltage is applied is within the range of the oblique line in FIG. 1, a high transmittance is obtained, and the change in the transmittance and the chromaticity due to the unevenness of the thickness d of the liquid crystal layer is small. I could not see it.

Embodiment 4 FIG. The rubbing direction was performed at an angle of 5 ° on the array substrate side with respect to the long side direction of the comb-shaped electrode. The color filter side substrate was rubbed in a direction antiparallel to the rubbing direction on the array substrate side. A liquid crystal display device was manufactured in the same manner as in Example 1, except for the rubbing direction. The transmission spectrum at the time of applying the driving voltage was within the range of the oblique line in FIG. 1, and a high transmittance was obtained. Further, changes in transmittance and chromaticity due to unevenness in the thickness d of the liquid crystal layer could not be visually recognized. However, there is a pixel in which reverse twist has occurred when driving voltage is applied,
The transmissivity of the generated pixel is reduced.

Embodiment 5 FIG. The rubbing direction was parallel to the long side direction of the comb-shaped electrode on the array substrate side. The color filter side substrate was rubbed antiparallel at an angle of 15 ° with respect to the rubbing direction on the array substrate side. A liquid crystal display device was manufactured in the same manner as in Example 1, except for the rubbing direction. The transmission spectrum at the time of applying the driving voltage was within the range of the oblique line in FIG. 1, and a high transmittance was obtained. Further, changes in transmittance and chromaticity due to unevenness in the thickness d of the liquid crystal layer could not be visually recognized. However, even when no voltage was applied, light leakage occurred due to the birefringence effect, and the contrast was reduced.

The alignment treatment in the above embodiment may be a photo-alignment method or a combination of a rubbing method and a photo-alignment method. As a method for controlling the thickness d of the liquid crystal layer, a spacer other than the polymer beads, for example, an inorganic spacer such as silica may be used.

Hereinafter, comparative examples with the above-mentioned Examples 1 to 5 will be described. Comparative Example 1 The thickness d of the liquid crystal layer is set to 3.0 μm (= 0.55 /
A liquid crystal display device was manufactured in the same manner as in Example 1, except that (2 × 0.091 × 1)). The transmission spectrum when the driving voltage was applied was out of the range of the hatched portion in FIG. 1, and a high transmittance could not be obtained. Further, changes in transmittance and chromaticity due to unevenness in the thickness d of the liquid crystal layer were visually recognized, and a low-quality liquid crystal display device was obtained.

[0023]

According to the first aspect of the present invention, when the thickness d of the liquid crystal layer is represented by Δn where the birefringence of the liquid crystal layer is Δn, d =
0.55 / (2 × Δn × r), 0.75 ≦ r ≦ 0.85
By setting so as to satisfy, when the driving voltage is applied, the transmittance is increased in the range of green light with high visibility,
In addition, there is an effect that the transmittance can be hardly changed by the change in the thickness d.

According to the second aspect of the present invention, the liquid crystal molecules are aligned so that the liquid crystal molecules of the liquid crystal layer are parallel to the substrate and the twist angle when no voltage is applied is 10 ° or less. Light leakage in black display when no voltage is applied can be suppressed. Thereby, there is an effect that display characteristics with high contrast can be obtained.

According to the third aspect of the invention, the angle between the alignment direction of the liquid crystal molecules when no voltage is applied and the long side of the comb-shaped electrode is set to 1
By setting the angle in the range of 0 to 20 °, it is possible to prevent a decrease in transmittance due to the occurrence of reverse twist. Thereby, there is an effect that display characteristics with high luminance can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a transmission spectrum of a liquid crystal display device of the present invention when a driving voltage is applied.

FIG. 2 is a schematic diagram illustrating a configuration of an IPS liquid crystal display device.

FIG. 3 is a cross-sectional view showing a state of a liquid crystal layer of the IPS type liquid crystal display device when no voltage is applied.

FIG. 4 is a cross-sectional view illustrating a state of a liquid crystal layer when a voltage is applied in the IPS mode liquid crystal display device.

[Explanation of symbols]

Reference Signs List 1 liquid crystal layer, 2 liquid crystal molecule, 3, 4 substrate, 5, 6 polarizing plate, 7 backlight, 8, 9 comb-shaped electrode, 10 switching element (TFT), 11 alignment film, 12 spacer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Satoshi Saito 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsui Electric Co., Ltd. (72) Inventor Tetsuyuki Kurata 2-3-2 Marunouchi, Chiyoda-ku, Tokyo 3 Rishi Electric Co., Ltd. F term (reference) 2H092 GA14 JA24 JA29 JA38 JB11 MA55 NA07 NA25 NA27 NA29 NA30 QA18

Claims (3)

[Claims] 1. A pair of transparent substrates, a comb-shaped electrode group formed on at least one of the pair of substrates and mainly for applying a voltage parallel to the substrates, and sandwiched between the pair of substrates to be capable of torsional deformation. Liquid crystal display device having a liquid crystal layer, the thickness d of the liquid crystal layer is as follows: d = 0.55 / (2 × Δn × r), 0.75 ≦ r ≦ 0.
85. A liquid crystal display device satisfying the following. 2. The liquid crystal layer according to claim 1, wherein the liquid crystal molecules in the liquid crystal layer are oriented so as to be parallel to the substrate and have a twist angle of 10 ° or less when no voltage is applied. Liquid crystal display device. 3. An angle between the alignment direction of the liquid crystal molecules in the liquid crystal layer when no voltage is applied and the long side of the comb-shaped electrode is 10 to 20 °.
The liquid crystal display device according to claim 1, wherein: