JPH04265948A - Liquid crystal display element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a field effect type liquid crystal display device having excellent time division drive characteristics.

0002

2. Description of the Related Art A liquid crystal display element which has excellent time-division drive characteristics and can be used to display monochrome or color images requires a reverse twist liquid crystal element or a birefringent plastic film (phase plate). In the twisted nematic type, the background or displayed image is yellow or blue due to the interference color caused by the birefringence and optical rotation of the liquid crystal molecules, and the reverse twisted liquid crystal element and birefringent plastic film are It is used to cancel this interference color and realize black and white image display.

However, the use of the reversely twisted liquid crystal element and the birefringent plastic film has caused a problem in that the light transmittance decreases, resulting in a dark display screen.

[0004]Furthermore, liquid crystal elements generally have a slow response speed;
There is a problem that it is not possible to follow the high-speed movement of the displayed image (using the mouse or scrolling the screen). An effective method for improving response speed is to make the liquid crystal layer thinner. However, in order to obtain sufficient light transmittance, the value of Δn・d (where Δn is the anisotropy of the refractive index of the liquid crystal, and d is the thickness of the liquid crystal layer (μm)) must be maintained at a predetermined value. It is necessary to increase the orientation Δn, and this has been difficult to achieve with current liquid crystal materials.

Furthermore, there is also the problem that a crosstalk phenomenon occurs and the contrast deteriorates. This crosstalk phenomenon occurs because non-display dots other than display dots do not completely become non-display when the liquid crystal element is time-divisionally driven using matrix electrodes or segment electrodes.

FIG. 9 shows the threshold characteristics of the drive voltage and light transmittance of display dots and non-display dots when a liquid crystal element is driven in a time division manner. The contrast is the ratio of the light transmittance Ton of the display dots to the light transmittance Toff of the non-display dots, and the optimum driving voltage V at which the contrast is maximized is
c exists. When the steepness of the threshold characteristic is small, the light transmittance Toff of the non-display dot becomes larger than the light transmittance To when the drive voltage is "0", and a complete non-display dot state can be achieved. It disappears. This difference in light transmittance ΔT (ΔT=Toff−To) becomes a crosstalk phenomenon.

[0007] This type of liquid crystal display element is disclosed in US Pat. No. 4,844,569.

[0008]

[Problems to be Solved by the Invention] As described above, conventional liquid crystal elements that have excellent time-division drive characteristics and are used for displaying black-and-white or color images have low light transmittance, resulting in dark display screens, and There was a problem of low contrast due to the talk phenomenon.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a liquid crystal display element capable of displaying bright and high-contrast images.

0010

[Means for Solving the Problems] In order to achieve this object, the present invention provides a spiral structure formed by sandwiching a liquid crystal between a pair of electrode substrates arranged opposite to each other, and a spiral structure formed by sandwiching a liquid crystal between a pair of electrode substrates arranged opposite to each other. In a liquid crystal display element equipped with a pair of polarizing plates arranged, one invention is such that the product of the thickness dμm of the liquid crystal layer and the anisotropy Δn of the refractive index of the liquid crystal is 0.2 μm to 0.35 μm. Another invention is characterized in that the liquid crystal layer has a thickness of 2 μm to 5 μm, and another invention is characterized in that when the relative dielectric constant of the liquid crystal in the long axis direction of the molecules is ε1, and the relative permittivity in the short axis direction of the molecules is ε2. , (2×ε1+ε2)/3 is set to 5 to 15.

[0011]

[Operation] A liquid crystal element with a Δn・d value of 0.2 μm to 0.35 μm transmits zero-order interference color well and has little wavelength dependence, making it possible to display bright black and white images with high contrast. . This means that by using a color filter in combination, it is possible to display a bright color image with high contrast. Further, by setting the thickness of the liquid crystal layer to 2 μm to 5 μm, a response speed that can rapidly follow image movement can be obtained. When the dielectric constant in the long axis direction of the liquid crystal molecules is ε1, and the dielectric constant in the short axis direction of the molecules is ε2, by setting (2×ε1+ε2)/3 to 5 to 15, the crosstalk phenomenon can be reduced and the contrast can be improved. will improve.

0012

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1 FIG. 1 is an exploded perspective view of a liquid crystal display element according to the present invention. The alignment film formed inside the rubbing direction arrow 4,
A pair of electrode substrates 3 and 6 rubbed in the direction 7 are placed facing each other at a predetermined distance by a spacer, and a nematic liquid crystal having positive dielectric anisotropy and added with an optically active substance is sandwiched between them. Ru. The twist direction and twist angle (twist angle) α of the liquid crystal molecules 5 of the nematic liquid crystal are determined by the upper electrode substrate 3.
It is determined by the rubbing direction 4 of the lower electrode substrate 6, the rubbing direction 7 of the lower electrode substrate 6, and the type and amount of the optically active substance added to the liquid crystal. The maximum value of the twist angle α is limited to 360 because the lighting state near the threshold value is an orientation that scatters light.
The upper limit is 180 degrees, and the lower limit is 180 degrees because the minimum value is restricted in order to obtain sufficient contrast. In this example, the twist angle α was set to 260 degrees in order to obtain a liquid crystal display element capable of displaying a black and white image with sufficient contrast using 200 or more scanning lines.

[0014] The polarizing plates 1 and 8 are G1220 manufactured by Nitto Denko.
Using DU (product name), the angle β formed by its absorption axes 2 and 9
The polarizing plates 1 and 8 are arranged on the outside of both electrode substrates 3 and 6 so that the polarizing plates 1 and 8 are substantially perpendicular or parallel to each other. Further, the angle β2 formed between the rubbing direction 7 of the lower electrode substrate 6 and the absorption axis 9 of the lower polarizing plate 8 is 30 degrees to 60 degrees (120 degrees to 150 degrees) considering contrast, brightness, color, etc. desirable. In this embodiment, the rubbing direction 7 of the lower electrode substrate 6 is
and the absorption axis 9 of the lower polarizing plate 8 is 45 degrees, and the rubbing direction 4 of the upper electrode substrate 3 and the absorption axis 2 of the upper polarizing plate 1 are 45 degrees.
The angle β3 formed by this was set to 135 degrees. The light source 10 is a fluorescent lamp (
(cold cathode tube) was used.

The liquid crystal display element having the above structure has a remarkable Δn
・Shows d dependence. FIG. 2 shows the relationship between the value of Δn·d and the light transmittance T, and FIG. 3 shows the value of Δn·d and the CIE color coordinates of the display image color and background color. In order to maximize the light transmittance T, the value of Δn・d should be 0.2 μm to 0.35 μm.
Alternatively, it is desirable to set it between 0.75 μm and 0.9 μm, more preferably between 0.25 μm and 0.3 μm, or between 0.8 μm and 0.85 μm.

FIG. 4 shows the wavelength dependence of transmitted light of the liquid crystal element. Conventional liquid crystal display elements use first-order interference color, so Δn is 0.13 and d is 6.35 μm.
Since Δn·d=0.825 μm, coloring occurs in the STN liquid crystal element alone, as can be seen from the illustrated characteristics. In this example, 0th-order interference color is used, and in order to utilize preferable wavelength-dependent characteristics of 0.2 μm≦Δn・d≦0.3 μm from the viewpoint of contrast, brightness, and color, Δn・d=
0.275 μm was adopted. If Δn・d is smaller than 0.2 μm, the light transmittance will decrease and the display screen will be dark.
If it exceeds 0.5 μm, the contrast will decrease, and if it exceeds 0.5 μm, the displayed image or background will be colored and a black and white image will not be displayed.

Furthermore, as described above, the response speed of a liquid crystal element depends on the thickness of the liquid crystal layer. FIG. 5 shows the relationship between the thickness of the liquid crystal layer and the response speed. In order to respond to image movement when using a mouse, a response speed of 150 ms is required, and in order to achieve this response speed, the thickness of the liquid crystal layer must be 5 μm or less.

By the way, the value of the anisotropy Δn of the refractive index of a liquid crystal is wavelength dependent, and tends to be large on the short wavelength side and small on the super wavelength side. The values used in this example are He
This is a value measured at 25° C. using -Ne laser light (wavelength: 6328 Å). The liquid crystal used was a nematic liquid crystal whose main components were biphenyl liquid crystal and ester cyclohexane liquid crystal, and the optically active substance was Merck's S8.
11 (trade name) was added in an amount of 0.5% by weight.

When using a liquid crystal with Δn=0.10 using Merck's MJ8922 (trade name), the thickness of the liquid crystal layer may be 2.75 μm, and Merck's ZLI
-3901 (product name) Δn using product name = 0.06
When using No. 7 liquid crystal, the thickness of the liquid crystal layer is 4.1μ.
It should be m.

The thickness of the liquid crystal layer is regulated by spacers such as polybeads made of polystyrene or acrylic resin that maintain the distance between the electrode substrates 3 and 6. The dielectric constant of the spacer is desirably 5 or more, and by making it approximately equal to the dielectric constant of the liquid crystal, crosstalk phenomena can be reduced.

[0021] According to this liquid crystal element, an achromatic (black and white) image can be displayed not only by the light source 10 using a cold cathode tube, but also by using external light using a hot cathode tube, electroluminescent tube, or reflector. can do.

Example 2 FIG. 6 shows the dielectric constant of the liquid crystal used in the long axis direction of the molecules as ε1.
, (2×ε
It shows the relationship between the value of 1+ε2)/3 and the crosstalk phenomenon. Referring to the figure, by using a liquid crystal material with a value of (2×ε1+ε2)/3 of 5 to 15, display characteristics with a crosstalk phenomenon of 10% or less can be obtained.
It can be seen that particularly good display characteristics can be obtained between 0 and 13. In this example, the structure of the previous example was followed, and the characteristics of the liquid crystal material used were (2×ε1+ε2)/3=12.

Furthermore, the occurrence of the crosstalk phenomenon is caused by the ratio ε of the dielectric constant εLC of the liquid crystal used and the dielectric constant εO of the alignment film.
Affected by LC/εO. Figure 7 shows crosstalk and εL
It shows the relationship of C/εO, and indicates that the crosstalk phenomenon is small in a range where the value of εLC/εO is 2 or more. By forming the alignment film with Nissan Chemical's LQ1800 (trade name), polyimide, polyamide, PVA, etc. to a thickness of 1000 Å or less and making the dielectric constant 5 or more, display characteristics with less crosstalk phenomenon can be obtained. It will be done.

Example 3 A color liquid crystal display element is obtained by forming color filters on the surfaces of the electrode substrates 3 and 6 of the liquid crystal display element shown in FIG. The color filter can be formed on the surfaces of the electrode substrates 3 and 6 by pigment printing, electrodeposition, or photolithography.

FIG. 8 shows the range of display image colors of the color liquid crystal display element in this embodiment using CIE color coordinates. Color reproducibility is achieved using commercially available TFT (thin film transistor)
It is equivalent to a built-in LCD TV.

Further, a contrast of 10:1 was obtained. Each of the embodiments described above is a liquid crystal element with a twist angle α of 260 degrees, but the value of Δn・d is 0.2 μm to 0.35 μm.
If m, the same effect can be obtained when the twist angle α is in the range of 180 degrees to 360 degrees.

[0027]

As described above, according to the present invention, the product of the thickness dμm of the liquid crystal layer and the anisotropy Δn of the refractive index of the liquid crystal is 0.2
μm to 0.35 μm and the thickness of the liquid crystal layer to 2
μm to 5 μm, or (2×ε1+ε2)/3 is 5 to 15, where the relative permittivity of the liquid crystal in the long axis direction of the molecules is ε1 and the relative permittivity of the liquid crystal in the short axis direction of the molecules is ε2. A liquid crystal display element capable of displaying images with high contrast can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view of a liquid crystal display element according to the present invention.

FIG. 2 is a characteristic curve diagram showing the relationship between the value of Δn·d and the light transmittance T.

FIG. 3 is a characteristic curve diagram showing the value of Δn·d, display image color, and background color using CIE color coordinates.

FIG. 4 is a diagram of a wavelength dependence curve of transmitted light of a liquid crystal element.

FIG. 5 is a graph showing the thickness and response speed of a liquid crystal layer.

FIG. 6 is a characteristic curve diagram showing the relationship between the dielectric constant of the liquid crystal used and the crosstalk phenomenon.

FIG. 7 is a characteristic curve diagram showing the relationship between the ratio of the dielectric constant of the liquid crystal used and the dielectric constant of the alignment film and crosstalk.

FIG. 8 is a characteristic curve diagram showing the range of display image colors of a color liquid crystal display element in another example using CIE color coordinates.

FIG. 9 is a threshold characteristic curve diagram of driving voltage and light transmittance of display dots and non-display dots when a liquid crystal element is driven in a time-division manner.

[Explanation of symbols]

1,8　Polarizing plate 3,6　Electrode substrate 5 Liquid crystal molecules

Claims (9)

[Claims] 1. A helical structure formed by sandwiching a nematic liquid crystal between a pair of electrode substrates disposed opposite to each other, and a pair of polarizing plates disposed sandwiching the helical structure, the thickness of the liquid crystal layer being dμm and the anisotropy Δn of the refractive index of the liquid crystal is 0.2 μm to 0.35 μm, characterized in that the thickness of the liquid crystal layer is 2 μm to 5 μm. . 2. In claim 1, when the relative dielectric constant of the liquid crystal in the direction of the long axis of the molecules is ε1, and the relative permittivity of the liquid crystal in the direction of the short axis of the molecules is ε2, (2×ε1+ε2)/3 is set to 5 to 15. A liquid crystal display element characterized by: 3. A liquid crystal display element comprising: a spiral structure formed by sandwiching a liquid crystal between a pair of electrode substrates disposed opposite to each other; and a pair of polarizing plates disposed sandwiching the spiral structure. When the relative dielectric constant in the long axis direction of the liquid crystal molecules is ε1, and the relative permittivity in the short axis direction of the molecules is ε2, (2×ε1+ε
2) A liquid crystal display element characterized in that /3 is 5 to 15. 4. A liquid crystal display element according to claim 1, wherein the thickness of the alignment film formed on the electrode substrate is 1000 Å or less. 5. In claim 4, when the dielectric constant of the alignment film in the long axis direction of the liquid crystal molecules is ε1, and the dielectric constant in the short axis direction of the molecules is ε2, (2×ε1+ε2)/3 is 5 to 5. 1
5. A liquid crystal display element characterized by: 6. The liquid crystal display element according to claim 1, wherein the ratio of the dielectric constant of the liquid crystal to the dielectric constant of the alignment film formed on the electrode substrate is 2:1 or more. 7. The liquid crystal display element according to claim 1, wherein the spacer for ensuring the distance between the electrode substrates has a dielectric constant of 5 or more. 8. A liquid crystal display element according to claim 7, wherein the spacer is a polybead. 9. According to claim 7 or 8, the ratio of the dielectric constant of the spacer to the dielectric constant of the liquid crystal is 0.8 to 1.
．． 2. A liquid crystal display element characterized by: