JPS63301023A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To eliminate unequal color tones and display colors by incorporating a dichromatic dye into a liquid crystal compsn. CONSTITUTION:The dye of the same color as a background color by double refraction, for example, a blue dichromatic dye is added to the liquid crystal compsn. layer which contains a nematic liquid crystal and cholesteric liquid crystal and is disposed between a pair of oriented substrates 11, 12 having electrodes 12, 35 for display. Dyes of an anthraquinone system, azo system, etc., are particularly advantageously used for the dichromatic dye. Absorption by the dye at the time of impressing an electric field to the element is substantially obviated by using such dichromatic dye. The high contrast is thereby maintained and the uneven display is decreased.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a so-called 5TN liquid crystal composition layer arranged between substrates in which liquid crystal molecules in a liquid crystal composition layer have a twist angle in a range of more than 90° and less than 270°. This invention relates to a (super twisted nematic) type liquid crystal display element.

[Prior art]

The liquid crystal display method that has been mainly used in the past is called the TN (twisted nematic) type, in which a liquid crystal layer made of nematic liquid crystal or a mixture of nematic liquid crystal and cholesteric liquid crystal is arranged between a pair of substrates having display electrodes. In this case, the liquid crystal molecules have a structure twisted by about 90 degrees. In this method, polarizing plates are arranged on the outside of each of the upper and lower substrates so that their polarization axes are perpendicular or parallel to each other, and the alignment direction of liquid crystal molecules on the substrate and the polarization axis of the polarizer are perpendicular or parallel to each other. , and utilizes the change in optical rotation of polarized light caused by the electric field. This display method has been sufficient for low time division driving in watches, calculators, etc., but when high time division driving is used to increase display capacity, it has the drawbacks of reduced contrast and narrow viewing angles. there were. This is because the ratio of the voltage applied to one selected point and non-selected point approaches 1 when high time-division driving is performed, and in order to obtain a display element with high contrast and a wide viewing angle, the relative transmittance of the element must be 50%. The voltage Vso changes and the voltage V□ changes by 10%. The ratio of (V
S. /■,. ) It is necessary to make the steepness γ as small as possible.

In the case of twisted nematic type, this γ value is 1.13
That's about it. In order to reduce this γ value, a display method called super twisted nematic (STN) mode has been proposed in recent years.

This method utilizes a change in color tone due to a change in birefringence by setting the twist angle of the liquid crystal molecules to 90° or more and shifting the preferential orientation of the liquid crystal on the substrate from the polarization axis.

With such a configuration, the γ value can be reduced to 1.1 or less, and high time-division driving of about 1/400 duty is possible, but in this case, since birefringence is used, minute There is a drawback that color unevenness occurs due to a change in cell thickness (liquid crystal layer thickness) (0.2 μm or more), and display quality deteriorates. In addition, in this STN type display system, by adjusting the cell thickness, the refractive index anisotropy of the liquid crystal, the orientation angle of liquid crystal molecules on the substrate relative to the polarizing plate, etc., it is possible to create a polarizing plate with a neutral gray tone, for example. When used, it is possible to display black to navy blue on a bright background color such as yellow to green (yellow mode), or display pale yellow on a background color of purple-blue (blue mode off).However, In the case of the conventional STN type system, in principle, it is extremely difficult to produce blue with high color purity, especially in blue mode display.

〔the purpose〕

SUMMARY OF THE INVENTION An object of the present invention is to overcome the drawbacks of conventional STN type liquid crystal display elements and provide a liquid crystal display element with improved single color tone and display color unevenness.

〔composition〕

According to the present invention, a liquid crystal composition layer containing a nematic liquid crystal and a cholesteric liquid crystal is provided between a pair of alignment-treated substrates having display electrodes, and the liquid crystal composition is approximately flat against the substrates when no voltage is applied. A liquid crystal cell arranged horizontally and configured such that liquid crystal molecules have a twist angle in a range of more than 90" and less than 270 degrees between the substrates, and a pair of polarizers arranged to sandwich the liquid crystal cell. Provided is a liquid crystal display element comprising the above liquid crystal composition, wherein the liquid crystal composition contains a dichroic dye.

Next, the present invention will be explained with reference to the drawings.6 Note that the effect of the present invention is particularly remarkable in the blue mode described above, so this will be mainly explained.

FIG. 1 is a sectional view showing an example of the structure of a liquid crystal display element of the present invention. A first substrate 11 having a transparent electrode 13 on which an alignment film 15 has been formed and which has been subjected to an alignment treatment, and a similar second substrate 21 are placed apart and facing each other, and a liquid crystal 33 is placed between them.
is sealed to form a liquid crystal cell. z3 is a transparent electrode, 25 is an alignment film, and 35 is a sealant. This liquid crystal cell 31 is sandwiched between a first polarizer 17 and a second polarizer 27 to constitute a liquid crystal display element 31.

In FIG. 1, a first substrate 11 and a second substrate 21
The liquid crystal molecules are aligned so that they are aligned substantially horizontally, and the liquid crystal molecules are preferentially aligned along the direction of the alignment process. The angle between the preferential alignment direction of liquid crystal molecules on the first substrate 11 and the preferential alignment direction of liquid crystal molecules on the second substrate 21 is as follows:
Range of 90<ω≦270°.

Preferably, 160≦ω≦260°, and the liquid crystal molecules have a helical structure twisted at this angle between the peripheral substrates 11 and 21. In this way, the twist angle ω is determined by the alignment process, and this alignment can be controlled by conventionally known oblique deposition of Sin, etc., or by rubbing with cotton cloth after forming a film of silane coupling agent, polyimide, etc. This can be done by Note that in order to prevent reverse rotation twist, it is preferable to add cholesteric liquid crystal or chiral nematic liquid crystal to the nematic liquid crystal. In this case, if ω is small, the steepness will worsen.

The time division drive characteristics deteriorate. Furthermore, if ω is too large, a scattering structure is generated when an electric field is applied, which deteriorates display quality, which is not preferable. In the present invention, the term "substantially horizontal orientation of liquid crystal molecules" means that the tilt angle of the liquid crystal molecules with respect to the substrate is approximately in the range of 0° to 30°.

In the present invention, the angle formed between the polarization transmission axis of a polarizer and the alignment direction of liquid crystal molecules on a substrate adjacent to the polarizer is +300 to +6, with the twist direction of the liquid crystal from the substrate being positive.
The angle is preferably in the range of 0°, preferably in the range of +35° to +55°. The angle formed between the polarization transmission axis of the other polarizer and the alignment direction of liquid crystal molecules on the substrate adjacent to the polarizer is 60° to 30° in the case of blue mode, with the twisted direction of the liquid crystal from the substrate as positive. in the range of 35°, preferably from 35°
In yellow mode, the range is preferably between -60° and -30°, preferably between -35° and 55°.
It is best to set it to 5°.

In the present invention, in order to obtain a liquid crystal display element with high contrast, the refractive index anisotropy Δn of the liquid crystal and the liquid crystal layer thickness d are as follows:
The color tone due to birefringence in the absence of dichroic dye is selected to be blue, violet, or bluish-violet in the case of blue mode, and yellow-green to yellow in case of yellow mode. The actual color tone is determined by the optical phase difference Δnd and the twist angle ω of the liquid crystal.
is about 0.95, (, when J = 240°, it is preferable to set Δn-d to be about 0.86. However, under such conditions.

In blue mode display, it is difficult in principle to display blue with high color purity. Adding blue pigments to liquid crystals makes it possible to significantly improve the color purity of blue. Furthermore, it was confirmed that the change in hue due to the change in Δn-d was approximately half that of the case where no dye was added, near blue, and that the unevenness in one display could be reduced. However, when this blue dye is added, the blue color of the dye remains when an electric field is applied, resulting in a decrease in contrast. Therefore, in the present invention, in order to overcome such drawbacks, a dye having a similar color to the background color due to birefringence, for example, a blue dichroic dye, is added to the liquid crystal.

The dichroic dye used in the present invention is preferably a blue or blue-green dye having a maximum absorption wavelength in the range of 580 nm to 720 nm in the case of blue mode. Dichroic dyes include those whose light absorption in the long axis direction of the molecule is higher than that in the short axis direction, such as anthraquinone-based, azo-based dyes (Dp type),
Although there are the opposite dyes, for example, tetrazine dyes (Dn type), the former ones are particularly advantageously used in the present invention. By using such a dichroic dye, absorption by the dye is almost eliminated when an electric field is applied, so the drawbacks seen with the non-dichroic dyes are eliminated, and high contrast can be maintained.

The dichroic dyes used in the present invention may be used alone or in the form of a mixture. When used in the form of a mixture 1 For example, a mixture of a blue pigment and a pigment of another color or a mixture of blue pigments, etc. In the case of the former four, the absorbance of the other color pigment is It is necessary to make the absorbance smaller than the absorbance of the dye. In the case of blue mode, the absorption maximum wavelength of the dichroic dye used in the present invention is preferably in the range of 580 nm to 750 nm, and outside this range, color purity is reduced and display quality is degraded. Especially from 600n+m
It is preferred to use a dichroic dye having a maximum absorption wavelength in the range of 700 rrn+. Examples of dichroic dyes preferably used in the present invention include the following.

(R: alkyl!ri(R: alkyl group or (CH2Cl, O)n CH2C
H=CH-R'.

R': alkyl group) (R1, R2, R3 = alkyl group, (R1; alkyl group, hydrogen atom R2; alkyl group, alkoxyalkyl group, alkoxyaryl group, aralkyl group) X: S, O, NH, NR (R :alkyl))(Rl
, R2, R3, R4 = alkyl group, alkoxyalkyl group, cycloalkyl group, aryl group, X, Y: halogen atom, hydrogen atom) (R', R2: alkyl group, arylalkyl group, alkylaryl group, aryl group X:○ or 5) (R: alkyl group, alkoxyalkyl group, phenyl group, substituted phenyl group) (R: alkyl) (X: H or alkyl group) (B, B': alkyl group alkoxy group) R1, R2 ,R
3. Alkyl group) (R: alkyl, alkylaryl group, aryl group) (R1, R'': hydrogen atom, alkyl group 5Y: substituted phenyl group) (R1: alkyl group, hydrogen atom, R: alkyl group, amino group ) (R: alkyl group, aryl group) (R: alkyl group, or alkoxyalkyl group) (RL
, R2, alkylamino group) (R', Rs: alkyl group, R2, R,: halogen, hydrogen atom, cyano group, R1: alkyl group, alkoxy group, phenyl group) (R: alkyl group (R1: alkyl group) , R2: alkyl group, alkoxy group) Y (R: alkyl group, phenyl group, substituted phenyl group,
, Y; NH2, OH.

Z: H, CN) (R1: alkyl group, R2: alkyl carboxyl group, aryl carboxyl group, X: hydrogen, halogen atom (R: alkyl group, alkoxy group).

A: hydrogen or NH,) (R: alkyl group, alkoxy group, X, Y: NH,, OH, hydrogen atom) (R: alkyl group, alkoxy group, X: ○ or 5) (R: alkyl group, (alkoxy group, Another advantage is that it can be done easily. In this case, in order to improve color purity and color unevenness, if the background color is yellow, use a dye that has a maximum absorption wavelength between 390 and 450 nm, and when the background color is yellow-green, use a dye that has maximum absorption wavelength between 610 and 750 nm. Preferably, a dye is used. The above is a case where a polarizing plate with a neutral play color tone is used, but it is also possible to use a colored polarizing plate such as purple or blue.

The liquid crystal display element of the present invention has the advantage that, especially in blue mode display, it can improve color purity, reduce color tone changes due to variations in cell thickness, that is, color unevenness, and have a high contrast ratio of displayed colors when an electric field is applied. has.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 A liquid crystal material having a refractive index anisotropy Δn=0.12 was used, and an anthraquinone dichroic dye having an absorption maximum wavelength of 43 nm (R:nC3H□ in the Nol compound above) was used.
1) was added at 1.5 wt%, the twist angle was 200°,
A liquid crystal cell with a liquid crystal layer thickness of 7.6 μm was manufactured. The alignment treatment was performed by polyimide rubbing treatment. This cell is sandwiched between a pair of polarizing plates arranged at angles such that the liquid crystal orientation direction of the upper and lower substrates and the angle of the polarizing plates are 45° and -45°, respectively (the twisting direction is the positive angle of the polarizing plates). I let it happen. This cell has a chromaticity coordinate of x=0 when no voltage is applied.
17, exhibited a blue color with high color purity of y: 0.12, and became almost achromatic (transparent) upon voltage application. In addition, almost no color unevenness was observed within the cell, even an unevenness in the thickness of the liquid crystal layer of about 0.5 μm. Also, the steepness at this time is 1.07
It shows a good value, and the duty is 1/100 to 1/200.
It was confirmed that high duty time division driving is possible.

Example 2 Using the same liquid crystal system and dye as in Example 1, the twist angles were 160°, 180°, 220', 240', and 2, respectively.
60" cells were prepared. In this case, the cell thickness and polarizing plate angle were adjusted so that the colors were blue, bluish-violet, and violet without adding any dye at each twist angle. These cells were also prepared in the same manner as in Example 1. When the voltage was turned off, it exhibited a blue color with high color purity, and when voltage was applied, it became achromatic (transparent).Comparative Example A cell was produced in the same manner as in Example 1.2 without mixing the dye. Although the cells were almost achromatic and transparent when voltage was applied, they took on a purplish tone when no voltage was applied, impairing the display quality. Even variations in the dust layer caused color irregularities.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the structure of a liquid crystal display element of the present invention. 11.21...Substrate, 13.25...Transparent electrode, 1
5.25...Alignment film, 17, 27...Polarizer, 3
3...LCD.

Claims (1)

[Claims] (1) A liquid crystal composition layer containing nematic liquid crystal and cholesteric liquid crystal is provided between a pair of alignment-treated substrates having display electrodes, and the liquid crystal composition is aligned approximately horizontally with respect to the substrates when no voltage is applied. and a liquid crystal cell configured such that liquid crystal molecules have a twist angle in a range of more than 90° and less than 270° between the substrates, and a pair of polarizers arranged to sandwich the liquid crystal cell. 1. A liquid crystal display element, wherein the liquid crystal composition contains a dichroic dye.