JPH07333599A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To obtain a liquid crystal display element which is bright at the time of on and has sufficient shieldability at the time of off as well by adopting specific constitution for a supertwisted nematic element with a compensation film. CONSTITUTION:The twist angle of the liquid crystal layer of the liquid crystal display element consisting of a liquid crystal cell formed by clamping a liquid crystal layer between a pair of substrates with electrodes is specified to 160 to 300 deg.. The dependency alphaon wavelengths of the refractive index anisotropy (DELTAn) of the liquid crystals, designated as DELTAn(450nm)/DELTAn(590mn), is alpha=1.12 to 1.23 and the dependency on wavelengths of the refractive index anisotropy of the compensation film laminated on the liquid crystal cell is alpha=l.05 to 1.15. The angle formed by the absorption axes of two sheets of the polarizing plates of this compensation film is specified to 35 to 75 deg.. There is a possibility that the light shieldability at the time of off is not assured if the dependency alpha on wavelengths of the refractive index anisotropy of the compensation film is smaller than this range. There is a possibility that the brightness at the time of on is imparted when the dependency described above is larger than this range. There is a tendency that the light shieldability at the time of off is low and the contrast is low when the angle formed by the absorption axes of two sheets of the polarizing plates deviates from this range.

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 suitable for a liquid crystal which responds at high speed.

[0002]

2. Description of the Related Art Conventionally, an STN element has been known as a liquid crystal display element suitable for multiplex driving a large number of scanning lines.

Since the STN element utilizes the interference of light, it is colored as it is and the display quality is low.
It is generally practiced to use a compensation film together for black and white. That is, by laminating a film having a birefringence amount substantially equal to the birefringence amount of the liquid crystal layer on the liquid crystal cell, optical compensation is performed and black and white is obtained.

Polycarbonate is often used for this compensation film because it can be mass-produced and the wavelength dispersion of refractive index anisotropy is almost equal to that of liquid crystal.

[0005]

In recent years, there has been an increasing demand for driving STN elements at high speed. There is a TFT as a liquid crystal element that can be driven at high speed, but there is a problem that the process is complicated and the yield does not increase especially when the area is increased. In this respect, the STN element is advantageous in terms of yield because it is not necessary to form a transistor on the substrate.

To increase the speed, it is necessary to reduce the gap of the liquid crystal cell. It is generally known that the response of liquid crystal depends on the viscosity of the liquid crystal and the thickness of the liquid crystal layer, and it is difficult to greatly change the viscosity of the liquid crystal. Therefore, reducing the cell gap is the most convenient and effective method. Is.

On the other hand, in order to secure a sufficient birefringence amount required for STN in a low gap, it is necessary to use a liquid crystal having a high refractive index anisotropy. Since STN is a display utilizing interference due to birefringence of liquid crystal, appropriate birefringence of the liquid crystal layer is required. Since the birefringence of the liquid crystal is defined by the product of the refractive index anisotropy Δn of the liquid crystal and the thickness d of the liquid crystal layer,
This is because the smaller the thickness d is, the more required Δn is. In particular, difluorostilbene-based liquid crystals and tolan-based liquid crystals have a relatively large Δn,
The increase in the viscosity of the liquid crystal is small, and the liquid crystal is suitable for such applications.

However, in general, a liquid crystal having a high refractive index anisotropy tends to have a large wavelength dispersion of the refractive index anisotropy. Therefore, when an ordinary compensation film is used for the STN element, the wavelength dispersion is not balanced between the liquid crystal and the film, and optical compensation tends to be weakened at some wavelengths. This is because when the liquid crystal is used in a negative STN, the light transmittance at the time of off is increased and the contrast ratio is lowered.

In such a case, use of a film such as polysulfone having a large wavelength dispersion can be considered in order to increase the light-shielding rate at the time of off. When polysulfone is used as the compensation film, the amount of wavelength dispersion between the liquid crystal and the film is balanced, so that optical compensation can be achieved over a wide wavelength range.

However, when a film having a large wavelength dispersion is used, the light-shielding rate at the time of off is improved, but conversely, at the time of on, problems such as deterioration of light transmittance and coloring occur. This is because, when the liquid crystal is on, the liquid crystal almost rises and the wavelength dispersibility of the refractive index anisotropy almost disappears, so that the compensation becomes insufficient depending on the wavelength.

The present invention has been made in order to solve the above-mentioned problems, and is intended to realize a liquid crystal display element which is bright when turned on and has a sufficient light-shielding degree when turned off.

[0012]

According to the present invention, in a liquid crystal display device comprising a liquid crystal cell in which a liquid crystal layer is sandwiched between a pair of substrates with electrodes, the liquid crystal layer has a twist angle of 160 to 300 ° and a refractive index of the liquid crystal. The wavelength dependence α of anisotropy (Δn) is Δn
When defined by (450 nm) / Δn (590 nm), α = 1.1
2 to 1.23, and the wavelength dependence of the refractive index anisotropy of the compensation film laminated on the liquid crystal cell is α = 1.05 to 1.1.
The angle formed by the absorption axes of the two polarizing plates is 35 to 7
Provided is a liquid crystal display device characterized by being set at 5 °.

The present invention realizes a liquid crystal display element which is bright in the ON state and has a sufficient light-shielding degree in the OFF state by adopting the above structure in the STN element with the compensation film.

In the present invention, the refractive index anisotropy of liquid crystal (Δ
The range of n) is 0.13 or more, particularly preferably 0.17.
As described above, it is 0.28 or less, particularly preferably 0.24 or less. If it is larger than this, it is necessary to narrow the gap, which makes manufacturing difficult. If it is smaller than this, it is necessary to increase the thickness of the cell, which may make it difficult to increase the speed. The wavelength dependence of the refractive index anisotropy of such a liquid crystal is α = 1.12 to 1.23, preferably α = 1.2, where α is Δn (450 nm) / Δn (590 nm).
It becomes 16-1.21.

In the present invention, the birefringence (Δn · d) range of the compensation film may be such that the coloring of the liquid crystal can be compensated. Outside this range, the compensation may be insufficient and the display may be colored. Specifically 300-600
It is appropriately selected between nm.

1 (a) and 1 (b) show the element design conditions in the present invention. As shown, 11 is the direction of the absorption axis of the polarizing plate on the front surface of the cell, 12 is the direction of the absorption axis of the polarizing plate on the rear surface of the cell, 21 is the direction of the optical axis of the retardation plate on the front surface of the cell, and 22 is the rear surface of the cell. The direction of the optical axis of the retardation plate, 31 is the direction of the rubbing axis of the liquid crystal on the front surface of the cell, and 32 is the direction of the rubbing axis of the liquid crystal on the rear surface of the cell.

The angle between the direction 31 of the rubbing axis of the liquid crystal on the front surface of the cell and the direction 21 of the optical axis of the retardation plate on the front surface of the cell is θ _Ff , and the direction 32 of the rubbing axis of the liquid crystal on the rear surface of the cell and the retardation plate on the rear surface of the cell. The angle formed by the optical axis direction 22 of the cell is θ _Fr , the angle formed by the optical axis direction 22 of the retardation plate on the rear surface of the cell and the absorption axis direction 12 of the polarizing plate on the rear surface of the cell is θ _A , and the retardation plate on the front surface of the cell is The angle between the direction 21 of the optical axis and the direction 11 of the absorption axis of the polarizing plate on the front surface of the cell is defined as θ _P. All angles are measured counterclockwise.

In the present invention, the wavelength dependence α of the refractive index anisotropy of the compensation film is set to about 1.05 to 1.15, preferably about 1.09 to 1.11. If it is smaller than this, there is a possibility that the light-shielding degree at the time of off may not be secured, and if it is larger than this, the brightness at the time of on may be impaired.

In the present invention, the angle between the optical axis of the compensation film and the rubbing direction is as follows. θ _Ff is 70 ° or more, preferably 90 ° or more, and 13
It is 0 ° or less, preferably 110 ° or less. On the other hand, θ _Rf
Is 50 ° or more, preferably 70 ° or more, and 9
It is 0 ° or less, preferably 110 ° or less.

In the present invention, the angle formed by the optical axis of the compensation film and the absorption axis of the polarizing plate is as follows. θ _A
Is 15 ° or more, preferably 30 ° or more, and 6
It is 0 ° or less, preferably 45 ° or less. θ _Ff , θ _Rf ,
If the conditions of θ _A and θ _P deviate from these, optical compensation tends to be weakened.

In the present invention, the angle formed by the two polarizing plates is 35 to 75 °, preferably 45 to 65 °.
If it deviates from this range, the light-shielding degree in the off state tends to be low and the contrast tends to be low.

Further, in the present invention, the twist angle of the liquid crystal is 160 ° or more, preferably 220 ° or more, and 3
The angle is 00 ° or less, preferably 270 ° or less.

The compensating film in the present invention is not necessarily limited to a uniaxial one. For example, the refractive index in the film stretching direction is n _x , the refractive index in the film plane in the direction perpendicular to the stretching direction is n _y , the film is When the refractive index in the thickness direction is n _z, it may be a biaxial compensation film such as a _{n x> n z> n y} . Further, 2 instead of uniaxial compensation film, used by laminating a _{n x> n y = n z} 1 biaxial films and n _z having a relationship of> n _x = n 1 biaxial films having a relationship of _y You can also do it.

Instead of the uniaxial or biaxial film, a polymer liquid crystal film having a twist structure in which the twist direction is opposite to that of the liquid crystal cell can be used in the cell structure of the present invention.

In order to give the refractive index anisotropy a predetermined wavelength dispersion characteristic, the following method is effective.

(1) The wavelength dispersion characteristic of a polymer film is unique to its molecular structure and can be expressed as in Equation 1.

[0027]

## EQU1 ## Δn = A + B / (λ ² −λ ₀ ² ) where A and B are constants specific to the substance, λ is the wavelength of light passing through the polymer, and λ ₀ is the absorption edge wavelength.

When using a compensation film composed of a single polymer material, it is necessary to select a material having desired wavelength dispersion characteristics.

(2) When a desired property cannot be obtained with a single material, a plurality of polymer materials having different wavelength dispersion properties can be blended and used. In this case, it is necessary to combine polymers having good compatibility with each other in order to avoid formation of a micro-layer separation structure.

(3) As another method of using a plurality of materials in combination, the additivity of the phase difference is utilized.
The wavelength dispersion characteristics of refractive index anisotropy can also be controlled by laminating a plurality of retardation films.

Polycarbonate is preferable as the material of the compensation film, and one having the structure of Chemical formula 1 is particularly preferable. However, it is not limited to this.

[0032]

[Chemical 1]

Known structures can be applied to other structures of the STN element in the present invention.

[0034]

Example As a first substrate, a transparent electrode of ITO provided on a glass substrate was patterned in a stripe shape,
A short-circuit preventing insulating film made of SiO ₂ was formed by a vapor deposition method, a polyimide overcoat was spin-coated on the insulating film, and this was rubbed to prepare a substrate on which an orientation control film was formed.

As a second substrate, a transparent electrode of ITO provided on a glass substrate is patterned in a stripe shape so as to be orthogonal to the first substrate, and an insulating film for preventing a short circuit is formed by SiO ₂ by a vapor deposition method. After that, a polyimide overcoat was spin-coated on this, and this was rubbed to prepare a substrate on which an orientation control film was formed.

The periphery of the two substrates was sealed with a sealing material to form a layer for injecting liquid crystal, and nematic liquid crystal having positive dielectric anisotropy was injected into this layer to seal the inlet. .
A plurality of birefringent plates were laminated on one side of this liquid crystal cell, and polarizing plates were arranged on both sides to form a negative type liquid crystal display element.

At this time, the liquid crystal layer was a left helix with a twist of 240 °, and as shown in each example of Table 1, the refractive index anisotropy of the liquid crystal,
The wavelength dependence of the refractive index anisotropy of the compensation film, the optical axis direction of the compensation film arranged on one side of the liquid crystal layer, the long axis direction (rubbing direction) of the liquid crystal molecules of this liquid crystal display element, and the absorption of the polarizing plate. The relative relationship with the axial direction was set.

A backlight with a cold cathode tube is arranged on the back side of the liquid crystal display device, and the duty ratio is 1/240, 1/1.
Table 1 also shows the contrast ratio, the light transmittance when off, and the light transmittance when on when driven with 6 bias.

In Table 1, PC is polycarbonate, P
S represents polysulfone.

FIG. 2 is a graph showing the relationship between voltage and light transmittance under the respective conditions of Example 1 and Example 6 in Table 1. It can be seen that when the polysulfone film is used, the on-luminance tends to be low. In Table 1 and FIG. 2, PC indicates polycarbonate and PS indicates polysulfone.

[0041]

[Table 1]

[0042]

According to the present invention, in a liquid crystal cell using a liquid crystal of high refractive index anisotropy suitable for high-speed driving, a liquid crystal display element which is bright when turned on and has a sufficient light-shielding degree when turned off is realized. it can.

[Brief description of drawings]

1 (a) and 1 (b) are explanatory views showing device design conditions in the present invention.

FIG. 2 is a graph showing the relationship between voltage and light transmittance between the present invention and a comparative example.

[Explanation of symbols]

Claims (2)

[Claims] 1. A liquid crystal display device comprising a liquid crystal cell in which a liquid crystal layer is sandwiched between a pair of substrates with electrodes, wherein the twist angle of the liquid crystal layer is 160 to 300 °, and the refractive index anisotropy of the liquid crystal (Δ
n) wavelength dependence α is Δn (450nm) / Δn (590nm)
, Α = 1.12 to 1.23, and the wavelength dependence of the refractive index anisotropy of the compensation film laminated on the liquid crystal cell is α = 1.05 to 1.15. The liquid crystal display element is characterized in that the angle formed by the absorption axes of the polarizing plates is 35 to 75 °. 2. The liquid crystal display device according to claim 1, wherein the material of the compensation film is polycarbonate.