JPH09269472A - Liquid crystal display element and its drive method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide liquid crystal material property, cell constitution and a drive method realizing high speed responsiveness and high contrast. SOLUTION: By providing a material property value of liquid crystal material as follows, the high speed responsiveness and the high contrast are obtained. By providing the elasticity constant K22 >=12.5pN, the rotation viscosity coefficient 120<=η twist <= 180cpa.s of the liquid crystal material, a response speed nearly 150ms or below is realized. Further, by providing the elasticity constant K22 <=9pN, the rotation viscosity coefficient ηtwist <= 120cpa.s of the liquid crystal material, the response speed nearly 100ms or below is realized. Further, by providing the elasticity constant ratio of the liquid crystal material to (K33 /K11 )>=1.5, the sufficient high contrast is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material property of a liquid crystal, a cell structure, and a driving method for realizing a high-speed response while maintaining a high contrast.

[0002]

2. Description of the Related Art Conventionally, in order to realize a high-speed response, from the viewpoint of the physical properties of liquid crystal materials, the elastic constant ratio of the liquid crystal materials (K ₃₃ / K
₁₁ ) is decreased, the nematic-isotropic phase transition temperature Tni is decreased, the threshold voltage is set high, and the cell thickness d is usually decreased from the aspect of the liquid crystal device configuration. Was common.

When the steepness “γ = V ₅₀ / V ₁₀ ” (where Vx is the voltage at relative transmittance x%, where saturation luminance is 100%) is designed to be small in order to increase the contrast. When the angle (pretilt angle) θp of the conventional alignment film-liquid crystal molecule interface is 4 ° or less, a sufficient alignment margin for production cannot be obtained.

Further, due to the high-speed response, in the normal Alt-Presico driving system, the on-luminance is lowered due to the response phenomenon within the frame, and the display with low liquid crystal panel surface luminance is obtained.

[0005]

The response speed τ and the cell thickness d
The relationship is expressed by the following first equation, where η is the viscosity of the liquid crystal material.

[0006]

[Equation 1]

From this equation, since the response speed is proportional to the square of the cell thickness d, high-speed response can be obtained by reducing the cell thickness when the response speed τadd (sum of rise time and fall time) is 200 ms or more. It was easy to deal with. However, in order to obtain a response speed of 200 ms or less, a cell thickness of around 4 μm is required. With a cell thickness of around 4 μm, the birefringence Δn must be 0.20 or more within the optical compensation design range for obtaining the contrast ratio and the brightness. As a result, the ratio of the liquid crystal component having a long molecular length (particularly the rigid portion) increases, and the viscosity η of the liquid crystal material remarkably increases beyond the effect of reducing the cell thickness d, which makes it difficult to achieve high-speed response.

In addition, a liquid crystal material having low viscosity may be added to the method of reducing the cell thickness. However, the conventional low-viscosity liquid crystal material has a rigid structure in the liquid crystal molecule structure in order to obtain a thinning effect. The birefringence Δn was small in order to give the portion flexibility.

That is, in the STN, the optical phase difference Δn · d is designed to be constant in order to obtain contrast and ON luminance. Therefore, the birefringence Δn needs to be large when the cell thickness d, which is effective for high-speed response, is made small, and in the conventional viscosity reducing material, the effective addition amount is limited and it is difficult to increase the speed.

In order to obtain a high contrast, the elastic constant ratio (K ₃₃ / K ₁₁ ) of the liquid crystal material may be designed to be large.
Since the angle formed at the liquid crystal molecule interface (pretilt angle θp) is 4 ° or less, a sufficient alignment state was not obtained. Further, since the alignment margin is narrow, the selection range of the liquid crystal material is narrow.

Further, in the case where the high-speed response liquid crystal is driven by Alto-Presico, a response within a frame (a high-speed response liquid crystal material whose ON luminance is lowered during the non-selection period because the discharge time of the liquid crystal layer is short with respect to the selection signal period) Phenomenon caused by using
As a result, the on-brightness decreases.

An object of the present invention is to provide a liquid crystal material physical property, a cell structure and a driving method which realize a high speed response and a high contrast.

[0013]

From the first equation, in order to achieve a high-speed response, a high-speed response can be realized by using a liquid crystal material having a large birefringence Δn and a small ηtwist.
As an example, there is a difluorostilbene type liquid crystal material of Seimi Chemical Co., Ltd. having a birefringence Δn of “0.20” or more and an extremely low viscosity, which is used for confirming the physical properties and characteristics of the present invention. The molecular structure is

[0014]

Embedded image

## EQU1 ## Further, by setting the ratio (K ₃₃ / K ₁₁ ) of the elastic constants (bend deformation mode and splay deformation mode), which is one of the physical properties of the liquid crystal material, to be “1.5” or more, the steepness γ can be reduced and high contrast can be achieved. Is possible.

However, the elastic constant ratio (K ₃₃ / K ₁₁ )
When the value is set to “1.5” or more, the pretilt angle θp (angle formed by the alignment film surface and liquid crystal molecules) that is generally used is 4
A sufficient alignment margin cannot be obtained with an alignment film having a temperature of less than or equal to °. By combining an alignment film having a pretilt angle θp of 4 ° or more with a liquid crystal material having the above-mentioned physical property values, a sufficient alignment margin can be obtained even in a liquid crystal material having high contrast.

Regarding the on-luminance, it is possible to raise the luminance which has been attenuated in the frame by raising the frame frequency in the normal Alt-Presico driving. In particular, the degree of in-frame response has a strong correlation with the response speed of the liquid crystal display element, and it is necessary to set the frame frequency at which the rising and falling response times of the liquid crystal molecules are optimal. Further, as another method for suppressing the intra-frame response, the bias ratio, which is the ratio of the selection voltage and the non-selection voltage, is reduced.

According to the structure of the present invention, the physical properties of the response speed and the contrast are regulated, the physical properties of both are regulated, the alignment margin is wider, and the reduction of the on-luminance due to the response within the frame is suppressed. It has the drive structure.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

 (Embodiment 1) First, the dynamic light scattering method used in the present invention is described.
Elastic constant (twist deformation mode) K _{twenty two}And rotational viscosity
Number (twist deformation mode) η twist measurement method
Briefly described.

Light scattering in nematic liquid crystals is described as a superposition of two long wavelength modes (Orsay).
L. C. G. FIG. : Mol. Cryst. and Li
q. Cryst. 13 (1971) 187). Mode 1
Is a combination of splay deformation and bend deformation, and mode 2 is
It is a combination of twist deformation and bend deformation. Mode 2
The relaxation frequency Γ _{2 at} is expressed by the following second equation only by twist deformation.

[0021]

[Equation 2]

Thus, (K ₂₂ : elastic constant (twist deformation mode), γ 1: rotational viscosity coefficient η twist (twist deformation mode), q: scattering vector perpendicular to the director)
expressed. In the applied state of the electric field, the relaxation frequency Γ ₂ is expressed by the following third equation. ε 0: Dielectric constant of vacuum,
εa: dielectric anisotropy, V: applied voltage, d: cell thickness).

[0023]

(Equation 3)

At this time, if εa can be measured independently, Γ ₂
And V ² have a linear relationship, slope: γ 1, y intercept: K
₂₂ can be decided. Next, regarding the method for producing a measurement cell used in the dynamic light scattering method, using a test cell in which homeotropic (vertical) orientation treatment is applied to the electrode surface, the sample liquid crystal has a nematic-isotropic phase transition temperature Tni or higher. After injecting capillaries while heating, the test cell was gradually cooled to room temperature so that the entire surface was vertically aligned.

As the sample liquid crystals used in this experiment, a liquid crystal material A having a normal structure and liquid crystal materials B and C in consideration of lowering the viscosity were prepared. The elastic constant K ₂₂ and rotational viscosity coefficient γ 1 (= ηtwist) measured by the dynamic light scattering method are shown in the following (Table 1).

[0026]

[Table 1]

Next, the test cell used for the optical response evaluation will be described. Two substrates were prepared by forming an indium tin oxide thin film electrode (ITO electrode) on a glass substrate. An aromatic polyamic acid solution (PSI-A-2201 manufactured by Chisso Petrochemical Co., Ltd.) was applied on each substrate by a spin coating method so as to have a film thickness of 800 Å. 80
After pre-curing heat treatment at 15 ° C for 15 minutes, heat curing treatment at 220 ° C for 1 hour was performed to form a polyimide alignment film.

Next, the alignment film surface of each substrate (substrates 1 and 2) was subjected to an alignment treatment by a usual rubbing method so as to have a 250 ° twist structure. Next, 0.1 wt% of glass fiber having a predetermined diameter was mixed with a thermosetting resin outside the ITO electrode pixel of the substrate 1 and screen-printed with a predetermined line width. Further, on the rubbing-treated surface of the substrate 2, plastic beads for ensuring a required gap thickness were sprayed in a required amount. After that, the substrate 1 and the substrate 2 were laminated so that the surface subjected to the rubbing orientation treatment was the inner side, vacuum packed using a heat resistant film, and then heat cured under predetermined conditions. As a result, an empty cell was completed as a test cell condition.

Further, the liquid crystal materials A, B, shown in (Table 1),
C was injected into the above-mentioned test cell using the vacuum injection method,
Test cells A, B and C are completed. Test cells A, B,
The optical response of each C was evaluated by the static drive method. The evaluation of optical response characteristics was performed under the cell constitutional conditions shown in (Table 2) below.

[0030]

[Table 2]

The driving conditions are the on-voltage and off-voltage ratio (Von
/ Voff) = 1.067 (corresponding to 1/240 duty) was used, and the evaluation results are shown in (Table 3) below.

[0032]

[Table 3]

The elastic constant K ₂₂ and the rotational viscosity coefficient η twist of the liquid crystal material A are outside the claims of the present invention, and in this case,
Contrast ratio is "18" but response speed is 180ms
And a little slow.

On the other hand, the elastic constant K ₂₂ and the rotational viscosity coefficient η twist of the liquid crystal material sample B satisfy the claims of the present invention, the contrast ratio is “20” or more and the response speed is 100 ms, which is a satisfactory characteristic. Obtained.

The liquid crystal material C has an elastic constant K ₂₂ of 10
pN or less, but the rotational viscosity coefficient η twist is 100 cPa
・ S is less than or equal to s, but (K ₃₃ / K ₁₁ ) <1, so
It has a high-speed response, but low contrast due to lack of steepness. The contrast ratio is "8", which is quite low.
The response speed was 80 ms, which was quite fast.

From the test cell evaluation using these liquid crystal materials A, B and C, the relationship between the response speed and the contrast is obtained. The relationship between the response speed and the elastic constant K ₂₂ is shown in FIG. When the relationship between the required response speed and the elastic constant K ₂₂ is obtained from the measured data, the elastic constant K ₂₂ needs to be 9 pN or less to make the response speed 100 ms or less. Response speed is 150m
In order to make it s or less, the elastic constant K ₂₂ needs to be 12.5 pN or less.

FIG. 2 shows the relationship between the response speed and the rotational viscosity coefficient ηtwist. Obtaining the relationship between the required response speed and the rotational viscosity coefficient ηtwist from the measured data gives a response speed of 100
The rotational viscosity coefficient η twist is 120 cPa to be less than ms.
・ It must be less than s. To make the response speed 150 ms or less, the rotational viscosity coefficient ηtwist must be 180 cPa · s or less.

The contrast and elastic constant ratio (K ₃₃ /
The relationship of K ₁₁ ) is shown in FIG. In the evaluation using the on-brightness / off-brightness with the black-and-white optical compensation configuration, the contrast ratio, which is sufficient under normal use conditions, is considered to be "18" or more. Therefore, the contrast and elastic constant ratio (K ₃₃ / K ₁₁ ), the elastic constant ratio (K ₃₃ / K ₁₁ ) needs to be “1.5” or more.

From the above results, a liquid crystal material having a high speed response
The design is the elastic constant K_{twenty two}And the rotational viscosity coefficient η twist are important.
Yes, and the elastic constant ratio (K₃₃
/ K ₁₁)is important. These things within the scope of the invention
The desired characteristics can be obtained by specifying the sex value.
I understood.

(Embodiment 2) In addition, test cells A and B
Absolute on-luminance evaluation was performed for each of the above by the Alto-Presico driving method. Driving condition is duty: 1/2
40, 1/300, frame frequency: 74, 160 Hz, bias ratio: optimum value (√ (240) +1 [for 1/240 duty]), 75% of the optimum value are combined and statically driven The results of evaluation by the on-brightness ratio with the on-brightness of No. 1 as "1" are shown in (Table 4) below.

[0041]

[Table 4]

With respect to the frame frequency, when the frequency is set to 160 Hz, the ON luminance is about 1.5 times higher than that at 74 Hz. Moreover, when the bias ratio was set to 75% of the optimum value, about 1.2 times the on-luminance was obtained.

By defining the physical properties of the liquid crystal material of the present invention and the driving conditions of the present invention, a liquid crystal display device having sufficient on-luminance could be realized. (Third Embodiment) As for the alignment margin, the definition of the alignment margin is shown in the following fourth formula.

[0044]

(Equation 4)

Normally, in order to stably produce a liquid crystal display device, the alignment margin (Δd / p) needs to be “0.05” or more. Using the liquid crystal material B having high contrast and high-speed response, the alignment margins of the alignment films D and E having different pretilt angles θp were evaluated. The evaluation results are shown in (Table 5) below.

[0046]

[Table 5]

FIG. 4 shows the relationship between the pretilt angle and the alignment margin. A sufficient alignment margin was obtained when the pretilt angle θp was 4 ° or more. The definition of the pretilt angle of the present invention greatly contributes to a liquid crystal display device aiming at high-speed response and high contrast.

In addition, although a vacuum injection cell is used as the test cell in each of the above embodiments, the present invention is still effective regardless of the cell configuration.

[0049]

As described above, according to the present invention, it is possible to realize a high-speed response and a high contrast by defining the physical property values of the liquid crystal material, and the elastic constant K ₂₂ ≦ 1 of the liquid crystal material.
2.5 pN, rotational viscosity coefficient 120 ≤ η twist ≤ 180 cPa ・ s
According to the standard, a high speed response with a response speed of 150 ms or less can be realized. In addition, the response speed is 100 when the elastic constant K ₂₂ ≦ 9 pN and the rotational viscosity coefficient η twist ≦ 120 cPa · s of the liquid crystal material are specified.
High-speed response of ms or less can be realized. Further, by defining the elastic constant ratio of the liquid crystal material as (K ₃₃ / K ₁₁ ) ≧ 1.5, it is possible to realize a sufficiently high contrast.

[Brief description of drawings]

FIG. 1 is a relationship diagram between response speed τ and elastic constant K ₂₂ .

[Fig. 2] Relationship between response speed τ and rotational viscosity coefficient η twist

[Fig. 3] Relationship between contrast and elastic constant ratio (K ₃₃ / K ₁₁ ).

FIG. 4 is a relationship diagram between a pretilt angle and an alignment margin.

[Explanation of symbols]

 A, B, C Test cell D, E Alignment film with different pretilt angle

Claims (6)

[Claims] 1. A liquid crystal material having an elastic constant (twist deformation mode) K ₂₂ of 12.5 pN or less and a rotational viscosity coefficient (twist deformation mode) η twist of 120 cPa · s or more.
A liquid crystal display element that uses a liquid crystal material that is 180 cPa · s or less. 2. A liquid crystal using a liquid crystal material having an elastic constant (twist deformation mode) K ₂₂ of 9.0 pN or less and a rotational viscosity coefficient (twist deformation mode) η twist of 120 cPa · s or less of physical properties of the liquid crystal material. Display element. 3. The ratio (K ₃₃ / K ₁₁ ) of the elastic constants (bend deformation mode and splay deformation mode) of the physical properties of the liquid crystal material is
The liquid crystal display device according to claim 1, wherein a liquid crystal material having a ratio of "1.5" or more is used. 4. The ratio (K ₃₃ / K ₁₁ ) of the elastic constants (bend deformation mode and splay deformation mode) of the physical properties of the liquid crystal material is
The liquid crystal display device according to claim 2, wherein a liquid crystal material having a ratio of "1.5" or more is used. 5. The angle θp formed by the alignment film surface and liquid crystal molecules is 4
The liquid crystal display element according to claim 3, wherein an alignment film having an angle of at least 0 is used. 6. A bias voltage ratio (selection signal voltage / non-selection) when the liquid crystal display device according to claim 1 is used to perform an Alto-presico drive which is a drive system of a super twist nematic liquid crystal display device. Signal voltage) less than the optimum value (√ (number of scanning lines) +1) 75-90%
Set the drive frequency to 70-200Hz and set the drive frequency to 70-200Hz.