JPH03243920A - Liquid crystal electrooptical device - Google Patents

Abstract

PURPOSE:To simultaneously obtain a high contrast ratio and a high transmittance by allowing two uniform states and at least one twist state to exist stably and maintaining orientation in a twist state at times exclusive of use. CONSTITUTION:The projections of the major axes n1 17, n1 18 of the liquid crystal molecules in contact with two sheets of the substrates 12 in the chiral nematic phase or smectic phase of a ferroelectric liquid crystal 16 are parallel with each other. The major axes n1 17, n1 18 of the liquid crystal molecules respectively form angles thetaD1 19, thetaD2 20 (=0) with the substrate surfaces. Two sheets of the substrates are so combined that the major axes n1 17, n1 18 of the liquid crystal molecules form angle alpha=¦thetaD1¦+¦thetaD2¦ with each other. The two uniform states and at least one twist state are, therefore, made to exist stably and the orientation is kept in the twist state at times exclusive of the use. The liquid crystal electrooptical device with which the high contrast ratio and the high transmittance are simultaneously obtd. over a long period of time and the reliability is high is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal electro-optical device, and more particularly to a ferroelectric liquid crystal electro-optical device.

[Prior Art] As is well known, ferroelectric liquid crystals have spontaneous polarization. The relationship between the molecular axis n (21) and the spontaneous polarization P (22) is orthogonal to each other as shown in FIG.

n is always tilted by θ with respect to the Z axis, and P and the xy plane are parallel. The projection of n onto the xy plane is represented by C(23), and the angle (azimuth angle) between C and the X axis is represented by φ. The substrate surface that supports the liquid crystal layer is in the xz plane, and the angle θ formed between n and the XZ plane. (
The pretilt is determined by the interaction between the liquid crystal molecules and the substrate surface (alignment film). If horizontal alignment treatment is applied, θ, doo'
It is.

If no external field is applied, φ increases or decreases with 2 at a sufficient distance from the substrate surface, forming a helical structure. However, if the liquid crystal layer becomes thinner to a certain extent, the helix will unravel due to the anchoring effect of the substrate supporting the liquid crystal layer, resulting in a twisted state as shown in FIGS. 3(a) and 3(b). However, here, the upper and lower boards (12)
Both have the property of attracting the negative side of P, and the smectic layer is assumed to be perpendicular to the substrate surface (XZ plane). If the thickness of the liquid crystal layer is further reduced, the elastic energy in the twisted state increases, as shown in FIG. 3(c).

A uniform state as shown in (d) is obtained.

Generally, in a liquid crystal electro-optical device under certain conditions, only two states can stably exist in the absence of an electric field, resulting in bistability (memory effect). The two states are two twisted states or two uniform states. Teswitching between two twist states or between two uniform state quantities can then be performed.

[Problems to be Solved by the Invention] By utilizing switching between two uniform states, a complete extinction position can be obtained, so that both high contrast ratio and high transmittance can be achieved.

However, if left in the on (off) state for several days, the off (on) state becomes more unstable than the on (off) state, and bistability often disappears, making it impossible to display normally.

The uneven distribution of impurity ions in the liquid crystal is thought to be the cause of the loss of bistability. Figure 4 shows the model.

Here, in the initial state (Fig. 4(a)), positive ions (41
) and negative ions (42) are randomly distributed, and the azimuth angle of all liquid crystal molecules is assumed to be φ = 0 (all P points upward and in the OFF state). However, it is assumed that the liquid crystal layer and orientation At the upper and lower interfaces with the membrane, there is a density P (C7
Since a positive surface charge (43) and a negative surface charge (44) of m) are generated, a downward internal electric field E+ (45) is generated inside the liquid crystal as shown. Therefore, if left in this state for a long time, the internal electric field will cause the positive (negative) ions to move downward (upward), causing the lower (upper) substrate and the liquid crystal to move. Accumulates at the interface with the layer. Thereafter, as shown in FIG. 4(C), the on state (P
is downward), the internal electric field E2 (>El:
If the amount of ions is sufficient, E2=2EI) will occur, so the energy of the on-state will be higher than that of the initial off-state. If the internal electric field is large enough, the on state will be reversed to the off state, and bistability will be lost, making display impossible.

On the other hand, when two twisted states are used, as shown in Figure 5, the signs of the surface charges appearing at the upper and lower interfaces are the same, so unlike the uniform state, ions with different signs are biased at the upper and lower interfaces. Never. Therefore, ions have little effect on bistability, but since a complete extinction position cannot be obtained in the twisted state, it is not possible to achieve both high contrast ratio and high transmittance.

The present invention solves the above problems and provides a highly reliable liquid crystal electro-optical device that can simultaneously obtain high contrast ratio and high transmittance over a long period of time.

[Means for Solving the Problems] The present invention provides an element in which a ferroelectric liquid crystal is sandwiched between an electrode and two substrates A1 and A2 on which an alignment film is formed, which is formed by an oblique evaporation method. An inorganic film or a polymer film subjected to rubbing treatment is used as the alignment film, and the long axes of liquid crystal molecules in contact with the two substrates A1 and A2 in the chiral nematic phase or smectic A phase of the ferroelectric liquid crystal the substrate surface A1 of n1, n2;
The projections onto A2 are substantially parallel to each other, and the long axes na and n2 of the liquid crystal molecules are aligned with the substrate surfaces A1 and A2, respectively.
Forms an angle θl, 6.2 (≠0) with A2,
And, the long axes n1 and n2 of the liquid crystal molecules form an angle α=I with respect to each other.
θ. By combining the two substrates so as to form 11+lθ, 21, two uniform states and at least one twisted state are made to stably exist, and the orientation is kept in the twisted state except when in use. .

[Example] (Example 1) A schematic cross-sectional view of a liquid crystal electro-optical device used in the present invention is shown in FIG. 11 is a polarizing plate, 12 is a glass substrate, 13 is a transparent electrode, 14 is an insulating layer (SiO2), 15 is a columnar alignment film (5 inches), 6 is a liquid crystal layer, 17.18 is in contact with a glass substrate The long axis direction (n1, n2) of the liquid crystal molecule in the smectic A phase, 19.20, is the angle θ that n1 and n2 make with the glass substrate in contact with each other. hθ. 2
It is. The thickness of the insulating layer is 500, and the thickness of the SiO film is 1.
000 people.

The SiO film was formed by oblique deposition at an angle of 85 degrees to the normal line of the substrate, so it had a column shape as shown. Also, regarding the column inclination direction,
As shown in the figure, the upper and lower substrates were stacked so that they were both in the negative direction of the Z axis. The liquid crystal layer thickness is 2 μm.

A ferroelectric liquid crystal (CS-1011 manufactured by Chisso Corporation) was sealed in the liquid crystal electro-optical device thus prepared, and the temperature was lowered from the isotropic phase temperature to the smectic A phase at a rate of 2°C/min. Obtained orientation.

The orientation state in the smectic A phase can be determined from the inclination angle of the smectic layer determined by X-ray diffraction in the -Z direction.
Quasi-ni tibae, + ”= 32”, e −a
"=-32", and as shown in the figure, it is estimated that n1 and n2 form an angle of 64° (=lθ, 11+1θ.21) with each other.

Figure 6 shows the threshold characteristics at 25°C. For this measurement, the voltage waveform shown in FIG. 7 was applied, and the changes in Stingray and ■ with respect to Vfl were measured. The transmittance was based on the intensity of light transmitted through the polarizing plate. Transmittance is 0% and 90
% status is shown in Figure 3(c).

In the uniform state shown in (d), the transmittance is 25.
% state is the twisted state shown in FIG. 3(a) or (b). As shown in Figure 6, if V8 is made high enough, Im
: Ip, and almost complete memory effect was obtained.

Also, v[! Any of these three states can be selected by adjusting .

This liquid crystal electro-optical device was driven at a duty ratio of 1/1000 using the multiplex drive waveform shown in FIG. V1::4v, V2:8v, V3
= IOV.

By switching between the two uniform states with V4 = -18v, the on transmission is 80%.
, a contrast ratio of 1:20 was obtained.

Further, when the device was left in the twisted state for 6 months and then driven again under the same conditions, no deterioration in display characteristics was observed. As a comparative example, after being left in a uniform state (off state) with a transmittance of 0% for a week, we drove it again under the same conditions.The transmittance in the on state was 60% at a duty ratio of 1/200, and 60% at a duty ratio of 1/200. At 400, it was 30%. This is because, as already mentioned, the stability of the on-state decreases due to the influence of ions, and the transmittance of the on-state to the pulse voltage decreases over time as shown in Figure 9, making the memory effect incomplete. be. Once such a situation occurs, the initial characteristics cannot be reproduced even if the driving voltage is changed.

(Example 2) In this example, TiO2 with a thickness of 500A was used as the alignment film. The deposition angle and the like are the same as in Example 1. In this case as well, two uniform states and one twisted state were obtained, and almost the same threshold characteristics as in FIG. 6 were obtained.

This liquid crystal electro-optical device was driven at a duty ratio of 171,000 using the multiplex drive waveform shown in FIG. V+: 4.5v, V2: 9v, V3:
By setting 10V and V t = -19V, an on-state transmittance of 80% and a contrast ratio of 1:18 were obtained.

Further, when the device was left in the twisted state for 6 months and then driven again under the same conditions, no deterioration in display characteristics was observed. As a comparative example, after being left in a uniform state (off state) with a transmittance of 0% for - weeks, we drove it again under the same conditions.The on transmittance was 5 at a duty ratio of 1/200.
5%, and at a duty ratio of 1/400, it became 30%. Further attempts were made to reproduce the initial characteristics by changing the driving voltage, but there was little improvement.

(Example 3) In this example, SiO2 with a thickness of 1500 mm was used as the alignment film.The evaporation angle and the like were the same as in Example 1.

In this case as well, two uniform states and one twisted state were obtained, and almost the same threshold preferential properties as in FIG. 6 were obtained.

This liquid crystal electro-optical device was driven at a duty ratio of 171,000 using the multiplex drive waveform shown in FIG. V+= 4v, V2:10V, V3=
By setting IOV and Va to -20v, an on-state transmittance of 80% and a contrast ratio of 1:16 were obtained.

Further, when the device was left in the twisted state for 6 months and then driven again under the same conditions, no deterioration in display characteristics was observed. As a comparative example, after being left in a uniform state (off state) with a transmittance of 40% for a week, it was driven again under the same conditions.The on transmittance was 55% at a duty ratio of 1/200, and 1/400 at a duty ratio of 1/400. The result was 20%.Although attempts were made to reproduce the initial characteristics by changing the driving voltage, there was almost no improvement.

(Example 4) In this example, polyimide having a thickness of 900 mm and made by a spin code method was used as the alignment film. After applying rubbing treatment and enclosing Chisso CS-1011,
In smectic A phase θ, I"F15@, θ, 2"
The knee was 15". In this case as well, two uniform states and one twisted state were obtained, and almost the same threshold preferential properties as in FIG. 6 were obtained.

This liquid crystal electro-optical device was driven at a duty ratio of 171,000 using the multiplex drive waveform shown in FIG. V+=4.5v, ■2=9v,
By setting V3=10v and Va=19v, an on-state transmittance of 80% and a contrast ratio of 1:18 were obtained.

Further, when the device was left in the twisted state for 6 months and then driven again under the same conditions, no deterioration in display characteristics was observed. As a comparative example, after being left in a uniform state (off state) with a transmittance of 0% for - weeks, we drove it again under the same conditions.The on transmittance was 5 at a duty ratio of 1/200.
5%, and at a duty ratio of 1/400, it became 30%. Further attempts were made to reproduce the initial characteristics by changing the driving voltage, but there was little improvement.

(Example 5) In this example, the alignment film was made of polyimide/
- A basic chromium complex mixed film was used. In this case, in smectic physiognomy, θ*+';25@, θ, 2-25
@Met. Here again, two uniform states and one twisted state were obtained, and almost the same threshold preferential properties as in FIG. 6 were obtained.

This liquid crystal electro-optical device was driven at a duty ratio of 1/1000 using the multiplex drive waveform shown in FIG. V+=4N/S V2=8v, Vs=10v.

By switching between two uniform states with Va=-18v, the on-transmittance is 80%.
, a contrast ratio of 1:19 was obtained.

Furthermore, when the device was left in the twisted state for 6 months and then driven again under the same conditions, no deterioration was observed in the display characteristics. As a comparative example, Transparent 1! After being left in the uniform state (off state) at 0% for a week, we drove it again under the same conditions, and the transmittance in the on state was 1/200 of the duty ratio.
When the duty ratio was 1/400, it was 55%, and 20% when the duty ratio was 1/400. Further attempts were made to reproduce the initial characteristics by changing the driving voltage, but there was little improvement.

Although polyimide and a polyimide/basic chromium complex mixed film were used as the organic alignment film, the material thereof is not limited. Furthermore, the ferroelectric liquid crystal material is not limited either.

[Effect of the invention] As described above, by stably existing two uniform states and at least one twisted state, and keeping the orientation in the twisted state except during use, a high contrast ratio and high transmittance can be achieved at the same time. Furthermore, since the influence of the internal electric field generated by biased ions during storage is reduced, bistability does not disappear.

The present invention relates to liquid crystal displays, liquid crystal printer heads,
It can be applied to spatial light modulators, etc.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a liquid crystal electro-optical device according to the present invention, FIG. 2 is a diagram showing the relationship between the molecular axis and spontaneous polarization of a ferroelectric liquid crystal, and FIG. 3 is a diagram showing a twisted state and a uniform state. Figure 4 shows the distribution of ions when held in a uniform state, Figure 5 shows the distribution of ions when held in a twisted state, and Figure 6 shows the distribution of ions obtained in the liquid crystal electro-optical device of the present invention. FIG. 7 is a diagram showing the applied voltage waveform for determining the threshold preferential property.
FIG. 8 is a diagram showing the applied voltage waveform for multiplex driving, and FIG. 9 is a diagram showing the memory effect after long-term storage in a uniform state. 11...Polarizing plate, 12...Glass substrate, 13...Transparent electrode, 14...Insulating film, 15...Alignment film, 16...Liquid crystal layer, 17.18...Liquid crystal molecules in contact with the glass substrate Long axis direction in smectic A phase (B1, B2),
19.20...B+. The angle θ that B2 makes with the glass substrate that it is in contact with,
1. θ, 2.21...Liquid crystal molecular axis n, 22...Spontaneous polarization P, 23...Projection of n onto the xy plane C141...Positive ions, 42...Negative ions, 43...Positive surface charge, 44・・・
Negative surface charge, 45... internal electric field. that's all

Claims (5)

[Claims] (1) In an element in which a ferroelectric liquid crystal is sandwiched between an electrode and two substrates A_1 and A_2 on which alignment films are formed, two uniform states and at least one twisted state stably exist. A liquid crystal electro-optical device featuring: (2) The liquid crystal electro-optical device according to claim 1, wherein the orientation state of the ferroelectric liquid crystal is kept in the twisted state except when in use. (3) The liquid crystal electro-optical device according to claim 1, wherein the alignment film is an inorganic film formed by oblique evaporation. (4) The liquid crystal electro-optical device according to claim 1, wherein the alignment film is made of a polymer film that has been subjected to a rubbing treatment. (5) In the chiral nematic phase or smectic A phase of the ferroelectric liquid crystal, the two substrates A_1, A
The projections of the liquid crystal molecule long axes n_1 and n_2 in contact with the substrate surfaces A_1 and A_2 are substantially parallel to each other,
Further, the long axes n_1 and n_2 of the liquid crystal molecules are at angles θ_D_1 and θ_D with the substrate surfaces A_1 and A_2, respectively.
_2 (≠0), and the long axis n of the liquid crystal molecules
_1 and n_2 are at an angle α=|θ_D_1|+|θ_
The liquid crystal electro-optical device according to claim 1, wherein the liquid crystal electro-optical device has a shape D_2|.