JPH07181495A - Ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To realize a uniform orientation state and to make a contrast high and display characteristic excellent by setting the directions of rubbing treatments parallel with each other between a pair of upper and lower substrates so as to twist within the specific angle from the same direction. CONSTITUTION:This ferroelectric liquid crystal element is constituted by clamping ferroelectric liquid crystals which exhibit two stable states in an oriented state and increase the apparent tilt angle by impressing AC electric fields thereto between upper and lower substrates having electrodes and orientation control films subjected to a rubbing treatment. The directions of the rubbing treatments are so set as to be parallel with each other between the upper and lower substrates and to twist at <=15 pts.wt. from the same direction. Namely, the rubbing direction of the lower substrate is preferably twisted clockwise with the rubbing direction of the upper substrate, when viewed from the upper substrate side if the spontaneous polarization PS of the ferroelectric liquid crystals is negative. The rubbing direction of the lower substrate is preferably twisted counterclockwise with the rubbing direction of the upper substrate when the spontaneous polarization PS is positive.

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, a liquid crystal device used in a liquid crystal optical shutter or the like, and more particularly to a ferroelectric liquid crystal device. More specifically, it improves display characteristics by improving the alignment state of liquid crystal molecules. The present invention relates to an improved liquid crystal device.

[0002]

2. Description of the Related Art A display device of the type in which the amount of transmitted light is controlled by combining it with a polarizer by utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules has been proposed by Clark and Lagerwall. (JP-A-56-107216, U.S. Pat. No. 4,336,792)
No. 4, etc.). This ferroelectric liquid crystal generally exhibits a non-helical chiral smectic C phase (SmC ^* phase) or H phase (SmH ^* phase) in a specific temperature region, and responds to an applied electric field in these phase states. Then, it has one of the first optical stable state and the second optical stable state, and has the property of retaining that state when no electric field is applied, that is, bistability. Further, since the ferroelectric liquid crystal has a characteristic that it responds quickly to changes in the electric field, it is expected to be applied to a large-screen, high-definition display as a memory type display medium driven at high speed.

In order for the optical modulator using this ferroelectric liquid crystal having bistability to exhibit desired driving characteristics,
It is necessary that the liquid crystal arrayed between the pair of parallel substrates is in a molecular alignment state that switches between the two stable states in a stable, efficient and reproducible manner.

On the other hand, in the case of a liquid crystal element utilizing the birefringence of liquid crystal, the light transmittance under the crossed Nicols is I / I ₀ = s
in ² 4θ sin ² (Δndπ / λ), where I ₀ is the incident light intensity, I is the transmitted light intensity, θ is the apparent tilt angle of two stable states, Δn is the refractive index anisotropy, and d Is the thickness of the liquid crystal layer, and λ is the wavelength of the incident light. In the above equation, in order to achieve high contrast, the apparent tilt angle θ, that is, the angle between the average positions in the long-axis direction of each molecule in the two states in the non-helical structure should be close to 22.5 °. is necessary.

It is generally known that the above-mentioned non-helical structure of the ferroelectric liquid crystal has a chevron structure in which the layer structure in the SmC ^* phase is bent. In this structure, the apparent tilt angle θ of the liquid crystal is smaller than the true tilt angle θ of the liquid crystal molecules. Therefore, by applying an AC electric field, the apparent tilt angle θ is changed to the true tilt angle θ. A method of approaching Θ has been proposed (Japanese Patent Laid-Open No. 62-16112).
3).

By this technique, the tilt angle θ in the non-helical structure can be increased, and the alignment of the liquid crystal can be changed from splay alignment to parallel alignment (uniform alignment) having C-directors in substantially the same direction between the upper and lower substrates. Therefore, the transmittance in the dark state under crossed nicols can be reduced, and high contrast display performance can be expected.

[0007]

However, in order to improve the display performance of the device using uniform alignment by applying the above-mentioned AC electric field, it is necessary to improve the uniaxiality of the liquid crystal molecules, and for that reason, the alignment regulation is required. It is necessary to use a strong alignment film material (polyimide or the like). When such an alignment film material is used, a part of the above-mentioned uniform alignment has a splay alignment part, or a splay alignment region appears during driving, and the electro-optical characteristics of the device are significantly deteriorated. This phenomenon becomes a very serious problem especially when gradation display is performed.

[0008]

Means for Solving the Problems and Actions The present invention, which was made to solve the above-mentioned problems, provides an electrode and a rubbing-treated alignment control film between a pair of upper and lower substrates, respectively. In a ferroelectric liquid crystal device having at least two stable states in an aligned state and sandwiching a ferroelectric liquid crystal capable of increasing an apparent tilt angle by applying an alternating electric field, a direction of the rubbing treatment is performed. Are parallel to each other between a pair of upper and lower substrates and twisted by 15 ° or less from the same direction. Specifically, the spontaneous polarization P _{s of the} ferroelectric liquid crystal is If negative, the rubbing direction of the lower substrate to the rubbing direction of the upper substrate when viewed from the upper substrate side is twisted clockwise, when the P _s is positive, versus the rubbing direction of the upper substrate when viewed from the upper substrate side The rubbing direction of the lower substrate is twisted counterclockwise Te is preferred.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 schematically shows an example of the ferroelectric liquid crystal device of the present invention. In the figure, 11a and 11b
Are In ₂ O ₃ and ITO (Indium Tin), respectively.
An alignment control film 13a made of polyimide, polyamide or the like and having an average film thickness of about 3 nm, which is a substrate (glass substrate) covered with transparent electrodes 12a and 12b such as oxide).
And 13b are laminated respectively. Blend control film 13a
And 13b are subjected to rubbing treatment (arrow directions) so that the orientation directions (directions A and A'in FIG. 1) are parallel to each other and slightly offset from the same direction. Board 11
A ferroelectric smectic C liquid crystal 14 is arranged between a and 11b, and the distance between the substrates 11a and 11b is
A distance small enough to suppress the formation of the helical alignment structure of the ferroelectric smectic C liquid crystal 14 (for example, 0.1 to 3).
μm), and the ferroelectric smectic C liquid crystal 14
Results in a bistable orientation state. The sufficiently small distance described above is held by the bead spacers 15 (silica beads, alumina beads, etc.) arranged between the substrates 11a and 11b. Reference numerals 16a and 16b denote polarizing plates placed in crossed Nicols.

In such a structure, when a DC or AC electric field (for example, a rectangular wave of ± 20 V / μm, 10 Hz) is applied, the apparent tilt angle θ _a of the element is larger than that before the electric field is applied. Become. The region in which the apparent tilt angle θ _a is large is uniform in the entire pixel, and a liquid crystal element in which the entire pixel has uniform alignment can be obtained.

FIG. 2 shows a liquid crystal at each position between the upper and lower substrates in the alignment state of the ferroelectric liquid crystal, which can be taken in the alignment in which the apparent tilt angle θ _a is increased by applying the above-mentioned DC or AC electric field. FIG. 3 is a diagram schematically showing a view in which the molecular director is projected onto the bottom surface of the cone and viewed from the bottom surface direction. In general, the ferroelectric smectic liquid crystal in the cell has a chevron structure in which the layers are bent in a V shape in the center of the upper and lower substrates in the as-grown state. It is considered to have a bookshelf structure with six orientations or a pseudo bookshelf structure similar to the bookshelf structure.

In the figure, 31 and 32 are orientations in which the C-directors (arrows in the figure) of liquid crystal molecules are oriented in the same direction and are called uniform orientations. 33, 34, 35 and 36 are states called splay alignment in which the C-directors of liquid crystal molecules are twisted between the upper and lower substrates. Generally, in order to obtain good electro-optical characteristics or display characteristics, it is necessary to uniformly form 31, 32 uniform orientations within a pixel. In order to realize uniform orientation 31 or 32, it is desirable to use polyvinyl alcohol (PVA), polyamide, or the like, which has a relatively weak alignment regulating force between the substrate surface and the liquid crystal molecules. At this time, an AC electric field is particularly applied. Then, when a technique of increasing the apparent tilt angle θ _a is used, the uniformity within the pixel, that is, the uniaxial orientation is likely to deteriorate. On the other hand, if an alignment material such as polyimide, which has a strong alignment regulating force, is used,
Although the uniaxial orientation is improved, a region in the splay orientation state appears. This splay alignment state increases the transmittance in the dark state compared to the uniform alignment, and
The switching characteristics between the two dark states are deteriorated.

Therefore, in the present invention, as shown in FIG. 3, the rubbing directions of the upper substrate and the lower substrate are offset from each other by θ _{cross when} viewed from the direction normal to the substrate plane, whereby excellent uniaxial orientation is achieved. Even when using an alignment material such as polyimide having a strong alignment control force as an alignment control film, when an apparent tilt angle θ _a is increased by applying an AC electric field, the splay alignment state as described above does not appear, It is possible to realize a uniform alignment state having excellent electro-optical characteristics and display characteristics.

In the present invention, the above-mentioned rubbing cross angle θ _cross cannot be unequivocally determined because the optimum angle varies depending on the alignment control force of the alignment control film, the magnitude of the spontaneous polarization P _s of the ferroelectric liquid crystal, and the like. Is usually 15 ° or less and P _s <0
3A, the rubbing direction of the lower substrate may be shifted clockwise with respect to the rubbing direction of the upper substrate as shown in FIG. 3A. When P _s > 0, It is preferable to shift the rubbing direction of the lower substrate counterclockwise with respect to the rubbing direction of the upper substrate as viewed from the upper substrate side as in b). As a result, the ferroelectric liquid crystal element has excellent electro-optical characteristics and display characteristics.

[0016]

EXAMPLES Examples of the present invention and comparative examples will be described below.

Example 1 First, an ITO film having a thickness of about 150 nm was formed on two glass substrates by a sputtering method, and 0.4% N of polyamic acid (Semicofine SP710) manufactured by Toray Industries, Inc. was formed thereon.
Apply the MP / NBC (1: 1) mixed solution with a spinner,
It was baked at 300 ° C. for 1 hour to form a polyimide film having an average film thickness of 3 nm. Next, the two substrates were rubbed under the following conditions (roller pressing amount: 0.4 mm, roller rotation speed:
16.7 times per second, roller feed speed: 12 mm / se
c, roller radius: 100 mm), and when viewed from the upper substrate side, with respect to the rubbing direction of the upper substrate, the rubbing direction of the lower substrate is twisted clockwise by 2 °, and the rubbing direction is 1.5 μm. A liquid crystal cell was produced by bonding with a gap maintained. The cell is a multi-component mixed liquid crystal containing a phenylbenzoate liquid crystal as a main component and has a tilt angle of 30 ° C.
A 2.2 ° ferroelectric liquid crystal was injected. The phase transition sequence of the liquid crystal was as follows, and P _s was -24 nC / cm ² .

[0018]

[Equation 1]

The liquid crystal cell of this embodiment after the above liquid crystal injection is
During the slow cooling process from the isotropic phase to the SmC ^* phase, an orientation state having a chevron structure was obtained, and an apparent tilt angle was 8.5 °.

Next, the above cell was subjected to ± 20 V / μ at 30 ° C.
When an alternating electric field of m, 10 Hz was applied for 1 min to change the layer structure, a stripe-shaped texture was formed through the roof-shaped texture, the apparent tilt angle expanded to 19.5 °, and the spray was spread over the entire pixel area. No regions of orientation appeared. Further, it was possible to suppress the appearance of splay alignment regions during display driving, and it was a device having high contrast and excellent display characteristics.

Comparative Example 1 A liquid crystal cell was produced in the same manner as in Example 1 except that the rubbing directions of the upper and lower substrates were the same.

When the ferroelectric liquid crystal used in Example 1 was injected into this liquid crystal cell and the same AC electric field application process as in Example 1 was performed, a splay alignment region remained in a part of the pixel and an apparent tilt was observed. The corners were smaller than in Example 1.

Further, a splay alignment area appeared during display driving, and the contrast ratio was lowered.

Comparative Example 2 The same as Example 1 except that the rubbing direction of the lower substrate was twisted counterclockwise by 2 ° with respect to the rubbing direction of the upper substrate as seen from the upper substrate side. Then, a liquid crystal cell was produced.

When the ferroelectric liquid crystal used in Example 1 was injected into this liquid crystal cell and the same AC electric field application process as in Example 1 was performed, a splay alignment region remained in a part of the pixel and an apparent tilt was observed. The corners were smaller than in Example 1.

Further, the splay alignment region appeared during display driving, and the contrast ratio was lowered.

Embodiment 2 In this embodiment, when viewed from the upper substrate side, the rubbing direction of the lower substrate is 2 ° counterclockwise with respect to the rubbing direction of the upper substrate.
A liquid crystal cell was produced in the same manner as in Example 1 except that the twisted direction was adopted.

Ferroelectric liquid crystal with P _s > 0 was injected into this liquid crystal cell.

The liquid crystal cell of this embodiment after the above liquid crystal injection is
During the slow cooling process from the isotropic phase to the SmC ^* phase, an oriented state having a chevron structure was obtained, and an apparent tilt angle was 9.0 °.

Next, the above cell was subjected to ± 20 V / μ at 30 ° C.
When an alternating electric field of m, 10 Hz was applied for 1 min to change the layer structure, a stripe-shaped texture was formed through the roof-shaped texture, and the apparent tilt angle expanded to 19.0 °, and the spray was spread over the entire pixel area. No regions of orientation appeared. Further, similarly to Example 1, the appearance of the splay alignment region during display driving could be suppressed, and the display characteristics were high and the display characteristics were excellent.

[0031]

As described above, according to the present invention,
Even if an alignment control film with excellent uniaxial alignment property and strong alignment control force is used, when a treatment to increase the apparent tilt angle by applying an AC electric field, a splay alignment state does not appear, and a uniform alignment state is obtained. Since it can be realized, it becomes a liquid crystal element with high contrast and excellent display characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an example of a ferroelectric liquid crystal device of the present invention.

FIG. 2 is a diagram for explaining an alignment state of smectic liquid crystals.

FIG. 3 is a diagram for explaining a relationship between rubbing processing directions between upper and lower substrates in the present invention.

[Explanation of symbols]

 11a, 11b Glass substrate 12a, 12b Transparent electrode 13a, 13b Alignment control film 14 Ferroelectric smectic C liquid crystal 15 Spacer 16a, 16b Polarizing plate

Claims (2)

[Claims] 1. At least two stable states in an alignment state are shown between a pair of upper and lower substrates each provided with an electrode and an alignment control film that has been subjected to a rubbing treatment, and an apparent electric field is obtained by applying an AC electric field. In a ferroelectric liquid crystal element sandwiching a ferroelectric liquid crystal capable of increasing a tilt angle, the rubbing directions are parallel to each other between a pair of upper and lower substrates and twisted by 15 ° or less from the same direction. A ferroelectric liquid crystal device characterized by the above. 2. When the spontaneous polarization P _s of the ferroelectric liquid crystal is negative, the rubbing direction of the lower substrate is twisted clockwise with respect to the rubbing direction of the upper substrate as viewed from the upper substrate side, and the P _s is positive. In the case, the ferroelectric liquid crystal element according to claim 1, wherein the rubbing direction of the lower substrate is twisted counterclockwise with respect to the rubbing direction of the upper substrate when viewed from the upper substrate side.