JPH04140723A - Liquid crystal electrooptical device - Google Patents

Abstract

PURPOSE:To attain the good displaying or driving characteristics of the liquid crystal electrooptical device by putting the initial orientation characteristics of a mixed liquid crystal material exhibiting a ferroelectric property into a multimicrodomain state in which many microdomains exist. CONSTITUTION:Electrodes 3, 3' for driving liquid crystals are so patterned and formed on transparent substrates (for example, glass) 2, 2' as to be formed in the form of a matrix in a line direction and a row direction. Orientation controls 4, 4' are provided on these electrodes and the one side thereof is subjected to a known rubbing treatment that the liquid crystal molecules are arrayed. The mixed liquid crystal material 5 which exhibits the ferroelectric property forms the microdomain orientation. The phase series of the mixed liquid crystal material 5 forms the direct smectic phase without having a nematic phase next to an isotropic phase.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to liquid crystal electro-optical devices such as liquid crystal display elements and liquid crystal optical shutter arrays. The present invention relates to a liquid crystal electro-optical device with improved driving characteristics.

[Conventional technology]

As a conventional liquid crystal electro-optical device, one using twisted nematic liquid crystal is known. This TN liquid crystal has a problem in that crosstalk occurs during time-division driving using a matrix electrode structure with high pixel density, so the number of pixels is limited.

Also, is there an active matrix type display element that connects a switching element using a thin film transistor to each pixel and switches each pixel individually?The process of forming a thin film transistor on a substrate is extremely complicated, and the manufacturing cost is low. Due to these factors, it is difficult to create a display element with a large area.

To solve these problems, Clark proposed a ferroelectric liquid crystal device in US Pat. No. 4,367,924.

FIG. 4 schematically depicts an example of a liquid crystal cell for explaining the operation of a ferroelectric liquid crystal. 11 and 11゛ are I
n 20 or ITO (Indium-Tin
A substrate (glass plate) coated with a transparent electrode made of a thin film such as -Oxide), between which a liquid crystal molecular layer 12 is oriented perpendicularly to the glass surface and exhibits SmC'' phase or other ferroelectric properties. In this liquid crystal electro-optical device, the liquid crystal molecules exhibiting ferroelectricity are encapsulated in a liquid crystal phase.
The tilted first state N) and the −〇 tilted second state (I[)
I take the.

Display was performed by applying an external electric field to these two state quantities and switching the ferroelectric liquid crystal molecules, thereby making use of the difference in the birefringence effect generated.

At this time, in order to change the ferroelectric liquid crystal molecules from the first state (I) to the second state (II), for example, a positive electric field is applied in a direction perpendicular to the smectic layer. On the other hand, in order to reverse the second state (C) to the first state (I), a negative electric field is applied.

That is, by changing the direction of an externally applied electric field, the two-dimensional structure of the ferroelectric liquid crystal molecules is changed, and the resulting difference in electro-optical effects is utilized.

Furthermore, even when this externally applied electric field was removed, the ferroelectric liquid crystal molecules maintained their state stably and had the first and second bistable memory properties.

Therefore, the signal waveform that drives a liquid crystal electro-optical device using this ferroelectric liquid crystal is a bipolar pulse train, and the direction in which the pulse polarity switches switches between the two states of the ferroelectric liquid crystal molecules. was.

[Problem that the invention attempts to solve]

In an electro-optical device using such a liquid crystal exhibiting ferroelectricity, uniform drive characteristics are naturally required throughout the device. To this end, technology has conventionally been developed with the goal of forming a defect-free and uniform liquid crystal alignment state, that is, a monodomain over the entire liquid crystal electro-optical device.

However, liquid crystal materials, especially ferroelectric liquid crystals, are prone to defects due to various causes such as minute scratches on the alignment film, irregularities in the electrodes for driving the liquid crystal, spacers used to maintain a constant distance between the substrates of the liquid crystal device, etc. However, it is not possible to obtain uniform monodomains. Therefore, attempts have been made to grow monodomains over the entire cell by one-dimensional crystal growth (temperature gradient) from the edge of the liquid crystal electro-optical device. Ta.

However, when a liquid crystal electro-optical device has a large area, the second method cannot be applied. That is, the size of the monodomain realized by this method is at most several tens of millimeters square, and it has been impossible to increase the area and use it industrially.

Furthermore, even if a usable monodomain were to be realized, the property of ferroelectric liquid crystal materials is that they do not align parallel to the substrate, but rather have a certain inclination (pretilt). The liquid crystal layer is bent or broken. Therefore, there was a zig-sag defect or a problem occurring in the domain.

When the liquid crystal material changes its possible states due to an external electric field, a phenomenon can be observed in which the state is inverted or reversed with this zig-sag defect as the boundary. For this reason, there was a problem in that uniform display and driving characteristics could not be obtained throughout the device.

The present invention solves the problem of not being able to obtain uniform display and drive characteristics by using a technical idea that is different from the conventional idea of obtaining monodomains. The present inventors have focused on the initial alignment that the liquid crystal material in the cell can take, and have created a state that is completely different from the state of alignment that was conventionally thought, thereby improving the display or driving of liquid crystal electro-optical devices. This characteristic has been realized.

That is, the present invention provides an isotropic phase, a smectic A phase, and a smectic C9 phase, without having a nematic phase, between a pair of parallel substrates formed only on one of the substrates or the alignment portion where the electrode liquid crystal material is initially aligned. In a liquid crystal electro-optical device in which a liquid crystal mixed material exhibiting ferroelectricity that exhibits a phase change is sandwiched, the initial orientation characteristic of the liquid crystal composite material exhibiting ferroelectricity is not a monodomain but a multi-domain consisting of a large number of microdomains or multiple domains. It is characterized by being in a microdomain state.

Multi-microdomains can be generated in a liquid crystal device having the above-mentioned configuration. The problem is the form of precipitation when the liquid crystal phase is precipitated from the isotropic phase. The liquid crystal mixture material is injected into the liquid crystal cell, and in order to achieve the initial alignment of the liquid crystal, the temperature of the liquid crystal material is gradually lowered from the high temperature isotropic phase, and the liquid crystal molecules begin to align in the direction defined by the alignment part. . At this time, when precipitation occurs from an isotropic phase via a nematic phase, the nematic phase is in a soft state, so the liquid crystal molecules are aligned uniformly within the plane.

From this state, it becomes a smectic A phase and further changes to a smectic C'' phase.

However, in the smectic C phase, since the upward direction of the liquid crystal molecules from the substrate is not uniform, zig-sag defects occur where the discrepancy occurs, making the switching contrast of the liquid crystal extremely low. In order to control this, it was necessary to use rotating oblique evaporation of silicon oxide or a special alignment film made of polyimide containing fluorine.

However, neither method was easy. In this case including a nematic phase, since the nematic phase is a phase close to a low-order liquid, at first glance the alignment may appear to be good as if a -plane mono-domain was formed, or it may end up suffering from zig-sag defects.

Here, the formation of multi-microdomains related to the present invention will be described. If the phase series of the liquid crystal does not include a nematic phase, the smectic A phase will precipitate directly from the isotropic phase. Since the smectic phase is a higher-order phase that is closer to a solid than the nematic phase, it is not only the direction of the long axis of the molecules, but also the strength and alignment of the molecules, that is, the formation of a layer that allows adjacent molecules to The phenomenon of twisting between comrades does not occur.

When the smectic A phase is directly deposited from the isotropic phase, it is sufficient to form an alignment layer on only one substrate side. More specifically, when alignment layers are provided on both substrates, liquid crystal molecules may be reversed in a certain direction when an electric field is applied due to the movement of the molecules being hindered by the alignment layers, or they may be reversed in a certain direction after no electric field is applied. It will return to the state as before, and the memory property will not be generated.

Since the smectic A phase thus precipitated from the isotropic phase is a liquid crystal phase with a hard layer structure, it becomes a uniform monodomain in each divided region.

The liquid crystal is in a state of microdomains (about several tens of micrometers in size) or multi-microdomains in which many domains are gathered. As can be seen from these methods, the multi-microdomain orientation of the present invention can be obtained by using materials or orientation treatments that have been considered bad methods for obtaining conventional monodomain orientation. In such a multi-microdomain state, alignment defects in the liquid crystal are alleviated by the boundaries of the domains, so that zigzag defects and the like do not occur in the entire cell of the liquid crystal electro-optical device.

In addition, since the inside of this minute domain is in a good monodomain state, there is no difference in the display or driving characteristics of the liquid crystal in each minute domain, and the device as a whole has the following characteristics:
It is possible to achieve uniform display or drive characteristics. An example is shown below.

[Example 1] FIG. 1 shows a schematic cross-sectional view of a cell of a liquid crystal electro-optical device used in this example. The figure shows a portion of the end portions of electrode portions arranged in a matrix in the row and column directions.

Furthermore, since this is a schematic diagram, its dimensions are arbitrary. The liquid crystal cell used in this example is exactly the same as that used conventionally. In other words, a transparent substrate (
For example, glass) 2.2 degrees above the electrodes 3, 3' for driving the liquid crystal.
They are formed by 12 legs arranged in a matrix in the row and column directions. Further, alignment controls 4, 4 are provided on the electrodes, and one side of the electrodes is subjected to a known laching process so that the liquid crystal molecules are aligned. The alignment control films 4 and 4' may both be made of the same material, or one side may be made of a different material. A substrate on which no alignment film was provided was used.

In this way, when the type of alignment control film is changed, a relatively large threshold value can be obtained when the liquid crystal molecules are driven by an external signal, which is advantageous in a matrix liquid crystal electro-optical device.

A liquid crystal cell is formed by stacking 2.2'' of such substrates on top of each other and maintaining a constant spacing with a spacer (not shown) in between.

In such a cell, the polyimide film 4 of the alignment control film
On the other hand, the rubbing treatment was carried out under the following conditions.

Table 1. Rubbing treatment conditions These conditions are described as a relative comparison with Ca5e3 as the standard.

A liquid crystal material was injected into the cell subjected to such treatment using a known vacuum injection method.

Then, the alignment state of each Ca5e liquid crystal was observed using a polarizing microscope. The liquid crystal material is oriented in a direction that relieves the strain energy generated in the alignment film 4 by the rubbing process, and the size of the domain is balanced with the size of the strain energy.

The actual value of the domain size is from Ca5el to Ca5e.
In any case of 3, the width is 10 to 50 μm, and the length is 200 to 70 μm.
0 μm, multi-microdomains were generated almost over the entire surface of the cell, and there was no partial deviation over the entire surface of the cell.

In addition, when examining these display characteristics, one pixel (
400 μm square), there were 20 or more minute domains, and even the inversion of the display of one pixel was not biased and uniform inversion characteristics were obtained.

When we observed the multi-microdomain alignment state obtained with Ca5e2 using an optical microscope, we found that, as shown in Figure 2, there were no zig-sag defects, etc., which were observed in large numbers in conventional liquid crystal devices. Boundary 9 of domain 8
It was in a state where it contained scratches and defects, and because the defects were small, it appeared that a uniform orientation was obtained throughout the cell. This FIG. 2 is a diagram schematically showing a micrograph showing the crystal structure of the liquid crystal in the aligned state shown in FIG. 3.
It is mainly drawn near the upper left side of the photo in Figure 3.

For such a liquid crystal, an external voltage was applied between the upper and lower electrodes 3 and 3' at a temperature near room temperature to drive the liquid crystal.
When a 10V triangular wave was applied and the state of inversion was observed, the liquid crystal inverted using the multi-microdomain 8 as the minimum unit, and did not invert by forming matrix domains within monodomains as in the conventional case. In addition, each domain is inverted almost simultaneously, and when looking at the entire liquid crystal cell,
The entire cell is inverted at the same time, and near the center of the cell,
The reversal of the edges and the liquid crystal was almost the same, and there was no difference in the reversal state depending on the location.

[Example 2] In this example, alignment states in cells with different combinations of liquid crystal materials with different phase series and alignment treatments were observed. The cell manufacturing method is the same as in Example 1. The rubbing condition used was that of Ca5e 3 of Example 1.

The results are shown in Table-2.

Table 2: Orientation Results of Example 2 The liquid crystals used herein include those manufactured by Merck, those manufactured by Chisso, and those of ester type. Also, in the description of the phase series, INAC
indicates that the phase sequence is isotropic phase - nematic phase - smectic A phase - smectic C1 phase, and 1. AC means that it takes a phase series of isotropic phase - smectic A phase and smectic C° phase. Even when a liquid crystal having a nematic phase is combined with a liquid crystal cell that has undergone one-sided alignment treatment, zig-sag defects occur. A lot of them occurred, and the contrast was not good. This is thought to be because the pretilt of the liquid crystal molecules cannot be imaged because a normal low pretilt type polyimide is used as the alignment film.

On the other hand, in the case of a liquid crystal that directly transitions from an isotropic phase to a smectic phase, when placed in a liquid crystal cell treated on both sides with alignment, many defects occurred in the normal direction perpendicular to the layer direction. In the case of one-sided alignment treatment, the result is multi-microdomains, and no zig-sag defects occur, resulting in good contrast.In this way, the entire liquid crystal cell can be filled with multi-microdomains. The size of this multi-microdomain was several tens of micrometers in width and several hundred micrometers in length.

〔effect〕

According to the present invention, it was possible to perform multi-microdomain alignment, which is the opposite direction to the direction of conventional technological progress, and as a result, it was possible to obtain a uniform alignment state as a whole.

Uniform display characteristics and a high contrast ratio were achieved without the occurrence of defects such as zig-sag defects that have a large optical effect.

Furthermore, since the inversion characteristics of each of the multi-microdomains are the same, it is possible to drive the liquid crystal uniformly throughout.

In addition, there is no need for complicated technical steps to form monodomains, making it easier to produce industrially.

[Brief explanation of the drawing]

FIG. 1 schematically shows a liquid crystal electro-optical device cell used in the present invention. FIG. 2 shows a diagram schematically depicting the micrograph of FIG. 3. Figure 3 is a micrograph showing the crystal structure of liquid crystal in an aligned state. FIG. 4 shows a schematic diagram of a ferroelectric liquid crystal. FIG. 5 shows possible states of liquid crystal molecules. Substrate electrode alignment control film liquid crystal mixed material microdomain

Claims (1)

[Claims] 1. A liquid crystal electric device in which a liquid crystal mixed material exhibiting ferroelectricity is sandwiched between parallel substrates with a pair of electrodes on which only one substrate is subjected to an alignment treatment to initially align the liquid crystal material. 1. A liquid crystal electro-optical device, wherein the liquid crystal mixed material exhibiting ferroelectricity forms a multi-microdomain alignment. 2. The liquid crystal electro-optical device according to claim 1, wherein the phase series of the liquid crystal mixed material exhibiting ferroelectricity is an isotropic phase, followed by a smectic phase without a nematic phase.