JP2564567B2 - Liquid crystal electro-optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal electro-optical device such as a liquid crystal display element or a liquid crystal optical shutter array, and more particularly, to a display by improving an initial alignment state of liquid crystal molecules. The present invention also relates to a liquid crystal electro-optical device having improved driving characteristics.

[Conventional technology]

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

In addition, an active matrix type display element in which a switching element using a thin film transistor is connected to each pixel and switching is performed for each pixel is known, but the process of forming a thin film transistor on a substrate is extremely complicated, and its manufacturing cost is low. There is a problem that it is difficult to manufacture a large-area display element due to factors such as yield.

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

FIG. 3 schematically shows an example of a cell for explaining the operation of the ferroelectric liquid crystal. 11 and 11 'are substrates (glass plates) covered with a transparent electrode made of a thin film such as In ₂ O ₂ or ITO (Indium-Tin-Oxide), and the liquid crystal molecular layer 12 is perpendicular to the glass surface between them. Oriented SmC ^*
Phase or another ferroelectric liquid crystal phase.
In this liquid crystal electro-optical device, the ferroelectric liquid crystal molecules
As shown in the figure, there are a first state (I) inclined + θ and a second state (II) inclined −θ with respect to the normal direction of the layer of the smectic layer.

Display is performed by the difference in the birefringence effect generated by switching the ferroelectric liquid crystal molecules between the two states by applying an electric field from the outside.

At this time, the ferroelectric liquid crystal molecules are shifted from the first state (I) to the second state.
This is done by adding, for example, a positive electrode in the direction perpendicular to the smectic layer in order to change the state (II). On the contrary, in order to reverse the state from the second state (II) to the first state (I), conversely, a negative electric field was applied. That is, the two states taken by the ferroelectric positive liquid crystal molecules are changed by changing the direction of the electric field applied from the outside, and the difference in the electro-optical effect caused by the change is used.

Further, even if the electric field applied from the outside was removed, the ferroelectric liquid crystal molecules kept their state in a stable state and had the first and second bistable memory properties.

Therefore, a bipolar pulse train is used as a signal waveform for driving the liquid crystal electro-optical device using the ferroelectric liquid crystal, and the two states taken by the ferroelectric liquid crystal molecules are switched depending on the switching direction of the pulse polarities. It was

[Problems to be solved by the invention]

In an electro-optical device using such a ferroelectric liquid crystal, uniform driving characteristics are naturally required in the entire device. For this reason, technical development has been conventionally made with the goal of forming a uniform liquid crystal phase, that is, a monodomain, in the entire liquid crystal electro-optical device without any defects.

However, liquid crystal materials, especially ferroelectric liquid crystal, have defects caused by minute scratches on the alignment film, unevenness of the electrodes for driving the liquid crystal, spacers for keeping the substrate spacing of the liquid crystal device constant, and various other causes. Since a uniform monodomain cannot be obtained, it has been conventionally attempted to grow a monodomain over the entire cell by a method (temperature gradient) of one-dimensional crystal growth of liquid crystal from the edge of the liquid crystal electro-optical device cell. It was

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

Even if a usable monodomain is realized, the ferroelectric liquid crystal material has a property that the liquid crystal material is not aligned parallel to the substrate but is aligned (pretilt) with a certain inclination, so that the ferroelectric liquid crystal is not aligned. The liquid crystal layer bends or breaks. Therefore, there is a problem that a zigzag defect occurs in the domain.

When the liquid crystal material changes its possible state by an external electric field, a phenomenon is observed in which the inversion process is reversed at the boundary of the zigzag defect. For this reason, there has been a problem that uniform display and drive characteristics cannot be obtained in the entire device.

[Means for solving problems]

The present invention solves the problem that uniform display and driving characteristics cannot be obtained by a technical idea that is different from the conventional idea of obtaining a monodomain. The present inventors have paid particular attention to the initial alignment of the liquid crystal material in the temperature lowering process in which the liquid crystal material shifts from the isotropic liquid state (high temperature state) to the liquid crystal state (low temperature state), and the alignment characteristic is regarded as a conventional idea. The above state is a state in which the display or drive characteristics of the liquid crystal electro-optical device are excellent by realizing a completely different state.

That is, the present invention provides a liquid crystal electro-optical device in which a ferroelectric liquid crystal is sandwiched between a pair of parallel substrates in which an alignment portion for initially aligning an electrode liquid crystal material is formed. Since the transition temperature range from liquid to liquid crystal state is narrow, it is recommended to use a ferroelectric liquid crystal that slowly transitions from the isotropic state to the liquid crystal state near the transition temperature. It is a feature.

[Work]

With the above configuration, the liquid crystal material is injected into the liquid crystal cell, and the temperature of the liquid crystal material is gradually lowered from the isotropic liquid state (high temperature state) in order to realize the initial alignment of the liquid crystal. Then, liquid crystal molecules start to be aligned in the direction defined by the alignment portion. At this time, since the transition temperature is wide, a portion in which the liquid crystal phase is changed and a portion in the isotropic liquid state are mixed in the cell. When the temperature is further lowered, the liquid crystal phase region increases and finally the entire cell becomes the liquid crystal phase.

At this time, since the transition temperature width of the liquid crystal is wide, it is impossible to form a uniform monodomain, and the liquid crystal is in a multi-domain state in which a large number of minute domains (the size is about several tens of μm) are gathered. ing.

In such a multi-domain state, the alignment defect of the liquid crystal is mitigated by the boundary between the domains, so that a zigzag defect or the like does not occur in the entire liquid crystal electro-optical device cell.

In addition, since the inside of the minute domain is in a good mono-domain state, there is no difference in the display or driving characteristics of the liquid crystal in each minute domain.
It is possible to realize uniform display or drive characteristics.

In addition, in the present specification, the monodomain means that the alignment directions of liquid crystal molecules are aligned in substantially the same direction in the entire panel.

In addition, the multi-domain refers to a state in which a plurality of domains exist within a panel. In the multi-domain, the alignment direction of liquid crystal molecules is the same in each domain, but the alignment direction of liquid crystal molecules in each domain is slightly different.

In addition, the microdomain refers to a multidomain in which each domain has a size of several μm to several 100 μm.

 Examples will be shown below.

〔Example〕

FIG. 1 shows a schematic sectional view of a cell of the liquid crystal electro-optical device used in the present invention. The figure shows a part of the end portion of the electrode portions arranged in a matrix in the row and column directions.

Since it is a schematic diagram, its dimensions are arbitrary.
The cell used in this embodiment is exactly the same as that used conventionally. That is, liquid crystal driving electrodes 3, 3'are patterned and formed on a transparent substrate (for example, glass) 2, 2'in a matrix in the row and column directions. Further, alignment control 4, 4 'is provided on the electrode, and one side thereof is subjected to a known rubbing treatment so that liquid crystal molecules are arranged. This alignment control film 4,
Both 4 'may use the same material or different materials on one side, but in the present embodiment, the alignment control film of 4 is a polyimide film, and the other alignment control film 4'is SiO 2. _Two membranes were used.

As described above, when the type of the alignment control film is changed, a relatively large threshold 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 sandwiching a spacer (not shown) between two substrates 2 and 2'which are superposed on each other and sandwiching a spacer (not shown).

Liquid crystal is injected into such a cell according to a known liquid crystal injection method. As the liquid crystal, a liquid crystal exhibiting ferroelectricity was used, and a liquid crystal material having a wide range of transition temperatures from the isotropic liquid to the liquid crystal phase was used.

As an example, a phase sequence of isotropic liquid crystal-smectic A phase-smectic B phase-smectic I phase-crystal phase is adopted, and the transition from the isotropic liquid to the smectic A phase is between 130 ° C and 98 ° C. A mixed liquid crystal having a transition temperature of 32 ° C. was used.

This mixed liquid crystal is a mixture of eight types of ester-based ferroelectric liquid crystals. The largest proportion of the single liquid crystal material in the mixed liquid crystal is 20%, and the mixing ratio of the eight types is 5 to 5%, respectively.
There is no main liquid crystal between 20%, and it is characterized by mixing multiple types in almost equal amounts.

Therefore, the range of the phase transition temperature is wide, and the range of the transition temperature when the first liquid crystal phase appears from the isotropic liquid state is wide.

The transition temperatures of the mixed liquid crystal used in this example are as follows.

Such a mixed liquid crystal was injected into the cell by a known injection method. At the time of injection, since the mixed liquid crystal is injected in an isotropic liquid state, when cooling is performed, a part in a liquid crystal state and a part in an isotropic liquid state exist. At this time, the portion in the liquid crystal state follows the rubbing treatment applied to the alignment control film 4.
Align to form microdomains. When the temperature is further lowered, a liquid crystal portion is newly generated from a liquid state portion, and similarly, a micro domain is formed.

In this way, the entire cell can be filled with microdomains. The size of this microdomain is several μm in width.
It was a long and narrow one with a length of several 100 μm.

When such a multi-domain orientation state was observed with an optical microscope, as shown in FIG. 2, the zigzag defects and the like conventionally seen do not exist, but rather the boundaries 9 of each domain 8 all show defects. Since it was in the state of containing, and its defects were small, it seemed that uniform alignment was obtained in the entire cell. To such a liquid crystal, at a temperature near room temperature, a ± 10 V triangular wave that drives the liquid crystal by externally applying a voltage between the upper and lower electrodes 3 and 3'is applied and the inversion state is observed. The reversal was carried out in 8 units, and the reversal was not performed by forming the boat-shaped domain within the monodomain as in the conventional case. Moreover, the inversion of each domain was also performed almost at the same time, and it was observed that the whole cell was inverted when viewed in the whole cell.

In addition, the inversion of the liquid crystal near the center of the cell, the edge, and the liquid crystal was almost the same, and there was no difference in the inversion state depending on the location.

[Effect]

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

Zigzag defects We were able to realize uniform display characteristics and high contrast ratio without the occurrence of defects that have a large optical influence.
Furthermore, since the inversion characteristics of each multi-domain are uniform, it is possible to drive the liquid crystal uniformly throughout.

In addition, complicated technical steps for forming a monodomain are not required, and industrial production is facilitated.

[Brief description of drawings]

FIG. 1 schematically shows a liquid crystal electro-optical device cell used in the present invention. FIG. 2 shows the alignment state of the liquid crystal of the present invention. FIG. 3 shows a diagram schematically showing the ferroelectric liquid crystal. FIG. 4 shows possible states of liquid crystal. 2,2 '... Substrate 3,3' ... Electrode 4,4 '... Alignment control film 5 ... Liquid crystal 8 ... Microdomain

Claims (2)

(57) [Claims] 1. A liquid crystal electro-optical device in which a ferroelectric liquid crystal is sandwiched between a pair of parallel substrates on which an alignment portion for initial alignment of an electrode and a liquid crystal material is formed, and an isotropic liquid of the ferroelectric liquid crystal. The liquid crystal electro-optical device is characterized in that the transition temperature range from the to smectic A phase exists between 10 ° C. and 50 ° C., and the ferroelectric liquid crystal forms multi-domains. 2. The liquid crystal electro-optical device according to claim 1, wherein the smectic C phase exists in a temperature range of 10 ° C. to 60 ° C.