JPH1096965A - Liquid crystal element and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal element having good orientability and high contrast by creating an orientation state of a good extinction characteristic in an initial stage of liquid crystal cell manufacture when antiferroelectric liquid crystals or a ferroelectric liquid crystal phase having the spiral pitch shorter than the spiral pitch of visible light is used. SOLUTION: A pair of substrates having at least transparent electrodes and orientation control layers are disposed to face each other apart a specified cell thickness spacing. A liquid crystal material is held between these substrates. The liquid crystal material exhibits the antiferroelectric liquid crystal phase or the ferroelectric liquid crystal phase of the spiral pitch shorter than the spiral pitch of the visible light. The liquid crystal material exhibits a chiral nematic liquid crystal phase on a high temp. side and the directions of the uniaxial orientation treatments of the orientation control layers of both substrates are approximately parallel when viewed from the direction perpendicular to the substrate surfaces. The liquid crystals have a chevron layer structure at the temp. at which the liquid crystals exhibit the liquid crystal phase. In addition, the direction of this chevron structure and the direction of the pretilt of the liquid crystal molecules are the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device using an antiferroelectric liquid crystal or a liquid crystal material exhibiting a ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light, and a method for manufacturing the liquid crystal device. More specifically, the present invention relates to a liquid crystal element for improving alignment and contrast and a method for manufacturing the liquid crystal element.

[0002]

2. Description of the Related Art At present, a nematic liquid crystal phase is used in most of commercialized TFT type liquid crystal displays, but as a development stage, an antiferroelectric liquid crystal phase or a helix from visible light is used. A device using a ferroelectric liquid crystal phase having a short pitch has been studied.

For example, FIG. 7 shows the molecular arrangement and switching principle of an antiferroelectric liquid crystal. In a state where no electric field is applied to the liquid crystal, the liquid crystal molecules are tilted to the opposite side for each layer as shown in FIG. Since the liquid crystal molecules have a spontaneous polarization in a direction perpendicular to the layer (with respect to the paper surface) and each layer has a spontaneous polarization in the opposite direction (upward and downward directions perpendicular to the paper surface), Spontaneous polarization as a liquid crystal layer has been canceled. When an electric field is applied to the liquid crystal molecules in this state (applied perpendicular to the paper surface), the molecules tend to tilt in either direction because the spontaneous polarization tends to be aligned in the direction of the electric field. Then, when a sufficient electric field is applied, as shown in FIG. 7B or FIG. 7C, the liquid crystal molecules are aligned in the same direction in each layer. Therefore, by combining the polarizer P having a polarization direction in the vertical direction with the smectic layer and the analyzer A having the polarization direction in the horizontal direction, the switching that is dark when there is no electric field and light when applying the electric field can be performed. Can be done.

Further, recently, a ferroelectric liquid crystal showing a voltage-transmittance curve without a threshold as shown in FIG. 8 in a voltage-transmittance characteristic has been reported. The liquid crystal molecules when the helical pitch of the ferroelectric liquid crystal is shorter than the visible light will be described below in detail with reference to FIG. When no electric field is applied, the liquid crystal molecules are arranged in a helical structure as shown in FIG. If the helical pitch is shorter than the wavelength of visible light, there is an extinction position in the normal direction of the layer. When a voltage is applied to the liquid crystal molecules, as shown in FIG. 9B, the optic axis changes according to the polarity of the voltage, and the directions of spontaneous polarization are aligned. Therefore, for example, if the liquid crystal cell is sandwiched between a pair of polarizing plates installed in crossed Nicols and the polarization axis of one of the polarizing plates is aligned with the extinction position (normal direction of the layer) in the absence of an electric field, the darkness in the absence of an electric field can be obtained. The state becomes bright when an electric field is applied, and display becomes possible. The liquid crystal mode as described above is D
HF (Deformed Helix Ferroel
ectric).

[0005]

The antiferroelectric liquid crystal and the DHF mode liquid crystal have a wide viewing angle, a high response speed,
However, it has the disadvantage of low contrast. For example, Saishu et al.
A 5-inch TFT liquid crystal display is manufactured using an antiferroelectric liquid crystal that does not have a systematic transmittance curve, but the contrast is 10: 1 or slightly higher. 10 when used
High contrast such as 0: 1 or 200: 1 has not been obtained [T. Saishu et al. , SID '
96 Digest, 703 (1996)]. Also, V
have prototyped an active-drive 0.75-inch display using a ferroelectric liquid crystal phase with a helical pitch shorter than visible light, but with a contrast of 25: 1.
[A. G. FIG. H. Verhulst et a
l. Proc. IDW'95, 35 (199
5). ].

At present, various causes can be considered as a cause of such a low contrast, but attention is paid to the fact that the initial alignment state is an important factor. That is, in the initial stage of liquid crystal cell fabrication (especially, before driving), unless a good quenching alignment state is created, a good black state that hardly transmits light cannot be created, and high contrast can never be obtained. It is not possible.

In view of the above, the present invention uses an antiferroelectric liquid crystal or a ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light to form an alignment state having good extinction property in an initial stage of manufacturing a liquid crystal cell. It is an object of the present invention to obtain a liquid crystal element having good orientation and high contrast.

[0008]

In order to achieve the above-mentioned object, according to the present invention, a pair of substrates having at least a transparent electrode and an orientation control layer are opposed at a constant cell thickness interval. Sandwiching a liquid crystal material in between, the liquid crystal material shows an antiferroelectric liquid crystal phase or a ferroelectric liquid crystal phase having a helical pitch shorter than visible light, and the liquid crystal material shows a chiral nematic liquid crystal phase at a high temperature side, The direction of the uniaxial alignment treatment of the alignment control layers in both substrates is substantially parallel when viewed from a direction perpendicular to the substrate surface, and the liquid crystal has a chevron layer structure at a temperature indicating the liquid crystal phase, and The direction of the chevron layer structure is the same as the direction of the pretilt of the liquid crystal molecules. In particular, preferably, the liquid crystal molecules in contact with the alignment control layer on the substrate surface have a pretilt angle of 2 to 10 ° with respect to the substrate.
Further, an antiferroelectric liquid crystal phase or a ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light exhibits a voltage-transmittance characteristic having no threshold.

According to a fourth aspect of the present invention, in the above-described method for manufacturing a liquid crystal element, the method is characterized in that the liquid crystal element is heated to a temperature at which a chiral nematic liquid crystal phase is exhibited, and then gradually cooled.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a liquid crystal device according to the present invention,
The mechanism for obtaining good orientation is discussed below. Conventional antiferroelectric liquid crystal materials can exhibit a smectic A phase on the high temperature side of the antiferroelectric liquid crystal phase, but do not exhibit a chiral nematic liquid crystal phase. In addition, liquid crystal materials exhibiting a ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light,
Only those showing no chiral nematic liquid crystal phase have been reported. The nematic liquid crystal phase has the same average molecular long axis direction, but the position of the center of gravity is random, and the liquid crystal phase has a low degree of order and is easily oriented. On the other hand, the smectic A liquid crystal phase has a layer structure, and has a higher degree of order than the nematic liquid crystal phase. Therefore, it is difficult to obtain a uniform orientation even when cooled from an isotropic liquid.

At this stage, there are not many studies and reports on the molecular arrangement of the antiferroelectric liquid crystal and the orientation in the liquid crystal cell, and many points have not been elucidated yet. The present inventors consider the following mechanism as a mechanism for obtaining good orientation from the device and the method for manufacturing the device. As an orientation method, consider a process of cooling from an isotropic liquid. First, as shown in FIG. 1A, in the chiral nematic phase, the liquid crystal molecules are aligned in the rubbing direction. Note that when rubbing is performed, pretilt occurs, but the pretilt is omitted in FIG. The pretilt will be described in detail with reference to FIGS.

First, when the chiral nematic phase is cooled,
As a first step, a smectic A phase, ie a layer structure, is formed, the layers being perpendicular to the substrate, as shown in FIG. Further, upon cooling, a ferroelectric liquid crystal phase or an antiferroelectric liquid crystal phase is formed as a second stage, and the molecules are inclined with respect to the layers as shown in FIG.
However, since the layer interval at the substrate interface is hard to change, the layer is tilted in accordance with the decrease in the layer interval, and further, a defect occurs at a boundary portion where the bending direction is different, and uniform alignment cannot be obtained. The plan view of FIG. 1C shows the case of an antiferroelectric liquid crystal. In the case of the DHF mode, a helical structure having a helical pitch shorter than the cell thickness as shown in FIG.

Now, when the antiferroelectric liquid crystal layer is formed from the isotropic liquid state, if the liquid crystal layer does not pass through the nematic liquid crystal phase, the liquid crystal transitions directly from the isotropic liquid state to the smectic phase. The phenomenon of aligning and the phenomenon of forming a layer occur simultaneously, and since there are many stages of layer change, it is difficult to finally orient. However, by passing through the nematic liquid crystal phase,
It is considered that the degree of orientational order can be gradually increased, and good orientation can be easily obtained.

In addition, when an antiferroelectric liquid crystal layer is formed from an isotropic liquid state, a method has been proposed in which only one of the substrates is subjected to a uniaxial alignment treatment to obtain good alignment. In the case of this method, the molecular arrangement is twisted in the chiral nematic phase, and when the phase is cooled to the smectic A phase in this state, it is difficult to obtain uniform orientation with uniform layer normal.

Therefore, a method of rubbing both substrates in an anti-parallel manner is adopted. When an antiferroelectric liquid crystal layer is formed from an isotropic liquid state, as shown in FIG. 2 or FIG. A pretilt angle occurs in the phase. When this chiral nematic phase is cooled, a smectic A phase, that is, a layer structure is formed as a first step, and the layer is perpendicular to the substrate as shown in FIG. Furthermore, upon cooling, a ferroelectric liquid crystal phase or an antiferroelectric liquid crystal phase is formed as a second stage, and in this case, as shown in FIG. In contrast to the smectic A phase in which the layer is perpendicular to the substrate, if the layer is inclined, molecules must move at the substrate interface, and the phenomenon cannot always be explained well. Rather, the layers bend,
Although there is a high possibility that it is as shown in FIG. 2D, in this case, a defect occurs at a boundary portion having different bending directions, and uniform alignment cannot be obtained.

The bending of the liquid crystal layer shown in FIGS. 2C and 2D will be described below. When both substrates holding the liquid crystal are rubbed in parallel, the result is as shown in FIG. That is, in the ferroelectric liquid crystal or the antiferroelectric liquid crystal, the liquid crystal layer bends in the shape of a "ku". In this case, if the pretilt is absent or very small, the energy difference depending on the direction of the bend is small, and both bends are mixed. In this case, uniform alignment is not achieved, and high contrast cannot be realized.

Therefore, it is necessary to align the bending direction of the layer in one direction, and the present inventors provide a pretilt angle of 2 to 10 ° to generate an energy difference depending on the bending direction, so that only one direction is stable. I found out that they can be aligned. When the pretilt angle is 2 ° or less, the energy difference caused by the direction in which the layer is bent is small, and it is difficult to align the layers in only one stable direction. By giving a pretilt angle of 2 ° or more, the energy difference can be increased to some extent depending on the bending direction of the layer,
It is possible to align in a stable direction. On the other hand, 10 °
It is not easy to obtain an alignment film exhibiting the above pretilt angle stably and without change over time.

As described above, the present inventors have found that uniform alignment can be achieved by using a liquid crystal material having a chiral nematic phase, using parallel rubbing, and giving an appropriate pretilt angle. The following examples will specifically explain that the present invention is effective for a liquid crystal device exhibiting good orientation and obtaining high contrast. However, the present invention is not limited to these examples.

[0019]

【Example】

<Example 1> ITO (Indium tin oxid)
e) An insulating film A-2014 (manufactured by Nissan Chemical Industries, Ltd.) and an alignment film SE-779 are respectively formed on two glass substrates with electrodes.
2 (manufactured by Nissan Chemical Industries) was applied and the surface was rubbed.
The pretilt angle of this alignment film (according to catalog values) is 7 to 8 °. The two substrates were bonded at an interval of 1.5 μm so that the rubbing directions were parallel. Liquid crystal 1 was injected into this, and the periphery was sealed. The liquid crystal 1 refers to a liquid crystal composition in which seven types of liquid crystal materials shown in Table 1 are mixed at a mixing ratio shown in the table. Table 1 shows the composition and the transition temperature.

[0020]

[Table 1]

The liquid crystal cell was once heated to an isotropic liquid state and then cooled. Thereafter, when the liquid crystal cell was sandwiched between a pair of polarizing plates in a crossed Nicols state and observed, a uniform dark state with good extinction properties was obtained.

Further, when the direction of the zigzag defect was observed, it was found that the direction of the chevron layer structure was the same as the direction of the pretilt of the liquid crystal molecules. The direction of the pretilt of the liquid crystal molecules is the same as the rubbing direction (FIG. 3),
The direction of the chevron layer structure was determined from the direction of minute zigzag defects observed with a polarizing microscope (see FIG. 4).

A rectangular wave of 60 Hz was applied to this, and the amount of transmitted light was measured. FIG. 5 shows the results. As can be seen from FIG. 5, it can be seen that the gradation is obtained by changing the amount of transmitted light according to the applied voltage. FIG. 6 shows the viewing angle dependence of the contrast at an applied voltage of 3.5 V and 25 ° C. Although not shown in FIG. 6, when viewed from the front (both X and Y are 0 °), a maximum contrast of 40: 1 was obtained.

<Example 2> An insulating film A-2014 (manufactured by Nissan Chemical Industries) and an alignment film JALS-212 (manufactured by Nippon Synthetic Rubber) were applied on two glass substrates with ITO electrodes, respectively, and the surfaces were coated. I rubbed. According to the catalog value, the pretilt angle of this alignment film is 4 to 5 °. This 2
1.5 μm so that the rubbing directions are parallel
At the following intervals. Liquid crystal 1 was injected into this, and the periphery was sealed.

The cell was once heated to an isotropic liquid state and then cooled. Observation of the cell sandwiched between a pair of polarizing plates in a crossed Nicols state revealed a uniform dark state with good extinction. In addition, observation of the direction of the zigzag defect revealed that the direction of the chevron layer structure was the same as the direction of the pretilt of the liquid crystal molecules.

Comparative Example 1 An alignment film LX-1400 (manufactured by Hitachi Chemical) was applied onto two glass substrates with ITO electrodes, and the surfaces were rubbed. The pretilt angle of this alignment film is about 0.5 °. The two substrates were bonded at an interval of 1.5 μm so that the rubbing directions were parallel. Liquid crystal 1 was injected into this, and the periphery was sealed. The cell was once heated to an isotropic liquid state and then cooled.

When this cell was observed by sandwiching it between a pair of polarizing plates in a crossed Nicols state, a uniform extinction state was obtained in a 70 to 80% region. However, 20 to 30% of the non-quenching region which occurs in a band shape in the direction perpendicular to the rubbing direction remains. Zigzag defects were found at the boundary between the non-quenched region and the quenched region. Therefore, regions having different chablon layer structure directions coexist. Also,
Many fine line defects along the rubbing direction were observed in the quenched region.

When a rectangular wave of 60 Hz is applied to this,
The amount of transmitted light varied depending on the voltage value, and the gradation was confirmed. However, the contrast was about 10: 1 visually.

Comparative Example 2 An alignment film LX-1400 (manufactured by Hitachi Chemical) was applied onto two glass substrates with ITO electrodes, and the surfaces were rubbed. The pretilt angle of this alignment film is about 0.5 °. The two substrates were bonded at an interval of 1.5 μm so that the rubbing directions were antiparallel. Liquid crystal 1 was injected into this, and the periphery was sealed. The cell was once heated to an isotropic liquid state and then cooled.

When this cell was observed by sandwiching it between a pair of polarizing plates in a crossed Nicols state, a number of fine linear defects were observed in the rubbing direction, and no quenching occurred. To this, 60Hz
When the rectangular wave of is applied, the amount of transmitted light changes depending on the voltage value, and the gradation can be confirmed.
It was about 1.

Comparative Example 3 An alignment film LX-1400 (manufactured by Hitachi Chemical Co., Ltd.) was applied on two glass substrates with ITO electrodes, and only one surface was rubbed. The pretilt angle of this alignment film is about 0.5 °. The two substrates were bonded at an interval of 1.5 μm. Liquid crystal 1 was injected into this, and the periphery was sealed. The cell was once heated to an isotropic liquid state and then cooled.

When this cell was observed by sandwiching it between a pair of polarizing plates in a crossed Nicols state, the layer directions were not uniform, and thus no uniform orientation was obtained. This is probably because the nematic liquid crystal phase has a helical structure.

[0033] <Comparative Example 4> The liquid crystal cell of the configuration example 1, antiferroelectric Chisso Corporation crystal CS-4000 (S <sub>C
* 82 ℃, Sc A * 84</sub> ℃, S A 101 ℃, was injected I). Here, S _C * is the smectic C of the ferroelectric liquid crystal.
The phase, Sc _A *, is an antiferroelectric liquid crystal phase, S _A is a smectic A phase, and I is an isotropic liquid.

After the cell was once heated to an isotropic liquid state, and then cooled, and observed while sandwiched between a pair of polarizing plates in a crossed Nicols state, there were many defects along the rubbing direction. No quenching was obtained.

Comparative Example 5 A DHF liquid crystal FLC5679 manufactured by Roche (S _C * 6) was added to the liquid crystal cell having the structure of Example 1.
0 ℃ was injected S _A 62 ℃ I). Once this cell
After heating to an isotropic liquid state and then cooling, the mixture was observed between a pair of polarizing plates in a crossed Nicols state. As a result, there were many defects along the rubbing direction, and sufficient quenching was not obtained. It goes without saying that the liquid crystal element according to the present invention can be driven actively, and can be colored by combining with a color filter.

[0036]

According to the present invention, in the antiferroelectric liquid crystal element and the DHF mode liquid crystal element, a uniform dark state with good extinction property can be obtained, and a high contrast liquid crystal display device can be obtained. Can be.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an alignment method by cooling from an isotropic liquid and a molecular alignment state at that time.

FIG. 2 is an explanatory view of an alignment method by cooling from an isotropic liquid and a molecular alignment state at that time.

FIG. 3 is a diagram showing a relationship between a rubbing direction, a direction of a pretilt angle, and a bending direction of a layer.

FIG. 4 is a diagram showing the relationship between the direction of the chevron layer structure and the direction of minute zigzag defects observed with a polarizing microscope.

FIG. 5 is a diagram showing a voltage-transmitted light curve of the liquid crystal element according to the present invention.

FIG. 6 is a view showing the viewing angle dependence of the contrast of the liquid crystal element according to the present invention.

FIG. 7 is a diagram showing the molecular arrangement and switching principle of an antiferroelectric liquid crystal.

FIG. 8 is a diagram showing an example of an antiferroelectric liquid crystal showing a voltage-transmittance curve without a threshold.

FIG. 9 is a diagram illustrating a device using a ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light.

[Explanation of Signs] 1. Liquid crystal composition

Claims (4)

[Claims] 1. A pair of substrates having at least a transparent electrode and an orientation control layer are opposed at a constant cell thickness interval, and a liquid crystal material is sandwiched between the substrates. The liquid crystal material exhibits a ferroelectric liquid crystal phase having a helical pitch shorter than that of light, and the liquid crystal material exhibits a chiral nematic liquid crystal phase at a high temperature, and the direction of the uniaxial orientation treatment of the orientation control layers in both substrates is perpendicular to the substrate surface. When viewed from above, the liquid crystal has a chevron layer structure at a temperature indicating the liquid crystal phase, and the direction of the chevron layer structure and the direction of the pretilt of the liquid crystal molecules are the same. Liquid crystal element. 2. The liquid crystal molecule according to claim 1, wherein the liquid crystal molecule in contact with the alignment control layer on the substrate surface has a pretilt angle of 2 to 10 ° with respect to the substrate. 3. The liquid crystal device according to claim 1, wherein the antiferroelectric liquid crystal phase or the ferroelectric liquid crystal phase having a helical pitch shorter than that of visible light has a voltage-transmittance characteristic having no threshold. 4. A method for manufacturing a liquid crystal device, comprising heating the liquid crystal device according to claim 1 to a temperature at which a chiral nematic liquid crystal phase is exhibited, and then gradually cooling the liquid crystal device.