JP2550054B2 - Ferroelectric smectic liquid crystal electro-optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to an electro-optical device using a ferroelectric smectic liquid crystal.

[Prior Art] Conventionally, as a practical means for orienting a smectic liquid crystal exhibiting ferroelectricity, an organic polymer film, particularly a polyimide polymer film is formed, and a cloth is rubbed in a certain direction to control the orientation. Using a film or a film with an orientation control film formed by oblique vapor deposition of SiO ₂ etc., after injecting liquid crystal, heating to a temperature showing a cholesteric phase or an isotropic phase, then slowly cooling the smectic There is a method in which the A phase is grown as a monodomain and the temperature is further lowered to obtain a desired smectic phase monodomain exhibiting ferroelectricity. [JS
Patel, TMLeslie and JWGoodby; Ferroelectrics, 59,
137 (1984)], [Hideo Takezoe, Atsuo Fukuda, Eiichi Kuze; Industrial Materials, Volume 31, No. 10, 22]. There are two bistable states depending on the direction of the electric field in the smectic phase monodomain showing ferroelectricity obtained by these methods, and the electric field is applied by means of applying a voltage between the transparent electrodes of the upper and lower substrates. Is applied, and one of the two bistable states is controlled by the direction of the electric field. Further, by combining the polarizing plates in such an arrangement that one of the two bistable states is bright and the other is dark, it is used as a display device.

However, the two bistable states are controlled by such a method only in the display pixel portion where the transparent electrodes of the upper substrate and the lower substrate face each other, and in the portion other than the display pixel, the bistable state obtained by the initial alignment is obtained. Since the two stable states are mixed, there is a serious drawback that the appearance of the display device is significantly impaired.

As described above, in the conventional alignment control method for the ferroelectric smectic liquid crystal electro-optical device, there is no method for controlling the portion other than the display pixel to one of the two bistable states, and the portion other than the display pixel is not controlled. However, there is a problem in that a display device having an appearance with good uniformity cannot be obtained.

[Problems to be Solved by the Invention] The present invention makes it possible to selectively control a portion other than a display pixel, which has not been solved by a conventional method, to one of bistable states. The purpose is to provide a device.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes an upper substrate and a lower substrate each having an alignment control film on which a transparent electrode having a desired shape is formed. In a liquid crystal electro-optical device, in which a smectic liquid crystal exhibiting ferroelectricity is sandwiched, and a means for applying a voltage between the transparent electrodes of the upper and lower substrates is provided, a surface potential control film is provided on at least one of the upper and lower substrates. , The alignment control films of the upper and lower substrates are formed of the same material, the alignment control films are uniaxially oriented, and the surface potential difference generated by the surface potential control film is 50 mV or more and 500 m or more.
A ferroelectric smectic liquid crystal electro-optical device characterized by being V or less.

FIG. 1 is a sectional view of a basic liquid crystal electro-optical device of the present invention. The transparent conductive films (2a) and (2b) and the alignment control film (3) are formed on the surfaces of the two transparent substrates (1a) and (1b), respectively.
Form a) and (3b). Conductive (2a), (2b) is an electrode for applying an electric field to the liquid crystal layer held by the substrate (4), in which are provided for the purpose of causing electrochemical response, an In ₂ A predetermined pattern is formed of O ₃ , SnO _{2 and the} like.

The alignment control films (3a) and (3b) are for aligning the layer structure of the liquid crystal in the smectic phase so that the layer normal direction is substantially horizontal to the upper and lower substrates. As a typical example, a method of forming an organic polymer film and rubbing with a cloth in a certain direction, or a method of controlling orientation used in nematic liquid crystals such as an oblique vapor deposition method of SiO 2 is used. Further, when the ferroelectric smectic liquid crystal used is a phase transition from an isotropic phase to a smectic phase without passing through a cholesteric phase, the above rubbing or oblique vapor deposition is performed on only one substrate, and the other is a liquid crystal molecule. Although it is almost horizontal to the substrate, it may be preferable not to perform orientation by rubbing or the like. Further, it is preferable to orient the upper and lower substrates in the same direction in order to reduce alignment defects.

The orientation control film thus formed is composed of the same material for the upper and lower substrates, and by providing a surface potential control film on both the upper and lower substrates, a surface potential difference of 50 mV or more is applied between the upper and lower substrates, A liquid crystal electro-optical device having a uniform appearance of parts other than display pixels, which is selectively controlled to one of two bistable states that appear in a smectic phase exhibiting ferroelectricity, is obtained over the entire surface of the liquid crystal cell. The present inventors have found that this is possible.

In the present invention, by appropriately selecting the control film of the surface potential, the alignment control films of the upper and lower substrates can be formed of the same material, and a surface potential difference of 50 mV can be imparted between the upper and lower substrates. Reliability is also improved. The surface potential control film can be formed between the alignment control film and the substrate on which the transparent electrode is formed, or can be formed on the surface of the alignment control film in contact with the liquid crystal. It is preferable to form it on the surface of the alignment control film which is in contact with the liquid crystal, because deterioration due to heat or the like can be avoided.

The surface potential control film formed on the surface of the orientation control film that contacts the liquid crystal is 100Å by vacuum deposition, spin coating, etc.
Films coated with various inorganic oxides, organic polymers, etc. and adsorbed films with surfactants are used below, but if the orientation control film is uniaxially oriented, it should not impair temporary orientation. Therefore, a surfactant adsorption film is preferable. It is preferable that the alignment control film is formed of the same material on both substrates, because the difference in the surface potentials of the separately formed surface potential control films appears as it is because it is easy to control.

Examples of the surfactant include a silane coupling agent, a titanate coupling agent, and the like, and it is preferable to use aminosilane, epoxysilane, or a mixture thereof in terms of alignment stability. In addition, as a method of forming the adsorption film,
After immersing the substrate in a dilute solution of a surfactant, a heat treatment may be performed if necessary.

The effect of the present invention can be obtained if the polarity difference between the upper and lower substrates provided as a result of providing the surface potential control film is 50 mV or more. However, the ferroelectric smectic liquid crystal has a small energy barrier when moving from one to the other in the two bistable states, and it is difficult to obtain a clear threshold characteristic.
Therefore, when the difference in polarities between the upper and lower substrates becomes large, the bistability in the display pixel portion is significantly impaired. We have
It was found that the difference in polarity between the upper and lower substrates should be 50 mV or more and 500 mV or less from the viewpoint of bistability.

This is because the control of the cell gap cannot be set to 0 completely.
If it is less than 50 mV, it may not be sufficiently controlled to one side, and if it exceeds 500 mV, it is necessary that the variation of the cell gap easily deviates from the range allowed by the voltage margin, and it is necessary to be within the range of 50 to 500 mV.

After such an alignment treatment, spacers, such as organic beads and alumina particles, are sandwiched and the periphery is fixed with a sealant (5) so that the substrates are held in parallel and at regular intervals. And At this time, the orientation control directions of the two substrates are set to be parallel to each other.

Then, the ferroelectric liquid crystal composition is heated to a cholesteric phase or an isotropic phase, injected into the cell, and then sealed. Two polarizing plates (6a) and (6b) are arranged outside the cell so that the polarizing plates are orthogonal to each other and form a certain angle with the orientation control direction of the substrate. This angle changes depending on the liquid crystal material, the operating temperature of the device, the driving method, etc., and the angle with the best contrast characteristics may be selected. In some cases, the polarization axes of the two polarizing plates are arranged so as to be slightly offset from the orthogonal. In some cases.

The light source (7) is placed on the side of the substrate (1b) so that the light is transmitted to the opposite side. In the case of using the reflection type, a reflection plate may be provided outside the polarizing plate (6b).

FIG. 2 shows a pattern example of the conductive films (2a) and (2b), which is used for a dot matrix display element and the like. One of the substrates has a horizontal stripe-shaped scan electrode group C _{1 to} Cn.
Patterning is performed on the other substrate, and vertical striped signal electrode groups S _{1 to} Sm are patterned on the other substrate. The intersections A _{11 to} Amn of the two electrode groups are pixels.

As the ferroelectric smectic liquid crystal used in the present invention, several kinds are known including the chiral smectic C phase (hereinafter abbreviated as SmC ^* ), but SmC is used because of easy alignment control or quick response. ^* It is preferable to use a liquid crystal having a phase. As a specific example, 4-
(4-n-decyloxybenzylideneamino) cinnamic acid ester and the like. Further, the characteristics may be realized by mixing some materials instead of the single materials, and for example, a mixture as shown in Table 1 is used.

The liquid crystal used in the present invention has a smectic phase (Sm) in a temperature range higher than the liquid crystal phase exhibiting ferroelectricity.
A liquid crystal having an (A phase) is preferable in terms of bistable symmetry.
When a liquid crystal that changes directly from an isotropic phase (I phase), a nematic phase (Ne phase) or a cholesteric phase (Ch phase) to a ferroelectric liquid crystal phase such as SmC ^* without passing through the SmA phase is usually used. There are two types of alignment states in which the direction of the liquid crystal molecular layer is different from the alignment control direction. When these two kinds of orientation states are mixed, the contrast is deteriorated. Therefore, when cooling from I phase, Ne phase or Ch phase to ferroelectric liquid crystal phase such as SmC ^* phase, a DC electric field with a unidirectional polarity is applied. However, it is necessary to take measures such as aligning only one of the two kinds of alignment states. In the stability of the element thus produced, the stable state, which is the same as the polarity of the electric field applied during cooling, is more stable among the first stable state and the second stable state. This leads to a decrease in bistability. On the other hand, in the liquid crystal having the SmA phase, there is only one direction of the liquid crystal molecular layer and no means for applying an electric field is required, and therefore the bistability is symmetrical with respect to the voltage and the bistability is good. .

Further, the liquid crystal used in the present invention preferably has a Ch phase in a temperature range higher than that of a liquid crystal phase exhibiting ferroelectricity, from the viewpoint of alignment uniformity. With respect to the method for producing the alignment of the liquid crystal, the element can be produced in a very good orientation by using the method disclosed in JP-A-61-153623.

It has a SmC ^* phase as a ferroelectric liquid crystal composition, has a Ch phase at a higher temperature, and has a helical pitch length (p) in the Ch phase between the substrates (1a) and (1b) (d). Liquid crystal that is four times longer than Also Ch phase and SmC ^*
Having an SmA phase between the phases is desirable in terms of orientation uniformity. Such a liquid crystal can be obtained by mixing an optically active substance, a smectic liquid crystal compound and a nematic liquid crystal compound in an appropriate ratio, and a non-liquid crystal additive may be added if necessary. In particular, in order to lengthen the pitch in the Ch phase, it is effective to mix an optically active substance that causes a left helix and an optically active substance that causes a right helix according to the magnitude of the force that causes the helix. .

Usually, the length of the helical pitch in the Ch phase changes with temperature. Cholesteric-
It is necessary to satisfy the condition of p> 4d just above the smectic phase transition point.

However, when the temperature range satisfying this condition is limited to the vicinity of the transition point, the spiral structure unintentionally transitions to the smectic phase when the temperature drop rate is high. In this case, since uniform orientation cannot be obtained, it is necessary to keep the temperature at which p> 4d is satisfied until the helical structure is unwound, or to slow the temperature drop rate. For this reason, the temperature range in which the helical pitch p is four times or more the distance d between the substrates is 5 ° C. from the cholesteric-smectic phase transition point.
It is preferable to cover the above range, and it is more preferable to cover the entire Ch phase temperature range.

It should be noted that the Ch phase referred to here also includes a Ne phase formed by a nematic liquid crystal in which an optically active substance is added to the nematic liquid crystal so as to have a unique pitch.

Further, it is preferable to provide a means for raising the temperature of the liquid crystal layer in case of alignment failure due to crystallization of liquid crystal or application of high voltage. As a means for this, a heater for increasing the temperature may be provided outside, but a simpler device can be obtained by applying a current to the electrode inside or outside the cell and heating directly.

[Operation] According to the present invention, by providing the surface of the alignment control film provided on the upper and lower substrates with the surface potential control film on at least one of the upper and lower substrates, a difference in polarity is imparted between the upper and lower substrates, and the ferroelectric property is improved. Since the interaction of the liquid crystal with the surface of the alignment control film is different between the upper substrate and the lower substrate, it is selectively controlled to one of two bistable states.

Example Next, an example of the present invention will be described.

In Fig. 2, a polyimide film (PIX-5400, Hitachi Chemical Co., Ltd.) was formed on the surface of a pair of substrates with 32 signal electrode groups on the upper substrate and 32 scanning electrode groups on the lower substrate. After baking for a minute, rubbing was performed in a certain direction with gauze to form an orientation control film.

Next, for the upper substrate, dilute epoxysilane solution (Shin-Etsu Chemical Co., Ltd. KBM-402 diluted with isopropyl alcohol)
The sample was immersed in the solution for 3 minutes, pulled up, and dried at 150 ° C. for 20 minutes to form a surface potential control film by an adsorption film.

Then, the upper and lower substrates were combined so that the rubbing directions were the same to form a cell having a cell interval of 1.5 μm, and the mixed liquid crystal shown in Table 1 was heated to about 100 ° C. and injected. Then, when the cell was cooled to 55 ° C. showing the SmC ^* phase, it was observed that the orientation was good and the cell was controlled to one of two bistable states over the entire surface. Further, in the case where the adsorption film is formed only on the lower substrate, the two bistable states are controlled to be different from the state where the adsorption film is formed only on the upper substrate. Furthermore, when the adsorption film is not provided, the orientation is good,
Two bistable states coexist, and even when electrolysis is applied, the bistable state of the display pixel portion cannot be controlled. Also, for cells with an adsorption film formed only on one side of the substrate, applying a ± 3 V, 0.5 Hz triangular wave, the transmitted light intensity was measured, and the hysteresis curve was examined with the horizontal axis as voltage and the vertical axis as transmitted light intensity. , The hysteresis loop was offset from V = 0 by 120 mV.

Example 2 A cell was prepared in the same manner as in Example 1 except that a dilute solution of aminosilane (KBM-602, Shin-Etsu Chemical Co., Ltd., diluted with isopropyl alcohol) was used instead of the epoxysilane of Example 1 to form a surface potential control film. Was prepared, it was controlled to one of two bistable states over the entire cell surface as in Example 1.

The hysteresis loop of this cell is deviated from V = 0 by 530 mV, the bistability deteriorates, and the voltage margin at the time of driving is reduced. Therefore, a good display can be obtained in a cell with a large variation in cell gap. There wasn't.

EFFECTS OF THE INVENTION The present invention has an excellent effect that a ferroelectric smectic liquid crystal electro-optical device having an excellent appearance can be obtained by adding a surface potential control technology to the conventional nematic liquid crystal alignment control technology. However, particularly when applied to a large dot matrix device, the bistable state of a fine portion between display pixels or a portion where electrodes do not face each other such as a portion where electrodes are drawn outside the cell is controlled to one state. Therefore, a black dot pattern can be displayed on a bright background. Further, even when bistability states are mixed due to static electricity or the like outside the cell, it is naturally recognized that the bistability state is controlled in advance within a few seconds to a few minutes and no permanent defect occurs. Furthermore, although the bistability necessary for driving for display is secured, when the power of the device is turned off, the display content automatically disappears within a few seconds to a few minutes. The effect of being obtained is also recognized.

In addition to the above, the present invention can be applied in various ways within the range of not impairing the effects of the present invention.

[Brief description of drawings]

FIG. 1 is a sectional view of a liquid crystal electro-optical device used in the present invention, and FIG. 2 is a plan view of an example of an electrode pattern used in the present invention. 1a, 1b: Transparent substrate 2a, 2b: Conductive film 3a, 3b: Alignment control film 4: Liquid crystal layer 5: Sealant 6a, 6b: Polarizing plate 7: Light source

Claims (7)

(57) [Claims] 1. A smectic liquid crystal exhibiting ferroelectricity is sandwiched between an upper substrate and a lower substrate each having an alignment control film on which a transparent electrode having a desired shape is formed, and a voltage is applied between the transparent electrodes of the upper and lower substrates. In a liquid crystal electro-optical device provided with a means for applying a voltage, a surface potential control film is provided on at least one of the upper and lower substrates, the alignment control films of the upper and lower substrates are formed of the same material, and the alignment control film is uniaxially oriented. A ferroelectric smectic liquid crystal electro-optical device, wherein a surface potential difference generated between the upper and lower substrates by the surface potential control film is 50 mV or more and 500 mV or less. 2. The ferroelectric liquid crystal is chiral smectic C.
The ferroelectric smectic liquid crystal electro-optical device according to claim 1, which is a liquid crystal. 3. The ferroelectric smectic liquid crystal electro-optical device according to claim 1, wherein the alignment control film is subjected to a rubbing treatment. 4. A ferroelectric smectic liquid crystal electro-optical device according to claim 3, wherein the rubbing directions of the upper and lower substrates are the same. 5. The ferroelectric smectic liquid crystal electro-optical device according to claim 1, 2 or 3, wherein the surface potential control film is formed on a surface of the alignment control film in contact with liquid crystal. 6. A ferroelectric smectic liquid crystal electro-optical device according to any one of claims 1 to 5, wherein a surfactant is used in the surface potential control film. 7. The ferroelectric smectic liquid crystal electro-optical device according to claim 6, wherein the surfactant is a silane surface treatment agent.