JP3378038B2 - Driving method of electro-optical device - Google Patents

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Application] A liquid crystal material having spontaneous polarization is sandwiched between transparent substrates having electrodes, and is driven and displayed with a voltage constantly applied without utilizing the memory property. The present invention relates to a liquid crystal electro-optical device used for a high-speed and high-gradation liquid crystal display, which is driven by a liquid crystal light shutter or a thin film transistor.

[0002]

2. Description of the Related Art Electronic devices using liquid crystals are expected to be applied not only to clocks and thermometers, but also to a wide range of video equipment such as word processors, laptop computers and television receivers.

The appeal of ferroelectric liquid crystals having spontaneous polarization in the liquid crystal itself has been widely known by Clark and Lagerval. The antiferroelectric liquid crystal, which is the opposite of this, was widely introduced by Chandani et al. These have characteristics different from those of nematic liquid crystals such as a twisted nematic (abbreviated as TN) liquid crystal display and a super twisted nematic (abbreviated as STN) liquid crystal display which have been widely used until now.

In the schematic diagram shown in FIG. 1, the ferroelectric liquid crystal has liquid crystal molecules 10 according to the orientation control of the surface of the substrate 100.
2 are arranged in a fixed direction. A layered structure 101 is formed between the liquid crystal molecules, and they are highly ordered in the three-dimensional direction. The orientation of the long axis of the liquid crystal molecule is
State 102 and second state 103.

Ferroelectric liquid crystals have a spontaneous polarization Ps (C /
m ² ), which is indicated by the arrow in FIG. The direction of the spontaneous polarization is antiparallel to the direction of the electric field due to the torque Ps · E which is the product of the spontaneous polarization and the electric field intensity E (V / m) generated in the direction perpendicular to the substrate surface generated by applying a voltage to the liquid crystal cell. To move. In accordance with the movement, the liquid crystal molecule long axis switches between the two states of the first state 102 and the second state 103. In other words, which of the two states is taken can be controlled by the direction of the electric field applied thereto.

It is possible to perform high-speed rewriting between two states as shown in FIG. 1 by switching using spontaneous polarization, to be able to stably maintain the state even after application of an electric field is finished, and through a polarizing plate. Since it has excellent properties such as distinguishing the two states over a wide viewing angle when observed, it is highly expected as a liquid crystal material that can realize a high-speed and large-capacity screen.

Generally, the ferroelectric liquid crystal is displayed by simple matrix driving in which it is sandwiched between substrates on which a large number of strip-shaped electrodes are arranged. A characteristic of the ferroelectric liquid crystal, which is not provided in the nematic liquid crystal that has a memory property that can stably hold the state of applying an electric field, is used there.

Utilizing this characteristic, for example, the number of pixels is 1000
It has been applied to very high density displays of x1000 or more. However, at that time, a driving method such as a 2-field method or a 4-pulse method is usually used. According to the driving method, an alternating voltage, which is always a small voltage, is applied, but the optical response of the liquid crystal fluctuates due to the waveform, and the contrast ratio is considerably lowered.

When the ferroelectric liquid crystal is actually sandwiched between the substrates and observed with a microscope, not only those in which the spontaneous polarization shows a uniform orientation indicating either of the substrates as shown in FIG. A splay orientation with an inward or outward orientation is observed. At this time, the direction of the long axis of the liquid crystal molecules is twisted between the substrates.
Those exhibiting such a twisted orientation cannot attain an extinction position, have a low contrast ratio, and have a problem that a memory property cannot be obtained, and thus the orientation is not practical.

This orientation is in a twisted state when viewed from the long axis of the molecule, and is in a splayed state when viewed from the spontaneous polarization. In the case of a ferroelectric liquid crystal, both means the same orientation state. .

When obtaining a display with a high contrast ratio, it is a major premise that the uniform orientation is exhibited and the memory property is maintained, but it is difficult to maintain this over a wide temperature range. Orientation appears.

It has become necessary to display more stably by another driving method without being obsessed with driving only a simple matrix to drive the ferroelectric liquid crystal having such a memory property.

[0013]

In order to stably drive a liquid crystal material having spontaneous polarization, bright and dark states may be created by direct driving in which a voltage is always applied during display. At this time, the spontaneous polarization and the optical response can be completely controlled by the direction of the electric field applied from the outside without utilizing the memory property.

Considering such a matter, it cannot be displayed in a simple matrix panel, but it is used as a simple shutter having only one pixel. This is used for a shutter or the like that controls on / off of a large amount of light for projection. This driving method also includes driving a thin film transistor (hereinafter abbreviated as TFT) provided in the substrate pixel. In any case, it is highly useful as it can fully utilize the region of high-speed response and high contrast ratio that cannot be realized with nematic liquid crystals.

However, in order to display the ferroelectric liquid crystal by such a driving method, it is not always good to simply divert the liquid crystal material used in the simple matrix.

That is, a liquid crystal that is driven by a simple matrix has a memory property, and an angle formed by a layer structure formed by a substrate and a ferroelectric liquid crystal is called a pretilt angle. It is as small as 15 °.
The angle formed by the liquid crystal at this time between the first state and the second state is often narrow. This angle (referred to as a cone angle) becomes a cone angle of approximately 10 to 38 °.

In order to increase the contrast ratio, it is required that the bright transmittance in the bright state is high and the dark transmittance in the dark state is low. In order to maximize the bright transmittance of the ferroelectric liquid crystal, the cone angle needs to be 45 °.

Therefore, the cone angle of the material for the simple matrix is too small to be suitable for a direct drive panel aiming at high transmittance. Therefore, it is necessary to design the material for direct drive.

[0019]

When a liquid crystal material having spontaneous polarization is used by direct driving controlled by an externally applied electric field, a high contrast ratio and high speed response are required. To increase the contrast ratio, it is necessary to maximize the light transmittance and minimize the dark transmittance.

In the present invention, as shown in FIG. 2, a liquid crystal material 194 having spontaneous polarization is sandwiched between transmissive substrates 190 and 192 having electrodes 195, and one substrate 1 in contact with the liquid crystal is provided.
A device having a means 191 for arranging a liquid crystal material in a uniaxial orientation direction only on the 90th surface, and when no voltage is applied from the electrode to the liquid crystal material, or in a portion, spontaneous polarization is generated with respect to the substrate spacing direction. The present invention relates to a liquid crystal electro-optical device, which is in a splay state that is inward or outward, and has a uniform spontaneous polarization when a voltage is applied from the electrodes to the liquid crystal material.

Further, the present invention relates to a liquid crystal electro-optical device characterized by applying a voltage equal to or higher than a voltage for maintaining the uniform state of liquid crystal in order to form the uniform state.

Further, the present invention relates to a liquid crystal electro-optical device characterized in that when the uniform of the liquid crystal material is formed, the angle formed by the liquid crystal molecules in the first state and the second state is 40 to 50 °. .

Furthermore, the liquid crystal electro-optical device is characterized in that the polarization direction of the polarizing plate 197 and the direction of the stable state indicated by the electro-optical characteristics of the device are matched.

[0024]

According to the present invention, there is provided a means for sandwiching a liquid crystal material having spontaneous polarization between transparent substrates having electrodes and for arranging the liquid crystal material in a uniaxial orientation direction only on one substrate surface in contact with the liquid crystal. Use the device. At this time, the dark state is not observed by the polarization microscope, and the spontaneous polarization is a splay state 198 inward or outward in the substrate spacing direction.

When a voltage is applied to this cell and the optical characteristics are measured, the characteristics show monostable characteristics. What is a monostable state?
Positioning in one of the states that liquid crystal molecules have is that it has electro-optically stable characteristics. A low dark transmittance can be obtained by aligning the polarization axis with the long axis of the liquid crystal molecules aligned on the side of this stable state.

Next, when a voltage is applied to the liquid crystal material, spontaneous polarization becomes uniform state 199, and a good dark state 200 is obtained.

In order to create the uniform state described above, it is necessary to apply a voltage higher than the voltage for maintaining the uniform state without causing the splay alignment of the liquid crystal. When examined in various systems, a voltage of 3 to 7 V or higher is required.

As described above, it is considered that a weak splay state is actually formed because the uniform orientation is easily changed by applying a voltage. It can be understood by looking at the electro-optical properties when it has orientation means on both sides.

Further, the cone angle of the material that can obtain a uniform state only when a voltage is applied is wide over a wide temperature range, and the angle formed by the liquid crystal molecules in the first state and the second state is 40 to 50. It becomes easy to take °.

This makes it easy to directly drive a liquid crystal material having spontaneous polarization by always applying a voltage.
It can be fully used for driving with a simple shutter or TFT.

[0031]

【Example】

[Example 1] A ferroelectric liquid crystal cell in which an alignment means is formed only on one substrate will be described. FIG. 2 shows the result of evaluating the state-maintaining voltage characteristics of the liquid crystal cell having spontaneous polarization.
It describes using. Indium-tin-oxide (abbreviated as ITO) 195 was formed on the blue plate glass 190 by a sputtering method or a vapor deposition method to a film thickness of 500 to 2000 Å, and patterned by a normal photolithography process. 2 this board
Sheets 190 and 192 were prepared, and polyimide 191 was applied onto one of the substrates 190 by a spin coating method, and baked at 280 ° C. As the polyimide, Hitachi Chemical LQ5200 or Toray LP-64 was used. Thickness is 100-300
It was Å. This substrate was subjected to rubbing treatment to obtain uniaxial orientation treatment. This was opposed to the other glass substrate 192 not provided with an alignment film. On the orientation film side substrate, a spacer (not shown in the drawing) of silica gel particles made by Catalysts and Chemicals Co., Ltd., 1.5 μm in diameter, is scattered, and a sealing agent 193 made of epoxy resin is formed on the counter electrode side by screen printing. did. Both substrates were attached to each other to a thickness of 1.5 μm according to the diameter of the spacer. Phenylpyrimidine-based ferroelectric liquid crystal 194 was injected into the cell. The phase sequence was isotropic phase-smectic A phase-smectic C ^* phase-crystalline phase. The magnitude of spontaneous polarization is 6 nC / cm ² , the relative dielectric constant is 4.4, and the response speed when a voltage of 14 V is applied to the cell is 100 μ.
It was seconds. The transition temperature to the isotropic phase was about 90 ° C., the liquid crystal material was heated to 110 ° C., and vacuum injection was performed while the liquid crystal was in the isotropic phase.

The alignment state was confirmed by a polarization microscope without applying a voltage. Regarding the orientation of the electrode portion and the portion other than the electrode, the position in the dark state was not obtained even when the cell was rotated, and the splay orientation was exhibited.

The cone angle was measured by applying a voltage of 20V. The results are shown in Fig. 8. The cone angle showed a stable angle of 43 ° ± 2 ° in the temperature range of 0 to 50 ° C.

A square wave was applied to this from 20 to 0.1 V, and the optical response at that time was measured. The transmittance of the liquid crystal cell changes rapidly when a high voltage is applied, and it takes 1 second or more to saturate when a low voltage is applied. The transmittance at that time was examined. The results are shown in Fig. 3. At this time, the position of the extinction position obtained by matching the long axis of the liquid crystal molecule with that of the polarizing plate is 150 when the polarizer is adjusted to the first state.
(O plot) and 151 (□ plot) when combined with the second state were measured.

When the polarizer was adjusted to the first state, a stable dark state was obtained from a high voltage to a low voltage, and a uniform state 199 was obtained. The contrast ratio became about 100.

However, when the polarizer was adjusted to the second state, the increase in the dark state was large at about 5 V or less. The state maintaining voltage is 5V. A large voltage of 15V was required to make the contrast ratio when the polarizer was adjusted to the first state and the contrast ratio when the polarizer was adjusted to the second state. Molecules pointing in this direction are not stable.

From the above, the polarizing plates 197 and 20 shown in FIG.
When the polarizing axis of the polarizing plate is arranged in the first state of No. 2, the contrast ratio is high and a good panel can be formed.

[Embodiment 2] A case where the liquid crystal material having the characteristics shown in Embodiment 1 is injected into a cell provided with TFTs will be described with reference to FIG. A self-aligned n-channel polysilicon TFT 281 was formed on a Corning 7059 substrate 280 by a normal low temperature process. The source side 292 is connected to the lead electrode 287, and the drain side 28
In No. 8, the pixel electrode 282 was arranged with ITO. Similarly, a Corning 7059 substrate 283 was used as the opposite substrate, and after forming the pixel common electrode 286 with ITO on the substrate, polyimide 289 was applied by spin coating and baked at 280 ° C. As polyimide, Hitachi Chemical LQ1500,
Alternatively, Toray LP-64 was used. The thickness is 100-30
It was 0Å. This substrate was subjected to rubbing treatment to obtain uniaxial orientation treatment. At this time, when rubbing the TFT substrate side, each lead electrode was dropped to the ground, and sufficient consideration was given so that the device would not be destroyed by static electricity.

Next, a spacer of 1.5 μm of spherical particles made by Catalysts and Chemicals made of silica, which is a silica particle, was sprinkled on the TFT substrate side, and a sealant, in this case an epoxy resin, was screen printed on the counter electrode side. Both substrates were attached to each other to a thickness of 1.5 μm according to the diameter of the spacer. Phenylpyrimidine-based ferroelectric liquid crystal 290 was injected into the cell. The phase sequence was isotropic phase-smectic A phase-smectic C ^* phase-crystalline phase. The transition temperature to the isotropic phase was about 80 ° C., the liquid crystal material was heated to 100 ° C., and vacuum injection was performed while the liquid crystal was in the isotropic phase.

The gate-drain characteristics of a typical TFT are shown in FIG. An on / off ratio of about 7 digits was obtained.

When no voltage was applied, a splay alignment was exhibited, but when a voltage higher than the state maintaining voltage was applied from the TFT, the direction of spontaneous polarization between the substrates was uniform.
Changed to uniform orientation. The on / off state of the liquid crystal was completely controlled by the direction of the electric field generated in the pixel electrode, and the contrast ratio at that time was about 100.

"Comparative Example 1" Using the same liquid crystal material as in Example 1, the characteristics when the alignment films were formed not on one of the two substrates but on both sides were examined. As shown in FIG. 6, since the alignment means is symmetric, the characteristics were the same regardless of the extinction position.

The state maintaining voltage was about 3 V, and a rise in the dark state was observed below that voltage.

Even when a high voltage was applied, good dark transmittance was not obtained, and the contrast ratio was 20 to 40, which was not good characteristics.

[Embodiment 3] The simple shutter used in Embodiment 1 is 3 panels and the TFT cell used in Embodiment 2 is 1.
A liquid crystal projector was created by using one piece. This will be described with reference to FIG. Using the dichroic mirror 171, red,
The light is divided into three colors, green and blue, and each light is made incident on the simple shutter 172. The light is introduced into the TFT cell 173 and projected onto the wall 175 using the lens system 174.

At this time, the light of three colors is incident on the TFT cell by a simple shutter and only one specific color, for example, red light, and at that time, a red image signal is formed in the TFT cell. The red shutter image, the green shutter image, and the blue shutter image are sequentially switched between the simple shutter and the TFT cell. When the formation time of the image of one color is switched in 5 ms, a color image of the projected image of 60 Hz is formed. The simple shutter was turned on and off by applying a voltage of -10 V for 5 msec and turned off by applying 10 V for 10 msec.
The response speed of the liquid crystal is 100 μsec at this voltage, and it optically changes corresponding to the response waveform. The image on the TFT panel was switched in correspondence with the projection of any one of these three colors. With this driving frequency, it is possible to display a stable TV image without flicker in the eyes of the observer.

[0047]

According to the present invention, a liquid crystal material having spontaneous polarization is driven in a state where a voltage is always applied, and unlike the conventional driving utilizing a memory property in which an AC waveform is applied, a wide temperature range is applied. It can be displayed stably. The present invention can be widely applied to things such as those driven by simple shutters and TFTs, and can also be applied to liquid crystal televisions and projections.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a ferroelectric liquid crystal.

FIG. 2 is a diagram showing the structure of a cell of the present invention.

FIG. 3 is a diagram showing the effect of the present invention.

FIG. 4 is a diagram in which the present invention is applied to a TFT cell.

FIG. 5 is a diagram showing TFT characteristics.

FIG. 6 is a diagram showing a comparative example with respect to the present invention.

FIG. 7 is a diagram showing an application example of liquid crystal projection using the present invention.

FIG. 8 is a diagram showing temperature dependence of a cone angle of liquid crystal.

Continued front page       (56) Reference JP-A-3-287231 (JP, A)                 JP 63-284525 (JP, A)                 JP-A-3-243915 (JP, A)                 JP-A-4-278914 (JP, A)                 JP 62-235927 (JP, A)                 JP-A-2-148022 (JP, A)                 JP-A-2-219821 (JP, A)

Claims (5)

(57) [Claims] 1. A pair of substrates, an electrode formed on each of the pair of substrates, a liquid crystal having spontaneous polarization sandwiched between the pair of substrates, and the liquid crystal formed on only one of the pair of substrates. It has a means for uniaxially aligning the liquid crystal and a polarizing plate, and when a voltage is applied to the liquid crystal, the spontaneous polarization becomes the first uniform state or the second uniform state depending on the direction of the electric field, and the voltage is applied to the liquid crystal. when but not applied, prior Symbol spontaneous polarization Ri is do the splay state to the interval direction of the pair of substrates, the spontaneous polarization of the first uniform states or the
A driving method of an electro-optical device, comprising: aligning a molecular long axis of the liquid crystal with a polarization axis of the polarizing plate in any one of the second uniform states , wherein the spray state is not used when driving. Always voltage
Is applied to use only the first uniform state and the second uniform state, and the cone angle of the liquid crystal is 43 ° ± 2 ° in the temperature range of 0 to 50 ° C.
The method for driving an electro-optical device is characterized in that 2. A pair of substrates, an electrode formed on each of the pair of substrates, a thin film transistor connected to an electrode formed on one of the pair of substrates, and a spontaneous sandwiched between the pair of substrates. A polarized liquid crystal, a means for uniaxially aligning the liquid crystal formed on only one or the other of the pair of substrates, and a polarizing plate, and when a voltage is applied to the liquid crystal, the spontaneous polarization becomes the first uniform state or a second uniform state, when a voltage to the liquid crystal is not applied, Ri do the previous SL spontaneous polarization splay state in the interval direction of the pair of substrates, the spontaneous polarization The first uniform state or the
A driving method for an electro-optical device comprising combining the polarization axis of the polarizing plate and the molecular long axis of the liquid crystal in any one of the states of the second uniform state, when driving, use the splay state Not always with voltage
Is applied to use only the first uniform state and the second uniform state, and the cone angle of the liquid crystal is 43 ° ± 2 ° in the temperature range of 0 to 50 ° C.
The method for driving an electro-optical device is characterized in that 3. The driving of the electro-optical device according to claim 1, wherein a voltage when a voltage is applied to the liquid crystal is a voltage of 3 V or more that can maintain the uniform state. Method. 4. The method according to any one of claims 1 to 3,
A method of driving an electro-optical device, wherein the liquid crystal is a phenylpyrimidine-based ferroelectric liquid crystal. 5. The method according to any one of claims 1 to 4,
A method of driving an electro-optical device, wherein a distance between the pair of substrates is 1.5 μm.