JPH10301089A - Liquid crystal element and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the liquid crystal display element utilizing an anti- ferroelectric liquid crystal material which provides a stable gradation display. SOLUTION: A liquid crystal layer 21 is sealed in between substrates 11 and 12. The layer 21 shows an anti-ferroelectric phase, in a bulk condition, depending on an applied voltage. The layer 21 shows the anti-ferroelectric phase while the layer 21 is sealed between one substrate and the other substrate and no voltage is applied. Moreover, the layer 21 is sealed to show the mixed phase, in which liquid crystal molecules of the intermediate phase oriented in the anti-ferroelectric, a ferroelectric and other conditions are mixed, while a voltage is applied. Depending on the applied voltage, the mixed phase is generated in the layer 21, the percentage of the liquid crystal molecules of the ferroelectric, the anti-ferroelectric and the intermediate phases varies and the optical axis of the layer 21 is continuously varied in accordance with the applied voltage. Thus, a gradation display is made possible by arranging polarizing plates 23 and 24 holding the substrates 11 and 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using an antiferroelectric liquid crystal (AFLC), and more particularly to an antiferroelectric liquid crystal display device capable of gradation display and a driving method thereof. .

[0002]

2. Description of the Related Art A ferroelectric liquid crystal display device using a ferroelectric liquid crystal has attracted attention in that a high-speed response and a wide viewing angle can be obtained as compared with a TN mode liquid crystal display device using a nematic liquid crystal. I have. As a ferroelectric liquid crystal display element, a ferroelectric liquid crystal display element using a ferroelectric liquid crystal and an antiferroelectric liquid crystal display element using an antiferroelectric liquid crystal are known. The antiferroelectric liquid crystal display element displays an image by utilizing the stability of the alignment state of the antiferroelectric liquid crystal.

More specifically, an antiferroelectric liquid crystal has three stable states in the alignment of liquid crystal molecules.
Is applied to the liquid crystal, the liquid crystal molecules are arranged in the first direction according to the polarity of the applied voltage.
(2) When a voltage equal to or lower than a second threshold lower than the first threshold is applied to the first ferroelectric phase or the second ferroelectric phase arranged in the second direction, And the second ferroelectric phase is oriented to an antiferroelectric phase that is in a different arrangement state. By setting the direction of the transmission axis of a pair of polarizing plates disposed on both sides of the liquid crystal display element with reference to the optical axis in the antiferroelectric phase, the light transmittance is controlled by the applied voltage to display an image. be able to.

[0004] The antiferroelectric liquid crystal has a first or second ferroelectric phase or an antiferroelectric phase as long as the applied voltage changes within a range between the first and second threshold values. Is maintained. That is, it has a memory property. The conventional antiferroelectric liquid crystal display device is driven by a simple matrix utilizing this memory property.

[0005] The memory properties of antiferroelectric liquid crystals are as follows.
Alternatively, it is determined by a voltage difference between a voltage at which a phase transition from the second ferroelectric phase to the antiferroelectric phase and a voltage at which the phase transition from the antiferroelectric phase to the first or second ferroelectric phase. The larger the voltage difference, the higher the memory property of the alignment state. That is, the larger the hysteresis of the optical characteristic, the higher the memory property.

For this reason, in the conventional anti-ferroelectric liquid crystal display element driven by simple matrix, the liquid crystal having a large voltage difference is used as the anti-ferroelectric liquid crystal.

[0007]

However, a conventional antiferroelectric liquid crystal display device using an antiferroelectric liquid crystal having a high memory property cannot arbitrarily control the light transmittance. That is, the control of the display gradation is almost impossible, and the gradation display cannot be realized.

The present invention has been made in view of the above circumstances, and has as its object to provide an antiferroelectric liquid crystal display device capable of realizing a clear gradation display.

[0009]

In order to achieve the above object, a liquid crystal display device according to a first aspect of the present invention is a liquid crystal display device in which a plurality of pixel electrodes and active elements connected to the pixel electrodes are arranged in a matrix. And the other substrate on which a common electrode opposed to the pixel electrode is formed, and sealed between the substrates to form a chiral smectic CA phase in a bulk state, and the one substrate and the other substrate In the meantime, in the absence of an electric field in which no voltage is applied between the pixel electrode and the common electrode, an antiferroelectric phase is formed, and in the electric field in which a voltage is applied between the pixel electrode and the common electrode, It is characterized by comprising a liquid crystal layer showing a mixed phase in which an antiferroelectric phase, a ferroelectric phase and an intermediate phase oriented in another state are mixed.

The liquid crystal used in this liquid crystal display element shows a mixed phase. That is, the liquid crystal molecules in the ferroelectric phase, the liquid crystal molecules in the antiferroelectric phase, and the liquid crystal molecules that behave along the cone drawn by themselves (behave by drawing the cone) and are aligned with a tilt with respect to the substrate. It exhibits a mixed phase in which liquid crystal molecules in a state are mixed. In this mixed phase, the ratio of the molecules of the liquid crystal in the ferroelectric phase and the liquid crystal in the antiferroelectric phase and the average orientation direction of the liquid crystal molecules in the state of being tilted with respect to the substrate surface are continuously changed according to the applied voltage. Changes to Therefore, the liquid crystal display device using the liquid crystal exhibiting the antiferroelectricity can display an image at an arbitrary gradation by continuously changing the display gradation.

In order to achieve the above object, a second aspect of the present invention is provided.
The liquid crystal display element according to the aspect of the present invention, one substrate on which an electrode is formed, the other substrate on which an electrode opposed to the electrode is formed, and a liquid crystal molecule that is sealed between the substrates and forms a smectic phase. Having a spontaneous polarization, forming an antiferroelectric phase arranged to draw a double helical structure in a bulk state, between the one substrate and the other substrate, in an electric field-free state where no electric field is applied, Antiferroelectricity, in which the helical structure is unwound and the liquid crystal molecules oriented in the first direction and the liquid crystal molecules oriented in the second direction are alternately arranged, so that the spontaneous polarization of the liquid crystal molecules in adjacent smectic layers cancel each other out. In a state where a phase is formed and a sufficiently large electric field is applied, the liquid crystal molecules exhibit a ferroelectric phase in which the liquid crystal molecules are oriented in the first direction or the second direction depending on the polarity, and in a state where an intermediate voltage is applied. , The first
And the liquid crystal molecules oriented in the second direction, and the liquid crystal molecules acted along the cone drawn by the liquid crystal molecules and tilted with respect to the principal surfaces of the one and other substrates. And a liquid crystal layer exhibiting a mixed phase in which liquid crystal molecules oriented in a state are mixed.

Also in this configuration, the ratio of the molecules of the ferroelectric phase liquid crystal and the antiferroelectric phase liquid crystal and the average orientation direction of the liquid crystal molecules in the state of being tilted with respect to the substrate surface depend on the applied voltage. Change continuously. For this reason, the image can be displayed at an arbitrary gradation by continuously changing the display gradation.

The antiferroelectric phase and the mixed phase formed in the liquid crystal layer form a chiral smectic CA phase in a bulk state, and a phase transition precursory phenomenon at the time of a phase transition from the antiferroelectric phase to the ferroelectric phase. Is obtained by using a liquid crystal material having a large value. That is, chiral smectic C in bulk state
The liquid crystal forming the A phase receives the alignment regulating force by the alignment treatment of the alignment film in a state of being sealed between the one substrate and the other substrate, but the intermolecular force of the liquid crystal is regulated by the alignment treatment by the alignment treatment. Since the force is larger than the force and the thickness of the liquid crystal layer is equal to or relatively larger than the helical pitch of the chiral smectic CA phase, the liquid crystal layer is oriented to the helically unwound antiferroelectric phase. Then, when a voltage is applied between the opposing electrodes,
A liquid crystal having a large phase transition precursor phenomenon has a small phase transition energy gap between the antiferroelectric phase and the ferroelectric phase, and therefore moves along the virtual cone of the chiral smectic CA (tilts) to an intermediate state. Liquid crystal molecules to be aligned are generated. Therefore, the director of the liquid crystal layer changes continuously, and a gray scale display element becomes possible.

Further, according to a third aspect of the present invention, there is provided a method of driving a liquid crystal display element, wherein a bulk is provided between one substrate on which an electrode is formed and the other substrate on which an electrode opposed to the electrode is formed. A chiral smectic CA phase is formed in the state described above, and an antiferroelectric phase is formed between the one substrate and the other substrate in an electric field-free state in which no voltage is applied between the electrodes facing each other. In a state where an electric field is applied with a voltage applied between the electrodes, a liquid crystal layer formed by a mixed phase in which an antiferroelectric phase, a ferroelectric phase, and an intermediate phase oriented in another state are sealed, and the opposing electrodes are sealed. The method is characterized in that the mixed phase is generated by applying a voltage therebetween, and the director of the liquid crystal is changed, thereby performing a gradation display. According to this driving method, a liquid crystal that generates a mixed phase is used and its director is changed, so that an arbitrary gradation can be displayed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An antiferroelectric liquid crystal display device capable of displaying a halftone according to an embodiment of the present invention will be described below with reference to the drawings.

The antiferroelectric liquid crystal display device is of an active matrix type and includes a pair of transparent substrates (eg, glass substrates) 11 and 12. In FIG. 1, a lower substrate (hereinafter, referred to as a lower substrate) 11 includes a transparent pixel electrode 13.
And active elements 14 connected to the pixel electrodes 13 are arranged in a matrix.

The active element 14 is composed of, for example, a thin film transistor (hereinafter, TFT). TFT14
A gate electrode formed on the substrate 11, a gate insulating film covering the gate electrode, a semiconductor layer formed on the gate insulating film, a source electrode and a drain electrode formed on the semiconductor layer, Consists of

Further, as shown in FIG. 2, a gate line (scan line) is provided between the rows of the pixel electrodes 13 on the lower substrate 11.
15 are wired. Further, a data line (gradation signal line) 16 is wired between columns of the pixel electrodes 13.
The gate electrode of each TFT 14 corresponds to the corresponding gate line 15
And the drain electrode is connected to the corresponding data line 16.
It is connected to the.

The gate line 15 is connected to a row driver (row drive circuit) 31 via an end 15a. The data line 16 is connected to a column driver (column driving circuit) 32 via an end 16a. The row driver 31 scans the gate line 15 by applying a gate signal described later. On the other hand, the column driver 32 receives the display data (gradation data) and applies a data signal corresponding to the display data to the data line 16.

The gate line 15 is covered with a gate insulating film (transparent film) of the TFT 14 except for the terminal portion 15a.
The data line 16 is formed on the gate insulating film. The pixel electrode 13 is made of ITO or the like, is formed on a gate insulating film, and has a TFT 14 at one end thereof.
Are connected to the source electrode of

In FIG. 1, a transparent common electrode 17 facing each pixel electrode 13 of a lower substrate 11 is formed on an upper substrate (hereinafter, upper substrate) 12. Common electrode 17
Is made of ITO or the like, is made up of one electrode having an area covering the entire display area, and is applied with a reference voltage V0. The pixel electrode 13 and the common electrode 17 control the orientation direction of the liquid crystal molecules by applying a voltage to the liquid crystal layer 21 therebetween, thereby changing the direction of the director (the average direction of the long axis of the liquid crystal molecules) continuously. To thereby control the optical axis of the liquid crystal layer continuously, thereby controlling the display gradation.

Alignment films 18 and 19 are provided on the electrode formation surfaces of the lower substrate 11 and the upper substrate 12, respectively. The alignment films 18 and 19 are horizontal alignment films, which have been subjected to an alignment process by rubbing in the same direction (a third direction 21C in FIG. 3 described later), so that liquid crystal molecules in the vicinity are aligned in the alignment direction 21C.
It has an alignment regulating force to be arranged in a matrix. Alignment film 1
8 and 19 preferably have a small surface energy and a relatively small alignment regulating force, for example, a thickness of 25 to 35.
consisting of an organic polymer compound such as polyimide of about nm,
Rubbing has been applied. As the alignment films 18 and 19, those having a dispersion force esd of 38 to 41 and a polar force esp relatively weak and about 4 to 10 are desirable.

The lower substrate 11 and the upper substrate 12 are bonded to each other at the outer peripheral edge thereof via a frame-shaped sealing material 20.
A liquid crystal layer 21 is sealed in a region surrounded by the sealant 20 between the substrates 11 and 12. The thickness of the liquid crystal layer 21 is
It is regulated by the transparent spacer 22. Spacer 2
Numerals 2 are scattered in the liquid crystal sealing area.

The liquid crystal layer 21 includes (1) a chiral smectic CA ^* (SmCA ^* ) phase in a bulk state, and (2) a substrate 1
The liquid crystal molecules are sealed between Nos. 1 and 12 in an electric field-free state in which no voltage is applied. (3) When a sufficiently large voltage is applied, the liquid crystal molecules are changed according to the polarity of the applied voltage as shown in FIG.
And the first direction 21A or the second direction 21B shown in FIG.
And (4) a mixed phase in which an anti-ferroelectric phase, a ferroelectric phase, and a liquid crystal molecule of an intermediate phase different from these phases are mixed when an intermediate electric field is applied. It is composed of a liquid crystal material. The details of the liquid crystal layer 21 will be described later.

A pair of polarizing plates 2 are provided above and below the liquid crystal display element.
3, 24 are arranged. As shown in FIG. 3, the optical axis (hereinafter referred to as the transmission axis) 23A of the lower polarizing plate 23 is set substantially parallel to the normal direction of the smectic layer, which substantially coincides with the third direction 21C. An optical axis (hereinafter referred to as a transmission axis) 24A of the upper polarizing plate 24 is set substantially perpendicular to a transmission axis 23A of the lower polarizing plate 23.

In the antiferroelectric liquid crystal display device in which the transmission axes of the polarizing plates 23 and 24 are set as shown in FIG. 3, the director of the liquid crystal layer 21 is substantially oriented in the first or second orientation direction 21A or 21B. In the ferroelectric phase, the transmittance becomes almost maximum (display is brightest). When the director of the liquid crystal layer 21 is in an antiferroelectric phase substantially oriented so as to face the third direction 21C, the transmittance becomes substantially minimum (display is darkest).

That is, the liquid crystal molecules move in the first or second direction 2
1A and 21B, the light in the linearly polarized state that has passed through the transmission axis 23A of the polarizing plate 23 on the incident side is the liquid crystal layer 21.
The polarization state changes due to the birefringent action of
4, the light having a component parallel to the transmission axis 24A of the output side polarizing plate 24 is transmitted, and the display becomes bright. In a state where the director faces the third direction 21C, the polarizing plate 23 on the incident side is used.
Of the liquid crystal layer 21 hardly receives the birefringent action of the liquid crystal layer 21. For this reason, the polarizing plate 2 on the incident side
The linearly polarized light passing through 3 passes through the liquid crystal layer 21 as linearly polarized light, is almost absorbed by the polarizing plate 14 on the emission side, and the display becomes dark. When the liquid crystal layer 21 is in the optically intermediate state,
A gradation corresponding to the direction of the director is obtained.

Next, the alignment films 18 and 19 and the liquid crystal layer 21 will be described in more detail. The liquid crystal layer 21 is, for example, a liquid crystal containing a liquid crystal composition having a skeletal structure represented by Chemical Formula 1 as a main component, and has physical properties as shown in Table 1.

[0029]

Embedded image

[0030]

Table 1 Phase Series Crystal -30 ° C-SmCA ^* -69 ° C-SmA-80 ° C-I
SO Spontaneous polarization 229 nC / cm ² Cone angle θ 32 ゜ Spiral pitch 1.5 microns

Here, the cone angle is the angle between the cone axis drawn by the liquid crystal and the cone, and the intersection angle between the first direction 21A and the second direction 21B is 2θ which is twice the cone angle θ. Equivalent to.

A liquid crystal having such a structure and physical properties is as follows.
The barrier of the potential energy between the antiferroelectric phase and the ferroelectric phase is small, the order of the antiferroelectric phase is easily disturbed, and the phase transition precursor phenomenon is large as compared with ordinary antiferroelectric liquid crystals. The phase transition precursor phenomenon is shown in FIG. 3 before the phase transition from the antiferroelectric phase to the ferroelectric phase occurs when the electric field applied to the liquid crystal molecules forming the antiferroelectric phase is gradually increased. This refers to a phenomenon in which the transmittance of a liquid crystal element in an optical arrangement increases, and an increase in the transmittance means that liquid crystal molecules behave before phase transition. The behavior of the liquid crystal molecules before the phase transition means that the barrier of the potential energy between the antiferroelectric phase and the ferroelectric phase is small.

In a bulk state, the liquid crystal material of the liquid crystal layer 21 has a layer structure and a helical structure of a molecular arrangement as shown in FIG. 4, and adjacent liquid crystal molecules are formed on a virtual cone for each layer. Has a double helical structure in which a helix is shifted by approximately 180 °, and the spontaneous polarization is canceled between liquid crystal molecules of adjacent smectic layers.

The thickness (cell gap) of the liquid crystal layer 21 is substantially equal to one pitch (natural pitch) of the spiral structure of the liquid crystal material (1.5 microns). Further, the liquid crystal molecules receive an alignment regulating force by the alignment treatment of the alignment films 18 and 19, and the intermolecular force of the liquid crystal is larger than the alignment regulating force by the alignment treatment. Therefore, as schematically shown in FIGS. 5 and 6A, the liquid crystal molecules form an antiferroelectric phase in which the double helical structure has disappeared and are sealed between the substrates 11 and 12. I have.

Here, the liquid crystal molecules have an intermolecular force for maintaining the antiferroelectric phase, while the alignment films 18 and 19 are oriented for regulating the liquid crystal in the vicinity in the alignment direction 21C. Have power. Therefore, when the liquid crystal molecules have a small alignment control force due to the interface effect compared to the intermolecular force for maintaining the antiferroelectric phase, the liquid crystal maintains the antiferroelectric phase as in the case of the bulk. I do. When the alignment regulating force is sufficiently larger than the intermolecular force, the liquid crystal molecules form a ferri phase. When the orientation regulating force is larger than the intermolecular force and smaller than the orientation regulating force for forming the ferri phase, a mixed phase described later is formed.

In this embodiment, when the intermolecular force for maintaining the antiferroelectric phase of the liquid crystal molecules of the liquid crystal layer 21 is larger than the alignment control force, and when no voltage is applied, the liquid crystal molecules of the adjacent smectic layer are not applied. Turn alternately in the first direction 21A and the second direction 21B in FIG. In this state, the director (the average direction of the major axis of the liquid crystal molecules) is almost aligned with the normal direction (or the third direction 21C) of the layer (smectic layer) in which the SmCA ^* phase is formed. The spontaneous polarizations Ps of the liquid crystal molecules of the adjacent smectic layers are opposite to each other as shown in FIG. 5, and the spontaneous polarizations of the adjacent smectic layers cancel each other. The spatially averaged optical axis of the liquid crystal layer 21 substantially coincides with the normal direction of the smectic layer (or the third direction 21C).

On the other hand, by applying a sufficiently positive voltage (voltage equal to or higher than the saturation voltage) to the liquid crystal layer 21,
As shown in (B), the liquid crystal molecules are aligned so as to be substantially aligned in the first direction 21A. In this state, the spontaneous polarization of the liquid crystal molecules is oriented substantially in the same direction, and the liquid crystal exhibits a first ferroelectric phase. On the other hand, by applying a sufficiently negative voltage (voltage equal to or lower than the saturation voltage) to the liquid crystal layer 21,
As shown in (C), the liquid crystal molecules are aligned so as to be substantially aligned in the second direction 21B. In this state, the spontaneous polarization of the liquid crystal molecules is oriented in substantially the same direction, and the liquid crystal exhibits a second ferroelectric phase. In these states, the optical axis of the liquid crystal layer 21 is the first axis.
21A or the second direction 21B.

As described above, the liquid crystal molecules of the liquid crystal layer 21 are:
Due to the alignment regulating forces of the alignment films 18 and 19, the order of the molecular arrangement is weakened at the interface with the liquid crystal layer 21, and as shown in FIG. 7, the liquid crystal molecules easily move along a virtual cone.
On the other hand, the barrier between the potential energy of the antiferroelectric phase and the ferroelectric phase of the liquid crystal layer 21 is small, and the order of the molecular arrangement of the antiferroelectric phase is relatively easily disturbed by the action of the interface with the alignment films 18 and 19. Therefore, when an intermediate voltage is applied to the liquid crystal layer 21, a part of the liquid crystal molecules behaves (moves) along a virtual cone of the chiral smectic CA ^* phase as shown in FIG. As shown in (2), the substrate 11 and 12 are tilted (with a tilt) with respect to the main surfaces thereof.

Therefore, when a low voltage is applied,
The liquid crystal molecules of the antiferroelectric phase (the liquid crystal maintaining the antiferroelectric phase order shown in FIG. 5) and the tilt with respect to the substrate surface as shown in FIGS. 8 and 6 (D) and (E). It becomes a state where the held liquid crystal is mixed.

When the applied voltage is further increased, the number of liquid crystal molecules in the antiferroelectric phase (the liquid crystal molecules maintaining the antiferroelectric phase arrangement shown in FIG. 6A) decreases, and the The number of tilted liquid crystal molecules increases. Further, the orientation direction of some of the molecules is reversed, and the molecules become ferroelectric phases (liquid crystal molecules having a ferroelectric phase order). For this reason, the liquid crystal of the antiferroelectric phase, the liquid crystal of the ferroelectric phase, and the liquid crystal of the intermediate phase oriented with a tilt are mixed.

In this state, a minute region composed of an antiferroelectric phase liquid crystal, a minute region composed of a ferroelectric phase liquid crystal, and a minute region composed of an intermediate phase liquid crystal are mixed in the visible light wavelength size region. Optically, a characteristic in which the characteristics of these domains are averaged is obtained.

In this specification, the state in which the intermediate voltage is applied is called a mixed phase in the sense that an antiferroelectric phase, a ferroelectric phase, and liquid crystal molecules in an intermediate alignment state are mixed.

FIG. 8 schematically shows a state in which the first direction 21A, the second alignment direction 21B, and the liquid crystal molecules having a tilt with respect to the substrate surface are mixed in the mixed phase.

In this mixed phase, the ratio of the liquid crystal molecules in the antiferroelectric phase, the liquid crystal molecules in the ferroelectric phase and the liquid crystal molecules in the intermediate state,
The average alignment direction of the liquid crystal molecules in the intermediate state (the alignment direction projected on the main surface of the substrate) continuously changes according to the polarity and value of the applied voltage. Therefore, the director of this liquid crystal (the average orientation direction of the liquid crystal molecules) is shown in FIGS.
(E), the first direction 21 according to the applied voltage.
A continuously changes between A and the second direction 21B.

For this reason, the optical characteristics of the liquid crystal display device having the above-described structure are such that there is no flat portion near the applied voltage of 0 V.
As the absolute value of the applied voltage increases, the optical characteristics also change continuously and smoothly. Further, the transmittance curve is also symmetric with respect to the polarity of the applied voltage. When a voltage whose absolute value is equal to or higher than the saturation voltage is applied, the transmittance is saturated. Furthermore, the hysteresis is very small.

As an example, a liquid crystal composition containing a liquid crystal having a skeletal structure represented by Chemical Formula 1 as a main component and having physical properties shown in Table 1 is prepared, and this liquid crystal composition is used as a liquid crystal layer 21 to reduce a cell gap. FIG. 9 shows the relationship between the transmittance and the applied voltage of the liquid crystal display element (Example 1) when the helical structure drawn by the molecules of the liquid crystal layer 21 is solved by setting the liquid crystal layer 21 to 1.5 microns.
It is shown in (A). As a comparative example, FIG. 9B shows the relationship between the transmittance and the applied voltage of a liquid crystal display element (Comparative Example 1) in which a liquid crystal is sealed while maintaining a helical structure drawn by liquid crystal molecules with a cell gap of 5 μm. .

The characteristics shown in FIGS. 9A and 9B are obtained by applying a triangular wave between the electrodes 13 and 17 facing each other. As shown in FIG. 9A, the applied voltage-transmittance characteristic of the liquid crystal display element of Example 1 does not have a definite threshold value, the transmittance continuously changes, and the polarity of the applied voltage varies. Symmetrical with very low hysteresis and high contrast. Therefore, the transmittance with respect to the applied voltage is almost uniquely determined, and the intermediate gradation can be stably displayed.
It can be understood that a high-contrast image can be stably displayed.

On the other hand, as shown in FIG. 9B, in Comparative Example 1, the interaction between the alignment films 18 and 19 and liquid crystal molecules in the middle of the liquid crystal layer 21 in the thickness direction is weak, and the total thickness of the liquid crystal layer is small. , The applied voltage-transmittance characteristic has a threshold value, has a large hysteresis, and a smooth applied voltage-transmittance characteristic cannot be obtained. Also, the contrast is small.

The fact that the liquid crystal molecules behave as described above in accordance with the applied voltage in the liquid crystal display element of Example 1 is, for example, shown in FIG. 10 and FIG.
The determination can be made from the enlarged views of the display surface shown in (A) to (C).

FIG. 10 shows a conoscopic image of a liquid crystal material in a bulk state. In this figure, melanops (bright spots)
Two are generated in a direction substantially perpendicular to the electric field E.
It is almost symmetrical. This indicates that the liquid crystal molecules are an antiferroelectric phase having a double helical structure.

On the other hand, in the liquid crystal layer 21 in which the liquid crystal material is sealed between the substrates, substantially no black is displayed in the absence of an electric field, as shown in FIG. Next, when the voltage is increased, as shown in FIG. 11C, almost the whole becomes white, and it can be seen that the liquid crystal molecules are aligned in the first or second direction. On the other hand, in the intermediate state, as shown in FIG. 11B, the whole becomes darker or brighter depending on the applied voltage. Therefore, it is understood that a mixed phase is generated at an intermediate voltage.

As described above, the liquid crystal layer 21 of this embodiment is
Liquid crystal molecules behave along the cone from the first or second alignment state to the second or first alignment state according to the applied voltage. Therefore, the average alignment direction of the liquid crystal layer 21 changes continuously according to the applied voltage, and the transmittance changes continuously. Therefore, an arbitrary gradation can be displayed.

In FIG. 3, the transmission axis 23A of the lower polarizing plate 23 is arranged substantially parallel to the normal direction of the smectic layer of the liquid crystal layer 21, and the transmission axis 24A of the upper polarizing plate 24 is arranged perpendicular to the transmission axis 23A. For example, the transmission axis 23A of the lower polarizing plate 23 and the transmission axis 24A of the upper polarizing plate 24 are determined in various arrangements according to the required electro-optical characteristics of the liquid crystal display device.

For example, as shown in FIG. 12A, the transmission axis 23A of the lower polarizing plate 23 is parallel to the second direction 21B, and the transmission axis 24A of the upper polarizing plate 24 is May be arranged so as to be orthogonal to. In this configuration, when a sufficiently large negative voltage (above the threshold value) is applied to the liquid crystal layer 21, the director moves in the second direction 21.
The display is darkest because it faces B. On the other hand, when a sufficiently large voltage (more than the threshold value) of the positive polarity is applied, the display is brightest because the director faces the first direction 21A.

When a liquid crystal material having a cone angle larger than 22.5 ° is used, one of the transmission axis 23A of the lower polarizing plate 23 and the transmission axis 24A of the upper polarizing plate 24 is adjusted by the method of forming the smectic layer of the liquid crystal layer 21. It may be arranged within a range of an angle smaller than the cone angle θ with respect to the line, and the other optical axis may be arranged substantially orthogonal to one optical axis. By using such an optical arrangement, it is possible to drive the liquid crystal without setting the liquid crystal to a ferroelectric phase, to prevent display burn-in and the like, and to suppress flicker.

For example, when a liquid crystal material having a cone angle of 32 ° as shown in Chemical Formula 1 is used, as shown in FIG. 12B, the transmission axis 23 A of the lower polarizing plate 23 is changed to the liquid crystal layer 2.
1 Normal direction of smectic layer (almost 21C direction)
, For example, in a direction intersecting at 22.5 °.
Further, the transmission axis 24A of the upper polarizing plate 24 is disposed substantially orthogonal to the transmission axis 23A.

Then, the director of the liquid crystal layer formed of this liquid crystal material has an angle range of 22.5 ° with respect to the normal direction of the smectic layer (approximately 21C direction) (range between 23A and 21D). ), The amount of transmitted light is controlled by applying a voltage between the opposing electrodes in a voltage range lower than that in which the liquid crystal layer forms a ferroelectric phase. With this configuration, the display becomes darkest when the director coincides with the direction 23A of the transmission axis, and the direction 21D in which the director is inclined by 45 ° with respect to the direction 23A.
Brightest when facing. Therefore, in order to obtain the maximum gradation from the minimum gradation, the directors are moved in the first directions 21A and 2A.
There is no need to set it to 1B. That is, the liquid crystal can be driven without setting the ferroelectric phase.

Even when this optical arrangement is adopted, the behavior of molecules and the phase change in the liquid crystal layer 21 with respect to the applied voltage are as described above, and the director of the liquid crystal layer 21 is in the first direction 21A and the second direction 21A. It changes continuously between the direction 21B. Therefore, an arbitrary gradation can be displayed. Further, as compared with the optical arrangement shown in FIG. 3, flicker is reduced and a ferroelectric phase is not formed in the liquid crystal layer 21.
Screen burn-in is suppressed, and the contrast of the display screen and the display quality can be increased.

FIG. 12 (B) shows the above-mentioned liquid crystal cell (a cell in which a liquid crystal composition containing a liquid crystal having a skeletal structure shown in Chemical Formula 1 as a main component and having the physical properties shown in Table 1 was sealed in a cell gap of 1.5 μm). FIG. 13A shows the relationship between the transmittance and the applied voltage of a liquid crystal display element (Example 2) to which the optical arrangement shown in FIG. As a comparative example, FIG. 1 shows the relationship between the transmittance and the applied voltage of a liquid crystal display device (Comparative Example 2) having the same configuration as that of Example 2 except that the cell gap was 5 μm.
3 (B).

The characteristics shown in FIGS. 13A and 13B are obtained by applying a triangular wave between the electrodes 13 and 17 facing each other. As shown in FIG. 13A, the applied voltage-transmittance characteristic of the liquid crystal display element of Example 2 does not have a definite threshold value, the transmittance continuously changes, and the With low hysteresis and high contrast. On the other hand, as shown in FIG. 13B, in Comparative Example 2, the applied voltage-transmittance characteristic has a threshold value and the hysteresis is large, so that a smooth applied voltage-transmittance characteristic cannot be obtained. Also, the contrast is small.

From FIGS. 13A and 13B, it can be confirmed that the liquid crystal display device of this embodiment has excellent gradation display capability.

Next, a method of driving the display element having the above configuration will be described with reference to FIGS. FIG. 14 (A)
14 shows a gate signal applied by the row driver 31 to the gate line 15 of an arbitrary row, and FIG. 14B shows a data signal applied to each data line 16 by the column driver 32 in synchronization with a gate pulse. The voltage of the data signal is set to a voltage that does not align the liquid crystal layer 21 in the ferroelectric phase, that is, a voltage corresponding to the transmittance to be displayed between VTmax and VTmin. FIG. 14C shows a change in transmittance when the data pulse shown in FIG. 14B is applied.

Each gate signal is turned on as a gate pulse during the selection period of the corresponding row. The TFT 14 in the selected row is turned on by the gate pulse. During the period when the TFT 14 is on, that is, during the writing period, the TFT 1
The data signal corresponding to the display gray scale via the pixel electrode 1
3 and the counter electrode 17. When the gate pulse is turned off, the TFT 14 is turned off, and the pixel electrode 13
The voltage applied between the pixel electrode 13 and the counter electrode 17 is held in the pixel capacitance formed by the pixel electrode 13, the counter electrode 17, and the liquid crystal layer 21 therebetween. For this reason, FIG.
As shown in (1), the display gradation corresponding to the hold voltage is held until the next selection period of this row. Therefore, according to this driving method, an arbitrary gradation image can be displayed by controlling the voltage of the data pulse.

The liquid crystal display device of Example 2 is shown in FIG.
Driving by the driving method shown in FIG.
It increases sequentially from 5 V to +5 V, and further, from +5 V to −5
FIG. 15 shows a change in transmittance when sequentially decreasing to V. From FIG. 15, it can be understood that an arbitrary gradation can be stably displayed by using the driving method in FIG. 14.

Next, an example of the configuration of the column driver 32 that enables such driving will be described with reference to FIG. As shown in FIG. 16, the column driver 32 includes a first sample / hold circuit 41 and a second sample / hold circuit 42.
, An A / D (analog / digital) converter 43, a timing circuit 44, and a voltage conversion circuit 45.

The first sample-and-hold circuit 41 samples and outputs a signal component (one image data) VD 'for a corresponding pixel from an externally supplied analog display signal.
Hold. The second sample-and-hold circuit 42 receives the hold signal VD 'from the first sample-and-hold circuit 41.
Sample hold. The A / D converter 43 is connected to the second
A / D-converts the hold signal of the sample-and-hold circuit 42 to digital gradation data. The timing circuit 44 supplies a timing control signal for instructing the first and second sample and hold circuits 41 and 42 to perform sampling and holding during each selection period TS.

The voltage conversion circuit 45 is a voltage corresponding to the digital gradation data output from the A / D converter 43 (a driving system voltage required to display the gradation specified by the digital gradation data) VD And outputs it to the corresponding data line 16. The voltage conversion circuit 45 separates the power supply system of the signal processing system from the power supply system of the drive system. The output voltage VD of the voltage conversion circuit 45 is applied to the liquid crystal layer 21 during the writing period when the TFT 14 in the corresponding row is on, and is held between the opposing electrodes 13 and 17 while the TFT 14 is off.

The first sample and hold circuit 41, the second sample and hold circuit 42, and the A / D converter 43
And the voltage conversion circuit 45 are arranged for each column of pixels, and the timing circuit 44 is commonly arranged for a plurality of columns.

The configuration of the column driver 32 is not limited to the configuration shown in FIG. For example, a sample and hold circuit included in the A / D converter 43 may be used as the second sample and hold circuit 42. Furthermore, A /
After performing a certain process on the output data of the D converter 43, the processed data may be supplied to the voltage conversion circuit 45 and converted into the voltage of the driving system. Further, after the processed data is once converted into a gradation signal having a voltage of a signal processing system, the voltage conversion circuit 45 may convert the data into a voltage of a driving system. Various timing signals may be supplied from outside the column driver 32.
Further, the image data itself may be constituted by digital data.

The present invention is not limited to the above embodiment,
Various modifications and applications are possible. For example, the antiferroelectric liquid crystal of the present invention is not limited to a liquid crystal having a skeleton structure shown in Chemical Formula 1 as a main component, and any liquid crystal that forms another mixed phase can be used. The same applies to its physical properties. Further, the material, thickness, and the like of the alignment film can be appropriately changed. The combination of the liquid crystal material and the alignment film is arbitrary as long as the alignment control force is smaller than the intermolecular force of the liquid crystal molecules forming the liquid crystal layer and a mixed phase can be formed. The thickness of the liquid crystal layer 21 is also arbitrary as long as the mixed phase can be formed over the entire thickness. Furthermore, even if a region where the mixed phase is not formed partially occurs, it is acceptable as long as it does not substantially affect the display.

In the embodiment, the transmission axis 23A of the polarizing plate 23 and the transmission axis 24A of the polarizing plate 24 are arranged at right angles. However, the polarizing plates 23 and 24 may be arranged so that they are parallel to each other. . The optical axis of the polarizing plate may be an absorption axis.

The present invention can be applied not only to an antiferroelectric liquid crystal display device using a TFT as an active element, but also to an antiferroelectric liquid crystal display device using an MIM as an active element. Further, according to the present invention, as shown in FIG. 17, a simple matrix type (passive matrix type) display in which a scanning electrode 71 and a signal electrode 72 orthogonal to the scanning electrode 71 are arranged on the opposing surfaces of the opposing substrates 11 and 12 is provided. It is also applicable to devices.

[0073]

As described above, according to the present invention, although the liquid crystal display device uses a liquid crystal exhibiting antiferroelectricity, the display gradation is continuously changed to provide an arbitrary gradation. Images can be displayed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a structure of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a plan view showing a configuration of a lower substrate of the liquid crystal display element shown in FIG.

FIG. 3 is a diagram illustrating a relationship between a transmission axis of a polarizing plate and an alignment direction of liquid crystal molecules.

FIG. 4 is a diagram for explaining a double helical structure drawn by liquid crystal molecules of a liquid crystal in a bulk state.

FIG. 5 is a diagram for explaining an alignment state of liquid crystal molecules sealed between substrates.

FIGS. 6A to 6E are diagrams showing the relationship between applied voltage and alignment of liquid crystal molecules.

FIG. 7 is a diagram for explaining the behavior of liquid crystal molecules when an intermediate voltage is applied.

FIG. 8 is a diagram for explaining an alignment state of liquid crystal molecules when an intermediate voltage is applied.

FIG. 9 (A) is a first embodiment employing the optical arrangement of FIG. 3;
When applying a low-frequency triangular wave voltage to the antiferroelectric liquid crystal display element of the above, is a graph showing the applied voltage-transmittance characteristics,
(B) is a graph showing the applied voltage-transmittance characteristics of Comparative Example 1 in which the gap length was 5 microns.

FIG. 10 is a conoscopic image of a liquid crystal in a bulk state.

FIGS. 11A to 11C are microscopic photographs of the liquid crystal display device.

FIGS. 12A and 12B are diagrams showing another example of the relationship between the transmission axis of a polarizing plate and the orientation direction of liquid crystal molecules.

FIG. 13A is a graph showing applied voltage-transmittance characteristics when a low-frequency triangular wave voltage is applied to the antiferroelectric liquid crystal display element of Example 2 employing the optical arrangement of FIG. (B) shows Comparative Example 2 in which the gap length was 5 microns.
5 is a graph showing an applied voltage-transmittance characteristic of FIG.

FIGS. 14A to 14C are timing charts for explaining a method of driving the antiferroelectric liquid crystal display device of the present invention.

FIG. 15 is a diagram showing an applied voltage-transmittance characteristic when the liquid crystal display element of Example 2 is driven using the driving methods shown in FIGS. 14 (A) to (C).

FIG. 16 is a block diagram showing a configuration example of a driver circuit for realizing the driving methods shown in FIGS.

FIG. 17 is a diagram illustrating a configuration of a simple matrix type liquid crystal display element.

[Explanation of symbols]

11: transparent substrate (lower substrate), 12: transparent substrate (upper substrate), 13: pixel electrode, 14: active element (TF
T), 15: gate line (scan line), 16: data line (gradation signal line), 17: common electrode, 1
8 ... alignment film, 19 ... alignment film, 20 ... sealing material, 21.
..Liquid crystal layer, 22 spacer, 23 polarizing plate (lower polarizing plate), 24 polarizing plate (upper polarizing plate), 25 liquid crystal cell,
31 ... row driver, 32 ... column driver, 71 ... scanning electrode, 72 ... signal electrode

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Manabu Takei 5 Casio Computer Co., Ltd., Hachioji Research Laboratory, 2951 Ishikawacho, Hachioji-shi, Tokyo

Claims (8)

[Claims] A substrate including a plurality of pixel electrodes and a plurality of active elements connected to the pixel electrodes arranged in a matrix; a second substrate provided with a common electrode opposed to the pixel electrodes; And a chiral smectic CA phase is formed in a bulk state, and between the one substrate and the other substrate, when no voltage is applied between the pixel electrode and the common electrode, the electric field is reduced. A mixed phase in which an antiferroelectric phase, a ferroelectric phase, and an intermediate phase oriented in another state are mixed in a state where a dielectric phase is formed and an electric field in which a voltage is applied between the pixel electrode and the common electrode is applied. A liquid crystal display device comprising a liquid crystal layer. 2. The liquid crystal layer comprises a ferroelectric liquid crystal, an antiferroelectric liquid crystal, and an intermediate phase in which liquid crystal molecules are arranged with a tilt with respect to the main surfaces of the one and the other substrates. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is made of a liquid crystal material exhibiting a mixed phase in which liquid crystal is mixed. 3. The liquid crystal layer is made of a liquid crystal material in which an alignment state of an intermediate phase is formed by moving liquid crystal molecules along a cone drawn by molecules of a chiral smectic CA phase by application of an electric field. The liquid crystal display device according to claim 1, comprising: 4. A substrate on which an electrode is formed, another substrate on which an electrode opposed to the electrode is formed, and liquid crystal molecules sealed between the substrate and forming a smectic phase have spontaneous polarization. Then, an antiferroelectric phase is formed to draw and arrange a double helical structure in a bulk state. Between the one substrate and the other substrate, the helical structure is melted in an electric field-free state where no electric field is applied. The liquid crystal molecules oriented in the first direction and the liquid crystal molecules oriented in the second direction are alternately arranged to form an antiferroelectric phase in which the spontaneous polarization of the liquid crystal molecules in the adjacent smectic layer cancel each other; In a state where a sufficiently large electric field is applied, the liquid crystal molecules exhibit a ferroelectric phase in which the liquid crystal molecules are oriented in the first direction or the second direction according to the polarity thereof.
And the liquid crystal molecules oriented in the second direction, and the liquid crystal molecules acted along the cone drawn by the liquid crystal molecules and tilted with respect to the principal surfaces of the one and other substrates. A liquid crystal display device comprising: a liquid crystal layer exhibiting a mixed phase in which liquid crystal molecules aligned in a state are mixed. 5. An opposing surface of the one substrate and the other substrate is subjected to an alignment process, wherein the liquid crystal has a liquid crystal material having an intermolecular force larger than an alignment control force by the alignment process. The liquid crystal display device according to any one of claims 1 to 4, comprising: 6. An alignment film is formed on each of opposing surfaces of said one substrate and said other substrate, and said alignment film exerts an alignment control force smaller than an intermolecular force of said liquid crystal layer on said liquid crystal layer. 5. The liquid crystal device according to claim 1, wherein the liquid crystal molecules have a surface energy.
A liquid crystal display element according to the item. 7. The liquid crystal device according to claim 1, wherein each of the opposing surfaces of the one substrate and the other substrate is subjected to an alignment treatment for giving an alignment regulating force smaller than an intermolecular force of the liquid crystal to the liquid crystal layer. The liquid crystal display device according to claim 1. 8. A chiral smectic CA phase is formed in a bulk state between one substrate on which an electrode is formed and another substrate on which an electrode facing the electrode is formed, and the one substrate and the other An anti-ferroelectric phase is formed in a non-electric field state where no voltage is applied between the opposing electrodes, and the anti-ferroelectric phase is formed in an electric field applied state where a voltage is applied between the opposing electrodes. And a ferroelectric phase and an intermediate phase oriented in another state are mixed with each other to form a mixed liquid crystal layer, and a voltage is applied between the opposing electrodes to generate the mixed phase. A method of driving a liquid crystal display element, wherein gradation display is performed by changing the director of (1).