JPH0730320B2 - Ferroelectric liquid crystal composition and method of driving electro-optical element using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a ferroelectric liquid crystal composition showing ferroelectricity used in matrix display devices, optical shutters for printers, and the like, and a liquid crystal electro-optical element using the same. The present invention relates to a driving method.

[Prior Art] An electro-optical element using a ferroelectric liquid crystal has a response 10 to 1000 times faster than an electro-optical element using a conventional liquid crystal,
It is expected to be applied to a high-speed optical shutter device, and is also expected to be applied to a large-sized and high-density display device because it is possible to have bistability against an electric field.

Ferroelectric liquid crystal is sandwiched between a substrate having a plurality of scanning electrode groups and a substrate having a plurality of signal electrode groups, and regarding an electro-optical element utilizing the bistability, a liquid crystal composition already used or Several proposals have been made regarding the structure of the electro-optical element and its driving method. (For example, JP-A-60-
(No. 33535) Ferroelectric liquid crystal has a very small energy barrier when moving from one to the other in two bistable states, and exhibits extremely fast response compared to conventional nematic liquid crystals, but has a clear threshold value. It has a drawback that it is difficult to obtain characteristics.
That is, when the scan electrode of the pixel is not selected,
The voltage of the signal electrode when writing to another pixel is applied.
As shown in FIG. 4, the so-called crosstalk voltage changes the state A that should be held originally to the other state B, or
Further, there is a phenomenon that the contrast is lowered because the tilt angle is changed from the state A to the state A ′.
Several proposals have been made in terms of a driving method in order to improve the defects of the electro-optical element using such a ferroelectric liquid crystal, enhance its bistability, and obtain high contrast.

In the conventional method for driving a ferroelectric liquid crystal electro-optical element, when the integrated value of the electric field applied to one pixel is taken, the DC component thereof is not 0 and greatly differs depending on the display pattern, and thus the following problems occur. First, the electrodes and the liquid crystal material are oxidized or reduced by the direct current electric field, so that the reliability is lowered. Secondly, when the orientation control film for orienting the liquid crystal molecules in one direction is an insulating film, charged particles such as ions in the liquid crystal are adsorbed on the surface thereof, so that the voltage effectively applied to the liquid crystal layer. However, there is a problem that the threshold characteristics are greatly changed and the bistability is impaired, because the voltage gradually changes depending on the DC electric field.

As a driving method for eliminating this DC component, Japanese Patent Application Laid-Open No. 60-123825 filed by Seiko Electronics Co., Ltd.
60-203920, JP-A-60-156046, JP-A-60-201325
However, in these driving methods, the crosstalk voltage in the non-selected state affects the light intensity that should be originally held, and there is a problem that sufficient contrast cannot be obtained in the lighting state and the non-lighting state. was there.

As a method for solving this problem, a method of driving a liquid crystal having a negative dielectric anisotropy with a waveform in which a high frequency signal is superimposed (Japanese Patent Application No. 60-219110) has been proposed. In this method, the interaction between the superposed high-frequency signal and the negative dielectric anisotropy of the liquid crystal molecules prevents the bisafety of the ferroelectric liquid crystal from being impaired by the crosstalk voltage when it is not selected, resulting in high contrast. Has been successful. However, this method has a drawback that the driving circuit is complicated and the current value is increased by applying a high frequency signal.

Therefore, if a high contrast can be obtained by a driving method that does not use a high frequency signal, 1) the structure of the driving circuit can be simplified, and 2) the current value flowing in the cell during driving can be suppressed, and energy saving can be achieved. Due to the above advantages, it has been desired to employ a driving method that does not superimpose high frequency signals.

[Problems to be Solved by the Invention] A conventional ferroelectric liquid crystal and an electro-optical element using the same are
There is a problem that there is sufficient bistability even in the non-selected state, no high contrast can be obtained even by a driving method that does not superimpose a high frequency signal, and it is impossible to increase the size and density of the device.

[Means for Solving Problems] The present invention has been made to solve the above problems,
Dielectric anisotropy is -1 to -6 and its spontaneous polarization value is 5
To 70, the ferroelectric liquid crystal composition and the ferroelectric liquid crystal composition are sandwiched between substrates provided with a pair of electrodes, and a polarizing plate is disposed on at least one of the outer sides of the liquid crystal layer. In a method of driving an electro-optical element, a composition in which a ferroelectric liquid crystal composition has a dielectric anisotropy of -1 to -6 and a spontaneous polarization value of 5 to 70 is used and AC driving is performed. A method of driving a characteristic electro-optical element is provided.

FIG. 1 is a sectional view of a ferroelectric liquid crystal electro-optical element driven by the present invention. Transparent conductive films (2a) and (2b) and alignment control films (3a) and (3b) are formed on the surfaces of two transparent substrates (1a) and (1b), respectively. The conductive films (2a) and (2b) are
Electrodes for applying an electric field to the liquid crystal layer (4) held between the substrates, which constitute a scanning electrode group and a signal electrode group respectively,
It is provided for the purpose of producing an electro-optical response, and is made of In ₂ O ₃ , SnO ₂ or the like and formed in a predetermined pattern.

The alignment control films (3a) and (3b) are for horizontally aligning the liquid crystal, and typically, an organic polymer film, particularly a polyimide polymer film is formed and rubbed in a certain direction with a cloth. However, a polyamide-based polymer film, a polyisidamide-based polymer film, a rubbing polymer film such as polyparaxylylene, and an obliquely evaporated film such as SiO ₂ are also effective and form an overcoat film. Instead, the alignment control film may be formed by directly rubbing the conductive films (2a) and (2b).

After performing such an alignment treatment, spacers, for example, spacers, are formed so that the substrates are held in parallel and at regular intervals.
A cell is prepared by sandwiching organic beads and alumina particles and fixing the periphery with a sealant (5). 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 polarizing plates may be arranged so as to deviate from each other at right angles.

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.
Is patterned, 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. One scan electrode group Ci among the scan electrode groups is selected by the method described below, and the pixels Ai are selected by the signals applied to the signal electrode groups S _{1 to} Sm at that time.
All pixels are written by writing _{1 to} Aim, selecting Ci + ₁ , and repeating this.

FIG. 3 shows another pattern example of the conductive films (2a) and (2b), which is used as an optical shutter element for a printer head. In this example, a pattern example in the case of driving at 1/4 duty is shown. A _{11 to} A ₄ m indicate openings, and the other portions are used by forming a light shielding film.

Next, the usefulness of the ferroelectric liquid crystal composition of the present invention and the electro-optical element using the same will be specifically described.

FIG. 4 shows two bistable states (A, B) of the liquid crystal, which are determined by the polarity of the electric field when a DC voltage higher than the threshold voltage is applied.
And its tilt angle θ (referred to as a static tilt angle) and two bistable states (A ′, A ′, of the liquid crystal obtained when a so-called crosstalk voltage below the threshold voltage is applied.
B ′) and its tilt angle θ ′ (referred to as a dynamic tilt angle) are shown. Θ between both tilt angles
There is a relation of> θ ′, and the contrast of the electro-optical element using the ferroelectric liquid crystal is determined by the dynamic tilt angle θ ′. Under the crossed Nicols condition, when θ '= 22.5 °, bright display with maximum contrast is obtained, but generally θ' =
This is a cause of deterioration of contrast because it is narrow at 5 to 10 °.

Therefore, the dynamic tilt angle θ'during driving is set to θ '> 10.
The contrast can be increased by setting the angle θ to be preferably θ′≈22.5 °. The aforementioned Japanese Patent Application Sho 60-
The method described in No. 219110 enlarges the dynamic tilt angle θ ′ by a high frequency signal which is non-selected, and is applied in particular, and aims to improve the contrast by devising a drive waveform.

In the present invention, the difference between the dielectric constant ∈ _L of the ferroelectric liquid crystal in the molecular major axis direction and the dielectric constant ∈ _{S in} the molecular minor axis direction ∈ _{L −} ∈ _S (this is generally known as the dielectric anisotropy Δ∈). It has been found that when a liquid crystal composition having a negative value and a large absolute value is used, a high contrast can be obtained even when the liquid crystal composition is driven by a bipolar pulse waveform that does not superimpose a high frequency signal.

As the ferroelectric liquid crystal used in the present invention, a liquid crystal composition having a liquid crystal phase exhibiting bistability depending on the polarity of an electric field can be used, and specifically, a chiral smectic C phase (SmC ^* phase).
A liquid crystal composition having a can be used.

The point of the present invention is that the composition has a dielectric anisotropy of -1 to -6.
It is to use the liquid crystal material in the range of. Even if the dielectric anisotropy is positive or negative, if the absolute value is smaller than this, high contrast cannot be obtained. When the absolute value of the negative dielectric anisotropy is gradually increased, the contrast tends to increase, and the dielectric anisotropy Δε (ε _L − varies depending on the liquid crystal used and the driving condition.
When ε _S ) exceeds −1, the contrast shows a remarkable increasing tendency. However, at this time, the energy barrier at the time of switching between the two bistable states also becomes high, and the content of the compound having a high negative dielectric anisotropy increases, resulting in the viscosity of the composition itself. Therefore, if the negative dielectric anisotropy is made too large, the response is drastically reduced, which is not practical.

Therefore, the dielectric anisotropy is also preferably -1 to -6, and more preferably -2 to -5, and high contrast can be obtained.

The spontaneous polarization Ps is preferably in the range of 3-100. If it is smaller than this range, the response speed is insufficient, and if it exceeds this range, the stability of orientation and the memory property may be impaired, which is not preferable. Especially the spontaneous polarization is 5
High-speed response and stable orientation can be obtained in the range of up to 70.

The liquid crystal composition used in the present invention may be a mixture of several materials, or may be a single material. For example, there are the compounds shown below. Among these, the halogen or cyano group is present in the minor axis direction of the molecule (1)
Since the compounds represented by (8) to (8) exhibit negative dielectric anisotropy, they are particularly useful compounds for constituting the liquid crystal composition of the present invention.

In the following examples, R ^* represents an optically active alkyl group or alkoxy group, R represents a linear or branched alkyl group or alkoxy group, and one compound has the same R ^* and R groups.
Is not necessarily the same group.

In addition to these, various known liquid crystal or non-liquid crystal liquid crystal additives can be used in combination, and examples thereof include the following.

And compounds obtained by substituting a part of hydrogen atoms of these benzene ring, cyclohexane ring and the like with halogen, cyano group, methyl group and the like.

For example, there are the following compounds.

In addition, R shows a linear or branched C1-C12 alkyl group, and even if the same R is shown to one compound, it is not necessarily the same group.

The liquid crystal used in the present invention has a smectic phase (SmA) in a temperature range higher than the liquid crystal phase exhibiting ferroelectricity.
A liquid crystal having a phase) is preferable in terms of bistability symmetry. Directly from the isotropic phase (I phase), nematic phase (Ne phase) or cholesteric phase (Ch phase) without passing through the SmA phase
When a liquid crystal that changes to a ferroelectric liquid crystal phase such as mC ^* phase is used, two kinds of alignment states in which the direction of the liquid crystal molecular layer is different from the normal alignment control direction are taken. 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, as the liquid crystal used in the present invention, it is more preferable to have a Ch phase in a temperature range higher than that of a liquid crystal phase exhibiting ferroelectricity, from the viewpoint of alignment uniformity. As for the method for producing the alignment of the liquid crystal, a device having an extremely good orientation can be produced by using the method of Japanese Patent Application No. 59-274073.

When a liquid crystal having a SmC ^* phase as the ferroelectric liquid crystal composition and having a Ch phase at a higher temperature is used, the length (p) of the helical pitch in the Ch phase is the substrates (1a) and (1b).
A liquid crystal that is four times as long as the distance (d) is used. In addition, it is desirable to have an SmA phase between the Ch phase and the SmC ^* phase from the viewpoint of orientation uniformity. Such liquid crystals include optically active substances,
It is obtained by mixing the smectic liquid crystal compound and the nematic liquid crystal compound in an appropriate ratio, and a non-liquid crystal additive may be added as necessary. In particular, to increase the pitch in the Ch phase, an optically active substance that causes a left helix,
It is effective to mix the optically active substance that causes the 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. In order to obtain uniform orientation, it is necessary to satisfy the condition of p> 4d just above the cholesteric-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 maintain 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.

[Operation] First, the operation of the electro-optical element using the liquid crystal composition having negative dielectric anisotropy according to the present invention will be described.

FIG. 5 is an explanatory view of the principle of the present invention.

In the ferroelectric liquid crystal, the liquid crystal molecules (41) form a layered structure, and the long axis direction of the molecules is inclined by a certain angle with respect to the layer vertical line direction. It has a spontaneous polarization (43) in a direction perpendicular to this molecule and included in the layer plane (42).

Two effects can be considered as the effect of changing the direction of the liquid crystal molecules by an external electric field. One is a coupling action between the spontaneous polarization and the electric field, and in this action, the orientation of the molecule changes so as to align the directions of the spontaneous polarization with the polarity of the electric field. That is, in the positive electric field direction and the negative electric field direction,
The direction of inclination from the layer normal of the molecule is reversed. As shown in FIG. 5, the electric field polarity of E ₁ is arranged as shown in (a), and the electric field polarity of E ₂ is arranged as shown in (b).

Another action is a coupling action between the dielectric anisotropy and the electric field. In this action, the orientation direction of the molecules does not depend on the polarity of the electric field, but is determined only by the direction. When the liquid molecule has a negative dielectric anisotropy, it tends to have an arrangement state of (a) or (b) with respect to the electric field direction of E ₃ .

The feature of the present invention is to obtain a high contrast by setting the dielectric anisotropy in a specific range to enhance the coupling action between the dielectric anisotropy and the electric field.

When a voltage is applied to the ferroelectric liquid crystal, the liquid crystal molecules invert their spontaneous polarization depending on the polarity of the electric field, and are arranged from (a) to (b) or (b) to (a) in FIG. Change. At this time, in order for the molecule to change its arrangement state from (a) to (b) or (b) to (a) in FIG. 5, it is necessary to go through the arrangement state of (c) or (d). To increase the negative dielectric anisotropy, the energy level of (c) and (d) is increased by utilizing the coupling action of the dielectric anisotropy and the electric field. It will raise the barrier. As a result, it is possible to prevent inversion of liquid crystal molecules due to the crosstalk voltage pulsed with an AC pulse when not selected, and thus it is possible to obtain high contrast. Further, at the time of writing, by raising the voltage of the write signal above a certain level or setting the pulse width appropriately, it is possible to easily perform high-speed writing and obtain high contrast.

[Example] A substrate in which electrodes were patterned in a stripe pattern was overcoated with polyimide and rubbed was used, and the periphery was sealed with a sealing material so that the cell gap was 2 μm. A ferroelectric liquid crystal composition having a phase, a cholesteric phase, a smectic A phase, and a chiral smectic C phase in this order was enclosed.

The specific composition of Example 1 is shown below.

The compositions of Examples 2 to 5 and Comparative Examples 1 and 2 were obtained by changing the composition ratio and changing the dielectric anisotropy and the spontaneous polarization. The dielectric anisotropy and spontaneous polarization of these are shown in Table 1.

This dielectric anisotropy (ε _{L −} ε _S ) was measured in the smectic A phase and extrapolated to the chiral smectic C phase transition temperature. The spontaneous polarization (Ps) was 30 ° C. and the triangular wave method (20 Hz−
50V).

A pair of polarizing films were formed on both surfaces of the liquid crystal cells of Examples 1 to 5 and Comparative Examples 1 and 2 manufactured in this way by shifting them by 22.5 ° from the alignment treatment direction and by making both polarizing films cross Nicol. I placed it.

1/32 duty, 1/4 as shown in Fig. 6 at 30 ℃
Driving was performed by applying a pulse having a pulse width of 200 μsec with a bias. In FIG. 6, (1) shows a waveform in which the pixel turns on, and (2) shows a waveform in which the pixel turns off. When one of the scan electrode groups is sequentially selected to perform a write operation, two scans form one cycle of write. At this time, a pair of pulses oscillating to the plus side and pulses oscillating to the minus side are applied to the liquid crystal of each pixel in one scan, and the pulses oscillating to the plus side in the first selection and the second selection are applied. And the order of the pulses swinging to the negative side is reversed. And 1
A voltage that changes the stable state of the liquid crystal is applied either during the second selection or during the second selection.

In FIG. 6, a pulse swinging to the plus side is applied after the first scan, and a pulse swinging to the minus side is applied after the application of the pulse swinging to the minus side during the second scan, and a pulse swinging to the plus side is applied after the application of the pulse swinging to the minus side. To be done. In the case of the (1) ON waveform, the voltage that changes the stable state of the liquid crystal is applied the first time, and in the case of the OFF waveform of (2), the voltage that changes the stable state of the liquid crystal is applied the second time. Has been done. That is,
In (1), the stable state of the liquid crystal is determined by the applied voltage on the negative side in the latter half of the first scanning, and in (2), the stable state of the liquid crystal is determined by the applied voltage on the positive side in the latter half of the second scanning.
The resulting contrast and response speed are shown in Table 1.

There is an optimum range for negative dielectric anisotropy.

Comparative Example 1 and Examples 1 to 4 are the results when the magnitude of spontaneous polarization was made substantially constant and only the negative dielectric anisotropy was increased. It can be seen that the contrast increases as the negative dielectric anisotropy increases. On the contrary, the responsiveness tended to gradually decrease. Example 5 is an example in which the negative dielectric anisotropy is further increased to improve the contrast.
Since the spontaneous polarization is also increased, the response is improved on the contrary. On the other hand, in the case of Comparative Example 2, as compared with Example 2, the response is remarkably reduced due to the smaller spontaneous polarization.

[Advantages of the Invention] The present invention exhibits negative dielectric anisotropy of -1 to -6, and its spontaneous polarization is set to 3 to 100, particularly 5 to 70, thereby enabling driving without superposition of high frequencies, It has the advantages that the drive circuit is simplified, its design is facilitated, and its power consumption is reduced.

Further, increasing the negative dielectric anisotropy tends to improve the contrast, and conversely tends to reduce the response speed. However, in the present invention, since the spontaneous polarization is increased, it is possible to prevent the decrease in the response speed and obtain an electro-optical element having a high contrast and a high response speed.

Furthermore, it has sufficient bistability against crosstalk voltage, and has the effect of enabling the size and density of the electro-optical element to be increased.

In addition, the present invention has an effect of preventing deterioration of the liquid crystal and the electrodes by not applying a DC electric field component to the liquid crystal. Also,
As a result, an insulating film can be used as the alignment film, or another insulating film can be formed between the liquid crystal and the electrodes, which has an effect of preventing a short circuit between the electrodes in the liquid crystal cell.

In addition, the present invention can provide sufficient bistability even in a cell in which the distance between cell substrates is increased, so that it is possible to reduce the difficulty in producing a liquid crystal cell and to reduce the difficulty between electrodes facing each other in the liquid crystal cell. The effect of reducing the occurrence of short circuits is also recognized.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the basic structure of an electro-optical element driven by the present invention, and FIGS. 2 and 3 show examples of preferable electrode patterns when driving the liquid crystal composition of the present invention. It is a top view. FIG. 4 is a schematic diagram showing the magnitude relationship between the static tilt angle and the dynamic tilt angle. FIG. 5 is a principle diagram illustrating the principle of the present invention. FIG. 6 is a waveform diagram of an AC signal used to drive the liquid crystal in the embodiment of the present invention. 1a, 1b: Transparent substrate 2a, 2b: Conductive film 3a, 3b: Alignment control film 4: Liquid crystal layer 6a, 6b: Polarizing plate

Claims (9)

[Claims] 1. A ferroelectric liquid crystal composition having a dielectric anisotropy of -1 to -6 and a spontaneous polarization value of 5 to 70. 2. The ferroelectric liquid crystal composition according to claim 1, which has a dielectric anisotropy of −2 to −5. 3. The ferroelectric liquid crystal composition according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic C liquid crystal. 4. The ferroelectric liquid crystal composition according to claim 3, which has a smectic phase (SmA phase) in a temperature range higher than that of the chiral smectic C phase. 5. The ferroelectric liquid crystal composition according to claim 4, which has a cholesteric phase in a temperature range higher than the smectic phase (SmA phase). 6. The ferroelectric liquid crystal composition according to claim 4, wherein the length of the helical pitch in the cholesteric phase is 4 times or more the distance between the substrates. 7. A method of driving an electro-optical element, comprising a ferroelectric liquid crystal composition sandwiched between substrates provided with a pair of electrodes, and a polarizing plate disposed on at least one of the outer sides of the liquid crystal layer.
A ferroelectric liquid crystal composition having a dielectric anisotropy of −1 to −6 and a spontaneous polarization value of 5 to 70 is used.
A method for driving an electro-optical element, which is characterized by being driven by alternating current. 8. The method of driving an electro-optical element according to claim 7, wherein the substrate provided with the scanning electrode group and the substrate provided with the signal electrode group are used as a pair of substrates. 9. When sequentially selecting one of the scan electrode groups to perform a write operation, two scans constitute one cycle of write, and the liquid crystal of each pixel is moved to the positive side by one scan. The swing pulse and the pulse swinging to the negative side are applied as one set, and the order of the pulse swinging to the plus side and the pulse swinging to the minus side are reversed at the time of the first selection and the second selection, and at the time of the first selection. 9. The method for driving an electro-optical element according to claim 8, wherein a voltage that changes the stable state of the liquid crystal is applied at any one of the second selection and the second selection.