JPS62240378A - Ferroelectric liquid crystal composition and driving of electrooptical element using said composition - Google Patents

Abstract

PURPOSE:The titled composition, having negative dielectric anisotropy and the spontaneous polarization thereof within a specific range, capable of driving without superposing high frequency and with high contrast, high speed of response and useful for matrix display devices, etc. CONSTITUTION:A composition having -1--6, preferably -2--5 difference in dielectric constant between the major axis direction of molecules and the minor axis direction thereof and 3-100 spontaneous polarization value. The above-mentioned composition is preferably obtained by using compounds expressed by formulas I-III, etc., in combination so as to satisfy the above- mentioned condition. The composition is used as a liquid crystal layer 4 of an AC driving electrooptical display device prepared by nipping the liquid crys tal layer 4 between transparent substrates (1a) and (1b) having a pair of electrodes consisting of an electrically conductive membranes (2a) and (2b) and placing polarizing plates (6a) and (6b) on the outside of the above-mentioned liquid crystal layer 4.

Description

[Detailed description of the invention] [Industrial application field] End of the chapter 1! + refers to a ferroelectric liquid crystal composition exhibiting ferroelectric properties used in matrix display devices, optical shutters for printers, etc., and a method for driving a liquid crystal electro-optical element using the same.

[Prior Art] Electro-optical elements using ferroelectric liquid crystals have a response that is 10 to 1000 times faster than conventional electro-optical elements using liquid crystals, and are expected to be applied to high-speed optical shutter devices. Since it is also possible to provide bistability to electric fields, it is expected to be applied to large-sized, high-density display devices.

Regarding electro-optical elements that utilize the bistability of a single ferroelectric liquid crystal sandwiched between a substrate having a plurality of scanning electrode groups and a substrate having a plurality of signal electrode groups, there are already used liquid crystal compositions and Several proposals have been made regarding the configuration of electro-optical elements and their driving methods. (For example, Japanese Patent Application Laid-open No. 8゜-33535) Ferroelectric liquid crystal has a very small energy barrier when moving from one state to the other in two bistable states, and has an extremely fast response compared to conventional nematic liquid crystals. However, it has the disadvantage that clear threshold 1 characteristics are difficult to obtain.

That is, if the scan electrode of that pixel is not selected,
A signal electrode voltage is applied when writing other pixels.

Due to the so-called crosstalk voltage, as shown in FIG. 4, the state A that should originally be maintained changes to the other state J! A phenomenon in which the contrast decreases is observed because the tilt angle decreases when changing to BB or changing from state Jli A to state A°.

Several proposals have been made in terms of driving methods in order to improve the drawbacks of such electro-optical elements using ferroelectric liquid crystals, increase their bistability, and obtain high contrast.

The conventional driving method for a ferroelectric liquid crystal electro-optical element causes a first-order problem because when an integral value of an electric field applied to one pixel is taken, the DC component thereof is not zero and varies greatly depending on the display pattern.

First, reliability is reduced because the electrodes and liquid crystal material are oxidized or reduced by the DC electric field. Second, when the alignment control film for aligning liquid crystal molecules in one direction is an insulating film, charged particles such as ions in the liquid crystal are adsorbed on its surface, so the voltage effectively applied to the liquid crystal layer is gradually differs depending on the DC electric field, so that the threshold 1
There was a problem in that it caused a large change in characteristics and impaired bistability.

Driving methods to eliminate this DC component include JP-A-80-123825 filed by Seiko Electronics, JP-A-80-203920 filed by Canon, and JP-A 60-1560.
There are many such driving methods, such as No. 48 and Japanese Patent Application Laid-Open No. 80-201325, but in these driving methods, the light intensity that should originally be maintained is reduced due to the crosstalk voltage in the non-selected state! # There was a problem in that sufficient contrast could not be obtained in the 1 lighting state and the non-lighting state i5 due to the sound.

Negative dielectric light is a way to solve this problem.

A method has been proposed (Japanese Patent Application No. 1977-ZTSTO) of driving a liquid crystal with a waveform on which a high-frequency signal is superimposed using a liquid crystal having polarity. In this method, the interaction between the superimposed 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 crosstalk voltage during non-selection, and achieves high contrast. It has been extremely successful. However, this method requires a complicated drive circuit;
It has a drawback that the current value increases when a high frequency signal is applied.

Therefore, if high contrast can be obtained by a driving method that does not use high-frequency signals, it will be possible to achieve various practical uses, such as 1) simplifying the configuration of the driving circuit 2) suppressing the current flowing through the cell during driving, and saving energy. Because of the above advantages, it has been desired to adopt a driving method that does not superimpose high-frequency signals.

[Problems to be solved by the invention] Conventional ferroelectric liquid crystals and electro-optical devices using them have the following problems:
Even in the non-selected state, there is sufficient bistability, and there is no driving method that does not superimpose a high-frequency signal that provides high contrast, which poses the problem that it is not possible to increase the size and density of the device.

[Means for solving the problems] The present invention was made to solve the problems mentioned above.
A liquid crystal with lithium-ion properties, characterized in that the difference between the dielectric constant in the long axis direction of the molecule and the dielectric constant in the short axis direction of the molecule is -1 to -6, and the value of spontaneous polarization is 3 to 100. In a method for driving an electro-optical element, the composition and the ferroelectric liquid crystal composition are sandwiched between a pair of substrates provided with electrodes, and a polarizing plate is disposed on at least one of the outer sides of the liquid crystal layer. As a substance, the difference between the dielectric constant in the long axis direction of the molecule and the dielectric constant in the short axis direction of the molecule is -1 to -B, and the value of the spontaneous polarization is 3.
The present invention provides a method for driving an electro-optical element, which is characterized by using a composition having a molecular weight of 100 to 100 and driving an electro-optical element using an alternating current.

FIG. 1 is a cross-sectional view of a ferroelectric liquid crystal electro-optical element driven by the present invention. Transparent IJJ conductive films (2a) and (2b) are placed on the surfaces of two transparent substrates (la) and (lb), respectively.
and alignment control films (3a) and (3b) are formed. conductivity 1
f! ! (2a) and (2b) are electrodes for applying an electric field to the letter liquid crystal layer (4) held between the substrates, which constitute a scanning electrode group and a signal electrode group, respectively, and generate an electro-optic response. It is established for the purpose of In? 0+ or
It is made of 5n02 or the like and is formed in a predetermined pattern.

The alignment control films (3a) and (3b) horizontally align the liquid crystal, and typical examples include an organic polymer film,
In particular, it is preferable to use a polyimide polymer film formed and rubbed in a certain direction with a cloth, but other materials include those formed by rubbing a polymer film such as a polyamide polymer film, polyisidamide polymer film, polyparaxylylene, etc. Oblique vapor deposition such as 5402 is also effective, and the orientation control film may be formed by directly rubbing the conductive films (2a) and (2b) without forming an overcoat film.

After performing such an alignment process, a spacer, for example,
Organic beads and alumina particles are sandwiched and the surrounding area is fixed with a sealant (5) to form a cell. At this time, the orientation control directions of the two substrates are made parallel to each other.

Thereafter, the ferroelectric liquid crystal composition is heated to a cholesteric phase or an isotonic phase, injected into a cell, and then subjected to R1.

Two polarizing plates (8a) and (eb) are arranged outside the cell so that the polarizing plates are perpendicular to each other and at a constant angle with the orientation control direction of the substrate. This angle varies depending on the liquid crystal material, operating temperature of the device, driving method, etc., and the angle that provides the best contrast characteristics should be selected. In some cases, the polarization axes of the two polarizing plates may be arranged with the polarization axes shifted from orthogonal. be.

A light source (7) is placed on the substrate (1b) side so that light is transmitted to the opposite side. Note that when using a single reflection type, a reflection plate may be provided outside the polarizing plate (8b).

The second factor is an example of conductor (2a) and (2b) which is used in dot matrix display elements and the like. One substrate is patterned with a horizontal striped scanning electrode group C1''Cn, and the other substrate is patterned with a vertical ring-shaped signal electrode group St to S. Two sets of electrodes The intersections All to A n of the groups become pixels. One of the scanning electrode groups Ci is selected by the method described later, and at that time, the signal electrode groups 81 to S
One pixel Ail to Aim is written by the signal applied to s, and then Ci◆1 is selected, and this is repeated to write all pixels.

FIG. 3 shows another example of a pattern of conductive films (2a) and (2b) used as an optical shutter element for a printer head. This example shows a pattern example when driven at 1/4 duty.

AIL to AI indicate openings, and a light shielding film is formed and used in other parts.

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

Figure 4 shows two bistable states m (A, B) of the liquid crystal determined by the polarity of the electric field when a DC voltage higher than the threshold voltage is applied.
) and its tilt angle 0 (expressed as static tilt angle)
and two bistable states J3 (A') of the liquid crystal obtained when a so-called crosstalk voltage below the threshold voltage is applied.
.

B') and its tilt angle θ' (denoted as dynamic tilt angle).There is a relationship of 0>θ'' between both tilt angles, and this shows that the relationship between the two tilt angles is 0>θ''. Contrast improves when the dynamic tilt angle is 0゛. Under crossed nicol conditions, when 0' = 22.5@, a bright and maximum contrast display can be obtained, but generally the contrast is narrow at θ゛; 5 to lθ°, so this contrast is This is the cause of a decline in

Therefore, the dynamic tilt angle θ' during driving is set to 0'>1.
By setting the angle to 0@preferably 0゛ζ22.5°, it becomes possible to increase the contrast. The method described in Japanese Patent Application No. 80-219110 is a method in which a dynamic tilt angle of 0° is expanded by applying a non-selective high-frequency signal, and aims to improve contrast by devising a drive waveform. It is.

The present invention makes the difference ε[-εS (this is generally known as dielectric anisotropy Δε) between the permittivity in the long axis direction of the molecule ((() and the permittivity εS in the short axis direction of the molecule) of a ferroelectric liquid crystal negative. It has been discovered that when a liquid crystal composition with a large absolute value is used, high contrast can be obtained even when driving with a hebola pulse waveform without superimposing 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) can be used.
A liquid crystal composition having the following can be used.

The key point of the invention is to use a liquid crystal material with an induced anisotropy in the range of -1 to -6. dielectric anisotropy is positive,
Even if it is negative, if the absolute value is smaller than this, high contrast cannot be obtained.As the absolute value of negative dielectric anisotropy is gradually increased, the contrast tends to increase accordingly, so use Although there are differences depending on the liquid crystal and driving conditions, the contrast shows a remarkable tendency to increase from the point where the dielectric anisotropy Δε(ε[-εS) exceeds -l.
However, at this time, the energy barrier when switching between two bistable states increases, and as a result of the increase in the content of compounds with high viscosity and negative dielectric anisotropy, the viscosity of the composition increases. Therefore, if the negative dielectric anisotropy is increased too much, the response will drop sharply, making it impractical.

For this reason, it is also preferable that the dielectric anisotropy is -1 to -6, especially -2 to -5.

High contrast can be obtained.

The spontaneous polarization Ps is preferably in the range of 3 to 100. If it is smaller than this, the response speed is insufficient, and if it exceeds this range, the stability of orientation and memory properties may be impaired. Neither is preferable.

The liquid crystal composition used in the present invention may be a mixture of several materials or may be a single material6.For example, there are the following compounds, but among these, molecular Has a halogen or cyano group in the short axis direction (1)
Since the compounds shown in (8) to (8) exhibit negative dielectric anisotropy, they are particularly useful compounds as L constituting the liquid crystal composition of the present invention.

In the following examples, R' represents an optically active alkyl group or alkoxy group, and R represents a linear or branched alkyl group or alkoxy group.

Even if the same R'' and R are shown in one compound, they are not necessarily the same group.

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

and compounds in which some of the hydrogen atoms of these benzene rings, cyclohexane rings, etc. are replaced with halogens, cyano groups, methyl groups, etc.

For example, there are the following compounds.

In addition, R represents a linear or branched alkyl group having 1 to 12 carbon atoms, and even if the same R is shown in one compound, it is not necessarily the same group.

In addition, the liquid crystal used in the final IJ1 has a smectic phase (
A liquid crystal having a 0-isotropic phase (■ phase), a nematic phase (Me phase), or a cholesteric phase (ch phase), which is preferable in terms of the symmetry of bistability, has a liquid crystal with a SmA phase). When using a liquid crystal that changes to a liquid crystal phase with a strong, d'iff, or similar phase, two types of alignment states are obtained in which the direction of the liquid crystal molecular layer differs with respect to the direction of alignment control.

This 2. ! ! If such orientation states coexist, this will lead to a decrease in contrast. Therefore, when cooling from the I phase, Me phase, or ch phase to a ferroelectric liquid crystal phase such as the Sac'' phase, a DC electric field with a unidirectional polarity is applied. It is necessary to take measures such as orienting the device in only one of the 2!& types of orientation states.In the element thus created, the stability is different between the first stable state !out and the second stable state. Among the stable states 7g, stable state 7g, which matches the polarity of the electric field applied during cooling, is more stable.

leading to low F of bistability. On the other hand, in a liquid crystal with an SmA phase, there is only one direction of the liquid crystal molecular layer, and there is no need for means such as applying an electric field, so the bistability is symmetrical with respect to the voltage and the bistability is good. .

In addition, it is more preferable for the liquid crystal used in the present invention to have a ch phase in a higher temperature range than a liquid crystal phase exhibiting ferroelectricity in terms of alignment uniformity. By using the method of No. 59-274073, an element with extremely good orientation can be produced.

When using a liquid crystal having a Sac'' phase as a ferroelectric liquid crystal composition and having an ah phase at a higher temperature, c
The helical pitch length p) in the h phase is the substrate (la)
A liquid crystal that is longer than the distance it (d) between and (lb) by at least four times is used. In addition, it is desirable to have an SmA phase between the ch phase and the SmG" phase from the viewpoint of alignment uniformity. Such a liquid crystal can be produced by mixing an optically active substance, a smectic liquid crystal compound, and a nematic liquid crystal compound in an appropriate ratio. It can be obtained by
Non-liquid crystal additives may be added as necessary. especially,
In order to lengthen the pitch in the cb phase, it is effective to mix an optically active substance that produces a left helix and an optically active substance that produces a right helix depending on the magnitude of the force that produces the helix.

Usually, the length of the helical pitch in the Ch phase changes with temperature. To obtain uniform orientation, cholesteric
It is necessary to satisfy the bridge of p>4d at a point directly E of the smectic phase transition point.

However, if the temperature range that satisfies this condition is limited to the very vicinity of the transition point, and if the temperature drop rate is fast, the helical structure will not unravel and will transition to the smectic phase. Since orientation cannot be obtained, it is necessary to maintain the temperature at a temperature that satisfies p>4d until the helical structure is unwound, or to slow down the temperature drop rate. For this reason, the temperature range in which the helical pitch p is 4 times or more the intersubstrate pressure fid is preferably 5°C or more above the cholesteric-smectic phase transition point, and moreover, it is preferable to cover the entire temperature range of the ch phase. More preferred.

Note that the ch phase referred to herein also includes the Me phase of nematic liquid crystal that is made to have a specific pitch by adding an optically active substance to the nematic liquid crystal.

[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 a detailed explanatory diagram of the present invention.

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

There are two possible effects of changing the direction of the liquid crystal molecules by an external electric field.

One is the combined action of spontaneous polarization and electric field, and in this action, the orientation of molecules changes so that the direction of spontaneous polarization aligns with the polarity of the electric field. That is, in the direction of the positive electric field and the direction of the negative electric field, the direction in which the molecules are inclined from the layer perpendicular is opposite.

As shown in FIG. 9, for the electric field polarity of El.

Arranged as shown in (a), for the electric field polarity of E2 (
Try to arrange as in b).

Another effect is the coupling effect between the dielectric anisotropy and the electric field. In this action, the direction in which the molecules are arranged does not depend on the polarity of the electric field, but is determined only by the direction. When liquid crystal molecules have negative dielectric anisotropy, they tend to take either (a) or (b) alignment state with respect to the electric field direction of E3.

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

When a voltage is applied to a ferroelectric liquid crystal, the liquid crystal molecules reverse their spontaneous polarization according to the polarity of the electric field, and the molecules change their alignment from (a) to (b) or from (b) to (a) in Figure 5. change. At this time, the molecules change their arrangement state from (a) in Figure 5 to (
To change to b) or from (b) to (a), use (C
) or (d), and to increase the negative dielectric anisotropy, the energy of (c) and (d) can be increased by utilizing the coupling effect between the dielectric anisotropy and the electric field. By increasing the level, the energy barrier required for one reversal will be increased. This prevents liquid crystal molecules from being inverted due to AC pulsed crosstalk voltage when not selected.

Therefore, high contrast can be obtained. Also,
At the time of reading, high-speed writing and high contrast can be easily achieved by raising the voltage of the write signal above a certain level or setting the pulse 10 appropriately.

[Example] A substrate with electrodes patterned in a stripe shape was overcoated with polyimide and rubbed, the periphery was sealed with a sealant so that the cell gap was 2 μl, and Tokichi phase was applied inside. A ferroelectric liquid crystal composition having a cholesteric phase, a smectic A phase, and a smectic C phase in this order was sealed.

The specific composition of Example 1 is shown below.

Chemical Structure Weight % By changing this composition ratio, dielectric anisotropy and spontaneous polarization were changed to obtain compositions of Examples 2 to 5 and Comparative Example 1.2. Table 1 shows their dielectric anisotropy and spontaneous polarization.

This dielectric anisotropy (←1-εS) was measured in the smectic A phase and extrapolated to the chiral smectic C phase transition temperature, and the spontaneous polarization (Ps) was measured by the triangular wave method (20
Hz-50V).

Examples 1 to 5 and Comparative Examples 1 and 2 manufactured in this way
A pair of polarizing films were arranged on both sides of the liquid crystal cell, 22.5' apart from the alignment treatment direction, and both polarizing films were in a crossed nicol configuration.

This was driven by applying a pulse of 11,200 .mu.sec at 1/32 duty and 1/4 bias as shown in FIG. 6 at 30.degree. (1) in FIG. 6 shows the waveform when the pixel turns on, and (2) shows the waveform when the pixel turns off. Table 1 shows the contrast and response speed of the results.

Table 1 There is an optimum range for negative dielectric anisotropy.

Comparative example 1. Examples 1 to 4 are results obtained when the magnitude of spontaneous polarization is kept almost constant and only the negative dielectric anisotropy is increased. Negative? It can be seen that as the A dielectric anisotropy increases, the contrast also increases. On the contrary, responsiveness showed a tendency to gradually decrease. Example 5 is an example in which the negative dielectric anisotropy was further increased to improve the contrast, but since the spontaneous polarization was also increased, the response was conversely improved. On the other hand, in the case of Comparative Example 2, the response was significantly lowered due to the smaller spontaneous polarization compared to Example 2.

[Effects of the Invention] The present invention exhibits a negative dielectric anisotropy of -1 to -8 and has a spontaneous polarization of 3 to 100, thereby enabling driving without superimposing high frequencies and simplifying the driving circuit. It has the advantage that its design is easy and its energy consumption is reduced.

Furthermore, increasing the negative dielectric anisotropy tends to improve the contrast, but conversely tends to reduce the response speed. However, in the present invention, since the spontaneous polarization is increased, this decrease in response speed can be prevented, and an electro-optical element with high contrast and high response speed can be obtained.

Furthermore, it has sufficient bistability against crosstalk voltages, making it possible to increase the size and density of electro-optic elements.

Further, the present invention has the effect of preventing deterioration of the liquid crystal and electrodes by not applying a DC electric field component to the liquid crystal.
This allows the use of an insulating film as the alignment film. Alternatively, it is also possible to form another insulating film between the liquid crystal and the electrodes, which is effective in preventing short circuits between the electrodes within the liquid crystal cell.

Furthermore, the present invention can provide sufficient bistability even in a cell with a large distance between cell substrates, which reduces the difficulty in creating a liquid crystal cell, and also reduces the difficulty in creating a liquid crystal cell between opposing electrodes. The effect of reducing the occurrence of short circuits is also recognized.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the basic structure of an electro-optical element driven by the present invention, and FIGS. 2 and 3 are examples of preferred electrode patterns when driving the liquid crystal composition of the present invention. FIG. FIG. 4 is a schematic diagram showing the magnitude relationship between the static tilt angle and the dynamic tilt angle. FIG. 5 is a detailed explanation diagram of the principle of the present invention. FIG. 6 is a waveform diagram of an AC signal used to drive a liquid crystal in an embodiment of the present invention. la, lb: Transparent substrates 2a, 2b: Conductive 11 3a, 3b: Orientation control 11!2 4 = Liquid crystal layer 8a, 8b: Polarizing plate sense 1 Figure 2 Figure I! 2 Li ≧ Akimura ￥ 3rd field signal to pole group 4th part Figure (C), d. Figure 6 (1) On-liquid type (2) Offer TLyt type

Claims (12)

[Claims] (1) The difference between the dielectric constant in the long axis direction of the molecule and the dielectric constant in the short axis direction of the molecule is -1 to -6, and the value of the spontaneous polarization is 3 to 1.
00. A ferroelectric liquid crystal composition characterized in that: 00. (2) The difference between the permittivity in the long axis direction of the molecule and the permittivity in the short axis direction of the molecule is -2 to -5, and the value of the spontaneous polarization is 5 to 7.
0. The ferroelectric liquid crystal composition according to claim 1, wherein the ferroelectric liquid crystal composition is 0. (3) The ferroelectric liquid crystal composition according to claim 1, wherein the ferroelectric liquid crystal is a chilasmectic C liquid crystal. (4) In a method for driving an electro-optical element, in which a ferroelectric liquid crystal composition is sandwiched between substrates provided with a pair of electrodes, and a polarizing plate is disposed on at least one outside of the liquid crystal layer, the ferroelectric liquid crystal composition A composition is used in which the difference between the dielectric constant in the long axis direction of the molecule and the dielectric constant in the short axis direction of the molecule is -1 to -6, and the value of the spontaneous polarization is 3 to 100, and the composition is driven by alternating current. A method for driving an electro-optical element, characterized in that: (5) A method for driving an electro-optical element according to claim 4, wherein a substrate provided with a scanning electrode group and a substrate provided with a signal electrode group are used as a pair of substrates. (6) When performing a write operation by sequentially selecting one of the scanning electrode groups, two scans constitute one cycle of writing, and when one scanning electrode is selected for the first time, the selection time T
Of _1, the voltage of V_0+V_1 is applied to the above, and the voltage of V_1 is applied to the latter part.
A voltage of 0-V_1 is applied, and during the second selection, a voltage of V_0-V_1 is applied above, and a voltage of V_0+V_1 is applied in the latter half, and the signal electrode is The state includes V_0-V within time T_2.
In the second state, a voltage of V_0+V_3 is applied in the first half of time T_2, and a voltage of V_0-V_3 is applied in the second half.
6. The method of driving an electro-optical element according to claim 5, wherein a voltage of 3 is applied. (7) The method for driving an electro-optical element according to claim 6, wherein the first half of the selection time T_1 is 1/2T_1 and the second half is 1/2T_1. (8) The method for driving an electro-optical element according to claim 6, wherein the first period of time T_2 is 1/2T_2, and the latter period is 1/2T_2. (9) The method for driving an electro-optical element according to claim 6, wherein time T_1=T_2. (10) Claim 6 where time T_1>T_2
A method for driving an electro-optical element as described in . (11) Claim 6 where time T_1<T_2
A method for driving an electro-optical element as described in . (12) The method for driving an electro-optical element according to claim 6, wherein the time T_2 is sufficiently shorter than the time required for the ferroelectric liquid crystal to reverse its spontaneous polarization.