JPH0711632B2 - Voltage application method for chiral smectic liquid crystal device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a driving method of a ferroelectric chiral smectic liquid crystal element applied to liquid crystal light-shutters, displays and the like, and more specifically, driving characteristics and display characteristics. A method for driving a ferroelectric chiral smectic liquid crystal device having improved characteristics.

(Prior Art) Conventionally, various liquid crystal elements in which a liquid crystal is sandwiched between a pair of opposing electrodes have been proposed, but DSM (Dynamic Scatterin
Other than g Mode) type liquid crystal elements, it has not been recognized that it is necessary to control charged bodies such as positive ions such as sodium ions and negative ions such as chlorine ions in the liquid crystal layer.

The reason is TN (Twisted Nematic), which is currently popular.
-Type liquid crystal device [eg, by M. Schadt and W. Helf-rich, “App
lied Physics Letters ", Vol.18, No.4 (1971.2.15),
“Volt-age Dependent Optical Activity” on P.127-128
of a Twisted Nematic Liquid Crystal ”], (1) Excessive ion flow disturbs the alignment of liquid crystal molecules.

(2) The durability of the liquid crystal material is reduced.

(3) The time constant of the voltage applied to the liquid crystal layer becomes short.

It was thought that the effects such as the above might be caused by conductive substances such as ions, but in reality, by appropriately refining the liquid crystal, the volume resistance of the liquid crystal could be increased to 10 ⁹ Ωcm or more,
The above problems (1) and (2) can be sufficiently dealt with by means such as effective prevention of liquid crystal contamination in the process of forming the device, while the driving method is an AC driving method or a refresh storage driving method. Since the method is basic, the point (3) above was not a serious problem.

On the other hand, in the case of a ferroelectric liquid crystal device which is being developed worldwide in recent years, the behavior of charged particles such as ions in the liquid crystal layer has a significant influence on the characteristics of the ferroelectric liquid crystal device. It has been revealed.

For example, in the configuration of the ferroelectric liquid crystal device proposed by Clark and Lagavar, the directions of the dipoles of each liquid crystal molecule are aligned in the liquid crystal layer as shown in FIG. 1, and spontaneous polarization of the liquid crystal occurs. .

Since the existence of this spontaneous polarization is a condition for the switching characteristics of the ferroelectric liquid crystal element, the bias of electrification due to this spontaneous polarization is SSFLCD (Surface Stabi-lized Ferroelectric
Liquid Crystal Device) is inevitable.

(Problems to be Solved by the Invention) Although spontaneous polarization of liquid crystal molecules in a ferroelectric liquid crystal device as described above is inevitable, due to the influence of this polarization charge, the device is not driven (that is, in a memory state). It was found that there was a problem that the bistability of the liquid crystal layer was impaired and the liquid crystal molecules became monostable.

That is, there is a transparent electrode such as an ITO electrode in the element,
In general, the structure is in contact with the liquid crystal layer via a dielectric and an alignment film, and in that case, even in a memory state (applied voltage = 0), an electric field generated by polarization charge of liquid crystal molecules is generated in the liquid crystal layer. The presence of this electric field causes the ionic impurities present in the liquid crystal layer to migrate, resulting in uneven distribution of ions. The uneven distribution of the ions, on the contrary, constrains the liquid crystal molecules, disturbing the bistability of the liquid crystal molecules in the switching state, and further causing a serious problem of erasing the memory property of the device itself. Considering the present ferroelectric liquid crystal device as an optical shutter or a display, it is a big obstacle.

Therefore, in the ferroelectric liquid crystal element, it is desired to solve the problem caused by the ions existing in the liquid crystal layer.

(Means for Solving Problems) The present inventor has completed the present invention as a result of earnest research to solve the above-mentioned problems of the prior art.

That is, the present invention relates to a pair of electrode substrates, which are arranged at intervals that suppress the formation of a helical structure of a chiral smectic liquid crystal, and have one of the surfaces in contact with the liquid crystal made conductive, and the pair of electrode substrates. A chiral smectic liquid crystal device having a chiral smectic liquid crystal disposed between the pair of electrodes, wherein a voltage is applied between the pair of electrodes. A DC voltage necessary to cause a charged body to migrate to the conductive surface to neutralize the one charged body and to generate a bistable chiral smectic liquid crystal having two different stable alignment states in the absence of an electric field is applied. A method for applying a voltage to a chiral smectic liquid crystal device, which is characterized by applying a voltage.

Next, the present invention will be described in more detail.

The ferroelectric chiral smectic liquid crystal element used in the present invention is one that takes one of a first optical stable state and a second optical stable state depending on an applied electric field, that is, a biaxial electric field. It is a liquid crystal substance having stability.

Among the chiral smectic liquid crystals used in the present invention, particularly chiral smectic C layer (SmC ^* ) or H layer (SmC ^* )
A liquid crystal of mH ^* ) is suitable. These ferroelectric liquid crystals are
"LEJOURNAL DE PHYSIOUE LETTERS" 36 (L-69) 1975,
"Ferroelectric Liquid Crystals"; Applied Physics
Letters " 36 (11) 1980," Submicro Second Bistable E
lectrooptic Switching in Liquid Crystals ";" Solid State Physics "16 (141) 1981" Liquid Crystals "and the like, and more specifically, for example, desiloxybenzylidene-P'-
Amino-2-methylbutyl cinnamate (DOBAMBC),
Hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACPC) and 4-o-
(2-methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

The second illustrated example is a schematic illustration of an example of a ferroelectric liquid crystal device, and 1 and 1'in the figure are In ₂ O ₃ , SnO ₂ or ITO.
(Indium-Tin-Oxide) is a substrate (eg glass plate) coated with a transparent electrode, and at least one of the pair of substrates is provided with an insulating layer and an alignment film (not shown). A liquid crystal layer 2 made of liquid crystal as described above between the control films is oriented so that the liquid crystal layer 2 is oriented perpendicular to the substrate surface.
^* Enclosed as phase liquid crystal.

A thick line 3 represents a liquid crystal molecule, and the liquid crystal molecule 3 has a dipole moment (P⊥) 4 in a direction orthogonal to the molecule.

When a voltage above a certain threshold is applied between the electrodes on the substrates 1 and 1'of such a ferroelectric liquid crystal element, the helical structure of the liquid crystal molecules 3 is unraveled, and the dipole moment (P⊥) 4 is all in the electric field direction. The alignment direction of the liquid crystal molecules 3 can be changed so that the liquid crystal molecules 3 are oriented toward

The liquid crystal molecules 3 have an elongated shape and exhibit anisotropy of refractive index in the major axis direction and the minor axis direction thereof. Therefore, for example, polarizers arranged in a crossed Nicol positional relationship above and below the substrate surface. It is easy to understand that the liquid crystal optical modulation element having the optical characteristics that changes depending on the polarity of voltage application can be obtained by placing.

Furthermore, when the thickness of the liquid crystal element is sufficiently thin (for example, 1μ
As shown in FIG. 3, in (m), the helical structure of the liquid crystal molecule is unwound (non-helical structure) even when no electric field is applied, and its dipole moment P or P ′ is upward (4
Take either a) or downward (4b). As shown in FIG. 3, when an electric field E or E'having a polarity different from a certain threshold value is applied to such an element for a predetermined time, the dipole moment is directed upward 4a or downward depending on the electric field vector of the electric field E or E '. 4b, and the liquid crystal molecules are aligned in either the first alignment state 5 or the second alignment state 5 'accordingly. There are two advantages of using such a ferroelectric liquid crystal element as an optical modulation element.

Firstly, the response speed is extremely fast, and secondly, the alignment of the liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 3, for example, when the electric field E is applied, the liquid crystal molecules are aligned in the first alignment state 5, but in this state, it is stable even when the electric field is cut off. The opposite electric field E ′
When a voltage is applied, the liquid crystal molecules are oriented in the second alignment state 5'and change their orientation, but they remain in this state even when the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, the respective alignment states are also maintained. In order to effectively realize such a high response speed and bistability, the element is preferably as thin as possible, and generally 0.5 to 20 μm, particularly 1 to 5 μm is suitable. A ferroelectric liquid crystal device having a matrix electrode structure using a ferroelectric liquid crystal of this type has been proposed by Clark and Lagabal in US Pat. No. 4,367,924.

The above is one example of the structure of the ferroelectric liquid crystal element. However, as described above, these ferroelectric liquid crystal elements cause various problems due to the ions existing in the liquid crystal layer.

That is, the ferroelectric liquid crystal has spontaneous polarization derived from the molecular dipole as shown in FIG. 1, and in the liquid crystal element in which the ferroelectric liquid crystal has bistability, the internal electric field induced by the polarization is generated. is doing. This internal electric field is always present in the ferroelectric liquid crystal layer regardless of whether a voltage is applied or not.

Due to the internal electric field induced by the spontaneous polarization described above,
Ionic species in the ferroelectric liquid crystal migrate, and these ionic species are localized in the direction of each electrode. The uneven distribution of these ions is stable for a considerable time.

The above phenomenon can be confirmed by the following experiments. For example, as shown in FIG. 4, a ferroelectric liquid crystal is sealed in an element in which insulating films 6 and 6 ′ such as alignment films are provided on transparent electrodes 7 and 7 ′ to form a ferroelectric liquid crystal element. The molecule 3 is directed in one direction (spontaneous polarization direction, up → down) and the electrode 7,
When 7'is left at the same potential, only the polarization direction becomes stable, the other state cannot be realized, and the bistability of the liquid crystal molecule 3 disappears (Fig. 4a). Next, an external voltage E of several volts is applied to the device from the upper substrate to the lower substrate, the device is left for several minutes (Fig. 4b), and the upper and lower electrodes are made to have the same potential again (Fig. 4c). This time, the other state becomes stable, and the liquid crystal molecules 3 cannot be transferred to the opposite state even if the drive pulse is applied.

These phenomena can be easily explained by the uneven distribution of ions represented by plus and minus in FIG.

In the case of FIG. 4a, ion migration occurs due to the internal electric field due to the polarization of the liquid crystal molecules 3, and in the case of FIGS. 4b and c, ion migration occurs, and the polarization direction of the liquid crystal molecules 3 is stabilized due to the uneven distribution of each It can be seen that the bistability of the liquid crystal molecules is reduced and the phenomenon of monostabilization of the liquid crystal molecules occurs.

In the present invention, in order to solve the monostability of these liquid crystal molecules, one of the surfaces of the two electrode substrates holding the ferroelectric liquid crystal layer in contact with the ferroelectric liquid crystal layer is made conductive. By this, it is possible to eliminate the positive charge or the negative charge unevenly distributed in the vicinity of the substrate, and as a result, it is possible to prevent the monostability of liquid crystal molecules due to the uneven distribution of ions, thereby solving the above-mentioned drawbacks of the prior art.

The present invention as described above will be described more specifically with reference to FIG. 5 showing an embodiment of the present invention.

FIG. 5 is a diagram schematically showing a cross section of an embodiment of the ferroelectric liquid crystal device of the present invention, in which 1, 1 ′ is a glass substrate, 7,
7'is a transparent electrode layer made of ITO or the like formed on the glass substrate 1, 1 ', 8 is a charged body having a positive charge unevenly distributed near the upper substrate, and 9 is a negative charge body unevenly distributed near the lower substrate. is there. Reference numeral 2 denotes a liquid crystal layer, reference numerals 5 and 5'represent two possible liquid crystal states, and reference numeral 6 denotes an insulating layer.

As shown in the figure, 5 is an intramolecular dipole, and at the same time, the direction of spontaneous polarization is also directed downward (from the upper substrate to the lower substrate) by an arrow (from − to +). Existence is stabilized. In contrast,
The arrangement of the liquid crystal molecules shown in 5'is in an electrically unstable state because the dipoles are directed in the opposite direction to the uneven distribution of ions, which is not preferable for a liquid crystal display element having bistability. is there.

On the other hand, in the case of the ferroelectric liquid crystal element of the present invention, since the insulating layer is not formed on the upper substrate side, it is conductive, and it is positive or negative on the side where the insulating layer is not formed by DC bias. By making one of the charged bodies unevenly distributed, adsorbing ions on the electrode, and canceling the polarity, the electrical instability of the liquid crystal molecule alignment due to the uneven distribution of charged bodies can be reduced, and display using the memory state becomes possible. Become.

As described above, by making one of the surfaces of the two electrode substrates sandwiching the ferroelectric liquid crystal layer in contact with the ferroelectric liquid crystal conductive, the monostability of the liquid crystal molecules due to the uneven distribution of the ions of the element is achieved. It can be prevented and the bistability of liquid crystal molecules can be significantly improved.

In the liquid crystal device of the present invention, as a method of making one of the substrate surfaces in contact with the liquid crystal layer conductive, a method of providing an insulating layer only on one electrode substrate surface as shown in FIG. When providing an insulating layer on both of the electrode substrates of,
A method in which the conductivity of one of the insulating layers is set to a specific resistance value ρ = 10 ⁶ Ω.cm or less, preferably ρ = 10 ⁴ Ω.cm or less may be used.

Further, it is preferable to provide an alignment control film that has been subjected to a rubbing treatment on the surface of the substrate that is in contact with the ferroelectric liquid crystal layer. In such a case, for example, when the orientation control film is provided on both substrates, the conductivity of one of the orientation control films is ρ = 10 ⁶ Ω.cm or less, preferably ρ = 10 ⁴ Ω. cm or less, when an insulating layer and an orientation control film are provided on one electrode substrate, and when an orientation control film is provided on the electrode without providing an insulating layer on the other electrode substrate, this orientation control film Conductivity ρ = 10 ⁶ Ω.cm
Hereafter, the method of setting ρ = 10 ⁴ Ω.cm or less is effective.

The orientation control film can be formed from polyvinyl alcohol, polyimide, polyamide, polyesterimide, cellulose, polyethylene or the like according to a conventional method, and in order to make the orientation control film conductive, include conductive particles or the like in these films. Just do it.

In addition, the measurement of the specific resistance in the above is performed by ASTM (AMERI-CAN NA
TIONAL STANDARD) This is the value measured by D-257.

In the above, the substrate, the liquid crystal, the transparent electrode, the insulating layer, the orientation control film, and the like that constitute the element may be formed according to the conventional technique.

(Operation / Effect) According to the present invention as described above, in constructing a ferroelectric chiral smectic liquid crystal element, by making one of the surfaces of the electrode substrate in contact with the liquid crystal layer conductive, the element is not driven. If a DC voltage is applied at times, it is possible to neutralize the uneven distribution of ions generated in the ferroelectric liquid crystal layer, and as a result, it is possible to prevent monostability of liquid crystal molecules when the device is driven and Is significantly improved, so that the response of the device can be made faster, and the display characteristics and the memory characteristics can be further improved.

Next, the present invention will be described more specifically with reference to Reference Examples and Examples.

Reference Example 1 CS-108 manufactured by Chisso Corporation was used as a liquid crystal material. An insulating layer consisting of a SiO ₂ film is formed to a thickness of 1000 Å on both of the _two glass substrates on which the ITO electrodes are formed by a sputtering method, and a polyvinyl alcohol film is spinner coated to a thickness of 400 Å on it and then dried and cured The substrate was unidirectionally rubbed with an acetate cloth. Such two substrates were bonded to face each other with a spacer interposed therebetween to form an element. The element thickness was 1 to 1.4 μm. The above-mentioned liquid crystal material was injected into this device at a temperature of 100 ° C., which is an isotropic phase. When the temperature of the device is controlled and gradually cooled at a speed of 5 ° C / hour, the liquid crystal in the device is Ch phase (cholesteric phase), Sm
After passing through the A phase (smectic A phase), the SmC ^* phase (chiral smectic C phase) is reached. These phase changes can be identified by disposing a polarizer and an analyzer on both sides of the liquid crystal element. In FIG. 5, a 9.0 V DC electric field was applied to the upper and lower substrates so that the spontaneous polarization was aligned in the 5'direction of the two stable states, and the substrates were allowed to stand for about 10 hours. as a result,
The 5 direction was stable, but the 5'direction was unstable and there was little contrast between the two stable states.

When observing the molecular state with and without a driving voltage under a microscope, when driving so that the spontaneous polarization is reversed from the direction of 5 to the direction of 5 ′, the voltage of the molecule reversed in the direction of 5 ′ is cut off while the voltage is being applied. After that, it turned out to be stable by reversing in the direction of 5 and becoming monostable in the direction of 5.

Example 1 In Reference Example 1, an insulating layer and an alignment film were not formed on any one electrode substrate of a liquid crystal element, and other elements were prepared under the same conditions as in Reference Example 1, and the same driving was performed. .

As a result, the direction 5 in FIG. 5 was stable and the direction 5'was unstable, but the contrast between the two stable states was significantly larger than that of the reference example. Observing that the spontaneous polarization is reversed from the direction 5 to the direction 5'with a microscope, the direction 5'is not unstable at all, and remains stable in the direction 5'even after the voltage is cut off. I understood it.

From this result, the insulating layer and the alignment film were not formed on one of the substrates of the liquid crystal element, and either the positive or negative charged body was unevenly distributed on the side where the insulating layer was not formed by DC bias. , By adsorbing ions on the electrodes and eliminating their polarities, the electrical instability of the liquid crystal molecular alignment due to the uneven distribution of charged bodies can be reduced, and display using the memory state is fully possible.

Example 2 Since the charged body is unevenly distributed by using the liquid crystal element of Example 1,
A DC electric field of 9.0 V was applied between the upper and lower substrates in the same direction as in Example 1 and allowed to stand. In Example 1, the change over time in the voltage (saturation voltage) for inverting all the liquid crystal molecular alignment from the stable direction to the unstable direction was measured. The sixth measurement result
As shown in the figure. In the sample in which the insulating layer and the alignment film were not formed on one of the electrode substrates of the liquid crystal element, the saturation voltage increased obviously slower than the sample in which the insulating layer and the alignment film were formed on the electrode substrates on both sides, And you can see that it is small. Such an increase in the saturation voltage is caused by the uneven distribution of charged bodies such as ions in the liquid crystal element, and the above results indicate that the insulating layer and the alignment film are not formed on one of the electrode substrates of the liquid crystal element. Then, it became clear that the influence of uneven distribution of the charged body can be reduced by applying the DC bias.

Example 3 In Reference Example 1, an insulating layer is provided on one electrode substrate,
An alignment control film (400 Å) was provided from polyvinyl alcohol on the other electrode substrate, and the liquid crystal element of the present invention was constructed in the same manner as in Reference Example 1 except for the above.

Incidentally, the specific resistance of one of the polyvinyl alcohol film of 400 Å used in this example was measured according to ASTM D-257,
ρ = 10 ⁶ Ω.cm.

When the above device was driven in the same manner as in Example 2 and similar results were obtained.

[Brief description of drawings]

1 to 4 are views schematically showing a part of a cross section of a ferroelectric liquid crystal device and also showing two states of polarization of liquid crystal molecules, and FIG. It is a figure which shows the cross section of a dielectric liquid crystal element diagrammatically, and FIG. 6 is a figure which shows the relationship between the saturation voltage and leaving time of the element of this invention, and the element of a reference example. 1, 1 '... Substrate 2 ... Liquid crystal layer 3 ... Liquid crystal molecule 4 ... Dipole moment 5, 5' ... Alignment state of liquid crystal molecule 6 ... Insulating layer 7 ... Electrode 8 ... Positive ion 9 ... …negative ion

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Claims (1)

[Claims] 1. A pair of electrode substrates, which are arranged at an interval that suppresses the formation of a helical structure of chiral smectic liquid crystal, and one of the surfaces in contact with the liquid crystal is made conductive, and between the pair of electrode substrates. A method of applying a voltage to a chiral smectic liquid crystal device, comprising: a chiral smectic liquid crystal arranged in a pair, wherein a voltage is applied between the pair of electrodes, wherein one of the ionic impurities is charged between the pair of electrodes when not driven. The body is migrated to the conductive surface to neutralize the one charged body,
A method of applying a voltage to a chiral smectic liquid crystal device, which comprises applying a DC voltage required to generate a bistable chiral smectic liquid crystal having two different stable alignment states in the absence of an electric field.