JPH0711637B2 - Ferroelectric liquid crystal element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ferroelectric liquid crystal device, and more specifically, the display characteristics are improved by setting the conductivity of an alignment film in contact with a liquid crystal layer to a specific value or more. And a ferroelectric liquid crystal device.

(Prior Art) Conventionally, various liquid crystal display elements in which a liquid crystal is sandwiched between a pair of opposing electrodes have been proposed. However, DSM (Dynamic Scatt
Other than the liquid crystal display device of the ering mode) type, 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 display device [for example, by M. Schadt and W. Helfrich,
"Applied Physics Letters", Vol.18, NO.4 (1971.2.1
5), P.127-128, “Voltage Dependent Optical Activ
In "ity 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, or in the process of forming the device. The above-mentioned problems (1) and (2) can be sufficiently addressed by means such as effective prevention of liquid crystal contamination.
This is because the point of (3) above did not become a serious problem because the refresh and storage types are basically used.

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

For example, in the structure of the ferroelectric liquid crystal device proposed by Clark and Lagavar, the dipole directions of the liquid crystal molecules are aligned in the liquid crystal layer as shown in FIG. 2, and spontaneous polarization of the liquid crystal occurs. .

Since the presence of this spontaneous polarization is a condition for the switching characteristics of the ferroelectric liquid crystal element, the deviation of the charge due to this spontaneous polarization causes SSFLCD (Surface Stabilized Ferroelectric L
iquid Crystal Display) is inevitable.

(Problems to be solved and used 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 has been found that there arises a problem that a change occurs which impairs the bistability of the liquid crystal layer.

That is, there is a transparent electrode such as an ITO electrode in the element,
In general, the image is in contact with the liquid crystal layer via a dielectric, but in that case, even in a memory state (applied voltage = 0), an electric field generated by the polarization charge of liquid crystal molecules exists in the liquid crystal layer. Due to this electric field, an electric field generated by the polarization charge of liquid crystal molecules exists in the liquid crystal layer, and the ionic impurities existing in the liquid crystal layer migrate due to this electric field, resulting in uneven distribution of ions.

The uneven distribution of the ions, on the contrary, constrains the liquid crystal molecules, disturbing the bistable state of the liquid crystal molecules in the switching state, and further causing a serious problem of inducing the disappearance of the memory property of the device. This is a major obstacle when considering the ferroelectric liquid crystal element as a display.

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) As a result of earnest research to solve the problems of the above-described conventional techniques, the present inventor has determined that the conductivity of an alignment film formed on a substrate and in contact with a liquid crystal layer is equal to or more than a specific value. As a result, the problems of the prior art as described above were solved, and the display characteristics of the ferroelectric liquid crystal element could be remarkably improved.

That is, the present invention relates to a ferroelectric liquid crystal device in which a ferroelectric liquid crystal layer is sandwiched between two facing electrode substrates, at least one of which has an alignment film, and the dielectric constant of at least one of the alignment films is ρ. = 1 × 10 ⁸ Ωcm or less, which is a ferroelectric liquid crystal device.

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

In the present invention, the conductivity of the alignment film formed on the substrate is ρ = 1 ×
It is a ferroelectric liquid crystal device characterized by being set to 10 ⁸ Ωcm or less.

The ferroelectric liquid crystal used in the present invention has one of a first optical stable state and a second optical stable state depending on an applied electric field, that is, it has bistability with respect to an electric field. It is a liquid crystal substance.

As the ferroelectric liquid crystal having bistability as described above, a chiral smectic liquid crystal having ferroelectricity is preferable, and among them, a chiral smectic C phase (Sm
C ^* ) or H phase (SmH ^* ) liquid crystals are suitable. These ferroelectric liquid crystals are "LEJOURNAL DE PHYSIOUE LETTERS"
36 (L-69) 1975, "Ferroelectric Liquid Crystals";
Applied Physics Letters " 36 (11) 1980," Submicro S
econd Bistable Electrooptic Switching in Liquid Cr
ystals ”;“ Solid State Physics ”16 (141) 1981“ Liquid Crystal ”and the like, and more specifically, for example, desiloxybenzylidene-P′-amino-2-methylbutylcinnamate (DOBAMBC), hexyl. Oxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACPC) and 4-o-
(2-methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

The third illustrated example is a ferroelectric liquid crystal device used in the present invention.
This is a schematic example, and 1 and 1'in the figure are In
A substrate (for example, a glass plate) coated with a transparent electrode such as ₂ O ₃ , SnO ₂ or ITO (Indium-Tin-Oxide), and an alignment film (not shown) is provided on at least one of the pair of substrates. The liquid crystal layer 2 made of the above-described liquid crystal is enclosed between these alignment films, and the SmC ^* phase liquid crystal oriented so as to be perpendicular to the substrate surface is enclosed.

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

When a voltage above a certain threshold value 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 unwound, and the dipole moment (P⊥) 4 is entirely the electric field. The alignment direction of the liquid crystal molecules 3 can be changed so as to face the direction.

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
In m), as shown in FIG. 4, 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. 4, when electric fields E or E'having different polarities, which are equal to or more than a certain threshold value, are applied to such a cell 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. 4, for example, when an 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, it is preferable that the cell is 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 ferroelectric liquid crystal device described above causes various problems due to the ions existing in the liquid crystal layer.

As a result of earnest studies to solve such problems, the present inventor has found that by increasing the conductivity of the alignment film in contact with these liquid crystal layers, ions generated in the liquid crystal layer are caused by the ions. It has been found that the uneven distribution of and the adverse effects on liquid crystal molecules due to the uneven distribution are eliminated, and the problems of the conventional techniques are solved.

A cross-sectional view of a preferred example of the ferroelectric liquid crystal device of the present invention is schematically shown in FIG. In the figure, 11 is a substrate such as a glass plate, 12 is a transparent electrode layer made of ITO or the like formed on the substrate 11, 13 is a conductive alignment film layer formed on the transparent electrode, 14 Is a positive ion and 15 is a negative ion. 18 has a liquid crystal layer, and 16 and 17 show two possible liquid crystal states.

The ferroelectric liquid crystal device of the present invention is mainly characterized in that the conductivity of at least one of the alignment films in contact with the liquid crystal layer is ρ = 1 × 10 ⁸ Ωcm or less, as schematically shown in FIG. Therefore, due to such characteristics, the uneven distribution of ions in the liquid crystal layer and the adverse effect thereof are eliminated, and by relaxing the internal electric field due to the polarization of the liquid crystal molecules, the bistability of the liquid crystal molecules is enhanced to improve the switching characteristics and the like. I was able to improve.

In the present invention, the conductivity of the alignment film is ρ = 1 × 10 ⁸ Ωcm
Any conductivity may be used as long as it is below, but if the conductivity is too high, a problem such as short-circuiting of a plurality of electrodes in the same substrate occurs, so that the preferable conductivity range is ρ = 1 × 10 ⁸ to It is in the range of 1 × 10 ⁶ Ωcm. One preferable method for adjusting the conductivity of the alignment film to ρ = 1 × 10 ⁸ Ωcm or less is to align the alignment film by adding a suitable conductive material to a resin material that is conventionally used as the alignment film. It is a method of forming a film.

For the conductivity (Ωcm) of the alignment film used in the present invention, the alignment film prepared separately can be measured by ASTM (AMERICAN NATIONAL STANDARD)
It is measured according to D-257.

As the material for forming the alignment film used in the present invention, conventionally known materials such as polyvinyl alcohol, polyimide, polyamide imide, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyvinyl acetate, polyamide, polystyrene, cellulose resin,
Resins such as melamine resin, urea resin, acrylic resin,
Alternatively, photosensitive polyimide, photosensitive polyamide, cyclized rubber photoresist, phenol novolac photoresist or electron beam photoresist [(meth) acrylate monomer or oligomer, epoxidized-1,4-
Polybutadiene, etc.] and the like.

Since the materials as described above are generally insulative, it is preferable to use these materials as a coating solution by using a solvent, and to disperse a suitable conductive material in the coating solution to obtain a suitable conductivity. Can be granted.

As the conductive material, any conventionally known conductive material can be used, but when forming a conductive alignment film on the substrate of the display surface of the liquid crystal element, it is preferable to use a transparent conductive material, for example, , Sn, In, Ni, Ti and other transition metal oxides, and oxides of these having different valences in solid solution are preferable, and examples thereof include SnO ₂ —Sb ₂ O ₃ system. These conductive materials have a finer particle size than the visible light wavelength (400 to 700 nm) so as not to interfere with light transmission, that is, a particle size of 0.4 μm or less, preferably 0.2 μm or less. Is preferably used.

Further, when forming a conductive alignment film on the side opposite to the display surface, the conductive material used does not necessarily need to be transparent, for example, conductive carbon, gold, silver, copper or other metal powder. Alternatively, fibers of these metals or whiskers can be used.

The above-mentioned conductive material may be dispersed in the above-mentioned coating liquid for forming an alignment film, or may be previously dispersed at a high concentration in a resin liquid, and the latter is particularly preferable. The amount of the conductive material added to the coating liquid may be such that the conductivity of the formed alignment film is ρ = 1 × 10 ⁸ Ωcm or less.
Generally, it is added in an amount of 5 parts by weight or more, preferably 5 to 50% by weight, based on 100 parts by weight of the resin. By changing the type and the amount of such a conductive material used, the conductivity of the obtained conductive alignment film can be arbitrarily changed.

Using the coating liquid containing the conductive material as described above, the surface of the substrate on which the electrode is formed is applied by any method such as a spinner coating method, dried or cured, and subjected to a rubbing treatment if necessary. Thus, a desired conductive alignment film can be formed.

These alignment films are preferably thin layers of, for example, about 50 to 1000Å, more preferably 100Å or less.

The above is a method using a synthetic resin, but in the present invention, an inorganic film may be used as long as it is conductive, for example,
The conductive alignment film may be formed of any of the conductive inorganic oxides and metals such as gold, silver, copper and aluminum as described above by an arbitrary method such as a sputtering method or a vapor deposition method.

The ferroelectric liquid crystal device of the present invention can be obtained by forming a conductive alignment film on at least one of the substrates on which the electrodes are formed as described above and then constructing the device according to a conventional method.

(Operation / Effect) According to the present invention as described above, in the conventional ferroelectric liquid crystal element, the conductivity of the orientation film is set to ρ = 1 × 10 ⁸ Ωcm or less, whereby the uneven distribution of ions in the liquid crystal layer is achieved. The present invention provides a ferroelectric liquid crystal device having excellent display characteristics, because variations in liquid crystal molecules and changes with time are not caused, and bistability of liquid crystal molecules is improved.

The present invention will be described more specifically with reference to the following examples.

Example 1 SiO ₂ (sputter) was formed as a dielectric layer on a glass substrate having an electrode layer, and a polyvinyl alcohol film containing 7% by weight of conductive carbon black was applied thereon by a spinner coating method and dried. After the curing, the surface was rubbed with an acetate cloth (hair length 1.5 mm) to form a conductive alignment film. Using this substrate as a lower substrate, and using a substrate having an alignment film containing no conductive carbon black as an upper substrate, liquid crystal CS-1014 manufactured by Chisso Corporation in the meantime, the cell thickness is controlled by alumina beads. , 1.
The ferroelectric liquid crystal display device of the present invention has a liquid crystal layer thickness of 0 μm to 1.6 μm.

The conductivity of the conductive alignment film in this case was about ρ = 3 × 10 ⁷ Ωcm when measured at room temperature according to ASTM D-257. On the other hand, the conductivity of the alignment film containing no conductive carbon formed on the upper substrate was ρ = 1 × 10 ¹⁰ Ωcm.

Comparative Example 1 A ferroelectric liquid crystal device for comparison was obtained in the same manner as in Example 1 except that the conductive carbon was not used.

When this device was gradually cooled and oriented, and then ± 20 V and 50 Hz alternating current was applied for 20 sec. And left for 24 hr, the device of Example 1 did not deteriorate, but the device of this comparative example had a threshold value of 0.5 ms. Pulse caused deterioration of 5V increase.

Example 2 In preparation for the alignment film in Example 1, an orthorhombic system vapor-deposited film of SiO was formed to a thickness of 700 Å, and a conductive alignment film was used on which gold was further evaporated to a thickness of 50 Å, A ferroelectric liquid crystal device of the present invention was formed in the same manner as in Example 1 except for the above.

The conductivity of the conductive alignment film in this case was measured at room temperature according to ASTM D-257, and was about ρ = 1 × 10 ⁸ Ωcm.

Comparative Example 2 A ferroelectric liquid crystal device for comparison was obtained in the same manner as in Example 2 except that gold was not deposited.

When this device was gradually cooled and oriented, and then ± 20 V and 50 Hz alternating current was applied for 20 sec. And left for 5 hr, the device of Example 1 did not deteriorate, but the device of this comparative example had a threshold value of 0.5 ms. The pulse caused deterioration of 1V increase.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a part of a cross section of a ferroelectric liquid crystal device of the present invention, and FIG. 2 is a diagram schematically showing two states of polarization of liquid crystal molecules of the ferroelectric liquid crystal device. FIG. 3 and FIG. 4 are diagrams schematically showing the operation of the ferroelectric liquid crystal element. 1, 1 ', 11 ... substrate 2, 18 ... liquid crystal layer 3, 16, 17 ... liquid crystal molecule 4 ... dipole moment 5, 5' ... alignment state 12 ... electrode 13 ... conductive alignment film 14, 15 …… AEON

Claims (3)

[Claims] 1. A ferroelectric liquid crystal device comprising a ferroelectric liquid crystal layer sandwiched between two opposing electrode substrates, at least one of which has an alignment film, wherein the conductivity of at least one of the alignment films is ρ = Ferroelectric liquid crystal device characterized by being set to 1 × 10 ⁸ Ωcm or less. 2. The ferroelectric liquid crystal device according to claim 1, wherein the alignment film is a resin film containing 5% by weight or more of a conductive material. 3. The conductive material is conductive carbon or gold,
The ferroelectric liquid crystal element according to claim (1), which is a metal particle such as silver.