JP3083017B2 - Liquid crystal element and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device used for a liquid crystal display device or a liquid crystal optical shutter, and more particularly to a ferroelectric liquid crystal device and a method of manufacturing the same. The present invention relates to a liquid crystal element having improved gradation display performance by improving instability, and a method for manufacturing the same.

[0002]

2. Description of the Related Art A display device of the type in which the amount of transmitted light is controlled in combination with a polarizer by utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules is disclosed by Clark and Lagerwall in Japanese Patent Application Laid-Open No. H10-163, 1988. 56-1072
No. 16 and U.S. Pat. No. 4,367,924. In general, the ferroelectric liquid crystal exhibits a chiral smectic C phase (SmC ^* phase) or an H phase (SmH ^* phase) having a non-helical structure in a specific temperature range. It has one of the first optically stable state and the second optically stable state, and has the property of maintaining that state even when the electric field is removed, that is, has bistability. Furthermore, since the ferroelectric liquid crystal has a feature that it responds quickly to changes in the electric field, it is expected to be applied to a large-screen, high-definition display device as a high-speed storage storage medium capable of simple matrix driving. ing.

[0003]

As described above, the ferroelectric liquid crystal element is characterized by switching between two different stable states by an external electric field. Therefore, it is necessary to make the switching (drive) characteristics stable and have good reproducibility. However, in the switching process of the ferroelectric liquid crystal, the process of relaxing the strong electric current of the device immediately after the application of the external electric field, the process of reversing the spontaneous polarization Ps, the displacement process of the impurity ions in the ferroelectric liquid crystal or the alignment film, and the process of turning off the external electric field Relaxation phenomena in various time regions, such as a strong electric current relaxation process of the element, a stabilization (latch) process of the inverted spontaneous polarization Ps, and a relaxation process of impurity ions, interfere with each other to form a complex time constant system. .

Further, an interface between an electrode for applying an electric field to the ferroelectric liquid crystal and an alignment film for aligning the liquid crystal and an interface between the alignment film and the ferroelectric liquid crystal layer are provided with electrons, ions and the like. There is a barrier to charges and a trap level. As a result, the electro-optical characteristics of the ferroelectric liquid crystal element can be stored in the white state and the black state in the previous state because the internal electric field caused by the display state written immediately before can be easily stored as a history. In some cases, the write threshold voltage differs, and so-called hysteresis or stability may occur. Therefore, when binary display (white / black) is performed by simple matrix driving, the amount of change (hysteresis) within the range of the difference (drive margin) between the voltage value applied when not selected and the voltage value applied when selected is selected. It is necessary to hold down.

On the other hand, one method of displaying a gray scale of a ferroelectric liquid crystal element by simple matrix driving is to change the area of a switching region in one pixel by applying a writing applied voltage or an applied pulse width modulation. There is a domain area gradation that adjusts the transmittance inside. In this case, it is necessary to provide an appropriate threshold distribution in the range of the voltage or the pulse width from the minimum transmittance (all black state) to the maximum transmittance (all white state), and the distribution of the electric field in one pixel is required. There are techniques such as a potential gradient, a cell thickness gradient, and a method of modifying a nucleus of an inversion domain. However, in the case of such gradation driving, a method of suppressing fluctuations using a driving margin as in binary display cannot be applied. Therefore, a specific halftone must always be obtained for a specific write voltage or pulse width, and ideally, hysteresis and instability must be completely removed. In other words, the number of gray levels that can be displayed is limited by the amount of hysteresis / instability.

[0006] In order to solve the above-mentioned technical problems, a method of applying a reset pulse for making a complete black state once during a driving waveform, and an anti-electric field effect caused by spontaneous polarization of a ferroelectric liquid crystal are considered. Although the use of a conductive alignment film has been devised to reduce this, even if these are used alone or in combination, there is still a limit in reducing the hysteresis, which limits the improvement in gradation display characteristics.

(Summary of the Invention) The present invention has been made in view of the above technical problems, and provides a liquid crystal element suitable for easily forming and driving high-density pixels over a wide area. It is in.

Another object of the present invention is to provide a liquid crystal element which can suppress a hysteresis and an afterimage phenomenon in a switching process.

Another object of the present invention is to provide a liquid crystal element suitable for performing grayscale driving.

[0010]

It is another object of the present invention to provide a ferroelectric liquid crystal and an electrode for applying a voltage to the ferroelectric liquid crystal on the opposing surface while holding the ferroelectric liquid crystal therebetween. A pair of substrates formed and subjected to a uniaxial alignment process for aligning the ferroelectric liquid crystal, wherein the ferroelectric liquid crystal in the alignment state shows at least two stable states, The alignment treatment is performed by rubbing a conductive polymer film, and the liquid crystal element is provided with means for applying a DC voltage or an AC voltage including a DC voltage component to the liquid crystal element during a pause. This is achieved by a liquid crystal element characterized by the following.

[0012]

According to the present invention, it is possible to provide a liquid crystal device in which hysteresis is suppressed by using a rubbed conductive polymer film as an alignment film and applying a DC voltage or an AC voltage including a DC voltage component. it can.

(Description of the Preferred Embodiment) An embodiment of the present invention will be described below.

It is considered that the hysteresis and instability observed during gradation driving of a ferroelectric liquid crystal display element is caused by an electric field (anti-electric field) generated by a bias of a charge distribution in an electric field application direction of the element. . As shown in FIG. 1, the spontaneous polarization of the ferroelectric liquid crystal is downward (Ps-dow).
Assuming that the display in the case of n) is black and the display in the upward direction (Ps-up) is white, the direction of the internal electric field generated by the spontaneous polarization of the ferroelectric liquid crystal differs in each display state. In order to cancel the internal electric field, various charges in the device, such as liquid crystal, impurity ions in the alignment film, and trapped charges in the vicinity of the interface move, causing an anti-electric field. Generally, in the gradation driving of the ferroelectric liquid crystal element, a pulse for once resetting the display history immediately before writing is applied, but this pulse is set to about 300 μsec or less in consideration of flicker on display. There is a need. However, the relaxation time of the electric field movement in the above-described device is several hundred msec to several sec, and
Since it is longer than 0 μsec, it is difficult to completely reset the uneven distribution of charges.

On the other hand, by using a conductive polymer as the alignment film, the anti-electric field can be reduced. This is presumably because the low impedance of the alignment film makes it possible to supply charges from the outside to the interface between the ferroelectric liquid crystal and the alignment film, so that the uneven distribution of the charges is quickly alleviated. However, even when a conductive alignment film is simply used, it is not sufficient to remove hysteresis and instability.

Therefore, in the present invention, reduction of hysteresis was realized by applying a DC voltage to a liquid crystal cell using this conductive polymer alignment film for a certain period of time. It is unclear what mechanism this method is effective for, but it is necessary to improve the consistency of the interface between the ferroelectric liquid crystal and the alignment film, reduce the interface trap density, and reduce the amount of ions in the liquid crystal or alignment film. Is considered to be the cause.

The above-described DC voltage application process (hereinafter referred to as DC
Aging can be performed during the process of manufacturing a ferroelectric liquid crystal element. However, a device using a ferroelectric liquid crystal (for example, a ferroelectric liquid crystal display device) can perform DC aging in advance. May be provided so that DC aging can be performed during an idle time of the apparatus. In this case, for example, when the display operation ends (display switch off), this is sensed, DC aging is started, and aging is performed for a certain period of time. By taking such measures, it is possible to reduce the change over time in hysteresis and instability.

FIG. 2 is a schematic view of a liquid crystal element according to one embodiment of the present invention, and shows one pixel portion in an enlarged manner.

On one substrate 31, a first electrode 32 is provided. On the first electrode 32, a first layer 33 having a main chain structure of a π-electron conjugated polymer is provided. Is provided.

Similarly, a second electrode 36 and a second layer 37 made of a π-electron conjugated polymer having a main chain structure are sequentially provided on a second substrate 35 serving as an opposite substrate.

Then, the first and second alignment layers 34, 38
A ferroelectric liquid crystal 1 is injected between them.

Reference numeral 39 denotes a spacer for keeping the distance between the substrates constant.

Reference numeral 30 denotes voltage applying means for applying a DC voltage or an AC voltage including a DC voltage component, and is provided in a manufacturing apparatus or a part of a control circuit of the liquid crystal element.

FIG. 3 is a diagram showing waveforms of three typical applied voltages used in the present invention. (A) applies a constant DC voltage during the aging period, (b) applies a DC voltage intermittently, (c) integrates DC voltages of different polarities during the aging period and the integration does not become zero. As shown in FIG.

FIG. 4 is a flowchart showing an example of a method for manufacturing a liquid crystal device including an aging step.

After sequentially forming an electrode and a conductive polymer alignment film on the substrate, a rubbing process is performed to assemble the two substrates with a substrate interval of about 1 μm.

After the liquid crystal is injected between the substrates, the temperature is lowered and the phase is changed to a chiral smectic C phase.

Thereafter, the aging process for applying the above-described voltage is performed. Here, the phase transition and the aging may be reversed.

FIG. 5 is a flowchart showing an aging processing mode set in a display device using a completed liquid crystal element.

An example is shown in which the aging process is performed at a desired timing after the display operation is completed, and then the next display operation is performed.

The aging time may be a time sufficient to obtain the required characteristics, and depends on the aging voltage, but is at least 1 minute or more, preferably 10 minutes or more, and most preferably 1 minute or more.
It is more than 00 minutes.

The polymer layer 3 used in the present invention
3, 37 may be any organic polymer material whose main chain structure is made of a π-electron conjugated system, such as polyacetylene, polyparaphenylene, polyparaphenylene vinylene, polypyrrole, polythiophene, polyquinoline,
Polypyridine, polyaniline, polycarbazole, polythienylene, polyparaphenylene sulfide, polythiazyl, polydibenzothiophene sulfide, polyfuran, and the like can be used. Further, various substituents may be introduced into the side chain for the purpose of enhancing the film forming property at the time of deposition or a mixture thereof. Further, a polymer mixture containing one or more of these components may be used. These organic polymer materials are applied to the first and second electrodes 32 and 36.
As a method of forming on the top, a well-known dip (immersion)
Method, spin coating method, Langmuir Blodgett (L
In addition to the wet method such as the B) method, a vapor deposition method or the like can be used. Further, I ₂ , Br ₂ , Na, AsF ₅ , and Cl are directly applied to the polymer material or after forming the π-electron conjugated polymer layer.
A material doped with a dopant such as O ₄ ⁻ , PF ₆ ⁻ , or Cl ⁻ may be used. Such a doping operation is not indispensable, but it is preferable to perform it in advance (before liquid phase injection) as necessary in order to sufficiently eliminate or reduce the hysteresis at the time of driving the liquid crystal. The lower limit of the conductivity of the π-electron conjugated polymer layer after doping is 10 ⁻¹⁰ S / c
m or more, preferably 10 ^-6 S / cm or more, more preferably 10 ^-4 S / cm or more, the upper limit of the conductivity 10 ⁰ S
/ Cm or less. For the purpose of stabilizing the dopant after doping, post-baking may be performed after doping as necessary.

As the first and second electrodes, transparent conductive materials such as tin oxide, indium oxide, and indium tin oxide (ITO) are preferably used. These materials can be preferably formed by a sputtering method, an ion plating method, or the like.

From the viewpoint of the liquid crystal driving characteristics, particularly, the elimination of hysteresis and generation of a reverse electric field, it is preferable to increase the electric capacitance component of the liquid crystal alignment layer, that is, to reduce the thickness of the liquid crystal alignment layer as much as possible. . On the other hand, from the viewpoint of liquid crystal alignment characteristics, if the thickness of the liquid crystal alignment layer is extremely thin,
In some cases, a good product cannot be obtained, and in addition, uniformity of the film thickness and the surface is indispensable. Therefore, the liquid crystal alignment layer of the present invention desirably has a thickness of 1 nm or more and 50 nm or less, more preferably 3 nm or more and 7 nm or less, and must have excellent uniformity. Therefore, at present, it is most preferable to form such a liquid crystal alignment layer by the LB method.

The surface of the liquid crystal alignment layer is subjected to a rubbing treatment so that the liquid crystal alignment layer has an alignment regulating force for controlling the uniaxial alignment of the liquid crystal and the inclination (pretilt angle) of the molecular axis of the liquid crystal molecules with respect to the substrate surface.

The liquid crystal used in the present invention is most preferably a chiral smectic liquid crystal having ferroelectricity as a liquid crystal having at least two stable states. Among them, chiral smectic C phase (SmC ^* ) and H phase ( S
mH ^* ), I phase (SmI ^* ), F phase (SmF ^* ) and G phase (SmG ^* ) liquid crystals are suitable. Hereinafter, such a ferroelectric liquid crystal will be briefly described.

As for the ferroelectric liquid crystal, R.S. B. Mey
"Ferroelectric Liquid"
Crystals "(Le Journal de
Physique Lettre, Vol. 36, L-69
P. 1975), N.M. A. Clark and S.M. T. L
agerwall's "SubmicrosecondB
istable Electro-Optical Swi
tching inLiquid Crystals "
(Applied Physics Letters
Journal, Vol. 36, No. 11, p. 899, 1980), Atsuo Fukuda and Hideo Takezoe, "Structure and Physical Properties of Ferroelectric Liquid Crystals" (Corona Co., 1990). The disclosed ferroelectric liquid crystal can be used.

Specifically, examples of the ferroelectric liquid crystal compound used in the present invention include decyloxybenzylidene-
p-Amino- (2-methyl) -butylcinnamate (D
OBAMBC), hexyloxybenzylidene-p-amino- (2-chloro) -propylcinnamate (HOB
ACPC) and 4- (2-methyl) -butyl-o-resorcylidene-4′-octylaniline (MBRA8). When a liquid crystal element is formed using these materials, the liquid crystal compound is composed of SmC ^* , SmH ^* , SmI ^* , and SmC ^* .
mF ^*, in order to hold the temperature state such that the SmG ^*,
If necessary, the element can be held by a copper block or the like in which a heater is embedded.

In the liquid crystal driving characteristics of the liquid crystal element having the above-described configuration, the conventional liquid crystal element, that is, the first and second π
Differences from the liquid crystal device having a structure in which an electron conjugated polymer layer does not exist and a liquid crystal alignment layer made of polyimides is deposited on the electrode are listed as follows. (1) The threshold of the liquid crystal drive voltage (2) Reduction of hysteresis at the time of driving the liquid crystal. (3) Reduction of the afterimage phenomenon at the time of driving of the liquid crystal.

In the liquid crystal device of the present invention, the mechanism why the above-mentioned change is observed is not scientifically known, but the electrical mechanism formed between the electrode and the layer adjacent to the electrode is unknown. It is thought to be based on the properties of the narrow membrane. The electrical properties of such a narrow film should greatly depend on the type of the adjacent layer, and when the adjacent layer is the π-electron conjugated polymer layer in the present invention, the adjacent layer has a main chain of polyimide or the like. π
It is conceivable that the resistance component is small and the capacitance component is large as compared with the case where the polymer layer is not composed only of the electron conjugated system.

[0041]

【Example】

(Embodiment 1) Embodiment 1 of the present invention will be described below. In this example, a ferroelectric liquid crystal cell was manufactured according to the following procedure. First, a pair of glass substrates coated with ITO as a transparent electrode by a sputtering method were prepared, and polyaniline was formed as an alignment film on each of the substrates by a spin coating method. The polyaniline used in this example is of a type soluble in an organic solvent such as N-methylpyrrolidone (NMP). After spin coating, the polyaniline is immersed in various acids or placed in an atmosphere to be about 10 ^-1 S / cm. Is a kind of conductive polymer that expresses conductivity. This polyaniline was dissolved in a mixed solvent of NMP: n-butyl cellosolve (n-BC) = 1: 1 at 0.5 wt% and filtered using a filter paper and a microfilter to obtain a coating solution. The spin condition was 2000 revolutions / minute for 20 seconds. After coating, the coating was dried at 80 ° C. for 15 minutes, and the resulting film thickness was 30 ° C.
Met.

Next, this substrate was immersed in 1N sulfuric acid for one hour to perform doping with polyaniline. After immersion, it was washed with water, dried at 80 ° C. for 10 minutes, and then post-baked at 65 ° C. for about 13 hours in a vacuum oven (under reduced pressure). This post-bake is performed for the purpose of reducing the change over time in the conductivity of polyaniline, and is not essential to the present invention. The conductivity of the polyaniline thus obtained was about 10 ^-1 S / cm.

Next, a rubbing treatment was performed on the surface of the polyaniline coating film as an alignment treatment. The cloth used for rubbing is made of nylon, and the rubbing conditions are as follows: the number of rotations of the roller is 1000 rotations / minute, the substrate feeding speed is 16 mm / second, and the roller is pushed 0.4 mm.

The pair of substrates processed in the above procedure were bonded to each other so as to have the same rubbing direction and to have a gap of 1.4 μm. Here, silica beads were used for the spacer. A liquid crystal exhibiting ferroelectricity at room temperature was injected into the gap in an isotropic phase (80 ° C.).

The ferroelectric liquid crystal used in this embodiment is a short-pitch liquid crystal having a chiral C pitch of about 0.35 μm.
m, Ps 71 nC / cm ² , tilt angle 26 ° (25 ° C.)
And has the following phase sequence:

Isotropic phase → SmA phase → Sm ^* C phase → Crystalline phase This liquid crystal has 20 V / u from the isotropic phase to the Sm ^* C phase.
By applying an AC electric field of about 10 Hz and slowly cooling, a uniform orientation having a striped texture can be obtained, and a high contrast display characteristic having an apparent tilt angle θa = 26 ° is obtained.

Aging was performed by applying a unipolar rectangular pulse of +12 V and 10 Hz to the ferroelectric liquid crystal cell manufactured as described above at room temperature for a certain period of time. After aging, the cell was placed under crossed Nicols, and an experimental drive waveform was applied to measure the transmittance. FIG. 6 shows a drive waveform. In the figure, 21 is an auxiliary pulse, 22 is a bipolar reset pulse, and 23 is a write pulse. The auxiliary pulse 21 is the write pulse 2
3 is a reverse polarity pulse for making the DC component of the drive waveform zero. The reset pulse 22 is for resetting the display state once (black state) before writing. The electro-optical response of the cell was measured by changing the voltage values of the write pulse 23 and the auxiliary pulse 21. The hysteresis characteristic was evaluated by evaluating the difference between the characteristic when left in a black state (for 3 seconds) before applying the driving waveform of FIG. 6 and the characteristic when left in a white state. The transmittance to be measured is a transmittance in a stable (latch) state 30 msec or more after the application of the drive waveform.

FIG. 7 shows a measurement result of a cell without DC aging, and FIG. 8 shows a measurement result of a cell with DC aging for 20 hours. 7 and 8, it can be seen that by performing DC aging, the amount of hysteresis ΔV is almost halved. FIG. 9 plots the amount of hysteresis with respect to the DC aging time.
As shown, the hysteresis decreased so as to be approximately proportional to the logarithm of the aging time. In addition, in a cell subjected to DC aging, when driven at a video rate, instability when continuously driven at a write voltage for the same gradation display was also reduced. Note that the same result was obtained regardless of the polarity of the DC aging voltage. The higher the DC aging voltage, the better, but there is a limit due to short-circuiting of the cells.

Example 2 Using a cell having the same configuration as the ferroelectric liquid crystal cell manufactured in Example 1, DC aging was performed.
The test was performed at a temperature (80 ° C.) where the liquid crystal was in an isotropic phase. Then, 20 V / um, 10 from the isotropic phase to the Sm ^* C phase.
The sample was gradually cooled while applying an AC electric field of Hz to obtain a uniform orientation having a striped texture. In this cell, as a result of the same measurement as in Example 1, the hysteresis and the instability were also reduced as compared with the case without DC aging.

[0050]

As described above, by performing DC aging, there is an effect of reducing the hysteresis and the instability, which are problems when performing gradation driving.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a polarization state of a liquid crystal element according to the present invention.

FIG. 2 is a schematic view for explaining a liquid crystal element and a method for manufacturing the same according to the present invention.

FIG. 3 is a diagram showing an applied voltage used in the present invention.

FIG. 4 is a flowchart showing an example of a manufacturing process of the liquid crystal element of the present invention.

FIG. 5 is a flowchart showing an example of the operation of the liquid crystal element of the present invention.

FIG. 6 is a view for explaining driving waveforms used in the liquid crystal element according to the embodiment of the present invention.

FIG. 7 is a diagram showing characteristics before aging according to the first embodiment of the present invention.

FIG. 8 is a graph showing characteristics after aging according to the first embodiment of the present invention.

FIG. 9 is a diagram showing a relationship between aging time and hysteresis according to the present invention.

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Claims (1)

(57) [Claims] An electrode for applying a voltage to the ferroelectric liquid crystal is formed on each of the opposing surfaces; and an electrode for applying a voltage to the ferroelectric liquid crystal. A liquid crystal device comprising a pair of substrates on which uniaxial alignment processing for aligning ferroelectric liquid crystal is performed, wherein the ferroelectric liquid crystal in the alignment state shows at least two stable states. Is a rubbed conductive polymer film, and the liquid crystal element is provided with means for applying a DC voltage or an AC voltage including a DC voltage component to the liquid crystal element during a pause. Liquid crystal element.