JPS6310130A - Ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To increase the quantity of transmittance light by setting a tilt angle imparted by a pulse voltage impression at the angle larger than the tilt angle when the pulse voltage impression is not executed. CONSTITUTION:The ferroelectric liquid crystal of a ferroelectric liquid crystal element disposed with the ferroelectric liquid crystal having two stable molecular arrangement states between a pair of substrates 11 and 11' has the tile angle theta1 (2theta1; the angle between the two optical main axes corresponding to the two stable molecular arrangement states after the pulse voltage impression) imparted by the pulse voltage impression. The relation theta1>theta2 is held between the tilt angle theta1 and the tilt angle theta2 (2theta2; the angle between the two optical main axes corresponding to the two stable molecular arrangement states when the pulse voltage impression is not executed) when the pulse voltage impression is not executed. The certain specified bistable state is thereby attained with good productivity. The display element or high-speed shutter element having high responsiveness, high picture element density and large area is realized by combining the specified stable state and polarizers.

Description

Detailed Description of the Invention [Field of the Invention] The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal-optical shutter, etc., and more specifically, it improves display and driving characteristics by improving the initial alignment state of liquid crystal molecules. This invention relates to a liquid crystal element with improved characteristics.

[Conventional technology]

The use of a liquid crystal element having bistability has been proposed by Clark et al. (JP-A-56-107216, US Pat. No. 4,367,924, etc.). Bistable liquid crystals are generally chiral smectic C phase (SmC*
) or H phase (SmH phase) is used. This liquid crystal has a bistable state consisting of a first optically stable state and a second optically stable state with respect to an electric field, and therefore, unlike the optical modulation element used in the above-mentioned TN type liquid crystal, for example, The liquid crystal is aligned in a first optically stable state with respect to one electric field vector, and the liquid crystal is aligned in a second optically stable state with respect to the other electric field vector. Furthermore, this type of liquid crystal has the property of very quickly taking one of the above two stable states in response to an applied electric field, and maintaining that state when no electric field is applied. By utilizing these properties, considerable substantial improvements can be obtained over many of the problems of conventional TN type devices.

[Problem that the invention seeks to solve]

However, in order for an optical modulation element using this bistable liquid crystal to exhibit predetermined drive characteristics, the liquid crystal placed between a pair of parallel substrates must be able to convert between the two stable states. It is necessary that the molecules be in such a state that they occur effectively. For example, SmC* or SmH*
For ferroelectric liquid crystals with phases, SmC* or S
The liquid crystal molecular layer having mH main phase is perpendicular to the substrate surface,
Therefore, it is necessary to form a region (monodomain) in which the liquid crystal molecular axes are arranged substantially parallel to the substrate surface. In order to form this monodomain stably, the substrate in contact with the liquid crystal must be subjected to uniaxial alignment treatment (rubbing method or oblique evaporation method).
This is the most effective method when considering productivity, but in reality, in optical modulation elements using ferroelectric liquid crystals that have bistability using uniaxial alignment treatment, liquid crystal molecules are It was found that the ideal arrangement state in which the axes were arranged almost parallel to the perpendicular direction of the board surface was not obtained, but that they were arranged in a twisted state with respect to the perpendicular direction of the board surface, which affected the display quality and response characteristics. was found to be significantly impaired.

[Means for Solving the Problems] and [Operation] In view of the above-mentioned circumstances, the object of the present invention is to achieve a specified bistable state with high productivity as described below, and to achieve the specified bistable state. An object of the present invention is to provide a liquid crystal optical element that can realize a display element or a high-speed shutter element having high-speed response, high pixel density, and large area by combining a stable state and a polarizer.

The present invention provides a ferroelectric liquid crystal element in which a ferroelectric liquid crystal having two stable molecular alignment states is arranged between a pair of substrates, in which the ferroelectric liquid crystal is given a tilt angle θ by a pulse voltage application process. , (2θ1; angle formed by two optical principal axes corresponding to two stable molecular arrangement states after pulse voltage application processing), and the tilt angle θ1 is the tilt angle θ2 when the pulse voltage application processing is not performed. (2θ2; angle formed by two principal optical axes corresponding to two stable molecular alignment states when no pulse voltage is applied) It has characteristics.

〔Example〕

The present invention will be explained below with reference to the drawings. The optical modulation substance used in the driving method of the present invention has a first optically stable state (for example, a bright state) and a second optically stable state (for example, a dark state) depending on the applied electric field. A substance having at least two stable states with respect to an electric field, in particular a liquid crystal having such properties, is used.

As the liquid crystal having bistability that can be used in the driving method of the present invention, chiral smectic liquid crystal having ferroelectricity is most preferable, and among these, chiral smectic liquid crystal
Phase ('SmC), H-phase (SmH*), I-phase (SmI), F-phase (SmF''), and G-phase (SmG*) liquid crystals are suitable.For this ferroelectric liquid crystal, “LE JOUR
NAL DE PHYSIQUE LETTRE") Volume 36 (L-69) 1975 "Ferroelectric Liquid Fist Ristars J ("Ferroele
ctricLiquid Crystals');
“Applied Physics Letters”
Lied Physics Letters”) No. 36
Volume, No. 11, 1980 “Submicro Second φ
Bistiple Electro-Optic Switching in Liquid Crystals J (rsubmi
cro 5econd B15tableElec
Trooptic Switching in
LiquidCrystals'); ``Solid State Physics 1
6 (141) 1981 r Liquid Crystal", and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

FIG. 1 schematically depicts an example of a cell for explaining the operation of a ferroelectric liquid crystal. Hereinafter, S as the desired phase
This will be explained using an mC tree as an example. 11 and 11' are I
n2O3, 5n02 or ITO (Indium-T
A substrate (glass plate) coated with a transparent electrode made of a thin film such as
SmC* phase liquid crystal oriented so that 2 is perpendicular to the glass surface is sealed. A thick line 13 represents a liquid crystal molecule, and the liquid crystal molecule 13 continuously forms a helical structure in the plane direction of the substrate. The angle formed between the central axis 15 of this helical structure and the axial direction of the liquid crystal molecules 13 is expressed as a tilt angle {circle around (2)}.

This liquid crystal molecule 13 has a dipole moment (P) 14 in a direction perpendicular to the molecule. Boards 11 and 11
When a voltage higher than a certain threshold is applied between the electrodes on
The liquid crystal molecules 13 are arranged so that all Pf) 14 are oriented in the direction of the electric field.
can change the orientation direction. The liquid crystal molecules 13 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the optical polarity can be changed depending on the voltage applied polarity. It is easily understood that this results in a liquid crystal optical element whose characteristics change.

The liquid crystal cell preferably used in the liquid crystal optical element of the present invention can have a sufficiently thin thickness (for example, 10 μm or less). As the liquid crystal layer becomes thinner, the helical structure of the liquid crystal molecules unwinds and becomes a non-helical structure even when no electric field is applied, as shown in Figure 2, and the dipole moment P or P' is directed upward. (24) or downward (24'). The 1/2 angle formed by the molecular axis of the liquid crystal molecular axis 23 and 23' is called the tilt angle θ. When an electric field E or E' with a different polarity, which is equal to or higher than a certain threshold value, is applied to such a cell by the voltage applying means 21 and 21' as shown in FIG. The direction changes upward 24 or downward 24' in response to the vector, and accordingly the liquid crystal molecules are aligned in either the first stable state 23 or the second stable state 23'.

As mentioned earlier, there are two advantages to using such ferroelectricity as a liquid crystal optical element. The first is that the response speed is extremely fast, and the second is that the alignment of liquid crystal molecules has bistability. To further explain the second point, for example, with reference to FIG. 2, when an electric field E is applied, the liquid crystal molecules are oriented in a first stable state 23, and this state remains stable even when the electric field is turned off. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 23' and change their orientation, but they remain in this state even after the electric field is turned off.

In order to effectively realize such fast response speed and bistability, it is preferable that the cell be as thin as possible.

As mentioned earlier, the biggest problem in forming devices using liquid crystals with ferroelectric properties is SmC.
*It is difficult to form a highly monodomain cell in which the phase-containing layer is aligned perpendicularly to the substrate surface and the liquid crystal molecules are aligned substantially parallel to the substrate surface.

By the way, in manufacturing a liquid crystal cell with a larger area than before,
A method is known in which a substrate surface is subjected to uniaxial alignment treatment. Examples of the -axial alignment treatment include a method of rubbing the substrate surface in one direction with velvet, cloth, or paper, and a method of obliquely depositing SiO or 5in2 on the substrate surface.

As a result of intensive studies by the present inventors, we found that by applying appropriate uniaxial alignment treatment to the substrate surface,
It has become clear that it is possible to achieve a specified bistable state, and that by aligning the polarizer with the specified axis direction, it is possible to achieve drive that takes advantage of memory properties. The pulse voltage application process used in the present invention is performed before the display panel is actually operated. Specifically, immediately after forming a chiral smectic liquid crystal that produces two stable molecular alignment states in the initial alignment process, the pulse voltage application process can be performed, and the pulse voltage application process can be applied to a ferroelectric liquid crystal panel for display purposes. Immediately before applying the drive pulse, the pulse voltage application process can be performed.

The pulse width of the pulse voltage used in the pulse voltage application process described above is suitably in the range of 1 μsec to 10 msec, preferably in the range of 10 μsec N1 msec. Further, the peak value of the above-mentioned pulse voltage is suitably 5V to 100V, preferably 10V to 50V, and the pulse interval is suitably 1 to 100 times the pulse width, preferably 2 to 50 times the pulse width.

FIG. 3(a) is a plan view of the liquid crystal cell used in the present invention.
FIG. 3(b) is a sectional view taken along the line xx'.

Striped electrode groups 34a and 34b having terminals 32al to 32a5 and 32b1 to 32b, respectively, are formed on glass or plastic substrates 33a and 33b using ITO (ITO).
ndium-Tin Oxide) to a thickness of 1,000 layers, and on top of that, alignment control films 35a and 35b are formed.
A polyvinyl alcohol film was formed with a thickness of 1000. Furthermore, bead-like spacers with a particle size of 1 μm were sprinkled on the upper layer in order to maintain the thickness of the liquid crystal layer. This spacer keeps the ferroelectric liquid crystal layer 31 constant over a wide range. After rubbing these two substrates, they were assembled into cells, and a biphenyl ester liquid crystal (C8IOII; manufactured by Chisso Corporation; Tokiyoshi phase 91°C←Cholesteric phase 78°C←Smectic A phase 556°C←Chiral smectic C phase) was introduced. , we created a liquid crystal cell with a chiral smect C phase in which the helical structure was released.

Incidentally, in the above-mentioned liquid crystal cell having the C5IOII helical structure, the tilt angle (2) was 22.3'.

Once the liquid crystal cell created as described above is in the isokyoshi phase state (9
1° or more), and then from the isotropic phase to 0.1'
Orientation treatment was performed by lowering the temperature to the chiral smectic C phase in 7 minutes.

Figure 4 schematically shows the alignment state of liquid crystal molecules.
It is a view seen from above from the substrate surface 405.

In the figure, 400 corresponds to the direction of monopoly orientation treatment, that is, the rubbing direction in this example. In SmA (smectic physiognomy), liquid crystal molecules are aligned with the average molecular axis direction 401 of the liquid crystal coinciding with the rubbing direction 400. S m
In C Primer, the average molecular axis direction of liquid crystal molecules is 4
02 direction, and the rubbing direction 400 and the average molecular axis direction 402 of SmC'' form an angle θ, resulting in a first stable orientation state. When a voltage is applied to the upper and lower substrates in this state,
The average molecular axis direction of the liquid crystal molecules of the SmC tree changes to an angle larger than the tilt angle θ, and the third direction is saturated at the tilt angle ■.
It assumes a stable orientation state. The average molecular axis direction at this time is 40
Set it to 3. Next, when the voltage is returned to zero, the liquid crystal molecules return to their original state in the first molecular axis direction 402. Therefore, in the state of the first molecular axis direction 402, the liquid crystal molecules have memory properties. Moreover, in the state of the molecular axis direction 402,
When a voltage in the opposite direction is applied, if the voltage is high enough, the average molecular axis direction of the liquid crystal molecules will be saturated and the angle
It is transferred to the average molecular axis direction 403' of a fourth stable orientation state in which . Then, when the voltage is returned to zero again, the liquid crystal molecules move to the average molecular axis direction 4 in the second stable alignment state forming an angle θ.
It settles down to the state of 02'.

Therefore, as shown in the figure, one polarization axis direction 40 of the polarizer
4 coincides with the molecular axis direction 402 forming an angle θ, when using a driving method that produces the orientation transition between the first and second stable orientation states caused by the electric field and this memory property as described above. Optical contrast between the on state and the off state can be improved.

The angle θ detects the average direction of the molecular axis in one stable state. Although it is not clear why this is smaller than the angle {circle over (2)}, it is thought that it is because the liquid crystals in the SmCmC inner layer do not take a completely parallel alignment, and the average molecular axis direction of this orientation is the direction of the tilt angle θ.

Increasing the value of θ has a great effect in increasing the transmittance. When the incident light To and the transmitted light L are assumed, the transmittance is expressed by the following formula [θ: tilt angle, Δn: refractive index anisotropy, d: film thickness,
λ: Transmittance when one average molecular axis direction and one polarization axis are made to coincide with one polarization axis and transferred to the other molecular axis direction under the above-mentioned crossed nicols on the wave wavelength. From the above equation, the transmittance is maximum when the tilt angle θ is 22.5°.

In the liquid crystal cell mentioned above, the cell thickness d is 1.2 μm and 1.6 μm.
When we conducted an experiment with m cells and measured the angle corresponding to each tilt angle θ2, we found that θd(1,2) = 7.1
°, θd (1,6) = 8.1', which was less than the optimum value.

The present inventors further conducted experiments in order to bring the tilt angle θ in the bistable state closer to the maximum value.

The reversal between bistable states was performed with the following rectangular pulses.

A pulsed electric field larger than the pulsed electric field required for switching between bistable states was applied to these cells as shown below.

Pulse width: 1 m5 ec Voltage value: 30 V Pulse interval: 100 m5 ec Such a pulse voltage was applied for 3 seconds while changing the polarity alternately.

Thereafter, it was confirmed by texture observation that the bistable state had transitioned from the initial state to another state, and it was also confirmed that the optical principal axis of the liquid crystal had changed from before the pulse voltage was applied. Therefore, when we measured the tilt angle θ1 in this bistable state, we found that g−− d=1.2 p m ; 19.9' d= 1.6 μm ; 21.1', and the value of the maximum tilt angle ■ , and the light transmittance of the liquid crystal cell in the bright state increased as shown below compared to before the pulse voltage was applied.

*The unit is an arbitrary unit, and was normalized by the amount of transmitted light after application of a pulse voltage of d=1.6μ.

〔Effect of the invention〕

As described above, by applying a pulse voltage to a ferroelectric liquid crystal cell that has a bistable state, the tilt angle of the bistable state after the electric field is removed expands, and the amount of transmitted light increases. effective.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a ferroelectric liquid crystal element having a spiral structure. FIG. 2 is a perspective view of the ferroelectric liquid crystal element of the present invention. FIG. 3(a) is a plan view of the ferroelectric liquid crystal element of the present invention, and FIG. 3(b) is its x-x' cross-sectional view. FIG. 4 is a plan view schematically showing the alignment state of liquid crystal molecules. 3rd concave

Claims (9)

[Claims] (1) In a ferroelectric liquid crystal element in which a ferroelectric liquid crystal having two stable molecular alignment states is arranged between a pair of substrates, the ferroelectric liquid crystal is given a tilt angle θ_1( 2θ_1; the angle formed by the two optical principal axes corresponding to the two stable molecular arrangement states after the pulse voltage application process), and the tilt angle θ_1 is the tilt angle θ_2 (2θ_2 θ_1>
A ferroelectric liquid crystal element characterized by having a relationship of θ_2. (2) The ferroelectric liquid crystal device according to claim 1, wherein the pulse width of the pulse used in the pulse application process is 1 μsec to 10 msec. (3) The peak value of the pulse used in the pulse application process is 5
The ferroelectric liquid crystal element according to claim 1, which has a voltage of V to 100V. (4) The ferroelectric liquid crystal element according to claim 1, wherein the pulse interval of the pulse voltage used in the pulse application process is 1 to 100 times the pulse width. (5) The ferroelectric liquid crystal device according to claim 1, wherein at least one of the pair of substrates has a uniaxial alignment treatment axis. (6) The ferroelectric liquid crystal device according to claim 1, wherein the pair of substrates have uniaxial alignment axes that are parallel to each other. (7) The ferroelectric liquid crystal element according to claim 5 or 6, wherein the uniaxially aligned axis is an axis formed by rubbing treatment. (8) The ferroelectric liquid crystal element according to claim 1, wherein the tilt angle θ_1 is an angle of 18° or more. (9) A ferroelectric liquid crystal device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal.