JPS6191631A - Driving method of liquid crystal element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a liquid crystal element, and more particularly to a novel method for driving an element using ferroelectric liquid crystal.

Conventionally, liquid crystal display elements that display images or information by configuring scanning Ti, electrode groups, and signal electrode groups in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels are well known. It is being

The method for driving this display element is to selectively apply an address signal to the scanning electrode group in one cycle, and selectively apply a predetermined information signal to the signal electrode group in parallel in synchronization with the address signal. Although time-division driving has been adopted, this display element and its driving method have major and fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, most of them are practically used as display elements, for example, M, 5ch, because of their relatively high response speed and low power consumption.
adt and W. He1frich, “Appl.
ied PhySics Letters”, Vo 1
．． 18. No. 4 (1971, 2, 15), P.
127-128 c7) “Voltage-Dep
endOptical Activity
of a TwistedNematic Liq
TN (twist
It uses an ed nematic) type liquid crystal,
This type of liquid crystal has positive dielectric anisotropy in the absence of an electric field.
The molecules of nematic liquid crystal have a twisted structure in the thickness direction of the liquid crystal layer (
The liquid crystal molecules form a structure in which the liquid crystal molecules are arranged parallel to each other on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in optical modulation. When this type of liquid crystal is used to construct a display element with a matrix electrode structure.

A voltage higher than the threshold required to align liquid crystal molecules perpendicularly to the electrode surface is applied to the region where both the scanning electrode and the signal electrode are selected (selection point), and the region where both the scanning electrode and the signal electrode are not selected (the non-selection point) is applied. No voltage is applied to the selected point), so the liquid crystal molecules maintain a stable alignment parallel to the electrode plane.

By arranging linear polarizers above and below such a liquid crystal cell in a cross Nicol relationship, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. becomes. However, when a matrix electrode structure is configured, scan electrodes are selected and areas where signal electrodes are not selected or scan electrodes are not selected.

The voltage applied to the 0 selection point where a finite electric field is also applied to the region where the signal electrode is selected (so-called "half selection point"),
If the difference in voltage applied to the half-selection point is sufficiently large and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to an intermediate voltage value, the display element will operate normally. However, in this method, the number of scanning lines (N
), the time during which an effective electric field is applied to one selected point (duty ratio) while scanning the entire screen (one frame) decreases at a rate of 1/N. Furthermore, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, resulting in a reduction in image contrast and crosstalk. This is a serious drawback. This kind of phenomenon.

Temporal accumulation of liquid crystals that do not have a bistable state (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). This is essentially an unavoidable problem that arises when driving using the effect (that is, repeatedly scanning). In order to improve this point, voltage averaging method, dual frequency drive method, multiple matrix method, etc. have already been proposed, but all of these methods are insufficient, and it is difficult to increase the screen size and density of display elements. Currently, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently.

An object of the present invention is to provide a novel method for driving a bistable liquid crystal, particularly a ferroelectric liquid crystal element, which solves all the problems of conventional liquid crystal display elements as described above.

Such an object of the present invention is to provide a pixel having a counter electrode, a dielectric material 11 formed on at least one of the counter electrodes, and a bistable liquid crystal oriented in either the stable state of fSl or the stable state of :52. A driving method for a liquid crystal element having a plurality of arrayed electrodes, in which a voltage having an electric field in the same direction is applied from an external power supply between the opposing electrodes, and the pulse width and/or voltage value of the electric field voltage in the same direction is controlled. By doing this, the bistable liquid crystal of the selected pixel is aligned in the first stable state, and the bistable liquid crystal of the other pixels is aligned in the second stable state! This is achieved by a driving method that causes the beams to orient in the same direction.

That is, in the present invention, a ferroelectric liquid crystal having a fast voltage response speed and state memory is driven by a unipolar signal, and the setting of an external drive circuit can be simplified.

Furthermore, liquid crystals exhibiting ferroelectricity tend to have regions where the state is reversed when a voltage below the threshold is applied for a long period of time; however, with a unidirectional electric field, two stable alignment states can be created. As a result, time-division driving has become possible by avoiding the phenomenon of applying a weak electric field for a long time.

The present invention will be explained below with reference to the drawings.

FIG. 1 schematically depicts an example of a ferroelectric liquid crystal cell. l and l' are I n203.

5n02 and ITO (Indium-TinOx
It is a substrate (glass plate) coated with a transparent electrode such as i d e), and a SmC main phase 0 liquid crystal in which the liquid crystal molecular layer 2 is oriented perpendicular to the glass surface is sealed between the substrates (glass plates). A thick line 3 represents a liquid crystal molecule, and this liquid crystal molecule 3 has a dipole moment (on P) 4 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 1 and 1', the helical structure of the liquid crystal molecules 3 is unraveled, and the liquid crystal molecules 3 are aligned so that all dipole moments (P) 4 are oriented in the direction of the electric field. You can change direction. The liquid crystal molecule 3 has an elongated shape and exhibits refractive index anisotropy in its long axis direction and short axis direction. , it is easily understood that the liquid crystal optical variable element is a liquid crystal optical variable element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (
For example, as shown in Figure 2, the helical structure of the liquid crystal molecules is released (non-helical structure), and its dipole moment hp or P' is directed upward (4a). Or, it takes either the downward (4b) state. As shown in FIG. 2, when an electric field E or E' with a different polarity above a certain threshold is applied to such a cell for a predetermined period of time, the dipole moment will move upward 4a, corresponding to the electric field vector of the electric field E or E'. or downward 4b, and accordingly the liquid crystal molecules are aligned in either the first alignment state 5 or the second alignment state 5'.

There are two advantages to using such a ferroelectric liquid crystal as a liquid crystal. Firstly, the response speed is extremely fast, and secondly, the liquid crystal molecules have a bistable orientation 78L. To explain the second point using, for example, Figure 2,
When an electric field E is applied, the liquid crystal molecules change to the first orientation ju'+
5, and this state is 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 alignment state 5' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is generally preferable for the cell to be as thin as possible.

0.5 to 20 pots, especially 1 to 5 pots are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal is an example.

For example, Clark and Ragabal, U.S. Pat. No. 43,679.
This is proposed in Specification No. 24.

When a dielectric layer (insulating layer) is formed on the counter electrode of the liquid crystal described in #11j, the equivalent circuit is shown in FIG.

When, for example, a positive pulse Va (electric field in the direction of external power supply a4-b in Fig. 3) is applied to this liquid crystal element, the liquid crystal layer is initially Tonne pressure is applied.

When the pulse voltage value is Va, the liquid crystal layer has VaC1/
(C1+C,2) (7) After the initial voltage vb is applied,
Assuming that the resistance value of the insulating layer is sufficiently high (ρ21016: volume resistance value Ω・cm), the time constant is = R2 (C1 + 02) (
R2 is the resistance value of the liquid crystal layer), causing a voltage drop,
At the time of pulse switching (grounding), the voltage corresponding to this voltage drop becomes a reverse polarity voltage and is applied to the liquid crystal layer. Therefore, if this voltage of opposite polarity exceeds the threshold voltage, the liquid crystal phase will be oriented in one of the stable states accordingly.

Therefore, the present invention applies the above-mentioned phenomenon to driving.
Specifically, it is necessary to apply a pulse width sufficient to generate a reverse polarity voltage above the threshold value, or to apply a pulse width such that even if the voltage drop occurs as a reverse polarity electric field, the voltage value will be below the threshold value. By making the pulses short, it is possible to align in either one of the stable states.

In order to obtain the above-mentioned effect more effectively, it is preferable to set the volume resistance -44 of the liquid crystal phase to 2X1010 Ωcm.Specifically, it is preferable to set the volume resistivity of the liquid crystal phase to 2×1010Ω·cm.Specifically, it is preferable to set the volume resistance of the liquid crystal phase to 2×1010Ω・cm. By incorporating a surfactant in an amount of 0.05 wt % or more, the volume resistivity can be adjusted to 2×10 10 Ω·0 m or less.

Ionic impurities exhibit ionicity in liquid crystals, and it does not matter whether they are solid or liquid.

When used in an actual display element, p is 2XIQ10Ω
・It can withstand use if kept at 0 m or less, but the response speed becomes slow, so it is preferably 3xio'3 Ω/0 m or less, particularly preferably 1 x 10 8 Ω/cm to 1 x 10 3 Ω medium cm
It is necessary to set it to .

The "volume resistivity" described in this specification can be measured by applying a rectangular pulse using the two-frequency method using the circuit shown in FIG. 4, and calculating ρ (volume resistivity) from the following equation. can.

■ I=I (+I scale=4f@CeV+- V: Measured voltage f: Frequency of square wave IC: Current value of capacitive component 1 scale: Current value of R component C: Capacitance of liquid crystal R: Resistance of liquid crystal (Ω) By changing f, p=R3/dd d: Liquid crystal film thickness (cell gap) S: Electrode area In reality, consider driving a ferroelectric liquid crystal using an X-Y matrix structure as shown in Figure 5. Can be done.

In Fig. 5(a) and (b), Sm, Sm+l+Sm
+2.3m+3 are the X-axis electrodes, which are used as scanning electrodes, respectively, and In + In + 1 .

In+2. In+3 is a Y electrode, which is used as a display signal and electrode.

FIG. 5(C) shows a lighting pattern using eight pixels in two columns on the left side of FIGS. 5(a) and 5(b), and FIG. 6 shows the driving voltage waveform when displaying this pattern. Figure 7iS6 shows a driving example in which the polarity of the scanning signal voltage is changed every cycle.

In FIG. 6, A-H indicates voltage waveforms applied to each pixel in FIG. 5(C). Further, A' and C indicate voltage waveforms applied to the liquid crystal phases of pixels A and C, respectively. The cell is made by forming a polyimide layer of 2,000 to 5,000 layers on an ITO electrode (Toshiba, 5P-510/spinner formation), and applying DOB between substrates that have been oriented by rubbing.
AMBC (desyloxyhensilite>'-P'-amino-2-methylbutyl cinnamate) was sandwiched and formed in a layer thickness of 1.3 Bm.

L no cell f: O+-j le D OB A M B
C nol"J (fj 4'j properties are as shown in FIG. 7.

The voltage v1v2. shown on the graph of FIG. By using v3 as pulses Tl, , T2, cases as shown in Table 1 can be obtained. The two states of ferroelectric liquid crystal are “
If we express it in terms of 0 and 1, the electric field in the ■ direction alone.

By changing the width of the pulse, a state of "0° or 1" can be obtained, and the same can be said for the electric field in the e direction.

Table 1 Therefore, in FIG. 6, the pixel can be brought into the "bright" state (orientated to the first stable state) by applying a signal of V2 with a pulse width of T1 or by applying a signal of -V3 with a pulse width of T2. On the other hand, a "dark" state (orientation to the second stable state) can be achieved by applying a signal of -V2 with a pulse width T1 or by applying a signal of V3 with a pulse width T2.

Further, FIG. 8 shows another driving example in which the polarity of the scanning signal voltage is changed every two cycles.

In the figure, A-H represents the driving waveform of the signal applied to each pixel.

The ferroelectric liquid crystal used in the driving method of the present invention takes either a first optically stable state or a second optically stable state depending on the applied electric field. That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property, is used.

It has bistable properties that can be used in the driving method of the present invention.
As the ferroelectric liquid crystal, a chiral smectic liquid crystal having ferroelectricity is most preferable.
phase (SmI book), J phase (S m J tree), K phase (SmK
*), G phase (5m0 lines), and F phase (SmF*) liquid crystals are suitable.

Regarding this ferroelectric liquid crystal, please refer to LE JOURNAL
E PHYSIOUE LETTER3″ embryo (L-69
) 1975゜Ferroelectric Liq
uid CrystalsJ:”Applied P
hysics Letters 36 (11)19
80, rsubmicro 5econd B15t
ableElectrooptic Switchin
The ferroelectric liquid crystal disclosed in these publications can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybendiυdenoP'-amino-2-methylbutylcinnamate (DOBAMBC
), hexyloxybenzylidene-P'-amino-2-
Gloropropyl runnamate (HOBACPC) and 4-o-(2-methyl)-butylresorsilitin-4'
-Octylaniline (MBRA8) a.

When constructing an element using these materials, the element is supported by a copper block with a heater embedded, etc., as necessary, in order to maintain the temperature state such that the liquid crystal compound becomes an SmCmC tree line SmH tree phase. be able to.

In addition, the dielectric material forming the dielectric layer mentioned above is not particularly limited, but includes silicon nitride, silicon nitride containing hydrogen, silicon carbide, silicon nitride containing hydrogen, and silicon oxide. , boron nitride, boron nitride containing hydrogen, cerium oxide, aluminum oxide, zirconium oxide, inorganic insulating materials such as titanium oxide and magnesium fluoride, or polyvinyl alcohol, polyimide, polyamideimide, polyesterimide, Polyparaxylylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride.

Polyvinyl acetate, polyamide, polystyrene.

Organic insulating materials such as cellulose resin, melamine resin, urea resin, acrylic resin and photoresist resin are used.

Hereinafter, the present invention will be explained according to examples.

Example 1 A polyimide film having a film thickness of 1000 nm was formed on each of the opposing matrix electrodes formed of crossed strips of ITO (5 wt of polyamic acid resin from a condensate of pyromellitic anhydride and 4°4'-diaminodiphenyl ether). %
N-Methylpyrrolidone solution was applied and a ring-closing reaction was performed by heating at a temperature of 250° C.), and the surfaces of this polyimide film were rubbed so as to be parallel to each other to create a cell with a cell thickness of 1 μm.

Next, a small amount of (C4H9)aN"Br- was added to DOBAMBC prepared in Canon Co., Ltd., and this DO
B AMB C was injected into the above-mentioned cell by vacuum injection method under Tokichi phase, and the cell was sealed. After that, slow cooling (5℃/Makima)
A liquid crystal cell made of SmC wood was created using the following method. Using this liquid crystal cell, the volume resistivity of the liquid crystal was determined to be 5.8×10 8 Ω·cm by measuring the current value when a voltage of 10 V and 1 Ov at 64 Hz was applied at 32H2.

A crossed Nicol polarizer and an analyzer were arranged on both sides of this liquid crystal cell, and a signal having the waveform shown in FIG. 6 was applied between the opposing matrix electrodes. At this time, Vl=20V, v2=4ov,
v3=sov, T1 (long pulse width)=3msec, T2
When driven at (short pulse width) = 0.6 m5ec, a good moving image display was obtained.

Example 2 HOBACP instead of DOBAMBC used in Example 1
When a liquid crystal element was fabricated in the same manner except that C was used, and the same driving waveform was applied, a good moving image display was obtained.

[Brief explanation of the drawing]

1 and 2 are perspective views schematically showing a liquid crystal element used in the present invention. FIG. 3 is an explanatory diagram showing an equivalent circuit of a pixel. FIG. 4 is an explanatory diagram showing an equivalent circuit when measuring one volume resistance value. :JSS diagram (a)~
(c) is an explanatory diagram showing a matrix electrode structure. FIG. 6 is an explanatory diagram of driving waveforms showing the driving method of the present invention in chronological order. FIG. 7 is an explanatory diagram showing the relationship between pulse width and voltage. FIG. 8 is an explanatory diagram of drive waveforms showing another drive method of the present invention in chronological order. 1.1'...Transparent electrode, 2...
... Liquid crystal molecule layer, 3 ... Ferroelectric liquid crystal molecules, 4 ...
... Multipole moment of the molecule. 4 a (P), 4 b (P') --・--
--The multipole moment of the molecule. 5.5' ...... Two stable states of liquid crystal molecules. Mu 5 figure 57+!
G5mft

'eSm middle class 2------
−−−−−−−−1−−−−−−−1、Eng: E=

eHe

Claims (8)

[Claims] (1) A plurality of pixels each having a counter electrode, a dielectric film formed on at least one of the counter electrodes, and a bistable liquid crystal oriented in either a first stable state or a second stable state are arranged. A method for driving a liquid crystal element, in which a voltage having an electric field in the same direction is applied from an external power supply between the opposing electrodes, and the pulse width and/or voltage value of the electric field voltage in the same direction is controlled, thereby driving the pixel. A method for driving a liquid crystal element, comprising aligning bistable liquid crystals of selected pixels to a first stable state, and aligning bistable liquid crystals of other pixels to a second stable state. (2) The method for driving a liquid crystal element according to claim 1, wherein the bistable liquid crystal has a volume resistivity of 2×10^1^0 Ω·cm or less. (3) The method for driving a liquid crystal element according to claim 1, wherein the bistable liquid crystal has a volume resistivity of 3×10^9 Ω·cm or less. (4) The bistable liquid crystal is 1×10^8Ω・cm ~ 1×
A method for driving a liquid crystal element according to claim 1, which has a volume resistivity of 10^9 Ω·cm. (5) The method for driving a liquid crystal element according to claim 1, wherein the bistable liquid crystal is a ferroelectric liquid crystal. (6) The method for driving a liquid crystal element according to claim 5, wherein the ferroelectric liquid crystal is a chiral smect liquid crystal. (7) The chiral smect liquid crystal is a chiral smect C phase, H phase, I phase, J phase, K phase, F phase or G phase.
7. A method for driving a liquid crystal element according to claim 6, wherein the liquid crystal element is a phase. (8) The method for driving a liquid crystal element according to claim 6, wherein the chiral smect liquid crystal has a non-helical structure.