JPS62275223A - Liquid crystal element and its driving method - Google Patents

Abstract

PURPOSE:To obtain a liquid crystal element which displays characteristics of ferroelectric liquid crystal to its maximum by obtaining a certain specified bistable state. CONSTITUTION:This a liquid crystal element of cell structure having the ferroelectric liquid crystal between a couple of substrates provided with electrodes and oriented films and the ferroelectric liquid crystal has a single stable state induced on an oriented film interface in the absence of an impressed electric field. Then, when a DC electric field (downward) is impressed on the liquid crystal element which is in a stable molecule orientation state in one direction so that the polarization direction of liquid crystal molecules becomes stable in the opposite direction, a bistable state can be realized artificially. Namely, the DC electric field in this opposite direction is impressed while the liquid crystal element is driven to generate a state where an electric field more than a certain value is impressed at any time, thereby obtaining the stable bistable state regardless of the phase structure of the liquid crystal, components of the oriented films, a combination of orientation processing methods, etc.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal-light shutter, etc., and a method for driving the same. The present invention relates to a liquid crystal element with improved display and driving characteristics by improving the alignment state of liquid crystal molecules, and a method for driving the same.

[Summary of the Disclosure] This specification and drawings describe a liquid crystal element used in a liquid crystal display element, a liquid crystal-light shutter, etc., and a method for driving the same, in which the alignment films of the upper and lower substrates and the alignment treatment thereof are respectively asymmetrically configured, and a single By applying a DC electric field that stabilizes the polarization direction of liquid crystal molecules in a stable state in the opposite direction, a pseudo-bistable state is induced, and the memory property, which is the most important feature of ferroelectric liquid crystals, is achieved. This makes it possible to utilize the power of the vehicle.

[Prior art] As a conventional liquid crystal element, for example, M-Shirt) (M
, 5chadt) and W. Helfrich (W.
, He1frich) °°Applied Physics Letters'
s Letters” Volume 18, No. 4 (1971
“Voltage Dependent Optical Activity of a Twisted Nematic Liquid”, pp. 127-128
Crystalpa ('Voltage Dependent
0ptical Activit7 of aTwi
sted Ne1Ilatic Liquid Cry
The twisted nematic (t
A device using twisted nematic (twisted nematic) liquid crystal is known. This TN liquid crystal has a problem in that crosstalk occurs during time division driving using a matrix electrode structure with high pixel density, so the number of pixels is limited.

In addition, display elements are known in which a switching element using a thin film transistor is connected to each pixel and switching is performed for each pixel, but the process of forming the thin film transistor on the substrate is extremely complicated, and it is difficult to create a display element with a large screen. There are some problems that make it difficult to do so.

Clark (C1
ark) and Lagerwal I
(#Open and Close Publication No. 58-107218, U.S. Patent No. 4,367,924, etc.), liquid crystals having bistability generally include chiral smectic C phase (SIIICo) or H phase ( A ferroelectric liquid crystal is used, which has a ferroelectric liquid crystal (SmH"). 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 is similar to the above-mentioned TN type liquid crystal. Unlike the optical modulation element used in 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, many of the problems of the conventional TN type elements mentioned above can be solved.
A considerable woody improvement is obtained.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell using a helical structure. lla and llb are In2
O3, SnO2 and ITO (Lndium Tin 0x
Substrate (glass plate) coated with a transparent electrode such as
A liquid crystal of SmC'' (chiral smectic C phase), which is oriented so that a liquid crystal molecule layer 12 is perpendicular to the glass surface, is sealed in between.The thick line 13 represents the liquid crystal molecules. This liquid crystal molecule 13 has a dipole moment (P) 14 in the direction perpendicular to the molecule.At this time, the tilt in the chiral smectifcation phase of the helical structure where the angle forming the right angle of the triangular pyramid is When a voltage higher than a certain threshold is applied between the electrodes on the substrates 11a and llb, the helical structure of the liquid crystal molecules 13 is unraveled, and the dipole moments (P) 14 are all oriented in the direction of the electric field. , it is possible to change the alignment direction of the liquid crystal molecules 13.The liquid crystal molecules 13 have an elongated shape and exhibit curvature anisotropy in the long axis direction and the short axis direction. It is easy to understand that if a polarizer is placed in Nicol positional relationship, it becomes a liquid crystal optical modulation 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 3, even when no electric field is applied, the helical structure of the liquid crystal molecules unravels and becomes a non-helical structure.
Its dipole moment) Pa or pb is upward (24a)
or downward (24b), and a bistable state is formed. When an electric field Ea or Eb of different polarity above a certain threshold is applied to such a cell as shown in FIG. 3, the dipole moment electric field Ea or Eb is directed upward 24a or downward 24b in accordance with the electric field vector and accordingly the liquid crystal molecules are in the first stable state 23
a or the second stable state 23b. 1/2 of the angle formed by the first and second stable states at this time
corresponds to the tilt angle θ.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. To explain the second point with reference to FIG. 3, for example, the electric field E
When a is applied, the culture crystal molecules are oriented in the first stable state J923a, and this state remains stable even when the electric field is turned off. or,
When an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 23b and change their orientation, but they remain in this state even after the electric field is turned off. Further, the respective orientation states are maintained as long as the applied electric field Ea does not exceed a certain threshold value. Such fast response speed and
How can bistability be effectively realized?

It is preferable that the cell be as thin as possible, and generally 0.5 ru 20 turns, particularly 1 ru to 5 ru are suitable.

Next, specific examples of driving methods for ferroelectric liquid crystals are shown in Figures 4 to 6.
This will be explained using figures.

FIG. 4 is a schematic diagram of a cell 31 having a matrix electrode structure in which a ferroelectric liquid crystal compound (not shown) is sandwiched between. 32 is a scanning electrode group, and 33 is a signal electrode group. First, the case where scanning electrode Sl is selected will be described. FIG. 5(a) and FIG. 5(b) are scanning signals, which are electric signals applied to the selected scanning electrode S1 and other scanning electrodes (unselected scanning electrodes) S1, respectively.
2. S3. It has an electric signal applied to Sa... FIG. 5(c) and FIG. 5(d) show information signals of selected signal electrodes fl+ 13+ Is.
This shows an electric signal given to the unselected signal electrode 12, In.

In FIGS. 5 and 6, the horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode groups 32 are sequentially and periodically selected. Now, for a liquid crystal cell having bistability for a predetermined voltage application time of 1° or t2, the threshold voltage for providing the first stable state is set as -Vtb+, and the threshold voltage for providing the second stable state is set as -Vtb+. is +Vthz, the electrode signal given to the selected scanning electrode 32 (S+) is as shown in FIG.
As shown in a), for phase (time) 1+, 2v is
Phase C time) At t2, the voltage is alternating to -2V.

When electrical signals having multiple phase intervals with mutually different voltages are applied to the scanning electrodes selected in this way, optical "
The important effect is that a state change between the first or second stable state of the liquid crystal corresponding to the "dark" or "bright" state can be caused quickly.

On the other hand, the other scanning electrodes S2 to S5 are in a grounded state as shown in FIG. 5(b), and receive the electrical signal O.
It is. In addition, the selected signal electrode 11+13°I5
The electric signal applied to the unselected signal electrode 12°I4 is <−V as shown in FIG. 5(d), and the electric signal applied to the unselected signal electrode 12°I4 is
Each voltage value above 0 is set to a desired value that satisfies the following relationship.

V<Vth2<3V -3V<-Vthl<-V Figure 6 shows the voltage waveforms applied to each pixel, for example, pixels A and B in Figure 4 when such an electrical signal is applied. Shown in (a) and (b). In other words, Fig. 6 (
As is clear from a) and (b), a voltage of 3v exceeding the threshold value vth2 is applied to the pixel A on the selected scanning line at phase t2. Furthermore, a voltage of -3V exceeding the threshold value -Vthl is applied to the pixel B existing on the same scanning line at the phase tl. Therefore, depending on whether or not a signal electrode is selected on the selected scanning electrode line, if the signal electrode is selected, the liquid crystal molecules are aligned in the first stable state, and if not selected, the liquid crystal molecules are aligned in the first stable state. Align the orientation to the stable state of 2.

On the other hand, as shown in FIGS. 6(C) and (d), on unselected scanning lines, the voltage applied to all pixels is V.
or -V, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules in each pixel other than those on the selected scanning line maintain the orientation corresponding to the signal state when scanned last time without changing the orientation state. , is kept as is. That is, when a scanning electrode is selected, a signal for one line is written, and that signal state can be maintained until the next selection after one frame ends. Therefore, even if the number of scanning electrodes increases, the actual duty ratio does not change and the contrast does not deteriorate at all.

[Problems to be Solved by the Invention] However, in order for an optical modulation element using this bistable liquid crystal to exhibit desired operating characteristics, the liquid crystal disposed between a pair of parallel substrates must meet the above two conditions. It is necessary that the molecules be in such a state that conversion between stable states can occur effectively. For example, 5raC,” (smectic C
) or SwH” (smectic H) phase, the liquid crystal molecular layer having the SmC or Sd phase is perpendicular to the substrate surface, and therefore the liquid crystal molecular axes are aligned almost parallel to the substrate surface. However, in conventional optical modulators using bistable liquid crystals, the alignment state of liquid crystals with monodomain structure cannot always be formed satisfactorily. The reality is that sufficient characteristics could not be obtained because the liquid crystal phase structure
There are several types of alignment film components and alignment processing methods, and depending on how they are combined, it is possible to maintain a uniform stable state even when an electric field is applied to the liquid crystal and it is switched to the first stable state. In some cases, this is not possible, and all pixels return to the first stable state within a finite period of time.

An object of the present invention is to provide a liquid crystal element that can maximize the characteristics of a ferroelectric liquid crystal by achieving a specified bistable state, and a method for driving the same.

c. Means for Solving Problems] The liquid crystal element of the first invention has a cell structure in which a ferroelectric liquid crystal is sandwiched between a pair of substrates provided with electrodes and an alignment film, and The ferroelectric liquid crystal is characterized in that the stable state induced by the alignment film interface has a single stable state when no electric field is applied.

The driving method of the second invention is a driving method of a liquid crystal element, which optically modulates a liquid crystal layer having a monostable state by applying a voltage between electrodes in a matrix structure consisting of a scanning electrode group and an information electrode group. , a constant DC voltage is applied between the electrodes during driving. Note that the DC electric field applied between the electrodes is preferably about 0.5 to IOV/μm.

[Function] FIG. 1 is a projected diagram of the molecular axis components of liquid crystal molecules and their polarization directions in the smectic layer shown in FIG. 3. 1st
FIG. 1(a) shows a monostable state produced by the alignment film according to the present invention, and FIG. 1(b) shows both ideal bistable states. In FIGS. 1(a) and 1(b), the shaded areas indicate alignment films. In the liquid crystal element according to the present invention, since the upper and lower alignment films are bipolar alignment films, the liquid crystal molecules are stable only in one polarization direction, as shown in FIG. 1(a). That is, in FIG. 1(a), the alignment film formed on the upper substrate side has the property of aligning the polarization direction of liquid crystal molecules near the interface toward the alignment film side, and the alignment film formed on the lower substrate side has the property of aligning the polarization direction of liquid crystal molecules near the interface. It has the property of orienting the polarization direction of the liquid crystal molecules toward the liquid crystal layer side. When a DC electric field (downward electric field in Fig. 1(a)) is applied to a liquid crystal element in which the molecular orientation state is stable in one direction, the polarization direction of the liquid crystal molecules is stabilized in the opposite direction. , a bistable state as shown in FIG. 1(b) can be realized in a pseudo manner. In other words, by applying a DC electric field in the opposite direction while driving the liquid crystal element and creating a state in which an electric field above a certain level is always applied, the combination of the phase structure of the liquid crystal, the components of the alignment film, the alignment treatment method, etc. Ideal bistability can be achieved regardless of the

[Example] Hereinafter, an example of a liquid crystal element and a driving method thereof according to the present invention will be described.

The alignment film used in the present invention is one that orients the polarization directions of liquid crystal molecules in opposite directions on the upper and lower substrates, and in this example, those shown in Table 1 below were used.

Table 1 Two types of liquid crystal compositions represented by the following formulas were used as the ferroelectric liquid crystal.

Liquid crystal composition A: Mixing ratio Iso
−+SmA −+ 5tsC”−→Sm3−〉Cr7
st liquid crystal composition B: mixing ratio 5
4° 50'-1o' Iso -onS+5A-)StaC" -+ Cr1
st Example 1 Striped ITO (India+-Tin-Ox
ide) After applying the polyimide resin 'PIQ'0 to 100OA on the glass substrate on which the electrode group was formed,
Fired.

On the other hand, the silane coupling agent' 5H60 was placed on a glass substrate on which a group of striped ITO electrodes was also formed.
After coating several OA of 20°, it was fired. next,'
PIQI2O3 was rubbed, the electrode surfaces of the peripheral substrates were made to face each other, and the electrode groups were assembled so as to be orthogonal to each other. Spacers with a particle size of 1.5 pm were scattered over the entire surface and the periphery was bonded. Then, a liquid crystal composition was prepared. A was injected and slowly cooled at 1° C./h to obtain a 5tsC phase liquid crystal cell.

A crossed Nicol polarizer and an analyzer were arranged on both sides of the liquid crystal cell, and a signal having the waveform shown in FIG. 5 described above was applied between the matrix electrodes. At this time, when a DC voltage of +3.2v was applied to the electrode group on the side where 'PIQ' was applied,
Uniform monodomain alignment could be obtained. In addition, when the scanning signal was driven with alternating voltages of +12V and -12V from the DC voltage, and the information signal was driven with +6V for selection and -6V for non-selection, memory-driven time division driving could be performed. A normal display image was obtained.

Example 2 Liquid crystal composition B was used, and the other structure was the same as that of Example 1.
It was exactly the same. In this example as well, uniform monodomain alignment could be obtained by applying a DC voltage of +2.4 V to the electrode group on the 'PIQ' coated side of the liquid crystal cell. Moreover, the scanning signal is +
When driving was performed using an alternating voltage of 15 V and an information signal of +7.5 V for selection and -7.5 V for non-selection, memory-driven time-division driving could be performed as in the previous embodiment.

Example 3 Liquid crystal composition B was used, and the alignment film was set to 'EC25°. The rest of the structure is exactly the same as that of the first embodiment. In this example as well, uniform monodomain alignment could be obtained by applying a DC voltage of +3 V to the alignment film on the "EG25" side. Moreover, the scanning signal is +
When driving was performed using an alternating voltage of 12V and an information signal of +6V for selection and -6V for non-selection, memory ff drive type time division driving could be performed as in the previous embodiment.

In each of the above embodiments, a case has been described in which the alignment films of the upper and lower substrates are asymmetric, but even if the alignment films of the upper and lower substrates have a symmetrical configuration, if a monostable state can be obtained, the driving method described above can be used. Bistable drive can be achieved.

[Effects of the Invention] As explained above, according to the present invention, by constantly applying a DC electric field to a liquid crystal element whose initial stable state is a single state and inducing a pseudo bistable state, a uniform monochrome Domain orientation can be achieved. As a result, it becomes possible to drive the ferroelectric liquid crystal by taking advantage of its memory properties, which is its greatest feature, and it is possible to obtain a high-quality, large-screen display device.

[Brief explanation of the drawing]

Figure 1 is a projected diagram of the molecular axis components of liquid crystal molecules and their polarization directions, Figures 2 and 3 are schematic diagrams of a liquid crystal cell, Figure 4 is a diagram showing the matrix electrode structure, and Figures 5 and 6. is a diagram showing each voltage waveform. 11a, llb: substrate, 12: liquid crystal molecule layer. 13: Liquid crystal molecule, 14: Dipole moment. 23a: first stable state. 23b: Second stable state. 24a: Upward dipole moment. 24b = Downward dipole moment. 32 (Sl, S2....): Scanning electrode group (scanning electrode). 33 (II, I2....): Signal electrode group (signal electrode). 7 Fu 77 Fu γ 77 Also, the molecular axis of Kyouko and the division of talent "Br1li1's Shifumi, the 1st figure is the revenge ceremony, 2 the 2nd figure is also the Figure 3 33 A Blind Den Oseshi Group II h I3 r4Is -----
Figure 4: Matrix electrical equipment

Claims (3)

[Claims] (1) In a liquid crystal element with a cell structure in which a ferroelectric liquid crystal is sandwiched between a pair of substrates provided with electrodes and an alignment film, the stable state of the ferroelectric liquid crystal induced by the interface of the alignment film is A liquid crystal element characterized by having a single stable state when no voltage is applied. (2) As an alignment film that induces a single stable state of the ferroelectric liquid crystal, the alignment film formed on one electrode side aligns the polarization direction of the ferroelectric liquid crystal molecules near the interface toward the alignment film side. Claim 1, characterized in that the alignment film formed on the other electrode side has the property of aligning the polarization direction of ferroelectric liquid crystal molecules near the interface toward the liquid crystal layer side. The liquid crystal element described in . (3) In a method for driving a liquid crystal element in which a liquid crystal layer having a monostable state is optically modulated by applying a voltage between electrodes in a matrix structure consisting of a scanning electrode group and an information electrode group,
A method for driving a liquid crystal element, characterized in that a constant DC voltage is applied between the electrodes during driving.