JPS62220930A - Ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To control a part other than a picture element part in a uniform orientation direction over the entire ferroelectric liquid crystal cell by applying an AC electric field to the ferroelectric liquid crystal cell of matrix electrode structure which has a monostable state. CONSTITUTION:Matrix electrodes (scanning electrode and signal electrode) are formed on a couple of substrates to form a monostable ferroelectric crystal element wherein orientation (initial orientation) in smectic phase (specially in chiral smectic phase) generated by raising temperature from another phase on a high temperatue side has one stable state and one semistable state, and the angle between the mean molecule axial direction of the stable state 102 and semistable state 102' is set to 2theta. Then, this ferroelectric liquid crystal element has relation theta<theta'<=, where the angle between two mean molecule axial direction s appearing when a desired bipolar DC electric field is applied to electrodes is 2H and the angle between the mean molecule axail directions in two bistable states after the AC voltage is ceased after being applied between electrodes for about several - 10 minutes is 2theta'.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ferroelectric liquid crystal element, and more specifically, to a ferroelectric liquid crystal element that improves density unevenness in non-pixel areas and achieves high contrast.

The present invention relates to a ferroelectric liquid crystal element that enables high-quality display.

[Conventional technology]

Conventionally, a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display one image or information.
well known. The driving method for this front element is to selectively and periodically apply address signals to the scanning electrode group.
Time-division driving is employed in which a predetermined information signal is selectively applied in parallel to the address signal in synchronization with the address signal.

- In the field of printers, ``laser beam printers, which provide electric image signals in the form of light to a photoreceptor in the form of light, are useful in terms of both pixel density and speed as a means of obtaining hard copies by inputting electrical signals. is currently the best. As an element that converts that electrical signal into an optical signal,
A liquid crystal shutter array has been proposed.

Most of these were put to practical use, such as “Applied Fix Letters” (“AppliedPhys Letters”).
sics Letters”) 1971, 18 (
4) M, Sheet (M, published in issue 127-128)
5chadt) and X.

"Voltage Dependent Optical Activity - φ of a Twisted Urn Nematic Liquid Crystal" co-authored by W. Helfrich.
ntOptical Activity of a T
twisted Nematic LiqnidCrys
tal') shown in TN (twisted n
It was a ``ematic'' type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both C1ark and Lagewal in Japanese Patent Application Laid-open No. 107216/1983 and US Pat. No. 436
This is proposed in the specification of No. 7924. As a bistable liquid crystal.

Generally, chiral smectic C phase (SrrC or H
A ferroelectric liquid crystal having a phase (Srrf) is used.

This liquid crystal exhibits a first optically stable state and a second optically stable state with respect to an electric field.
It has a bistable state consisting of an optically stable state of , 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 '4 field, and maintaining that state when no electric field is applied.

[Problem that the invention seeks to solve]

However, in a structure in which this bistable liquid crystal is sandwiched between a cell in which a scanning electrode group and a signal electrode group are formed in a matrix on upper and lower substrates, scanning polarization and signals are generated at the intersection of the rods, that is, at the 0 pixel area. (The part where opposing electrodes are formed) can be controlled to @1 or the second stable orientation state depending on the polarity of the applied electric field as described above, but! In areas where the poles do not intersect, that is, in non-pixel areas (areas where opposing electrodes are not formed), it is only possible to control the stable state of 10,000 by applying voltage. In this case, the ferroelectric liquid crystal in the non-pixel area has a domain structure in which liquid crystal aligned in the first stable state and liquid crystal aligned in the second stable state coexist.

In particular, the domains of the first stable-state oriented ferroelectric liquid crystal and the second stable-state oriented ferroelectric liquid crystal domains tend to be distributed separately over a wide range. In the non-pixel area at the upper right of the screen, a domain oriented in the first stable state is formed, and the non-ii! In the i-element part, a case may occur in which a domain oriented in the second stable state is formed. Therefore, polarizers are arranged above and below the liquid crystal element (crossed nicols with each other), and the polarization axes of the -10,000 polarizers are arranged to match the molecular axis direction of the liquid crystal molecules oriented in the first stable state, for example. When this happens, the non-pixel area at the top right of the 1st display screen will take on a "black" display state, and the 1st non-pixel part at the bottom left of the 1000th display screen will take on a "white" display state, and this display element will There was a problem that the display quality was low and the contrast was low.

In this way, a strong n-electroconductor liquid crystal exhibiting a bistable state or a multistable state forms domains with different orientation states in non-pixel areas,
As these different domains are distributed within the display screen, non-pixel areas within the display screen exhibit partially different display states, more specifically different transmittance characteristics, which impairs display quality. .

[Means for solving the problem] and [effect] Therefore 1
An object of the present invention is to provide a ferroelectric liquid crystal element with improved display quality, and in particular, to provide a ferroelectric liquid crystal element that provides a uniform display state in non-pixel areas within the display screen over the entire screen. The purpose of the present invention is to provide a liquid crystal element.

The present invention provides a ferroelectric liquid crystal element having a ferroelectric liquid crystal disposed between a pair of parallel substrates, a pixel portion having opposing electrodes, and a non-pixel portion having no opposing electrodes. The ferroelectric liquid crystal element is characterized in that the ferroelectric liquid crystal in the pixel portion and the non-pixel portion is brought into a monostable state, and then the ferroelectric liquid crystal in the pixel portion is brought into a bistable state or a multistable state.

That is, the present invention provides a method for controlling the tilt angle θ that lowers the initial alignment state of the ferroelectric liquid crystal on which the IJ screen electrode is formed.
After the ferroelectric liquid crystal in the non-pixel area is brought into a monostable state, an AC voltage is applied between opposing electrodes in the pixel to bring the ferroelectric liquid crystal in the pixel into a bistable or multistable state. In addition to making it into one orientation state based on the monovalent state,
The ferroelectric liquid crystal in the pixel portion can be in a bistable state or a multistable state with a large tilt angle.

〔Example〕

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

In a preferred embodiment of the present invention, matrix electrodes (scanning electrodes and signal electrodes) are formed on a pair of substrates, and a snap phase formed by cooling a different phase on the high temperature side [
In particular, it is a monostable ferroelectric liquid crystal element in which the orientation state in the chiral smectic phase has one stable state (referred to as the initial orientation state) and one metastable state, and The angle between the average molecular axis directions is 20,
(2) The angle between the two average molecular axis directions that occurs when a bipolar desired DC electric field is applied to the electrodes is 20, and (2) After applying and removing an AC voltage between the electrodes for about a few seconds to 10 minutes, The angle between the average molecular axis directions of two bistable states 2
When set to 0', 6. A ferroelectric liquid crystal element having a relationship of θ and θI≦ between θ′ and ■ can be obtained. The frequency of the AC voltage applied between the electrodes at this time is 20Hz to 20Hz.
0Hz, and its voltage is preferably 3V to 100V.

The monostable ferroelectric liquid crystal used in the present invention is oriented in one stable state when no voltage is applied, but when a DC pulse exceeding the threshold voltage is applied, it becomes oriented in another state different from the stable state described above. It is oriented in a stable state, and can be relaxedly transferred back to the original stable state when no voltage is applied. Therefore, the angle formed by the average molecular axis direction in the above-mentioned stable state and metastable state corresponds to the above-mentioned angle 2θ.

Further, the monostable ferroelectric liquid crystal used in the present invention can form a bistable state or a multistable state by applying the above-mentioned alternating current voltage. This point will be explained in detail below.

A particularly suitable liquid crystal material for use in the present invention is a chiral smectic liquid crystal having ferroelectricity. Specifically, chiral smectic C phase (8m
C”), chiral smectic C phase (8mG”),
Liquid crystals in chiral smectic F phase (in SmF), chiral smectic E phase (8 mI) or chiral smectic H phase (8 mH) can be used. For more information on ferroelectric liquid crystals, please refer to “Le Journal de Fuisique Rutile” (@IJ JOURNAL DB
PHYSIOUE Lg'r'rgas") 1975.

36 (L-69) No. 7 ERROELECTRIC LIQUID CRYSTA A/SUJ (R Ferroe
electricLiquid Crystals J
); “Applied Physics-L/Tars-(
-Applied physics Letters”
) 1980, issue 36 (11), ``Submicro Second Bistiple Electro-Optic - Switching Fist in Liquid e-Crystals'' (r
8ubmicro 5econd B15table
Bleetrooptic Switching I
nLiquidCrystals J); “Liquid Crystals” published in “Solid State Physics = 1981, No. 16 (141)”
etc., and these disclosed ferroelectric liquid crystals can be used in the present invention.

Specific examples of ferroelectric liquid crystal compounds include desiloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino-2-chloropropylcinnamate (H
OBACPC), 4-0-(2-methyl)-butylresol silidan-4'-octylaniline (MBR)
can be mentioned. In particular, preferred ferroelectric liquid crystals include:
A liquid crystal exhibiting a cholesteric phase at a higher temperature than this can be used, for example, a biphenyl ester liquid crystal exhibiting a phase transition temperature listed in the Examples below can be used.

When constructing an element using these materials, the element can be supported by a copper block or the like in which a heater is embedded, if necessary, in order to maintain the temperature at which the liquid crystal compound becomes a desired phase.

FIG. 4 schematically depicts an example of a cell for explaining the operation of a ferroelectric liquid crystal. Hereinafter, S as the desired phase
An explanation will be given using "mC" as an example.

11 and 11' are In, 0. , 8nO, or IT
It is a substrate (glass plate) covered with a transparent electrode made of a thin film of O (Indium-TinOxide), etc., and 8mC phase liquid crystal in which the liquid crystal molecular layer 12 is oriented perpendicular to the glass surface is sealed between the substrates. . 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 between the central axis 15 of this helical structure and the axial direction of the liquid crystal molecules 13 is expressed as 0.

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 above a certain threshold is applied between the upper electrodes, the helical structure of the liquid crystal molecules 13 is unraveled and the dipole moment) (
P, ) 14 are all 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 major and minor axis directions. Therefore, for example, if cross-niffle polarizers are placed above and below the glass surface, the polarizer will change depending on the voltage applied polarity. It is easily understood that this results in a liquid crystal optical element whose optical properties 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 in this way, 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 5, and the dipole moment P or P' is directed upward ( 24) or downward (24'). This liquid crystal molecule 1
1/2 of the angle formed by the molecular axis of 3 and 23' is called the tilt angle (0), and this tilt angle (■) is equal to 1/2 of the apex angle formed by the cone when it takes a helical structure. .

To such a cell, as shown in FIG.
The dipole moment is given by the electric field E or E
′ upward 24 or downward 2ν corresponding to the electric field vector
Accordingly, the liquid crystal molecules are oriented to 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. 5, when the 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 the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off.

The biggest problem when forming an element with such a liquid crystal having ferroelectricity is, as mentioned earlier, that relatively thick donine structures with different alignment states occur in non-pixel areas. This cannot be controlled in non-pixel areas.

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. As an example of this -axial orientation treatment method, polyimide (P
I)-? Methods include applying a polymer film such as polyvinyl alcohol (PVA) as an alignment film to the substrate surface and then rubbing the surface in one direction with velvet, cloth, or paper, or obliquely depositing 8i0 or S102 on the substrate surface. Can be mentioned.

When the ferroelectric liquid crystal is subjected to such alignment treatment, a bistable initial alignment state with a tilt angle θ is realized. however,
This bistable state is different from the ideal molecular arrangement shown in FIG. 5, as will be described later.

FIG. 1 is a schematic diagram showing the orientation state of molecules in a liquid crystal element according to the present invention. FIGS. 2 and 3 are a plan view showing an example of a liquid crystal cell used in the present invention, and its X-X/
FIG. In FIGS. 2 and 3, the liquid crystal cell 1 includes glass or plastic substrates 3a and 3b.
Above, striped electrode groups 4a and 4b are made of ITO (In
dium-Tln0xide) to a film thickness of tooK, and the upper layer is coated with PVA.
was formed with a film thickness of about 500A. Furthermore, in order to maintain the thickness of the liquid crystal layer on the upper layer, a group 6 of 20 μm square dot-shaped negative resist material spacers is provided only on one side of the substrate, and this group 6 of spacers allows the liquid crystal layer 2 to have a constant thickness over a wide range. is maintained. After rubbing the above two substrates, they are assembled into cells and liquid crystal is introduced.

Hereinafter, as a liquid crystal material, ester-based mixed liquid crystal C made by Chisso Co., Ltd.
Taking 81011 as an example, the present invention will be explained in accordance with the above figures. This ester-based mixed liquid crystal exhibits the following phase transition state (this phase transition point was determined by microscopic observation).

Iso (Tokichi phase) → ch (Cholesteric phase) 90
℃ → 8mA (Smectic physiognomy) → 8mC → Cr
y75℃ 50℃
Below 0° C. (Crystalline Phase) First, the cell structure 1 in which the above-mentioned piphenyl ester liquid crystal is sealed is set in a heating case (not shown) in which the entire cell 1 is heated uniformly.

Next, the temperature at which the compound in cell 1 becomes the tokiyoshi phase (approximately 95°C
). Thereafter, the temperature of the heating case is lowered, and the compound in the isokyoshi phase in the cell 1 is transferred to the temperature lowering process. During this temperature-lowering process, the compounds in the Tokichi phase are reduced to approximately 90°C.
undergoes a phase transition to the cholesteric phase of the Grand Shuan structure,
If the temperature-lowering process is further continued, a phase transition can occur at about 75° C. from the cholesteric phase to SmA, which is a uniaxial auspicious phase. At this time, the 8 mA liquid crystal molecular axes are aligned in the rubbing direction.

Thereafter, when the temperature is lowered from this 8 mA, a phase transition occurs in SmC, and if the cell thickness is reduced to, for example, about 3 μL't1 or less, a monodomain SmC with a non-helical structure can be obtained.

Here, FIG. 1 will be explained again. FIG. 1 is a schematic diagram of the "p" state as seen from above the substrate surface.

In the figure, 10G corresponds to the direction of uniaxial alignment treatment, that is, the rubbing direction in this example. In the 8 mA phase, the liquid crystal molecules are aligned with the average molecular axis direction 101 of the liquid crystal coinciding with the rubbing direction 100.

In the 8mC” phase, the average molecular axis direction of liquid crystal molecules is
The rubbing direction 100 and the average molecular axis direction 102 of 8 mC form an angle θ, resulting in one stable alignment state. In this state, when a DC pulse voltage of a predetermined threshold value or more is applied to the upper and lower substrates, , switching of the liquid crystal molecules occurs to the average molecular axis direction in a metastable state where the average molecular axis direction of the liquid crystal molecules forms an angle θ2 with the rubbing direction 100.

The threshold value is expressed by the pulse width of a rectangular pulse and the voltage peak value, and is approximately 5V for a pulse width of 1 m5ec, for example. Although θ and θ differ depending on the alignment film used and the rubbing conditions, it is assumed that θ is about 9° and θ2 is about 4°, and the relationship θ≦02 holds in a monostable state.

Now, the liquid crystal molecules are switched from the stable state to the metastable state by the switching pulse voltage, but after the voltage is removed, the liquid crystal molecules transition from the metastable state to the original stable state. This transition time varies depending on the alignment treatment method, but is from several m5ec to several minutes. Although the detailed reason why such two monostable states appear is not clear, it is thought that this is because the two originally bistable states are no longer energetically equivalent due to some kind of asymmetrical orientation control effect of the upper and lower substrates. ing.

Therefore, at least in the initial alignment state, the liquid crystal molecules are aligned throughout the cell in the average molecular axis direction 102 in a stable state with lower energy.

As a method for realizing such a monostable state in which the helical structure is unwound in the absence of an electric field, in addition to the method of providing a spacer material only on one side of the substrate when creating the cell, as described above,
There is a method of coating different types of alignment films on upper and lower substrates, for example, a method of using a substrate on which an organic polymer film such as PI or PV film is formed and a substrate on which an inorganic film such as a silane coupling agent is formed.

Furthermore, as another method for realizing a monostable state, a method can be used in which the orientation method used when creating cells is different. For example, a method of shifting the rubbing direction of the upper and lower substrates by a certain angle from parallel or antiparallel directions, a method of applying rubbing treatment to only one side of the substrate, a method of reducing the thickness of the alignment film formed on the upper and lower substrates, or one of the methods. ITO/insulating film/alignment film is provided on one substrate, and ITO is provided on the other substrate.
/ A method of providing an alignment film, etc. can be used.

In this way, by making the upper and lower substrates asymmetric when creating a cell, you can easily create a monostable ferroelectric liquid crystal element! can be created.

Next, an alternating current electric field of 3 to IOV and 20 to 10 GHz was applied to the cell for several minutes.

After the application of alternating current, the average molecular axis direction changed in a bistable state to directions forming an angle θI with the rubbing direction 101, respectively. This tilt angle θI almost corresponds to the angle 0, which is 1/2 of the apex angle of the cone when it takes the helical structure described above, and is about 1
It was 8°.

Although it is not clear how the "conversion" from a monostable state with a small tilt angle to a bistable state with a large tilt angle occurs due to such AC application, we speculate as follows.

That is, in the monostable state obtained as the initial alignment state, there is some "twisting" of the liquid crystal molecules in the thickness direction of the cell, and due to the alignment control action of the asymmetric substrate interface, two types of twisting with different energy states are created. It is thought that this corresponds to a stable state and a metastable state, and it is thought that this "twisting" causes the average molecular axis direction to be optically observed to be smaller than that in an untwisted state.

On the other hand, the state after application of the AC electric field almost matches the molecular arrangement in the ideal bistable state shown in Figure 6, and is not affected much by the asymmetry of the orientation control effect at the substrate interface, resulting in a bistable state. We believe that two states are realized.

This switching between bistable states is the same as before application of alternating current.
This is done using a pulse voltage that is higher than a predetermined threshold, and the threshold is 1.
The voltage was about 15V with a pulse width of m5ec.

Unlike the monostable state before the application of alternating current, each stable state is maintained even after the electric field is removed.

Here, such a conversion occurs within the cell,
Needless to say, only the pixel portion is applied with the AC electric field, and the parts other than the pixels maintain the monostable state before application of the AC electric field.

〔Effect of the invention〕

As explained above, according to the present invention,! By applying an alternating current electric field to a ferroelectric liquid crystal cell in a monostable state with a trix electrode structure, it is possible to control the alignment direction of parts other than the pixel part to be uniform throughout the cell, and
In the pixel portion, a bistable state with an enlarged tilt angle is realized, making it possible to increase the amount of transmitted light and contrast, and providing a ferroelectric liquid crystal element with excellent display characteristics.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of one embodiment of a liquid crystal cell according to the present invention, FIGS. 2 and 3 are a plan view and a sectional view of the liquid crystal cell, and FIGS. 4 and 5 are schematic diagrams of the liquid crystal cell. 1! Hu J: 1. Full O. Hatadan 3: Board
4: Electrode 5: Coating 6: Spacer 101 Nilaping direction average molecular axis 102: Stable state average molecular axis 103: Saturated average molecular axis Patent applicant Canon Co., Ltd. Fu1 (21) 1055 Penetration hole: 1M view of night crystal

Claims (5)

[Claims] (1) A ferroelectric liquid crystal element having a ferroelectric liquid crystal disposed between a pair of parallel substrates, a pixel portion having opposing electrodes, and a non-pixel portion having no opposing electrodes formed therein. 1. A ferroelectric liquid crystal element, characterized in that the ferroelectric liquid crystal in a pixel portion and a non-pixel portion is brought into a monostable state, and then the ferroelectric liquid crystal in the pixel portion is brought into a bistable state or a multistable state. (2) The bi- or multistable ferroelectric liquid crystal of the pixel portion is a ferroelectric liquid crystal formed by applying an alternating current voltage to a monostable ferroelectric liquid crystal. The ferroelectric liquid crystal element described in . (3) Claim 2, wherein the frequency of the AC voltage is 20Hz to 200Hz, and the voltage is 3V to 100V.
The ferroelectric liquid crystal element described in . (4) The ferroelectric liquid crystal device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (5) The ferroelectric liquid crystal device according to claim 4, wherein the chiral smectic liquid crystal is a liquid crystal formed by cooling another phase having a higher temperature than the chiral smectic liquid crystal.