JPH083582B2 - Driving method for ferroelectric liquid crystal cell - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for driving a ferroelectric liquid crystal cell applied to a display or the like, and more specifically, to a difference in threshold value or bistability after being left for a long time. The present invention relates to a method for driving a ferroelectric liquid crystal cell, which does not cause the problem of deterioration.

(Prior Art) Conventionally, various liquid crystal cells in which a liquid crystal is sandwiched between a pair of opposing electrodes have been proposed. However, DSM (Dynamic Scatter)
ing Mode) type liquid crystal cells, except for positive ions such as sodium ions and negative ions such as chlorine ions in the liquid crystal layer is not required to be controlled.

The reason is that TN (Twisted Nemati), which is currently popular.
c) type liquid crystal cell [eg M. Schadt and W. Helfrich, “A
pplied Physics Letters ", Vol.18, No.4 (1971.2.1
5), P.127-128, “Voltage Dependent Optical Activ
In "ity of a Twisted Nematic Liquid Crystal"], (1) excessive ion flow disturbs the alignment of liquid crystal molecules.

(2) The durability of the liquid crystal material is reduced.

(3) The time constant of the voltage applied to the liquid crystal layer becomes short.

It was thought that the effects such as the above might be caused by conductive substances such as ions, but in reality, by appropriately refining the liquid crystal, the volume resistance of the liquid crystal could be increased to 10 ⁹ Ωcm or more,
The above-mentioned problems (1) and (2) can be sufficiently dealt with by means such as effective prevention of liquid crystal contamination in the process of constructing the cell, while the drive system is an AC drive, refresh, storage type. This is because the above point (3) did not become a serious problem because it is the basis.

On the other hand, in the case of a ferroelectric liquid crystal cell that has been developed worldwide in recent years, the behavior of charged particles such as ions in the liquid crystal layer has a significant influence on the characteristics of the ferroelectric liquid crystal cell. It has been revealed.

For example, in the structure of the ferroelectric liquid crystal cell proposed by Clark and Lagavar, the directions of the dipoles of each liquid crystal molecule are aligned in the liquid crystal layer as shown in FIG. 4, and spontaneous polarization of the liquid crystal occurs. .

Since the existence of this spontaneous polarization is a condition for the switching characteristics of the ferroelectric liquid crystal cell, the deviation of the charge due to this spontaneous polarization is caused by the SSFLCD (Surface Stabilized Ferroelectric).
Liquid Crystal Display) is inevitable.

(Problems to be solved and used by the invention) Spontaneous polarization of liquid crystal molecules in a ferroelectric liquid crystal cell as described above is inevitable, but due to the effect of this polarization charge, the cell is not driven (that is, in a memory state). It was found that there is a problem in that a change that impairs the bistability of the liquid crystal layer occurs.

That is, a structure in which a transparent electrode such as an ITO electrode is present in the cell and is in contact with a liquid crystal layer through a dielectric and an alignment film is generally used, but in that case, a memory state (applied voltage = 0) However, an electric field generated by the polarization charge of the liquid crystal molecules exists in the liquid crystal layer, and the ionic impurities existing in the liquid crystal layer migrate due to the electric field, resulting in uneven distribution of ions. The uneven distribution of the ions, on the contrary, constrains the liquid crystal molecules, disturbing the bistable state of the liquid crystal molecules in the switching state, and further causing a serious problem of erasing the memory property of the cell itself. This is a major obstacle when considering the ferroelectric liquid crystal cell as a display.

Therefore, in the ferroelectric liquid crystal cell, it is desired to solve the problem caused by the ions existing in the liquid crystal layer.

(Means for Solving Problems) The present inventor has completed the present invention as a result of earnest research to solve the above-mentioned problems of the prior art.

That is, the present invention provides a ferroelectric liquid crystal cell in which a ferroelectric liquid crystal layer having bistability against an electric field is sandwiched between two opposing electrode substrates each having an electrode and an alignment film. Is a method of driving a ferroelectric liquid crystal cell, which is characterized in that an electric field whose magnitude is approximately equal to and opposite to a counter electric field due to the spontaneous polarization Ps of liquid crystal molecules is applied from the outside during non-display.

 Next, the present invention will be described in more detail.

The ferroelectric liquid crystal cell used in the present invention may be any conventionally known ferroelectric liquid crystal cell, and the present invention can be applied to any cell.

That is, the ferroelectric liquid crystal used in the prior art has one of a first optical stable state and a second optical stable state depending on an applied electric field, that is, a ferroelectric liquid crystal that has a two-dimensional structure with respect to the electric field. It is a liquid crystal substance having stability.

As a ferroelectric liquid crystal having bistability as described above,
A chiral smectic liquid crystal having ferroelectricity is preferable, and among them, a chiral smectic C phase (Sm
C ^* ) or H-phase (SmH ^* ) liquid crystals are suitable. These ferroelectric liquid crystals are based on the "LE JOURNAL DE PHYSIOUE LETTER
S " 36 (L-69) 1975," Ferroelectric Liquid Crystal
s "; Applied Physics Letters" 36 (11) 1980, "Submic
ro Second Bistable Electrooptic Switching in Liqui
d Crystals ”;“ Solid State Physics ”16 (141) 1981“ Liquid Crystal ”and the like, and more specifically, for example, desiloxybenzylidene-P′-amino-2-methylbutylcinnamate (DOBAMBC), Hexyloxybenzylidene-
P'-amino-2-chloropropyl cinnamate (HOBA
CPC) and 4-o- (2-methyl) -butylresorcylidene-4'-octylaniline (MBRA8).

The fifth example shown schematically shows one example of a ferroelectric liquid crystal cell of the prior art, and 1 and 1'in the figure are In ₂ O ₃ , S.
A substrate (for example, a glass plate) coated with a transparent electrode such as nO ₂ or ITO (Indium-Tin Oxide), and an alignment film (not shown) is provided on at least one of the pair of substrates. A liquid crystal layer 2 made of liquid crystal as described above between the films is oriented to be perpendicular to the substrate surface.
^* Enclosed as phase liquid crystal.

A thick line 3 represents a liquid crystal molecule, and the liquid crystal molecule 3 has a dipole moment (P⊥) 4 in a direction orthogonal to the molecule.

When a voltage above a certain threshold is applied between the electrodes on the substrates 1 and 1'of such a ferroelectric liquid crystal cell, the helical structure of the liquid crystal molecules 3 is unraveled, and the dipole moment (P⊥) 4 is all in the electric field direction. The alignment direction of the liquid crystal molecules 3 can be changed so that the liquid crystal molecules 3 face each other.

The liquid crystal molecule 3 has an elongated shape and exhibits anisotropy of refractive index in the major axis direction and the minor axis direction thereof, and therefore, for example,
It is easily understood that a liquid crystal optical modulation cell whose optical characteristics change depending on the polarity of voltage application can be easily obtained by disposing polarizers arranged in a crossed Nicol positional relationship above and below the substrate surface.

If the liquid crystal cell is made sufficiently thin (for example, 1
As shown in FIG. 6, the helical structure of the liquid crystal molecule is unwound (non-helical structure), and its dipole moment P or P ′ is upward (4 μm).
Take either a) or downward (4b). As shown in FIG. 6, when an electric field E or E'having a polarity different from a certain threshold value is applied to such a cell for a predetermined time, the dipole moment is directed upward 4a or downward depending on the electric field vector of the electric field E or E '. 4b, and the liquid crystal molecules are aligned in either the first alignment state 5 or the second alignment state 5 'accordingly. There are two advantages of using such a ferroelectric liquid crystal cell as an optical modulation cell.

Firstly, the response speed is extremely fast, and secondly, the alignment of the liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 6, for example, the liquid crystal molecules are aligned in the first alignment state 5 when the electric field E is applied, but in this state, it is stable even when the electric field is cut off. When a reverse electric field E'is applied, the liquid crystal molecules are oriented in the second alignment state 5'to change the orientation of the molecules, but they remain in this state even when the electric field is cut off. Further, as long as the applied electric field E does not exceed a certain threshold value, the respective alignment states are also maintained. In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, generally 0.5 to 20 μm, and particularly 1 to 5 μm.
Is suitable. A ferroelectric liquid crystal cell having a matrix electrode structure using a ferroelectric liquid crystal of this kind has been proposed by Clarke and Lagabal in US Pat. No. 4,367,924.

The above is one example of the configuration of a conventionally known ferroelectric liquid crystal cell. However, these conventional ferroelectric liquid crystal cells are as described above.
Ions present in the liquid crystal layer cause various problems.

As a result of earnest research aimed at solving such problems, the present inventor has found that the spontaneous polarization of liquid crystal molecules in the liquid crystal layer when the cell is not displayed.
By applying an electric field whose magnitude is approximately equal to and opposite in direction to the counter electric field due to Ps, the uneven distribution of ions and the adverse effect on the liquid crystal molecules are eliminated, and the difference in threshold value after leaving the cell for a long time and the liquid crystal molecule The inventors have found that the problems of the prior art such as the decrease of bistability can be solved.

Next, the present invention will be described more specifically with reference to the accompanying drawings showing an embodiment of the present invention.

 FIG. 1 is a diagram schematically showing an embodiment of the present invention.

In the driving method of the present invention, the substrate 12 having the electrode 12 and the alignment film 13 is vertically opposed to each other, and the ferroelectric liquid crystal layer 14 is sandwiched between the direction of the molecular axis when an electric field is applied and the direction of the molecular axis when no electric field is applied. In this cell, a voltage V ₀ is externally applied to the cell to cancel the counter electric field 16 due to the spontaneous polarization Ps15 of the liquid crystal molecules, which is a method for driving a ferroelectric liquid crystal cell.

Hereinafter, the electric fields of the layers 12 to 14 when the voltage V ₀ is applied from the outside will be described.

Now, as shown in FIG. 2, the relative permittivity of each layer is ε ₁ , ε _{2, the} layer thickness is d ₁ , d ₂ and the electric field is E ₁ , E ₂ , E ₃ and the electric flux density is D ₁ , D ₂ , Assume D ₃ . Further, the direction of the spontaneous polarization Ps is determined to be the direction in which the positive charges have moved, and here, from the bottom to the top as shown in FIG. At this time, the demagnetizing field due to Ps is generated from top to bottom. In addition, the dielectric constants of the 14 liquid crystal layers were assumed to include the effects of other dielectric polarizations, except for the spontaneous polarization Ps. At this time, the electric flux density of each layer is expressed as follows.

D ₁ = ∈ ₀ ∈ ₁ E ₁ (1) D ₂ = ∈ ₀ ∈ ₂ E ₂ + Ps (2) D ₃ = ∈ ₀ ∈ ₁ E ₃ (3) where ∈ ₀ is the dielectric constant of the vacuum.

From the continuation of the vertical component of the electric flux density, that is, D ₁ = D ₂ = D ₃ , equations (1) to (3) are as follows.

Ε ₀ ε ₁ E ₁ = ε ₀ ε ₂ E ₂ + Ps (4) E ₁ = E ₃ (5) On the other hand, since the voltage V ₀ is applied from the outside, the following formula is established.

E ₃ d ₁ + E ₂ d ₂ + E ₁ d ₁ = V ₀ (6) However, V ₀ is the potential difference from the upper electrode to the lower electrode, and the direction of V ₀ shown in FIG. 2 is positive.

From the expressions (4) to (6), the electric field of each layer is as follows.

Therefore, from equation (8), By applying this voltage, the demagnetizing field due to the spontaneous polarization Ps is canceled. At this time, the voltage is applied so that the direction from the high potential side to the low potential side matches the Ps direction.
Now for example Then, a voltage of V ₀ = 0.45 [V] may be applied.

On the other hand, the electric field E ₂ of the liquid crystal layer when the upper electrode and the lower electrode are short-circuited is given by V ₀ = 0 in the equation (8), Becomes Now, for example, if d ₂ = 1 × 10 ⁻⁶ [m] ∈ ₂ = 5 in addition to the condition of equation (9), then E ₂ = −3.4 × 10 ⁵ [V / m] (11) An electric field exists inside the liquid crystal layer due to spontaneous polarization.

As described above, the electric field inside the liquid crystal layer causes various problems. In particular, when left for a long time, since the electric field always exists in the same direction, the movement of ions cannot be ignored.
Due to the electric field in the same direction, the positive ions and the negative ions contained in the liquid crystal layer move toward the opposite boundary directions and are separated by the electric field. The electric field generated by the separation of the positive ions and the negative ions gradually cancels the de-electric field due to Ps.However, such separation of the ions is caused by the fact that all the ions in the liquid crystal layer move to the liquid crystal alignment film boundary or Ps. It continues until the de-electric field due to is completely canceled. Therefore, if left for a long time, positive or negative ions are attached to the boundary surface.

Therefore, there is a difference in the threshold value when switching between the first stable state and the second stable state of the liquid crystal molecules after leaving for a long time. Assuming that the electric field generated inside by the ions attached to the boundary surface is Eion and the voltage due to the external voltage is Eex, even if the same voltage is applied, one becomes Eex + Eion and the other becomes Eex−Eion depending on the application direction. That is, the external voltage corresponding to the electric field of 2 Eion appears as a difference in threshold. In this way, there is a difference in threshold between the inversion of the liquid crystal molecules from the first stable state to the second stable state and the inversion of the liquid crystal molecules from the second stable state to the first stable state. If it changes depending on the standing time, it becomes a great obstacle to driving the ferroelectric liquid crystal cell. This is because the threshold value of the liquid crystal layer originally depends on the pulse width ΔT and the voltage V, and in order to prevent pixel inversion due to crosstalk, it is necessary to adopt an appropriate driving method having a magnitude related to the threshold value Vth. In addition, especially when Eion is large, the bistability of liquid crystal molecules cannot be realized.

As explained above, the electrical measures for the cell are:
There are three main types: (1) open upper and lower electrodes, (2) short circuit between upper and lower electrodes (V ₀ = 0), and (3) apply voltage to upper and lower electrodes (V ₀ ≠ 0). In the case of (2), an electric field is generated inside the liquid crystal layer due to a counter electric field due to Ps, which causes a problem.
Here, (1) and (2) have heretofore been carried out without much consideration. On the other hand, (3) corresponds to the present invention. Especially V ₀
By applying a voltage of about = 2d ₁ Ps / ∈ ₀ ∈ ₁ so as to cancel the de-electric field due to Ps, it is possible to improve the problem of the difference in threshold value and the decrease of bistability after leaving the cell for a long time. It was

Up to this point, a cell having a symmetrical alignment film without an insulating layer such as a SiO ₂ film on the substrate has been shown. However, in practice, an insulating film such as SiO _{2 is used} to prevent short circuit between the electrodes.
It is often provided between and the alignment film 13. In addition, an asymmetric alignment film may be provided to improve the alignment state.

However, even in such a case, generally, in a cell in which a ferroelectric liquid crystal is sandwiched between electrodes, the molecular width direction when an electric field is applied is approximately the same as the molecular axis direction when no electric field is applied, an appropriate voltage is externally applied to the cell. Then, by canceling the anti-electric field due to the spontaneous polarization Ps, it is possible to solve the problems of the difference in threshold value and the decrease of bistability after being left for a long time.

FIG. 3 shows that the permittivity between electrodes is ε ₁ ′,
Ε ₂ ′, ε ₃ ′, ε ₄ ′, ε ₅ ′, thicknesses d ₁ ′, d ₂ ′,
An external voltage is applied to the cell sandwiching the media d ₃ ′, d ₄ ′ and d ₅ ′.
The case where V ₀ is applied is shown. Let the electric fields of each medium be E ₁ ′, E ₂ ′,
E ₃ ′, E ₄ ′, E ₅ ′, and the electric flux density is D ₁ ′, D ₂ ′, D ₃ ′,
Assuming D ₄ ′ and D ₅ ′, the electric field E ₃ ′ in the ferroelectric liquid crystal layer can be expressed as follows in the same manner as above.

Therefore, from equation (16) By applying the following external voltage to the cell and canceling the anti-electric field due to the spontaneous polarization Ps, it is possible to improve the problem of the difference in threshold value and the decrease of bistability after being left for a long time.

Also, from the above, it is clear that, if necessary, an appropriate external voltage V ₀ for canceling the demagnetizing field due to the spontaneous polarization Ps can be similarly shown in a more general case.

(Operation / Effect) According to the present invention as described above, in the conventional ferroelectric liquid crystal cell, when the cell is not displayed, an electric field whose magnitude is approximately equal to and opposite to the demagnetizing field due to the spontaneous polarization Ps of the liquid crystal molecules. By externally applying, even if the cell is left for a long time, the cell can be effectively driven without a difference in threshold value and a decrease in bistability.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 3 are diagrams schematically showing a driving method of a ferroelectric liquid crystal cell of the present invention, and FIG. 4 is a diagram showing two methods of polarization of liquid crystal molecules of the ferroelectric liquid crystal cell. It is a figure which shows a state diagrammatically, and
FIG. 6 and FIG. 6 are diagrams schematically showing an example of a conventionally known ferroelectric liquid crystal cell. 1, 1 ', 11 ... substrate 2, 14 ... liquid crystal layer 3 ... liquid crystal molecule 4 ... dipole moment 5, 5' ... alignment state 12 ... electrode 13 ... alignment film 15 ... spontaneous polarization Ps 16 …… Ps demagnetization field V ₀ …… External voltage

Claims (1)

[Claims] 1. A ferroelectric liquid crystal cell in which a ferroelectric liquid crystal layer having bistability against an electric field is sandwiched between two facing electrode substrates each having an electrode and an alignment film. A method for driving a ferroelectric liquid crystal cell, characterized in that an electric field having substantially the same magnitude and opposite direction is applied from the outside to a repulsive electric field due to spontaneous polarization Ps of liquid crystal molecules during non-display.