JPH0833537B2 - Liquid crystal device and driving method thereof - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a liquid crystal device, and more particularly to a liquid crystal device using a ferroelectric liquid crystal.

[Prior art]

Clark and Lager Wale, Applied Physics Letter
s Vol. 36, No. 11 (Published June 1, 1980), P.899-90
1, or U.S. Pat.No. 4,367,924, U.S. Pat.No. 4,563,059
Issue, surface-stabilized liquid crystal (Surface-stabilized f
A bistable ferroelectric liquid crystal with an erroelectric liquid crystal was clarified. This bistable ferroelectric liquid crystal
A vertical molecular layer composed of a plurality of liquid crystal molecules is arranged between a pair of substrates set at a distance small enough to suppress the formation of a helical alignment structure of liquid crystal molecules in the chiral smectic phase in the bulk state. It was realized by arranging in one direction.

In the ferroelectric liquid crystal device described above, the average direction (n) of the molecular long axes that is stable in the projected component of the substrate is limited to two directions, and has the dipole moment () of the molecule parallel to the vertical molecular layer, It forms its own polarization (Ps) on average. This spontaneous polarization (Ps) and the applied electric field form strong coupling. When a unidirectional electric field is applied to the ferroelectric liquid crystal, the dipole moment () in the vertical molecular layer is aligned with the electric field direction. Tilt angle at this time is equivalent to half the apex angle of the helical array structure, resulting in maximum tilt angle (refer to molecular arrangement state at this time is Yunifuomu orientation state U _1). When the above-mentioned electric field is released, after a relaxation period for a while (it depends on the type of the ferroelectric liquid crystal, it is generally about ₁ μs to 2 μs), and then the degree of molecular order is higher than that of the uniform alignment state U _1. Is low, the optical uniaxiality is low, and the tilt angle is small, another molecular alignment state (this state is referred to as a splay alignment state S ₁ ) is stabilized. The dipole moments of the molecules in the splay orientation state S ₁ are not in the same direction, but the direction of spontaneous polarization (Ps) is the same as in the uniform orientation state U ₁ . or,
By applying an electric field in the opposite direction, a uniform orientation state (U ₂ ) and a splay orientation state (S ₂ ) are similarly generated.

Therefore, when the above-mentioned ferroelectric liquid crystal element is applied to a display panel, the brightness of the panel is uniquely determined by the transmittance in the splay alignment states S ₁ and S ₂ . That is, assuming that the molecular arrangement state is uniaxial, the amount of transmitted light is the intensity of the incident light I ₀ under crossed Nicols, (Here, θa is a tilt angle, Δn is a refractive index anisotropy, d is a cell thickness, and λ is a wavelength of incident light.). Experiments conducted by the present inventors have revealed that the tilt angle θa in the splay alignment states S ₁ and S ₂ is generally 5 ° to 8 °.

In order to solve the above-mentioned problems, liquid crystal devices using a high frequency AC applying means (AC stabilization effect) are disclosed in, for example, JP-A-61-246722, JP-A-61-246723, and JP-A-61-24672.
No. 4, No. 61-249024, No. 61-249025, etc. In such an apparatus, a means for applying a high-frequency alternating current is used in addition to the driving switching pulse, so that there is a problem that power consumption increases.

The AC stabilization effect is determined by the correlation between the torque acting on the molecule due to the spontaneous polarization Ps and the torque acting on the molecule due to the dielectric anisotropy Δε. However, when multi-divided matrix driving is performed in the AC stabilized state. Is desired to have a wide margin, that is, a so-called margin, in the drive voltage feasible range or the drive frequency movable range, but there is a problem that this margin is very narrow as compared with the drive method which does not use the AC stabilization effect.

[Outline of Invention]

An object of the present invention is to provide a liquid crystal device capable of applying an AC voltage to a ferroelectric liquid crystal pixel without superimposing a high frequency AC voltage for increasing a tilt angle and without reducing a driving voltage margin. Especially.

That is, the present invention determines the optical states of a plurality of pixels configured by arranging a ferroelectric liquid crystal at the intersection of the scan electrode group and the signal electrode group and the optical state of the pixel corresponding to the selected scan electrode. A driving means for applying a bipolar pulse signal to the pixel, and the ferroelectric liquid crystal is
The current response time τ ₀ exhibits a minimum value τ _min and is a ferroelectric liquid crystal having negative dielectric anisotropy, and the bipolar pulse signal applied to the selected pixel has a predetermined peak value. A unit pulse having a peak value, another unit pulse having a peak value higher than the predetermined peak value, and a pulse having a pulse width of a polarity opposite to that of the unit pulse equal to or smaller than the minimum value τ _min are arranged in time series. A liquid crystal device characterized by being a pulse train, and a ferroelectric having a current response time τ ₀ having a minimum value τ _min and a negative dielectric anisotropy at the intersection of the scanning electrode group and the signal electrode group. A driving method of a liquid crystal device comprising a plurality of pixels configured by arranging liquid crystals, wherein a unit pulse having a predetermined peak value and a predetermined peak value are applied to a pixel corresponding to a selected scanning electrode. Another unit pulse with a high peak value and a pulse width of the opposite polarity to the unit pulse A pulse is less than the small value tau _min, it is characterized by a driving method of a liquid crystal device according to claim but applying a bipolar pulse signal arranged in time series.

[Detailed Description of Embodiments of the Invention]

The torque Γ _{Ps of the} liquid crystal molecule generated by the combination of the applied electric field (E) and the dipole moment, and the torque Γ of the liquid crystal molecule generated by the combination of the applied electric field (E) and the dielectric anisotropy (Δε).
_Δε is expressed by the following equations.

Γ _Ps ∝Ps ・ E ………… (1) (Here, ε ₀ is the vacuum permittivity) From the above formula (2), it is understood that the larger the dielectric anisotropy Δε of the liquid crystal molecules, the more easily the helical arrangement structure is suppressed or erased. Moreover, in the case of Δε <0, the liquid crystal molecules are predominantly arranged in the projected component of the substrate under the applied electric field,
As a result, the helical sequence structure is suppressed.

FIG. 1 shows tilts of the liquid crystal (I) with Δε = −5.5, the liquid crystal (II) with Δε = −3.0, the liquid crystal (III) with Δε = 0, and the liquid crystal (IV) with Δε = 1.0 with respect to the effective voltage value V _rms . It shows the dependence of the angle θa. In the measurement shown in FIG. 1, a rectangular alternating current of 60 KHz was used to eliminate the influence from spontaneous polarization Ps. In the figure, ○, ×, △ and □ are measured values.

As is clear from FIG. 1, the larger the dielectric anisotropy Δε, the larger the tilt angle θa. Liquid crystal (I)
The maximum transmittances under the crossed Nicols in the cells using (III) and (III) were 15% [liquid crystal (I)] and 6% [liquid crystal (III)], respectively.

2 to 4 are examples of drive waveforms used in the present invention. In the figure, S ₁ , S ₂ , and S ₃ are scanning signals, I is an information signal, and A (S ₁
-I) represents a composite waveform when the information signal I is applied to the pixel on the selected scanning line.

As the ferroelectric liquid crystal used in the present invention, it is preferable to use a chiral smectic liquid crystal having a negative dielectric anisotropy Δε. For example, "CS1011" (trade name) manufactured by Chitso Corp. is known. Further, the film thickness of the ferroelectric liquid crystal is preferably set to be sufficiently thin so as to suppress the formation of the chiral smectic phase helical molecular alignment structure (in the bulk state) (for example, 0.5 μm to 10 μm, preferably 1.0 μm to 5 μm). This ferroelectric liquid crystal is preferably contacted at the interface of a rubbing-treated polyimide film, polyamide film, polyamideimide film, polyesterimide film or polyvinyl alcohol film, or obliquely vapor-deposited SiO film or SiO ₂ film, As a result, a mono domain can be formed.

Further, the ferroelectric liquid crystal used in the present invention generates a polarization reversal current by an applied pulse as shown in FIG. The current response time τ ₀ depends on the applied voltage (pulse peak value), where the time from the rise of the applied pulse to the peak P of the polarization reversal current is the current response time τ ₀ .
FIG. 6 clarifies the applied voltage dependence on the current response time τ ₀ of the liquid crystals A and B described below. According to Figure 6, the liquid crystal A when the applied pulse is near 20V, although the minimum value τ _min ≒ 110μsec the current response time tau ₀ is appeared, for the liquid crystal B does not occur a minimum value tau _min. The above-mentioned current response time τ ₀ can be measured by the current response time measuring device shown in FIG. 7. In the figure, 71 is a 5 Hz pulse generator, 72 is a resistance of 1 KΩ, and 73 is a ferroelectric liquid crystal cell. ch1 corresponds to the oscillograph ch1 shown in FIG.
ch2 corresponds to the oscillograph ch2 shown in FIG.
In a preferred embodiment of the present invention, the above-mentioned minimum value τ _min
Assuming that the electric field strength E ₁ (about 20 V in the case of the liquid crystal A described below) that causes the maximum pulse width ΔT of the information signal pulse train
When is set to a minimum value τ _min or less, crosstalk can be prevented by applying a voltage of electric field strength E ₁ or more to the half-selected point on the write line. The reason is that a high-frequency alternating current is applied at the half-selection point to cause dielectric anisotropy Δε coupling, and when a voltage of electric field strength E ₁ or more is applied, the molecular orientation of the liquid crystal is reversed or It is presumed that it is based on the fact that the fluctuation of the molecule does not occur. Therefore, in the preferred embodiment of the present invention, the electric field strength E ₀ applied to the half-selected point can be set based on the following equation (3).

E ₀ > E ₁ (3) In this formula, E ₁ is the electric field strength (V / V / V) corresponding to the minimum value τ _min.
m) and E ₀ are electric field strengths (V / m) applied to the half-selected points.

The present invention can also be applied to static driving using a common signal and an information signal pulse train, in addition to the multiplexing driving using the scan selection signal and the information pulse train described above.

FIG. 8 shows a C-director for an axis 84 parallel to the substrate.
The angle θ formed by 81 (referred to as “C-Director angle θ”)
Is represented. The C-director represents the projection of the long axis of a liquid crystal molecule onto a vertical molecular layer organized by a plurality of chiral liquid crystal molecules. Further, the increasing direction of the angle θ of the C-director is represented by the positive torque 82, and the decreasing direction of the angle θ of the C-director is represented by the negative torque 83.

FIG. 9 shows the relationship between the applied voltage and the torque when the angle θ of the C-director is used as a parameter.

FIG. 8 shows that the larger the positive torque 82 is, the easier the reverse switching is, and the larger the negative torque 83 is, the harder the reverse switching is. However, according to FIG. 9, the C-director angle θ is 50 °. The smaller the value is, the larger the negative torque 83 becomes as the applied voltage increases, and the coupling of the dielectric anisotropy acts predominantly so that the reverse switching does not occur. On the other hand, when the angle of the C-director is 60 °, the maximum positive torque is produced when the applied voltage is about 10V, and therefore, the reverse switching is produced at the applied voltage of about 10V. Further, increasing the C-director angle .theta. To 80.degree. Produces a maximum positive torque near an applied voltage of about 25V and, therefore, reverse switching at an applied voltage of about 25V.

Therefore, according to the present invention, the ferroelectric liquid crystal in the alignment state (where θ of the C-director is again set to a small value) generated under the state of applying the alternating voltage which causes the dielectric anisotropy coupling is time-consuming. The drive voltage margin can be expanded by applying a pulse having a low peak value along the axis and subsequently applying a pulse having a high peak value to perform the inversion switching. Further, in a preferred embodiment of the present invention, a high peak value pulse is applied along the time axis at a half-selection point which is an intersection of a selected scan electrode and a non-selected signal electrode, and then a low peak value pulse is applied. Therefore, reverse switching can be prevented.

As a method of producing an orientation state in which the angle θ of the C-director can be set to a small value, a method of applying a high frequency, for example, an alternating voltage of a relaxation frequency or higher to the non-selected pixels during driving (Japanese Patent Laid-Open No. 61-246722) JP, 61-246723, JP, 61
No. 246724, No. 61-249024, No. 61-249025, US Pat. No. 4668051, etc.) or a method of applying a high frequency alternating current in advance before driving (eg 62-220930, JP-A-62-223729)
Can be used.

FIG. 10 shows a driving device of the ferroelectric liquid crystal panel 101 in which the matrix electrodes used in the present invention are arranged. First
In the panel 101 of FIG. 0, the scanning lines 102 and the data lines 103 intersect each other, and the ferroelectric liquid crystal is arranged between the scanning lines 102 and the data lines 103 at the intersections. or,
In FIG. 10, 104 is a scanning circuit, 105 is a scanning side driving circuit, and 106.
Is a signal side drive voltage generation circuit, 107 is a line memory, 108
Is a shift register, 109 is a scan side drive voltage generating power supply, 100
Stands for Micro Processor Unit (MPU).

The scanning-side drive voltage generating power supply 109 is provided with voltages V ₁ , V ₂ and Vc. For example, the voltages V ₁ and V ₂ are used as the power supply for the scan selection signal and the voltage Vc is used as the power supply for the scan non-selection signal. be able to.

 Next, the present invention will be described according to examples.

〔Example〕

A 1000 Å thick SiO ₂ film was formed by a sputtering method on a glass substrate on which an ITO (indium-thein-oxide) film to be a transparent electrode was patterned in a stripe shape, and a 500 Å thick polyimide film was formed on it. To form the polyimide film, a polyamic acid solution SP-710 (trade name) manufactured by Toray Industries, Inc. was used). Next, the polyimide film was rubbed with an acetate flocked cloth.

Two glass substrates each subjected to the above rubbing treatment were prepared, and the two glass substrates were bonded so that the stripe electrodes intersect each other and the rubbing treatment axes are parallel to each other.
At this time, silica beads having an average particle size of 1.5 μm were scattered on one of the glass substrates so that the distance between the two glass substrates was about 1.5 μm when they were bonded.

Chiral smectic liquid crystal (liquid crystal A) was injected into the cell thus formed. The characteristics of this liquid crystal A were as follows. Liquid crystal A (Measurement temperature; 25 ° C) Spontaneous polarization Ps; 12.9nc / cm ² τ _min ; 110μsec (at 20V) Δε ； -5.8 Vertical angle in spiral array structure; 23 ° Threshold at 18V rectangular wave; 120μsec Phase series; Iso → ch → SmA → SmC ^* (In the table, Iso-isotropic phase, ch-cholesteric phase, SmA-smectic A phase, SmC ^* -chiral smectic C phase) The characteristics of liquid crystal B are as follows. there were. Liquid crystal B (Measurement temperature; 25 ° C) Spontaneous polarization Ps; 6.6nc / cm ² τ _min ； None Δε ； －0.1 Apical angle ； 23 ° 18V Square wave threshold ； 50μsec Phase series ； Iso → ch → SmA → SmC ^* FIG. 11 shows the threshold characteristics of the liquid crystals A and B. In the figure, Δ and ○ indicate threshold voltage values, and ▲ and ● indicate saturation voltage values. FIG. 11 (A) is a characteristic diagram when a bipolar pulse of V and -V is applied, and FIG. 11 (B) is a characteristic diagram when a unipolar pulse of V is applied.

Next, when the drive waveform shown in FIG. 3 was applied under the following condition A, a good display image was obtained. Condition A ΔT ₁ = 30μsec ΔT ₂ = 60μsec ΔT ₃ = 30μsec ｜ ± 17V | ＜ ｜ ± (V ₁ + V ₃ ) ｜ ＜ ｜ ± 31V | V ₁ = V ₂ Bias ratio: constant Also, drive shown in Fig. 2 When the waveform was applied under the following condition B, good display was obtained. Condition B V ₁ = 14V V ₂ = 10V V ₃ = 14V V ₄ = 10V 36 μsec ΔT 54 μsec Further, when the drive waveforms shown in FIG. 4 were applied under the following conditions C and D, good display was obtained. Condition C V ₁ = 16V V ₂ = 16V V ₃ = 8V 52μsec ΔT ₂ 92μsec When driving under this condition C, the optical state is converted by the application of the front pulse A and the optical state is converted by the application of the rear pulse B. There wasn't. Condition D V ₁ = 16V V ₂ = 16V V ₃ = 8V 112μsec ΔT ₂ 132μsec When driving under this condition D, the optical state was not changed by the application of the front pulse A, but by the application of the rear pulse B. A transformation of the optical state occurred.

〔The invention's effect〕

According to the present invention, it is possible to arbitrarily adjust the DC bias component, and preferably set the DC bias component to zero. Moreover, according to the present invention, it is possible to realize a display surface that does not cause crosstalk.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the applied voltage dependency of the tilt angle θa on the dielectric anisotropy Δε. 2, 3, and 4 are waveform diagrams of driving examples used in the present invention. FIG. 5 is an explanatory diagram showing an oscillograph ch1 of an applied pulse waveform and an oscillograph ch2 of a polarization reversal current. FIG. 6 shows the current response time τ ₀ corresponding to the variable pulse crest value when the time from the rise of the applied voltage pulse to the peak value of the polarization inversion current generated by the pulse application is the current response time τ ₀ . It is a characteristic view which shows the minimum value (tau) _min . FIG. 7 is a circuit diagram of the polarization reversal current measuring device. FIG. 8 is an explanatory diagram of the angle θ of the C-director. FIG. 9 is a characteristic diagram showing the relationship between applied voltage and torque when the angle θ of the C-director is used as a parameter. FIG. 10 is a block diagram of the device of the present invention. FIGS. 11A and 11B are threshold characteristic diagrams of the ferroelectric liquid crystal cell used in the present invention.

Claims (10)

[Claims] 1. A plurality of pixels configured by arranging a ferroelectric liquid crystal at an intersection of a scan electrode group and a signal electrode group, and for determining an optical state of a pixel corresponding to a selected scan electrode. In a liquid crystal device comprising a driving means for applying a bipolar pulse signal to the pixel, the ferroelectric liquid crystal has a current response time τ _{0 of which} is a minimum value τ.
_The bipolar pulse signal applied to the selected pixel is a ferroelectric liquid crystal exhibiting _min and a negative dielectric anisotropy, and a unit pulse having a predetermined peak value and a predetermined pulse A liquid crystal device characterized in that another unit pulse having a peak value higher than a high value and a pulse having a pulse width of a polarity opposite to that of the unit pulse is the minimum value τ _min or less are pulse trains arranged in time series. . 2. A pulse train in which at least a unit pulse having a predetermined crest value and another unit pulse having a crest value lower than the crest value are time-sequentially applied to the pixel of the half-selected point. The liquid crystal device according to claim 1, wherein the liquid crystal device is a liquid crystal device. 3. The driving means applies an AC voltage to a pixel corresponding to a non-selected scanning electrode by applying an AC signal to the signal electrode group. The liquid crystal device according to item 1. 4. The liquid crystal device according to claim 1, wherein the pixel at the half-selected point is applied with an electric field strength E _{0 which} is equal to or higher than the electric field strength E ₁ which produces the minimum value τ _min . 5. The liquid crystal device according to claim 1, further comprising an MPU for controlling the driving means. 6. The liquid crystal device according to claim 1, wherein the liquid crystal is a chiral smectic liquid crystal. 7. A ferroelectric liquid crystal having a current response time τ ₀ having a minimum value τ _min and a negative dielectric anisotropy is arranged at the intersection of the scan electrode group and the signal electrode group. A method of driving a liquid crystal device including a plurality of pixels, wherein a pixel corresponding to a selected scan electrode has a unit pulse having a predetermined peak value and another pulse having a peak value higher than the predetermined peak value. A driving method for a liquid crystal device, comprising applying a bipolar pulse signal in which a unit pulse and a pulse having a pulse width of a polarity opposite to that of the unit pulse are a minimum value τ _min or less are arranged in time series. 8. A pulse train in which at least a unit pulse having a predetermined crest value and another unit pulse having a crest value lower than the predetermined crest value are time-sequentially applied to the half-selected pixel. The method for driving a liquid crystal device according to claim 7, wherein the liquid crystal device is driven. 9. The liquid crystal device according to claim 7, wherein an AC voltage is applied to a pixel corresponding to a non-selected scanning electrode by supplying an AC signal to the signal electrode group. Driving method. 10. The method for driving a liquid crystal device according to claim 7, wherein the pixel at the half-selected point is applied with an electric field intensity E ₀ which is equal to or higher than the electric field intensity E ₁ which produces the minimum value τ _min .