JP2644528B2 - Liquid crystal device and driving method thereof - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device and a driving method thereof, and more particularly, to a liquid crystal device using a ferroelectric liquid crystal and a driving method thereof.

(Prior art)

Clark and Lager Walk are Applied Physics Letter
s Volume 36, Issue 11, June 1, 1980, P.899-90
1, or U.S. Patent 4,367,924, U.S. Patent 4,563,059
No., surface-stabilized liquid crystal (Surface-stabilized
Bistable ferroelectric liquid crystal by ferroelectric liquid crystal) was revealed. This bistable ferroelectric liquid crystal
A vertical molecular layer composed of a plurality of liquid crystal molecules is arranged in one direction between a pair of substrates set at an interval small enough to suppress the formation of a helical arrangement of liquid crystal molecules in the chiral smectic phase in the bulk state. It was realized by arranging in.

In the above-described ferroelectric liquid crystal element, the average direction () of the long axis of molecules stable in the projected component of the substrate is limited to two directions, and has a dipole moment () of molecules parallel to the vertical molecular layer. It forms spontaneous polarization (Ps). 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 direction of the electric field. 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-described electric field is released, after a short relaxation period (which varies depending on the type of ferroelectric liquid crystal, it is generally about ₁ μs to 2 μs), the degree of molecular order is lower than that of the uniform alignment state U _1. low, optically uniaxial low and stabilized to another molecule alignment state tilt angle is small (say this state splay alignment state S _1). Although the dipole moment of the molecule in the splay alignment state S ₁ is not in the same direction, the direction of the spontaneous polarization (Ps) is the same as in the uniform alignment state U ₁ and the molecular axis to the vertical molecular layer. Are arranged in a non-parallel manner between the substrates, especially along a circular orbit of a spiral arrangement between the substrates. At this time, the C-directors are not necessarily arranged along a one-cycle circular orbit, and the C-directors adjacent to the upper substrate and the C-directors adjacent to the lower substrate are not arranged.
Directors may be arranged on a circular orbit at an angle of 180 ° or more or less. In addition, the application of an electric field in the opposite direction causes a uniform orientation state (U ₂ ) and a splay orientation state (S ₂ ).

Therefore, in the case of applying the ferroelectric liquid crystal device described above in Deisupurei panel, the brightness of the panel is uniquely determined by the transmittance in the spray orientation state S ₁ and S _2. That is, assuming that the molecular arrangement state is uniaxial, the amount of transmitted light with respect to the intensity of the incident light I ₀ under crossed Nicols Is determined by According to the experiments of the present inventors, it has been found 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 problem, a liquid crystal device using a high-frequency AC applying means (AC stabilizing effect) is disclosed in, for example, JP-A-61-246722, JP-A-61-246723, and JP-A-61-24.
These are disclosed in JP-A-6724, JP-A-61-249024, JP-A-61-249025 and the like.

The AC stabilizing effect described above is based on the dielectric anisotropy (△ ε)
By applying an AC electric field to the negative ferroelectric liquid crystal, a torque for orienting the liquid crystal molecular axis perpendicular to the electric field direction is generated, and the tilt angle θa can be increased.

However, the above-mentioned AC stabilizing effect changes in accordance with the temperature change, and in particular, the tilt angle θa
However, there is a problem that the number is reduced.

That is, the present invention provides a liquid crystal device having a matrix electrode and a ferroelectric liquid crystal having a negative dielectric anisotropy in which an intersection of a scanning electrode and a signal electrode is a pixel, and a selected pixel on a selected scanning electrode. Means for applying a pulse having sufficient intensity to reverse the direction of spontaneous polarization of the ferroelectric liquid crystal,
The liquid crystal device has a feature of increasing the peak value of the pulse in accordance with the temperature rise.

[Detailed Description of Embodiments of the Invention] The torque Γ _{Ps of} liquid crystal molecules generated by the coupling between the applied electric field (E) and the dipole moment (spontaneous polarization) and the applied electric field (E) and the dielectric anisotropy (△ ε) the torque gamma _{△ epsilon} of the liquid crystal molecules caused by binding, respectively represented by the following formula.

Γ _Ps ∝Ps ・ E …… (1) (Here, ε ₀ is the vacuum dielectric constant) From the above equation (2), it can be seen that the larger the dielectric anisotropy 液晶 ε of the liquid crystal molecules is, the more 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, and as a result, the helical arrangement structure is suppressed.

FIG. 1 shows the effective voltage values 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. It shows the dependence of the tilt angle θa on V _rms . In the measurement shown in FIG. 1, a short alternating current of 60 KHz was used to remove the influence from the spontaneous polarization Ps. ○, ×, △, and □ in the figure are measured values.

As is clear from FIG. 1, it can be seen that the larger the dielectric anisotropy Δ △, the larger the tilt angle θa. Liquid crystal (I)
The maximum transmittance under crossed Nicols in the cells using (III) and (III) was 15% [liquid crystal (I)] and 6% [liquid crystal (III)], respectively.

The characteristics of the ferroelectric chiral smectic liquid crystal cell used in this example were as follows.

Dielectric anisotropy Δε; -0.1 Spontaneous polarization Ps; 12.9 Maximum tilt angle; 23.3 ゜ Spontaneous polarization was measured in a 100 μm cell by the following triangular wave application method (measurement temperature; 27 ° C.).

The measurement of spontaneous polarization by this triangular wave application method is described in "Japanese Journal of Applied Physics" (1983).
, 22 (10), pages 661-663, co-authored by K. Miyasato et al.
With "" Triangle Waves, Fourth, Medium, Spontenious, Pottery Essence on Full Electric Liquid Crystals "
(“Direct Method with” “Trianguler Waves for Meas
uring Spontanious Polarization on Ferroelectric Li
quid Crystal ").

 The dielectric anisotropy (△ ε) (measuring temperature 27 ° C)
Electricity (ε ) And vertical permittivity (ε_⊥) And the difference (△ ε = ε
−ε_⊥).

For the measurement of the vertical dielectric constant ( _ε⊥ ), the cell thickness was set to 1.5 μm, and “FS-116” manufactured by Daikin Industries, Ltd. was used as a vertical alignment film.
The capacity of an empty cell (empty cell capacity C _0⊥ ) using “S” and the capacity when a ferroelectric liquid crystal is injected into the cell (liquid crystal cell capacity C _LC⊥ ) are determined by “4192” of the US Hewlett-Packard Company.
A ". Ratio of these capacities C _LC⊥ / C
To determine the vertical dielectric constant ε _⊥ from _0⊥.

 Parallel permittivity (ε ) For the measurement, the cell thickness is set to
μm, and Toray's polyimide as a parallel alignment film
Empty cell capacity using “SP-710” (empty cell capacity
C₀) And the capacitance when ferroelectric liquid crystal is injected into the cell
(Liquid crystal cell capacity C_LC) Is the same as the “4192A” device
Was measured by Ratio C of these capacities_LC/ C₀Or
Parallel permittivity ε I asked.

In addition, the frequency at the time of capacity measurement was set to 100 KHz. or,
The distance d between the electrodes of the cell used in this example was 1.4 μm,
A rubbed polyimide film was used as an orientation control film.

When the above-described ferroelectric smectic liquid crystal was operated with the drive waveform shown in FIG. 2, the operation margin shown in FIG. 3 was obtained. On this occasion, Met.

The horizontal axis in FIG. 3 indicates the pulse width ΔT ₂ , and the vertical axis indicates the write voltage V _OP (= V ₁ + V ₃ ). According to FIG. 4, an operation margin between the temperatures of 23 ° C. to 35 ° C. is shown, and it can be seen that the operable area shifts to the upper left side of the figure as the temperature rises.

Such a margin shape is generally seen in a ferroelectric smectic liquid crystal cell with Δε <0, and the higher the driving voltage, the wider the pulse width margin.

Therefore, by increasing the drive voltage as the temperature rises, the pulse width margin is maintained, and a high-quality drive state can be obtained even in a large-area cell.

Increasing the pulse width margin increases the permissible range with respect to temperature changes, so that it is more resistant to temperature variations within the cell.

In FIG. 3, the bias voltage is kept constant (1/3) and the applied voltage of both the scanning line and the signal line is changed. However, the applied voltage of the signal line is kept constant and only the peak value of the scanning signal is changed. FIG. 4 shows a pulse width margin in the case where is changed. As in FIG. 3, the vertical axis represents the peak value (V _OP ) at the time of selection, and the horizontal axis represents the pulse width ΔT _2, and the range in which the matrix can be driven is shown.
Also in this case, it can be seen that the pulse width margin increases as the voltage increases.

Therefore, similar to the case where the bias ratio is constant in FIG. 3,
It can be seen that a method of increasing the peak value at the time of selection with the temperature rise is also effective.

 It should be noted that shoulder numbers in FIG. 4 indicate bias ratios.

On the other hand, application of an electric field to the liquid crystal layer is desirably as small as possible from the viewpoint of storage stability and power consumption. Therefore, in a low temperature region where a relatively wide margin can be secured even at a low voltage, it is desirable to lower the drive voltage.

FIG. 5 shows an example of a change in temperature and a change in write voltage. This is set at 23 ° C. on the line AA ′ in FIG. 3, and when it changes to 27 ° C., it is moved to the line BB ′ and
In ° C., this corresponds to the movement to the CC ′ line.

FIG. 6 shows a driving device of the ferroelectric liquid crystal panel 61 in which the matrix electrodes used in the present invention are arranged. In the panel 61 of FIG. 6, scanning lines 62 and data lines 63 are wired so as to cross each other, and a ferroelectric liquid crystal is disposed between the scanning lines 62 and the data lines 63 at the intersections. . Also, in FIG.
64 is a scanning circuit, 65 is a scanning side drive circuit, 66 is a signal side drive voltage generation circuit, 67 is a line memory, 68 is a shift register, 69 is a scan side drive voltage generation power supply, and 60 is a microprocessor unit (MPV) Is represented.

The scanning-side drive voltage generation power supply 69 is provided with voltages V ₁ , V ₂ , and V _C. For example, the voltages V ₁ and V ₂ are used as the power supply for the above-described scan selection signal, and the voltage V _C is used as the power supply for the scan non-selection signal. It can be.

〔The invention's effect〕

According to the present invention, the operating margin can be increased by reducing the temperature dependency at the time of driving under AC stabilization.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the dependency of the tilt angle θa on the applied voltage with respect to the dielectric anisotropy Δε. FIG. 2 is a waveform diagram of the drive used in this embodiment. 3 and 4 are explanatory diagrams showing an operation margin. FIG. 5 is an explanatory diagram showing a sequence based on a temperature change. FIG. 6 is a block diagram of the device of the present invention.

Claims (9)

(57) [Claims] A liquid crystal device having a matrix electrode and a ferroelectric liquid crystal having a negative dielectric anisotropy, wherein a pixel is formed at an intersection of a scanning electrode and a signal electrode; A pulse having a sufficient intensity to change the molecular alignment state of the ferroelectric liquid crystal is applied to the pixel, and the peak value of the pulse is increased in accordance with the temperature rise, and the AC is applied to the pixel on the unselected scan electrode. A liquid crystal device having voltage applying means for applying a voltage pulse. 2. The liquid crystal device according to claim 1, wherein the AC voltage pulse has a pulse train having a constant peak value. 3. The ratio of the peak value of a pulse having a sufficient intensity to change the molecular alignment state of the ferroelectric liquid crystal to the peak value of the AC voltage pulse is a constant value. The liquid crystal device according to the above. 4. A liquid crystal device having a matrix electrode and a chiral smectic liquid crystal having a negative dielectric anisotropy having a pixel at an intersection of a scanning electrode and a signal electrode, and a selected pixel on a selected scanning electrode. A pulse having an intensity sufficient to change the molecular orientation state of the chiral smectic liquid crystal is applied, and the peak value of the pulse is increased in accordance with the temperature rise. A liquid crystal device having a voltage applying means for applying a voltage. 5. The liquid crystal device according to claim 4, wherein the AC voltage pulse has a pulse train having a constant peak value. 6. The ratio of the peak value of a pulse having a sufficient intensity to change the molecular alignment state of the chiral smectic liquid crystal to the peak value of the AC voltage pulse is a constant value. Liquid crystal device. 7. A method of driving a liquid crystal device having a matrix electrode and a chiral smectic liquid crystal having a negative dielectric anisotropy, wherein a pixel is formed at an intersection of a scanning electrode and a signal electrode. A pulse having an intensity sufficient to change the molecular alignment state of the chiral smectic liquid crystal is applied to the pixel, and the peak value of the pulse is increased in accordance with the temperature rise, and the AC is applied to the pixel on the unselected scanning electrode. A method for driving a liquid crystal device, comprising applying a voltage pulse. 8. The method according to claim 7, wherein the AC voltage pulse has a pulse train having a constant peak value. 9. The ratio of the peak value of a pulse having a sufficient intensity to change the molecular alignment state of the chiral smectic liquid crystal to the peak value of the AC voltage pulse is a constant value. For driving liquid crystal devices.