JP3149451B2 - Driving method of liquid crystal electro-optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a display, a light valve and the like, and more particularly, to a method for driving a display using a liquid crystal material.

[0002]

BACKGROUND OF THE INVENTION Between a stable state in an antiferroelectric phase of an antiferroelectric liquid crystal and two orientation states of an electric field induced ferroelectric phase,
So-called tristable switching is expected as one of the methods to solve some essential problems found in the conventional surface-stabilized ferroelectric liquid crystal device (SSFLC), and is being actively studied ( ADLChandani et al .: Jpn.J.Appl.
Phys., 27, L729 (1988), A. D. L. Chandani et al .: J
pn. J. Appl. Phys., 28, L1265 (1988), etc.) The main features of tristable switching are the following four points.

(1) The antiferroelectric-ferroelectric phase transition caused by the application of a voltage has a sharp threshold characteristic with respect to a DC voltage (FIG. 7).

(2) Since the phase transition has a wide optical hysteresis, the state can be maintained if a maintenance voltage VH is applied after selecting an antiferroelectric phase or a ferroelectric phase. .

(3) Two orientation states in the ferroelectric phase can be made optically equivalent.

(4) Since the bias of the charges in the liquid crystal layer can be prevented, there is no change with time in the electro-optical characteristics as seen in SSFLC.

By using these features, a high-definition liquid crystal display can be manufactured by simple matrix driving.
In the present application, in order to distinguish two orientation states in an electric field induced ferroelectric phase, they are referred to as a ferroelectric phase (+) and a ferroelectric phase.
I will call it (-).

As an example of a known driving method, a method shown in FIG. 8 is disclosed in Japanese Patent Application Laid-Open No. 2-137724. Vt and Vd in FIG. 8A are voltage waveforms applied to the scanning electrode and the signal electrode, respectively, and FIG. 8B is a composite waveform thereof, and this composite voltage waveform is applied to the liquid crystal layer. One frame includes a selection period and a non-selection period indicated by [S] and [NS], and the selection period [S] includes a reset period and a writing period indicated by [R] and [W].

In this driving method, the liquid crystal layer is reset to the OFF state by applying 0 volt to the liquid crystal layer during the reset period. When selecting the ON state, a voltage larger than V (AF) s is applied in the latter half of the writing period to
After switching to the N state, the unipolar sustain voltage waveform (V0
−V2 to V0 + V2) to apply the ON state (ferroelectric phase)
To maintain. When the OFF state is selected, after applying a voltage equal to or lower than V (AF) t in the latter half of the writing period,
An OFF state (antiferroelectric phase) is maintained by applying a unipolar sustain voltage waveform.

[0010] The display principle according to the driving method will be described with reference to FIG. The optical axis OA in the antiferroelectric phase is orthogonal to the smectic layer 34. Sandwiching the liquid crystal layer in the two glass substrates 38 in which the liquid crystal alignment layer 310 and the transparent electrode 37 is provided as shown in FIG. 6 (b), the further polarization axis 39 is set parallel or perpendicular to the optical axis OA If the polarizing plate 35 is interposed between the polarizing plate 35 and the analyzing plate 36 orthogonal to the polarizing plate, the light transmittance is 0 (OFF).
State). Here, even if a voltage equal to or lower than V (AF) t is applied, the change in light transmittance is slight, and the OFF state can be maintained. On the other hand, when a positive polarity voltage having an absolute value of V (AF) s or more is applied, a phase transition from an antiferroelectric phase to a ferroelectric phase (+) occurs. At this time, the liquid crystal molecule alignment direction (optical axis) is OF (+), and the spontaneous polarization is Ps (+). If a negative voltage having an absolute value of V (AF) s or more is applied, the optical axis is OF (-) and the spontaneous polarization is P
Phase transition to the other ferroelectric phase (-), which is s (-). OF
(+), The angle between OF (-) and the polarization axis is θ (+), θ (-), respectively.
Then, since they are not 0, light is transmitted, and they are turned on. Then, the state can be maintained as long as the sustain voltage VH is applied. Furthermore, θ (+) and θ (-)
Are equal to each other, and since the two ferroelectric phases (+) and (-) have the same light transmittance, they are optically equivalent. Therefore, in order to select the ON state, the ferroelectric phase whose optical axis is OF (+) or OF (-) may be selected.

However, in order to maintain the ferroelectric phase,
The sustain voltage must be continuously applied. If a voltage of one polarity is continuously applied, impurity ions in the liquid crystal layer are swept to the interface between the liquid crystal layer and the alignment film, which adversely affects the electro-optical characteristics of the liquid crystal. Therefore, in order to prevent the electro-optical characteristics from changing over time due to the bias of the polarity of the externally applied voltage, the time average value of the voltage within a unit time must be set to zero. Therefore, in this driving method, the data voltage waveform is set to AC, and the polarity of the write voltage waveform and the sustain voltage waveform which are not AC in one frame is inverted for each frame, so that the voltage in a unit time is changed. Is set to zero.

[0012]

However, the conventional driving method has the following two problems.

FIG. 8 (b) shows the voltage waveform of the second frame as OF
It is a voltage waveform applied to a liquid crystal layer when F state is selected. As described above, a voltage VNS =-(V0 + V1) + V2 having an absolute value equal to or less than the threshold value is applied at the end of the writing period, and immediately thereafter, a sustain voltage smaller than VNS is applied. It is difficult to obtain a high contrast ratio by such a driving method. The reason will be described with reference to FIG.

For the sake of simplicity, the maintenance voltage is set to a constant value VH,
The light transmittance when VNS is applied is represented by INS, and the light transmittance when VH is applied immediately thereafter is represented by IH. A loop C drawn by a broken line in FIG. 3 shows a hysteresis characteristic when the maximum value of the applied voltage is VNS.
As can be seen from this figure, the voltage is set to V lower than the threshold.
Even when going down from NS, the light transmittance changes as shown by the arrow and shows hysteresis characteristics. Therefore, the voltage V
The light transmittance IH when the maintenance voltage VH is applied immediately after the application of NS is as shown in the figure. Contrast ratio is IH
Therefore, it is difficult to obtain a high contrast ratio by the conventional driving method in which IH cannot be sufficiently reduced.

The second problem is that the reset period must be lengthened because the relaxation speed of the phase transition from the ferroelectric phase to the antiferroelectric phase is generally slower than the relaxation speed of the phase transition in the opposite direction. That is, it takes time to scan the screen.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to obtain a high contrast ratio and a high light transmittance by fully utilizing the features of tristable switching. It is an object of the present invention to provide a multiplex drive method capable of preventing charge bias.

[0017]

The driving method of the liquid crystal electro-optical device of the present invention According to an aspect of the liquid crystal layer is sandwiched between the substrate having a substrate and a signal electrode having a scan electrode, the liquid crystal layer <br / > Is a method for driving a liquid crystal electro-optical element having an orientation state of an antiferroelectric phase and an orientation state of a ferroelectric phase, wherein a voltage for selecting one of the orientation states of the liquid crystal layer within a selection period. Applying the voltage to the liquid crystal layer, and applying a voltage for maintaining the selected alignment state to the liquid crystal layer during a non-selection period, the liquid crystal layer within the selection period
When selecting the orientation state of the antiferroelectric phase, the liquid crystal layer
The applied voltage has two levels whose polarities are different from each other.
Voltage, and the absolute value of each voltage level is the antiferroelectric phase.
Absolute value of threshold voltage for phase transition from ferroelectric phase to ferroelectric phase
It is characterized in that it is set as follows .

As described above, the liquid crystal layer is strongly induced during the selection period.
When selecting the orientation state of the electric phase, the voltage applied to the liquid crystal layer
The voltage must include two levels of voltage with different polarities.
The light transmittance of the liquid crystal layer in the antiferroelectric phase alignment state is
For example, as shown by the solid line in FIG.
It is improved as compared to motion.

[0019] In the present invention, further, in the selection period,
When selecting the liquid crystal layer in the orientation state of the ferroelectric phase, the liquid
The voltage applied to the crystal layer is two voltages whose polarities are different from each other.
And the voltage applied to the liquid crystal layer first is
Saturation during phase transition from antiferroelectric phase to ferroelectric phase
Value is set to be equal to or greater than the absolute value of the voltage, and later applied to the liquid crystal layer.
The applied voltage causes a phase transition from the ferroelectric phase to the antiferroelectric phase.
May be set higher than the absolute value of the threshold voltage when
preferable.

[0020]

EXAMPLES (Example 1) As shown in FIG. 6 (b), a transparent electrode on a glass substrate (I
TO), and a liquid crystal alignment film (polyimide) is further formed thereon. The transparent electrodes formed on the upper and lower substrates correspond to a scanning electrode and a signal electrode, respectively. Then, a rubbing treatment is performed on the liquid crystal alignment film. The liquid crystal material 4- (1-methylheptyloxycarbonyl) phenyl-4'-octylo
xybiphenyl-4-carboxylate (MHPOBC)
A sample whose ambient temperature was kept within the temperature range of the antiferroelectric chiral smectic C phase was used as a sample. The liquid crystal layer thickness is
1.7 μm. This sample was sandwiched between two orthogonal polarizing plates, and the polarization axis of one polarizing plate was orthogonal to the smectic layer surface. Fig. 7 shows the hysteresis characteristics of the light transmittance at 70 ° C.
Shown in V (AF) t = 14 [v], V (AF) s = 20
[V], V (FA) t = 6 [v].

FIG. 1 shows a drive voltage waveform according to the present invention.
FIG. 1A shows a scanning voltage waveform, and Vd (OFF) and Vd (ON) in FIG. 1B.
Are the antiferroelectric phase (OFF state) and the ferroelectric phase (ON
9 is a data voltage waveform for selecting (state). The upper part of FIG. 2 shows a voltage waveform applied to the liquid crystal layer, which is a composite waveform of the scanning voltage waveform and the data voltage waveform. The lower part of FIG. 2 shows the electro-optical response of the liquid crystal to that. The reset voltage is VSE = 0 [v], and the data voltage is | VD1 | = | V
= 2 [v], the peak value of the penultimate write voltage pulse in the selection period is VS1 = 17 [v], and the peak value of the last write voltage pulse is VS2 = -4 [v]. The sustain voltage waveform was an AC voltage pulse of ± 9 [v] starting from negative polarity. As the compensation voltage waveform, the pulse width and the peak value are PW2 and VC = − (VS1 + VS2 +
A voltage pulse of (VSE) =-13 [v] is applied at the beginning of the reset period. The drive duty ratio and pulse widths PW1 and PW2 are 1/1000 and 480 μsec, respectively.
0 μsec, and the frequency of the sustain voltage waveform is 1 / (1.991 × 10
-3) Hz.

The case of applying the ON data voltage waveform Vd (ON) to the signal electrode, the voltage applied from the end of the selection period in the second to the liquid crystal layer is VS1-VD1 = 20 [v] and Do Ri, the absolute
Saturation when the pair transitions from an antiferroelectric phase to a ferroelectric phase
The value V (AF) absolute value than the Do because of S, phase transition to the ferroelectric phase from the anti-ferroelectric phase (+) occurs. The last voltage that follows is -7 [v]. As described above, when the peak value of the pulse voltage directly changes from +20 [v] to −7 [v], 7
Since [v] is equal to or greater than | V (FA) t |, it switches over to the other ferroelectric phase (−) through the antiferroelectric phase. Thereafter, during the non-selection period, -6 to -12 [v] and 6 to 12
Since the sustain voltage pulse [v] is alternately applied to alternately enter the ferroelectric phase (-) and the ferroelectric phase (+),
The ON state is maintained.

Next, the OFF data voltage waveform V is applied to the signal electrode.
When d (OFF) is applied, the voltage applied to the liquid crystal layer from the end of the selection period is 14 [v]. This value is |
V (AF) t |
No phase transition to (+) occurs. The light transmittance at this time is shown in FIG.
As shown in FIG. The last voltage that follows is
-1 [v]. Since this voltage has the opposite polarity to the penultimate voltage, the light transmittance falls to a value close to zero during this period. Thereafter, during the non-selection period, -6 to -12
Sustain voltage pulses of [v] and 6 to 12 [v] are applied alternately. In this case, the light transmittance is the loop B shown in FIG.
To almost follow. However, in this figure, only the positive side is shown.

The change over time of the actual light transmittance by such a driving method is shown by a solid line in FIG. For comparison, the light transmittance when driven by the conventional method is shown by a broken line in FIG. From this, there is no difference between the two in the light transmittance in the ON state, but a clear difference is recognized in the light transmittance in the OFF state. The average light transmittance in the OFF state according to the present invention is almost 2/3 times that of the conventional method. Since the contrast ratio is inversely proportional to the light transmittance in the OFF state, the contrast ratio is almost 3/2 times that of the conventional one, and is improved from 1:25 to 1:35. Further, as described above, since one compensation voltage pulse is applied, the time average value of the voltage applied to the liquid crystal layer in one frame is 0, and the electric charge is not biased in the liquid crystal layer.

Next, the display speeds of the driving method according to the present invention and the driving method according to the prior art will be compared. Since the relaxation time from the ferroelectric phase to the antiferroelectric phase is about 420 μsec, the length of the selection period is 80 × 2 + 420 = 58 in the conventional method.
0 μsec. On the other hand, in the method according to the present invention, the length of the selection period (writing period) is 160 μsec. Therefore, if the driving method according to the present invention is used,
About 3.5 times higher speed than the conventional driving method is achieved. However, how much the speed is increased depends on the relaxation time from the ferroelectric phase to the antiferroelectric phase, and the longer the relaxation time, the greater the effect.

[0026]

(Embodiment 2 ) In this embodiment, | VD1 |
And the upper limit V2 of | VD2 | is set to 3 [v] and changed within that range. However, as in the first embodiment, VD1 = −
VD2. By modulating the data voltage in this way, a gradation display could be performed.

Embodiment 3 In this embodiment, as shown in FIG. 4, a compensation voltage waveform is superimposed on a sustain voltage waveform applied during a non-selection period. VS1 =
17 [v], VS2 = -4 [v], VH = ± 9 [v], VC =
−13 [v], | VD1 | = | VD2 | = 3 [v]. Therefore, the voltage (VH + VC) at the portion where the compensation voltage waveform is superimposed on the sustain voltage waveform is -4 [v]. Also in this embodiment, the same display characteristics as those of the first embodiment were obtained.

Example 4 The same sample as in Example 1 was driven by a voltage waveform whose maintenance voltage waveform was DC as shown in FIG. VS1
= 17 [v], VS2 = -4 [v], VH = -9 [v],
| VD1 | = | VD2 | = 3 [v] and VSE = 0 [v]. Since the average value of the applied voltage within one frame period is not 0, the average value within a unit time is set to 0 by inverting the polarity of all the voltage waveforms for each frame. As for the display characteristics, a contrast ratio of 1:17 was obtained as in Example 1. Further, as in the case of the fourth embodiment, gradation display could be performed by modulating the data voltage.

[0030]

(Embodiment 5 ) In the same configuration as in Embodiment 1, the environmental temperature was set to 100 ° C. Since the temperature was higher than in Example 1, the relaxation rate from the ferroelectric phase to the antiferroelectric phase was increased. Therefore, it can be driven even when PW1 = 160 μsec,
A contrast ratio of 1:31 was obtained.

[0032]

As described above, according to the present invention, in a multiplex drive of a liquid crystal electro-optical element using switching between an antiferroelectric phase and an electric field induced ferroelectric phase,
Since the light transmittance in the OFF state can be reduced as compared with the conventional method, a higher contrast ratio can be obtained. INDUSTRIAL APPLICABILITY The present invention can be applied to a large-sized and high-definition liquid crystal display and a light valve.

[0033]

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment according to the present invention.

FIG. 2 is a diagram showing a voltage waveform applied to a liquid crystal layer and an electro-optical response of the liquid crystal in the first embodiment.

FIG. 3 is a diagram showing hysteresis characteristics in a low voltage region.

FIG. 4 is a diagram showing a third embodiment according to the present invention.

FIG. 5 is a diagram showing a fourth embodiment according to the present invention.

FIG. 6 is a diagram showing a display principle.

FIG. 7 is a diagram showing hysteresis characteristics obtained in a sample used in an example.

FIG. 8 is a diagram showing a conventional driving method.

Claims (2)

(57) [Claims] A liquid crystal layer is sandwiched between a substrate having a scanning electrode and a substrate having a signal electrode, wherein the liquid crystal layer has an orientation state of an antiferroelectric phase and an orientation state of a ferroelectric phase. Applying a voltage to the liquid crystal layer for selecting one of the alignment states of the liquid crystal layer during a selection period, and maintaining the selected alignment state during a non-selection period. Applying a voltage to the liquid crystal layer to perform the operation, the voltage applied to the liquid crystal layer when the liquid crystal layer is selected to be in the antiferroelectric phase alignment state during the selection period, It includes two levels of voltages whose polarities are different from each other, and the absolute value of each voltage level is set to be equal to or less than the absolute value of the threshold voltage when the phase transition from the antiferroelectric phase to the ferroelectric phase is performed. Method for driving a liquid crystal electro-optical element. 2. The two-level voltage according to claim 1, wherein the voltage applied to the liquid crystal layer when the liquid crystal layer is selected to be in the ferroelectric phase alignment state during the selection period is different in polarity. The absolute value of the voltage previously applied to the liquid crystal layer is set to be equal to or greater than the absolute value of the saturation voltage when the phase transition from the antiferroelectric phase to the ferroelectric phase is performed, and the absolute value of the voltage applied later to the liquid crystal layer is set. the driving method of the liquid crystal electro-optical element is the absolute value of that voltage, characterized in that it is set to more than the absolute value of the threshold voltage at which to phase transition to an antiferroelectric phase from the ferroelectric phase.