JP3204702B2 - Driving method of liquid crystal display 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 liquid crystal display using a chiral smectic liquid crystal, and more particularly to a technique effective when applied to a time division driving method.

[0002]

2. Description of the Related Art In this type of liquid crystal display device, a matrix system in which a matrix-like electrode is formed on a substrate and liquid crystal is driven in a time-division manner, and a TFT system in which a thin film transistor is formed in each pixel and each pixel is driven for each pixel are widely used. It is used.

[0003] Among them, the TFT type is currently producing a 15-inch diagonal panel as a trial. However, since a circuit is required for each pixel, there is a problem that the cost is high and it is difficult to increase the capacity.

[0004] On the other hand, the matrix type has an advantage that it is not necessary to form a transistor in each pixel, and a large-capacity liquid crystal panel can be manufactured at a relatively low cost. In addition, there is a problem that the response time is as slow as several tens of ms, and further, when the duty ratio is 1/500 or less, there is a problem that crosstalk easily occurs and the viewing angle is narrow.

In recent years, a ferroelectric liquid crystal display device (for example, Japanese Patent Laid-Open Publication No. (Patent Publication No. 56-107216).

The liquid crystal display device of this system utilizes the fact that two stable states of spontaneous polarization up (up) and down (down) are obtained by the alignment regulating force at the substrate interface.
Lighting and darkness are displayed by switching using the memory property by applying an electric field between these two states.

However, the ferroelectric liquid crystal exhibiting the bistable state is difficult to obtain the memory effect expected at the beginning, so that it is difficult to improve the contrast, and the spontaneous polarization is fixed at the substrate interface, resulting in an afterimage. Problems such as ease of use have been pointed out.

On the other hand, recently, a liquid crystal exhibiting three stable states and exhibiting antiferroelectric properties (hereinafter referred to as a tristable ferroelectric liquid crystal) has been found in the chiral smectic phase. It is expected to be applied to a new display device that can overcome the problems of the device. Hereinafter, the properties of the tristable ferroelectric liquid crystal display device will be described.

FIG. 2 shows the electric flux density-voltage hysteresis curve when a triangular wave is applied to this type of tristable ferroelectric liquid crystal display, and a double hysteresis curve peculiar to antiferroelectricity is drawn. It is understood that it has the feature of.

FIG. 3 shows a transmission light amount-voltage hysteresis curve of this liquid crystal display element. As shown in the drawing, when no voltage is applied, the first stable state (I0) is obtained, and when a positive polarity voltage is applied (voltage V2 or more is applied), a bright state (I3) is applied. It can be seen that the device has three stable states of a bright state (I3 ') when voltage is applied (voltage application of -V2 or less).

In driving the liquid crystal display element, a positive (negative) pulse is applied while being superimposed on the bias voltage VB shown in FIG. 3 to change the I1 (I2) dark (bright) state to the I2 (I1) bright state. Light / dark display is performed using the transition to the (dark) state.
Further, a negative (positive) pulse is applied superimposed on the negative bias voltage -VB, and I1 '(I2') is changed from the dark (bright) state to I1 '(I2').
The same light / dark display can be performed by using the transition to the light (dark) state of 2 ′ (I1 ′).

In the time-division driving of the matrix type tristable ferroelectric liquid crystal display element, a scanning voltage is sequentially applied to the scanning electrodes, and a voltage for applying bright and dark writing to the signal electrodes in synchronization with the scanning voltage. Driving is performed sequentially.

The first screen writing scan is performed by a positive / negative pulse voltage superimposed on the bias voltage VB shown in FIG. 3, and bright and dark display is performed by I1 and I2. On the other hand, the subsequent second screen writing is performed by the positive and negative pulse voltages superimposed on the bias voltage −VB shown in FIG.
Bright and dark display is performed by I1 'and I2'.

At this time, the DC component can be completely canceled between the first scan and the second scan, and the liquid crystal and the electrodes are prevented from being deteriorated.

[0015]

However, this type of 3
It has been found by the present inventors that when performing a matrix display using a stable ferroelectric liquid crystal display element, the change from the bright state to the dark state is significantly delayed with respect to the change from the dark state to the bright state. Was.

FIG. 5 is a plot of a change in the amount of transmitted light when a voltage is applied to the selected pixel. First of applied waveform
In the period (P1), zero volts is applied to reset the state from the previous state to the dark state, and the writing to the bright state is performed in the second period (P2). The third period (P
In 3), reset to the dark state is performed again.

As described above, the rise time from dark to light is as fast as about 100 μs, but the time (t) required for fall is about 2 ms, which is extremely slow. The reason for this is that, when the voltage rises, the electric field reverses spontaneous polarization and the torque is very large, whereas when the voltage falls, the voltage simply drops. This is considered to be due to the natural relaxation of the liquid crystal molecules, which does not add any forcing to the spontaneous polarization.

[0018] Therefore, in the driving by the above-mentioned waveform, since the falling time is slow, the line writing time is as long as several milliseconds. As a result, the writing time on the screen becomes extremely long, and a large amount of information is displayed or displayed. It could not be used for moving images with large changes in content.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a technique capable of displaying a large amount of information at a high speed by increasing the voltage falling speed in a matrix type liquid crystal display. It is in.

[0020]

According to the present invention, when a matrix display is performed using a three-stable liquid crystal display element in which a chiral smectic liquid crystal is sealed, a second stable state is obtained immediately before a write voltage is applied. Alternatively, an erasing pulse voltage that is equal to or less than a threshold value for transition to the third stable state and includes at least one pulse having a polarity opposite to one of the pulses constituting the write voltage is applied.

The erasing pulse voltage may be composed of one to three pulses or a pair of positive and negative pulses. The erasing pulse voltage may be applied at the beginning of the scanning electrode selection period. Good.

[0022]

In the liquid crystal display device having the above-mentioned structure, by applying an erasing pulse voltage including a pulse having a polarity opposite to that of one of a pair of pulses constituting the write voltage immediately before the application of the write voltage, the liquid crystal display device becomes particularly bright. The reset switching between the state and the dark state can be performed at high speed.

This is because, as a restoring force at the time of transition from the second or third stable state to the first stable state, a large torque due to the coupling between the spontaneous polarization and the applied voltage (see Equation 1 below). This is because it can be used.

[0024]

## EQU1 ## F1 = Ps.E

In the above equation (1), F1 is a torque, and this force is represented by a spontaneous polarization vector Ps and an external electric field Es.
Can be obtained by the cross product of

[0026]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a main part showing a structure of a liquid crystal cell of a tristable ferroelectric liquid crystal display element according to an embodiment of the present invention.

As shown in FIG. 1, a pair of transparent substrates 2 and 5 made of glass are opposed to each other with an interval of about 2 μm. A polymer film such as polyimide is formed on the surfaces of the transparent substrates 2 and 5 in advance, and an orientation process such as rubbing is performed in one direction.

Deflection plates 1 and 6 are provided on the non-opposite surfaces of the transparent substrates 2 and 5, respectively, and the polarization axis of one of the polarization plates is substantially aligned with the rubbing direction. Each of the transparent substrates 2 and 5
Transparent electrodes 7 and 10 are formed on the sides facing each other, and the surfaces thereof are covered with alignment films 3 and 9.

One of the transparent electrodes 7 and 10 is used as a signal electrode, and the other is used as a scanning electrode.
The two electrodes have a structure that intersects with each other at a predetermined interval when viewed from the plane (see FIG. 6).

A spacer 8 is provided between the transparent substrates 7 and 10.
The spacer 8 keeps the distance between the substrates of 2 μm. The spacer 8 is made of, for example, SiO 2 or polymer-based particles.

In the space formed by the spacer 8, for example, an antiferroelectric liquid material (MH) represented by the following structural formula (Formula 1)
POBC; 4- (1-methyl-heptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate) is encapsulated.

[0032]

Embedded image

The phase transition temperature of this material is as follows.

[0034]

Embedded image

In addition, various antiferroelectric liquid crystal materials can be used, and for example, there is one disclosed in JP-A-3-163048.

FIG. 4 shows the molecular arrangement of the liquid crystal molecules 4 of the tristable ferroelectric liquid crystal display device. In the figure,
(A) is a liquid crystal molecule arrangement in the absence of an electric field, and the long axis of the molecule is aligned in a "ku" shape corresponding to the rubbing direction.

Next, when an electric field is applied between the two electrodes 7 and 10 of the liquid crystal cell (upward from the bottom in FIG. 4),
Since the direction of the spontaneous polarization of the liquid crystal molecules 4 is aligned with the direction of the electric field, it becomes as shown in FIG.

When an electric field of the opposite polarity (downward from top in FIG. 4) is applied, the orientation shown in FIG. The combination of such an orientation direction and the direction of the deflection axis of the deflecting plates 1 and 6 enables bright and dark display on the display matrix.

FIG. 6 shows scanning electrodes X1, X2,... XN and signal electrodes Y1, Y2,.
FIG. The intersection of the scanning electrode X and the signal electrode Y corresponds to a display pixel.

FIG. 7 shows a voltage waveform of the scanning electrode and a voltage waveform of the signal electrode. The selection voltage 1 is applied to the scan electrode X when the scan electrode X is selected, and is applied to the first period T of the selection period.
1, the second period T2, the third period T3, and the fourth period T4
It is composed of

Here, the period T1 is V2 volts, the period T2 is -V2 volts, the period T3 is V1 volts, and the period T4 is 2VB-V1.
And The non-selection voltage 2 is a voltage applied to the scan electrode X when the scan electrode X is not selected.

Like the first selection period of the non-selection period, the period includes a period T1, a second period T2, a third period T3, and a fourth period T4. The voltage applied to the non-selection electrode was VB in all of the periods T1, T2, T3, and T4.

An ON voltage 3 or an OFF voltage 4 is applied to the signal electrode Y. When the display information of the display pixel at the intersection of the selected scanning electrode X and signal electrode Y is in the low transmission light display state, an on-voltage 3 is applied to the signal electrode, and in the high transmission light display state, An off voltage 4 is applied. As the ON voltage 3, zero volt is applied during the periods T1 and T2, V3 volt is applied during the period T3, and -V3 volt is applied during the period T4. The off voltage 4 is applied during the period T1
And period T2 is zero volts, period T3 is -V3 volts, period T4
Applies V3 volts.

In the waveform (7-1), a pair of voltages centered on VB, that is, V1 and 2VB-V1 applied at T3 and T4 are write voltages, and are the beginning of the selection period in the scan electrode X, that is, T1 and A pair of positive and negative pulses of V2 and -V2 applied at T2 is an erasing pulse voltage. Of these, -V2 at T2 functions as an erasing pulse.

When the above voltage is applied to the scanning electrode X and the signal electrode Y, the voltage waveform applied to the display pixel becomes (7-5), (7-6), (7-7), (7-7) shown in FIG. 7-8) -4
The combinations are as follows. The waveform (7-5) is a voltage when the selection voltage (7-1) is applied to the scan electrode X and the voltage (7-3) is applied to the signal electrode. At this time, V2 volts during the period T1, -V2 volts during the period T2, V1 -V3 volts during the period T3, and 2VB-V1 + V3 volts during the period T4.

The waveform (7-6) shows the selection voltage (7
-1), the off voltage (7-4) is applied to the signal electrode. V2 volts during the period T1, −V2 volts during the period T2, V1 + V3 volts during the period T3, and 2VB−V1−V3 volts during the period T4.

The waveform (7-7) is a voltage when the non-selection voltage (7-2) is applied to the scanning electrode and the voltage (7-3) is applied to the signal electrode. At this time, VB in the period T1 and the period T2
Volts, VB-V3 volts in period T3, 2VB + in period T4
V3 volts.

In the waveform (7-8), the non-selection voltage (7-2) is applied to the scanning electrodes, and the off voltage (7-4) is applied to the signal electrodes. At this time, VB volts in the period T1 and the period T2,
VB + V3 volts during the period T3 and 2VB-V3 volts during the period T4.

Here, the applied voltage of the scanning electrode X and the signal electrode Y and the threshold voltage for switching the antiferroelectric liquid crystal are 2VB-V1 + V3> V2 VB <2VB-V1-V3 <V2 VB + V3 <V1 It is set so that VB−V3> V1. From this, the waveform (7-5)
During the period T2, the display pixel is temporarily turned off, and during the period T4, the display pixel is turned on again.

The voltage in the period T1 and the voltage in the period T3 are applied for the purpose of removing a DC component during driving, and do not directly affect the display state. Period T2 of waveform (7-6)
, The display pixel becomes dark, and during the period T4, the display pixel is kept dark.

Note that the display state before that is not changed by the application of the waveform (7-7) and the waveform (7-8). When the drive waveform of this embodiment is used, as described in the waveform (7-6), since the erase pulse is applied in the period T2, the fall delay time (t) in FIG. 5 can be reduced. Thus, writing in the off state can be performed at high speed.

Next, a specific display method using this embodiment will be described. FIG. 8 shows a temporal change of a driving waveform for obtaining the display shown in FIG.

In FIG. 6, □ indicates a bright state, and Δ indicates a dark state. Here, a voltage is applied to the scanning electrodes in order of the waveforms X1, X2,. FIG. 8 shows only a time chart of the applied voltage in the first column of the signal electrode. X1-Y1, X1-Y2, and X1-YN indicate waveforms applied to each pixel.

The voltage applied to the pixel formed at the intersection of the scanning electrode X1 and the signal electrode Y1 is equal to the voltage applied during the period T of the first frame.
At 1 ', the display pixel is in a dark state (■), and the applied voltage in the subsequent first frame period does not change the light transmission state of the display pixel, so that it is formed at the intersection of the scanning electrode X1 and the signal electrode Y1. The pixel maintains the dark state (■).

In the subsequent second frame, a waveform obtained by inverting the polarity of the waveform applied to the scanning electrode and the signal electrode in the first frame is applied. In this case, light and dark are displayed using the one-sided hysteresis curve shown in FIG.

In the figure, the second frame displays exactly the same display as the first frame, but of course it may display completely different. Also in this case, the DC components applied to the pixels in the first and second frames are canceled.

The voltage applied to the pixel formed at the intersection between the scanning electrode X2 and the signal electrode Y1 is equal to the voltage applied during the period T of the first frame.
At 1 ', the display pixel is in a bright state (□), and since the applied voltage in the subsequent first frame does not change the light transmission state of the display pixel, it is formed at the intersection of the scanning electrode X3 and the signal electrode Y1. The pixel keeps a bright state (□).

In the subsequent second frame, a waveform obtained by inverting the polarity of the waveform applied to the scanning electrode and the signal electrode in the first frame is applied. In this case, light and dark are displayed using the one-sided hysteresis curve shown in FIG.

In the prior art shown in FIG. 5, the fall time (t) is 2 ms, whereas according to the present embodiment, the rise time (dark → bright) is 100 μs, and the fall time (bright → bright). Dark) was 500 μs, indicating that extremely high-speed display switching can be performed.

[0060]

According to the present invention, it is possible to rapidly change the amount of transmitted light in response to the fall of the applied voltage, and in particular, to rapidly switch from a bright state to a dark state. In the display device, large-capacity display can be performed at high speed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a liquid crystal display device according to the present invention.

FIG. 2 is a graph for explaining a DE hysteresis curve of an antiferroelectric liquid crystal.

FIG. 3 is a graph illustrating the relationship between the amount of transmitted light and the applied voltage of the liquid crystal element having the configuration shown in FIG.

FIG. 4 is a schematic diagram illustrating an arrangement of liquid crystal molecules in the liquid crystal display device of FIG.

FIG. 5 is a graph showing a change in transmitted light amount of the liquid crystal display device of FIG. 1;

FIG. 6 is a schematic diagram illustrating a pixel that is an intersection of a scanning electrode and a signal electrode.

FIG. 7 is a graph showing waveforms applied to scanning electrodes and signal electrodes.

FIG. 8 is a timing chart of a waveform applied to a pixel.

[Explanation of symbols]

 1, Polarizing plate 2, Transparent substrate 3, Alignment film 4, Liquid crystal molecule 5, Transparent substrate 6, Polarizing plate 7, Transparent electrode 8, Spacer 9, Alignment film 10, Transparent electrode X ..Scanning electrodes Y..Signal electrodes

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/133 560 G09G 3/36

Claims (5)

(57) [Claims] 1. A first transparent substrate having a scanning electrode on its surface and a second transparent substrate having a signal electrode on its surface are opposed to each other so that the scanning electrode and the signal electrode are substantially orthogonal to each other. A chiral smectic liquid crystal is sealed in, an intersection of the scanning electrode and the signal electrode is used as a display pixel, the scanning electrodes are sequentially scanned to be in a selected state for one scanning period, and the signal electrode is synchronized with a selection voltage of the scanning electrode, When performing display by applying a signal voltage for turning on or off the display state of the selected display pixel, at least three stable states in relation between the amount of transmitted light and the applied voltage, that is, the first stable state in which no voltage is applied. State, a second stable state to which a transition is made by applying a positive polarity voltage from the first stable state, and a third stable state to which a transition is made by applying a negative polarity voltage from the first stable state. A pair of substantially perpendicular to the outside of the transparent substrate so that the first stable state is a low transmitted light state (OFF) and the second and third stable states are a high transmitted light state (ON). A selected pixel is turned on during a selection period in line-sequential scanning using a liquid crystal display element having a polarizing plate
When the selected pixel is OFF, a write voltage equal to or higher than the threshold value for transition from the first stable state to the second or third stable state is applied to the selected pixel. And a write voltage equal to or lower than the threshold value for the transition from the first to the second or third stable state, and the write voltage comprises a pair of pulses centered on an appropriately selected bias voltage. Immediately before erasing, one or more pulses having a polarity equal to or less than a threshold value for transition from the first stable state to the second or third stable state and having a polarity opposite to one of the pulses constituting the write voltage are used. A method for driving a liquid crystal display device, comprising applying a pulse voltage. 2. The method according to claim 1, wherein the erasing pulse comprises one to three pulses. 3. The method according to claim 1, wherein the erasing pulse comprises a pair of positive and negative pulses. 4. The method according to claim 1, wherein the erasing pulse voltage is applied at an early stage of a scanning electrode selection period. 5. The method according to claim 1, wherein the chiral smectic liquid crystal is an antiferroelectric liquid crystal.