JPH06242419A - Method for driving liquid crystal device - Google Patents

Abstract

PURPOSE:To maintain excellent optical characteristics at the time of storing operation for a ferroelectric liquid crystal device which can not be expected to be held in a sufficient surface stabilized state when no voltage is applied between electrodes. CONSTITUTION:The liquid crystal device controls double refraction based upon the optical anisotropy of ferroelectric liquid crystal 14 by applying a voltage between a couple of opposite electrodes 10 and 11 of the liquid crystal which has the electrodes 10 and 11 and a liquid crystal cell having the ferroelectric liquid crystal 14 between the electrodes 10 and 11. For the driving method of this liquid crystal device, a state is written in the liquid crystal cell by applying a write pulse voltage and the state written in the liquid crystal cell is held with a DC hold voltage following the write pulses; and the hold voltage has the same polarity with the write pulses and are much lower than the write voltage. In a period wherein a select signal is applied to a scanning line to select the liquid crystal cell, the write pulse voltage is applied through a data line, and then the hold voltage is applied successively, and then the scanning line is unselected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method of a liquid crystal device using a ferroelectric liquid crystal such as an optical shutter, an optical switch or an image display device.

[0002]

2. Description of the Related Art A liquid crystal device in which a ferroelectric liquid crystal is sandwiched between transparent electrodes and which utilizes an electro-optical effect is required when a cell thickness is small.
It has been found that it has characteristics not found in other liquid crystal devices, such as the speed of response observed when a voltage is applied and the memory property of optical anisotropy due to the surface stabilization state observed when a voltage is removed. . Utilizing this property, a simple matrix type liquid crystal device which turns on / off light by combining with birefringence due to polarized light is disclosed in US Pat.
924 are presented.

Regarding the memory property based on the surface-stabilized state of the ferroelectric liquid crystal, the response characteristic to an electric field, and the principle of the birefringent liquid crystal device, the publication "Structure and Physical Properties of Ferroelectric Liquid Crystal" (Atsuo Fukuda) , Hidezo Takezoe, Published by Corona Publishing Co., Ltd. on May 25, 1990), "Liquid Crystal Device Handbook"
(The 142nd Committee of Japan Society for the Promotion of Science, September 2, 1989)
9th, published by Nikkan Kogyo Shimbun), and the description is omitted here.

It is well known that manifestation of the memory state essential for the above-mentioned liquid crystal device is actually accompanied by considerable restrictions and technical difficulties in manufacturing. For example, the occurrence of a surface-stabilized state is
The materials currently used are limited to the case where the cell thickness is 2 μm or less, and above that, the bistable state cannot be maintained when the voltage is removed. Further, even if the thickness is 2 μm or less, there is generally a difference between the states when the voltage is applied and when the voltage is removed (at the time of memory), and it is extremely difficult to use in application to a system. Further, this difference greatly changes depending on the properties of the liquid crystal material used, the method of alignment treatment, and the conditions, which makes the manufacture of the ferroelectric liquid crystal device difficult.

FIG. 7 is a diagram showing a time-dependent change in drive voltage waveform and transmitted light intensity of a birefringent liquid crystal device in which a liquid crystal cell is sandwiched by two parallel polarizing plates. The typical variation of the transmitted light intensity when 53 and 54 are applied is shown. Assuming that the transmitted light intensity when a positive voltage is applied is Ion and the transmitted light intensity when a negative voltage is applied is Ioff, the extinction ratio R = Ion / Ioff, which represents the performance of the liquid crystal device, can be selected at a wavelength corresponding to a half-wave plate. For example, a value of 1000 or more has been achieved.

However, the intensity of the transmitted light changes as shown in FIG. 7 as the voltage is removed, and after a certain time, Imon and Imof, respectively.
becomes f. Therefore, there is a problem that the extinction ratio Rm = Imon / Imoff is significantly reduced as compared to when a voltage is applied. As a method of reducing this change, an improvement plan by a manufacturing technique such as a method of using a conductive resin for the alignment film has been proposed, but the process is complicated, the compatibility with the liquid crystal material is good, and it can be used for general purposes. Absent.

Therefore, a method of paying attention only to the high-speed property of the ferroelectric liquid crystal and realizing the memory property by adding an active element is presented in, for example, Japanese Patent Application Laid-Open No. 60-230121. An active matrix type liquid crystal device in which active elements are arranged in a matrix to drive a TN liquid crystal has been a well-known technique for a long time, and an AC voltage V is applied to the TN liquid crystal.
Two optical states are realized depending on whether rms is applied and the electric charge is retained or whether no voltage is applied (0 electric charge is retained).

When the TN liquid crystal is changed to a ferroelectric liquid crystal, it is well known that there are two optical methods depending on whether a positive voltage is applied to hold a positive charge or a negative voltage is applied to hold a negative charge. The state will be realized. However, if one of the states is continuously maintained, a positive or negative unipolar voltage is constantly applied to the liquid crystal cell during memory.
In general, it is well known that a liquid crystal cell suffers from burn-in or deterioration of the extinction ratio due to deterioration of orientation when a high DC voltage component is continuously applied for a long time, and such a situation is not desirable in terms of device reliability.

Therefore, a method of applying a pulse voltage (reset voltage) of opposite polarity at the time of writing a voltage has been proposed, but it is difficult to cancel the direct current component in the matrix type liquid crystal device, and the optical characteristics Deteriorates.

[0010]

As described above, when the ferroelectric liquid crystal is used, the optical characteristics at the time of memory are deteriorated. Therefore, the excellent characteristics at the time of applying a voltage are applied to the performance of the simple matrix type liquid crystal device. Could not be reflected. Further, there is a problem in that reliability cannot be maintained when a voltage is constantly applied due to the active matrix configuration.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a ferroelectric liquid crystal device in which a sufficient surface stabilization state cannot be expected when the voltage between electrodes is removed. Another object of the present invention is to provide a method of driving a liquid crystal device that maintains good optical characteristics in memory.

Another object of the present invention is to provide a driving method which gives good optical characteristics especially when a cell thickness of 2 μm or more is required such as an optical shutter in the infrared region.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0014]

In order to achieve the above object, the present invention provides a space between the electrodes of a liquid crystal device having a pair of electrodes facing each other and a liquid crystal cell in which a ferroelectric liquid crystal is sandwiched between the electrodes. In a method of driving a liquid crystal device, which controls birefringence based on optical anisotropy of the ferroelectric liquid crystal by applying a voltage to the liquid crystal cell, writing of a state of the liquid crystal cell is performed by applying a write pulse voltage, The state of being written in the cell is held by a DC holding voltage following the writing pulse, and the holding voltage has the same polarity as the writing pulse and is a voltage sufficiently lower than the writing voltage. .

Further, a plurality of pixel electrode groups arranged in a matrix and active elements connected to each of them.
Liquid crystal in which a ferroelectric liquid crystal is sandwiched between an active matrix substrate having scanning lines and data lines for selecting the active elements and an opposing substrate having an opposing electrode provided to face the pixel electrode group. In a method of driving a liquid crystal device, in which a birefringence based on optical anisotropy of the ferroelectric liquid crystal is controlled by applying a voltage between the pixel electrode group and a counter electrode of the liquid crystal device forming a cell, a scan line is While the liquid crystal cell is being selected by applying the selection signal, the write pulse voltage is applied via the data line, and subsequently, the holding voltage having the same polarity as the pulse voltage and a sufficiently low voltage is applied, and thereafter. It is characterized in that the scanning line is not selected.

[0016]

According to the above-mentioned means, the writing of the state of the ferroelectric liquid crystal cell is carried out with the same polarity of DC following the writing pulse and with a low holding voltage, so that the optical characteristics at the time of memory may deteriorate. Therefore, when the voltage between the electrodes is removed, good optical characteristics at the time of memory can be maintained for the ferroelectric liquid crystal device in which a sufficient surface stabilization state cannot be expected. Especially 2 μm for optical shutter in infrared region
Good optical characteristics can be provided when the above cell thickness is required.

[0017]

Embodiments of the present invention will be described in detail below with reference to the drawings. In all the drawings for explaining the embodiments, those having the same functions are designated by the same reference numerals, and the repeated description thereof will be omitted.

(Embodiment 1) FIGS. 1 and 2 are schematic configuration diagrams showing a schematic configuration of Embodiment 1 of a liquid crystal device according to the present invention. Specifically, when applied to a display device or an optical shutter device. It is a principle block diagram of. FIG. 3 shows the first embodiment.
FIG. 3 is a diagram showing a drive voltage waveform and the amount of light transmitted through the liquid crystal device of FIG.

1 to 3, 1 is a drive waveform, 2 is
Is the amount of light transmitted, 3 is a positive voltage pulse, 4 is a negative voltage pulse,
5 is the maximum amount of transmitted light, 6 is the minimum amount of light, 7 is the DC holding voltage V
st and 8 are dotted lines, 10 and 11 are transparent electrodes, 12 and 13 are glass substrates, 14 is a ferroelectric liquid crystal, 15 is a layer normal line, 16,
17 is a polarizing plate, 18 is an external drive circuit, 19 is a transmission axis, 2
Reference numeral 0 is a light beam, 21 is outgoing light, and 22 is a plane of polarization.

As shown in FIG. 1, the liquid crystal device according to the first embodiment includes a space between two glass substrates 1 having transparent electrodes 10 and 11 formed of, for example, an indium tin film (ITO).
A ferroelectric liquid crystal 14 (symbolically representing the molecular shape) is sandwiched between 2 and 13, and a layer normal line 15 of the ferroelectric liquid crystal is oriented so as to be parallel to the substrate surface to form a liquid crystal cell.

The thickness d of the liquid crystal layer is such that between the refractive index anisotropy Δn of the ferroelectric liquid crystal and the wavelength λ of light used,

[0022]

## EQU1 ## The relation of Δnd = λ / 2 (1) is established. A positive voltage or a negative voltage is supplied from the external drive circuit 18 between the transparent electrodes. 1 shows a state when a positive voltage pulse 3 equal to or more than the threshold value shown in FIG. 3 is applied, and FIG. 2 shows a state when a negative voltage pulse 4 shown in FIG. 3 is applied.

As is well known, when a voltage higher than a threshold value is applied, the ferroelectric liquid crystal molecules are realigned in the directions of tilt angles of ± θ with respect to the layer normal 15 by spontaneous polarization. Polarizing plates 16 and 17 are arranged outside the substrates 12 and 13 so that the transmission axis 19 is equal to the tilt angle + θ (or −θ) with respect to the layer normal 15.

As shown in FIGS. 1 and 2, when the molecular axis and the transmission axis are made to coincide with each other when the positive voltage pulse is applied, the molecular axis and the transmission axis are deviated by 2θ when the negative voltage pulse is applied. Therefore, when the light beam 20 is incident, when a positive voltage pulse is applied (Fig. 1)
In, the polarization plane 22 of the emitted light 21 does not change, the light is transmitted as it is, and the maximum transmitted light amount 5 is obtained (FIG. 3). On the other hand, when a negative voltage pulse is applied (FIG. 2), the polarization plane of the polarized light incident on the liquid crystal cell rotates 4θ on the exit surface. Now θ = 2
When the ferroelectric liquid crystal of 2.5 degrees is used, the plane of polarization is rotated by 90 degrees, so that the transmitted light is blocked by the polarizing plate 17 and the emitted light quantity gives the minimum light quantity 6 (FIG. 3).

Next, the operation (action) of the ferroelectric liquid crystal in the memory state will be described. In the prior art, the voltage was set to 0 V after the application of the write pulse voltage at the time of memory, but in the present embodiment, the DC holding voltage Vst7 having an appropriate polarity as shown in FIG. 3 is not set to 0 V after the application of the write voltage pulse. Apply. By applying the DC holding voltage Vst, it becomes possible to hold the alignment state of the liquid crystal molecules with almost no change in the writing state.

The write pulse voltage is usually 10 V / μm
While the DC holding voltage Vst is 1/5 to 1/100 of the writing voltage, it depends on the material used, the cell structure, and the process. Even if it is lower than the memory threshold voltage, it is sufficiently effective. For example, when the ferroelectric liquid crystal layer is sealed in a cell having a thickness of 5 μm, when a writing pulse voltage of ± 40 V is applied, high-speed and stable switching to an optically anisotropic state is exhibited when the voltage is applied. However, when voltage is removed (Vst = 0V)
(Corresponding to), the electrically oriented molecule immediately returns to a helical structure that does not exhibit an optically anisotropic state. Therefore, the amount of transmitted light is almost the same between positive voltage and negative voltage, as shown by the dotted line 8 in FIG. Therefore, when ± 2 V is applied as the holding voltage by the driving method of the present embodiment, the previously oriented state is maintained in almost the same state, and the transmitted light amounts become waveforms 5 and 6 that do not change when the pulse voltage is applied.

As described above, the present invention is effective for a ferroelectric liquid crystal cell having a large cell thickness such that the memory property due to the surface-stabilized state cannot be expected during non-writing. For example, when the wavelength of light to be controlled is in the infrared region of 1 μm or more, Δn of the current ferroelectric liquid crystal is 0.1 to 0.2,
According to the equation (1), the cell thickness needs to be 2.5 μm or more, and sufficient memory property due to the surface stable state cannot be expected. Of course, even when applied to an electro-optical device that uses a memory property due to a surface-stabilized state of 2 μm or less, an excellent effect of improving the extinction ratio during memory is exhibited.

The DC holding voltage Vst is preferably low in terms of reliability of the liquid crystal cell when a unipolar voltage is constantly applied. However, this is not the case in the case of driving in which rewriting is periodically performed and a reset pulse voltage of reverse polarity can be applied.

FIG. 4 shows such a driving waveform 24 and a transmitted light amount 25, which is composed of a write pulse 26, a holding voltage 27, and a reset pulse 28, and the negative polarity component (area) of the reset pulse is the write pulse. And the holding voltage can be made equal to the positive polarity component (area) to cancel the DC component and ensure high reliability.

(Embodiment 2) FIG. 5 is a schematic configuration diagram showing a schematic configuration of Embodiment 2 of a liquid crystal device according to the present invention, and specifically, it is applied to an active matrix type liquid crystal device using a thin film transistor as an active element. It is an example. In FIG. 5, 31 is a pixel electrode made of a transparent electrode such as indium tin film (ITO), 32 is a thin film transistor for switching, 33 is a gate line, 34 is a data line, and 35 is a counter electrode, which is electrically grounded here. There is. 38 is a glass substrate, 39 is a polarizing plate, 40 is a ferroelectric liquid crystal, 41 is a data line drive circuit, and 42 is a gate line drive circuit.

FIG. 6 is a diagram showing the drive waveform of the liquid crystal device of the second embodiment and the amount of light transmitted through the polarizing plate similar to that of the first embodiment. 43 is the data line 34 connected to the pixel to be written. With a voltage waveform, a positive or negative write signal voltage Vw is supplied at time t0. A voltage waveform 44 of the gate line 33 connected to the pixel to be written is a high voltage at time t1.

The operation (action) of the liquid crystal device of the second embodiment will be described with reference to FIG. At time t1, the thin film transistor 32
Is turned on, and the voltage 45 of the pixel electrode 31 is charged to the write signal voltage Vw. At the same time, the ferroelectric liquid crystal 40 is oriented in the electric field, for example, when the write signal voltage Vw is positive, it is oriented in the transmission axis direction of the polarizing plate, and when it is negative, it is oriented in the direction orthogonal to the transmission axis. Therefore, the transmitted light amount 46 of the pixel is
The maximum and minimum values are given when positive and negative voltages are applied.

At time t2, the data line voltage is lowered to the holding voltage Vst. As described in the first embodiment, the amount of transmitted light is maintained even in that state. Next, when the gate line voltage is set to a low voltage and the thin film transistor is turned off at time t3, electric charges for applying the voltage Vst are accumulated and held between the pixel electrodes. Therefore, the amount of transmitted light is maintained as it is, and a high extinction ratio can be maintained.

The holding time of the optical state depends on the discharge time constant determined by the capacitance of the liquid crystal layer, the resistance value of the liquid crystal layer and the thin film transistor, and the memory property of the ferroelectric liquid crystal. It is possible for several hours or more due to an increase in specific resistance, an increase in off resistance of the thin film transistor, and the like.

As described above, in the second embodiment, the write pulse voltage is applied through the data line and the holding voltage is continuously applied during the period in which the liquid crystal cell is selected by applying the selection signal to the scanning line. , And then the scanning line is deselected.

In the prior art, since the write voltage was stored in the pixel electrode as it was, there was a problem that the liquid crystal cell deteriorates when held for a long time. However, the holding voltage Vst becomes a sufficiently low voltage independently of the write voltage by the above driving method. Therefore, deterioration of the liquid crystal cell could be minimized. Conversely, since the writing voltage can be set to a higher voltage independently of the holding voltage, the switching speed of the ferroelectric liquid crystal can be further increased.

For example, when an active matrix type liquid crystal device having a liquid crystal layer with a thickness of 2 μm is formed by using a ferroelectric liquid crystal, the voltage which can be applied to the pixel by the conventional driving method is ± 7 V at most for ensuring reliability. , The response speed was about 300 μsec, but using the second embodiment, the write voltage was ±
The voltage can be set to 30 V and the response speed can be shortened to 50 μsec. The minimum holding voltage was ± 0.5 V, which was a sufficient value for ensuring long-term reliability. As described above, the application of the holding voltage is extremely effective as a driving method in the case of the active matrix configuration for utilizing only the high speed property of the ferroelectric liquid crystal, and the high reliability can be secured by the simple method. The reliability is further improved by applying a reset pulse of opposite polarity just before the write voltage is applied to cancel the DC component.

In the above-described embodiments, the display device or the optical shutter is used as an example of the applicable liquid crystal device, but the present invention can be similarly applied to other liquid crystal devices utilizing birefringence due to polarized light and has the same excellent performance. It is clear that the For example, in a beam shift type switch in which a liquid crystal cell and a polarization beam splitter are laminated, or in an optical processor that performs optical calculation by controlling the polarization plane of a liquid crystal cell in multiple layers, a high extinction ratio at the time of memory is ensured, It is effective as a method for improving switching and calculation accuracy and improving reliability. Especially, like the optical path switching optical switch for communication,
If the interruption of the optical path due to the reset pulse cannot be tolerated,
Since the DC voltage is continuously applied for a long time, the holding voltage has a great effect of improving the reliability.

Although the present invention has been specifically described based on the embodiments, it is needless to say that the present invention is not limited to the above embodiments and can be modified without departing from the scope of the invention.

[0040]

As described above, according to the present invention, the writing of the state of the ferroelectric liquid crystal cell is performed at the same time as the writing pulse with the same DC polarity and a low holding voltage. Since the optical characteristics of No. 1 are not deteriorated, good optical characteristics at the time of memory can be maintained for the ferroelectric liquid crystal device in which the surface stabilization state cannot be expected when the voltage between the electrodes is removed. That is, the present invention is effective in maintaining the optical state of a ferroelectric liquid crystal device having a thick cell thickness and a memory property which is unlikely to exhibit the memory property due to the surface-stabilized state when not writing.

Further, even when applied to a liquid crystal device using a memory property, an excellent effect of improving the extinction ratio at the time of memory is exhibited. Further, since the holding voltage Vst can be set to a sufficiently low voltage independently of the writing voltage, deterioration of the liquid crystal cell can be suppressed to the minimum. In other words, since the writing voltage can be set to a higher voltage independently of the holding voltage, the switching speed of the ferroelectric liquid crystal can be further increased.

This is particularly effective as a driving method in the case of an active matrix structure for making use of only the high speed of the ferroelectric liquid crystal, and has an advantage that high reliability and high speed can be realized by a simple method.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a schematic configuration of a first embodiment of a liquid crystal device according to the invention,

FIG. 2 is a schematic configuration diagram showing a schematic configuration of Example 1 of the liquid crystal device according to the invention,

FIG. 3 is a diagram showing a drive voltage waveform and an amount of light transmitted through the liquid crystal device according to the first embodiment of the present invention;

FIG. 4 is a diagram showing a drive voltage waveform and a transmitted light amount of the liquid crystal display device of the first embodiment,

FIG. 5 is a schematic configuration diagram showing a schematic configuration of a second embodiment of the liquid crystal device according to the present invention,

FIG. 6 is a diagram showing drive voltage waveforms and transmitted light amounts of the liquid crystal device of the second embodiment,

FIG. 7 is a diagram showing a drive voltage waveform and a change with time of transmitted light intensity according to a conventional typical drive method.

[Explanation of symbols]

1 ... Drive waveform, 2 ... Amount of transmitted light, 3 ... Positive voltage pulse, 4
... Negative voltage pulse, 5 ... Maximum transmitted light amount, 6 ... Minimum light amount, 7
... DC holding voltage Vst, 8 ... Dotted line, 10, 11 ... Transparent electrode, 12, 13 ... Glass substrate, 14 ... Ferroelectric liquid crystal, 1
5 ... Layer normal line, 16, 17 ... Polarizing plate, 18 ... External drive circuit, 19 ... Transmission axis, 20 ... Light beam, 21 ... Outgoing light, 2
2 ... Polarization plane, 24 ... Drive waveform, 25 ... Transmitted light amount, 26 ...
Write pulse, 27 ... Holding voltage, 28 ... Reset pulse, 31 ... Pixel electrode, 32 ... Thin film transistor, 33 ...
Gate line, 34 ... Data line, 35 ... Counter electrode, 38 ... Glass substrate, 39 ... Polarizing plate, 40 ... Ferroelectric liquid crystal, 41 ...
Data line drive circuit 42 ... Gate line drive circuit 43, 4
4 ... Voltage waveform, 45 ... Pixel electrode voltage, 46 ... Transmitted light amount, 53 ... Positive pulse, 54 ... Negative pulse

Claims (2)

[Claims] 1. An optical anisotropy of the ferroelectric liquid crystal by applying a voltage between a pair of electrodes facing each other and the electrodes of a liquid crystal device having a liquid crystal cell in which a ferroelectric liquid crystal is sandwiched between the electrodes. In the method of driving a liquid crystal device for controlling birefringence based on the property, the writing of the state of the liquid crystal cell is performed by applying a writing pulse voltage, and the holding of the state written in the liquid crystal cell is performed by a direct current following the writing pulse. A method of driving a liquid crystal device, which is performed by a holding voltage, the holding voltage having the same polarity as the writing pulse and a voltage sufficiently lower than the writing voltage. 2. An active matrix substrate having a plurality of pixel electrode groups arranged in a matrix, active elements connected to each of them, a scanning line and a data line for selecting the active elements, and the pixel electrode. By applying a voltage between the pixel electrode group and the counter electrode of the liquid crystal device that constitutes the liquid crystal cell by sandwiching the ferroelectric liquid crystal between the counter substrate and the counter substrate that has the counter electrode that is installed facing the group. In the method of driving a liquid crystal device for controlling birefringence based on the optical anisotropy of a ferroelectric liquid crystal, writing is performed via a data line while a liquid crystal cell is selected by applying a selection signal to a scanning line. A method of driving a liquid crystal device, comprising applying a pulse voltage, subsequently applying a holding voltage having the same polarity as the write pulse and a sufficiently low voltage, and then deselecting a scanning line.