JPS63143530A - Method for driving liquid crystal electrooptic device - Google Patents

Abstract

PURPOSE:To easily select liquid crystal to be used and to reduce power consumption by applying a voltage to liquid crystal molecules which do not have bistability from outside and applying a voltage to electrodes so that a produced electric field is in only one direction. CONSTITUTION:The liquid crystal molecules which do not have the bistability in a liquid crystal electrooptic device are applied with the voltage so that the electric field produced in the liquid crystal electrooptic device with the voltage applied from outside is in only one direction and thus the liquid crystal molecules are changed in state and the resulting electrooptic effect is utilized. In the cell structure of, for example, the 3X3 matrix liquid crystal electrooptic device, the electric field is produced in a part where different orienting films are formed on upper and lower substrates to make the ferroelectric liquid crystal molecules monostable, a unipolar pulse signal for placing X1-Y1 and X1-Y2 in an ON bright state is applied to electrodes X and Y to make a display, and even liquid crystal which is slow in response speed is usable.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a novel method for driving an electro-optical device using a ferroelectric liquid crystal.

rConventional technology Solid-state display devices that replace CRT are those that use liquid crystal materials,
A wide variety of devices have been developed, including those that utilize electrochromic phenomena and those that use gas discharge.

In particular, liquid crystal display devices are suitable for practical use due to their low power consumption and fast response speed, and their development has been particularly active.

However, recently, Jo! As the number of pixels increases, the number of pixels in one screen continues to increase. TN for a small number of pixels
Although display quality could be ensured even with display devices using liquid crystal materials, for example, in the case of matrix liquid crystal display devices with a large number of pixels, such as 640 x 400 pixels, the image quality deteriorates due to crosstalk etc. Image quality has been improved by using liquid crystals, by using SBE mode even when using TN liquid crystals, and by driving using semiconductor elements as switches for each pixel.

In a TN active matrix display device using semiconductor elements, the production cost for forming the semiconductor elements is high;
Furthermore, since the manufacturing yield of the device is low, it has been difficult to reduce the price of the display device itself. However, although the display image quality itself was good and the production price could be reduced through efforts such as mass production, the response speed of the liquid crystal material was slow,
It was unsuitable for displaying content that required high speed.

In addition, instead of this TN type liquid crystal, a liquid crystal electro-optical device using a ferroelectric liquid crystal was proposed by N. A. C1ark et al. As shown in FIG. 1, the liquid crystal molecules assume a first state, nja (I), which is tilted by +θ with respect to the normal direction of the smectic layer, and a second state (It), which is tilted by −θ.

Display was performed by applying an external electric field to switch between these two states to switch the ferroelectric liquid crystal molecules, thereby making use of the difference in birefringence effect generated.

At this time, in order to change the ferroelectric liquid crystal molecules from the first state (I) to the second state fi (n), for example, a positive electric field is applied in a direction perpendicular to the smectic layer.

Conversely, the second state M (II) makes the first state l (I)
In order to reverse it, it was done by applying a negative electric field. In other words, by changing the direction of the externally applied electric field, the 2
This method utilized the difference in electro-optical effect that occurs as a result of changing the state.

Furthermore, even when this externally applied electric field was removed, the ferroelectric liquid crystal molecules maintained their state stably and had the first and second bistable memory properties.

Therefore, the signal waveform that drives a liquid crystal electro-optical device using this ferroelectric liquid crystal is a bipolar pulse train, as shown in Figure 2, and the direction in which the pulse polarity switches changes the direction of the ferroelectric liquid crystal molecules. It was switching between two states.

This switching is performed much faster than in TN type liquid crystals, and even if this signal is removed, the state of the ferroelectric liquid crystal molecules is retained in memory.

However, in the method of matrix driving the ferroelectric liquid crystal using such a signal waveform, the response speed required of the ferroelectric liquid crystal must be 1/2 or less of the scanning selection time.

Furthermore, in order to switch between two states, two scans constitute one cycle, and for example, a screen with 400 scanning lines is scanned in one cycle.
In order to display 30 frames per second, the response time of the ferroelectric liquid crystal had to be 20 μsec or less.

Although such an ultra-high-speed response was theoretically possible with ferroelectric liquid crystals, it was extremely difficult to actually achieve it.

As described above, in the conventional driving method, the ferroelectric liquid crystal is required to have high-speed response, so there are very few usable ferroelectric liquid crystal materials.

Furthermore, the operating frequency of the driving IC also increases, resulting in an increase in power consumption. This was originally contrary to low power consumption, which is one of the characteristics of liquid crystal electro-optical devices.

Furthermore, in conventional liquid crystal electro-optical devices using ferroelectric liquid crystals, the ferroelectric liquid crystal molecules were required to have bistability, so the structure of the device was also designed to achieve bistability. Certain conditions had to be met. In other words, it was necessary to narrow the distance between the substrates that sandwich the ferroelectric liquid crystal to a distance that would achieve bistability.

The present invention provides a method for driving a liquid crystal electro-optical device that uses a liquid crystal exhibiting ferroelectricity and utilizes the electro-optic effect generated due to the difference in states that the liquid crystal molecules can take. Liquid crystal molecules do not have bistability within the liquid crystal electro-optical device, and the direction of the electric field generated within the liquid crystal electro-optical device is made to be in only one direction by the voltage applied to the liquid crystal from the outside. , which is characterized by changing the state of liquid crystal molecules by applying a voltage to the electrodes and utilizing the electro-optic effect generated accordingly.
A light-emitting display such as an RT has a pre-fluorescent response characteristic in which the rising response time to a scanning voltage pulse is fast (and the falling response time is relatively slow), and it does not flicker if the period is shorter than a certain period for periodic light stimulation. This can be said to take advantage of the characteristic of the human eye, which cannot be sensed.

By the way, the electro-optical response characteristics of ferroelectric liquid crystal are shown in Figure 4 (
It is as shown in A). This characteristic was measured using the measurement system shown in Figure 3, and the detector (
4) is displayed on an oscilloscope (5).

The characteristics shown in FIG. 4(A) were obtained when the signal waveform shown in FIG. 4(C) was added. If the pulse width of the signal waveform to be added is shortened, the memory property will no longer be exhibited as shown in FIG. 2(B).

In such a region, the signal waveform to be applied is shown in Figure 4 (
When the liquid crystal is unipolar as shown in D), the response characteristics of the ferroelectric liquid crystal are very similar to those of a CRT, as shown in FIG. 4(E).

In other words, it exhibits characteristics such that the rising response time to the applied signal waveform is fast and the falling response time is slow.

By using such a mode, it is possible to perform a display similar to that of a CRT using a ferroelectric liquid crystal.

In other words, ferroelectric liquid crystal molecules do not need to have bistability because they take advantage of the slow falling response time.
Furthermore, since the response speed only needs to be fast enough that the human eye does not perceive flicker, it has industrially useful features such as not requiring an ultra-high response speed of 20 μsec or less.

Furthermore, since the ferroelectric liquid crystal molecules are influenced weaker than the substrate interface, the response time for falling may be too slow. In such a case, by applying a bias voltage to the ferroelectric liquid crystal drive signal to make the ferroelectric liquid crystal molecules monostable, it is possible to speed up the falling response time.

In addition, instead of applying this bias voltage, the cell structure of the liquid crystal electro-optical device can be made into a special structure (for example, by using different types of alignment films on the top and bottom to generate an internal electric field inside the cell), ferroelectric liquid crystal Monostability of the molecule may be achieved.

Examples are shown below.

Embodiment j FIG. 5 schematically shows a liquid crystal electro-optical device used in this embodiment.

This is a 3×3 matrix liquid crystal electro-optical device. In the cell structure of the liquid crystal electro-optical device used in this example, different alignment films were formed on the upper and lower substrates to generate an electric field inside.

Therefore, ferroelectric liquid crystal molecules were monostable.

At this time, unipolar pulse signals as shown in FIG. 6 were applied to the X and Y electrodes, respectively, in order to turn X+-Y and X+-Yz into an ON (bright) state, and a display was performed.

The ferroelectric liquid crystal used in this example has a response speed of 0.
．． 5m5e, which had characteristics that made it impossible to use with conventional ferroelectric liquid crystal driving methods, but it became fully usable with the driving method of the present invention.

r effect" The present invention relates to a method for driving a liquid crystal electro-optical device that uses a liquid crystal exhibiting ferroelectricity and utilizes an electro-optic effect generated due to the difference in states that the liquid crystal molecules can take. A voltage is applied to the electrodes so that the device does not have bistability and the electric field generated in the liquid crystal electro-optical device by a voltage applied externally to the liquid crystal is in only one direction. It is characterized in that the state of the liquid crystal molecules is changed by adding it, and the electro-optic effect generated accordingly is utilized. That is, since bistability is not required, it has become possible to obtain a large industrial margin when manufacturing a liquid crystal electro-optical device.

By using the driving method of the present invention, the required response speed of the ferroelectric liquid crystal can be used even if it is slower than that using the conventional driving method, making it easier to select a usable ferroelectric liquid crystal. .

Further, since the driving method of the present invention does not require a high driving frequency, power consumption can be reduced and a special driving IC is not required.

[Brief explanation of the drawing]

FIG. 1 shows the appearance of ferroelectric liquid crystal molecules. FIG. 2 shows signal waveforms when using a conventional liquid crystal display driving method. FIG. 3 shows a measurement system for electro-optic effects. FIG. 4 shows the electro-optic effect on the drive signal waveform of the ferroelectric liquid crystal. FIG. 5 schematically shows a ferroelectric liquid crystal electro-optical device according to the present invention. FIG. 6 shows an example of the drive signal of the present invention.

Claims (1)

[Scope of Claims] 1. A method for driving a liquid crystal electro-optical device using a liquid crystal exhibiting ferroelectricity and utilizing electro-optic effects generated due to differences in possible states of the liquid crystal molecules, the liquid crystal electro-optical device Since liquid crystal molecules do not have bistability, a voltage is applied to the electrodes so that the direction of the electric field generated within the liquid crystal electro-optical device is only in a single direction due to the voltage applied externally to the liquid crystal. A method 2 for driving a liquid crystal electro-optical device, characterized in that the state of liquid crystal molecules is changed by adding a voltage to the electrode, and an electro-optic effect generated thereby is utilized. Liquid crystal electro-optical device driving method 3, characterized in that the liquid crystal molecules are aligned only in a single direction before applying , a method for driving a liquid crystal electro-optical device characterized by adding a bias voltage to an externally applied voltage.