JPH0199031A - Driving method of electro-optical device - Google Patents

Abstract

PURPOSE:To erase the display in a short time even in case of a liquid crystal panel for showing an abnormal hysteresis by executing the erasion by applying both of a pulse voltage and '0' or about '0' volt, in an electro-optical device having a hysteresis characteristic. CONSTITUTION:In a device which has utilized a fact that an applied voltage and transmittivity cause a hysteresis phenomenon in a high twist state exceeding 90 deg., in an area where an abnormal hysteresis is shown especially when sweeping speed becomes high, a period te in which a holding voltage Vk is dropped to '0' from an ON state and erased becomes very long. Therefore, a high pulse for erasing the voltage being higher than the holding voltage is applied to the head of an erasion period for a time ts (5-0.2msec), and subsequently, an erasing waveform for applying zero volt is used. According to this erasing waveform, the erasing time te is shortened surely.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for driving an electro-optical device that can be used as a display device, a transmitted light amount control device, etc.

[Prior Art J] Conventionally, various electro-optical devices have been developed. Among these, electro-optical devices using liquid crystals have a relatively simple structure and are widely used in applications such as display devices and optical shutters due to their features such as small size, light weight, and low power consumption.

In manufacturing a liquid crystal electro-optical device, when increasing capacity/area, using a simple 7 matrix is more advantageous than using an active matrix, but in a conventional liquid crystal electro-optical device using a simple matrix, It is difficult to manufacture large-capacity devices due to limitations in electro-optical properties. For example, in one using twisted nematic mode, 1/2
00 duty, about 1/40 even in S TN/S B E mode, which has high performance by increasing the angle.
When the duty is about 0, there is a problem that display quality or response speed deteriorates, making it impractical for practical use.

Therefore, memory is created using liquid crystals and their orientation control (
J, A that utilizes hysteresis characteristics due to applied voltage
ppl, Phys, , sword, 3087. This method takes advantage of the fact that depending on the liquid crystal material, a hysteresis phenomenon occurs between the applied voltage and the transmittance in a highly twisted state exceeding 90 degrees. This is achieved by applying a holding voltage that falls within the hysteresis loop.Hereinafter, in the present invention, this method will be abbreviated as the HTN method.

In an electro-optical device using this HTN method, various conditions must be satisfied in order for hysteresis characteristics to appear in the electro-optical characteristics. That is, the electro-optical device (the twist angle, cell thickness, pretilt angle, spontaneous pitch of the liquid crystal composition, elastic constant,
The state of expression of hysteresis characteristics differs depending on the state of interaction between the alignment film and liquid crystal molecules. In general, the pretilt angle is large, the twist angle is large, the interfacial regulating force is large, the ratio of elastic constants K ''s/K + +, Kri3/ and 22 are both small, Δε/ε is small, and the liquid crystal cell ΔP (= Pc/Ps −
There is a tendency that hysteresis characteristics are more likely to occur when 1) is in the small direction.

[Problems to be solved by the invention] However, it depends on other conditions such as interfacial regulating force and elastic constant, but for example, it is possible to make △P extremely small, or in other words, reduce the amount of chiral dopant. If the cell thickness is increased, the twisting force of the liquid crystal composition will no longer reach the twist angle regulated by the cell side as a container, and a so-called low twist domain will occur, making it unusable as an electro-optical device. It disappears.

The region between the region where this low twist domain occurs and the region where normal hysteresis characteristics are expressed becomes an unstable region, which may exhibit an abnormal hysteresis phenomenon (hereinafter referred to as abnormal hysteresis).

Normally, when measuring the electro-optical characteristics of a liquid crystal panel having hysteresis characteristics, the sweep speed of the driving voltage is made as small as possible. However, hysteresis characteristics are often affected by the sweep speed of the drive voltage during measurement. Even in the case of HT N type liquid crystal panels,
In the region showing the hysteresis characteristic of Tsuka, if the sweep speed is made very slow, the driving voltage rise process will be as shown in Figure 2a,
The driving voltage falling process exhibits the characteristics shown in Figure 2 (S), but when the sweep speed is increased, the hysteresis characteristics change as shown in Figure 2 (C) in the driving voltage rising process and Figure 2 (D) in the driving voltage falling process. It shows characteristics such that the width of the width is different. However, in the above-mentioned abnormal hysteresis region, this tendency is further exacerbated, and if the sweep speed of the drive voltage is slow enough, normal hysteresis characteristics as shown in Figure 2 a and b are exhibited, but the sweep speed of the drive voltage is If is fast, then
In the process of increasing the driving voltage, it shows almost the same characteristics as in Figure 2C, but in the process of decreasing the driving voltage, it shows characteristics not shown in Figure 2e, and even if you try to erase it by turning the voltage to zero from the ON state, the display will not work. It takes a very long time to disappear.

As shown in Figure 3, the concept of the drive waveform in the conventional HT N method is as follows: a holding period th1 during which the previous display state is maintained;
an erasure period te in which the previous display state is erased by applying zeropol); a write period tw in which a new display state is written;
Furthermore, the holding period consists of ++2 during which the display state is maintained by applying the holding voltage vh that falls within the hysteresis loop.

Therefore, from the ON state of the holding voltage vh, apply zero volts? 111 corresponds to the case where the sweep speed is high as described above, and the erasing time becomes extremely long for liquid crystal panels exhibiting hysteresis characteristics, which is extremely difficult to use as an electro-optical device. There was a problem.

Therefore, an object of the present invention is to provide a method for driving an electro-optical device using the HTN method, which can widen the allowable range of ΔP and facilitate the manufacture of liquid crystal panels.

[Means for Solving the Problems] In the present invention, in a method of driving an electro-optical device having a hysteresis characteristic, which is driven by applying a holding voltage to hold written display contents, erasing is performed using a pulse voltage. Embodiment The present invention will be described in detail below based on embodiments.

(Example 1) As an electro-optical device, HTN with 750' x 1120 pixels
We prototyped a liquid crystal panel using this method. For orientation treatment, SiO
The pretilt angle of liquid crystal molecules is set to 25 degrees by oblique evaporation of
The twist angle of the liquid crystal between the upper and lower substrates was set to 270 degrees, and the liquid crystal was injected to form a liquid crystal panel.

This panel was filled with Z L I-3187 manufactured by Merck and CB-15 manufactured by BDH as a chiral dopant, thereby preparing a liquid crystal panel.

The appearance of hysteresis characteristics differs depending on the cell manufacturing conditions such as interfacial regulation force, but in the inventors' experiments, for example, Merck's ZL was used in a liquid crystal panel with a cell thickness of 4.4 μm.
For I-3187, a liquid crystal containing 2.43 to 2.17 wt% of CB-15 from BDH as a chiral dopant was injected, and the value of ΔP was set to -0.05 to -0.15. Normal hysteresis characteristics were obtained. In addition, the amount of CB-15 added is 2.05w-1% or less,
When ΔP was made smaller than -0.2, low twist domains were often generated. The value of △P is -0,15
Although no low twist domain occurs in the range of ~-0.2, abnormal hysteresis may or may not occur depending on the manufacturing conditions of the liquid crystal panel.

Among these liquid crystal panels, for example, liquid crystal panels that exhibit normal hysteresis characteristics with ΔP of -0 and 1 have a temperature of 30°C.
In the conventional HTN erasure method of applying zero volts shown in FIG. 3, erasure was possible in 13 m5ec. On the other hand, for liquid crystal panels exhibiting abnormal hysteresis with ΔP of -0 and 2, the erasing waveform of the conventional HTN method requires an erasing time of several hundred m5ec at the fastest, and several seconds at the slowest. On the other hand, Fig. 1 (a) or (b)
Not shown in J: By applying a high voltage erase pulse higher than the holding voltage at the beginning of the erase period, and then applying zero volts to the erase waveform, we were able to make the erase time extremely short. . The high pulse application time ts for erasing is set to 5
The value of the erasing time τe (msec) for the high pulse voltage Ve for erasing when 0, 2 m5ec is the first
Shown in the table.

At this point, we are not at the stage where we can confirm the mechanism, but O
The N state, that is, the state in which the liquid crystal molecules stand up due to writing, is a metastable state, and in order to go from this state to the OFF state, that is, the state in which the liquid crystal molecules are horizontally aligned, some kind of potential wall must be overcome. Although we have to overcome this problem, we believe that by applying an erase pulse, the energy is given from the outside, and erasing due to natural relaxation may be faster.

From Table 1, it can be seen that it is better for the erasing high pulse to have a high voltage and a short application time.

As described above, although the principle of the phenomenon remains in the realm of speculation, it is a fact that according to the present invention it is possible to shorten the erasing time.

[Example 2] As an electro-optical element, a 400 x 640 pixel (TN type) panel was prototyped. The orientation treatment was the same as in Example 1.
SiO oblique evaporation, pre-tilt angle of 25 degrees, twist angle of 270 degrees, cell thickness of 6.0 μm, Merck &Co.'s Z L
I-1132 was filled with liquid crystal to which CB-15 from BDH was added as a chiral dopant.

In this liquid crystal panel, the amount of CB-15 added was 1.
90~1.1.1wt%, △P value is 0.3~-0
．． In the region 2, normal hysteresis characteristics are obtained, and CB
-15 addition amount is 1.07~1゜02wt%, △
Hysteresis occurs in the range of P from -0.25 to -0.3. In addition, the amount of CB-15 added was 1.02wt.
% or less, and when ΔP is -0.3 or less, it becomes a region where low twist domains occur. △P is -0,2~-0
The region of °25 is unstable and shows hysteresis,
It may not be shown.

When the erasing time was measured in the same manner as in Example 1 using a liquid crystal panel exhibiting hysteresis with ΔP of -0.3 or more,
At 25°C, under the conditions of erase pulse voltage Ve=20V and erase pulse application time te=0.5 m5ec, the erase waveform of the conventional HTN method shown in FIG. With the erasing waveforms of the present invention shown in FIGS. (a) and (b), erasing was possible in 40 m5ec.

Although the embodiments have been described above, the present invention is not limited to the above embodiments, but can be applied to any device that applies a voltage other than zero volts during the erase period, and can be applied to any voltage, pulse width, etc. Needless to say, this does not impose any restrictions.

[Effects of the Invention] As described above, according to the present invention, in a method for driving an electro-optical device having a hysteresis characteristic that is driven by a method of applying a holding voltage to hold written contents, Even if a liquid crystal panel exhibits abnormal hysteresis, which cannot be used because practical erasing cannot be performed, it becomes possible to erase the display in a short time, increasing the manufacturing margin of the liquid crystal panel and making it easier to manufacture. It has the great effect of becoming.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the concept of drive waveforms of the present invention. Figure 2 shows the hysteresis characteristic +1. FIG. 12 is a diagram illustrating electro-optical characteristics 11: of an electro-optical device having the following. FIG. 3 is a diagram showing the concept of drive waveforms in the conventional HTN method. Above Figure 1 7i Figure 2-12 = Figure 3

Claims (1)

[Claims] (1) In a method of driving an electro-optical device having a hysteresis characteristic, which is driven by applying a holding voltage to hold written display contents, erasing is performed by applying both a pulse voltage and a voltage of zero or approximately zero volts. 1. A method for driving an electro-optical device, characterized in that the driving method is performed by: