JPH05203913A - Ferroelectric liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the degradation in the durability occurring in the transfer of a ferroelectric liquid crystal by providing a means for heating up this liquid crystal up to the next high-temp. side liquid crystal phase by a planar heating element. CONSTITUTION:The planar heating element (heater) 10 is fixed to, for example, either of a pair of substrates. The temp. of a liquid crystal display section 101 is put as temp. data through a driving control circuit 108 into a scanning signal control circuit 106 and a heater control circuit 105 by a temp. detecting element 111. The ferroelectric liquid crystal is heated from, for example, a smectic C phase up to at least the transition phase of the next high-temp. side, for example, a smectic A phase, by a heating up means 104. The transfer of the liquid crystal is then eliminated when the liquid crystal is slowly cooled. The change in the cell thickness by the transfer of the liquid crystal molecules and the change in the threshold value by writing of a fixed pattern are corrected and the normal control of the liquid crystal is restored. The heating up may be up to the high-temp. side liquid crystal phase at this time. Since the heating up and the slow cooling are executed in a relatively low temp. region, the restoration is executed relatively early.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device having a liquid crystal element used for a liquid crystal display element or a liquid crystal optical shutter, and more particularly to a ferroelectric liquid crystal device having improved durability.

[0002]

2. Description of the Related Art A ferroelectric liquid crystal device is disclosed in Japanese Patent Laid-Open No.
As shown in 1-94023, etc., 1 to 3 μm
It is known that a ferroelectric liquid crystal is injected into a cell formed by facing two glass substrates, each of which has a transparent electrode formed on the inner surface of the m-side cell gap and is subjected to an orientation treatment.

The features of the above device using a ferroelectric liquid crystal are as follows:
Since the ferroelectric liquid crystal has spontaneous polarization, the coupling force between the external electric field and the spontaneous polarization can be used for switching, and the long-axis direction of the ferroelectric liquid crystal molecule is the polarization direction of the spontaneous polarization.
Since it corresponds to the pair 1, it is possible to switch depending on the polarity of the external electric field.

Since a chiral smectic liquid crystal (SmC *, SmH *) is generally used as the ferroelectric liquid crystal, the long axis of the liquid crystal molecules is twisted in the bulk state. Twisting of the long axis of the liquid crystal molecule can be eliminated by putting it in the cell of the gap.

Ferroelectric liquid crystals have characteristics such as high-speed response and bistability, but also have the following problematic characteristics. That is, the ferroelectric liquid crystal has a switching threshold value that changes significantly with temperature, and the response becomes slower as the temperature decreases.The ferroelectric liquid crystal has a different structure depending on temperature, and has a layered structure in the chiral smectic phase. However, in the low temperature region, it changes from the chiral smectic phase to the crystalline state, and once it is crystallized, it does not return to the chiral smectic phase even at room temperature, but after heating to the cholesteric phase or the isotropic liquid crystal phase. , Must be slowly cooled (this heat treatment is called reorientation),
When a liquid crystal panel is subjected to mechanical pressure, the layer structure of the ferroelectric liquid crystal collapses, and alignment defects such as sandet texture tend to occur.Displaying an image in this state impairs the regular switching of the ferroelectric liquid crystal. A good image cannot be obtained, and in this case as well, there is no self-healing property, and reorientation processing is required.

Therefore, in order to solve the above-mentioned problems, a liquid crystal display device incorporating a flat plate-shaped heating element (hereinafter referred to as a heater) as described in JP-A-1-237521, has been conventionally used. A liquid crystal display device incorporating a heater as shown in FIG. 6 has been considered.

[0007]

However, when the conventional cell structure is used, the durability of the liquid crystal cell further has the following problems. It is known that liquid crystal molecules also move to some extent by a non-selection signal during matrix driving. This is also clear from the fact that when the optical response of the pixel to which the non-selection signal is applied is examined, the light amount changes in synchronization with the applied pulse. In the so-called splay orientation (orientation in which the angle of the long axis of the molecule is greatly twisted between the upper and lower substrates), such fluctuations in the molecule retain the displayed content unless the stable position of the molecule changes (switching). Since it is possible, there is no problem except a slight decrease in contrast.

However, in a cell with uniform alignment, in which the change in the angle of the long axis of the molecule between the upper and lower substrates is relatively small, the phenomenon that liquid crystal molecules move in the layer when a voltage (for example, a non-selection signal) is applied. Can be seen. A prominent example of this phenomenon will be described in detail with reference to FIG. Figure 4 (a)
FIG. 3B is a front view of the ferroelectric liquid crystal cell showing a cell state before voltage application, and FIG. In this cell, the alignment treatment is performed by rubbing a polyimide thin film. In both FIGS. 4A and 4B, the upper and lower substrates are parallel to each other from the bottom to the top of the drawing. When such alignment treatment is performed, the smectic layer 4 of the ferroelectric liquid crystal 44 is formed.
2 is generated in a direction orthogonal to the rubbing direction 41, as shown in FIG.

When the cell thickness is thin enough to release the helical pitch, the liquid crystal molecules 43 can be in two stable states, but one of them can be brought into one state so that all the molecules 43 in the cell are aligned. Once this state is shown in FIG.
As shown in (d), if a + θ state is formed by forming an angle of + θ with the normal vector 46 of the smectic layer 42, another stable state exists at a position of −θ that is almost symmetrical with respect to the layer line. To do. If an electric field (for example, a rectangular wave of 10 Hz, ± 8 V) is applied to the entire surface of the cell under this state (+ θ),
The liquid crystal molecules keep the inclination with respect to the + θ layer line, as shown in FIG.
Start moving in the layer from point A to point B in (a).
As a result, when the voltage application is continued for a long time, the thickness of the liquid crystal cell changes, and finally, as shown in FIG. Is thicker than part A.

When the liquid crystal molecules are in the state of −θ, such a phenomenon is caused by the liquid crystal moving in the layer from the B end to the A end, and the void like the E part where there is no liquid crystal. Part occurs at the B end.

In addition, although the cause is not clear, a change in Vth (threshold voltage) due to each fixed pattern (pattern sticking of liquid crystal molecules, monostable) is observed, and the write pattern is changed even if the drive frequency and drive voltage are changed. You can also see the phenomenon that remains.

The existence of such an electro-optically uncontrollable portion is not desirable in terms of display quality, and since the cell thickness or Vth changes with time, it is difficult to control the driving of the entire liquid crystal panel and the ferroelectricity is increased. It becomes a problem as an optical element using liquid crystal, and it is + in practical use.
Durability is a problem in fixed patterns such as non-display areas around the display area that are driven only in either θ or −θ and semi-fixed pattern areas such as the word processor menu screen. There is.

In view of the above problems of the prior art, it is an object of the present invention to prevent the deterioration of the durability of the ferroelectric liquid crystal display device due to the movement of the liquid crystal.

[0014]

In order to achieve the above object, a ferroelectric liquid crystal display device of the present invention includes a ferroelectric liquid crystal between a pair of substrates having a matrix electrode formed of a scan electrode and an information electrode. And a means for heating the ferroelectric liquid crystal up to the next high temperature side liquid crystal phase by means of a plate-shaped heating element.

The high temperature side liquid crystal phase is, for example, a smectic A phase, and after the temperature is raised to this phase, the ferroelectric liquid crystal is gradually cooled again to the ferroelectric liquid crystal phase. Therefore, it is preferable to provide a means for performing such temperature rise and subsequent gradual cooling at the end of driving the apparatus, for example. The flat heating element is fixed to, for example, one of the pair of substrates. The alignment state of the ferroelectric liquid crystal element is, for example, uniform alignment.

[0016]

In this structure, when the temperature is raised from the ferroelectric liquid crystal phase, for example, the smectic C phase, to at least the next transition phase on the high temperature side, for example, the smectic A phase, the liquid crystal moves when gradually cooled. And the change in cell thickness due to the movement of liquid crystal molecules and the change in threshold Vth due to writing of a fixed pattern (pattern burn-in of liquid crystal molecules, monostabilization).
Is restored and normal control of the LCD is restored. At that time, the temperature may be raised up to the next liquid crystal phase on the high temperature side, and the temperature is raised and gradually cooled in a relatively low temperature region, so that the recovery is relatively early. Such recovery processing can be performed, for example, when the system (or soft application) switch is turned off.

According to this, practically sufficient durability performance of the device can be obtained. Further, it is also possible to use a conventional recovery control system in a relatively high temperature region for realignment treatment in which the temperature is raised to a cholesteric phase or an isotropic liquid crystal phase on the higher temperature side than the smectic A phase and gradually cooled.

[0018]

1 is a block diagram of a liquid crystal display device according to an embodiment of the present invention. In the figure, 109 is a graphic controller, 108 is a drive control circuit, 105 is a heater control circuit, 106 is a scanning control circuit, 107 is an information control circuit, 104 is a heater (a flat plate-shaped heating element), 102 is a scanning signal applying circuit, Reference numeral 103 is an information signal application circuit, 101 is a liquid crystal display unit, 110 is a temperature detection element, and 111 is a temperature detection circuit. The data sent from the graphic controller 109 passes through the drive control circuit 108 to the heater control circuit 1
05, the scanning signal circuit 106, and the information signal circuit 107, and are converted into address data and display data, respectively. Further, the temperature of the liquid crystal display unit enters the temperature detection circuit 111 through the temperature detection element 110, and enters the scanning signal control circuit 106 and the heater control circuit 105 through the drive control circuit 108 as temperature data. Then, the scanning signal applying circuit 103 generates a scanning signal according to the address data and the display data, and applies the scanning signal to the scanning electrodes of the liquid crystal display unit 101. Further, the information signal applying circuit 103 applies the information signal to the information electrode according to the display data.

FIG. 6 is a sectional view of this device. 6 in the figure
00 is a liquid crystal panel, and 601 and 602 are liquid crystal panels 6.
Glass substrate 003, 603 an upper polarizing plate, 604
Is a glass for bonding the upper polarizing plate, 605 is a lower polarizing plate,
Reference numeral 606 is a glass for laminating the lower polarizing plate, 607 is a backlight, and 608 is a diffusion plate, and these constitute the liquid crystal display unit 101. 609 is a printed wiring film, 610 is a circuit board, 611 is a panel fixing board, 612 is an adhesive, 613 is a supporting member, 614 is an elastic member, 615 is a housing, 616 is a frame-shaped supporting member, 104 is a heater, and 618 is Adhesive, 619 is heater control circuit 10
A heater control board having 5 and 110 is a temperature detecting element.

With this structure, multiplexing driving is performed using the driving waveform shown in FIG. 3, and after continuously displaying an image as shown in FIG. 5 for a predetermined time, the temperature of the panel surface is controlled by a heater to about 75 ° C. A process of performing a recovery process of gradually increasing the temperature and then gradually cooling was performed several times to confirm a change in cell thickness and a change in threshold Vth. However,
Among them, S1, S2, and S3 represent voltage waveforms applied to scanning signals, and I1, I2, and I3 represent voltage waveforms applied to typical information lines. In FIG. 5, reference numeral 51 (area within the broken line) is a display area, 53 is a frame display portion, 41 is a rubbing direction, 42 is a smectic layer, and 43 is a liquid crystal molecule. The driving voltage is 15V, and the liquid crystal used is
It has the characteristics shown in the following chemical formula 1.

[0021]

[Chemical 1] The results are shown in FIGS. 2 (a) and 2 (b). Figure 2 (a) shows
Change rate (%) of threshold Vth with respect to endurance display time (days)
The black circles represent the values after continuous durability display, and the white circles represent the values after the recovery process. From FIG. 2 (a), the rate of change is 1
At 0% or more (NG points), it can be seen that the drive margin including various variations disappears and the display quality deteriorates.

FIG. 2B is a graph showing the change in cell thickness (μm) with respect to the durability display time (days). Figure 2 (b)
It can be seen from the results that when the cell thickness changes by 0.2 μm or more, empirically, color unevenness that is noticeable to the user's eyes occurs, and the display quality deteriorates.

2 (a) and 2 (b), the heater temperature is raised within the continuous operation within 48 hours (here, the panel surface is 75 ° C.).
It is understood that practically sufficient performance can be satisfied by carrying out the recovery process by the set smectic A phase state).

Further, as shown by the curve 21 in FIG. 2C, if the reorientation is usually carried out at about 95 ° C. (in the ISO phase), it takes 45 minutes from the temperature rise to the gradual cooling. However, when it is carried out at about 75 ° C. in the state of smectic A phase as shown by the curve 23 in FIG. 2 (c), it can be carried out in about 15 minutes. Thus, the recovery time can be further increased depending on the temperature characteristics of the ferroelectric liquid crystal element.

[0025]

As described above, according to the present invention, means for raising the temperature of the ferroelectric liquid crystal to the next high temperature side liquid crystal phase by the flat heating element is provided. By raising the temperature to the phase and gradually cooling it, the cell thickness change due to the movement of the liquid crystal molecules and the change in the threshold Vth due to the fixed pattern writing (pattern burning of the liquid crystal molecules, monostabilization) are repaired, and normal control of the liquid crystal is performed. You can recover. At that time, the temperature may be raised up to the next high temperature side liquid crystal phase, and since the temperature raising and the incipient cooling can be performed in a relatively low temperature region, the recovery process can be performed relatively quickly. And this
The durability performance of the device can be improved to a level where there is no practical problem.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a ferroelectric liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a graph showing a recovery effect and a recovery control time in the device of FIG.

FIG. 3 is a waveform diagram of a drive signal used when performing multiplexing driving on the apparatus of FIG.

FIG. 4 is an explanatory diagram of a movement phenomenon of liquid crystal molecules.

5 is a schematic diagram showing a display pattern used in the apparatus of FIG.

6 is a cross-sectional view of the device of FIG.

[Explanation of symbols]

101: liquid crystal display unit, 102: scanning signal application circuit, 10
3: information signal application circuit, 104: heater (planar heating element), 105: heater control circuit, 106: scanning control circuit, 107: information control circuit, 108: drive control circuit, 1
09: graphic controller, 110: temperature detecting element, 111: temperature detecting circuit, 600: liquid crystal panel, 60
1, 602: glass substrate, 603: upper polarizing plate, 604:
Glass, 605: lower polarizing plate, 606: glass, 607:
Backlight, 608: Diffusion plate, 609: Printed wiring film, 610: Circuit board, 611: Panel fixed board, 619: Heater control board

Claims (5)

[Claims] 1. A ferroelectric liquid crystal device having a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix electrode formed of a scanning electrode and an information electrode, and a plate-shaped heat generation of the ferroelectric liquid crystal. A ferroelectric liquid crystal display device comprising: a means for raising the temperature to the next high temperature side liquid crystal phase by the body. 2. The high temperature side liquid crystal phase is a smectic A phase, and after the temperature is raised to this phase, the ferroelectric liquid crystal is gradually cooled again to the ferroelectric liquid crystal phase. 1. The ferroelectric liquid crystal display device according to 1. 3. A means for performing the temperature rise and the subsequent gradual cooling at the end of driving the apparatus,
The ferroelectric liquid crystal display device according to claim 1. 4. The ferroelectric liquid crystal display device according to claim 1, wherein the flat heating element is fixed to one of the pair of substrates. 5. The ferroelectric liquid crystal display device according to claim 1, wherein the alignment state of the ferroelectric liquid crystal element is uniform alignment.