JPH07294933A - Liquid crystal electro-optic element - Google Patents

Abstract

PURPOSE:To realize a liquid crystal electro-optic element which has a relatively low cost and is adapted to large-capacity display and is capable of gradation display and has a memory function. CONSTITUTION:With respect to the liquid crystal electro-optic element which is provided with chiral nematic liquid crystal having a twist structure in the initial state and has two arbitrarily selectable orientation states different from the initial state as relaxation states after the occurrence of Frederiks dislocation due to voltage application, plural areas A and B different by pretilt angles exist in each picture element, or not only plural areas A and B different by pretilt angles but also plural areas different by liquid crystal layer thickness exist in each picture element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal electro-optical element having a memory property using a chiral nematic liquid crystal.

[0002]

2. Description of the Related Art As one of liquid crystal electro-optical elements having a memory property, there is a system disclosed in Japanese Patent Publication No. 1-51818. It is equipped with a chiral nematic liquid crystal having a 180 ° twisted structure in the initial state, and has two freely selectable orientations different from the initial state as relaxed states after the Fredericks transition is generated once by applying a voltage. Have states (360 ° twisted state and 0 ° twisted state),
That is. In this liquid crystal electro-optical element, the alignment state used for display is 360 ° twisted state and 0
There are two twisted states.

Since the switching between these two states is such that the Freedericksz transition is generated once and then relaxed to either state, there is no so-called intermediate state that exists in, for example, a super twist nematic liquid crystal. Therefore, it is impossible to perform gradation display by adjusting the applied voltage.

In general, a dither method (Japanese Patent Laid-Open No. 62-70815) and a time-division gray-scale method (Japanese Patent Laid-Open No. 62-9320) are used as means for displaying gradation in such a display device having a memory property.
Would be used.

[0005]

However, the dither method has the drawbacks that the voltage waveform is distorted and the cost of the circuit becomes high. This is because the dither method is to divide each pixel (each stripe-shaped electrode) into a plurality of dots (a plurality of stripe-shaped electrodes) and apply a different voltage to each dot. The width of the narrowest one is at least 1/2 the width of one pixel
The reason for this is as follows, and the resistance of the electrode becomes higher than that in the case where no division is made. Further, if the scanning electrode is divided into n, the number of drivers on the scanning side becomes n times.

Next, the time division gray scale method is to display gray scales on a time average by changing the time for selecting the bright state per unit time, so that scanning must be performed at a sufficiently high speed. Flicker will occur. Alternatively, in order to prevent flicker from occurring, the number of scanning lines must be extremely reduced, which is not applicable to large-capacity display.

The present invention is intended to solve the above problems, and its object is to have a relatively low cost.
There is a realization of a liquid crystal electro-optical element having a memory property and capable of gradation display, which is also suitable for large-capacity display.

[0008]

The invention according to claim 1 is
A liquid crystal having a chiral nematic liquid crystal having a twisted structure in an initial state and having two arbitrarily selectable alignment states different from the initial state as a relaxation state after the Fredericks transition is generated once by applying a voltage. In the electro-optical element, each pixel has a plurality of regions having different pretilt angles θp (the angle between the major axis direction of the liquid crystal molecules in contact with the liquid crystal alignment film and the glass substrate surface). .

In addition to the invention described in claim 1, the invention described in claim 2 is characterized in that a plurality of regions having different liquid crystal layer thicknesses are present inside each pixel.

[0010]

According to the first aspect of the present invention, since a plurality of regions having different pretilt angles θp exist inside each pixel, the threshold voltage in each region has a different value.
Therefore, if the applied voltage is changed stepwise, gradation display can be performed similarly to the dither method without dividing the pixel into a plurality of dots.

According to the second aspect of the present invention, in addition to the first aspect of the present invention, a plurality of regions having different liquid crystal layer thicknesses are present inside each pixel, so that the threshold voltage in each region is only the effect of θp. Not only that, but also due to the effect of the liquid crystal layer thickness, the values become different. Therefore, as compared with the case where only the effect of θp is used, it is possible to prevent the difference in threshold voltage between the regions from becoming larger than necessary. If the effect of θp and the liquid crystal layer thickness are used at the same time, the number of regions having different threshold voltages, that is, the number of gradation levels can be increased as compared with the case of using only the effect of θp.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

(Embodiment 1) The operation principle of the liquid crystal electro-optical element according to the present application will be briefly described. However, the case where the twist structure in the initial state is a 180 ° twist state will be described. First, a relatively large voltage pulse (reset pulse) is applied in the reset period to cause the Freedericksz transition, and then a write pulse is applied in the selection period following an appropriate delay time (including 0 second). If the absolute value of the write pulse is smaller than the threshold value, the 360 ° twist state (assuming a dark state) is selected, and if the absolute value of the writing pulse is greater than the threshold value, the twist angle is almost 0 ° in a uniform state (a temporary state is assumed to be a bright state). ) Is selected.

An important point in the present invention is that the threshold voltage is greatly influenced by the pretilt angle (θp) as shown in FIG. FIG. 3 is a diagram showing the relationship between the threshold voltage for selecting the uniform state and θp. As can be seen from this figure, if θp changes by several degrees, the threshold voltage changes by several volts. . By utilizing this feature, gradation display by voltage modulation becomes possible.

FIG. 1 is a schematic sectional view of a liquid crystal electro-optical element according to the first aspect of the invention. 1 is a glass substrate, 2 is a striped transparent electrode (ITO), 3 is a polarizing plate, 4 is a sealing material, 5 is a liquid crystal layer, 6 is an insulating layer (SiO ₂ ), 7 is a liquid crystal alignment film A, and 8 is a liquid crystal. This is the alignment film B. A pixel is a portion where stripe-shaped transparent electrodes provided on two glass substrates intersect each other. This figure is a schematic view of a liquid crystal electro-optical element in which pixels are arranged in 2 rows × 2 columns, but actually, a liquid crystal electro-optical element in which pixels are arranged in 480 rows × 640 columns was prepared. Two types of liquid crystal alignment films are provided for each pixel on the glass substrate, which can be formed by photolithography or a printing technique. However, the area ratio between the area A and the area B in each pixel is drawn as 1: 1, but the ratio is arbitrary. Although an insulating layer is provided on both of the two glass substrates, this is also optional.

Since θ p has a different value depending on the material of the liquid crystal alignment film, the regions A and B have different θ p. Therefore, the threshold voltage of each region has a different value according to FIG. Specifically, as liquid crystal alignment film A and liquid crystal alignment film B, liquid crystal alignment film materials SE7111 and SE3140 (both manufactured by Nissan Chemical Co., Ltd.) that give θp of 6.9 ° and 4.2 °, respectively.
Was used. The film thickness is 150Å in all cases.

Rubbing treatment was performed after forming two types of liquid crystal alignment films on both substrates, and the directions were made antiparallel to each other between the upper and lower substrates. The thickness of the liquid crystal layer
1.84 μm, the optical activator (E. Merck
Manufactured by: S811 as a nematic liquid crystal (manufactured by E. Merck: ZLI-33)
The composition in which an appropriate amount was mixed with 29) was used as a liquid crystal material.

FIG. 2 shows an example of a drive voltage waveform of this liquid crystal electro-optical element. (A) is a voltage waveform applied to the scan electrodes, and (b) is a voltage waveform applied to the signal electrodes. A voltage pulse with a peak value ± Vr is a reset pulse, a voltage pulse with a peak value ± VS is a selection pulse, and a peak value ± Vd
The voltage pulse of is a data pulse for selecting ON / OFF. The signal electrode has Vd depending on the selected state.
Is applied in synchronization with the selection pulse, and the peak value VW of the write pulse applied to the liquid crystal layer during the selection period is ± (VS-Vd). In this embodiment, tr
= 1 msec, td = 84 μsec, ts = 42 μsec, and Vr = 25V. Since tS = 42 μsec and the number of scanning lines is 480,
The frame frequency is about 50Hz, and flicker does not occur.

Region A (θp = 6.9 °) and Region B (θp =
The threshold voltages at 4.2 ° were 7.5 V and 9.1 V, respectively, so that VS = 9.3 V, Vd = 0.0 V when selecting a perfect bright state, and Vd when selecting an intermediate state.
d = 1.0V, and Vd when selecting a completely dark state
= 2.0V realizes 3 gradation display.
Specifically, when Vd = 0.0V, both areas are in a bright state, when Vd = 1.0V, only area A is in a bright state, and when Vd = 2.0V, both areas are in a dark state and halftone is performed. Was displayed.

According to the above-mentioned structure, since it is not necessary to divide the pixel into a plurality of dots, it is possible to avoid the problem that the voltage waveform is distorted due to the increase in the resistance of the transparent electrode and the cost of the circuit is increased due to the increase in the number of drivers. By changing the applied voltage stepwise, gradation display can be performed similarly to the dither method. Further, since the present invention is not a time division gradation method, there is no problem of flicker.

(Embodiment 2) FIG. 4 is a schematic sectional view of a first example of a liquid crystal electro-optical element according to the invention described in claim 2. The difference from Example 1 is that two types of liquid crystal alignment films are laminated. In this case, since the thickness of the liquid crystal layer is different between the regions A and B, the threshold voltage is affected not only by θp but also by the difference in the liquid crystal layer thickness. FIG. 5 is a diagram showing the relationship between the threshold voltage and the liquid crystal layer thickness. As can be seen from this figure, the increase amount of the threshold voltage with the increase of the liquid crystal layer thickness is more than the compensation of the decrease of the electric field with the increase of the liquid crystal layer thickness. This means that the range of the liquid crystal layer thickness that enables bistable switching between the 360 ° twist state and the uniform state is limited, and if the liquid crystal layer is too thin, it becomes monostable only in the uniform state, and the liquid crystal layer becomes This is due to the fact that if it is too thick, it will be monostable only in the 360 ° twist state. Therefore, the thickness of the liquid crystal layer is several hundreds.
Å Even if it changes, the threshold voltage changes by several volts.

In FIG. 4, since the region A has a thicker liquid crystal layer than the region B, the threshold voltage of the region A is higher than that of the region B if θp in each region is equal. When a liquid crystal alignment film A having a larger θp than that of the liquid crystal alignment film B is used and the liquid crystal layer thicknesses are equal to each other, the threshold voltage of the region A becomes lower than that of the region B due to the effect of θp.
Θ depending on the material of the liquid crystal alignment film, its film thickness, and rubbing strength
Although it is possible to control p to some extent, it is not possible to freely set θp to a desired value.Therefore, if the thickness of the liquid crystal layer is also changed depending on the region, the threshold voltage of each region can be set to a desired value. it can.

Specifically, in this embodiment, as the liquid crystal alignment film A and the liquid crystal alignment film B, liquid crystal alignment film materials SE7111 and SE3140 which give θp of about 6.9 ° and 4.2 °, respectively, were used. Further, since the liquid crystal alignment film has a thickness of 150 Å, the liquid crystal layer thickness difference between the regions is 0.03 μm. However, the thickness of the liquid crystal layer in the region A is 1.84 μm. Area A (θp = 6.
Since the threshold voltages in 9 °) and region B (θp = 4.2 °) were 7.5 V and 6.9 V, respectively, in the drive waveform shown in FIG. Vd = 0.0V when selecting, and V when selecting an intermediate state
d = 0.5V, and Vd when selecting a completely dark state
= 1.0V realizes 3 gradation display.
Specifically, when Vd = 0.0V, both areas are in a bright state, when Vd = 0.5V, only area B is in a bright state, and when Vd = 1.0V, both areas are in a dark state and halftone is performed. Was displayed.

The same type of liquid crystal alignment film as in Example 1 was used, but the maximum value of Vd was smaller than that in Example 1. This is because the difference in threshold voltage between the regions A and B is smaller than that in the first embodiment because the effect of the thickness of the liquid crystal layer is used together. As Vd increases, the effective value of the voltage applied during the non-selection period also increases, and the stability of the selected state tends to decrease. This is
When a large-sized display body is created, unevenness due to the thickness of the liquid crystal layer or the location of the temperature, or scratches or dust on the surface of the liquid crystal alignment film due to rubbing may lead to defective display. Therefore, if the effect of the liquid crystal layer thickness is also used as in this embodiment,
Even if the same type of liquid crystal alignment film is used, the stability of the selected state can be further improved.

Here, the thickness of the liquid crystal layer is changed by laminating two kinds of liquid crystal alignment films, but the thickness of the liquid crystal layer is changed by forming a step in the insulating layer or the glass substrate. You may let me.

(Embodiment 3) FIG. 6 shows a first example of the second example of the liquid crystal electro-optical element according to the invention described in claim 2.
3 is a schematic diagram of a pixel. FIG. 6A is a perspective view.
(B) is a plan view of one pixel. In this figure,
The insulating layer and the polarizing plate are not shown for simplification. In the above-described embodiment, the two types of liquid crystal alignment films A and B respectively provided on the upper substrate and the lower substrate are completely overlapped with each other so that the liquid crystal layer is vertically sandwiched by the liquid crystal alignment films of the same type. However, in this embodiment, a region in which the liquid crystal layer is sandwiched by different kinds of liquid crystal alignment films is also formed.
One pixel is divided into four regions, but if it is distinguished by a combination of liquid crystal alignment films, it is divided into three regions. Therefore, these areas are set to AA, A
Let us call them B and BB. The region AA is a region in which the liquid crystal layer is sandwiched between the liquid crystal alignment film A in the upper and lower sides, the region AB is a region in which it is sandwiched between the liquid crystal alignment film A and the liquid crystal alignment film B, and the region BB is in the liquid crystal alignment both in the upper and lower directions. Membrane B
It is the area sandwiched by. Since the threshold voltage in the region AB is a value between the thresholds in the regions AA and BB, 4-gradation display can be performed.

As the liquid crystal alignment film A and the liquid crystal alignment film B, a liquid crystal alignment film material SE71 which gives θp of about 6.9 ° and 4.2 °, respectively.
When 11 and SE3140 were used and the thickness of the liquid crystal layer was set to 1.84 μm, the threshold voltages in the regions AA, AB, and BB were 7.5 V, 8.2 V, and 9.1 V, respectively. Therefore, in the drive waveform shown in FIG. 2, VS = 9.3V and Vd is 0.0
4 gradation display was realized by selecting from 4 types of V, 0.6V, 1.4V and 2.0V. Specifically, Vd = 0.0V
, All areas are in the bright state (level 1), and V
When d = 0.6V, only area BB is in dark state (level 2)
When Vd = 1.4V, the regions BB and AB are in the dark state (level 3), and when Vd = 2.0V, all the regions are in the dark state (level 4).

By simultaneously using the effect of θp and the liquid crystal layer thickness as described above, the number of gradation levels can be increased as compared with the case of using only the effect of θp (Example 1).

(Embodiment 4) FIG. 7 shows a first example of the third example of the liquid crystal electro-optical element according to the invention described in claim 2.
It is a schematic sectional view and a schematic plan view of a pixel. Also in this figure, the insulating layer and the polarizing plate are not shown for simplification. The difference from the third embodiment is that one pixel has three areas (AA, AB,
It is divided into three equal parts in BB). for that reason,
Even in terms of brightness, it is possible to display four gradations equally divided.

Since the same kind of liquid crystal alignment film A and liquid crystal alignment film B as in Example 3 were used, equal gradation four-gradation display was realized under the same driving conditions as in Example 3.

(Embodiment 5) FIG. 8 is a view showing a liquid crystal electro-optical device according to a second embodiment of the present invention in the fourth example.
3 is a schematic diagram of a pixel. 8A is a perspective view, and FIG.
(B) is a plan view. Also in this figure, the insulating layer and the polarizing plate are not shown for simplification. Here, two kinds of liquid crystal alignment films are laminated, and a region where the liquid crystal layer is sandwiched by different kinds of liquid crystal alignment films is also formed. Further, the film thicknesses of the liquid crystal alignment films B provided on the upper and lower substrates are different from each other. In a plan view, one pixel has four areas AA, BA, AB,
It is divided into BB. Regions AA, BA, AB, and BB are regions in which the liquid crystal alignment films sandwiching the liquid crystal layer vertically are A and A, B and A, A and B, and B and B, respectively.

If the liquid crystal layer thickness in the region AA is dAA and the film thicknesses of the liquid crystal alignment film B provided on the upper and lower substrates are respectively represented by dBU and dBL, the liquid crystal layer thicknesses dBA, dAB in the regions BA, AB, BB,
dBB is dBA = dAA-dBU and dAB = dAA-dB, respectively.
L, dBB = dAA- (dBU + dBL). Therefore, dBU
If ≠ dBL, the liquid crystal layer thicknesses of the four regions have different values. Further, the display of x: y in FIG.
(Θp on upper substrate): (θp on lower substrate) is shown. In general, since θp is different when the film thickness of the liquid crystal alignment film is different, in the present embodiment in which the film thickness of the liquid crystal alignment film B is different between the upper and lower sides, there is no region in which the upper and lower θp combinations are completely the same. Therefore, in this embodiment, all four regions have different threshold voltages, so that five-level gradation display is realized.

SE3140 and SE7111 were used as the liquid crystal alignment film A and the liquid crystal alignment film B, respectively. The thickness of the liquid crystal alignment film A is
Since dBU = 80Å and dBL = 160Å as 150Å, the combination of θp in each region is as shown in FIG. 8 (b). Also, since dAA = 1.84 μm, dBA = 1.832
μm, dAB = 1.824 μm, and dBB = 1.816 μm.

The threshold voltages in the regions AA, BA, AB and BB were 9.1V, 7.9V, 7.1V and 5.9V, respectively, so that Vs is -1.8V, -1.0V, 0.0 with VS = 7.5V.
By selecting from 5 types of V, 1.0V and 1.8V, 5 gradation display was realized. Specifically, when Vd = -1.8V, all areas are in the bright state (level 1), and Vd =
When -1.0V, only area AA is in the dark state (level 2), when Vd = 0.0V, area AA and area BA are in the dark state (level 3), and when Vd = 1.0V, area A
A, BA and AB are in a dark state (level 4), Vd = 1.8
At V, all areas were in the dark state (level 5).

In this embodiment, the liquid crystal layer thickness is changed for each combination of the upper and lower liquid crystal alignment films by stacking two kinds of liquid crystal alignment films, but there is a step difference in the insulating layer / transparent electrode or the glass substrate. If the liquid crystal layer is provided so as to have a change in thickness, the liquid crystal layer can have a change in thickness even in the combined region of the liquid crystal alignment films. For example, as shown in FIG. 9, if the glass substrate provided with a step in the insulating layer is replaced with the lower glass substrate of the liquid crystal electro-optical element shown in FIG. 8A, both the area AA and the area BA are formed. , And is further divided into two regions having different liquid crystal layer thicknesses. Since one pixel is divided into a total of 6 areas, 7-level gradation display is possible. Furthermore, if the upper glass substrate is also replaced with the glass substrate shown in FIG. 9, gray scale display of 10 levels becomes possible.

In all of the above-mentioned examples, two kinds of liquid crystal alignment films were used, but both were rubbed with the same strength, and θp was changed depending on the material of the liquid crystal alignment film. However, since θp varies depending on the baking temperature and the rubbing strength of the liquid crystal alignment film, one type of liquid crystal alignment film may be used to adjust these to form regions having different θp. For example, as a means of changing the baking temperature of the liquid crystal alignment film depending on the region, a method of first preparing a liquid crystal alignment film that is baked at high temperature and then partially forming a liquid crystal alignment film that is baked at low temperature is used. is there. As a means for changing the rubbing intensity depending on the region, there is a method of covering with a mask and rubbing with different rubbing intensity depending on the region. In any case, it suffices that one pixel can be divided into a plurality of regions having different θp, and any means therefor does not matter.

[0037]

As described above, according to the present invention, a liquid crystal electro-optical element having a memory property, which has a relatively low cost and does not cause flicker, is suitable for a large-capacity display and is capable of gradation display. Can be realized. The present invention can be applied not only to a display device but also to an optical shutter such as a printer-head.

[Brief description of drawings]

FIG. 1 is a diagram showing a liquid crystal electro-optical element according to the present invention.

FIG. 2 is a diagram showing drive waveforms of a liquid crystal electro-optical element according to the present invention.

FIG. 3 is a diagram showing a relationship between a threshold voltage and a pretilt angle of a liquid crystal electro-optical element according to the present invention.

FIG. 4 is a sectional view of a liquid crystal electro-optical element used in Example 2 of the present invention.

FIG. 5 is a diagram showing the relationship between the threshold voltage and the liquid crystal layer thickness of the liquid crystal electro-optical element according to the present invention.

FIG. 6 is a diagram showing a liquid crystal electro-optical element used in Example 3 of the present invention.

FIG. 7 is a diagram showing a liquid crystal electro-optical element used in Example 4 of the present invention.

FIG. 8 is a diagram showing a liquid crystal electro-optical element used in Example 5 of the present invention.

FIG. 9 is a diagram showing an example of a glass substrate used in the liquid crystal electro-optical element according to the present invention.

[Explanation of symbols]

 1 ... Glass substrate 2 ... Transparent electrode 3 ... Polarizing plate 4 ... Seal material 5 ... Liquid crystal layer 6 ... Insulating layer 7 ... Liquid crystal alignment film A 8 ... Liquid crystal alignment film B

Claims (2)

[Claims] 1. A chiral nematic liquid crystal having a twisted structure in an initial state is provided, and two arbitrarily selectable different from the initial state are provided as a relaxation state after the Fredericks' transition is generated once by applying a voltage. A liquid crystal electro-optical element having an alignment state, wherein each pixel has a plurality of regions having different pretilt angles. 2. A chiral nematic liquid crystal having a twisted structure in an initial state is provided, and two arbitrarily selectable different from the initial state are set as a relaxed state after the Fredericks transition is generated once by applying a voltage. In the liquid crystal electro-optical element having an alignment state, each pixel has a plurality of regions having different pretilt angles, and each pixel has a plurality of regions having different liquid crystal layer thicknesses. Liquid crystal electro-optical element that can