JPS61254932A - Liquid crystal electrooptic device - Google Patents

Abstract

PURPOSE:To increase a response speed and to obtain high contrast by applying alternately a liquid crystal composite with electric fields whose frequencies are lower and higher than a cross frequency at which dielectric anisotropy is zero. CONSTITUTION:When an electric field of frequency fL lower than the cross frequency where the dielectric anisotropy is zero is applied to a cholesteric phase having the planar structure of the liquid crystal composite whose dielectric anisotropy is positive at the lower frequency and negative at a higher frequency fH, a force operates to direct axes of molecules in the direction of the electric field, so that a nematic phase is entered. When a voltage of frequency fH is applied, on the other hand, the axes of molecules are directed at right angles and transition to not focal conic structure, but the planar structure is made. Thus, electric fields of different frequencies are applied alternately to obtain high-speed switch operation having not memory property and the spiral pitch of the cholesteric phase is made smaller to increase the contrast of a phase shift type guest-host mode display.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a liquid crystal electro-optical device.

[Summary of the invention]

The present invention provides a liquid crystal electro-optical device in which a liquid crystal composition forming a cholesteric phase is held between a pair of substrates, and means for controlling the cholesteric-nematec structure transition of the liquid crystal composition on the substrate is provided. Using a liquid crystal whose dielectric anisotropy is positive at frequencies lower than the crossover frequency at which its dielectric anisotropy becomes zero and negative at frequencies higher than the crossover frequency, an electric field at a frequency lower than the crossover frequency and an electric field at a frequency higher than the crossover frequency are used. is applied to control the cholesteric-nematec phase transition of the liquid crystal composition with an electric field, thereby making possible reversible optical switching with a high contrast ratio and a fast response speed.

[Conventional technology]

It is generally known that applying an electric field to a cholesteric phase having a planar structure causes a phase transition to a nematic phase. Optical switching that utilizes this phase transition has been extensively studied. For example, there are known methods that apply the fact that circular dichroism in the cholesteric phase disappears in the nematic phase, and methods that incorporate dichroic dyes into liquid crystals and apply changes in light absorption rate accompanying phase transition to switching. ing. In particular, a liquid crystal optical switch mode that does not use a polarizing plate and uses an electric field-induced liquid crystal phase transition (cholesteric-nematec phase transition) that combines a host liquid crystal and a dichroic dye (guest) is attracting attention as a practical device. In the cholesteric phase with a Graenner structure (Fig. 5 (α)), dichroic dye molecules are distributed with their molecular axes oriented in arbitrary directions on a plane parallel to the substrate surface, so they are difficult to absorb incident light. Efficient absorption.On the other hand, the electric field-induced homeotropically oriented nematic phase (Fig. 3(b))
Since the dichroic dye molecules have their molecular axes oriented in a direction perpendicular to the substrate surface, incident light is transmitted according to the dichroic ratio. The above-mentioned principle enables optical switching. The phase change type guest-host display is characterized in that it does not require a polarizing plate and has almost no viewing angle dependence.

[Problems and Objectives to be Solved by the Invention] However, the above-mentioned prior art has the following problems.

(1) The response from an electric field-induced nematic phase to a cholesteric phase with a planar structure is extremely slow.

(2) It is not possible to increase the contrast ratio.

The problem with (1) is explained as follows. During the phase transition from the electric field-induced nematic phase ((α) in Figure 5) to the Graenar structure ((b) in Figure 5), the planar structure passes through the focal conic structure as shown in C in Figure 3. Return to organization. In the 7-ohal chalconic tissue, absorption of light by pigments decreases, and a phenomenon called light scattering also occurs.

Since it remains in this focal conic structure for a long time (memory phenomenon), the response speed becomes extremely slow (2 in Fig. 4). On the other hand, the above problem (1) becomes more pronounced as the helical pitch of the cholesteric phase becomes shorter. Therefore, in order to obtain a relatively fast response speed, the number of spirals in the cholesteric phase cannot be increased. This means that the light absorption efficiency of the dichroic dye in the precursor structure cannot be increased, and for this reason it is difficult to increase the contrast ratio. The present invention solves the above-mentioned problem, and aims to provide fast response speed.
The purpose of the present invention is to provide a phase change type liquid crystal electro-optical device with high contrast.

[Means to solve the problem]

A liquid crystal electro-optical device of the present invention is a liquid crystal electro-optical device in which a liquid crystal composition forming a cholesteric phase is held between a pair of substrates, and means for controlling the cholesteric-nematec phase transition of the liquid crystal composition with an electric field is provided on the substrates. The liquid crystal composition is a liquid crystal in which the dielectric anisotropy is positive at frequencies lower than the crossing frequency at which the dielectric anisotropy of the liquid crystal composition becomes zero, and negative at higher frequencies; The method is characterized in that the phase transition of the liquid crystal composition is controlled by the electric field (double-frequency driving) by applying an electric field with a higher frequency and an electric field with a lower frequency than the crossing frequency. Furthermore, the liquid crystal composition is characterized in that it contains a dichroic dye. Furthermore, the present invention is characterized in that an active switching element is used as the means for controlling the electric field.

[Effect]

The operation of the electro-optical device of the present invention will be explained using FIG. 1 and FIG. 6. According to the present invention, the liquid crystal composition held between a pair of substrates exhibits positive dielectric anisotropy at frequencies lower than the crossover frequency and negative dielectric anisotropy at frequencies higher than the crossover frequency, depending on the frequency of the applied electric field. Respond as if you have. (
Figure 6).

For example, consider the case where a liquid crystal composition having dielectric dispersion as shown in FIG. 6 is used. /L is the frequency at which a liquid crystal with positive dielectric anisotropy responds.

Let fH be the frequency at which the liquid crystal responds as having negative dielectric anisotropy. Cholesteric phase with Brenner structure (
When an electric field of /L is applied to Fig. 5 (1), a force that directs the molecular axis parallel to the direction of the electric field acts to transform it into a nematic phase (Fig. 5). On the other hand, when an electric field of frequency/H is applied, the liquid crystal molecules behave as a dielectric with negative dielectric anisotropy, and thus tend to orient the molecular axes in a direction perpendicular to the direction of the electric field. Furthermore, since the present liquid crystal composition is optically active, it forms a helical structure oriented perpendicular to the direction of the electric field and returns to the original Graenner structure. In this way, the transition from the nematic phase to the Graenner structure can be made without passing through the focal conic structure, so that the switching speed can be greatly improved. Furthermore, in the guest-host mode, the nematic-cholesteric phase transition can be reversibly controlled regardless of the helical pitch, so the light absorption efficiency of the guest dye in the planar structure 1 can be increased by increasing the number of spirals. As described above, in the cholesteritter nematic phase change type liquid crystal electro-optical device, a liquid crystal that can have either positive or negative dielectric anisotropy depending on the magnitude of the applied electric field frequency is used.

By alternately applying two different frequency electric fields corresponding to negative dielectric anisotropy, high-speed switching without memory properties is possible. Furthermore, since the number of spirals can be increased (the spiral pitch can be shortened), high contrast can be achieved.

[Example 1] Transparent electrodes were provided on a pair of transparent substrates, a polyimide film was formed on the substrates, and rubbing treatment was performed so that the rubbing direction was 180° between the upper and lower substrates. The orientation method is not limited to the above-mentioned rubbing method, but since planarization torque is applied by high frequency, the orientation does not matter much, for example, 2, 1.
1. Tilt orientation, combinations thereof, etc. are possible. Further, an oblique vapor deposition method may be used instead of the rubbing treatment. A cell having a cell thickness of 8 μm was created on both of the above substrates with a spacer interposed therebetween. A chiral nematic liquid crystal composition containing a dye was sealed in the liquid crystal cell under vacuum. The chiral nematic liquid crystal composition used was 3514 (Roche) containing an optically active agent Z.
Add 4 wt% of L Any 11 and further add 6 wt% of guest dye.
It contains wt%. The helical pitch of the chiral nematic liquid crystal composition is approximately 2 μm, which means that the composition is twisted by 8π within the cell.

FIG. 1 shows the characteristics of the liquid crystal electro-optic cell. The response characteristic when input signal 1 is applied to the liquid crystal cell is 2. The input signal 1 consists of an alternating current signal with a low frequency of 50 Hz and a high frequency of 10 KHz. The response characteristics were a rise time of 110m8 and a fall time of 30m8.

The fall time was extremely fast because the transition from the nematic phase to the cholesteric phase occurred without passing through the focal conic structure. Also, for the high/# modification component, there is no need to apply the non-selected period raw metal in the time domain, and here at least 30
If ms is applied, the structure shifts to a planar structure.

During the remaining time, even in the absence of an electric field, the planar structure stably exists due to the interfacial orientation force. The transmittance 5 when selected (when a low frequency electric field is applied) is 60%, and the transmittance 4 when not selected (when a high frequency electric field is applied) is 7%, and a contrast ratio of about 10 was obtained. Next, when the low frequency voltage value of the input signal is raised from OV to 20V and then returned to Ov as shown in 1 in Figure 1, the transmittance when selected is 5 and the transmittance when not selected is 4.
was investigated (Figure 2). When no high-frequency electric field is applied during non-selection (Fig. 2 (α)), that is, in the case of single-frequency drive, when the low-frequency voltage is increased, the change in transmittance 1 during selection and the transmission during non-selection are shown. The rate change 5 is different from the transmittance change 2 during selection and the transmittance change 4 during non-selection when the low frequency voltage is lowered. In other words, it showed hysteresis. When a high-frequency electric field is applied during non-selection and dual-frequency driving is performed (Figure 2 (b)), the transmittance change is 6 when non-selected and 5 when selected.
showed a reversible change even when the low peripheral 81M pressure was increased or decreased. That is, the liquid crystal electro-optical device of the present invention is capable of not only a simple optical switch but also analog modulation that continuously changes the transmitted light.

[Example 2] Next, an anti-glare mirror as shown in FIGS. 7(C) and (d) was created. A reflective surface 4 was formed on a substrate by thinning aluminum, chromium, nickel, etc. by physical vapor deposition such as vacuum deposition or sputtering. A polyimide film was formed on the transparent glass substrate with transparent electrodes and the above-mentioned substrate with a reflective surface, and subjected to rubbing treatment to obtain an alignment film. A chiral nematic liquid crystal composition was encapsulated in an 8 μm liquid crystal cell via a spacer under vacuum to obtain an anti-glare mirror. The chiral nematic liquid crystal composition contains 52 wt % of a black dye using 5514 (Roche Co., Ltd.) containing 4 vrt % of an optically active agent as a guest dye. This anti-glare mirror can handle low frequency voltage from 0 to 12
7, the reflected light 12i1 for the emitted light 1
It can be arbitrarily changed within a wide range from 0% to 45%.

In particular, from the non-glare state Fig. 7 (α) to the anti-glare state Fig. 7 (b)
It has the characteristics that the switching can be performed at high speed, and that the reflected image is extremely easy to see even in the anti-glare state because there is no light scattering associated with the focal conic structure.

[Example 3] The liquid crystal electro-optical device of the present invention was applied to a color display for characters. FIG. 8 shows the structure and driving method of the color character display of this embodiment. This embodiment will be explained below using FIG. 8.

A polysilicon thin film transistor (TIFT) was formed as a switching element on a transparent quartz glass 4 in the shape of an IJ rack. A color filter layer 2 was formed on a counter substrate 1 by a photolithography technique, a printing method 1, a dye evaporation layer method 1, etc., and a transparent common electrode 5 was provided thereon. A cell having a thickness of approximately 8 .mu.m was assembled using both of the above substrates via a spacer and a gap agent, and a liquid crystal composition was sealed in the cell under vacuum to obtain a character color display. The liquid crystal composition used contained 5421 (Roche) as a host liquid crystal, 4.2 wt % of 2L Knee 811 as an optically active agent, and 6 wt % of black pigment as a guest dye.

FIG. 8Cb) shows a driving waveform for character display. 8 is a video signal input to the source 6, 9 is a signal input to the gate section 5, and 10' is a signal input to the common electrode 5 (Nikkei Electronics Volume 551.?2)
11.1984) Here, when a display pixel switches from a selected state to a non-selected state, or when the display screen changes, a high-frequency electric field is applied from the common electrode 3 using one frame period 11. . In this example, one frame period is 35
It was set as m5. By applying a high-frequency electric field, all display pixels become temporarily in an inhuman state, but after one frame (55y, e
(after), new information is glanced at, so the screen changes to a new information screen without any visual unnaturalness. If the above information is to be displayed as it is for a long time, it is not necessary to apply a high frequency electric field to non-selected pixels every frame, and it is sufficient to apply no field. This is because the cholesteric phase having a Grainner structure formed by the high-frequency electric field is stably held by the alignment force at the substrate interface.

The contrast ratio of this display was 8. It also has one layer of color filters in which the five primary colors of red, green, and blue are arranged in a mosaic pattern, making it possible to display multiple colors. Full-color display is also possible by appropriately modulating the value of the video voltage. In addition, since the display mode of this display does not require a polarizing plate, the light transmission in the on state is 25%, which is 5% of the normal TN color display.
%, it is much brighter. That is, no special backlight is required, and color information can be displayed on a suitable scattering plate. Furthermore, since this display is a guest-host display, there is no viewing angle dependence.As described above, when the liquid crystal electro-optic device of this research was applied to a display, a color display with no viewing angle dependence and a contrast ratio that poses no problem in practical use was achieved. It has been found that this can be provided without the need for a special light source.

[Example 4] The liquid crystal electro-optical device of the present invention was applied to a display for displaying characters. FIG. 9 shows the structure of the liquid crystal display of this embodiment. A data line 5 was formed of tantalum on a transparent substrate 3, and an insulating layer 6 was provided on the data line using tantalum oxide. The pixel electrode 4 was formed using chromium nickel nickel gold, and an M element was fabricated in the form of a matrix. A transparent electrode 2 was formed on the counter substrate 1 as a source line. After forming a polyimide film on the above pair of substrates and performing a rubbing treatment, a 7.5 μm liquid crystal cell was assembled via a spacer and a gap agent, and a liquid crystal composition was sealed in a vacuum to obtain a character display display. The liquid crystal composition used 3421 (Roche) as a host liquid crystal and 3wt of 0B-15 (Merck & Co.) as an optically active agent.
%, and 6 wt % of black pigment was used as a dichroic pigment.

FIG. 10 shows a method of driving the M-engine M-character liquid crystal display of this embodiment. 1 is the signal input to the source line,
2 indicates a signal input to the data line, and 3 indicates the electric field value applied to the M element and the liquid crystal by the input signal 2.2. A driving waveform shown in 4 is applied from the pixel electrode to the liquid crystal layer on the pixel electrode in accordance with the waveform 3. In this embodiment, as in Embodiment 2, one frame cycle (
After applying a high-frequency electric field for 53 mθ) and deselecting all pixels, new information is input to the data line and source line.

This display displays two colors, black and white, but in order to display in color, a single layer of color filters may be formed on the substrate on the source line side by photolithography, printing, dye distillation, or the like. The contrast ratio of the present y-tech M display was approximately 6. Furthermore, there is no viewing angle dependence, and the light transmittance when selected is as high as 60%, so information can be displayed very easily even in indoor light. It is also possible to form a reflective surface on one of the substrates and use it as a reflective display; in this case, the contrast ratio will be 30, and a very vivid display will be possible. In this way, by using a liquid crystal for dual-frequency driving as the liquid crystal composition and driving the steric-nematic phase transition of the liquid crystal composition at two frequencies using an active element, there is no viewing angle dependence and it can be easily achieved even in indoor light. It has been found that it is possible to display an image that is visible and has a contrast ratio that poses no practical problems.

〔Effect of the invention〕

As described above, according to the present invention, the cholesteric-nematec phase transition of a liquid crystal composition in which the dielectric anisotropy is positive at frequencies lower than the crossover frequency at which the dielectric anisotropy becomes zero and negative at higher frequencies is suppressed at the above-mentioned high temperature. By controlling with a low frequency electric field, especially by applying a high frequency electric field, it is possible to transition from a nematic phase to a cholesteric phase having a planar structure without passing through a focal conic structure. This has made it possible to create a high-speed reversible optical switch without memory properties. As a result of the above effect, the helical pitch of the cholesteric phase can be made smaller, which has the effect of increasing the contrast of the phase change type guest-host mode display. When the liquid crystal electro-optical device of the present invention was applied to an anti-glare mirror, it was found that it has the effect of making it possible to vary the amount of reflected light over a wide range. Furthermore, when the electro-optical device of the present invention is applied to a display that controls an electric field using an active switching element, a color display that can be easily viewed even in indoor light, has no viewing angle dependence, and has a contrast ratio that poses no problem in practical use can be obtained. I found out that it is possible

[Brief explanation of the drawing]

Figure 1; Electro-optic response characteristic diagram of liquid crystal electro-optical device 1... Input waveform 2... Electro-optic response characteristic 5... When selected (when low frequency electric field is applied) Transmittance of 4
・・・・・・Transmittance when not selected (when high frequency electric field is applied) Figure 2: Switching characteristics diagram of liquid crystal electro-optical device Figure 2 (α): Switching characteristics diagram when driving with a single frequency Figure 2 ( b): Switching characteristics during dual frequency drive Figure 1: Low frequency voltage dependence of transmittance when selected during single frequency drive (Low frequency voltage increase 2: Single frequency drive) -Low frequency voltage dependence of transmittance when selected during frequency drive (low frequency voltage drop) 5...Low frequency voltage dependence of transmittance when not selected during single frequency drive (low frequency voltage drop) voltage increase) 4...
Low-frequency voltage dependence of transmittance when not selected during single-frequency drive (low-frequency voltage drop) 5... Low-frequency voltage dependence of transmittance when selected during three-frequency drive 6... ...Low frequency voltage dependence of transmittance during non-selection during three-frequency drive Figure 5 (α)*(b)*(C): Conventional cholesteritter nematic phase transition Figure 1...・Planar structure 2...Nematic phase 5...Focal conic structure tube 4FA: Conventional electro-optic response characteristic diagram 1...Input signal 2...Electro-optic Response characteristics Figure 5 (α), (b): Orientation state diagram of liquid crystal layer 1...
...Grenner group M (cholesteric phase) 2...
Homeotropic structure (nematic phase) Fig. 6: D-electrodispersion characteristic diagram of liquid crystal Fig. 7 (α, (j), (c): Structure diagram of anti-glare mirror 1... Incident light 2... ...Reflected light 5 ...Transparent glass substrate 4 ...Reflection layer #!8 Figure 8: Schematic diagram of TPT character display No. 8
Figure (α): Perspective view of T1FT character display gray Figure 8 (b): Drive signal formation diagram 1...Glass substrate 2...Color filter layer 5...Common Electrode 4...Quartz base 5...Gate line 6...Source line 7...Art collection electrode 8...Source line input video Signal 9...
・Gate line input signal 10...Common electrode input signal Figure 9: Perspective view of M engineering M liquid crystal display 1...
・Transparent substrate 2...Transparent electrode 5...Transparent substrate 4...Pixel electrode 5...Data line 6...Insulating layer 10th Figure: Driving formation of M liquid crystal display Diagram 1... Source line input signal 2... Data line input signal 3... Mark 7 JFl field signal 4...・More than the applied electric field signal of M-engine M-seiko

Claims (3)

[Claims] (1) A liquid crystal electro-optical device in which a liquid crystal composition forming a cholesteric phase is held between a pair of substrates, and a means for controlling the cholesteric-nematec phase transition of the liquid crystal composition with an electric field is provided on the substrate. A liquid crystal composition in which the dielectric anisotropy is positive at frequencies lower than the crossing frequency at which the dielectric anisotropy becomes zero and negative at frequencies higher than the crossing frequency, A liquid crystal electro-optical device characterized in that a frequency electric field and a high frequency electric field are applied to control phase transition of the liquid crystal composition. (2) The liquid crystal electro-optical device according to claim 1, wherein the liquid crystal composition contains a dichroic dye. (3) The electro-optical device according to claim 1, wherein an active switching element is provided on the substrate as means for controlling the electric field.