JPH01284834A - Liquid crystal electrooptical device - Google Patents

Abstract

PURPOSE:To improve the yield of production by using a liquid crystal mixture, the liquid crystal molecules of which have a spiral axis in the direction perpendicular to a smectic layer in a liquid crystal cell and which exhibits a ferroelectric property and changing the state that the liquid crystal molecules attains by the inversion of the direction of the electric field to be impressed to this liquid crystal material. CONSTITUTION:Electrodes 3, 3' for driving the liquid crystal are patterned and formed on transparent 1, 1' to a matrix shape in a line direction and a row direction. Orientation control films 4, 4' are provided on the electrodes and one side thereof is subjected to a rubbing treatment in such a manner that the major axes of the liquid crystal molecules are arrayed in one direction. The spacing between the substrates 1, 1' superposed on each other is maintained in the range where the spiral formation of the liquid crystal mixture is not suppressed. The liquid crystal molecules form the spirals so as to have the spiral axis in the direction perpendicular to the smectic layer in the liquid crystal cell. The state that the liquid crystal molecules attain is changed by the inversion of the direction of the electric field to be impressed to the liquid crystal mixture exhibiting the ferroelectric property.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a liquid crystal electro-optical device such as a liquid crystal display element and a liquid crystal optical shutter array, and more particularly to a liquid crystal electro-optical device using ferroelectric liquid crystal. It is.

[Conventional technology]

As a conventional liquid crystal electro-optical device, one using twisted nematic liquid crystal is known. This TN liquid crystal has a problem in that crosstalk occurs during time division driving using a matrix electrode structure with high pixel density, so the number of pixels is limited.

In addition, active matrix type display elements are known in which a switching element using a thin film transistor is connected to each pixel and switching is performed for each pixel, but the process of forming the thin film transistor on the substrate is extremely complicated, and the manufacturing cost is high. There is a problem in that it is difficult to produce large-area display elements due to factors such as yield.

Furthermore, liquid crystal display devices using these T''N type liquid crystals cannot obtain sufficient display contrast between ON and OFF states, and in addition, the viewing angle of the display device is narrow, making it difficult for multiple workers to work near the display device. could not see the display.

To solve these problems, Clark et al. proposed an electro-optical device using ferroelectric liquid crystal in US Pat. No. 4,367,924.

This liquid crystal electro-optical device is a highly anticipated liquid crystal electro-optical device because it has a high-speed response of several hundred microseconds or less to externally applied signals and the liquid crystal molecules have bistability.

The liquid crystal electro-optical device proposed by Clark et al. will be explained with reference to FIG.

FIG. 1 schematically depicts an example of a cell for explaining the operation of a ferroelectric liquid crystal. 1 and 1' are parallel substrates for sealing liquid crystal, and between them, a chiral smectic C phase (
A liquid crystal material having a liquid crystal phase exhibiting ferroelectric properties is sealed.

This ferroelectric liquid crystal generally has liquid crystal molecules uniaxially aligned and has a smectic layer structure. Furthermore, in the smectic C phase, the liquid crystal molecules are tilted at a certain angle unique to the liquid crystal material with respect to the normal direction of the smectic layer, and have a helical structure.

This helical structure can be seen when observing a liquid crystal cell filled with ferroelectric liquid crystal with an optical microscope under several hundred times magnification.
It can be confirmed by the striped pattern corresponding to the helical pitch.

-a, this helical pitch is said to be from a little less than 1 μm to about 5 μm. Clark et al. found that when the spacing between the substrates of a liquid crystal cell filled with ferroelectric liquid crystal was narrowed to near the helical pitch, the helix unraveled due to the influence of the substrate surface.
This is what I used.

When the distance between the substrates is made smaller than the pitch of the liquid crystal and the helix of the liquid crystal is unwound, the ferroelectric liquid crystal molecules form the first layer tilted by +θ with respect to the normal direction of the smectic phase layer, as shown in Figure 2. A second state (If) tilted by -θ from the state (I) is taken. This state is called 5SFLC (surface stabilized state) by Clark et al., and by applying an external electric field between these two states,
Display was performed using the difference in birefringence effect generated by switching ferroelectric liquid crystal molecules.

At this time, in order to change the ferroelectric liquid crystal molecules from the first state (I) to the second state (II), for example, a positive electric field is applied in a direction perpendicular to the smectic layer. On the other hand, in order to reverse the second state (II) to the first state N), it was accomplished by applying a negative electric field.

That is, by changing the direction of an externally applied electric field, the two states of ferroelectric liquid crystal molecules are changed, and the resulting difference in electro-optical effects is utilized.

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, and the ferroelectric liquid crystal molecules switch between the two states depending on the direction in which the pulse polarity switches. Ta.

Regarding the switching mechanism between these two states, many models have been proposed to date, including the model proposed by Clark et al. in the above-mentioned U.S. patent, but none of them utilize the helical state that ferroelectric liquid crystals inherently have. , it was a switching mechanism when the spiral was changed to an uncoiled state in some way.

In order to unwind this helix, it is a well-known method to make the distance between the substrates of the liquid crystal cell sandwiching the ferroelectric liquid crystal close to the helical pitch, that is, an extremely narrow distance of about 1 μm. I was worried. However, when considering industrial production technology, etc., this interval is clearly narrow, which causes difficulties in the production process.

In particular, manufacturing large-area liquid crystal cells such as A4 size with uniform spacing of about 1 μm was possible at the research and development level, but almost impossible on an industrial scale.

[Structure of the invention]

The inventors of the present invention have conducted extensive studies and arrived at the present invention in order to solve the problems of conventional liquid crystal electro-optical devices using ferroelectric liquid crystals.

While conventional liquid crystal electro-optical devices using ferroelectric liquid crystals are always used in an uncoiled spiral state, the present invention realizes an electro-optical device using ferroelectric liquid crystals in a spirally formed state. The mode is completely different from conventional electro-optical devices using ferroelectric liquid crystals.

In other words, in a liquid crystal cell formed by a pair of parallel substrates, the spacing between the substrates is maintained at a distance that does not inhibit the spiral formation of the ferroelectric liquid crystal material injected into the liquid crystal cell, and in this state, the ferroelectric The liquid crystal is in a state where it can form a helix with the helical axis in the normal direction of the smectic layer, and in this state, an electric field is applied to the ferroelectric liquid crystal from the outside, and the direction of the applied electric field is reversed, causing the liquid crystal molecules to take on a spiral shape. It takes advantage of the fact that the state changes. At this time, the liquid crystal molecules are aligned in a specific state (first state) according to the direction of the electric field applied due to the spontaneous polarization of the molecules.

Next, when an electric field in the opposite direction is applied, a different second state is achieved. This is an electro-optical device that utilizes the difference between the first state and the second state.

Furthermore, the present inventors have confirmed that, depending on the direction of this externally applied electric field, liquid crystal molecules always pass through a helical state when switching from the second state to the first state.

Furthermore, the present inventors have confirmed that liquid crystal molecules once in the first and second states continue to maintain these states even when no externally applied electric field is applied.

That is, to summarize the characteristics, in the liquid crystal electro-optical device of the present invention, the distance between the substrates of the liquid crystal cell is sufficiently wide compared to the distance between the substrates of a conventional liquid crystal cell, so that the ferroelectric liquid crystal can form a spiral within the liquid crystal cell. state, and depending on the direction of the electric field applied from the outside, the liquid crystal molecules are in the first or second state.
When switching from the first state to the second state or from the second state to the first state, the state goes through a spiral state.

In this way, the present invention aims to provide a new configuration of an electro-optical device using ferroelectricity, in particular a new switching method, and as a result, achieves the high-speed response, contrast, etc. that ferroelectric liquid crystals inherently have. The purpose of the present invention is to provide a liquid crystal electro-optical device that does not interfere with the production process without sacrificing its characteristics.

The electro-optical device of the present invention may include a display device, an optical shutter, and other devices with various functions, but a specific explanation will be given below using a display device as an example. However, the present invention is limited only to this display device. It is not something that will be done.

In addition, as the liquid crystal material used in the present invention, a material exhibiting ferroelectricity is used, and in particular, a liquid crystal having a smectic C* phase in a temperature range of 110"C to around 70"C is the most practical. Met.

In addition, although any phase series could be used, ferroelectric liquid crystals that have a smectic A phase on the higher temperature side than the smectic C1 phase have a good initial orientation state and are free from defects. This was preferable because a clean alignment with no oxidation can be obtained.

EXAMPLE 1 The present invention will be described with reference to examples below. [Example 1] In this example, a matrix type liquid crystal display device was used as the liquid crystal electro-optical device.

FIG. 1 shows a schematic cross-sectional view of a cell of a liquid crystal display device used in this example. The figure shows a portion of the end portion of the electrode portion arranged in a Madrisk pattern in the row direction and the column direction.

Furthermore, since this is a schematic diagram, its dimensions are arbitrary. The cell used in this embodiment has exactly the same configuration as that used conventionally. That is, an electrode 3 for driving the liquid crystal is placed 1°1° above a transparent substrate (for example, glass).
, 3° are patterned and formed in a matrix shape in the row and column directions. Further, alignment control films 4, 4'' are provided on the electrodes, and one side of the alignment control films 4, 4'' is subjected to a known rubbing treatment so that the long sleeves of liquid crystal molecules are aligned in one direction. , 4'' may be made of the same material or different materials may be used for each side, but in this embodiment, a polyimide film is used as the orientation control film of 4, and a polyimide film is used for the other orientation control film 4'. A SiO□ film was used.

In this way, when the type of alignment control film is changed, a relatively large threshold value can be obtained when liquid crystal molecules are driven by an external signal, which is advantageous in a matrix liquid crystal electro-optical device.

A liquid crystal cell is formed by stacking such substrates 1 and 1' on top of each other and maintaining a constant spacing with a glass fiber spacer (not shown) interposed between them. However, in this embodiment, the spacing is made sufficiently wider than in the conventional case, and is wider than the helical pitch of the liquid crystal material, so as not to suppress the formation of the helix.

In this example, the interval was approximately 20 μm.

The ferroelectric liquid crystal material used in this example has the general formula (in the above formula, R represents an alkyl group having 6 to 12 carbon atoms, and Ro represents an optically active group having an asymmetric carbon atom having 9 to 15 carbon atoms. ,l
is 1 or 2. ), and in this example, the following mixed liquid crystal material was used.

The phase series and phase transition temperature of the ferroelectric liquid crystal material containing these substances ABC are shown below.

79 °C 64 °C093 °C13 Q ←
−−+ 3 mA 4−−−+ 3 m” C←−−h
Further, the ferroelectric liquid crystal material used in this example had a wide helical pitch of about 2.8 μm near room temperature.

In addition to the liquid crystal materials used in this example, it was also possible to use other liquid crystal materials such as biphenyl-based and pyrimidine-based liquid crystal materials.

At this time, the state of the liquid crystal inside the liquid crystal cell immediately after the liquid crystal material was injected was observed using a polarizing optical microscope.

Around room temperature, which is exactly the smectic C0 phase, about 23
When observing at temperature 'C), only a striped pattern corresponding to the helical pitch was seen, and although I tried rotating the microscope stage to align it with the extinction position, the extinction position could not be obtained. Ta. Next, when a direct current electric field was applied to the liquid crystal through electrodes 3.3' of the liquid crystal cell, an extinction position was obtained. That is, a dark state was obtained. At this time, even if the application of the electric field was stopped, the extinction state B (dark state) remained.

For such a liquid crystal cell, changes in transmitted light intensity with respect to an electric field applied to the cell were measured using an electro-optical characteristic measuring system as shown in FIG.

FIG. 4(a) shows the waveform of the triangular wave applied to the cell, and FIG. 4(b) shows the change in the intensity of transmitted light obtained.

In addition, FIG. 6(C) is a photograph of an actual oscilloscope waveform.

Further, FIG. 5(b) shows how the transmitted light intensity changes when a pulse waveform as shown in FIG. 5(a) is applied.

Moreover, the same figure (C) is the waveform photograph of the oscilloscope obtained at that time.

All measurements are taken at a temperature of approximately 25°C.

In particular, FIGS. 6(a) to 6(d) show polarized light micrographs showing the state of arrangement of liquid crystal molecules during switching when a triangular wave as shown in FIG. 4(a) is applied. A striped pattern (b) corresponding to the helical pitch is formed between the first state (a) and the second state (C) or between the second state (C) and the first state (a).
)(d), indicating that the state passes through a helical formation state during the switching process.

Furthermore, when a pulse voltage as shown in FIG. 5(a) is applied, even after the voltage is no longer applied as shown in FIG. 5(b), the transmitted light intensity does not change and remains in the first state or It can be seen that the second state continues to be maintained. or,
The contrast ratio between this period and the dark was a large value of 23.5. This value is approximately the same as the contrast ratio obtained with display devices using conventionally known ferroelectric liquid crystal materials, and the display viewing angle is also comparable to that of conventional devices. .

[Example 2] In this example, a liquid crystal cell having the same structure as in Example 1 was used, and the distance between the substrates was changed. The width is 10μ as the interval
Three types of cells were created: m, 20 μm, and 50 μm.

Furthermore, the same ferroelectric liquid crystal material as in Example 1 was used as the material injected into the liquid crystal cell.

Immediately after injecting the liquid crystal material in any of these cells,
An extinction position could not be obtained using a polarizing microscope, and a striped pattern corresponding to the helical pitch was observed.

However, only in the 10 μm cell, no clear striped pattern was observed, and although it was blurred, the extinction position could not be obtained, so it is thought that it forms a spiral like the other cells.

The following table shows the relationship between the contrast ratio value and the cell spacing when the pulse shown in FIG. 5(a) is applied to these cells.

As described above, when the cell spacing is around 10 μm, the liquid crystal is slightly influenced by the substrate of the liquid crystal cell, so that the orientation state of the liquid crystal molecules becomes blurred, and the contrast ratio becomes low. On the other hand, when the thickness of the liquid crystal cell is around 50 μm, the smectic layer of the liquid crystal material shifts in the thickness direction of the liquid crystal cell, so that the contrast ratio similarly becomes low.

This tendency is also seen in cases other than this example, and the thickness at which the contrast ratio begins to be affected is slightly different due to the influence of the substrate due to the difference in the characteristics of the liquid crystal used.

However, sufficient display quality was obtained even with liquid crystal cells having an interval of 10 μm, and there were no practical problems.

Further, the critical cell thickness for completely suppressing the spiral formation of the ferroelectric liquid crystal material used in the examples was around 2.0 μm.

〔effect〕

The present invention is an electro-optical device in a completely different mode from conventionally known electro-optical devices using ferroelectric liquid crystals.

In particular, since there is no need to suppress the helical formation of ferroelectric liquid crystal materials, which was essential in conventional electro-optical devices using ferroelectric liquid crystals, manufacturing yields can be improved during industrial mass production. In addition, it has become possible to have a wide tolerance range for various conditions during production.

Therefore, the present invention makes it possible to improve productivity without sacrificing the characteristics of ferroelectric liquid crystal materials, such as high-speed response and high contrast.

[Brief explanation of the drawing]

FIG. 1 schematically shows a liquid crystal electro-optical device cell. Figure 2 shows the appearance of ferroelectric liquid crystal molecules. FIG. 3 shows a measurement system for electro-optical characteristics. FIG. 4(a) shows the waveform of the triangular wave applied to the cell, and FIG. 4(b) shows the change in the intensity of transmitted light obtained. Figure (C) is a photograph of an actual oscilloscope waveform. FIG. 5(a) shows the pulse waveform applied to the cell, and FIG. 5(b) shows how the transmitted light intensity changes at that time. Figure (C) is an oscilloscope waveform photograph obtained at that time. Figure 6 (a), (b), (c), and (d) are the switching process.

Claims (1)

[Scope of Claims] 1. A liquid crystal electro-optical device in which a ferroelectric liquid crystal mixture containing at least one type of liquid crystal exhibiting ferroelectricity is injected into a liquid crystal cell formed by a pair of parallel substrates, The spacing between the parallel substrates is maintained within a range that does not inhibit spiral formation of the liquid crystal mixture, and the liquid crystal molecules are in a state in which they can form a spiral with a helical axis perpendicular to the smectic layer in the liquid crystal cell, and . A liquid crystal electro-optical device characterized in that it utilizes changing the state of the liquid crystal molecules by reversing the direction of an electric field applied to the liquid crystal mixture exhibiting ferroelectricity. 2. In the liquid crystal electro-optical device having the structure described in claim 1, the liquid crystal mixture is characterized in either the first or second molecular orientation state depending on the direction of the electric field applied to the liquid crystal mixture. A liquid crystal electro-optical device 3 according to claim 1 or 2, wherein the liquid crystal electro-optical device is a matrix type liquid crystal display element.