JPS59105679A - Liquid crystal display element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a liquid crystal display element.

[Technical background of the invention and its problems]

Conventionally, twisted nematic devices and dot matrix time-division display devices using a dynamic scattering mode are known as large-sized, large-information display devices using liquid crystals. However, such a display element has the disadvantage that as the amount of information increases, the difference between the voltage applied to a pixel in a selected state and the voltage applied to a pixel in an unselected or half-selected state becomes smaller. was there.

Therefore, there is a need for a liquid crystal material that responds sensitively even to the small voltage difference. However, the type of liquid crystal used in the display element has a limit in the steepness of the change in transmitted light with respect to the applied voltage, and its threshold voltage is easily affected by temperature changes, resulting in a decrease in contrast. This dot matrix time division display element has a problem in displaying a large amount of information.

For these reasons, thermo-electro-optic display elements have recently been developed as elements capable of displaying a high amount of information.

Such a display element consists of a substrate having a plurality of heater electrodes for heating the liquid crystal, for example in the column direction, and a substrate having a plurality of writing signal electrodes in rows perpendicular to the heaters, and the opposite surface of these substrates. It has a structure in which a liquid crystal cell with a vertical alignment treatment layer is sandwiched between liquid crystals that exhibit a smectic phase at room temperature and change to a nematic phase and an isotropic liquid as the temperature is raised from room temperature, with positive dielectric anisotropy. ing.

In such a liquid crystal display device, when a voltage is applied in a pulsed manner to the heater electrode, the temperature of the liquid crystal locally rises from the smectic temperature to the isotropic liquid phase due to Joule heat.
After the current is cut off, it is cooled by the ambient temperature and returns to the smectic phase temperature. If this cooling process is carried out in a short time, the fluctuations of the liquid crystal molecules in an isotropic liquid state are frozen and become a scattering state (opaque state). This scattering state is memorized and retained when the smecti is mixed and cooled. On the other hand, when a voltage larger than the threshold voltage of the liquid crystal is applied to the signal electrode as a signal voltage to the liquid crystal layer in the nematic phase between the isotropic liquid phase and the smectic phase during the cooling process, the liquid crystal molecules are connected to the substrate. Since the liquid crystal cell is oriented with its long axis directed in the vertical direction, it becomes transparent, and when it is further cooled and enters a smectic phase, the transparent state is memorized and retained.

However, even if a voltage larger than the threshold voltage is applied to the liquid crystal that has entered the smectic phase, the liquid crystal molecules do not respond. In this way, the liquid crystal display element receives two kinds of external forces: heat generated by a plurality of heaters arranged in the column direction on one substrate, and voltage generated by a plurality of signal electrodes arranged in the row direction on the other substrate. Since the display is selected or non-selected at a predetermined intersection between the heater and the signal electrode, there is, in principle, no losstalk unlike when display is performed using only the signal electrode, and it has the ability to display a large amount of information. . Also, since it has a memory effect, it has the advantage of retaining the display even when the power is turned off.

By the way, in the above-mentioned thermoelectro-optic liquid crystal display element, in order to increase the contrast between the scattering part (when cloudy) and the transparent part, the isotropic liquid phase is rapidly cooled, and the liquid crystal molecules are It is desirable to efficiently freeze fluctuations and increase the degree of scattering. To this end, it is conceivable to construct a liquid crystal display element by sandwiching a liquid crystal material having a nematic phase in a narrow temperature range on the high temperature side where the difference from ambient temperature is large between liquid crystal cells. However, such display elements require a large amount of heat to raise the temperature from the ambient temperature to the isotropic liquid phase, so the power consumption of the heater increases, and if the voltage value of the heater is fixed, the temperature will increase. requires a long time. As a result, there is a problem that the writing speed is slow.

Conversely, in order to reduce power consumption and improve the writing speed, it is conceivable to use a liquid crystal material that has a nematic phase temperature that does not differ much from the ambient temperature. But from Hiko,
When such a liquid crystal material is used, cooling is not performed rapidly and the liquid crystal molecules tend to be parallel to each other during the nematic phase during the cooling process, resulting in poor scattering and a decrease in contrast. However, if the ambient temperature fluctuates, the rate of change in the amount of heat required to raise the temperature to the temperature of the isotropic liquid phase will increase. As a result, it is necessary to largely change the heater voltage and pulse width necessary to obtain the desired degree of scattering, and the margin for setting the heater voltage and heater cell width becomes small. Therefore, in any of the conventional liquid crystal display devices, it has been impossible to achieve high-speed writing with low power consumption and high-contrast display.

[Purpose of the invention]

The present invention uses a liquid crystal display element that is capable of high-speed writing with low power consumption, high-contrast display, has a large margin for setting heater voltage and heater voltage pulse width, and is suitable for low signal voltage driving. This is what we are trying to provide.

[Summary of the invention]

The present invention can impart a chiral substance to a smectic liquid crystal that has positive dielectric anisotropy and exhibits a nematic phase at high temperatures, and a helical pitch length that is 1 to 6 times the distance between the substrates of the liquid crystal cell to be sandwiched. The present invention is characterized in that it uses a smectic liquid crystal composition which is added in a large amount to force the liquid crystal composition, and this liquid crystal composition is sandwiched between liquid crystal cells. By using a liquid crystal composition in which a predetermined amount of a chiral substance is added to a smectic liquid crystal, the torsion force between liquid crystal molecules caused by the helical structure of the molecules of the chiral substance can be used to suppress scattering in an isotropic liquid. The freezing property of the state is improved, and a more stable scattering state can be obtained. As a result, it is possible to sufficiently increase the degree of scattering even if the temperature difference between the transition temperature of the isotropic liquid phase and the ambient temperature is small, and high-contrast display is possible. It is possible to reduce the power consumption necessary to write data and to realize high-speed writing. Furthermore, since the degree of scattering is not improved by using only thermal effects, there is a greater degree of leeway in setting the voltage (current value) of the heater and the width of the heater.

As the smectic liquid crystal, for example, biphenyl liquid crystal, azomethine liquid crystal, azoxy liquid crystal, ester liquid crystal, bi-1,1-midine liquid crystal, or a mixture of these liquid crystals can be used.

Examples of the chiral substance include 4-(2-methylbutyl)-4'-(4-n-pentylcyclohexyloxycarbonyl)biphenyl, 4-n-hebutoxy-4'-
(2-methylbutyroxycarbonyl)biphenyl, 4-
Examples include (4-(2-methylbutyl)pentyloxy)benzoic acid-4'-n-pentylphenyl ester.

The reason for limiting the amount of chiral substance added as described above is that if the amount is added so that the helical pitch length is less than one time the distance between the substrates, it often becomes cloudy in the initial state, and during heating and cooling. When electricity is applied to the signal electrode in the nematic phase state, it does not become transparent and becomes cloudy! , the responsiveness deteriorates. However, if the amount is added so that the helical pitch length exceeds six times the distance between the substrates, the effect of adding the chiral substance will not be obtained.

[Embodiments of the invention]

Next, the present invention will be explained in detail.

Example A biphenyl-based smectic liquid crystal (manufactured by BDH Co., Ltd., part name: K24) and a similar smectic liquid crystal (BDH
(Part name: CM-2 manufactured by Chisso Co., Ltd.) and 30) are mixed in an equimolar fraction, and a chiral substance (Part name manufactured by Chisso Co., Ltd.: CM-2) is mixed in an equimolar fraction.
The helical pitch length is 13.7 by adding 2% by weight of 0).
A μm smectic liquid crystal composition was prepared.

On the other hand, a vertical alignment agent (manufactured by Sumitomo 3M Co., Ltd., part name: FC-805) is applied to the opposing surfaces of the glass substrate to which the heater electrode made of metal film is attached and the glass substrate to which the signal electrode made of indium oxide is attached. A vertical alignment layer was provided. Next, the above-described smectic liquid crystal composition was sandwiched between the substrates arranged in parallel with an interval of 9 μm so that the vertical alignment layers faced each other, and the periphery between the substrates was sealed with a sealing material to produce a liquid crystal display element. .

Comparative Example 1 A biphenyl-based smectic liquid crystal (part name: K24 manufactured by BDH Corporation) and a similar smectic liquid crystal (BDH
A liquid crystal display element (10) was fabricated by using a Sumekdic liquid crystal material mixed with KO (part name: K2O) manufactured by Co., Ltd. in an equimolar proportion and sandwiching it between substrates and sealing with a sealing material in the same manner as in the example.

Comparative Example 2 A liquid crystal display element was manufactured using a smectic liquid crystal material having a high nematic phase transition temperature by sandwiching the material between substrates and sealing with a sealing material in the same manner as in the example.

Therefore, the smectic-nematic phase transition temperature, the nematic-isotropic liquid phase transition temperature, the contrast ratio with respect to Comparative Example 1, and the minimum value of the write signal voltage were investigated for the liquid crystal display elements of this example and Comparative Examples 1 and 2. Ta.

The results are shown in the table below.

The table also shows the voltage at which a scattering state can be achieved by applying a gust-like voltage to the heater electrodes of the liquid crystal display elements of Example 11/IJ and Comparative Examples 1 and 2, and changing the voltage value and pulse width. When we investigated the relationship between this and the heater pulse width, we obtained the characteristic diagram shown in the figure. In the figure, A is a characteristic line of the liquid crystal display element of this example, B is a characteristic line of the same element of Comparative Example 1, and C is a characteristic line of the same element of Comparative Example 2. Note that the shaded portion of each characteristic line indicates the allowable range.

As is clear from the illustrated characteristic diagram, the liquid crystal display element of Comparative Example 1 can achieve a scattering state with a low voltage and a short voltage pulse. This is because, as shown in the table above, the difference between the transition temperature to an isotropic liquid and the ambient temperature is small, so the temperature can be raised from a smectic phase to an isotropic liquid phase with a small amount of heat. However, the liquid crystal display element of Comparative Example 1 had low contrast as shown in the above table, and as shown in the same figure, the margin for setting the heater voltage value and its pulse width was very low.

Further, in the liquid crystal display element of Comparative Example 2, it is necessary to apply a voltage with a high voltage and a long pulse width in order to realize a scattering state. This is because there is a large difference between the phase transition temperature to isotropic liquid and the ambient temperature, so a large amount of heat is required to raise the temperature from the numectic phase to the isotropic liquid phase. Moreover, although the liquid crystal display element of Comparative Example 2 is capable of high contrast display, as shown in the figure, the margin of latitude in setting the heater voltage value and its pulse width is very low.

On the other hand, the liquid crystal display element of this example, which had a smectic liquid crystal composition doped with a chiral substance, had a somewhat larger heater voltage and pulse width than the element of comparative example 1;
A scattering state could be achieved with a voltage of 1,4+Rus width, which was much smaller than in Comparative Example 2. Furthermore, by adding a chiral substance to the smectic liquid crystal, the contrast of the display element of this example was improved by 2.6 times compared to Comparative Example 1 using a smectic liquid crystal of the same composition without the addition of a chiral substance.
Moreover, a high contrast display exceeding that of Comparative Example 2, which had a high nematic phase temperature, could be achieved. Furthermore, as shown in the figure, the liquid crystal display element of this embodiment has extremely high latitude in setting the heater voltage value and its pulse width. Therefore, even if there was a change in the ambient temperature, a stable scattering state could be obtained without greatly changing the heater voltage value or /4' pulse width setting. In addition, the liquid crystal display element of this example is Comparative Example 1
, 2, a transparent state was achieved with a signal voltage value to the signal electrode that was almost the same as that of 2.

〔Effect of the invention〕

As described in detail above, according to the present invention, high-speed writing is possible with low power consumption, high-contrast display is possible, there is a large degree of latitude in setting the heater voltage pulse width and heater voltage pulse width, and furthermore, it is possible to perform high-speed writing with low power consumption. It is possible to provide a liquid crystal display element having excellent characteristics such as being able to be driven by voltage.

[Brief explanation of drawings]

The figure is a characteristic diagram showing the relationship between heater voltage and heater pulse width that can realize a scattering state in the liquid crystal display element of the present invention and two types of conventional liquid crystal display elements.

Claims (1)

[Claims] In a thermo-electro-optical liquid crystal display element in which a smectic liquid crystal composition is sandwiched between substrates each having a signal electrode for heating and a vertical alignment treatment layer, the smectic liquid crystal composition is Use a smectic liquid crystal that has positive dielectric anisotropy and exhibits a nematic liquid crystal phase at high temperatures, with a chiral substance added in an amount capable of imparting a helical pitch length of 1 to 6 times the distance between the substrates. A liquid crystal display element featuring: