JPS5838767B2 - liquid crystal display device - Google Patents

Description

[Detailed description of the invention] The present invention relates to a display device using liquid crystal.

It is well known that nematic liquid crystals with negative dielectric anisotropy cause light scattering when a voltage is applied.

This effect is generally called dynamic scattering mode (DSM) and is applied to display devices.

However, displays based on the DSM effect of conventional nematic liquid crystals require an applied voltage of about 15 V or more to obtain sufficient display contrast, and when time-division driving such as matrix display is performed, cross-stack occurs, resulting in static It was limited to powered vehicles.

Such drawbacks in conventional nematic liquid crystal display devices are mainly due to the slow voltage dependence of the light scattering intensity of nematic liquid crystals.

In a normal nematic liquid crystal, the light scattering start voltage is about 7V, but the light scattering saturation voltage is 30 to 50V or more.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new liquid crystal display device that improves the drawbacks of the conventional liquid crystal display device described above.

According to the present invention, there is provided a liquid crystal display panel in which a mixed liquid crystal in which a very small amount of cholesteric liquid crystal is added to a nematic liquid crystal is sandwiched between a pair of electrode substrates which have been subjected to rubbing treatment, and a projection line of an incident light beam onto the panel is aligned with the rubbing direction. A liquid crystal display device consisting of a light source arranged almost at right angles and a driving power source for applying voltage to the liquid crystal exhibits sharp voltage dependence and high contrast characteristics that greatly improve the above-mentioned drawbacks. A liquid crystal display device having the above structure can be obtained.

Next, the present invention will be described in detail with reference to the drawings.

In a normal nematic liquid crystal cell that produces DSM, when a voltage is applied, a light scattering domain called a Williams domain appears at about 6 to IOV, and at about 10 V or more, dynamic light scattering due to turbulence is often exhibited. Are known.

In addition, in a nematic liquid crystal cell that has been subjected to a so-called rubbing treatment in which the inner surface of the cell's electrode substrate is rubbed in one direction with cotton, etc., the liquid crystal molecules are arranged in parallel with their long axes parallel to the rubbing direction, and when a voltage is applied, It is also known that Williams domains take the form of stripes perpendicular to the rubbing direction.

This rubbing cell exhibits relatively sharp voltage-dependent light scattering characteristics compared to liquid crystal cells that are not rubbed, when the incident light is made to enter the cell so that the projection line of the light beam onto the cell surface is parallel to the rubbing direction. show.

A typical example of this characteristic is shown in curve 1 of FIG.

However, in a liquid crystal cell that has been subjected to a unidirectional rubbing process and is filled with a mixed liquid crystal that is a mixture of normal nematic liquid crystal and a very small amount of cholesteric liquid crystal, the voltage dependence of light scattering is even sharper, and it is also extremely high. It was discovered that it has contrasting properties.

Typical examples of the light scattering properties of a rubbing cell filled with this mixed liquid crystal are shown in curves 2 and 3 in FIG.

Curve 2 is the case where the incident light is incident on the liquid crystal cell so that the projected line of the light onto the surface of the rubbing cell is perpendicular to the rubbing direction, and curve 3 is the case where the projected line of the incident light is parallel to the rubbing direction. This is the case.

In other words, in a rubbing cell filled with a mixed liquid crystal in which a very small amount of cholesteric liquid crystal is added to a nematic liquid crystal, the line of projection of incident light onto the rubbing cell is in the rubbing direction, contrary to the case of the rubbing cell containing only nematic liquid crystal described above. Sharp, high-contrast light scattering properties can be obtained when the light is incident at right angles to the light.

When a rubbing cell filled with this mixed liquid crystal is observed under a microscope, striped domains similar to Williams domains appear at a low voltage of about 5 cm.

It was found that the direction of this stripe domain axis is parallel to the rubbing direction, unlike in the case of rubbing cells filled with nematic liquid crystals.

In order to generate such striped domains parallel to the rubbing direction, the helical pitch P of the molecular arrangement of the mixed cholesteric liquid crystal obtained by mixing cholesteric liquid crystal and nematic liquid crystal must be approximately twice the liquid crystal layer thickness d. It has also been found that it is necessary to adjust the mixing ratio.

When the applied voltage is further increased a little, grid-like or matrix-like domains appear instead of stripe-like domains.

When the voltage is further increased, the domain pattern shifts, causing liquid crystal flow.

Therefore, it is considered that the first peak 4 of light scattering in the curve 2 in FIG. 1 is based on the stripe-like domains, and the second peak 5 is based on the matrix-like domains.

The liquid crystal used to make such a mixed liquid crystal may be any ordinary nematic liquid crystal or cholesteric liquid crystal.

However, nematic liquid crystals have a suitable electrical resistivity, i.e., 108ΩcIn, which is sufficient to produce a normal DSM.
It is preferable to have a resistance of about 100Ωα.

The sharp, high-contrast light scattering characteristics caused by the generation of striped domains parallel to the rubbing direction due to the application of a low voltage in the above-mentioned cell in which a small amount of cholesteric liquid crystal is added to the nematic liquid crystal depend mainly on the concentration of cholesteric liquid crystal added, and This appropriate concentration value depends on the type of cholesteric liquid crystal and the thickness of the liquid crystal layer of the cell.

The former is involved in the ability to form a helical alignment of cholesteric liquid crystals, and it is necessary to increase the concentration of materials that have a low ability to form a helical alignment.

The latter is usually about 10μ door, but depending on the device, 5μ
~50 μn in some cases, and the larger the liquid crystal layer thickness, the greater the cholesteric liquid crystal addition concentration needs to be.

In general, when using a typical cholesteric liquid crystal that is commonly used, for example, cholesteric nonanoate in the following example, and when the liquid crystal layer thickness is about 10 μm, which is usually adopted, the appropriate concentration of cholesteric liquid crystal to be added is extremely small. That is, it is approximately 0.1 to 1.0% (weight ratio).

If the doping concentration is below this level, the striped domains parallel to the rubbing direction due to the voltage application described above will not be formed, and therefore sharp and high contrast light scattering characteristics will be obtained under low voltage application as shown by curve 2 in Figure 1. cannot be obtained, and if it is higher than this, the above-mentioned striped domains that produce sharp threshold characteristics and high contrast will not be formed, and the threshold voltage will be higher than that of nematic liquid crystal alone, and the threshold voltage will be higher than that of the nematic liquid crystal alone. The characteristics will no longer be sharp, and the response speed will also become very slow.

Note that when a vertically aligned cell is filled with a mixed liquid crystal in which a very small amount of cholesteric liquid crystal is added to a nematic liquid crystal, the light scattering characteristics are sharper than in a cell with only nematic liquid crystal, but the contrast is lower than that of the mixed liquid crystal. considerably lower than that of a rubbing cell filled with

Furthermore, the fall time after voltage removal is longer than that of a rubbing cell.

As described above, a liquid crystal cell in which a rubbing cell is filled with a mixed liquid crystal in which a very small amount of cholesteric liquid crystal is added to a nematic liquid crystal has excellent performance such as sharp threshold characteristics, high contrast characteristics, and high-speed response.

Next, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a schematic diagram of a liquid crystal display device according to an embodiment of the present invention to which the above discovery is applied.

The liquid crystal display panel 6 consists of a pair of glass electrode substrates 7 coated with transparent electrodes such as Nesa film in an appropriate pattern.
It has a sandwich-like structure.

The thickness of the liquid crystal layer may be approximately several to several tens of microns.

The liquid crystal 8 is a mixture of a nematic liquid crystal having an electrical resistivity of about 10'Ωα and a cholesteric liquid crystal at a weight ratio of about 0.5%.

In this example, methoxybenzylidene-phthylaniline was used as the nematic liquid crystal, but any liquid crystal that exhibits the DSM effect may be used.

Of course, in order to widen the operating temperature, a composition containing several types of nematic liquid crystals may be used.

Although cholesteryl nonanoate was used as the cholesteric liquid crystal, any cholesteric liquid crystal may be used.

However, the optimum mixing ratio differs somewhat depending on the liquid crystal material.

The inner surface of the glass electrode substrate 7 is subjected to a rubbing treatment.

Rubbing may be a conventional method of rubbing the surface of the substrate in one direction with cotton or the like.

Further, instead of rubbing, a technique may be used in which fine grooves are formed on the substrate surface using an obliquely vapor-deposited film of metal or the like.

In FIG. 2, the rubbing direction of the liquid crystal display panel 6 is the direction indicated by the arrow 9 in the figure.

The light source 10 that illuminates the liquid crystal display panel 6 is set at a position such that the projection line 12 of the incident light 110 onto the panel surface is perpendicular to the rubbing direction 9.

The angle between the incident light 11 and the liquid crystal display panel 60 is 20 to 6
00 position is appropriate, but it is set so that it is not visible to the viewer 13.

The electrode end of the liquid crystal display panel 6 is connected to a drive power source 14.

The drive voltage is preferably an alternating current voltage of several tens of Hz, but a direct current voltage may also be used.

In the liquid crystal display device having the above configuration, a high contrast of 1:40 or more was obtained when a 50 Hz AC voltage of 8 V was applied.

The starting voltage for light scattering is approximately 5V.

A linear polarizing plate may be inserted between the light source 10 and the liquid crystal display panel 60.

The polarization direction of the polarizing plate is the rubbing direction 9 of the liquid crystal display panel 6.
High contrast can be obtained when the polarization direction is the same as that of the polarization direction, but the response speed can also be changed by appropriately changing the polarization direction.

As described in detail above, according to the present invention, a rubbing cell is filled with a mixed liquid crystal of nematic liquid crystal and cholesteric liquid crystal, and light is made to enter the cell so that the projection line is perpendicular to the rubbing direction. Accordingly, a liquid crystal display device having sharp voltage dependence and high contrast can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing light scattering characteristics of liquid crystal for explaining the present invention. FIG. 2 is a schematic diagram of a liquid crystal display device according to an embodiment of the present invention. In FIG. 2, 6...Liquid crystal display panel, 7...Glass electrode substrate, 8...Mixed crystal, 9...Rubbing direction, 10・・・・・・
Light source, 11... Incident light, 12... Projection line, 13... Viewer, 14... Drive power source.

Claims (1)

[Claims] 1 A mixed liquid crystal obtained by adding cholesteric liquid crystal to a nematic liquid crystal at a weight ratio of approximately 0.1 to 1.0% was filled between a pair of electrode substrates, at least one of which was transparent and whose inner surface was subjected to a unidirectional rubbing treatment. It is characterized by comprising a liquid crystal display panel, a light source arranged so that a line of incident light projected onto the panel is substantially perpendicular to the rubbing direction, and a drive power source for applying a voltage to the liquid crystal display panel. LCD display device.