JPH0377491B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to eliminating uneven coloration, so-called color unevenness, in liquid crystal elements, particularly twisted nematic liquid crystal elements (hereinafter abbreviated as TN elements).

Liquid crystal cells are becoming increasingly popular, for example, in display boards for numbers and characters in electronic desktop calculators, clocks, and the like.
FIG. 1 is a cross-sectional view showing the general structure of a TN element used in these devices. In this figure, transparent electrodes 2a and 2a made of a thin film of indium oxide, tin oxide, etc. are mounted on transparent substrates 1a and 1b such as glass.
2b, and a spacer 3 is placed between these transparent electrodes 2a and 2b so that the long axis of the liquid crystal molecules is aligned with the transparent electrode 2.
parallel to the planes a and 2b and between these electrodes
The dielectric anisotropy is positive when the dielectric anisotropy is twisted by 90 degrees.
That is, a P-type nematic liquid crystal 4 is injected to form a liquid crystal cell 100, and the liquid crystal cell 100 is arranged on transparent electrodes 2a and 2b with their polarization directions perpendicular to each other (orthogonal Nicols) or parallel to each other (parallel Nicols). A TN element is constructed by sandwiching the polarizing plates 5a and 5b between two polarizing plates 5a and 5b aligned with the long or short axes of liquid crystal molecules. The thickness of the four layers of nematic liquid crystal is usually 1 μm.
~1 mm.

When a voltage ratio is applied to a TN element, a bright field can be obtained in the case of crossed Nicols, and a dark field can be obtained in the case of parallel Nicols.
In this state, transparent electrodes 2a, 2. When a predetermined voltage is applied to the four layers of nematic liquid crystal through
In the case of crossed Nicols, a dark field is obtained, and in the case of parallel Nicols, a bright field is obtained, which is an inverted state from the state in which no voltage is applied. Therefore, the transparent electrodes 2a, 2b
If it is formed into a predetermined pattern by, for example, etching, it becomes possible to display letters, numbers, symbols, etc.

Generally, nematic liquid crystals have a parallel structure, but when sandwiched between two substrates, the parallel structure of the nematic liquid crystal is strongly influenced by the condition of the surfaces of the substrates with which it comes into contact. When the inner surface of a substrate is rubbed with cotton or the like, the long axes of the liquid crystal molecules in contact with the surface follow the direction of the friction. This direction of friction is called the wall direction. When a nematic liquid crystal is sandwiched between two substrates with wall orientations, the liquid crystal molecules in contact with the substrate surface align according to the wall orientation, and the orientation of the liquid crystal molecules in the middle layer changes from one wall orientation to the other wall orientation. It will be a continuous transition. Therefore, the liquid crystal molecules of the nematic liquid crystal are twisted by 90 degrees along an arbitrary line perpendicular to the substrate.

A liquid crystal cell having such a liquid crystal molecular orientation can be regarded as a uniaxial crystal having an optical axis parallel to the substrate. Ideally, the polarization direction of plane-polarized light passing through a nematic liquid crystal layer is thought to change according to the rotation of the 90-degree twist structure. If the relationship between crossed Nicols or parallel Nicols and the optical axis is ideal, color unevenness will not occur, but in reality it is not perfect. You will see unevenness. In the case of parallel Nicols, color unevenness appears where a uniform dark field should normally be visible. This color unevenness is interference color due to retardation caused by ordinary rays and extraordinary rays passing through the uniaxial liquid crystal. ordinary rays,
The retardation δ of the extraordinary ray is expressed as δ=π/λd ₁ ·Δn=π/λR, R=d ₁ ·Δn. Here, _d1 is the thickness of the nematic liquid crystal layer of the liquid crystal cell, Δn is the difference in refractive index between the ordinary ray and the extraordinary ray, and λ is the wavelength of light. The smaller the letter dation δ (in other words, R), the more likely color unevenness will occur (see Journal of the Japanese Horological Society No. 90, pages 60 to 70'79).

The occurrence of color unevenness is undesirable because it reduces the display contrast of the TN element and significantly impairs the commercial value. If the thickness _d1 of the nematic liquid crystal layer or Δn of the liquid crystal material is increased, color unevenness will be reduced. However, due to the need to speed up the response time of liquid crystal elements or other electro-optical property problems, _d1 cannot be made very large, and furthermore, it is difficult to develop a P-type nematic liquid crystal with a large Δn, so R tends to become small, and there has been a demand for some method of improving color unevenness.

This invention was made to alleviate the above-mentioned conventional drawbacks, and provides a liquid crystal element that reduces the occurrence of color unevenness by overlapping a conventional liquid crystal cell with another liquid crystal cell that does not have an electrode. It is intended to. Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 2 is a sectional view of a liquid crystal element showing an embodiment of the present invention. A conventional liquid crystal cell 100 is used as a first cell, and this and two transparent substrates 6 without electrodes are used.
A second cell 200 having eight layers of nematic liquid crystal is superimposed between a and 6b via a spacer 7, and these are sandwiched between two polarizing plates 9a and 9b. FIG. 3 shows a schematic diagram showing the direction of wall orientation and the alignment state of liquid crystal molecules in the first cell 100 and the second cell 200 at this time.

In FIG. 3, arrows 10, 11, 12, 13
indicates the direction of wall orientation, and the liquid crystal molecules are schematically illustrated in the shape of a cylinder. 14a, 14b correspond to the transparent substrates 1a, 1b in FIG. 2, 15a,
15b corresponds to the transparent substrates 6a and 6b in FIG. Note that illustration of electrodes and spacers is omitted in this figure. The polarizing plates 9a and 9b in FIG. 2 are arranged in crossed Nicols or parallel Nicols, and aligned with the long axis or short axis of the liquid crystal molecules on the transparent electrodes 2a and 2. A feature is that the wall orientations applied to the electrode-equipped transparent substrate 14b constituting the first cell 100 and the electrode-less transparent substrate 15a constituting the second cell 200 are in the same direction. The direction of the wall orientation may be determined depending on the nematic liquid crystals 4 and 8 used so that twist disclination does not occur within the first cell 100 and the second cell 200.

In the liquid crystal device of the present invention, which is wall-oriented as shown in FIG. 3, the ordinary rays in the four nematic liquid crystal layers in the first cell 100 also become ordinary rays in the eight nematic liquid crystal layers in the second cell 200. 1st cell 10
The extraordinary ray in the 4 layers of nematic liquid crystal in cell 200 also becomes an extraordinary ray in the 8 layers of nematic liquid crystal in the second cell 200. Therefore, R of the liquid crystal element of the present invention is R=(d ₁ +d ₂ )Δn. Here, d ₁ and d ₂ are each the first cell 10
0, the thickness of the 4th and 8th nematic liquid crystal layers of the second cell 200.

In the case of the conventional TN element described above, R=d ₁ ·Δn, and the liquid crystal element of the present invention has a larger retardation δ (in other words, R) and is less likely to cause color unevenness. In this case, the second cell 20
The thickness of the eight layers of 0 nematic liquid crystal is suitably in the range of 1 .mu.m to 1 mm.

In the liquid crystal element of the present invention, for example, when the two polarizing plates 9a and 9b are crossed Nicols, the first cell 10
If no voltage is applied between the transparent electrodes 2a and 2b, a dark field will be obtained, and if a predetermined voltage is applied, a bright field will be obtained. That is, the pattern shape of the transparent electrodes 2a, 2b of the first cell 100 is displayed as a bright field in a dark field. In the case of parallel Nicols, this is reversed.

In the case of the above example, the liquid crystal molecular arrangement of the second cell 200 had a structure twisted by 90 degrees like the first cell 100, but as shown in FIG. The effect of reducing this can be obtained.

Further, in the above embodiment, the same nematic liquid crystals 4 and 8 were used for the first cell 100 and the second cell 200. However, this does not necessarily have to be the same. The nematic liquid crystal 8 in the second cell 200 does not need to respond to voltage, and may be a nematic liquid crystal with negative dielectric anisotropy other than a P-type nematic liquid crystal. For the nematic liquid crystal 8 used in the second cell 200, selecting a liquid crystal material with a large Δn as much as possible is more effective in reducing color unevenness in the liquid crystal element of the present invention.

Furthermore, as shown in FIG.
The transparent substrate 1b of the cell 100 and the transparent substrate 6a of the second cell 200 may be integrated to form a two-layer liquid crystal element having two nematic liquid crystal layers and three transparent substrates.

As explained in detail above, the present invention overlaps a conventional liquid crystal cell with another liquid crystal cell without electrodes that compensates for the retardation of the liquid crystal cell with electrodes. There is an advantage in that the occurrence of color unevenness can be reduced without deteriorating the response time and other electro-optical characteristics of a certain TN element.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the configuration of a conventional TN element.
FIG. 2 is a sectional view showing one embodiment of the present invention, and FIG.
The figure is a schematic diagram showing an example of the direction of wall orientation and the alignment state of liquid crystal molecules in the first cell and second cell of the liquid crystal element of the present invention, and FIG. 4 is a schematic diagram of the first cell in another embodiment of the present invention , a schematic diagram showing the direction of wall orientation and the alignment state of liquid crystal molecules in the second cell, and FIG. 5 is a sectional view showing still another embodiment of the present invention. In the figure, 1a and 1b are transparent substrates, 2a and 2b are transparent electrodes, 3 and 7 are spacers, 4 and 8 are nematic liquid crystals, 6a and 6b are transparent substrates, 9a and 9b are polarizing plates, 10, 11, 12, 13 is an arrow indicating the direction of wall orientation. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. The walls of first and second transparent substrates each having an electrode are oriented perpendicular to each other on the surface having the electrode, and the nematic liquid crystal is sandwiched between the substrates so as to face inward to display the pattern of the electrode. The first cell having the above configuration and the second cell having the nematic liquid crystal sandwiched between the third and fourth transparent substrates having no electrodes with their walls oriented perpendicularly or parallel to each other and facing inward. A liquid crystal device characterized in that second and third transparent substrates are stacked so that their respective wall orientations are in the same direction. 2. The liquid crystal element according to claim 1, wherein the second and third transparent substrates are integrated.