JPH02291526A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To obtain good visibility even if a visual angle deviates to some extent by arranging ferroelectric liquid crystal molecules in such a manner that the pretilt angle thereof opens toward the top direction of an effective screen. CONSTITUTION:The liquid crystal display element is formed by further crimping a pair of substrates which crimp the ferroelectric liquid crystal 2 at a prescribed pretilt angle phi by a pair of polarizing plates 4, 5, the axes of polarization of which intersect orthogonally with each other. One of the bistable directions of the ferroelectric liquid crystal 2 is aligned to the axis of polarization of the one polarizing plate 4 and the other is aligned to the vertical direction of the effective screen of the liquid crystal display element. The respective molecules of the ferroelectric liquid crystal are so oriented that the pretilt angle phi opens toward the upper side of the effective screen with respect to the substrate disposed on the rear surface side of a pair of the substrates. The generation of a trouble, such as degradation in contrast or generation of the color inversion in a display part and a background part is substantially prevented in this way and the excellent visibility is obtd.

Description

Detailed Description of the Invention (Summary of the Invention) The present invention provides a liquid crystal display in which a pair of substrates holding a ferroelectric liquid crystal at a predetermined pretilt angle is further held between a pair of polarizing plates whose polarization axes are orthogonal to each other. In the element, by appropriately setting the alignment direction of the ferroelectric liquid crystal molecules in relation to the orientation treatment direction of the substrate and the relative position with the polarizing plate in relation to the installation direction of the liquid crystal display element, the colors of the display area and the background area can be reversed. This prevents a decrease in contrast.

[Conventional technology]

In recent years, in the field of liquid crystal devices, liquid crystal display elements. With the commercialization of liquid crystal shutters used in optical writing printers, television receivers that display dot matrix images, etc., high responsiveness is required. Ferroelectric liquid crystals exhibiting chiral smectic C phase (hereinafter referred to as S06 phase) or chiral smectic H phase (hereinafter referred to as SH'' phase) have been proposed as liquid crystal materials that meet these requirements. This liquid crystal exhibits ferroelectric behavior in response to a DC electric field, and can respond at high speeds on the microsecond order by reversing its spontaneous polarization depending on the direction of the electric field. This is due to the following properties.

Since ferroelectric liquid crystals have a large spontaneous polarization in a direction perpendicular to the long sleeve of the molecules, it is difficult to align them uniformly in a predetermined alignment direction. For example, even if alignment treatment such as rubbing is performed, unlike the case where nematic liquid crystal molecules are aligned in one direction, there are two stable, almost symmetrical directions that are shifted by an angle θ to the left and right with respect to the alignment direction of nematic liquid crystal molecules. Take. A display element is constructed by making the difference between these two stable directions (bistable directions) correspond to the contrast between brightness and darkness. The angle θ at this time is called the cone angle. In addition, ferroelectric liquid crystal has a spiral molecular arrangement structure from one substrate to the other, but as mentioned above, it can take two stable directions, so it A state in which surrounding molecules and counterclockwise molecules are mixed occurs,
So-called jittery defects tend to appear on the screen. In order to prevent this, liquid crystal molecules are aligned in advance by tilting them at a predetermined angle with respect to the substrate. The angle at this time is called the pretilt angle φ.

[Problem to be solved by the invention]

However, when a ferroelectric liquid crystal is provided with a pretilt angle φ, there are disadvantages such as a marked decrease in contrast depending on the viewing direction of the display screen, or color reversal between the display area and the background area.

Therefore, an object of the present invention is to provide a liquid crystal display element that is less likely to suffer from these disadvantages and has excellent visibility.

[Means to solve the problem]

As a result of extensive studies to achieve the above object, the inventors of the present invention found that the visibility of the liquid crystal display element can be improved if the direction in which contrast ratio decreases and color inversion occurs is directed in a direction that is not normally used by the viewer. They have found that this is a significant improvement and have completed the present invention.

That is, the liquid crystal display element according to the present invention is a liquid crystal display element in which a pair of substrates holding a ferroelectric liquid crystal at a predetermined pretilt angle is further held between a pair of polarizing plates whose polarization axes are orthogonal to each other, One of the bistable directions of the ferroelectric liquid crystal is aligned with the polarization axis of one polarizing plate,
The other of the bistable directions of the ferroelectric liquid crystal described above coincides with the vertical direction of the effective screen of the liquid crystal display element. The ferroelectric liquid crystal is characterized in that each molecule of the ferroelectric liquid crystal is oriented such that a pretilt angle opens toward the upper side of the effective screen with respect to the substrate placed on the rear side of the pair of substrates. be.

[Function] The direction in which visibility decreases in a liquid crystal display element using a ferroelectric overnight product depends on the viewing direction of liquid crystal molecules. According to the inventor's experiments, a liquid crystal display element in which one of the bistable directions is aligned with the polarization axis of one polarizing plate,
It was found that the contrast change was greatest when viewed while changing the viewing angle in a field that included the long axis of the ferroelectric liquid crystal molecules that took the other of the bistable directions and was perpendicular to the substrate, making it difficult to use. Furthermore, if the ferroelectric liquid crystal molecules are oriented with a pretilt angle φ with respect to the substrate,
It has been found that if the upper and lower sides of the effective screen are set in such a direction that the viewer sees a cross section perpendicular to the long sleeve of the ferroelectric liquid crystal molecules on the rear substrate, the contrast is significantly reduced.

In the optical/crystalline display element according to the present invention, the directions in which these inconveniences occur are all directed above the effective screen, and are excluded from the normal viewing angle range of the viewer. Therefore, a liquid crystal display element that maintains good visibility even when the viewing angle is slightly shifted is available.

[Embodiments] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

First, the directional relationship of each element in the liquid crystal display element according to the present invention will be explained using FIG. Ferroelectric liquid crystal molecules (
2) is a substrate (1) that has been subjected to a rubbing process in the direction R in advance.
It is oriented in directions A and B, which are tilted left and right by a cone angle θ, with the center being the center. The above ferroelectric liquid crystal molecules (
2), when holding the liquid crystal cell containing the polarizing plate between two polarizing plates, the polarizing axis P1 of the first polarizing plate is aligned with the direction A, and the polarizing axis P1 of the second polarizing plate is aligned with the direction A. is set in a direction perpendicular to direction A.

Note that the direction A and the direction B may be opposite to each other.

In any case, the reason why the polarization axis of one polarizing plate is aligned with either direction A or direction B is to maximize the contrast ratio. Light incident on a liquid crystal cell is refracted in the long axis direction due to the refractive index anisotropy of ferroelectric liquid crystal molecules.
When the polarization axes of the two polarizing plates are orthogonal as described above,
If the polarization plane of the incident light rotates 90 degrees while passing through the liquid crystal cell, that is, if direction A and direction B are perpendicular, the intensity of the transmitted light will be maximum, but if the rotation angle of the polarization plane is 90 degrees, If the angle is less than 1, only the vector component corresponding to the rotation angle is transmitted, and the intensity of the transmitted light is reduced.

Therefore, in the case of Figure 1, the darkest field of view is in the part where the ferroelectric liquid crystal molecules (2) are arranged along direction A, and the brightest field is in the part where the ferroelectric liquid crystal molecules (2) are arranged along direction B. This is the part that is being done. The area within the square angle formed by direction A and direction B becomes a visual field having intermediate brightness.

As mentioned above, the ferroelectric liquid crystal molecules (2) aligned along directions A and B are further oriented at a pretilt angle φ with respect to the plane of the substrate (1).
When displayed three-dimensionally in the yz coordinate system, it looks like Figure 2. For simplicity, illustration of the board is omitted here. In this figure, the surface of the liquid crystal layer (3) that corresponds to the back side when viewed from the viewer (6) is placed in the xy plane, and the vertical direction (vertical direction) of the effective screen is the y axis, The thickness direction is aligned with the two axes. Two polarizing plates whose optical axes are orthogonal to each other,
That is, the first polarizing plate (4) and the second polarizing plate (5) are
It is placed so as to sandwich the liquid crystal layer (3) and to be parallel to the xy plane.

Now, a ferroelectric liquid crystal molecule (2) is placed at the origin 0 of the coordinate system. The above ferroelectric liquid crystal molecules (2) are arranged in the direction C and 2
Taking direction D that forms an angle of θ, the projection of direction C onto the xy plane is the projection of direction A and direction D onto the xy plane in Fig. 1 (
2) corresponds to the direction B in FIG. 1. In other words, the angle formed by direction A and direction C (hereinafter referred to as
It is called ZAOC. ), and the angle formed by direction B and direction D (hereinafter referred to as /BOD) is the pretilt angle φ. Direction A coincides with the polarization axis P2 of the second polarizing plate (5) that corresponds to the back side from the observer (6), and the direction A coincides with the polarization axis P2 of the second polarizing plate (5) that corresponds to the back side when viewed from the observer (6). The polarization axis P1 of is orthogonal to the polarization axis P2. The above-mentioned direction B coincides with the vertical direction (vertical direction) of the effective screen. As mentioned above, if you look at a liquid crystal display element with one of the bistable directions aligned with the polarization axis of one of the polarizing plates from various directions, you will notice that it is a ferroelectric liquid crystal that has the other bistable direction. The plane containing the long sleeve of the molecule and perpendicular to the substrate (
This is based on the fact that the contrast change was greatest when viewed while changing the viewing angle within the plane (which is the locus when changing /BOD in Figure 2). In other words, direction B is an observation direction with a narrow permissible viewing angle range in which good image quality can be guaranteed. If the direction B is oriented in a direction where the viewing angle can vary greatly in the normal usage of equipment with a liquid crystal display element (generally in the left-right direction of the effective screen), the visibility of the liquid crystal display element will be significantly reduced. Therefore, this direction is directed toward the vertical direction, where there is relatively little variation in visual angle. Next, the ferroelectric liquid crystal molecules (2) are not aligned parallel to the substrate, but at a predetermined pretilt angle φ, so that
The spatial arrangement of the ferroelectric liquid crystal molecules (2) differs depending on which of the vertical directions the above-mentioned direction B is actually directed to.
The optical characteristics of the liquid crystal display element are also affected. In the present invention, the ferroelectric liquid crystal molecules (2) are arranged so that the pretilt angle φ opens toward the top of the effective screen on the xy plane. This is a device to eliminate the possibility of viewing the ferroelectric liquid crystal molecules (2) from a cross-sectional direction perpendicular to the long sleeve, and to increase the contrast ratio. In the first place, ferroelectric liquid crystal can be applied to such a liquid crystal display element because of the refractive index n along the long sleeve direction of the molecule and the refractive index n perpendicular to the long axis direction of the molecule. This is because there is a difference Δn between the two, that is, refractive index anisotropy.

The magnitude of the refractive index anisotropy Δn varies depending on the direction in which the molecule is observed, and is minimum when the molecule is observed in a cross-sectional direction perpendicular to the long sleeve, and maximum when the molecule is observed in a plane that includes its long axis. Therefore, it is preferable for the observer (6) to observe from a direction in which the long axis appears to be as long as possible, and for this purpose, the pretilt angle φ must not open toward the observer. Generally, when looking at the displays, billboards, bulletin boards, etc. of various OA devices, there is little chance of looking down at the displayed information from the sky, but rather in many cases looking up. In particular, when the liquid crystal display is relatively close to a horizontal surface and is integrated with an operating section such as a keyboard, as is the case with some portable word processors, the viewing direction is extremely limited. Therefore, it is almost unthinkable to turn the display upside down and look at it from the sky. Therefore, by setting the direction in which the pretilt angle φ on the back side substrate opens as the upward direction of the effective screen as described above, a good contrast ratio can always be ensured.

By the way, in order to achieve high-speed response and uniform image quality in a liquid crystal display element using ferroelectric liquid crystal, molecules are arranged parallel to each other on the inner surfaces of two substrates that sandwich the liquid crystal layer. It is desirable that Figure 3 shows an example of a liquid crystal cell with such molecular alignment. This liquid crystal cell is made in advance! A liquid crystal layer (15) is sandwiched between two substrates on which a transparent electrode layer (13) made of To (indium tin oxide) or the like and an alignment film (l4) having sawtooth surface irregularities are formed in sequence. It has the following configuration. Here, the substrate facing the observer (17) side is the front substrate (11), and the substrate that is not directly visible from the observer (17) side is the back substrate (l2).
). On the surface of the alignment film (14), the ferroelectric liquid crystal molecules (16) are aligned with a constant pretilt angle φ due to the shape effect. Here, the pretilt angle φ is the acute angle formed between the center line of the cone, which is the locus of the precession of the individual ferroelectric liquid crystal molecules (16), and the substrate surface.
The ferroelectric liquid crystal molecules (16) on the inner surfaces of the front substrate (1l) and the rear substrate (12) are parallel to each other. Therefore, if the pretilt angle φ opens upwards on the screen on the inner surface of the rear substrate (12), the pretilt angle φ opens downwards on the inner surface of the front substrate (11). In such a liquid crystal display element, characters. Naturally, information such as images should be displayed in accordance with the vertical direction of the active screen. For example, as shown in FIG.
When character A is recognized through i(4), the top and bottom of the screen and the top and bottom of character A are matched, eliminating the possibility that the top and bottom will be observed with the top and bottom reversed. As a result, the visibility of the liquid crystal display element is always maintained at a good level.

〔Effect of the invention〕

As is clear from the above description, by applying the present invention, it is possible to realize a liquid crystal display element that has high-speed response, bistable memory properties, and good visibility even if the viewing angle is slightly shifted.

The above-mentioned directionality can be controlled by simple substitutions such as changing the orientation treatment direction or the cutting direction of the polarizing plate, which greatly increases the complexity of the manufacturing process, increases costs, and degrades productivity. It can be provided without any additional charges.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing the directional relationship of each element in the liquid crystal display element of the present invention, Fig. 2 is a schematic perspective view showing the orientation state of ferroelectric liquid crystal molecules in three dimensions, and Fig. 3 is a plan view showing the directional relationship of each element in the liquid crystal display element of the present invention. FIG. 2 is an enlarged cross-sectional view of a main part showing the alignment state of liquid crystal molecules. 1,11.12 Substrate Ferroelectric liquid crystal molecule Liquid crystal layer First polarizing plate Second polarizing plate

Claims (1)

[Scope of Claims] A liquid crystal display element comprising a pair of substrates holding a ferroelectric liquid crystal at a predetermined pretilt angle, which is further held between a pair of polarizing plates whose polarization axes are perpendicular to each other. One of the stable directions is aligned with the polarization axis of one of the polarizing plates, and the other of the bistable directions of the ferroelectric liquid crystal is aligned with the vertical direction of the effective screen of the liquid crystal display element, and the ferroelectric liquid crystal A liquid crystal display device characterized in that each of the molecules is oriented such that a pretilt angle opens upward of an effective screen with respect to the substrate disposed on the rear side of the pair of substrates.