JPH04349424A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To improve a visual angle characteristic and visibility by disposing a liquid crystal cell for driving and a specific liquid crystal layer for compensation between two sheets of polarizing plates. CONSTITUTION:This display element is constituted by inserting the liquid crystal layer 2 for compensation and the liquid crystal 3 for driving between two sheets of the polarizing plates 1 and 4 disposed to respectively parallel absorption axes 1, 1, 4, 1. The liquid crystal layer 2 for compensation has a liquid crystal structure interposed with a liquid crystal between transparent substrates. The liquid crystal cell 3 for driving is made into such arrangement that the twist angle is >=360 deg. and the spiral axis thereof is inclined with the basic normal of the liquid crystal 3 for driving, i.e., inclines at a prescribed angle with the negative X-axis direction in the biaxial positive directions, by which the visual angle characteristic is improved and the visibility is improved. The high-grade liquid crystal display element us thus obtd.

Description

[Detailed description of the invention]

0001 [Purpose of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which viewing angle dependence of contrast ratio is controlled.

0003

2. Description of the Related Art In recent years, liquid crystal display devices, which have the great advantages of being thin, lightweight, and low power consumption, have been actively used as display devices for personal office automation equipment such as Japanese word processors and desktop personal computers. Most liquid crystal display elements (hereinafter abbreviated as LCD) use twisted nematic liquid crystal, and the display method is as follows:
It can be roughly divided into two modes: birefringence mode and optical rotation mode.

[0004] Birefringence mode display type LCDs generally have a molecular arrangement twisted by 90° or more (called the ST method) and have steep electro-optic characteristics, so each pixel is equipped with a switching element (thin film transistor or diode). Even without a simple matrix-like electrode structure, a large-capacity display can be easily obtained by time-division driving. However, since this ST method uses the birefringence effect, the display colors are yellow and dark blue (so-called yellow mode display) and white and blue (so-called blue mode display) due to light interference.
Black and white or color display was not possible. As a means to eliminate such discoloration of the display, it was discovered in Japanese Patent Publication No. 63-53528 that a black and white display could be realized by placing a second liquid crystal cell twisted in reverse between the polarizing plate and the liquid crystal cell.
(Applicant: Sharp). The principle behind this black-and-white conversion is that the ordinary light component and the extraordinary light component, which have become elliptically polarized light in a display liquid crystal cell in which liquid crystal molecules are arranged in a twisted arrangement, are exchanged with each other by a second liquid crystal cell, which is an optical compensator. elliptically polarized light is converted into linearly polarized light. the result,
Coloring caused by light interference is eliminated, and black and white display can be achieved. In order to convert elliptically polarized light into linearly polarized light as described above, the optical compensator must have almost the same retardation value as the display liquid crystal cell, and have opposite twist directions. The arrangement is such that the alignment directions of liquid crystal molecules closest to each other are orthogonal. Moreover, in such an LCD, the display color is colored at an oblique angle deviating from the normal line of the display surface, and a black and white display cannot be obtained.

[0005] In addition to the ST method, there is a birefringence mode LCD that uses a liquid crystal cell in which liquid crystals with negative dielectric anisotropy are arranged approximately perpendicular to the substrate (ECB).
method). This method attempts to perform display by applying a voltage to the liquid crystal cell to tilt the liquid crystal molecules with respect to the normal line of the substrate, thereby changing the retardation value of the liquid crystal cell. However, even with this ECB method, Depending on the angle at which the liquid crystal cell is viewed, the retardation value changes significantly, resulting in phenomena such as the displayed image becoming invisible or appearing inverted.

On the other hand, optically rotating mode LCDs have a molecular arrangement twisted by 90°, for example, and have a fast response time (several tens of milliseconds), exhibiting a high contrast ratio and good gradation display, and are therefore used in watches, calculators, and even other devices. are equipped with switching elements for each pixel and are combined with color filters to display full-color LCD TVs (TFT-LCD and MIM-L).
CD) (TN method).

Regarding color display, a CN liquid crystal cell that exhibits a certain hue using selective scattering when no voltage is applied is placed between a polarizing plate and a liquid crystal cell, and a combination of the CN liquid crystal cell and the presence or absence of voltage application to the liquid crystal cell Mol. Cryst. Liq. Cryst. ，
1977, VOL. 39, PP. 127-138.

However, these liquid crystal display elements that utilize birefringence mode, optical rotation mode, and selective scattering have viewing angle dependence in that the displayed color and contrast ratio change depending on the viewing angle and orientation, It has not yet reached the point where it completely exceeds the display performance of .

[0009]

It is generally known that liquid crystal molecules have different refractive indices in the major axis direction and the minor axis direction of the liquid crystal molecules. When light in a certain polarization state is incident on liquid crystal molecules exhibiting such anisotropy of refractive index, the polarization state of the light changes depending on the angle of the liquid crystal molecules. Therefore, the polarization state of the light propagating through the liquid crystal cell is different depending on whether the light is incident perpendicularly to the liquid crystal cell or obliquely. This appears as a phenomenon in which the pattern appears reversed, the display pattern becomes completely invisible, or the display becomes discolored, which is undesirable from a practical standpoint.

The present invention solves the above-mentioned disadvantages.

[0011] [Structure of the invention]

0012

[Means for Solving the Problems] The present invention provides a driving liquid crystal cell between two polarizing plates, which has a twist angle of 360° or more and whose helical axis is inclined with respect to the normal line of the substrate of the driving liquid crystal cell. It is characterized by the arrangement of an aligned compensation liquid crystal layer.

[0013]

[Function] Liquid crystal cells generally used for liquid crystal display elements are
In order to obtain a uniform molecular arrangement over the display area, the substrate surface is subjected to alignment treatment. Such alignment treatment not only aligns liquid crystal molecules on the substrate surface in a specific direction, but also has the effect of tilting the liquid crystal molecules obliquely with respect to the substrate surface. The angle of inclination of the substrate surface relative to the substrate surface is called the pretilt angle, and this value varies depending on the combination of the alignment film used in the liquid crystal cell, the conditions and method of alignment treatment, and the liquid crystal material, and is approximately 1° to 10°. The value of is common. The tilt of the liquid crystal molecules on the substrate surface is
It is also very important to maintain a uniform molecular alignment state when a voltage higher than the threshold voltage is applied to the liquid crystal cell, and also has the effect of prioritizing the direction in which the liquid crystal molecules in the cell are tilted when voltage is applied to the cell. Combined with motsu. If this slope does not exist at all,
When a voltage is applied to the cell, there is no preferred direction in which the liquid crystal molecules tilt, so the liquid crystal molecules in the cell tilt in different directions, resulting in problems such as a decrease in contrast ratio and a display image remaining even after the voltage is removed. occurs. Therefore, the pretilt angle has the advantage of making the molecular arrangement state uniform when voltage is applied. Furthermore, the pretilt angle greatly affects the display characteristics of the liquid crystal display element. Next, using a TN mode liquid crystal display element as an example, the influence of the pretilt angle on display characteristics will be explained.

When the molecular arrangement state is calculated when a voltage value that provides a dark state is applied to a 90° twisted TN type liquid crystal cell, the state is shown in FIG. 1. Here, 7 and 8 in the figure are the tilt angle and twist angle, respectively, and the tilt angle is the xy plane when the substrate surface of the liquid crystal cell is the xy plane in the coordinate system shown in FIG. The twist angle is the angle between the axis of the liquid crystal molecules 6 projected from the z-axis to the xy plane and the x-axis. Even when voltage is applied, liquid crystal molecules do not tilt much near the upper and lower substrate surfaces due to the influence of the alignment regulating force of the substrate surfaces. Further, the twist angle has an S-shaped distribution. When a voltage is applied, the liquid crystal molecules near the center of the liquid crystal cell do not stand completely vertically, but tilt at a certain angle. When Figure 1 is shown three-dimensionally as shown in Figure 3, the liquid crystal molecules 6 near the center of the liquid crystal cell
The direction in which the liquid crystal molecules are tilted depends on the direction in which the liquid crystal molecules on the surfaces of the upper and lower substrates 3a and 3b are tilted, and when the pretilt angles θp on the upper and lower substrates 3a and 3b are equal, the direction is approximately halfway between the directions in which the surfaces of the upper and lower substrates are tilted. become. Place the polarizing plates 1 and 4 on the upper and lower substrates respectively in such an oriented state, and align the absorption axis 1 of the polarizing plates with
When 5 and 16 are arranged to match the orientation directions 13 and 14 of the respective substrates, the direction in which the contrast ratio (bright state brightness/dark state brightness) is highest is the direction of the liquid crystal molecules near the center of the liquid crystal cell. It almost matches the direction of inclination.

FIG. 4 shows the measurement results of the equal contrast ratio-azimuth angle characteristics of a TN type liquid crystal display element. Here, the equal contrast ratio-azimuth angle characteristic indicates the viewing angle characteristic of the liquid crystal display element, and the point at which the liquid crystal display element 10 is observed is defined by the azimuth angle Φ and the viewing angle θ as shown in FIG. The visual angles that show a certain equal contrast ratio when changing the orientation are measured and are shown in polar coordinates as shown in FIG. Therefore, the viewing angle θ is large in directions with a high contrast ratio (indicated numerically as 5, 10, 20, 30, etc. in the figure), and an ideal liquid crystal display element should have a large viewing angle at any direction. is desired. As can be seen in the measurement results in FIG. 4, the orientation directions 13 and 14 of the upper and lower substrates are as shown by the arrows in FIG. If the angle of view is set to 90°, the viewing angle will be widest at 90°.

As described above, the pretilt angle θp greatly affects the viewing angle characteristics of the liquid crystal display element. As described above, currently, liquid crystal display elements do not have the characteristic of exhibiting a substantially uniform contrast ratio no matter where they are viewed, unlike cathode ray tubes (CRTs), and the contrast ratio varies greatly depending on the viewing angle and direction. That is, liquid crystal display elements have viewing angle characteristics, and therefore, it is necessary to expand the viewing angle characteristics of the liquid crystal display element or to design the viewing angle characteristics according to the specifications depending on the use of the liquid crystal display element. A method is required to improve the viewing angle characteristics only in a specific direction.

The present invention achieves the above object, and the principle and method for achieving the object will be explained below.

As described above, liquid crystal display elements have a pretilt angle for manufacturing purposes and for stabilizing alignment, which affects viewing angle characteristics. When a voltage equal to or higher than a threshold voltage is applied to a liquid crystal cell, most of the liquid crystal molecules in the liquid crystal cell, except near the substrate surface, are tilted with respect to the substrate surface. FIG. 6 shows the orientation state of the liquid crystal simply using a three-dimensional refractive index ellipsoid. The birefringence phenomenon is a phenomenon related to the difference in refractive index within a two-dimensional plane when the refractive index ellipsoid 6a is viewed from a certain direction. (when viewed from the front)
The refractive index body (6.2) in the two-dimensional plane is an ellipse, which is different from the refractive index body (6.4) when viewed from the long axis direction (6.3) of the three-dimensional refractive index ellipsoid. Therefore, the place where the contrast ratio is maximum is where the refractive index body in the two-dimensional plane becomes a complete circle, and this is in the long axis direction of the three-dimensional refractive index ellipsoid.

Therefore, as shown in FIG. 7, by changing the major axis direction of the three-dimensional refractive index ellipsoid of the liquid crystal cell using the refractive index ellipsoid 9 for optical compensation, the azimuth and viewing angle at which the contrast ratio is maximized can be determined. can change. As the refractive index ellipsoid 9 for optical compensation, a disk-like shape is preferred, corresponding to the shape of the three-dimensional refractive index ellipsoid 6a of the liquid crystal cell, and the long axis direction (6.3) of the ellipsoid is shown in FIG. By arranging it diagonally to the z-axis as shown in
(direction with a large viewing angle) can be easily changed.

[0020] Such a refractive index ellipsoid for optical compensation is
It has a characteristic that it has an optically negative sign and its optical axis is oblique with respect to the display surface.

A cholesteric liquid crystal cell is an example of a cell exhibiting negative optical anisotropy. Cholesteric liquid crystal has a spirally twisted arrangement of liquid crystal molecules, and while ordinary liquid crystals have positive optical anisotropy, cholesteric liquid crystal has a negative optical anisotropy due to the spirally twisted arrangement. Show your gender. When the twist angle in a cholesteric liquid crystal cell is very large, its optical anisotropy becomes almost symmetrical. Here, for example, by providing a predetermined alignment layer on the substrate surface of a cholesteric liquid crystal cell, the helical axis can be tilted from the normal direction of the substrate. By doing this, the optical anisotropy of the cholesteric liquid crystal cell can be made asymmetrical with respect to the normal direction of the cell, and the viewing angle characteristics of the driving liquid crystal cell can be adjusted to the optical axis tilt of the driving liquid crystal cell. can be fully compensated. Therefore, a liquid crystal display element with a wide viewing angle and high contrast can be obtained. At this time, the alignment layer used on the surface of the substrate may be, for example, an obliquely vapor deposited layer of SiO. At this time, the viewing angle is expanded by tilting the helical axis of the cholesteric liquid crystal cell in a direction that compensates for the optical characteristics of the driving liquid crystal cell.
In addition, the viewing angle direction can be tilted in any direction by tilting the driving liquid crystal cell in a direction that enhances the optical characteristics.

[0022] The explanation has been given above using a TN liquid crystal cell as an example, but not only the TN type but also the ST type, LCD with a display type with a small twist angle of 90° or less, and furthermore, the vertically arranged ECB type liquid crystal display element can be used. A similar effect can be obtained.

It goes without saying that the above-mentioned cholesteric liquid crystal cell can also obtain the same function by using a polymeric liquid crystal layer with twisting properties, and in this case, for example, at least one of the substrates of the driving liquid crystal cell obtained by coating such a polymer liquid crystal layer on the
It is easier to manufacture and a more desirable liquid crystal display element can be obtained. In this case, the inclination of the helical axis can be changed by performing a predetermined alignment treatment on the substrate used or by changing the structure of the polymeric liquid crystal material used. As the polymer liquid crystal to be used, for example, a polymer copolymer liquid crystal having a polysiloxane main chain and side chains containing biphenyl benzoate and cholesteryl groups in an appropriate ratio can be used.

[0024]

EXAMPLES Examples of the liquid crystal display element of the present invention will be described in detail below.

(Embodiment 1) FIGS. 8 and 9 show cell configurations in this embodiment. The liquid crystal display element includes two polarizing plates 1,
4, and a compensating liquid crystal cell 2 and a driving liquid crystal cell 3 are sandwiched therebetween. Polarizing plate 1 is transparent substrate 1a
A polarizing film 1b is attached to the inside of the transparent substrate 4a, and the polarizing plate 4 is similarly formed by attaching a polarizing film 4b to a transparent substrate 4a. Also, the absorption axes (1.1), (4.1) of these polarizing plates 1 and 4
are arranged parallel to each other.

The compensating liquid crystal cell 2 consists of these polarizing plates 1 and 4.
It has a liquid crystal cell structure in which a liquid crystal 2c is interposed between transparent substrates 2a and 2b.

The driving liquid crystal cell 3 is arranged between the compensation liquid crystal cell 2 and the polarizing plate 4. The upper substrate 3a and the lower substrate 3b form transparent electrodes 3c and 3d, respectively, and are connected to a drive power source 3f. A twisted nematic liquid crystal is introduced between the substrates 3a and 3b with a twist angle of 90°, and its state changes depending on the voltage applied from the driving power source 3f.

The optical axis of the liquid crystal of the driving liquid crystal cell 3 is twisted counterclockwise from the lower substrate 3b to the upper substrate 3a (
left-handed twist). (3.1) and (3.2) are the rubbing axes of the upper and lower substrates, respectively, and these are orthogonal to each other.
The y-axis and the rubbing axis (3.1) in the figure are arranged at 45 degrees clockwise when viewed from the z-direction. The retardation value of cell 3 is 450 nm. When a voltage is applied to such a TN cell, the optical axis is in the positive z-axis direction and in the negative x-axis direction.

The compensation liquid crystal cell 2 is a liquid crystal cell twisted by 5490° (15.25 rotations to the right). A film of SiO is obliquely deposited on each substrate surface at an angle of 85° from the normal to the substrate, and (2.1) and (2.2) represent the deposition of SiO on the upper and lower substrates 2a and 2b, respectively. The helical axis in the compensating liquid crystal cell 2 is inclined by 31 degrees from the substrate normal in the positive z-axis direction and in the negative x-axis direction.

The figure shows the equal contrast ratio-azimuth characteristics of the liquid crystal display element of this configuration (bright state: 1V applied from the drive power supply 3f between the liquid crystal cell electrodes; dark state: 5V applied from the drive power supply 3f between the liquid crystal cell electrodes). 10 shows a conventional example along with FIG. Compared to the conventional example, a contrast ratio of 35:1 or more can be obtained up to a viewing angle of 30° in any direction, and the viewing angle characteristics are significantly improved. Furthermore, in contrast to the conventional normally closed system, where the display color in the dark state was colored, a good black color was obtained.

(Embodiment 2) In the configuration of Embodiment 1, the driving liquid crystal cell 3 is an ST type liquid crystal cell with a twist angle of 240° and equipped with a transparent electrode for applying voltage to the liquid crystal layer. The pretilt angle of the liquid crystal is 5°.

The rubbing axes (3.1) are arranged at an angle of +60° counterclockwise as viewed from the z-axis, and the angle between the rubbing axes is 120°. The retardation value of cell 3 is 845 nm. The absorption axis (1.1) of the polarizing plate 1 is 60° counterclockwise when viewed from the x-axis or the z-axis, and the absorption axis (4.1) of the polarizing plate 4 is 150° from the x-axis.

The compensation liquid crystal cell 2 rotates 15.25 times clockwise from the lower substrate to the upper substrate, and each substrate surface has an alignment film, and the surfaces of the two substrates are rubbed so as to be perpendicular to each other. has been done. The helical axis of the compensation liquid crystal layer in the compensation liquid crystal cell 2 is inclined by 5° from the substrate normal.

[0034] When we created a 640 x 400 dot super twist type LCD using this configuration and displayed 16 gradations using the frame thinning method at a duty of 1/200, we found that the contrast was high enough to distinguish between the 16 gradations even when the viewpoint was changed. We were able to realize a LCD that When viewing angle characteristics were measured, 6
A contrast ratio of 12:1 or more was obtained with a 0° cone, and a good black and white display without inversion of the display image or change in display color was obtained even at an incident angle of 60° or more.

(Embodiment 3) In Embodiment 1, a homeotropically aligned liquid crystal cell is arranged as the driving liquid crystal cell 3. The pretilt angle of the liquid crystal is 2° from the substrate normal.

The absorption axis (1.1) of the polarizing plate 1 was arranged counterclockwise when viewed from the z-axis at an angle of 45° to the x-axis.

The compensation liquid crystal cell 2 is a liquid crystal cell twisted by 5490° (15.25 rotations to the right). Each substrate surface has an alignment film, and rubbing is performed so that the two substrate surfaces are perpendicular to each other. The helical axis in the compensating liquid crystal cell 2 is inclined at 2° from the substrate normal.

[0038] When we created an ECB type LCD with 640 x 480 dots using this configuration and drove it in a simple multiplex at a duty of 1/240, we were able to realize an LCD with a high contrast display in which the display pattern could be discerned even when the viewpoint was changed. . When viewing angle characteristics were measured, a contrast ratio of 10:1 or more was obtained at a 60° cone, and good display was obtained without inversion of the display image or change in display color even at an incident angle of 60° or more.

[0039]

According to the present invention, the viewing angle characteristics of the liquid crystal display element are improved, and it is possible to provide a liquid crystal display element with high quality display and excellent visibility. Further, it goes without saying that excellent effects can be obtained even when the present invention is applied to active matrix liquid crystal display elements using three-terminal or two-terminal elements such as TFTs and MIMs.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the molecular arrangement in the thickness direction of a liquid crystal cell when a voltage is applied to the liquid crystal cell.

FIG. 2 is a diagram showing a coordinate system of tilt angles and twist angles of liquid crystal molecules in FIG. 1;

FIG. 3 is a tilt diagram showing the molecular arrangement state of a 90° twisted liquid crystal cell when voltage is applied.

FIG. 4 is a diagram illustrating equal contrast characteristics of a conventional TN type liquid crystal display element.

FIG. 5 is a diagram illustrating the coordinate system of the observation point in FIG. 4.

FIG. 6 is a diagram showing a three-dimensional refractive index ellipsoid with liquid crystal molecules standing upright.

7 is a diagram illustrating a refractive index ellipsoid that optically compensates for the refractive index ellipsoid in FIG. 6. FIG.

FIG. 8 is a cross-sectional view showing a liquid crystal display element of Example 1 of the present invention.

FIG. 9 is an exploded perspective view showing the configuration of a liquid crystal display element according to Example 1 of the present invention.

FIG. 10 is a diagram illustrating the effects of Example 1.

[Explanation of symbols]

1, 4...Polarizing plate 2...Compensation liquid crystal cell 3...Drive liquid crystal cell

Claims (1)

[Claims] 1. A compensating liquid crystal layer arranged between two polarizing plates and having a twist angle of 360° or more with respect to a driving liquid crystal cell and whose helical axis is inclined with respect to the normal line of the substrate of the driving liquid crystal cell. A liquid crystal display element characterized by arranging.