JPH03215830A - 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 (Field of Industrial Application) The present invention relates to improvements in liquid crystal display elements, particularly so-called DAP type liquid crystal display elements. Specifically, the present invention relates to a simple matrix-driven DAP type liquid crystal display element.

(Prior Art) There are various types of liquid crystal display elements, such as TN type liquid crystal display elements, STN type liquid crystal display elements, and DAP type liquid crystal display elements, and their low voltage operation and low power consumption have made them suitable for a wide range of applications. being done.

Among these, TN type liquid crystal display elements have a black and white display and have a simple structure, so they are widely used mainly in watches and calculator displays. Without it, it cannot be used for large capacity display.

The STN type liquid crystal display element has excellent voltage transmittance steepness, but although it is capable of displaying a large capacity, it has the problem that the display is colored. A method of displaying black and white using a compensation plate has also been developed, but the structure is complicated and the cost is high.

In particular, the precision of cell thickness is strict, which leads to poor productivity.

The DAP type liquid crystal display element is a display method that has been known for a long time.It has excellent steep voltage transmittance characteristics and can display a large capacity. It has the feature of being able to display colors.

Next, the structure and principle of a DAP type liquid crystal display device will be explained. FIG. 5 shows a cross-sectional view of the element. Transparent electrode 1
2. A pair of transparent substrates 11.22 such as glass or plastic. A liquid crystal layer 3 made of a liquid crystal composition having negative dielectric anisotropy is formed between the outer peripheral seal 14. It is sealed by 24. Substrate 11.
21 and transparent electrode 12. The liquid crystal is placed on the side of the substrate 11.22 that is in contact with the liquid crystal. Alignment films 15 and 25 are formed so as to be oriented perpendicularly to 21. Also, liquid crystal cell 30
A pair of polarizing plates 16. 26 are provided. When no voltage is applied, the liquid crystal is oriented perpendicularly to the substrates 11 and 21, and at this time, no birefringence occurs due to the liquid crystal, and the upper and lower polarizing plates 16. When the polarization axes of 26 are perpendicular to each other, a black display is obtained. When a voltage is applied between the upper and lower electrodes, liquid crystal molecules 4
0 is the substrate 11.0 as shown in FIG. 6(b). θ for 21
If the direction of the tilt is other than parallel or perpendicular to the transmission axis or absorption axis of the polarizing plates 16, 26, birefringence occurs;
Transmits light. The spectrum of transmitted light corresponds to retardation, and by adjusting the thickness of the liquid crystal layer 3, the birefringence of the liquid crystal, and the applied voltage, it is possible to display white or colored.

(Problems to be Solved by the Invention) However, the conventional DAP type liquid crystal display element has the following problems.

(1) The viewing angle is narrow.

Optically, it is considered a uniaxial crystal, so its birefringence changes greatly depending on the viewing angle. Therefore, even if a black color is obtained when viewed from the front when no voltage is applied (or when an effective value voltage below the threshold voltage is applied), light leakage occurs when viewed from an angle. Similarly, the color and brightness when voltage is applied also depends on the field of view.

Methods have been developed to reduce field-of-view dependence when no voltage is applied (or when an effective value voltage below the threshold voltage is applied) using a compensation plate, but it is hardly effective when a voltage above the threshold voltage is applied. It's fleeting.

(1) It is difficult to control the alignment of liquid crystals.

In order to obtain a homogeneous display, it is necessary to control the direction in which the liquid crystal tilts when voltage is applied. For this reason, it is necessary to control the orientation so that the orientation when no voltage is applied is not completely vertical, but tilted from the vertical direction.

FIG. 6(a) is a diagram showing the tilt angle, in which the liquid crystal molecules 40 are tilted at an angle of θ from the vertical direction. It is tilted in advance by -3. If this tilt angle is small, the direction in which the liquid crystal tilts when a voltage is applied is not uniform, causing display unevenness due to an orientation defect called reverse tilt, resulting in a problem of reduced contrast. If the tilt angle is increased, the above-mentioned alignment defects will not occur, but the viewing angle will become narrower, the steepness of the voltage transmittance characteristics will worsen, and the contrast will decrease when moving in a simple matrix display, resulting in impaired display performance. The problem arises that

(3) Poor productivity.

Furthermore, conventional DAP type liquid crystal display elements have drawbacks in terms of manufacturing. Conventionally, the preferred tilt angle is 1° to 3'', but a method for controlling such alignment is to apply a vertical alignment agent such as alkoxysilane having a long-chain alkyl group onto the substrate and then perform a rubbing treatment. Method, Si
A method is known in which O is obliquely evaporated onto a substrate, subjected to horizontal alignment treatment, and then the surface is treated with a vertical alignment agent. However, the reproducibility of the tilt angle of the liquid crystal obtained by these methods is poor, and therefore there is a problem in that the yield is likely to decrease.

The objects of the present invention are to provide a wide viewing angle, especially the viewing angle of the display section, which has been difficult to improve with conventional methods, to avoid alignment defects even at small tilt angles, to provide high contrast, and to facilitate productivity by facilitating alignment processing. An object of the present invention is to provide a DAP type liquid crystal display element having high alignment stability.

(Means for Solving the Problems) In order to achieve the above object, the present invention comprises two substrates subjected to vertical alignment treatment, and a nematic liquid crystal and an optically active substance sealed between the substrates. It is composed of a liquid crystal composition having negative dielectric anisotropy and a pair of polarizing plates, and is configured such that when a voltage is applied, liquid crystal molecules take a twisted orientation in the thickness direction in an angle range of 100' to less than 400'. A liquid crystal display element having characteristics is provided.

FIG. 1 shows a cross-sectional view of an example of the structure of a liquid crystal display element according to the present invention.

The embodiment shown in FIG. 1 basically consists of a liquid crystal cell 63 and a pair of polarizing plates 5!11. The liquid crystal cell 63 has a nematic liquid crystal and an optically active material between a pair of transparent substrates 52, 53 such as glass or plastic having transparent electrodes 50, 51. A liquid crystal layer 54 having negative dielectric anisotropy is arranged, and an outer peripheral seal 55.
56, the structure is sealed. Transparent substrate 52. 53 and transparent electrode 50. On the sides of 51 in contact with the liquid crystal layer 54, alignment films 57 and 58 are formed, in which the liquid crystal is aligned perpendicularly to the substrate, preferably slightly inclined.

When no voltage is applied, the liquid crystal is on the transparent substrate 52. 53, and at this time, birefringence due to the liquid crystal does not occur,
Upper and lower polarizing plates 59. When the polarization axes of 60 are orthogonal, a black display is obtained. When a voltage is applied, the liquid crystal molecules are tilted with respect to the substrate, and the direction of the tilt is aligned with the polarizing plate 59. 6
If the axis is not parallel or perpendicular to the zero transmission axis or absorption axis, birefringence occurs and light is transmitted. When this voltage is applied, in this embodiment, the liquid crystal molecules 40 are arranged at 10 mm in the thickness direction of the substrate.
It is configured to have a twisted orientation in an angular range of 0° or more and less than 400°.

-7-Such an alignment can be achieved by using the following composition (shown in (■)) as a liquid crystal composition, and by aligning the liquid crystal on a substrate by tilting it at a slight angle in a specific direction ((■ )).

(1) Liquid crystal composition: A mixed composition of nematic liquid crystal and optically active substance with negative dielectric anisotropy, specifically nematic liquid crystal with negative dielectric anisotropy and a small amount of cholesteric liquid crystal. It is a mixed composition of Alternatively, a mixed composition of a nematic liquid crystal with negative dielectric anisotropy and a small amount of a non-liquid crystal optically active substance may be used.

A twisted structure is induced in liquid crystals by optically active substances.

Induced natural pitch (pitch without orientation control)
There are favorable conditions for the relationship between P0 and the thickness d of the liquid crystal layer.

This relationship is expressed as d/Po, where 0.1≦d/P0≦1.
It is preferable that it is 2.

However, the more preferable range depends on the twist angle.

For example, when the twist angle during voltage application is 180', d/Po is preferably 0.2 to 0.8.

If d/P is smaller than this range, the desired -8-angle of twist cannot be obtained, and the effects of the present invention will be reduced. On the contrary, d/P
If o is large, a scattering structure will be generated when a voltage is applied, which will significantly impair display quality.

A more preferable upper limit of d/P is 0.9. Also,
The reason why the twist angle is set to 100° or more and less than 400° in this example is that if the twist angle when voltage is applied is smaller than 100°, the effect of improving the viewing angle is difficult to be achieved. This is because if the angle is larger than 400°, the steepness of the voltage transmittance characteristics deteriorates, the contrast tends to decrease, and the above-mentioned scattering structure tends to occur.

The amount of the optically active substance to be added depends on the thickness of the liquid crystal, the type of nematic liquid crystal, and the type of the optically active substance, so it cannot be stated unconditionally, but it is generally in the range of 0.5% to 0%.

(n) Orientation treatment: A vertical alignment treatment, preferably a slightly inclined vertical alignment treatment, is employed. As alignment agents, alkoxysilanes having long-chain alkyl groups, alkoxytitanium, alkoxyzirconium, organometallic compounds such as long-chain alkylcarboxylic acids or chromium complexes of fluorine W-substituted alkylcarboxylic acids, fluorine-substituted polyalkylene resins, etc. are generally used for vertical alignment treatment. Materials used in can be used.

The alignment film formed from the above material is preferably subjected to a rubbing treatment in one direction. Further, an oblique vapor deposition method of Si◯ can also be adopted. A preferred pretilt angle is in the range of 0.1° to 5°, more preferably 0.
．． It ranges from 1° to 3°. The liquid crystal display element of this example has the characteristic that tilt disclination is less likely to occur than the conventional DAP type liquid crystal display element, and cells can be manufactured without alignment defects even at very small tilt angles. Therefore, the scope of use of the alignment agent can be greatly expanded. Furthermore, when a liquid crystal cell is manufactured with a small pretilt angle, the viewing angle can be further increased, which is an extremely excellent feature in terms of display quality. Note that the present invention does not limit the method of alignment treatment.

The verte tilt direction of liquid crystal molecules on the substrate surface is determined by the rubbing direction in the rubbing method, and by the deposition direction in the oblique deposition method. As shown in FIG. 2, in this embodiment, the liquid crystal molecules 40 take a twisted orientation with a tilt angle when a voltage is applied, but at this time, it is preferable that the liquid crystal molecules 40 take a splay orientation as shown in FIG. Without,
The reverse tilt suppressing effect will be reduced. Therefore, in this embodiment, it is preferable to control this pellet tilt direction. Therefore, the angle (α) formed by the pretilt directions of the upper and lower substrates defined in Figure 4 is in the same direction as the natural twist angle (= d / P, x360°) determined by the twist direction d / Po determined by the optically active material. It is necessary that the angle is within 1006 to 400°, and preferably within twice the natural helix angle. Note that FIG. 4 is a projection view of the liquid crystal molecules on the substrate onto the substrate surface, where the solid line arrow indicates the upper substrate side, the broken line arrow indicates the lower substrate side, and the arrow side at the angle α indicates the top surface of the paper.

Note that polarizing plates 59. The transmission axis of 60 is preferably set to form an angle of approximately 30'' to 60' with the direction of pretilt of liquid crystal molecules on adjacent substrates.

In the conventional DAP type liquid crystal display element, the birefringence changes greatly depending on the viewing angle, so the viewing angle dependence was large.However, in the liquid crystal display element of the present invention, when a voltage is applied, the liquid crystal changes as shown schematically in Figure 2. Since the orientation direction has a twisted structure, changes in birefringence are averaged over the viewing angle, resulting in a wide viewing angle.

(Example) Examples of the present invention will be shown below.

Example 1: A silane-based vertical alignment agent ○DS-E manufactured by Chisso Corporation was applied to a glass substrate having a transparent electrode made of ITO (Indium Tin Oxide), dried at 120°, and then rubbed in one direction with a cotton cloth. Processed. Perform the same process on the other substrate, and place both substrates so that the alignment film surfaces face each other.
The spacers were bonded together so that the rubbing directions were parallel. A mixed liquid crystal composition of a liquid crystal composition EN37 manufactured by Chisso Corporation, which has negative dielectric anisotropy, and 8811 manufactured by Merck, which is a cholesteric liquid crystal, is injected into the gap between both substrates,
A liquid crystal cell was created.

-12 - The pitch of the liquid crystal used here was 15 μm, and the thickness of the liquid crystal was 7.5 μm. The pretilt angle was 0.2°. A pair of neutral gray linear polarizing plates were placed above and below this liquid crystal cell so that their polarization axes were perpendicular to each other and formed an angle of 45° with the rubbing direction.

This liquid crystal display element is black when no voltage is applied;
．． It became colorless by applying a voltage of 8V.

No alignment defects such as tailed disclination were observed, and an extremely uniform display was obtained. When the viewing angle during voltage application was compared with that of a comparative example without twisting, the liquid crystal display of the present invention showed less change in both color and brightness, indicating that the liquid crystal display of the present invention has superior display performance. This was confirmed.

On a glass substrate with a transparent electrode made of ITO (Indium Tin Oxide), SiO was obliquely deposited to a thickness of about 300 angstroms from the direction 60' from the normal line of the substrate, and then SiO was deposited with a silane-based film manufactured by Chisso Corporation. A vertical alignment agent ○DS-E was applied and dried at 120°C. A similar process was applied to another substrate, and both substrates were bonded together via a spacer so that the alignment film surfaces faced each other and the vapor deposition directions were antiparallel. A liquid crystal composition EN37 manufactured by Chisso Corporation and having negative dielectric constant anisotropy was injected into the gap between both substrates to produce a liquid crystal cell. The thickness of the liquid crystal layer is 7.5 μm. The pretilt angle was 2°. A pair of neutral gray linear polarizing plates were placed above and below this liquid crystal cell so that their polarization axes were perpendicular to each other and formed at an angle of 45° with the vapor deposition direction.

This liquid crystal display element is black when no voltage is applied. 2
．． It becomes colorless by applying a voltage of 8V. Although no tilt disclination occurred and a uniform display was obtained,
The viewing angle was narrower than that of the liquid crystal display element of the example.

Comparative Example 2 A silane-based vertical alignment agent ODS-E manufactured by Chisso Corporation was applied to a glass substrate having a transparent electrode made of IT○ (Indium Tin Oxide), dried at 120°C, and then rubbed in one direction with a cotton cloth. did. The other substrates were subjected to the same treatment, and both substrates were bonded together via a spacer so that the alignment film surfaces faced each other and the rubbing directions were antiparallel. A liquid crystal composition EN37 manufactured by Chisso Corporation and having negative dielectric constant anisotropy was injected into the gap between both substrates to produce a liquid crystal cell. The thickness of the liquid crystal layer is 7.5 μm. A pair of neutral gray linear polarizing plates were placed above and below this liquid crystal cell so that their polarization axes were perpendicular to each other and formed an angle of 45° with the rubbing direction.

This liquid crystal display element is black when no voltage is applied;
．． Although it became colorless by applying a voltage of 8 V, many tilt disclinations and the like occurred, resulting in extremely non-uniform display.

(Effects of the Invention) As explained above, the present invention adds an optically active substance to the liquid crystal composition of a DAP type liquid crystal display element and gives the liquid crystal molecules a twisted structure when a voltage is applied. It effectively improves dependence. In addition, it provides a liquid crystal display element with uniform display without alignment defects. Furthermore, since no alignment defects occur even at a small pretilt angle, a liquid crystal display with high contrast and a wide viewing angle can be obtained. Furthermore, since the alignment process is easy, productivity is high and a liquid crystal display element with excellent alignment stability can be easily provided.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display element showing one embodiment of the present invention, FIG. 2 is a diagram showing twisted orientation according to this embodiment, FIG. 3 is a diagram showing splay orientation, and FIG. 4 is a diagram showing this embodiment. FIG. 5 is a cross-sectional view of the liquid crystal display element, and FIGS. 6(a) and (b) show the state of liquid crystal molecules when no voltage is applied and when voltage is applied, respectively. FIG. 52. 53... Transparent substrate, 54... Liquid crystal layer, 5
9. 60...Polarizing plate. -16- 勺（） （） 梠

Claims (1)

[Claims] It consists of two substrates that have been subjected to vertical alignment treatment, a liquid crystal composition with negative dielectric anisotropy made of nematic liquid crystal and an optically active substance sealed between the substrates, and a pair of polarizing plates. 1. A liquid crystal display element characterized in that, when an electric current is applied, liquid crystal molecules are twisted in an angle range of 100° or more and 400° or less in the thickness direction.