JPH07175065A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To obtain a wide region of high contrast in the visual direction while making the visual characteristics uniform by making the outermost film of a substrate such a surface state that liquid crystal molecules are tilted in the direction of the normal line of the substrate. CONSTITUTION:The liquid crystal display element is formed to satisfy the condition of 0<=d/p (< or ) 0.75, wherein (p) is the chiral pitch of a chiral nematic liquid crystal and (d) is thickness of the liquid crystal layer in a direction where it is held between glass substrates. Namely, when expressed in terms of an angle, (p) and (d) are determined to have an optical rotatory power of 0 deg. to about 270 deg.. The liquid crystal with a specified chiral pitch (p) is injected and sealed between transparent glass substrates 1, 2 arranged parallel to each other with a gap (d). In this case, the outermost film of the substrate is preferably made of a film having a similar mol. number and molecular structure to the liquid crystal molecules to be used, for example, a polymer film such as polyimide film. It is preferable that the film has such a surface state that the liquid crystal molecules are tilted to the normal line of the substrate at several degrees (0.1 deg. to 27 deg.).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device,
In particular, it relates to a liquid crystal display element that does not require alignment treatment.

[0002]

2. Description of the Related Art A liquid crystal display element used for a display or the like changes the state of a specific molecular arrangement of liquid crystal to another different molecular arrangement by an external action such as an electric field, and visually recognizes the optical characteristics between them. It is used for display as a change. In order to bring the liquid crystal molecules into a certain specific alignment state, it is usual to perform an alignment treatment on the surface of the glass substrate sandwiching the liquid crystal.

In a conventional twisted nematic (TN) type liquid crystal display element, a so-called rubbing method is employed as an alignment treatment in which glass sandwiching liquid crystal is rubbed in one direction with cloth or nylon.

In practice, the rubbing directions are such that the rubbing directions are orthogonal to each other between the upper and lower substrates, and when the liquid crystal display element is a negative display, a parallel Nicol polarizing plate sandwiching the element is used as its polarization axis. Is placed so that it is parallel to one of the rubbing directions, and in the case of positive display,
Polarizing plates of crossed Nicols are arranged so that their polarization axes are parallel to the rubbing direction of the substrate.

[0005]

When such a rubbing alignment treatment is performed, since the alignment state of the liquid crystal molecules is uniform on the display screen, the angle at which the display is easy to see when the observer looks at the screen is specified. The viewing angle characteristic is limited to the angular range.

FIG. 3A is an example of a plot of contrast value and contour value representing the viewing angle characteristic of the TN type liquid crystal display element. In FIG. 3A, the normal direction of the liquid crystal display element is θ.
= 0, the angle θ from the normal line is taken as a center with respect to that, and the observation direction in the horizontal plane is indicated by the azimuth angle φ. The definition is shown in FIG.

The plot of FIG. 3A shows the distribution of isocontrast lines. Three contrast values are shown by varying the shape of each plot. Figure 3
As shown in (A), it can be seen that the viewing angle region with high contrast is biased to a specific angle region. Therefore, such a display element is easy to see from a certain direction,
It has a viewing angle dependency that it is difficult to see from another direction.

In the liquid crystal display device having such a viewing angle dependency, a certain angle (φ in FIG. 3A) with respect to the display screen.
= 180 °), the contrast is extremely reduced, and in extreme cases, the brightness of the display is reversed.

The view angle characteristic as shown in FIG.
This is because the rubbing process causes the liquid crystal molecules to be aligned in a substantially constant direction while having a pretilt. The direction in which the liquid crystal has pretilt coincides with the rubbing vector direction indicated by the arrow in FIG.

When a voltage is applied to the liquid crystal, the liquid crystal molecules rise in the pretilt direction, so that the optical rotatory power is easily eliminated when observed from that direction. Therefore, the direction in which the vector ends is the easiest to see.

A schematic sectional view of a conventional liquid crystal display device is shown in FIG.
It is shown in FIG. Liquid crystal molecules 3 are formed on the surface of the lower substrate 2.
Indicates a rising upward pretilt. Therefore,
This liquid crystal display element is easy to see when viewed from the right side in the figure, but there is a direction in which contrast deteriorates in other directions.

Further, during rubbing, static electricity due to friction may be generated to cause dielectric breakdown in the alignment film, or display defects may be caused by defective alignment in that portion. Further, in a liquid crystal display element adopting an active drive system, a drive element such as a TFT (thin film transistor) or MIM diode or wiring may be destroyed by rubbing.

Further, a large amount of minute dust is generated during rubbing, and the dust adheres to the substrate due to static electricity, which may cause defective display such as gap defect of the liquid crystal display element and black spots and white spots.

As a liquid crystal display element which does not require such rubbing and a method for manufacturing the same, Japanese Patent Application No. 4-274 that positively utilizes a multi-domain as a set of micro-domains.
There is 236652.

However, when an interface film having a liquid crystal pretilt of 0 ° is used in this method, the region of high contrast in the viewing angle direction is narrow as shown in FIG. 5 and is high to improve the display quality. There is a drawback that a driving voltage is required.

The object of the present invention is to eliminate the need for rubbing treatment which causes non-uniformity in the viewing angle characteristics and causes product defects such as display defects and element destruction, and which makes the viewing angle characteristics uniform and provides a high contrast region in the viewing angle direction. An object of the present invention is to provide a liquid crystal display device that can be widened.

[0017]

A liquid crystal display device of the present invention comprises a pair of substrates having no unidirectional alignment structure with respect to liquid crystal molecules and having a gap d, and a chiral nematic liquid crystal layer sandwiched between the substrates. In the liquid crystal display element having, the film on the outermost surface of the substrate has a surface state in which the film is inclined with respect to the liquid crystal molecules in the direction normal to the substrate.

[0018]

Since the film on the outermost surface of the substrate does not have an alignment structure, liquid crystal molecules on the surface of the substrate are oriented in various directions with respect to the in-plane direction of the substrate. Since no rubbing is performed, various problems due to rubbing do not occur.

On the other hand, due to the surface condition of the film on the outermost surface of the substrate, the liquid crystal molecules are, on average, slightly inclined toward the normal line direction of the substrate. As a result, the display contrast with respect to a constant drive voltage is increased due to a decrease in the threshold voltage of the liquid crystal display element.

[0020]

Embodiments of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is a schematic cross-sectional view of a liquid crystal display device according to the present invention, in which 1 and 2 are transparent glass substrates and 3 is a chiral nematic liquid crystal molecule.

As the method of manufacturing the liquid crystal cell, the process according to the conventional technique can be used as it is. However, the liquid crystal display device is manufactured under the following conditions in order to eliminate the need for alignment treatment, that is, rubbing.

When the chiral pitch of the chiral nematic liquid crystal is p and the thickness of the liquid crystal layer in the direction sandwiched by the glass substrates is d, 0 ≦ d / p (<or ≈) 0.75
The liquid crystal display element is formed so as to satisfy the following condition. That is, in terms of angles, p and d are determined so as to have an optical rotatory power of 0 ° to about 270 °.

For example, a liquid crystal having a chiral pitch p of d / p = 0.25 (corresponding to a 90 ° twist) is injected between the transparent glass substrates 1 and 2 having a gap d arranged in parallel and sealed. Stop.

Of course, in the case of the active drive system, it goes without saying that drive elements such as TFTs, wirings, pixel electrodes, etc. are formed on the glass substrates 1 and 2.
However, it is omitted in the drawing.

When injecting the liquid crystal, the temperature of the liquid crystal and the substrate before injecting the liquid crystal is higher than the phase transition point (NI point) of the liquid crystal NI (N: nematic, I: isotropic). Inject in the isotropic phase while keeping
When the liquid crystal display element is manufactured by gradually lowering the temperature to below the NI point, the display as the display element becomes clearer.

As for the arrangement of the polarizing plates, the angle of the polarization axis is arbitrary as long as the relationship of orthogonal Nicols in the case of positive display and parallel Nicols in the case of negative display are maintained. As the film on the outermost surface of the substrate, a film having a molecular number and a molecular structure similar to those of the liquid crystal molecules used, for example, a polymer film such as a polyimide film is desirable. It is desirable that the surface state (surface structure, surface energy, etc.) of the film be such that the liquid crystal molecules are inclined several degrees (0.1 ° to 27 °) toward the normal line direction of the substrate. The optimum value of this angle varies depending on the twist angle of the liquid crystal display element and the like. To obtain such a surface state, for example, a polymer film having a side chain may be used.

For example, there are various types of polyimide, but the one used in general TN-LCDs, which is called the high tilt type, is used. The high tilt type changes the molecular structure of the polyimide and is devised so that the pretilt easily occurs in the number and angle of side chains closely related to the occurrence of the pretilt angle.

As a coating method, polyimide is dissolved in a solvent such as thinner, and spin coating by a spinner, or
Apply by a printing method such as flexographic printing, 150 ℃ ~ 30
A method of removing the solvent by heat treatment at 0 ° C. can be used.

Since no rubbing is performed, the substrate 1,
On the two surfaces, liquid crystal molecules can be aligned in any in-plane direction.
Therefore, a large number of microdomains having a diameter of several μm to several tens of μm are formed on the surface of the substrate, and a multidomain is formed as a whole. The orientation directions within each microdomain are random and, as a whole, are oriented in all directions. Furthermore, the long axis direction of the liquid crystal molecules is inclined toward the substrate normal direction.

In FIG. 2, a polymer film having a pretilt angle of about 6 ° when subjected to a rubbing treatment is used, and d / p =
A viewing angle characteristic of the contrast of a liquid crystal display element of 0.25, that is, TN-LCD is shown. The polarizers are arranged in a crossed Nicols state, and the front and rear polarization axes are set to the (0 ° -180 °) and (90 ° -270 °) directions for positive display.

As is clear from FIG. 2, this TN-LC
In D, the viewing angle characteristics are almost the same in all directions, and the contrast from a specific angle is not deteriorated as seen in the conventional liquid crystal display element of FIG. . This is probably because the orientation directions of the multi-domains are evenly generated.

When the structure of this liquid crystal display element was observed with a polarization microscope, minute multi-domains were observed over the entire cell surface. This is considered to be the reason why the viewing angle dependency is eliminated in the present invention.

Also, comparing the viewing angle characteristics of FIGS. 2 and 5,
The contrast in FIG. 2 is higher, and when compared with contrast 10, the maximum viewing angle is 4 ° in the θ direction. One of the causes for this is that in the liquid crystal display element of FIG. 2, the liquid crystal molecules are slightly tilted in the direction normal to the substrate surface when no voltage is applied. It is thought that this makes it easier to stand up. This helps prevent the occurrence of reverse tilt disclination in halftone display.

The example of FIG. 2 is not limited to the optimum condition, and there is a possibility that the contrast can be further increased by selecting various interface films. Although the present invention has been described above with reference to the embodiments, the present invention is not limited thereto. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

[0035]

According to the present invention, since rubbing is unnecessary, destruction of elements and wiring due to static electricity does not occur,
Display defects due to dust generation and adhesion can be reduced, and the manufacturing cost can be reduced.

Further, since the viewing angle characteristics are substantially uniform in all directions, there is no inconvenience that the contrast is lowered depending on the specific position of the observer. In particular, the contrast when this liquid crystal display device is viewed from an angle is high,
High-quality display can be obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display device according to an exemplary embodiment of the present invention.

FIG. 2 is a view angle characteristic of a liquid crystal display device according to an embodiment of the present invention.

FIG. 3 is a view angle characteristic of a liquid crystal display device according to a conventional rubbing technique.

FIG. 4 is a diagram illustrating pretilt by rubbing and a cross-sectional view of a liquid crystal display device according to a conventional technique.

FIG. 5 is a view angle characteristic of a liquid crystal display device according to a conventional non-rubbing technique.

[Explanation of symbols]

 1, ... Glass substrate 3 Liquid crystal molecules

Continuation of the front page (72) Inventor Yasuo Toko 205-1 Heimachi, 1-1-15 Sakuramachi, Koganei-shi, Tokyo

Claims (2)

[Claims] 1. A liquid crystal display device comprising a pair of substrates which do not have a unidirectional alignment structure with respect to liquid crystal molecules and which has a gap d, and a chiral nematic liquid crystal layer sandwiched between the substrates. A liquid crystal display device having a surface state in which the film is inclined with respect to liquid crystal molecules in the direction normal to the substrate. 2. When the gap between the pair of substrates is d and the chiral pitch of the chiral nematic liquid crystal is p, 0 ≦ d / p (<or ≈) 0.75 is satisfied. Liquid crystal display element.