JPH06281939A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To provide a liquid crystal display element capable of obtaining good display quality free from the initial orientation defect of a liquid crystal and reverse tilt discrimination, etc., at the time of applying a driving voltage by forming horizontally oriented films as rough surfaces films having microruggedness and horizontally orienting liquid crystal molecules with the pretilt angle corresponding to the surface roughness of the oriented films. CONSTITUTION:The horizontally oriented films 8, 9 formed by the LB method (Langmuir-Blodgett technique) on both substrates 1, 2 are formed as the rough surfaces films having the microruggedness and the liquid crystal molecules A are horizontally oriented at the pretilt angle phi corresponding to the surface roughness of the oriented films 8, 9. The horizontally oriented films 8, 9 are formed as the rough surface films by roughening the film surfaces of insulating films 6, 7 formed to cover transparent electrodes 4, 5 on the ground surfaces, i.e., both substrates 1, 2 on which the oriented films are formed. Then, the liquid crystal molecuels are oriented with the desired pretilt angle if the surface roughness of the oriented films 8, 9 is controlled. The display quality free from the initial orientation defect of the liquid crystal and the reverse tilt discrimination, etc., at the time of applying the driving voltage is obtd.

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 which liquid crystal molecules are horizontally aligned.

[0002]

2. Description of the Related Art Liquid crystal display devices in which liquid crystal molecules are horizontally aligned include TN (twisted nematic) mode, STN (super twisted nematic) mode, and ECB (electric field control birefringence). Among the modes, there are horizontal alignment type, modes using ferroelectric liquid crystal or anti-ferroelectric liquid crystal, and the like.

In these liquid crystal display elements, a transparent electrode and a horizontal alignment film for horizontally aligning liquid crystal molecules are formed on the surfaces of a pair of transparent substrates which face each other with a liquid crystal layer in between and which face each other. It is composed.

The horizontal alignment film provided on the substrate of these liquid crystal elements is conventionally a method of applying polyimide or the like on the substrate and rubbing the film surface, or silicon oxide (SiO ₂ ) or the like on the substrate. Was formed by the method of oblique vapor deposition, but recently, Langmuir-Blodgett (La
A method of forming a horizontal alignment film made of a polyimide film by the ngumuir-Blodgett method has been adopted.

In the Langmuir-Blodgett method (hereinafter referred to as the LB method), a monomolecular film is formed on a still water surface, and a substrate previously immersed in water vertically is pulled up at a constant speed while being pulled up at a constant speed. It is a method of depositing a monomolecular film on a substrate.

The formation of the polyimide film by the LB method is
An amphipathic compound having a hydrophobic group and a hydrophilic group, for example, a monomolecular film of a polyamic acid derivative compound obtained by subjecting a polyamic acid and an amine having a long-chain alkyl group to an ionic bond reaction is deposited on a substrate by the LB method. This step is repeated a plurality of times to stack the monomolecular film on the substrate, and the stacked film is imidized to form a polyimide film.

In the monomolecular film of the amphipathic compound deposited on the substrate by the above-mentioned LB method, since molecules having a long molecular length are arranged along the pulling direction of the substrate, a laminated film of this monomolecular film is formed. The imidized polyimide film has an orientation property of uniformly aligning liquid crystal molecules in one direction, and therefore,
This polyimide film can be directly used as a horizontal alignment film without rubbing the film surface.

In the liquid crystal display element, the alignment state of the liquid crystal molecules has a great influence on the electro-optical characteristics of the liquid crystal display element. Therefore, in order to obtain a good display, it is necessary to improve the stability of the alignment of the liquid crystal molecules. is there.

An important factor that influences the stability of the alignment of the liquid crystal molecules is the pretilt angle of the liquid crystal molecules with respect to the substrate surface (the angle formed by the long axis of the liquid crystal molecules with respect to the substrate surface) in the absence of applied voltage. Therefore, the larger the pretilt angle, the better the stability of the alignment of the liquid crystal molecules.

[0010]

However, the above L
In the case of the horizontal alignment film formed by the B method, the liquid crystal molecules can be uniformly aligned in one direction, but it is difficult to give the liquid crystal molecules a desired pretilt angle.

Therefore, in the conventional liquid crystal display element in which the horizontal alignment film is formed by the above-mentioned LB method, the initial alignment defect of the liquid crystal occurs and the reverse tilt disclination occurs when the drive voltage is applied. I had.

According to the present invention, although a horizontal alignment film is formed by the LB method, liquid crystal molecules are aligned with a desired pretilt angle, and initial misalignment of liquid crystal or a reverse tilt discretization when a drive voltage is applied. An object of the present invention is to provide a liquid crystal display device that can obtain good display quality without nation and the like.

[0013]

A liquid crystal display element of the present invention comprises a transparent electrode, a hydrophobic group and a hydrophilic group by the LB method, on the surfaces of a pair of transparent substrates facing each other across a liquid crystal layer. And a horizontal alignment film formed of a polyimide film obtained by imidizing the laminated film by laminating monomolecular films of an amphipathic compound having The liquid crystal molecules are horizontally aligned with a pretilt angle according to the surface roughness of the alignment film. In one embodiment of the invention,
The horizontal alignment film is a rough surface film by making the underlying surface on which the alignment film is formed a rough surface.

[0014]

As described above, when the horizontal alignment film is a rough surface film having minute irregularities, liquid crystal molecules are horizontally aligned with a pretilt angle according to the surface roughness of the alignment film, so that the surface roughness of this alignment film is controlled. By doing so, it is possible to orient the liquid crystal molecules with a desired pretilt angle, and obtain good display quality without defective initial alignment of the liquid crystal, reverse tilt disclination when a drive voltage is applied, and the like.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a sectional view of the liquid crystal display element. This liquid crystal display element includes a pair of transparent substrates 1 made of glass or the like.
2 are joined via a frame-shaped sealing material 3, and both substrates 1,
Liquid crystal is sealed in a region surrounded by the seal material 3 between the two substrates 1.
Transparent electrodes 4 and 5 made of a transparent conductive film such as TO are formed. The electrode formation surfaces of the substrates 1 and 2 are covered with transparent insulating films 6 and 7 made of silicon oxide (SiO ₂ ), and the horizontal alignment film 8 is formed on the insulating films 6 and 7. , 9 are formed.

This liquid crystal display element is of TN mode or STN mode, and the molecules A of the liquid crystal enclosed between the substrates 1 and 2 are oriented films 8 and 9 on the substrates 1 and 2 side.
The respective alignment directions of the liquid crystal molecules A in the vicinity of are regulated by the alignment films 8 and 9, and the substrates 1 and 2 are twist-aligned at a predetermined twist angle. However, in FIG. 2, for convenience, the liquid crystal molecules A are shown in a state where they are not twisted.

Each of the horizontal alignment films 8 and 9 is an amphipathic compound having a hydrophobic group and a hydrophilic group, for example, an amine having a polyamic acid and a long-chain alkyl group (hereinafter referred to as a long-chain alkylamine). It is composed of a polyimide film obtained by imidizing a film obtained by stacking several monolayers of a compound obtained by reacting with and several layers to several tens of layers.

The horizontal alignment films 8 and 9 are formed by the following method. Although the formation of the horizontal alignment film 8 provided on one substrate 1 will be described here, the horizontal alignment film 9 provided on the other substrate 2 is also formed in the same manner.

The above polyamic acid is represented by the following structural formula [Chemical Formula 3], and this polyamic acid is composed of the tetracarboxylic acid dianhydride represented by the structural formula [Chemical Formula 1] and [Chemical Formula 2]. It is obtained by synthesizing the diamine represented by the structural formula.

[0021]

[Chemical 1]

[0022]

[Chemical 2]

[0023]

[Chemical 3]

The long-chain alkylamine is for imparting hydrophobicity to the polyamic acid having hydrophilicity, and the long-chain alkylamine is represented by the following structural formula [Chemical formula 4].

[0025]

[Chemical 4]

A solution prepared by dissolving the polyamic acid in a solvent and a solution prepared by dissolving the long-chain alkylamine in the same solvent are mixed at a predetermined ratio to cause an ionic bond reaction between the polyamic acid and the long-chain alkylamine, A solution of a polyamic acid derivative compound (polyamic acid salt) represented by the following structural formula [Chemical Formula 5] is prepared. The solvent for the polyamic acid and the long-chain alkylamine is NM.
P (N-methyl-2-pyrrolidinone) and benzene 1:
A mixed solvent mixed at a ratio of 1 is used.

[0027]

[Chemical 5]

The horizontal alignment film 8 is formed of the transparent electrode 4
LB is formed on the substrate 1 on which the insulating film 6 is formed.
A monomolecular film of the above polyamic acid derivative compound is laminated on a predetermined layer by the method, and the laminated film of the monomolecular film is imidized to form the film. FIG. 3 shows a method of depositing a monomolecular film of a polyamic acid derivative compound on the substrate 1 by the LB method. The deposition of this monomolecular film is performed as follows. First, the monomolecular film adhered surface of the substrate 1 (the insulating film 6
The substrate 1 is subjected to hydrophilic treatment, and the substrate 1 is vertically immersed in water in the water tank 10.

Next, the solution of the above polyamic acid derivative compound is dropped onto the water surface between the bar-shaped moving barrier 11 provided at the water surface height and the substrate 1 to spread the monomolecular film a on the water surface. Let

Next, the moving barrier 11 is moved in the direction of the substrate so that the single molecules on the water surface are densely packed, and the surface pressure of the monomolecular film a is adjusted to a constant pressure (25 dyn / cm). The monomolecular film a is moved in a predetermined direction (2 mm / min) while pushing the monomolecular film a toward the substrate, and the substrate 1 is pulled up in synchronization with this to deposit the monomolecular film a on the water surface onto the substrate 1. .

At this time, the monomolecules on the water surface are pulled up by adhering to the substrate 1 whose hydrophilic portion has been subjected to the hydrophilic treatment, so that the molecules of the monomolecular film a are aligned in almost one direction. In this state, it is deposited on the substrate 1. The following is the monolayer a
The above-mentioned deposition process is repeated to laminate the monomolecular film a on the substrate 1 into a predetermined layer.

After laminating the monomolecular film a of the polyamic acid derivative compound on the substrate 1 in this manner, a heat treatment of heating at 200 ° C. or higher for about 1 hour or a chemical treatment with a solution of an acid anhydride or the like is performed. Then, the laminated film on the substrate 1 is imidized to form a polyimide film (horizontal alignment film 8). It should be noted that the imidization of this laminated film may be performed by using both the heat treatment and the chemical treatment in combination.

This polyimide film is obtained by removing the alkylamine of the polyamic acid derivative compound, which is a compound in which a polyamic acid and a long-chain alkylamine are ion-bonded, and is imidized. It has a structure.

[0034]

[Chemical 6]

By the way, the horizontal alignment film made of the polyimide film formed by the LB method does not uniformly align the liquid crystal molecules in one direction, as described in the section [Problems to be Solved by the Invention]. However, it is difficult to give liquid crystal molecules a desired pretilt angle.

Therefore, in the liquid crystal display element of this embodiment,
Horizontal alignment films 8 and 9 formed on both substrates 1 and 2 by the LB method are roughened films having minute irregularities, and liquid crystal molecules A are horizontally aligned with a pretilt angle φ corresponding to the surface roughness of the alignment films 8 and 9. It is oriented.

The horizontal alignment films 8 and 9 are rough surfaces of the underlying surfaces on which the alignment films are formed, that is, the insulating films 6 and 7 formed on the substrates 1 and 2 to cover the transparent electrodes 4 and 5. By doing so, a rough surface film is formed.

FIG. 2 is an enlarged view of the portion I in FIG. 1. In this embodiment, the transparent electrode 5 formed on the substrate 2 is covered with a roughened surface to cover the electrode 5. The film surface of the insulating film 7 (base surface on which the alignment film 9 is formed) is roughened, and the alignment film 9 formed thereon by the LB method is a roughened film. In this way, when the horizontal alignment films 8 and 9 of both substrates 1 and 2 are roughened films having fine irregularities, the liquid crystal molecules A have a pretilt angle φ corresponding to the surface roughness of the alignment films 8 and 9.

FIG. 4 shows the relationship between the surface roughness of the transparent electrodes 4 and 5 (height difference of the unevenness) and the pretilt angle φ of the liquid crystal molecule A. Here, insulation is provided on the transparent electrodes 4 and 5. The figure shows the value when the number of laminated monomolecular films a is 11 when the monomolecular films a are directly laminated on the electrodes without providing the films 6 and 7.

As shown in FIG. 4, when the surface roughness of the electrodes 4, 5 is smaller than 5 nm, the pretilt angle φ of the liquid crystal molecule A is 0 °, but the surface roughness of the electrodes 4, 5 is 5 nm or more. Then, the liquid crystal molecules A are aligned in a state of being pretilted with respect to the surfaces of the substrates 1 and 2, and the pretilt angle φ is
It increases as the surface roughness of the electrodes 4 and 5 increases.

Therefore, if the surface roughness of the electrodes 4 and 5 is set to 5 nm or more and the surface roughness of the alignment films 8 and 9 is controlled within this range, the liquid crystal molecules A are aligned with a desired pretilt angle φ. be able to.

The surface roughness of the electrodes 4 and 5 changes depending on the film thickness of the electrodes 4 and 5, that is, the film thickness of the ITO film or the like to be the electrodes 4 and 5, and As the film thickness is increased, the surface roughness increases and the surface resistance of the electrodes 4 and 5 decreases. FIG. 5 shows the relationship between the sheet resistance and the surface roughness when the electrodes 4 and 5 are formed of an ITO film.

In the above liquid crystal display device, the horizontal alignment films 8 and 9 formed on both substrates 1 and 2 by the LB method are rough surface films having fine irregularities, so that the liquid crystal molecules A are aligned. Since the liquid crystal is horizontally aligned with the pretilt angle φ corresponding to the surface roughness of 8 and 9, it is possible to obtain good display quality without initial alignment defect of the liquid crystal, reverse tilt disclination when a drive voltage is applied, and the like.

In the above-described embodiment, since the film surfaces of the insulating films 6 and 7 which are the underlying surfaces on which the alignment films 8 and 9 are formed are rough, the surfaces of the transparent electrodes 4 and 5 below are rough. The surface of the insulating films 6 and 7 may be roughened by roughening etching, but in that case, the surfaces of the electrodes 4 and 5 may not be rough.

In the liquid crystal display element of the above embodiment, the transparent electrodes 4 and 5 are covered with the insulating films 6 and 7 and the horizontal alignment films 8 and 9 are formed thereon by the LB method. Can also be applied to a liquid crystal display element in which horizontal alignment films 8 and 9 are formed directly on the transparent electrodes 4 and 5 by the LB method. In that case, on the underlying surface on which the alignment films 8 and 9 are formed, The surfaces of certain transparent electrodes 4 and 5 may be roughened as in the above-described embodiment, and the alignment films 8 and 9 may be roughened films.

Furthermore, in the above-mentioned examples, the polyamic acid derivative compound obtained by reacting polyamic acid with a long-chain alkylamine was used as the amphipathic compound having a hydrophobic group and a hydrophilic group. The functional compound is not limited to the above polyamic acid derivative compound as long as it becomes a polyimide by imidization treatment.

Further, the liquid crystal display device of the above-mentioned embodiment is TN
Mode or STN mode, the present invention
The present invention can also be applied to a horizontal alignment type liquid crystal display element of an ECB (electric field control type birefringence) mode, a liquid crystal display element using a ferroelectric liquid crystal or an antiferroelectric liquid crystal, and the like.

[0048]

In the liquid crystal display device of the present invention, the horizontal alignment film is a rough surface film having fine irregularities, and the liquid crystal molecules are horizontally aligned with a pretilt angle corresponding to the surface roughness of the alignment film. , A horizontal alignment film is formed by the LB method, the liquid crystal molecules are aligned with a desired pretilt angle, and there is no initial alignment failure of the liquid crystal or reverse tilt disclination when a drive voltage is applied. Display quality can be obtained.

[Brief description of drawings]

FIG. 1 is an enlarged view of part I of FIG.

FIG. 2 is a sectional view of a liquid crystal display element showing an embodiment of the present invention.

FIG. 3 is a diagram showing a method for laminating a monomolecular film on a substrate by the LB method.

FIG. 4 is a diagram showing the relationship between the surface roughness of electrodes and the pretilt angle of liquid crystal molecules.

FIG. 5 is a diagram showing a relationship between surface resistance and surface roughness of electrodes.

[Explanation of symbols]

 1, 2 ... Substrate 4, 5 ... Electrode 6, 7 ... Insulating film 8, 9 ... Horizontal alignment film formed by LB method A ... Liquid crystal molecule φ ... Pretilt angle of liquid crystal molecule

Claims (2)

[Claims] 1. A single molecule of an amphipathic compound having a transparent electrode and a hydrophobic group and a hydrophilic group by the Langmuir-Blodgett method on the surfaces of a pair of transparent substrates that face each other with a liquid crystal layer between them, facing each other. A liquid crystal display device comprising a horizontal alignment film made of a polyimide film obtained by stacking films and imidizing the stacked film, wherein the horizontal alignment film is a rough surface film having minute unevenness, and liquid crystal molecules are aligned in the alignment direction. A liquid crystal display device characterized by being horizontally aligned with a pretilt angle according to the surface roughness of the film. 2. The liquid crystal display element according to claim 1, wherein the horizontal alignment film is a rough surface film by roughening a base surface on which the alignment film is formed.