KR19990084546A - Liquid Crystal Display Device and Liquid Crystal Alignment Method - Google Patents

Abstract

The present invention relates to an optical alignment method of a liquid crystal using circularly polarized light without a rubbing process, and more particularly, a liquid crystal display device and a liquid crystal, which can obtain a high pretilt angle of a liquid crystal using a material having no polarity as a polymer thin film. It relates to an orientation method.

The present invention provides a liquid crystal display device comprising a polymer alignment film containing a photoactive compound formed on the upper and lower substrates, and an upper and lower substrates, and a liquid crystal injected between the upper and lower substrates. Polarization is made of small polymer material.

In the present invention, the polymer alignment film having a small polarization is made of polystyrene containing methyl orange.

Description

Liquid Crystal Display Device and Liquid Crystal Alignment Method

The present invention relates to a method of optical alignment of a liquid crystal without a rubbing process, and more particularly to a liquid crystal alignment method using circularly polarized light that can control the pretilt angle of the liquid crystal using polystyrene having no polarity as a polymer thin film. will be.

In the future, flat panel displays will be important devices for handling information in small TVs, large screen TVs and portable terminals. In particular, the liquid crystal display of the flat panel display device is widely used as an information display due to the low driving voltage and current, high contrast ratio, low price, reliability in solar light and interface with the integrated circuit.

In order to manufacture a liquid crystal display device having uniform brightness and high contrast, the liquid crystal molecules injected between the upper and lower substrates must be uniformly arranged in a predetermined direction. In general liquid crystal display devices, liquid crystal alignment layers are formed on opposite surfaces of upper and lower substrates to align liquid crystal molecules in a predetermined direction.

The rubbing method is used as a surface treatment method as a conventional method of forming a liquid crystal aligning film and orientating a liquid crystal. The rubbing method is currently the most widely used method, and is a method of orienting liquid crystals along a straight groove formed in the alignment film in a constant direction.

However, since this rubbing method performs rubbing after forming a polyimide on a glass substrate on which a color filter or TFT array is arranged, there is a problem of causing electrical and mechanical damage to the TFT array arranged under the polyimide during rubbing. In addition, the liquid crystal display device manufactured by the rubbing method has a disadvantage in that a viewing angle is small. In addition, the alignment method has a problem that it is difficult to manufacture a multi-domain LCD to increase the viewing angle.

In addition, in the liquid crystal display device, it is important to incline and adjust the pretilt angle which acts as important variables such as the reaction time and contrast ratio of the liquid crystal. Recently, a method of controlling the pretilt angle without rubbing has been actively studied recently. In addition, high pretilt angles are required for fast response time and stable image quality in STN-LCDs.

It is an object of the present invention to provide a method for orienting a liquid crystal to have a high pretilt angle using polystyrene containing methyl orange as a polymer alignment layer without a rubbing process.

Another object of the present invention is to provide a liquid crystal display device which can obtain a high pretilt angle by using polystyrene containing a methyl orange having no polarity as a polymer alignment layer.

Another object of the present invention is to provide a liquid crystal display device and a liquid crystal alignment method capable of obtaining a uniform alignment film with excellent reproducibility.

1 is a cross-sectional view of a liquid crystal cell for explaining the principle of optical alignment according to an embodiment of the present invention;

2A and 2B are diagrams illustrating transmittance curves according to an incident angle of a polymer alignment layer of methyl orange / polystyrene (MO / PS) of the present invention.

Figure 2a is a case of the MO / PS polymer alignment film having a MO concentration of 1wt.%,

Figure 2b is a case of the MO / PS polymer alignment film having a MO concentration of 10wt.%,

FIG. 3 is a graph illustrating transmittance curves according to incident angles of light with respect to a polymer alignment layer of methyl orange / polyvinyl alcohol (MO / PVA);

(Explanation of symbols for the main parts of the drawing)

10, 20: substrate 11: polyimide alignment film

21: polymeric alignment film (polystyrene) containing a pigment

30: liquid crystal 1: liquid crystal cell

In order to achieve the above object, the present invention provides a liquid crystal display device comprising an upper and a lower substrate, a polymer alignment film containing a photoactive compound formed on each of the upper and lower substrates, and a liquid crystal injected between the upper and lower substrates. The polymer alignment film containing the compound is characterized by providing a liquid crystal display device made of a polymer material having a small polarization.

According to an embodiment of the present invention, the polymer alignment layer having a small polarization is made of polystyrene containing methyl orange.

In addition, the present invention is a method for orienting the liquid crystal injected between the upper and lower substrates each formed with a polymer alignment film containing a photoactive compound, the polymer alignment film to form a low polarized polymer thin film, the circularly polarized It is characterized by providing a liquid crystal alignment method for inclining the liquid crystal by entering the light to have a predetermined pretilt angle.

According to the exemplary embodiment of the present invention, the polymer alignment layer is made of polystyrene containing methyl orange, and the pretilt angle of the liquid crystal is changed according to the concentration of methyl orange.

The present invention can not only tilt the liquid crystal using circularly polarized light, but also control the pretilt angle of the liquid crystal by controlling the polarity of the polymer alignment layer, thereby improving the viewing angle as well as the liquid crystal of the multi-domain. It is easy to manufacture the display element.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a cross-sectional structural view of a liquid crystal cell for explaining the principle of control of the liquid crystal alignment and pretilt angle according to an embodiment of the present invention.

Referring to FIG. 1, a substrate 10 in which a polyimide is formed of an alignment layer 11 is provided as one substrate of the liquid crystal cell 1, for example, an upper substrate, and another substrate, for example, contains a pigment as a lower substrate. A substrate 20 on which a polymer alignment film 21 is formed is provided.

As the polymer alignment film 21 containing the dye, methyl orange, which exhibits a photoisomerization reaction, is used for the dye. In the present invention, polystyrene (PS) is used as the matrix.

In the liquid crystal cell 1 of FIG. 1, the alignment layer 11 of the upper substrate 10 is already oriented so as to have a pretilt angle by a rubbing method before fabrication of the liquid crystal cell by the bonding of the upper and lower substrates. The alignment film 21 at 20 is pumped by polarized light before cell production and is oriented to have a predetermined pretilt angle.

When the pretilt angle of the liquid crystal cell 1 as mentioned above is measured by the crystal rotation method, the pretilt angle (alpha) of the whole cell is represented by following formula (1).

α ≒ = … … … (One)

Here, α _PI and α _LC are the pretilt angles of the liquid crystal near the polyimide alignment layer 11 and the polymer alignment layer 21 near the reference plane, respectively, θ _x is the symmetry angle of the transmittance curve, and n _e and n _o are The normal refractive index and the extraordinary refractive index of the liquid crystal are shown, respectively.

From the above formula, the pretilt angle α _LC of the liquid crystal oriented by the polarized light of the polymer alignment film 21 can be obtained.

In the liquid crystal cell 1 of FIG. 1, when the circularly polarized pump wave is inclined to the surface of the substrate 20, the methyl orange (MO) molecules contained in the polymer alignment layer 20 are aligned with respect to the traveling direction of light. Arranged inclined with respect to the substrate surface. Accordingly, the liquid crystal molecules injected between the upper and lower substrates 10 and 20 are diagonally aligned along the methyl orange (MO) molecules.

When the circularly polarized light is incident on the polymer alignment layer 21 of MO / PVA containing methyl orange in this manner, the liquid crystal injected between the upper and lower substrates 10 and 20 along the methyl orange (MO) molecules is pre-tilted. An angle oblique orientation technique is disclosed in Korean Patent Application No. 96-42514.

When the liquid crystal is tilted using the circularly polarized light as described above, the polarity of the polymer alignment layer affects when the MO molecules are arranged to interact with the liquid crystal.

Accordingly, the present invention is a photo-alignment method of tilting a polymer alignment film containing methyl orange using circularly polarized light and thereby tilting the liquid crystal, and polarizing the polymer alignment film as a polymer alignment film in order to reduce the influence of polarization of the polymer alignment film. Polystyrene containing methyl orange with little polarity is used.

In the present invention, an MO / PS polymer alignment layer using polystyrene (PS, PolyStyrene) having almost no polarization as a matrix is used. The method of forming the MO / PS polymer alignment layer is performed by using N-Methyl-2-Pyrrolidone (NMP) as the polymer PS. Dimethyl Sulfoxide is dissolved in a 2: 1 solution and an appropriate amount of MO is added to the solution to make a MO / PS solution.

The MO / PS solution was coated at 1000 rpm for 1 minute using a spin-coater to prepare the MO / PS thin film 21 on the glass substrate 20.

FIG. 3 shows the transmittance with respect to the incident angle under a circularly polarized pump when polyvinyl alcohol containing methyl orange is used as the polymer alignment film.

When the MO / PVA polymer alignment film is used as the polymer alignment film, a transmittance curve as shown in FIG. 3 is obtained, and the symmetry angle θ _X is measured about 10 °. Therefore, 3.1 degree was obtained by the pretilt angle (alpha _LC ) of a liquid crystal. At this time. The incident angle of the circularly polarized pump wave is 60 °, the cell thickness is 24 μm, and the concentration of methyl orange in the polymer alignment layer is 15 wt.%.

On the other hand, when MO / PVA was used as the polymer alignment film, other conditions were the same as in the above case, and when the concentration of the MO molecules was 1 wt.%, The liquid crystal was hardly oriented in the liquid crystal cell 1.

This is because in the MO / PVA alignment film, polyvinyl alcohol (PVA) used as a matrix has a very large polarity, which affects the arrangement of MO molecules to be arranged by circularly polarized pump waves into which the MO molecules are incident. Because.

That is, since the polarization of the polymer PVA, which is a matrix, is large, the anisotropy of the MO molecules arranged by the circularly polarized pump wave screens the polarization of the PVA to reduce the interaction between the MO molecules and the liquid crystal molecules. For this reason, when the MO concentration contained in the PVA polymer was low, the liquid crystal was hardly oriented along the MO molecule. When the MO concentration was high, the liquid crystal was inclined, but a small pretilt angle was obtained.

Figure 2 shows the transmission curve according to the incident angle measured by the crystal rotation method when using the MO / PS almost no polarization as the polymer alignment film according to an embodiment of the present invention. 2A is a transmittance curve according to an incident angle of an MO / PS polymer alignment layer having an MO concentration of 1 wt.%, And FIG. 2B is a transmittance curve of an incident angle of an MO / PS polymer alignment layer having an MO concentration of 10 wt.%.

As shown in FIG. 2A and FIG. 2B, it can be seen that the symmetry angle of the curve is larger than that of using MO / PVA as the alignment layer. The incident angle of the pump wave is 60 °, and the other pump conditions are the same as those in the case where MO / PVA is used as the polymer alignment film.

The liquid crystal cell of which MO / PS polymer thin film having an MO concentration of 1 wt.% Was used as an alignment layer was well aligned, and as shown in FIG. 2A, since the symmetry angle was 24 °, the pretilt angle of the liquid crystal was calculated to be 14 ° from the above equation. do. Larger pretilt angles were obtained compared to MO / PVA polymer alignment films containing MO molecules at the same concentration. At this time, α _PI = 1 °.

In addition, as shown in FIG. 2B, in the case of the MO / PS polymer alignment film having a MO concentration of 10wt.%, A very large value is obtained at 36 ° or more, which is out of the range that can be measured by the crystal rotation method. Can be. At this time, looking at the transmittance curve, the symmetry angle θ _X is 60 ° or more.

From the above results, when the polymer thin film containing methyl orange was used as the alignment layer, the use of polystyrene having almost no polarity as the matrix resulted in a relatively larger pretilt angle than that of the highly polar material. Able to know. In addition, it can be seen that as the concentration of methyl orange contained in the polymer thin film increases the pretilt angle.

At this time, looking at the polarization of the polystyrene and polyvinyl alcohol, polarity (P) is expressed by the following formula (2).

… … … (2)

As shown in Equation (2), when the same electric field is applied, the polarization is proportional to the permittivity ε, and the permittivity ε varies depending on the material. Polystyrene has a dielectric constant (ε) of 3x10 ^-4 F / m and an electric polyvinyl alcohol of 8 F / m when subjected to an electric field at 1 KHz. Thus, it can be seen that the polarization of polystyrene is relatively small compared to polyvinyl alcohol.

Therefore, in the present invention, a polymer alignment film containing azo dye is used as the alignment film without performing a rubbing step as a step for orienting the liquid crystal, and the anisotropy of the azo dye molecule array is induced using circularly polarized light to align the liquid crystal. At this time, it is possible to adjust the pretilt angle of the liquid crystal by orienting the liquid crystal by the anisotropy of the MO molecules and by controlling the anisotropy of the molecular arrangement of the azo dyes included in the alignment layer by the polarity of the polymer thin film serving as the matrix. .

As described in detail above, according to the present invention as described above, the present invention can obtain the following effects.

First, since the rubbing method is not used, not only a uniform alignment film without defects can be formed, but also excellent reproducibility can be obtained.

Second, the pretilt angle may be obtained by tilting the liquid crystal molecules by irradiating circularly polarized light to the polymer alignment layer. In addition, it is possible to adjust the pretilt angle of the liquid crystal by using MO / PS having almost no polarity as the polymer alignment film. Therefore, not only the viewing angle can be improved but also the manufacturing of the multi-domain liquid crystal display device is easy.

Third, in the case of manufacturing a liquid crystal cell using the liquid crystal alignment method of the present invention, the loss of TFTs due to the rubbing process is prevented to improve the production yield as well as to improve the reliability of the product.

Claims (6)

In the liquid crystal display device comprising an upper and lower substrate, a polymer alignment film containing a photoactive compound formed on each of the upper and lower substrates, and a liquid crystal injected between the upper and lower substrates, The polymer alignment layer containing the photoactive compound is made of a polymer material having a small polarization. The liquid crystal display device according to claim 1, wherein the polymer alignment film having a small polarization is made of polystyrene having a dielectric constant? Of 3x10 ^-4 when an electric field is applied at 1 KHz. The liquid crystal display device according to claim 1 or 2, wherein the polymer alignment film is made of polystyrene containing methyl orange. In the method of orienting the liquid crystal injected between the upper and lower substrates each having a polymer alignment film containing a photoactive compound, And forming a polymer thin film having a low polarization as the polymer alignment layer, and inclining the liquid crystal to have a predetermined pretilt angle by injecting circularly polarized light into the polymer thin film. The liquid crystal alignment method as claimed in claim 4, wherein the polymer alignment layer is made of polystyrene containing methyl orange. 6. The liquid crystal alignment method of claim 5, wherein the pretilt angle of the liquid crystal is changed according to the concentration of methyl orange.