WO1998008136A1

WO1998008136A1 - Liquid crystal display device

Info

Publication number: WO1998008136A1
Application number: PCT/EP1997/004501
Authority: WO
Inventors: Takamasa Harada; Fumie Nozawa; Masami Aizawa
Original assignee: Hoechst Research & Technology Deutschland Gmbh & Co. Kg
Priority date: 1996-08-22
Filing date: 1997-08-18
Publication date: 1998-02-26
Also published as: JPH1062788A

Abstract

To provide an IPS-TFT liquid crystal display device having a wide viewing angle. Means for solution: an IPS-TFT liquid crystal display device characterized in that at least two regions differing in alignment direction from each other are formed in an alignment layer on the substrate of the electrode side.

Description

Liquid Crystal Display Device

This invention relates to a process for producing a liquid crystal alignment layer and a liquid crystal display device with the use of the same. More particularly, it relates to a liquid crystal display device capable of achieving a wide viewing angle.

A liquid crystal display device is an electro-optical device containing a liquid crystal material which undergoes changes in the optical characteristics when an electrical field is externally applied thereto. Recently, there has been proposed an operating mode called IPS(ln-Plane Switching)-TFT for widening the viewing angle of liquid crystal display devices. This IPS-TFT mode is characterized by placing parallel electrodes on one substrate and rotating liquid crystal molecules located horizontally to the substrate within this plane to generate "on" state and "off" state (Fig. 1 ) (Nikkei Electronics, Dec. 4, 1995, p. 184). Since the liquid crystals are always aligned in the direction horizontal to the substrate in this mode, the contrast scarcely varies depending on the viewing direction, which makes it possible to achieve a wide viewing angle.

Even in this IPS-TFT mode, however, there still remains a region wherein the contrast is low and a gray-scale inversion arises depending on viewing angle (Fig. 2). This problem is caused by the fact that good optical characteristics cannot be obtained in the direction vertical to the longitudinal direction of molecules under applying an electric voltage.

Accordingly, it is an object of the present invention to provide a liquid crystal alignment layer which is capable of providing a liquid crystal display device having excellent viewing angle characteristics and thus solving the above-mentioned problems and a process for producing the same, as well as a liquid crystal display device with the use of said liquid crystal alignment layer. The present inventors have now found that the viewing angle characteristics of an IPS-TFT liquid crystal display device can be further improved by forming at least two regions differing in alignment direction from each other in an alignment layer, thus completing the present invention.

The present invention provides an IPS-TFT liquid crystal display device characterized in that at least two regions differing in alignment direction from each other are formed in an alignment layer on the substrate of the electrode side. Preferably, in the liquid crystal display device of the present invention, a part of the alignment layer has been irradiated with UV light or an electron beam after completion of the aligning process, whereby the alignment direction of the alignment layer within the irradiated region is shifted by about 90° to the alignment direction established by the aligning process.

The present invention further provides a process for producing an IPS-TFT liquid crystal display device. The process comprises the steps of: preparing an alignment layer made of organic polymers; aligning said alignment layer; irradiating a part of the alignment layer with UV light or an electron beam to shift the alignment direction of the alignment layer within the irradiated region by about 90° to the alignment direction established by the aligning process; and assembling the liquid crystal display device using the alignment layer thus obtained as the alignment layer on the substrate of the electrode side.

The present invention furthermore relates to use of an alignment layer having at least two regions differing in alignment direction from each other in an IPS-TFT liquid crystal display device. The alignment layer employed in the method of the present invention is preferably characterized in that a part of said alignment layer has been irradiated with UV light or an electron beam after completion of the aligning process, whereby the alignment direction of the alignment layer within the irradiated region is shifted by about 90° to the alignment direction established by the aligning process. According to the present invention, an IPS-TFT liquid crystal display device having good contrast and less viewing angle dependency of display characteristics can be produced by forming at least two regions differing in alignment direction from each other in an alignment layer on the substrate of the electrode side.

The IPS-TFT liquid crystal display device of the present invention can be produced by partly irradiating the surface of an alignment layer with UV light or an electron beam so that the alignment direction of the irradiated region is shifted by about 90°, more particularly 85 to 95°, to the alignment direction established by the aligning process. The term "alignment direction of the alignment layer" as used herein means the direction along which the liquid crystal is to be aligned on the surface of the alignment layer. As shown in Example 1 and Fig. 3, for example, irradiated regions and unirradiated regions are alternately arranged along the direction of the electrical field in the alignment layer on the substrate of the electrode side. Such an arrangement allows to form two regions differing in alignment direction by about 90° from each other (i.e. differing in viewing direction by about 90° from each other) in a single pixel, thus widening the viewing angle. Alternatively, as shown in Example 2 and Fig. 5, it is also possible to arrange two regions differing in alignment direction from each other vertically to the direction of the electrical field. In this case, it is preferable to arrange these regions in such a manner that the adjacent regions in adjacent dots differ from each other in alignment direction.

It is also possible, as shown in Example 3 and Fig. 7, to arrange regions differing in alignment direction from each other by 90° in combination horizontally and vertically to the direction of the electrical field, thus further improving the viewing angle characteristics. In this case, every dot should have at least one pair of each of (a-b) and (c-d) regions. It is preferable that the total area of the (a-b) regions is identical with that of the (c-d) regions.

As the alignment layer material employed in the present invention in which the alignment direction of the irradiated region is shifted by about 90° to the alignment direction established by the aligning process by irradiating with UV light or an electron beam, use can be made of photosensitive polymers having unsaturated bonds in the molecular chain such as polyvinyl cinnamate. When irradiated with UV light after the aligning process, such a polymer undergoes dimerization or isomerization and thus its alignment direction is shifted by about 90°.

Alternatively, the alignment layer employed in the present invention can be obtained by mixing a polymer [polymer (A)], which has such properties as to align the liquid crystal in the direction of the aligning process after completion of the aligning process, with another polymer [polymer (B)], which has such properties as to align the liquid crystal in the direction shifted by about 90° to the direction of the aligning process.

Examples of the polymer (A) include but are not limited to backbone type polymers such as polyimide, polyamide, polyether, polyester and polyurethane. It is still preferable to use polyimide or polyamide, in particular, those bearing a fluorine and siloxane group. When irradiated with UV light, etc., many of these polymers suffer from a decrease in pretilt angle and thus alignment regulation tends to be weakened.

Examples of the polymer (B) include polyvinyl, polymethacrylate and polyacrylate having bulky groups in side chain such as polystyrene and polycyclohexyl methacrylate. It is preferable to use polyacenaphthylene, poly-4-vinylpyridine, polyvinyl diphenyl, polyvinyl naphthalene, polyvinyl formal, polyvinylpyridine-N- oxide, polyphenyl methacrylate, polyvinyl carbazole, etc. Polyvinyl cinnamate can also be used. Although many of these polymers (B) would be slightly or not aligned by usual aligning processes, they can establish uniform alignment after irradiation with UV light, etc. It still remains unknown why such uniform alignment can be established by irradiation with UV light, etc. It is assumed that the irradiation with energy beams such as UV light might cause a rearrangement of the molecular chains of such a polymer in the direction established by the aligning process. Alternatively, crosslinking might occur in some chemical structures. It is also possible that the terminal group in the side chain of a branched polymer is substituted by vinyl, acrylate, etc. and then the polymer is crosslinked upon UV-irradiation to fix the alignment. By mixing the polymers (A) and (B) at an appropriate ratio, which is easily established by routine methods known to someone skilled in the art, it is possible to form an alignment layer which has the alignment direction established by the aligning process after completion of the aligning process, and has the alignment direction shifted by about 90° to the direction established by the aligning process after irradiation with UV light or an electron beam Thus, after the aligning process, the alignment regulation by the polymer (B) is weaker than the alignment regulation by the polymer (A) and, therefore, the liquid crystal is aligned in the direction established by the aligning process On the other hand, when irradiated with UV light or an electron beam, the alignment regulation by the polymer (A) is weakened while that by the polymer (B) is strengthened As a result, the alignment direction of the alignment layer is shifted by about 90° to the alignment direction established by the aligning process

The appropriate mixing ratio of the polymers (A) to (B) and the irradiation dose of UV light, etc are determined depending on the difference in alignment regulation between these polymers The mixing ratio of the polymers (A) to (B) preferably ranges from 1 50 to 50 1 When the amount of the polymer (A) is too large, the alignment regulation by the polymer (A) becomes dominant continuously, even if the alignment layer is irradiated with UV light, etc In this case, the alignment direction is not shifted by about 90° The polymers (A) and (B) may be each a mixture of two or more polymers The irradiation dose of UV light, etc preferably ranges from 0 01 mJ to 50 J, still preferably from 0.1 mJ to 10 J. If the dose of UV light, etc was too large, the polymers would be oxidized and thus the alignment would be disordered The wavelength of UV light, etc preferably ranges from 150 to 450 nm, still preferably from 200 to 400 nm

The alignment layer employed in the present invention can be produced by forming a polymer film on a glass or plastic substrate provided with a transparent electrode by spin coating or printing in accordance with a method commonly known in the art, then the film is subjected to the aligning process followed by the irradiation with UV light or an electron beam In the present invention, it is also possible to use nonpolarized UV light which facilitates the production process It is preferable that the aligning process is carried out by the rubbing method, though the present invention is not restricted thereto. By using a mask in the step of irradiation with UV light or an electron beam, regions with different viewing directions can be formed in a single pixel. These regions can be arbitrarily varied in size and shape by appropriately selecting the mask pattern. The mask size is determined depending on the display size. Namely, the mask may have the same size as the display, or a smaller mask may be used by using steppers. The pattern size of the mask is preferably several ten to 500 μm depending on the size of a pixel.

Alternatively, the alignment layer employed in the present invention can be produced by other multi-domain formation methods known in the art. For example, it is possible to form two regions differing in alignment direction from each other in a single pixel by using photolithographic techniques. By mixing two or more polymers differring in SP value of at least 1 , preferably at least 2, it is also possible to form an alignment layer which can provide at least two different states of liquid crystal alignment by the aligning process in a single direction.

The SP value is a solubility parameter (SP - value δ = (Δ E/V)^{1 2} wherein Δ E is the molar heat of vaporization and V is the volume fraction (see, e.g, I.H. Hildebrand, R.L Scott, The Solubility of Nonelectrolytes, 3rd Ed., Reinhold, New Yor, 1949). It is the preferable that the liquid crystal alignment layer has microprotrusions on the surface thereof. It is still preferable in these cases that in the liquid crystal alignment layer, at least one of the polymers is a polymer containing siloxane or one containing fluorine.

It is preferable that the alignment layer has microprotrusions with arbitrary sizes and shapes on the surface thereof. These microprotrusions are from several hundred A to several μm in size, i.e., in diameter, and from several hundred A to several μm in height, preferably from several hundred A to several thousand A in height. Size and height of the microprotrusions can be measured by methods well known in the art, e.g., atomic force microscopy or scanning electron microscopy. When at least one polymer, which differs in SP value by 1 or more from other polymers, serves as a base material amounting to 50 % or more of the alignment layer materials while other polymer(s) are used as so-called dopant(s) to be blended with the base, a phase separation is generated by mixing these polymers. As a result, it is believed that the polymer(s) added form microphase separation structures on the base polymer, thus giving microprotrusions.

The alignment layer can be produced by mixing two or more polymers differing in SP value by at least 1, and forming a film of the resulting mixture on a glass or plastic substrate by spin coating or print coating in accordance with a method commonly known in the art. By forming such alignment layers on both substrates, a liquid crystal display device having two or more alignment states with a high stability and a good reproducibility can be established.

The polymers to be used as the material for the alignment layer can be selected from, but are not limited to, those commonly employed in organic alignment layers such as polyimide, polyamide, polyurethane, polyester, polycarbonate, polyurea, polyether, polyimidoamide, polypeptide, polyolefins, cellulose and derivatives thereof, polyacrylates, polymethacrylates, polyvinyl such as polystyrenes and polyvinyl alcohol. It is preferable that at least one of the polymers to be used as the material for the alignment layer of the present invention is a polymer containing siloxane or one containing fluorine.

Polyimide, polyamide, polyurethane, polyester, polycarbonate, polyether, polyimidoamide and polyurea can be obtained by polymerizing monomers (for example, diisocyanate, diol, dicarboxylic acid, diamine, tetracarboxylic anhydride) by a method which has been commonly known by those skilled in the art.

A polymer containing fluorine can be obtained by using a monomer substituted with fluorine atom(s).

Examples of cellulose and derivatives thereof include hydroxypropyl cellulose, cellulose, hydroxymethyl cellulose, cellulose acetate butyrate, cellulose acetate phthalate, cellulose triacetate, methyl cellulose, cellulose acetate, cellulose hydroxypeptide, p-aminobenzyl cellulose, polyethyleneimine cellulose, triethylaminoethyl cellulose, ethyl cellulose, cyanoethylated cellulose, carboxymethylated cellulose, diethylaminohydroxypropylated cellulose, sulfohydroxypropylated cellulose, trimethylaminohydroxy-propylated cellulose and bromoacetyl cellulose.

Examples of polyvinyl and derivatives thereof include polystyrene, sodium poly-4- styrene sulfonate, polymethylstyrene, dicarboxy- terminated polystyrene, monocarboxy-terminated polystyrene, polystyrene divinylbenzene, polystyrene methyl methacrylate, 3-trifluoromethylstyrene, polyvinylalcohol, polyvinylbiphenyl, polyvinylbiphenylether, polyvinylcinnamate, polyvinylformal, polyacenaphthylene, polyvinylcarbazole, polyvinylcyclohexyl, polyvinylmethlketone, polyvinylnaphthalene, polyvinylphenol, polyvinylpyridine, polyvinylbutyral, polyvinylidenefluoride, polyvinylpyhdine-N-oxide, polyvinylchloride, polyvinylfluoride and polystyrenesulfonylfluoride.

Examples of the polyacrylates or polymethacrylates include poly(methyl acrylate), poly(methyl methacrylate), poly(ethyl acrylate), poly(ethyl methacrylate), poly(butyl acrylate), poly(butyl methacrylate), poly(isobutyl acrylate), poly(isobutyl methacrylate), poly(t-butyl acrylate), poly(t-butyl methacrylate), poly(hexyl acrylate), poly(hexyl methacrylate), poly(2-ethylbutyl acrylate), poly(2-ethylbutyl methacrylate), poly(benzyl acrylate), poly(benzyl methacrylate), poly(cyclohexyl acrylate), poly(cyclohexyl methacrylate), poly(norbomyl acrylate), poly(norbornyl methacrylate), poly(isobornyl acrylate), poly(isobomyl methacrylate), poly(biphenyl acrylate), poly(biphenyl methacrylate) and copolymers thereof.

A fluorine-containing polymer can be obtained by introducing fluorine atom(s) into such a polyacrylate or a polymethacrylate.

As the polyolefins, use can be made of, for example, polyethylene, polypropylene, polyacetylene, polybutadiene, polyvinylidene fluoride and copolymers thereof. In addition thereto, polysilastyrene, etc. are also usable therefor.

As a siloxane-containing polymer, use can be made of polymers obtained by reacting a compound represented by the following general formula as a siloxane component. It is preferable to use a block polymer of siloxane with other polymer(s).

wherein m is an integer of 1 or above;

R² represents a divalent hydrocarbon group;

R³ represents a monovalent, linear or branched, aliphatic hydrocarbon group having from 1 to 5 carbon atoms or a alicyclic or aromatic hydrocarbon group having from 4 to 14 carbon atoms; and R⁴ represents -NH₂, -OH, -COOH, Ar(COOH)₂, Ar(CO)₂O or

-NHSi(CH₃)₃, wherein Ar represents an aromatic group.

It is preferable that R² is a linear alkylene group having from 1 to 10 carbon atoms. Ar is preferably an aromatic group having from 4 to 14 carbon atoms.

Examples of the aliphatic hydrocarbon group usable as R³ include methyl, ethyl, propyl, isopropyl, n-butyl, t-butyl and pentyl groups. Examples of the alicyclic hydrocarbon group include cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl groups. Examples of the aromatic hydrocarbon group include phenyl, tolyl, xylyl, biphenyl, naphthyl, anthryl and phenanthryl groups. These aromatic rings may be substituted by, for example, halogen atoms, nitro groups or alkyl groups. R³ may be different from each other. It is preferable that R³ is a methyl group.

In the above formula, it is preferable that m is 2 or above, still preferably 8 or above. When the polymerization degree of polysiloxane is excessively high, however, there arises a tendency toward deterioration in the strength of the alignment layer material. It is therefore preferable that m is not more than 100. Examples

To further illustrate the present invention in greater detail, and not by way of limitation, the following Examples will be given.

Example 1

A polyaramide/polysiloxane copolymer and polyvinyl carbazole were dissolved in N-methylpyrrolidone at a ratio of 1 : 1 to give a concentration of 2.5 % by weight. The solution thus obtained was applied onto glass substrates by using a spinner at 2,000 r.p.m. for 20 seconds, and dried at 180°C for 1 hour to form a polymer film having a thickness of about 50 nm. Then the film was aligned by rubbing with a nylon fabric in one direction. Next, the alignment layer was irradiated with UV light for 5 minutes by using a lattice-patterned mask (300 μm x 50 μm) under a high pressure mercury lamp (105 W) to form an alignment layer having two types of regions adjacent to each other, namely, the UV-irradiated regions (300 μm x 50 μm) and unirradiated regions. Then an IPS-TFT cell was assembled using the substrate having the alignment layer thus obtained as the substrate of the electrode side. Fig. 3 shows the constitution of the alignment layer and the alignment directions of liquid crystal molecules. In Fig. 3, a pair of an UV-irradiated region (b) and an unirradiated region (a) corresponds to a dot regulated by a TFT device. A dot of one (a-b) pair usually corresponds to one of colors R (red), G (green) and B (blue).

A nematic liquid crystal material (LIXON 5043LC, manufactured by Chisso Petrochemical Co.) was filled into the cell and subjected to a heat treatment. Evaluation of the viewing angle characteristics of this liquid crystal cell indicated that the region with low contrast was narrowed and thus the viewing angle of the cell was widened (Fig. 4).

Example 2 An IPS-TFT cell was assembled in the same manner as that described in Example 1 , except for using a mask of a pattern size of 150 μm x 100 μm and arranging the alignment layer so that the electrical field was applied vertically thereto. The alignment layer was arranged so that the adjacent regions in adjacent dots differed from each other in alignment direction. Fig. 5 shows the constitution of the alignment layer and the alignment directions of liquid crystal molecules.

A nematic liquid crystal material (LIXON 5043LC, manufactured by Chisso Petrochemical Co.) was filled into the cell and subjected to a heat treatment.

Evaluation of the viewing angle characteristics of this liquid crystal cell indicated that the region with low contrast was narrowed and thus the viewing angle of the cell was widened (Fig. 6).

Example 3

An alignment layer was assembled in the same manner as that described in Example 1 and aligned by rubbing in one direction. Next, the alignment layer was irradiated with UV light for 5 minutes by using a mask (100 μm x 37.5 μm and 150 μm x 25 μm) under a high pressure mercury lamp (105 W) so that the UV-irradiated regions and the unirradiated regions were arranged as shown in Fig. 7. Then an

IPS-TFT cell was assembled using the substrate as the substrate on the electrode side.

A nematic liquid crystal material (LIXON 5043LC, manufactured by Chisso Petrochemical Co.) was filled into the cell and subjected to a heat treatment. Evaluation of the viewing angle characteristics of this liquid crystal cell indicated that the region with low contrast substantially disappeared and the viewing angle of the cell was further widened (Fig. 8).

Fig. 1 shows the structure of an IPS-TFT liquid crystal display device.

Fig. 2 shows the viewing angle characteristics of a conventional IPS-TFT cell.

Fig. 3 shows the constitution and alignment directions of liquid crystal molecules of the alignment layer of the present invention produced in Example 1. Only the regions b were irradiated with UV light. Fig. 4 shows the viewing angle characteristics of the liquid crystal display device of the present invention produced in Example 1.

Fig. 5 shows the constitution and alignment directions of liquid crystal molecules of the alignment layer of the present invention produced in Example 2. Only the region d was irradiated with UV light.

Fig. 6 shows the viewing angle characteristics of the liquid crystal display device of the present invention produced in Example 2.

Fig. 7 shows the constitution of the alignment layer of the present invention produced in Example 3. The regions b and d were irradiated with UV light.

Fig. 8 shows the viewing angle characteristics of the liquid crystal display device of the present invention produced in Example 3.

Claims

1. An IPS-TFT liquid crystal display device characterized in that at least two regions differing in alignment direction from each other are formed in an alignment layer on the substrate of the electrode side.

2. A liquid crystal display device as claimed in Claim 1 , characterized in that a part of said alignment layer has been irradiated with UV light or an electron beam after completion of the aligning process, whereby the alignment direction of the alignment layer within the irradiated region is shifted by about 90° to the alignment direction established by the aligning process.

3. A process for producing an IPS-TFT liquid crystal display device comprising the steps of: preparing an alignment layer made of organic polymers; aligning said alignment layer; irradiating a part of the alignment layer with UV light or an electron beam to shift the alignment direction of the alignment layer within the irradiated region by about 90° to the alignment direction established by the aligning process; and assembling the liquid crystal display device using the irradiated alignment layer as the alignment layer on the substrate of the electrode side.

4. Use of an alignment layer having at least two regions differing in alignment direction from each other in an IPS-TFT liquid crystal display device.

5. Use as claimed in Claim 4, characterized in that a part of said alignment layer has been irradiated with UV light or an electron beam after completion of the aligning process, whereby the alignment direction of the alignment layer within the irradiated region is shifted by about 90° to the alignment direction established by the aligning process.