FIELD OF THE INVENTION

The present invention relates to a high-quality liquid crystal display device that improves viewing angle characteristics and display contrast and a manufacturing method thereof.

BACKGROUND OF THE INVENTION

Since liquid crystal display devices have merits such as high display quality, reduced thickness, reduced weight, and low power consumption, the use applications of the devices are expanding, and the devices are used for various use applications including mobile device monitors such as a mobile telephone monitor, digital still camera monitor, personal computer monitor, monitor intended for printing and design, medical monitor, and liquid crystal television. In association with the expansion of these use applications, it is demanded to further improve the image quality and the quality of the liquid crystal display device, and it is strongly demanded to improve luminance and to decrease power consumption by achieving higher transmittances specifically. Moreover, in association with the spread the liquid crystal display device, a decrease in costs is also demanded.

In general, images are displayed on the liquid crystal display device in which an electric field is applied to the liquid crystal molecules of a liquid crystal layer sandwiched between a pair of substrates to change the alignment direction of the liquid crystal molecules and the change causes changes in the optical properties of the liquid crystal layer for displaying images. The alignment direction of the liquid crystal molecules when the electric field is not applied defined by an alignment film that the surface of a polyimide thin film is rubbed. Conventionally, in an active matrix liquid crystal display device having a switching element such as a thin film transistor (TFT) for each pixel, an electrode is individually provided on a pair of substrates between which a liquid crystal layer is sandwiched, an electric field is set to a so-called vertical electric field that the direction of the electric field applied to the liquid crystal layer almost perpendicular to the substrate surface, and images are displayed using the optical rotatory power of liquid crystal molecules forming the liquid crystal layer. For representative liquid crystal display devices in a vertical field mode, liquid crystal display devices in a twisted nematic (TN) mode and a vertical alignment (VA) mode are known.

In liquid crystal display devices in the TN mode and the VA mode, one of large problems is a narrow viewing angle. Therefore, as display modes to achieve wider viewing angles, an in-plane switching (IPS) mode and a fringe-field switching (FFS) mode, which is one type of the IPS mode, are known.

The IPS mode and the FFS mode are a so-called transverse electric field display mode in which an electrode is formed on one of a pair of substrates and an electric field to be generated has a component nearly in parallel with the substrate surface. Liquid crystal molecules forming a liquid crystal layer are rotated in a plane nearly in parallel with the substrate, and images are displayed using the birefringence of the liquid crystal layer. The IPS mode and the FFS mode are advantageous in that the viewing angle is wide and the load capacity is low as compared with the previously existing TN mode because of the in lane switching of the liquid crystal molecules, for example. The liquid crystal display devices in the IPS mode and the FFS mode are regarded as new promising devices that replace liquid crystal display devices in the TN mode, and are in a rapid progress in these years.

In the liquid crystal display device, the orientation state of the liquid crystal molecules in the liquid crystal layer is controlled by the presence or absence of an electric field. In other words, upper and lower polarizers provided on the outer sides of the liquid crystal layer are set in the completely orthogonal state, a phase difference is generated due to the orientation state of the liquid crystal molecules between the polarizers, and light and dark states are formed. In order to control the orientation state in which no electric field is applied to the liquid crystal molecules, this control is achieved in which a polymer thin film called an alignment film is formed on the surface of the substrate and the liquid crystal molecules are arrayed in the array direction of polymers due to an intermolecular interaction caused by van der Waals force between a polymer chain and the liquid crystal molecule on the interface. This interaction is also referred to as alignment regulating force, the provision of a liquid crystal aligning function, or an alignment process.

Polyimide is often used for an alignment film of a liquid crystal display device. In a forming method for the alignment film, polyamic acid that is a polyimide precursor is dissolved in various solvents, and coated on a substrate by spin coating or printing, the substrate is heated at high temperature at a temperature of 200° C. or more, the solvents are removed, and the polyamic acid is imidized to polyimide by cyclization. The thin film has a thickness of about 100 nm in the imidization. The surface of this polyimide thin film is rubbed in a certain direction using a rubbing cloth, polyimide polymer chains on the surface are aligned in the rubbing direction, and then it is achieved that polymers on the surface are in a high anisotropic state. However, there are problems such as the occurrence of static electricity and foreign substances caused by rubbing and ununiform rubbing caused by irregularities on the surface of the substrate, and a photo-alignment method is becoming adopted in which polarized light is used to control molecular orientations with no need to contact a rubbing cloth.

The photo-alignment method for a liquid crystal alignment film includes photoisomerization type photo-alignment that the geometry in a molecule is changed by applying a polarized ultraviolet ray like azo dye and photodimerization type photo-alignment that molecular frameworks generate a chemical bond caused by a polarized ultraviolet ray such as cinnamic acid, coumalin, and chalcone, and other types. Photodecomposition type photo-alignment is suited to the photo-alignment of polyimide that is reliable and achieves results as a liquid crystal alignment film, in which a polarized ultraviolet ray is applied to polymers, only polymer chains arranged in the polarization direction are broken and decomposed and molecular chains in the direction perpendicular to the polarization direction are left.

This method is studied in various liquid crystal display modes. For the IPS mode in the various modes, Japanese Patent Application Laid-Open No. 2004-206091 discloses a liquid crystal display device that decreases the occurrence of display failures caused by changes in the initial alignment direction, stabilizes liquid crystal alignment, and improves mass production, a contrast ratio, and image quality. In Japanese Patent Application Laid-Open No. 2004-206091, the function of controlling molecular orientations is provided by performing an alignment process in which at least one secondary treatment of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure is applied to polyimide or polyamic acid formed of aromatic diamine, cyclobutanetetracarboxylic dianhydride, and a derivative of cyclobutanetetracarboxylic dianhydride, polyamic acid formed of aromatic diamine and cyclobutanetetracarboxylic dianhydride, or polyamic acid formed of aromatic diamine and a derivative of cyclobutanetetracarboxylic dianhydride. It is noted that a configuration in which the alignment film is formed in a two-layer structure is described in Japanese Patent Application Laid-Open No. 2010-72011.

More specifically, Japanese Patent Application Laid-Open No. 2004-206091 describes that the effect is further effectively exerted when at least one process of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure is performed in a temporal overlap of a polarized light irradiation process, and that the effect is also effectively exerted when an alignment control film is subjected to an imidization baking process and the polarized light irradiation process in a temporal overlap. More specifically, Japanese Patent Application Laid-Open No. 2004-206091 describes that in the case where a liquid crystal alignment film is subjected to at least one process of heating, infrared irradiation, far infrared irradiation, electron beam irradiation, and radiation exposure in addition to polarized light irradiation, the temperature of the alignment control film is desirably in a range of a temperature of 100 to 400° C., and more desirably in a range of a temperature of 150 to 300° C. The processes of heating, infrared irradiation, and far infrared irradiation can be combined with the imidization baking process of the alignment control film, which is effective.

However, the liquid crystal display device using these photo-alignment films has a short history as compared with the case of using rubbed alignment films, and sufficient findings are not available for long-term display quality over several years as a practical liquid crystal display device. In other words, the fact s that the relationship between image quality failures and problems unique to the photo-alignment film, which are not obvious in the initial stage of manufacture, are rarely reported.

SUMMARY OF THE INVENTION

The present inventors thought that in order to implement a liquid crystal display device of high quality and high definition in future, photo-alignment techniques became important, and conducted detailed studies on problems in the application of the photo-alignment techniques to liquid crystal display devices. As a result, the following was revealed. In the previously existing photo-alignment techniques, ultraviolet rays used for photo-alignment processes are effective in producing liquid crystal alignment regulating force on the surface of the alignment film. However, ultraviolet rays also work in the inside of the film, for which a long-term structural stability is necessary, and the ultraviolet rays optically degrade the inside of the film, and decrease the mechanical strength of the film. Thus, the ultraviolet rays affect the display stability of a liquid crystal display device when vibrations, impacts, thermal strain, and the like are externally applied, leading to problems to cope with products in future.

It is an object of the present invention to provide a liquid crystal display device that can stably provide excellent display characteristics for a long time even in the case where photo-alignment techniques are used and a manufacturing method thereof.

In the present application, a brief description of a representative configuration of some aspects to be disclosed is as follows.

A liquid crystal display device includes: a TFT substrate having a pixel electrode and a TFT and formed with an alignment film on a pixel; a counter substrate disposed opposite to the TFT substrate and formed with an alignment film on a top surface on the TFT substrate side; and a liquid crystal sandwiched between the alignment film of the TFT substrate and the alignment film of the counter substrate. In the liquid crystal display device, the alignment film is configured of a first alignment film layer contacting the liquid crystal layer and having at least one kind of polyimide and a second alignment film layer formed below the first alignment film layer and having at least one kind of polyimide, the first alignment film layer is a material that is enabled to provide liquid crystal alignment regulating force by applying polarized light, and the first alignment film layer and the second alignment film layer contain a common polyimide structure.

Moreover, in the liquid crystal display device, the common polyimide structure includes a part of a repetition unit structure forming a polyimide structure contained only in the first alignment film layer and a part of a repetition unit structure forming a polyimide structure contained only in the second alignment film layer.

Furthermore, in the liquid crystal display device, the alignment film is in a state of a liquid solution uniformly dissolved in a solvent immediately before the alignment film is coated on the substrate, and a phase separation state appears in the liquid solution state by heating after the alignment film is coated on the substrate.

In addition, in the liquid crystal display device, immediately before the alignment film is coated on the substrate, the polyimide is not imidized and in a state of a liquid solution uniformly dissolved in a solvent, and a phase separation state appears in the liquid solution state by heating after the alignment film is coated on the substrate.

Moreover, in the liquid crystal display device, the common polyimide structure is present in a range of 1 to 20 wt % to the entire first alignment film layer, and the common polyimide structure is present in a range of 1 to 20 wt % to the entire second alignment film layer.

Furthermore, in the liquid crystal display device, the alignment film is a photodecomposition type photo-alignment film.

In addition, in the liquid crystal display device, the first alignment film layer is a photodecomposition type photo-alignment film containing polyimide given by Chemical formula 1,


where a formula in brackets expresses a chemical structure of a repetition unit, numerical subscript n expresses a number of the repetition unit, N expresses a nitrogen atom, O expresses an oxygen atom, A expresses a quadrivalent organic group containing a cyclobutane ring, and C expresses a divalent organic group.

Moreover, in the liquid crystal display device, the alignment film has a structure in which two types of alignment films are stacked in a two-layer structure formed of a photo-alignable photo-alignment upper layer and a low resistive under layer having a resistivity lower than a resistivity of the photo-alignment upper layer. The specific resistivity of the under layer alignment film is decreased, so that it is possible to eliminate electric charges charged on the alignment film in an early stage, and it is possible to reduce an afterglow phenomenon.

Furthermore, in the liquid crystal display device, the liquid crystal display device is an IPS mode liquid crystal display device.

In addition, a manufacturing method for a liquid crystal display device is a manufacturing method for a liquid crystal display device including a TFT substrate having a pixel electrode and a TFT and formed with an alignment film on a pixel, a counter substrate disposed opposite to the TFT substrate and formed with an alignment film on a top surface on the TFT substrate side, and a liquid crystal sandwiched between the alignment film of the TFT substrate and the alignment film of the counter substrate, the method including the steps of: preparing the TFT substrate having the pixel electrode and the TFT; preparing an alignment film material containing a blend of a first polymeric material having a first solubility parameter, a second polymeric material having a second solubility parameter, and a third polymeric material having a third solubility parameter between the first solubility parameter and the second solubility parameter; subjecting a surface of the TFT substrate or the counter substrate to ozone water processing; forming an alignment film on the TFT substrate or the counter substrate using the alignment film material after the ozone water processing; applying a polarized ultraviolet ray to the alignment film to provide a state of liquid crystal alignment regulating force on a top surface layer of the alignment film; attaching the TFT substrate attached with the alignment film provided with alignment regulating force to the counter substrate; and filling a liquid crystal between the TFT substrate and the counter substrate in the attaching step or after the attaching step.

Moreover, in the manufacturing method for a liquid crystal display device, after applying the ultraviolet ray, the alignment film is oxidized.

Furthermore, in the manufacturing method for a liquid crystal display device, after the step of forming the alignment film on the TFT substrate or the counter substrate, the substrate is held at a temperature of 100 to 150° C.

Polyimide referred here is a polymer compound expressed by Chemical formula 1, where a formula in brackets expresses the chemical structure of a repetition unit, numerical subscript n expresses the number of the repetition unit, N expresses a nitrogen atom, O expresses an oxygen atom, A expresses a quadrivalent organic group containing a cyclobutane ring, and D expresses a divalent organic group. Examples of the structure of A can include: an aromatic cyclic compound such as a phenylene ring, naphthalene naphthalene ring, and anthracene ring; an aliphatic cyclic compound such as cyclobutane, cyclopentane, and cyclohexane; or a compound that a substituent group is bonded to these compounds, for example. In addition, examples of the structure of D can include: an aromatic cyclic compound such as phenylene, biphenylene, oxybiphenylene, biphenyleneamine, naphthalene, and anthracene; an aliphatic cyclic compound such as cyclohexene and bicyclohexene; or a compound that a substituent group is bonded to these compounds, for example.



These polyimides are coated on various base layers held on a substrate in a state of a polyimide precursor.

Moreover, the polyimide precursor referred here is polyamic acid or a polyamic acid ester polymer compound expressed by Chemical formula 2, where H expresses a hydrogen atom, R1and R2express hydrogen or an alkyl chain —CmH2m+1, and m=1 or 2.



In order to form such an alignment film, a thin film is formed using a typical forming method for a polyimide alignment film, for example, in which a base layer is purified using various surface treatment methods such as a UV/ozone method, excimer UV method, and oxygen plasma method, the precursor of the alignment film is coated using various printing methods such as screen printing, flexographic printing, and ink jet printing, the film is subjected to a leveling process to provide a uniform film thickness under predetermined conditions, and then the film is heated at a temperature of 180° C. or more, for example, to imidize a precursor polyamide to polyimide. In the formation, it is also possible to add various additives in advance in order to improve the wettability to the base layer and to promote the imidization reaction, for example.

In the formation, the provision of a strong polarity to the base layer is specifically effective in forming an alignment film having a plurality of types of alignment film layers like the alignment film according to an embodiment of the present invention by only a single coating step. In other words, it is possible to use the effect that the alignment film is a liquid solution uniformly mixed with polyimide structures forming a plurality of types of the alignment film layers immediately after the liquid solution is coated and the liquid solution is naturally separated into two layers based on the difference between the surface free energy to the air layer of the topmost layer and the surface free energy of the base layer in the processes of drying the solvent after the liquid solution is coated.

However, since the effect of the cleanness of the base layer is lost at a moment at which the base layer contacts the liquid solution containing the solvent, the surface free energy difference is smaller than the surface free energy difference in the state immediately before the liquid solution is coated in the midway point in which the concentration of the liquid solution becomes denser for drying the solvent. When a polar group is generated by surface treatment to provide a strong polarity on the base layer to maintain surface free energy, ozone water cleaning, for example, so that it is possible to maintain the surface free energy difference for a long time, and it is possible to keep the properties that cause the liquid solution to be naturally separated into two layers for a long time. Moreover, for such two-layer separation properties, the polyimide structures used for forming a plurality of types of alignment film layers desirably substantially have phase separation properties.

In the case where it is supposed that the liquid solution is a polyimide blend with no phase separation properties, it is not possible to efficiently separate the polymer blend into two layers even though the surface free energy difference is present between the air layer and the base layer, and specific component polymers can be induced only to near the interface contacting the air layer or the base layer. In general, for the phase separation properties, some polymer blends exhibit phase separation at a temperature lower than a critical temperature and some exhibit phase separation at a temperature higher than a critical temperature. In forming the alignment film, phase separation properties desirably appear in the processes of increasing temperature to a temperature of 180° C. or more, at which the thermal imidization reaction proceeds, after the polymer blend is coated at ambient temperature.

More desirably, it is important to select a polymer blend having phase separation properties at a high temperature at which two-layer separation properties are promoted by performing a process that the polymer blend is held at a temperature of 100 to 150° C. for certain hours before thermal imidization. Since the solvent is left too much on the substrate after the blend is coated at a temperature lower than a temperature of 100 to 150° C., the phase is a mixed phase of the polyimide precursor and the solvent. When the solvent is being dried to some extent, the phase separation properties of the polyimide precursor blend can appear. Moreover, alignment regulating force can be produced on the surface of the polyimide alignment film by applying polarized ultraviolet rays or by moderate postprocessing using desired schemes.

Two substrates attached with the alignment film thus formed are attached to each other with a certain gap maintained, and the gap portion is filled with a liquid crystal. Alternatively, a liquid crystal is dropped before the substrates are attached to each other, and then the substrates are attached to each other. After attaching the substrates, the end portions of the substrates are sealed, and a liquid crystal panel is completed. Optical films such as a polarizer and a retardation plate are attached to the panel, a drive circuit, a backlight, and other components are mounted, and then a liquid crystal display device is obtained.

According to an aspect of the present invention, it is possible to provide a high quality liquid crystal display device that achieves the mechanical strength of a photo-alignment film and has excellent long-term storage stability, viewing angle characteristics, high display contrast, and less afterglow.

