JP2016095537A - Liquid crystal alignment film material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an optical alignment film having large anchoring strength in a liquid crystal display device of an IPS system and to suppress an AC residual image of the liquid crystal display device.SOLUTION: In alignment film material on which optical alignment treatment is applied, a polyimide to be a rigid high polymer and a polyimide to be a soft high polymer are mixed to be used. The material easily generates rotation of an oligomer after emission of polarizing ultraviolet, and can improve dichroic ratio of UV absorption of an alignment film. Thus, anchoring strength by the alignment film to liquid crystal is strong, and an AC residual image can be suppressed.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which an alignment film is provided with an alignment control ability by light irradiation.

  In a liquid crystal display device, there are a TFT substrate in which pixel electrodes and thin film transistors (TFTs) are formed in a matrix, and a counter substrate in which color filters are formed at locations corresponding to the pixel electrodes of the TFT substrate, facing the TFT substrate. The liquid crystal is sandwiched between the TFT substrate and the counter substrate. An image is formed by controlling the light transmittance of the liquid crystal molecules for each pixel.

  Since the liquid crystal display device is flat and lightweight, the application is expanding in various fields such as a large display device such as a TV, a mobile phone, and a DSC (Digital Still Camera). On the other hand, viewing angle characteristics are a problem in liquid crystal display devices. The viewing angle characteristic is a phenomenon in which luminance changes or chromaticity changes when the screen is viewed from the front and when viewed from an oblique direction. The viewing angle characteristic is excellent in an IPS (In Plane Switching) system in which liquid crystal molecules are operated by a horizontal electric field.

  As a method for imparting an alignment treatment, that is, an alignment control ability, to an alignment film used in a liquid crystal display device, there is a conventional method of rubbing. The alignment treatment by rubbing is performed by rubbing the alignment film with a cloth. On the other hand, there is a technique called a photo-alignment method that imparts alignment control ability to the alignment film in a non-contact manner. Since the IPS method does not require a pretilt angle, a photo-alignment method can be applied. “Patent Document 1” to “Patent Document 7” are known examples of photo-alignment films, and by irradiating linearly polarized ultraviolet rays, a molecular crosslinking reaction, a cleavage reaction, and a dimerization reaction are induced in the thin film. It has been described to impart molecular anisotropy in thin films.

JP 2004-86047 A JP 2004-20658 A JP 2004-163646 A JP 2004-341030 A Japanese Patent Laid-Open No. 2004-346311 JP 2005-215029 A JP 2006-17880 A

  Conventional photo-alignment processing tends to cause image sticking called an afterimage, compared to rubbing processing. The AC afterimage is an afterimage caused by the cause when the liquid crystal display device is operated for a long time, and the direction of the initial alignment is deviated from the original direction of the liquid crystal display device. AC afterimage is irreversible and cannot be recovered.

  Such an AC afterimage includes (1) improvement in alignment order of the alignment film, (2) improvement in mechanical strength using parameters such as elastic modulus and hardness of the alignment film, and (3) affinity between the alignment film and liquid crystal. It can be improved by improving. Among them, the improvement of the alignment order of the alignment film is particularly effective for reducing the AC afterimage.

  However, in the photo-alignment method, an effective method for improving the alignment order has not been found. An object of the present invention is to improve the alignment order of the alignment film and suppress the occurrence of AC afterimage in the photo-alignment treatment.

  The present invention overcomes the above problems, and specific means are as follows. That is, a TFT substrate in which an alignment film is formed on a pixel having a pixel electrode and a TFT, a counter substrate in which an alignment film is formed on a color filter so as to face the TFT substrate, and an alignment film on the TFT substrate And a liquid crystal display device in which a liquid crystal is sandwiched between the alignment films of the counter substrate, the alignment films have undergone a photo-alignment treatment, and the alignment films are 1,2-biss represented by (Chemical Formula 1). An alignment film material obtained by esterifying a dehydration condensate or a dehydration condensate of (4-aminophenyl) ethane and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3); A liquid crystal display device comprising: a phenyldiamine and a dehydrated condensate of 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) or an alignment film material obtained by esterifying the dehydrated condensate. is there.

  The wt% with respect to the entire flexible structure is greater than 0 wt% and not more than 80 wt%, more preferably not less than 60 wt% and not more than 80 wt%.

  According to the present invention, since the order parameter (OP) can be increased and the anchoring strength can be increased in the photo-alignment film, a liquid crystal display device having a photo-alignment film with suppressed AC afterimage is realized. I can do it.

1 is a cross-sectional view of an IPS liquid crystal display device. It is a top view of the pixel electrode of FIG. It is process drawing which shows the liquid process of an alignment film. In the photoalignment process by a prior art example and this invention, it is a schematic diagram which shows the film | membrane structure for every process. It is a graph which shows the relationship between wt% of a flexible structure, and the order parameter (OP) of alignment film.

  Before describing embodiments of the present invention, the structure of an IPS liquid crystal display device to which the present invention is applied will be described. FIG. 1 is a cross-sectional view showing a structure in a display region of an IPS liquid crystal display device. Various electrode structures of IPS liquid crystal display devices have been proposed and put into practical use. The structure shown in FIG. 1 is a structure that is widely used at present. To put it simply, a comb-like pixel electrode 110 is formed on a counter electrode 108 formed of a flat solid with an insulating film interposed therebetween. Yes. Then, an image is formed by controlling the light transmittance of the liquid crystal layer 300 for each pixel by rotating the liquid crystal molecules 301 by the voltage between the pixel electrode 110 and the counter electrode 108. The structure of FIG. 1 will be described in detail below. Although the present invention will be described by taking the configuration of FIG. 1 as an example, it can also be applied to IPS type liquid crystal display devices other than FIG.

  In FIG. 1, a gate electrode 101 is formed on a TFT substrate 100 made of glass. The gate electrode 101 is formed in the same layer as the scanning line. The gate electrode 101 has a MoCr alloy laminated on an AlNd alloy.

  A gate insulating film 102 is formed of SiN so as to cover the gate electrode 101. A semiconductor layer 103 is formed of an a-Si film on the gate insulating film 102 at a position facing the gate electrode 101. The a-Si film is formed by plasma CVD. The a-Si film forms the channel portion of the TFT, and the source electrode 104 and the drain electrode 105 are formed on the a-Si film with the channel portion interposed therebetween. Note that an n + Si layer (not shown) is formed between the a-Si film and the source electrode 104 or the drain electrode 105. This is for making ohmic contact between the semiconductor layer and the source electrode 104 or the drain electrode 105.

  The source electrode 104 is also used as a video signal line, and the drain electrode 105 is connected to the pixel electrode 110. The source electrode 104 and the drain electrode 105 are simultaneously formed in the same layer. In this embodiment, the source electrode 104 or the drain electrode 105 is made of a MoCr alloy. In order to reduce the electrical resistance of the source electrode 104 or the drain electrode 105, for example, an electrode structure in which an AlNd alloy is sandwiched between MoCr alloys is used.

  An inorganic passivation film 106 is formed of SiN so as to cover the TFT. The inorganic passivation film 106 protects the TFT, particularly the channel portion, from the impurities 401. An organic passivation film 107 is formed on the inorganic passivation film 106. The organic passivation film 107 has a role of flattening the surface at the same time as protecting the TFT, and thus is formed thick. The thickness is 1 μm to 4 μm.

  A photosensitive acrylic resin, silicon resin, polyimide resin, or the like is used for the organic passivation film 107. In the organic passivation film 107, it is necessary to form a through hole 111 at a portion where the pixel electrode 110 and the drain electrode 105 are connected. However, since the organic passivation film 107 is photosensitive, the organic passivation film 107 is not used without using a photoresist. The through hole 111 can be formed by exposing and developing itself.

  A counter electrode 108 is formed on the organic passivation film 107. The counter electrode 108 is formed by sputtering ITO (Indium Tin Oxide), which is a transparent conductive film, over the entire display region. That is, the counter electrode 108 is formed in a planar shape. After the counter electrode 108 is formed on the entire surface by sputtering, the counter electrode 108 is removed by etching only in the through hole 111 portion for conducting the pixel electrode 110 and the drain electrode 105.

  An upper insulating film 109 is formed of SiN so as to cover the counter electrode 108. After the upper insulating film 109 is formed, a through hole 111 is formed by etching. The through hole 111 is formed by etching the inorganic passivation film 106 using the upper insulating film 109 as a resist. Thereafter, ITO that becomes the pixel electrode 110 covering the upper insulating film 109 and the through hole 111 is formed by sputtering. The pixel electrode 110 is formed by patterning the sputtered ITO. ITO serving as the pixel electrode 110 is also deposited on the through hole 111. In the through hole 111, the drain electrode 105 extending from the TFT and the pixel electrode 110 become conductive, and a video signal is supplied to the pixel electrode 110.

  FIG. 2 shows an example of the pixel electrode 110. The pixel electrode 110 is a comb-like electrode. A slit 112 is formed between the comb teeth. A planar counter electrode 108 is formed below the pixel electrode 110. When a video signal is applied to the pixel electrode 110, the liquid crystal molecules 301 are rotated by electric lines of force generated between the counter electrode 108 through the slit 112. As a result, light passing through the liquid crystal layer 300 is controlled to form an image.

  FIG. 1 illustrates this as a cross-sectional view. A slit 112 shown in FIG. 1 is formed between the comb-shaped electrode and the comb-shaped electrode. A constant voltage is applied to the counter electrode 108, and a voltage based on a video signal is applied to the pixel electrode 110. When a voltage is applied to the pixel electrode 110, as shown in FIG. 1, the lines of electric force are generated, and the liquid crystal molecules 301 are rotated in the direction of the lines of electric force to control the transmission of light from the backlight. Since transmission from the backlight is controlled for each pixel, an image is formed.

  In the example of FIG. 1, a counter electrode 108 formed in a planar shape is disposed on the organic passivation film 107, and a comb electrode 110 is disposed on the upper insulating film 109. However, conversely, the pixel electrode 110 formed in a planar shape may be disposed on the organic passivation film 107, and the comb-like counter electrode 108 may be disposed on the upper insulating film 109. An alignment film 113 for aligning liquid crystal molecules 301 is formed on the pixel electrode 110. The alignment film 113 is subjected to a photo-alignment process.

  In FIG. 1, a counter substrate 200 is provided with a liquid crystal layer 300 interposed therebetween. A color filter 201 is formed inside the counter substrate 200. The color filter 201 is formed with red, green, and blue color filters 201 for each pixel, and a color image is formed. A black matrix 202 is formed between the color filters 201 to improve the contrast of the image. Note that the black matrix 202 also has a role as a light shielding film of the TFT, and prevents a photocurrent from flowing through the TFT.

  An overcoat film 203 is formed to cover the color filter 201 and the black matrix 202. Since the surface of the color filter 201 and the black matrix 202 is uneven, the surface is flattened by the overcoat film 203. On the overcoat film 203, an alignment film 113 for determining the initial alignment of the liquid crystal is formed. This alignment film 113 is also subjected to a photo-alignment process.

  The present invention is to improve the alignment order of the photo-alignment film in FIG. 1 and reduce the AC afterimage. The inventor theoretically analyzes physical properties such as molecular extinction coefficient and photolysis quantum yield in alignment film materials, and the relationship between process parameters (heating temperature, heating time, exposure time, etc.) and alignment order. As a result, the molecular extinction coefficient ratio between the major and minor molecular directions of the alignment film material and the change in the molecular direction of the oligomer generated by the photodegradation of polyimide by polarized light exposure are important for improving the alignment order. Clarified that he plays a role. Here, the molecular extinction coefficient ratio is εp / εv, εp is the molecular absorption ratio of polyimide in the molecular long axis direction, and εv is the molecular absorption ratio of polyimide in the molecular short axis direction.

  The larger the molecular extinction coefficient ratio between the molecular major axis direction and the molecular minor axis direction, the greater the difference in concentration of polyimide in the orthogonal method with the electric field vector due to polarized light exposure. Is possible. In general, a polyimide having a large ratio of molecular extinction coefficient between the major axis direction and the minor axis direction is a rigid polymer having high molecular linearity.

  On the other hand, the direction change amount of the oligomer becomes larger as the flexibility of the polyimide (polyimide having an alkyl chain in the main chain and in which molecular axis rotation is induced by heating after polarization exposure). For this reason, in order to achieve both improved alignment order in photolysis and improved alignment order in polarized light exposure, it was necessary to eliminate the conflicting properties of polyimide.

  The inventor can eliminate the reciprocal relationship between rigidity and flexibility required for the photo-alignment film by mixing polyimide (rigid) with a high molecular extinction coefficient ratio and polyimide with high flexibility. It has been found that an alignment film having alignment order can be realized. The contents of the present invention will be described in detail below using examples.

  FIG. 3 is a diagram illustrating a process of forming an alignment film using photo-alignment processing. The process of FIG. 3 is common to the TFT substrate and the counter substrate. An alignment film is applied to the TFT substrate on which the pixel electrode in FIG. 1 is formed or the counter substrate on which the overcoat film is formed. The alignment film is applied by spin coating, inkjet, spray coating, rod coating, or the like.

  As an alignment film material, esterification of a dehydration condensate of 1,2-bis (4-aminophenyl) ethane represented by (Chemical Formula 1) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) And an alignment material obtained by esterifying the dehydration condensate of paraphenyldiamine represented by (Chemical Formula 2) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) in a weight ratio of 1: 1 is included.

  The applied alignment film is baked at 230 ° C. to imidize the alignment film. At this time, the alignment material obtained by esterifying the dehydration condensate of paraphenyldiamine represented by (Chemical Formula 2) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) has a rigid structure (hereinafter, a rigid structure) Alignment film), an orientation obtained by esterifying a dehydration condensate of 1,2-bis (4-aminophenyl) ethane represented by (Chemical Formula 1) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) The material has a flexible structure (hereinafter referred to as an alignment film having a flexible structure).

Thereafter, the temperature of the substrate is lowered to near room temperature. In addition, since a board | substrate is thin, when it takes out from a baking furnace, temperature will fall in a short time. In this state, the alignment film is irradiated with linearly polarized ultraviolet rays for photoalignment. Uniaxiality is imparted by linearly polarized ultraviolet light by cutting the main chain of the polymer alignment film in the polarization direction. At this time, a volatile low-molecular substance or an oligomer is generated by cutting the polymer. The ultraviolet rays were irradiated to the alignment film at 3 J / cm ² using a polarization exposure apparatus combining a Ushio Deep-UV lamp (ultra-high pressure He-Xe) and a polarizer.

  After the ultraviolet irradiation, the substrate is heated to 230 ° C. to volatilize the volatile low-molecular substance. At this time, the nonvolatile oligomer in the alignment film having a rigid structure does not move in the alignment film. On the other hand, the non-volatile oligomer in the alignment film having a flexible structure can be rotated in the alignment film, and the alignment order can be improved.

  FIG. 4 is a schematic view showing a comparison between the alignment film structure in the conventional example and the alignment film structure in the present invention. The upper side is a conventional structure, and the lower side is a structure according to the present invention. The material in the conventional example is formed only of a rigid alignment film, and the material in the present invention is a mixture of a rigid structure and a flexible structure. In FIG. 4, such alignment film materials are mixed at a weight ratio of 1: 1 and applied onto a quartz substrate.

  In FIG. 4, the left column is a schematic diagram of the structure of the alignment film in a state where the alignment film is applied and then heated at 230 ° C. for 10 minutes. In the conventional example, polyimide is regularly formed in a lattice shape and has a rigid structure. On the other hand, in the present invention, a rigid structure in which polyimide is formed in a lattice shape and a flexible structure in which flexible polyimide intersects flexibly are mixed.

  Then, the state which cooled the board | substrate to near normal temperature and exposed using polarized ultraviolet rays is shown in the middle column of FIG. In the middle column of FIG. 4, the main chain of polyimide is cut in the electric field vector direction of polarized light of ultraviolet light by polarized ultraviolet light, and the alignment order of the alignment film appears. This is similar to the conventional structure and the structure of the present invention.

  Then, the state baked at 230 degreeC for about 10 minutes is shown in the right column of FIG. In the conventional example, the oligomer is immobile because it is made of only rigid polyimide. On the other hand, in the present invention, a rigid structure and a flexible structure are mixed, and both the oligomer in the rigid structure and the oligomer in the flexible structure rotate to improve the alignment order.

  The above description is summarized as follows. That is, in the conventional structure shown in the upper part of FIG. 4, the alignment order is made as large as possible by using rigid polyimide, which has a large difference in molecular extinction coefficient between the molecular long axis direction and the short axis direction, as the alignment film material. Things were going on. This is because the photodegradation rate of polyimide is proportional to the molecular extinction coefficient of polyimide, so the greater the difference in molecular extinction coefficient, the greater the difference in polyimide concentration between E‖ direction and E⊥ direction after polarized UV irradiation. This is based on the idea of increasing orientational ordering. Here, E‖ is a component parallel to the electric field vector of polarization, and E⊥ is a component perpendicular to the electric field vector of polarization.

  However, a polyimide having a large difference in molecular extinction coefficient is generally a rigid polymer having high molecular linearity. For this reason, even if it heats after polarization exposure, since the molecular direction of a photolysis product is being fixed in the direction as produced | generated by photolysis, orientation orderness does not become large.

  On the other hand, in the present invention shown in the lower part of FIG. 4, an alignment film material mixed with flexible polyimide whose main chain is cut by polarized ultraviolet rays is used. As a result, as shown in FIG. 5, the concentration difference between the alignment molecules in the E‖ direction and the E⊥ direction becomes larger than that in the alignment film using each alignment film material alone, and the alignment order can be increased.

  FIG. 5 shows the relationship between the mixing ratio and the alignment order when a rigid polyimide and a flexible polyimide are mixed. As can be seen from FIG. 5, the alignment order is higher when the mixed alignment film material is used than when each alignment film material is used alone.

  FIG. 5 shows the following measurement performed on the sample shown in FIG. The polarized ultraviolet spectrum of the alignment film obtained by heating was measured, and the UV absorption dichroic ratio, ie, (A 配 向 −A‖) / (A⊥ + A‖), which is an index of alignment order, was obtained from the absorbance. . Here, A‖ is the absorbance of the alignment film in the E‖ direction, and A⊥ is the absorbance of the alignment film in the E⊥ direction. When A is absorbance, there is a relationship of A = εCt. ε is the molecular extinction coefficient, C is the concentration of molecules in a specific direction of the polyimide, and t is the thickness of the polyimide.

  In FIG. 5, the vertical axis represents OP (order parameter) in the ultraviolet ray having a wavelength of 245 nm, and the horizontal axis represents wt% of the alignment film having a flexible structure in the alignment film. OP (order parameter) is synonymous with UV absorption dichroic ratio. As can be seen from FIG. 5, the value of OP when the proportion of the flexible structure is 50 wt% is 0.51. When the anchoring strength of the liquid crystal by the alignment film at this time was measured, it was found to be 4.1 mJ / m 2.

  As a comparative example, an alignment film material is obtained by esterifying a dehydration condensate of paraphenyldiamine represented by (Chemical Formula 2) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) into a rigid structure. When the alignment film was formed using only the material and the OP was measured, the result was 0.48. Also, the dehydrated condensate of 1,2-bis (4-aminophenyl) ethane represented by (Chemical Formula 1) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) is esterified into the alignment film material. The result of measuring the OP after forming the alignment film using only the alignment material having the flexible structure was 0.44. That is, OP, that is, the UV absorption dichroic ratio can be improved by using a mixture of a flexible structure and a rigid structure film as in the present invention.

  In FIG. 5, OP, that is, the UV absorption dichroic ratio can be 0.5 or more by using 20 wt% to 60 wt% of the alignment film material having a flexible structure. Moreover, by setting the alignment film material having a flexible structure to 80% or less, it is possible to exceed 0.48 which is OP when the flexible structure is zero. In this case, the material of the flexible structure is larger than 0 wt% and 80 wt% or less.

  As described above, according to the present invention, OP (order parameter), that is, UV absorption dichroic ratio can be improved, the anchoring strength of liquid crystal by the alignment film can be improved, and AC afterimage can be suppressed. .

  In the above embodiment, as the alignment film material that gives a flexible structure, 1,2-bis (4-aminophenyl) ethane represented by (Chemical Formula 1) and 1,3-dimethylcyclobutanetetracarboxylic acid dicarboxylic acid represented by (Chemical Formula 3) are used. An alignment material obtained by esterifying an anhydrous dehydration condensate was used, but in addition to this, 4,4′-diaminodiphenyl ether represented by (Chemical Formula 4) and derivatives thereof, and 1,3-dimethyl represented by (Chemical Formula 3) An alignment film material having a flexible structure can be formed using an alignment material obtained by esterifying a dehydration condensate of cyclobutanetetracarboxylic dianhydride.

  Further, in the above examples, as an alignment film material giving a rigid structure, a dehydration condensate of paraphenyldiamine represented by (Chemical Formula 2) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) is esterified. In addition to this, a dehydrated condensate of a paraphenylenediamine derivative represented by (Chemical Formula 5) and 1,3-dimethylcyclobutanetetracarboxylic dianhydride represented by (Chemical Formula 3) was esterified. A rigid alignment film material can be formed using the alignment material. In addition to the esterified product, a condensate of 1,3-dimethylcyclobutanetetracarboxylic dianhydride and diamine can also be suitably used as the alignment material of the present invention.

  DESCRIPTION OF SYMBOLS 100 ... TFT substrate 101 ... Gate electrode 102 ... Gate insulating film 103 ... Semiconductor layer 104 ... Source electrode 105 ... Drain electrode 106 ... Inorganic passivation film 107 ... Organic passivation film 108 ... Counter electrode 109 ... Upper insulating film, 110 ... pixel electrode, 111 ... through hole, 112 ... slit, 113 ... alignment film, 200 ... counter substrate, 201 ... color filter, 202 ... black matrix, 203 ... overcoat film, 210 ... surface conductive film, 300 ... Liquid crystal layer, 301 ... Liquid crystal molecule

Claims (8)

A first material that has an alkyl chain in the main chain and becomes a first polyimide by imidization;
A second material that becomes a second polyimide by imidization,
The second material is a compound obtained by reacting paraphenylenediamine and 1,3-dimethylcyclobutanetetracarboxylic dianhydride, or an ester of the compound, and an alignment film for an IPS liquid crystal display device Liquid crystal alignment film material used in A first material that has an alkyl chain in the main chain and becomes a first polyimide by imidization;
A second material that becomes a second polyimide by imidization,
The second material is a compound obtained by reacting a paraphenylenediamine derivative with 1,3-dimethylcyclobutanetetracarboxylic dianhydride or an ester of the compound, and the orientation of an IPS liquid crystal display device Liquid crystal alignment film material used for the film.   The liquid crystal alignment film according to claim 1, wherein the alignment film material includes a first ester that is an ester of the first material and a second ester that is an ester of the compound. material. 4. The liquid crystal alignment film material according to claim 3, wherein the first ester is greater than 0 wt% and equal to or less than 80 wt% with respect to the total weight of the liquid crystal alignment film material.   4. The liquid crystal alignment film material according to claim 3, wherein the first ester is 20 wt% to 60 wt% with respect to the total weight of the liquid crystal alignment film material. When the molecular extinction coefficient in the major axis direction of the molecule is εp, the molecular extinction coefficient in the minor axis direction of the molecule is εv, and the molecular extinction coefficient ratio is εp / εv,
The liquid crystal alignment film material according to claim 1, wherein the second polyimide has a higher molecular extinction coefficient ratio than the first polyimide.   The liquid crystal alignment film material according to claim 1, wherein the liquid crystal alignment film material is a photo alignment film material.   The liquid crystal alignment film material according to claim 1, wherein the second polyimide is harder than the first polyimide.

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