JP5400213B2 - Alignment film material for photo-alignment film - Google Patents

Description

  The present invention relates to a display device, and more particularly to a liquid crystal display device in which screen burn-in during operation is small and screen defects such as black unevenness do not occur even after long-time operation.

  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, its 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.

  An example of such an IPS liquid crystal display device is described in “Patent Document 1”.

JP 09-73101 A

  The liquid crystal display device is used for a long time. When the liquid crystal display device is used for a long time, a screen defect may occur. This screen defect can be divided into two types. The first defect is that the display screen changes irreversibly after long-term use. For example, the first defect is called black unevenness in a normally black mode in which black is displayed when no voltage is applied. The second defect is a phenomenon in which the same image remains on the screen when the same image is displayed for a long time, and is called, for example, a DC afterimage.

  An example of black unevenness is shown in FIG. In FIG. 12, black unevenness is indicated by BB. Black unevenness is a phenomenon in which an area on the screen becomes darker than other areas. This phenomenon may occur when the liquid crystal display device is operated for a long time, for example, 1000 hours or more. The cause of this is considered to be that the liquid crystal is contaminated with impurities by operating the liquid crystal display device for a long period of time, and the insulation resistance of the liquid crystal is lowered.

  The DC afterimage is a phenomenon in which, for example, when a pattern as shown in FIG. 10 is displayed for a certain period of time and then a gray gray solid pattern is displayed, the pattern as shown in FIG. 10 remains thin on the screen. As an example of the explanation for such a phenomenon, when an impurity adheres to the alignment film sandwiching the liquid crystal, it is considered that the impurity is charged by the video signal, and this charge is maintained for a specific time, thereby leaving an afterimage. The DC afterimage is a reversible phenomenon because it disappears when the impurities on the alignment film are not charged.

  An object of the present invention is to eliminate black unevenness and DC afterimage as described above.

  The present invention overcomes the above problems, and specific means are as follows.

  (1) A TFT substrate in which an alignment film is formed on a pixel electrode and a pixel having TFTs, a counter substrate that is opposed to the TFT substrate and has an alignment film formed on a color filter, and the alignment of the TFT substrate A liquid crystal display device in which a liquid crystal is sandwiched between a film and an alignment film of the counter substrate, wherein the alignment film has a polyamic acid ester having a structure of chemical formula (1), and the liquid crystal is oxidized A liquid crystal display device comprising an inhibitor, wherein the liquid crystal has a dielectric anisotropy of 5 or less.

Here, R1 is each independently an alkyl group having 1 to 8 carbon atoms, and R2 is independently a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a phenyl group, an alkyl group having 1 to 6 carbon atoms, An alkoxy group having 1 to 6 carbon atoms, a vinyl group (— (CH 2) m —CH═CH 2, m = 0, 1, 2) or an acetyl group (— (CH 2) m —C≡CH, m = 0, 1, 2), Ar is an aromatic compound.

  (2) The liquid crystal display device according to (1), wherein the alignment film uses the polyamic acid ester as a precursor.

  (3) The liquid crystal display device according to (1), wherein the counter electrode facing the pixel electrode is an IPS liquid crystal display device formed on the TFT substrate.

  (4) The liquid crystal display device according to (1), wherein the alignment film is subjected to alignment treatment by photo-alignment.

  (5) A first electrode formed in a planar shape, a second electrode disposed on the first electrode via an insulating film, and an alignment film formed on the pixel on which the TFT is formed A TFT substrate, a counter substrate facing the TFT substrate and having an alignment film formed on the color filter, and a liquid crystal sandwiched between the alignment film of the TFT substrate and the alignment film of the counter substrate In the display device, the alignment film includes a polyamic acid ester having a structure of the chemical formula (1), the liquid crystal includes an antioxidant, and the dielectric anisotropy of the liquid crystal is 5 or less. There is a liquid crystal display device.

Here, R1 is each independently an alkyl group having 1 to 8 carbon atoms, and R2 is independently a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a phenyl group, an alkyl group having 1 to 6 carbon atoms, An alkoxy group having 1 to 6 carbon atoms, a vinyl group (— (CH 2) m —CH═CH 2, m = 0, 1, 2) or an acetyl group (— (CH 2) m —C≡CH, m = 0, 1, 2), Ar is an aromatic compound.

  (6) The liquid crystal display device according to (5), wherein the alignment film uses the polyamic acid ester as a precursor.

  (7) The liquid crystal display device according to (5), wherein the alignment film is subjected to alignment treatment by photo-alignment.

  (8) The liquid crystal display device according to (5), wherein the first electrode is a counter electrode, and the second electrode is a pixel electrode formed in a comb shape.

  (9) The liquid crystal display device according to (5), wherein the first electrode is a pixel electrode, and the second electrode is a counter electrode formed in a comb shape.

  According to the present invention, since the liquid crystal is prevented from being oxidized or decomposed, it is possible to prevent the liquid crystal oxide or decomposition product from becoming impurities in the liquid crystal and lowering the insulation resistance of the liquid crystal. Black unevenness caused by operating the apparatus for a long time can be prevented.

  In the present invention, since the alignment film is made of a material having a polyamic acid ester as a precursor that hardly adsorbs impurities to the interface, it is possible to prevent an afterimage due to residual DC caused by charging of the impurity at the interface of the alignment film. I can do it.

  The present invention uses a liquid crystal having a relative dielectric constant Δε of 5 or less, so that there is little mixing of impurities in the liquid crystal, and since a material having a polyamic acid ester alignment film as a precursor is used, the impurities are adsorbed to the alignment film interface. Hard to do. Therefore, afterimages due to residual DC and black unevenness that occurs after the liquid crystal display device has been operated for a long time can be prevented.

  The contents of the present invention will be described in detail by the following examples.

  FIG. 1 is a cross-sectional view showing a structure in a display region of a 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 electrode 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 with both ends closed. A slit 112 is formed between the comb teeth. A planar counter electrode 108 (not shown) 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 reference 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. Note that an alignment film 113 for aligning the liquid crystal molecules 301 is formed on the pixel electrode 110.

  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.

  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. 2 is IPS, the counter electrode 108 is formed on the TFT substrate 100 side, and is not formed on the counter substrate 200 side.

  As shown in FIG. 1, in IPS, a conductive film is not formed inside the counter substrate 200. Then, the potential of the counter substrate 200 becomes unstable. Further, external electromagnetic noise enters the liquid crystal layer 300 and affects the image. In order to eliminate such a problem, a surface conductive film 210 is formed outside the counter substrate 200. The surface conductive film 210 is formed by sputtering ITO, which is a transparent conductive film.

  When the liquid crystal of the liquid crystal layer 300 shown in FIG. 1 is oxidized or decomposed, the display characteristics of the liquid crystal display device deteriorate. This is particularly noticeable due to the presence of light and heat. Accordingly, the antioxidant 400 is mixed in the liquid crystal layer 300 to prevent the liquid crystal from being oxidized. Examples of the antioxidant contained in the liquid crystal material include phenol-based, phosphite-based, phosphite-based, and sulfur-based substances. Although the effect can be obtained by containing these substances in the range of 0.0005 to 10 wt%, in order to obtain a higher effect, it is desirable to be in the range of 0.001 to 5 wt%. It is not limited to the substance and range.

  As shown in FIG. 1, the liquid crystal layer 300 is sandwiched between alignment films 113. Conventionally, a material having polyamic acid 1132 as a precursor has been used as the material of the alignment film 113. Part of the polyamic acid 1132 becomes polyimide by heating at around 200 ° C., which is called imidization baking, and part of the polyamic acid 1132 remains as unreacted polyamic acid 1132. However, as will be described later, the remaining polyamic acid 1132 is highly polar due to the nature of the material, and easily adsorbs the antioxidant 400.

  When the antioxidant 400 is adsorbed on the alignment film 113, the antioxidant 400 in the liquid crystal layer 300 decreases. As a result, the liquid crystal is easily oxidized or decomposed in the presence of light or heat, and impurities 401 are generated. If the impurities 401 are present in the liquid crystal layer 300, the insulation resistance of the liquid crystal layer 300 is reduced and the voltage holding characteristics are deteriorated. On the other hand, when the impurities are adsorbed on the alignment film 113, they are charged up during the operation of the liquid crystal display device, causing a DC afterimage.

  The present invention prevents the antioxidant 400 from being adsorbed by the alignment film 113 by using a material having the polyamic acid ester 1131 as a precursor for the alignment film 113. This is shown in FIG. FIG. 3A is a schematic cross-sectional view showing a configuration of the present invention, and FIG. 3B is a schematic cross-sectional view showing a conventional configuration. In FIG. 3A and FIG. 3B, the detailed structure of the liquid crystal display device shown in FIG. 1 is omitted. 3A and 3B, white arrows indicate light from the backlight that promotes oxidation and decomposition of the liquid crystal.

  In FIG. 3A, a material having a polyamic acid ester 1131 as a precursor is used for the alignment film 113. Similarly to the polyamic acid, this polyamic acid ester 1131 also becomes a part of polyimide by imidization baking, and a part remains as unreacted polyamic acid ester 1131. However, as will be described later, the remaining polyamic acid ester 1131 is less polar than the polyamic acid 1132 due to the nature of the material and hardly adsorbs the antioxidant 400. Therefore, since sufficient antioxidant 400 is present in the liquid crystal layer 300, the liquid crystal molecules 301 can be prevented from being oxidized. On the other hand, in the conventional example, since the alignment film 113 uses a material having polyamic acid 1132 as a precursor, the antioxidant 400 is easily adsorbed. Then, the amount of the antioxidant 400 in the liquid crystal layer 300 is relatively reduced, and the liquid crystal is oxidized or decomposed, causing problems such as black unevenness and DC afterimage.

  Chemical formula (1) is a structural formula of the polyamic acid ester 1131 used in FIG.

In the chemical formula (1), each R1 is independently an alkyl group having 1 to 8 carbon atoms, and each R2 is independently a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a phenyl group, or a carbon atom having 1 to 6 carbon atoms. An alkyl group, an alkoxy group having 1 to 6 carbon atoms, a vinyl group (— (CH 2) m —CH═CH 2, m = 0, 1, 2) or an acetyl group (— (CH 2) m —C≡CH, m = 0 , 1, 2), and Ar is an aromatic compound.

  A feature of the polyamic acid ester 1131 is R1 in the chemical formula (1). In the polyamic acid ester 1131, R1 is CnH2n-1, and n is 1 or more. The polyamic acid 1132 used in the conventional alignment film 113 shown in FIG. 3B has n = 0 in the chemical formula (1), and only hydrogen exists at the position of R1. The degree of hydrogen varies depending on the amount of water and the type and amount of water present in the vicinity, but a portion of the polyamic acid 1132 is ionized as hydrogen ions, and the polyamic acid 1132 after the hydrogen ions are ionized is a carboxylic acid having a negative charge Become an ion. As a result, the antioxidant 400 and various impurities 401 in the liquid crystal layer 300 are easily adsorbed. On the other hand, in the present invention, the use of the polyamic acid ester 1131 that does not ionize hydrogen ions as the material of the alignment film 113 prevents the antioxidant 400 from being adsorbed on the alignment film 113.

  On the other hand, various impurities 401 are mixed in the liquid crystal layer 300 in addition to the antioxidant. The impurities 401 may exist in the liquid crystal before being sealed in the liquid crystal display panel, or may be mixed from various materials that form a structure in the liquid crystal display panel after the liquid crystal is injected. If the relative dielectric constant of the liquid crystal is high, such impurities 401 are likely to be mixed into the liquid crystal. On the other hand, when the relative dielectric constant of the liquid crystal is low, it is difficult to capture such impurities 401.

  If such an impurity 401 is included in the liquid crystal layer 300, black unevenness, a DC afterimage, and the like are caused. In the present invention, attention is paid to the dielectric anisotropy of the liquid crystal as a parameter having a high correlation with the relative dielectric constant of the liquid crystal. By using the dielectric anisotropy having a dielectric anisotropy of 5 or less, the impurities 401 incorporated into the liquid crystal. By reducing the amount of light, black unevenness, DC afterimage, and the like are suppressed. This is shown in FIG. FIG. 4A shows the case where the dielectric anisotropy of the liquid crystal is 5 or less as in the present invention, and FIG. 4B shows the case where the dielectric anisotropy is larger than 5. FIGS. 4A and 4B show that even when the same alignment film material is used, the liquid crystal layer 300 has a large amount of impurities 401 when the dielectric anisotropy of the liquid crystal is large.

  In order to reduce the black unevenness and the DC afterimage, not only the oxidation or decomposition of the liquid crystal is prevented to reduce the oxide or decomposition product of the liquid crystal, but also the impurities 401 incorporated into the liquid crystal must be reduced. The present invention reduces black unevenness and DC afterimage by using both of these means. FIG. 5 is a schematic sectional view showing this state.

  FIG. 5A is a schematic cross-sectional view showing the configuration of the present invention. In FIG. 5A, a material having a polyamic acid ester 1131 as a precursor is used as the alignment film material. A liquid crystal material having a dielectric anisotropy of 5 or less is used. On the other hand, FIG.5 (b) is a cross-sectional schematic diagram which shows the structure of a prior art example. In FIG. 5B, a material having polyamic acid 1132 as a precursor is used for the alignment film 113. In addition, a liquid crystal having a dielectric anisotropy larger than 5 is used.

  In the conventional example shown in FIG. 5B, the amount of the impurities 401 sandwiched between the alignment films 113 is large, and the impurities 401 are also present in the liquid crystal layer 300 and are also adsorbed to the alignment films 113. On the other hand, in the present invention shown in FIG. 5A, the alignment film 113 does not adsorb impurities 401, and the liquid crystal layer 300 has fewer impurities 401 than in the conventional example. Therefore, in FIG. 5A showing the structure of the present invention, both the black unevenness caused by the impurities 401 in the liquid crystal and the DC afterimage caused by the impurities 401 at the interface of the alignment film 113 can be reduced.

  FIG. 6 is a graph showing an evaluation related to occurrence of black unevenness after the liquid crystal display device is operated for a long time. In FIG. 6, the vertical axis represents the voltage holding ratio SVR, and the horizontal axis represents the backlight lighting time. FIG. 6 shows a sample in which the sample shown in FIG. 7 is set on a plurality of backlights, and the voltage holding ratio SVR is measured every time. This voltage holding ratio SVR is a measure of liquid crystal deterioration.

  FIG. 7 shows an outline of a method for measuring the voltage holding ratio SVR. In FIG. 7, a conductive film 501 serving as an electrode is formed inside two glass substrates 500. An alignment film 113 is formed over the conductive film 501. An alternating pulse voltage is applied to the conductive film 501 with the liquid crystal layer 300 interposed therebetween. The liquid crystal molecules 301 are rotated by this AC pulse voltage. The voltage between the terminals of the conductive film 501 of the sample becomes the holding voltage SV. When the insulation resistance of the liquid crystal decreases, the holding voltage SV between the conductive film terminals decreases. This is because an internal resistance R exists in the power supply. If the liquid crystal deteriorates due to oxidation or the like after being operated for a long time or being exposed to a backlight for a long time, the insulation resistance of the liquid crystal is lowered. As a result, the holding voltage SV decreases. A voltage holding ratio SVR is a ratio of the decrease in the holding voltage SV. Thereby, it is possible to evaluate how much the liquid crystal has deteriorated by measuring the voltage holding ratio SVR for each operation time.

  In FIG. 6, a plurality of samples having different specifications are placed on a liquid crystal backlight, and the backlight is turned on as in a normal liquid crystal display device. The voltage holding ratio SVR is measured every lighting time. It can be said that the smaller the decrease in the voltage holding ratio SVR, the less the occurrence of black unevenness in the actual liquid crystal display device. In FIG. 6, as the alignment film 113, a case where a material having a polyamic acid ester 1131 as a precursor is used is compared with a case where a material having a polyamic acid 1132 as a precursor is used. As the directivity Δε, the cases of 4 and 7 are compared.

  In FIG. 6, the alignment film 113 using a material having a polyamic acid ester 1131 as a precursor has better lifetime characteristics of the voltage holding ratio than a material using a material having a polyamic acid 1132 as a precursor. The voltage holding ratio is better when Δε is 4 than when Δε is 7. As shown in FIG. 6, a material having a polyamic acid ester 1131 as a precursor is used for the alignment film 113 and a material with a dielectric anisotropy smaller than 5 is used for the alignment film 113 as shown in FIG. In this case, the life characteristics of the voltage holding ratio are superior to those of other specifications. Therefore, according to the present invention, black unevenness hardly occurs even after long-time operation.

  FIG. 8 evaluates whether or not a DC afterimage is generated due to the specific resistance of the liquid crystal when a material having a polyamic acid ester 1131 as a precursor is used as the alignment film 113.

  The DC afterimage was evaluated as follows. That is, an 8 × 8 checker flag pattern in black and white as shown in FIG. 10 is displayed for 12 hours, and then returned to a gray solid halftone. The halftone gradation is 64/256. Immediately after returning to the halftone, the intensity of the DC afterimage was evaluated by measuring the difference between the luminance of the checker flag pattern where black was displayed and the luminance of the portion where white was displayed. In FIG. 8, the horizontal axis ρ represents the specific resistance of the liquid crystal, and the vertical axis LD represents the change rate of the luminance as a percentage. As can be seen from FIG. 8, it was found that the difference in luminance, that is, the intensity of the DC afterimage, has a correlation with the specific resistance of the liquid crystal. The actual afterimage is determined as NG if the checker flag pattern can be recognized 10 minutes after returning to the gray solid halftone, and OK if not recognized.

As shown in FIG. 8, if the specific resistance ρ of the liquid crystal is 10 ¹⁴ or more, it can be seen that the DC afterimage is OK. On the other hand, as shown in FIG. 9, the specific resistance ρ of the liquid crystal changes depending on the dielectric anisotropy of the liquid crystal. When the dielectric anisotropy Δε of the liquid crystal is 5 or less, the specific resistance ρ of the liquid crystal can be 10 ¹⁴ or more. If the dielectric anisotropy of the liquid crystal is small, the impurities 401 mixed in the liquid crystal can be reduced. Furthermore, by using a material having polyamic acid ester 1131 as a precursor for the alignment film 113, the degree to which the impurities 401 are adsorbed to the alignment film 113 is reduced. Therefore, the amount of impurities 401 adsorbed on the alignment film 113 can be reduced, or residual DC can be suppressed.

  As described above, by using the present invention, it is possible to reduce black unevenness, which is an irreversible phenomenon after long-term operation, and reversible residual DC that causes afterimages, and a liquid crystal display having excellent life characteristics. A device can be realized.

  In general, in the IPS system, an interface tilt with the substrate surface is not required in principle, unlike the vertical electric field system typified by the conventional TN system, and the viewing angle characteristics are better as the interface tilt angle is smaller. In particular, by setting the tilt angle to 1 degree or less, it is effective because the color change and the brightness change depending on the viewing angle of the liquid crystal display device can be made below the allowable limit. Therefore, the optical alignment that can make the interface tilt angle substantially 0 ° is an effective process in the IPS system.

The material having the polyamic acid ester 1131 as a precursor described in Example 1 is suitable as the alignment film 113 for photo-alignment. As shown in FIG. 11A, the polyimide obtained by imidization baking has a network structure in a film-formed state. Such a film is irradiated with polarized ultraviolet light with an energy of, for example, 6 J / cm ² . If it does so, the structure of the polarization direction of the polarized ultraviolet rays in a polyimide will be decomposed | disassembled by an ultraviolet-ray, as shown in FIG.11 (b).

  When a liquid crystal display device is manufactured using the alignment film 113 formed in this manner, the liquid crystal molecules 301 are aligned in a direction orthogonal to the polarization direction of polarized ultraviolet light, as shown in FIG. become. One of the problems with photo-alignment is that the portion of the alignment film 113 decomposed by ultraviolet rays has polarity and may become impurities 401 and be taken into the liquid crystal display device.

  Such a problem can be reduced by using a material having a polyamic acid ester 1131 as a precursor for the alignment film 113 as in the present invention. That is, since the polyamic acid ester 1131 remaining after the imidization baking is difficult to have a polarity as a material as described in the first embodiment, it is difficult to adsorb such a decomposition product of the alignment film 113. Hard to get inside.

  Further, by setting the dielectric anisotropy Δε of the liquid crystal to 5 or less as in the present invention, the possibility of taking the alignment film decomposition product into the liquid crystal can be reduced. Therefore, a phenomenon in which black unevenness is caused by the influence of the impurities 401 in the liquid crystal layer 300 can be reduced.

  As described above, by using a material having a polyamic acid ester 1131 as a precursor for the alignment film 113 and using a liquid crystal having a dielectric anisotropy Δε of 5 or less, optical alignment with excellent life characteristics can be achieved. The liquid crystal display device used can be realized.

  In the above embodiment, the IPS liquid crystal display device has been described. However, the present invention can be applied not only to the IPS system but also to a TN (Twisted Nematic) system or a VA (Vertical Alignment) system liquid crystal display device.

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 a schematic diagram which compares this invention and a prior art example. It is another schematic diagram which compares this invention with a prior art example. It is another schematic diagram which compares this invention with a prior art example. It is a comparison figure of the characteristic of this invention and a prior art example. This is a method for measuring the voltage holding ratio. This is the relationship between the residual DC and the specific resistance of the liquid crystal. This is the relationship between the relative dielectric constant and the specific resistance of the liquid crystal. It is a checker flag pattern used for afterimage evaluation. It is a schematic diagram which shows the principle of optical orientation. This is an example of black unevenness.

  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, DESCRIPTION OF SYMBOLS 300 ... Liquid crystal layer, 301 ... Liquid crystal molecule, 400 ... Antioxidant, 401 ... Impurity, 500 ... Test glass substrate, 501 ... Test conductive film, 1131 ... Polyamic acid ester, 1132 ... Polyamic acid, BB ... Black unevenness, LD ... Brightness change rate, SV ... Holding voltage, SVR: Voltage holding ratio, ρ: Specific resistance of liquid crystal.

Claims (4)

A TFT substrate having a photo-alignment film formed on a pixel electrode and a pixel having TFTs, a counter substrate facing the TFT substrate and having a photo-alignment film formed on a color filter, and photo-alignment of the TFT substrate An alignment film material for a photo-alignment film used in a liquid crystal display device in which a liquid crystal is sandwiched between a film and a photo-alignment film of the counter substrate,
The alignment film material for a photo-alignment film is a polyamic acid ester having a structure of the chemical formula (1), and
The polyamic acid ester is used for a liquid crystal display device having a liquid crystal in which the liquid crystal contains an antioxidant and the dielectric anisotropy of the liquid crystal is 5 or less. .

Here, R1 is each independently an alkyl group having 1 to 8 carbon atoms, and R2 is independently a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a phenyl group, an alkyl group having 1 to 6 carbon atoms, An alkoxy group having 1 to 6 carbon atoms, a vinyl group (— (CH 2) m —CH═CH 2, m = 0, 1, 2) or an acetyl group (— (CH 2) m —C≡CH, m = 0, 1, 2), Ar is an aromatic compound.   The alignment film material for a photo-alignment film according to claim 1, wherein the polyamic acid ester serves as a precursor of the photo-alignment film.   The alignment film material for a photoalignment film according to claim 1, wherein the polyamic acid ester becomes the photoalignment film when irradiated with polarized ultraviolet rays.   The alignment film material for a photo-alignment film according to claim 1, wherein the polyamic acid ester is used for an IPS liquid crystal display device.

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