JP2014167643A - Alignment layer material for liquid crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of a second afterimage after generating an initial DC afterimage in an IPS (in-plane switching) system liquid crystal display device.SOLUTION: An alignment layer 113 is formed to have a two-layer structure of an upper alignment layer 1131 and a lower alignment layer. The upper alignment layer 1131 is a photo-alignment layer and is formed using a polyamide acid ester as a precursor. The lower alignment layer 1132 has resistance lower than that of the upper alignment layer 1131 as well as lower photoconductivity characteristics. The lower alignment layer 1132 is formed, not using polyamide acid derived from PMDA (pyromellitic dianhydride) as the precursor but using the polyamide acid having a sulfonic acid group or a carboxyl group as the precursor. Thereby, in a pixel, the difference of resistance between a portion irradiated with light and a portion not irradiated with the light is decreased, to prevent generation of a second afterimage.

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device provided with a liquid crystal display panel 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 is superior in performance when the pretilt angle is small, the photo-alignment method is advantageous.

  In “Patent Document 1”, the alignment film has a two-layer structure, the layer in contact with the liquid crystal layer is made of polyimide having a polyamic acid ester excellent in photo-alignment property as a precursor, and the lower layer is used to release charges. It is described that a polyimide having a precursor of polyamic acid which is easily reduced in resistance is used.

  In “Patent Document 2”, the alignment film has a two-layer structure, the layer in contact with the liquid crystal layer is a polyimide having a polyamic acid ester excellent in photo-alignment as a precursor, and the lower layer is decomposed by light. In addition, it is described that a polyimide having a high mechanical strength and a polyamic acid having no cyclobutane as a precursor is used. According to “Patent Document 2”, since the mechanical strength of the lower alignment film is high, alignment stability can be maintained high in the photo-alignment.

JP 2010-072011 A Japanese Patent Application No. 2010-032443

  In the liquid crystal display device, when the same pattern is displayed for a long time, the pattern burns on the screen. For example, as shown in FIG. 8, the checker flag pattern is displayed in white and black for about 100 hours. At this time, white in the checker flag pattern has the maximum luminance. Thereafter, when the entire screen is made a gray pattern of, for example, 31 gradations / 256 gradations, the checker flag pattern remains as an afterimage. If the rate of change in luminance between the portion displaying white and the portion displaying black is 1% or more, the human eye can recognize it as an afterimage. This phenomenon includes an afterimage called a DC afterimage.

  This is because even if the image is switched, the charge of the previous pattern remains in the alignment film and the insulating film, and this charge disappears with time. Therefore, when time elapses, this DC afterimage disappears. However, the presence of a DC afterimage degrades the image quality, so it is desirable that the DC afterimage disappear quickly.

  In “Patent Document 1”, in the IPS system, the alignment film is made into two layers, and a low resistance layer is arranged as a lower layer, thereby lowering the electrical resistance of the entire alignment film and accelerating the elimination of charges accumulated in the alignment film. ing. At this time, it is described that if the lower layer has photoconductivity, electric charges can be released more efficiently. FIG. 9 is a diagram showing this effect. B shown by a dotted line is a diagram showing a change in afterimage when the electrical resistance of the alignment film is high, and a solid line A is a change in afterimage when the electrical resistance of the lower layer film is low. FIG. As shown in FIG. 9, the afterimage disappears early when the electrical resistance of the lower layer film is low.

  However, after the afterimage disappeared early, a phenomenon that a new afterimage appeared. Hereinafter, this is referred to as a second DC afterimage. FIG. 10 shows this phenomenon. FIG. 10 is a diagram in which afterimages of four samples are evaluated. FIG. 10 shows an evaluation of the level of afterimage remaining on the screen when the checker flag pattern shown in FIG. 8 is displayed for 100 hours and then a gray pattern of 31 gradations / 256 gradations is displayed.

  The horizontal axis in FIG. 10 represents the time t after the checker flag pattern is displayed for 100 hours and then switched to the gray pattern. The vertical axis MD represents the rate of change in luminance between the white portion and the black portion in the checker flag pattern. In FIG. 10, it may be considered that the afterimage has disappeared when the luminance change rate is about 1% or less. As shown in FIG. 10, in all four samples, the luminance change rate becomes about 1% or less in about 15 minutes, and the afterimage disappears once.

  However, after that, as shown in FIG. The generation of the second DC afterimage occurs after 30 minutes, and the intensity increases up to about 300 minutes. Such a second DC afterimage becomes particularly strong in about 240 minutes to 480 minutes. This second DC afterimage has been found to occur if the checker flag display time is not 100 hours but is about 10 hours or longer. An object of the present invention is to prevent the occurrence of such second DC afterimage.

  The present invention overcomes the above-described problems, and specific means are as follows. That is, the alignment film for aligning the liquid crystal has a two-layer structure of an upper layer and a lower layer. An upper layer in contact with the liquid crystal is formed as a photo-alignment film, and a polyamic acid ester is formed as a precursor. The lower layer has a lower electrical resistance than the upper layer, but also has a low photoconductive property. The lower layer is formed using a polyamic acid containing a sulfonic acid group or a carboxyl group as a precursor without using a polyamic acid made of PMDA as a raw material. When neither a sulfonic acid group nor a carboxyl group is contained, the thickness of the interlayer insulating film existing between the counter electrode and the pixel electrode is set to 770 nm or more.

  According to the present invention, it is possible to prevent the occurrence of the second DC afterimage after the first DC afterimage has occurred, so that a high-quality liquid crystal display device can be realized.

1 is a cross-sectional view of an IPS liquid crystal display device. It is a top view of the pixel of FIG. It is a perspective view which shows the structure of the alignment film by this invention. This is an equivalent circuit near the pixel. It is explanatory drawing of DC brightness relaxation time constant. It is a manufacturing process of a lower alignment film. It is a table | surface which shows the effect of this invention. This is a DC afterimage inspection pattern. It is a figure which shows the change of DC afterimage. It is an evaluation result of an afterimage. It is a cross-sectional schematic diagram which shows the mechanism of 2nd afterimage generation. It is an equivalent circuit which shows the mechanism of 2nd afterimage generation.

  A mechanism of generation of the second DC afterimage shown in FIG. 10 will be described. FIG. 11 is a schematic cross-sectional view of a liquid crystal display device. In FIG. 11, a backlight BL is arranged at the bottom. A liquid crystal panel is present on the backlight BL. As the liquid crystal panel, in the pixel portion, a light shielding film 130, a counter electrode 108, an interlayer insulating film 109, a pixel electrode 110, a lower alignment film 113, a liquid crystal layer 300, and an upper alignment film 113 are described in order from the bottom. Other components of the liquid crystal panel are omitted. 11, the thickness of the interlayer insulating film 109 is, for example, 500 nm, the thickness TA of the alignment film is, for example, 100 nm, and the thickness TL of the liquid crystal layer is, for example, 4 μm.

  In FIG. 11, the light L from the backlight BL is irradiated to the back surface of the liquid crystal panel. Light L from the backlight BL is shielded by the light shielding film 130. The alignment film 113 has photoconductive properties, and the portion irradiated with the light L from the backlight BL is reduced in resistance to be ρL. If the electrical resistance of the alignment film 113 is low, the charge charged on the surface disappears early and the DC afterimage disappears.

  On the other hand, in the portion where the light L from the backlight BL is not transmitted, the electrical resistance of the alignment film 113 remains high and is ρH. Therefore, it takes time for the charges charged in this portion of the alignment film 113 to disappear. However, since the shaded portion does not form an image, the DC afterimage is not affected.

  However, the charge charged in the light-shielding portion moves to the alignment film 113 in the transmissive portion of the backlight, and after a predetermined time has elapsed, the alignment film 113 in the transmissive portion is charged again to generate a second DC afterimage. I understood. FIG. 12 is an equivalent circuit showing this. In FIG. 12, a pixel electrode 110 is formed on the counter electrode 108 with an insulating layer 109 interposed therebetween. The pixel electrode 110 is connected to the counter substrate through the alignment film 113 and the liquid crystal layer 300 and further through a capacitor. The right side in FIG. 12 is the transmission part TR, and the left side is the light shielding part SH.

  In FIG. 12, the liquid crystal layer is represented by a circuit in which a capacitor CL and a resistor RL are connected in parallel, and the alignment film is represented by a resistor RA. The capacitance between the alignment film and the counter electrode is CAC, and the leakage resistance of CAC is RAC. The charge charged in the CAC causes a DC afterimage.

By the way, the resistance RA is reduced in the transmission portion due to photoconductive characteristics, for example, 10 ¹³ Ωcm. On the other hand, the resistance RA is, for example, 10 ¹⁴ Ωcm because there is no decrease in resistance due to photoconductivity in the light shielding portion. Therefore, the electric charge in the transmission part disappears early through the resistor RAC, and the afterimage disappears early.

  However, in the light shielding portion, the resistance of the alignment film 113 is high, and the charge is charged on the alignment film for a long time. As indicated by the arrow, the charge moves to the alignment film 113 in the transmissive portion, and the alignment film in the transmissive portion is charged again, thereby generating a second DC afterimage. The movement of the charge from the light shielding part to the transmission part is very large in resistance because the charge moves not in the thickness direction but in the surface direction, and the movement of the charge takes time. Appears after the DC afterimage disappears.

  The present invention prevents such a second DC afterimage from being generated by the invention as will be described in the following examples.

  FIG. 1 is a cross-sectional view showing a structure in a display region of an IPS liquid crystal display device. 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 with a flat solid surface with an insulating film 109 interposed therebetween. ing. 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.

  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 forms the channel portion of the TFT, and the drain electrode 104 and the source 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 drain electrode 104 or the source electrode 105. This is to make ohmic contact between the semiconductor layer and the drain electrode 104 or the source electrode 105.

  The drain electrode 104 is also used as a video signal line, and the source electrode 105 is connected to the pixel electrode 110. The drain electrode 104 and the source electrode 105 are simultaneously formed in the same layer. In this embodiment, the drain electrode 104 or the source electrode 105 is made of a MoCr alloy. In order to lower the electrical resistance of the drain electrode 104 or the source 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 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 source electrode 105.

  An interlayer insulating film 109 is formed of SiN so as to cover the counter electrode 108. After the interlayer 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 interlayer insulating film 109 as a resist. Thereafter, an ITO film that covers the interlayer insulating film 109 and the through hole 111 and becomes the pixel electrode 110 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 source 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. Video signal lines 1041 exist on both sides of the pixel electrode 110. 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.

  In FIG. 2, the pixel electrode 110 is connected to the TFT source electrode 105 through a through hole 111. The source electrode 105 and the pixel electrode 110 overlap. Since the source electrode 105 is formed of metal, the alignment film 113 formed on the pixel electrode 110 is not irradiated with backlight light in a portion where the source electrode 105 and the pixel electrode 110 overlap. Since the alignment film 113 has photoconductive characteristics, the resistance of the alignment film 113 is higher in the portion overlapping the source electrode 105 than in the portion not overlapping.

  An alignment film 113 for aligning liquid crystal molecules 301 is formed on the pixel electrode 110. In the present invention, the alignment film 113 has a two-layer structure of a photo-alignment film 1131 in contact with the liquid crystal layer 300 and a low-alignment film 1132 formed under the photo-alignment film 1131. The configuration of the alignment film 113 will be described in detail later.

  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. Similarly to the alignment film 113 on the TFT substrate side, the alignment film 113 on the counter substrate side has a two-layer structure of a photo-alignment film 1131 in contact with the liquid crystal layer 300 and a low-resistance alignment film 1132 formed under the photo-alignment film 1131. It has become. An external conductive film 210 is formed outside the counter substrate 200 in order to stabilize the potential inside the liquid crystal panel, and a predetermined voltage is applied to the external conductive film 210.

  FIG. 3 is a schematic view showing an alignment film 113 according to the present invention. In FIG. 3, the alignment film 113 is formed on the pixel electrode 110, and is formed of an upper alignment film 1131 that is a photo alignment film 113 and a lower alignment film 1132 that has a lower resistance than the upper alignment film 1131.

  The upper alignment film 1131 is formed of polyimide having a precursor of a polyamic acid ester having excellent photo-alignment properties. Chemical formula (1) is a structural formula of a polyamic acid ester having excellent photoalignment properties.

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.

  The polyamic acid ester having excellent photo-alignment property has a photodegradable site, and when irradiated with polarized ultraviolet light to the polyimide having the polyamic acid ester as a precursor, the light of the polyimide parallel to the polarization direction of the ultraviolet light is irradiated. The decomposable part is decomposed, and the alignment film has uniaxial anisotropy. The photo-alignment film thus formed has almost no pretilt angle. However, when the pretilt on the surface of the alignment film is measured, a numerical value of about -0.1 degrees to +0.1 degrees is obtained, and this degree may be considered that the pretilt angle is zero. Particularly in the IPS system, if the pretilt of the alignment film can be made zero, light control by liquid crystal molecules can be performed efficiently.

  As described above, the alignment film 1131 made of polyimide having a polyamic acid ester as a precursor exhibits excellent characteristics in photo-alignment, but has high electrical resistance, and it is difficult to eliminate the DC afterimage early. Therefore, in the alignment film according to the present invention, polyimide having a precursor of polyamic acid capable of lowering electrical resistance is used as the lower alignment film 1132.

  This alignment film made of polyimide with polyamic acid as a precursor usually has photoconductive properties, and its electrical resistance is lowered when irradiated with light. The photoconductive property is advantageous for quickly eliminating the DC afterimage because the charge charged in the alignment film can be quickly released. However, if the difference in the electrical resistance of the alignment film between the portion irradiated with light and the portion not irradiated with the photoconductive characteristic effect is greatly different, the second DC afterimage as described above is generated.

  Therefore, in order to prevent the occurrence of the second DC afterimage, it is preferable that the lower conductive film 1132 formed of polyimide having a polyamic acid as a precursor has a small photoconductive characteristic. In other words, in the present invention, the electrical resistance of the lower alignment film 1132 should be lower than the electrical resistance of the upper alignment film 1131, but the photoconductive characteristics of the lower alignment film 1132 should also be small at the same time.

The photoconductive property is dependent on the intensity of light from the irradiated backlight, that is, luminance. For example, more of the electrical resistance when the luminance is irradiated with light of 10000 cd / ^{m 2} is lower than the electrical resistance when the luminance is irradiated with light of 1000 cd / ^{m 2.} In the present invention, the ratio of the electrical resistance of the alignment film when irradiated with light of 1000 cd / m ² to the electrical resistance of the alignment film when irradiated with light of 10000 cd / m ² needs to be a predetermined value or less. And

  In order to specifically define this, a parameter that is a DC luminance relaxation time constant is introduced. The DC luminance relaxation time constant can be obtained, for example, based on an equivalent circuit as shown in FIG. 4 is a partial cross-sectional view of the liquid crystal panel shown in FIG. On the TFT substrate 100 side in FIG. 4, a counter electrode 108 is formed on the organic passivation film 107, and a pixel electrode 110 is formed thereon via an interlayer insulating film 109. An alignment film 113 is formed to cover the pixel electrode 110 and the interlayer insulating film 109. In addition, an alignment film 113 is formed over the overcoat film 203 in the counter substrate 200, and a liquid crystal layer exists between the alignment film 113 and the alignment film 113.

  In FIG. 4, when the switch is turned on and a voltage is applied between the pixel electrode 110 and the counter electrode 108, lines of electric force as indicated by arrows in FIG. 4 are generated. The liquid crystal molecules are rotated by the lines of electric force, and the amount of light passing through the liquid crystal layer 300 is controlled. It can be considered that an equivalent circuit as shown in FIG. 4 is formed along the electric field from the pixel electrode 110 to the counter electrode 108. The equivalent circuit shown in FIG. 4 and the equivalent circuit shown in FIG. 17 are based on different models.

  In FIG. 4, it can be considered that the charge from the pixel electrode 110 reaches the counter electrode 108 by moving the alignment film 113, the liquid crystal layer 300, and the interlayer insulating film 109 in series. Each layer is a parallel circuit of capacitance and resistance. When a DC voltage is applied to the pixel electrode, the DC voltage is first divided according to the capacitance of each layer and generated on the surface of each layer. In this case, the capacitance CL of the liquid crystal is smaller than other capacitances, that is, the capacitance CA of the alignment film and the capacitance CI of the interlayer insulating film. Then, a large voltage is applied to the liquid crystal layer at the moment when the DC voltage is applied. Therefore, if the liquid crystal panel is normally black, the screen becomes bright.

  On the other hand, since leakage resistance exists in each layer, the potential of each layer is converged to a potential determined by the leakage resistance RA of the alignment film 113, the leakage resistance RL of the liquid crystal layer, and the leakage resistance CI of the interlayer insulating film 109 as time passes. To do. That is, the voltage applied to the liquid crystal layer 300 gradually decreases. Therefore, if the liquid crystal panel is normally black, the luminance increases immediately after the DC voltage is applied, and thereafter, the luminance gradually decreases and approaches a certain level of brightness.

  This is shown in FIG. In FIG. 5, the horizontal axis represents time t, and the vertical axis represents the luminance of the liquid crystal display device. That is, when a DC voltage is applied between the pixel electrode and the counter electrode shown in FIG. 4 at time 0, the liquid crystal molecules rotate and light from the backlight is transmitted to increase the luminance of the liquid crystal display device.

  As described in the circuit of FIG. 4, immediately after the DC voltage is applied, the DC voltage is divided according to the capacitance of each element such as the alignment film 113, the liquid crystal layer 300, the interlayer insulating film 109, etc. The voltage is large. Therefore, the luminance of the liquid crystal display device at that time is high, for example, B1 in FIG. However, the voltage applied to each element gradually settles at a voltage determined by the leakage resistance of each element, and finally the luminance becomes B2 shown in FIG. In the transient phenomenon shown in FIG. 5, the time constant from the luminance B1 to B2 is the DC luminance relaxation time constant T. That is, when the change in luminance in FIG. 5 is approximated as a capacitor discharge, the time constant T is defined as the time when the initial luminance B1 becomes B2 + (B1−B2) × 0.368. Here, 0.368 = 1 / e.

  As shown in FIG. 4, the DC luminance relaxation time constant T varies depending on the magnitude of the leakage resistance of each element. Since the alignment film has photoconductivity, the leakage resistance of the alignment film differs between when irradiated with light that easily causes photoconductivity and when irradiated with light that does not easily cause photoconductivity. The DC luminance relaxation time constant T is also different.

Photoconductivity high intensity of light, for example, towards the case of irradiation with light of 10000 cd / ^{m 2} is under a low intensity of light, for example, is larger effect than the case of irradiation with light of 1000 cd / ^{m 2.} That is, the resistance of the alignment film is reduced by irradiating light with high luminance. That is, the DC luminance relaxation time constant T shown in FIG. 5 is large when irradiated with light of 1000 cd / m ² , for example, T1, and small when irradiated with light of 10000 cd / m ² , for example, T2. .

In the present invention, the photoconductivity of the alignment film when irradiated with visible light from a backlight is better. The method for evaluating the photoconductivity of the alignment film includes a DC luminance relaxation time constant T1 when irradiated with low luminance, for example, 1000 cd / m ² , and DC luminance when irradiated with high luminance, for example, 10000 cd / m ² . This is a method of evaluation based on the ratio to the relaxation time constant T2. That is, it can be said that the greater the difference between T1 and T2, the more remarkable is the photoconductivity.

  As a result of evaluating the second DC afterimage based on the above knowledge, the DC luminance relaxation time constant T1 when irradiated with light of luminance I and the DC luminance relaxation time constant when irradiated with light of luminance I × 10 are obtained. It has been found that the use of an alignment film having a ratio of T2 to T1 / T2 of 3 or less can prevent the phenomenon of the second afterimage.

On the other hand, the second DC afterimage is detected prominently when the normal DC afterimage is as short as 30 minutes or less. This phenomenon is evaluated by representing light with low brightness. That is, the present invention is particularly effective when the DC luminance relaxation time constant T1 when irradiated with light of 1000 cd / m ² is 30 minutes or less.

  In the present invention, the alignment film has two layers, but the above description is an evaluation of the entire two-layer alignment film. However, in reality, the upper alignment film has little room for greatly changing the material because of the demand for photo-alignment characteristics. On the other hand, the lower alignment film has a large room for changing the material so as to reduce the second DC afterimage.

  The lower alignment film is an alignment film made of polyimide having polyamic acid as a precursor. FIG. 6 is a structural formula showing the formation of the alignment film. As shown in FIG. 6, when acid dianhydride and diamine are mixed, polyamic acid is formed. By heating this polyamic acid, it imidizes and a polyimide is produced | generated and this becomes a lower alignment film. In a normal heating process, the polyamic acid is not 100% imidized and 10% to 50% remains unreacted. The upper alignment film 1131 and the lower alignment film 1132 do not need to be prepared separately, and a liquid in which the polyamic acid ester that is the precursor of the upper alignment film and the polyamic acid that is the precursor of the lower alignment film is mixed is applied. Then, in the subsequent drying (leveling) step, since they are divided into a lower layer and an upper layer, the upper layer 1131 and the lower layer 1132 can be formed simultaneously.

  In FIG. 6, the portion A in the structural formula of acid dianhydride is preferably a substance represented by chemical formula (2) or chemical formula (3).

  Conventionally, a benzene ring as shown in chemical formula (4), that is, PMDA (pyromeric dihydride) has been used as the A portion of acid dianhydride.

However, when PMDA is used as the acid dianhydride, the formed alignment film has high photoconductivity and tends to generate a second afterimage. Therefore, in the lower alignment film in the present invention, a benzene ring, that is, PMDA is not used as A shown in the structural formula of acid dianhydride in FIG.

  The portion B in the diamine of FIG. 6 is, for example, a benzene ring, and an example of a specific structural formula of diamine is shown in chemical formula (5).

  In the present invention, as another example of a preferable polyamic acid, a diamine having a sulfonic acid group or a carboxyl group introduced is used. The structure of such a diamine is represented by chemical formula (6), chemical formula (7), chemical formula (8), chemical formula (9), chemical formula (10), and chemical formula (11).

  By using such a polyamic acid, it is possible to form a lower alignment film having a lower resistivity and a lower photoconductivity than an alignment film made of polyimide having a polyamic acid ester as an upper layer as a precursor.

  As described with reference to FIG. 12, in the second afterimage, the charge charged in the alignment film not irradiated with the backlight moves to the alignment film whose resistance is reduced by receiving the light irradiated from the backlight. Caused by. This phenomenon is affected by the thickness of the interlayer insulating film.

  When the thickness of the interlayer insulating film is large, the charge charged in the alignment film easily moves, and as a result, the second afterimage is hardly generated. Conventionally, the interlayer insulating film has a thickness of about 500 nm. However, if the thickness of the interlayer insulating film is 770 nm or more, the charge charged in the alignment film can be easily moved, and the second afterimage is hardly generated.

  FIG. 7 shows the effect of the configuration described above on the second afterimage. In FIG. 7, 13 samples with various parameters changed were evaluated. In the table of FIG. 7, the low-resistance component material is a material having a polyamic acid as a precursor constituting the lower layer of the alignment film. As a low-resistance material component, the case where the precursor polyamic acid is made of PMDA as a raw material is compared with the case where it does not contain sulfonic acid groups or carboxyl groups.

  For photo-alignment, it is necessary to irradiate the alignment film with ultraviolet rays and to heat the alignment film. As this process, there are a case where the alignment film is irradiated with ultraviolet rays and heating at the same time (simultaneous heating in FIG. 7), and a case where the alignment film is irradiated with ultraviolet rays and then heated (post-heating in FIG. 7). Further, the alignment film for evaluation is formed by changing the heating temperature to 180 ° C., 200 ° C., and 230 ° C. As the liquid crystal, the same liquid crystal is used. As the interlayer insulating film, a film having a thickness of 500 nm, which is a conventional example, is compared with a film having a thickness of 770 nm which is thicker than this.

For the 13 samples prepared as described above, the DC luminance relaxation constant shown in FIG. 5 is the DC luminance relaxation time constant T1 when the light having the luminance of 1000 cd / m ² is irradiated, and the luminance is 10,000 cd / m. comparing the DC luminance relaxation time constant T2 when irradiated with light of m ^2. FIG. 7 also shows the value of T1 / T2. And the presence or absence of the 2nd afterimage was evaluated about each sample.

  In FIG. 7, the case where the second afterimage occurs is indicated by x, and the case where the second afterimage does not occur is indicated by ◯. As shown in FIG. 7, the second afterimage appears when polyamic acid using PMDA as a raw material is used as a precursor for the alignment film lower layer (low resistance component material). Even when polyamic acid made of PMDA is not used as a precursor for the alignment film lower layer (low resistance component material), there is no sulfonic acid group or carboxyl group, and the interlayer insulating film is 500 nm as in the prior art. In this case, a second afterimage is generated.

  On the other hand, the second afterimage does not occur in all of the sample Nos. 9, 10, 11, and 13 in which the polyamic acid using PMDA as a raw material is not used as a precursor and the sulfonic acid group or the carboxyl group exists. Even when the polyamic acid using PMDA as a raw material was not used as a precursor and no sulfonic acid group or carboxyl group was present, the second afterimage did not occur if the interlayer insulating film had a thickness of 770 nm.

  In addition, there was no significant difference in the photo-alignment process, that is, whether or not the irradiation of polarized ultraviolet rays was performed before heating, the heating temperature, or the like with respect to the second DC afterimage.

  As described above, when the polyamic acid containing PMDA as a precursor is not used as the precursor and the polyamic acid containing a sulfonic acid group or a carboxyl group is used for the lower alignment film (low resistance component material), the second is all No afterimage occurs. On the other hand, even when the lower alignment film (low resistance component material) does not use polyamic acid made of PMDA as a precursor and uses polyamic acid containing neither a sulfonic acid group nor a carboxyl group, the interlayer insulating film has a thickness of 770 nm. If this is the case, the second afterimage does not occur.

  DESCRIPTION OF SYMBOLS 100 ... TFT substrate 101 ... Gate electrode 102 ... Gate insulating film 103 ... Semiconductor layer 104 ... Drain electrode 105 ... Source electrode 106 ... Inorganic passivation film 107 ... Organic passivation film 108 ... Counter electrode 109 ... Interlayer 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 | numerator, 1041 ... Video signal line, 1131 ... Upper layer alignment film, 1132 ... Lower layer alignment film

Claims (8)

An alignment film material used for an alignment film of a liquid crystal display device,
A polyamic acid ester;
A liquid crystal alignment film material comprising an acid dianhydride having no benzene ring and a polyamic acid formed from a diamine. The alignment film material for liquid crystal according to claim 1, wherein the acid dianhydride having no benzene ring is PMDA. The alignment film material for liquid crystal according to claim 1, wherein the acid dianhydride having no benzene ring has a chemical formula 2 or a chemical formula 3.

4. The alignment film material for liquid crystal according to claim 1, wherein the diamine contains a sulfonic acid group or a carboxyl group. 5. The alignment film material for liquid crystal according to claim 1, wherein the diamine is represented by chemical formulas (6) to (11).

The alignment film is a photo-alignment type alignment film,
6. The alignment film material for liquid crystal according to claim 1, wherein the polyamic acid ester has a photodegradable portion. The alignment film material for liquid crystal according to claim 6, wherein the polyamic acid ester is represented by the chemical formula (1).

However, in Chemical formula (1), R1 is each independently a C1-C8 alkyl group, R2 is respectively independently a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, a phenyl group, C1-C1 An alkyl group having 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a group represented by (— (CH 2) m —CH═CH 2, m = 0, 1, 2) or (— (CH 2) m —C≡CH, m = 0, 1, 2), and Ar is an aromatic compound. The alignment film material for a liquid crystal according to claim 1, wherein the alignment film material is a liquid in which the polyamic acid ester and the polyamic acid are mixed.

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