JP2016066053A - Liquid crystal display device and material for alignment film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent flickers or uneven luminance in a low-frequency driving state in a liquid crystal display device having an alignment film subjected to an optical alignment process.SOLUTION: A liquid crystal used has negative dielectric anisotropy and has an absolute value of the dielectric anisotropy of 5 or less. An alignment film is composed of: a first film that has an aligning ability by an optical alignment process and is present on a side in contact with the liquid crystal; and a second film that has no aligning ability by the optical alignment process and is under the first film. The alignment film is formed by applying a mixture material of a first material to form the first film and a second material to form the second film on a substrate. The proportion of the material for the first alignment film is more than 10 wt.% and less than 40 wt.% with respect to the total weight of the first material and the second material.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a display device, and more particularly to a liquid crystal display device having an alignment film using photo-alignment.

  In a liquid crystal display device, a TFT substrate in which pixels having pixel electrodes and thin film transistors (TFTs) are formed in a matrix and a counter substrate are arranged opposite the TFT substrate, and liquid crystal is sandwiched between the TFT substrate and the counter substrate. ing. An image is formed by controlling the light transmittance of the liquid crystal molecules for each pixel.

  In the liquid crystal display device, it is necessary to initially align the liquid crystal using an alignment film. Conventionally, a rubbing method has been used as an alignment treatment for the alignment film. The rubbing method causes problems such as generation of bright spots due to shavings of the alignment film during rubbing, and dielectric breakdown of wiring due to static electricity generated during the rubbing process. On the other hand, the so-called photo-alignment that gives uniaxial anisotropy to the alignment film by cutting the polymer chain in a predetermined direction using polarized ultraviolet rays has no problems due to rubbing as described above. It is becoming popular.

  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. The IPS system is particularly suitable for photo-alignment because a so-called pretilt angle is not required.

  Patent Document 1 describes a configuration that prevents the occurrence of an afterimage by using an alignment film having a polyamic acid ester as a precursor in photo-alignment. Patent Document 2 discloses an alignment film in which a polyamic acid ester and a polyamic acid are used as materials, an alignment film using a polyamic acid ester as a precursor, and an alignment film using a polyamic acid as a precursor are formed in layers. The structure which suppresses the minute unevenness | corrugation of the surface by making the weight average molecular weight of a polyamic acid ester smaller than the weight average molecular weight of a polyamic acid is described. Furthermore, Patent Document 3 describes an example of an imidization accelerator that assists the formation of an alignment film.

JP 2009-288298 A WO2011 / 115078 WO2011 / 114103 Publication

  The liquid crystal used in the liquid crystal display device includes a positive type and a negative type. Positive liquid crystal has a positive dielectric anisotropy Δε of liquid crystal molecules, and negative liquid crystal has a negative dielectric anisotropy Δε of liquid crystal molecules. In other words, the positive type liquid crystal is such that the major axis of the liquid crystal molecule is oriented in the direction of the electric field, and the negative type liquid crystal is such that the minor axis of the liquid crystal molecule is oriented in the direction of the electric field.

  Taking an IPS liquid crystal display device as an example, the liquid crystal molecules rotate in accordance with the direction of the electric field to control the light transmittance in each pixel, and control the light transmittance for each pixel to form an image. . By the way, in the display region, a region in which reverse rotation occurs in the pixel occurs depending on the shape of the pixel electrode or the counter electrode. Although this area is called disclination, since this area does not transmit light, the brightness of the screen is deteriorated and the contrast is lowered. In the positive type liquid crystal, disclination is likely to occur particularly at the end of the electrode pattern.

  Another problem of the positive type liquid crystal is that when the surface of the liquid crystal display panel is pressed, the liquid crystal molecules tend to rise in the normal direction of the substrate. Such rise of liquid crystal molecules is likely to occur in a region where disclination occurs. In the IPS liquid crystal display device, when such a rise of liquid crystal molecules occurs, blurring of the screen tends to occur.

  On the other hand, the negative type liquid crystal is less likely to cause the above problems than the positive type liquid crystal. Therefore, a negative type liquid crystal is used particularly in a liquid crystal display device in which it is desired to prevent disclination. However, when negative liquid crystal is used, the voltage holding ratio may be lowered. The voltage holding ratio is an evaluation of how much the previous signal voltage is held from writing a signal to a pixel until writing the next signal.

  When low frequency driving is performed to reduce power consumption of the liquid crystal display device in a state where the voltage holding ratio is low, flicker occurs in the display area. In addition, when the voltage holding ratio is partially reduced on the screen, screen unevenness occurs in the display area. An object of the present invention is to prevent occurrence of flicker and screen unevenness due to a decrease in voltage holding ratio in a liquid crystal display device using negative liquid crystal and optically aligning an alignment film.

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

(1) A liquid crystal display device having a liquid crystal between a first substrate having an alignment film and a second substrate having an alignment film, wherein the liquid crystal has a negative dielectric anisotropy and a dielectric constant Absolute value of anisotropy is 5
The alignment film is subjected to a photo-alignment process using polarized ultraviolet rays, the alignment film has an alignment capability by the photo-alignment process, and the first film exists on the side in contact with the liquid crystal, and Alignment ability by photo-alignment treatment is not given, and it is formed by the second film existing on the first substrate or the second substrate side, and the alignment film is a first material for forming the first film And the second material forming the second film is applied to the first substrate or the first substrate, and the first material is a sum of the first material and the second material. A liquid crystal display device characterized by being larger than 10 wt% and smaller than 40 wt% with respect to the weight.

  (2) The liquid crystal display device according to (1), wherein the first material is a polyamic acid ester represented by (Chemical Formula 1), and the second material is a polyamic acid.

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. 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 acetyl group _{(- (CH 2) m-} C≡CH, m = 0, 1, 2), and Ar is an aromatic compound.

  (3) The first material is a polyamic acid ester, and the polyamic acid ester is formed using a first diamine as a precursor, the first diamine has an aromatic ring, and two The liquid crystal display device according to (1) or (2), which is a diamine having no nitrogen atom other than the nitrogen atom in the amino group, a fluorine atom, and an oxygen atom.

  (4) The second material is a polyamic acid, and the polyamic acid is formed using a second diamine as a precursor, and the second diamine is nitrogen other than nitrogen atoms in two amino groups. 4. The liquid crystal display device according to any one of (1) to (3), which is a diamine having an atom, a fluorine atom, or an oxygen atom.

  The mixed material contains an imidization accelerator, and the imidization accelerator is picoline, quinoline, isoquinoline, a secondary or tertiary amine having a pyridine skeleton, or an amino acid having an alkoxycarbonyl group. The liquid crystal display device according to any one of (1) to (4), wherein:

It is sectional drawing of the liquid crystal display device with which this invention is applied. It is a flowchart which shows the process of optical orientation. This is a circuit for measuring the voltage holding ratio. It is an equivalent circuit for measuring the voltage holding ratio. It is a figure which shows the motion of the electric potential of one electrode of a test cell. It is a graph which shows the relationship between the ratio of the 1st material which forms the 1st film | membrane which receives a photo-alignment process, and the 2nd material which forms the 2nd film | membrane which does not receive a photo-alignment process, and a voltage holding ratio. It is a schematic cross section of a two-layer alignment film. It is sectional drawing which shows the example of an actual two-layer alignment film. It is a graph which shows the example of the ratio of the 1st material which forms the 1st film | membrane which receives a photo-alignment process in the depth direction of an alignment film, and the 2nd material which forms the 2nd film | membrane which does not receive a photo-alignment process. It is a graph which shows the relationship between the ratio of the 1st material which forms the 1st film | membrane which receives a photo-alignment process, and the 2nd material which forms the 2nd film | membrane which does not receive a photo-alignment process, and display unevenness.

  The contents of the present invention will be described in detail below using examples.

  The present invention can be applied to all liquid crystal display devices using photo-alignment. Hereinafter, the present invention will be described by taking an IPS liquid crystal display device as an example. For example, a TN (Twisted Nematic) method or a VA (Virtual Alignment) method liquid crystal display device may be used.

  FIG. 1 is a cross-sectional view of an IPS liquid crystal display device. The TFT in FIG. 1 is a so-called top gate type TFT, and LTPS (Low Temperature Poly-Silicon) is used as a semiconductor to be used. On the other hand, when an a-Si semiconductor is used, a so-called bottom gate type TFT is often used. In the following description, a case where a top gate type TFT is used will be described as an example. However, the present invention can also be applied to a case where a bottom gate type TFT is used.

In FIG. 1, a first base film 101 made of SiN and a second base film 102 made of SiO ₂ are formed on a glass substrate 100 by CVD (Chemical Vapor Deposition). The role of the first base film 101 and the second base film 102 is to prevent the semiconductor layer 103 from being contaminated by impurities from the glass substrate 100.

  A semiconductor layer 103 is formed on the second base film 102. The semiconductor layer 103 is obtained by forming an a-Si film on the second base film 102 by CVD and converting it into a poly-Si film by laser annealing. The poly-Si film is patterned by photolithography.

A gate insulating film 104 is formed on the semiconductor film 103. This gate insulating film 104 is a SiO ₂ film made of TEOS (tetraethoxysilane). This film is also formed by CVD. A gate electrode 105 is formed thereon. The gate electrode 105 also serves as the scanning line 10 shown in FIG. The gate electrode 105 is formed of, for example, a MoW (molybdenum, tungsten) film. When it is necessary to reduce the resistance of the gate electrode 105 or the scanning line 10, an Al alloy is used.

  The gate electrode 105 is patterned by photolithography. At the time of this patterning, impurities such as phosphorus or boron are doped into the poly-Si layer by ion implantation to form a source S or drain D in the poly-Si layer. Further, an LDD (Lightly Doped Drain) layer is formed between the channel layer of the poly-Si layer and the source S or the drain D using a photoresist when patterning the gate electrode 105.

Thereafter, a first interlayer insulating film 106 is formed of SiO _{2 so as} to cover the gate electrode 105. The first interlayer insulating film 106 is for insulating the gate wiring 105 and the contact electrode 107. A through hole 120 for connecting the source portion S of the semiconductor layer 103 to the contact electrode 107 is formed in the first interlayer insulating film 106 and the gate insulating film 104. Photolithography for forming the through hole 120 in the first interlayer insulating film 106 and the gate insulating film 104 is performed simultaneously.

  A contact electrode 107 is formed on the first interlayer insulating film 106. The contact electrode 107 is connected to the pixel electrode 112 through the through hole 130. The drain D of the TFT is connected to the video signal line 20 shown in FIG.

  The contact electrode 107 and the video signal line are formed in the same layer at the same time. For the contact electrode 107 and the video signal line (hereinafter represented by the contact electrode 107), for example, an AlSi alloy is used to reduce the resistance. However, the AlSi alloy generates hillocks or Al diffuses into other layers. Therefore, for example, a structure in which AlSi is sandwiched between a barrier layer and a cap layer of MoW material (not shown) is employed.

  The contact electrode 107 and the TFT are covered and protected by an inorganic passivation film (insulating film) 108. The inorganic passivation film 108 is formed by CVD in the same manner as the first base film 101. For example, silicon nitride or silicon oxide is used for the inorganic passivation film 108. Further, an organic passivation film 109 is formed so as to cover the inorganic passivation film 108. The organic passivation film 109 is made of a photosensitive acrylic resin. The organic passivation film 109 can be formed of silicone resin, epoxy resin, polyimide resin, or the like in addition to acrylic resin. Since the organic passivation film 109 has a role as a planarizing film, it is formed thick. The thickness of the organic passivation film 109 is 1 to 4 μm, but in many cases is about 2 μm.

  In order to establish conduction between the pixel electrode 110 and the contact electrode 107, a through hole 130 is formed in the inorganic passivation film 108 and the organic passivation film 109. The organic passivation film 109 uses a photosensitive resin. When this resin is exposed after application of a photosensitive resin, only the portion exposed to light is dissolved in a specific developer. That is, the formation of a photoresist can be omitted by using a photosensitive resin. After the through-hole 130 is formed in the organic passivation film 109, the organic passivation film 109 is completed by baking the organic passivation film at about 230 ° C.

  Thereafter, ITO (Indium Tin Oxide) to be the common electrode 110 is formed by sputtering and patterned so as to remove the ITO from the through hole 130 and its periphery. The common electrode 110 can be formed in a planar shape common to each pixel. Thereafter, SiN to be the second interlayer insulating film 111 is formed on the entire surface by CVD. Thereafter, a through hole is formed in the second interlayer insulating film 111 and the inorganic passivation film 108 in order to make the contact electrode 107 and the pixel electrode 112 conductive in the through hole 130. Thereafter, ITO is formed by sputtering and patterned to form the pixel electrode 112.

An alignment film material is applied on the pixel electrode 112 by flexographic printing or inkjet, and is baked to form the alignment film 113. In the present invention, since the alignment film is subjected to polarized ultraviolet treatment, an alignment film material suitable for photo-alignment is used. There are various methods of photo-alignment. In the case of a so-called photodimerization type photo-alignment film, alignment treatment is performed by irradiating polarized ultraviolet rays having a wavelength of 313 nm with an intensity of 100 mJ / cm ² . In the case of a so-called photoisomerization type photo-alignment film, alignment treatment is performed by irradiating polarized ultraviolet rays having a wavelength of 365 nm with an intensity of 200 mJ / cm ² . In the case of a so-called photodegradable photo-alignment film, the alignment treatment is performed by irradiating polarized ultraviolet rays having a wavelength of 254 nm with an intensity of 1000 mJ / cm ² .

  FIG. 2 is a flowchart showing the formation of a photolytic photo-alignment film. In FIG. 2, an alignment film is applied to the TFT substrate, dried, and leveled. In this leveling step, the alignment film is flattened. Thereafter, the alignment film is imidized by heating at a high temperature of 200 ° C. or higher. Thereafter, polarized ultraviolet rays are irradiated to give uniaxial anisotropy to the alignment film. Thereafter, heating is performed at a higher temperature to evaporate remaining monomers and oligomers. In the present invention, as will be described later, as the alignment film material, a first material that is subjected to photo-alignment treatment and a second material that is not subjected to photo-alignment treatment are used, but the photo-alignment treatment is the same as described above. is there.

  Returning to FIG. 1, when a voltage is applied between the pixel electrode 112 and the common electrode 110, lines of electric force as shown in FIG. 1 are generated. The liquid crystal molecules 301 are rotated by this electric field, and an image is formed by controlling the amount of light passing through the liquid crystal layer 300 for each pixel.

  In FIG. 1, a counter substrate 200 is disposed 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 for each pixel, thereby forming a color image. A light shielding film 202 is formed between the color filters 201 and the contrast of the image is improved. The shape of the light shielding film 202 is not particularly limited, but a matrix shape is preferable.

  An overcoat film 203 is formed to cover the color filter 201 and the light shielding film 202. Since the surfaces of the color filter 201 and the light shielding film 202 are uneven, the surface is flattened by the overcoat film 203. An alignment film 113 for determining the initial alignment of the liquid crystal is formed on the overcoat film. The alignment treatment of the alignment film 113 is the same as the alignment film 113 on the TFT substrate 100 side described in FIG.

  The above configuration is an example. For example, the inorganic passivation film 108 in the TFT substrate 100 may not be formed depending on the type. In addition, the formation process of the through hole 130 may differ depending on the type.

  In the case of an alignment film that has been subjected to a photo-alignment process, the alignment film in a range where ultraviolet rays are transmitted is subjected to an alignment process, unlike the rubbing method. As a result, for example, in the case of the photolysis type, the alignment film is decomposed by ultraviolet rays, so that the film strength becomes weak. When the film strength is weakened, a phenomenon in which the initial alignment of the liquid crystal becomes unstable, such as an afterimage phenomenon, occurs.

  In order to prevent this, the present invention adopts a two-layer structure of a first film that is subjected to photo-alignment treatment and a second film that is not subjected to photo-alignment treatment. As a result, both liquid crystal alignment and film strength are achieved. Such an alignment film having a two-layer structure is formed, for example, by applying a mixture of a polyamic acid ester represented by (Chemical Formula 1) and a polyamic acid represented by (Chemical Formula 2) as an alignment film material.

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.

In the chemical formula (2), each R2 independently represents 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 .

  Unlike the chemical formula (1), the chemical formula (2) does not have a cyclobutane skeleton. Chemical formula (2) has no cyclobutane and is not easily affected by ultraviolet rays. In addition, the difference between the chemical formula (1) and the chemical formula (2) is that in the chemical formula (2), R1 is replaced with H in the chemical formula (1) indicating the polyamic acid ester.

  When the alignment film material in which (Chemical Formula 1) and (Chemical Formula 2) are mixed is applied, the layers are separated, and the first material gathered on the upper side (liquid crystal side) is subjected to photo-alignment treatment using a polyamic acid ester as a precursor. The second material that is not subjected to photo-alignment treatment with a polyamic acid as a precursor is collected.

  In other words, it was found that even if the layers are separated, they are not clearly separated at the boundary between the upper alignment film and the lower alignment film. That is, it has been found that the distribution of components exists so that the upper photo-aligning component gradually increases from the lower side. It is desirable that the outermost surface has a distribution that is completely covered with the components that are photoaligned, but the lower alignment film component that is not photoaligned is distributed at a ratio of several nm below the outermost surface. ing. That is, the first film on the side in contact with the liquid crystal and the second film on the second substrate side do not exist as clearly separated films.

  The polyamic acid represented by (Chemical Formula 2) is an example, and can be formed into a mixture with the polyamic acid ester. After applying the mixture, the polyamic acid is separated from the polyamic acid ester and disposed in the lower layer. If it exists, it may be a polyamic acid having another structure.

  Recently, liquid crystal display devices have been used to reduce power consumption of a drive circuit by performing low-frequency driving (for example, a frequency of 20 kHz or less) or intermittent driving in still image display or the like. When performing low-frequency driving or intermittent driving (hereinafter represented by low-frequency driving), the voltage holding ratio of the pixel from signal writing to writing is important. When the time from signal writing to writing becomes long, the potential of the pixel decreases due to leakage. A decrease in the resistivity of the liquid crystal is a major cause of such leakage. If the potential of the pixel fluctuates between signal writing, it causes flicker. In addition, when a leak occurs due to local accumulation of impurities in the liquid crystal, a local luminance change occurs, which appears as luminance unevenness. That is, maintaining the liquid crystal resistivity and the pixel voltage holding ratio at high values is important for high-quality image display.

  When the alignment film is formed of the first film whose upper layer is photo-aligned and the lower layer is formed of the second film that is not photo-aligned, the material of the first film that is photo-aligned and the second film that is not photo-aligned Depending on the ratio of these materials, the voltage holding ratio of the pixel electrode of the liquid crystal display device changes. The resistivity of the liquid crystal has a great influence on the voltage holding ratio.

  FIG. 3 is a schematic diagram showing a voltage holding ratio measuring circuit. In FIG. 3, an electrode 112 is formed on the lower substrate 100 of the test cell, and an alignment film 113 is formed thereon. An electrode 110 is formed on the upper substrate 200, and an alignment film 113 is formed thereon. Note that both the electrode 112 and the electrode 110 are made of ITO in the same manner as the pixel electrode or the common electrode. Liquid crystal is sandwiched between the upper substrate and the lower substrate.

  In FIG. 3, the voltage holding ratio (VHR) is determined by the voltage V between the upper electrode and the lower electrode when an AC voltage is applied between the electrode 112 and the electrode 110. Since the power supply has an internal resistance R, the voltage V decreases when the resistance of the liquid crystal decreases. If the resistance is reduced, for example, when the liquid crystal takes in impurities, the voltage V decreases. The voltage holding ratio also decreases when the liquid crystal deteriorates during long-time operation. When the voltage holding ratio decreases, flicker and screen brightness unevenness occur.

  The voltage holding ratio shown in FIG. 3 has different values depending on the voltage waveform to be applied and the measurement temperature. The voltage holding ratio in this specification is determined by the voltage V when a voltage as shown in FIG. 4 is applied. In FIG. 4, SV is the waveform of the power supply voltage, R is the internal resistance of the power supply, CL is the capacitance of the liquid crystal, and RL is the leakage resistance of the liquid crystal. When the leak resistance RL is decreased, V is decreased and flicker or the like is likely to occur. The waveform SV of the power supply shown in FIG. 4 applies a pulse with a specific period.

  FIG. 5 shows a change in the voltage of one of the electrodes of the test cell when the power supply waveform in the circuit of FIG. 4 is applied. In FIG. 5, the power supply applies a pulse of 5 m and a width of 4 msec at intervals of 1 sec. At this time, the voltage of one electrode of the test cell keeps 5 V for 4 msec during which the pulse is applied, and then decreases by a time constant determined by the leakage resistance of the liquid crystal and the capacitance of the liquid crystal. One second after the first pulse is applied, a pulse having a width of -5 V and a width of 4 msec is applied. The reason why the positive and negative pulses are alternately applied in this way is to perform the test under conditions close to the driving of the liquid crystal.

  In FIG. 5, after applying a pulse voltage, one electrode of the test cell is lowered by an exponential function determined by a time constant. If there is no leak in the liquid crystal, one electrode of the test cell keeps 5V as shown by a dotted line in FIG. In this specification, the area of 1 sec × 5 V when there is no voltage leakage, that is, the ratio of the area of the hatched area in FIG. 5 to the rectangular area is defined as the voltage holding ratio. That is, a liquid crystal display device having no voltage leak has a higher voltage holding ratio and can suppress the occurrence of flicker or the like in a low frequency operation.

  In other words, the voltage holding ratio can be increased as the resistance of the liquid crystal is higher. The resistance of the liquid crystal varies depending on various conditions. In the present invention, as a liquid crystal material, a so-called negative liquid crystal in which the dielectric anisotropy of the liquid crystal is negative is targeted. Negative type liquid crystals tend to have lower resistance than so-called positive type liquid crystals in which the dielectric anisotropy of the liquid crystals is positive. This is presumably because negative liquid crystals have a property of easily incorporating impurities into the liquid crystals.

  When photo-alignment is performed, the alignment film is decomposed by ultraviolet irradiation, and the decomposition product is not completely evaporated by heating after ultraviolet irradiation, and a residue is often present. When such a residue is mixed in the negative type liquid crystal, the resistance of the liquid crystal is lowered and the voltage leakage is increased. When the alignment film has a two-layer structure, a first film that receives photo-alignment is disposed in the upper layer, and a second film that does not receive photo-alignment exists in the lower layer.

  The first film that undergoes photo-alignment is decomposed by ultraviolet rays, and the probability that the decomposed product remains without being completely evaporated increases. On the other hand, the second film that does not undergo photo-alignment is much less decomposed by ultraviolet light than the first film that undergoes photo-alignment, and the decomposed product is much less than the first film that undergoes photo-alignment. The inventor has found that in the alignment film material forming the alignment film having a two-layer structure, the resistance of the liquid crystal changes, that is, the voltage holding ratio changes depending on the ratio of the material that undergoes photo-alignment.

  FIG. 6 is a graph showing this state. In FIG. 6, the horizontal axis indicates the ratio of the alignment film material that forms the alignment film that undergoes photo-alignment in the alignment film material that forms the two-layer structure. The vertical axis represents the voltage holding ratio (VHR). The measurement temperature at this time is 60 ° C. The alignment film material is a mixture of the polyamic acid ester (first material) represented by (Chemical Formula 1) and the polyamic acid (second material) represented by (Chemical Formula 2), and the horizontal axis of FIG. Indicates.

  FIG. 6 shows a case where the dielectric anisotropy ε of the liquid crystal is positive and a case where ε is negative. As can be seen from FIG. 6, when ε is positive, the voltage holding ratio has a very small dependence on the ratio of the polyamic acid ester (PE). On the other hand, when ε is negative, the voltage holding ratio greatly depends on the ratio of the polyamic acid ester (PE). That is, the voltage holding ratio decreases as the ratio of the polyamic acid ester increases.

  That is, since the alignment film having a polyamic acid ester as a precursor undergoes photo-alignment, a residue of decomposition products after ultraviolet irradiation is generated. Although this is taken into the liquid crystal, the negative type liquid crystal has a tendency to take in the residue, so that the resistance of the liquid crystal is reduced. And it seems that this residue increases as the amount of polyamic acid ester increases.

  It was found that even in the case of a negative liquid crystal, it is difficult to incorporate impurities into the liquid crystal if the absolute value of dielectric anisotropy | ε | If there are few impurities in the liquid crystal, the resistance of the liquid crystal can be kept high, and a decrease in voltage holding ratio can be prevented. Therefore, in the present invention, it is desirable to use a negative liquid crystal having | ε | of 5 or less.

  By the way, the alignment film has a certain thickness, but the action of aligning the liquid crystal is the outermost layer. That is, it is only necessary to perform photo-alignment centering on the outermost surface of the alignment film. Therefore, as shown in FIG. 7, the alignment film is mostly an alignment film 1132 having a polyamic acid which is not subjected to photo-alignment as a precursor, and a polyamic acid ester that is subjected to photo-alignment only on the surface as a precursor. 1131 is the most efficient. In FIG. 7, the hatched portion is a region that receives a photo-alignment and generates a large amount of decomposition products.

  That is, the film that undergoes photo-alignment decomposes to the extent that polarized ultraviolet rays reach. Therefore, in proportion to the irradiation amount of polarized ultraviolet rays, the number of decomposition products increases and the amount of residues increases. If the film undergoing photo-alignment is only a thin layer with an extremely thin surface, the amount of residue is reduced. Therefore, generation of a residue due to polarized ultraviolet rays can be reduced while maintaining the alignment ability of the liquid crystal.

  However, the actual two-layer alignment film has the same thickness as the dotted line in FIG. That is, it has a distribution both in the plane direction and in the depth direction. FIG. 8 is a cross-sectional view of an alignment film showing an example in which the ratio of the first film that undergoes photo-alignment varies depending on the location. As suggested in FIG. 8, when the ratio of the first film 1131 that undergoes photo-alignment is extremely reduced, a region where no alignment film that undergoes photo-alignment exists is generated. An orientation failure occurs.

  FIG. 9 is an example showing the ratio of the alignment film that undergoes photo-alignment in the thickness direction of the alignment film. In FIG. 9, the horizontal axis represents the depth direction of the alignment film, and the vertical axis represents the ratio of the first film 1131 that undergoes photo-alignment. In FIG. 9, the first film 1131 that receives photo-alignment exists on the surface, but the second film 1132 that does not receive photo-alignment is located closer to the substrate than the first film 1131 exists. When the alignment film has a certain depth t1, the ratio of the first film 1131 that undergoes photo-alignment becomes substantially zero. In addition, the thickness of the alignment film in FIG. 9 is t0. The example of FIG. 9 is an example in which the entire surface of the alignment film is only the first film that undergoes photo-alignment. However, since the amount of the first film is not sufficient, if the ratio of the alignment film that undergoes photo-alignment is slightly smaller than that in FIG. 9, there is a possibility that alignment failure may occur.

  FIG. 10 shows the ratio of the first material that forms the first film that undergoes photo-alignment and the second material that forms the second film that does not receive photo-alignment and display unevenness in the alignment film material in an actual liquid crystal display device. It is a table | surface which shows the result of having evaluated the relationship. In FIG. 10, “◯” indicates a state where display unevenness is not visible, “Δ” indicates a state where it appears light, and “×” indicates a state where display unevenness is visible.

  In FIG. 10, when the first material is too small, such as 10 wt% or less, display unevenness occurs. This is because there is a case where there is no first film that undergoes photo-alignment on the outermost surface of the alignment film, and this is because of the alignment failure at that portion. On the other hand, when the material forming the first film subjected to photo-alignment is 40 wt% or more, display unevenness occurs. This is presumably because the voltage holding ratio decreases as shown in FIG.

  A more detailed evaluation of the relationship between the voltage holding ratio and display unevenness in accordance with FIG. 6 is as follows. When the ratio of the first material is more than 10 wt% and less than 40 wt%, the voltage holding ratio can be ensured to be 92% or more, and the occurrence of display unevenness can be prevented. Further, if the ratio of the first material forming the alignment film that undergoes photo-alignment is 18 wt% or more and less than 40 wt%, the occurrence of display unevenness can be suppressed more stably. Furthermore, if the ratio of the first material forming the first film is 24 wt% or more and 35 wt% or less, the occurrence of display unevenness can be reliably suppressed. Although FIG. 10 is evaluated based on display unevenness, flicker is caused by the same cause, and therefore, countermeasures can be taken in the same manner for flicker.

From the above description, the alignment film material and the alignment film preferably have the following material characteristics.
When the alignment film is in contact with the sealing material that bonds the TFT substrate 100 and the counter substrate 100, the alignment film is required to have high film strength and strong adhesive strength to the sealing material. In that case, it is preferable to have a silane coupling agent represented by (Chemical Formula 3) or (Chemical Formula 4). Further, other additives may be included.

  The alignment film material preferably contains an imidization accelerator as another additive. By including an imidization accelerator, imidation of polyamic acid or polyamic acid ester can be efficiently advanced. And after performing a photo-alignment process, a favorable 1st film | membrane can be formed in the surface which touches the liquid crystal of an alignment film. However, the imidization accelerator may be decomposed into small molecules by heat treatment or ultraviolet treatment in the process of forming the alignment film, and may become an impurity that lowers the resistance of the liquid crystal. Even if the amount of the impurity is small, the resistance of the liquid crystal may be affected. Therefore, the imidization accelerator should be selected in consideration of the imidization promoting effect and decomposability.

  In the present specification, preferred imidization accelerators are secondary or tertiary amines having a skeleton of picoline, quinoline, isoquinoline or pyridine, or amino acids having an alkoxycarbonyl group. As the above-mentioned amines, tertiary amines are preferable. Further, the alkoxycarbonyl group of the amino acid is preferably a butoxycarbonyl group. The above amino acid preferably has two or more alkoxycarbonyl groups or has a fluorene skeleton.

  When a polyamic acid ester is used as the first material subjected to the photo-alignment treatment, it is preferable that two of the two R2s represented by (Chemical Formula 1) are methyl groups and two are hydrogen. A structure of (Chemical Formula 5) is more preferable. R3 and R4 in the formula are each independently an alkyl group having 1 to 8 carbon atoms.

  The polyamic acid and the polyamic acid ester preferably use diamine as a precursor. The structure of the diamine (first diamine) as the precursor of the polyamic acid ester is not particularly limited. A preferred diamine is a diamine having an aromatic ring and having no nitrogen atom, fluorine atom, or oxygen atom other than the nitrogen atoms in the two amino groups. More preferred is a diamine having the structure of (Chemical Formula 6). R5 and R6 in the formula are each independently hydrogen or an alkyl group having 3 or less carbon atoms.

  When the diamine described above is used, the polarity of the polyamic acid ester is lowered. As a result, when the mixed material obtained by mixing the polyamic acid ester as the first material and the second material is applied to the TFT substrate 100, the first material is likely to be present on the liquid crystal layer 300 side. Thereby, even if the ratio of the first material in the mixed material is lowered, an alignment film having good characteristics can be formed.

  On the other hand, the structure of diamine (second diamine) as a precursor of polyamic acid is not particularly limited. Preferred diamines preferably have a nitrogen atom other than the nitrogen atoms in the two amino groups, a fluorine atom, or an oxygen atom. When a diamine having these elements in the main skeleton is used, the polarity of the polyamic acid ester is increased. As a result, when the mixed material obtained by mixing the first material and the polyamic acid as the second material is applied to the TFT substrate 100, the second material is likely to be present on the TFT substrate 100 side. Thereby, even if the ratio of the first material in the mixed material is lowered, an alignment film having good characteristics can be formed.

  As described above, according to the present invention, in a liquid crystal display device using a negative liquid crystal and using a photo-alignment process, it is possible to prevent occurrence of display unevenness or flicker even when low frequency driving is performed. .

  DESCRIPTION OF SYMBOLS 100 ... TFT substrate, 101 ... 1st foundation film, 102 ... 2nd foundation film, 103 ... Semiconductor layer, 104 ... Gate insulating film, 105 ... Gate electrode, 106 ... 1st interlayer insulation film, 107 ... Contact electrode, 108 ... Inorganic passivation film, 109 ... Organic passivation film, 110 ... Common electrode, 111 ... Second interlayer insulating film, 112 ... Pixel electrode, 113 ... Alignment film, 120 ... First through hole, 130 ... Second through hole, 140 ... First 3 through holes, 200 ... counter substrate, 201 ... color filter, 202 ... light-shielding film, 203 ... overcoat film, 300 ... liquid crystal layer, 301 ... liquid crystal molecule, 1131 ... first film, 1132 ... second film, D ... drain Part, S ... source part, SV ... power supply waveform for voltage holding ratio measurement, R ... power supply internal resistance, RL ... Liquid crystal leakage resistance, CL ... Capacitance of liquid crystal

Claims (12)

A liquid crystal display device having a liquid crystal between a first substrate having an alignment film and a second substrate having an alignment film,
The liquid crystal has a negative dielectric anisotropy and an absolute value of the dielectric anisotropy is 5 or less,
The alignment film has been subjected to a photo-alignment treatment with polarized ultraviolet rays,
The alignment film has alignment ability by the photo-alignment treatment, and is not provided with the first film present on the side in contact with the liquid crystal and the alignment ability by the photo-alignment treatment. Formed by the second film present on the second substrate side,
The alignment film is formed by applying a mixed material of a first material forming the first film and a second material forming the second film to the first substrate or the first substrate,
The liquid crystal display device, wherein the first material is larger than 10 wt% and smaller than 40 wt% with respect to a total weight of the first material and the second material. The liquid crystal display device according to claim 1, wherein the first material is a polyamic acid ester represented by (Chemical Formula 1), and the second material contains polyamic acid.

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. 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 acetyl group _{(- (CH 2) m-} C≡CH, m = 0, 1, 2), and Ar is an aromatic compound. The first material is a polyamic acid ester;
The polyamic acid ester is formed using a first diamine as a precursor,
The said 1st diamine is a diamine which has an aromatic ring and does not have nitrogen atoms other than the nitrogen atom in two amino groups, a fluorine atom, and an oxygen atom. Liquid crystal display device. The second material is polyamic acid;
The polyamic acid is formed using a second diamine as a precursor,
4. The liquid crystal display device according to claim 1, wherein the second diamine is a diamine having a nitrogen atom other than nitrogen atoms in two amino groups, a fluorine atom, or an oxygen atom. 5. The mixed material includes an imidization accelerator;
The imidization accelerator is picoline, quinoline, isoquinoline, a secondary or tertiary amine having a pyridine skeleton, or an amino acid having an alkoxycarbonyl group. The liquid crystal display device according to any one of the above. 6. The liquid crystal display device according to claim 1, wherein the alignment film material further contains a silane coupling agent represented by (Chemical Formula 3) or (Chemical Formula 4).

  The liquid crystal display device according to claim 1, wherein the liquid crystal display device drives a liquid crystal at a frequency of 20 kHz or less. An alignment film material used for forming an alignment film of a liquid crystal display device,
The alignment film material is a mixture of a first material and a second material,
The first material includes a polyamic acid ester represented by (Chemical Formula 1), and the second material includes a polyamic acid represented by (Chemical Formula 2),
An alignment film material for liquid crystal, wherein the first material is larger than 10 wt% and smaller than 40 wt% with respect to the total weight of the first material and the second material.

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. 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 acetyl group _{(- (CH 2) m-} C≡CH, m = 0, 1, 2), and Ar is an aromatic compound.

In the chemical formula (2), each R2 independently represents 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 acetyl group _{(- (CH 2) m-} C≡ The polyamic acid ester is formed using a first diamine as a precursor,
9. The liquid crystal according to claim 8, wherein the first diamine is a diamine having an aromatic ring and having no nitrogen atom, fluorine atom, and oxygen atom other than nitrogen atoms in two amino groups. Alignment film material. The polyamic acid is formed using a second diamine as a precursor,
The alignment film material for liquid crystal according to claim 8 or 9, wherein the second diamine is a diamine having a nitrogen atom other than nitrogen atoms in two amino groups, a fluorine atom, or an oxygen atom. Including an imidization accelerator,
The imidization accelerator is picoline, quinoline, isoquinoline, a secondary or tertiary amine having a pyridine skeleton, or an amino acid having an alkoxycarbonyl group. The liquid crystal display device according to any one of the above. The alignment film material for liquid crystal according to claim 8, wherein the alignment film material further contains a silane coupling agent represented by (Chemical Formula 3) or (Chemical Formula 4).

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