JP2017167563A - Varnish for alignment film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a light alignment film with a DC residual image suppressed in an IPS liquid crystal display device.SOLUTION: An alignment film 113 has a two-layer structure including a light alignment film 1131 capable of light alignment and a low-resistance alignment film 1132 with lower resistivity than the light alignment film. The light alignment film 1131 is formed of polyimide in which polyamide acid alkyl ester serves as a precursor, and the number average molecular weight is large and the alignment stability by the light alignment is excellent. The low-resistance alignment film 1132 is formed of polyimide in which polyamide acid serves as a precursor, and the number average molecular weight is small and the resistivity is low. When the alignment film has the two-layer structure, the excellent light alignment characteristic can be maintained and the resistivity of the entire alignment film can be reduced. Thus, the DC residual image can be suppressed.SELECTED DRAWING: Figure 3

Description

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

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. Viewing angle characteristics are IPS (In Plane Switchc) in which liquid crystal molecules are operated by a horizontal electric field.
Hing) system has excellent characteristics.

As a method of imparting alignment treatment, that is, alignment control ability, to an alignment film used in a liquid crystal display device,
As a conventional technique, there is a method of processing by 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 capability to an alignment film in a non-contact manner. Since the IPS method does not require a pretilt angle, a photo-alignment method can be applied.

Patent Document 1 discloses a photo-decomposition type photo-alignment process by irradiation of light typified by ultraviolet rays. According to this, in photo-decomposition type photo-alignment process, (1) a complicated step structure in a pixel portion is disclosed. (2) Resolving display defects caused by damage to thin film transistors due to static electricity in the alignment treatment by rubbing, disturbance of the rubbing tip of the rubbing cloth, or alignment disturbance due to dust, and uniform alignment control ability This eliminates the complexity of the process due to the frequent replacement of the rubbing cloth.

JP 2004-206091 A

However, it is known that the optical alignment treatment generally has lower alignment stability than the rubbing treatment in terms of imparting alignment control ability to the alignment film. If the alignment stability is low, the initial alignment direction varies, causing display defects. In particular, in a liquid crystal display device using a horizontal electric field type liquid crystal display panel that requires high alignment stability, a display defect symbolized by an afterimage tends to occur due to low alignment stability.

In the photo-alignment process, the process of stretching the main chain of the polymer, such as rubbing, to make it linear is LC
Not present in the D process. For this reason, in the photo-alignment treatment, a synthetic polymer alignment film represented by polyimide irradiated with polarized light is given uniaxiality by cutting the main chain in a direction parallel to the polarization direction. The liquid crystal molecules are not cut and are aligned along the long main chain direction that remains on the straight line, but if the length of this main chain is shortened, the uniaxiality decreases and the interaction with the liquid crystal weakens. Since the orientation deteriorates, the above-mentioned afterimage is likely to occur.

Therefore, in order to improve the uniaxiality of the alignment film and improve the alignment stability, it is necessary to increase the molecular weight of the alignment film. A photo-alignment film material can be used. According to this, the polyamic acid alkyl ester material does not involve a decomposition reaction into a diamine and an acid anhydride at the time of the imidation reaction that occurs in the conventional polyamic acid material, and can maintain a large molecular weight even after imidization. Orientation stability equivalent to that of rubbing treatment can be obtained.

In addition, since the polyamic acid alkyl ester material does not contain a carboxylic acid in its chemical structure, the voltage holding ratio of the LCD is higher than that of the polyamic acid material, and improvement in long-term reliability can be ensured.

In order to obtain the alignment stability and long-term reliability of the photo-alignment film, it is effective to apply a polyamic acid alkyl ester material, but this material generally has a higher specific resistance of the alignment film than the polyamic acid material. Therefore, when a DC voltage is superimposed on a signal waveform for driving liquid crystal molecules to form a residual DC, the time constant until the residual DC is relaxed is large, and it is likely to cause image sticking (DC afterimage).

An object of the present invention is to use a liquid crystal display panel that can reduce the disappearance time of a DC afterimage without impairing the alignment stability and long-term reliability of the polyamic acid alkyl ester material in the photo-alignment treatment, and enables high-quality display. The present invention provides a liquid crystal display device.

The present invention overcomes the above problems, and specific means are as follows. In other words, the liquid crystal display device of the present invention has T on the uppermost layer of the main surface on which the active element for pixel selection is formed.
TFT substrate having FT-side alignment film, counter substrate having color filter-side alignment film on the uppermost layer of the main surface on which the color filter is formed, TFT-side alignment film of TFT substrate, and color filter of counter-substrate A liquid crystal display panel composed of liquid crystal sealed between side alignment films is provided. The TFT side alignment film and the color filter side alignment film have a liquid crystal alignment control function provided by light irradiation.

An alignment film material for imparting a liquid crystal alignment control function by light irradiation is aligned with light typified by ultraviolet rays to at least one of the alignment film on the TFT substrate and the alignment film on the opposite substrate on which the color filter is formed. A two-component material composed of a material to be formed and a material having a low specific resistance, and after forming the alignment film, has a two-layer structure of a photo-alignment film for alignment and a low-resistance film that does not contribute to alignment.

As the material oriented by light, a material obtained by polyimidizing a photodegradable polyamic acid alkyl ester material can be used. Moreover, what has this chemical polyimidation 70% or more can be used.

As the low resistance material, a material obtained by polyimidizing a polyamic acid material can be used, and this does not need to be photodegradable. Moreover, this chemical imidation can use what is 40% or more.

According to the present invention, a material composed of a two-component system is phase-separated to form an alignment film with a two-layer structure, and a photo-alignment component having high alignment stability is arranged on the liquid crystal layer side. By arranging the resistance component on the substrate side, it becomes possible to satisfy both the alignment stability and the reduction of the time constant of the DC afterimage due to the low resistance of the alignment film. As a result, the afterimage characteristics of the photoalignment film are greatly improved. Is done.

Further, since the low resistance component arranged on the substrate side does not contribute to the alignment, the molecular weight can be lowered to the limit. Therefore, the tolerance for adjusting the concentration and viscosity of the alignment film varnish in which the polyamic acid alkyl ester or the polyamic acid is dissolved in an organic solvent is expanded, and the alignment film forming method is not limited to the conventional flexographic printing but also the low varnish. Even in the case of inkjet coating, in which it is difficult to increase the thickness of the alignment film due to the necessity of increasing the viscosity, the tolerance for increasing the thickness of the alignment film is improved.

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 figure which shows the structure of the alignment film by this invention. It is a figure which shows the formation process of a photo-alignment film. It is sectional drawing of the alignment film of this invention. It is a DC afterimage evaluation pattern. It is an evaluation result of DC afterimage.

  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 an IPS liquid crystal display device. IPS
Various electrode structures for liquid crystal display devices of the type 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. The present invention will be described by taking the configuration of FIG. 1 as an example, but an IPS type liquid crystal display device other than FIG. 1, for example, the counter electrode is located on the upper insulating film and the pixel electrode is below the upper insulating film. The present invention can also be applied to the case where the counter electrode and the pixel electrode are on the same plane.

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 is A
A MoCr alloy is laminated on the lNd 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 a channel portion of the TFT, and the source electrode 10 is formed on the a-Si film with the channel portion interposed therebetween.
4 and the drain electrode 105 are formed. 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. When it is desired to lower the electrical resistance of the source electrode 104 or the drain electrode 105, for example, AlNd
An electrode structure in which the 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. 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. This upper insulating film 109
The through hole 111 is formed by etching the inorganic passivation film 106 using as a resist. Thereafter, the upper insulating film 109 and the through hole 111 are covered to cover the pixel electrode 110.
ITO to be formed is formed by sputtering. The pixel electrode 110 is formed by patterning the sputtered ITO. ITO serving as the pixel electrode 110 is a through hole 111.
Also attached to. In the through hole 111, the drain electrode 1 extending from the TFT
05 and the pixel electrode 110 are conducted, 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. Below the pixel electrode 110,
A planar counter electrode 108 (not shown) is formed. When a video signal is applied to the pixel electrode 110, the liquid crystal molecules 301 are rotated by electric lines of force generated between the counter electrode 108 through the slit 112. As a result, light passing through the liquid crystal layer 300 is controlled to form an image.

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

In the example of FIG. 1, the counter electrode 10 formed in a planar shape on the organic passivation film 107.
8 and the comb electrode 110 is disposed on the upper insulating film 109. However, conversely, the pixel electrode 110 formed in a planar shape may be disposed on the organic passivation film 107, and the comb-like counter electrode 108 may be disposed on the upper insulating film 109.

An alignment film 113 for aligning liquid crystal molecules 301 is formed on the pixel electrode 110. In the present invention, the alignment film 113 includes a photo-alignment film 1131 in contact with the liquid crystal layer 300,
It has a two-layer structure of a low resistance 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. Counter substrate 20
A color filter 201 is formed inside 0. 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.

Overcoat film 20 covering color filter 201 and black matrix 202
3 is formed. 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. 2 is IPS, the counter electrode 108 is T
It is formed on the FT substrate 100 side and not 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.

FIG. 3 is a schematic view showing an alignment film 113 according to the present invention. 3A is a plan perspective view of the alignment film 113, and FIG. 3B is a cross-sectional perspective view. The alignment film 113 of the present invention has a two-layer structure. The upper side in contact with the liquid crystal layer is a photo-alignment film 1131 and the lower side is a low-resistance alignment film 1132. The photo-alignment film 1131 is formed by the photodegradable polymer 10 having a large molecular weight, and the low-resistance alignment film 1132 is formed by the low-resistance polymer 11 having a small molecular weight.

Since FIG. 3A is a perspective view, the photodegradable polymer 10 and the low-resistance polymer 11 can be seen through. Actually, the photodegradable polymer 10 exists above the low-resistance polymer 11. In FIG. 3B, the alignment film 113 is formed on the pixel electrode 110 or the organic passivation film 107 in FIG. In FIG. 3B, the alignment film 113 is formed on the pixel electrode 110. The thickness t1 of the upper photoalignment film 1131 is about 50 nm, and the thickness t2 of the lower low-resistance alignment film 1132 is about 50 nm. Since the boundary between the photo-alignment film 1131 and the low-resistance alignment film 1132 is not clear, it is indicated by a dotted line.

FIG. 4 is a schematic diagram showing a process for aligning liquid crystal with the photo-alignment film 1131. In FIG. 4, the low resistance alignment film 1132 is omitted. FIG. 4A shows the photo-alignment film 113.
1 shows the applied state. The photo-alignment film 1131 is formed of the photodegradable polymer 10.

For the photo-alignment film 1131 shown in FIG. 4A, polarized ultraviolet light is, for example, 6 J / cm ^2.
Irradiate with the energy of. Then, the photodegradable polymer 10 in the polarization direction of the ultraviolet light polarized in the photo-alignment film 1131 is destroyed by the ultraviolet light as shown in FIG. That is, the cut part 15 by ultraviolet rays is formed along the polarization direction of ultraviolet rays. Then, the liquid crystal molecules are aligned in the direction of arrow A in FIG.

As shown in FIG. 4, when the main chain of the photodegradable polymer 10 is short, the uniaxiality is lowered, the interaction with the liquid crystal is weakened, and the orientation is lowered. Therefore, in FIG. 4B, it is desirable that the photodegradable polymer 10 extends in the direction of arrow A as long as possible even after photo-alignment. That is, in order to improve the uniaxiality of the alignment film 113 and improve the alignment stability, it is necessary to increase the molecular weight of the alignment film 113.

The molecular weight of the alignment film 113 can be evaluated based on several molecular weights. The number molecular weight is an average molecular weight when polymers having various molecular weights exist in the alignment film 113. In the photo-alignment film 1131, the number molecular weight needs to be 5000 or more in order to obtain sufficient alignment stability.

In order to obtain such a photoalignment film 1131 having a large number molecular weight, an imidized polyamic acid alkyl ester can be used. The structure of the polyamic acid alkyl ester is as shown in chemical formula (1).

化学式（１）において、Ｒ１は、それぞれ独立に炭素数１〜８のアルキル基であり、Ｒ２
は、それぞれ独立に水素原子、フッ素原子、塩素原子、臭素原子、フェニル基、炭素数１
〜６のアルキル基、炭素数１〜６のアルコキシ基、アルケニル基（−（ＣＨ2）ｍ−ＣＨ＝Ｃ
Ｈ2，ｍ＝０，１，２）又はアルキニル基（−（ＣＨ2）ｍ−Ｃ≡ＣＨ，ｍ＝０，１，２）で
あり、Ａｒは芳香族化合物である。

In the chemical formula (1), each R1 is independently an alkyl group having 1 to 8 carbon atoms, and R2
Are each independently a hydrogen atom, fluorine atom, chlorine atom, bromine atom, phenyl group, carbon number 1
-6 alkyl group, C1-C6 alkoxy group, alkenyl group (-(CH2) m-CH = C
H2, m = 0,1,2) or an alkynyl group (— (CH2) m—C≡CH, m = 0,1,2), and Ar is an aromatic compound.

The feature of the polyamic acid alkyl ester is R1 in the chemical formula (1). In the polyamic acid alkyl ester, R1 is CnH2n-1, and n is 1 or more. When the polyamic acid alkyl ester is used as a precursor of the photo-alignment film 1131, it does not involve a decomposition reaction into a diamine and an acid anhydride at the time of an imidization reaction that occurs in a conventional polyamic acid material, and has a molecular weight after imidization. Can be kept large, and alignment stability equivalent to the rubbing treatment can be obtained.

However, the photo-alignment film 1131 obtained by imidizing the polyamic acid alkyl ester has a very high specific resistance, for example, about 10 ¹⁵ Ωcm. Thus, when the specific resistance of the alignment film 113 is large, the charge charged in the liquid crystal layer cannot be released, and this charge causes a DC afterimage.

In a liquid crystal display device, AC driving is performed so that charges are not charged in the liquid crystal layer. However, when a specific image is displayed for a specific time, a DC component remains on one substrate for a certain time, and the liquid crystal layer is charged. In some cases, an afterimage is generated due to the charged electric charge. If the resistance of the alignment film 113 is not extremely high, the charge in the liquid crystal can flow through the alignment film 113 to the pixel electrode 110 or the like.

However, the photo-alignment film 1131 has a very large specific resistance, and the charge in the liquid crystal cannot be released in a short time. In the present invention, the alignment film 113 is divided into a photo-alignment film 1131 and a low-resistance alignment film 1132.
A layered structure solves this problem. That is, the photo-alignment film 1131 in contact with the liquid crystal layer is formed of the photodegradable polymer 1 having a large molecular weight in order to obtain sufficient alignment stability.
0 is used.

On the other hand, the lower alignment film 113 is a low resistance alignment film 1132 having a small molecular weight but a small specific resistance.
And The resistivity of the low resistance alignment film 1132 is, for example, about 10 ¹² to 10 ¹⁴ Ωcm. Such a low resistance alignment film 1132 can be formed by imidizing polyamic acid.

  Polyamic acid is represented by the structure represented by chemical formula (2).

化学式（２）において、Ｒ２は、それぞれ独立に水素原子、フッ素原子、塩素原子、臭素
原子、フェニル基、炭素数１〜６のアルキル基、炭素数１〜６のアルコキシ基、アルケニル基
（−（ＣＨ2）ｍ−ＣＨ＝ＣＨ2，ｍ＝０，１，２）又はアルキニル基（−（ＣＨ2）ｍ−Ｃ
≡ＣＨ，ｍ＝０，１，２）であり、Ａｒは芳香族化合物である。

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, an alkenyl group (-( CH2) m-CH = CH2, m = 0, 1, 2) or an alkynyl group (-(CH2) m-C
≡CH, m = 0, 1, 2), and Ar is an aromatic compound.

The polyamic acid is different from the polyamic acid alkyl ester in that R1 is replaced with H in the chemical formula (1) indicating the polyamic acid alkyl ester. Since the polyamic acid is accompanied by a decomposition reaction into diamine and acid anhydride during the imidation reaction for forming the polyimide, the polyimide cannot have a sufficiently large molecular weight. Therefore, the photo-alignment film 113
For example, sufficient characteristics cannot be obtained. On the other hand, the specific resistance of polyimide formed from polyamic acid is not so high.

Thus, by making the alignment film 113 into a two-layer structure, it is possible to ensure alignment stability by photo-alignment and to control the specific resistance of the alignment film 113 as a whole to an appropriate value.
C afterimage can be reduced.

In order to form the alignment film 113 having a two-layer structure, the alignment film 113 can be formed without increasing the number of processes. That is, as shown in FIG. 5 (a), when a material in which the photodegradable polymer 10 and the low-resistance polymer 11 are mixed is applied to the substrate, the leveling effect results in FIG. 5 (b).
As shown in, a material that is familiar with the substrate is formed in the lower layer, and other materials are formed on the upper layer.
So-called phase separation occurs. Note that the substrate in FIG. 5A is represented by the pixel electrode 110.

In this embodiment, the substrate corresponds to the ITO or the organic passivation film 107 that forms the pixel electrode 110. In FIG. 5, a pixel electrode 110 is shown as a substrate. Since the polyamic acid is easier to adjust to the ITO or the organic passivation film 107 forming the pixel electrode 110 than the polyamic acid alkyl ester, the polyamic acid is always the lower layer.

The resin film thus formed is polyimideized by applying heat of about 200 ° C. Polyimidation is performed simultaneously on both the lower polyamic acid and the upper polyamic acid alkyl ester. Accordingly, the two-layer alignment film 1 is formed by the same process as the formation of the one-layer alignment film 113.
13 can be formed.

Since the upper photoalignment film 1131 needs to increase the molecular weight of the photodegradable polymer 10 in order to stabilize the alignment characteristics, it is necessary to increase the imidization rate. The imidation ratio in the photo-alignment film 1131 is 70% or more, more preferably 80% or more. This rest is
The polyamic acid alkyl ester as a precursor is present in the photo-alignment film 1131.

On the other hand, the low resistance alignment film 1132 in the lower layer is not related to the optical alignment characteristics of the liquid crystal, so the imidization rate may be low. For example, an imidation rate of 40% or more is sufficient. That is, the imidization condition may be set mainly on imidization of the upper polyamic acid alkyl ester.

The boundary between the upper layer and the lower layer of the alignment film 113 is not clear. In FIG. 5, this boundary is indicated by a dotted line. In FIG. 5B, the upper layer photo-alignment film 1131 has a molecular weight larger than that of the lower layer low-resistance alignment film 1132. When comparing the molecular weight of the photo-alignment film 1131 and the low-resistance alignment film 1132,
The molecular weight at the surface of the photo-alignment film 1131 may be compared with the molecular weight at the interface between the low-resistance alignment film 1132 and the substrate.

The DC afterimage characteristics were evaluated when the alignment film 113 having a two-layer structure of a photo-alignment film 1131 and a low-resistance alignment film 1132 and an alignment film having only a photo-alignment film were used. The afterimage was evaluated as follows. That is, an 8 × 8 checker flag pattern in black and white as shown in FIG. 6 is displayed for 12 hours, and then returned to a gray solid halftone. The halftone gradation is 64/256.

FIG. 7 shows the evaluation result of the DC afterimage. In FIG. 7, the horizontal axis represents the time after returning to a gray solid halftone. The vertical axis represents the afterimage level. On the vertical axis, RR is a state in which the checker flag pattern is clearly visible when the tone is returned to the halftone, and is NG. R is a state in which the checker flag pattern is thin but visible when the tone is returned to the halftone. In FIG.
A curve A is a DC afterimage characteristic when the alignment film according to the present invention is used. Curve B is an example of DC afterimage characteristics when only one photo-alignment film is used as the alignment film.

Even if the afterimage level is R when the halftone is restored, it can be said that there is no practical problem if it disappears in a short time. In the case of a single photo-alignment film, the level R at the time of returning to the halftone continues for a long time, so that there remains a practical problem. On the other hand, in the alignment film 113 having a two-layer structure according to the present invention, DC
After the afterimage rapidly decreases and returns to the halftone, the DC afterimage disappears completely in about 17 minutes.

Thus, the major difference between the case of the single layer of the photo-alignment film and the case of the alignment film of the present invention is that the photo-alignment film 1
In the case of the layer, the DC afterimage lasts for a long time, whereas when the alignment film of the present invention is used, the DC afterimage rapidly decreases. In FIG.
Comparing the level of DC afterimage 10 minutes after returning to halftone, which is a measure of DC afterimage,
When the alignment film is only one photo-alignment film, the DC afterimage is 90%, whereas the DC afterimage in the present invention is 25% or less, which shows that the effect of the present invention is very large.

By the way, in order to stabilize the alignment characteristics in the photo-alignment film 1131, it is better that the molecular weight of the photodegradable polymer 10 is large. However, when the molecular weight is large, the viscosity of the alignment film varnish increases. When the viscosity increases, flexographic printing and ink jet coating become difficult, so the concentration of the material is lowered to lower the viscosity. If it does so, a coating film will become thin. Therefore, the coating conditions for forming the alignment film 113 with only one layer of the photo-alignment film are limited.

In contrast, the present invention has the following advantages. That is, since the low resistance component arranged on the substrate side does not contribute to the alignment, the molecular weight can be lowered to the limit. Therefore, the tolerance for the adjustment of the concentration and viscosity of the alignment film varnish in which the polyamic acid alkyl ester or the polyamic acid is dissolved in the organic solvent is expanded, and the formation method of the alignment film 113 is not limited to the conventional flexographic printing, Application by ink jet, which has been considered difficult to increase the thickness of the alignment film 113 because it is necessary to reduce the viscosity, is also possible.

In Example 1, a film obtained by imidizing polyamic acid is used as the low resistance alignment film 1132 in the lower layer of the two alignment films 113. Since the lower resistance alignment film 1132 in the lower layer is not related to the photo-alignment, the material can be selected mainly for reducing the resistance of the alignment film 113.

In a liquid crystal display device, a backlight is disposed on the back of a liquid crystal display panel, and an image is formed by controlling light from the backlight for each pixel. As a backlight light source,
Cold cathode tubes and light emitting diodes are used. Therefore, the alignment film 113 is always exposed to light from the backlight while the liquid crystal display device is in operation. Underlying low resistance alignment film 113
If a photoconductive material is used as 2, the charge charged in the liquid crystal layer can be released more quickly, and the DC afterimage can be further reduced.

As a material in which the alignment film material exhibits photoconductive properties, for example, Nissan Chemical's SE6414 can be cited. Also in this case, when mixed with the polyamic acid alkyl ester and the mixed material is applied onto the substrate, the phases are separated to form a two-layer structure. Also in this case, the polyamic acid alkyl ester is the upper layer. Then, although imidation is performed by heating, this imidation can also be performed simultaneously.

In the above description, the alignment film 113 on the TFT substrate 100 side has been described.
The same applies to the alignment film 113 on the 0 side. The alignment film 113 on the counter substrate 200 side is formed on the overcoat film 203. In this case as well, the low resistance polymer 11 forming the low resistance alignment film 1132 is familiar with the overcoat film 203. The low resistance alignment film 1132 is formed on the overcoat film 203 side by leveling, and the photo alignment film 113 is formed thereon.
1 will be formed.

It should be noted that the two-layer alignment film 113 according to the present invention can be effective even when applied only to the alignment film 113 of the TFT substrate 100 or the counter substrate 200, for example. The reason why the charge in the liquid crystal layer is released is that a certain effect can be obtained from only one of the substrates.

As described above, according to the present invention, sufficient alignment regulation by photo-alignment can be stably performed, and the problem of DC afterimage due to the high resistivity of the photo-alignment film 1131 can be solved. .

Note that the pretilt angle of the liquid crystal display device to which the present invention is applied is 0.5 degrees or less according to the measurement using the crystal rotation method.

10 ... Photodegradable polymer, 11 ... Low resistance polymer, 15 ... Cut by ultraviolet rays,
DESCRIPTION OF SYMBOLS 100 ... TFT substrate 101 ... Gate electrode 102 ... Gate insulating film 103 ... Semiconductor layer 104 ... Source electrode 105 ... Drain electrode 106 ... Inorganic passivation film 107 ... Organic passivation film 108 ... Counter electrode 109 ... Upper insulating film, 110 ... pixel electrode, 111 ... through hole, 112 ... slit, 113 ... alignment film, 200 ... counter substrate, 201 ... color filter, 202 ... black matrix,
203 ... Overcoat film, 210 ... Surface conductive film, 300 ... Liquid crystal layer, 301 ... Liquid crystal molecule, 1131 ... Photo-alignment film, 1132 ... Low-resistance alignment film

Claims (8)

An alignment film varnish for a photo-alignment film of a liquid crystal display device comprising a first polymer, a second polymer, and an organic solvent,
The average molecular weight of the first polymer is greater than the average molecular weight of the second polymer;
The alignment film varnish is characterized in that the second polymer is a polyamic acid that is more compatible with ITO (Indium Tin Oxide) or acrylic resin than the first polymer.   The alignment film varnish according to claim 1, wherein the film formed by imidizing the first polymer has photodegradability.   The resistivity of the film formed by imidizing the second polymer is smaller than the resistivity of the film formed by imidizing the first polymer. Alternatively, the alignment film varnish according to 2.   4. The alignment film varnish according to claim 1, wherein the film formed by imidizing the second polymer has photoconductivity. 5. An alignment film varnish for a photo-alignment film of a liquid crystal display device comprising a first polymer, a second polymer, and an organic solvent,
The second polymer is a polyamic acid that is more compatible with ITO (Indium Tin Oxide) or acrylic resin than the first component,
The film formed by imidizing the first polymer has photodegradability,
An alignment film varnish characterized in that a film formed by imidizing the second polymer has photoconductivity.   The alignment film varnish according to claim 5, wherein an average molecular weight of the first polymer is larger than an average molecular weight of the second polymer.   6. The resistivity of the film formed by imidizing the second polymer is smaller than the resistivity of the film formed by imidizing the first polymer. Alternatively, the alignment film varnish according to 6.   The alignment film varnish according to any one of claims 1 to 7, wherein the alignment film varnish is for forming a photo alignment film of an IPS liquid crystal display device.

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