JPH0355523A - Active matrix type liquid crystal display panel - Google Patents

Abstract

PURPOSE:To satisfy both of the prevention of the deterioration in the off characteristics of thin-film transistors TFT and liquid crystal orientability by forming liquid crystal oriented films into at least two-layered structures of polyimide films which vary in at least any of structure, compsn. or imidization rate. CONSTITUTION:The liquid crystal oriented films formed on substrates provided with switching elements for each of picture elements are made into at least the two-layered structures of the polyimide films which vary in at least any of the structure, compsn. or imidization rate. Namely, the lower layer film near the switching elements is formed of the polyimide with which polarization hardly arises and which is low in polarity by forming the oriented films into the two-layered constitution, by which the shift of the TFT characteristics at the time of continuous driving is suppressed. The liquid crystal orientability is improved as well by providing the polyimide film having some polarity and the excellent liquid crystal orientability as the upper layer. The deterioration in the off characteristics of the TFTs is prevented in this way without using a vapor deposition process and the active type liquid crystal display panel having the good liquid crystal orientability is obtd.

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention relates to active matrix liquid crystal display panels used for image display, etc., and particularly relates to improved alignment films. Although highly reliable polyimide is exclusively used as a practical material, polyimide films are usually made by coating a polyamic acid film obtained by reacting a tetracarboxylic dianhydride component and a diamine component on a substrate and baking it at high temperatures. The polyimide obtained by forming a polyimide and generally used as an alignment film is a wholly aromatic polyimide. Recently, liquid crystal alignment films using non-aromatic tetracarboxylic dianhydrides have also been commercially available, but active matrix liquid crystals with switching elements such as thin film transistors (TPTs) and diodes installed in each pixel have recently become commercially available. Display panel {Yo) Even though this active matrix type liquid crystal display panel is applied to various display devices such as liquid crystal televisions and is provided with an alignment film, aromatic polyimide is used as a practical material as the alignment film material, as mentioned above. This invention is intended to solve the problem that the invention aims to solve, even though liquid crystal uniaxial alignment is imparted by a rubbing process, which is commonly used and in which the surface of an alignment film formed on a substrate is rubbed in a fixed direction with a soft cloth, etc. In active matrix liquid crystal panels, the TPT's OFF characteristics deteriorate during continuous use, which is a problem in terms of reliability. ○When the FF characteristics deteriorate, the leakage current flowing between the source and drain increases when the TPT is turned OFF. △ The effective voltage applied to the liquid crystal layer decreases (＼ This may appear as a deterioration of the displayed image.Although this phenomenon is also related to the alignment film formed on the TFT side substrate, in general, an inorganic vapor-deposited film such as silicon nitride is used. It is possible to prevent OFF deterioration of the TPT by forming it as a bashivation film on the TFT and then forming an alignment film on top of it. However, forming such a deposited film requires a vacuum evaporation device and the manufacturing process is complicated. The problem is that the manufacturing cost increases.Although the deterioration of TPT's OFF characteristics also depends on the molecular structure of the polyimide used as the alignment film, if the polyimide used is selected appropriately, the deterioration of the OFF characteristics can be prevented. However, when polyimide, which can suppress the deterioration of OFF characteristics, is used as an alignment film, there is a problem that the liquid crystal alignment deteriorates and the display contrast decreases. OF TPT without using
A method for solving this problem by providing an active matrix type liquid crystal display panel that prevents deterioration of F characteristics and has good liquid crystal alignment.A liquid crystal alignment film formed on a substrate with a switching element for each pixel. The polyimide film is formed into at least two layers with different structures or imidization ratios.

Function The present invention has an alignment film composed of at least two layers of polyimide.
The lower layer film close to the switching element suppresses shifts in TPT characteristics during continuous driving, and the upper layer film makes it possible to produce a panel with good liquid crystal orientation.

The polarization of the alignment film may be the cause of the deterioration of the OFF characteristics of TPT.The polarization mechanism may be due to charge injection from the polar groups present in the alignment film polyimide electrode. It is possible that polarization of the alignment film occurs due to the influence of the DC voltage component generated when driving the liquid crystal panel, and that the electric field generated as a result of this polarization shifts the characteristics of the TPT. Looking at the cause of this polarization from the perspective of molecular structure, it is thought that the polar groups contained in the molecular structure are oriented. In terms of structure, it is possible to suppress the shift in TPT characteristics during continuous driving by forming the lower layer near the switching element with a polyimide that does not cause such polarization, that is, has low polarity. Although the liquid crystal orientation is generally inferior in terms of liquid crystal orientation, the alignment of the liquid crystal by a polymer alignment film is achieved by the interaction between the liquid crystal and the polymer on the surface that has been stretched and oriented by rubbing, etc. Molecular field theory Coulomb force does not work between the liquid crystal and alignment film, and only dispersion force, which is weaker than Kronka's, works.

Therefore, it is thought that the liquid crystal alignment force is weak and good alignment cannot be obtained, but by providing a polyimide film with a certain degree of polarity and excellent liquid crystal alignment in the upper layer, it is possible to create a panel with good liquid crystal alignment. The polyimide used as the material for the water alignment film provided in the active matrix type liquid crystal display panel of the example has a structure of condensation polymerization of tetracarboxylic dianhydride and diamine, but in general polyimide, both of these are aromatic. Also, recently, products using non-aromatic tetracarboxylic dianhydride are commercially available for use in semiconductor passivation films and liquid crystal alignment films. (1) Polyimide has excellent properties such as printing flaw resistance, heat flaw resistance, liquid crystal alignment layer, and long-term reliability, and the advantages of using it as an alignment film material are significant. Or, it is characterized by a thick or at least two-layer structure in which at least one of the imidization ratios is different. In the Examples and Comparative Examples shown below, as an example of a switching element, An active matrix type full color liquid crystal panel with staggered structure amorphous silicon TPT is used, its size is 1.1 inches diagonally, 220 gate lines and 352 source lines.
In the book, the counter electrode is in the form of a full-surface electrode on the R, G, and B color filters, and the gap between the TFT side substrate and the counter substrate is filled with liquid crystal.

The liquid crystal display mode uses a negative type TN mode that blocks light when no voltage is applied. Figure 1 shows the equivalent circuit of one pixel of the liquid crystal panel used in the example and comparative example.
Figure 2 shows the drive voltage waveform of the gate line.The TPT is turned on and off by changing the gate voltage Vg.
- Charge the source signal VS to the liquid crystal and auxiliary capacitor.As shown in Figure 2, the TPT's ○N voltage is 14V, the OFF voltage is -7V, and the &TFT is designed to be turned off at 0■. The OFF'F voltage is set to -7■.If the setting allows for an OFF margin of 7V, but if the drive signal shown in Figure 2 is continuously applied, the OFF margin will decrease. In other words, although the voltage level at which the TPT can actually be turned off shifts to the negative side, it eventually becomes smaller than -7■, and the TPT cannot be turned off at the set OFF voltage level. If this happens, the charge charged when the TPT is turned on will not be retained, and the voltage applied to the liquid crystal will drop~＼ Normal display may no longer be possible, but this may be due to the deterioration of the TPT characteristics when the liquid crystal panel is continuously driven. The voltage level for turning on the TPT hardly changes due to continuous driving of the resistor.As a measure to investigate the deterioration of the OFF characteristic of the TPT, the liquid crystal panel with Vg(-) defined as follows is set to the maximum transmission state and L. , a line that changes the OFF voltage level of Vg from the negative side to the positive side. The lowest voltage value that does not affect the image is Vg (-). Vg (-) shifts to the negative side lA lower than -7V If this happens, the TPT cannot be completely turned off, and even if the brightness decreases, the shift in Vg (-) when the drive waveform shown in Figure 2 is continuously applied is related to the imidization rate. The higher the imidization rate, the smaller the shift amount.However, in polyimide made of aromatic diamine, when the imidization rate is lOO%, Vg(-) becomes smaller than -7V due to L continuous driving ( ＼ Even though there is a problem with reliability, the present invention will be explained in more detail by giving Examples and Comparative Examples below. Using wholly aromatic polyimide A,
The acid dianhydride component of polyimide A was added as a lower layer film on a substrate with TPT.
A film with an imidization rate of 95% (sintered at 250°C) was formed to a thickness of IOOOA by a printing method, and then a film of polyimide A with an imidization rate of 60% (sintered at 170°C) was formed on top of it.
was formed to a thickness of 1000A by a printing method, and subjected to alignment treatment by rubbing to produce the above liquid crystal panel.
The imidization rate of polyimide A is also 60 for the alignment film on the counter electrode.
% film was used, and Vg(-) was investigated when this liquid crystal panel was driven continuously at 60°C for 10 hours. Comparative Example 1 A film with an imidization rate of 60% of polyimide A was formed on a substrate with a TFT as an alignment film to a thickness of 1000A by a printing method. Table 1 shows Vg(-) and contrast when continuously driven at 60°C for 10 hours in the same manner as in Example 1. Comparative Example 2 Polyimide A was imidized on a substrate with TFT as an alignment film. Table 1 shows the Vg(-) and contrast when a film with a 95% yield was formed to a thickness of 1000A by a printing method and continuously driven at 60°C for 10 hours in the same manner as in Example 1.
Comparative Example 3: A film with a polyimide A imidization rate of 60% was formed to a thickness of 100 OA by a printing method on a TPT-equipped substrate as an alignment film, and then a polyimide A film with an imidization rate of 95% was printed on top of it. The above liquid crystal panel was fabricated by alignment treatment by rubbing, and a film with an imidization ratio of polyimide A of 60% was used for the alignment film on the counter electrode. This is because it is not possible to increase the baking temperature of the polyimide in which the filter is formed, and the imidization rate cannot be increased.Generally, the heat resistance temperature of color filters is 200℃ or less, so it is difficult to obtain a high imidization rate. (Similarly in Comparative Example 2, the imidization rate of the film on the counter electrode is 60%. 1. The contrast is very small for high force <.Vg(-), which indicates that the OFF characteristic deterioration of TPT is large.
Contrary to Comparative Example 1, the shift of Vg(-) is caused by a small force t.
Even though the contrast is low due to poor liquid crystal alignment, the panel with the two-layer alignment film structure of Example 1 has a G
Both Vg(-) and contrast showed good values.
In Comparative Example 3, which had a two-layer structure opposite to Example 1, even though the contrast was poor (&Vg(-)), the imidization rate of the alignment film in contact with the liquid crystal was different between the TFT-equipped substrate side and the counter substrate side. In the panels of Comparative Examples 2 and 3, which differ in (
An internal electric field is generated, which is thought to be due to the difference in the properties of the interface between the liquid crystal and the alignment film.This causes flicker and a decrease in brightness.Similar phenomena also occur when polyimides of different compositions are used. Hiroshi Alignment film in contact with liquid crystal + friend TFT
It is preferable to make the same imidization rate of the same magnet on the side of the attached substrate and the side of the counter substrate.
．． It is extremely difficult to satisfy the required characteristics with a single layer structure of alignment film, even though it is related not only to the deterioration of the OFF characteristics of TFTs but also to the asymmetric nature of the panel and the dielectric loss of negative liquid crystal panels. Inventive alignment film laminated structure (like
Whales are very effective in solving such problems.

These results show that although it is clear that the alignment film structure of Example 1 is superior, in Example 1, examples are shown for cases where the imidization rate is 60% and 95%. A film with a high imidization rate is provided in the lower layer, and a film with a low imidization rate is provided in the upper layer.
Although good properties were obtained, Example 2 Polyimide B consisting of an aromatic tetracarboxylic dianhydride component and a non-aromatic diamine component having the following structural formula was used as the alignment film.
Using the acid dianhydride component of polyimide B, the diamine of polyimide B is separated using IOO as a lower layer film on a substrate with TFT by a printing method.
A film of 60% imidization rate of polyimide A used in Example 1 was formed on the OA thickness by a printing method.
The above-mentioned liquid crystal panel was manufactured by performing alignment treatment by rubbing formed to OOA thickness, and the imidization rate of polyimide A was 60 in the alignment film on the counter electrode.
% film was used, and Vg(-) was investigated when this liquid crystal panel was driven continuously at 60°C for 10 hours. The photocurrent output ratio of the multi-briar was also measured and shown in Table 2. Comparative Example 4 A film with an imidization rate of 90% of polyimide B was formed to a thickness of 1000A by a printing method on a substrate with a TFT as an alignment film. Table 2 shows Vg(-) and contrast when t was continuously driven at 60°C for 10 hours in the same manner as in Example 2.
Example 3 A film with an imidization rate of 100% of polyimide C consisting of the following non-aromatic tetracarboxylic dianhydride component and aromatic diamine component was coated with IOOOA by a printing method on a substrate equipped with a TFT as an alignment film. Regarding the thickness, a film of 60% imidization rate of polyimide A used in Example 1 was formed on top of the acid dianhydride component of polyimide C to a thickness of 100 OA using a printing method. Table 2 shows Vg(-) and contrast when continuously driven at 60°C for 10 hours in the same manner as t. +'i'rFT OFF in Comparative Example 4
However, in this case, the single-layer structure does not have sufficient liquid crystal conformation and the contrast is low (~ By using a two-layer structure as in Example 2, TPT OFF
Prevention of property deterioration Even if the liquid crystal orientation is satisfied, it is also the same in Example 3, but it should be noted here (Above) In Example 2, polyimide B containing a non-aromatic diamine component was used as the lower layer. When used in T
The degree of deterioration in the OFF characteristics of PT is particularly small compared to Examples 1 and 3. From these results, it is clear that the use of polyimide containing a non-aromatic diamine component in the lower layer film is &TPT is found to be particularly effective in preventing deterioration of the OFF characteristics.Here, as a specific example, we will show an example in which the non-aromatic diamine component contains an alicyclic diamine, particularly a cyclohexane ring. Similarly, TP can be applied to alicyclic diamines other than rings or chain aliphatic diamines.
Polyimide for alignment films is often copolymerized with non-aromatic diamines containing silicon to improve adhesion to the substrate. Since such diamines are also non-aromatic, they are effective in preventing deterioration of the OFF characteristics of TPT.Also, depending on the purpose, copolymerization or mixing with other substances is possible, but they are non-aromatic. When group diamines are used, the glass transition temperature generally tends to decrease, and a high imidization rate can be achieved by low-temperature annealing. If a polyimide containing a non-aromatic diamine is used as an alignment film and the L imidation rate is low, the effect of preventing the deterioration of TPT's OFF characteristics cannot be said to be sufficient. Also from the viewpoint of stability of liquid crystal alignment, etc. It is preferable to use it at an imidization rate of at least 50%.Furthermore, it is also possible to provide a third layer for the purpose of improving adhesion, etc., and the effects of the present invention can also be achieved in that case. 《The effects of the invention are not impaired in any way.《The active matrix type liquid crystal display panel of the present invention
The liquid crystal alignment film formed on the substrate provided with a switching element for each pixel has a polyimide film Q having at least two layers with different configurations or imidization rates.& TPT OFF It is possible to provide an active matrix liquid crystal display panel that prevents characteristic deterioration and has good liquid crystal coordination.

[Brief explanation of drawings]

Figure 1 shows the equivalent circuit of one pixel of the liquid crystal panel used in the example, and Figure 2 shows the pulse application state of the gate line drive voltage waveform Vg.

Claims (3)

[Claims] (1) An active liquid crystal alignment film formed on a substrate provided with a switching element for each pixel is composed of at least two layers of polyimide films having different structures, compositions, or imidization rates. Matrix type liquid crystal display panel. (2) The active matrix liquid crystal display panel according to claim 1, wherein the lower layer film close to the switching element has a higher imidization rate than the upper layer film. (3) The lower layer film close to the switching element is formed of polyimide containing a non-aromatic diamine as a diamine component which is one of the structural units of polyimide. active matrix type liquid crystal display panel.