JPH11144537A - Method for forming transparent conductive film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transparent conductive film forming method by which a transparent conductive film of small specific resistance can be obtained using wet application method. SOLUTION: A dispersion liquid in which crystalline conductive oxide microparticles are dispersed in a solvent is applied on a substrate and a layer made of the crystalline conductive oxide microparticles is formed by heat treatment. An application liquid which forms a transparent conductive oxide film on being thermally decomposed is applied on the layer made of the crystalline conductive oxide microparticles and is baked. The crystalline conductive oxide microparticles have particle diameters of 5 to 100 nm converted in terms of specific surface area measured by BET method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a transparent conductive film.

[0002]

2. Description of the Related Art Transparent conductive materials (antimony-doped tin oxide (ATO), tin-doped indium oxide (ITO), etc.)
Is widely used as a thin film, particularly for display elements and surface heating elements. Above all, ITO is used in a very wide field because of its high transparency and conductivity, and most of them are manufactured by a dry film forming method such as a CVD method and a sputtering method. However, these methods require equipment costs and also have a problem in productivity. As a simpler method, there have been proposed many methods of applying a solution such as an indium salt and subjecting the solution to thermal decomposition to obtain a film ( For example, JP-A-5-314830).

However, the transparent conductive film obtained by this thermal decomposition method has a specific resistance about one digit or more higher than that of a deposited film manufactured by a sputtering method, a CVD method or the like. This is because 1) the density is low due to many voids formed by decomposition of organic substances in the film, and 2) the crystallinity of the film is low, that is, the mobility is low because the mean free path of electrons is narrow. The reason is that it does not increase.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is intended to form a transparent conductive film having a small specific resistance, particularly an ITO film, by a wet coating method. The purpose is to provide a method.

[0005]

According to the present invention, a layer comprising crystalline conductive oxide fine particles is formed on a substrate by applying a dispersion of crystalline conductive oxide fine particles dispersed in a solvent and subjecting the dispersion to heat treatment. Thereafter, a method for forming a transparent conductive film, comprising applying a coating solution on which a transparent conductive oxide film is formed by thermal decomposition onto the layer made of the crystalline conductive oxide fine particles, followed by firing. provide.

In the present invention, the use of crystalline conductive oxide fine particles is an important component in improving the crystallinity of a finally obtained transparent conductive film. In the present invention, the layer composed of the crystalline conductive oxide fine particles, which is the base, is not a continuous layer but a layer having a gap between the crystalline conductive oxide fine particles.

After forming a layer composed of fine particles of crystalline conductive oxide, a transparent conductive oxide film is formed on the layer by thermal decomposition, whereby a transparent conductive oxide film formed by thermal decomposition is formed. It is considered that the crystallinity of the resulting transparent conductive film is improved by the presence of the crystalline particles in the lower layer, and as a result, the crystallinity of the entire transparent conductive film finally obtained is increased, and the conductivity is increased.

[0008] The method for producing the crystalline conductive oxide fine particles is not particularly limited, but a wet synthesis method such as a coprecipitation method is preferable because a uniform fine powder is easily obtained.

[0009] The crystalline conductive oxide fine particles are dispersed in a solvent. A known method can be used for the dispersion method, and a ball mill,
Sand mills, jet mills, homogenizers and the like can be used.

As a dispersion medium, a solvent containing water as a main component is preferably used because a stable dispersion having a high dielectric constant is easily obtained. In particular, the crystalline conductive oxide fine particles are made of crystalline indium-tin oxide fine particles (hereinafter referred to as crystalline ITO).
Pulverization in a low pH region where the surface of the ITO particles is positively charged is preferable. In the case where a solvent composed of only water (100% water) may deteriorate the wettability to the substrate due to its high surface tension, a small amount of an organic solvent having a boiling point higher than that of water may be added. The organic solvent may be added at the time of dispersion or after dispersion.

The solid content concentration of the dispersion is appropriately determined according to the coating method, desired film thickness and the like. Usually, the ratio of the crystalline conductive oxide fine particles to the dispersion is preferably 0.1 to 50% by weight, particularly preferably 0.5 to 20% by weight.

The method of applying the dispersion to the substrate is not particularly limited, and any known method can be used. Dip coating, spin coating, spray coating, flow coating, die coating, roll coating, etc. , Transfer printing, screen printing and the like.

The size of the crystalline conductive oxide fine particles is determined from the viewpoint of the crystallinity of the finally obtained transparent conductive film.
The particle size calculated from the specific surface area by the BET method is 5-1.
It is preferably 00 nm. If the particle size exceeds the above range, the transparency of the finally obtained transparent conductive film may be impaired, and if the particle size is less than the above range, defects in the particles increase, resulting in crystallinity and conductivity. Tends to decrease. In particular, the particle diameter is preferably 5 to 50 nm.

When the crystalline conductive oxide fine particles are dispersed in the dispersion, the average dispersed particle diameter of the crystalline conductive oxide fine particles is preferably 500 nm or less. This is also from the viewpoint of the transparency, crystallinity, and conductivity of the finally obtained transparent conductive film. In particular, the thickness is preferably 200 nm or less. The average dispersed particle diameter can be usually measured by a centrifugal sedimentation method, a laser scattering method, or the like.

As for the conductivity of the crystalline conductive oxide fine particles, it is preferable that the green compact resistivity after applying a pressure of 100 kgf is 1 Ω · cm or less. Since it is difficult to measure the true specific resistance of the particles, it is usually measured as a green compact by a DC four-terminal method as a green compact. When the resistance of the particles exceeds 1 Ω · cm, the conductivity of the finally obtained transparent conductive film may be inferior. In particular, it is preferably 0.5 Ω · cm or less.

The layer composed of the fine particles of the crystalline conductive oxide and the transparent conductive oxide film formed by thermal decomposition on the layer are preferably the same oxide because good results can be obtained. .

As the material of the crystalline conductive oxide fine particles,
It is preferably a crystalline ITO fine particle. In addition, the crystalline ITO fine particles preferably have an indium / tin atomic ratio of 97.5 / 2.5 to 90/10 from the viewpoint of the crystallinity and conductivity of the finally obtained transparent conductive film. .

In the present invention, it is necessary to apply a dispersion of crystalline conductive oxide particles on a substrate and then remove the solvent by heating to fix the dispersion as a particle layer. The heating temperature is 100 ° C. or higher, particularly 12 ° C., at which the solvent can be completely removed.
0 ° C. or higher is preferred. On the other hand, practically, a temperature of 500 ° C. or less is preferable. Raising the temperature beyond this has little effect. The heating time depends on the temperature, but is usually 5 minutes to 24 hours.

A coating solution for forming a transparent conductive oxide film by thermal decomposition (hereinafter referred to as a coating solution for forming a transparent conductive oxide film)
It is preferable to use a coating liquid for forming a film made of indium oxide and tin oxide (hereinafter referred to as an ITO film forming coating liquid). In addition, the coating liquid for forming an ITO film has an atomic ratio of indium / tin in the coating liquid of 97.5.
/2.5 to 85/15, particularly preferably 95/5 to 90/10.

The indium compound in the coating solution for forming the ITO film is not particularly limited as long as it is a compound which can be converted into indium oxide by thermal decomposition. Inorganic salts such as indium nitrate, indium sulfate and indium chloride, and other organic acid salts can be used. Alkoxide and the like. Availability, price,
In view of solubility in various solvents, inorganic salts such as indium nitrate and indium chloride are preferable.

The tin compound in the coating solution for forming an ITO film is not particularly limited as long as it is a compound which can be converted into tin oxide by thermal decomposition.
Tin chloride, tin sulfate and the like. Oxalates and acetates are preferred from the viewpoints of solubility and reactivity.

A complexing agent may be added to the coating solution for forming a transparent conductive oxide film in order to improve thermal and chemical stability. As complexing agents, β diketones such as acetylacetone, β ketoesters such as oxobutanoate,
Examples include aminoethanols such as triethanolamine, polyhydric alcohols such as ethylene glycol, and polybasic acids such as oxalic acid. These complexing agents may be used alone or as a mixture of two or more.

The concentration of the coating liquid for forming a transparent conductive oxide film is appropriately determined according to the coating method, the desired film thickness and the like. Usually, the ratio of the compound for forming a transparent conductive oxide film to the coating solution (solid concentration in terms of oxide) is 0.1 to 30.
% By weight.

Known methods can be used for applying the coating liquid for forming a transparent conductive oxide film, such as dip coating, spin coating, spray coating, flow coating, die coating, roll coating, and the like. Transfer printing method,
Screen printing method and the like can be mentioned.

The firing temperature after the application of the coating liquid for forming a transparent conductive oxide film is preferably 350 ° C. or higher. If the temperature is lower than 350 ° C., the decomposition of the compound for forming a transparent conductive oxide film (for example, an indium compound or a tin compound) in the coating solution does not proceed, and an organic substance may remain in the film, so that sufficient conductivity may not be obtained. There is.

The thickness of the layer composed of the fine particles of crystalline conductive oxide (meaning the geometric thickness, the same applies hereinafter) is preferably 10 to 100 nm as the thickness after firing. 100n
If it exceeds m, transparency may be impaired, and if it is less than 10 nm, it is difficult to obtain the effect of the crystalline particles.

The thickness of the layer composed of the crystalline conductive oxide fine particles is determined by calculating the total thickness of the transparent conductive film obtained after applying and firing the coating liquid for forming the transparent conductive oxide film. The thickness is determined by subtracting the film thickness of the transparent conductive film obtained through the same process without forming a layer composed of oxide fine particles.

The thickness of the transparent conductive oxide film formed from the coating liquid for forming a transparent conductive oxide film is preferably from 5 to 300 nm as a thickness after firing. If it exceeds 300 nm, the transparency may be impaired, and the coating may be cracked due to shrinkage during firing. If it is less than 5 nm, it is difficult to obtain the desired conductivity.

The thickness of the finally obtained transparent conductive film is preferably from 15 to 400 nm after firing from the viewpoint of transparency and crack conductivity due to shrinkage.

As the substrate used in the present invention,
A glass substrate such as soda lime glass, non-alkali glass, and quartz glass, and a substrate having a heat resistance of 350 ° C. or more, such as ceramics, are preferable. When using a glass containing alkali such as soda lime glass as the substrate,
Since there is a possibility that problems such as a decrease in conductivity and devitrification due to elution of an alkali component from the substrate may occur,
It is preferable to keep the temperature at 0 ° C. or lower, or a silica coat or the like to prevent elution of the alkaline component. The firing time when soda-lime glass is used depends on the temperature, but is preferably 5 minutes to 5 hours.

The heat-treated film is preferably heat-treated in a non-oxidizing atmosphere in order to develop higher conductivity. The non-oxidizing atmosphere is an atmosphere containing no oxygen, and specifically means one or more gas atmospheres selected from the group consisting of nitrogen, an inert gas (eg, argon), hydrogen, and ammonia, and a vacuum. I do. In order to exhibit conductivity, it is preferable to use an inert gas (such as argon) atmosphere containing 3% by volume or more of hydrogen. The firing temperature is
300-500 ° C, particularly preferably 400-500 ° C.

As a preferred example of the present invention, a soda-lime glass substrate has a particle diameter calculated from a specific surface area by a BET method of 5 to 100 nm and an average dispersed particle diameter of 5
After applying a dispersion in which crystalline ITO fine particles having a size of 00 nm or less are dispersed in a solvent containing water as a main component, and performing a heat treatment to form a layer composed of the crystalline ITO fine particles,
A coating liquid for forming an ITO film is applied on the layer made of O fine particles, and then baked, and further, is applied in a non-oxidizing atmosphere.
The heat treatment is performed at a temperature of 00 to 500C.

[0033]

EXAMPLES The details of the present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

(Example 1) The powder obtained by the coprecipitation method was calcined at 700 ° C and then subjected to a reduction treatment.
ET-equivalent particle size 24 nm, green compact resistance 0.15 Ω · c
m) was added to 0.001 N aqueous nitric acid, pulverized by a sand mill using quartz beads, and the beads and coarse particles were separated by filtration to obtain a dispersion of crystalline ITO fine particles having a solid content of 6%. The average dispersed particle size of the crystalline ITO fine particles in the obtained dispersion was 110 nm.

To 50 g of this dispersion, 38 g of water, 10 g of butyl cellosolve and 2 g of N-methyl-2-pyrrolidone were added in this order to obtain dispersion (A). This dispersion is washed with a well-washed alkali-free glass (100 × 100 × 1.
1 mm) by spin coating,
By heating at 120 ° C. for 5 minutes and at 350 ° C. for 10 minutes, the crystalline I
A layer composed of TO fine particles (hereinafter simply referred to as an ITO particle layer) was formed.

Next, 10 g of indium nitrate trihydrate and 0.4 g of stannous oxalate were added to a 100 cc eggplant-shaped flask containing 14 g of ethylene glycol and 3.5 g of acetylacetone, and the mixture was heated at 75 ° C. for 3 hours to give a pale solution. A clear yellow solution was obtained. 17 g of methanol and 7 g of normal propanol were added to this solution, and the mixture was thoroughly stirred to obtain an ITO film-forming coating solution (B).

This coating solution was applied on the ITO particle layer by spin coating, formed into a film, dried at 60 ° C. for 10 minutes, and then baked in an electric furnace maintained at 500 ° C. for 1 hour. The thickness of the ITO particle layer is 15 nm, and the total thickness of the finally obtained ITO film (hereinafter simply referred to as ITO film) is 1 nm.
25 nm.

Thereafter, the ITO film is formed by adding 5% by volume of hydrogen.
The treatment was performed at 450 ° C. for 1 hour in an argon gas atmosphere containing the mixture. Table 1 shows the evaluation results.

Example 2 B as crystalline ITO fine particles
ET-equivalent particle diameter 59 nm, green compact resistance 0.01 Ω · c
m was performed in the same manner as in Example 1 except that fine particles of m (average dispersed particle diameter was 141 nm) were used. Table 1 shows the evaluation results of the obtained ITO films.

Example 3 B as crystalline ITO fine particles
ET-equivalent particle size 11 nm, green compact resistance 0.29 Ω · c
m was performed in the same manner as in Example 1, except that m fine particles (average dispersed particle diameter was 109 nm) were used. Table 1 shows the evaluation results of the obtained ITO films.

Example 4 The thickness of the ITO particle layer was 30 n.
Except having set it as m, it carried out similarly to Example 1. Table 1 shows the evaluation results of the obtained ITO films.

(Embodiment 5) The total thickness of the ITO film is 200 n
The procedure was performed in the same manner as in Example 1 except that the coating liquid (B) for forming an ITO film was applied so that m was obtained. Table 1 shows the evaluation results of the obtained ITO films.

Example 6 The procedure of Example 1 was repeated, except that the heating time at 120 ° C. after the application of the dispersion (A) was changed to 1 hour. IT obtained
Table 1 shows the evaluation results of the O film.

Comparative Example 1 The coating liquid (B) for forming an ITO film of Example 1 was directly applied to glass without forming an ITO particle layer, and treated in the same manner as in Example 1 (total film thickness was 100 n).
m). Table 1 shows the evaluation results of the obtained ITO films.

(Comparative Example 2) The total thickness of the ITO film was 125 n
m (same as in Example 1), except that the treatment was the same as in Comparative Example 1. Table 1 shows the evaluation results of the obtained ITO films.

(Comparative Example 3) After forming only the ITO particle layer through the same steps as in Example 1, it was treated at 450 ° C. for 1 hour in an argon gas atmosphere containing 5% by volume of hydrogen as in Example 1. . Table 1 shows the evaluation results of the obtained ITO films.

[0047]

[Table 1]

[0048]

According to the present invention, a transparent conductive film having low specific resistance and excellent transparency can be obtained at low cost by a simple method called wet coating.

Claims (9)

[Claims] 1. A method in which a dispersion of crystalline conductive oxide fine particles dispersed in a solvent is applied on a substrate and heat-treated to form a layer composed of the crystalline conductive oxide fine particles. A method for forming a transparent conductive film, which comprises applying a coating solution for forming a transparent conductive oxide film by thermal decomposition on a layer made of fine particles, and then firing. 2. A crystalline conductive oxide fine particle having a particle diameter calculated from a specific surface area by a BET method of 5 to 100 nm.
The method for forming a transparent conductive film according to claim 1, wherein the crystalline conductive oxide fine particles are used. 3. The method for forming a transparent conductive film according to claim 1, wherein a dispersion having an average dispersed particle size of the crystalline conductive oxide fine particles in the dispersion of 500 nm or less is used as the dispersion. 4. The method for forming a transparent conductive film according to claim 1, wherein crystalline indium-tin oxide fine particles are used as the crystalline conductive oxide fine particles. 5. The coating liquid according to claim 1, wherein a coating liquid for forming a film composed of indium oxide and tin oxide is used as the coating liquid for forming the transparent conductive oxide film by thermal decomposition. A method for forming a transparent conductive film. 6. The method for forming a transparent conductive film according to claim 1, wherein a solvent containing water as a main component is used as the solvent. 7. The heating temperature in the heat treatment is 100 to 50.
The method for forming a transparent conductive film according to any one of claims 1 to 6, wherein the temperature is set to 0 ° C. 8. The method for forming a transparent conductive film according to claim 1, wherein the temperature in the firing is 350 ° C. or higher. 9. The method for forming a transparent conductive film according to claim 1, wherein after the firing, a heat treatment is further performed at a temperature of 300 to 500 ° C. in a non-oxidizing atmosphere.