JP7640626B2 - Copper-containing silver powder, conductive paste, conductive film, and solar cell - Google Patents

Description

The present invention relates to copper-containing silver powder, conductive paste, conductive film, and solar cell.

A method of forming a conductive film such as an electrode or electrical wiring by applying or printing a conductive paste containing conductive metal powder onto a substrate such as a film, board, or electronic component, and then heating it to dry, harden, or bake it has been widely used for some time. However, with the increasing performance of electronic devices in recent years, conductive films formed using conductive pastes are required to have lower resistance, and this requirement is becoming stricter every year.

In response to the above demands, for example, Patent Document 1 proposes the use of a silver powder consisting of flaky silver powder and spherical silver powder, in which a polyvalent carboxylic acid is attached to the surface of at least one of the flaky silver powder and the spherical silver powder, as a conductive metal powder, in order to obtain a conductive paste composition having high conductivity (low resistance).

JP 2011-100573 A

Here, the silver powder (silver powder) that can be used in conventional conductive pastes has room for further improvement in terms of reducing the line resistance of the conductive film formed using the conductive paste.

Therefore, an object of the present invention is to provide a copper-containing silver powder capable of reducing the line resistance of a conductive film.
Another object of the present invention is to provide a conductive paste capable of reducing the line resistance of a conductive film.
Another object of the present invention is to provide a conductive film with reduced line resistance.
Another object of the present invention is to provide a solar cell having excellent performance.

As a result of extensive research into solving the above problems, the inventors have completed the present invention, which is described below.

In other words, the gist of the present invention to solve the above problems is as follows:

[1] A copper-containing silver powder,
In the differential curve of the TMA curve from 100 ° C. to 600 ° C. obtained by thermomechanical analysis in which the copper-containing silver powder is heated at a heating rate of 10 ° C. / min,
A copper-containing silver powder having an expansion peak and a first shrinkage peak having a dTMA of -0.10%/min or less at a temperature lower than the expansion peak.

[2] The copper-containing silver powder described in [1] has a second shrinkage peak in the differential curve, with a dTMA of -0.30%/min or less, between the temperature of the expansion peak and 300°C.

[3] The copper-containing silver powder according to [1] or [2], in which the differential curve between 300°C and 600°C meanders.

[4] A conductive paste comprising the copper-containing silver powder described in [1] or [2], aluminum, and a solvent.

[5] A conductive film formed using the conductive paste described in [4].

[6] A solar cell having the conductive film described in [5].

According to the present invention, it is possible to provide a copper-containing silver powder capable of reducing the line resistance of a conductive film.
Moreover, according to the present invention, it is possible to provide a conductive paste capable of reducing the line resistance of a conductive film.
Moreover, according to the present invention, it is possible to provide a conductive film with reduced line resistance.
Furthermore, according to the present invention, a solar cell having excellent performance can be provided.

1 is a scanning electron microscope photograph of a cross section of the copper-containing silver powder according to Example 1. 1 is a dTMA curve of the copper-containing silver powder according to Example 1. 4 is a dTMA curve of the copper-containing silver powder according to Example 2. 4 is a dTMA curve of the copper-containing silver powder according to Example 3. 4 is a dTMA curve of the silver powder of Comparative Example 1. 4 is a dTMA curve of the mixed silver powder of Comparative Example 2.

The copper-containing silver powder of the present invention is suitable for use as a conductive filler for conductive paste. A conductive paste using the copper-containing silver powder of the present invention can be used to form a conductive pattern on a substrate or to form an electrode. A conductive paste using the copper-containing silver powder of the present invention can be printed on a substrate by, for example, screen printing, offset printing, photolithography, or the like to form a conductive film such as a conductive pattern or an electrode.

In recent years, in solar cells (particularly n-type solar cells) that have a silicon substrate and a conductive film such as electrodes and electrical wiring formed on the surface of the silicon substrate, attempts have been made to add aluminum, such as aluminum powder, to the conductive paste used to form the conductive film in order to reduce the contact resistance (interface resistance) at the interface between the semiconductor layer and the electrodes, etc. However, when aluminum is added to the conductive paste, the bulk resistance of the formed conductive film increases, which can result in a problem that the line resistance of the conductive film may increase. The copper-containing silver powder of the present invention can reduce the line resistance of the conductive film, and therefore is particularly suitable for use in solar cells, specifically, in aluminum-containing conductive pastes used to form the conductive film provided in solar cells.

(Terminology and Measurement Methods)
Prior to describing the embodiments, the terms and measurement methods used in this specification will be described.

<Thermo-mechanical analysis of copper-containing silver powder>
Thermomechanical analysis (TMA) of the copper-containing silver powder was carried out as follows.
First, 0.3 g of copper-containing silver powder was weighed. Next, the copper-containing silver powder was put into a predetermined mold with a diameter of 5 mmφ, and pressed with a press machine for 1 min under a load of 50 kg to prepare a disk-shaped measurement sample. This measurement sample was set in a sample holder of a thermomechanical analysis (TMA) device (Thermo plus EVO 2 series TMA8311), and a measurement load of 98 mN was applied by a measurement probe, and the temperature was raised from 25 ° C to 900 ° C at a heating rate of 10 ° C / min to measure the expansion rate and contraction rate of the measurement sample. The following evaluation was performed on the measurement results from 100 ° C to 600 ° C.
The expansion rate and contraction rate, as well as the temperature at that time, were recorded every second from the start of the measurement (heating), and a TMA curve was obtained. The obtained TMA curve was differentiated with respect to time to obtain a differential curve of the TMA curve (hereinafter, the differential curve of the TMA curve may be referred to as a "dTMA curve"). Then, using the obtained dTMA curve, the temperature and dTMA of the expansion peak, the temperature and dTMA of the first contraction peak, the temperature and dTMA of the second contraction peak, and the presence or absence of meandering of the dTMA curve between 300°C and 600°C were confirmed. The first contraction peak, the expansion peak, and the second contraction peak are present between 100°C and 300°C.
In this specification, the term "expansion peak" refers to a peak having a dTMA of 0.1%/min or more among the upwardly convex peaks in the dTMA curve, and is usually the peak having the maximum dTMA in the dTMA curve.
In this specification, the term "wandering" of a dTMA curve refers to a state in which the dTMA curve has multiple valleys or peaks, rather than having 0 to 1 valley (a downwardly convex curve) or peak (an upwardly convex curve).

<Confirmation of the presence or absence of voids inside silver particles in copper-containing silver powder>
In this specification, the presence or absence of voids was confirmed by observing silver particles using the following method.
First, the copper-containing silver powder was put into a resin and a hardener, and the resin was solidified. The solidified resin was then cut. The cut surfaces were then polished with a cross-section polisher to expose the cross sections of the silver particles. Each silver particle was then observed with a scanning electron microscope to confirm the presence or absence of voids inside the silver particles.
As the resin and hardener, for example, "Epofix Resin" and "Epofix Hardener" manufactured by Struers can be used. As the cross-section polisher, for example, "ArBlade5000" manufactured by Japan High-Technologies Corporation can be used. As the scanning electron microscope, for example, "JSM-IT800SHL" manufactured by JEOL Ltd. can be used.
In this specification, the "voids inside the silver particles" do not refer to the spaces created by the irregularities on the surface of the silver particles, but rather to the closed spaces inside the silver particles.
In this specification, silver particles having voids therein may be referred to as "void-containing silver particles".

As used herein, "void diameter (i.e., diameter of the enclosed space inside a void-containing silver particle)" refers to the diameter of the smallest circle drawn in a scanning electron microscope image that will contain all of the voids.
In this specification, the term "void" refers to a void having a diameter of 15 nm or more as observed in an image of the cross section of a silver particle photographed at 10,000 to 40,000 magnifications using the above method.

<Measurement of copper content in copper-containing silver powder>
The amount of copper in the copper-containing silver powder was determined by the following analytical method.
1) 1 g of a sample was precisely weighed, and 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added thereto, followed by heating at 200° C. for 30 minutes.
2) After allowing the sample heated in 1) to cool, the volume was adjusted to 100 mL using pure water, 5 mL of the supernatant was removed, and the volume was adjusted again to 100 mL using pure water to prepare a sample for ICP analysis.
3) The copper standard solution used to create the calibration curve was prepared by adjusting the silver concentration using 5N silver to the same level as the sample for ICP analysis.
4) ICP quantification was performed using Agilent 5800 ICP-OES manufactured by Agilent Technologies.

<Carbon content, oxygen content, and nitrogen content in copper-containing silver powder>
In this specification, the "carbon content" was measured using a carbon/sulfur analyzer (EMIA-810W manufactured by Horiba, Ltd.) to measure the amount of carbon in an oxygen stream at 1350° C. The "oxygen content" and "nitrogen content" were measured using an oxygen/nitrogen/hydrogen simultaneous analyzer (ONH836 manufactured by LECO) to measure the amounts of oxygen and nitrogen in an Ar gas atmosphere with an impulse furnace power value of 3500 W.

<True density of copper-containing silver powder>
In this specification, the "true density of the copper-containing silver powder" means the density of the copper-containing silver powder taking into account the voids present inside the silver particles (i.e., the above-mentioned "closed spaces inside the silver particles").
In this specification, the true density of the copper-containing silver powder was measured by a dry automatic density meter (also called a true density measuring device) using helium gas. Specifically, the volume of the copper-containing silver powder was measured from the gas volume when the container was filled with helium gas until the pressure inside the container reached a certain level, and the mass of the copper-containing silver powder was calculated by dividing the mass by the volume. Note that the helium gas at this time can reach the uneven spaces on the particle surface and the spaces between the particles, but cannot reach the closed spaces (i.e., voids) inside the particles that are not connected to the outside. Therefore, usually, the higher the proportion of voids inside the silver particles, the lower the true density of the copper-containing silver powder tends to be.

<Average diameter (SEM average diameter)>
The average diameter (SEM average diameter) was determined by measuring the Heywood diameter of 100 or more particles whose outer shapes could be recognized in a 5,000x SEM image using image processing software Mac-View (Ver. 4) manufactured by MOUNTECH, and taking the average value of the Heywood diameters.

<BET specific surface area>
In this specification, the "BET specific surface area" was measured by a BET single point method using nitrogen adsorption with a Macsorb HM-model 1210 (manufactured by MOUNTECH Co., Ltd.) In the measurement of the BET specific surface area, the degassing conditions before the measurement were 60°C and 10 minutes.

<Particle size distribution>
In this specification, the volume-based minimum particle size (D _MIN ), cumulative 10% particle size (D ₁₀ ), cumulative 50% particle size (D ₅₀ ), cumulative 90% particle size (D _{90 ), cumulative 95% particle size (D 95 )} , and cumulative 100% (i.e., maximum particle) particle size (D _MAX ) of the copper-containing silver powder were measured using a laser diffraction/scattering type particle size distribution measuring device (Microtrac MT-3300 EXII, manufactured by Microtrac-Bell Corporation).

<Measurement of copper content in conductive paste>
The amount of copper in the conductive paste was determined by the following analytical method.
1) 1 g of the paste was precisely weighed, washed with acetone, and then dried to obtain a sample.
2) 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added to the dried sample, and the sample was heated at 200° C. for 30 minutes.
3) After allowing the sample heated in 2) to cool, the volume was adjusted to 100 mL using pure water, 5 mL of the supernatant was removed, and the volume was adjusted again to 100 mL using pure water to prepare a sample for ICP analysis.
4) The copper standard solution used to create the calibration curve was prepared by adjusting the silver concentration using 5N silver to the same level as the sample for ICP analysis.
5) ICP volumetric analysis was performed using an Agilent 5800 ICP-ES manufactured by Agilent Technologies.

<Measurement of silver content in conductive paste>
The amount of silver in the conductive paste was determined by the following analytical method.
1) 1 g of the paste was precisely weighed, washed with acetone, and then dried to obtain a sample.
2) 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added to the dried sample, and the sample was heated at 200° C. for 30 minutes.
3) 15 mL of pure water and 10 mL of hydrochloric acid (for precision analysis) were added to the sample heated in 2), and the mixture was heated at 150° C. for 30 minutes.
4) The silver chloride produced in 3) was ripened for 12 hours or more.
5) The silver chloride was filtered through a precisely weighed glass filter and washed with pure water.
6) The glass filter containing silver chloride was dried in a dryer at 120° C. for about 3 hours until it reached a constant weight.
7) After cooling in the desiccator for at least 1 hour, the glass filter containing silver chloride was precisely weighed, and the silver content was calculated from the mass of silver chloride produced.

(Copper-containing silver powder)
The copper-containing silver powder of the present invention has an expansion peak and a first contraction peak on the lower temperature side of the expansion peak, the dTMA of which is -0.10%/min or less (absolute value of 0.10%/min or more in the negative direction) in a dTMA curve from 100°C to 600°C obtained by thermomechanical analysis in which the copper-containing silver powder is heated at a heating rate of 10°C/min.
The above-mentioned copper-containing silver powder can reduce the line resistance of the conductive film. In particular, the effect of reducing bulk resistance is remarkable in the aluminum-containing conductive paste. The reason for this is that the conductive film formed using the conductive paste is usually a film formed by applying or printing the conductive paste and firing it. When the copper-containing silver powder used in the conductive paste shrinks at a certain rate at a temperature before it expands, gaps are formed between the silver particles, and the presence of these gaps makes it easier for gas components in the gaps to escape outside the conductive paste, so that the excessive expansion of the conductive paste itself due to subsequent firing is effectively suppressed, and the resulting conductive film becomes dense, which is presumably why the bulk resistance of the conductive film can be effectively reduced.

In the dTMA curve, the dTMA of the first shrinkage peak is −0.10%/min or less, preferably −0.30%/min or less, more preferably −0.50%/min or less, and even more preferably −0.80%/min or less.
If the dTMA of the first shrinkage peak is −0.10%/min or less, the line resistance of the conductive film can be effectively reduced.
On the other hand, the dTMA of the first shrinkage peak is, for example, −3.00%/min or more, may be −2.00%/min or more, or may be −1.50%/min or more.

In the dTMA curve, the temperature of the first contraction peak is not particularly limited as long as it is lower than the temperature of the expansion peak, but may be, for example, 150°C or higher, 170°C or higher, or 180°C or higher, or, for example, 210°C or lower, 200°C or lower, 195°C or lower, or 190°C or lower.

In the dTMA curve, the dTMA of the expansion peak is 0.1%/min or more, preferably 0.50%/min or more, more preferably 1.00%/min or more, even more preferably 2.00%/min or more, and even more preferably 3.00%/min or more.
If the dTMA of the expansion peak is 0.1%/min or more, the line resistance of the conductive film can be effectively reduced.
On the other hand, the dTMA of the expansion peak is, for example, 10.00%/min or less, and may be 5.00%/min or less.

In the dTMA curve, the temperature of the expansion peak is not particularly limited as long as it is higher than the temperature of the first contraction peak, but may be, for example, 200°C or higher, or 210°C or higher, or may be, for example, 270°C or lower, or 250°C or lower.

The difference between the dTMA of the expansion peak and the dTMA of the first shrinkage peak ("dTMA of the expansion peak" - "dTMA of the first shrinkage peak") is not particularly limited as long as each dTMA satisfies the above requirements, but is preferably 0.50%/min or more, more preferably 2.00%/min or more, and even more preferably 3.50%/min or more.
If the difference between the dTMA of the expansion peak and the dTMA of the first contraction peak is 0.50%/min or more, the line resistance of the conductive film can be effectively reduced.
On the other hand, the difference between the dTMA of the expansion peak and the dTMA of the first contraction peak is, for example, 10.00%/min or less, and may be 7.00%/min or less.

The difference between the temperature of the expansion peak and the temperature of the first shrinkage peak ("temperature of the expansion peak" - "temperature of the first shrinkage peak") is not particularly limited as long as it satisfies the relationship of "temperature of the expansion peak">"temperature of the shrinkage peak", but is preferably 3°C or more, more preferably 10°C or more, even more preferably 20°C or more, and even more preferably 30°C or more.
If the difference between the temperature of the expansion peak and the temperature of the first contraction peak is 3° C. or more, the line resistance of the conductive film can be effectively reduced.
On the other hand, the difference between the temperature of the expansion peak and the temperature of the first contraction peak is, for example, 70° C. or less, and may be 50° C. or less.

The copper-containing silver powder of the present invention preferably has a second shrinkage peak in the dTMA curve, with dTMA of -0.30%/min or less (absolute value of 0.30%/min or more in the negative direction from zero) between the temperature of the expansion peak and 300° C. If the copper-containing silver powder has a second shrinkage peak, the line resistance of the conductive film can be effectively reduced.
Here, the dTMA of the second shrinkage peak is not particularly limited as long as it is −0.30%/min or less, but is preferably −0.50%/min or less.
On the other hand, the dTMA of the second shrinkage peak is, for example, −3.00%/min or more, may be −2.00%/min or more, or may be −1.50%/min or more.

The difference between the dTMA of the expansion peak and the dTMA of the second contraction peak ("dTMA of the expansion peak" - "dTMA of the second contraction peak") is not particularly limited as long as each dTMA satisfies the above requirements, but is preferably 0.50%/min or more, more preferably 2.00%/min or more, and even more preferably 3.50%/min or more.
If the difference between the dTMA of the expansion peak and the dTMA of the second contraction peak is 0.50%/min or more, the line resistance of the conductive film can be effectively reduced.
On the other hand, the difference between the dTMA of the expansion peak and the dTMA of the second contraction peak is, for example, 10.00% or less, and may be 7.00%/min or less.

The copper-containing silver powder of the present invention preferably has a meandering dTMA curve between 300°C and 600°C. Such copper-containing silver powder can effectively reduce the line resistance of the conductive film. In conventional silver powders, after the above-mentioned contraction peak or expansion peak occurs (at 300°C or higher), only gentle valleys and peaks appear in the dTMA curve, but the copper-containing silver powder of the present invention has a meandering dTMA curve at 300°C or higher. In the conductive paste, glass softens at 500 to 600°C and aluminum powder can melt at 500 to 650°C, so it is thought that an interaction occurs when the copper-containing silver powder, which has a meandering dTMA curve at temperatures of 300°C or higher, coexists with glass and aluminum powder in the conductive paste.

The copper content in the copper-containing silver powder means the mass ratio of copper to the total mass of silver and copper in the copper-containing silver powder. The copper content in the copper-containing silver powder is preferably 10 ppm or more, more preferably 30 ppm or more, and is preferably 10,000 ppm or less, more preferably 3,000 ppm or less, even more preferably 1,000 ppm or less, and even more preferably 100 ppm or less.
When the copper content in the copper-containing silver powder is 10 ppm or more, the bulk resistance of the conductive film can be effectively reduced.
On the other hand, if the copper content in the copper-containing silver powder is 10,000 ppm or less, it is possible to relatively increase the silver content in the copper-containing silver powder, and as a result, the line resistance of the conductive film can be effectively reduced.

The carbon content in the copper-containing silver powder means the mass ratio of carbon to the total mass in the copper-containing silver powder. The carbon content in the copper-containing silver powder is, for example, 0.06 mass% or more, may be 0.2 mass% or more, or may be 0.3 mass% or more, for example, 1 mass% or less, may be 0.7 mass% or less, or may be 0.5 mass% or less.

The oxygen content in the copper-containing silver powder means the mass ratio of oxygen to the total mass in the copper-containing silver powder. The oxygen content in the copper-containing silver powder is, for example, 0.11 mass% or more, or may be 0.2 mass% or more, or 0.3 mass% or more, for example, 1 mass% or less, or 0.8 mass% or less, or 0.6 mass% or less.

The nitrogen content in the copper-containing silver powder means the mass ratio of nitrogen to the total mass of the copper-containing silver powder. The nitrogen content in the copper-containing silver powder is, for example, 0.01 mass% or more, or may be 0.06 mass% or more, or 0.09 mass% or more, or, for example, 0.5 mass% or less, or 0.3 mass% or less, or 0.2 mass% or less.

The true density of the copper-containing silver powder is preferably 10 g/cm ³ or less, more preferably 9.8 g/cm ³ or less, and even more preferably 9.7 g/cm ³ or less.
If the true density of the copper-containing silver powder is 10 g/cm ³ or less, the line resistance of the conductive film can be effectively reduced.
On the other hand, the true density of the copper-containing silver powder is, for example, 9 g/cm ³ or more, or may be 9.2 g/cm ³ or more, or may be 9.4 g/cm ³ or more.

The minimum particle size (D _MIN ) of the copper-containing silver powder on a volume basis is, for example, 0.05 μm or more, or may be 0.1 μm or more, or 0.2 μm or more, and may be, for example, 1 μm or less, 0.7 μm or less, or 0.5 μm or less.

The volume-based cumulative 10% particle diameter (D ₁₀ ) of the copper-containing silver powder is, for example, 0.1 μm or more, or may be 0.5 μm or more, or 0.8 μm or more, and may be, for example, 2 μm or less, 1.5 μm or less, or 1.2 μm or less.

The volume-based cumulative 50% particle diameter ( _D50 ) of the copper-containing silver powder is preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 1.5 μm or more, and is preferably 6 μm or less, more preferably 2.5 μm or less, and even more preferably 1.9 μm or less.
If _D50 is within the above range, the line resistance of the conductive film can be effectively reduced.

The volume-based cumulative 90% particle diameter (D ₉₀ ) of the copper-containing silver powder is, for example, 1.5 μm or more, or may be 2 μm or more, or 2.5 μm or more, and may be, for example, 5 μm or less, 4 μm or less, or 3.5 μm or less.

The volume-based cumulative 95% particle diameter (D ₉₅ ) of the copper-containing silver powder is, for example, 2 μm or more, or may be 2.3 μm or more, or 3 μm or more, and may be, for example, 10 μm or less, 7 μm or less, or 4 μm or less.

The particle diameter (D _MAX ) of 100% cumulative (i.e., the largest particle) based on volume of the copper-containing silver powder is, for example, 3 μm or more, or 4 μm or more, or 5 μm or more, for example, 15 μm or less, 10 μm or less, or 8 μm or less.

The BET specific surface area of the copper-containing silver powder is preferably 0.1 m ² /g or more, more preferably 0.3 m ² /g or more, and even more preferably 0.45 m ² /g or more, and is preferably 1 m ² /g or less, more preferably 0.8 m ² /g or less, and even more preferably 0.6 m ² /g or less.
When the BET specific surface area of the copper-containing silver powder is within the above range, the line resistance of the conductive film can be effectively reduced.

In one embodiment, the copper-containing silver powder of the present invention contains void-containing silver particles, and may optionally contain silver particles that do not have internal voids. In addition, when the copper-containing silver powder of the present invention is observed in cross section, it is preferable that the void-containing silver particles account for at least half of the total silver particles.

<Porous Silver Particles>
The void-containing silver particles that can be contained in the copper-containing silver powder of the present invention are silver particles having voids therein, and preferably contain copper.
Incidentally, as a method for confirming that the void-containing silver particles contain copper, for example, EDS, XPS, or the like can be used.

In void-containing silver particles, the average void diameter is determined by measuring the diameter of voids of 15 nm or more contained in at least 10 silver particle cross sections in the particle cross section images at 10,000 to 40,000 times magnification described above. The average void diameter is, for example, 30 nm or more, or may be 50 nm or more, or 100 nm or more, or, for example, 500 nm or less, or 300 nm or less, or 200 nm or less.

When the void-containing silver particles contain copper, the content of silver particles made of silver is preferably small from the viewpoint of suppressing uneven distribution of copper in the conductive paste. The content of silver particles made of silver in the copper-containing silver powder means the mass ratio of silver particles made of silver to the total mass of the copper-containing silver powder. The content of silver particles made of silver in the copper-containing silver powder is, for example, 50 mass% or less, may be 20 mass% or less, may be 10 mass% or less, or may be 0 mass%, i.e., the copper-containing silver powder may not contain silver particles made of silver.

<Method of manufacturing copper-containing silver powder>
The copper-containing silver powder of the present invention can be produced, without being limited to any particular method, by using a method including, for example, a reduction step in which a reducing agent is added to a mixed solution containing a silver compound and the above-mentioned copper-containing compound (e.g., a copper compound, etc.) to precipitate void-containing silver particles, and a separation step in which the void-containing silver particles are separated from the mixed solution and dried to obtain the copper-containing silver powder.
In addition, in addition to the reduction step and the separation step, the above-mentioned manufacturing method may further include a surface treatment agent addition step in which a surface treatment agent is added to the mixed liquid containing the void-containing silver particles obtained in the reduction step to obtain surface-treated void-containing silver particles.

Below, an example of a method for producing copper-containing silver powder is described, but the method for producing copper-containing silver powder is not limited to this.

[Reduction step]
In the reduction step, a reducing agent is added to a mixture containing a silver compound and a copper compound to precipitate void-containing silver particles. The mixture is usually an aqueous solution or suspension containing a silver compound and a copper compound, and contains water as a solvent. The mixture containing a silver compound and a copper compound may optionally contain a pH adjuster or the like.
The mixture obtained after the void-containing silver particles have been precipitated is usually a suspension (so-called slurry) or dispersion in which the void-containing silver particles are dispersed.

The reducing agent added to the mixture in the reduction step preferably contains a compound having an aldehyde group. By using a reducing agent containing a compound having an aldehyde group, void-containing silver particles can be obtained efficiently. Examples of such reducing agents include an aqueous formaldehyde solution (so-called formalin) and an aqueous acetaldehyde solution, but it is preferable to use formalin because it allows void-containing silver particles to be obtained more efficiently. In addition, from the viewpoint of effectively generating voids, ultrasound or the like may be used during reduction and precipitation.

The silver compound is not particularly limited, and examples thereof include silver nitrate and silver oxide. In addition, it is preferable to use, as the silver compound, a silver ammine complex obtained by reacting silver nitrate or silver oxide with ammonia water or an ammonium salt.

The copper compound is not particularly limited, and examples thereof include copper(II) nitrate trihydrate and copper oxide. It is also preferable to use, as the copper compound, a copper ammine complex obtained by reacting copper(II) nitrate trihydrate or copper oxide with ammonia water or an ammonium salt.

In the mixed solution, the ratio of copper to silver is preferably 0.001 mass% or more, preferably 0.003 mass% or more, preferably 3 mass% or less, more preferably 1 mass% or less, even more preferably 0.1 mass% or less, and even more preferably 0.01 mass% or less.
If the ratio of copper to silver is 0.001 mass % or more, the bulk resistance of the conductive film can be effectively reduced.
On the other hand, if the ratio of copper to silver is 3 mass % or less, it is possible to relatively increase the ratio of silver, and as a result, the line resistance of the conductive film can be effectively reduced.

The mixed solution may contain a pH adjuster such as a common acid or base, for example, nitric acid or sodium hydroxide. In the reduction step, it is preferable to adjust the mixed solution to an alkaline state.

[Surface treatment agent addition step]
In the optional surface treatment agent addition step, a surface treatment agent is added to the mixed liquid containing the void-containing silver particles obtained in the reduction step to obtain surface-treated void-containing silver particles (hereinafter sometimes referred to as "surface-treated void-containing silver particles").
The mixed liquid containing the surface-treated void-containing silver particles is usually a suspension (so-called slurry) or a dispersion in which the surface-treated void-containing silver particles are dispersed.

Examples of surface treatment agents include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, linolenic acid, benzotriazole, etc. These may be used alone or in combination of two or more.

The amount of surface treatment agent added in the surface treatment agent addition process is usually 0.05% by mass or more and 0.5% by mass or less relative to the mass of silver contained in the mixed solution obtained in the reduction process.

The amount of surface treatment agent that adheres to the surface-treated void-containing silver particles by the surface treatment agent addition process and remains after the separation process is a value measured on the copper-containing silver powder after the separation process described below, with the type of surface treatment agent specified.
The type of the surface treatment agent can be identified by qualitative analysis by gas chromatography of the surface treatment agent volatilized by heating the copper-containing silver powder.

The amount of the surface treatment agent attached to the copper-containing silver powder can be determined according to the quantitative analysis method for fatty acids described in Japanese Patent No. 5,622,543.
Specifically, first, the copper-containing silver powder is dissolved in an acid, and then an organic solvent is mixed therein. The entire amount of the surface treatment agent is extracted into the organic solvent phase. A predetermined amount of the organic solvent phase is then separated and evaporated to dryness. The carbon content of the remaining solid matter is measured using a carbon-sulfur analyzer, and the carbon content can be calculated.

For example, if the surface treatment agent is specified as stearic acid and the copper-containing silver powder does not contain any carbon sources other than stearic acid, the measurement method for stearic acid is as follows.

When the carbon amount (strength) of each of the standard solutions with different stearic acid contents (mg) is measured by a carbon-sulfur analyzer to obtain a calibration curve, the slope of the curve is A (strength/mg).The mass X (mg) and concentration Y (%) of stearic acid in the copper-containing silver powder can be calculated by the following formulas (A) and (B), respectively, when the carbon amount obtained by measuring the remaining solid matter is C (strength) from the treatment agent extracted into the total organic solvent amount a (mL) by the treatment of the copper-containing silver powder, and the amount of the copper-containing silver powder dissolved in the acid is M (g).
X (mg) = (C/A x a/b)...(A)
Y(%)=X/(M×1000)×100...(B)

Even when oleic acid is used as a treatment agent, the carbon content is measured and determined in the same manner as above. The oleic acid concentration Y' is calculated using the calibration curve of stearic acid. The molecular weight of stearic acid is 284.48, with a carbon content of 216.19, and the molecular weight of oleic acid is 282.46, with a carbon content of 216.19, so the oleic acid concentration Y' can be calculated by the following formula (C).
Oleic acid concentration Y' (%) = Y x (216.19/284.48) x (282.46/216.19) ... (C)

The amount of the surface treatment agent applied is usually 0.05% by mass or more and 0.5% by mass or less based on the mass of the copper-containing silver powder.

[Separation step]
In the separation step, the void-containing silver particles or the surface-treated void-containing silver particles (hereinafter, collectively referred to as "void-containing silver particles, etc.") are separated from the mixed liquid and dried to obtain a copper-containing silver powder. The copper-containing silver powder obtained in the separation step can be the copper-containing silver powder of the present invention.
In addition, the separation step may optionally include a washing and recovery step of recovering and washing the separated void-containing silver particles and the like.

In the washing and recovery process, for example, the aggregates of the separated void-containing silver particles, etc. are formed into a cake-like form, and the cake of the aggregates of the void-containing silver particles, etc. is washed. Washing in the washing and recovery process may be performed using, for example, pure water. Dehydration in the washing and recovery process may be performed, for example, by decantation or a filter press. The end point of washing may be determined using the electrical conductivity of the washing water. Specifically, the end of washing may be determined when the electrical conductivity of the washing water becomes equal to or lower than a predetermined value. After washing, the void-containing silver particles, etc., in an aggregated state such as a cake-like form, may be subjected to a drying process.

In the drying process, aggregates of void-containing silver particles and the like that contain moisture and are in an agglomerated state are dried. The drying process may be performed by vacuum drying or using an airflow dryer. In the drying process, a high-pressure air flow may be blown onto the aggregates of void-containing silver particles and the like, or the cake or copper-containing silver powder in the drying process may be put into a stirrer having a stirring rotor and stirred to impart a dispersing force to the cake or copper-containing silver powder that is dried and has voids, thereby promoting dispersion and drying.

During the drying process, the temperature of the copper-containing silver powder is usually 100°C or less. If the temperature of the copper-containing silver powder is 100°C or less, sintering of void-containing silver particles in the copper-containing silver powder can be effectively suppressed.

Since the copper-containing silver powder after drying may be in the form of clumps, a dry crushing process or classification operation may be performed simultaneously with or after the drying process in order to improve the handleability of the copper-containing silver powder. Here, improving the handleability of the copper-containing silver powder means, for example, ensuring a degree of fluidity that does not interfere with the supply operation into the device, or loosening the copper-containing silver powder to an appropriate degree so that processing in the device proceeds efficiently.

There are no particular limitations on the method of dry crushing and it can be selected appropriately depending on the purpose, but it is preferable to use a crusher that rotates the stirring blades to crush and fluidize the copper-containing silver powder, for example, a Henschel mixer, a sample mill, a blender, a coffee mill, etc.

(Conductive paste)
The conductive paste of the present invention contains the above-mentioned copper-containing silver powder of the present invention, aluminum, and a solvent. Since the conductive paste of the present invention contains the copper-containing silver powder of the present invention, the line resistance of the conductive film can be reduced.

Here, the silver content in the conductive paste means the mass ratio of silver to the total mass of all components in the conductive paste. The silver content in the conductive paste is preferably 50% by mass or more, more preferably 80% by mass or more, preferably 98% by mass or less, and more preferably 95% by mass or less.

The copper content in the conductive paste means the mass ratio of copper to the total mass of all components in the conductive paste.
The copper content in the conductive paste is preferably 5 ppm or more, and more preferably 10 ppm or more.
The copper content in the conductive paste is preferably 5000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less.
If the copper content in the conductive paste is within the above range, the line resistance of the conductive film can be effectively reduced.

<Filler>
The conductive paste may contain a portion of silver powder that does not contain copper. Hereinafter, the copper-containing silver powder and the copper-free silver powder may be collectively referred to as a filler.

The content of the filler in the conductive paste means the mass ratio of the filler to the total mass of all components in the conductive paste. The content of the filler in the conductive paste is preferably 50 mass% or more, more preferably 80 mass% or more, and is preferably 98 mass% or less, and more preferably 95 mass% or less.
In addition, in the conductive paste, since this can suppress the uneven distribution of copper caused by the filler, it is preferable that the proportion of the copper-containing silver powder of the present invention in the filler is high, preferably 50 mass% or more, and most preferably 100 mass%.

The copper content in the filler means the mass ratio of copper to the total mass of silver and copper in the filler.
The copper content in the filler is preferably 5 ppm or more, and more preferably 10 ppm or more.
The copper content in the filler is 10,000 ppm or less, preferably 5,000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less.
If the copper content in the filler is within the above range, the line resistance of the conductive film can be effectively reduced.

<Aluminum>
The aluminum contained in the conductive paste of the present invention may be either metallic aluminum or an aluminum compound, but is preferably metallic aluminum. Examples of the form of metallic aluminum include aluminum (metallic aluminum) and an aluminum alloy.
Here, in the conductive paste, the aluminum is preferably in the form of powder, and examples of the powder include aluminum powder (powder made of metallic aluminum), aluminum alloy powder, silver-coated aluminum powder, aluminum-adhered silver powder, etc. As the aluminum, it is particularly preferable to use aluminum powder.
In addition, since the aluminum powder is a powder made of metallic aluminum, it does not contain any components other than impurities (e.g., iron, silicon, etc.) that are inevitably mixed in during production. The content of metallic aluminum in the aluminum powder is usually 99 mass% or more, preferably 99.5 mass% or more, and more preferably 99.8 mass% or more.
The surfaces of the aluminum powder, aluminum alloy powder, silver-coated aluminum powder, aluminum-adhered silver powder, etc. may be oxidized. For example, the aluminum powder may have an aluminum oxide film on the surface.

When the aluminum is in the form of powder, the average diameter (SEM average diameter) of the powder is preferably 3.0 μm or less, and more preferably 2.0 μm or less.
When the powdered aluminum has an average diameter (SEM average diameter) of 3.0 μm or less, the conductive paste can be suitably used for forming a thin conductive film.

The aluminum content in the conductive paste means the mass ratio of aluminum to the total mass of all components in the conductive paste. The aluminum content in the conductive paste is, for example, 0.1 mass% or more, 0.5 mass% or more, or 1 mass% or more, for example, 5 mass% or less, 3 mass% or less, or 2 mass% or less.

<Solvent>
The solvent (i.e., dispersion medium) of the conductive paste is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include texanol, butyl carbitol acetate, tributyl citrate, 1-octanol, terpineol, butyl carbitol, etc. These may be used alone or in combination of two or more.

The content of the solvent in the conductive paste means the mass ratio of the solvent to the total mass of all components in the conductive paste. The content of the solvent in the conductive paste is preferably 1 mass% or more and 19 mass% or less.

<Other ingredients>
The conductive paste may further contain components other than the above-mentioned copper-containing silver powder, aluminum, solvent, and any copper-free silver powder (hereinafter, sometimes referred to as "other components").
Other components that may be optionally contained in the conductive paste include, for example, glass frit, binder, dispersant, viscosity adjuster, etc. These may be used alone or in combination of two or more.

Glass frit is a component that can improve the adhesion between the conductive film obtained by firing the conductive paste and the substrate. The glass frit can be appropriately selected depending on the purpose, and any glass frit that can be generally used for solar cells and the like can be used without any particular limitations, but it is preferable that the glass frit contains components such as Si, B, Al, Bi, Li, Na, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, and Cr. Such components can be present in the glass frit in the form of oxides, compounds that produce oxides when heated, other compounds, and mixtures thereof.

The content of the glass frit in the conductive paste means the mass ratio of the glass frit to the total mass of all components in the conductive paste. The content of the glass frit in the conductive paste is preferably 0.1 mass% or more and 10 mass% or less.
Here, preferred oxides are alkali metal oxides, alkaline earth metal oxides, rare earth oxides, Group 14 to Group 16 oxides, other oxides, or combinations thereof.
The glass frit is usually in the form of glass powder.

Examples of binders include epoxy resins, acrylic resins, polyester resins, polyimide resins, polyurethane resins, phenoxy resins, silicone resins, and cellulose. These may be used alone or in combination of two or more.

Dispersants include oleic acid, triacetin, stearic acid, myristic acid, palmitic acid, linoleic acid, lauric acid, linolenic acid, etc. These may be used alone or in combination of two or more.

Viscosity adjusters include methylphenylpolysiloxane, hydrogenated castor oil, fatty acid amides, etc.

<Conductive paste properties>
The viscosity of the conductive paste is not particularly limited and can be appropriately selected depending on the purpose, but under conditions of a paste temperature of 25°C and a rotation speed of 1 rpm, it is preferably 150 Pa·s or more, more preferably 200 Pa·s or more, and is preferably 800 Pa·s or less, and more preferably 750 Pa·s or less.
When the viscosity of the conductive paste is 150 Pa·s or more, printing defects such as "bleeding" and "printing sagging" that may occur when forming a conductive film by printing or the like can be effectively suppressed.
On the other hand, if the viscosity of the conductive paste is 800 Pa·s or less, breakage of the resulting conductive film can be effectively prevented, making it possible to form a thin conductive film.

<Applications of conductive paste>
The conductive paste of the present invention is suitable for forming a conductive film, that is, for forming a conductive pattern on a substrate or for forming an electrode. For example, it can be applied directly to various substrates such as silicon wafers for solar cells, films for touch panels, and glass for EL devices, or can be applied or printed on a substrate on which a transparent conductive film is further provided as required, to form a conductive film. The conductive film formed using the conductive paste of the present invention can be suitably used for electrodes or electrical wiring applications such as collector electrodes for solar cells, external electrodes for chip-type electronic components, RFID, electromagnetic shields, vibrator bonding, membrane switches, and electroluminescence.

<Method of manufacturing conductive paste>
The method for producing the conductive paste of the present invention is not particularly limited, and can be obtained, for example, by mixing each material that can be contained in the conductive paste described above and dispersing and/or kneading the resulting mixture. For dispersion and kneading, for example, ultrasonic dispersion, a disperser, a three-roll mill, a ball mill, a bead mill, a two-axis kneader, a self-revolving mixer, etc. can be used.

(Conductive film)
The conductive film of the present invention is formed by using the conductive paste of the present invention described above. Since the conductive film of the present invention is formed by using the conductive paste of the present invention described above, the line resistance is reduced.
The conductive film of the present invention is usually a film formed by applying or printing the above-mentioned conductive paste of the present invention onto a substrate or the like, and then baking the same.

(Solar cell)
The solar cell of the present invention includes the conductive film of the present invention described above. The solar cell of the present invention has excellent performance because it includes a conductive film with reduced line resistance. The conductive film included in the solar cell can function as an electrode (collecting electrode, etc.) or electrical wiring of the solar cell.
The conductive film provided in the solar cell of the present invention is usually a film formed by applying or printing the above-mentioned conductive paste of the present invention onto a substrate or the like, and then firing the same.

The solar cell of the present invention is not particularly limited as long as it has the conductive film of the present invention, and may appropriately have a semiconductor substrate, a diffusion layer, an anti-reflection layer, a BSF (Back Surface Field) layer, a front electrode, a back electrode, etc.

The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples.

(Preparation of Silver Powder X1)
40.8 g of a 5% by weight aqueous solution of copper (II) nitrate trihydrate was added to 3.5 L of an aqueous solution of silver nitrate with a silver ion concentration of 0.14 mol/L to obtain a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (1% by weight of copper relative to silver). Next, while stirring the mixed solution, 113.2 g of 28% by weight of ammonia water was added, and the liquid temperature of the mixed solution was adjusted to 45°C. Next, 15 g of a 20% by weight aqueous solution of sodium hydroxide was added to the mixed solution to adjust the pH, and 250 g of a 26% by weight aqueous solution of formaldehyde was added as a reducing agent to precipitate void-containing silver particles. Next, 15 seconds after adding the reducing agent to the mixed solution, a 0.18% by weight isopropanol solution containing stearic acid relative to silver was added. Stirring was stopped, and the solid matter was filtered using a Nutsche filter and washed with water, and the obtained solid matter was dried in a vacuum dryer at 73°C for 10 hours. After drying, the mixture was subjected to a dry crushing treatment using a sample mill to obtain a copper-containing silver powder, Silver Powder X1.

[Measurement of copper content in silver powder X1]
The quantitative determination of copper in silver powder X1 was carried out by the following analytical method.
1) 1 g of a sample was precisely weighed, and 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added thereto, followed by heating at 200° C. for 30 minutes.
2) After allowing the sample heated in 1) to cool, the volume was adjusted to 100 mL using pure water, 5 mL of the supernatant was removed, and the volume was adjusted again to 100 mL using pure water to prepare a sample for ICP analysis.
3) The copper standard solution used to create the calibration curve was prepared by adjusting the silver concentration using 5N silver to the same level as the sample for ICP analysis.
4) ICP quantification was performed using Agilent 5800 ICP-OES manufactured by Agilent Technologies.

[Measurement of carbon, oxygen and nitrogen content]
The carbon amount was measured using a carbon/sulfur analyzer (EMIA-810W manufactured by Horiba, Ltd.). The oxygen and nitrogen amounts were measured using an oxygen/nitrogen/hydrogen simultaneous analyzer (ONH836 manufactured by LECO). The results are shown in Table 1.

(Preparation of Silver Powder X2)
Except for changing the amount of 5% by mass aqueous solution of copper (II) nitrate trihydrate to 4.1 g to obtain a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (containing 0.1% by mass of copper relative to silver), various operations were carried out in the same manner as in the preparation of silver powder X1 to obtain silver powder X2, which is a copper-containing silver powder. Then, various measurements were carried out using the obtained silver powder X2 in the same manner as in the preparation of silver powder X1. The results are shown in Table 1.

(Preparation of Silver Powder X3)
Except for changing the amount of 5% by mass aqueous solution of copper (II) nitrate trihydrate to 0.2 g to obtain a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (containing 0.005% by mass of copper relative to silver), various operations were carried out in the same manner as in the preparation of silver powder X1 to obtain silver powder X3, which is a copper-containing silver powder. Then, various measurements were carried out using the obtained silver powder X3 in the same manner as in the preparation of silver powder X1. The results are shown in Table 1.

(Preparation of silver powder X4 to X6)
As silver powder X4, silver powder having internal voids and not containing copper (manufactured by Dowa Electronics, "AG-4-8FD") was prepared.
Silver powder X5 was prepared which had no internal voids and did not contain copper (manufactured by Dowa Electronics, "AG-4-54F").
As silver powder X6, an atomized silver-copper alloy powder (SEM average diameter 1.2 μm, manufactured by Dowa Electronics Co., Ltd.) containing 72 mass% silver and 28 mass% copper was prepared. The atomized silver-copper alloy powder is a silver-copper alloy powder produced by an atomization method. The atomized silver-copper alloy powder does not have voids inside.

Example 1
<Preparation of copper-containing silver powder>
As a filler, a copper-containing silver powder made of the silver powder X1 obtained above was prepared. The following measurements were performed using the copper-containing silver powder. Note that a scanning electron microscope image of a cross section of the copper-containing silver powder according to Example 1 is shown in FIG.

[Evaluation of thermomechanical analysis]
Thermomechanical analysis (TMA) of the copper-containing silver powder was carried out as follows.
First, 0.3 g of copper-containing silver powder was weighed. Next, the copper-containing silver powder was put into a predetermined mold with a diameter of 5 mmφ, and pressed with a press machine for 1 min under a load of 50 kg to prepare a disk-shaped measurement sample. This measurement sample was set in a sample holder of a thermomechanical analysis (TMA) device (Thermo plus EVO 2 series TMA8311), and a measurement load of 98 mN was applied by a measurement probe, and the temperature was raised from 25 ° C to 900 ° C at a heating rate of 10 ° C / min to measure the expansion rate and contraction rate of the measurement sample. The following evaluation was performed on the measurement results from 100 ° C to 600 ° C.
The expansion rate, contraction rate, and temperature at that time were recorded every second from the start of measurement (heating), and a TMA curve was obtained. The obtained TMA curve was differentiated with respect to time to obtain a differential curve of the TMA curve (dTMA curve). The dTMA curve of the copper-containing silver powder of Example 1 is shown in FIG. 2.
Using the above dTMA curve, the temperature and dTMA of the maximum expansion peak, the temperature and dTMA of the first contraction peak, the temperature and dTMA of the second contraction peak, and the presence or absence of meandering in the dTMA curve between 300° C. and 600° C. were confirmed. The results are shown in Table 2.

[Observation of void-containing silver particles]
Using the above copper-containing silver powder, the silver particles were observed by the following method to confirm the presence or absence of voids inside the silver particles.
Specifically, first, the copper-containing silver powder was placed in a resin (Epofix resin, manufactured by Struers) and a hardener (Epofix hardener, manufactured by Struers) and solidified, and the solidified resin was cut. Next, the cut surface was polished with a cross-section polisher (ArBlade5000, manufactured by Japan High-Technologies Corporation) to expose the cross section of the silver particles, and each silver particle was observed with a scanning electron microscope (JSM-IT800SHL, manufactured by JEOL Ltd.) to confirm the presence or absence of voids inside the silver particles, and the presence or absence of void-containing silver particles having voids inside the copper-containing silver powder was determined. The results are shown in Table 2. It should be noted that more than half of the observed silver particles had voids.

[Measurement of copper content in copper-containing silver powder]
The amount of copper in the copper-containing silver powder was determined by the following analytical method. The results are shown in Table 2.
1) Weigh out 1 g of sample, add 15 mL of pure water and 10 mL of nitric acid (for precision analysis), and dilute to 200 mL.
The mixture was heated at 30° C. for 30 minutes.
2) After allowing the sample heated in 1) to cool, the volume was adjusted to 100 mL using pure water, 5 mL of the supernatant was removed, and the volume was again adjusted to 100 mL using pure water to prepare a sample for ICP analysis.
3) The copper standard solution used to create the calibration curve was prepared by adjusting the silver concentration using 5N silver to the same level as the sample for ICP analysis.
4) ICP quantification was performed using Agilent 5800 ICP-OES manufactured by Agilent Technologies.

[Measurement of true density]
The true density of the copper-containing silver powder was measured using a dry automatic densimeter. The results are shown in Table 2.
Specifically, first, the copper-containing silver powder was filled into a 10 cc platinum crucible, and the mass of the filled copper-containing silver powder was precisely measured. Then, using a dry automatic density meter (Micromeritics' Accupypic II1340), the volume of the copper-containing silver powder was measured from the gas volume when helium gas was filled into the crucible until the pressure inside the crucible reached a certain level, and the mass of the copper-containing silver powder was divided by the volume to calculate the true density.

[Measurement of BET specific surface area]
The BET specific surface area of the copper-containing silver powder was measured by a BET specific surface area measuring device (Macsorb HM-model 1210, manufactured by MOUNTECH) using a BET single-point method with nitrogen adsorption. In the measurement of the BET specific surface area, the degassing conditions before the measurement were 60° C. and 10 minutes. The results are shown in Table 2.

[Measurement of particle size distribution]
The minimum particle size ( _DMIN ), cumulative 10% particle size ( _D10 ), cumulative 50% particle size ( _D50 ), cumulative 90% particle size ( _D90 ), cumulative 95% particle size ( _D95 ), and cumulative 100% (i.e., maximum particle) particle size ( _DMAX ) of the copper-containing silver powder on a volume basis were measured by the following methods. The results are shown in Table 2.
0.1 g of the copper-containing silver powder was added to 40 mL of isopropyl alcohol (IPA) and dispersed for 2 minutes using an ultrasonic homogenizer (apparatus name: US-150T, manufactured by Nippon Seiki Seisakusho Co., Ltd.; 19.5 kHz, tip diameter 18 mm), and then the particle size distribution was measured using a laser diffraction/scattering type particle size distribution measuring device (Microtrac MT-3300 EXII, manufactured by Microtrac-Bell Corporation).

<Preparation of Conductive Paste>
The above copper-containing silver powder, aluminum powder (SEM average diameter: 2.0 μm) having a metallic aluminum content of 99.87% by mass and an impurity content of 0.13% by mass (iron 0.09% by mass, silicon 0.04% by mass), glass powder (containing PbO as the main component and containing B ₂ O ₃ , SiO ₂ and other oxides) as glass frit, ethyl cellulose, texanol, butyl carbitol acetate, tributyl citrate, 1-octanol, oleic acid, triacetin, methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amide were mixed to obtain a mixture having the composition shown in Table 3.
Next, the obtained mixture was premixed under conditions of a planetary centrifugal mixer (revolution 1000 rpm) and then kneaded with a three-roll mill (manufactured by Esact) to obtain a conductive paste.
The silver content in the conductive paste was 85.305% by mass, and the copper content was 0.145% by mass.

[Line resistance value]
A straight line shape was printed by screen printing using the conductive paste obtained above. The straight line had a design line width of 12 μm and a length of 150 mm. A Microtec printer was used for printing, and printing was performed at a skiing speed of 350 mm/s. A silicon substrate (for solar cell use, texture formation and SiN _x film formation) with a thickness of about 170 μm was used for printing. After printing, the substrate was dried for 5 minutes in a dryer set at a temperature of 200° C., and then baked in a solar cell baking furnace (made by NGK) under conditions such that the peak temperature of the upper surface of the wafer was 750° C. to prepare a sample. The resistance value of the electrode after baking (the line resistance value of the conductive film) was measured with a digital multimeter (made by ADC Co., Ltd.) by applying measuring terminals to both ends of the printed electrode. The results are shown in Table 2.

Example 2
Various operations and measurements were carried out in the same manner as in the examples, except that the copper-containing silver powder made of silver powder X2 was used as the filler instead of the copper-containing silver powder made of silver powder X1. The results are shown in Table 2. The dTMA curve of the copper-containing silver powder according to Example 2 is shown in FIG.
The silver content in the conductive paste of Example 2 was 85.420 mass %, and the copper content was 0.030 mass %.
In addition, in the observation of the void-containing silver particles, more than half of all the silver particles observed had voids.

Example 3
Various preparations and measurements were carried out in the same manner as in the examples, except that the copper-containing silver powder made of silver powder X3 was used as the filler instead of the copper-containing silver powder made of silver powder X1. The results are shown in Table 2. The dTMA curve of the copper-containing silver powder according to Example 3 is shown in FIG.
The silver content in the conductive paste of Example 3 was 85.446 mass %, and the copper content was 0.004 mass %.
In addition, in the observation of the void-containing silver particles, more than half of all the silver particles observed had voids.

(Comparative Example 1)
Various operations and measurements were carried out in the same manner as in the examples, except that the copper-containing silver powder made of silver powder X1 was replaced with the silver powder made of silver powder X4 as the filler. The results are shown in Table 2. The dTMA curve of the silver powder according to Comparative Example 1 is shown in FIG.

(Comparative Example 2)
Various operations and measurements were carried out in the same manner as in the examples, except that a mixed silver powder consisting of silver powder X5 and silver powder X6 was used as a filler instead of the copper-containing silver powder consisting of silver powder X1. The results are shown in Table 2. The dTMA curve of the mixed silver powder according to Comparative Example 2 is shown in FIG.

As is clear from Table 2, the copper-containing silver powder of the embodiment can reduce the line resistance of the conductive film.

According to the present invention, it is possible to provide a copper-containing silver powder capable of reducing the line resistance of a conductive film.
Moreover, according to the present invention, it is possible to provide a conductive paste capable of reducing the line resistance of a conductive film.
Moreover, according to the present invention, it is possible to provide a conductive film with reduced line resistance.
Furthermore, according to the present invention, a solar cell having excellent performance can be provided.

Claims (6)

A copper-containing silver powder comprising:
In the differential curve of the TMA curve from 100 ° C. to 600 ° C. obtained by thermomechanical analysis in which the copper-containing silver powder is heated at a heating rate of 10 ° C. / min,
and a first contraction peak at a temperature lower than the expansion peak and having a dTMA of -0.10%/min or less;
The copper-containing silver powder has a copper content of 10 ppm or more and 10,000 ppm or less . The copper-containing silver powder according to claim 1, wherein the differential curve has a second shrinkage peak with a dTMA of -0.30%/min or less between the temperature of the expansion peak and 300°C. The copper-containing silver powder according to claim 1 or 2, in which the differential curve between 300°C and 600°C is meandering. A conductive paste comprising the copper-containing silver powder according to claim 1 or 2, aluminum, and a solvent. A conductive film formed using the conductive paste according to claim 4. A solar cell comprising the conductive film according to claim 5.

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