TW202504701A - Silver powder containing copper and its manufacturing method, conductive paste, conductive film and solar cell - Google Patents

Abstract

The object of the present invention is to provide a copper-containing silver powder capable of reducing the line resistance of a conductive film. The present invention is a copper-containing silver powder having a true density of less than 10 g/cm ³ and a copper content of 10 ppm or more and 10,000 ppm or less.

Description

Silver powder containing copper and its manufacturing method, conductive paste, conductive film and solar cell

The present invention relates to a silver powder containing copper and a manufacturing method thereof, a conductive paste, a conductive film and a solar cell.

The method of applying or printing a conductive paste containing conductive metal powder on a base material such as a film, substrate, or electronic component and then drying and curing or sintering the paste by heating to form a conductive film such as an electrode or an electric wiring has been widely used. However, with the recent improvement in the performance of electronic devices, the conductive film formed using the conductive paste is required to have lower resistance, and this requirement is becoming more stringent year by year.

In response to the above requirements, for example, Patent Document 1 proposes the following: for the purpose of obtaining a conductive paste composition having high conductivity (low resistance), a silver powder containing a flake-shaped silver powder and a spherical silver powder and having a polycarboxylic acid attached to the surface of at least one of the silver powders of the flake-shaped silver powder and the spherical silver powder is used as a conductive metal powder. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2011-100573

[Problem to be solved by the invention] Here, there is room for further improvement in reducing the line resistance of a conductive film formed using the conductive paste with respect to the silver powder (silver powder) that can be used in the existing conductive paste.

Therefore, the object of the present invention is to provide a copper-containing silver powder capable of reducing the line resistance of a conductive film and a method for producing the same. In addition, the object of the present invention is to provide a conductive paste capable of reducing the line resistance of a conductive film. In addition, the object of the present invention is to provide a conductive film having reduced line resistance. In addition, the object of the present invention is to provide a solar cell having excellent performance. [Means for Solving the Problem]

In order to solve the above problems, the inventors have repeatedly conducted diligent research, and as a result, they have completed the present invention described below.

That is, the main structure of the present invention for solving the above-mentioned problems is as follows.

[1] A silver powder containing copper, having a true density of less than 10 g/cm ³ and a copper content of not less than 10 ppm and not more than 10,000 ppm.

[2] The copper-containing silver powder as described in [1], wherein when observing a cross section, particles having voids inside account for more than half of all silver particles.

[3] The copper-containing silver powder according to [1] or [2], wherein the Brunauer-Emmett-Teller (BET) specific surface area is 0.1 m ² /g or more and 1 m ² /g or less.

[4] The copper-containing silver powder according to any one of [1] to [3], wherein the cumulative 50% particle size on a volume basis is 0.5 μm or more and 6 μm or less.

[5] A conductive paste comprising the copper-containing silver powder as described in any one of [1] to [4], aluminum, and a solvent.

[6] A conductive film formed using the conductive paste of [5].

[7] A solar cell comprising the conductive film as described in [6].

[8] A method for producing a copper-containing silver powder, wherein the copper-containing silver powder as described in any one of [1] to [4] is produced, the method comprising: a reduction step, wherein a reducing agent is added to a mixed solution containing a silver compound and a copper compound to precipitate silver particles containing voids; and a separation step, wherein the silver particles containing voids are separated from the mixed solution and dried to obtain a copper-containing silver powder, wherein the reducing agent comprises a compound having an aldehyde group. [Effect of the invention]

According to the present invention, a copper-containing silver powder capable of reducing the line resistance of a conductive film and a method for producing the same can be provided. In addition, according to the present invention, a conductive paste capable of reducing the line resistance of a conductive film can be provided. In addition, according to the present invention, a conductive film with reduced line resistance can be provided. In addition, according to the present invention, a solar cell with excellent performance can be provided.

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

Here, in recent years, in a solar cell (particularly an n-type solar cell) including a silicon substrate and a conductive film such as an electrode or electric wiring formed on the surface of the silicon substrate, in order to reduce the contact resistance (interface resistance) at the interface between the semiconductor layer and the electrode, etc., attempts have been made to add aluminum such as aluminum powder to the conductive paste used in forming the conductive film. However, if aluminum is added to the conductive paste, the bulk resistance in the formed conductive film increases, and as a result, there is 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 can be particularly suitably used in a solar cell, specifically, a conductive paste containing aluminum used in forming a conductive film included in a solar cell.

(Terms and measurement methods) First, before explaining the implementation form, the terms and measurement methods used in this manual are explained.

<Confirmation of the presence or absence of voids inside silver particles> In this specification, confirmation of the presence or absence of voids is performed by observing silver particles in the following method. First, silver powder containing copper is placed in a resin and a hardener and cured, and the cured resin is cut. Then, the cross-section of the silver particle is exposed by polishing the cut surface using a cross-section polisher, and each silver particle is observed using a scanning electron microscope to confirm the presence or absence of voids inside the silver particle. As the resin and hardener, for example, "EpoFix resin" and "EpoFix hardener" manufactured by Struers can be used. In addition, as the cross-sectional polisher, for example, "ArBlade 5000" manufactured by Japan High-technologies Corporation can be used. In addition, as the scanning electron microscope, for example, "JSM-IT800SHL" manufactured by JEOL Ltd. can be used. In addition, in this specification, "voids inside silver particles" do not refer to the concave and convex spaces on the surface of silver particles, but refer to the closed spaces inside silver particles. In addition, in this specification, silver particles having voids inside are sometimes referred to as "void-containing silver particles".

In this specification, "the diameter of the void (i.e., the diameter of the closed space inside the silver particle containing the void)" refers to the diameter of the smallest circle in which all the voids are drawn in the scanning electron microscope image. Furthermore, in this specification, the void refers to a void having a diameter of 15 nm or more observed in the method using an image obtained by photographing a cross section of a silver particle at 10,000 to 40,000 times.

<Determination of copper content in copper-containing silver powder> The quantitative method of copper in copper-containing silver powder is implemented by the following analytical method. 1) Accurately weigh 1 g of the sample, add 15 mL of pure water and 10 mL of nitric acid (for precision analysis), and heat at 200°C for 30 minutes. 2) After cooling the heated sample in 1), make it 100 mL with pure water, take 5 mL of the supernatant, and make it 100 mL again with pure water to prepare the sample for inductively coupled plasma (ICP) analysis. 3) The copper standard solution used in making the calibration curve is prepared by using 5 N silver and adjusting the silver concentration to the same as that of the sample for ICP analysis. 4) ICP quantification was performed using an Aglent 5800 ICP-OES manufactured by Agilent Technologies.

<Contents of carbon, oxygen, and nitrogen in copper-containing silver powder> In this manual, "carbon content" is measured at 1350°C in an oxygen flow using a carbon-sulfur analyzer (EMIA-810W manufactured by Horiba, Ltd.). In addition, "oxygen content" and "nitrogen content" are measured in an Ar gas environment using an oxygen-nitrogen-hydrogen simultaneous analyzer (ONH836 manufactured by LECO) with the power value of the pulse furnace set to 3500 W.

<True density of copper-containing silver powder> In this specification, "true density of copper-containing silver powder" means the density of copper-containing silver powder taking into account the voids existing inside the void-containing silver particles (i.e., the "closed space inside the void-containing silver particles"). In addition, in this specification, the true density of copper-containing silver powder is measured by using a dry automatic density meter (also called a true density measuring device) using helium. Specifically, the volume of copper-containing silver powder is measured based on the gas volume when helium is filled until a certain pressure is reached in the container, and the mass of copper-containing silver powder is divided by its volume to calculate. Furthermore, the helium at this time can reach the concave and convex spaces on the particle surface and the spaces between particles, but cannot reach the closed spaces (i.e., voids) inside the particles that are not connected to the outside. Therefore, the greater the proportion of voids inside the copper-containing silver powder, the lower the true density of the copper-containing silver powder tends to be.

<Average diameter (Scanning Electron Microscope (SEM) average diameter)> The average diameter (SEM average diameter) is the average value of the Heywood diameters of more than 100 particles whose shapes can be identified in the SEM image with a magnification of 5,000 times, measured using the image processing software Mac-View (Ver. 4) manufactured by MOUNTECH.

<BET specific surface area> In this manual, "BET specific surface area" can be measured using Macsorb HM-model 1210 (manufactured by MOUNTECH) using the BET single-point method based on nitrogen adsorption. In the measurement of BET specific surface area, the degassing conditions before the measurement are set to 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., largest particle) particle size (D _MAX ) of the silver powder containing copper are measured using a laser diffraction-scattering particle size distribution measuring apparatus (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.).

<Determination of copper content and aluminum content in conductive paste> The quantitative method of copper and aluminum in conductive paste is implemented by the following analysis method. 1) Accurately weigh 1 g of paste, wash it with acetone, and dry it to obtain a sample. 2) Add 15 mL of pure water and 10 mL of nitric acid (for precision analysis) to the dried sample, and heat it at 200°C for 30 minutes. 3) After cooling the heated sample in 2), make it up to 100 mL with pure water, take 5 mL of the supernatant, and make it up to 100 mL again with pure water to prepare the sample for ICP analysis. 4) The copper and aluminum standard solutions used to prepare the calibration curve were prepared using 5 N silver and adjusted to the same silver concentration as the sample for ICP analysis. 5) ICP volume determination was performed using an Agilent 5800 ICP-OES manufactured by Agilent Technologies.

<Determination of silver content in conductive paste> The quantitative method of silver in conductive paste is implemented by the following analysis method. 1) Accurately weigh 1 g of paste, wash with acetone, and dry to obtain a sample. 2) Add 15 mL of pure water and 10 mL of nitric acid (for precise analysis) to the dried sample, and heat at 200°C for 30 minutes. 3) Add 15 mL of pure water and 10 mL of hydrochloric acid (for precise analysis) to the sample heated in 2), and heat at 150°C for 30 minutes. 4) Aging the silver chloride generated in 3) for more than 12 hours. 5) Filter the silver chloride using a accurately weighed glass filter and wash with pure water. 6) Dry the glass filter containing silver chloride in a dryer at 120°C for about 3 hours until it becomes constant. 7) Cool it in a desicator for more than 1 hour, accurately weigh the glass filter containing silver chloride, and calculate the silver content based on the mass of silver chloride generated.

(Silver powder containing copper) The silver powder containing copper of the present invention may include silver particles having voids inside (silver particles containing voids), and may arbitrarily include silver particles without voids inside. Moreover, the true density of the silver powder containing copper of the present invention is less than 10 g/ ^cm3 , and the copper content is greater than 10 ppm and less than 10,000 ppm. If it is the above-mentioned silver powder containing copper, the line resistance of the conductive film can be reduced. In particular, in a conductive paste containing aluminum, the effect of reducing the bulk resistance is remarkable. The reason is presumed to be that by forming a conductive film using a conductive paste containing the above-mentioned silver powder containing copper, the bulk resistance of the conductive film can be effectively reduced. The density of silver without voids inside is 10.49 g/cm ³ , and the density of copper is 8.93 g/cm ^3. When silver contains copper, the density decreases, but even if the copper content is about 10,000 ppm, the density is only reduced to 10.47 g/cm ^3. Therefore, the true density is less than 10 g/cm ³ when observing the cross section of the copper-containing silver powder, and the state of the void-containing silver particles with voids inside is fully observed. For example, the ratio of the void area to the particle cross-sectional area of the void-containing silver particles when observing the cross section is preferably 1% or more on average. Furthermore, the copper-containing silver powder of the present invention is preferably such that the void-containing silver particles are more than half of all the silver particles when observing the cross section.

The copper content in the copper-containing silver powder refers to 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 10 ppm or more, preferably 30 ppm or more, and 10,000 ppm or less, preferably 3,000 ppm or less, more preferably 1,000 ppm or less, and further preferably 100 ppm or less. If 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, the silver content in the copper-containing silver powder can be set to a certain level or more, and as a result, the line resistance of the conductive film can be effectively reduced.

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

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

The nitrogen content in the copper-containing silver powder refers to 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, 0.06 mass% or more, or 0.09 mass% or more, and, for example, 0.5 mass% or less, 0.3 mass% or less, or 0.2 mass% or less.

<Silver particles containing voids> In the copper-containing silver powder, the silver particles containing voids are silver particles having voids inside, and preferably contain copper. In addition, as a method for confirming that the silver particles containing voids contain copper, for example, energy dispersive spectroscopy (EDS) or X-ray photoelectron spectroscopy (XPS) can be used.

The average value of the diameter of the voids of the silver particles containing voids is obtained by measuring the diameters of voids of 15 nm or more contained in at least ten or more silver particle cross sections in the particle cross section images magnified 10,000 to 40,000 times. The average value of the diameter of the voids is, for example, 30 nm or more, 50 nm or more, or 100 nm or more, and, for example, 500 nm or less, 300 nm or less, or 200 nm or less.

In the case where the void-containing silver particles contain copper, the content of the silver particles containing silver in the copper-containing silver powder is preferably small from the viewpoint of suppressing the partial presence of copper in the conductive paste. The content of the silver particles containing silver in the copper-containing silver powder refers to the mass ratio of the silver particles containing silver to the total mass of the copper-containing silver powder. The content of the silver particles containing silver in the copper-containing silver powder is, for example, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 0% by mass, that is, the copper-containing silver powder does not contain the silver particles containing silver.

<Properties of the copper-containing silver powder> The true density of the copper-containing silver powder is 10 g/cm ³ or less, preferably 9.8 g/cm ³ or less, and 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, 9.2 g/cm ³ or more, and 9.4 g/cm ³ or more.

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

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

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

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

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

The volume-based cumulative 100% (ie, largest particle) particle size (D _MAX ) of the copper-containing silver powder is, for example, 3 μm or more, 4 μm or more, or 5 μm or more, and, 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 further 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 further preferably 0.6 m ² /g or less. If 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.

(Method for producing copper-containing silver powder) The method for producing copper-containing silver powder of the present invention comprises: a reduction step, adding a reducing agent to a mixed solution containing a silver compound and a copper compound to precipitate silver particles containing voids; and a separation step, separating the silver particles containing voids from the mixed solution and drying them to obtain copper-containing silver powder, wherein the reducing agent contains a compound having an aldehyde group. If the production method is as described above, the copper-containing silver powder of the present invention can be obtained. Furthermore, in addition to the reduction step and the separation step, the manufacturing method of the present invention may also arbitrarily include a surface treatment agent adding step, wherein the surface treatment agent is added to the mixed solution containing the silver particles containing voids obtained in the reduction step to obtain the silver particles containing voids after surface treatment.

<Reduction step> In the reduction step, a reducing agent is added to a mixed solution containing a silver compound and a copper compound to precipitate silver particles containing voids. The mixed solution is usually an aqueous solution or a suspension containing water as a solvent and containing a silver compound and a copper compound. The mixed solution containing a silver compound and a copper compound may arbitrarily contain a pH adjuster, etc. As a pH adjuster that can be contained in the mixed solution, a common acid or alkali can be used, for example, nitric acid, sodium hydroxide, etc. can be listed. Furthermore, in the reduction step, it is preferred to adjust the mixed solution to be alkaline. Furthermore, the mixed solution after the silver particles containing voids are precipitated is usually a suspension (so-called slurry) or a dispersion in which the silver particles containing voids are dispersed.

The reducing agent added to the mixed solution in the reduction step includes a compound having an aldehyde group. If a reducing agent including a compound having an aldehyde group is used, silver particles containing voids inside can be efficiently obtained. Examples of such reducing agents include aqueous formaldehyde solution (so-called formalin) and aqueous acetaldehyde solution. For more efficient acquisition of silver particles containing voids inside, formalin is preferably used. In addition, from the perspective of effectively generating voids, ultrasound or the like can also be used during reduction precipitation.

The silver compound is not particularly limited, and examples thereof include silver nitrate, silver oxide, etc. In addition, for example, a silver ammine complex obtained by reacting silver nitrate or silver oxide with ammonia water or an ammonium salt is preferably used as the silver compound.

The copper compound is not particularly limited, and examples thereof include copper (II) nitrate trihydrate, copper oxide, etc. In addition, for example, a copper ammine complex obtained by reacting copper (II) nitrate trihydrate or copper oxide with ammonia water or an ammonium salt is preferably used as the copper compound.

In the mixed solution, the ratio of copper to silver is preferably 0.001 mass % or more, preferably 0.003 mass % or more, and preferably 3 mass % or less, more preferably 1 mass % or less, further preferably 0.1 mass % or less, and further 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, the ratio of silver in the copper powder containing silver can be set to a certain value or more, and as a result, the line resistance of the conductive film can be effectively reduced.

<Surface treatment agent adding step> In any surface treatment agent adding step, a surface treatment agent is added to the mixed solution containing 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"). In addition, the mixed solution containing 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 the surface treatment agent include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linolenic acid, linolenic acid, and benzotriazole. These may be used alone or in combination of two or more.

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

The amount of the surface treatment agent that is attached to the silver particles containing voids after the surface treatment is completed by the surface treatment agent addition step and remains after the separation step is a value measured in a state where the type of the surface treatment agent is determined for the copper-containing silver powder after the separation step described later. In addition, the type of the surface treatment agent can be determined by qualitative analysis of the surface treatment agent volatilized by heating the copper-containing silver powder based on gas chromatography.

The amount of surface treatment agent attached to the copper-containing silver powder can be determined according to the quantitative analysis method of fatty acids described in Japanese Patent No. 5622543. Specifically, first, the copper-containing silver powder is dissolved with an acid, then mixed with an organic solvent, and the total amount of the surface treatment agent is extracted from the organic solvent phase. A predetermined amount of the organic solvent phase is then separated, and the carbon content of the residual solids after evaporation and drying is measured using a carbon-sulfur analysis device, which can be calculated.

For example, when the surface treatment agent is determined to be stearic acid and the copper-containing silver powder does not contain a carbon source other than stearic acid, the method for determining stearic acid is as follows.

In standard solutions with different stearic acid contents (mg), the calibration curve is obtained by measuring each carbon amount (strength) using a carbon-sulfur analyzer, and its slope is set as A (strength/mg). In addition, regarding the mass X (mg) and concentration Y (%) of stearic acid in the copper-containing silver powder, a predetermined amount b (mL) is taken from the total organic solvent amount a (mL) obtained by extracting the treatment agent by treating the copper-containing silver powder, and the carbon amount obtained by measuring the residual solid matter is set as C (strength), and the amount of copper-containing silver powder dissolved in the acid is set as M (g). In this case, the mass X and concentration Y of stearic acid can be calculated by the following formula (A) and formula (B), respectively. X(mg)=(C/A×a/b)…(A) Y(%)=X/(M×1000)×100…(B)

When oleic acid is used as the treatment agent, the carbon content is measured in the same manner as described above. For oleic acid, the calibration curve of stearic acid is also used for calculation. The molecular weight of stearic acid is 284.48, and the carbon content is 216.19. The molecular weight of oleic acid is 282.46, and the carbon content is 216.19. Therefore, the oleic acid concentration Y' can be calculated by the following formula (C). Oleic acid concentration Y' (%) = Y × (216.19/284.48) × (282.46/216.19) ... (C)

The amount of the surface treatment agent deposited relative to the mass of the copper-containing silver powder is usually 0.05 mass % or more and 0.5 mass % or less.

<Separation step> In the separation step, silver particles containing voids or silver particles containing voids that have been arbitrarily surface treated (hereinafter, sometimes collectively referred to as "silver particles containing voids, etc.") are separated from the mixed solution and dried to obtain copper-containing silver powder. The copper-containing silver powder obtained in the separation step can be used as the copper-containing silver powder of the present invention. Furthermore, in the separation step, a cleaning and recovery step can be arbitrarily performed to recover and clean the separated silver particles containing voids, etc.

In the cleaning and recovery step, for example, the separated aggregate of silver particles containing voids, etc. is made into a filter cake shape, and the filter cake of the aggregate of silver particles containing voids, etc. is cleaned. The cleaning in the cleaning and recovery step can be performed, for example, using pure water. The dehydration in the cleaning and recovery step can be performed, for example, by decanting or a filter press. The end point of the cleaning can be determined by the conductivity of the cleaning water. Specifically, the cleaning can be determined to be completed when the conductivity of the cleaning water becomes below a specified value. The cleaned silver particles containing voids, etc. can be provided to the drying step in a coagulated state such as a filter cake shape.

In the drying step, the aggregate of silver particles containing voids in a coagulated state containing water is dried. In the drying step, a vacuum drying or an air flow type dryer can be used. In the drying step, the following operation can also be performed: a high-pressure air flow is blown to the aggregate of silver particles containing voids, or the filter cake or the silver powder containing copper in the drying process is put into a stirrer with a stirring rotor and stirred, thereby giving a dispersing force to the filter cake or the silver powder containing copper in the drying process to promote dispersion or drying.

In the drying step, the temperature of the copper-containing silver powder is usually 100° C. or lower. If the temperature of the copper-containing silver powder is 100° C. or lower, sintering of silver particles containing voids in the copper-containing silver powder can be effectively suppressed.

The dried copper-containing silver powder may be in a lump state, so a dry crushing treatment or classification operation may be performed at the same time as the drying step or after the drying step for the purpose of improving the handleability of the copper-containing silver powder. Here, improving the handleability of the copper-containing silver powder means, for example, appropriately loosening the copper-containing silver powder in order to ensure fluidity to a degree that does not hinder the supply operation into the device or to perform the processing in the device efficiently.

The dry crushing method is not particularly limited and can be appropriately selected according to the purpose. It is preferred to use a crusher that rotates the stirring blades to crush and flow the silver powder containing copper. For example, a Henschel mixer, a sample mill, a blender, a coffee grinder, etc. can be used.

(Conductive paste) The conductive paste of the present invention contains the copper-containing silver powder of the present invention, aluminum, and a solvent. The conductive paste of the present invention contains the copper-containing silver powder of the present invention, and thus can reduce the line resistance of the conductive film.

Here, the silver content in the conductive paste refers to 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 mass % or more, more preferably 80 mass % or more, and preferably 98 mass % or less, more preferably 95 mass % or less.

In addition, the copper content in the conductive paste refers to 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, more preferably 10 ppm or more. In addition, the copper content in the conductive paste is preferably set to 5000 ppm or less, more preferably 500 ppm or less, and further 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 also contain copper-free silver powder in part. Hereinafter, the copper-containing silver powder and the copper-free silver powder may be collectively referred to as filler.

The content of filler in the conductive paste refers to the mass ratio of the filler to the total mass of all components in the conductive paste. The content of filler in the conductive paste is preferably 50% by mass or more, more preferably 80% by mass or more, and preferably 98% by mass or less, and more preferably 95% by mass or less. Furthermore, in the conductive paste, in order to suppress the partial presence of copper caused by the filler, the proportion of the copper-containing silver powder of the present invention in the filler is preferably high, preferably 50% by mass or more, and optimally 100% by mass.

The copper content in the filler refers to 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. In addition, the copper content in the filler is 10000 ppm or less, preferably 5000 ppm or less, more preferably 500 ppm or less, and further 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 any of metallic aluminum and aluminum compounds, preferably metallic aluminum. Examples of the form of metallic aluminum include aluminum (metallic aluminum) and aluminum alloys. Here, in the conductive paste, the aluminum is preferably in the form of a powder, and examples of the powder include aluminum powder (powder containing metallic aluminum), aluminum alloy powder, silver-coated aluminum powder, and aluminum-attached silver powder. As the aluminum, aluminum powder is particularly preferably used. Furthermore, the aluminum powder is a powder containing metallic aluminum, and therefore does not contain components other than impurities (for example, iron or silicon) that are inevitably mixed during production. Here, 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. Furthermore, the surface of aluminum powder, aluminum alloy powder, silver-coated aluminum powder, aluminum-attached silver powder, etc. can also be oxidized. For example, the aluminum powder can have an aluminum oxide film on the surface.

When the aluminum is in the form of a 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. If the average diameter (SEM average diameter) of the powdered aluminum is 3.0 μm or less, the conductive paste can be suitably used for forming a thinned conductive film.

The content of aluminum in the conductive paste refers to the mass ratio of aluminum to the total mass of all components in the conductive paste. The content of aluminum in the conductive paste is, for example, 0.1 mass % or more, 0.5 mass % or more, or 1 mass % or more, and is, 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 according to the purpose. For example, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (texanol), butyl carbitol acetate, tributyl citrate, 1-octanol, terpineol, butyl carbitol, etc. These can be used alone or in combination of two or more.

The content of the solvent in the conductive paste refers to 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 preferably 19 mass % or less.

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

Glass glue is a component that can improve the adhesion between the conductive film obtained by calcining the conductive paste and the substrate. As glass glue, it can be appropriately selected according to the purpose. Generally, as long as it can be used for solar cells, etc., it can be used without particular limitation. It is preferably composed of Si, B, Al, Bi, Li, Na, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, Cr and the like. Such components can exist in the glass glue in the form of oxides, compounds that generate oxides when heated, other compounds, and mixtures thereof.

The content of glass glue in the conductive paste refers to the mass ratio of glass glue to the total mass of all components in the conductive paste. The content of glass glue in the conductive paste is preferably 0.1 mass % or more and preferably 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. Furthermore, the glass glue is usually in the form of glass powder.

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

Examples of 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.

As viscosity adjusters, there are: methylphenyl polysiloxane, hydrogenated castor oil, fatty acid amides, etc.

<Properties of conductive paste> The viscosity of the conductive paste is not particularly limited and can be appropriately selected according to the purpose. Under the 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 preferably 800 Pa･s or less, more preferably 750 Pa･s or less. If the viscosity of the conductive paste is 150 Pa･s or more, it is possible to effectively suppress printing defects such as "bleeding" and "printing flow" that may occur when forming a conductive film by printing, etc. On the other hand, if the viscosity of the conductive paste is 800 Pa･s or less, it is possible to effectively suppress the disconnection of the obtained conductive film, thereby achieving thinning of the conductive film.

<Application of conductive paste> The conductive paste of the present invention is suitable for forming a conductive film, that is, forming a conductive pattern on a substrate, or forming an electrode. For example, it can be suitably used to form a conductive film by coating or printing directly on various substrates such as silicon wafers for solar cells, films for touch panels, and glass for electroluminescence (EL) elements, or on a transparent conductive film that is further provided on the substrate as needed. The conductive film formed using the conductive paste of the present invention can be suitably used, for example, as a collector electrode of a solar cell, an external electrode of a chip-type electronic component, radio-frequency identification (RFID), electromagnetic wave shielding, vibrator bonding, diaphragm switches, electrodes for electroluminescence, or electrical wiring, etc.

<Method for producing 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 the various materials that can be included in the conductive paste, and dispersing and/or kneading the obtained mixture. Furthermore, in the dispersion and kneading, for example, ultrasonic dispersion, disperser, three-roll mill, ball mill, bead mill, double-shaft kneader, rotation and revolving mixer, etc. can be used.

(Conductive film) The conductive film of the present invention is formed using the conductive paste of the present invention. Since the conductive film of the present invention is formed using the conductive paste of the present invention, the line resistance is reduced. In addition, the conductive film of the present invention is usually a film formed by applying or printing the conductive paste of the present invention on a substrate or the like and calcining it.

(Solar cell) The solar cell of the present invention includes the conductive film of the present invention. The solar cell of the present invention has excellent performance because it includes a conductive film that reduces line resistance. The conductive film included in the solar cell can function as an electrode (collector electrode, etc.) or electrical wiring of the solar cell. Furthermore, the conductive film included in the solar cell of the present invention is usually a film formed by applying or printing the conductive paste of the present invention on a substrate, etc. and calcining it.

The solar cell of the present invention is not particularly limited as long as it includes the conductive film of the present invention, and may also appropriately include a semiconductor substrate, a diffusion layer, an anti-reflection layer, a back surface field (BSF) layer, a surface electrode, a back electrode, etc. [Example]

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

(Preparation of silver powder X1) 40.8 g of a 5 mass % aqueous solution of copper (II) nitrate trihydrate was added to 3.5 L of a 0.14 mol/L silver nitrate aqueous solution as a silver ion to obtain a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (1 mass % of copper relative to silver). Then, while stirring the mixed solution, 113.2 g of 28 mass % ammonia water was added, and the liquid temperature of the mixed solution was adjusted to 45°C. Then, 15 g of a 20 mass % sodium hydroxide aqueous solution was added to the mixed solution to adjust the pH value, and 250 g of a 26 mass % formaldehyde aqueous solution was added as a reducing agent to precipitate silver particles containing voids. Then, 15 seconds after the reducing agent was added to the mixed solution, an isopropyl alcohol solution containing 0.18 mass % of stearic acid relative to silver was added. The stirring was stopped, and the solid was filtered using a nutsche filter, washed with water, and the obtained solid was dried at 73°C for 10 hours using a vacuum dryer. After drying, a sample mill was used to perform dry crushing treatment to obtain silver powder containing copper, namely silver powder X1.

[Determination of copper content in silver powder X1] The quantitative method of copper in silver powder X1 is implemented by the following analytical method. 1) Accurately weigh 1 g of the sample, add 15 mL of pure water and 10 mL of nitric acid (for precision analysis), and heat at 200°C for 30 minutes. 2) After the sample heated in 1) is left to cool, it is fixed to 100 mL with pure water, and 5 mL of the supernatant is taken from it and fixed to 100 mL again with pure water to prepare the sample for ICP analysis. 3) The copper standard solution used in making the calibration curve is prepared using 5 N silver and adjusted to the same silver concentration as the sample for ICP analysis. 4) ICP quantification was performed using an Agilent 5800 ICP-OES manufactured by Agilent Technologies.

[Measurement of carbon content, oxygen content, and nitrogen content] The carbon content was measured using a carbon-sulfur analyzer (EMIA-810W manufactured by Horiba, Ltd.). In addition, the oxygen content and nitrogen content 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) The amount of 5 mass % aqueous solution of copper nitrate (II) trihydrate added was changed to 4.1 g to obtain a mixed solution containing silver nitrate and copper nitrate (II) trihydrate (containing 0.1 mass % copper relative to silver). Various operations were performed in the same manner as in the preparation of silver powder X1, and a silver powder containing copper, namely, silver powder X2, was obtained. Then, various measurements were performed using the obtained silver powder X2 in the same manner as for silver powder X1. The results are shown in Table 1.

(Preparation of Silver Powder X3) The addition amount of 5 mass % aqueous solution of copper nitrate (II) trihydrate was changed to 0.2 g to obtain a mixed solution containing silver nitrate and copper nitrate (II) trihydrate (containing 0.005 mass % copper relative to silver). Various operations were performed in the same manner as in the preparation of silver powder X1 to obtain a silver powder containing copper, namely, silver powder X3. Then, various measurements were performed using the obtained silver powder X3 in the same manner as for silver powder X1. The results are shown in Table 1.

(Preparation of silver powder X4 to silver powder X6) As silver powder X4, silver powder having voids inside and containing no copper (manufactured by DOWA Electronics, "AG-4-8FD") was prepared. As silver powder X5, silver powder having no voids inside and containing no copper (manufactured by DOWA Electronics, "AG-4-54F") was prepared. As silver powder X6, atomized silver-copper alloy powder containing 72 mass % of silver and 28 mass % of copper (SEM average diameter 1.2 μm, manufactured by DOWA Electronics) was prepared. Note that atomized silver-copper alloy powder is a silver-copper alloy powder manufactured by an atomization method. Moreover, the atomized silver-copper alloy powder has no voids inside. Using silver powder X4 to silver powder X6, various measurements were performed in the same manner as silver powder X1. The results are shown in Table 1.

(Preparation of silver powder X7) To 3.3 L of 0.12 mol/L silver nitrate aqueous solution as silver ions, 112.4 g of 25.7% by mass ammonia water was added, and the temperature of the mixed solution was 35°C. Then, 1.017 g of 5% by mass sodium carbonate aqueous solution and 0.508 g of 5% by mass polyethyleneimine (PEI) aqueous solution (manufactured by Nippon Catalyst Co., Ltd., weight average molecular weight 600) were added, and stirring was continued. Then, 3 minutes after the addition of ammonia water, 397 g of 1.9% by mass hydrazine aqueous solution was added. 20 seconds after the addition of hydrazine, 3.866 g of 5 mass% copper (II) nitrate trihydrate (0.1 mass% copper relative to silver) was added, and 70 seconds after the addition of hydrazine, 10 g of a 10 mass% sodium borohydride aqueous solution was added to precipitate silver particles. Then, 10 seconds after the addition of sodium borohydride to the mixed solution containing the silver particles, a stearic acid emulsion containing 0.16 mass% stearic acid relative to silver was added. Thereafter, stirring was stopped, the solid was filtered using a Nuchi filter, washed with water, and the obtained solid was dried at 73°C for 10 hours using a vacuum dryer. After drying, a sample mill was used to perform dry crushing treatment to obtain silver powder containing copper, namely silver powder X7. In addition, various measurements were performed using the obtained silver powder X7 in the same manner as silver powder X1. The results are shown in Table 1.

(Example 1) <Preparation of copper-containing silver powder> As a filler, a copper-containing silver powder including the obtained silver powder X1 was prepared. The following measurements were performed using the copper-containing silver powder. In addition, a scanning electron microscope image of the copper-containing silver powder of Example 1 is shown in FIG1 .

[Observation of silver particles containing voids] Using the above-mentioned 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 put into a resin (EpoFix resin manufactured by Struers) and a hardener (EpoFix hardener manufactured by Struers) and cured, and the cured resin was cut. Next, the cross-section was polished using a cross-section polisher (ArBlade 5000 manufactured by Japan High-technologies) to expose the cross section of the silver particles, and each silver particle was observed using a scanning electron microscope (JSM-IT800SHL manufactured by JEOL Ltd.) to determine whether there were void-containing silver particles in the copper-containing silver powder. The results are shown in Table 2. Furthermore, more than half of the observed silver particles had voids.

[Determination of copper content in copper-containing silver powder] The quantitative method of copper in copper-containing silver powder is implemented by the following analytical method. The results are shown in Table 2. 1) Accurately weigh 1 g of the sample, add 15 mL of pure water and 10 mL of nitric acid (for precision analysis), and heat at 200°C for 30 minutes. 2) After the heated sample in 1) is left to cool, it is fixed to 100 mL with pure water, and 5 mL of the supernatant is taken from it and fixed to 100 mL again with pure water to prepare the sample for ICP analysis. 3) The copper standard solution used in preparing the calibration curve is prepared using 5 N silver and adjusted to the same silver concentration as the sample for ICP analysis. 4) ICP quantification was performed using an 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 density meter. The results are shown in Table 2. Specifically, first, a 10 cc platinum crucible was filled with copper-containing silver powder, and the mass of the filled copper-containing silver powder was precisely measured. Then, a dry automatic density meter (AccuPyc II1340 manufactured by Micromeritics) was used to measure the volume of the copper-containing silver powder based on the gas volume when helium was filled until the crucible reached a certain pressure, and the mass of the copper-containing silver powder was divided by its 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 using a BET specific surface area measuring device (Macsorb HM-model 1210, manufactured by MOUNTECH) and a BET single-point method based on nitrogen adsorption. In the measurement of the BET specific surface area, the degassing conditions before the measurement were set to 60°C and 10 minutes. The results are shown in Table 2.

[Measurement of particle size distribution] 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., largest particle) particle size (D _MAX ) of the copper-containing silver powder were measured by _the following method. The results are shown in Table 2. 0.1 g of silver powder containing copper was added to 40 mL of isopropyl alcohol (IPA), dispersed for 2 minutes using an ultrasonic homogenizer (device name: US-150T, manufactured by Nippon Seiki Co., Ltd.; 19.5 kHz, chip tip diameter 18 mm), and then measured using a laser diffraction-scattering particle size distribution measuring device (Microtrac Bell Co., Ltd., Microtrac MT-3300 EXII).

<Preparation of conductive paste> The copper-containing silver powder, aluminum powder (SEM average diameter: 2.0 μm) having a metallic aluminum content of 99.87 mass % and an impurity content of 0.13 mass % (iron: 0.09 mass % and silicon: 0.04 mass %) as aluminum, glass powder (containing PbO as a main component and containing B ₂ O ₃ , SiO ₂ and other oxides) as glass glue, ethyl cellulose, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, butyl carbitol acetate, tributyl citrate, 1-octanol, oleic acid, triacetin, methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amide were mixed so as to have the composition shown in Table 3 to obtain a mixture. Then, the obtained mixture was pre-mixed in a rotary mixer (revolution 1000 rpm) and then kneaded using three rolls (manufactured by Esact) to obtain a conductive paste. The conductive paste contained 85.305% by mass of silver and 0.145% by mass of copper.

[Line resistance] The conductive paste obtained as described above was used to print a straight line shape by screen printing. The designed line width of the straight line was 12 μm, and the length of the straight line was set to 150 mm. During printing, a printer manufactured by Microtech was used, and printing was performed at a squeegee speed of 350 mm/s. During printing, a silicon substrate with a thickness of about 170 μm was used (for solar cell use, texture formation, SiN _x film formation completed). After printing, it was dried in a dryer set at a temperature of 200°C for 5 minutes, and then calcined in a solar cell calciner (manufactured by NGK) under the condition that the peak temperature on the upper surface of the wafer became 750°C to produce a sample. The resistance value of the calcined electrode (line resistance value of the conductive film) was measured using a digital multimeter (manufactured by ADC Co., Ltd.) by placing measurement terminals at both ends of the printed electrode. The results are shown in Table 2.

(Example 2) As a filler, a copper-containing silver powder including silver powder X2 was used instead of the copper-containing silver powder including silver powder X1. Except for this, various operations and measurements were performed in the same manner as in Example 2. The results are shown in Table 2. In addition, 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 silver particles containing voids, more than half of the entire silver particles observed had voids.

(Example 3) As a filler, a copper-containing silver powder including silver powder X3 was used instead of the copper-containing silver powder including silver powder X1. Except for this, various operations and measurements were performed in the same manner as in Example 3. The results are shown in Table 2. In addition, 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 silver particles containing voids, more than half of the entire silver particles observed had voids.

(Comparative Example 1) As a filler, silver powder containing silver powder X4 was used instead of the copper-containing silver powder containing silver powder X1. Various operations and measurements were performed in the same manner as in the example. The results are shown in Table 2.

(Comparative Example 2) As a filler, a mixed silver powder containing silver powder X5 and silver powder X6 was used instead of the copper-containing silver powder containing silver powder X1. Various operations and measurements were performed in the same manner as in the embodiment. The results are shown in Table 2.

(Comparative Example 3) As a filler, a mixed silver powder containing silver powder X4 and silver powder X6 was used instead of the copper-containing silver powder containing silver powder X1. Various operations and measurements were performed in the same manner as in the embodiment. The results are shown in Table 2.

(Comparative Example 4) As a filler, the copper-containing silver powder including the silver powder X7 was used instead of the copper-containing silver powder including the silver powder X1. Various operations and measurements were performed in the same manner as in Example 1. The results are shown in Table 2.

[Table 1] X1 (Silver powder containing copper) X2 (Silver powder containing copper) X3 (Silver powder containing copper) X4 (Silver powder without copper) X5 (Silver powder without copper) X6 (Silver-Copper Alloy Powder) X7 (Silver powder containing copper) Preparation method or type of silver powder Ratio of copper to silver [mass %] 1 0.1 0.005 - - - 0.1 Reducing agent Formalin Formalin Formalin - - - Hydrazine Type - - - AG-4-8FD AG-4-54F Atomizing powder - Properties of Silver Powder Copper content [ppm] 1700 350 50 0 0 280000 520 True density [g/cm ³ ] 9.57 9.48 9.69 9.66 10.43 10.05 10.45 The presence or absence of voids within the particles have have have have without without without Carbon content in silver particles [mass %] 0.34 0.43 0.24 0.23 0.13 0.03 0.12 Oxygen content in silver particles [mass %] 0.36 0.47 0.35 0.05 0.01 0.02 0.05 Nitrogen content in silver particles [mass %] 0.09 0.13 0.09 0.32 0.05 0.63 <0.01

[Table 2] Embodiment Comparison Example 1 2 3 1 2 3 4 filler Type of silver powder used [mass %] X1 100 - - - - - - X2 - 100 - - - - - X3 - - 100 - - - - X4 - - - 100 - 99.01 - X5 - - - - 99.87 - - X6 - - - - 0.13 0.99 - X7 - - - - - - 100 The presence or absence of silver particles with internal voids have have have have without have without Copper content [ppm] 1700 350 50 0 350 2772 520 True density [g/cm ³ ] 9.57 9.48 9.69 9.66 10.43 9.67 10.45 BET specific surface area [m ² /g] 0.83 0.45 0.57 0.44 0.32 0.43 0.29 Particle size distribution Dmin[µm] 0.3 0.4 0.4 0.4 0.4 0.5 0.6 D10[µm] 0.7 1.1 1.0 1.1 1.2 1.3 1.3 D50[µm] 1.4 1.9 1.7 1.8 2.0 2.0 2.1 D90[µm] 2.1 3.0 2.6 2.8 3.3 3.0 3.4 D95[µm] 2.4 3.5 3.1 3.2 4.0 3.4 4 Dmax[µm] 4.6 7.8 6.5 6.5 9.3 6.5 9.3 Linear resistance of conductive film [Ω] 18.7 17.0 16.7 20.9 19.1 19.9 23.8

[Table 3] Composition of conductive paste Material Content [mass%] filler 85.45 Aluminum powder 1.22 Glass powder 4.04 Ethyl cellulose 0.20 2,2,4-Trimethyl-1,3-pentanediol monoisobutyrate 1.26 Butyl Carbitol Acetate 4.95 Tributyl citrate 0.25 1-Octanol 1.19 Oleic acid 0.25 Triacetin 0.25 Methylphenyl polysiloxane 0.50 Hydrogenated castor oil 0.36 Fatty acid amides 0.08

As is clear from Table 2, it is known that the copper-containing silver powder of the embodiment can reduce the line resistance of the conductive film. [Industrial Applicability]

According to the present invention, a copper-containing silver powder capable of reducing the line resistance of a conductive film and a method for producing the same can be provided. In addition, according to the present invention, a conductive paste capable of reducing the line resistance of a conductive film can be provided. In addition, according to the present invention, a conductive film with reduced line resistance can be provided. In addition, according to the present invention, a solar cell with excellent performance can be provided.

without

FIG. 1 is a scanning electron microscope photograph of a cross section of the copper-containing silver powder of Example 1.

Claims (8)

A copper-containing silver powder has a true density of less than 10 g/cm ³ and a copper content of not less than 10 ppm and not more than 10,000 ppm. The copper-containing silver powder as described in claim 1, wherein when observing a cross section, particles having voids inside account for more than half of all silver particles. The copper-containing silver powder according to claim 1, wherein the Buerter specific surface area is 0.1 m ² /g or more and 1 m ² /g or less. The copper-containing silver powder according to claim 1, wherein a cumulative 50% particle size on a volume basis is 0.5 μm or more and 6 μm or less. A conductive paste comprising: the copper-containing silver powder as described in any one of claims 1 to 4, aluminum, and a solvent. A conductive film is formed using the conductive paste as described in claim 5. A solar cell comprising the conductive film as described in claim 6. A method for producing a silver powder containing copper, wherein the silver powder containing copper as described in any one of claims 1 to 4 is produced, the method comprising: a reduction step, wherein a reducing agent is added to a mixed solution containing a silver compound and a copper compound to precipitate silver particles containing voids; and a separation step, wherein the silver particles containing voids are separated from the mixed solution and dried to obtain the silver powder containing copper, and the reducing agent comprises a compound having an aldehyde group.