Copper-containing silver powder, method for producing same, electroconductive paste, electroconductive film, and solar battery cell

Abstract

The purpose of the present invention is to provide a copper-containing silver powder capable of reducing line resistance of an electroconductive film. The present invention is a copper-containing silver powder having a true density of less than 10 g / cm3 and a copper content of 10 to 10,000 ppm.

Description

Copper-containing silver powder and its manufacturing method, conductive paste, conductive film, and solar cell

[0001] The present invention relates to a copper-containing silver powder and a method for producing the same, a conductive paste, a conductive film, and a solar cell.

[0002] A method of forming a conductive film such as an electrode or electrical wiring by applying or printing a conductive paste containing a conductive metal powder onto a substrate such as a film, a board, or an electronic component, and then heating it to dry, harden, or bake it has been widely used. However, with the recent increase in the performance of electronic devices, conductive films formed using conductive pastes are required to have lower resistance, and this requirement is becoming stricter every year.

[0003] In response to the above demands, for example, Patent Document 1 proposes the use of a silver powder consisting of a flaky silver powder and a spherical silver powder, with a polyvalent carboxylic acid 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).

[0004] JP 2011-100573 A

[0005] Here, with regard to the silver powder (silver powder) that can be used in conventional conductive pastes, there is room for further improvement in terms of reducing the line resistance of the conductive film formed using the conductive paste.

[0006] 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, and a method for producing the same. 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 with excellent performance.

[0007] As a result of extensive research by the present inventors to solve the above-mentioned problems, the present inventors have completed the present invention described below.

[0008] That is, the gist and configuration of the present invention for solving the above-mentioned problems is as follows.

[0009] [1] True density is 10 g / cm ３ and the copper content is 10 ppm or more and 10,000 ppm or less.

[0010] [2] The copper-containing silver powder according to [1], wherein when the cross section is observed, particles having voids inside account for at least half of all the silver particles.

[0011] [3] BET specific surface area is 0.1 m ２ / g or more 1m ２ / g or less.

[0012] [4] The copper-containing silver powder according to any one of [1] to [3], wherein the cumulative 50% particle diameter on a volume basis is 0.5 μm or more and 6 μm or less.

[0013] [5] A conductive paste comprising the copper-containing silver powder according to any one of [1] to [4], aluminum, and a solvent.

[0014] [6] A conductive film formed using the conductive paste of [5].

[0015] [7] A solar cell comprising the conductive film according to [6].

[0016] [8] A method for producing a copper-containing silver powder according to any one of [1] to [4], comprising: a reduction step of adding a reducing agent to a mixed solution containing a silver compound and a copper compound to precipitate void-containing silver particles; and a separation step of separating and drying the void-containing silver particles from the mixed solution to obtain copper-containing silver powder, wherein the reducing agent comprises a compound having an aldehyde group.

[0017] 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. Furthermore, according to the present invention, a conductive paste capable of reducing the line resistance of a conductive film can be provided. Furthermore, according to the present invention, a conductive film with reduced line resistance can be provided. Furthermore, according to the present invention, a solar cell with excellent performance can be provided.

[0018] 1 is a scanning electron microscope photograph of a cross section of the copper-containing silver powder according to Example 1.

[0019] The copper-containing silver powder of the present invention is suitable for use as a conductive filler for conductive pastes. The conductive paste using the copper-containing silver powder of the present invention can be used to form conductive patterns or electrodes on a substrate. The 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 conductive films such as conductive patterns or electrodes.

[0020] In recent years, in solar cells (particularly n-type solar cells) comprising a silicon substrate and a conductive film such as an electrode or 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 electrode, etc. However, adding aluminum to the conductive paste increases the bulk resistance of the formed conductive film, 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 is capable of reducing the line resistance of the conductive film and is therefore particularly suitable for use in solar cells, specifically in aluminum-containing conductive pastes used to form the conductive film provided in solar cells.

[0021] (Terminology and Measurement Methods) First, prior to describing the embodiments, the terminology and measurement methods used in this specification will be described.

[0022] <Confirmation of the Presence or Absence of Voids Inside Silver Particles> In this specification, the presence or absence of voids was confirmed by observing the silver particles using the following method. First, copper-containing silver powder was placed in a resin and a curing agent, solidified, and the solidified resin was cut. Next, the cut surface was polished with a cross-section polisher to expose the cross-section of the silver particle, and each silver particle was observed under a scanning electron microscope to confirm the presence or absence of voids inside the silver particle. As the resin and curing agent, for example, "Epofix Resin" and "Epofix Hardener" manufactured by Struers can be used. As the cross-section polisher, for example, "ArBlade 5000" 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. Note that, in this specification, "voids inside silver particles" does not refer to the spaces created by the irregularities on the surface of the silver particle, but rather to the closed spaces inside the silver particle. In this specification, silver particles having voids therein may be referred to as "void-containing silver particles."

[0023] As used herein, the term "void diameter (i.e., the diameter of the closed space inside a void-containing silver particle)" refers to the diameter of the smallest circle that can contain all of the voids when drawn in a scanning electron microscope image. Note that, 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-mentioned method.

[0024] <Measurement of Copper Content in Copper-Containing Silver Powder> The copper content in the copper-containing silver powder was quantified using the following analytical method. 1) 1 g of sample was precisely weighed, and 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added, followed by heating at 200°C for 30 minutes. 2) After allowing the heated sample in 1) to cool, the volume was adjusted to 100 mL using pure water, and 5 mL of the supernatant was removed from the sample and adjusted to 100 mL again using pure water to prepare a sample for ICP analysis. 3) The copper standard solution used to create the calibration curve was prepared using 5N silver to have a silver concentration equivalent to that of the sample for ICP analysis. 4) ICP quantification was performed using an Aglent 5800 ICP-OES manufactured by Agilent Technologies.

[0025] <Carbon, oxygen, and nitrogen contents 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 Corporation) to measure the amount of oxygen and nitrogen in an Ar gas atmosphere at an impulse furnace power value of 3500 W.

[0026] <True Density of Copper-Containing Silver Powder> In this specification, the "true density of copper-containing silver powder" refers to the density of the copper-containing silver powder taking into account the voids present inside the void-containing silver particles (i.e., the above-mentioned "closed spaces inside the void-containing silver particles"). In this specification, the true density of the copper-containing silver powder was measured using a dry automatic densimeter (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 a constant pressure was reached, and the mass of the copper-containing silver powder was calculated by dividing the mass by this volume. Note that the helium gas can reach the uneven spaces on the particle surfaces 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, generally, 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.

[0027] <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 an SEM image at a magnification of 5,000 times using image processing software Mac-View (Ver. 4) manufactured by MOUNTECH, and using this as the average value of the Heywood diameters.

[0028] <BET Specific Surface Area> In this specification, the "BET specific surface area" was measured by a BET single-point method using nitrogen adsorption using a Macsorb HM-model 1210 (manufactured by MOUNTECH Co., Ltd.) In measuring the BET specific surface area, the degassing conditions before measurement were 60°C and 10 minutes.

[0029] <Particle size distribution> In this specification, the minimum particle diameter (D ＭＩＮ ), cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), and cumulative 95% particle diameter (D ９５ ), and the particle diameter of cumulative 100% (i.e., the largest particle) (D ＭＡＸ ) was measured using a laser diffraction / scattering particle size distribution measuring device (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.).

[0030] <Measurement of Copper and Aluminum Contents in Conductive Paste> The copper and aluminum contents in the conductive paste were quantified using the following analytical method. 1) 1 g of paste was precisely weighed, washed with acetone, and 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 heated at 200°C for 30 minutes. 3) After cooling, the sample heated in 2) was adjusted to a constant volume of 100 mL using pure water, and 5 mL of the supernatant was removed from the sample and adjusted to a constant volume of 100 mL using pure water to prepare a sample for ICP analysis. 4) The copper and aluminum standard solutions used to create the calibration curve were prepared using 5N silver to have the same silver concentration as the sample for ICP analysis. 5) ICP volumetric analysis was performed using an Agilent 5800 ICP-OES manufactured by Agilent Technologies.

[0031] <Measurement of Silver Content in Conductive Paste> The silver content in the conductive paste was quantified using the following analytical method. 1) 1 g of paste was precisely weighed, washed with acetone, and 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 mixture 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 aged for 12 hours or more. 5) The silver chloride was filtered using 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 approximately 3 hours until it reached a constant weight. 7) After cooling in a desiccator for 1 hour or more, the glass filter containing silver chloride was precisely weighed, and the silver content was calculated from the mass of silver chloride produced.

[0032] (Copper-containing silver powder) The copper-containing silver powder of the present invention contains silver particles having voids therein (void-containing silver particles), and may optionally contain silver particles having no voids therein. The copper-containing silver powder of the present invention has a true density of 10 g / cm ３ The copper content is less than 10 ppm and 10,000 ppm. The copper-containing silver powder described above can reduce the line resistance of the conductive film. In particular, the effect of reducing bulk resistance is remarkable in aluminum-containing conductive pastes. The reason for this is presumably that the bulk resistance of the conductive film can be effectively reduced by forming a conductive film using a conductive paste containing the copper-containing silver powder. The density of silver without internal voids is 10.49 g / cm. ３ and the density of copper is 8.93 g / cm ３ When silver contains copper, its density decreases, but even when the copper content is about 10,000 ppm, the density is 10.47 g / cm ３ The true density is 10 g / cm ３"Less than" means that when the cross section of the copper-containing silver powder is observed, void-containing silver particles having voids therein can be sufficiently observed. For example, when the cross section is observed, the ratio of the void area to the particle cross-sectional area of ​​the void-containing silver particles is preferably 1% or more on average. 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 more than half of the total silver particles.

[0033] 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 even more 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.

[0034] The carbon content in the copper-containing silver powder means 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, or may be 0.2 mass% or more, or 0.3 mass% or more, or, for example, 1 mass% or less, or 0.7 mass% or less, or 0.5 mass% or less.

[0035] The oxygen content in the copper-containing silver powder means 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, or may be 0.2 mass% or more, or 0.3 mass% or more, or, for example, 1 mass% or less, or 0.8 mass% or less, or 0.6 mass% or less.

[0036] 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.

[0037] <Void-containing silver particles> In the copper-containing silver powder, the void-containing silver particles are silver particles having voids therein and preferably contain copper. In addition, as a method for confirming that the void-containing silver particles contain copper, for example, EDS or XPS can be used.

[0038] The average void diameter of void-containing silver particles 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 image at 10,000 to 40,000 magnifications. 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.

[0039] When the void-containing silver particles contain copper, the content of silver particles made of silver in the copper-containing silver powder 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, 30 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.

[0040] <Properties of copper-containing silver powder> The true density of the copper-containing silver powder is 10 g / cm ３ or less, and 9.8 g / cm ３ It is preferable that the density is 9.7 g / cm or less. ３ It is more preferable that the true density of the copper-containing silver powder is 10 g / cm or less. ３ On the other hand, the true density of the copper-containing silver powder is, for example, 9 g / cm ３ or more, and 9.2 g / cm ３ or more, 9.4 g / cm ３ More than that is fine.

[0041] The minimum particle size (D) of the copper-containing silver powder based on volume ＭＩＮ) is, for example, 0.05 μm or more, may be 0.1 μm or more, or may be 0.2 μm or more, and is, for example, 1 μm or less, may be 0.7 μm or less, or may be 0.5 μm or less.

[0042] Cumulative 10% particle diameter (D １０ ) is, for example, 0.1 μm or more, may be 0.5 μm or more, or may be 0.8 μm or more, and is, for example, 2 μm or less, may be 1.5 μm or less, or may be 1.2 μm or less.

[0043] Cumulative 50% particle diameter (D ５０ ) 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. ５０ If is within the above range, the line resistance of the conductive film can be effectively reduced.

[0044] Cumulative 90% particle diameter (D ９０ ) is, for example, 1.5 μm or more, may be 2 μm or more, or may be 2.5 μm or more, and is, for example, 5 μm or less, may be 4 μm or less, or may be 3.5 μm or less.

[0045] Cumulative 95% particle diameter (D) of the volume-based copper-containing silver powder 9５ ) is, for example, 2 μm or more, may be 2.3 μm or more, or may be 3 μm or more, and is, for example, 10 μm or less, may be 7 μm or less, or may be 4 μm or less.

[0046] The particle diameter (D) of the cumulative 100% (i.e., the largest particle) based on the volume of the copper-containing silver powder ＭＡＸ ) is, for example, 3 μm or more, may be 4 μm or more, or may be 5 μm or more, and may be, for example, 15 μm or less, may be 10 μm or less, or may be 8 μm or less.

[0047] The BET specific surface area of ​​the copper-containing silver powder is 0.1 m ２ / g or more, and ２ / g or more, and 0.45m ２ / g or more, and２ / g or less, and ２ / g or less, and more preferably 0.6m ２ 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.

[0048] (Method for producing copper-containing silver powder) The method for producing copper-containing silver powder of the present invention includes a reduction step in which a reducing agent is added to a mixed solution containing a silver compound and a copper compound 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 copper-containing silver powder, wherein the reducing agent includes a compound having an aldehyde group. The above-mentioned production method can produce the copper-containing silver powder of the present invention. In addition to the reduction step and separation step, the production method of the present invention may optionally further include a surface treatment agent addition step in which a surface treatment agent is added to the mixed solution containing the void-containing silver particles obtained in the reduction step to obtain surface-treated void-containing silver particles.

[0049] <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 void-containing silver particles. The mixed solution usually contains water as a solvent and is an aqueous solution or suspension containing a silver compound and a copper compound. The mixed solution containing a silver compound and a copper compound may optionally contain a pH adjuster or the like. As the pH adjuster that may be contained in the mixed solution, a common acid or base may be used, for example, nitric acid, sodium hydroxide, etc. In the reduction step, it is preferable to adjust the mixed solution to alkaline. In addition, the mixed solution after precipitating the void-containing silver particles is usually a suspension (so-called slurry) or dispersion in which the void-containing silver particles are dispersed.

[0050] The reducing agent added to the mixed solution in the reduction step contains a compound having an aldehyde group. By using a reducing agent containing a compound having an aldehyde group, void-containing silver particles having voids inside can be efficiently obtained. Examples of such reducing agents include an aqueous formaldehyde solution (so-called formalin) and an aqueous acetaldehyde solution, but formalin is preferably used because it allows for more efficient production of void-containing silver particles having voids inside. Furthermore, from the viewpoint of effectively generating voids, ultrasound or the like may be used during reduction and precipitation.

[0051] The silver compound is not particularly limited, and examples thereof include silver nitrate, silver oxide, etc. Furthermore, 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.

[0052] The copper compound is not particularly limited, and examples thereof include copper(II) nitrate trihydrate, copper oxide, etc. Furthermore, it is 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.

[0053] In the mixed solution, the ratio of copper to silver is preferably 0.001% by mass or more, preferably 0.003% by mass or more, preferably 3% by mass or less, more preferably 1% by mass or less, even more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less. If the ratio of copper to silver is 0.001% by 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% by mass or less, the silver ratio in the silver-containing copper powder can be made to be a certain level or more, and as a result, the line resistance of the conductive film can be effectively reduced.

[0054] <Surface Treatment Agent Addition Step> In the optional surface treatment agent addition step, a surface treatment agent is added to a 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 dispersion in which the surface-treated void-containing silver particles are dispersed.

[0055] 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.

[0056] The amount of the surface treatment agent added in the surface treatment agent addition step 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 step.

[0057] The amount of surface treatment agent that adheres to the surface-treated void-containing silver particles in the surface treatment agent addition step and remains after the separation step is a value measured on the copper-containing silver powder after the separation step described below, with the type of surface treatment agent specified. The type of surface treatment agent can be specified by qualitative analysis by gas chromatography of the surface treatment agent that has been vaporized by heating the copper-containing silver powder.

[0058] The amount of the surface treatment agent attached to the copper-containing silver powder can be determined according to the method for quantitative analysis of fatty acids described in Japanese Patent No. 5622543. Specifically, the copper-containing silver powder is first dissolved in acid, and then an organic solvent is mixed therewith to extract the entire amount of the surface treatment agent into the organic solvent phase. A predetermined amount of the organic solvent phase is then separated and evaporated to dryness, and the remaining solid matter is measured for its carbon content using a carbon-sulfur analyzer, whereby the amount of the surface treatment agent attached can be calculated.

[0059] For example, if the surface treatment agent is specified to be stearic acid and no carbon source other than stearic acid is contained in the copper-containing silver powder, the method for measuring stearic acid is as follows.

[0060] When a calibration curve is obtained by measuring the carbon amount (strength) of each standard solution containing different amounts (mg) of stearic acid using a carbon-sulfur analyzer, the slope of the curve is defined as 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: X (mg) = (C / A x a / b) (A); Y (%) = X / (M x 1000) x 100 (B).

[0061] When oleic acid is used as a treatment agent, the carbon content is measured and determined in the same manner as above. Calculations for oleic acid are also performed using the calibration curve for stearic acid. Since the molecular weight of stearic acid is 284.48, of which the carbon content is 216.19, and the molecular weight of oleic acid is 282.46, of which the carbon content is 216.19, the oleic acid concentration Y' can be calculated using the following formula (C): Oleic acid concentration Y' (%) = Y x (216.19 / 284.48) x (282.46 / 216.19) (C)

[0062] The amount of the surface treatment agent attached 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.

[0063] <Separation step> In the separation step, the void-containing silver particles or optionally surface-treated void-containing silver particles (hereinafter collectively referred to as "void-containing silver particles, etc.") are separated from the mixed solution 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. Note that the separation step may optionally include a washing and recovery step in which the separated void-containing silver particles, etc. are recovered and washed.

[0064] In the washing and recovery step, for example, aggregates of the separated void-containing silver particles, etc. are formed into a cake, and the cake of aggregates of the void-containing silver particles, etc. is washed. Washing in the washing and recovery step may be performed using, for example, pure water. Dehydration in the washing and recovery step may be performed by, for example, 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 a predetermined value or less. The void-containing silver particles, etc. after washing may be subjected to a drying step in an aggregated state, such as a cake.

[0065] In the drying step, aggregates of void-containing silver particles and the like that contain moisture and are in an agglomerated state are dried. The drying step may be performed by vacuum drying or using an airflow dryer. In the drying step, 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, thereby applying a dispersing force to the cake or copper-containing silver powder in the drying process and promoting dispersion and drying.

[0066] 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 void-containing silver particles and the like in the copper-containing silver powder can be effectively suppressed.

[0067] Since the copper-containing silver powder after drying may be in the form of lumps, a dry crushing treatment or classification operation may be carried out simultaneously with or after the drying step 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 fluidity to the extent that it does not interfere with the supply operation into the apparatus, or loosening the copper-containing silver powder to an appropriate extent so that the processing in the apparatus proceeds efficiently.

[0068] The method of dry crushing treatment is not particularly limited and can be appropriately selected depending on the purpose, but it is preferable to use a crusher that rotates a stirring blade to crush and fluidize the copper-containing silver powder, and for example, a Henschel mixer, a sample mill, a blender, a coffee mill, etc. can be used.

[0069] (Conductive Paste) The conductive paste of the present invention contains the copper-containing silver powder of the present invention described above, aluminum, and a solvent. Because 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.

[0070] 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 is preferably 98 mass% or less, more preferably 95 mass% or less.

[0071] 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. 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.

[0072] <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.

[0073] 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 preferably 98 mass% or less, and more preferably 95 mass% or less. In order to suppress uneven distribution of copper due to the filler in the conductive paste, the proportion of the copper-containing silver powder of the present invention in the filler is preferably high, preferably 50 mass% or more, and most preferably 100 mass%.

[0074] 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, 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.

[0075] <Aluminum> The aluminum contained in the conductive paste of the present invention may be either metallic aluminum or an aluminum compound, but metallic aluminum is preferred. Examples of metallic aluminum include aluminum (metallic aluminum) and aluminum alloys. In the conductive paste, the aluminum is preferably in the form of a powder, and examples of such powders include aluminum powder (powder composed of metallic aluminum), aluminum alloy powder, silver-coated aluminum powder, and aluminum-adhered silver powder. Aluminum powder is particularly preferred. Since the aluminum powder is composed of metallic aluminum, it contains no components other than impurities (e.g., iron, silicon, etc.) inevitably mixed in during production. The content of metallic aluminum in the aluminum powder is typically 99% by mass or more, preferably 99.5% by mass or more, and more preferably 99.8% by 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 its surface.

[0076] 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, more preferably 2.0 μm or less. When 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 thin conductive film.

[0077] The aluminum content in the conductive paste refers to the mass ratio of aluminum to the total mass of all components in the conductive paste, and may be, for example, 0.1 mass % or more, 0.5 mass % or more, or 1 mass % or more, or, for example, 5 mass % or less, 3 mass % or less, or 2 mass % or less.

[0078] <Solvent> The solvent (i.e., dispersion medium) for 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.

[0079] 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.

[0080] <Other Components> The conductive paste may further contain components other than the copper-containing silver powder, aluminum, solvent, and optional copper-free silver powder (hereinafter, sometimes referred to as "other components"). Examples of other components that may be optionally contained in the conductive paste include glass frit, binders, dispersants, viscosity modifiers, etc. These may be used alone or in combination of two or more.

[0081] The glass frit is a component that can improve the adhesion between the obtained conductive film and the substrate when the conductive paste is fired. The glass frit can be appropriately selected depending on the purpose, and any glass frit that can be generally used for solar cells or the like can be used without any particular limitation. However, 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.

[0082] The content of glass frit in the conductive paste refers to the mass ratio of glass frit to the total mass of all components in the conductive paste. The content of glass frit in the conductive paste is preferably 0.1 mass% or more and 10 mass% or less. Preferable oxides here include 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.

[0083] Examples of binders include epoxy resins, acrylic resins, polyester resins, polyimide resins, polyurethane resins, phenoxy resins, silicone resins, cellulose, etc. These may be used alone or in combination of two or more.

[0084] 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.

[0085] Examples of viscosity modifiers include methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amides.

[0086] <Properties of Conductive Paste> The viscosity of the conductive paste is not particularly limited and can be selected appropriately depending on the purpose. However, under conditions of a paste temperature of 25°C and a rotation speed of 1 rpm, the viscosity is preferably 150 Pa·s or more, more preferably 200 Pa·s or more, preferably 800 Pa·s or less, and more preferably 750 Pa·s or less. If 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, disconnection of the resulting conductive film can be effectively suppressed, thereby enabling the conductive film to be thinned.

[0087] <Uses of Conductive Paste> The conductive paste of the present invention is suitable for forming a conductive film, i.e., for forming a conductive pattern on a substrate or for forming an electrode. For example, the conductive paste of the present invention can be suitably used to form a conductive film by applying or printing it directly onto various substrates such as silicon wafers for solar cells, films for touch panels, and glass for EL devices, or, if necessary, onto a transparent conductive film further provided on a substrate. The conductive film formed using the conductive paste of the present invention is suitably used for electrodes or electrical wiring applications such as collecting electrodes for solar cells, external electrodes for chip-type electronic components, RFID, electromagnetic wave shielding, vibrator bonding, membrane switches, and electroluminescence.

[0088] <Method for producing conductive paste> The method for producing the conductive paste of the present invention is not particularly limited, and the conductive paste can be obtained, for example, by mixing the materials that can be contained in the conductive paste described above and dispersing and / or kneading the resulting mixture. Note that for dispersion and kneading, for example, ultrasonic dispersion, a disperser, a three-roll mill, a ball mill, a bead mill, a biaxial kneader, a planetary stirrer, etc. can be used.

[0089] (Conductive film) The conductive film of the present invention is formed using the conductive paste of the present invention described above. Since the conductive film of the present invention is formed using the conductive paste of the present invention described above, the line resistance is reduced. Note that the conductive film of the present invention is usually a film formed by applying or printing the conductive paste of the present invention described above onto a substrate or the like and firing the applied paste.

[0090] (Solar Cell) The solar cell of the present invention comprises the conductive film of the present invention described above. The solar cell of the present invention has excellent performance because it comprises a conductive film with reduced line resistance. The conductive film provided 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 conductive paste of the present invention described above onto a substrate or the like, and then firing the same.

[0091] The solar cell of the present invention is not particularly limited as long as it comprises the conductive film of the present invention, and may appropriately comprise a semiconductor substrate, a diffusion layer, an anti-reflection layer, a BSF (Back Surface Field) layer, a front electrode, a back electrode, etc.

[0092] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples in any way.

[0093] (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 a silver nitrate aqueous solution containing 0.14 mol / L silver ions, yielding a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (1% by weight copper relative to silver). Next, while stirring the mixed solution, 113.2 g of 28% by weight aqueous ammonia was added, and the temperature of the mixed solution was adjusted to 45°C. Next, 15 g of a 20% by weight aqueous sodium hydroxide solution was added to the mixed solution to adjust the pH, and 250 g of a 26% by weight aqueous formaldehyde solution 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 stearic acid-containing isopropanol solution relative to silver was added. Stirring was stopped, and the solid was filtered using a Nutsche filter and washed with water. The resulting solid 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.

[0094] [Measurement of Copper Content in Silver Powder X1] The copper content in Silver Powder X1 was quantified using the following analytical method. 1) 1 g of sample was weighed out, and 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added. The mixture was heated at 200°C for 30 minutes. 2) After allowing the heated sample to cool, the volume was adjusted to 100 mL using pure water. 5 mL of the supernatant was then removed and the volume was adjusted to 100 mL again using pure water to prepare a sample for ICP analysis. 3) The copper standard solution used to create the calibration curve was prepared using 5N silver to have a silver concentration equivalent to that of the sample for ICP analysis. 4) ICP quantification was performed using an Aglent 5800 ICP-OES from Agilent Technologies.

[0095] [Measurement of Carbon, Oxygen, and Nitrogen Amounts] The carbon amount was measured using a carbon / sulfur analyzer (EMIA-810W manufactured by Horiba, Ltd.). The oxygen and nitrogen amounts were also measured using an oxygen / nitrogen / hydrogen simultaneous analyzer (ONH836 manufactured by LECO Corporation). The results are shown in Table 1.

[0096] (Preparation of Silver Powder X2) Various operations were carried out in the same manner as in the preparation of Silver Powder X1, except that the amount of 5 mass% aqueous solution of copper (II) nitrate trihydrate added was changed to 4.1 g, and a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (containing 0.1 mass% copper relative to silver) was obtained, thereby obtaining Silver Powder X2, a copper-containing silver powder. Various measurements were then carried out using the obtained Silver Powder X2 in the same manner as Silver Powder X1. The results are shown in Table 1.

[0097] (Preparation of Silver Powder X3) Various operations were carried out in the same manner as in the preparation of Silver Powder X1, except that the amount of 5 mass% aqueous solution of copper (II) nitrate trihydrate added was changed to 0.2 g, and a mixed solution containing silver nitrate and copper (II) nitrate trihydrate (containing 0.005 mass% copper relative to silver) was obtained, thereby obtaining Silver Powder X3, a copper-containing silver powder. Various measurements were then carried out using the obtained Silver Powder X3 in the same manner as Silver Powder X1. The results are shown in Table 1.

[0098] (Preparation of Silver Powders X4 to X6) As Silver Powder X4, a silver powder having internal voids and not containing copper ("AG-4-8FD" manufactured by Dowa Electronics) was prepared. As Silver Powder X5, a silver powder having internal voids and not containing copper ("AG-4-54F" manufactured by Dowa Electronics) was prepared. As Silver Powder X6, an atomized silver-copper alloy powder (SEM average diameter 1.2 μm, manufactured by Dowa Electronics) containing 72 mass% silver and 28 mass% copper was prepared. Note that 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 internal voids. Using Silver Powders X4 to X6, various measurements were performed in the same manner as for Silver Powder X1. The results are shown in Table 1.

[0099] (Preparation of Silver Powder X7) 112.4 g of 25.7 mass% ammonia water was added to 3.3 L of a silver nitrate aqueous solution containing 0.12 mol / L silver ions, and the temperature of the mixture was 35 ° C. Next, 1.017 g of a 5 mass% sodium carbonate aqueous solution and 0.508 g of a 5 mass% polyethyleneimine (PEI) aqueous solution (manufactured by Nippon Shokubai Co., Ltd., weight average molecular weight 600) were added, and stirring was continued. Then, 3 minutes after the addition of the ammonia aqueous solution, 397 g of a 1.9 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. Next, 10 seconds after adding sodium borohydride to this mixed solution containing silver particles, a stearic acid emulsion containing 0.16% by mass of stearic acid relative to the silver was added. Stirring was then stopped, and the solid matter was filtered using a Nutsche filter and washed with water. The resulting solid matter was dried in a vacuum dryer at 73°C for 10 hours. After drying, the solid matter was subjected to a dry crushing process using a sample mill to obtain Silver Powder X7, a copper-containing silver powder. Various measurements were then performed using the resulting Silver Powder X7 in the same manner as for Silver Powder X1. The results are shown in Table 1.

[0100] Example 1 Preparation of Copper-Containing Silver Powder A copper-containing silver powder consisting of the silver powder X1 obtained above was prepared as a filler. The following measurements were carried out using this copper-containing silver powder. A scanning electron microscope image of the copper-containing silver powder according to Example 1 is shown in FIG.

[0101] [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, the copper-containing silver powder was first 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 surfaces were polished with a cross-section polisher (ArBlade 5000, manufactured by Japan High-Technologies Corporation) to expose the cross sections of the silver particles, and each silver particle was observed with a scanning electron microscope (JSM-IT800SHL, manufactured by JEOL Ltd.) to determine the presence or absence of void-containing silver particles having internal voids in the copper-containing silver powder. The results are shown in Table 2. Note that more than half of the observed silver particles had voids.

[0102] [Measurement of Copper Content in Copper-Containing Silver Powder] The copper content in the copper-containing silver powder was quantified using the following analytical method. The results are shown in Table 2. 1) 1 g of sample was weighed out, and 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added. The mixture was heated at 200°C for 30 minutes. 2) After allowing the heated sample to cool, the volume was adjusted to 100 mL using pure water. 5 mL of the supernatant was removed from the sample and the volume was adjusted to 100 mL again using pure water to prepare a sample for ICP analysis. 3) The copper standard solution used to create the calibration curve was prepared using 5N silver to have a silver concentration equivalent to that of the sample for ICP analysis. 4) ICP quantification was performed using an Aglent 5800 ICP-OES manufactured by Agilent Technologies.

[0103] [Measurement of True Density] The true density of the copper-containing silver powder was measured using a dry automatic densitometer. 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 densitometer (Micromeritics Accupyc II1340), the volume of the copper-containing silver powder was measured from the gas volume when helium gas was filled into the crucible until a constant pressure was reached, and the true density was calculated by dividing the mass of the copper-containing silver powder by that volume.

[0104] [Measurement of BET specific surface area] The BET specific surface area of ​​the copper-containing silver powder was measured by the BET single-point method using nitrogen adsorption using a BET specific surface area measuring device (Macsorb HM-model 1210, manufactured by MOUNTECH Co., Ltd.). In measuring the BET specific surface area, the degassing conditions before measurement were 60°C and 10 minutes. The results are shown in Table 2.

[0105] [Measurement of particle size distribution] The minimum particle diameter (D ＭＩＮ ), cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), and cumulative 95% particle diameter (D ９５ ), and the particle diameter of cumulative 100% (i.e., the largest particle) (D ＭＡＸ ) was measured by the following method. The results are shown in Table 2. 0.1 g of copper-containing silver powder was added to 40 mL of isopropyl alcohol (IPA) and dispersed for 2 minutes using an ultrasonic homogenizer (device 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 particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.).

[0106] <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% by mass and an impurity content of 0.13% by mass (iron 0.09% by mass, silicon 0.04% by mass), and glass powder (containing PbO as the main component, B ２ O ３ , SiO ２and other oxides), 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 the composition shown in Table 3, to obtain a mixture. Next, the obtained mixture was premixed using a planetary 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.

[0107] [Line Resistance Value] Using the conductive paste obtained above, a straight line shape was printed by screen printing. The straight line had a designed line width of 12 μm and a straight line length of 150 mm. A Microtec printer was used for printing, and printing was performed at a ski speed of 350 mm / s. A silicon substrate (for solar cell applications, texture formation, SiN) with a thickness of approximately 170 μm was used for printing. ｘ After printing, the wafer was dried for 5 minutes in a dryer set at 200°C, and then baked in a solar cell baking furnace (manufactured by NGK) under conditions such that the peak temperature on the top surface of the wafer reached 750°C to produce a sample. The resistance value of the electrode after baking (the line resistance value of the conductive film) was measured with a digital multimeter (manufactured by ADC Corporation) by placing measurement terminals on both ends of the printed electrode. The results are shown in Table 2.

[0108] (Example 2) Various operations and measurements were carried out in the same manner as in Example 2, except that 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 silver content in the conductive paste of Example 2 was 85.420 mass% and the copper content was 0.030 mass%. In addition, when observing the void-containing silver particles, more than half of the observed silver particles had voids.

[0109] (Example 3) Various operations and measurements were carried out in the same manner as in Example 3, except that 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 silver content in the conductive paste of Example 3 was 85.446 mass %, and the copper content was 0.004 mass %. In addition, when observing the void-containing silver particles, more than half of the observed silver particles had voids.

[0110] (Comparative Example 1) Various operations and measurements were carried out in the same manner as in Example 1, 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.

[0111] (Comparative Example 2) Various operations and measurements were carried out in the same manner as in Example 1, except that a mixed silver powder consisting of Silver Powder X5 and Silver Powder X6 was used as the filler instead of the copper-containing silver powder consisting of Silver Powder X1. The results are shown in Table 2.

[0112] (Comparative Example 3) Various operations and measurements were carried out in the same manner as in Example 1, except that a mixed silver powder consisting of Silver Powder X4 and Silver Powder X6 was used as the filler instead of the copper-containing silver powder consisting of Silver Powder X1. The results are shown in Table 2.

[0113] (Comparative Example 4) Various operations and measurements were carried out in the same manner as in Example 1, except that the copper-containing silver powder made of Silver Powder X7 was used as the filler instead of the copper-containing silver powder made of Silver Powder X1. The results are shown in Table 2.

[0114]

[0115]

[0116]

[0117] As is clear from Table 2, the copper-containing silver powders of the examples can reduce the line resistance of the conductive film.

[0118] 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. Furthermore, according to the present invention, a conductive paste capable of reducing the line resistance of a conductive film can be provided. Furthermore, according to the present invention, a conductive film with reduced line resistance can be provided. Furthermore, according to the present invention, a solar cell with excellent performance can be provided.

Claims

1. True density is 10g / cm ３ and the copper content is 10 ppm or more and 10,000 ppm or less.

2. The copper-containing silver powder according to claim 1, in which, when observed in cross section, particles having internal voids account for more than half of all silver particles.

3. BET specific surface area is 0.1m ２ / g or more 1m ２ The copper-containing silver powder according to claim 1, wherein the copper content is 0.01 to 0.15 wt %.

4. The copper-containing silver powder according to claim 1, 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 according to any one of claims 1 to 4, aluminum, and a solvent.

6. A conductive film formed using the conductive paste according to claim 5.

7. A solar cell comprising the conductive film according to claim 6.

8. A method for producing copper-containing silver powder according to any one of claims 1 to 4, comprising: a reduction step of adding a reducing agent to a mixed solution containing a silver compound and a copper compound to precipitate void-containing silver particles; and a separation step of separating the void-containing silver particles from the mixed solution and drying them to obtain copper-containing silver powder, wherein the reducing agent comprises a compound having an aldehyde group.

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