Aluminum oxide-coated silver powder, method for manufacturing the same, and conductive paste

Abstract

When used in forming electrodes for solar cells, the open-circuit voltage (V) OC Silver powder capable of improving the open-circuit voltage (V) and a method for manufacturing the same. Furthermore, the open-circuit voltage (V) can be improved by the formed conductive film. OC The present invention provides a sintered conductive paste for solar cell electrodes that can be improved. [Solution] When depositing metallic silver particles from an aqueous solution containing silver ions, an aluminum compound is coexisted in the aqueous solution, and a reducing agent that has the effect of lowering the pH of the aqueous solution when added as a reducing agent, or a reducing agent whose oxidation products have the effect of lowering the pH of the aqueous solution is used. By depositing aluminum oxide on the surface of the deposited silver particles at a concentration of 400 ppm to 3000 ppm by mass relative to the mass of the aluminum oxide-coated silver powder, it is possible to obtain aluminum oxide-coated silver powder that has low resistance when the deposited silver powder is made into a paste and used to form electrodes.

Description

[Technical Field]

[0001] The present invention relates to aluminum oxide-coated silver powder and a method for producing it, as well as a baked conductive paste, suitable for use in conductive pastes for forming electrodes of solar cells. [Background technology]

[0002] Traditionally, resin-type or fired silver paste has been widely used to form the electrodes and circuits of solar cells. The conversion efficiency of solar cells is related to the short-circuit current (I sc (A)) and the open-circuit voltage (V OC It can be calculated by dividing the product of three parameters, (V), fill factor (shape factor FF (%)), by the light-receiving area and irradiance. Here, the fill factor is one of the indicators that represent the performance of a solar cell, and it represents the degree of rectangularity of the output characteristic curve of the solar cell, and the conversion efficiency of the solar cell is I sc , V OC If FF can be maximized, it can be greatly improved. In the case of conductive paste using silver powder as a filler, by making the electrodes thinner, I sc This increases the line resistance of the conductive film formed using the conductive paste and the contact resistance at the interface between the conductive film and silicon, thereby increasing the FF (fastest filament).

[0003] For example, Patent Document 1 discloses that by using a firing-type silver paste containing silver powder with closed voids inside the particles, it is possible to draw fine wiring and form electrode wiring that has even lower resistance than conventional wiring after firing. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-056050 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, in Patent Document 1, although the drawing property of fine wiring and the improvement of the resistance value when using silver powder containing silver particles having the above-mentioned closed voids inside the particles are disclosed, the effect of improving the open voltage (V OC ) is not mentioned. Since the resin type and sintered type conductive pastes have different solar cell structures, the effects of the silver powder used on V OC are different.

[0006] The resin type conductive paste is mainly used for heterojunction solar cells. In a heterojunction solar cell, an i - amorphous Si layer is laminated on the surface of an n - type semiconductor, and this amorphous Si layer forms a hydrogen bond with a portion having surface defects to passivate the defects, thereby having a structure capable of obtaining a high open voltage (V OC ). Therefore, it can be said that the effect on V OC of the silver powder used for the conductive paste is small.

[0007] On the other hand, the sintered type conductive paste is mainly used for PERC (passivated emitter rear contact) solar cells and TOPCon (tunnel oxide passivated contact) solar cells. In the case of these solar cells, in order to reduce the contact resistance (interface resistance) at the interface between the semiconductor layer and the conductive film, glass powder is added to the conductive paste together with the silver powder to decompose the passivation layer of the substrate. This glass powder may decompose the passivation layer and reach not only the n - type semiconductor layer but also the p - type semiconductor layer. In that case, silver dissolved in the glass ensures conduction with the n - type semiconductor layer while short - circuiting with the p - type semiconductor layer, increasing the carrier recombination loss and causing a problem that the open voltage (V OC ) decreases. Therefore, silver powder used for the sintered type conductive paste is required to be able to reduce the contact resistance at the interface between the semiconductor layer and the electrode and to be difficult to short - circuit with the p - type semiconductor layer.

[0008] The technical problem to be solved in this invention is the open-circuit voltage (V) when used for forming electrodes in solar cells. OC The objective is to provide silver powder that can improve the open-circuit voltage (V). OC The objective is to provide a sintered conductive paste for solar cell electrodes that is advantageous for forming conductive films that can improve the performance of the material. [Means for solving the problem]

[0009] As a result of diligent research to achieve the above-mentioned objectives, the inventors have found that by depositing a predetermined amount of aluminum oxide on the surface of silver powder, the open-circuit voltage (V) can be increased. OC We discovered that it is possible to increase ) and completed the present invention described below.

[0010] In other words, in order to achieve the above-mentioned objectives, the present invention provides: (1) Aluminum oxide coated silver powder consisting of silver particles coated with aluminum oxide on the surface, wherein the amount of aluminum is 400 ppm by mass or more and 3000 ppm by mass or less relative to the mass of the aluminum oxide coated silver powder, and the volume-based cumulative 50% diameter D measured by a laser diffraction scattering particle size distribution analyzer. 50 The particle size is between 0.2 μm and 5.0 μm, and the true density is 10.00 g / cm³. 3 The following aluminum oxide-coated silver powder is provided. (2) The aluminum oxide-coated silver powder in item (1) above preferably has an aluminum content of 600 ppm by mass or more and 2500 ppm by mass or less relative to the mass of the aluminum oxide-coated silver powder. (3) When the aluminum oxide-coated silver powder described in items (1) and (2) above is observed in cross-section, it is preferable that the silver particles having voids inside the particles account for 70 percent or more of the total silver particles. (4) The aluminum oxide-coated silver powder described in items (1) to (3) above is D 50 and BET diameter D BET The ratio (D 50 / D BET It is preferable that the ratio is between 1.00 and 2.50. (5) The aluminum oxide-coated silver powder described in items (1) to (4) above has a volume-based cumulative 90% diameter D measured by a laser diffraction scattering particle size distribution analyzer. 90 and volume-based cumulative 10% diameter D 10 The difference (D 90 -D 10 ) and the aforementioned D 50 The ratio ((D 90 -D 10 ) / D 50 It is preferable that the ratio is between 0.50 and 2.00. (6) The aluminum oxide-coated silver powder described in items (1) to (5) above preferably has an ignition loss (Ig-loss) value of 0.30% or more and 3.00% or less. (7) The aluminum oxide-coated silver powder referred to in items (1) to (6) above shall be prepared in a quantity of 2.5 kg / mm 2 The sample obtained by compression molding was given 5 mN / mm 2 In the TMA curve obtained by performing thermomechanical analysis (TMA) while applying a load and heating from room temperature to 900°C at a heating rate of 10°C / min, it is preferable that the temperature at which the expansion coefficient relative to room temperature reaches 0.50% is between 150°C and 350°C. (8) The aluminum oxide-coated silver powder described in items (1) to (7) above can be used in a fired conductive paste for forming solar cell electrodes.

[0011] Furthermore, the following inventions are provided. (9) A baked conductive paste for forming solar cell electrodes using the aluminum oxide-coated silver powder described in items (1) to (8) above.

[0012] As a method for producing the above aluminum oxide-coated silver powder, (10) A process to produce a silver-ammine complex aqueous solution containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" to obtain a silver-ammine complex aqueous solution by adding ammonia to a silver ion-containing aqueous solution to form a silver-ammine complex, and a "pH adjustment step" to adjust the pH of the aqueous solution to 10.0 or higher by adding a pH adjusting agent to the silver-ammine complex aqueous solution, wherein one or more methods selected from a method using the silver ion-containing aqueous solution containing an aluminum compound in the complex formation step, a method adding the aluminum compound to the aqueous solution during the period until the completion of the pH adjustment step, and a method adding the aluminum compound to the aqueous solution after the pH adjustment step has been completed, The process involves adding a reducing agent to the aforementioned aluminum-containing silver-ammine complex aqueous solution with a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously lowering the pH of the aqueous solution to 4.0 or higher and 9.0 or lower, thereby precipitating aluminum oxide. A method for producing aluminum oxide-coated silver powder, The amount of aluminum supplied to the liquid by the above-mentioned aluminum compound is such that the amount of aluminum relative to the amount of silver in the aqueous solution at the start of the addition of the reducing agent is between 400 ppm by mass and 3000 ppm by mass. A method for producing aluminum oxide-coated silver powder is provided, wherein the reducing agent is an organic compound containing one or more of the COOH group, CHO group, and OH group in its molecule. (11) In the method for producing aluminum oxide-coated silver powder described in item (10) above, the reducing agent is preferably formaldehyde. (12) The manufacturing method described in items (10) to (11) above may further include a step of adding a surface treatment agent to a slurry containing silver powder precipitated by adding the reducing agent. (13) In the manufacturing method described in item (12), the amount of the surface treatment agent added is preferably 0.05% by mass or more and 0.50% by mass or less, relative to the amount of silver in the silver-ammine complex. [Effects of the Invention]

[0013] By using the aluminum oxide-coated silver powder of the present invention, when the aluminum oxide-coated silver powder is made into a paste and a conductive film is formed on a solar cell substrate, coupling loss due to the conductive film is suppressed, and a high open-circuit voltage (V) is achieved. OC ) can be obtained. [Brief explanation of the drawing]

[0014] [Figure 1] This is a design drawing for a screen printing plate used for alternative evaluation of open-circuit voltage (VOC). [Figure 2] This is the XPS spectrum (Al2p) obtained for aluminum oxide-coated silver powder obtained according to the examples and comparative examples of the present invention. [Figure 3] This is the TMA curve of aluminum oxide-coated silver powder obtained by the examples and comparative examples of the present invention. [Figure 4] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 1 of the present invention. [Figure 5] These are SEM images of the aluminum oxide-coated silver powder obtained in Example 1 of the present invention, along with the Lα characteristic X-ray image of silver and the Kα characteristic X-ray image of aluminum. [Figure 6] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 2 of the present invention. [Figure 7] These are SEM images of the aluminum oxide-coated silver powder obtained in Example 2 of the present invention, along with the Lα characteristic X-ray image of silver and the Kα characteristic X-ray image of aluminum. [Figure 8] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 3 of the present invention. [Figure 9] These are SEM images of the aluminum oxide-coated silver powder obtained in Example 3 of the present invention, along with the Lα characteristic X-ray image of silver and the Kα characteristic X-ray image of aluminum. [Figure 10] This is a cross-sectional SEM image of commercially available silver powder (4-8FD manufactured by DOWA High-Tech Co., Ltd.) obtained in Comparative Example 1 of the present invention. [Figure 11] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Comparative Example 2 of the present invention. [Figure 12] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Comparative Example 3 of the present invention. [Figure 13] These are SEM images of the aluminum oxide-coated silver powder obtained in Comparative Example 3 of the present invention, along with the Lα characteristic X-ray image of silver and the Kα characteristic X-ray image of aluminum. [Modes for carrying out the invention]

[0015] The aluminum oxide-coated silver powder of the present invention is suitable for use as a filler in conductive pastes for forming electrodes in solar cells. Conductive pastes using the silver powder of the present invention are printed on solar cell substrates, for example, by screen printing, offset printing, or photolithography, to form conductive films such as conductive patterns and electrodes. Since the aluminum oxide-coated silver powder of the present invention can reduce carrier recombination losses due to silver in the conductive film, it can be suitably used in sintered conductive pastes used to form conductive films in solar cells.

[0016] [Silver powder coated with aluminum oxide] The aluminum oxide-coated silver powder of the present invention is a silver powder containing silver particles coated with aluminum oxide on their surface, and has a true density of 10.00 g / cm³. 3 The silver powder is as follows: The amount of aluminum deposited is 400 ppm by mass or more and 3000 ppm by mass or less relative to the total mass of the aluminum oxide-coated silver powder. Furthermore, the aluminum oxide-coated silver powder of the present invention has a volume-based cumulative 50% diameter D measured by laser diffraction scattering particle size distribution measurement. 50 The particle size is in the range of 0.2 μm to 5.0 μm. By forming a paste of aluminum oxide-coated silver powder that satisfies these conditions and creating a conductive film on a substrate, the open-circuit voltage (V) of the final solar cell can be determined. OC This makes it possible to suppress the decline of ( ).

[0017] In this specification, silver particles coated with aluminum oxide on their surface refer to silver particles on which aluminum oxide is coated so as to cover part or all of the surface. Whether or not a silver particle has aluminum oxide coated on part or all of its surface can be confirmed by measuring the distribution of aluminum on the surface of the silver particle using energy-dispersive X-ray fluorescence spectroscopy (EDX), as described later, and by measuring the chemical bonding state of aluminum using X-ray photoelectron spectroscopy (XPS), as described later. It should be noted that, as long as the effects of the present invention are achieved, it is permissible for some of the particles in the silver powder to be silver particles that do not have aluminum oxide coated on their surface. For example, the proportion of silver particles with aluminum oxide coated on part or all of their surface to the total number of particles in the silver powder should be 80% or more, preferably 90% or more, and the upper limit should be 100% or more. If the proportion of silver particles with aluminum oxide coated on part or all of their surface is within the above range, the effect of suppressing the uneven distribution of aluminum in the conductive paste is excellent.

[0018] In the present invention, the deposition of aluminum oxide onto the surface of silver particles is carried out by depositing aluminum oxide onto the surface of the silver particles during the reduction deposition of the silver particles, as described later. This allows the deposited material to be present together with the silver particles even when the aluminum oxide-coated silver powder is mixed into a conductive paste, and makes it possible to suppress the uneven distribution of silver particles and aluminum oxide within the conductive paste.

[0019] The aluminum distribution area ratio per silver powder particle of the present invention can be determined by binarizing the aluminum distribution image obtained by the aforementioned EDX using image processing software. The aluminum distribution area ratio can be calculated by estimating the average value of the aluminum distribution area ratio per particle in a sample of 10 particles. The detailed measurement method will be described later. The average value of the aluminum distribution area ratio per silver particle of the present invention is preferably 50% or more, more preferably 70% or more, and even more preferably 85% or more. A value of 50% or more is preferable because it suppresses the uneven distribution of aluminum in the conductive paste, as aluminum oxide is distributed to a certain extent on the surface of the silver particles.

[0020] In this specification, "aluminum content" in aluminum oxide-coated silver powder refers to the amount of aluminum relative to the mass of the aluminum oxide-coated silver powder, measured using inductively coupled plasma atomic emission spectroscopy (ICP-OES) after completely dissolving the aluminum oxide-coated silver powder with an acid. In this invention, aluminum is abundant on the surface side of the silver particles, but it is considered to exist in two forms: aluminum present within the silver particles and aluminum coated on the surface as aluminum oxide. In this specification, "aluminum content" is the sum of the amount of aluminum present within the silver particles and the amount of aluminum coated on the surface.

[0021] Since ICP-OES cannot perform state analysis, whether or not the aluminum present on the surface of the silver particles exists in the form of aluminum oxide, that is, whether or not the particles are "silver particles coated with aluminum oxide," is determined by state analysis of the aluminum using X-ray photoelectron spectroscopy (XPS). In this invention, if a peak exists in the range of 74.3 ± 1 eV, which is the binding energy of oxidized aluminum, in the obtained XPS spectrum, the aluminum is determined to be aluminum oxide. In this invention, it is preferable that the aluminum present on the surface of the silver particles is aluminum oxide, but it may also contain a small amount of peaks representing metallic aluminum, for example. In the XPS measurement described later, the ratio of the height of the peak corresponding to metallic aluminum to the peak corresponding to aluminum oxide is preferably 10% or less.

[0022] The amount of aluminum is preferably 400 ppm by mass or more relative to the total mass of the silver powder. If the amount of aluminum is less than 400 ppm by mass, the V of the electrode film of the present invention OC The improvement effect is insufficient. Furthermore, since aluminum oxide is a type of insulator, if the amount of aluminum exceeds 3000 ppm by mass, the line resistance of the conductive film deteriorates, which worsens the characteristics of the solar cell and is therefore undesirable. The amount of aluminum is more preferably 600 ppm by mass or more and 2500 ppm by mass or less, and even more preferably 1000 ppm by mass or more and 2000 ppm by mass or less.

[0023] The aluminum oxide-coated silver powder of the present invention has a true density of 10.00 g / cm³. 3 The following is the value: 9.90 g / cm³ 3 Preferably, it is 9.85 g / cm³. 3 The following is more preferable. In this invention, the true density value in the silver powder refers to the density of the silver powder taking into account the voids present inside the silver particles (i.e., the "closed spaces inside the silver particles" mentioned above). The true density of the silver powder is 10.00 g / cm³. 3The following conditions are preferable because, when observing the cross-section of the silver powder, sufficient silver particles with internal voids can be observed, which facilitates expansion behavior when the conductive paste is fired, and furthermore, the sintering of the entire silver particle proceeds sufficiently quickly due to the reaction of gas components in the internal voids of the particles after the sintering of the particle surface begins. On the other hand, a true density of, for example, 9.00 g / cm³ is preferable. 3 The above is 9.40 g / cm³. 3 The above, or 9.70 g / cm³ 3 That's fine too.

[0024] In the aluminum oxide-coated silver powder of the present invention, when the cross-section of the silver particles constituting the silver powder is observed with a scanning electron microscope (SEM), it can be confirmed that there are fine voids inside the silver particles that are closed from the outside. In the aluminum oxide-coated silver powder, the proportion of silver particles having one or more voids inside the particle is preferably 70% or more of the total silver particles, more preferably 75% or more, even more preferably 80% or more, and most preferably 85% or more. A value of 70% or more is preferable because it makes expansion behavior more likely to occur when the conductive paste is fired, and furthermore, after the sintering of the particle surface begins, the reaction of gas components in the voids inside the particles accelerates, allowing the sintering of the entire silver particle to proceed quickly, and a dense conductive film to be formed. The upper limit of the proportion is 100%.

[0025] In this specification, voids are defined as particles with a diameter of 15 nm or more, as observed using cross-sectional SEM images of silver particles taken at 10,000 to 40,000 times magnification. In this invention, "void diameter (i.e., diameter of the closed space inside the void-containing silver particle)" means the diameter of the outer circle when drawing the smallest circle that contains all the voids in a scanning electron microscope image. The proportion of silver particles with voids inside in the silver powder can be confirmed by observing cross-sectional SEM images of silver particles (containing 10 or more particles) taken at 10,000 times magnification in 5 or more fields of view, and calculating the ratio of the number of particles with one or more voids inside to the number of silver particles observed.

[0026] The aluminum oxide-coated silver powder of the present invention has a volume-based cumulative 50% diameter D measured by a laser diffraction scattering particle size distribution analyzer. 50 It is preferable that the particle size is 0.2 μm or more and 5.0 μm or less, and more preferably 1.0 μm or more and 3.0 μm or less. 50 If the thickness is less than 0.2 μm, the viscosity of the conductive paste increases, making it difficult to increase the silver concentration in the conductive paste. This is undesirable because it may cause disconnections when wiring is drawn using the conductive paste. Also, D 50 If the thickness exceeds 5.0 μm, it becomes difficult to draw fine wiring when using conductive paste, which is undesirable.

[0027] The aluminum oxide-coated silver powder of the present invention has a maximum volume-based diameter D measured by a laser diffraction scattering particle size distribution analyzer. max It is preferable that the particle size is 15.0 μm or less, more preferably 12.0 μm or less, and even more preferably 10.0 μm or less. max If the thickness is 15.0 μm or less, it is possible to draw fine wiring using conductive paste.

[0028] The cumulative 10% diameter D of the aluminum oxide-coated silver powder measured by the laser diffraction scattering particle size distribution analyzer of the present invention is measured by the present invention. 10 The diameter is preferably 0.1 μm or larger, more preferably 1.0 μm or larger. It is also preferably 3.0 μm or smaller, more preferably 2.0 μm or smaller, and even more preferably 1.5 μm or smaller. Cumulative 10% diameter D 10 If the value is within the above range, it is preferable because it prevents the viscosity of the conductive paste from becoming too high.

[0029] The cumulative 90% diameter D of the aluminum oxide-coated silver powder measured by the laser diffraction scattering particle size distribution analyzer of the present invention is determined by volume. 90The particle size is preferably 1.5 μm or larger, more preferably 2.0 μm or larger, and even more preferably 2.5 μm or larger. Furthermore, it is preferably 6.0 μm or smaller, more preferably 5.0 μm or smaller, and even more preferably 4.0 μm or smaller. Cumulative 90%D 90 If the value is within the above range, it is preferable because it makes it easier to draw fine wiring when using conductive paste to draw wiring and other patterns.

[0030] The aluminum oxide-coated silver powder of the present invention has a volume-based cumulative 50% diameter D measured by a laser diffraction scattering particle size distribution analyzer. 50 And the BET diameter D calculated from the BET specific surface area value. BET The ratio (D 50 / D BET ) is preferably 1.00 or more and 2.50 or less. 50 / D BET When the aggregated state is small and the state is close to monodisperse, the value tends to be close to 1, more preferably 2.00 or less, and even more preferably 1.50 or less. If the above value is 2.50 or less, it is effective from the viewpoint of forming fine wiring when wiring is drawn using conductive paste. Note D BET The definition will be explained later.

[0031] The aluminum oxide-coated silver powder of the present invention has a volume-based cumulative 90% diameter D measured by a laser diffraction scattering particle size distribution analyzer. 90 and cumulative 10% diameter D 10 The difference (D 90 -D 10 ) and cumulative 50% diameter D 50 The ratio ((D 90 -D 10 ) / D 50 (D 90 -D 10 ) / D 50This value is an indicator of particle size distribution, and the smaller this value, the sharper the particle size distribution of the aluminum oxide-coated silver powder, meaning that the particle size is uniform. In the present invention, if this value is within the above range, it is preferable because it makes it easier to draw fine wiring when using conductive paste to draw wiring. Setting this value to 2.00 or less is advantageous in suppressing the phenomenon in which fine wiring becomes difficult to form due to the influence of coarse particles when using conductive paste to draw wiring.

[0032] The aluminum oxide-coated silver powder of the present invention preferably has an ignition loss (Ig-loss) of 0.30% to 3.00%, more preferably 0.50% to 1.50%, and even more preferably 0.70% to 1.20%. If the ignition loss is 0.30% or more, aggregation of silver powder particles can be suppressed and dispersibility can be improved. On the other hand, if the ignition loss is 3.00% or less, deterioration of the resistance value due to excess impurities can be prevented.

[0033] The aluminum oxide-coated silver powder of the present invention preferably has a temperature of 150°C to 350°C at which the expansion rate relative to room temperature reaches 0.50% (hereinafter sometimes referred to as the "0.50% expansion temperature") in the TMA curve obtained from room temperature (25°C ± 5°C) to 900°C, obtained by thermomechanical analysis (TMA) with a heating rate of 10°C / min. The maximum value of the expansion rate reached (hereinafter sometimes referred to as the "maximum expansion rate") is 0.50% or more, preferably 1.00% or more, and there is no particular upper limit, but it may be, for example, 5.00% or less, or 3.00% or less. Furthermore, the aluminum oxide-coated silver powder of the present invention preferably has a temperature of 550°C to 900°C at which the expansion rate relative to room temperature becomes 0.00% or less (i.e., less than or equal to the sample length before measurement) after the expansion rate initially increases with heating and then begins to decrease.

[0034] The expansion behavior of the aluminum oxide-coated silver powder is more likely to occur due to the presence of voids within the silver particles. However, even with a similar void ratio, the aluminum oxide-coated silver powder of the present invention tends to exhibit more pronounced expansion behavior compared to silver powder without aluminum oxide coating. As shown in Comparative Example 1 described later, in silver powder with voids and no aluminum oxide coating, the expansion rate relative to room temperature does not reach 0.50% in the temperature range of 150°C to 350°C, and the open-circuit voltage (V OC The effect of reducing the ) is minimal. Generally, it has been tried to avoid the large expansion of silver particles when heated, but in the present invention, by having voids and being coated with aluminum oxide, the open-circuit voltage (V) is reduced in relation to the above-mentioned expansion behavior. OC The effect of reducing ) is greatly improved.

[0035] By using the aluminum oxide-coated silver powder of the present invention, the open-circuit voltage (V) of the solar cell can be increased. OC The mechanism by which the open-circuit voltage (V) decreases is currently unknown, but the inventors believe it to be as follows: In other words, they estimate that the silver powder of the present invention has aluminum oxide adhering to the surface of the silver particles, which reduces the frequency of contact between silver and glass powder in the paste, and thus reduces the amount of silver dissolved in the glass. That is, in the case of PERC type solar cells, glass powder is added to the conductive paste to decompose the passivation layer of the substrate, and ideally, after decomposing the passivation layer, it should stop progressing in the n-type semiconductor layer, but in some cases it penetrates the n-type semiconductor layer and proceeds to the p-type semiconductor layer. In that case, the silver dissolved in the glass short-circuits with the p-type semiconductor, increasing the carrier recombination loss, and the open-circuit voltage (V), which is one of the characteristics evaluations of solar cells, decreases. OC It is known that the open-circuit voltage (V) decreases and the characteristics deteriorate. By using the aluminum oxide-coated silver powder of the present invention, the amount of silver dissolved in the glass can be optimized, so that the silver penetrates to the n-type semiconductor layer, ensuring conductivity while suppressing carrier coupling loss due to short circuits with the p-type semiconductor, thus reducing the open-circuit voltage (V). OC It is believed that this can be improved.

[0036] [Confirmation of the presence or absence of voids within silver particles] In the examples described below, the presence or absence of voids inside the silver particles was confirmed by the following method. First, aluminum oxide-coated silver powder was placed in resin and a hardener and allowed to solidify. The solidified resin was then cut. Next, the cut surface was polished with a cross-section polisher to expose the cross-section of the silver particles. A scanning electron microscope (SEM) was then used to observe the cross-section of each silver particle and confirm the presence or absence of voids inside the particles. For example, the resin and curing agent mentioned above can be "Epofix resin" and "Epofix curing agent" manufactured by Storuas. For example, the cross-section polisher can be "ArBlade5000" manufactured by Nippon High Technologies. For example, the scanning electron microscope can be "JSM-IT800SHL" manufactured by JEOL Ltd. In this specification, "voids within silver particles" refers to enclosed spaces within silver particles, not spaces in the surface irregularities of the silver particles.

[0037] [Manufacturing method] The present invention provides a method for producing aluminum oxide-coated silver powder, which utilizes a wet process that offers advantages in terms of manufacturing cost and mass productivity.

[0038] [Starting materials] In the method for producing aluminum oxide-coated silver powder of the present invention, an aqueous solution containing monovalent silver ions (silver ion-containing aqueous solution) is used as a starting material. As a source of silver ions, known inorganic silver salts used industrially, such as silver(I) nitrate, silver(I) sulfate, silver(I) carbonate, silver(I) chloride, and silver(I) oxide, can be used. Alternatively, an aqueous solution containing monovalent silver ions and aluminum ions may be obtained by adding an aluminum compound to the above aqueous solution. The timing of adding the aluminum compound may be before the addition of ammonia, after the addition of ammonia, or after the addition of the pH adjuster; there are no particular restrictions as long as it is before the addition of the reducing agent.

[0039] Although not specifically defined in this invention, the silver ion concentration in the starting aqueous solution is preferably 1.0% by mass or more and 2.0% by mass or less at the solution preparation stage. If the silver ion concentration is less than 1.0% by mass, the amount of silver powder that can be produced in one reaction will be small, which may be disadvantageous in terms of productivity. If the silver ion concentration exceeds 2.0% by mass, the viscosity of the reaction solution after silver particle deposition will increase, and depending on the manufacturing equipment, it may be difficult to uniformly stir the reaction solution.

[0040] [Complex formation process] In the present invention's method for producing aluminum oxide-coated silver powder, silver ions are complexed with ammonium ions to form a silver-ammine complex, which is then converted into an aqueous solution of the silver-ammine complex. As a source of ammonium ions, ammonia water, ammonium chloride, ammonium carbonate, or other ammonium salts can be used. When ammonium ions are used as a complexing agent, a silver-ammine complex is formed in the aqueous solution. In this case, since the coordination number of the ammine complex is 2, at least 2 moles of ammonium ions are added per mole of silver ions. The reaction temperature for forming the silver-ammine complex is preferably between 10°C and 40°C. The reaction temperature for forming the silver-ammine complex is preferably within the above range, considering that an exothermic reaction occurs during the formation of the silver-ammine complex, in order to set the temperature of the reduction deposition step described later to a desired temperature. If the reaction temperature is too low or too high, temperature adjustment will take time, leading to increased energy costs.

[0041] [pH adjustment process] Subsequently, a pH adjusting agent is added to the silver-ammine complex aqueous solution obtained in the above step to adjust the pH of the aqueous solution to 10.0 or higher. The pH is raised to 10.0 or higher in order to sufficiently increase the reducing power of the reducing agent. As the pH adjusting agent, alkali metals such as sodium hydroxide and calcium hydroxide, or alkali earth metal hydroxides and carbonates can be used. In the method for producing aluminum oxide-coated silver powder of the present invention, there is no particular upper limit to the pH of the aqueous solution, but it is preferable to keep the pH at 13.0 or lower to avoid excessive use of the pH adjusting agent. The temperature of the pH adjustment step is preferably between 10°C and 40°C. If the reaction temperature is too low or too high, it will take time to adjust the temperature, which will increase energy costs.

[0042] [Aluminum compounds] In the present invention, the timing of adding the aluminum compound may be before the addition of ammonia, after the addition of ammonia, or after the addition of the pH adjuster, as described above. In other words, the aluminum compound may be added before the addition of the reducing agent in the silver particle precipitation step described later. The present invention includes a step of adding the aluminum compound to an aqueous solution containing silver ions, an aqueous solution of the silver-ammine complex, or an aqueous solution to which the pH adjuster has been added.

[0043] As for the aluminum compound, any compound that can be added to the aqueous solution in an ionic state depending on the pH range of the aqueous solution at the timing of addition is acceptable. For example, inorganic aluminum salts such as aluminum nitrate, aluminum acetate, and aluminum oxalate can be used. It is also preferable to match the type of acid or base used. For example, it is preferable to use aluminum nitrate in an aqueous silver nitrate solution. Note that since aluminum oxides are amphoteric oxides, aluminum ions (Al) are present in the low pH range. 3+ ), in the high pH range, aluminate ions (AlO 2- It dissolves in the state shown above, and at a pH near neutral, it enters the solid phase stable region of aluminum oxide (Al2O3). Here, aluminum oxide is a concept that includes aluminum hydroxide (Al(OH)3 or Al2O3·3H2O).

[0044] The amount of aluminum supplied into the liquid by the aluminum compound (or the total amount of aluminum supplied if supplied in multiple stages) is preferably such that, at the start of the addition of the reducing agent described later, the amount of aluminum relative to the amount of silver ions in the aqueous solution is 400 ppm by mass or more and 3000 ppm by mass or less. If the amount of aluminum relative to the amount of silver is less than 400 ppm by mass, the effects of the present invention may be insufficient. Furthermore, if the amount of aluminum relative to the amount of silver exceeds 3000 ppm by mass, it is undesirable because aluminum oxide is a type of insulator, which worsens the line resistance. It is more preferable that the amount of aluminum relative to the amount of silver be 600 ppm by mass or more and 2500 ppm by mass or less, and even more preferable that be 1000 ppm by mass or more and 2000 ppm by mass or less.

[0045] Furthermore, when adding an aluminum compound as an aqueous solution, the concentration is preferably 0.5% by mass or more and 10.0% by mass or less as aluminum. If the aluminum concentration is less than 0.5% by mass, the amount of aqueous aluminum compound solution added to obtain the desired aluminum oxide-coated silver powder increases, which in turn increases the volume of the reaction solution and the amount of reagents used, making it uneconomical. If the aluminum concentration exceeds 10.0% by mass, the amount of aqueous aluminum compound solution added to obtain the desired aluminum oxide-coated silver powder becomes very small, and depending on the operating conditions, the error in the amount added may become large.

[0046] [Silver particle deposition process] In the present invention's method for producing aluminum oxide-coated silver powder, a reducing agent is added to the aluminum-containing silver-ammine complex aqueous solution with a pH of 10.0 or higher obtained in the above step to reduce the silver-ammine complex and precipitate silver particles in the aqueous solution. At this time, the reducing agent used is one that has the effect of lowering the pH of the aqueous solution when added. The reason for using a reducing agent that has the effect of lowering the pH of the aqueous solution is that by adding the reducing agent, the pH of the aqueous solution is lowered to the solid-phase stable range of the aluminum oxide, and aluminum oxide is deposited on the surface of the precipitated silver particles. If a reducing agent that does not have the effect of lowering the pH of the aqueous solution is used, it is possible to lower the pH of the aqueous solution by adding a pH adjusting agent after the silver particles have precipitated and deposit aluminum oxide on the surface of the silver particles, but this method is undesirable because it increases the number of steps. Furthermore, it is undesirable because it becomes difficult to uniformly deposit aluminum oxide on the particle surface. Furthermore, it is preferable that the reaction temperature when precipitating silver particles using the aforementioned reducing agent be between 10°C and 40°C. If the reaction temperature is too low or too high, it will take time to adjust the temperature, which can lead to increased energy costs.

[0047] [Reducing agent] The greatest technical feature of the present invention's method for producing aluminum oxide-coated silver powder is the use of a reducing agent that, upon addition, lowers the pH of an aqueous solution. The mechanism by which the pH of an aqueous solution decreases upon addition of a reducing agent includes cases where the reducing agent itself has a pH-lowering effect, and cases where the oxidation products of the reducing agent have a pH-lowering effect.

[0048] As reducing agents having such effects, organic compounds containing one or more of the COOH group, CHO group, and OH group in their molecule are preferred. Specifically, examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde. In the case of an organic compound reducing agent having a COOH group in its molecule, the pH of the aqueous solution decreases upon addition of the reducing agent. In the case of a reducing agent where the CHO group has reducing properties, the CHO group reduces silver ions, and the CHO group is oxidized to a COOH group, thus decreasing the pH of the aqueous solution. In the case of a reducing agent where the OH group has reducing properties, the OH group is oxidized to a COOH group via the CHO group, thus decreasing the pH of the aqueous solution. Furthermore, it is presumed that the presence of voids within the silver particles constituting the aluminum oxide-coated silver powder of the present invention is due to the gas generated by the decomposition of the aforementioned organic compound being trapped inside the silver particles.

[0049] In the method for producing aluminum oxide-coated silver powder according to the present invention, the pH of the aqueous solution is adjusted to 4.0 or higher and 9.0 or lower by adding the reducing agent. By setting the pH within the above range, aluminum oxide can be coated onto silver particles from the mixed aqueous solution.

[0050] The pH value that decreases by adding the aforementioned reducing agent can be the pH value after the precipitation of silver powder is complete, in which case the pH can be the measured pH of the filtrate after the silver powder has been filtered and recovered. This pH value can be controlled by the pH value in the pH adjustment step described above, the amount of pH adjusting agent added, the equivalent amount of ammonia in the complex formation step, or the amount of reducing agent. It is more preferable that the above pH be 6.0 or higher. If the above pH falls below 4.0, aluminum will be Al 3+ This is undesirable because it may cause the aluminum oxide on the surface to redissolve due to the formation of ions. Furthermore, if the pH is greater than 9.0, it is undesirable because aluminum will hardly be detectable in the measurement of the aluminum coated with the resulting silver powder. Furthermore, when the reducing agent in Example 1, described later, was changed from formalin to hydrazine, the pH of the filtrate became 10.0 or higher, and almost no aluminum was detected in the measurement of the amount of aluminum coated on the resulting silver powder.

[0051] From this, we believe that when the pH of the mixed aqueous solution decreases from above 10.0 to approach 9.0, aluminum oxide begins to precipitate on the surface of the silver particles. Then, as the particles grow until the final pH is reached, a layer containing aluminum (e.g., aluminum oxide) is formed near the surface of the silver particles, and the aluminum oxide adheres to the surface of the silver particles.

[0052] In the manufacturing method of the present invention, aluminum ions are included in the mixed solution beforehand, and aluminum oxide is deposited on the surface of the silver particles by pH transition during the precipitation of the reducing agent, thereby uniformly depositing aluminum oxide on the surface of the silver particles.

[0053] When aldehydes are used as reducing agents, silver particles precipitate to some extent, and after the concentration of carboxylic acid, which is an oxidation product, increases, the pH of the aqueous solution reaches the aforementioned pH range. This is preferable from the viewpoint of making it easier for aluminum oxide to adhere to the surface of the precipitated silver particles. As for the aldehyde, formaldehyde is more preferable from the viewpoint of ease of availability and reducing power. The amount of reducing agent added is preferably 1 equivalent or more relative to silver in order to increase the yield of silver, and may also be 2 equivalents or more relative to silver, for example, 10 equivalents or more and 20 equivalents or less.

[0054] [Surface treatment agent addition process] In the method for producing aluminum oxide-coated silver powder of the present invention, a surface treatment agent may be added to the solution containing the aluminum oxide-coated silver particles obtained in the silver particle precipitation step. This yields aluminum oxide-coated silver particles coated with the surface treatment agent. Examples of surface treatment agents to be added to the solution containing the aluminum oxide-coated silver particles include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, and protective colloids. Here, from the viewpoint of being easily and uniformly adhered to the silver powder surface, it is preferable that the surface treatment agent be one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

[0055] [Surface treatment agent] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used individually or in combination of two or more. Examples of fatty acid salts include salts of the fatty acids listed above. Examples of salts include sodium salts and potassium salts. Examples of compounds having an azole structure include benzotriazole, benzotriazole sodium salt, and benzotriazole potassium salt. These may be used individually or in combination of two or more.

[0056] The amount of surface treatment agent added to the solution containing aluminum oxide-coated silver particles is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.15% by mass or more, relative to the amount of silver in the silver-ammine complex aqueous solution. An amount of 0.05% by mass or more is effective in maintaining good dispersibility of the resulting aluminum oxide-coated silver powder. On the other hand, the amount of surface treatment agent added is preferably 1.00% by mass or less, more preferably 0.30% by mass or less, and even more preferably 0.20% by mass or less. When the amount of surface treatment agent added is 1.00% by mass or less, it is easier to suppress the amount of organic matter in the resulting aluminum oxide-coated silver powder from becoming too high, which is advantageous in suppressing the increase in resistance when a conductive film is obtained using a conductive paste containing aluminum oxide-coated silver powder.

[0057] It is preferable to add the surface treatment agent to the solution containing aluminum oxide-coated silver particles after a predetermined time has elapsed since adding the reducing agent to the silver-ammine complex aqueous solution. In this invention, since the growth of silver particles stops when the surface treatment agent is added, the silver particles precipitate, the precipitation of aluminum oxide due to the change in pH progresses, and the timing of addition can be arbitrarily determined so that the silver in the solution precipitates as silver particles.

[0058] [Separation and Recovery Process] The silver powder coated with aluminum oxide obtained through the above-described series of steps is separated and recovered using known solid-liquid separation methods, then washed with water as necessary and dried. Known solid-liquid separation methods include, for example, decantation or a filter press. The end of the washing process may be determined using the electrical conductivity of the washing water. Specifically, washing is determined to be complete when the electrical conductivity of the washing water falls below a predetermined value. The silver particles after washing may be subjected to the drying process in an aggregated state such as a cake.

[0059] [Drying process] The drying process can be carried out using vacuum drying or an airflow dryer. In the drying process, operations may be performed to promote dispersion and drying by blowing a high-pressure airflow onto the aggregate of silver particles, or by introducing the cake or the spherical silver powder in the drying process into a stirrer with a stirring rotor and stirring it. The drying temperature of the silver powder should be 100°C or lower. If the temperature of the silver powder is 100°C or lower, sintering of the silver particles in the silver powder can be effectively suppressed. [Examples]

[0060] [Measurement of aluminum content in aluminum oxide-coated silver powder] Inductively coupled plasma atomic emission spectrometry (ICP-OES) was used to measure the aluminum content of aluminum oxide-coated silver powder. 1 g of the sample (silver powder) was accurately weighed, 15 mL of pure water and 10 mL of nitric acid (for precision analysis) were added, and the mixture was heated at 200°C for 30 minutes. After the heated sample cooled, it was diluted to 100 mL using pure water, 5 mL of the supernatant was taken, and the volume was again diluted to 100 mL using pure water to prepare the sample for ICP analysis. The standard addition method was used for the measurement. A calibration curve was created by adding aluminum to a standard solution using 5N silver, with a silver concentration similar to that of the sample. For quantitative analysis, an Agilent 5800 ICP-OES from Agilent Technologies was used.

[0061] [Analysis of the state of aluminum] The state of aluminum adhering to the surface of silver particles was analyzed by X-ray photoelectron spectroscopy (XPS). A scanning X-ray photoelectron spectrometer (PHI5000 Versa Prove III) manufactured by ULVAC-PHI, Inc. was used for the XPS measurements. Monochromatic AlKα rays were used as the X-ray source, with an acceleration voltage of 15kV, an output of 25W, an X-ray incidence angle of 90 degrees, and a photoelectron extraction angle of 45 degrees.

[0062] For the chemical state analysis of aluminum, an Al2p spectrum was used, with a pulse energy of 69 eV, integration time of 80 ms, measurement energy interval of 0.125 eV / step, and 500 integration cycles. Charge correction was performed using a CC bond energy of 284.8 eV. In this specification, the peak with a bond energy of 74.6 ± 1 eV is referred to as aluminum oxide. In the state analysis, aluminum oxide is, for example, Al2O3. In this invention, the aluminum present on the surface of the silver particles is preferably aluminum oxide, but it may also contain a small amount of peaks representing, for example, metallic aluminum. In the XPS measurement described later, the ratio of the height of the peak corresponding to metallic aluminum to the peak corresponding to aluminum oxide is preferably 10% or less.

[0063] [Analysis of the distribution of aluminum on the surface of silver particles] The aluminum oxide coating on the surface of silver particles was analyzed by capturing characteristic X-ray (Kα) images of silver and aluminum using an energy-dispersive X-ray fluorescence analyzer (EDX) attached to the SEM. The obtained characteristic X-ray (Kα) images of aluminum were examined using ImageJ (public domain image processing software), which is capable of binarization. Specifically, using the software, the outer perimeter of 10 particles randomly selected from the aluminum distribution image was manually set, and the aluminum distribution area per particle was determined by binarization. The average ratio of the aluminum distribution area per particle was then calculated by dividing this by the total area of ​​the single particle. The characteristic X-ray (Kα) images of aluminum were adjusted to 8 bits, and Brightness and Contrast were set to their maximum values ​​to highlight the areas where aluminum was detected by the energy-dispersive X-ray fluorescence analyzer attached to the SEM before binarization. The threshold value for binarization was set to 1 lower than the maximum value (254 in this case), and the white areas were considered to be the aluminum distribution areas. Furthermore, a scanning electron microscope, JSM-IT800SHL, manufactured by JEOL Ltd., was used for observation.

[0064] [Measurement of particle size distribution] The cumulative 10% particle size (D) of silver powder by volume. 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), cumulative 100% particle diameter (D maxThe particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.). For the measurement, 0.1 g of the sample was dispersed in 40 mL of isopropyl alcohol (IPA). An ultrasonic homogenizer (US-150T: 19.5 kHz, tip diameter 18 mm, manufactured by Nippon Seiki Seisakusho Co., Ltd.) was used for dispersion. The dispersion time was 2 minutes. The dispersed sample was subjected to the above-mentioned apparatus, and the particle size distribution was determined using the accompanying analysis software. During the measurement, an SDC (Sample Delivery Controller) device was used in the circulation chamber of the laser diffraction / scattering particle size distribution analyzer, and the "flow velocity (%)" setting of the circulation chamber was set to 60.

[0065] [BET specific surface area] The specific surface area of ​​the BET sample was measured using a single-point BET method with nitrogen adsorption using a Macsorb HM-model 1210 (MOUNTECH). For the BET specific surface area measurement, a sample weight of 3.0 g was used, a mixed gas of N2:He=30:70 was employed, with a gas flow rate of 25 mL / min, and degassing conditions of 60°C for 10 minutes before measurement.

[0066] [BET diameter (D BET )] The BET diameter is the particle size calculated from the BET specific surface area, and in this specification, it is referred to as "D BET It is written as "D". BET This was calculated by substituting the BET specific surface area measured by the BET single-point method and the true density described below into the right-hand side of equation (1) below. D BET (μm)=6 / (BET specific surface area x true density) …(1) Here, the "BET specific surface area" on the right-hand side of equation (1) has the unit m. 2 The BET specific surface area value, expressed in g / g, is substituted into the "True density" field, with the unit g / cm² being used. 3 The value of the true density represented by is substituted.

[0067] [True density of aluminum oxide-coated silver powder] In this specification, the "true density" of silver powder means the density of silver powder considering the voids existing inside silver particles, that is, the "closed space inside the void-containing silver particles" described above. In this specification, for the measurement of the true density of silver powder, a dry automatic densitometer using helium gas (manufactured by Micromeritics, device name: AccuPyc II 1340) was used, and it was measured by the constant-volume expansion method (the "gas pycnometer method" in the Japanese Pharmacopoeia). Specifically, the volume of silver powder was measured from the gas volume when filling helium gas until the inside of the container reached a constant pressure, and it was calculated by dividing the mass of silver powder by that volume. Note that the helium gas at this time cannot reach the closed space (that is, voids) on the particle surface. Therefore, usually, as the proportion of voids inside silver powder increases, the true density of silver powder tends to be smaller than the density of silver, 10.50 g / cm 3 and becomes smaller.

[0068] [Expansion rate and shrinkage rate of aluminum oxide-coated silver powder] The expansion rate and shrinkage rate of aluminum oxide-coated silver powder were measured by thermomechanical analysis (TMA) according to the following procedure. First, 0.3 g of silver powder was weighed. Next, the silver powder was put into a predetermined mold with a diameter of 5 mmφ, and using a press machine, it was compacted under a load of 50 kg for 1 minute to prepare a disc-shaped measurement sample. This measurement sample was set in the sample holder of a thermomechanical analysis (TMA) device (Thermo plous EVO 2 series TMA8311), and a load of 98 mN was applied by a measurement probe, and the temperature was raised from room temperature (25°C ± 5°C) to 900°C at a heating rate of 10°C / min to obtain a TMA. Regarding the profile of the length L (mm) of the disc-shaped measurement sample at each temperature, the "expansion rate in thermomechanical analysis" and the "shrinkage rate in thermomechanical analysis" were calculated by the following equations (2) and (3), respectively. <00​​​​​​​​​​​​​​is the length (mm) of the disk-shaped measurement sample at room temperature (25°C ± 5). Starting from room temperature, the temperature at which the expansion rate first reaches 0.50% (0.50% expansion temperature), the temperature at the maximum expansion point (the peak point of the TMA curve), and the temperature at which the shrinkage rate first reaches 1.00% (1.00% shrinkage temperature) were determined respectively. When the shrinkage rate does not reach 1.00%, the shrinkage rate at 900°C was determined.

[0069] [Ignition loss (Ig-loss) value] In this specification, the "ignition loss (Ig-loss) value" is the amount of change in the mass of the aluminum oxide-coated silver powder when heated from room temperature (25°C ± 5°C) to 800°C. This ignition loss value serves as an indicator showing the amount of components other than silver contained in the silver powder, such as residual components of the treatment agents and additives used in the production process of the silver powder. In this specification, the ignition loss (Ig-loss) value is obtained by precisely weighing the silver powder sample (weighing value: w1), placing it in a magnetic crucible, heating it to 800°C, holding it at 800°C for 30 minutes as a sufficient time to reach a constant weight, then cooling it and weighing again (weighing value: w2), and calculating from w1 and w2 according to the following formula (4). Ignition loss (Ig-loss) value (mass%) = 100×(w1 - w2) / w1 …(4)

[0070] [Open-circuit voltage (V OC ) alternative evaluation] In the present invention, the presence or absence of improvement in the open-circuit voltage (V OC ) was evaluated by an open-circuit voltage (V OC ) alternative evaluation. The open-circuit voltage (V OC ) alternative evaluation was carried out by the following method. As described in Reference 1 (Journal of the Institute of Electronics Packaging Vol.3 No.2 (2000) "Basic Science Series" No.10 Solar Cells and Their Performance, Kenta Itoh), in a solar cell, the following relationship of formula (5) holds between the open-circuit voltage (V OC ) and the saturation current density (J o ). V OC =(n D kT / q)ln[(Jph / J0)+1] ···(5) Here, n D is the diode index, k is the Boltzmann constant, T is the absolute temperature, q is the electric charge, J ph is the photocurrent density. From equation (5), the saturation current density (J) o The smaller the value of (V), the lower the open-circuit voltage (V). OC It can be seen that the value of ) becomes larger.

[0071] Next, reference 2 ("Challenges for the Quantification of metal induced recombination losses," D. Hermann, Doctoral Dissertation (2021)) states that the change in the saturation current density of a solar cell (ΔJ) is... o A method for determining the open-circuit voltage (V) obtained by the QSSPC measurement is described. In the PLI method, the average PLI intensity φ1 in the region where QSSPC measurement (pseudo-steady-state photoconductivity measurement) was performed and the open-circuit voltage (V) obtained by the QSSPC measurement are described. OC ), thermal voltage (V) at 25℃ t By setting the measurement reference constant C using ), we can obtain equation (6) below. Further expanding equation (6) yields equation (7). C = φ1 / exp(V) OC / V t ) …(6) exp (V OC / V t ) = φ1 / C …(7)

[0072] Next, the saturation current density of the part of the electrode is J. o,metal The saturation current density in the area without electrodes is J o,non-metal Therefore, the change in saturation current density originating from the electrodes (ΔJ) o ) can be expressed by the following equation (8). ΔJ o =J o,metal -J o,non-metal …(8) Here, the behavior of an ideal diode is such that the generated current density (J) rec It can be expressed by equation (9) using ). J rec =J o ·exp(V OC / V t ) …(9) Using equation (9), J o,metal , J o,non-metal This can be expressed as equations (10) and (11). J o,metal =J rec / exp(V OC / V t ) metal …(10) J o,non-metal =J rec / ecp(V OC / V t ) non-metal …(11) Here, if we let the PLI intensity in the area with the silver electrode be φ3 and the PLI intensity in the area without the silver electrode be φ2, then equations (9), (10), and (11) can be expressed as equations (12) and (13). J o,metal =J rec ×C / φ3…(12) J o,non-metal =J rec ×C / φ2…(13) Substituting equations (12) and (13) into equation (8), we get the change in saturation current density (ΔJ). o ) can be expressed by equation (14). ΔJ o =J rec ×C×(1 / φ3-1 / φ2) …(14)

[0073] Change in saturation current density (ΔJ) o This value indicates the change in a portion of the cell between the presence and absence of electrodes, and it can change depending on the silver powder used in the conductive paste. In the PLI method, the saturation current density is J. o Since the value cannot be directly determined, here we simply use the change in saturation current density (ΔJ) from the change in luminescence intensity (1 / φ3 - 1 / φ2) due to the presence or absence of electrodes. o ) and calculate the saturation current density (J) of equation (5). o The open-circuit voltage (V) due to the change in ) OCFrom the change in ), V can be indirectly calculated. OC The improvement effect was evaluated. Specifically, for each silver powder obtained in the examples and comparative examples described later, a conductive paste was prepared as described later, and a substrate containing areas with and without electrodes was fabricated using the printing conditions and printing plates described later. By plotting the value of (1 / φ3-1 / φ2) against the ratio of electrodes in a unit area, the V was indirectly evaluated. OC The following evaluation was performed. Here, the larger the value of (1 / φ3-1 / φ2), the greater the ΔJ. o The value of ΔJ increases. o The larger the value of V OC Since it becomes smaller, the open-circuit voltage (V OC In alternative evaluation, the change in saturation current density (ΔJ) o A smaller value for V is preferable. OC This demonstrates an improvement effect. In other words, to show that the smaller the slope of (1 / φ3 - 1 / φ2) with respect to the ratio of electrodes in a unit area, the smaller the change in saturation current density (ΔJ0), the open-circuit voltage (V OC The improvement effect of ) is considered to be significant. Below, the value of the slope of (1 / φ3-1 / φ2) with respect to the ratio of electrodes in a unit area is given by "V OC This is called the "alternative evaluation slope value," and it results in an open-circuit voltage (V OC The improvement effect of ) was evaluated.

[0074] [Method for manufacturing conductive paste] A mixture was obtained by mixing aluminum oxide-coated silver powder, aluminum powder (metallic aluminum: 99.87% by mass, iron: 0.09% by mass, silicon: 0.04% by mass, SEM average diameter: 2.0 μm), glass frit (glass powder: mainly PbO, containing B2O3, SiO2 and other oxides), ethylcellulose, texanol, butyl carbitol acetate, tributyl citrate, oleic acid, triacetin, methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amide. The composition of the mixture is shown in Table 1. The obtained mixture was placed in a propellerless self-rotating agitator defoaming device (V-mini300 manufactured by EME Co., Ltd.), pre-mixed for 30 seconds under conditions of rotation at 1000 rpm, and then kneaded using a three-roll system (80S manufactured by EXAKT Corporation), passing the mixture through a roll gap from 100 μm to 20 μm to obtain a conductive paste.

[0075] [Table 1]

[0076] [Printing conditions and PLI measurement] Using the conductive paste obtained by the above procedure, screen printing was performed using the printing plate shown in Figure 1. The printing plate measures 125 mm x 125 mm and has 25 sections measuring 25 mm x 25 mm. Each section for forming electrodes is designed to have four sections each, where lines with a line width of 15 μm and a line length of 22 μm are spaced 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm apart. There are also nine sections where electrodes are not formed (sections without slits). Printing was performed using a Microtec printing press at a squeegee speed of 350 mm / s. After printing, the samples were dried for 5 minutes in a dryer set to 200 °C, and then fired in a solar cell firing furnace (NGK) under conditions where the peak temperature of the wafer surface reached 720 °C to prepare samples for PLI measurement. For PLI measurement, a POPLI-Octa LED light source manufactured by AITEC Corporation was used, with a center wavelength of 830 nm. A POPLI-Lcta manufactured by AITEC Corporation was used as the PL excitation source, with a center wavelength of 340 nm. A PVX1000 camera manufactured by AITEC Corporation was used, with an exposure time of 3 seconds, a rate of 2 MHz, a long-pass filter, and a cut-on wavelength of 990 nm.

[0077] [Example 1] To 3476.0 g of an aqueous silver nitrate solution containing 53.7 g of silver, 7.3 g of a 5% by mass aqueous solution of aluminum nitrate notahydrate (Al(NO3)3·9H2O: manufactured by Kojun Kagaku Kenkyusho Co., Ltd., standard content 98.0+%) was added. While stirring at 332 rpm, 113.2 g of a 28% by mass aqueous ammonia solution (manufactured by Junsei Kagaku Co., Ltd.) was added to produce an aqueous silver ammine complex solution. The amount of aluminum in the added aluminum nitrate notahydrate solution was 480 ppm by mass relative to the amount of silver. One minute after adding the aqueous ammonia solution, 10.0 g of a 20% by mass aqueous solution of sodium hydroxide was added, and the solution temperature was adjusted to 26.5°C. At this time, the pH of the solution was 12.1. Then, three minutes after adding the aqueous ammonia solution to the aluminum-containing silver nitrate solution, 251.4 g of 25.9% by mass formalin (manufactured by Mitsubishi Chemical Corporation) was added all at once as a reducing agent. Fifteen seconds after adding the reducing agent, 6.1 g of an emulsion aqueous solution containing 1.55% by mass of stearic acid was added to the slurry containing the precipitated silver powder. The stirring was then stopped, the slurry containing the precipitated silver particles was filtered, and the filtrate was washed with water until its electrical conductivity was 0.5 mS / m or less. The mixture was then vacuum-dried at 73°C for 10 hours to obtain a cake-like silver powder. The pH of the filtrate was 6.4. A 5 L beaker was used for the reaction, and baffles and a three-stage turbine blade were used for stirring. 50 g of the silver powder obtained in the above process was placed in a sample mill (Kyōritsu Riko Co., Ltd., SK-M10) and crushed twice for 30 seconds using the dial setting of 100 to obtain the aluminum oxide-coated silver powder (test powder) according to Example 1.

[0078] When the aluminum content of the aluminum oxide-coated silver powder was measured, it was found to be 480 ppm relative to the mass of the silver powder, and the volume-based cumulative 50% diameter D was measured using a laser diffraction scattering particle size distribution analyzer. 50 It is 1.9 μm, D 50 and D BET The ratio (D 50 / DBET The glycemic index (Ig-loss) was 1.09, and the ignition loss (Ig-loss) was 0.93%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in Example 1. A peak was observed in the spectrum at a bond energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of aluminum oxide on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (at a bond energy of approximately 72.6 eV) was observed in the XPS measurement.

[0079] Figure 3 shows the TMA curve obtained under the above-described measurement conditions for the aluminum oxide-coated silver powder obtained in Example 1. The aluminum oxide-coated silver powder obtained from Example 1 began to expand at around 220°C, reaching an expansion rate of 0.50% (0.50% expansion temperature) at 225°C, with a maximum expansion rate of 1.60%. Subsequently, the expansion rate became 0.00% at around 605°C, returning to the sample length before measurement, and then contracted. The temperature at which the contraction rate reached 1.00% (1.00% contraction temperature) was 684°C.

[0080] Figure 4 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 1. The length of the white bar displayed in the lower right of the SEM image is 1 μm (similarly in Figures 5-13). The aluminum oxide-coated silver powder obtained in Example 1 contained silver particles with internal voids, and the proportion of silver particles with voids was 91 percent.

[0081] Figure 5 shows the SEM image (left), Kα characteristic X-ray image of aluminum (center), and Lα characteristic X-ray image of silver (right) of the aluminum oxide-coated silver powder obtained in Example 1. Each characteristic X-ray image is displayed as a monochrome version of the original color image, and areas with higher brightness (appearing whiter) have a higher concentration of the element in question (the same applies to Figures 7, 9, and 13 below). In the case of this aluminum oxide-coated silver powder, the average distribution area ratio of aluminum per particle was 95%. From this, it can be seen that aluminum oxide is widely distributed on the surface of the silver powder.

[0082] Using the aluminum silver oxide powder obtained in this embodiment, the open-circuit voltage (V) is obtained by following the procedure described above. OC )An alternative evaluation was performed, and the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes in a unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000862.

[0083] Table 2 shows the aluminum content of the silver powder used as the test powder and the cumulative 10% particle size (D) by volume. 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), cumulative 100% particle diameter (D max ), BET specific surface area, BET diameter (D BET ), D 50 and D BET The ratio (D 50 / D BET ), D 90 and D 50 The difference (D 90 -D 10 ) and D 50 The ratio ((D 90 -D 10 ) / D 50 ), ignition loss (Ig-loss), proportion of silver particles with voids, true density, average ratio of aluminum distribution area per particle, 0.50% expansion temperature, and V OC The slope values ​​of the alternative evaluations are shown (similarly in each of the following examples).

[0084] [Example 2] The aluminum oxide-coated silver powder (test powder) according to this example was obtained using the same procedure as in Example 1, except that 14.8 g of aluminum nitrate notahydrate aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 970 ppm by mass relative to the amount of silver. At that time, the pH of the solution before the addition of the reducing agent was 12.1, and the pH of the filtrate was 6.4. The aluminum content of the aluminum oxide-coated silver powder is 970 ppm by mass, and the cumulative 50% diameter D is based on volume. 50 It is 2.1 μm, D 50 / D BET The glycemic index was 1.24, and the ignition loss (Ig-loss) was 1.01%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in Example 2. A peak was observed in the spectrum at a bond energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of aluminum oxide on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (at a bond energy of approximately 72.6 eV) was observed in the XPS measurement. Furthermore, the proportion of silver particles containing voids in the aluminum oxide-coated silver powder was 89 percent. Figure 6 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 2.

[0085] Figure 3 shows the TMA curve of the aluminum oxide-coated silver powder obtained in Example 2. The silver powder began to expand at around 220°C, with a 1.00% expansion temperature of 228°C and a maximum expansion rate of 2.50%. Subsequently, it returned to the sample length before measurement at around 800°C, with a 1.00% contraction temperature of 878°C.

[0086] Figure 7 shows the SEM image (left), Kα characteristic X-ray image of aluminum (center), and Lα characteristic X-ray image of silver (right) of the aluminum oxide-coated silver powder obtained in Example 2. In this example, the average distribution area ratio of aluminum per particle was 96%. This indicates that aluminum oxide is widely distributed on the surface of the silver powder.

[0087] Using the aluminum oxide-coated silver powder obtained in this embodiment, the open-circuit voltage (V OC )When an alternative evaluation was performed, the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes in a unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000589.

[0088] [Example 3] The aluminum oxide-coated silver powder (test powder) according to this example was obtained using the same procedure as in Example 1, except that 29.3 g of aluminum nitrate notahydrate aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 1920 ppm by mass relative to the amount of silver. At that time, the pH of the solution before the addition of the reducing agent was 12.1, and the pH of the filtrate was 6.7. The aluminum content of the aluminum oxide-coated silver powder is 1920 ppm by mass, and the cumulative 50% diameter D is based on volume. 50 It is 1.8 μm, D 50 / D BET The ratio was 1.46, and the ignition loss (Ig-loss) was 0.93%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in Example 2. A peak was observed in the spectrum at a bond energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of aluminum oxide on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (at a bond energy of approximately 72.6 eV) was observed in the XPS measurement. Furthermore, the proportion of silver particles containing voids in the aluminum oxide-coated silver powder was 92%. Figure 8 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 3.

[0089] Figure 3 shows the TMA curve of the aluminum oxide-coated silver powder obtained in Example 3. The silver powder began to expand at around 220°C, with a 1.00% expansion temperature of 229°C and a maximum expansion rate of 2.50%. Subsequently, it returned to the sample length before measurement at around 860°C, and the shrinkage rate at 900°C was 0.60%.

[0090] Figure 9 shows the SEM image (left), Kα characteristic X-ray image of aluminum (center), and Lα characteristic X-ray image of silver (right) of the aluminum oxide-coated silver powder obtained in Example 3. In Example 3, the average distribution area ratio of aluminum per particle was 95%. This indicates that aluminum oxide is widely distributed on the surface of the silver powder.

[0091] Using the aluminum oxide-coated silver powder obtained in this embodiment, the open-circuit voltage (V OC )An alternative evaluation was performed, and the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes in a unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000395.

[0092] [Comparative Example 1] In this comparative example, commercially available silver powder (4-8FD manufactured by DOWA High-Tech Co., Ltd.) was used as the test powder. The volume-based cumulative 50% diameter D of the silver powder in this comparative example was measured using a laser diffraction scattering particle size distribution analyzer. 50 It is 1.9 μm, D 50 / D BET The saturation ratio was 1.32, and the ignition loss (Ig-loss) was 0.64%. Furthermore, when examining the cross-sectional SEM image of the silver powder of Comparative Example 1, it was found to contain voids, and the proportion of silver particles with voids was 90%. Figure 10 shows a cross-sectional SEM image of the silver powder of Comparative Example 1.

[0093] Figure 3 shows the TMA curve for the silver powder of Comparative Example 1. The silver powder of Comparative Example 1 began to expand at around 220°C, but the maximum expansion rate was 0.47%, and the expansion rate did not reach 0.50% between 150°C and 350°C. Subsequently, it returned to the sample length before measurement at around 360°C and began to contract, with the 1.00% contraction temperature being 380°C.

[0094] Using the silver powder from Comparative Example 1, the open-circuit voltage (V OC )When an alternative evaluation was performed, the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes in a unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000957. As shown in Table 2, the open-circuit voltage (V) obtained using the silver powder in the examples was calculated using the silver powder. OC The results of the alternative evaluation showed that, compared to the results obtained using the silver powder (commercial product) of Comparative Example 1, the slope value when plotting the value of (1 / φ3 - 1 / φ2) was smaller for the silver powder obtained in the example, and in the silver powder obtained in Comparative Example 1, V OCIt can be seen that no improvement was obtained. From this, as mentioned above, it can be inferred that in the case of aluminum oxide-coated silver powder obtained in the present invention, the amount of silver dissolved in the glass can be optimized, so carrier coupling loss can be suppressed, and as a result the open-circuit voltage can be improved.

[0095] [Comparative Example 2] The aluminum oxide-coated silver powder (test powder) for this comparative example was obtained using the same procedure as in Example 1, except that 3.2 g of aluminum nitrate notahydrate aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 210 ppm relative to the amount of silver. At that time, the pH of the solution before the addition of the reducing agent was 12.1, and the pH of the filtrate was 6.4. The aluminum content of the aluminum oxide-coated silver powder is 210 ppm by mass, and the cumulative 50% diameter D is based on volume. 50 It is 2.0 μm, D 50 / D BET The ratio was 1.11, and the ignition loss (Ig-loss) was 0.92%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in Comparative Example 2. A peak was observed in the spectrum at a bond energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of aluminum oxide on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (at a bond energy of approximately 72.6 eV) was observed in the XPS measurement. Furthermore, when examining the cross-sectional SEM image of the silver powder obtained in Comparative Example 2, it was found that it contained voids, and the proportion of silver particles with voids was 84 percent. Figure 11 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Comparative Example 2.

[0096] Figure 3 shows the TMA curve for the aluminum oxide-coated silver powder obtained in Comparative Example 2. The silver powder began to expand at around 220°C, reaching an expansion rate of 1.00% at 225°C, with a maximum expansion rate of 1.60%. Subsequently, it returned to the sample length before measurement at around 525°C and began to contract, with the 1.00% contraction temperature being 620°C.

[0097] Similar to Example 1, when the Lα characteristic X-ray images of silver and the Kα characteristic X-ray images of aluminum were measured for the aluminum oxide-coated silver powder obtained in Comparative Example 2, the distribution could not be measured because the values ​​were below the detection limit.

[0098] Using the aluminum oxide-coated silver powder obtained in Comparative Example 2, the open-circuit voltage (V OC )When an alternative evaluation was performed, the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes in a unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000947.

[0099] [Comparative Example 3] 3232.0 g of a silver nitrate aqueous solution containing silver 50.8 was stirred at 174 rpm, and 103.2 g of a 28% by mass ammonia aqueous solution (manufactured by Junsei Kagaku Co., Ltd.) was added to produce a silver ammine complex aqueous solution. 30 seconds after adding the ammonia aqueous solution, 0.3 g of a 5% by mass sodium carbonate aqueous solution was added, and another 30 seconds later, 0.5 g of a 5% by mass PEI600 aqueous solution was added to adjust the solution temperature to 33.5°C. At this time, the pH of the solution was 12.1. Next, 3 minutes after adding the ammonia aqueous solution, 397.0 g of a 1.86% by mass hydrazine aqueous solution (manufactured by Mitsubishi Gas Chemical Company, Inc.) was added all at once as a reducing agent. Ten seconds after adding the reducing agent, 23.5 g of a 5.0% by mass aqueous solution of aluminum nitrate novahydrate (Al(NO3)3·9H2O, manufactured by Kojun Chemical Laboratory Co., Ltd., standard content 98.0+%) was added relative to the silver. Twenty seconds after adding the reducing agent, 100.0 g of a 67.5% by mass aqueous solution of nitric acid was added. The amount of aluminum in the added aluminum nitrate aqueous solution was 1629 ppm by mass relative to the amount of silver. Furthermore, 30 seconds after adding the reducing agent, 5.1 g of an emulsion aqueous solution containing 1.55% by mass of stearic acid was added to the slurry containing the precipitated silver powder. Afterward, stirring was stopped, the silver particle slurry was filtered, washed with water until the electrical conductivity was 0.5 mS / m or less, and then vacuum-dried at 73°C for 10 hours to obtain silver powder. The pH of the filtrate was 8.9. A 5 L beaker was used for the reaction, along with baffles and a three-stage turbine blade. 50 g of the silver powder obtained in the above process was placed in a sample mill (Kyōritsu Riko Co., Ltd., SK-M10) and crushed twice for 30 seconds using the dial setting of 100 to obtain aluminum oxide-coated silver powder (test powder) according to Comparative Example 3.

[0100] The aluminum content of the aluminum oxide-coated silver powder is 1078 ppm by mass, and the cumulative 50% diameter D is based on volume. 50 It is 1.7 μm, D 50 / D BET The glycemic index was 1.20, and the ignition loss (Ig-loss) was 0.30%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in Comparative Example 3. A peak was observed in the spectrum at a bond energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of aluminum oxide on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (at a bond energy of approximately 72.6 eV) was observed in the XPS measurement. Furthermore, when examining the cross-sectional SEM image of the silver powder obtained in Comparative Example 3, it was found that there were no voids, and the proportion of silver particles with voids was 0 percent. Figure 12 shows a cross-sectional SEM image of the silver powder obtained in Comparative Example 3.

[0101] Figure 3 shows the TMA curve for the silver powder obtained in Comparative Example 3. The silver powder expanded immediately after the start of measurement, reaching an expansion of 0.50% or more at 373°C, with a maximum expansion rate of 1.00%. Subsequently, it returned to the sample length before measurement at around 890°C, and the contraction rate at 900°C was 0.30%.

[0102] Figure 13 shows the SEM image of the silver powder obtained in Comparative Example 3 (left), the Kα characteristic X-ray image of aluminum (center), and the Lα characteristic X-ray image of silver (right). In Comparative Example 3, the average distribution area ratio of aluminum per particle was 83%. This indicates that aluminum oxide is widely distributed on the surface of the silver powder.

[0103] Comparative Example 3: Using sw silver powder, V was performed using the procedure described above. OC When an alternative evaluation was performed, the value of (1 / φ3 - 1 / φ2) was plotted against the ratio of electrodes per unit area, and the slope value (V OC The slope value of the alternative evaluation was 0.000973.

[0104] [Table 2]

[0105] As is clear from Table 2, the aluminum oxide-coated silver powder in the examples has an open-circuit voltage (V OC In the alternative evaluation, the slope of (1 / φ3 - 1 / φ2) with respect to the ratio of electrodes in a unit area is small, which indicates that the change in saturation current density (ΔJ0) is small. Therefore, the silver powder in the example has an open-circuit voltage (V OC It can be seen that this has an improvement effect.

[0106] According to the present invention, the open-circuit voltage (V) originating from the conductive film is OC The present invention provides silver powder that can improve the open-circuit voltage (V) derived from the conductive film, and a method for manufacturing the same. OC A conductive paste can be provided that can improve the open-circuit voltage (V) originating from the conductive film. OC This allows us to provide a conductive film that can improve the performance of the material.

Claims

1. Aluminum oxide-coated silver powder consists of silver particles coated with aluminum oxide on their surface, wherein the amount of aluminum is 400 ppm by mass or more and 3000 ppm by mass or less relative to the mass of the aluminum oxide-coated silver powder, and the volume-based cumulative 50% diameter D is measured by a laser diffraction scattering particle size distribution analyzer. ５０ The particle size is between 0.2 μm and 5.0 μm, and the true density is 10.00 g / cm³. ３ The following is silver powder coated with aluminum oxide.

2. The aluminum oxide-coated silver powder according to claim 1, wherein the amount of aluminum is 600 ppm by mass or more and 2,500 ppm by mass or less relative to the mass of the aluminum oxide-coated silver powder.

3. The aluminum oxide-coated silver powder according to claim 1, wherein, when the particle cross-section is observed, the number of silver particles having voids inside the particle is 70% or more of the total silver particles.

4. The aforementioned D ５０ and BET diameter D ＢＥＴ The ratio (D ５０ / D ＢＥＴ The aluminum oxide-coated silver powder according to claim 1, wherein the ratio is 1.00 or more and 2.50 or less.

5. The cumulative 90% diameter D on a volume basis measured by a laser diffraction / scattering particle size distribution measuring device ９０ and the cumulative 10% diameter D on a volume basis １０ The difference (D ９０ − D １０ ), and the ratio of the above D ５０ ((D ９０ − D １０ ) / D ５０ ) is 0.50 or more and 2.00 or less. The aluminum oxide-coated silver powder according to claim 1.

6. The aluminum oxide-coated silver powder according to claim 1, wherein the ignition loss (Ig-loss) value is 0.30% or more and 3.00% or less.

7. The aluminum oxide-coated silver powder according to claim 1, wherein the aluminum oxide-coated silver powder is distributed at 2.5 kg / mm³ ２ The sample obtained by compression molding was given 5 mN / mm² ２ Aluminum oxide-coated silver powder, wherein, in a thermomechanical analysis (TMA) performed by heating from room temperature to 900°C at a heating rate of 10°C / min under the applied load, the temperature at which the expansion coefficient relative to room temperature reaches 0.50% is between 150°C and 350°C.

8. The aluminum oxide-coated silver powder according to claim 1, for use in a firing paste for forming electrodes of solar cells.

9. A baked paste for forming electrodes of a solar cell using aluminum oxide-coated silver powder as described in claim 1.

10. The process includes a "complex formation step" to obtain a silver-ammine complex aqueous solution by adding ammonia to a silver ion-containing aqueous solution to form a silver-ammine complex, and a "pH adjustment step" to adjust the pH of the aqueous solution to 10.0 or higher by adding a pH adjusting agent to the silver-ammine complex aqueous solution, wherein one or more methods selected from the following are used: using the silver ion-containing aqueous solution containing an aluminum compound in the complex formation step, adding the aluminum compound to the aqueous solution during the period until the pH adjustment step is completed, and adding the aluminum compound to the aqueous solution after the pH adjustment step is completed, to produce a silver-ammine complex aqueous solution containing aluminum and having a pH of 10.0 or higher. The process involves adding a reducing agent to the aforementioned aluminum-containing silver-ammine complex aqueous solution with a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously lowering the pH of the aqueous solution to 4.0 or higher and 9.0 or lower, thereby precipitating aluminum oxide. A method for producing aluminum oxide-coated silver powder, The amount of aluminum supplied to the liquid by the above-mentioned aluminum compound is such that the amount of aluminum relative to the amount of silver in the aqueous solution at the start of the addition of the reducing agent is 400 ppm by mass or more and 3000 ppm by mass or less. A method for producing aluminum oxide-coated silver powder, wherein the reducing agent is an organic compound containing one or more of the COOH group, CHO group, and OH group in its molecule.

11. The method for producing aluminum oxide-coated silver powder according to claim 10, wherein the reducing agent is formaldehyde.

12. A method for producing aluminum oxide-coated silver powder according to claim 10, further comprising the step of adding a surface treatment agent to a slurry containing silver powder precipitated by adding the aforementioned reducing agent.

13. The method for producing aluminum oxide-coated silver powder according to claim 12, wherein the amount of the surface treatment agent added is 0.05% by mass or more and 0.50% by mass or less relative to the amount of silver in the silver-ammine complex aqueous solution.

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