Alumina-coated silver powder, method for producing the same, and conductive paste

Abstract

Provided are a silver powder for solar cell electrode formation that can improve open-circuit voltage (V oc ) and a method for producing the same. Also provided are a sinterable conductive paste for solar cell electrode formation that can improve open-circuit voltage (V oc ) using the formed conductive film. When metal silver particles are precipitated from an aqueous solution containing silver ions, an aluminum compound can be co-present in the aqueous solution. As a reducing agent, a reducing agent whose addition has the effect of lowering the pH of the aqueous solution, or a reducing agent whose oxidation product has the effect of lowering the pH of the aqueous solution is used, and the precipitated silver particles are coated with 400 mass ppm or more to 3000 mass ppm or less of aluminum oxide relative to the mass of the silver powder coated with aluminum oxide on the surface, whereby an aluminum oxide-coated silver powder can be obtained, in which, in the case where the precipitated silver powder is made into a paste for electrode formation, the electrode will form low resistance.

Description

Technical Field

[0001] This invention relates to alumina-coated silver powder suitable for use in forming conductive pastes for solar cell electrodes, a method for manufacturing the same, and a sintered conductive paste. Background Technology

[0002] Traditionally, the electrodes and circuits of solar cells have been formed using resin-based or sintered silver paste. The conversion efficiency of a solar cell can be obtained by: [calculating the short-circuit current (I)]. sc (A) ), Open circuit voltage (V) oc The product of the three parameters (V) and the fill factor (shape factor FF, (%)) is divided by the light-receiving area and the irradiance. The fill factor, as one of the indicators characterizing solar cell performance, represents the rectangularity of the solar cell's output characteristic curve. If the fill factor can make I... sc V oc When the FF (field-to-electrical flux) is maximized, the conversion efficiency of the solar cell can be significantly improved. In the case of conductive pastes using silver powder as filler, by making the electrodes finer, I... sc The increase is achieved by reducing the line resistance of the conductive film formed using this conductive paste and the contact resistance between the conductive film and the silicon interface.

[0003] For example, Patent Document 1 discloses that by using a sintered silver paste containing silver particles with closed pores inside the particles, it is possible to draw fine wiring and form electrode wiring with lower resistance than conventional wiring.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-056050 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] However, while Patent Document 1 discloses improvements in the drawability and resistance of microwires when using silver powder containing silver particles with closed pores inside, it does not address the improvement of open-circuit voltage (V). oc The effect of ) was not mentioned. Resin-based conductive pastes and sintered conductive pastes differ in their use of different solar cell structures, and the silver powder used affects the open-circuit voltage (V). oc The effects caused by each are different.

[0009] Resin-based conductive pastes are primarily used in heterojunction solar cells. In heterojunction solar cells, an i-amorphous Si layer is stacked on the surface of an n-type semiconductor. This amorphous Si layer forms hydrogen bonds with sites of surface defects, passivating the defects and thus achieving a higher open-circuit voltage (V). oc The structure of ) . Therefore, silver powder used in conductive pastes is related to V oc The impact can be described as minimal.

[0010] In contrast, sintered conductive pastes are mainly used in PERC (Passivated Emitter Back Contact) solar cells and TOPCon (Tunneling Oxide Passivated Contact) solar cells. In these solar cells, 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 along with silver powder to decompose the passivation layer of the substrate. This glass powder decomposes the passivation layer, potentially reaching not only the n-type semiconductor layer but also the p-type semiconductor layer. In this case, while the silver dissolved in the glass ensures conduction with the n-type semiconductor layer, it can also form a short circuit with the p-type semiconductor layer, resulting in increased carrier recombination losses and an increased open-circuit voltage (V). oc The problem of reduced contact resistance exists. Therefore, for silver powder used in sintered conductive pastes, it is required to reduce the contact resistance at the interface between the semiconductor layer and the electrode, and to avoid short-circuiting with the p-type semiconductor layer.

[0011] The technical problem to be solved by this invention is: to provide a method for improving the open-circuit voltage (V) in the case of forming solar cell electrodes. oc The silver powder is also beneficial for forming an improved open-circuit voltage (V). oc Conductive paste for sintering conductive films suitable for solar cell electrodes.

[0012] Methods for solving problems

[0013] To achieve the above-mentioned goals, the inventors have conducted tireless and active research, and have discovered that by coating the surface of silver powder with a specified amount of aluminum oxide, the open-circuit voltage (V) can be reduced. oc The invention described below was completed by increasing the size of the components.

[0014] That is, in order to achieve the above-mentioned problem, in this invention, (1) an alumina-coated silver powder is provided, which is an alumina-coated silver powder containing silver particles that coat the surface with alumina, wherein the amount of aluminum relative to the mass of the alumina-coated silver powder is more than 400 ppm by mass and less than 3000 ppm by mass, and the cumulative 50% particle size D of the alumina-coated silver powder as measured by a laser diffraction scattering particle size distribution measuring device is... 50 Its thickness ranges from 0.2 μm to 5.0 μm, and its true density is below 10.00 g / cm³.

[0015] (2) Preferably, the amount of aluminum in the alumina-coated silver powder described in (1) above is between 600 ppm and 2500 ppm relative to the mass of the alumina-coated silver powder.

[0016] (3) Preferably, when observing the cross-section of the alumina-coated silver powder mentioned in (1) to (2), the number of silver particles with pores inside the particles is more than 70% of all silver particles.

[0017] (4) The alumina-coated silver powder described in items (1) to (3) above, preferably D 50 With BET particle size D BET The ratio (D) 50 / D BET The value ranges from 1.00 to 2.50.

[0018] (5) The alumina-coated silver powder mentioned in items (1) to (4) above preferably has a cumulative 90% particle size D based on the volume reference measured by a laser diffraction scattering particle size distribution measuring device. 90 Cumulative 10% particle size D based on volume 10 The difference (D) 90 -D 10 ) and the D 50 The ratio of (D) 90 -D 10 ) / D 50 The range is from 0.50 to below 2.00.

[0019] (6) Preferably, the heat loss (Ig-loss) of the alumina-coated silver powder described in (1) to (5) above is 0.30% to 3.00%.

[0020] (7) Preferably, the alumina-coated silver powder described in (1) to (6) above satisfies the condition that, under the condition of applying a load of 5 mN / mm² to the sample obtained by compressing the alumina-coated silver powder with 2.5 kg / mm², the temperature at which the expansion rate reaches 0.50% based on room temperature is above 150°C and below 350°C in the TMA curve obtained by heating from room temperature to 900°C at a heating rate of 10°C / min.

[0021] (8) The alumina-coated silver powder described in items (1) to (7) above can be used as a sintered conductive paste for forming solar cell electrodes.

[0022] In addition, the following invention is provided.

[0023] (9) A sintered conductive paste for forming solar cell electrodes, which uses silver powder coated with alumina as described in items (1) to (8) above.

[0024] As a method for manufacturing the above-mentioned alumina-coated silver powder, a method for manufacturing alumina-coated silver powder is provided (10), which includes the following steps:

[0025] A process for preparing an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" and a "pH adjustment step," wherein the complex formation step is a step in which a silver-ammonia complex is formed by adding ammonia to an aqueous solution containing silver ions, thereby obtaining an aqueous solution of the silver-ammonia complex; the pH adjustment step is a step in which a pH adjusting agent is added to the aqueous solution of the silver-ammonia complex to adjust the pH of the aqueous solution to 10.0 or higher; the method used is selected from one or more of the following methods to prepare the aqueous solution of the silver-ammonia complex: a method of using the aqueous solution containing an aluminum compound in the complex formation step, a method of adding an aluminum compound to the aqueous solution in the period before the pH adjustment step is completed, and a method of adding an aluminum compound to the aqueous solution after the pH adjustment step has been completed; and

[0026] The process involves adding a reducing agent to an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously adjusting the pH of the aqueous solution to between 4.0 and 9.0 to precipitate alumina.

[0027] in,

[0028] The amount of aluminum supplied to the liquid via the aluminum compound is controlled such that the amount of aluminum relative to silver in the aqueous solution at the initial point of addition of the reducing agent is between 400 ppm by mass and 3000 ppm by mass.

[0029] The reducing agent is an organic compound containing one or more of the following groups: COOH, CHO, and OH.

[0030] (11) In the method for manufacturing silver powder coated with alumina described in (10) above, the reducing agent is preferably formaldehyde.

[0031] (12) In the manufacturing method described in (10) to (11) above, a step of adding a surface treatment agent to a slurry containing silver powder precipitated by adding the reducing agent may be further included.

[0032] (13) In the manufacturing method of the preceding (12), it is preferred that the amount of the surface treatment agent added is more than 0.05% by mass and less than 0.50% by mass relative to the amount of silver in the aqueous solution of the silver-ammonia complex.

[0033] Invention Effects

[0034] By using the alumina-coated silver powder of the present invention, and forming a paste from the alumina-coated silver powder to form a conductive film on a solar cell substrate, recombination loss of the conductive film can be suppressed, thereby obtaining a high open-circuit voltage (V). oc ). Attached Figure Description

[0035] Figure 1 For use with open circuit voltage (V) oc (A design drawing for a screen printing plate used for alternative evaluation.)

[0036] Figure 2 XPS spectra (Al2p) obtained for silver powder coated with alumina obtained from the embodiments and comparative examples of the present invention.

[0037] Figure 3 The TMA curves of silver powder coated with alumina obtained from the embodiments and comparative examples of the present invention are shown.

[0038] Figure 4 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 1 of the present invention.

[0039] Figure 5 The images shown are SEM images of silver powder coated with alumina obtained in Example 1 of the present invention, as well as Lα characteristic X-ray images of silver and Kα characteristic X-ray images of aluminum.

[0040] Figure 6 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 2 of the present invention.

[0041] Figure 7 The images shown are SEM images of silver powder coated with alumina obtained in Example 2 of the present invention, as well as Lα characteristic X-ray images of silver and Kα characteristic X-ray images of aluminum.

[0042] Figure 8 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 3 of the present invention.

[0043] Figure 9 The images shown are SEM images of silver powder coated with alumina obtained in Example 3 of the present invention, as well as Lα characteristic X-ray images of silver and Kα characteristic X-ray images of aluminum.

[0044] Figure 10 This is a cross-sectional SEM image of commercially available silver powder (manufactured by Tongwa Technology Co., Ltd., 4-8FD) obtained in Comparative Example 1 of the present invention.

[0045] Figure 11 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Comparative Example 2 of the present invention.

[0046] Figure 12 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Comparative Example 3 of the present invention.

[0047] Figure 13 The images shown are SEM images of silver powder coated with alumina obtained in Comparative Example 3 of the present invention, as well as Lα characteristic X-ray images of silver and Kα characteristic X-ray images of aluminum. Detailed Implementation

[0048] The alumina-coated silver powder of the present invention is suitable for use as a filler in conductive pastes for forming electrodes of solar cells. The conductive paste using the silver powder of the present invention is printed onto a solar cell substrate by, for example, screen printing, offset printing, photolithography, etc., thereby forming a conductive film containing conductive patterns and electrodes. The alumina-coated silver powder of the present invention can reduce the carrier recombination loss of silver in the conductive film, and therefore is suitable for use as a sintered conductive paste for forming the conductive film of a single cell in a solar cell.

[0049] [Alumina-coated silver powder]

[0050] The alumina-coated silver powder of the present invention refers to silver powder containing silver particles that coat the surface of alumina, and has a true density of 10.00 g / cm³. 3 The following silver powder. The aluminum coating amount relative to the total mass of the alumina-coated silver powder is 400 ppm to 3000 ppm. Furthermore, the alumina-coated silver powder of the present invention has a cumulative 50% particle size D measured by laser diffraction scattering particle size distribution measurement. 50 The thickness ranges from above 0.2 μm to below 5.0 μm. By preparing a paste of silver powder coated with alumina that meets these conditions, a conductive film can be formed on the substrate, thus suppressing the open-circuit voltage (V) of the final solar cell. oc The decrease of ).

[0051] In this specification, silver particles coated with alumina refer to silver particles whose surface is partially or completely coated with alumina. Whether a silver particle is partially or completely coated with alumina can be confirmed by determining the aluminum distribution on the surface of the silver particle using energy-dispersive X-ray fluorescence spectroscopy (EDX) as described later, and by determining the chemical bonding state of aluminum using X-ray photoelectron spectroscopy (XPS) as described later. It should be noted that, as long as it is within the scope of the effects of this invention, it is permissible to include silver particles in the silver powder that are not coated with alumina. For example, the proportion of silver particles partially or completely coated with alumina to all particles in the silver powder is acceptable as long as it is 80% or more, preferably 90% or more, and the upper limit is 100%. If the ratio of silver particles partially or completely coated with alumina is within the above range, the effect of suppressing aluminum segregation (uneven distribution) in the conductive paste is excellent.

[0052] In this invention, the coating of silver particles with alumina is as described below. When the silver particles are reduced and precipitated, the silver particles are precipitated in such a way that the alumina is coated on the surface of the silver particles. Even when the silver powder coated with alumina is mixed into a conductive paste, the coating can coexist with the silver particles. Even in a conductive paste, the segregation of silver particles and alumina can be suppressed.

[0053] It should be noted that the aluminum distribution area ratio per silver particle in this invention can be obtained by binarizing the image of aluminum distribution obtained using the aforementioned EDX using image processing software. The aluminum distribution area ratio can be calculated as the average value of the aluminum distribution area ratio per particle in 10 particles. The detailed measurement method will be described later. The average value of the aluminum distribution area ratio per silver particle in this invention is preferably 50% or more, more preferably 70% or more, and even more preferably 85% or more. If it is 50% or more, it indicates that alumina has been distributed to a certain extent on the surface of the silver particles, thus suppressing the segregation of aluminum in the conductive paste, which is therefore preferred.

[0054] The "aluminum content" in the alumina-coated silver powder of this specification refers to the amount of aluminum relative to the mass of the alumina-coated silver powder, as determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after the alumina-coated silver powder has been completely dissolved in acid. In this invention, aluminum is mostly present on the surface of the silver particles, but it can be considered to exist in two forms: aluminum present in the silver particles and aluminum coated on the surface as alumina. The "aluminum content" in this specification is the sum of the amount of aluminum present in the silver particles and the amount of aluminum coated on the surface.

[0055] It should be noted that since ICP-OES cannot perform state analysis, whether the aluminum present on the surface of silver particles exists in the state of aluminum oxide, i.e., whether it is "silver particles with aluminum oxide coating the surface", can be determined by aluminum state analysis using X-ray photoelectron spectroscopy (XPS). In this invention, if a peak exists in the XPS spectrum within the binding energy range of 74.3 ± 1 eV for aluminum oxide, the aluminum can be identified as aluminum oxide. It should be noted that in this invention, the aluminum present on the surface of silver particles is preferably aluminum oxide, but even a small amount of peaks containing metallic aluminum is acceptable. In the XPS measurements described below, the ratio of the height of the peak corresponding to metallic aluminum to the height of the peak corresponding to aluminum oxide is preferably 10% or less.

[0056] The aluminum content relative to the total mass of the silver powder is preferably 400 ppm or more. If the aluminum content is less than 400 ppm, the V of the electrode film of the present invention... oc The improvement effect will be insufficient. Furthermore, since alumina is an insulator, when the aluminum content exceeds 3000 ppm by mass, the line resistance of the conductive film deteriorates, thereby reducing the performance of the solar cell, which is therefore undesirable. More preferably, the aluminum content is 600 ppm by mass or more to 2500 ppm by mass, and even more preferably 1000 ppm by mass or more to 2000 ppm by mass.

[0057] The true density of the silver powder coated with alumina in this invention is 10.00 g / cm³. 3 The following is more preferred: 9.90 g / cm³ 3 The preferred value is 9.85 g / cm³. 3 The following is an explanation of the concept of true density in this invention. In this invention, the true density of the silver powder as a whole refers to the density of the silver powder that takes into account the porosity (i.e., the "closed space inside the silver particles" mentioned above) that exists within the silver particles. For example, if the true density of the silver powder is 10.00 g / cm³... 3 The following describes the state of silver particles with internal pores, which can be fully observed when examining the cross-section of the silver powder. This allows for easy expansion during the firing of the conductive paste. Furthermore, after sintering begins on the particle surface, the gas components within the pores of the particles react rapidly, thereby fully promoting the sintering of the entire silver particle, making this method preferable. On the other hand, its true density can also be, for example, 9.00 g / cm³. 3 Above, 9.40 g / cm 3 Above, or 9.70 g / cm 3 above.

[0058] When observing the cross-section of the silver particles constituting the alumina-coated silver powder of the present invention using a scanning electron microscope (SEM), it can be confirmed that there are micropores sealed from the outside inside the silver particles. In this alumina-coated silver powder, the proportion of silver particles having one or more pores inside the particles is preferably 70% or more of all silver particles, more preferably 75% or more, further preferably 80% or more, and most preferably 85% or more. If this value is 70% or more, the conductive paste is more likely to expand during firing. Furthermore, after the surface sintering of the particles begins, the gas components in the pores inside the particles react rapidly, thereby easily promoting the sintering process of all silver particles and forming a dense conductive film, which is therefore preferred. It should be noted that the upper limit of the proportion is 100%.

[0059] It should be noted that in this specification, a pore with a diameter of 15 nm or more, observed using cross-sectional SEM images of silver particles taken at 10,000 to 40,000x magnification, is defined as a "pore". In this invention, "pore diameter (i.e., the diameter of the enclosed space inside the silver particle containing the pore)" refers to the outer diameter of the smallest circle that completely accommodates the pore when drawn in a scanning electron microscope image. The proportion of silver particles with pores inside the particles in this silver powder can be confirmed by the following ratio: the ratio of the number of particles with one or more pores inside to the number of silver particles observed by observing the aforementioned cross-sectional SEM images of silver particles taken at 10,000x magnification (containing more than 10 particles) in more than 5 fields of view.

[0060] The cumulative 50% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 50 Preferably, the micrometer size is 0.2 μm or more to 5.0 μm or less, more preferably 1.0 μm or more to 3.0 μm or less. 50 When the diameter 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 can sometimes lead to broken lines when using conductive paste to draw wiring, making it less desirable. Furthermore, when D... 50 When the diameter exceeds 5.0 μm, it becomes difficult to draw fine wirings using conductive paste, making it less desirable.

[0061] The maximum particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. max Preferably, it is 15.0 μm or less, more preferably 12.0 μm or less, and even more preferably 10.0 μm or less. If D max For diameters below 15.0 μm, conductive paste can be used to draw fine wiring.

[0062] The cumulative 10% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on the volume standard. 10 Preferably, the particle size is 0.1 μm or more, more preferably 1.0 μm or more. Furthermore, preferably 3.0 μm or less, more preferably 2.0 μm or less, and even more preferably 1.5 μm or less. If the cumulative particle size D is 10%,... 10 If the value is within the above range, the viscosity of the conductive paste can be prevented from becoming too high, and therefore it is preferred.

[0063] The cumulative 90% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 90 Preferably, the micrometer size is 1.5 μm or more, more preferably 2.0 μm or more, and even more preferably 2.5 μm or more. Furthermore, it is preferably 6.0 μm or less, more preferably 5.0 μm or less, and even more preferably 4.0 μm or less. If the cumulative D... 90 When the value is within the above range, it is easier to draw fine wiring when using conductive paste for wiring and other drawing tasks, and therefore it is preferred.

[0064] The cumulative 50% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 50 The BET particle size D calculated from the BET specific surface area value BET The ratio (D) 50 / D BET The preferred value is between 1.00 and 2.50. In cases where there are few aggregated states and the state is close to monodisperse, D... 50 / D BET The value tends to be close to 1, more preferably 2.00 or less, and even more preferably 1.50 or less. If the value is 2.50 or less, it is effective from the viewpoint of forming fine wiring when using conductive paste to draw wiring. Furthermore, D BET The definition will be described later.

[0065] The cumulative 90% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 90 With a cumulative 10% particle size D 10 The difference (D) 90 -D 10 ) and cumulative 50% particle size D 50 The ratio of (D) 90 -D 10 ) / D 50 The value is preferably 0.50 to 2.00 or less, more preferably 1.50 or less, further preferably 1.30 or less, and most preferably 1.00 or less. Wherein, (D 90-D 10 ) / D 50 As an indicator of particle size distribution, the smaller this value, the sharper the particle size distribution of the silver powder coated with alumina, meaning the more uniform the particle size. In this invention, if it is within the above range, it is easier to draw fine wirings when using conductive paste to draw wirings, which is therefore preferable. By controlling this value to below 2.00, it is beneficial to suppress the phenomenon that fine wirings are difficult to form due to the influence of coarse particles when using conductive paste to draw wirings.

[0066] The loss on heating (Ig-loss) of the alumina-coated silver powder of the present invention is preferably from 0.30% to 3.00%, more preferably from 0.50% to 1.50%, and even more preferably from 0.70% to 1.20%. If the loss on heating is 0.30% or more, agglomeration between silver powder particles can be suppressed, improving dispersibility. On the other hand, if the loss on heating is 3.00% or less, deterioration of resistivity due to excess impurities can be prevented.

[0067] In the thermomechanical analysis (TMA) curves of the alumina-coated silver powder of the present invention, obtained by heating at a rate of 10°C / min from room temperature (25°C ± 5°C) to 900°C, the temperature at which the expansion rate reaches 0.50% based on room temperature (hereinafter sometimes referred to as the "0.50% expansion temperature") is preferably 150°C or higher to 350°C or lower. Regarding the maximum expansion rate (hereinafter sometimes referred to as the "maximum expansion rate"), it is 0.50% or higher, preferably 1.00% or higher, and there is no particular upper limit, but it can be, for example, 5.00% or lower or 3.00% or lower. Furthermore, the expansion rate of the alumina-coated silver powder of the present invention preferably decreases after a brief increase with heating, and the temperature range in which the expansion rate reaches 0.00% or lower based on room temperature (i.e., below the length of the sample before measurement) is 550°C or higher to 900°C.

[0068] The aforementioned expansion behavior of alumina-coated silver powder is facilitated by the presence of pores within the silver particles. However, even at the same porosity, the alumina-coated silver powder of the present invention exhibits a more significant expansion behavior compared to uncoated silver powder. As shown in Comparative Example 1 described below, the uncoated, porous silver powder exhibits an expansion rate of less than 0.50% at room temperature within a temperature range of 150°C to 350°C, resulting in a decrease in open-circuit voltage (V). oc The effect is small. Generally, it is preferable to avoid significant expansion of silver particles upon heating, but in this invention, due to its porosity and alumina coating, the open-circuit voltage (V) is reduced due to the aforementioned expansion behavior. oc The effect is greatly improved.

[0069] By using the alumina-coated silver powder of the present invention, the final open-circuit voltage (V) of the solar cell is improved. oc The mechanism by which the silver powder in this invention decreases is currently unclear, but the inventors have considered the following: It is speculated that because the silver powder of this invention is coated with alumina on the surface of the silver particles, the contact frequency between the silver in the paste and the glass powder is reduced, thereby reducing the amount of silver dissolved in the glass. Specifically, in the case of PERC solar cells, glass powder is added to the conductive paste to decompose the passivation layer of the substrate. Ideally, after decomposing the passivation layer, the glass powder stops entering the n-type semiconductor layer. However, there are cases where the glass powder penetrates the n-type semiconductor layer and enters the p-type semiconductor layer. It is known that in this case, the silver dissolved in the glass short-circuits with the p-type semiconductor, increasing carrier recombination losses and reducing the open-circuit voltage (V), one of the evaluation criteria for solar cell characteristics. oc The open-circuit voltage (V) is reduced and its properties deteriorate. It can be considered that when using the alumina-coated silver powder of the present invention, the amount of silver dissolved in the glass can be optimized. Therefore, silver can enter the n-type semiconductor layer to ensure conduction, while suppressing carrier recombination losses caused by short circuits with the p-type semiconductor, thereby improving the open-circuit voltage (V). oc ).

[0070] [Confirm whether there are pores inside the silver particles]

[0071] In the embodiments described below, the presence or absence of pores inside the silver particles was confirmed by the following method.

[0072] First, silver powder coated with alumina is added to resin and a curing agent for curing, and then the cured resin is cut. Next, the cut surface is ground using a cross-section polisher to expose the cross-section of the silver particles. The cross-section of each silver particle is then observed using a scanning electron microscope (SEM) to confirm whether there are any pores inside the particles.

[0073] For example, "EpoFix resin" and "EpoFix curing agent" manufactured by Struers Co., Ltd. can be used as the resin and curing agent mentioned above. Alternatively, "ArBlade 5000" manufactured by Nippon High Technology Co., Ltd. can be used as the section polishing machine mentioned above. Furthermore, "JSM-IT800SHL" manufactured by Nippon Egis Corporation can be used as the scanning electron microscope mentioned above.

[0074] It should be noted that in this specification, "pores inside the silver particle" does not refer to the uneven space on the surface of the silver particle, but rather to the closed space inside the silver particle.

[0075] [Manufacturing Method]

[0076] The method for manufacturing alumina-coated silver powder of the present invention employs a wet process, which has excellent manufacturing cost and large-scale production efficiency.

[0077] [Starting material]

[0078] In the method for manufacturing alumina-coated silver powder of the present invention, an aqueous solution containing monovalent silver ions (aqueous solution containing silver ions) is used as the starting material. As the source of silver ions, industrially known inorganic silver salts such as silver nitrate (I), silver sulfate (I), silver carbonate (I), silver chloride (I), and silver oxide (I) can be used. It should be noted that an aluminum compound can also be added to the above aqueous solution to make it an aqueous solution containing both monovalent silver ions and aluminum ions. The aluminum compound can be added at any time, before the addition of ammonia, after the addition of ammonia, or after the addition of a pH adjuster, as long as it is before the addition of a reducing agent; there are no particular limitations.

[0079] Although not specifically specified in this invention, the concentration of silver ions in the aqueous solution used as the starting material is preferably between 1.0% by mass and 2.0% by mass during the solution preparation stage. When the silver ion concentration is less than 1.0% by mass, the amount of silver powder that can be produced in each reaction decreases, which is disadvantageous from the perspective of production efficiency. When the silver ion concentration exceeds 2.0% by mass, the viscosity of the reaction solution after the silver particles precipitate increases, and depending on the manufacturing equipment, it may be difficult to uniformly stir the reaction solution.

[0080] [Complex Formation Engineering]

[0081] In the method for manufacturing silver powder coated with alumina according to the present invention, silver ions are complexed with ammonium ions to form a silver-ammonia complex, thereby preparing an aqueous solution of the silver-ammonia complex. Ammonium salts such as ammonia, ammonium chloride, or ammonium carbonate can be used as the source of ammonium ions. When ammonium ions are used as the complexing agent, a silver-ammonia complex can be formed in the aqueous solution; however, in this case, since the coordination number of the ammonia complex is 2, more than 2 moles of ammonium ions are added per mole of silver ions. It should be noted that the reaction temperature for forming the silver-ammonia complex is preferably controlled between 10°C and 40°C. Since the formation of the silver-ammonia complex is an exothermic reaction, the reaction temperature for forming the silver-ammonia complex is preferably within the aforementioned range in order to set the temperature of the reduction precipitation process described later at the desired temperature. If the reaction temperature is too low or too high, temperature adjustment requires time, thus increasing energy costs.

[0082] [pH adjustment process]

[0083] Next, a pH adjuster is added to the aqueous solution of the silver-ammonia complex obtained from the aforementioned steps to adjust the pH of the aqueous solution to 10.0 or higher. Controlling the pH to 10.0 or higher is to sufficiently enhance the reducing power of the reducing agent. As the aforementioned pH adjuster, hydroxides or carbonates of alkali metals or alkaline earth metals such as sodium hydroxide or calcium hydroxide can be used. It should be noted that in the method for manufacturing alumina-coated silver powder of the present invention, the upper limit of the pH of the aqueous solution is not specifically specified, but to avoid excessive use of the pH adjuster, it is preferable to control the pH to 13.0 or lower. The temperature of the pH adjustment step is preferably controlled between 10°C and 40°C. If the reaction temperature is too low or too high, temperature adjustment requires time, thus increasing energy costs.

[0084] [Aluminum compounds]

[0085] In this invention, the aluminum compound is added at the aforementioned time, either before or after the addition of ammonia, or after the addition of the pH adjuster. That is, the aluminum compound is added only before the addition of the reducing agent in the silver particle precipitation process described later. This invention includes a step of adding the aluminum compound to the aforementioned aqueous solution containing silver ions, the aforementioned aqueous solution of the silver-ammonia complex, or the aforementioned aqueous solution with added pH adjuster.

[0086] As an aluminum compound, any compound that allows aluminum to be added to an aqueous solution in ionic form, depending on the pH range of the aqueous solution at the time of addition, is acceptable. For example, inorganic aluminum salts such as aluminum nitrate, aluminum acetate, and aluminum oxalate can be used. Furthermore, it is preferable to match the type of acid or base used; for example, using aluminum nitrate for silver nitrate aqueous solutions is also preferred. It should be noted that aluminum oxides are amphoteric oxides; therefore, aluminum is added in the low pH range in the form of aluminum ions (Al). 3+ It dissolves in the form of aluminate ions (AlO2) in a high pH range. - It dissolves in the form of aluminum hydroxide (Al(OH)3 or Al2O3·3H2O) at a pH near neutral, forming a solid-phase stable region of aluminum oxide (Al2O3). It should be noted that here, aluminum oxide includes aluminum hydroxide (Al(OH)3 or Al2O3·3H2O).

[0087] The amount of aluminum supplied to the liquid by the aluminum compound (the total amount of aluminum supplied in the case of multiple supplies) is preferably such that, at the starting point of the addition of the reducing agent described later, the amount of aluminum in the aqueous solution is 400 ppm by mass or more to 3000 ppm by mass or less relative to the amount of silver ions contained in the aqueous solution. If the amount of aluminum relative to silver is less than 400 ppm by mass, the effect of the present invention may be insufficient. In addition, when the amount of aluminum relative to silver exceeds 3000 ppm by mass, the line resistance deteriorates because alumina is an insulator, which is not preferred. It should be noted that it is more preferable to control the amount of aluminum relative to silver to 600 ppm by mass or more to 2500 ppm by mass, and even more preferably to control it to 1000 ppm by mass or more to 2000 ppm by mass or less.

[0088] Furthermore, when adding aluminum compounds in an aqueous solution, the concentration is preferably between 0.5% by mass and 10.0% by mass (calculated as aluminum). When the aluminum concentration is below 0.5% by mass, the volume of the reaction solution increases as the amount of aluminum compound aqueous solution added to obtain alumina-coated silver powder (the target material) increases, requiring more reagents and thus becoming uneconomical. When the aluminum concentration exceeds 10.0% by mass, the amount of aluminum compound aqueous solution added to obtain alumina-coated silver powder (the target material) becomes trace, and the error in the amount added can easily increase depending on the operating conditions.

[0089] [Silver particle precipitation process]

[0090] In the method for manufacturing alumina-coated silver powder of the present invention, a reducing agent is added to the aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher obtained in the aforementioned steps to reduce the silver-ammonia complex, thereby causing silver particles to precipitate in the aqueous solution. At this time, a reducing agent that lowers the pH of the aqueous solution by adding the reducing agent is used. The reason for using a reducing agent that lowers the pH of the aqueous solution is that by adding this reducing agent, the pH of the aqueous solution can be lowered to the solid-phase stable region of the aforementioned alumina, thereby causing alumina to coat the surface of the precipitated silver particles. If a reducing agent that does not lower the pH of the aqueous solution is used, adding a pH adjuster after the silver particles precipitate to lower the pH of the aqueous solution may result in alumina coating on the surface of the silver particles, but this increases the number of steps in the manufacturing method, and is therefore not preferred. Furthermore, it is difficult to uniformly coat the particle surface with alumina, which is also not preferred.

[0091] It should be noted that the reaction temperature for precipitating silver particles using the aforementioned reducing agent is preferably controlled between 10°C and 40°C. If the reaction temperature is too low or too high, temperature adjustment will take time, thus increasing energy costs.

[0092] [reducing agent]

[0093] The most significant technical feature of the method for manufacturing alumina-coated silver powder of the present invention is the use of a reducing agent that lowers the pH of an aqueous solution upon addition. The mechanism by which the pH of the aqueous solution is lowered by adding the reducing agent includes situations where the reducing agent itself has a pH-lowering effect, and situations where the oxidation product of the reducing agent has a pH-lowering effect.

[0094] As a reducing agent with such an effect, it is preferable to be an organic compound containing one or more of the following groups: COOH, CHO, and OH. Specifically, examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde.

[0095] When an organic compound containing a COOH group is used as a reducing agent, the pH of the aqueous solution decreases due to the addition of this reducing agent. In the case of a reducing agent with a CHO group that has a reducing effect, the CHO group reduces silver ions, and the CHO group is oxidized to form a COOH group, thus lowering the pH of the aqueous solution. In the case of a reducing agent with an OH group that has a reducing effect, the OH group is oxidized and converted to a COOH group via the CHO group, thus lowering the pH of the aqueous solution.

[0096] It should be noted that the presence of pores inside the silver particles constituting the aluminum oxide-coated silver powder of the present invention is presumed to be due to the gas generated by the decomposition of the aforementioned organic compounds being trapped inside the silver particles.

[0097] In the method for manufacturing alumina-coated silver powder of the present invention, the pH of the aforementioned aqueous solution is controlled to be between 4.0 and 9.0 by adding the aforementioned reducing agent. By setting the pH within the above-mentioned range, alumina can be coated onto silver particles from the mixed aqueous solution.

[0098] The pH value lowered by adding the aforementioned reducing agent can be used as the pH value after silver powder precipitation is complete. In this case, this pH value can be set as the measured pH value of the filtrate after filtering and recovering the silver powder. This pH value can be controlled by the pH value in the pH adjustment process described above, the amount of pH adjusting agent added, the equivalent of ammonia in the complex formation process, or the amount of reducing agent. The pH value is more preferably set to 6.0 or higher. When the pH value is lower than 4.0, aluminum may transform into Al. 3+ The ions cause the aluminum oxide on the surface to redissolve, which is not preferable. Furthermore, when the pH is greater than 9.0, almost no aluminum is detected in the determination of the aluminum coated with the silver powder, which is also not preferable.

[0099] It should be noted that when the reducing agent in Example 1 described below is changed from formalin to hydrazine, the pH of the filtrate becomes above 10.0, and almost no aluminum is detected in the determination of the amount of aluminum coated on the silver powder.

[0100] Therefore, it is believed that when the pH of the mixed aqueous solution decreases from above 10.0 to near 9.0, aluminum oxide begins to precipitate on the surface of the silver particles. Thus, as the particles grow until the final pH is reached, an aluminum-containing layer (e.g., aluminum oxide) forms near the surface of the silver particles, thereby gradually coating the surface of the silver particles with aluminum oxide.

[0101] In the manufacturing method of the present invention, the aforementioned mixed solution is pre-contained with aluminum ions, and the pH change during the precipitation process of the reducing agent causes aluminum oxide to coat the surface of the silver particles, thereby enabling aluminum oxide to uniformly coat the surface of the silver particles.

[0102] When aldehydes are used as reducing agents, silver particles precipitate to a certain extent. As the concentration of the oxidation product, carboxylic acid, increases, the pH of the aqueous solution reaches the aforementioned pH range. Therefore, from the viewpoint of facilitating the coating of alumina onto the surface of the precipitated silver particles, this is preferable. As for the aldehyde, formaldehyde is more preferred from the viewpoint of ease of acquisition and reducing power.

[0103] To improve the silver yield, the amount of the aforementioned reducing agent added is preferably 1 equivalent or more relative to silver, or 2 equivalents or more relative to silver, for example, 10 equivalents or more to 20 equivalents or less.

[0104] [Surface treatment agent addition process]

[0105] In the method for manufacturing alumina-coated silver powder of the present invention, a surface treatment agent may be added to the solution containing alumina-coated silver particles obtained in the aforementioned silver particle precipitation step. This yields alumina-coated silver particles coated with the surface treatment agent.

[0106] Examples of surface treatment agents added to the solution containing silver particles coated with alumina include fatty acids, compounds with azo structures, fatty acid salts, surfactants, organometallic chelate forming agents, and protective colloids. From the viewpoint of easy and uniform adhesion to the surface of silver powder, the surface treatment agent is preferably one or more selected from the group consisting of fatty acids, compounds with azo structures, and fatty acid salts.

[0107] [Surface treatment agent]

[0108] Examples of fatty acids include benzyl acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. They can be used alone or in combination of two or more.

[0109] Examples of fatty acid salts include the salts of the fatty acids listed above. Examples of salts include, for example, sodium salts and potassium salts.

[0110] Examples of compounds with azole structures include benzotriazole, sodium benzotriazole, and potassium benzotriazole. They can be used alone or in combination of two or more.

[0111] The amount of surface treatment agent added to the solution containing silver particles coated with alumina 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-ammonia complex aqueous solution. If it is 0.05% by mass or more, it is effective in maintaining the dispersibility of the obtained silver powder coated with alumina well. On the other hand, the amount of the aforementioned surface treatment agent 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 the aforementioned surface treatment agent is 1.00% by mass or less, it is easier to suppress the excessive organic matter content in the obtained silver powder coated with alumina, and when using a conductive paste containing silver powder coated with alumina to obtain a conductive film, it is beneficial to suppress the increase in resistance value.

[0112] A surface treatment agent is added to a solution containing silver particles coated with alumina, preferably after adding a reducing agent to an aqueous solution of a silver-ammonia complex and allowing a predetermined time for addition. In this invention, the addition of the surface treatment agent inhibits the growth of silver particles, thus causing silver particles to precipitate and promoting the precipitation of alumina due to pH changes. The timing of addition, which allows silver in the solution to precipitate as silver particles, can be arbitrarily determined.

[0113] [Separation and recycling process]

[0114] After separating and recovering the alumina-coated silver powder obtained from the aforementioned series of processes using known solid-liquid separation methods, it is washed and dried as needed. Known solid-liquid separation methods include, for example, decantation or a filter press. The endpoint of the washing process can be determined by the conductivity of the washing water. Specifically, the washing process is considered complete when the conductivity of the washing water reaches a predetermined value or below. The washed silver particles, etc., can be supplied to the drying process in a cake-like or other agglomerated state.

[0115] [Drying Engineering]

[0116] The drying process can be carried out using vacuum drying or airflow dryers. During the drying process, high-pressure air jets can be sprayed onto the silver particles or other aggregates, or the cake-shaped and spherical silver powder from the drying process can be added to a mixer equipped with a stirring rotor for stirring. This disperses the cake-shaped and spherical silver powder from the drying process, allowing for operations based on dispersion or drying. The drying temperature of the silver powder is controlled below 100°C. Maintaining the silver powder temperature below 100°C effectively inhibits the sintering between silver particles in the silver powder.

[0117]

Example

[0118] [Determination of Aluminum Content in Alumina-Coated Silver Powder]

[0119] The aluminum content in silver powder coated with alumina was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). 1 g of the sample (silver powder) was accurately weighed and added to 15 mL of pure water and 10 mL of nitric acid (for precision analysis). The mixture was heated at 200°C for 30 minutes. After the heated sample cooled naturally, it was brought to a final volume of 100 mL with pure water. 5 mL of the supernatant was collected and brought to a final volume of 100 mL with pure water to prepare the sample for ICP analysis. A standard addition method was used for the determination. Aluminum was added to a standard solution of 5N silver to form a silver concentration equivalent to that of the sample, thus establishing a calibration line. Quantitative analysis was performed using an Agilent 5800 ICP-OES system manufactured by Agilent Technologies.

[0120] [Aluminum Phase Analysis]

[0121] The state of aluminum coated on the surface of silver particles was analyzed using X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed using a scanning X-ray photoelectron spectroscopy system (PHI 5000Versa Prove III) manufactured by ULVAC Fiennes GmbH. The X-ray source was monochromatic Al Kα line, with an accelerating voltage of 15 kV, an output power of 25 W, an X-ray incident angle of 90 degrees, and a photoelectron extraction angle of 45 degrees.

[0122] Chemical state analysis of aluminum was performed using Al 2p spectroscopy, with a pulse energy of 69 eV, an integration time of 80 ms, an energy interval of 0.125 eV / step, and a cumulative count of 500. Charge correction was performed using a C / C bond binding energy of 284.8 eV. In this specification, the peak with a binding energy of 74.6 ± 1 eV is referred to as aluminum oxide. In state analysis, aluminum oxide is, for example, Al₂O₃. It should be noted that in this invention, the aluminum present on the surface of the silver particles is preferably aluminum oxide, but it may also contain small amounts of, for example, peaks of metallic aluminum. In the XPS measurements described below, the height ratio of the peak corresponding to metallic aluminum to the peak corresponding to aluminum oxide is preferably 10% or less.

[0123] [Analysis of the distribution of aluminum on the surface of silver particles]

[0124] The alumina coating state on the surface of silver particles was analyzed by capturing characteristic X-ray (Kα line) images of silver and aluminum using an energy-dispersive X-ray fluorescence (EDX) analyzer attached to a SEM. The obtained aluminum characteristic X-ray (Kα line) images were verified using ImageJ (public domain image processing software), an image processing software capable of binarization. Specifically, using the software, the periphery of 10 randomly selected particles from the aluminum distribution image was manually set. Binarization was performed to calculate the aluminum distribution area of ​​each particle, which was then divided by the total surface area of ​​the particle to obtain the average ratio of aluminum distribution area to each particle. It should be noted that the aluminum characteristic X-ray (Kα line) images were adjusted to 8 bits, and both brightness and contrast were set to their maximum values. The locations of aluminum detected by the aforementioned energy-dispersive X-ray fluorescence analyzer attached to the SEM were highlighted and then binarized. The binarization threshold was set to a value one lower than the maximum value (here, 254), and the white areas were considered as the aluminum distribution areas. In addition, the observation was performed using a scanning electron microscope (JSM-IT800SHL) manufactured by Nippon Electron Ltd.

[0125] [Particle size distribution determination]

[0126] The cumulative 10% particle size (D) of silver powder on a volume basis was determined using a laser diffraction scattering particle size distribution measuring device (MicroTrack MT-3300 EXII, manufactured by Microtrac BEL Corp.). 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ), cumulative 100% particle size (D max During the determination, 0.1 g of sample was added to 40 mL of isopropanol (IPA) for dispersion. An ultrasonic homogenizer (manufactured by Nippon Seiki Co., Ltd., US-150T: 19.5 kHz, probe tip diameter 18 mm) was used for dispersion. The dispersion time was set to 2 minutes. The dispersed sample was then fed into the aforementioned apparatus, and the particle size distribution was determined using the accompanying analytical software. It should be noted that the circulator of the aforementioned laser diffraction scattering particle size distribution determination apparatus used an SDC (sample delivery controller) device, and the "flow rate (%)" setting of this circulator was 60.

[0127] [BET specific surface area]

[0128] The BET specific surface area was determined using a Macsorb HM-model 1210 (manufactured by MOUNTTECH) according to the single-point method of BET adsorption. It should be noted that in the determination of the BET specific surface area, the sample weight was set to 3.0 g, a N2:He = 30:70 mixed gas was used, the gas flow rate was set to 25 mL / min, and the degassing conditions before the determination were 60℃ for 10 min.

[0129] BET particle size (D) BET )]

[0130] BET particle size is the particle size calculated from the specific surface area of ​​BET, and is denoted as "D" in this specification. BET ".D BET The BET specific surface area, determined by the BET single-point method, and the actual density described below are substituted into the right side of equation (1) below.

[0131] D BET (μm) = 6 / (BET specific surface area × true density)...(1)

[0132] Here, the BET specific surface area value expressed in m² / g is substituted into the "BET specific surface area" on the right side of equation (1), and the true density value expressed in g / cm³ is substituted into the "true density" value.

[0133] [True density of silver powder coated with alumina]

[0134] In this specification, the "true density" of silver powder refers to the density of silver powder that takes into account the porosity within the silver particles, i.e., the "closed space within the porous silver particles" mentioned above. In this specification, the true density of silver powder is determined using a dry automatic densitometer (manufactured by Microtrac, device name: AccuPyc II 1340) employing helium gas, via the constant volume expansion method (i.e., the "gas specific gravity bottle method" in the Japanese Pharmacopoeia). Specifically, helium gas is introduced into the container until a certain pressure is reached, and the volume of the silver powder is measured from the volume of the gas at that point. The true density of the silver powder is calculated by dividing the mass of the silver powder by this volume. It should be noted that the helium gas cannot reach the closed space (i.e., pores) on the particle surface at this pressure. Therefore, generally, the higher the proportion of porosity within the silver powder, the lower the true density of the silver powder tends to be compared to the density of silver, which is 10.50 g / cm³. 3 .

[0135] [Expansion and contraction rates of silver powder coated with alumina]

[0136] The expansion and contraction rates of silver powder coated with alumina were determined by thermomechanical analysis (TMA) following the steps below.

[0137] First, weigh 0.3 g of silver powder. Then, put the silver powder into a specified mold with a diameter of 5 mmφ, and press it with a 50 kg load for 1 minute to make a circular plate-shaped test sample. Install the test sample on the sample holder of the thermomechanical analysis (TMA) device (Thermo plus EVO 2 series TMA8311), apply a measuring load of 98 mN through the measuring probe, and heat from room temperature (25℃±5℃) to 900℃ at a heating rate of 10℃ / min to obtain the TMA curve. For the length L (mm) distribution of the circular plate-shaped test sample at each temperature, use the following equations (2) and (3) to calculate the "expansion rate in thermomechanical analysis" and "shrinkage rate in thermomechanical analysis" respectively.

[0138] Expansion rate (%) in thermomechanical analysis = 100 × (L - L) RT ) / L RT ...(2)

[0139] Shrinkage rate (%) in thermomechanical analysis = 100 × (L) RT -L) / L RT ...(3)

[0140] Here, L RT The length (mm) of the circular plate-shaped measurement sample is measured at room temperature (25℃±5).

[0141] Therefore, the temperatures at which the expansion rate initially reaches 0.50% (0.50% expansion temperature), the maximum expansion point (the maximum point of the TMA curve), and the initial shrinkage rate reaching 1.00% (1.00% shrinkage temperature) were calculated. It should be noted that the shrinkage rate at 900℃ was calculated when the shrinkage rate had not yet reached 1.00%.

[0142] [Ig-loss value]

[0143] In this specification, "Ig-loss" refers to the change in mass of alumina-coated silver powder when heated from room temperature (25℃±5℃) to 800℃. This Ig-loss serves as an indicator of the amount of residual components other than silver contained in the silver powder, such as processing agents or additives used in the silver powder manufacturing process. In this specification, the Ig-loss is calculated as follows: a silver powder sample is accurately weighed (weighing value: w1), placed in a magnetic crucible, heated to 800℃, and then held at 800℃ for 30 minutes as sufficient time to reach constant weight. After natural cooling, it is weighed again (weighing value: w2). The Ig-loss is calculated from w1 and w2 according to the following formula (4).

[0144] Loss on heating (Ig - loss) (mass%) = 100 × (w1 - w2) / w1...(4)

[0145] Open circuit voltage (V) oc Alternative evaluation of )

[0146] In this invention, through the open-circuit voltage (V) oc Alternative evaluation to determine open-circuit voltage (V) oc Has there been any improvement? The open-circuit voltage (V) oc The alternative evaluation is conducted using the following methods.

[0147] As described in Reference 1 (Electronics Journal of the Packaging Society, Vol. 3, No. 2 (2000), "Basic Science Series," Episode 10: "Solar Cells and Their Performance," Kentaro Ito), in a solar cell, the open-circuit voltage (V... oc ) and saturation current density (J o The following equation (5) applies between them.

[0148] V OC =(n D kT / q)ln[(J ph / J0)+1] ･･･(5)

[0149] Where, n D Here, k is the diode index, k is the Boltzmann constant, T is the absolute temperature, q is the charge, and J is the weight of the diode. ph Let J be the photocurrent density. From equation (5), it can be seen that the smaller the value of the saturation current density (J0), the lower the open-circuit voltage (V). oc The value of ) becomes larger.

[0150] Next, reference 2 (“Challenges for the Quantification of metal-induced recombination losses”, D. Hermann, doctoral dissertation (2021)) describes the use of PLI (photoluminescence imaging) to determine the change in saturation current density of solar cells (ΔJ). o The method described above uses the average PLI intensity φ of the region where QSSPC (quasi-steady-state photoconductivity measurement) was performed. １ The open-circuit voltage (V) obtained by QSSPC measurement oc ) and thermal voltage at 25℃ (V t When setting the reference constant C, we can obtain the following equation (6). Furthermore, when expanding equation (6), we can obtain equation (7).

[0151] C=φ1 / exp(V OC / V t …(6)

[0152] exp(V OC / V t =φ1 / C …(7)

[0153] Next, let the saturation current density of the electrode portion be J. o,metal Let the saturation current density of the electrodeless portion be J. o,non-metal At that time, the change in saturation current density from the electrode (ΔJ) o ) can be expressed as the following formula (8).

[0154] ΔJ o =J o,metal -J o,non-metal …(8)

[0155] Here, the behavior of an ideal diode can be described by the generated current density (J). rec It is represented by equation (9).

[0156] J rec =J o ·exp(V OC / V t …(9)

[0157] J is expressed using equation (9) o,metal J o,non-metal When we get equations (10) and (11), we get equations (10) and (11).

[0158] J o,metal =J rec / exp(V OC / V t ) metal …(10)

[0159] J o,non-metal =J rec / ecp(V OC / V t ) non-metal …(11)

[0160] Here, when the PLI intensity of the part with the silver electrode is φ3 and the PLI intensity of the part without the silver electrode is φ2, equations (9) to (10) and (11) can be expressed by equations (12) and (13).

[0161] J o,metal =J rec ×C / φ3 …(12)

[0162] J o,non-metal =J rec×C / φ2 …(13)

[0163] When equations (12) and (13) are applied to equation (8), the change in saturated current density (ΔJ) o ) can be represented by equation (14).

[0164] ΔJ o =J rec ×C×(1 / φ3-1 / φ2) …(14)

[0165] The change in saturation current density (ΔJ) o The value represents the change in the presence of electrodes in a portion of a single cell compared to the absence of electrodes. This value can vary depending on the silver powder used in the conductive paste.

[0166] Since the saturation current density J cannot be directly obtained in the PLI method o Therefore, the change in luminous intensity caused by the presence or absence of electrodes (1 / φ3 - 1 / φ2) can be used to easily calculate the change in saturation current density (ΔJ). o ), by utilizing the saturation current density (J) of equation (5) o Open-circuit voltage (V) caused by the change in quantity oc The change in V oc The improvement effect was indirectly evaluated. Specifically, for the various silver powders obtained in the examples and comparative examples described below, a conductive paste as described below was prepared, and a substrate including portions with formed electrodes and portions without formed electrodes was prepared using the printing conditions and printing plate described below. The V value was indirectly evaluated by plotting the (1 / φ3-1 / φ2) value relative to the ratio of electrodes per unit area. oc An evaluation was conducted. Here, the larger the value of (1 / φ3-1 / φ2), the greater the ΔJ. o The larger the value of ΔJ becomes, the more... o The larger the value of V, the better. oc The smaller it becomes, therefore, in the open-circuit voltage (V oc In the alternative evaluation, the change in saturated current density (ΔJ) is preferred. o The smaller the value of ), the more V-shaped the display. oc The improvement effect.

[0167] That is, the smaller the slope of the ratio of (1 / φ3-1 / φ2) to the proportion of electrodes per unit area, the greater the change in saturation current density (ΔJ). o The smaller the value of ), the more it can be evaluated as open-circuit voltage (V). oc The greater the improvement effect, the better. Hereinafter, the slope value of the above "(1 / φ3-1 / φ2) relative to the proportion of electrodes per unit area" will be referred to as "V". ocThe slope value of the alternative evaluation was used to evaluate the open-circuit voltage (V). oc The improvement effect of ).

[0168] [Manufacturing method of conductive paste]

[0169] A mixture was prepared by mixing alumina-coated silver powder, aluminum powder (metallic aluminum: 99.87% by mass, iron: 0.09% by mass, silicon: 0.04% by mass, SEM average particle size: 2.0 μm), glass frit (glass powder: containing PbO as the main component, and containing B2O3, SiO2 and other oxides), ethyl cellulose, TEXANOL, butyl carbitol acetate, tributyl citrate, oleic acid, triacetin, methylphenyl polysiloxane, hydrogenated castor oil, and fatty acid amide. The composition of the mixture is shown in Table 1. The obtained mixture was fed into a paddleless rotary stirring degassing device (V-mini300 manufactured by EME Corporation), premixed for 30 seconds at a rotation speed of 1000 rpm, and then kneaded using a three-roll mill (80S manufactured by EXAKT Corporation) through a roller gap of 100 μm to 20 μm to obtain a conductive paste.

[0170] Table 1

[0171]

[0172] [Printing conditions and PLI measurement]

[0173] Using the conductive paste obtained in the above steps, utilize Figure 1The printing plate shown was screen-printed. The aforementioned printing plate was 125 mm × 125 mm in size, containing 25 25 mm × 25 mm regions. Within these electrode-forming regions, lines with a width of 15 μm and a length of 22 μm were designed, arranged with four regions of each line spacing of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm. Additionally, nine regions without electrodes (groovless regions) were designed. Printing was performed using a Microtech printer at a squeegee speed of 350 mm / s. After printing, the sample was dried for 5 minutes in a dryer set to 200°C, and then fired in a solar cell firing furnace (NGK) at a peak surface temperature of 720°C to prepare a sample for PLI measurement. The PLI measurement used a POPLI-Octa LED light source manufactured by Aites Corporation, with a center wavelength set to 830 nm. The PL excitation source was a POPLI-Lcta manufactured by iTES Corporation, with a center wavelength set to 340 nm. Images were taken using a PVX1000 manufactured by iTES Corporation, under the following conditions: 3-second exposure, 2MHz sampling rate, long-pass filter, and cutoff wavelength of 990 nm.

[0174] [Example 1]

[0175] To 3476.0 g of a silver nitrate aqueous solution containing 53.7 g of silver, 7.3 g of a 5% by mass aluminum nitrate nonahydrate (Al(NO3)3·9H2O: manufactured by High Purity Chemical Research Institute Co., Ltd., specification purity 98.0+%) aqueous solution was added. While stirring at 332 rpm, 113.2 g of a 28% by mass ammonia aqueous solution (manufactured by Pure Science Co., Ltd.) was added, generating a silver-ammonia complex aqueous solution. The aluminum content in the added aluminum nitrate nonahydrate aqueous solution was 480 ppm by mass relative to the silver content. One minute after adding the ammonia aqueous solution, 10.0 g of a 20% by mass sodium hydroxide aqueous solution was added, and the solution temperature was adjusted to 26.5°C. At this point, the pH of the solution was 12.1. Next, after adding the above ammonia aqueous solution to the aluminum-containing silver nitrate aqueous solution, 3 minutes later, 251.4 g of 25.9% by mass formalin (manufactured by Mitsubishi Chemical Co., Ltd.) was added all at once as a reducing agent. Fifteen seconds after adding the reducing agent, 6.1 g of an emulsified aqueous solution containing 1.55% stearic acid was added to the slurry containing the precipitated silver powder. Then, stirring was stopped, and the slurry containing the precipitated silver particles was filtered and washed with water until the conductivity of the filtrate was below 0.5 mS / m. The filtrate was then vacuum-dried at 73°C for 10 hours to obtain a cake-like silver powder. At this point, the pH of the filtrate was 6.4. It should be noted that the reaction was carried out in a 5-liter beaker, and a baffle and a three-stage turbine blade were used for stirring. 50 g of the silver powder obtained in the above steps was loaded into a sample mill (manufactured by Kyoritsu Rikko Co., Ltd., SK-M10) and milled twice for 30 seconds at a dial memory value of 100 to obtain the alumina-coated silver powder (experimental powder) according to Example 1.

[0176] The aluminum content of the silver powder coated with alumina was determined. The results showed that, relative to the silver powder mass of 480 ppm, the cumulative 50% particle size D of the volume reference, measured using a laser diffraction scattering particle size distribution analyzer, was [value missing]. 50 It is 1.9 μm, D 50 With D BET The ratio (D) 50 / D BET The value of 1.09 and the loss on heating (Ig-loss) were 0.93%.

[0177] Figure 2 The figure shows the XPS spectrum of the alumina-coated silver powder obtained in Example 1. A peak was observed in the spectrum near the binding energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of alumina on the surface of the silver powder obtained in this example. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS determination.

[0178] Figure 3The TMA curves for the alumina-coated silver powder obtained in Example 1 under the above-described measurement conditions are shown. The alumina-coated silver powder obtained in Example 1 begins to expand around 220°C, reaching 0.50% expansion at 225°C, with a maximum expansion rate of 1.60%. Subsequently, around 605°C, the expansion rate becomes 0.00%, returning to the sample length before measurement, and then shrinks. The shrinkage rate reaches 1.00% at 684°C (1.00% shrinkage temperature).

[0179] Figure 4 This shows a cross-sectional SEM image of the silver powder coated with alumina obtained in Example 1. The white scale bar shown in the lower right corner of the SEM image has a length of 1 μm. Figures 5-13 (The same applies to the Chinese version). The alumina-coated silver powder obtained in Example 1 contains silver particles with internal pores, and the proportion of silver particles with pores is 91%.

[0180] Figure 5 The image shows a SEM image of silver powder coated with alumina obtained in Example 1 (left), a Kα characteristic X-ray image of aluminum (middle), and an Lα characteristic X-ray image of silver (right). Each characteristic X-ray image was converted from the original color image to monochrome for display; the higher the brightness (appearing as white), the higher the concentration of the element (hereinafter). Figure 7 , 9 (The same applies to 13). In the case of silver powder coated with alumina, the average area ratio of aluminum distribution per particle is 95%. This indicates that alumina is widely distributed on the surface of the silver powder.

[0181] Using the alumina silver powder obtained in this embodiment, the open-circuit voltage (V) was determined according to the steps described above. oc The alternative evaluation plotted the values ​​of (1 / φ3-1 / φ2) relative to the proportion of electrodes per unit area. The results showed that the slope value (V) oc The slope value of the alternative evaluation is 0.000862.

[0182] Table 2 lists the aluminum content and the cumulative 10% particle size (D) of the silver powder used for testing. 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ), cumulative 100% particle size (D max ), BET specific surface area, BET particle size (D) BET D 50 With D BET The ratio (D) 50 / D BET D 90 With D 50 The difference (D)90 -D 10 ) and D 50 The ratio of (D) 90 -D 10 ) / D 50 The following parameters were considered: Ig-loss (weight loss upon heating), the proportion of porous silver particles, true density, the average area ratio of aluminum distribution per particle, the 0.50% expansion temperature, and V0. oc The slope value of the alternative evaluation (the same applies to all the examples below).

[0183] [Example 2]

[0184] Except for the addition of 14.8 g of aluminum nitrate nonhydrate aqueous solution, alumina-coated silver powder (experimental powder) according to this embodiment was obtained using the same procedures as in Example 1. The amount of aluminum in the added aluminum nitrate nonhydrate aqueous solution was 970 ppm by mass relative to the amount of silver. At this 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.

[0185] The aluminum content of the silver powder coated with alumina is 970 ppm by mass, and the cumulative 50% particle size D on a volume basis is... 50 It is 2.1 μm, D 50 / D BET The value is 1.24, and the loss on heating (Ig-loss) is 1.01%.

[0186] Figure 2 The figure shows the XPS spectrum of the alumina-coated silver powder obtained in Example 2. A peak was observed in the spectrum near the binding energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of alumina on the surface of the silver powder obtained in this example. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS measurement.

[0187] In addition, the proportion of porous silver particles in this alumina-coated silver powder is 89%.

[0188] Figure 6 The image shows a cross-sectional SEM image of the silver powder coated with alumina obtained in Example 2.

[0189] Figure 3 The TMA curve of the alumina-coated silver powder obtained in Example 2 is shown. The silver powder begins 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 recovers to the sample length before measurement at around 800°C, with a 1.00% shrinkage temperature of 878°C.

[0190] Figure 7The image shows a SEM image (left) of silver powder coated with alumina, a Kα characteristic X-ray image (middle) of aluminum, and an Lα characteristic X-ray image (right) of silver obtained in Example 2. In this example, the average distribution area ratio of aluminum per particle is 96%. This indicates that alumina is widely distributed on the surface of the silver powder.

[0191] The open-circuit voltage (V) was determined using the alumina-coated silver powder obtained in this embodiment. oc The alternative evaluation results show that, relative to the proportion of electrodes per unit area, the value of (1 / φ3-1 / φ2) is plotted, and the slope value (V) is obtained. oc The slope value of the alternative evaluation is 0.000589.

[0192] [Example 3]

[0193] Except that the amount of aluminum nitrate nonhydrate aqueous solution added was 29.3 g, the alumina-coated silver powder (experimental powder) according to this embodiment was obtained by following the same steps as in Example 1. The amount of aluminum in the added aluminum nitrate nonhydrate aqueous solution was 1920 ppm by mass relative to the amount of silver. At this 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.

[0194] The aluminum content of the silver powder coated with alumina is 1920 ppm by mass, and the cumulative 50% particle size D on a volume basis is... 50 It is 1.8μm, D 50 / D BET The value was 1.46, and the loss on heating (Ig-loss) was 0.93%.

[0195] Figure 2 The figure shows the XPS spectrum of the alumina-coated silver powder obtained in Example 2. A peak was observed in the spectrum near the binding energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of alumina on the surface of the silver powder prepared in this example. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS measurement.

[0196] In addition, the proportion of porous silver particles in this alumina-coated silver powder is 92%.

[0197] Figure 8 A cross-sectional SEM image of the silver powder coated with alumina obtained in Example 3 is shown.

[0198] Figure 3The TMA curve of the alumina-coated silver powder obtained in Example 3 is shown. The silver powder begins 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 recovers to the sample length before testing at around 860°C, and the shrinkage rate at 900°C is 0.60%.

[0199] Figure 9 The image shows a SEM image (left) of silver powder coated with alumina, a Kα characteristic X-ray image (middle) of aluminum, and an Lα characteristic X-ray image (right) of silver obtained in Example 3. In Example 3, the average area ratio of aluminum distribution per particle was 95%. This indicates that alumina is widely distributed on the surface of the silver powder.

[0200] Using the silver powder coated with alumina obtained in this embodiment, the open-circuit voltage (V) was measured. oc The alternative evaluation was performed by plotting the ratio of electrodes per unit area to the value of (1 / φ3 - 1 / φ2). The resulting slope value (V) was then analyzed. oc The slope value of the alternative evaluation is 0.000395.

[0201] [Comparative Example 1]

[0202] In this comparative example, commercially available silver powder (manufactured by Tongwa Technology Co., Ltd., 4-8FD) was used as the test powder. The cumulative 50% particle size D of the silver powder in this comparative example was measured using a laser diffraction scattering particle size distribution measuring device as a volume reference. 50 It is 1.9 μm, D 50 / D BET The value was 1.32, and the heat loss (Ig-loss) was 0.64%. In addition, when examining the cross-sectional SEM image of the silver powder in Comparative Example 1, it was found that it had pores, and the proportion of silver particles with pores was 90%.

[0203] Figure 10 The image shown is a cross-sectional SEM image of the silver powder of Comparative Example 1.

[0204] Figure 3 The TMA curve of the silver powder of Comparative Example 1 is shown. 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. After that, it recovered to the length of the sample before the measurement at around 360°C and began to shrink. The 1.00% shrinkage temperature was 380°C.

[0205] The open-circuit voltage (V) was measured using silver powder from Comparative Example 1. oc The alternative evaluation results were obtained by plotting the ratio of electrodes per unit area to the value of (1 / φ3 - 1 / φ2), and the slope value (V) was obtained. ocThe slope value of the alternative evaluation is 0.000957.

[0206] As shown in Table 2, the open-circuit voltage (V) obtained using the silver powder from the examples was measured. oc The results of the alternative evaluation, compared with those obtained using the silver powder (commercially available product) of Comparative Example 1, show that the slope value of the silver powder obtained in the example is smaller when plotting the values ​​of (1 / φ3 - 1 / φ2). This indicates that the silver powder used in Comparative Example 1 did not achieve the desired V-value. oc Improved effect. It can be inferred that, as mentioned above, the alumina-coated silver powder obtained in this invention can optimize the amount of silver dissolved in the glass, thus suppressing carrier recombination loss and consequently improving the open-circuit voltage.

[0207] [Comparative Example 2]

[0208] Except that the amount of aluminum nitrate nonahydrate aqueous solution added was 3.2 g, the alumina-coated silver powder (test powder) according to this comparative example was obtained by following the same procedures as in Example 1. The amount of aluminum in the added aluminum nitrate nonahydrate aqueous solution was 210 ppm relative to the amount of silver. At this 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.

[0209] The aluminum content of the silver powder coated with alumina is 210 ppm by mass, and the cumulative 50% particle size D on a volume basis is... 50 It is 2.0 μm, D 50 / D BET The value is 1.11, and the loss on heating (Ig-loss) is 0.92%.

[0210] Figure 2 The XPS spectrum of the alumina-coated silver powder obtained in Comparative Example 2 is shown. In this spectrum, a peak is observed near the binding energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating that alumina is present on the surface of the silver powder obtained in this example. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS measurement.

[0211] In addition, when examining the cross-sectional SEM image of the silver powder obtained in Comparative Example 2, it was found that it had pores, and the proportion of silver particles with pores was 84%.

[0212] Figure 11 The image shown is a cross-sectional SEM image of silver powder coated with alumina obtained in Comparative Example 2.

[0213] Figure 3The TMA curve of the alumina-coated silver powder obtained in Comparative Example 2 is shown. The silver powder began to expand at around 220°C, and the expansion rate reached 1.00% at 225°C, with a maximum expansion rate of 1.60%. Subsequently, at around 525°C, it returned to the sample length before the measurement and began to shrink, with a 1.00% shrinkage temperature of 620°C.

[0214] Similar to Example 1, the Lα characteristic X-ray images of silver and the Kα characteristic X-ray images of aluminum were measured for the silver-coated alumina powder obtained from Comparative Example 2. As a result, the distribution could not be determined because it was below the detection limit.

[0215] The open-circuit voltage (V) was measured using the alumina-coated silver powder obtained in Comparative Example 2. oc The alternative evaluation results, the proportion of electrodes per unit area, were plotted as (1 / φ3-1 / φ2), and the slope value (V) was obtained. oc The slope value of the alternative evaluation is 0.000947.

[0216] [Comparative Example 3]

[0217] While stirring 3232.0 g of a silver nitrate aqueous solution containing 50.8 g of silver at 174 rpm, 103.2 g of a 28% by mass ammonia aqueous solution (manufactured by Junko Chemical Co., Ltd.) was added to generate a silver ammonia complex aqueous solution. After adding the ammonia aqueous solution for 30 seconds, 0.3 g of a 5% by mass sodium carbonate aqueous solution was added, followed by 0.5 g of a 5% by mass PEI600 aqueous solution after another 30 seconds, and the solution temperature was adjusted to 33.5°C. At this point, the pH of the solution was 12.1. Next, after adding the ammonia aqueous solution for 3 minutes, 397.0 g of a 1.86% by mass hydrazine aqueous solution (manufactured by Mitsubishi Gas Chemical Co., Ltd.) was added in a single batch as a reducing agent. Ten seconds after adding the reducing agent, 23.5 g of an aqueous solution of aluminum nitrate nonahydrate [Al(NO3)3·9H2O, manufactured by High Purity Chemical Research Institute Co., Ltd., specification purity 98.0+%] (5.0% by mass) relative to silver was added. Twenty seconds after adding the reducing agent, 100.0 g of an aqueous solution of nitric acid (67.5% by mass) was added. The amount of aluminum in the added aluminum nitrate aqueous solution relative to silver was 1629 ppm by mass.

[0218] Furthermore, after adding the reducing agent, 30 seconds later, 5.1 g of an emulsified aqueous solution containing 1.55% by mass stearic acid was added to the slurry containing the precipitated silver powder. Afterwards, stirring was stopped, the silver particle slurry was filtered, washed with water until the conductivity dropped to below 0.5 mS / m, and then vacuum dried at 73°C for 10 hours to obtain silver powder. At this point, the pH of the filtrate was 8.9. It should be noted that the reaction was carried out using a 5-liter beaker with baffles and a 3-stage turbine blade. 50 g of the silver powder obtained in the above steps was loaded into a sample mill (manufactured by Kyoritsu Rikko Co., Ltd., SK-M10), and milled twice for 30 seconds each time with the dial value memorized as 100, to obtain the alumina-coated silver powder (experimental powder) according to Comparative Example 3.

[0219] The aluminum content of the silver powder coated with alumina is 1078 ppm by mass, and the cumulative 50% particle size D on a volume basis is... 50 It is 1.7 μm, D 50 / D BET The value is 1.20, and the loss on heating (Ig-loss) is 0.30%.

[0220] Figure 2 The XPS spectrum of the alumina-coated silver powder obtained in Comparative Example 3 is shown. A peak was observed in the spectrum near a binding energy of approximately 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of alumina on the surface of the silver powder obtained in this example. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS measurement.

[0221] Furthermore, upon examining the SEM image of the silver powder cross-section obtained in Comparative Example 3, it was found that there were no pores, and the proportion of silver particles with pores was 0.

[0222] Figure 12 The image shown is a cross-sectional SEM image of the silver powder obtained in Comparative Example 3.

[0223] Figure 3 The TMA curve of the silver powder obtained in Comparative Example 3 is shown. The silver powder expanded immediately after the start of the measurement, reaching an expansion rate of over 0.50% at 373°C, with a maximum expansion rate of 1.00%. Subsequently, it returned to the length of the sample before the measurement at around 890°C, and the shrinkage rate at 900°C was 0.30%.

[0224] Figure 13 The image shows a SEM image (left) of the silver powder obtained in Comparative Example 3, a Kα characteristic X-ray image of aluminum (center), and an Lα characteristic X-ray image of silver (right).

[0225] In Comparative Example 3, the average distribution area of ​​aluminum relative to each particle was 83%. This indicates that aluminum oxide is widely distributed on the surface of the silver powder.

[0226] Using Comparative Example 3SW silver powder, V was prepared according to the steps described above. oc Alternative evaluation results: A graph was plotted showing the ratio of electrodes per unit area to the value of (1 / φ3 – 1 / φ2). The resulting slope (V) was... oc The slope value of the alternative evaluation is 0.000973.

[0227]

[0228] As shown in Table 2, the silver powder coated with alumina in the embodiments exhibits a high open-circuit voltage (V). oc In the substitution evaluation, the slope value of (1 / φ3-1 / φ2) relative to the proportion of electrodes per unit area is small, indicating that the change in saturation current density (ΔJ0) is small. Therefore, the silver powder in the embodiment has the effect of improving the open circuit voltage (V oc The effect of ).

[0229] According to the present invention, it is possible to provide an open-circuit voltage (V) that improves the conductivity of the conductive film. oc Silver powder and its manufacturing method are described below. Additionally, according to the present invention, a method for improving the open-circuit voltage (V) from a conductive film can be provided. oc A conductive paste. Additionally, according to the present invention, an open-circuit voltage (V) from the conductive film can be provided. oc ) conductive film.

Claims

1. Alumina-coated silver powder, which is alumina-coated silver powder containing silver particles that coat the surface of alumina, wherein, The aluminum content relative to the mass of the silver-coated alumina powder is between 400 ppm and 3000 ppm; the cumulative 50% particle size D of the silver-coated alumina powder, measured using a laser diffraction scattering particle size distribution measuring device, is... 50 Its thickness ranges from 0.2 μm to 5.0 μm; its true density is less than 10.00 g / cm³.

2. The alumina-coated silver powder according to claim 1, wherein, The amount of aluminum relative to the mass of the silver powder coated with alumina is between 600 ppm and 2500 ppm.

3. The alumina-coated silver powder according to claim 1, wherein, When observing the cross-section of the particles, more than 70% of all silver particles have pores inside.

4. The alumina-coated silver powder according to claim 1, wherein, The D 50 With BET particle size D BET The ratio (D) 50 / D BET The value ranges from 1.00 to 2.

50.

5. The alumina-coated silver powder according to claim 1, wherein, The cumulative 90% particle size D of the volume standard, measured by a laser diffraction scattering particle size distribution measuring device. 90 Cumulative 10% particle size D based on volume 10 The difference (D) 90 -D 10 ) and the D 50 The ratio of (D) 90 -D 10 ) / D 50 The range is from 0.50 to below 2.

00.

6. The silver powder coated with alumina according to claim 1 has a loss on heating (Ig-loss) value of more than 0.30% and less than 3.00%.

7. The alumina-coated silver powder according to claim 1, wherein, The thermomechanical analysis (TMA) curves obtained by compressing the silver powder coated with alumina at 2.5 kg / mm² and applying a load of 5 mN / mm² to the sample were obtained by heating from room temperature to 900℃ at a heating rate of 10℃ / min. The temperature at which the expansion rate reaches 0.50% based on room temperature is above 150℃ and below 350℃.

8. The alumina-coated silver powder according to claim 1, used as a sintering paste for forming solar cell electrodes.

9. A sintering paste for forming solar cell electrodes, comprising the alumina-coated silver powder as described in claim 1.

10. A method for manufacturing silver powder coated with alumina, comprising the following steps: A process for preparing an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" and a "pH adjustment step," wherein the complex formation step is a step in which a silver-ammonia complex is formed by adding ammonia to an aqueous solution containing silver ions, thereby obtaining an aqueous solution of the silver-ammonia complex; the pH adjustment step is a step in which a pH adjusting agent is added to the aqueous solution of the silver-ammonia complex to adjust the pH of the aqueous solution to 10.0 or higher; the method used is selected from one or more of the following methods to prepare the aqueous solution of the silver-ammonia complex: a method of using the aqueous solution containing an aluminum compound in the complex formation step, a method of adding an aluminum compound to the aqueous solution in the period before the pH adjustment step is completed, and a method of adding an aluminum compound to the aqueous solution after the pH adjustment step has been completed; and The process involves adding a reducing agent to an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously adjusting the pH of the aqueous solution to between 4.0 and 9.0 to precipitate alumina. in, The amount of aluminum supplied to the liquid via the aluminum compound is controlled such that the amount of aluminum relative to silver in the aqueous solution at the initial point of addition of the reducing agent is between 400 ppm by mass and 3000 ppm by mass. The reducing agent is an organic compound containing one or more of the following groups: COOH, CHO, and OH.

11. The method for manufacturing alumina-coated silver powder according to claim 10, wherein, The reducing agent is formaldehyde.

12. The method for manufacturing alumina-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 reducing agent.

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

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