Gallium-containing silver powder, method for producing gallium-containing silver powder, and conductive paste

Abstract

Provided are: a gallium-containing silver powder which can supply gallium, which is a p-type impurity, in an appropriate form, can achieve low resistance under low-temperature calcination, and can reduce resistance under high-temperature calcination compared to the case where aluminum is added; and a method for producing the gallium-containing silver powder. The gallium-containing silver powder according to the present invention has a volume-based median diameter (D50) of 0.2 [mu] m to 5.0 [mu] m (inclusive) as determined by a laser diffraction method.

Description

Technical Field

[0001] This invention relates to gallium-containing silver powder, a method for manufacturing gallium-containing silver powder, and a conductive paste. Background Art

[0002] Silver powder is used as a bonding material for bonding various electrodes. For example, solar cells exist in two types: an n-type solar cell, in which a p-type emitter layer with p-type dopant is formed on the surface of an n-type silicon substrate; and a p-type solar cell, in which an n-type emitter layer with n-type dopant is formed on the surface of a p-type silicon substrate. In recent years, due to reasons such as increasing energy demand, the demand for solar cells with high power generation efficiency has been increasing, and electrode materials have attracted much attention.

[0003] In the past, when forming electrodes by coating a conductive paste onto a semiconductor layer (the aforementioned p-type emitter layer or p-type substrate) that has been p-typed using p-type dopant, attempts were made to include elements in the conductive paste that are p-type impurities for the Si substrate.

[0004] For example, as known in Patent Document 1, a silver-aluminum paste is used in the electrode formation of the light-receiving surface of an n-type solar cell, employing a combination of Ag (silver) and Al (aluminum) as conductive fillers. In Patent Document 1, by including Al in the conductive paste, p-type dopant is added to the semiconductor layer, attempting to reduce the resistance at the interface between the p-type semiconductor layer and the electrode.

[0005] In addition, Patent Document 2 discloses silver powder coated with Al and In (indium), which is a member of the same group, a method for manufacturing the same, and a conductive paste containing the silver powder.

[0006] Existing technical documents

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Publication No. 2016-519839

[0009] Patent Document 2: Japanese Patent Application Publication No. 2015-132002 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] However, it is known that if the conductive paste contains Al as in Patent Document 1, even if the element, which is a p-type impurity for silicon (Si), is supplied to the interface between silicon and the p-type semiconductor layer, the resistance of the formed electrode sometimes increases. Furthermore, the indium-coated silver powder disclosed in Patent Document 2 is an unusual coating material for silver powder because indium has a very large atomic radius, making it uncommon as a coating material for silicon. Therefore, it is desirable to develop a powder that is equivalent to or better than aluminum in terms of reducing resistance. Additionally, in the method disclosed in Patent Document 2 of adding a reducing agent to a paste containing silver powder and metal ions, gallium's redox potential is lower than that of hydrogen, thus gallium oxide is easily formed. Furthermore, when attempting to obtain gallium-coated silver powder using this method, a very strong reducing agent such as aluminum must be used to reduce the gallium, and the oxidized aluminum may remain as oxides or hydroxides, causing contamination.

[0012] The inventors envisioned replacing aluminum with gallium in silver powder and attempted to realize this. Therefore, the object of the present invention is to provide gallium-containing silver powder and a method for manufacturing the same, as well as a conductive paste using the same, wherein the gallium-containing silver powder is particularly capable of being supplied in a suitable form as gallium, which is a p-type impurity, so that it reaches the interface with the p-type semiconductor layer of a solar cell, and that low resistance can be achieved under low-temperature calcination, and that resistance can be reduced under high-temperature calcination compared to the case of adding aluminum.

[0013] Solutions for solving problems

[0014] In order to achieve the above-mentioned objectives, the inventors have conducted repeated and in-depth research, and as a result, the inventors have completed the following invention.

[0015] That is, the main structure of the present invention for achieving the above-mentioned objectives is as follows.

[0016] (1) A gallium-containing silver powder, characterized in that it contains gallium,

[0017] The median particle size D50 of the aforementioned gallium-containing silver powder based on the volume standard of laser diffraction is above 0.2 μm and below 5.0 μm.

[0018] (2) The gallium-containing silver powder according to (1), wherein gallium is attached to the surface of the silver particles constituting the aforementioned gallium-containing silver powder.

[0019] (3) The gallium-containing silver powder according to (1) or (2), wherein the gallium is metallic gallium.

[0020] (4) The gallium-containing silver powder according to any one of (1) to (3) contains the aforementioned gallium at a rate of 0.01 wt% or more and 10 wt% or less relative to silver.

[0021] (5) The gallium-containing silver powder according to any one of (1) to (4), wherein a surfactant is attached to the surface of the gallium-containing silver powder, and the amount of organic matter contained in the gallium-containing silver powder is 0.01 wt% or more and 4 wt% or less.

[0022] (6) A method for manufacturing gallium-containing silver powder, characterized by comprising the following steps:

[0023] A dispersion process, which forms an emulsion in which gallium droplets are dispersed; and

[0024] The attachment process involves adding (a) the aforementioned emulsion or (b) a gallium powder dispersion obtained by cooling the aforementioned emulsion to a liquid containing silver powder, thereby causing gallium to adhere to the aforementioned silver powder.

[0025] (7) The method for manufacturing gallium-containing silver powder according to (6), wherein, in the aforementioned dispersion step, a surfactant selected from fatty acids, azole compounds, alkenyl succinic acid, aliphatic amines, or their salts or their anhydrides is used.

[0026] (8) The method for manufacturing gallium-containing silver powder according to (6) or (7), wherein the liquid temperature in the aforementioned attachment process is set to below 70°C.

[0027] (9) A conductive paste, characterized in that it comprises a filler, a solvent and a glass frit, wherein the filler contains gallium-containing silver powder as described in any one of (1) to (5).

[0028] (10) The conductive paste according to (9), wherein the aforementioned filler further comprises silver powder.

[0029] (11) The conductive paste according to (9) or (10), wherein the conductive paste contains 0.01 to 4 wt% of the aforementioned gallium relative to silver.

[0030] The effects of the invention

[0031] According to the present invention, gallium-containing silver powder and a method for manufacturing the same, as well as a conductive paste using the same, are provided. The gallium-containing silver powder is particularly capable of being supplied in a suitable form as gallium as a p-type impurity so that it reaches the interface with the p-type semiconductor layer of a solar cell. Furthermore, it can achieve low resistance during low-temperature calcination and can reduce resistance during high-temperature calcination compared to the case of adding aluminum. Attached Figure Description

[0032] Figure 1 This is a SEM image of gallium powder in Example 1 of the present invention.

[0033] Figure 2This is a SEM image of gallium-containing silver powder in Example 1 of the present invention.

[0034] Figure 3 This is a schematic diagram of SEM-EDS-based elemental mapping of gallium-containing silver powder in Embodiment 1 of the present invention.

[0035] Figure 4 This is a schematic diagram of the SEM image of gallium-containing silver powder based on SEM-EDS and the elemental mapping of each element in Embodiment 1 of the present invention. DETAILED DESCRIPTION

[0036] The silver powder targeted in this invention is gallium-containing silver powder. When referred to as "gallium" in this specification, it refers not only to elemental gallium but also to the oxidation of a portion of the surface of metallic gallium. First, before describing the embodiments, gallium as the target of this specification will be described.

[0037] -Metal gallium (Ga)-

[0038] First, the resistivity of elemental gallium (Ga) and its group III metals is shown in Table 1 below.

[0039] [Table 1]

[0040] Metal elements Resistivity (Ω·m) Silver (Ag) <![CDATA[1.59×10 -8 ]]> Aluminum (Al) <![CDATA[2.65×10 -8 ]]> Indium (In) <![CDATA[8.37×10 -8 ]]> Gallium (Ga) <![CDATA[1.36×10 -7 ]]>

[0041] Ga, like B, Al, and In, belongs to Group III elements, but its resistivity is higher than that of Al and In. Therefore, it has not been considered to contribute to the reduction of resistance in electrodes formed from Ag until now. However, the inventors believe that gallium-containing silver powder has a resistance-reducing effect, especially when used to form FF50 type electrodes for solar cells.

[0042] In addition, gallium has a high resistivity but an extremely low melting point.

[0043] [Table 2]

[0044] Metal elements Melting point (°C) Aluminum (Al) 660 Indium (In) 156.6 Gallium (Ga) 29.78

[0045] It can be assumed that Ga, with its low melting point, melts at the beginning of initial sintering, below the glass softening temperature, and migrates first onto the substrate. It can also be assumed that when calcining a conductive paste containing silver powder, if the silver powder contains Al, Al may sometimes remain between the Ag particles at the temperature at which the Ag particles begin to neck. On the other hand, when the silver powder contains Ga, at that temperature, gallium is no longer present between the Ag particles, and it is expected that gallium has already migrated to the vicinity of the interface with the semiconductor layer. This may be because it contributes to lower resistivity of the silver powder compared to using Al, especially during low-temperature calcination at peak temperatures below 500°C (e.g., 400°C).

[0046] -Contains gallium-

[0047] Here, the phrase "containing gallium" in gallium-containing silver powder can be confirmed by detecting gallium in a sample solution prepared by completely dissolving the gallium-containing silver powder using nitric acid and hydrochloric acid, followed by ICP analysis. When performing ICP analysis on silver powder, silver powder primarily detecting gallium, in addition to silver, is gallium-containing silver powder. Gallium-containing silver powder includes silver powder in which a portion of the silver and gallium have been alloyed.

[0048] Gallium adhesion-

[0049] Gallium adhering to the surface of silver powder is another way to identify gallium-containing silver powder. Gallium adhering to the silver powder surface can be confirmed by observing the surface of silver particles using SEM-EDS and mapping gallium based on elemental mapping. Even if only a portion of the silver particle surface is covered with gallium, it will be detected. When the particle size of gallium-containing silver powder is small, surface elemental analysis can be performed using methods with low detection depth, such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).

[0050] -Median particle size on a volumetric basis-

[0051] The median particle size of gallium-containing silver powder is the cumulative 50% diameter (D50) based on a volumetric reference using laser diffraction. The laser diffraction determination can be performed using a MICROTRAC particle size distribution measuring apparatus (MICROTRAC BEL, MT-3300EXII), which is used in this specification.

[0052] The details of the gallium-containing silver powder described in this invention will be explained below.

[0053] (Gallium-containing silver powder)

[0054] The gallium-containing silver powder of this invention contains gallium, and the median particle size D50 of the gallium-containing silver powder based on volume diffraction is 0.2 μm or more and 5.0 μm or less. Preferably, the aforementioned median particle size D50 is 1 μm or more and 3 μm or less. Furthermore, regarding the BET specific surface area of ​​the gallium-containing silver powder, measured using the BET single-point method, it is preferably 0.15 m² / s. 2 / g or more and 4m 2 / g or less, more preferably 0.25m 2 / g or more and 1m 2 / g or less.

[0055] In addition to silver and gallium, the elements in the gallium-containing silver powder may include, within the range that enables the effects of the present invention, elements that can become p-type impurities (e.g., B, Al, In, Zn, etc.), elements that contribute to burn-through (e.g., Te), and alloys of gallium with these elements, in addition to unavoidable impurities. Furthermore, alloys of silver and gallium may be included. Here, the gallium content in the gallium-containing silver powder may be set to 50% or more, preferably 80% or more, in terms of elemental concentration ratio of the total elements excluding silver.

[0056] In the silver particles constituting gallium-containing silver powder, gallium is preferably attached to the surface of at least a portion of the silver particles. More preferably, gallium-containing silver powder has metallic gallium attached to the surface of at least a portion of the silver particles. Regarding the gallium-containing silver powder, in SEM-EDS mapping, the proportion of silver particles in which gallium is observed in at least a portion is preferably 5% or more, more preferably 10% or more, relative to the total number of particles. Gallium or metallic gallium can be attached to the entire surface of the silver particles or to a portion of the silver particles. It should be noted that the metallic gallium particles used in the manufacturing process of the gallium-containing silver powder of the present invention can be partially mixed. When gallium is attached to the surface of the silver particles, after a conductive paste containing gallium-containing silver powder is applied to the semiconductor layer, the gallium is easily movable upon heating. Furthermore, it can be considered that, due to its low melting point, especially in order to function as a dopant in the semiconductor layer of a solar cell, it is preferable for gallium to exist in a metallic gallium state compared to a gallium oxide state.

[0057] The gallium-containing silver powder of the present invention preferably contains gallium at a concentration of 0.01 wt% to 10 wt% relative to silver, and more preferably 2 wt% or less relative to silver. If the gallium content in the paste is too high, the resistivity will increase, which is undesirable. However, the gallium content relative to the silver in the paste can be adjusted to an appropriate range by mixing gallium-containing silver powder with gallium-free silver powder. In particular, the proportion of gallium-containing particles that serve as gallium sources in gallium-containing silver powder with high gallium content will decrease. Therefore, to improve the uniformity of the electrode calcination film, it is preferable not to drastically increase the gallium content. This gallium content can be quantitatively determined by using a sample solution prepared by completely dissolving the gallium-containing silver powder in nitric acid and hydrochloric acid, and then analyzing it using ICP analysis.

[0058] The surfactant adheres to the surface of the gallium-containing silver powder, and the organic matter content of the gallium-containing silver powder is preferably 0.01 to 4 wt%. The organic matter content can be determined by solvent extraction using an organic solvent such as hexane after dissolving the gallium-containing silver powder with acid, and then evaluating the extracted solvent layer by GC / MS (Gas Chromatography-Mass Spectrometry) or carbon analysis. Furthermore, for highly polar organic compounds such as those with hydroxyl groups, the extraction ability (partition coefficient) into organic solvents is sometimes low. For example, as a pretreatment, the solvent extraction ability can be improved by mixing an acid with an alcohol and heating it, thereby esterifying the carboxyl groups, and quantitative analysis can be performed based on this. The type of organic matter adhering to the surface is preferably a surfactant that is free of sulfur and phosphorus and has a molecular weight of 100 to 1000, and also preferably a fatty acid or aliphatic amine with a carbon chain length of 10 or more. The type of organic matter adhering to the surface can be thermally desorbed by heating in a He gas stream using a pyrolysis device, and then identified by qualitative analysis using GC / MS. Furthermore, the thermal weight change of gallium-containing silver powder is preferably less than 0.6 wt%. It should be noted that the thermal weight change refers to the total amount of impurities such as moisture and organic matter contained in the gallium-containing silver powder. It is the value obtained by dividing the difference between the initial mass and the mass after heating (e.g., at 800°C for 30 minutes) by the initial mass, after sufficient heating to achieve a constant weight (e.g., at 800°C for 30 minutes) without further weight change.

[0059] Next, a detailed description will be given of the manufacturing method for obtaining the gallium-containing silver powder described in this invention.

[0060] (Method for manufacturing gallium-containing silver powder)

[0061] The method for manufacturing gallium-containing silver powder according to the present invention includes at least: a dispersion step, which forms an emulsion in which metallic gallium droplets are dispersed; and an adhesion step, which involves adding (a) the emulsion or (b) a gallium powder dispersion obtained by cooling the emulsion to a liquid containing silver powder and mixing them, thereby adhering gallium to the silver powder. Hereinafter, each step and optional step of the manufacturing method according to the present invention will be described sequentially.

[0062] <Distributed Processes>

[0063] In the dispersion process, an emulsion containing dispersed gallium droplets is formed. Preferably, molten gallium containing high-purity gallium (4N or higher) heated to a temperature above its melting point is added to a solvent containing a surfactant (sometimes also called a dispersant). The mixture is stirred using an ultrasonic vibrator such as a homogenizer to form small droplets of gallium and to allow the surfactant to adhere to the gallium surface of the droplets, thus creating an emulsion (liquid-liquid dispersion) dispersed in the liquid.

[0064] -surfactant-

[0065] Here, when gallium-containing silver powder is used in the conductive paste, the surfactant adhering to the gallium surface is also included in the conductive paste. Considering this, the surfactant is preferably one that does not easily remain as an impurity in the conductive film after coating the conductive paste and calcining. For example, surfactants selected from fatty acids, azole compounds, alkenyl succinic acid, aliphatic amines, or their salts or anhydrides are preferred. Various surfactants can be used. The surfactant can be a reagent with the selected component as the main component, and the reagent may contain other components. Fatty acids and aliphatic amines are more preferred as surfactants. A carbon chain length of 10 or more is more preferred as surfactants. The surfactant preferably contains little or no phosphorus and sulfur. Phosphoric acid, sulfonic acid, and thiol surfactants are very effective for dispersing gallium, but after calcination, they may remain in the conductive film as phosphates or sulfates, potentially reducing properties and deteriorating reliability. In particular, surfactants free of sulfur and phosphorus and with a molecular weight of 100 or more and 1000 or less are preferred, preferably fatty acids or aliphatic amines with a carbon chain length of 10 or more. This is because it can be assumed that in the calcination of conductive films in solar cells, the calcination time is only tens of seconds, which is very short. Therefore, depending on the type of surfactant, the debinding agent will not completely terminate in the case of polymers with a molecular weight of more than 1000.

[0066] When considering the use of gallium-containing silver powder obtained by this manufacturing method in conductive pastes, it is preferable to use known surface treatment agents used in silver powders as surfactants that are less likely to leave residues after calcination of the conductive paste. Examples include stearic acid, palmitic acid, oleic acid, hydroxystearic acid, castor oil acid, benzotriazole, dodecenyl succinic anhydride, pentadecenyl succinic anhydride, stearylamine, etc. Furthermore, it is preferable that the surfactant has a composition that is partially or entirely the same as the surface treatment agent used in the subsequent adhesion process that adheres to the surface of the silver powder.

[0067] -solvent-

[0068] The solvent used is not particularly limited, but it is preferred to use a solvent that can dissolve the above-mentioned surfactant. For example, alcohols such as ethanol and isopropanol or water can be used.

[0069] <Attachment Process>

[0070] In the attachment step following the dispersion step, gallium is attached to the silver powder by adding (a) an emulsion or (b) a gallium powder dispersion obtained by cooling the emulsion to the liquid containing silver powder. Here, as mentioned above, the melting point of metallic gallium is about 30°C; therefore, even if the emulsion is only cooled to, for example, room temperature, the gallium in the emulsion will solidify, and a gallium powder dispersion containing solid gallium powder can be obtained. In this step, the emulsion obtained in the previous step can be added directly in its emulsion state, or a gallium powder dispersion obtained by cooling the emulsion can be added. It is preferable that the liquid containing silver powder is stirred from the beginning before addition.

[0071] In the adhesion process, silver powder is dispersed in a solvent, and an emulsion or gallium powder dispersion containing gallium droplets obtained in a previous step is added dropwise to it. By mixing them while adhering gallium to the surface of the silver powder, a dispersion of gallium-containing silver powder can be obtained. In this process, the method of adhering gallium to the silver powder while mixing the liquid containing silver powder with the emulsion or the gallium powder dispersion obtained by cooling it is arbitrary, but ultrasonic vibration is preferred.

[0072] <<Liquid Temperature>>

[0073] Furthermore, it is preferable to set the liquid temperature in the attachment process to below 70°C, more preferably above the melting point of the solvent, and even more preferably above 20°C and below 60°C. Particularly preferred is to keep the liquid temperature within the range of not exceeding 70°C during stirring. If the liquid temperature is too high, the dispersed gallium droplets may aggregate. While it is impossible to measure the actual local temperature, it is considered ideal that the mixing process is carried out only when the temperature reaches above the melting point of gallium (above 30°C) in the localized area where gallium droplets or gallium particles impact silver particles. It also depends on the kinetic energy of ultrasonic vibration or the like applied during the mixing process. However, when the liquid temperature is below 20°C, it is difficult to achieve a local temperature of above 30°C even with the application of kinetic energy such as ultrasonic vibration. On the other hand, if the temperature exceeds 70°C, gallium tends to aggregate. Furthermore, it is preferable to solidify the droplets by allowing them to cool naturally or by using a refrigerator to cool them to room temperature after the attachment process, and then store them in the form of a gallium powder dispersion.

[0074] <<Gallium powder dispersion prepared by cooling emulsion>>

[0075] -SEM average particle size-

[0076] The SEM average particle size of gallium particles obtained by cooling and drying the emulsion, and the SEM average particle size of gallium particles obtained by drying the gallium dispersion, are preferably 0.01 to 10 μm, more preferably 0.2 to 2 μm. In the method for determining the SEM average particle size, for a 5000x SEM image, the Heywood diameter is measured using the image processing software Mac-View Ver. 4 manufactured by MOUNTECH, for more than 100 particles whose particle shape can be clearly identified, and the average value can be calculated.

[0077] <Optional Process>

[0078] The resulting dispersion of gallium-containing silver powder is subjected to filtration and drying processes to obtain gallium-containing silver powder. Further processing steps, such as crushing and classification, can also be performed.

[0079] The above-described method for manufacturing gallium-containing silver powder is one example. Another method is to directly heat and stir the emulsion or gallium powder dispersion containing gallium droplets onto the dried silver powder. Alternatively, during the wet manufacturing process of the silver powder (from the addition of a reducing agent to the silver complex solution until filtration), the emulsion or gallium powder dispersion containing gallium droplets can be mixed and stirred.

[0080] (Conductive paste)

[0081] Next, a conductive paste obtained using the gallium-containing silver powder described in this invention will be explained. The conductive paste of this invention comprises at least a filler, a solvent, and a glass frit. Furthermore, the filler contains the gallium-containing silver powder described in this invention. This conductive paste is suitable for use as a conductive paste for forming p-type electrodes in solar cells. Preferably, the conductive paste contains 0.01 wt% to 4 wt% gallium relative to silver, more preferably 0.2 wt% to 2 wt%. The method for confirming the presence of gallium in the conductive paste can be performed by ICP analysis. Since the conductive paste also contains organic solvents and / or resins, it is preferable to perform a pretreatment to remove them before performing ICP analysis. For example, ICP analysis can be performed by dissolving the solid components obtained by dispersing and / or dissolving the conductive paste in a large amount of organic solvent such as acetone, and then filtering and / or washing it, using nitric acid and / or hydrochloric acid. Silver and / or gallium do not dissolve in organic solvents; therefore, by performing ICP analysis on the solution of the solid components, the amount of gallium and silver can also be quantitatively analyzed, and the amount of gallium relative to silver can be determined.

[0082] <packing>

[0083] The filler may include silver powder (gallium-free silver powder) in addition to gallium-containing silver powder. Furthermore, it may include metal powders such as aluminum powder. The filler content in the conductive paste is preferably 65 wt% or more and 95 wt% or less.

[0084] Solvent

[0085] The solvent is not particularly limited, and can be, for example, terpineol, butylcarbitol, butylcarbitol acetate, TEXANOL, etc. Two or more solvents can be used simultaneously. The solvent content in the conductive paste is preferably set at 1 wt% or more and 40 wt% or less.

[0086] <Glass Material>

[0087] The glass frit is not particularly limited, but preferably has a softening point below 550°C. The shape of the glass frit is not particularly limited; it can be spherical or irregularly shaped. The content of the glass frit in the conductive paste is preferably 0.1 wt% or more and 10 wt% or less.

[0088] <Optional Contents>

[0089] Conductive pastes may also contain resins, dispersants, surfactants, and viscosity modifiers.

[0090] The aforementioned conductive paste uses gallium-containing silver powder as a filler, as described in this invention. Other than that, it can be manufactured using conventional methods. For example, the paste composition, which includes at least filler, solvent, and glass frit, can be appropriately adjusted. The paste composition is then mixed, pre-mixed using a mixer, and then kneaded using a three-roll mill or similar method to obtain the conductive paste. During dispersion and kneading, ultrasonic dispersion, dispersers, three-roll mills, ball mills, bead mills, biaxial kneaders, and rotary mixers can be used. By printing the obtained conductive paste onto a substrate using, for example, screen printing, offset printing, photolithography, or inkjet printing, a conductive film of the desired shape can be formed.

[0091] Example

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

[0093] (Example 1)

[0094] First, 99.99% pure solid gallium (manufactured by DOWA Electronics) was liquefied by heating in a 60°C bath. Separately, 20g of isopropanol (manufactured by FUJIFILM; Wako Grade 1) solution containing 1wt% stearic acid (used as a surfactant) was prepared. 1g of the liquefied gallium was added to this isopropanol solution to obtain a gallium-containing liquid. This gallium-containing liquid was then ultrasonically dispersed using an ultrasonic homogenizer (manufactured by Nippon Seiki Co., Ltd., model: US-600AT). To prevent the temperature of the gallium-containing liquid from exceeding 70°C due to the exothermic effect of ultrasonic irradiation, ultrasonic dispersion was performed intermittently for a total of 5 minutes while simultaneously cooling in a water bath, with intermediate cooling times, to obtain an emulsion containing dispersed gallium droplets. Ultrasonic dispersion was continued using an ultrasonic homogenizer at approximately 40°C until no gallium accumulated at the bottom of the beaker. Subsequently, the dispersion process was stopped and the mixture was allowed to stand at room temperature, thereby solidifying the dispersed gallium droplets to obtain a gallium powder dispersion. It should be noted that the output power of the ultrasonic dispersion device is affected by the dispersion state and the liquid temperature, and therefore is not constant, varying between 150W and 250W.

[0095] To evaluate the obtained gallium powder dispersion, it was separated in an emulsion state containing gallium droplets, cooled in a refrigerator, and dried using cold air to obtain gallium powder. SEM images of the gallium powder obtained at 5000x magnification are shown below. Figure 1In addition, for the obtained SEM images, the Heywood diameter of more than 100 particles with clear outlines was measured using image processing software (MOUNTECH, Mac-View Ver.4), and the average value was set as the SEM average particle size. The SEM average particle size of gallium powder is 0.9 μm.

[0096] Next, 100g of spherical silver powder (DOWA ELECTRONICS; AG4-8FD, SEM average particle size 1.3μm) was added to 20g of isopropanol (FUJIFILM; Wako Grade 1), and ultrasonic dispersion was performed for 5 minutes using an ultrasonic homogenizer (Nippon Seiki Co., Ltd., device name: US-600AT) at an output power of 100W. It should be noted that the main component of the surface treatment agent used in the spherical silver powder was stearic acid.

[0097] After dispersing the spherical silver powder for 5 minutes as described above, the previously prepared gallium powder dispersion (total amount 21g) was added dropwise using a dropper over 1 minute while continuing ultrasonic dispersion. Ultrasonic dispersion was then performed for another 5 minutes, thereby preparing a dispersion containing gallium-containing silver powder. During the dispersion process, the liquid temperature rose to 60°C. After stopping the dispersion process, the liquid was cooled to room temperature.

[0098] Furthermore, the dispersion containing the gallium-containing silver powder was pressure filtered using a membrane filter with a mesh size of 0.2 μm, and the resulting filter cake was dried at room temperature and in air for 12 hours, thereby obtaining the gallium-containing silver powder described in Example 1. A photograph of the obtained gallium-containing silver powder obtained by SEM observation at 5000x magnification is shown below. Figure 2 .

[0099] (SEM-EDS measurement)

[0100] The image of gallium-containing silver powder obtained by SEM-EDS at 10,000x magnification and elemental mapping by qualitative analysis is shown below. Figure 3 , Figure 4 . Figure 3 It is formed by superimposing the mappings of C (carbon)-KO (oxygen)-K, Ga (gallium)-L, and Ag (silver)-L (K refers to the K line, and L refers to the L line). Figure 4 The image shows the mapping between the SEM image and its elements. Figure 4 IMG1(1st) is the secondary electron image before EDS measurement, and IMG1 is the secondary electron image after EDS measurement. Figure 3 , Figure 4 It can be confirmed that gallium-bearing silver particles are attached to a portion of the silver surface. Figure 4The gray color in Ag(silver)-L is deep, and Ga(gallium)-L is also a dark gray region, and it contains gallium particles that are not attached to the silver particles. Figure 4 (The black areas in Ag(silver)-L and the gray areas in Ga(ga)-L). Considering the manufacturing method according to the present invention, it is not easy for Ga to be mixed into the interior of the silver particles; therefore, these Ga particles can be considered to exist on the surface of the silver particles. The amount of gallium particles remaining that are not attached to the silver particles can also be reduced by adjusting the manufacturing conditions, so that all of them are in an attached state.

[0101] (Particle size distribution determination)

[0102] Furthermore, the particle size distribution of the obtained gallium-containing silver powder was determined using a MICROTRAC particle size distribution measuring device (MICROTRAC BEL, device name: MT-3300EXⅡ) via wet laser diffraction. The sample used for the measurement was prepared by dispersing 0.1 g of gallium-containing silver powder in 40 mL of isopropanol (IPA). It should be noted that an ultrasonic homogenizer (US-150T, Nippon Seiki Co., Ltd.) with an 18 mm chip tip diameter was used for dispersion at a frequency of 19.5 kHz for 2 minutes. The dispersed sample was then fed into the aforementioned device, and the particle size distribution was determined using the accompanying analysis software. The results are shown in Table 3. According to the results in Table 3, there is almost no difference in particle size distribution between the silver powder before and after gallium addition (DOWAELECTRONICS, AG4-8FD).

[0103] [Table 3]

[0104]

[0105] (BET specific surface area determination)

[0106] The BET specific surface area of ​​gallium-containing silver powder was determined using a fully automated specific surface area measuring device (Macsorb HM-model1210, manufactured by MOUNTECH). Specifically, the gallium-containing silver powder was placed in the device, and after passing a He-N2 mixed gas (30% nitrogen) through it for 10 minutes at 60°C and then degassed, the specific surface area was determined using the BET single-point method, confirming it to be 0.49 m². 2 / g. The raw material used was manufactured by DOWA ELECTRONICS; the specific surface area of ​​the spherical silver powder AG4-8FD was 0.43m². 2 / g, therefore it can be confirmed that the BET specific surface area of ​​gallium-coated silver powder with gallium attached is slightly higher.

[0107] (Gallium content determination)

[0108] The quantitative analysis of gallium content in gallium-containing silver powder confirmed that it contains 0.95 wt% gallium, which is 0.96 wt% gallium relative to silver. Based on the theoretical value in manufacturing, gallium accounts for 1 wt% of the silver powder; therefore, almost all of the added gallium adheres to the silver. It should be noted that the quantitative method for gallium was implemented using the following analytical approach.

[0109] 1) Accurately weigh 1g of sample, add 15mL of pure water and 10mL of nitric acid (for precision analysis), and heat at 200℃ for 30 minutes.

[0110] 2) Add 15 mL of pure water and 10 mL of hydrochloric acid (for precise analysis), and heat at 150°C for 30 minutes.

[0111] 3) Adjust the volume of the heated sample to 100 mL, take 5 mL from it, and adjust the volume to 100 mL again to prepare the sample for ICP analysis.

[0112] 4) ICP quantification was performed using the Agilent 5800 ICP-OES manufactured by Agilent Technologies.

[0113] (Organic matter analysis)

[0114] 0.5g of gallium-containing silver powder was dissolved in nitric acid and then extracted with hexane. Qualitative analysis of the extraction solvent layer was performed using GC / MS (7890A / 5975C, Agilent Technologies), confirming that the organic matter contained in the gallium-containing silver powder was stearic acid. After the extraction solvent hexane was evaporated, the carbon content of the remaining components was determined using a carbon-sulfur analyzer (EMIA-810W, Horiba Manufacturing Co., Ltd.). All the organic matter contained in the gallium-containing silver powder was identified as stearic acid, and the stearic acid content was calculated based on the carbon content, yielding a result of 0.18 wt%.

[0115] (Measurement of weight change under intense heat)

[0116] Regarding the thermal weight change, 3g of gallium-containing silver powder was weighed and placed in a magnetic crucible. The temperature was gradually increased from room temperature to a constant temperature. After heating at 800°C for 30 minutes, the sample was cooled and weighed to determine the mass (w) after heating. The thermal weight change (wt%) was calculated using the following formula (1), and it was confirmed to be 0.37wt%.

[0117] Weight change due to intense heat (wt%) = (3-w) / 3×100(1)

[0118] (Preparation of conductive paste)

[0119] Prepare the conductive paste according to the paste composition shown in Table 4 below.

[0120] [Table 4]

[0121] composition Percentage (wt%) Filler (silver powder with gallium attached) 88.73 <![CDATA[Frit (main components: PbO, SiO2)]]> 1.98 Ethyl cellulose 0.20 TEXANOL 1.25 Butyl carbitol acetate 4.96 Tributyl citrate 0.25 1-Octanol 1.19 Oleic acid 0.25 Three vinegar essences 0.25 Methylphenyl polysiloxane 0.50 Hydrogenated castor oil 0.30 Fatty acid amide 0.14 total 100.00

[0122] The mixtures were combined and pre-mixed using a rotary mixer (1000 rpm). After pre-mixing, the mixture was kneaded using a three-roll mill (Exact Corporation) to obtain a conductive paste. The conductive paste contained 0.96 wt% gallium relative to silver.

[0123] (Resistance Measurement)

[0124] Linear shapes were printed using a prepared conductive paste via screen printing. The designed linewidths were 10μm, 12μm, 14μm, 16μm, 18μm, and 20μm, with a length of 150mm. Printing was performed using a MICROTEC printer at a squeegee speed of 350mm / s. A silicon substrate with a thickness of approximately 170μm (for solar cell applications, texture formation / SiNx film deposition already completed) was used for printing. After printing, the substrate was dried in a dryer set to 200°C for 5 minutes. The samples were calcined using a solar cell calcination furnace (NGK), set to peak temperatures of 400°C and 700°C on the wafer surface. The resistance of the printed electrodes after calcination was measured using a digital multimeter (ADC) by connecting the two ends of the printed electrodes to measurement terminals.

[0125] (Example 2)

[0126] The spherical silver powder was replaced with spherical silver powder (manufactured by DOWAELECTRONICS; AG4-54F), and an adhesion process was performed. Otherwise, gallium-containing silver powder was prepared using the same method as in Example 1. It should be noted that the main component of the surface treatment agent for the spherical silver powder was stearic acid.

[0127] The particle size distribution of the gallium-containing silver powder was determined by laser diffraction, following the same procedure as in Example 1. The results are shown in Table 5. As can be seen from the results in Table 5, there is almost no difference in particle size distribution before and after gallium attachment. The specific surface area was also measured using the same method as in Example 1, and was confirmed to be 0.43 m². 2 / g. The specific surface area of ​​the spherical silver powder used (manufactured by DOWA ELECTRONICS; AG4-54F) is 0.37m². 2 / g, therefore it can be confirmed that the BET specific surface area of ​​gallium-coated silver powder with gallium adhering to it is slightly higher. Quantitative analysis of the gallium content in the gallium-containing silver powder confirmed that it contains 0.75 wt% gallium, which is 0.76 wt% gallium relative to silver. Based on the theoretical value in manufacturing, gallium is 1 wt% relative to silver powder; therefore, it is assumed that slight losses occurred due to liquid splashing during ultrasonic dispersion in the process, adhesion to the container walls during mixing and addition, and losses caused by processing. Analysis of organic matter using the same method as in Example 1 showed that the organic matter contained in the gallium-containing silver powder was stearic acid, and the amount of organic matter in the gallium-containing silver powder was 0.14 wt%. Regarding the weight change due to intense heat, the increase due to gallium oxidation was greater than the decrease in impurities such as moisture and / or organic matter contained in the silver powder, reaching -0.12 wt%. Furthermore, using the gallium-containing silver powder of Example 2, a conductive paste was prepared and printed using the same procedure as in Example 1, and the resistance value of the printed electrode after calcination was measured. The conductive paste contains 0.76 wt% gallium relative to silver.

[0128] [Table 5]

[0129]

[0130] (Example for reference)

[0131] Regarding the filler used in the conductive paste, spherical silver powder AG4-8FD (made by DOWA ELECTRONICS) without gallium attachment, and gallium powder (SEM average particle size 0.9 μm) obtained by pressure filtration of the gallium powder dispersion in the above example using a 0.2 μm membrane filter and vacuum drying at room temperature for 30 minutes, were mixed with 0.89 wt% Ga powder and 87.84 wt% silver powder in a ratio of 1 wt% of gallium powder in the filler. Otherwise, the conductive paste was prepared, printed, and calcined as in Example 1, and the resistance value of the subsequently obtained printed electrode was measured. The gallium content relative to silver in the conductive paste was 1.01 wt%.

[0132] (Comparative Example)

[0133] Spherical silver powder AG4-8FD (without gallium attachment) manufactured by DOWA ELECTRONICS and Al powder (average particle size 5 μm) were used as fillers in the conductive paste. The Al powder was mixed in a ratio of 1 wt% to the filler. The conductive paste was prepared, printed, and calcined under the same conditions as in Example 1, and the resistance of the resulting printed electrode was measured. The aluminum content relative to silver in the conductive paste was 1.01 wt%.

[0134] The resistance values ​​for a linewidth of 18 μm in the above embodiments, reference examples, conventional examples, and comparative examples are shown in Table 6 below.

[0135] [Table 6]

[0136]

[0137] In the low-temperature calcination at 400°C, Examples 1, 2, and the Reference Example achieved low resistance compared to the Comparative Example. Furthermore, compared to the Reference Example, it is evident that the gallium-containing silver powder of the present invention exhibits superior low resistance performance.

[0138] During high-temperature calcination at 700°C, Al's melting point is 660°C, which is the temperature at which Al easily reaches the interface with the p-type semiconductor layer of the solar cell. As seen in the comparative example, the resistance increases significantly when Al powder is mixed with silver powder. However, it is observed that such a significant increase in resistance is not observed in Examples 1 and 2 and the reference example, where gallium is added.

[0139] Industrial availability

[0140] According to the present invention, gallium-containing silver powder and a method for manufacturing the same, as well as a conductive paste using the same, are provided. The gallium-containing silver powder is particularly capable of being supplied in a suitable form as gallium as a p-type impurity so that it reaches the interface with the p-type semiconductor layer of a solar cell. Furthermore, it can achieve low resistance under low-temperature calcination and can reduce resistance under high-temperature calcination compared to the case of adding aluminum.

Claims

1. A gallium-containing silver powder, characterized in that, It contains gallium. The median particle size D50 of the gallium-containing silver powder, based on the volume reference, is greater than 0.2 μm and less than 5.0 μm according to laser diffraction.

2. The gallium-containing silver powder according to claim 1, wherein, Gallium is attached to the surface of the silver particles that constitute the gallium-containing silver powder.

3. The gallium-containing silver powder according to claim 2, wherein, The gallium mentioned is metallic gallium.

4. The gallium-containing silver powder according to claim 1, wherein the gallium comprises 0.01 wt% or more and 10 wt% or less relative to silver.

5. The gallium-containing silver powder according to claim 1, wherein, A surfactant adheres to the surface of the gallium-containing silver powder, wherein the amount of organic matter in the gallium-containing silver powder is more than 0.01 wt% and less than 4 wt%.

6. A method for manufacturing gallium-containing silver powder, characterized in that, It has the following processes: A dispersion process, which forms an emulsion in which gallium droplets are dispersed; and The attachment process involves adding (a) the emulsion or (b) a gallium powder dispersion formed by cooling the emulsion to a liquid containing silver powder, thereby causing gallium to adhere to the silver powder.

7. The method for manufacturing gallium-containing silver powder according to claim 6, wherein, In the dispersion process, a surfactant selected from fatty acids, azole compounds, alkenyl succinic acid, aliphatic amines, or their salts or anhydrides is used.

8. The method for manufacturing gallium-containing silver powder according to claim 6, wherein, The liquid temperature in the adhesion process is set to below 70°C.

9. A conductive paste, characterized in that, It contains fillers, solvents, and glass frit. The filler contains gallium-containing silver powder as described in claim 1.

10. The conductive paste according to claim 9, wherein, The filler also contains silver powder.

11. The conductive paste according to claim 9 or 10, wherein, The conductive paste contains gallium in an amount of 0.01 to 4 wt% relative to silver.

Images