Low-silver copper graphite conductive paste, preparation method and application thereof

Abstract

The application relates to the technical field of conductive paste, and discloses a low-silver copper graphite conductive paste as well as a preparation method and application thereof. The paste comprises solid-phase conductive powder and an organic carrier, and the solid-phase conductive powder is composed of copper graphite composite powder, silver-coated copper powder and nano silver powder. The solid-phase conductive powder is prepared by mixing electrolytic copper powder and flake graphite powder, compression molding, pre-burning and sintering to obtain copper graphite carbon brushes, and then ball milling, drying and screening. The organic carrier comprises acrylic resin, a solvent, a dispersing agent, a thixotropic agent and a copper passivation agent. The paste is prepared by mixing, grinding and vacuum deaerating the solid-phase conductive powder and the organic carrier. The paste can form a conductive drainage layer at 230-270 DEG C for 30-90 s, the curing temperature is fully compatible with the passivation layer process of a TOPCon solar cell, the silver consumption is greatly reduced compared with traditional silver paste, and the paste can be used for preparing a conductive drainage layer on the back surface of a TOPCon solar cell.

Description

Technical Field

[0001] This invention relates to the field of conductive paste technology, specifically to a low-silver copper graphite conductive paste, its preparation method, and its application. Background Technology

[0002] TopCon (TOPCon) battery back electrodes typically employ a two-layer structure: a silver seed layer and a conductive guide layer. The silver seed layer achieves ohmic contact by etching a silicon nitride layer with a silver paste containing glass powder at high temperatures. The core function of the upper conductive guide layer is to reduce electrode series resistance and ensure battery conductivity. Currently, the industry mainly uses three types of upper conductive pastes: pure silver paste, silver-coated copper paste, and copper-based paste. However, all three suffer from significant drawbacks in balancing performance and cost, as detailed below: 1. Pure silver paste: It has excellent conductivity, but the silver content is extremely high (usually ≥90wt%), making the material cost expensive. This seriously restricts the cost reduction process of TOPCon battery industrialization and cannot meet the cost requirements of large-scale mass production. At the same time, the global silver resource reserves are limited, the demand for silver resources in the photovoltaic industry continues to grow, and the impact of silver price fluctuations on industry profits is becoming increasingly significant. 2. Conventional silver-coated copper paste: Although it replaces some silver powder to reduce costs, the silver content is still maintained at more than 15wt%, so there is limited room for cost reduction. In addition, the copper core is easy to oxidize and migrate, which can lead to battery performance degradation with long-term use. 3. Pure copper paste: It has a lower cost, but copper is easily oxidized to form copper oxide at high temperatures, and copper ions are prone to migrate to the semiconductor layer, causing battery leakage and passivation layer failure, which ultimately leads to a significant decrease in battery efficiency and cannot meet long-term reliability requirements. 4. Mechanically mixed copper-graphite powder slurry: Copper powder and graphite powder are only physically mixed, with loose interfacial bonding. After the slurry is cured, the porosity is high, the continuity of the conductive network is poor, and the surface of copper powder has no effective protection and weak oxidation resistance, making it difficult to adapt to the industrial application standards of TOPCon batteries.

[0003] Based on the aforementioned industry pain points, this invention proposes a low-silver conductive paste using sintered copper-graphite carbon brushes as raw materials. It fully preserves the original copper-graphite dense dual-continuous microstructure of the carbon brush and, combined with the low-silver component design, achieves comprehensive performance with high conductivity, strong oxidation resistance, low silver consumption, and no risk of copper migration. At the same time, it is compatible with the existing mass production process of TOPCon batteries, helping to reduce battery costs and increase efficiency. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a low-silver copper-graphite conductive paste, its preparation method, and its application. Through the synergistic design of a three-level conductive filler consisting of copper-graphite composite powder, silver-coated copper powder, and nano-silver powder, combined with a low-temperature crosslinking system of acrylic resin, the conductive paste can be rapidly cured at a low temperature of 230~270℃, making it suitable for the fabrication process of TOPCon solar cells.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a low-silver copper-graphite conductive paste, comprising the following steps: S1, mix electrolytic copper powder and flake graphite powder to obtain mixed powder, add a molding agent, press into shape, pre-fire in argon gas, then heat up to sinter, and cool to room temperature to obtain copper graphite carbon brush; S2, copper-graphite carbon brushes are dispersed in a dispersion medium, ball-milled, vacuum filtered, dried, and sieved to obtain copper-graphite composite powder. S3, mix copper-graphite composite powder, silver-coated copper powder and nano silver powder to obtain solid-phase conductive powder; S4, mix acrylic resin and solvent to react, after the reaction is complete, add dispersant, thixotropic agent and copper passivator, stir, cool to room temperature, and obtain organic carrier; S5. Solid conductive powder is added to an organic carrier, stirred, ground, and vacuum degassed to obtain a low-silver copper graphite conductive slurry.

[0006] Preferably, in step S1, the purity of the electrolytic copper powder is ≥99.5%.

[0007] Preferably, in step S1, the particle size of the flake graphite powder is 1~5μm.

[0008] Preferably, in step S1, the mass ratio of electrolytic copper powder to flake graphite powder is (25~35):(65~75).

[0009] Preferably, in step S1, the mass ratio of the mixed powder to the molding agent is 100:(1~2).

[0010] Preferably, in step S1, the molding agent is stearic acid.

[0011] Preferably, in step S1, the pressing pressure is 150~200MPa.

[0012] Preferably, in step S1, the pre-firing temperature is 300~400℃ and the pre-firing time is 30~60min.

[0013] Preferably, in step S1, the sintering temperature is 800~900℃ and the sintering time is 2~4h.

[0014] Preferably, in step S1, the density of the copper graphite carbon brush is ≥90%.

[0015] In S1, electrolytic copper powder and flake graphite powder are compounded at a mass ratio of (25~35):(65~75), which takes into account the conductivity of copper and the ductility of graphite, so that the copper-graphite composite powder provides a continuous low-impedance transmission path in the conductive network. After high-pressure compaction and high-temperature protective atmosphere sintering, the copper-graphite carbon brush can form a dense microstructure with copper-graphite double continuous interlocking. This structure has outstanding characteristics such as high density, excellent conductivity, excellent oxidation resistance, and low risk of copper migration.

[0016] Preferably, in step S2, the dispersion medium is anhydrous ethanol.

[0017] Preferably, in step S2, the mass-to-volume ratio of the copper graphite carbon brush to the dispersion medium is 1 g: (1~2) mL.

[0018] Preferably, in step S2, the ball milling speed is 200~300 r / min, and the ball milling time is 4~8 h.

[0019] Preferably, in step S2, the drying temperature is 80~100℃ and the drying time is 2~4h.

[0020] Preferably, in step S2, the sieve mesh size is 15 μm.

[0021] Preferably, in step S2, the D50 of the copper-graphite composite powder is 6~10μm.

[0022] Preferably, in step S3, the mass ratio of the copper-graphite composite powder, the silver-coated copper powder, and the nano silver powder is (85~92):(3~10):(1~5).

[0023] Preferably, in step S3, the silver-coated copper powder has a core-shell structure in which a silver layer encapsulates a copper core, the mass ratio of the silver layer to the copper core is (5~20):(80~95), and the particle size of the copper core is 1~3μm.

[0024] Preferably, in step S3, the particle size of the silver nanoparticles is 30~500nm.

[0025] Preferably, in step S3, the mixing speed is 800~1200 r / min, and the mixing time is 30~60 min.

[0026] The conductive powder uses copper-graphite composite powder as the main conductive framework. The copper-graphite composite powder has a dense microstructure with copper-graphite dual continuous interlocking, providing a continuous low-impedance current transmission path and undertaking the conductive function of most micron-sized silver powder in traditional slurries. Silver-coated copper powder serves as the conductive connecting phase. The silver layer is connected through solid-phase diffusion at a low temperature of 230~270℃, building highly conductive bridges between particles. Only a small amount of silver is needed to achieve effective electrical connection between particles. At the same time, the silver layer coating blocks the oxidation of copper cores, improving the utilization efficiency of silver. Nano-silver powder serves as the interstitial modification phase. Due to its small size effect, it significantly reduces the sintering activation energy and preferentially forms silver-silver sintering necks during the curing process. Only a trace amount of nano-silver is needed to fill the micro-gaps between conductive particles, greatly reducing the contact resistance.

[0027] Preferably, in step S4, the mass ratio of the acrylic resin, solvent, dispersant, thixotropic agent and copper passivator is (10~20):(70~85):(1~3):(1~3):(0.5~2).

[0028] Preferably, in step S4, the acrylic resin is obtained by polymerization of any one or more of hard monomers, soft monomers, and functional monomers.

[0029] Preferably, the hard monomer comprises methyl methacrylate and acrylonitrile.

[0030] Preferably, the soft monomer comprises butyl acrylate and ethyl acrylate.

[0031] Preferably, the functional monomers include acrylic acid, methacrylic acid, γ-methacryloyloxypropyltrimethoxysilane (KH-570), 3-acrylamidopropyltrimethoxysilane, and glycidyl methacrylate.

[0032] Acrylic resin rapidly crosslinks at low temperatures to form a three-dimensional network, which can firmly fix the conductive paste to the back substrate of the TOPCon battery. Adding carboxyl-containing monomers such as acrylic acid during the polymerization of the acrylic resin introduces a small amount of carboxyl groups into the resin backbone, improving dispersion stability.

[0033] Functional monomers such as KH-570 contain polymerizable carbon-carbon double bonds and hydrolyzable alkoxysilyl groups. The carbon-carbon double bonds can participate in free radical copolymerization reactions, allowing silane groups to be grafted onto the acrylic resin molecular chain as side groups. Under hydrolytic conditions, the alkoxysilyl groups are converted to silanols, which can react with the silanols on the surface of the silicon carbide layer to form a Si-O-Si covalently bridging structure. This chemical bonding enhances the interfacial bonding between the resin and the silicon carbide layer from purely physical adsorption to a covalent bond level. Furthermore, silanols can also undergo similar reactions with the silanols on the surface of lead borosilicate glass powder, simultaneously strengthening the interfacial bonding between the resin and the inorganic glass phase.

[0034] Adding glycidyl methacrylate during polymerization introduces epoxy groups into the resin side chains. During the slurry curing stage, the epoxy rings undergo a ring-opening reaction, chemically reacting with active hydrogen (silanol and boronol) on the glass powder surface and potentially with hydroxyl groups on the surface of metal oxides, forming stable covalent bonds. This reaction time window precisely corresponds to the critical stage of the slurry's transition from the printed state to the cured state, establishing a chemical anchor at the resin-inorganic phase interface before resin thermal decomposition.

[0035] Preferably, in step S4, the solvent is a mixture of terpineol and butyl carbitol acetate, and the mass ratio of terpineol to butyl carbitol acetate is (3~5):1.

[0036] Preferably, in step S4, the dispersant is a polycarboxylate dispersant.

[0037] Preferably, the polycarboxylate dispersant is tego710 or BYK-110.

[0038] Preferably, in step S4, the thixotropic agent is fumed silica.

[0039] Preferably, the average particle size of the fumed silica is 8~40 nm.

[0040] Preferably, in step S4, the copper passivating agent is benzotriazole and / or toluenetriazole.

[0041] Preferably, in step S4, the temperature of the mixing reaction is 60~80℃, the rotation speed of the mixing reaction is 300~500 r / min, and the mixing reaction time is 60~90 min.

[0042] Preferably, in step S4, the stirring temperature is 60~80℃, the stirring speed is 300~500r / min, and the stirring time is 30~40min.

[0043] Preferably, in step S5, the addition rate of the solid conductive powder is 2~4 g / min.

[0044] Preferably, in step S5, the mass ratio of the solid conductive powder to the organic carrier is (75~95):(5~25).

[0045] Preferably, in step S5, the stirring speed is 200~300 r / min, and the stirring time is 20~30 min.

[0046] Preferably, in step S5, the grinding process involves transferring the stirred material into a three-roll mill for grinding.

[0047] Preferably, the grinding gap of the three-roll mill is 5~7μm.

[0048] Preferably, in step S5, the fineness of the primary slurry is ≤7μm, and the viscosity of the primary slurry is 100~300Pa·s.

[0049] Preferably, in step S5, the vacuum degree of vacuum degassing is -0.08 to -0.1 MPa, and the vacuum degassing time is 15 to 20 minutes.

[0050] In a second aspect, the present invention provides a low-silver copper-graphite conductive paste prepared by the preparation method described in the first aspect.

[0051] The conductive paste prepared by this invention does not contain glass powder, thus avoiding the erosion of the silicon nitride passivation layer by glass powder. Combined with the double protection of copper passivator, it enables the battery power output to have excellent long-term reliability.

[0052] Thirdly, the present invention provides an application of the low-silver copper-graphite conductive paste described in the second aspect in the back of a TOPCon solar cell, wherein the low-silver copper-graphite conductive paste forms a conductive guide layer on the back of the TOPCon solar cell after curing.

[0053] A silver seed layer and a conductive current-guiding layer are sequentially stacked on the back of the TOPCon solar cell to form a double-electrode structure. The silver seed layer is prepared by printing silver paste containing glass powder onto the back of the TOPCon solar cell and then sintering it at a high temperature of 680–720°C. The silver seed layer is formed by etching the back silicon nitride layer with melted glass powder to create an ohmic contact with the underlying polycrystalline silicon. The silver seed layer can be a continuous grid line, a discontinuous grid line, or a dotted distribution. Part of the conductive current-guiding layer is in electrical contact with the silver seed layer, while another part directly covers the silicon nitride layer, without contacting the semiconductor layer or forming an ohmic contact.

[0054] The silicon nitride layer of the double-layer electrode structure serves as a copper diffusion barrier layer; the graphite phase in the copper-graphite composite powder encapsulates the copper phase in situ, blocking the continuous diffusion channels of copper; low-temperature curing inhibits the diffusion of copper atoms; trace amounts of silver and copper passivating agents passivate and pin the copper phase on the surface, synergistically inhibiting copper migration.

[0055] Preferably, the curing temperature is 230~270℃ and the curing time is 30~90s.

[0056] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: This invention provides a low-silver copper-graphite conductive paste, its preparation method, and its application. The resulting conductive paste has a silver content ≤10wt%, significantly reducing costs compared to traditional silver-containing pastes. This paste can be cured at 230~270℃, making it suitable for the fabrication process of TOPCon solar cells. This invention partially replaces the silver-containing components in traditional pastes with copper-based and carbon-based materials, resulting in an overall silver content of ≤10wt%. It also constructs a conductive network composed of copper-graphite composite powder, silver-coated copper powder, and nano-silver powder to compensate for conductivity loss due to the low silver content, allowing the conductive paste to maintain a low sheet resistance even with a sharp drop in silver content. This invention employs a low-temperature crosslinking system with acrylic resin, matching the low-temperature sintering range of the conductive network, enabling the conductive paste to cure at 230~270℃. This effectively avoids passivation damage caused by high-temperature processing, helping to maintain the surface passivation quality and conversion efficiency of TOPCon cells. Attached Figure Description

[0057] Figure 1 The image shows a SEM image of the mixture of electrolytic copper powder and flake graphite in S1, magnified 2000 times. Figure 2 The image shows a SEM image of the broken copper-graphite carbon brush in S1, magnified 2000 times. Figure 3 The image shows a SEM image of the broken copper-graphite carbon brush in S1, magnified 5000 times. Figure 4 This is a photograph of the low-silver copper-graphite conductive paste prepared in Example 1 of the present invention. Figure 5 This is a physical image of the conductive paste prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0058] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations thereof.

[0059] Example 1 This embodiment provides a method for preparing a low-silver copper-graphite conductive paste, comprising the following steps: S1: Electrolytic copper powder with a purity ≥ 99.5% was mixed with flake graphite powder with a particle size of 3μm at a mass ratio of 30:70 to obtain a mixed powder. The SEM image of the mixed powder is shown below. Figure 1As shown; stearic acid was added to the mixed powder, the mass ratio of the mixed powder to stearic acid being 100:1.5. The mixture was pressed into shape at 170 MPa, then placed in argon gas and pre-calcined at 350°C for 45 min, followed by sintering at 850°C for 3 h, and cooled to room temperature to obtain a copper-graphite carbon brush; the density of the copper-graphite carbon brush was ≥90%; the SEM image of the broken copper-graphite carbon brush is shown below. Figure 2 and Figure 3 As shown; S2: Add copper-graphite carbon brushes to a planetary ball mill, add anhydrous ethanol, the mass-to-volume ratio of copper-graphite carbon brushes to anhydrous ethanol is 1g:1.5mL; ball mill at 250r / min for 6h; after vacuum filtration, dry at 90℃ for 3h, the dried product is sieved through a sieve with a pore size of 15μm to obtain copper-graphite composite powder; the D50 of the copper-graphite composite powder is 8μm; S3: Copper-graphite composite powder, silver-coated copper powder, and nano-silver powder with a particle size of 200 nm are added to a high-speed mixer at a mass ratio of 89:10:1 and mixed at a speed of 1000 r / min for 45 min to obtain a solid-phase conductive powder; the silver-coated copper powder has a core-shell structure in which a silver layer encapsulates a copper core, the mass ratio of the silver layer to the copper core is 20:80, and the particle size of the copper core is 2 μm; S4: Acrylic resin and solvent are added to a reaction vessel, heated to 70°C, and stirred at 400 r / min for 75 min. The solvent is a mixture of terpineol and butyl carbitol acetate, wherein the mass ratio of terpineol to butyl carbitol acetate is 4:1. After the reaction is complete, a dispersant, a thixotropic agent, and a copper passivator are added, and the mixture is stirred at 400 r / min for 35 min at 70°C. The mixture is then cooled to room temperature to obtain an organic carrier. The mass ratio of acrylic resin, solvent, dispersant, thixotropic agent, and copper passivator is 16:79.5:2:2:0.5. The dispersant is BYK-110, the thixotropic agent is fumed silica with an average particle size of 10 nm, and the copper passivator is benzotriazole. The acrylic resin is formed by suspension copolymerization of butyl acrylate, methyl methacrylate, and acrylic acid in a mass ratio of 15:12:1.5. The specific polymerization process is as follows: Deionized water and polyvinyl alcohol are added to a reaction vessel at a mass ratio of 100:1. The deionized water and polyvinyl alcohol in the reaction vessel are stirred at 300 rpm. The temperature is raised to 80°C and stirred continuously for 5 min. Then, the temperature is lowered to 40°C, butyl acrylate, methyl methacrylate, and acrylic acid are added, and the temperature is raised to 80°C. Then, a 15 wt% azobisisobutyronitrile (AIBN) solution in acetone is added dropwise at a rate of 5 mL / min. After the addition is complete, the reaction is maintained at this temperature for 4 h. After the reaction is completed, the temperature is lowered to 40°C, filtered, and the filter cake is washed with deionized water. The acrylic resin is obtained by vacuum drying at 50°C for 12 h. The amount of AIBN used is 0.4% of the total mass of butyl acrylate, methyl methacrylate, and acrylic acid. The mass ratio of deionized water to the total mass of butyl acrylate, methyl methacrylate, and acrylic acid is 3:1. S5: Add solid conductive powder to the organic carrier at a rate of 3 g / min, with a mass ratio of solid conductive powder to organic carrier of 85:15; stir at 250 r / min for 25 min to obtain a primary mixture. Transfer the primary mixture to a three-roll mill, adjust the grinding gap to 6 μm, and grind twice to obtain a primary slurry with a fineness ≤7 μm and a viscosity of 200 Pa·s. After vacuum degassing at -0.09 MPa for 17 min, obtain the following... Figure 4 The low-silver copper-graphite conductive paste shown.

[0060] Example 2 This embodiment provides a method for preparing a low-silver copper-graphite conductive paste, comprising the following steps: S1: Electrolytic copper powder with a purity ≥99.5% is mixed with flake graphite powder with a particle size of 1μm at a mass ratio of 35:65 to obtain a mixed powder. Stearic acid is added to the mixed powder, and the mass ratio of the mixed powder to stearic acid is 100:1. The mixture is pressed into shape at 150MPa, then placed in argon gas and pre-fired at 400℃ for 30min. Subsequently, the temperature is raised to 900℃ and sintered for 2h. After cooling to room temperature, a copper graphite carbon brush is obtained; the density of the copper graphite carbon brush is ≥90%. S2: Add copper-graphite carbon brushes to a planetary ball mill, add anhydrous ethanol, the mass-to-volume ratio of copper-graphite carbon brushes to anhydrous ethanol is 1g:1mL; ball mill at 300r / min for 4h; after vacuum filtration, dry at 100℃ for 2h, the dried product is sieved through a sieve with a pore size of 15μm to obtain copper-graphite composite powder; the D50 of the copper-graphite composite powder is 6μm; S3: Copper-graphite composite powder, silver-coated copper powder, and nano-silver powder with a particle size of 30nm are added to a high-speed mixer at a mass ratio of 92:3:5 and mixed at a speed of 1200r / min for 30min to obtain solid-phase conductive powder; the silver-coated copper powder has a core-shell structure in which a silver layer encapsulates a copper core, the mass ratio of the silver layer to the copper core is 5:95, and the particle size of the copper core is 1μm; S4: Acrylic resin and solvent are added to a reaction vessel, heated to 60°C, and stirred at 300 r / min for 90 min. The solvent is a mixture of terpineol and butyl carbitol acetate, wherein the mass ratio of terpineol to butyl carbitol acetate is 3:1. After the reaction is complete, a dispersant, a thixotropic agent, and a copper passivator are added, and the mixture is stirred at 300 r / min for 40 min at 60°C. The mixture is then cooled to room temperature to obtain an organic carrier. The mass ratio of acrylic resin, solvent, dispersant, thixotropic agent, and copper passivator is 20:70:3:3:2. The dispersant is Tego710, the thixotropic agent is fumed silica with an average particle size of 40 nm, and the copper passivator is a mixture of benzotriazole and toluenetriazole, wherein the mass ratio of benzotriazole to toluenetriazole is 1:1. The acrylic resin is the same as that used in Example 1. S5: Add solid conductive powder to the organic carrier at a rate of 4 g / min, with a mass ratio of solid conductive powder to organic carrier of 95:5; stir at 300 r / min for 20 min to obtain a primary mixture; transfer the primary mixture to a three-roll mill, adjust the grinding gap to 5 μm, and grind twice to obtain a primary slurry with a fineness ≤7 μm and a viscosity of 300 Pa·s; after vacuum degassing at -0.08 MPa for 20 min, obtain a low-silver copper graphite conductive slurry.

[0061] Example 3 This embodiment provides a method for preparing a low-silver copper-graphite conductive paste, comprising the following steps: S1: Electrolytic copper powder with a purity ≥99.5% is mixed with flake graphite powder with a particle size of 5μm at a mass ratio of 25:75 to obtain a mixed powder. Stearic acid is added to the mixed powder, and the mass ratio of the mixed powder to stearic acid is 100:2. The mixture is pressed into shape at 200MPa, then placed in argon gas and pre-fired at 300℃ for 60min. Subsequently, the temperature is raised to 800℃ and sintered for 4h. After cooling to room temperature, a copper graphite carbon brush is obtained; the density of the copper graphite carbon brush is ≥90%. S2: Add copper-graphite carbon brushes to a planetary ball mill, add anhydrous ethanol, the mass-to-volume ratio of copper-graphite carbon brushes to anhydrous ethanol is 1g:2mL; ball mill at 200r / min for 8h; after vacuum filtration, dry at 80℃ for 4h, the dried product is sieved through a sieve with a pore size of 15μm to obtain copper-graphite composite powder; the D50 of the copper-graphite composite powder is 10μm; S3: Copper-graphite composite powder, silver-coated copper powder, and nano-silver powder with a particle size of 500nm are added to a high-speed mixer at a mass ratio of 85:10:5 and mixed at a speed of 800r / min for 60min to obtain solid-phase conductive powder; the silver-coated copper powder has a core-shell structure in which a silver layer encapsulates a copper core, the mass ratio of the silver layer to the copper core is 8:92, and the particle size of the copper core is 3μm; S4: Acrylic resin and solvent are added to a reaction vessel, heated to 80°C, and stirred at 500 r / min for 60 min. The solvent is a mixture of terpineol and butyl carbitol acetate, wherein the mass ratio of terpineol to butyl carbitol acetate is 5:1. After the reaction is complete, a dispersant, a thixotropic agent, and a copper passivator are added, and the mixture is stirred at 500 r / min for 30 min at 80°C. The mixture is then cooled to room temperature to obtain an organic carrier. The mass ratio of acrylic resin, solvent, dispersant, thixotropic agent, and copper passivator is 10:85:1:1:1. The dispersant is Tego710, the thixotropic agent is fumed silica with an average particle size of 8 nm, and the copper passivator is toluenetriazole. The acrylic resin is polymerized from butyl acrylate, methyl methacrylate, γ-methacryloyloxypropyltrimethoxysilane, and acrylic acid in a mass ratio of 12:10:2.5:1.0. The specific polymerization process is as follows: Ethylene glycol monobutyl ether and butyl acetate are added to a reaction vessel, and nitrogen gas is purged for 30 minutes to remove oxygen; the temperature is raised to 78°C; butyl acrylate, methyl methacrylate, γ-methacryloyloxypropyltrimethoxysilane, and acrylic acid are mixed evenly to obtain a monomer premix; 15% of the total mass of the monomer premix and 50% of the total mass of the initiator are added to the reaction vessel, and a pre-reaction is initiated for 30 minutes; the remaining monomer premix and the remaining initiator are added dropwise to the reaction vessel at a uniform rate over 2 hours, maintaining the temperature at 78°C during the dropwise addition; after the dropwise addition is completed, the reaction is kept at this temperature for 3 hours, and then the temperature is raised to 85°C and kept at this temperature for 1 hour; after the reaction is completed, the temperature is lowered to 45°C, and the reaction solution is slowly poured into deionized water stirred at 800 rpm, with a volume ratio of reaction solution to deionized water of 1:5; the solid is collected by filtration, washed with deionized water, and then vacuum dried at 50°C for 12 hours to obtain acrylic resin; S5: Add solid conductive powder to the organic carrier at a rate of 2 g / min, with a mass ratio of solid conductive powder to organic carrier of 75:25; stir at 200 r / min for 30 min to obtain a primary mixture; transfer the primary mixture to a three-roll mill, adjust the grinding gap to 7 μm, and grind 3 times to obtain a primary slurry with a fineness ≤7 μm and a viscosity of 100 Pa·s; after vacuum degassing at -0.1 MPa for 15 min, obtain a low-silver copper graphite conductive slurry.

[0062] Comparative Example 1 A method for preparing a low-silver copper-graphite conductive paste, differing from Example 1 in that the low-silver copper-graphite conductive paste is replaced with a pure silver-coated copper paste, wherein the silver content in the pure silver-coated copper paste is 20 wt%. Other process parameters and operating conditions are identical to those in Example 1. A photograph of the resulting paste is shown below. Figure 5 As shown.

[0063] Comparative Example 2 A method for preparing a low-silver copper-graphite conductive paste differs from Example 1 in that the electrolytic copper powder and flake graphite powder in S1 are mechanically mixed and used directly without pressing, pre-firing, and sintering. Other process parameters and operating conditions are exactly the same as in Example 1.

[0064] Comparative Example 3 A method for preparing a low-silver copper-graphite conductive paste, which differs from Example 1 in that the low-silver copper-graphite conductive paste is replaced with pure copper paste, while other process parameters and operating conditions are exactly the same as in Example 1.

[0065] The performance of the low-silver copper-graphite conductive pastes prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the specific process is as follows: First, a silver paste containing glass powder was coated onto the back of a TOPCon solar cell, and then sintered at 700°C to obtain a TOPCon solar cell containing a silver seed layer. The silver seed layer had a continuous grid line distribution. Next, the low-silver copper-graphite conductive paste prepared in Examples 1-3 and Comparative Examples 1-3 was coated onto the silver seed layer and cured at 250°C for 60 seconds to form a conductive guide layer. At this point, the silver seed layer and the conductive guide layer were sequentially stacked on the back of the TOPCon solar cell, forming a double-layer electrode structure. The sheet resistance of the conductive guide layer was tested using the four-probe method.

[0066] Table 1. Performance test results of the slurries prepared in Examples 1-3 and Comparative Examples 1-3 after curing.

[0067] As shown in Table 1, the sheet resistance of Examples 1 to 3 is less than 14.5 mΩ / sq, which is lower than that of Comparative Example 2.

[0068] Comparative Example 1 used pure silver-coated copper paste, and its sheet resistance after curing was only 4.1 mΩ / sq, far lower than that of Examples 1-3 and Comparative Examples 2-3. However, the silver content in the paste of Comparative Example 1 was as high as 20 wt%, while the silver content in the pastes prepared in Examples 1-3 was all less than 10 wt%, and the cost was much lower than that of Comparative Example 1. The copper graphite carbon brush in the paste of Comparative Example 2 was not subjected to pressing, pre-firing, and sintering treatment, so its oxidation resistance was poor, and its resistance increased after high-temperature aging, with a sheet resistance as high as 32.5 mΩ / sq, far higher than that of Examples 1-3, Comparative Example 1, and Comparative Example 3. Comparative Example 3 used pure copper paste instead of low-silver copper graphite conductive paste. After curing, copper ion migration was obvious, and battery leakage increased. Therefore, its sheet resistance was higher than that of Examples 2-3 and Comparative Example 1, close to that of Example 1, but far lower than that of Comparative Example 2.

[0069] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a low-silver copper graphite conductive paste, characterized in that, Includes the following steps: S1, mix electrolytic copper powder and flake graphite powder to obtain mixed powder, add a molding agent, press into shape, pre-fire in argon gas, then heat up to sinter, and cool to room temperature to obtain copper graphite carbon brush; S2, copper-graphite carbon brushes are dispersed in a dispersion medium, ball-milled, vacuum filtered, dried, and sieved to obtain copper-graphite composite powder. S3, mix copper-graphite composite powder, silver-coated copper powder and nano silver powder to obtain solid-phase conductive powder; S4, mix acrylic resin and solvent to react, after the reaction is complete, add dispersant, thixotropic agent and copper passivator, stir, cool to room temperature, and obtain organic carrier; S5. Solid conductive powder is added to an organic carrier, stirred, ground, and vacuum degassed to obtain a low-silver copper graphite conductive slurry.

2. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S1, the mass ratio of electrolytic copper powder to flake graphite powder is (25~35):(65~75); the mass ratio of mixed powder to molding agent is 100:(1~2).

3. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S1, the pressing pressure is 150~200MPa; the pre-firing temperature is 300~400℃ and the pre-firing time is 30~60min; the sintering temperature is 800~900℃ and the sintering time is 2~4h.

4. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S2, the mass-to-volume ratio of the copper-graphite carbon brush to the dispersion medium is 1 g: (1~2) mL; the ball milling speed is 200~300 r / min, and the ball milling time is 4~8 h.

5. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S3, the mass ratio of the copper-graphite composite powder, the silver-coated copper powder, and the nano-silver powder is (85~92):(3~10):(1~5); the silver-coated copper powder has a core-shell structure in which a silver layer encapsulates a copper core, and the mass ratio of the silver layer to the copper core is (5~20):(80~95), the particle size of the copper core is 1~3μm; and the particle size of the nano-silver powder is 30~500nm.

6. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S3, the mixing speed is 800~1200 r / min, and the mixing time is 30~60 min.

7. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S4, the mass ratio of the acrylic resin, solvent, dispersant, thixotropic agent, and copper passivator is (10~20):(70~85):(1~3):(1~3):(0.5~2); the solvent is a mixture of terpineol and butyl carbitol acetate, and the mass ratio of terpineol and butyl carbitol acetate is (3~5):

1.

8. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S4, the mixing reaction temperature is 60~80℃, the mixing reaction speed is 300~500r / min, and the mixing reaction time is 60~90min; the stirring temperature is 60~80℃, the stirring speed is 300~500r / min, and the stirring time is 30~40min.

9. The method for preparing a low-silver copper graphite conductive paste according to claim 1, characterized in that, In step S5, the mass ratio of the solid conductive powder to the organic carrier is (75~95):(5~25); the stirring speed is 200~300 r / min, and the stirring time is 20~30 min.

10. A low-silver copper graphite conductive paste, characterized in that, It is prepared according to any one of claims 1 to 9.

11. The application of the low-silver copper-graphite conductive paste as described in claim 10 on the back side of a TOPCon solar cell, characterized in that, The low-silver copper-graphite conductive paste is cured at 230~270℃ for 30~90s to form a conductive guide layer on the back of the TOPCon solar cell.

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