Low-temperature sintering type copper paste for photovoltaic and preparation method and application thereof

Abstract

The application provides a low-temperature sintering copper paste for photovoltaic cells and a preparation method and application thereof, and belongs to the technical field of photovoltaic electrode materials. The copper paste is prepared from 85-90% of composite copper powder, 1-5% of nitrogen-doped low-melting-point phosphate glass powder, 5-12% of an organic carrier and 0.1-2% of an auxiliary functional additive by mass percentage. The sintering temperature of the copper paste is 350-450 DEG C, which can avoid high-temperature damage to photovoltaic silicon wafers and passivation layers; the strong reducing property of N3- in the glass network is used to reduce copper ions in the system into elemental copper in the sintering process, inhibit the migration of copper ions, and effectively reduce the electrode resistivity. The copper paste has excellent printing performance, film adhesion and moisture and heat resistance, the preparation process is simple, and the copper paste can be widely applied to the preparation of electrodes of crystalline silicon photovoltaic cells and thin-film photovoltaic cells. ‑ ​

Description

Technical Field

[0001] This invention relates to the field of photovoltaic electrode materials technology, and in particular to a low-temperature sintering type of copper paste for photovoltaics, its preparation method and application. Background Technology

[0002] Photovoltaic cells are core components in the new energy field, and conductive electrodes play a crucial role in collecting and discharging current. Currently, the application of photovoltaic conductive copper paste mainly follows two technical routes: One method involves low-temperature epoxy resin-cured copper paste, with a curing temperature of approximately 200℃, relying on the resin to bond the copper powder. Because the copper particles are only in physical contact, the interface contact is poor, resulting in high electrode resistance and weak conductivity. Simultaneously, the resin matrix has poor heat and moisture resistance, making it prone to aging and failure with long-term use.

[0003] Secondly, high-temperature sintering of copper paste, using ordinary silicate or pure phosphate glass powder as the binder phase, with sintering temperatures reaching 500-600℃. High-temperature conditions easily lead to severe oxidation of the copper electrode and cause copper ions to migrate into the silicon substrate under the influence of an electric field, resulting in battery performance degradation or even short-circuit failure.

[0004] In recent years, some studies have proposed using phosphate nitride (phosphorus oxynitride) glass as the binder phase for copper paste. The N in this type of glass network... 3- It has reduction properties and can reduce high-valence copper ions to low-valence ions and elemental copper in the medium and low temperature range of 300~450℃. At the same time, the phosphate matrix has a low melting point and is suitable for low-temperature sintering process. However, the existing research still has the following shortcomings: (1) The preparation process of phosphate nitride glass is complicated and mostly stays in the laboratory stage, making large-scale production difficult; (2) Using this type of glass alone has problems such as high brittleness, insufficient electrode adhesion, and poor resistance to humid heat; (3) The rheological properties and printing adaptability of the paste need to be improved; (4) There is a lack of complete formulas and process schemes compatible with photovoltaic production lines. Summary of the Invention

[0005] The purpose of this invention is to provide a low-temperature sintering type of copper paste for photovoltaic applications, its preparation method, and its application, in order to solve the above-mentioned technical problems.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a low-temperature sintering type of copper paste for photovoltaic applications, which is prepared from the following raw materials by mass percentage: Composite copper powder 85%~90%; Nitrogen-doped low-melting-point phosphate glass powder, 1%–5%; Organic carriers: 5%–12%.

[0007] Furthermore, by weight percentage, the composite copper powder comprises: Spherical copper powder with a particle size of 200~300nm accounts for 50%~70%; 20%–30% spherical copper powder with a particle size of 1–3 μm; 5% to 10% of flake-shaped copper powder with a thickness of 0.1 to 0.3 μm and a diameter of 2 to 5 μm.

[0008] Furthermore, the nitrogen-doped low-melting-point phosphate glass powder, by mole percentage, comprises the following elements: P 10%~30%; Si 5%~20%; Li 5%~15%; Zn 5%~10%; Mn 5%~10%; Al 5%~10%; N 1%~5%; And one or more of Na, K, Mg, Ca, Ba, Fe, Cu, B, Pb, and Bi, in total of 0% to 5%.

[0009] Furthermore, the nitrogen-doped low-melting-point phosphate glass powder has a glass softening temperature of 350~460℃ and a D50 particle size of 0.5~2μm.

[0010] Furthermore, the nitrogen-doped low-melting-point phosphate glass powder is prepared by the following method: Oxide raw materials containing P, Si, Li, Zn, Mn and Al are mixed according to the target composition. Ammonium nitrate, accounting for 5% to 15% of the total mass of the mixed raw materials, is added as a nitrogen source. The mixture is melted at 1000 to 1200℃ for 0.5 to 2 hours under nitrogen protection. The resulting glass melt is then subjected to rapid cooling by rolling and water quenching, followed by ball milling to obtain nitrogen-doped low-melting-point phosphate glass powder with a D50 particle size of 0.5 to 2 μm.

[0011] Furthermore, by weight percentage, the organic carrier comprises: 10%~30% polymer resin; Mixed organic solvents 60%~80%; Plasticizer 1%~5%; 1-10% auxiliary functional agents.

[0012] Furthermore, the polymer resin is selected from one or more of ethyl cellulose, acrylic resin, terpene resin, polyamide resin and modified phenolic resin; The mixed organic solvent is selected from two or more of the following: terpineol, diethylene glycol monobutyl ether, triethylene glycol butyl ether, cyclohexanone, isophorone, and dioctyl phthalate; The plasticizer is selected from one or more of dibutyl phthalate, dioctyl sebacate and tributyl citrate; The auxiliary functional additives include dispersants, defoamers, leveling agents, thixotropic agents, and antioxidants; The dispersant is an organosilicon dispersant and / or a fatty acid amide dispersant; The thixotropic agent is fumed silica and / or hydrogenated castor oil; The antioxidant is benzotriazole and / or a phenolic antioxidant.

[0013] This invention also provides a method for preparing low-temperature sintering type photovoltaic copper paste, comprising the following steps: (1) Preparation of organic carrier: Mix the polymer resin with the mixed organic solvent, heat and stir until completely dissolved, add plasticizer and continue stirring, and cool to room temperature; (2) Mixing and grinding: Weigh the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, mix them evenly under inert gas protection to obtain dry mixed powder; (3) Grinding and molding: Add organic carrier and auxiliary functional additives to the dry mixed powder in batches, premix and then grind in a three-roll mill until the slurry fineness is ≤4μm. After filtration, the finished product is obtained.

[0014] Furthermore, the heating and stirring temperature in step (1) is 70~85℃, and stirring is continued for 30~70 minutes after adding the plasticizer; In step (3), the premixing speed is 100~300 rpm and the time is 15~30 min; the number of grinding passes is 3~6.

[0015] The present invention also provides an application of a low-temperature sintering type of photovoltaic copper paste, wherein the copper paste is used for the preparation of the front electrode and / or back electrode of a crystalline silicon photovoltaic cell or a thin-film photovoltaic cell, the sintering temperature is 350~450℃, and the holding time is 10~35min.

[0016] The beneficial effects of this invention are: Compared with the prior art, the present invention has the following beneficial effects: (1) Low-temperature sintering with good process compatibility. The sintering temperature of the copper paste of this invention is 350~450℃, which is much lower than the 500~600℃ of traditional high-temperature copper paste. This can effectively avoid damage to photovoltaic silicon wafers, passivation layers or thin film substrates caused by high temperature. It can be directly adapted to existing photovoltaic low-temperature sintering production lines with low process modification costs.

[0017] (2) Excellent electrical conductivity. This invention uses three types of copper powder with different morphologies and particle sizes to form a composite (nanoscale spherical copper powder fills the gaps between micron-scale spherical copper powder, and sheet-like copper powder overlaps to increase the contact area), which can construct a continuous and dense three-dimensional conductive network; at the same time, nitrogen-doped phosphate glass powder will condense the Cu on the surface of the copper powder during the sintering process. 2+ In-situ reduction to Cu + and elemental Cu 0 This further reduces electrode contact resistance. Example data shows that the volume resistivity of the copper paste of this invention can be as low as 2.2 μΩ·cm.

[0018] (3) Inhibit copper ion migration at its source. This invention utilizes N in the glass network 3- Its strong reducing properties will cause Cu in the system to... 2+ In-situ reduction to Cu + and elemental Cu 0 This eliminates the material basis that leads to ion migration at the source, thereby completely suppressing the migration of copper ions under the action of an electric field and greatly improving the long-term working reliability of the electrode.

[0019] (4) Excellent adhesion and resistance to damp heat. This invention, through the rational design of the glass powder composition (introducing Si and Al to improve network stability and water resistance) and softening temperature (350~460℃), ensures that the glass fully wets the copper powder and substrate during sintering, forming a strong chemical bond. After aging at 85℃ / 85%RH for 500 hours, the film layer remains intact, with a resistivity change rate ≤8%.

[0020] (5) Excellent printing performance. This invention achieves suitable viscosity and thixotropic properties in copper paste through the optimized combination of composite organic carrier (resin, mixed solvent, plasticizer) and auxiliary functional additives, resulting in excellent screen printing performance that meets the process requirements for precision printing of photovoltaic electrodes.

[0021] (6) The preparation process is simple and the raw materials are readily available. This invention uses a melt-quenching method to prepare nitrogen-doped phosphate glass powder in one step. The process is simple and easy to scale up. The raw materials used are all conventional industrial raw materials, the cost is controllable, and it has good prospects for industrial application. Detailed Implementation

[0022] This invention provides a low-temperature sintering type of copper paste for photovoltaic applications, which is prepared by mass percentage from 85% to 90% composite copper powder, 1% to 5% nitrogen-doped low-melting-point phosphate glass powder, and 5% to 12% organic carrier.

[0023] 1. Composite copper powder

[0024] Copper powder is a conductive functional phase, and its content, morphology, and particle size distribution directly affect the conductivity and printability of the paste. In this invention, the copper powder content is 85%~90%, which ensures the formation of a continuous and dense conductive network within the paste while avoiding the problem of excessive viscosity and poor printability caused by excessive copper powder content.

[0025] The composite copper powder of this invention is a composite powder with different morphologies and particle sizes, comprising, by mass percentage: (1) 50% to 70% of the spherical copper powder with a particle size of 200 to 300 nm. The nano-sized spherical copper powder has a large specific surface area and high surface activity. It is easy to sinter and densify during the sintering process, fill the gaps between the micron-sized copper powder particles, and increase the number of conductive paths.

[0026] (2) 20% to 30% of spherical copper powder with a particle size of 1 to 3 μm. Micron-sized spherical copper powder in this particle size range is the main skeleton of the conductive network. The uniform particle size and high sphericity are conducive to the close packing of particles and the formation of continuous conductive chains.

[0027] (3) 5% to 10% of flake copper powder with a thickness of 0.1 to 0.3 μm and a diameter of 2 to 5 μm. Flake copper powder has a large aspect ratio and overlaps in the paste in a surface contact manner, which can significantly increase the contact area between copper particles, reduce contact resistance, and improve conductivity. At the same time, the flake copper powder is oriented along the shear force direction during the printing process, which is beneficial to improving the printing resolution and film smoothness of the paste.

[0028] This invention, through the rational blending of three copper powder morphologies, can maximize the particle packing density and conductive network connectivity while reducing the electrode volume resistivity, all while maintaining the same amount of copper powder.

[0029] 2. Nitrogen-doped low-melting-point phosphate glass powder

[0030] The nitrogen-doped low-melting-point phosphate glass powder used in this invention is the core innovative component, serving the dual functions of a binder and a reducing agent. Its usage is 1% to 5% by mass. If the glass powder content is too low, the bonding ability is insufficient, resulting in poor adhesion between the electrode and the substrate, making it prone to detachment; if the content is too high, the proportion of the insulating phase increases, leading to a decrease in conductivity.

[0031] The glass powder contains the following elements in mole percentage: P 10%~30%, Si 5%~20%, Li 5%~15%, Zn 5%~10%, Mn 5%~10%, Al 5%~10%, N 1%~5%, and one or more of Na, K, Mg, Ca, Ba, Fe, Cu, B, Pb, and Bi in total of 0%~5%.

[0032] In this system, P is the glass network forging, forming the phosphate glass framework; the introduction of Si improves the stability of the glass network structure, enhances the adhesion between the glass and the silicon substrate, and also improves the water resistance of the glass; Li + As a network modifier, it can lower the melting and softening temperatures of glass, allowing it to soften and flow in the 350–460°C range. The addition of Zn and Mn optimizes the chemical stability and thermal expansion coefficient of the glass, making it more compatible with the thermal expansion coefficient of photovoltaic silicon wafers and reducing thermal stress during sintering and cooling. Al2O3 can further improve the glass's resistance to water vapor erosion. Nitrogen element partially replaces oxygen in the glass network through covalent bonds, forming PN, Si-N, and other structural units, giving the glass strong reducing properties.

[0033] The glass powder of this invention has a glass softening temperature of 350~460℃ and a D50 particle size of 0.5~2μm. The softening temperature is lower than the sintering temperature, ensuring that the glass powder softens and melts during sintering, fully wetting the copper powder and the substrate surface, thus achieving good adhesion. If the particle size is too coarse, the glass distribution will be uneven, resulting in poor adhesion; if the particle size is too fine, it will easily agglomerate, affecting the dispersibility of the slurry.

[0034] The preparation method of this glass powder is as follows: Oxide raw materials containing P, Si, Li, Zn, Mn, and Al (such as P2O5, SiO2, Li2CO3, ZnO, MnO2, Al2O3, etc.) are mixed uniformly according to the target element composition ratio. Ammonium nitrate (NH4NO3) is added as a nitrogen source, accounting for 5% to 15% of the total mass of the mixed raw materials. During the heating process, ammonium nitrate decomposes to produce active nitrogen species, which are incorporated into the phosphate glass network in the molten state.

[0035] The mixture is placed in a crucible and heated to 1000–1200 °C under a nitrogen protective atmosphere, then held at this temperature for 0.5–2 hours. This temperature range ensures complete melting of the raw materials and effective nitrogen doping while preventing excessive nitrogen escape due to overheating. After melting, the resulting molten glass is sequentially subjected to rapid cooling by rolling and water quenching to obtain glass fragments. Rapid cooling by rolling rapidly cools the molten glass, inhibiting crystal precipitation and maintaining the amorphous structure. Finally, the fragments are ball-milled to a D50 particle size of 0.5–2 μm to obtain nitrogen-doped phosphate glass powder.

[0036] 3. Organic carrier

[0037] Organic carriers serve as dispersion media for copper and glass powders, imparting suitable rheological properties and printing performance to the copper paste. By mass percentage, the organic carrier comprises: 10%–30% polymer resin, 60%–80% mixed organic solvents, 1%–5% plasticizer, and 1%–10% auxiliary functional additives. The total amount of organic carrier in the copper paste is 5%–12%.

[0038] The polymer resin is selected from one or more of ethyl cellulose, acrylic resin, terpene resin, polyamide resin and modified phenolic resin. Its main function is to provide the film-forming properties and thixotropy of the slurry, and to ensure that the film layer has a certain shape retention ability after printing.

[0039] The mixed organic solvent is selected from two or more of terpineol, diethylene glycol monobutyl ether, triethylene glycol butyl ether, cyclohexanone, isophorone and dioctyl phthalate, and is used to adjust the evaporation rate and viscosity of the paste to ensure that the solvent evaporates moderately during the printing process and does not produce defects such as pinholes and bubbles.

[0040] The plasticizer is selected from one of dibutyl phthalate, dioctyl sebacate, and tributyl citrate, which can improve the flexibility of the resin and improve the crack resistance of the film.

[0041] The organic carrier also contains two or more of the following: self-dispersant, defoamer, leveling agent, thixotropic agent, and antioxidant. The dispersant is an organosilicon dispersant and / or a fatty acid amide dispersant, used to improve the dispersion uniformity of copper powder and glass powder in the organic carrier and prevent particle agglomeration. The thixotropic agent is fumed silica and / or hydrogenated castor oil, used to adjust the thixotropic index of the slurry, reducing the viscosity of the slurry under printing shear, facilitating its passage through the screen, and ensuring rapid viscosity recovery after shearing is removed, thus guaranteeing the clarity and resolution of printed lines. The antioxidant is benzotriazole and / or phenolic antioxidants, used to prevent oxidation of the copper powder during storage and the early stages of sintering.

[0042] 4. Preparation method

[0043] The method for preparing copper paste according to the present invention includes the following steps: (1) Preparation of organic carrier: Mix polymer resin and mixed organic solvent in proportion, heat and stir in a water bath at 70~85℃ until the resin is completely dissolved, add plasticizer and continue stirring for 30~70min to fully homogenize the components, cool to room temperature to obtain organic carrier.

[0044] (2) Mixing and grinding: Weigh the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, put them into a mixing tank, and mix them for 30 to 60 minutes at a speed of 200 to 350 rpm under the protection of inert gas such as nitrogen or argon to obtain dry mixed powder. The inert gas protection can prevent the copper powder from oxidizing during the mixing process.

[0045] (3) Grinding and molding: Add organic carrier and auxiliary functional additives to dry mixed powder in batches. First, premix at a low speed of 100~300rpm for 15~30min to initially wet the powder and carrier. Then, transfer it to a three-roll mill and grind it repeatedly for 3~6 times. The agglomerates are broken up by the shear force between the rollers, so that each component is evenly dispersed. Control the fineness of the slurry to ≤4μm. After filtering through a 500~800 mesh sieve, the finished low-temperature sintered photovoltaic copper paste is obtained.

[0046] 5. Application Method

[0047] This invention's copper paste is suitable for preparing the front and / or back electrodes of crystalline silicon photovoltaic cells or thin-film photovoltaic cells. In application, the copper paste is printed onto the surface of the cell substrate using a screen printing process, with the printed film thickness controlled at 15-25 μm. Sintering is carried out in a protective atmosphere (such as nitrogen) or a reducing atmosphere, with the temperature increased to 350-450°C at a rate of 3-8°C / min, held at that temperature for 10-35 minutes, and then cooled in the furnace.

[0048] (II) Mechanism of Action

[0049] The core mechanism of this invention lies in the in-situ reduction mechanism of nitrogen-doped phosphate glass powder during the sintering process.

[0050] In nitrogen-doped phosphate glasses, nitrogen atoms covalently replace some of the bridging and non-bridging oxygen positions, forming PN, Si-N, Al-N, and other structural units. The electronegativity of N (3.04) is lower than that of O (3.44). 3- Lone pairs of electrons are more easily delocalized and released. Under high-temperature sintering conditions (350~450℃), N in the glass network... 3- It gains enough energy, loses electrons, and is oxidized to N2, which is released. The reaction is as follows: 2N 3- →N2↑+6e - The released electrons have strong reducing properties, and their effects are manifested in three aspects: First, it captures trace amounts of residual oxygen in the sintering atmosphere: O2 + 4e - →2O 2- This prevents the copper powder from being oxidized during high-temperature sintering and protects the metallic state of the copper powder surface.

[0051] Second, the copper powder surface may be affected by trace oxidation or residual Cu in the solution. 2+ Stepwise reduction: Cu 2+ +e - →Cu + Cu + +e-→Cu 0 Through this reduction reaction, Cu in the system... 2+ It was completely eliminated.

[0052] Third, Cu 2+ It is the root cause of the electrochemical migration of copper ions, under the action of an electric field, Cu² + Migration and deposition towards the cathode can lead to leakage or even short circuits between electrodes. This invention utilizes N... 3- The reducing effect removes Cu from the source. 2+ Completely converted to Cu + and elemental Cu 0 This completely suppresses the problem of copper ion migration.

[0053] Meanwhile, the glass powder softens and melts at the sintering temperature, forming a continuous glass phase bridge between copper powder particles and between the copper powder and the silicon substrate. Upon cooling, this forms a strong chemical bond, giving the electrode excellent adhesion. Elements such as Si and Al in the glass network can form chemical bonds with the SiO2 layer on the silicon substrate surface, further enhancing the interfacial bonding strength.

[0054] The technical solutions provided by the present invention will be described in detail below with reference to embodiments, but this should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention fall within the scope of protection of the present invention.

[0055] Example 1

[0056] This embodiment provides a low-temperature sintering type of copper paste for photovoltaic applications, the formulation of which is as follows (by mass percentage): Composite copper powder: 88% Nitrogen-doped low-melting-point phosphate glass powder: 3.5%; Organic carrier (including auxiliary functional agents): 8.5%; The composite copper powder consists of: 60% spherical copper powder with a particle size of 250nm, 30% spherical copper powder with a particle size of 1.5μm, and 10% flake copper powder with a thickness of 0.2μm and a diameter of 3μm.

[0057] The elemental composition (in mole percentage) of the nitrogen-doped low-melting-point phosphate glass powder is: P 22%, Si 12%, Li 10%, Zn 7%, Mn 8%, Al 6%, N 3%, with the remainder being unavoidable impurities. The glass powder has a softening temperature of 390℃ and a D50 particle size of 0.8 μm.

[0058] The organic carrier is an integrated compound system, and its composition, based on the total mass of the organic carrier, is as follows: High molecular weight resin: 20% ethyl cellulose; Mixed organic solvents: 15% terpineol, 55% diethylene glycol monobutyl ether; Plasticizer: 5% dibutyl phthalate; Dispersant: 2% organosilicon dispersant; Thixotropic agent: 2% fumed silica; Antioxidants: 1% phenolic antioxidants; The preparation method in this embodiment is as follows: (1) Preparation of nitrogen-doped phosphate glass powder Weigh out the oxide raw materials, including P2O5, SiO2, Li2CO3, ZnO, MnO2, and Al2O3, according to the target composition, and mix them in a V-type mixer for 60 minutes to ensure that the components are fully homogeneous. Add ammonium nitrate (NH4NO3) at 10% of the total mass of the raw materials as a nitrogen source to the mixture, and continue mixing for 30 minutes.

[0059] The above mixture was loaded into an alumina crucible and pushed into a tube furnace. Under a nitrogen protective atmosphere (nitrogen flow rate 2 L / min), the temperature was raised to 1100 °C at a rate of 10 °C / min and held at that temperature for 1.5 hours to melt. During the melting process, the active nitrogen species generated by the decomposition of ammonium nitrate diffused into the molten phosphate and incorporated into the glass network.

[0060] After melting, the molten glass in the crucible is slowly poured into a preheated double-roll mill, where it is rapidly cooled to a thickness of about 1-2 mm. This thin glass strip is then quenched in deionized water to obtain glass fragments. The glass fragments are placed in an oven and dried at 120°C for 4 hours. They are then fed into a planetary ball mill and milled at 300 rpm for 4 hours using anhydrous ethanol as the medium and zirconia balls as the grinding media. The resulting powder is sieved through a 500-mesh sieve to obtain nitrogen-doped phosphate glass powder with a D50 particle size of 0.8 μm.

[0061] (2) Preparation of organic carrier

[0062] Weigh out each component according to the described organic carrier composition: Mix ethyl cellulose with terpineol and diethylene glycol monobutyl ether, and add the mixture to a three-necked flask equipped with a stirrer and a reflux condenser. Heat in a water bath at 78°C and stir at 200 rpm until the resin is completely dissolved (approximately 1.5 hours). Add dibutyl phthalate, organosilicon dispersant, fumed silica, and phenolic antioxidant, and continue stirring for 45 minutes to ensure that the additives are uniformly dispersed in the resin solution. Stop heating and allow to cool naturally to room temperature with stirring to obtain a homogeneous and stable organic carrier.

[0063] (3) Mixing and grinding

[0064] Weigh out the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, and put them into a double planetary mixer. Purge the air in the mixer with high-purity nitrogen (nitrogen flow rate 3L / min, continuous purging for 10 minutes). Mix at 280 rpm for 40 minutes under nitrogen protection to obtain a dry mixed powder.

[0065] (4) Grinding and shaping

[0066] Add the organic carrier prepared in step (2) to the above dry mixed powder in batches (add in 3 batches, 10 minutes apart each time). First, premix at a low speed of 200 rpm for 20 minutes to fully wet the powder and organic carrier. Transfer the premixed slurry to a three-roll mill, adjust the roller spacing (initial gap is 30 μm, final gap is 5 μm), and grind repeatedly 5 times. During this period, use a scraper fineness gauge to check the fineness of the slurry to ≤3.5 μm. Finally, filter and degas through a 600-mesh stainless steel screen to obtain the finished low-temperature sintered photovoltaic copper paste.

[0067] (5) Electrode preparation and sintering

[0068] The obtained copper paste was printed onto the surface of a monocrystalline silicon photovoltaic cell substrate using screen printing technology, with the printing film thickness controlled at 20±2μm. The printed substrate was placed in a tube sintering furnace and heated to 400℃ at a heating rate of 5℃ / min under a high-purity nitrogen protective atmosphere (nitrogen flow rate 5L / min), held at that temperature for 20 minutes, and then cooled to room temperature with the furnace to obtain the copper electrode sample.

[0069] Example 2

[0070] This embodiment provides a low-temperature sintering type of copper paste for photovoltaic applications, the formulation of which is as follows (by mass percentage): Composite copper powder: 86%; Nitrogen-doped low-melting-point phosphate glass powder: 4.5%; Organic carrier (including auxiliary functional agents): 9.5%; The composite copper powder consists of: 55% spherical copper powder with a particle size of 220nm, 35% spherical copper powder with a particle size of 2μm, and 10% flake copper powder with a thickness of 0.15μm and a diameter of 4μm.

[0071] The elemental composition (in mole percentage) of the nitrogen-doped low-melting-point phosphate glass powder is: P 20%, Si 15%, Li 8%, Zn 6%, Mn 9%, Al 7%, N 4%, with the remainder being unavoidable impurities. The glass powder has a softening temperature of 375℃ and a D50 particle size of 1.2 μm.

[0072] The organic carrier is an integrated compound system, and its composition, based on the total mass of the organic carrier, is as follows: Polymer resins: 15% acrylic resin, 10% terpene resin; Mixed organic solvents: 20% triethylene glycol butyl ether, 45% cyclohexanone; Plasticizer: 5% tributyl citrate; Dispersant: 2% fatty acid amide dispersant; Thixotropic agent: 1.5% hydrogenated castor oil; Antioxidant: benzotriazole 1.5%; The preparation method in this embodiment is as follows: (1) Preparation of nitrogen-doped phosphate glass powder According to the target composition, oxide raw materials such as P2O5, SiO2, Li2CO3, ZnO, MnO2, and Al2O3 were weighed out and mixed evenly. Ammonium nitrate, accounting for 12% of the total mass of the mixed raw materials, was added as a nitrogen source. The mixture was placed in a crucible and heated to 1150℃ at a rate of 8℃ / min under nitrogen protection, and held at that temperature for 1 hour to melt. The resulting glass melt was rapidly cooled by rolling and water quenched to obtain glass fragments. After drying, the fragments were ball-milled to a D50 particle size of 1.2μm to obtain nitrogen-doped phosphate glass powder.

[0073] (2) Preparation of organic carrier

[0074] Acrylic resin, terpene resin, triethylene glycol butyl ether, and cyclohexanone were mixed and stirred in a water bath at 72°C until completely dissolved. Tributyl citrate, fatty acid amide dispersant, hydrogenated castor oil, and benzotriazole were added, and stirring was continued for 50 minutes. The mixture was then cooled to room temperature to obtain the organic carrier.

[0075] (3) Mixing and grinding

[0076] Weigh out the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, put them into a mixing tank, and mix them at 300 rpm for 35 minutes under nitrogen protection to obtain a dry mixed powder.

[0077] (4) Grinding and shaping

[0078] Organic carriers are added to dry mixed powders in batches. The mixture is first premixed at a low speed of 250 rpm for 25 minutes, and then transferred to a three-roll mill for repeated grinding 4 times to control the fineness of the slurry to ≤3.8μm. The finished copper slurry is obtained after filtration through a 700-mesh sieve.

[0079] (5) Electrode preparation and sintering

[0080] Copper paste was printed onto the surface of a monocrystalline silicon photovoltaic substrate using screen printing, with the film thickness controlled at 20±2μm. Under a nitrogen protective atmosphere, the temperature was increased to 380℃ at a rate of 4℃ / min, held for 25 minutes, and then cooled to room temperature in the furnace.

[0081] Example 3

[0082] This embodiment provides a low-temperature sintering type of copper paste for photovoltaic applications, the formulation of which is as follows (by mass percentage): Composite copper powder: 89% Nitrogen-doped low-melting-point phosphate glass powder: 2.8% Organic carrier (containing 1% auxiliary functional agents): 8.2% The composite copper powder consists of: 65% spherical copper powder with a particle size of 280nm, 25% spherical copper powder with a particle size of 1μm, and 10% flake copper powder with a thickness of 0.25μm and a diameter of 5μm.

[0083] The elemental composition (in mole percentage) of the nitrogen-doped low-melting-point phosphate glass powder is: P 25%, Si 10%, Li 12%, Zn 8%, Mn 6%, Al 5%, N 2%, Na 1%, K 1%. The softening temperature of this glass powder is 410℃, and the D50 particle size is 0.5μm.

[0084] The organic carrier is an integrated compound system, and its composition, based on the total mass of the organic carrier, is as follows: Polymer resins: 12% modified phenolic resin, 10% polyamide resin Mixed organic solvents: 25% isophorone, 46% dioctyl phthalate Plasticizer: 3% dioctyl sebacate Dispersant: 2% organosilicon dispersant Leveling agent: 1% Antioxidants: 1% phenolic antioxidants The preparation method in this embodiment is as follows: (1) Preparation of nitrogen-doped phosphate glass powder According to the target composition, oxide raw materials such as P2O5, SiO2, Li2CO3, ZnO, MnO2, Al2O3, Na2CO3, and K2CO3 were weighed out and mixed evenly. Ammonium nitrate, accounting for 8% of the total mass of the mixed raw materials, was added as a nitrogen source. The mixture was placed in a crucible and heated to 1200℃ at a rate of 12℃ / min under nitrogen protection, and held at this temperature for 0.8 hours to melt. The resulting glass melt was rapidly cooled by rolling and water quenched to obtain glass fragments. After drying, the fragments were ball-milled to a D50 particle size of 0.5μm to obtain nitrogen-doped phosphate glass powder.

[0085] (2) Preparation of organic carrier

[0086] Modified phenolic resin, polyamide resin, isophorone, and dioctyl phthalate were mixed and stirred in an 80°C water bath until completely dissolved. Dioctyl sebacate, organosilicon dispersant, leveling agent, and phenolic antioxidant were added, and stirring was continued for 60 minutes. The mixture was then cooled to room temperature to obtain the organic carrier.

[0087] (3) Mixing and grinding

[0088] Weigh out the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, put them into a mixing tank, and mix them at 320 rpm for 30 minutes under argon protection to obtain a dry mixed powder.

[0089] (4) Grinding and shaping

[0090] Organic carriers are added to dry mixed powders in batches. The mixture is first premixed at a low speed of 180 rpm for 15 minutes, and then transferred to a three-roll mill for repeated grinding 6 times to control the fineness of the slurry to ≤3.0 μm. The finished copper paste is obtained after filtration through an 800-mesh sieve.

[0091] (5) Electrode preparation and sintering

[0092] Copper paste was printed onto the surface of a monocrystalline silicon photovoltaic substrate using screen printing, with the film thickness controlled at 20±2μm. Under a nitrogen protective atmosphere, the temperature was increased to 430℃ at a rate of 6℃ / min, held for 15 minutes, and then cooled to room temperature in the furnace.

[0093] Comparative Example 1 (Low-Temperature Epoxy Resin Copper Paste)

[0094] This comparative example provides a commercially available typical low-temperature epoxy resin-cured copper paste, with the following formulation by mass percentage: 88% composite copper powder (same specifications as in Example 1), 10% epoxy resin binder, and 2% organic solvent. The preparation process involves conventional stirring and mixing, followed by grinding to a fineness ≤5μm using a three-roll mill. The curing process involves heating to 200℃ at a rate of 5℃ / min under a nitrogen atmosphere and holding at that temperature for 30 minutes.

[0095] Comparative Example 2 (Ordinary phosphate glass copper paste, without nitrogen doping)

[0096] This comparative example provides a copper paste using ordinary phosphate glass (without nitrogen doping), with the following formula by mass percentage: 88% composite copper powder (same specifications as in Example 1), 3.5% ordinary phosphate glass powder, and 8.5% organic carrier (without auxiliary functional additives). The elemental composition (by mole percentage) of the ordinary phosphate glass powder is: P 23%, Si 12%, Li 10%, Zn 7%, Mn 8%, Al 6% (without N doping), with a softening temperature of 395℃ and a D50 particle size of 0.8μm. The composition of the organic carrier is the same as in Example 1. The preparation method is the same as in Example 1. The sintering process is as follows: under a nitrogen protective atmosphere, the temperature is increased to 550℃ at a rate of 5℃ / min and held for 20 minutes.

[0097] Performance testing

[0098] The copper paste samples obtained in Examples 1-3 and Comparative Examples 1-2 were subjected to system performance tests. The test conditions and evaluation criteria are as follows: (1) Sample preparation All pastes were printed onto monocrystalline silicon photovoltaic substrates using standard screen printing technology (325 mesh screen, 70° squeegee angle, 0.3 MPa printing pressure), with a printing area of ​​20 mm × 20 mm and a film thickness controlled at 20 ± 2 μm. Each example and comparative example was processed according to its respective sintering / curing process conditions.

[0099] (2) Testing items and methods

[0100] Volume resistivity: The sheet resistance of the sintered electrode film was measured using an RTS-8 four-probe tester, and the volume resistivity was calculated by multiplying it by the film thickness. The average value of 5 test points was taken for each sample.

[0101] Copper ion migration tendency: Referring to the IPC-TM-650 2.6.25 standard, the accelerated test was conducted for 168 hours at 85℃ / 85%RH and an applied DC bias voltage of 5V. The presence of copper dendrites was observed between the electrodes and at the electrode edges using an optical microscope. The following criteria were used for evaluation: no migration, slight migration (sparse precipitation points are visible), and severe migration (obvious dendrites are visible).

[0102] Electrode adhesion: A 100-grid test was conducted according to ASTM D3359 standard. A 1mm × 1mm grid (100 grids) was cut on the film layer with a cutting tool. After applying 3M 600 tape, the grids were quickly peeled off, and the number of grids that peeled off was counted. The rating standards were: Grade 0 (peeling area ≤ 5%), Grade 1 (5% < peeling area ≤ 15%), Grade 2 (15% < peeling area ≤ 35%), Grade 3 (35% < peeling area ≤ 65%), and Grade 4 (peeling area > 65%).

[0103] Film integrity: The surface and cross-sectional morphology of the sintered film were observed using a metallographic optical microscope (500×) to evaluate whether the film was intact and whether there were cracks or pores.

[0104] Moisture and heat resistance: The printed and sintered sample was placed in a constant temperature and humidity test chamber and aged for 500 hours at 85℃ / 85%RH. After taking it out, the volume resistivity after aging was tested, and the resistance change rate was calculated: Change rate = |ρ_500h-ρ_0| / ρ_0×100%.

[0105] (3) Test results

[0106] The results of the various performance tests are summarized in Table 1.

[0107] Table 1 Performance test results of the examples and comparative examples

[0108] Results Analysis

[0109] The test results in Table 1 show that: (1) Sintering temperature and process compatibility The sintering temperatures of Examples 1-3 of this invention are 380-430℃, which is much lower than the 550℃ high-temperature sintering process of Comparative Example 2. This effectively avoids damage to photovoltaic silicon wafers, passivation layers, or thin film substrates caused by high temperatures. Furthermore, it is fully compatible with the temperature window (350-450℃) of the medium-low temperature sintering section of existing photovoltaic production lines, allowing for direct adaptation to mass production equipment without the need for production line modifications. Although Comparative Example 1 uses a lower temperature, this resin curing method sacrifices conductivity and is not comparable.

[0110] (2) Electrical conductivity

[0111] The volume resistivity of Examples 1-3 of this invention is 2.4, 2.7, and 2.2 μΩ·cm, respectively, which is much lower than that of Comparative Example 1 (18.6 μΩ·cm). This indicates that the three-dimensional conductive network constructed by the composite of three copper powder morphologies in this invention produces a significant synergistic effect with the in-situ reduction of nitrogen-doped phosphate glass: on the one hand, the composite of multi-morphology copper powder maximizes the contact area between particles and the number of conductive pathways; on the other hand, the N³ content in the glass powder... - Cu² on the surface of copper powder + In-situ reduction to highly conductive elemental copper lowers the contact resistance between particles.

[0112] (3) Inhibition of copper ion migration

[0113] No signs of copper ion migration were observed in Examples 1-3, while no signs of copper ion migration were observed in Comparative Example 1, but severe migration was observed in Comparative Example 2 (obvious copper dendrites were visible between the electrodes). This fully demonstrates that the present invention, through the strong reducing properties of nitrogen-doped phosphate glass, eliminates the material basis leading to ion migration—Cu—at its source. 2+ Its mechanism of action is as follows: N in the glass network 3- At high temperatures, it loses electrons and is oxidized to N2, releasing (2N). 3- →N2 ↑+ 6e - The released electrons will cause Cu in the system to... 2+ Stepwise reduction to Cu + and elemental Cu 0 (Cu) 2+ →Cu + →Cu 0 ), thereby enabling the migration of Cu 2+ It was completely eliminated.

[0114] (4) Adhesion

[0115] The adhesion ratings of Examples 1 and 3 were 0 (fractional area ≤ 5% in cross-cut adhesion test), and Example 2 was 1, both superior to Comparative Example 1's 4 and Comparative Example 2's 2. The introduction of elements such as Si and Al into the glass powder of this invention enhances the glass network structure. Simultaneously, the glass powder softens sufficiently at the sintering temperature, exhibiting excellent wettability on both copper powder and silicon substrates, and forming strong chemical bonds upon cooling.

[0116] (5) Stability against damp heat

[0117] After 500 hours of constant temperature and humidity aging at 85℃ / 85%RH, the resistance change rates of Examples 1-3 were 4.2%, 6.8%, and 3.5%, respectively, all ≤8%, indicating that the electrodes have excellent long-term stability under high temperature and high humidity conditions. In contrast, the change rate of Comparative Example 1 was as high as 52.3% (due to degradation caused by hydrothermal aging of the resin matrix), and that of Comparative Example 2 was 33.6% (due to the combined effects of copper ion migration and oxidation), both failing to meet the reliability requirements for a 25-year lifespan of photovoltaic cells.

[0118] (6) Membrane integrity

[0119] The films in Examples 1-3 and Comparative Example 2 were all intact and crack-free, indicating that the glass binder phase formed a continuous and dense film structure during sintering. Comparative Example 1 was a resin-cured system. During the curing and shrinkage process, the resin generated internal stress, which led to microcracks in the film. This is one of the reasons for its high resistivity and rapid performance deterioration after aging.

[0120] In summary, the low-temperature sintering photovoltaic copper paste and its preparation method provided by this invention, through the synergistic effect of nitrogen-doped low-melting-point phosphate glass powder with specific elemental composition and multi-morphology composite copper powder, achieves comprehensive performance of low resistivity (≤2.7μΩ·cm), no copper ion migration, high adhesion (0~1 grade), and excellent resistance to damp heat (500h aging resistance change rate ≤8%) under low-temperature sintering conditions of 350~450℃. This effectively solves the technical problems of high sintering temperature, easy copper ion migration, insufficient conductivity, and poor resistance to damp heat in existing photovoltaic copper paste technologies.

[0121] Industrial applicability

[0122] The low-temperature sintering photovoltaic copper paste of this invention has widely available raw materials, a simple and controllable preparation process, and is highly compatible with the equipment and process conditions of existing photovoltaic screen printing and sintering production lines, enabling industrial application without large-scale equipment modifications. This copper paste can be widely used in the preparation of front and back electrodes for monocrystalline silicon, polycrystalline silicon photovoltaic cells, and various thin-film photovoltaic cells, demonstrating promising industrial application prospects and significant economic and social benefits.

Claims

1. A low-temperature sintering type copper paste for photovoltaic applications, characterized in that, It is prepared from the following raw materials by mass percentage: Composite copper powder 85%~90%; Nitrogen-doped low-melting-point phosphate glass powder, 1%–5%; Organic carriers: 5%–12%.

2. The low-temperature sintering type photovoltaic copper paste according to claim 1, characterized in that, The composite copper powder comprises, by weight percentage: Spherical copper powder with a particle size of 200~300nm accounts for 50%~70%; 20%–30% spherical copper powder with a particle size of 1–3 μm; 5% to 10% of flake-shaped copper powder with a thickness of 0.1 to 0.3 μm and a diameter of 2 to 5 μm.

3. The low-temperature sintering type photovoltaic copper paste according to claim 1 or 2, characterized in that, The nitrogen-doped low-melting-point phosphate glass powder, by mole percentage, comprises the following elements: P 10%~30%； Si 5%~20%; Li 5%~15%; Zn 5%~10%; Mn 5%~10%; Al 5%~10%； N 1%~5%； And one or more of Na, K, Mg, Ca, Ba, Fe, Cu, B, Pb, and Bi, in total of 0% to 5%.

4. The low-temperature sintering type photovoltaic copper paste according to claim 3, characterized in that, The nitrogen-doped low-melting-point phosphate glass powder has a glass softening temperature of 350~460℃ and a D50 particle size of 0.5~2μm.

5. The low-temperature sintering type photovoltaic copper paste according to claim 4, characterized in that, The nitrogen-doped low-melting-point phosphate glass powder was prepared by the following method: Oxide raw materials containing P, Si, Li, Zn, Mn and Al are mixed according to the target composition. Ammonium nitrate, accounting for 5% to 15% of the total mass of the mixed raw materials, is added as a nitrogen source. The mixture is melted at 1000 to 1200℃ for 0.5 to 2 hours under nitrogen protection. The resulting glass melt is then subjected to rapid cooling by rolling and water quenching, followed by ball milling to obtain nitrogen-doped low-melting-point phosphate glass powder with a D50 particle size of 0.5 to 2 μm.

6. The low-temperature sintering type photovoltaic copper paste according to claim 1, characterized in that, The organic carrier comprises, by weight percentage: 10%~30% polymer resin; Mixed organic solvents 60%~80%; Plasticizer 1%~5%; 1-10% auxiliary functional agents.

7. The low-temperature sintering type photovoltaic copper paste according to claim 6, characterized in that, The polymer resin is selected from one or more of ethyl cellulose, acrylic resin, terpene resin, polyamide resin and modified phenolic resin; The mixed organic solvent is selected from two or more of the following: terpineol, diethylene glycol monobutyl ether, triethylene glycol butyl ether, cyclohexanone, isophorone, and dioctyl phthalate; The plasticizer is selected from one or more of dibutyl phthalate, dioctyl sebacate and tributyl citrate; The auxiliary functional additives include dispersants, defoamers, leveling agents, thixotropic agents, and antioxidants; The dispersant is an organosilicon dispersant and / or a fatty acid amide dispersant; The thixotropic agent is fumed silica and / or hydrogenated castor oil; The antioxidant is benzotriazole and / or a phenolic antioxidant.

8. The method for preparing low-temperature sintering photovoltaic copper paste according to any one of claims 1 to 7, characterized in that, Includes the following steps: (1) Preparation of organic carrier: Mix the polymer resin with the mixed organic solvent, heat and stir until completely dissolved, add plasticizer and continue stirring, and cool to room temperature; (2) Mixing and grinding: Weigh the composite copper powder and nitrogen-doped phosphate glass powder according to the formula, mix them evenly under inert gas protection to obtain dry mixed powder; (3) Grinding and molding: Add organic carrier and auxiliary functional additives to the dry mixed powder in batches, premix and then grind in a three-roll mill until the slurry fineness is ≤4μm. After filtration, the finished product is obtained.

9. The preparation method according to claim 8, characterized in that, The heating and stirring temperature in step (1) is 70~85℃, and stirring continues for 30~70 minutes after adding the plasticizer; In step (3), the premixing speed is 100~300 rpm and the time is 15~30 min; the number of grinding passes is 3~6.

10. The application of the low-temperature sintering type photovoltaic copper paste according to any one of claims 1 to 7, characterized in that, The copper paste is used to prepare the front electrode and / or back electrode of crystalline silicon photovoltaic cells or thin-film photovoltaic cells, with a sintering temperature of 350~450℃ and a holding time of 10~35min.

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