Silver-coated nickel conductive paste for photovoltaic cells and preparation method thereof

Abstract

This invention relates to the field of conductive paste technology, and discloses a silver-coated nickel conductive paste for photovoltaic cells and its preparation method. The conductive paste comprises 65-85 parts of nickel-based alloy core@Ag composite powder, 15-30 parts of auxiliary silver powder, 2-4 parts of glass powder, 25-35 parts of organic carrier, and 0.2-0.8 parts of interface adaptant. The alloying element in the nickel-based alloy core is selected from one or more of P, B, and Si, with P content of 0.5%-5.0%, B content of 0.1%-2.0%, and Si content of 1.0%-4.0%. The interface adaptant is selected from organosilver or organonickel compounds. Alloying reduces the Ag / Ni interface energy to inhibit dewetting, and the interface adaptant is used to repair micro-defects in situ, improve the integrity of the silver shell, reduce contact resistance, and improve cell efficiency and reliability.

Description

Technical Field

[0001] This invention relates to the field of conductive paste technology, and in particular to a silver-coated nickel conductive paste for photovoltaic cells and its preparation method. Background Technology

[0002] With the photovoltaic industry's increasingly urgent need for cost reduction and efficiency improvement, the removal of silver from electrode materials has become an important development direction for the industry. Silver-coated nickel conductive paste, which reduces the amount of silver while maintaining conductivity, is regarded as a powerful candidate material to replace traditional silver paste and reduce the non-silicon cost of batteries. It is screen-printed onto the surface of silicon wafers and formed into electrode grid lines after high-temperature sintering. The silver shell layer is responsible for building the conductive network, while the nickel core plays the role of reducing costs and inhibiting silver electromigration. However, the interface stability problem of silver-coated nickel paste during high-temperature sintering has always been a key bottleneck restricting its large-scale application.

[0003] To address the aforementioned issues, existing technologies have undertaken a series of studies. For example, Chinese Patent CN119626626A discloses a silver-coated nickel paste and its preparation method, as well as a high-temperature sintered silicon solar cell metallization method, which improves conductivity by optimizing powder ratio and resistivity parameters. Chinese Patent CN118367063A discloses a TOPCon solar cell metallization method and solar cell using silver-coated nickel paste, which reduces contact resistance degradation caused by nickel oxidation by introducing a nickel diffusion regulator to suppress the diffusion of nickel elements into the silver shell and silicon substrate during sintering. These solutions mainly focus on formula optimization or diffusion suppression, which improves the performance of the paste to a certain extent.

[0004] A recent paper by Zhang Rui and his team published in the journal *Surfaces and Interfaces*, titled "Nickel-intermediated copper core-silver shell architectures via facile synthesis: Synergistic enhancement of high-temperature stability and electrical conductivity," offers a new perspective. The paper reveals a high interfacial energy between silver and nickel. During high-temperature heat treatment, the silver layer undergoes dewetting, shrinking, spheroidizing, and even cracking. The exposed nickel core rapidly oxidizes after the silver shell cracks, forming an insulating nickel oxide layer. This oxide layer not only has extremely high resistance but, more seriously, physically blocks direct contact between silver and the silicon substrate and disrupts the conductive pathways between silver particles. This results in increased grid resistance and contact resistance, leading to battery efficiency far below theoretical expectations. Existing research has only focused on revealing the mechanism without proposing targeted solutions. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the existing silver-coated nickel conductive paste has the disadvantage of dewetting of the silver shell layer during high-temperature sintering due to the high silver-nickel interface energy, which leads to the exposure and oxidation of the nickel core and blocks the conductive path. To this end, we propose a silver-coated nickel conductive paste for photovoltaic cells and its preparation method.

[0006] To achieve the above objectives, this application adopts the following technical solution: a silver-coated nickel conductive paste for photovoltaic cells, characterized in that, by weight, it comprises: 65-85 parts of nickel-based alloy core@Ag composite powder, 15-30 parts of auxiliary silver powder, 2-4 parts of glass powder, 25-35 parts of organic carrier, and 0.2-0.8 parts of interface adaptant, wherein the core layer of the nickel-based alloy core@Ag composite powder is a nickel-based alloy, and the alloying element in the nickel-based alloy is selected from one or more of phosphorus, boron, and silicon; the interface adaptant is selected from organosilver compounds or organonickel compounds.

[0007] Preferably, the atomic percentage of the alloying elements in the nickel-based alloy is as follows: when the alloying element is phosphorus, its content is 0.5%-5.0%; when the alloying element is boron, its content is 0.1%-2.0%; and when the alloying element is silicon, its content is 1.0%-4.0%.

[0008] Preferably, the glass powder is selected from one or more of Pb-Te-O, Bi-B-Si-Zn-O, or Te-VO glass powders, and its softening point is 450-600℃.

[0009] Preferably, the organic carrier comprises a resin, a solvent, and an additive, wherein the resin is selected from one or more of ethyl cellulose, acrylic resin, and polyvinyl butyral, accounting for 5-15% of the organic carrier; the solvent is selected from one or more of terpineol, butyl carbitol, and butyl carbitol acetate, accounting for 80-90% of the organic carrier; and the additives include thixotropic agents and dispersants, accounting for 1-5% of the organic carrier.

[0010] Preferably, the organosilver compound is selected from one or more of silver citrate, silver tartrate, silver stearate, and silver oxalate, and the organonickel compound is selected from one or more of nickel acetylacetonate, nickel formate, and nickel oxalate.

[0011] A method for preparing a silver-coated nickel conductive paste for photovoltaic cells includes the following steps: S1: Dissolving nickel salt and alloy element source compound in water, adding a reducing solution to carry out a reduction reaction, obtaining a nickel-based alloy core precipitate, and obtaining nickel-based alloy core powder after washing and drying; S2: Dispersing the nickel-based alloy core powder obtained in S1 in water, adding silver ammonia solution and reducing agent to carry out a displacement silver plating reaction, and obtaining nickel-based alloy core@Ag composite powder after washing and drying; S3: Mixing resin, solvent and additives, heating and dissolving, and obtaining an organic carrier after filtration; S4: Adding the nickel-based alloy core@Ag composite powder obtained in S2, auxiliary silver powder, glass powder and interface adaptant to the organic carrier obtained in S3, and stirring and mixing evenly; S5: Rolling the premixed paste in S4 with three rollers, controlling the rolling pressure to increase step by step until the paste fineness is ≤10μm; S6: Filtering the paste after rolling in S5, and then performing vacuum degassing to obtain the finished conductive paste.

[0012] Preferably, in S1, the nickel salt is selected from one or more of nickel sulfate, nickel chloride, nickel nitrate, and nickel acetate; the alloying element source compound is determined according to the selected alloying element, and when the alloying element is phosphorus, it is selected from one or more of sodium hypophosphite, sodium phosphite, and hypophosphite; when the alloying element is boron, it is selected from one or more of sodium borohydride, dimethylamine borane, and boric acid; when the alloying element is silicon, it is selected from one or more of sodium silicate, aminosilane, and tetraethyl orthosilicate.

[0013] Preferably, in S1, the reducing solution is prepared by dissolving a reducing agent in an alkaline solution. The reducing agent is selected from one or more of hydrazine hydrate, sodium hypophosphite, sodium borohydride, and ascorbic acid. The alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 2-6 mol / L and a pH value of 11-13.

[0014] Preferably, in S2, the reducing agent is selected from one or more of glucose, ascorbic acid, and formaldehyde.

[0015] Preferably, in S5, the pressure of the three-roll rolling is set to 5MPa, 10MPa, and 15MPa in sequence, and the rolling is performed 1-2 times under each pressure, for a total of 3-5 times.

[0016] The technical effects and advantages of this invention are as follows:

[0017] In this invention, specific alloying elements are used to modify the nickel core. By utilizing the surface segregation or lattice regulation of P, B, and Si in the nickel-based alloy, the interfacial energy between silver and nickel is reduced, thereby suppressing the dewetting behavior of the silver shell during high-temperature sintering. Simultaneously, an organometallic precursor is introduced into the slurry system as an interfacial adaptor, which decomposes in situ in the sintering temperature range to release highly active metal atoms, dynamically repairing any local micro-defects that may exist in the silver shell. Through the synergistic effect of the alloy core's preventive function and the adaptor's repair function, the silver shell maintains a high degree of integrity during high-temperature sintering. Based on the above technical solution, this invention effectively avoids the problem of increased contact resistance caused by nickel core exposure and oxidation, improves the interfacial contact performance and conductivity of the electrode, thereby improving the photoelectric conversion efficiency of the photovoltaic cell and giving the electrode excellent long-term reliability. Attached Figure Description

[0018] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts:

[0019] Figure 1 This is a scanning electron microscope image of the electrode cross-section after sintering of the conductive paste prepared in Example 1 of the present invention;

[0020] Figure 2 This is a scanning electron microscope image of the electrode cross-section after sintering of the conductive paste prepared in Example 2 of the present invention;

[0021] Figure 3 This is a scanning electron microscope image of the electrode cross-section after sintering of the conductive paste prepared in Example 3 of the present invention;

[0022] Figure 4 This is a comparison chart of the battery efficiency degradation curves during the high temperature and high humidity aging process of the present invention.

[0023] Figure 5 This is a comparison chart of the rate of change of contact resistivity during the high temperature and high humidity aging process of the present invention.

[0024] Figure 6 This is a comparison chart of the battery efficiency decay curves during the photo-induced degradation test of this invention. Detailed Implementation

[0025] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention.

[0026] This invention provides a silver-coated nickel conductive paste for photovoltaic cells, which, by weight, comprises: 65-85 parts of nickel-based alloy core@Ag composite powder, 15-30 parts of auxiliary silver powder, 2-4 parts of glass powder, 25-35 parts of organic carrier, and 0.2-0.8 parts of interface adaptant.

[0027] The core layer of the nickel-based alloy core@Ag composite powder is a nickel-based alloy, and the alloying element in the nickel-based alloy is selected from one or more of phosphorus (P), boron (B), and silicon (Si); the interface adaptant is selected from organosilver compounds or organonickel compounds.

[0028] When elements such as P, B, and Si are introduced into the nickel core to form an alloy, the surface properties of the alloy core will change.

[0029] Taking P as an example, P tends to agglomerate on the surface of the alloy core during the alloy preparation process, forming a surface state with metal-nonmetal covalent characteristics. The electronic interaction between this surface state and metallic silver is enhanced, which significantly reduces the interface energy of silver-nickel-based alloys compared to the interface energy of silver-pure nickel. The reduction in interface energy means that the thermodynamic driving force for the silver shell to shrink and spheroidize at high temperatures is effectively weakened.

[0030] The mechanism of action of B differs from that of P, but it is equally effective. Due to the extremely small atomic radius of B, it tends to agglomerate at grain boundaries and free surfaces in nickel-based alloys, which can increase the energy barrier for Ag atoms to diffuse and migrate along the surface of the nickel core. Even if there are local thermodynamic fluctuations, it is difficult for large-scale atomic rearrangement to occur in the silver shell.

[0031] The mechanism of Si's role focuses on optimizing lattice matching. After entering the nickel lattice, Si exists in the form of substitutional solid solution, changing the lattice constant of the nickel-based alloy and reducing the lattice mismatch between it and the silver shell, thereby reducing the impact of interfacial thermal stress on the stability of the silver shell.

[0032] Since the above three alloying elements differ in their existence form, segregation behavior, and mechanism of action in nickel-based alloys, their content range needs to be limited according to their respective characteristics to ensure that the basic properties of the alloy core are not impaired while suppressing dewetting.

[0033] When the alloying element is P, its atomic percentage in the nickel-based alloy core needs to be controlled between 0.5% and 5.0%. If the P content is less than 0.5%, its segregation on the surface is insufficient to form an effective interface control layer, and the reduction in interface energy is limited. If the P content is higher than 5.0%, brittle phases such as Ni3P are easily generated, which leads to an increase in the intrinsic resistivity of the alloy core.

[0034] When the alloying element is B, its atomic percentage in the nickel-based alloy core needs to be controlled between 0.1% and 2.0%. B is an extremely efficient interface modifier. An addition of 0.1% can form effective segregation at grain boundaries and surfaces. If the B content is higher than 2.0%, excessive segregation may lead to grain boundary embrittlement, and the risk of B volatilization during high-temperature sintering increases, which is not conducive to process stability.

[0035] When the alloying element is Si, its atomic percentage in the nickel-based alloy core needs to be controlled between 1.0% and 4.0%. When the Si content is less than 1.0%, its effect on adjusting the nickel lattice is weak, and the improvement of lattice mismatch is not obvious. When the Si content is higher than 4.0%, it is easy to form high-resistivity nickel silicides, such as NiSi and Ni2Si, which degrades the conductivity of the alloy core.

[0036] A silver shell is coated on the surface of the nickel-based alloy core, and its thickness is controlled to be 50-200 nm. The selection of the silver shell thickness needs to take into account both the integrity of the coating and the control of the amount of silver used. If the silver shell is too thin, it is difficult to form a continuous and dense coating layer on the surface of the nickel core, and pinholes or local defects are likely to occur, causing the nickel core to be exposed to oxidation during sintering. If the silver shell is too thick, the amount of silver used will increase significantly, which is contrary to the original intention of this invention to reduce the silver content.

[0037] The auxiliary silver powder, selected from spherical or flake silver powder, serves as a supplementary component to the conductive network. The particle size D50 is controlled to be 0.5-2.0 μm. It is intended to fill the gaps between the nickel-based alloy core@Ag composite powder particles, forming a continuous and dense conductive path and reducing the bulk resistivity of the gate line.

[0038] Glass powder, as an inorganic binder phase, is selected from one or more of Pb-Te-O, Bi-B-Si-Zn-O, or Te-VO glass powders. Its softening point is controlled at 450-600℃. During the high-temperature sintering process, the glass powder melts and flows to the surface of the silicon wafer. By etching the SiNx passivation layer, the conductive phase forms a reliable ohmic contact with the silicon substrate, while the electrode grid lines are firmly adhered to the surface of the silicon wafer.

[0039] The organic carrier consists of three parts: resin, solvent, and additives. The resin is selected from one or more of ethyl cellulose, acrylic resin, and polyvinyl butyral, accounting for 5-15% of the organic carrier. Its main function is to give the paste appropriate viscosity and thixotropy, so that the printed grid lines maintain good shape stability and completely volatilize and decompose during the subsequent drying process, leaving no carbon residue.

[0040] The solvent is selected from one or more of terpineol, butyl carbitol, and butyl carbitol acetate, accounting for 80-90% of the organic carrier. Its main function is to dissolve the resin and adjust the overall viscosity and evaporation rate of the slurry.

[0041] The additives include thixotropic agents and dispersants, which together account for 1-5% of the organic carrier. The thixotropic agents keep the slurry at a high viscosity when it is standing to prevent sedimentation, and reduce the viscosity under the printing shearing action to facilitate the passage of the screen. The dispersants ensure that the powder components are evenly dispersed in the organic carrier and avoid agglomeration.

[0042] The interface adaptor is selected from one or more organosilver compounds or organonickel compounds;

[0043] The organic silver compound is selected from one or more of silver citrate, silver tartrate, silver stearate, and silver oxalate;

[0044] The organonitrile is selected from one or more of nickel acetylacetonate, nickel formate, and nickel oxalate;

[0045] The interface adaptor undergoes thermal decomposition in the mid-temperature zone of the sintering process, releasing atomic metals with high surface activity. These newly formed metal atoms preferentially migrate to weak areas, microcracks, or pinhole defects on the silver shell surface under the drive of interface energy, and achieve in-situ repair of the silver shell through deposition filling.

[0046] This invention also provides a method for preparing a silver-coated nickel conductive paste for photovoltaic cells, comprising the following steps:

[0047] S1: Dissolve nickel salt and alloy element source compound in deionized water to prepare a mixed salt solution with a total metal ion concentration of 0.5-2.0 mol / L. Dissolve reducing agent in alkaline solution to prepare reducing solution. Add reducing solution dropwise to mixed salt solution under stirring. Control reaction temperature at 60-90℃ and reaction time at 1-3h to allow nickel ions and alloy elements to co-reduction and deposit, forming nickel-based alloy core precipitate. After the reaction is complete, filter. Wash the precipitate with deionized water 3-5 times and then vacuum dry at 50-80℃ for 4-8h to obtain nickel-based alloy core powder.

[0048] S2: Dissolve silver nitrate in deionized water to prepare a solution with a silver ion concentration of 0.1-0.5 mol / L. Slowly add ammonia water under stirring to obtain a silver ammonia solution. Disperse the nickel-based alloy core powder in deionized water and ultrasonically disperse for 10-30 min to prepare a suspension. Under stirring, slowly add the silver ammonia solution to the suspension while simultaneously adding a reducing agent solution. Control the reaction temperature at 40-60℃, the pH value at 10-12, and the reaction time at 45-70 min. After the reaction is complete, filter the product and wash it 3-5 times with deionized water. Then, vacuum dry it at 50-70℃ for 4-6 h to obtain the nickel-based alloy core@Ag composite powder.

[0049] S3: Mix the solvent, resin and additives, heat and stir in a water bath at 50-80℃ for 1-2 hours until the resin is completely dissolved and the solution is uniform and transparent. Then filter with a 200-300 mesh stainless steel mesh to obtain the organic carrier.

[0050] S4: Add the nickel-based alloy core@Ag composite powder, auxiliary silver powder, glass powder, and interface adaptant to the organic carrier, place it in a planetary mixer, and stir at a speed of 500-1000 rpm for 30-60 minutes to make the components initially mixed evenly.

[0051] S5: Transfer the premixed slurry to a three-roll mill for rolling. The rolling pressure is gradually increased, set to 5MPa, 10MPa and 15MPa in sequence. Roll 1-2 times under each pressure, for a total of 3-5 times. After each rolling, use a scraper fineness gauge to check the fineness of the slurry until the fineness is ≤10μm.

[0052] S6: Filter the rolled slurry with a 300-400 mesh stainless steel mesh to remove large particles or impurities. Then place the filtered slurry in a vacuum degassing machine and degas it at -0.09MPa for 30-60 minutes until the slurry surface is smooth and free of bubbles. The finished slurry is then obtained.

[0053] It should be noted that in step S1, the nickel salt is selected from one or more of nickel sulfate, nickel chloride, nickel nitrate, and nickel acetate;

[0054] The source compound of the alloying element is determined according to the selected element. When the alloying element is P, it is selected from one or more of sodium hypophosphite, sodium phosphite, and hypophosphite. When the alloying element is B, it is selected from one or more of sodium borohydride, dimethylamine borane, and boric acid. When the alloying element is Si, it is selected from one or more of sodium silicate, aminosilane, and tetraethyl orthosilicate.

[0055] The reducing agent is selected from one or more of hydrazine hydrate, sodium hypophosphite, sodium borohydride, and ascorbic acid. The alkaline solution is a sodium hydroxide solution or potassium hydroxide solution with a concentration of 2-6 mol / L. The pH value of the reducing solution is controlled at 11-13.

[0056] In step S2, the reducing agent is selected from one or more of glucose, ascorbic acid, and formaldehyde, and is prepared into a solution of 0.1-0.5 mol / L.

[0057] The present invention also provides an application of silver-coated nickel conductive paste for photovoltaic cells in the preparation of photovoltaic cell electrodes. Through three process stages of screen printing, drying and sintering, the paste is transformed into electrode grid lines with excellent performance.

[0058] Conductive paste is printed onto the front or back of crystalline silicon solar cells using a screen printing machine. The screen mesh count is selected from 250 to 400 mesh depending on the required grid line resolution. For the front fine grid electrode, a 330-400 mesh screen is usually used, while for the back electrode, a 250-330 mesh screen can be used. The printing wet weight is controlled at 0.10-0.25g / piece. The uniformity of grid line width, height, and shape is ensured by adjusting the printing parameters.

[0059] The printed solar cells are placed in an infrared drying tunnel and dried at 150-250℃ for 2-5 minutes. The purpose is to allow the solvent in the organic carrier to fully evaporate, the grid lines to be initially shaped and to obtain a certain dry film strength, so as to prepare for the subsequent sintering process.

[0060] The dried battery cells are then fed into a chain sintering furnace for segmented heat treatment: in the low-temperature zone, the temperature is increased from room temperature to 280°C at a rate of 20-40°C / s; in the medium-temperature repair zone, the temperature is increased from 280°C to 480°C at a rate of 5-15°C / s, with a 5-15 minute holding period in this zone; in the high-temperature sintering zone, the temperature is increased from 480°C to the peak temperature at a rate of 30-50°C / s, with the peak temperature determined according to the battery type, typically 500-650°C for PERC batteries and 650-750°C for TOPCon batteries; and in the cooling zone, the temperature is naturally or forcibly cooled to room temperature. Throughout the sintering process, especially in the medium-temperature repair zone and the high-temperature sintering zone, a nitrogen mixture containing 3%-8% hydrogen is continuously introduced as a protective atmosphere, with a flow rate controlled at 5-20 L / min.

[0061] The aforementioned insulation section is designed to provide time for the decomposition, migration, and repair of the interface adaptant. The organometallic precursor undergoes thermal decomposition within this temperature range, releasing highly active atomic metals, which then migrate to the micro-defects in the silver shell under thermal drive and deposit to fill them.

[0062] To enable those skilled in the art to better understand and implement the present invention, specific embodiments are provided below. These embodiments are only used to illustrate the technical solutions of the present invention and do not constitute any limitation on the scope of protection of the present invention. Any adjustments or modifications made by those skilled in the art without departing from the concept of the present invention shall fall within the scope of protection of the present invention.

[0063] Example 1: This example provides a silver-coated nickel conductive paste for photovoltaic cells, and its preparation steps are as follows:

[0064] S1: Nickel sulfate (NiSO4·6H2O) and sodium hypophosphite (NaH2PO2·H2O) were dissolved in deionized water at a Ni:P molar ratio of 97:3 to prepare a mixed salt solution with a total metal ion concentration of 1.0 mol / L. Sodium hydroxide (NaOH) was dissolved in water to prepare a 4 mol / L alkaline solution. Sodium hypophosphite was added as an auxiliary reducing agent to prepare a reducing solution. The reducing solution was added dropwise to the mixed salt solution under stirring. The reaction temperature was controlled at 80℃, the pH value at 12, and the reaction time at 2 h. After the reaction was completed, the mixture was filtered. The precipitate was washed three times with deionized water and dried under vacuum at 60℃ for 6 h to obtain Ni-P alloy core powder with a P atom percentage of 3.0%.

[0065] S2: Dissolve silver nitrate in deionized water to prepare a silver ammonia solution with a silver ion concentration of 0.2 mol / L. Disperse Ni-P alloy core powder in deionized water and sonicate for 20 min to prepare a suspension. Slowly add the silver ammonia solution to the suspension while stirring, and simultaneously add 0.2 mol / L glucose solution as a reducing agent. Control the reaction temperature at 50℃, the pH value at 11, and the reaction time at 60 min. After the reaction is completed, filter and wash, and vacuum dry at 60℃ for 4 h to obtain Ni-P alloy core@Ag composite powder.

[0066] S3: Weigh 80 parts terpineol, 10 parts ethyl cellulose, 8 parts butyl carbitol, and 2 parts hydrogenated castor oil, mix them, and place them in a 70°C water bath with stirring to dissolve for 2 hours. Filter the mixture through a 300-mesh stainless steel screen to obtain the organic carrier.

[0067] S4: Weigh 75 parts of Ni-P alloy core@Ag composite powder, 20 parts of auxiliary silver powder, 3 parts of Bi-B-Si-Zn-O glass powder, and 0.5 parts of silver citrate. Add 30 parts of organic carrier obtained in step S3 and place them in a planetary mixer to premix at 800 r / min for 45 min.

[0068] S5: Transfer the premixed slurry to a three-roll mill and roll it once each at pressures of 5MPa, 10MPa and 15MPa, for a total of 4 times, until the slurry fineness is ≤10μm.

[0069] S6: Filter the rolled slurry with a 400-mesh stainless steel mesh, and then place it in a vacuum degassing machine to degas for 45 minutes at a vacuum degree of -0.09MPa to obtain the finished conductive slurry.

[0070] Example 2: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between the preparation method and that of Example 1 is that sodium hypophosphite in S1 is replaced with sodium borohydride (NaBH4) as the boron source, and the percentage of B atoms in the Ni-B alloy core is 1.0% by adjusting the feeding ratio.

[0071] Example 3: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between the preparation method and that of Example 1 is that sodium hypophosphite in S1 is replaced with sodium silicate (Na2SiO3) as the silicon source, and the percentage of Si atoms in the Ni-Si alloy core is 2.5% by adjusting the feeding ratio.

[0072] Example 4: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the ratio of Ni to P in S1 is adjusted so that the percentage of P atoms in the obtained Ni-P alloy core is 0.5%.

[0073] Example 5: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the ratio of Ni to P in S1 is adjusted so that the percentage of P atoms in the obtained Ni-P alloy core is 5.0%.

[0074] Example 6: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 2 is that the ratio of Ni to B in S1 is adjusted so that the percentage of B atoms in the resulting Ni-B alloy core is 0.1%.

[0075] Example 7: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 2 is that the ratio of Ni to B in S1 is adjusted so that the percentage of B atoms in the resulting Ni-B alloy core is 2.0%.

[0076] Example 8: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 3 is that the ratio of Ni to Si in S1 is adjusted so that the percentage of Si atoms in the obtained Ni-Si alloy core is 1.0%.

[0077] Example 9: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 3 is that the ratio of Ni to Si in S1 is adjusted so that the percentage of Si atoms in the obtained Ni-Si alloy core is 4.0%.

[0078] Example 10: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the amount of silver citrate in S4 is adjusted to 0.2 parts.

[0079] Example 11: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the amount of silver citrate in S4 is adjusted to 0.8 parts.

[0080] Example 12: This example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the interface adaptant in S4 is replaced by nickel acetylacetone instead of silver citrate, and the amount is 0.5 parts.

[0081] Comparative Example 1: This comparative example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the Ni-P alloy core prepared in S1 is replaced with a pure nickel core, that is, no alloy element source is added in S1.

[0082] Comparative Example 2: This comparative example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the ratio of Ni to P in S1 is adjusted so that the percentage of P atoms in the obtained Ni-P alloy core is 9.0%.

[0083] Comparative Example 3: This comparative example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that silver citrate is not added in S4.

[0084] Comparative Example 4: This comparative example provides a silver-coated nickel conductive paste for photovoltaic cells. The difference between its preparation method and that of Example 1 is that the Ni-P alloy core@Ag composite powder prepared in S1 and S2 is replaced with a physical mixture of pure nickel powder and pure silver powder. The mass ratio of nickel powder to silver powder is the same as the nickel to silver ratio of the Ni-P alloy core@Ag composite powder in Example 1.

[0085] To verify the effectiveness of the technical solution of the present invention, the slurries prepared in Examples 1-12 and Comparative Examples 1-4 were used to prepare photovoltaic cell electrodes, and the obtained slurries and electrode samples were subjected to systematic performance tests. For ease of description, the samples prepared in Examples 1-12 are referred to as A1-A12, and the samples prepared in Comparative Examples 1-4 are referred to as D1-D4.

[0086] The test results are illustrated below through specific experimental examples. It should be noted that these experimental examples are only used to illustrate the technical effects of the present invention and are not intended to limit the scope of protection of the present invention.

[0087] Experimental Example 1: Samples A1, A2, and A3 were selected as the observation objects in this experiment. The aim was to directly observe the influence of different alloying elements on the integrity of the silver shell in silver-coated nickel particles using a scanning electron microscope, and to verify the effectiveness of the present invention in using a nickel-based alloy core to suppress the dewetting of the silver shell.

[0088] After sintering, the electrode sample was cut into small pieces of approximately 5mm × 5mm along the direction perpendicular to the grid lines. The pieces were then fixed onto the SEM sample stage with conductive adhesive, so that the cross-section of the grid lines faced upwards. The cross-section was polished using an ion beam polisher to remove the damaged layer caused by cutting and expose the clear internal structure. The polished sample was then sputter-coated with gold to enhance its conductivity.

[0089] Scanning electron microscopy was used for observation. An accelerating voltage of 15 kV was set, and different fields of view were randomly selected for each sample. The focus was on observing the continuity and integrity of the silver shell, the presence of shrinkage and spheroidization, the absence of cracks and spalling, and the interfacial bonding state with the nickel core. The results are shown in [Figure number missing]. Figure 1-3 As shown.

[0090] SEM observation results of sample A1 are as follows Figure 1 As shown, the silver-coated nickel particles in sample A1 are evenly distributed, with clear particle outlines. The silver shell layer continuously and completely covers the surface of the nickel core. The interface between the silver shell and the nickel core is clear and dense, with no obvious gaps or separation phenomena observed.

[0091] SEM observation results of sample A2 are as follows Figure 2 As shown, the silver shell in sample A2 has a smooth and continuous surface, a uniform and dense coating, and a tight interface with the nickel core, with no voids or oxide layers at the interface.

[0092] SEM observation results of sample A3 are as follows Figure 3 As shown, the silver-coated nickel particles in sample A3 have a good overall morphology, and the silver shell layer is basically continuous and intact. Slight local unevenness of the silver shell was observed in some particles, and the silver shell of a few particles was slightly thinner.

[0093] Experimental Example 2: In this experimental example, A1-A12 and D1-D4 were selected as experimental objects. By testing contact resistivity and volume resistivity, the effects of different alloying elements, alloy content, interface adaptants, and core-shell structures on the contact performance and conductivity of the electrode interface were evaluated.

[0094] On the solar cell with completed electrode fabrication, a 10mm wide test strip was cut from the complete grid area, containing five parallel grid lines with spacings of 0.5, 1.0, 1.5, 2.0, and 2.5mm. A four-probe tester was used to measure the total resistance between adjacent grid lines. Each grid line was measured three times and the average value was taken. A linear fit was performed with the grid line spacing as the x-axis and the total resistance as the y-axis. Half of the intercept of the fitted line was taken as the contact resistance Rc, and the contact resistivity ρc was the product of the contact resistance and the grid line length. Five solar cells were tested for each sample, and three test strips were selected at different positions on each cell, and the arithmetic mean was taken.

[0095] The slurry to be tested was printed in a 2cm × 2cm pattern on a clean glass substrate. After undergoing the same drying and sintering process as the electrode preparation, the sheet resistance Rs of the sintered film was measured using a four-probe tester, and the film thickness t was measured using a step tester. The volume resistivity ρv is the product of Rs and t. Three parallel samples were prepared for each sample, and the arithmetic mean was taken.

[0096] The test results of contact resistivity and volume resistivity of each sample are summarized in Table 1.

[0097] Table 1. Test results of contact resistivity and volume resistivity for each sample: ;

[0098] According to the data in Table 1, the contact resistivity of the samples using alloy cores were 0.82, 0.71, and 1.18, respectively, all lower than that of the pure nickel core samples. The pure nickel cores showed severe silver shell cracking, and the exposed nickel oxidized to form a NiO insulating layer, blocking the conductive path. In contrast, the alloy cores effectively suppressed dewetting and avoided NiO formation, thus achieving low contact resistance. The differences in the effects of the three elements are consistent with their mechanisms of action. B had the best effect due to strong grain boundary segregation and inhibition of Ag diffusion; P was second best by reducing the interface energy through surface states; and Si was slightly less effective, mainly by relieving stress through lattice matching.

[0099] Comparing samples A1, A4, A5, and D2, it can be seen that when the P content is 0.5%, the contact resistivity is 1.46, which is better than D1 but worse than A1. When the P content is 5.0%, the contact resistivity is 1.12, which is good. When the P content increases to 9.0%, the contact resistivity rises back to 2.46, and the volume resistivity also rises to 6.9. This indicates that the excessively high P content leads to an increase in the resistance of the alloy core itself, which degrades the performance.

[0100] Comparing samples A1, A10, A11 and D3, the contact resistivity of D3 is higher than that of the sample with added adaptant, indicating that even with an alloy core, the silver shell may still have a small number of local micro-defects. Although these micro-defects do not cause the silver shell to crack, they will still locally increase the current transport barrier. The adaptant decomposes in the sintering temperature zone, releasing active metal atoms, which migrate to the defect site for in-situ repair, thereby reducing the contact resistance.

[0101] Experiment Example 3: In this experiment, A1-A12 and D1-D4 were selected as experimental subjects to determine the photoelectric conversion efficiency and silver content of the battery.

[0102] The prepared solar cell was placed in a solar cell testing system for IV characteristic testing. The testing system included an AAA-grade solar simulator, a Keithley 2400 digital source meter, and a standard silicon reference cell. Before testing, the light intensity was calibrated using the standard silicon reference cell. During testing, the solar cell was placed on a temperature-controlled sample stage, and the temperature was controlled at 25℃±1℃ using a circulating water bath. A four-wire connection was used to reduce the influence of contact resistance. The voltage scan range was -0.1V to 0.7V, with a step size of 0.01V, and the scan direction was from short circuit to open circuit.

[0103] Ten solar cells were tested for each sample, and each solar cell was tested three times. The average value was taken as the open-circuit voltage Voc, short-circuit current Isc, fill factor FF, and photoelectric conversion efficiency Eff for that sample.

[0104] The conversion efficiency is calculated using the following formula: ;

[0105] In the formula, Pin is the incident light power, and Area is the cell area.

[0106] The silver content in the sintered grid lines was determined by titration with a standard thiocyanate solution. Approximately 0.5 g of the sintered grid line sample was weighed and placed in a 250 mL Erlenmeyer flask. 20 mL of nitric acid was added, and the mixture was heated until the sample dissolved. Heating was continued to remove nitrogen oxides. After cooling, the solution was diluted to a 100 mL volumetric flask. 25 mL of the solution was transferred to a 150 mL beaker, and 5 mL of nitric acid was added. The beaker was placed on an automatic potentiometric titrator with a composite silver electrode as the indicator electrode. Titration was performed with a 0.1 mol / L potassium thiocyanate standard titration solution at a titration rate of 0.5 mL / min, and the titration volume V was recorded.

[0107] Silver content is calculated using the following formula: ;

[0108] Where c is the concentration of potassium thiocyanate standard titration solution, V is the titration volume consumed by the sample, V0 is the titration volume consumed by the blank, and m is the mass of the sample. Each sample is measured in parallel 3 times, and the arithmetic mean is taken.

[0109] The test results of photoelectric conversion efficiency and silver content of each sample battery are summarized in Table 2.

[0110] Table 2. Test results of battery performance and silver content for each sample: ;

[0111] According to the data in Table 2, the conversion efficiencies of the samples using alloy cores were 23.8%, 23.9%, and 23.5%, respectively, all of which were higher than those of the pure nickel core samples.

[0112] The silver content of samples A1-A12 in this invention is concentrated between 68.1% and 68.7%, while the silver content of pure nickel core samples is 69.2%, and the silver content of physically mixed samples is as high as 70.5%. This invention can obtain better electrode performance with lower silver content by inhibiting dewetting and ensuring that the silver shell can play an effective role.

[0113] Experimental Example 4: This experimental example selects samples A1, A2, A3, A10, A11, A12 and D1, D3, D4 as experimental objects. The performance stability of the electrode prepared by the technical solution of the present invention under long-term use conditions is evaluated through high temperature and high humidity aging, thermal cycling test and photo-induced degradation test.

[0114] The prepared electrode cells were placed in a constant temperature and humidity chamber, with the temperature set at 85℃±2℃ and the relative humidity at 85%±5%, and aged continuously for 1000 hours. Samples were removed after 100 hours of aging, and after cooling, the battery efficiency degradation and contact resistivity changes were tested according to the methods in Experiments 2 and 3. Five cells were tested for each group of samples, and the arithmetic mean was taken. The results are shown in [Figure 1]. Figure 4 , Figure 5 As shown in Table 3.

[0115] Table 3. Test results of battery efficiency and contact resistivity changes after high temperature and high humidity aging: ;

[0116] Temperature cycling tests were conducted according to JESD22-A104F standard. Each cycle consisted of cooling to -40℃±2℃ at a rate not exceeding 10℃ / min and holding for 30 min; then heating to 85℃±2℃ within 5 min and holding for 30 min; and finally cooling to -40℃ to complete one cycle. A total of 200 cycles were performed. The battery efficiency degradation and contact resistivity changes were measured before and after the test. Five battery cells were tested for each sample group, and the arithmetic mean was taken. The results are shown in Table 4.

[0117] Table 4. Test results of battery efficiency and contact resistivity changes after temperature cycling: ;

[0118] The solar cells were placed in a continuous illumination testing system and continuously illuminated for 1000 hours under standard test conditions. The sample stage temperature was controlled at 25℃±2℃. The cell efficiency degradation was tested at 100-hour intervals during this period. Five solar cells were tested for each sample group, and the arithmetic mean was taken. The results are shown in [Figure number missing]. Figure 6 As shown in Table 5.

[0119] Table 5. Battery efficiency degradation test results after illumination test: ;

[0120] Combination Figure 4-6 The data in Table 3-5 show that the efficiency decay rates of samples A1, A2, and A3 are all better than those of D1. Due to severe dewetting of the silver shell during sintering, a large number of nickel cores were exposed and oxidized to form NiO. During aging, water vapor seeped in along the cracks, thermal stress caused microcracks to propagate, and photogenerated carriers recombine at defects, accelerating the performance decay. In contrast, the sample of this invention has an intact silver shell, which effectively protects the nickel cores. The channels for further oxidation of nickel during aging are blocked, thus slowing down the decay.

[0121] Comparing A1, A10, A11, A12 and D3, the efficiency decay rate of D3 without the adapter was higher than that of the sample with the adapter. The decay rate of A10 was slightly higher than that of A1 and A11, indicating that the amount of adapter needed to reach a certain threshold to form an effective repair layer. The decay rate of A12 was comparable to that of A10, but slightly higher than that of the organic silver adapter. The micro-defect areas repaired by the adapter during the sintering process form a dense metal filling layer, which becomes a strengthening point during the aging process, preventing water vapor penetration and crack propagation.

[0122] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A silver over nickel conductive paste for photovoltaic cells, characterized in that, By weight, it includes: 65-85 parts nickel-based alloy core@Ag composite powder, 15-30 parts auxiliary silver powder, 2-4 parts glass powder, 25-35 parts organic carrier, and 0.2-0.8 parts interface adaptant; The core layer of the nickel-based alloy core@Ag composite powder is a nickel-based alloy, and the alloying elements in the nickel-based alloy are selected from one or more of phosphorus, boron, and silicon. The interface adaptant is selected from organosilver compounds or organonickel compounds.

2. The silver-nickel conductive paste for photovoltaic cells according to claim 1, characterized in that: The atomic percentage of the alloying elements in the nickel-based alloy is as follows: when the alloying element is phosphorus, its content is 0.5%-5.0%; when the alloying element is boron, its content is 0.1%-2.0%; and when the alloying element is silicon, its content is 1.0%-4.0%.

3. The silver-coated nickel conductive paste for photovoltaic cells according to claim 1, characterized in that: The glass powder is selected from one or more of Pb-Te-O, Bi-B-Si-Zn-O, or Te-VO glass powders, and its softening point is 450-600℃.

4. The silver-coated nickel conductive paste for photovoltaic cells according to claim 1, characterized in that: The organic carrier includes resin, solvent and additives, wherein the resin is selected from one or more of ethyl cellulose, acrylic resin and polyvinyl butyral, accounting for 5-15% of the organic carrier; the solvent is selected from one or more of terpineol, butyl carbitol and butyl carbitol acetate, accounting for 80-90% of the organic carrier; and the additives include thixotropic agents and dispersants, accounting for 1-5% of the organic carrier.

5. The silver-coated nickel conductive paste for photovoltaic cells according to claim 1, characterized in that: The organosilver compound is selected from one or more of silver citrate, silver tartrate, silver stearate, and silver oxalate, and the organonickel compound is selected from one or more of nickel acetylacetonate, nickel formate, and nickel oxalate.

6. A method for preparing a silver-coated nickel conductive paste for photovoltaic cells as described in any one of claims 1-5, characterized in that, Includes the following steps: S1: Dissolve nickel salt and alloy element source compound in water, add reducing solution to carry out reduction reaction, obtain nickel-based alloy core precipitate, and obtain nickel-based alloy core powder after washing and drying; S2: The nickel-based alloy core powder obtained in S1 is dispersed in water, silver ammonia solution and reducing agent are added to carry out a displacement silver plating reaction, and after washing and drying, nickel-based alloy core@Ag composite powder is obtained; S3: Mix the resin, solvent and additives, heat to dissolve, and filter to obtain the organic carrier; S4: Add the nickel-based alloy core@Ag composite powder, auxiliary silver powder, glass powder, and interface adaptant obtained in S2 to the organic carrier obtained in S3, and stir to mix evenly; S5: The slurry premixed in S4 is subjected to three-roll rolling, and the rolling pressure is gradually increased until the slurry fineness is ≤10μm; S6: Filter the slurry after S5 rolling and then perform vacuum degassing to obtain the finished conductive slurry.

7. The method for preparing a silver-coated nickel conductive paste for photovoltaic cells according to claim 6, characterized in that: In S1, the nickel salt is selected from one or more of nickel sulfate, nickel chloride, nickel nitrate, and nickel acetate. The alloying element source compound is determined according to the selected alloying element. When the alloying element is phosphorus, it is selected from one or more of sodium hypophosphite, sodium phosphite, and hypophosphite. When the alloying element is boron, it is selected from one or more of sodium borohydride, dimethylamine borane, and boric acid. When the alloying element is silicon, it is selected from one or more of sodium silicate, aminosilane, and tetraethyl orthosilicate.

8. The method for preparing a silver-coated nickel conductive paste for photovoltaic cells according to claim 6, characterized in that: In S1, the reducing solution is prepared by dissolving a reducing agent in an alkaline solution. The reducing agent is selected from one or more of hydrazine hydrate, sodium hypophosphite, sodium borohydride, and ascorbic acid. The alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 2-6 mol / L. The pH value of the reducing solution is 11-13.

9. The method for preparing a silver-coated nickel conductive paste for photovoltaic cells according to claim 6, characterized in that: In S2, the reducing agent is selected from one or more of glucose, ascorbic acid, and formaldehyde.

10. A method for preparing a silver-coated nickel conductive paste for photovoltaic cells according to claim 6, characterized in that: In S5, the pressure of the three-roll rolling is set to 5MPa, 10MPa and 15MPa in sequence, and the rolling is performed 1-2 times under each pressure, with a total number of rolling times of 3-5 times.

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