Direct copper electroplating process without silver paste bottoming

Abstract

The invention relates to the technical field of electroplating, in particular to a direct copper electroplating process without silver paste bottoming. Comprising the following steps: pretreating a crystalline silicon substrate to remove an oxide layer and activating; the method comprises the following steps: covalently grafting a directional modifier containing benzothiazole sulfydryl and a hydrolyzable silane end group on the surface of a base material in an alcohol / water-containing grafting solution, and carrying out thermocuring to form a localized coordination interface layer; preparing an electroplating solution containing a complexing agent, copper salt, chlorine salt and free benzothiazole sulfydryl, and ensuring that the solution is stable by adopting a specific charging sequence; and carrying out pulse plating starting and constant-current growth electroplating by taking the modified base material as a cathode. Uniform and compact nucleation of copper is promoted through the molecular interface layer, silver paste bottoming is replaced, the cost is remarkably reduced, and the coating is high in binding force, low in resistance, good in damp-heat reliability and suitable for metallization of bare silicon areas such as fine grids of crystalline silicon solar cells.

Description

Technical Field

[0001] This invention relates to the field of electroplating technology, and in particular to a copper electroplating process that does not require a silver paste base coat. Background Technology

[0002] Metallization of crystalline silicon solar cells is a key step in improving photoelectric conversion efficiency. Its core lies in forming highly conductive paths on the cell surface to collect and transport photogenerated charges. Traditional processes widely use silver paste as a pretreatment layer for copper electroplating, leveraging silver's excellent conductivity and ohmic contact properties with silicon to achieve low-resistance bonding. However, the silver paste pretreatment process has several inherent drawbacks, particularly in terms of cost, interface stability, and environmental adaptability.

[0003] Silver, as a precious metal, is scarce and its price fluctuates wildly, resulting in persistently high production costs for solar cells and making it difficult to meet the photovoltaic industry's continuous cost reduction needs. Silver paste preparation involves complex processes of powder dispersion, organic carrier formulation, and printing sintering, which not only increases process complexity but also introduces the risk of organic residues, affecting interface cleanliness. More importantly, the bonding between silver paste and the silicon surface mainly relies on physical adsorption and low-temperature eutectic reaction, resulting in weak interfacial adhesion. Under thermal or mechanical stress, it is prone to peeling, limiting the long-term reliability of the electroplated copper layer.

[0004] During copper electroplating, while the silver paste underlayer provides initial conductivity, the lattice mismatch and potential difference between silver and copper easily induce galvanic corrosion. During the migration and reduction of copper ions on the silver surface, the uneven distribution of nucleation sites tends to form island-like growth patterns, resulting in insufficient coating density and grain coarsening. These microstructural defects increase contact resistance and become the starting point for current congestion, reducing the cell fill factor and maximum output power. Furthermore, glassy phase components in the silver paste may migrate to the interface, forming an insulating layer that further hinders charge transport.

[0005] Reliability under humid and hot conditions presents another significant challenge. The interface between the silver paste underlayer and silicon contains micropores, allowing moisture and oxygen to easily penetrate and trigger electrochemical corrosion, leading to a significant shift in contact resistance over time. The heterogeneous interface between silver and copper is more prone to ion migration under high temperature and humidity, forming conductive filaments or voids, accelerating performance degradation. Existing processes improve adhesion by increasing the silver layer thickness or adding alloying elements, but this often sacrifices conductivity and fails to fundamentally solve the interface instability problem.

[0006] Furthermore, the environmental pressure of silver paste undercoating processes is increasing. Silver mining and paste preparation involve the emission of harmful substances, and the treatment of silver ions in electroplating wastewater is complex, increasing the environmental burden. The industry has attempted silver-free or reduced-silver solutions, such as directly electroplating copper onto bare silicon surfaces. However, the inherent oxide layer and hydrophobicity of the bare silicon region make copper nucleation difficult, easily leading to problems such as discontinuous plating and poor adhesion. How to achieve a stable, low-resistance connection between copper and silicon without relying on silver paste has become a technological bottleneck in solar cell metallization. Summary of the Invention

[0007] In view of this, the purpose of this invention is to propose a direct copper electroplating process without silver paste as a base, so as to achieve a stable and low-resistance connection between copper and silicon without relying on silver paste.

[0008] To achieve the above objectives, the present invention provides a copper electroplating process that does not require a silver paste undercoat, comprising the following steps: (1) Substrate pretreatment: The crystalline silicon substrate with open bare silicon regions is cleaned; the intrinsic oxide layer is removed using a fluorine-containing system, and then activated using an oxidant; (2) Constructing a directional modified interface layer: In an alcohol / water-containing grafting solution, a directional modifier A containing benzothiazole-thiol sites and having hydrolyzable silane end groups is covalently grafted onto a pretreated crystalline silicon substrate and then thermally cured to obtain a crystalline silicon substrate with a localized benzothiazole-thiol site interface layer. (3) Preparation of copper plating electrolyte: Dissolve the complexing agent in water and add an alkaline source to pre-constitute the complexing agent into a salt. Add free 2-mercaptobenzothiazole and stir until the solution is clear. Then add copper salt in batches and stir until the complexation is stable. Finally, add chloride salt to obtain copper plating electrolyte. (4) Copper electroplating: using a crystalline silicon substrate with a localized benzothiazole-thiol site interface layer as the cathode, copper is electroplated in a copper electroplating electrolyte; wherein, no silver paste is formed as the underlayer during the entire process.

[0009] Preferably, the crystalline silicon substrate is a crystalline silicon solar cell, and the open bare silicon area is one or more of the following: fine grid grooves, back contact holes / hole arrays, or other open structures that expose bare silicon.

[0010] Preferably, the fluorine-containing system in step (1) is a hydrofluoric acid or buffered ammonium fluoride system; the concentration of the fluorine-containing system is 0.2-5 wt%, the treatment time is 10-120 s, and the treatment temperature is 15-35 °C.

[0011] Preferably, the oxidant in step (1) is one or more of hydrogen peroxide, ozone water, or persulfate system; the concentration of the oxidant is 0.01-1 wt%, the treatment temperature is 20-60°C, and the treatment time is 0.5-10 min.

[0012] Preferably, the directional modifier A is obtained by reacting a precursor containing a benzothiazole thiol skeleton and an amino group with a silane coupling agent, wherein the silane coupling agent is one or more of isocyanate silane, epoxy silane, or chlorosilane.

[0013] More preferably, the precursor is 6-amino-2-mercaptobenzothiazole, the silane coupling agent is 3-(triethoxysilylpropyl)isocyanate, and the directional modifier A contains a urea bond linkage structure formed by the amino group and the isocyanate.

[0014] Preferably, the preparation of the directional modifier A is carried out in a polar aprotic solvent, which is one or more of DMF, DMSO or NMP; the reaction is carried out under nitrogen or argon protection, the reaction temperature is 30-80℃, and the reaction time is 1-8h; and the polar aprotic solvent is dehydrated under reduced pressure before the reaction.

[0015] Preferably, the volume ratio or mass ratio of alcohol to water in the grafting solution in step (2) is 85 / 15 to 97 / 3, the concentration of the directional modifier A in the grafting solution is 0.1 to 1 wt%, the grafting temperature is 30 to 45°C, the grafting time is 5 to 15 min, the thermosetting temperature is 100 to 130°C, and the thermosetting time is 10 to 30 min.

[0016] Preferably, the complexing agent in step (3) is one or more of amino acids, aminocarboxylate salts or hydroxycarboxylate salts; Preferably, the amino acid in step (3) is one or more of glycine, β-alanine, alanine or histidine; the aminocarboxylate is one or more of NTA, IDA, EDTA or HEDTA; and the hydroxycarboxylate is one or more of citrate or tartrate.

[0017] Preferably, the pH of the copper plating electrolyte in step (3) is 8.5 to 10.5, and the concentration of free 2-mercaptobenzothiazole in the copper plating electrolyte is 5 to 200 mg / L.

[0018] Preferably, the concentration of the complexing agent in step (3) is 0.6 to 1.8 mol / L, and the concentration of the copper salt is 0.1 to 0.4 mol / L.

[0019] Preferably, the chloride salt in step (3) is NaCl or KCl, and the chloride salt concentration in the copper plating electrolyte is 20-120 g / L.

[0020] Preferably, the electroplating temperature in step (4) is 15 to 35°C.

[0021] Preferably, the electroplating in step (4) includes a starting pulse stage and a growth stage: the pulse current density of the starting pulse stage is 20-40 mA / cm². 2 The power-on time is 100–1000 ms, the power-off time is 300–3000 ms, and the number of cycles is 20–200. During the growth stage, constant current or weak pulse growth is used, with a current density of 8–15 mA / cm². 2 The electroplating time is 5 to 20 minutes.

[0022] Preferably, after copper electroplating in step (4), a post-treatment is further included: the electroplated crystalline silicon substrate is treated with a purification solution containing an oxidant and an organic acid; wherein the concentration of the oxidant is 0.01-0.5 wt%, the concentration of the organic acid is 0.05-2 wt%, the treatment temperature is 20-60°C, and the treatment time is 0.5-10 min; subsequently, annealing is carried out in an inert atmosphere at a temperature of 150-250°C for 1-30 min.

[0023] The beneficial effects of this invention are: This invention constructs a directionally modified interface layer with benzothiazole thiol sites, forming covalently anchored molecular-level coordination sites on the bare silicon surface, effectively enhancing the wettability and chemical activity of the silicon surface. This interface layer, through the strong coordination between benzothiazole thiol groups and copper ions, reduces the copper nucleation overpotential, promoting uniform and dense nucleation of copper atoms on the silicon surface, thereby obtaining a fine-grained, dense, and continuous copper plating layer. The result is a significantly improved uniformity of plating thickness distribution, reduced surface roughness, and avoidance of porosity and defects caused by island growth, providing a solid foundation for low contact resistance.

[0024] A neutral to weakly alkaline amino acid complex electroplating system, combined with the introduction of free benzothiazole mercapto additives, enables controlled deposition of copper ions. The complexing agent forms a stable complex with copper ions, buffering the concentration of free copper ions and inhibiting copper hydroxide precipitation; while the free additive further refines the grains and fills micropores through a dynamic adsorption-desorption mechanism. Specific mixing sequences ensure solution stability and prevent coating degradation caused by local pH fluctuations. This synergistic effect allows the electroplating process to proceed under low stress, reducing internal stress in the coating, enhancing adhesion, and thus improving interfacial mechanical strength and electrical continuity.

[0025] The application of pulsed cathode activation promotes rapid reduction and nucleation of copper ions at the directionally modified interface by applying a high instantaneous current density during the initial plating stage. The pulsed power-off period allows for ion diffusion replenishment and hydrogen release, reducing concentration polarization and bubble retention, and preventing scorching or voids. This non-equilibrium deposition mode achieves high nucleation density and, after switching to constant current during the growth stage, maintains uniform coating growth, significantly improving coverage within trenches or pores and enhancing overall conductivity uniformity.

[0026] Through synergistic optimization of each step, the overall process achieves high-quality direct copper electroplating without the need for silver paste undercoating. This reduces costs while ensuring the electrical performance and reliability of the plating, providing an efficient and environmentally friendly solution for the metallization of solar cells. Detailed Implementation

[0027] The present invention will be described in detail below with reference to the embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0028] This invention provides a process for directly electroplating copper in an open bare silicon region without the need for silver paste undercoating. The process generally includes: substrate pretreatment (decontamination / deoxidation / surface activation), constructing a directional modified interface layer with benzothiazole thiol sites (localized MBT), electroplating copper in a neutral / weakly alkaline complex system (free MBT + specific mixing sequence + optional pulsed cathode activation), post-treatment purification, and inert atmosphere annealing stabilization. The localized thiol sites and the synergistic effect of trace sulfur-containing additives in the bulk phase reduce the nucleation overpotential of copper in the bare silicon region and make nucleation more uniform, thereby obtaining a fine-grained and dense coating with more stable adhesion / contact.

[0029] (A) Substrate and Opening Area: In the process provided by this invention, the substrate can be a crystalline silicon solar cell (including fine grid grooves, back contact holes / hole arrays, or other opening structures exposing bare silicon). Preferably, the opening is a bare silicon surface that can be fully wetted by a wet process to ensure that subsequent grafting and electroplating solutions can enter and be continuously renewed. Optionally, the cell may have a passivation / dielectric layer in advance, exposing silicon only at the opening; the exposed bare silicon area should be electrically connected to an external cathode conductor, preferably with current spread provided by an emitter / conductive layer to ensure a stable current distribution during the initial plating stage.

[0030] (B) Pretreatment: The pretreatment provided by the present invention includes: organic cleaning, deionized water rinsing, removal of the intrinsic oxide layer by a fluorine-containing system, and introduction of graftable surface hydroxyl groups / thin oxide layer with a mild oxidant at a low concentration / for a short time.

[0031] In the deoxidation step provided by the present invention, the fluorine-containing system may be hydrofluoric acid or buffered ammonium fluoride system; its concentration is preferably 0.2-5 wt%, more preferably 0.5-2 wt%, the time is preferably 10-120 s, more preferably 20-60 s, and the temperature is preferably 15-35°C.

[0032] In the mild re-oxidation / activation step provided by the present invention, the oxidant may be hydrogen peroxide, ozone water or persulfate system; preferably, an aqueous hydrogen peroxide solution, with a concentration preferably 0.01-1 wt%, more preferably 0.05-0.3 wt%, a temperature preferably 20-60°C, more preferably 35-55°C, and a time preferably 0.5-10 min, more preferably 1-5 min.

[0033] In the cleaning and drying process provided by the present invention, rinsing is preferably continuous rinsing with deionized water (multi-stage rinsing is optional), and drying is preferably nitrogen blowing or rotary drying; optionally, drying at 60-120°C is added before grafting to reduce the randomness of surface adsorbed water.

[0034] (C) Construction of directional modifier A (free MBT, localized MBT interface layer that can be covalently anchored): In the construction of the interface layer provided by the present invention, it is preferable to first prepare directional modifier A containing benzothiazole-thiol sites and hydrolyzable silane end groups, and then complete the covalent anchoring and thermal curing of the silicon surface in a grafting solution containing alcohol / water.

[0035] In the synthesis of the directional modifier A provided by this invention, the precursor containing the benzothiazole thiol skeleton can be 6-amino-2-mercaptobenzothiazole or its reactive derivatives; the silane coupling end can be selected from isocyanate silanes, epoxy silanes, chlorosilanes, etc. Preferably, 3-(triethoxysilylpropyl) isocyanate is used to react with the amino group to form a urea bond.

[0036] The solvent is preferably a polar aprotic solvent, such as one or more of DMF, DMSO, and NMP; DMF is more preferred. The reaction atmosphere is preferably nitrogen / argon protection; the reaction temperature is preferably 30–80°C, more preferably 40–60°C; and the reaction time is preferably 1–8 h, more preferably 2–6 h.

[0037] The dehydration step is preferably carried out under reduced pressure to reduce the solvent water content (e.g., to a process control level below 0.1 wt%). This parameter affects the probability of isocyanate end-group side reactions (reaction with water), and thus affects the effective silane end-group content and grafting efficiency of the product.

[0038] In the grafting solution provided by this invention, the alcohol / water volume ratio (or mass ratio) is preferably 70 / 30 to 99 / 1, more preferably 85 / 15 to 97 / 3; the concentration of the directional modifier A is preferably 0.05 to 5 wt%, more preferably 0.1 to 1 wt%. The grafting temperature is preferably 20 to 60°C, more preferably 30 to 45°C; the grafting time is preferably 2 to 30 min, more preferably 5 to 15 min; subsequently, heat curing is performed, with a curing temperature preferably 80 to 150°C, more preferably 100 to 130°C, and a curing time preferably 5 to 60 min, more preferably 10 to 30 min.

[0039] (D) Electroplating copper electrolyte (neutral / weakly alkaline complexing system + trace amounts of free MBT additives + specific mixing sequence): In the electroplating solution provided by the present invention, an amino acid salt / carboxylate salt complexing system is preferably used to stabilize Cu(II) under neutral to weakly alkaline conditions, and the local instantaneous generation of Cu(OH)2 and complexation mismatch are suppressed by the mixing sequence of first pre-forming salt with complexing agent, first dissolving trace amounts of sulfur-containing additives, and then adding copper salts in batches.

[0040] In the complexing agent provided by this invention, glycine is preferred; its substitutes may be one or more of other amino acids (such as β-alanine, alanine, histidine), aminocarboxylic acids (such as NTA, IDA, EDTA, HEDTA), and hydroxycarboxylic acids (such as citrate, tartrate); as long as a stable copper complex can be formed at the target pH and electroplatable free Cu can be maintained. 2+ Activity level is sufficient.

[0041] In the electroplating solution provided by the present invention, the concentration of the complexing agent (calculated as glycine) is preferably 0.3-2.5 mol / L, more preferably 0.6-1.8 mol / L; the concentration of the copper salt (calculated as CuSO4·5H2O) is preferably 0.05-0.8 mol / L, more preferably 0.1-0.4 mol / L. The pH is preferably 7.5-11, more preferably 8.5-10.5, and the alkaline source can be selected from NaOH, KOH or an amine buffer system.

[0042] The ratio of complexing agent to copper salt and pH directly affect the complexation morphology of copper and the stability of the solution, which in turn affect the plating window (too low pH may inhibit complexation, and too high pH may lead to the risk of hydroxide formation), grain size and plating stress.

[0043] MBT is preferred among the sulfur-containing free additives provided by this invention; its dosage is preferably 1-500 mg / L, more preferably 5-200 mg / L. Sulfur-containing / nitrogen-containing leveling or refining components that can be substituted or used in combination may specifically be benzotriazole derivatives, thiazole / triazoles, thiourea, sulfur-containing polymers, etc. (limited to those that do not impair the stability and electroplating properties of the neutral complexation system of this invention).

[0044] The amount of free MBT affects the cathode nucleation overpotential and grain refinement: too low a concentration results in insufficient refinement and delayed plating; too high a concentration may lead to excessive adsorption, inhibiting deposition and causing roughness or blistering, which in turn affects contact resistance and adhesion.

[0045] In the chloride ion system provided by the present invention, it can be provided by NaCl or KCl; the chloride ion concentration is preferably 10 to 200 g / L (calculated as NaCl), more preferably 20 to 120 g / L, to improve anodic dissolution / conductivity and coating morphology window.

[0046] In the mixing sequence provided by this invention, the preferred order is as follows: deionized water, dissolution of the complexing agent, use of an alkali source to pre-salt the complexing agent, complete dissolution of free MBT until clear, addition of copper salt in batches with thorough stirring until the complexation is stable, and finally addition of chloride salt; this sequence is used to reduce localized transient high pH / high Cu. 2+ The resulting Cu(OH)2 formation and coarse grain plating.

[0047] (E) Pulse cathode activation and electroplating process (initiation-growth segmentation): In the electroplating provided by the present invention, the cathode is a battery cell treated with a directional modified interface layer, and the anode can be a copper plate (soluble anode) or an inert anode with copper ion replenishment; the electroplating temperature is preferably 15-35°C, more preferably 20-30°C; stirring / circulation preferably maintains a stable solution flow rate (e.g., magnetic stirring or circulating spray) to suppress concentration polarization and bubble retention in the trench.

[0048] In the initial plating stage provided by this invention, a pulsed current is preferably used to promote uniform nucleation: the pulsed current density is preferably 10-60 mA / cm². 2 More preferably 20–40 mA / cm 2 The preferred power-on time is 100–1000 ms, the preferred power-off time is 300–3000 ms, and the preferred number of cycles is 20–200.

[0049] Pulse parameters affect instantaneous overpotential and adsorption layer renewal, which in turn affect nucleation density and initial grain size: higher peak current density and appropriate power-off time are conducive to rapid nucleation at localized thiol sites and replenishment of ions and release of hydrogen during the power-off period, reducing the risk of charring and voids.

[0050] In the growth stage provided by this invention, constant current or weak pulse growth is preferred: the current density is preferably 5-25 mA / cm². 2 More preferably 8-15 mA / cm 2 The electroplating time is preferably 3 to 30 minutes, more preferably 5 to 20 minutes, in order to obtain the target thickness and resistance.

[0051] (F) Post-treatment purification and annealing (forming Cu-S type ultrathin interface and stabilizing contact): In the post-treatment provided by the present invention, it is preferred to use a purification solution of low concentration oxidant + complexing / organic acid for short-time treatment to remove surface residues and weakly bound particles, followed by annealing in an inert atmosphere to reduce contact resistance and stabilize the interface.

[0052] The preferred oxidant in the purification solution is hydrogen peroxide, with a concentration preferably (calculated as H2O2) of 0.01–0.5 wt%, more preferably 0.05–0.2 wt%; the preferred organic acid is citric acid (malic acid, tartaric acid, etc.), with a concentration preferably of 0.05–2 wt%, more preferably 0.1–1 wt%; the preferred temperature is 20–60 °C, and the preferred time is 0.5–10 min.

[0053] This parameter affects the ability to remove loose copper particles / organic residues, which in turn affects the interface defects and peeling risk during subsequent annealing; too strong a value will cause excessive corrosion of the copper surface, while too weak a value will result in insufficient purification.

[0054] Annealing is preferably carried out in an inert atmosphere such as nitrogen / argon, with a temperature preferably of 150-250°C, more preferably 180-220°C, and a time preferably of 1-30 min, more preferably 5-15 min.

[0055] Annealing temperature / time affects the interface rearrangement and the tendency of sulfur-containing sites to become inorganic (increasing the possibility of forming Cu-S type ultrathin interface layers), thereby improving adhesion and contact stability; at the same time, controlling the upper limit can reduce the potential risk of copper diffusion into silicon. Example

[0056] Step S1: Take one crystalline silicon solar cell to be metallized (including the fine grid lines of the open bare silicon region), first ultrasonically clean it in 1000g of anhydrous ethanol for 3min, then rinse it continuously with deionized water for 1min and dry it with nitrogen; then immerse it in 2000g of hydrofluoric acid aqueous solution (concentration 1wt%) for 30s, take it out and immediately rinse it with deionized water for 30s and dry it with nitrogen; then immerse it in 3000g of hydrogen peroxide aqueous solution (concentration 0.1wt%) at 50℃ for 2min, take it out and rinse it with deionized water for 30s and dry it with nitrogen; obtain the pretreated crystalline silicon solar cell. Step S2: Add 100g of N,N-dimethylformamide to a dry reaction flask and dehydrate under reduced pressure at 120℃ for 2h, then lower the temperature to 50℃, add 4g of 6-amino-2-mercaptobenzothiazole and stir to dissolve, then add 5g of 3-(triethoxysilylpropyl)isocyanate and react at 50℃ for 4h under nitrogen protection. Subsequently, remove the solvent under reduced pressure to obtain directional modifier A; weigh 950g of anhydrous ethanol and mix with 50g of deionized water, then add 5g of directional modifier A and stir for 30min to obtain grafting solution; immerse the pretreated crystalline silicon solar cell in the grafting solution at 40℃ for 10min, then remove it, rinse it with anhydrous ethanol and deionized water in sequence and blow it dry with nitrogen, and finally heat-cur it at 110℃ for 20min to obtain crystalline silicon solar cell with benzothiazole-mercaptosite sites; Step S3: First, weigh 1480g of deionized water, add 240g of glycine and stir to dissolve. Then add 40g of sodium hydroxide, followed by 200mg of 2-mercaptobenzothiazole and stirring until the solution is clear. Next, add 160g of copper sulfate pentahydrate and stir for 30 minutes to form a copper-glycine complex system. Finally, add 80g of sodium chloride to obtain the copper plating electrolyte. Use the pretreated crystalline silicon solar cell as the cathode and the copper plate as the anode. At 25℃, first perform the initial plating pulse current density of 30mA / cm. 2 The cycle is 500ms on / 1500ms off, repeated 60 times, then switched to a growth current density of 10mA / cm². 2 Constant current electroplating for 10 minutes, followed by rinsing with deionized water for 30 seconds and drying with nitrogen to obtain electroplated copper-silicon solar cell. Step S4: Weigh 1980g of deionized water, add 10g of hydrogen peroxide aqueous solution (concentration 30wt%) and 10g of citric acid and stir to dissolve to obtain a purified solution; immerse the battery cell obtained in step S3 at 40℃ for 2min and then take it out, rinse it with deionized water for 30s and blow it dry with nitrogen; then place it under a nitrogen atmosphere at 200℃ for 10min and cool it naturally to obtain the finished product. Example

[0057] The only difference is that 240g of glycine in step S3 is replaced with 240g of β-alanine; the other conditions are the same as in Example 1. Example

[0058] The only difference is that the chelating agent in step S3 is a combination of glycine and histidine: 180g of glycine + 60g of histidine (the total amount is still in the range of 240g); the other conditions are the same as in Example 1. Example

[0059] The only difference was that the free MBT in step S3 was changed from 200 mg to 50 mg; the other conditions were the same as in Example 1. Example

[0060] The only difference was that the free MBT in step S3 was changed from 200 mg to 300 mg; the other conditions were the same as in Example 1. Example

[0061] The only difference is that the copper salt in step S3 is replaced with copper nitrate trihydrate instead of copper sulfate pentahydrate (to maintain a consistent copper ion concentration), and the amount of chloride salt added is adjusted accordingly to maintain the target chloride ion concentration; the other conditions are the same as in Example 1.

[0062] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that step S2 is omitted, the grafting solution is not prepared, and the pre-treated crystalline silicon solar cell is not immersed in the grafting solution, rinsed with anhydrous ethanol and deionized water, dried with nitrogen, and heat-cured at 110°C for 20 minutes; the pre-treated crystalline silicon solar cell obtained in step S1 is directly used as the cathode in step S3 for electroplating after being dried with nitrogen; the other conditions are the same as in Example 1.

[0063] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the plating start-up pulse program is canceled in step S3, and the current density of 30 mA / cm² is not executed. 2 The plating pulse consists of a 500ms power-on time, a 1500ms power-off time, and 60 cycles. A current density of 10mA / cm² is used directly only at 25℃. 2 Constant current electroplating for 10 minutes; other conditions are the same as in Example 1.

[0064] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that the order of adding materials to the electroplating solution in step S3 is changed. Specifically, 240g of glycine is added to 1480g of deionized water and stirred to dissolve. Then, 160g of copper sulfate pentahydrate is added and stirred for 30min. Next, 40g of sodium hydroxide is added and stirred for 10min. Then, 200mg of 2-mercaptobenzothiazole is added and stirred until the solution is clear. Finally, 80g of sodium chloride is added to obtain the electroplating solution. The remaining conditions are the same as in Example 1.

[0065] Comparative Example 4: The difference between Comparative Example 4 and Example 1 is that free 2-mercaptobenzothiazole is not added in step S3. Instead, 160g of copper sulfate pentahydrate is added directly after dissolving in 1480g of deionized water and 240g of glycine, adding 40g of sodium hydroxide, stirring for 30min, and then adding 80g of sodium chloride to form a bath, thus obtaining an electroplating solution without free 2-mercaptobenzothiazole. The other conditions are the same as in Example 1.

[0066] Comparative Example 5: The difference between Comparative Example 5 and Example 1 is that the amount of free 2-mercaptobenzothiazole added in step S3 is doubled; the other conditions are the same as in Example 1.

[0067] Performance testing: The samples used for performance testing were the finished products of the examples and the finished products of the comparative examples. Five crystalline silicon solar cells were prepared for each type of sample and obtained according to the same batch, the same equipment, the same pretreatment and the same cleaning process after electroplating. Except for the single difference clearly recorded in each comparative example, the other preparation conditions were completely consistent. For each test, five cells of each type of sample were taken for measurement and the data of each cell were recorded and the arithmetic mean was calculated.

[0068] Cross-sectional thickness measurement: For five samples each from the examples and comparative examples, 10mm × 10mm specimens were cut from the fine grid groove area. After being encapsulated with cold-mounted resin, the specimens were successively ground with 400-grit, 800-grit, 1200-grit, and 2000-grit sandpaper and polished with 1.0μm diamond polishing fluid until the cross-section was clear. Secondary electron images of the cross-section were acquired under the conditions of 10kV accelerating voltage and 10mm working distance. Ten measuring points were selected at equal intervals along the grid width direction on the cross-section of each sample to measure the copper layer thickness. The average thickness and thickness uniformity were calculated (expressed as thickness standard deviation / average thickness × 100%).

[0069] Surface roughness: Five samples from each of the examples and comparative examples were tested using a stylus-type surface profilometer in the copper-plated area of ​​the fine grid groove. The stylus radius was 2 μm, the measuring force was 0.75 mN, the sampling length was 0.8 mm, the evaluation length was 4.0 mm, and the scanning speed was 0.5 mm / s. Ra was measured on three parallel test lines (1 mm apart) for each sample and the average value was taken.

[0070] Electrical performance (IV and series resistance of photovoltaic devices): The electrical performance of the devices was evaluated according to GB / T 6495.1-1996. Five samples each from the examples and comparative examples were tested for IV under standard test conditions, with the light source conditions set as AM1.5G spectrum and irradiance of 1000 W / m². 2 The cell temperature was set at 25°C and stabilized for 10 minutes on the back temperature control platform before testing. The open-circuit voltage Voc (V), short-circuit current Isc (A), fill factor FF (%), and maximum power Pmax (W) were recorded. The series resistance Rs (Ω) was extracted from the IV curve using the standard method.

[0071] Damp heat reliability and performance retention rate: Damp heat aging and performance retention rate were evaluated according to the damp heat / environmental stress evaluation methods in GB / T 2423.3-2016 and GB / T 9535-1998, respectively. Five samples from each of the examples and comparative examples were exposed to constant damp heat conditions of 85℃ and 85% relative humidity for 1000h. After being removed, they were restored to 25℃ and 50% relative humidity for 2h. Then, the electrical performance test was repeated, and the change in series resistance ΔRs (Ω) was calculated. The test results are shown in Table 1.

[0072] Table 1 Performance Test Results

[0073] Data Analysis: As can be seen from the data in Table 1, the present invention can simultaneously achieve high thickness uniformity and low surface roughness during the formation of the micron-level copper plating layer, and keep the open-circuit voltage and short-circuit current of the solar cell stable. The fill factor and maximum power show a synchronous increasing trend as the series resistance decreases. After damp heat aging, the overall increase in series resistance is small, indicating good conductivity and interface stability. The possible reasons are: the silicon-oxygen bond network introduced by 3-(triethoxysilylpropyl)isocyanate and the sulfur-containing coordination sites provided by 6-amino-2-mercaptobenzothiazole synergistically construct the interface anchoring layer, which enhances the wetting and nucleation of the silicon surface and inhibits the formation of coarse copper grains in the early stage; at the same time, the complexation-deposition balance regulated by glycine and sodium hydroxide and the chloride ions provided by sodium chloride promote the mass transfer of copper ions, resulting in high nucleation density in the pulse current stage and dense filling in the DC stage, thereby reducing contact resistance and slowing down corrosion propagation under damp heat.

[0074] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, the absence of the interface anchoring layer constructed from 6-amino-2-mercaptobenzothiazole and 3-(triethoxysilylpropyl)isocyanate increases the thickness dispersion of the copper plating layer and makes the surface more prone to roughening, leading to an increase in series resistance and further reducing the fill factor and maximum power. Simultaneously, the increase in series resistance after humid heat is significantly amplified. The main reason for this is that insufficient nucleation sites at the interface cause copper to tend to grow in island-like formations, and pores and weak bonding areas are more likely to become the starting points for corrosion and current congestion under humid heat.

[0075] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, after eliminating the pulsed current plating stage, the uniformity and surface smoothness of the plating layer decreased simultaneously, while the series resistance and the increase in resistance after damp heat both increased significantly, ultimately resulting in a decrease in the fill factor and maximum power. The main reason for this is that the pulsed current can provide a high instantaneous current density in a short time, prompting a large number of crystal nuclei to form simultaneously and shortening the diffusion layer recovery time, thereby inhibiting edge thickening and the formation of coarse grains; when only a constant current is used, the initial nucleation density is low, and subsequent growth is more prone to grain coarsening and micro-defect accumulation.

[0076] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 3 and 4, when the order of adding materials during the electroplating bath construction is changed, and factors such as the absence of free 2-mercaptobenzothiazole occur, the coating thickness distribution and surface condition deteriorate, and the series resistance and the increase in resistance after wet heating generally increase. The main reasons may be that: sudden changes in the local environment during the bath construction process affect the complexation and stability of copper ions, leading to fluctuations in the effective deposition components; at the same time, the lack of dynamic adsorption-desorption of free 2-mercaptobenzothiazole makes it difficult to continuously refine the grains and seal the micropores. It can be seen that there is a synergistic relationship between the bath construction order and sulfur-containing additives that jointly determine the deposition window, exhibiting a typical 1+1 greater than 2 effect.

[0077] As can be seen from the data in Table 1 for Example 1 and Comparative Example 5, even with a significant increase in the amount of free 2-mercaptobenzothiazole in the electroplating solution, omitting step S2 and not constructing the interface layer of directional modifier A, it is still difficult to simultaneously achieve coating uniformity, low series resistance, and wet heat reliability. The overall performance is still significantly lower than that of Example 1. The main reason is that free 2-mercaptobenzothiazole can only participate in interface regulation through reversible adsorption. Without covalent anchoring, it is difficult to form a stable and localized sulfur-containing coordination framework in the bare silicon region, making it susceptible to fluctuations in nucleation sites and effective concentrations due to flow field renewal, competitive adsorption, and consumption. Therefore, it is evident that there is a significant synergistic effect between the benzothiazole-mercaptosites fixed by directional modifier A and free 2-mercaptobenzothiazole. Simply increasing the amount of free component is insufficient to achieve the synergistic effect of 1+1 greater than 2.

[0078] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A copper electroplating process that does not require a silver paste base coat, characterized in that, Includes the following steps: (1) Substrate pretreatment: The crystalline silicon substrate with open bare silicon regions is cleaned; the intrinsic oxide layer is removed using a fluorine-containing system, and then activated using an oxidant; (2) Constructing a directional modified interface layer: In an alcohol / water-containing grafting solution, a directional modifier A containing benzothiazole-thiol sites and having hydrolyzable silane end groups is covalently grafted onto a pretreated crystalline silicon substrate and then thermally cured to obtain a crystalline silicon substrate with a localized benzothiazole-thiol site interface layer. (3) Preparation of copper plating electrolyte: Dissolve the complexing agent in water and add an alkaline source to pre-constitute the complexing agent into a salt. Add free 2-mercaptobenzothiazole and stir until the solution is clear. Then add copper salt in batches and stir until the complexation is stable. Finally, add chloride salt to obtain copper plating electrolyte. (4) Copper electroplating: using a crystalline silicon substrate with a localized benzothiazole-thiol site interface layer as the cathode, copper is electroplated in a copper electroplating electrolyte; wherein, no silver paste is formed as the underlayer during the entire process.

2. The copper plating process without silver paste undercoat as described in claim 1, characterized in that, The fluorine-containing system in step (1) is hydrofluoric acid or ammonium fluoride buffer system; the concentration of the fluorine-containing system is 0.2-5 wt%, the treatment time is 10-120 s, and the treatment temperature is 15-35 ℃; the oxidant is one or more of hydrogen peroxide, ozone water, or persulfate system; the concentration of the oxidant is 0.01-1 wt%, the treatment temperature is 20-60 ℃, and the treatment time is 0.5-10 min.

3. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, The directional modifier A is obtained by reacting 6-amino-2-mercaptobenzothiazole with 3-(triethoxysilylpropyl)isocyanate.

4. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, In step (2), the alcohol / water volume ratio or mass ratio of the grafting solution is 85 / 15 to 97 / 3, the concentration of the directional modifier A in the grafting solution is 0.1 to 1 wt%, the grafting temperature is 30 to 45°C, the grafting time is 5 to 15 min, the thermosetting temperature is 100 to 130°C, and the thermosetting time is 10 to 30 min.

5. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, The complexing agent mentioned in step (3) is one or more of amino acids, aminocarboxylic acids or hydroxycarboxylic acids.

6. The copper plating process without silver paste undercoat as described in claim 1, characterized in that, The pH of the copper plating electrolyte in step (3) is 8.5 to 10.5, and the concentration of free 2-mercaptobenzothiazole in the copper plating electrolyte is 5 to 200 mg / L.

7. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, The concentration of the complexing agent in step (3) is 0.6 to 1.8 mol / L, and the concentration of the copper salt is 0.1 to 0.4 mol / L.

8. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, The chloride salt mentioned in step (3) is NaCl or KCl, and the chloride salt concentration in the copper plating electrolyte is 20-120 g / L.

9. The direct copper electroplating process without silver paste undercoat as described in claim 1, characterized in that, Step (4) of electroplating includes a starting pulse stage and a growth stage: the pulse current density of the starting pulse stage is 20-40 mA / cm². 2 The power-on time is 100–1000 ms, the power-off time is 300–3000 ms, and the number of cycles is 20–200. During the growth stage, constant current or weak pulse growth is used, with a current density of 8–15 mA / cm². 2 The electroplating time is 5 to 20 minutes.

10. The direct copper electroplating process without silver paste undercoat according to claim 1, characterized in that, The process after copper electroplating in step (4) includes post-treatment: the electroplated crystalline silicon substrate is treated with a purification solution containing an oxidant and an organic acid; wherein the concentration of the oxidant is 0.01-0.5 wt%, the concentration of the organic acid is 0.05-2 wt%, the treatment temperature is 20-60°C, and the treatment time is 0.5-10 min; then annealing is performed in an inert atmosphere at a temperature of 150-250°C for 1-30 min.

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