A method for recycling a copper seed layer of a heterojunction copper interconnect battery

Abstract

The application discloses a recycling method of a copper seed layer of a heterojunction copper interconnection battery, and relates to the technical field of photovoltaics. The method utilizes the difference in surface adsorption energy of organic corrosion inhibitor molecules in an organic corrosion inhibitor solution between electroplated copper (coarse grains) and a copper seed layer (fine grains) to form a dense monomolecular protective film on the surface of the electroplated copper grid lines, and the non-grid line areas are incompletely protected. In an electrolyte, a low-voltage anodic polarization is applied, and the current preferentially passes through the copper seed layer of the non-grid line areas which are not protected, so that the copper seed layer is selectively oxidized and dissolved, and the protected grid lines are not corroded due to the increase of the oxygen evolution overpotential. The method provided by the application has ultra-high selectivity, realizes the reaction only in the areas which need to be removed, and the electroplated copper grid lines have no size loss. In the process of the area-selective electrochemical stripping, the process endpoint can be judged simply and reliably based on the "self-stopping" mechanism of the current signal, and precise timing is not needed.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic technology, and more specifically, to a method for recovering the copper seed layer of a heterojunction copper interconnect cell. Background Technology

[0002] Current technologies and research on the treatment of copper seed layers after electroplating in heterojunction solar cells mainly focus on two categories: (1) Chemical etching: This is the most intuitive and common method. After electroplating, the battery is directly immersed in a strong oxidizing etching solution, such as ammonium persulfate, sulfuric acid-hydrogen peroxide system, or acidic copper chloride solution. These chemicals indiscriminately oxidize and dissolve the copper on the entire surface, including the electroplated copper grid lines that need to be retained and the copper seed layer in the non-grid line areas that need to be removed. Subsequent steps aim to remove the battery immediately when the copper seed layer in the non-grid line areas is just etched clean, by precisely controlling the etching time, in order to minimize lateral corrosion and thickness reduction of the electroplated copper grid lines.

[0003] (2) Patterned mask etching method: Before electroplating, a patterned mask (such as photoresist) is first made on the copper seed layer. After electroplating, the mask is not removed, but etching is performed directly. At this time, the mask protects the copper seed layer and the electroplated copper below the gate line, and the etching solution only attacks the unprotected copper seed layer in the non-gate line area. After etching is completed, the mask is then peeled off.

[0004] The above two types of existing technologies have the following drawbacks: (1) Poor selectivity and easy damage to the grid lines are the core drawbacks of chemical etching. Chemical etching is isotropic, which will inevitably etch the top and sidewalls of the grid lines at the same time, resulting in narrower grid line width and reduced height (deterioration of aspect ratio), which directly increases series resistance and reduces cell conversion efficiency. The process window is extremely narrow. If the etching time is too short, the seed layer will not be completely removed (leading to short circuit); if the etching time is too long, the grid lines will be severely damaged.

[0005] (2) High consumption of chemical solution, high cost and environmentally unfriendly: Strong oxidizing etching solution is not only consumed quickly, but also generates a large amount of copper-containing waste liquid after reaction, which is costly to treat and does not conform to the trend of green manufacturing.

[0006] (3) Risk of damage to TCO and passivation layer: Strong acid or strong oxidizing etching solution may damage or even penetrate the underlying TCO layer, further attacking the amorphous silicon passivation layer which is extremely sensitive to impurities, causing permanent degradation of battery performance.

[0007] (4) High process complexity (disadvantages of patterned mask etching): Patterned mask etching adds multiple processes such as photolithography and resist removal, which significantly increases process complexity and production cost, and extends the production cycle.

[0008] Therefore, there is an urgent need to optimize the processing technology to selectively, gently, and environmentally friendly remove the copper seed layer in the non-gate area of ​​the heterojunction cell while perfectly preserving the geometry and conductivity of the electroplated copper gate lines, and at the same time ensuring that the underlying TCO layer and amorphous silicon passivation layer are not damaged.

[0009] In view of this, the present invention is proposed. Summary of the Invention

[0010] The purpose of this invention is to provide a method for recycling the copper seed layer of a heterojunction copper interconnect cell, aiming to provide a highly selective processing technology to remove the copper seed layer in the non-gate region of the heterojunction cell without damaging the underlying TCO layer and amorphous silicon passivation layer.

[0011] This invention is implemented as follows: In a first aspect, the present invention provides a method for recovering the copper seed layer of a heterojunction copper interconnect battery, comprising: A heterojunction solar cell is provided: The heterojunction solar cell includes a substrate, on which an amorphous silicon layer, a TCO layer and a copper seed layer are sequentially deposited on at least one side, and electroplated copper grid lines are disposed on the copper seed layer. Interface pretreatment and selective modification: The surface with the copper seed layer is treated sequentially with an oxidizing solution and an organic corrosion inhibitor solution to form a selective protective film; Region-selective electrochemical stripping: The surface with the copper seed layer is immersed in an electrolyte with a pH of 5.0-6.0, and a voltage is applied using a heterojunction cell as the working electrode to dissolve the copper seed layer in the non-gate region.

[0012] In an optional embodiment, the oxidant in the oxidizing solution is hydrogen peroxide; Preferably, the preparation process of the oxidizing liquid includes: mixing water, oxidant and ammonia, controlling the mass fraction of the oxidant to be 0.5%-2% and the mass fraction of the ammonia to be 0.05%-0.20%.

[0013] In an optional embodiment, the process of using an oxidation solution includes: immersing the edge of the clamped battery cell in an oxidation solution at 20°C-25°C for 50-70 seconds, and then removing the battery cell for washing with water after the treatment is completed. Preferably, the washing process includes: immersing the battery cells in water, lifting them up and down 3 to 5 times, and then drying the surface.

[0014] In an optional embodiment, the organic corrosion inhibitor solution is obtained by mixing a corrosion inhibitor and an organic solvent; the corrosion inhibitor is selected from at least one of benzotriazole and imidazole; Preferably, the concentration of the corrosion inhibitor in the organic corrosion inhibitor solution is 0.05 mol / L to 0.20 mol / L; Preferably, the organic solvent is anhydrous ethanol; Preferably, during the treatment process using an organic corrosion inhibitor solution, the treatment temperature is controlled at 20℃-25℃, the treatment time is 150s-300s, and after treatment, the mixture is allowed to stand and dry, followed by purging.

[0015] In an optional implementation, the region-selective electrochemical stripping process includes: placing the assembled working electrode, reference electrode, and counter electrode together into an electrolytic cell and applying a voltage; when the copper seed layer is completely dissolved and the TCO layer is exposed, terminating the electrochemical stripping based on the change in current. Preferably, the counter electrode is a platinum sheet or platinum mesh with an area larger than that of the heterojunction solar cell; Preferably, the reference electrode is a saturated calomel electrode.

[0016] In an optional embodiment, the electrolyte preparation process includes: preparing a salt solution with a concentration of 0.01 mol / L to 0.20 mol / L, and then adjusting the pH value to 5.0 to 6.0; More preferably, the salt solution is selected from at least one of ammonium sulfate solution and ammonium citrate solution; More preferably, the pH value is adjusted using sulfuric acid solution or ammonia.

[0017] In an optional implementation, a region-selective electrochemical stripping method is used, either linear scanning voltammetry or chronoamperometry. Preferably, the linear scanning voltammetry method is used for testing. The starting potential is the stabilized open circuit potential, and the scanning direction towards the anode is carried out at a scanning rate of 5-15 mV / s until +1.0 V is reached. The potential plateau where selective dissolution occurs is determined by observing the current-voltage curve. Preferably, when using the chronoamperometry method, a constant anodic potential is applied to dissolve the copper seed layer, and the applied potential is lower than the decomposition potential of TCO and the oxygen evolution potential of water; Preferably, the operation time for region-selective electrochemical stripping is 90s-150s.

[0018] In an optional implementation, after terminating the electrochemical stripping, the battery cell is removed, washed with water, and then immersed in an acid solution to remove the selective protective film. Preferably, the acid solution is a dilute nitric acid solution with a volume fraction of 0.5%-2.0%.

[0019] In an optional embodiment, the thickness of the copper seed layer is 50nm-150nm, and the height of the electroplated copper gate line is 10μm-20μm. And / or, the substrate is an N-type substrate.

[0020] In an optional implementation, the heterojunction solar cell is surface-cleaned before interface pretreatment and selective modification. Preferably, the surface cleaning process includes: ultrasonic cleaning of the heterojunction solar cell using an alcohol solvent; More preferably, after ultrasonic cleaning, the device is purged and dried or dry-cleaned using a plasma cleaner.

[0021] This invention offers the following advantages: It utilizes the difference in adsorption energy between organic corrosion inhibitor molecules in the organic corrosion inhibitor solution and the copper seed layer (fine grains) to form a dense monomolecular protective film on the surface of the electroplated copper grid lines, while the non-grid line areas are not fully protected. In the electrolyte, low-voltage anodic polarization is applied, and the current preferentially passes through the unprotected non-grid line areas of the copper seed layer, selectively oxidizing and dissolving it, while the protected grid lines are protected from corrosion due to the increased oxygen evolution overpotential. The method provided by this invention exhibits ultra-high selectivity, achieving reaction only in the areas requiring removal, with no dimensional loss in the electroplated copper grid lines. During the region-selective electrochemical stripping process, the "self-stopping" mechanism based on the current signal makes process endpoint determination simple and reliable, eliminating the need for precise timing. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a process flow diagram of the copper seed layer recovery process for heterojunction copper interconnect cells according to the present invention. Figure 2 This is a schematic diagram of the structure of the heterojunction solar cell to be processed. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0025] Existing processes often damage the underlying TCO layer and amorphous silicon passivation layer when removing the copper seed layer in the non-gate area of ​​a heterojunction cell. In view of this, the present invention optimizes the processing technology, which enables the reaction to occur only in the area that needs to be removed, without any dimensional loss of the electroplated copper gate line, achieving ultra-high selectivity.

[0026] This invention provides a method for recovering the copper seed layer of a heterojunction copper interconnect cell, the main process steps of which are as follows: Figure 1 The following sections explain each step: S1. Sample preparation and initial status confirmation The heterojunction solar cell that has already undergone copper electroplating is the object to be processed. The heterojunction solar cell includes a substrate, on which an amorphous silicon layer, a TCO layer, and a copper seed layer are sequentially deposited in sequence. Electroplated copper grid lines are disposed on the copper seed layer. The surface of the heterojunction solar cell to be processed should have clear, short-circuit-free, and contamination-free copper grid lines (main grid and fine grid). The initial PVD copper seed layer in the area between the grid lines should completely cover the area.

[0027] like Figure 2 As shown, the substrate can be an N-type substrate, specifically an N-type silicon wafer. A texturing process is used to form a textured surface on the substrate. On the N-side, an N-side intrinsic amorphous silicon layer, an N-type doped layer, an N-side TCO conductive layer, and an N-side copper seed layer are sequentially formed. N-side electroplated gate lines are then formed on the N-side copper seed layer. On the P-side, a P-side intrinsic amorphous silicon layer, a P-type doped layer, a P-side TCO conductive layer, and a P-side copper seed layer are sequentially formed. P-side electroplated gate lines are then formed on the P-side copper seed layer.

[0028] In some embodiments, the thickness of the copper seed layer is 50nm-150nm, such as 50nm, 80nm, 100nm, 120nm, 150nm, etc.; the height of the electroplated copper gate line is 10μm-20μm, such as 10μm, 13μm, 15μm, 18μm, 20μm, etc.

[0029] Specifically, there are no restrictions on the specific fabrication processes and parameters of the N-side intrinsic amorphous silicon layer, N-type doped layer, N-side TCO conductive layer, N-side copper seed layer, N-side electroplated gate line, P-side intrinsic amorphous silicon layer, P-type doped layer, P-side TCO conductive layer, P-side copper seed layer, and P-side electroplated gate line; existing heterojunction copper interconnect cells can be referenced.

[0030] Before performing interface pretreatment and selective modification, the surface of the heterojunction solar cell is cleaned to remove any organic contaminants and particulate matter that may be present on the surface, ensuring consistency in subsequent processing.

[0031] In some embodiments, the surface cleaning process includes: ultrasonically cleaning the heterojunction solar cell using an alcohol solvent; and after ultrasonic cleaning, performing blow-drying or dry cleaning using a plasma cleaner.

[0032] In actual operation, it is divided into two steps: (1) Ultrasonic cleaning: Immerse the battery cell in a beaker containing analytical grade anhydrous ethanol or isopropanol, place it in an ultrasonic cleaner, and ultrasonically treat it for 3 minutes at room temperature. (2) Rinsing and drying: Blow the surface of the battery with a high-purity nitrogen gun to ensure that it is completely dry. Alternatively, dry cleaning can be performed using a plasma cleaner (O2 / Ar plasma, 100W, 30 seconds), which will have a better effect.

[0033] S2, Interface Preprocessing and Selective Modification A dense monomolecular protective film is formed on the surface of electroplated copper gate lines through interface pretreatment and selective modification, while the non-gate line areas are not fully protected, creating conditions for selective corrosion. The specific process includes sequentially treating the surface with the copper seed layer with an oxidizing solution and an organic corrosion inhibitor solution to form a selective protective film.

[0034] [Oxidation solution treatment] In some embodiments, the oxidant in the oxidation solution is hydrogen peroxide, but it is not limited thereto. The preparation process of the oxidation solution includes: mixing water, oxidant, and ammonia, controlling the mass fraction of the oxidant to be 0.5%-2%, such as 0.5%, 1.0%, 1.5%, 2.0%, etc.; and the mass fraction of the ammonia to be 0.05%-0.20%, such as 0.05%, 0.10%, 0.15%, 0.20%, etc. Ammonia can form a soluble complex with copper ions, achieving a chemical polishing effect, thereby ensuring the formation of a uniform, dense, and controllable thickness oxide film on the surface of the copper seed layer, laying a key interfacial foundation for subsequent selective adsorption of corrosion inhibitors and achieving area-specific protection.

[0035] Furthermore, the oxidation process involves immersing the edge of the solar cell in an oxidation solution at 20°C-25°C for 50-70 seconds, such as 50, 55, 60, 65, or 70 seconds. In practice, the edge of the solar cell is held with PTFE tweezers and completely immersed in the weak oxidation solution, ensuring the solution completely covers the cell. During the oxidation process, extremely fine bubbles will uniformly form on the surface of the solar cell, indicating that a uniform oxidation reaction is underway, forming a dense CuO / Cu2O oxide film.

[0036] Further, after treatment with the oxidation solution, the battery cells are removed and washed with water to remove residual reagents. The washing process includes immersing the battery cells in water and lifting them up and down 3-5 times, followed by blowing the surface dry. In actual operation, the battery cells are removed from the oxidation solution and immediately immersed vertically into a rinsing tank filled with deionized water, lifting them up and down 3-5 times to terminate the reaction and remove residual reagents. Finally, the surface is gently blown dry with a nitrogen gun. Note: The next step should be performed as soon as possible after removal from the oxidation solution to prevent oxide film aging.

[0037] [Treatment with organic corrosion inhibitor solution] The organic corrosion inhibitor solution is obtained by mixing a corrosion inhibitor and an organic solvent. The corrosion inhibitor is selected from at least one of benzotriazole and imidazole, and can be any one or more of these. The concentration of the corrosion inhibitor in the organic corrosion inhibitor solution is 0.05 mol / L-0.20 mol / L, such as 0.05 mol / L, 0.10 mol / L, 0.15 mol / L, 0.20 mol / L, etc. The organic solvent is anhydrous ethanol, but is not limited to this.

[0038] After drying, completely immerse the battery cells in an organic corrosion inhibitor solution within the concentration range mentioned above. Control the treatment temperature at 20℃-25℃ (e.g., 20℃, 23℃, 25℃, etc.); the treatment time at 150s-300s (e.g., 150s, 200s, 250s, 300s, etc.) to ensure sufficient self-assembly of BTA molecules to form a dense monolayer. It is recommended to operate in a glove box or fume hood to avoid ethanol evaporation and environmental pollution.

[0039] After processing, remove the battery cells and do not rinse them with water. Place them vertically on a drying rack and allow them to air dry naturally for about 30 seconds. Once there are no obvious droplets on the surface, blow the surface with a very gentle stream of nitrogen (pressure <0.1MPa) to ensure that there are no residual droplets but the protective film is not damaged.

[0040] S3, regional selective electrochemical stripping The surface with the copper seed layer is immersed in the electrolyte, and a voltage is applied using a heterojunction cell as the working electrode to dissolve the copper seed layer in the non-gate area, while the protected gate lines are not corroded due to the increased oxygen evolution overpotential.

[0041] In some embodiments, the region-selective electrochemical stripping process includes: placing the assembled working electrode, reference electrode, and counter electrode together into an electrolytic cell and applying a voltage; when the copper seed layer is completely dissolved, the exposed TCO layer stabilizes at a set potential, the current drops sharply, and the process automatically terminates. In other words, the method provided in this embodiment of the invention has an electrochemical self-stopping effect.

[0042] Custom-made electrolytic cells made of quartz or PTFE can be used to avoid metal ion contamination. The electrode system is as follows: Working electrode: The treated solar cell. A specially designed PTFE clamp is used to hold the unused areas at the edge of the solar cell, ensuring that only the front side to be treated contacts the electrolyte and conducts electricity well.

[0043] Counter electrode: A platinum sheet or platinum mesh with an area much larger than that of the solar cell is used.

[0044] Reference electrode: saturated calomel electrode.

[0045] Electrochemical workstation: A digital electrochemical workstation is used to connect to a three-electrode system.

[0046] In some embodiments, the pH value of the electrolyte is 5.0-6.0, such as 5.0, 5.3, 5.5, 5.8, 6.0, etc. The method provided in this embodiment of the invention can apply low-voltage anodic polarization in a mild, near-neutral electrolyte to achieve the purpose of gently and environmentally friendly removal of the copper seed layer in the non-gate region of the heterojunction cell. The electrolyte preparation process includes: preparing a salt solution with a concentration of 0.01 mol / L-0.20 mol / L, then adjusting the pH value to 5.0-6.0, and gently stirring with a magnetic stirrer to ensure uniformity. The salt solution is selected from at least one of ammonium sulfate solution and ammonium citrate solution; the salt solution can be any one or more of the above. Sulfuric acid solution or ammonia water can be used to adjust the pH value.

[0047] Furthermore, regioselective electrochemical stripping can be performed using linear sweep voltammetry or chronoamperometry: Option 1 (Linear Scan Voltammetry (LSV), for process development): The linear scan voltammetry method is used for testing. The starting potential is the stabilized open circuit potential (the "open circuit potential" is not a fixed value, but a starting potential under specific conditions that needs to be measured in real time by the instrument). The scan rate is 5-15 mV / s towards the anode until +1.0 V is reached. The potential plateau where selective dissolution occurs is determined by observing the current-voltage curve.

[0048] Option 2 (Chorometric method, for mass production): Apply a constant anodic potential, for example +0.8V (vs. SCE). This potential needs to be determined through preliminary experiments and must be sufficient to dissolve the copper seed layer, but far below the decomposition potential of TCO and the oxygen evolution potential of water.

[0049] Real-time monitoring and endpoint determination: The software interface will display the current-time curve in real time.

[0050] Initial stage: The current is high and stable (e.g., ~8 mA / cm). 2 This corresponds to the uniform dissolution of the copper seed layer in the non-gate region.

[0051] Endpoint signal: When the current drops sharply over time to a stable and very low value (e.g., 1-2% of the initial current, approximately 0.1 mA / cm²). 2 When the reaction reaches 0, it indicates that the copper seed layer has completely dissolved, the poorly conductive TCO layer is exposed, and the reaction stops automatically.

[0052] Process time: The entire process typically takes between 90 and 150 seconds, depending on the thickness of the copper seed layer and the applied potential.

[0053] S4. Post-processing and final inspection After terminating the electrochemical stripping, the battery cells are removed and washed with water, then immersed in an acid solution to remove the selective protective film.

[0054] Specifically, upon reaching the endpoint, immediately disconnect the circuit and remove the battery cells from the electrolytic cell. Immerse them in a flowing deionized water bath for approximately 30 seconds to thoroughly remove the electrolyte. Then, immerse the battery cells in an acid solution for 20-60 seconds (e.g., 30 seconds). This step effectively removes, decomposes, and washes away the BTA protective film. Afterward, rinse thoroughly with plenty of deionized water and dry the surface of the battery cells with a high-purity nitrogen gun.

[0055] In some embodiments, the acid solution is a dilute nitric acid solution with a volume fraction of 0.5%-2.0%, but is not limited thereto. The volume fraction of the dilute nitric acid solution can be 0.5%, 0.8%, 1.0%, 1.3%, 1.5%, 1.8%, 2.0%, etc.

[0056] After the above processing, quality inspection is carried out: (1) Optical microscope / scanning electron microscope: check whether the grid line morphology is complete and whether the edges are sharp; whether the area between the grid lines is clean and free of copper residue. (2) Four-probe tester / transmission line measurement: measure the resistivity of the grid line to verify that its conductivity is not damaged. (3) Electroluminescence test: ensure that the entire process does not damage the PN junction and passivation layer of the battery.

[0057] It should be noted that the method provided in this embodiment of the invention can transform "indiscriminate chemical attack" into "intelligent electrochemical selection controlled by interface properties", which applies the following principles: (1) Physicochemical principle: It utilizes the difference in adsorption energy of metal surfaces with different microstructures, which is an inherent physicochemical property, thereby achieving precise protection at the molecular level. (2) Electrochemical principle: It utilizes the current density distribution characteristics of anodic dissolution, where the current always preferentially flows to the path with the least resistance (most easily corroded), i.e., the unprotected area. (3) Materials science principle: It utilizes the chemical stability of TCO under a weak electric field as an ideal reaction stopping layer.

[0058] The method provided in this invention has the following advantages over existing processes: (1) Ultra-high selectivity: It achieves reaction only in the area that needs to be removed, and there is no dimensional loss in the electroplated copper grid lines. (2) Zero substrate damage: Mild electrochemical conditions ensure the safety of TCO and amorphous silicon passivation layers, and can better protect TCO and amorphous silicon passivation layers. (3) Wide process window: The "self-stop" mechanism based on current signals makes the determination of the process endpoint simple and reliable, without the need for precise timing. (4) Environmentally friendly: The electrolyte can be recycled and copper can be recovered, reducing waste liquid discharge from the source. (5) No new approach: It completely bypasses the traditional chemical etching and mask etching technology paths, forming a completely new technical solution.

[0059] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0060] It should be noted that all embodiments use the same heterojunction solar cell: an M6 size n-type silicon wafer, with ITO deposited on the front side as a TCO layer (100nm thick), a PVD copper seed layer with a thickness of 100nm, and a copper grid line height of 15μm after electroplating.

[0061] like Figure 1 As shown, the intrinsic amorphous silicon layer on the N-plane was deposited using plasma-enhanced chemical vapor deposition (PECVD), and is intrinsic (i-type) with a thickness of 6 nm. The N-type doped layer was deposited using PECVD and doped with phosphine, with a bulk doping concentration of approximately 5 x 10⁻⁶. 18 cm -3 The thickness is 10 nm; the intrinsic amorphous silicon layer on the P-side is deposited using plasma-enhanced chemical vapor deposition (PECVD), and is intrinsic (i-type) with a thickness of 6 nm; the P-type doped layer is deposited using PECVD and achieved by doping with borane, with a bulk doping concentration of approximately 5 x 10⁻⁶. 18 cm -3 The thickness is 8nm.

[0062] Example 1: Standard process (benzotriazole corrosion inhibitor) Step 1: Sample preparation and initial status confirmation Receive heterojunction solar cells that have already undergone copper electroplating.

[0063] Immerse the battery cells in a beaker containing analytical grade anhydrous ethanol or isopropanol, place it in an ultrasonic cleaner, and sonicate for 3 minutes at room temperature. Blow the battery surface with a high-purity nitrogen gun to ensure it is completely dry.

[0064] Step 2: Interface Preprocessing and Selective Enhancement (1) Solution preparation Weak oxidizing solution: Measure 500 mL of deionized water (resistivity ≥ 18.2 MΩ·cm) into a polypropylene or quartz beaker. Add 5.0 mL of 30% analytical grade hydrogen peroxide and 0.5 mL of 25% analytical grade ammonia solution, and stir slowly with a glass rod until homogeneous. Maintain the solution temperature at 25°C. The mass fraction of hydrogen peroxide in the weak oxidizing solution is 1%, and the mass fraction of the ammonia solution is 0.1%.

[0065] Corrosion inhibitor solution: Weigh benzotriazole powder (analytical grade), dissolve it in 100 mL of anhydrous ethanol, and prepare a 0.1 mol / L stock solution. No dilution is required before use.

[0066] (2) Pretreatment: forming a uniform oxide layer Holding the edge of the solar cell with PTFE tweezers, immerse it completely in the weak oxidation solution, ensuring the solution completely covers the cell. Maintain the weak oxidation solution temperature at 25°C for 60 seconds. Remove the cell and immediately immerse it vertically in a rinsing tank filled with deionized water, lifting it up and down four times to terminate the reaction and remove any residual solution. Finally, gently dry the surface with a nitrogen gun.

[0067] (3) Modification: Formation of a selective protective film After drying, completely immerse the solar cells in a 0.1 mol / L benzotriazole ethanol solution at 25°C for 180 seconds. Remove the solar cells and do not rinse them with water. Place them vertically on a drying rack and allow them to air dry naturally for about 30 seconds. Once there are no obvious droplets on the surface, blow the surface with a very gentle stream of nitrogen gas (pressure <0.1 MPa).

[0068] Step 3: Regioselective electrochemical stripping A custom-made electrolytic cell made of quartz or PTFE is used. The working electrode is the treated solar cell, with a special PTFE clamp holding the unused areas at the edge of the cell to ensure that only the front side to be treated contacts the electrolyte and conducts well. The counter electrode is a platinum sheet with an area much larger than the solar cell. The reference electrode is a saturated calomel electrode. A digital electrochemical workstation is used to connect the three electrodes. The assembled solar cell (working electrode), reference electrode, and counter electrode are placed together into the electrolytic cell.

[0069] Electrolyte: Dissolve ammonium sulfate in water to prepare a 0.05 mol / L solution. Fine-tune the pH to 5.5. Gently stir with a magnetic stirrer to ensure homogeneity.

[0070] Parameters: Constant potential anodic polarization, with a potential of +0.8 V (vs. SCE).

[0071] Process: Initial current density is ~8.5 mA / cm² 2 At approximately 105 seconds, the current density dropped sharply to ~0.15 mA / cm². 2 And it is stable, so it is determined to be the endpoint.

[0072] Step 4: Post-processing and final inspection Upon reaching the endpoint, immediately disconnect the circuit and remove the battery cells from the electrolytic cell. Immerse them in a flowing deionized water bath for 30 seconds to thoroughly remove the ammonium sulfate electrolyte. Then immerse the battery cells in a 1% vol. dilute nitric acid solution for 30 seconds to remove BTA, rinse with deionized water, and dry with nitrogen.

[0073] The test results are as follows: SEM images show that the electroplated copper grid lines have sharp edges and no lateral corrosion, and their dimensions are completely consistent with those after electroplating. The ITO surfaces between the grid lines are clean, with no copper residue.

[0074] Electrical performance: No change in grid resistivity, high cell fill factor of 85.2%, average photoelectric conversion efficiency of 25.0%, and open circuit voltage of up to 750 mV.

[0075] Example 2: Changing the corrosion inhibitor (imidazolium-based) Step 1: Sample preparation and initial status confirmation The specific steps are the same as in Example 1.

[0076] Step 2: Interface Preprocessing and Selective Enhancement Oxidation treatment: Same as in Example 1.

[0077] Protective layer formation: Soak in 0.2 mol / L imidazole aqueous solution (adjust pH to 9.0 with KOH) for 120 seconds, then dry with nitrogen.

[0078] Step 3: Regioselective electrochemical stripping Electrolyte: Same as in Example 1. Reference electrode and counter electrode are also the same as in Example 1.

[0079] Parameters: Constant potential +0.75 V (vs. SCE). The protective efficacy of imidazole differs slightly from that of BTA, and the optimal potential needs to be fine-tuned.

[0080] Process: Initial current density ~9.0 mA / cm² 2 It reached the finish line in 98 seconds.

[0081] Step 4: Post-processing and final inspection The specific steps are the same as in Example 1.

[0082] The test results are as follows: The results were comparable to those of Example 1, successfully removing the copper seed layer in the non-gate area while perfectly preserving the gate lines. This demonstrates that the method does not rely on a single specific corrosion inhibitor and has a certain degree of universality.

[0083] Example 3: Change of electrolyte (citrate system) Step 1: Sample preparation and initial status confirmation The specific steps are the same as in Example 1.

[0084] Step 2: Interface Preprocessing and Selective Enhancement The specific steps are the same as in Example 1.

[0085] Step 3: Regioselective electrochemical stripping Electrolyte: 0.1 mol / L ammonium citrate solution, pH=6.0. Citrate ions have a complexing effect on copper ions, which can promote dissolution. The reference electrode and counter electrode are the same as in Example 1.

[0086] Parameters: Constant potential +0.7 V (vs. SCE).

[0087] Process: Higher initial current density (~10 mA / cm²) 2 Because of its faster dissolution kinetics, the process takes only 80 seconds.

[0088] Step 4: Post-processing and final inspection The specific steps are the same as in Example 1.

[0089] The test results are as follows: perfect selective stripping was also achieved. This demonstrates that different mild electrolyte systems are applicable under near-neutral conditions, enhancing process flexibility.

[0090] Example 4: Excessive concentration of ammonia in the oxidizing liquid Reference standard: Example 1 (using 1% H2O2 + 0.1% NH3·H2O oxidizing solution) Parameter changes: The ammonia concentration in the oxidation solution was increased to 2.0% (ten times the optimized concentration), while the hydrogen peroxide concentration remained unchanged at 1%. All subsequent steps (BTA modification, electrochemical stripping) were exactly the same as in Example 1 to ensure the uniqueness of the variables.

[0091] Theoretical risk: High concentrations of ammonia water will strongly complex copper ions, changing the oxidation process from "surface transformation" to a complex process of "oxidation-complexation dissolution", thus destroying anisotropy.

[0092] Impact on electroplated copper grid lines: severe lateral corrosion. SEM images show: grid line top width reduced by ~2.5μm; obvious "grooving" or "necking" at the grid line root (at the junction with the seed layer); and reduced overall grid line height.

[0093] Impact on TCO / passivation layer: High-risk damage. TCO layer: Excessive etching may cause excessive lateral hollowing of the copper seed layer at the root of the gate line, resulting in prolonged exposure of the local TCO to the oxide solution, posing a risk of erosion. Passivation layer: Although not in direct contact, the rough, incomplete interface and potential metal ion contamination increase the risk of subsequent degradation. EL images may show sporadic dark spots in the corresponding over-etched areas.

[0094] Selectivity: Loss of selectivity, transitioning to overall corrosion. During electrochemical stripping, the current decay curve lacks a clear "self-stopping" plateau; the current remains at a high level after a slow decline. Reason: After pretreatment, the gate sidewalls have also lost their complete protection and will continue to be dissolved during the electrochemical steps.

[0095] Process stability: Extremely poor. Process window disappears: Oxidation time requires extremely precise timing (second-level control), otherwise significant differences will occur. Poor uniformity: Differences in solution flow will lead to different degrees of corrosion at the edges and center. Inability to determine the endpoint: The current curve lacks a characteristic inflection point, making it impossible to determine whether the seed layer has been completely removed using the current method; only empirical time can be relied upon, leading to over-etching or under-etching.

[0096] The comparative example used battery cells from the same batch as the example cells, with the same performance parameters before treatment.

[0097] Comparative Example 1: Conventional Ammonium Persulfate Chemical Etching Method: Skip steps two and three. Immerse the electroplated battery cells directly in a 10% (wt.) ammonium persulfate aqueous solution and shake continuously at 25°C. Through preliminary experiments, the "apparently optimal" etching time was determined to be 55 seconds.

[0098] The test results are as follows: SEM images show that the electroplated copper grid lines exhibit significant lateral corrosion, with the grid line width decreasing by approximately 1.5 μm and the height decreasing by approximately 0.8 μm, resulting in a deteriorated aspect ratio; the grid line edges are irregular in some areas.

[0099] Problem: Isotropic etching cannot avoid attacking the gate lines. The process window is extremely narrow, and even a slight deviation in timing can result in either incomplete removal (if the time is too short) or over-etching of the gate lines (if the time is too long).

[0100] Electrical performance: Due to the reduced grid cross-sectional area and increased series resistance, the battery fill factor drops to 83.5%.

[0101] Comparative Example 2: Electrochemical treatment without corrosion inhibitor protection Method: Skip the core step two (interface modification). After initial cleaning and weak oxidation treatment, the solar cells are directly subjected to the same electrochemical stripping process as in Example 1 (+0.8 V vs. SCE) without immersion in BTA solution.

[0102] The test results are as follows: Macroscopic phenomenon: After being energized, the entire surface of the battery reacts violently, producing a large number of bubbles, and the solution quickly turns blue (Cu). 2+ color).

[0103] SEM images showed that the electroplated copper grid lines were severely or even completely dissolved, leaving the battery surface flat with only residual copper and exposed ITO.

[0104] Comparative Example 2 demonstrates that without a selective protective film, the electrochemical process is indiscriminate overall corrosion, and interface modification is the prerequisite and core for achieving regional selectivity.

[0105] Comparative Example 3: Corrosion inhibitor treatment but using a strongly acidic electrolyte Step 1: Same as Example 1.

[0106] Step 2: Same as in Example 1, perform BTA protection.

[0107] Step 3: Use a highly corrosive electrolyte: 0.5 mol / L sulfuric acid solution (pH 0.3). Parameters: Apply an anodic potential of +0.5 V (vs. SCE).

[0108] Phenomenon: The current remains very high, and the BTA protective film is unstable in a strongly acidic environment and fails rapidly.

[0109] Step 4: Same as Example 1.

[0110] The test results are as follows: SEM results showed similarities to Comparative Example 2, with severely corroded grid lines. Meanwhile, EL testing revealed numerous dark areas in the cell.

[0111] Conclusion: A mild, near-neutral electrolyte environment is crucial for protecting the TCO and amorphous silicon passivation layer from damage. Strong acid environments not only damage the protective film but also corrode the TCO and poison the acid-sensitive heterojunction passivation layer.

[0112] The effects of the recycling methods provided in the examples and comparative examples were compared, and the results are shown in Table 1.

[0113] Table 1. Comparison of the effects of the recovery methods provided in the examples and comparative examples.

[0114] The comparison between the examples and comparative examples shows that the interface modification and mild electrochemical stripping of the present invention are two indispensable core elements. With the combined effect of the two, a series of key technical problems such as poor selectivity, damage to gate lines and substrate that cannot be overcome by traditional methods have been successfully solved, and the invention has very good application prospects.

[0115] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for recovering the copper seed layer of a heterojunction copper interconnect battery, characterized in that, include: A heterojunction solar cell is provided: the heterojunction solar cell includes a substrate, on which an amorphous silicon layer, a TCO layer and a copper seed layer are sequentially deposited on at least one side, and an electroplated copper grid line is disposed on the copper seed layer; Interface pretreatment and selective modification: The surface with the copper seed layer is treated sequentially with an oxidizing solution and an organic corrosion inhibitor solution to form a selective protective film; Region-selective electrochemical stripping: The surface with the copper seed layer is immersed in an electrolyte with a pH of 5.0-6.0, and a voltage is applied using the heterojunction cell as the working electrode to dissolve the copper seed layer in the non-gate region.

2. The recycling method according to claim 1, characterized in that, The oxidant in the oxidizing solution is hydrogen peroxide; Preferably, the preparation process of the oxidizing liquid includes: mixing water, oxidant and ammonia, controlling the mass fraction of the oxidant to be 0.5%-2% and the mass fraction of the ammonia to be 0.05%-0.20%.

3. The recycling method according to claim 2, characterized in that, The process of using the oxidation solution includes: immersing the edge of the clamped battery cell in the oxidation solution at 20℃-25℃ for 50s-70s, and then taking out the battery cell for washing after the treatment. Preferably, the washing process includes: immersing the battery cell in water, lifting it up and down 3-5 times, and then drying the surface.

4. The recycling method according to claim 1, characterized in that, The organic corrosion inhibitor solution is obtained by mixing a corrosion inhibitor and an organic solvent; the corrosion inhibitor is selected from at least one of benzotriazole and imidazole. Preferably, in the organic corrosion inhibitor solution, the concentration of the corrosion inhibitor is 0.05 mol / L to 0.20 mol / L; Preferably, the organic solvent is anhydrous ethanol; Preferably, during the treatment process using the organic corrosion inhibitor solution, the treatment temperature is controlled at 20℃-25℃, the treatment time is 150s-300s, and after treatment, the mixture is allowed to stand and dry, followed by purging.

5. The recycling method according to claim 1, characterized in that, The region-selective electrochemical stripping process includes: placing the assembled working electrode, reference electrode, and counter electrode together into an electrolytic cell and applying a voltage; when the copper seed layer is completely dissolved and the TCO layer is exposed, the electrochemical stripping is terminated according to the current change. Preferably, the counter electrode is a platinum sheet or platinum mesh with an area larger than that of the heterojunction solar cell; Preferably, the reference electrode is a saturated calomel electrode.

6. The recycling method according to claim 1 or 5, characterized in that, The preparation process of the electrolyte includes: preparing a salt solution with a concentration of 0.01 mol / L to 0.20 mol / L, and then adjusting the pH value to 5.0 to 6.0; More preferably, the salt solution is selected from at least one of ammonium sulfate solution and ammonium citrate solution; More preferably, the pH value is adjusted using sulfuric acid solution or ammonia.

7. The recycling method according to claim 6, characterized in that, The regioselective electrochemical stripping was performed using linear sweep voltammetry or chronoamperometry. Preferably, the linear scanning voltammetry method is used for testing. The starting potential is the stabilized open circuit potential, and the scanning direction towards the anode is carried out at a scanning rate of 5-15 mV / s until +1.0 V is reached. The potential plateau where selective dissolution occurs is determined by observing the current-voltage curve. Preferably, when using the chronoamperometry method, a constant anode potential is applied to dissolve the copper seed layer, and the applied potential is lower than the decomposition potential of TCO and the oxygen evolution potential of water; Preferably, the operation time for the region-selective electrochemical stripping is 90s-150s.

8. The recycling method according to claim 6, characterized in that, After terminating the electrochemical stripping, the battery cells are removed and washed with water, and then immersed in an acid solution to remove the selective protective film. Preferably, the acid solution is a dilute nitric acid solution with a volume fraction of 0.5%-2.0%.

9. The recycling method according to claim 1, characterized in that, The thickness of the copper seed layer is 50nm-150nm, and the height of the electroplated copper gate line is 10μm-20μm. And / or, the substrate is an N-type substrate.

10. The recycling method according to claim 1, characterized in that, Before performing the interface pretreatment and selective modification, the surface of the heterojunction solar cell is cleaned. Preferably, the surface cleaning process includes: ultrasonic cleaning of the heterojunction solar cell using an alcohol solvent; More preferably, after ultrasonic cleaning, the device is purged and dried or dry-cleaned using a plasma cleaner.

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