Electroplating solution and electroplating method for arched cross-section copper conductive grid lines of photovoltaic silicon plates

Abstract

This invention discloses an electroplating solution and method for copper conductive grid lines with arched cross-sections in photovoltaic silicon panels, belonging to the field of photovoltaic electroplating technology. Each liter of the electroplating solution contains: 150-250 g of copper sulfate pentahydrate, 40-70 g of concentrated sulfuric acid, 10-100 mg of soluble chloride, 30-300 mg of polyethylene glycol, 1-20 mg of sodium polydithiopropane sulfonate, 1-30 mg of methylene violet, and the balance being distilled water. A silicon wafer with patterned mask trenches is immersed in the electroplating solution at a current density of 1-5 A / dm³. 2 Electroplating for 5–90 minutes allows copper to be deposited in a raised pattern within the trench. After removing the mask, an arched cross-section of copper conductive grid lines is obtained. This invention utilizes the synergistic effect of chloride ions, polyethylene glycol, sodium polydisulfide dipropane sulfonate, and methylene violet to preferentially promote longitudinal growth of copper within the mask trench, forming smooth, dense, and minimally shadowed arched grid lines under oblique light. Simultaneously, pure copper exhibits low resistivity and significantly lower cost than silver paste, thus comprehensively improving the light utilization efficiency and economic viability of photovoltaic cells.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic electroplating technology, specifically relating to an electroplating solution and electroplating method for copper conductive grid lines with arched cross sections in photovoltaic silicon panels. Background Technology

[0002] Conductive grid lines are a key structure on the surface of solar cells for collecting and discharging photocurrent. Their linewidth accuracy, aspect ratio, conductivity, and light-shielding characteristics directly affect the photoelectric conversion efficiency of the cell. Currently, the mainstream grid line manufacturing technology is silver paste screen printing. However, this process has significant drawbacks: the linewidth of screen-printed silver paste is typically 40–50 μm, making it difficult to achieve finer linewidths; the silver paste contains glass powder and organic impurities, resulting in a resistivity as high as 5–10 μΩ·cm; and the high cost of silver materials restricts further improvements in cell efficiency and cost reduction.

[0003] Copper electroplating technology is considered an important alternative to silver paste printing. Copper has excellent conductivity (pure copper resistivity is about 1.7 μΩ·cm), costs only about 1% of silver, and the electroplating process can achieve finer linewidths. A typical process flow is as follows: a copper seed layer is physically vapor-deposited on a transparent conductive oxide film on the silicon wafer surface. Then, through masking, exposure, and development processes, a gate pattern is formed on the seed layer. Electroplating is then performed on the patterned area, depositing copper on the seed layer via electrolysis to form copper gate lines. Finally, the mask and excess seed layer are removed.

[0004] However, in existing copper plating technologies, the copper plating layer typically exhibits isotropic growth characteristics when filling patterned trenches, meaning it grows both vertically and horizontally simultaneously, resulting in square or rectangular grid lines. From an optical perspective, when light is incident at an oblique angle, the top corners of square grid lines produce significant shadow extension, causing light-blocking losses. In contrast, arched grid lines have no sharp corners, resulting in a smaller oblique light projection area and lower light-blocking losses for the same bottom width. Furthermore, the oblique sidewalls of arched grid lines can reflect some obliquely incident light to the cell absorption layer, further improving light utilization. Bunthof et al. found that oblique sidewalls can reflect obliquely incident light to the cell surface, maintaining high light utilization even under oblique light (Solar Energy, 2017, 144: 166-174); Cavalli et al. further reported that trapezoidal grid lines can improve photoelectric utilization efficiency by approximately 0.8% (Solar Energy Materials and Solar Cells, 2021, 231:111294). However, the trapezoidal or inclined sidewall grid lines used in the above studies all rely on electron beam evaporation combined with photolithography to prepare the grids. The equipment costs are high and the yield is low, which makes it difficult to meet the needs of large-scale production in the photovoltaic industry. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of existing copper electroplating grid lines with square or rectangular cross sections, which result in large shadows and high light loss under oblique light irradiation, as well as the defects of existing arched or trapezoidal grid line preparation which rely on complex electron beam evaporation combined with photolithography process, high equipment cost and low yield. The present invention provides an electroplating solution and electroplating method for arched cross section copper conductive grid lines of photovoltaic silicon panels.

[0006] To solve the above-mentioned technical problems, the present invention provides an electroplating solution for copper conductive grid lines with arched cross sections in photovoltaic silicon panels. Each liter of the electroplating solution contains the following components: 150-250 g of copper sulfate pentahydrate, 40-70 g of concentrated sulfuric acid, 10-100 mg of soluble chloride (calculated as chloride ions), 30-300 mg of polyethylene glycol, 1-20 mg of sodium polydisulfide dipropane sulfonate, 1-30 mg of methylene violet, and the balance being distilled water.

[0007] Further, preferably, each liter of the electroplating solution contains: 160-220 g of copper sulfate pentahydrate, 45-60 g of concentrated sulfuric acid, 60-100 mg of soluble chloride, 100-200 mg of polyethylene glycol, 1-5 mg of sodium polydisulfide dipropane sulfonate, 1-10 mg of methylene violet, and the balance being distilled water.

[0008] Furthermore, the average molecular weight of the polyethylene glycol is 4000 to 15000.

[0009] Furthermore, the soluble chloride is one or more of sodium chloride, potassium chloride, and calcium chloride.

[0010] This invention also provides a method for fabricating arched cross-section copper conductive grid lines on a photovoltaic silicon substrate. The specific method involves immersing a silicon wafer with patterned mask trenches into the electroplating solution described in any one of claims 1 to 3, wherein a copper seed layer is exposed at the bottom of the mask trenches; using the silicon wafer as the cathode and a phosphorus-containing copper plate or an inert electrode as the anode, an electroplating solution is applied at a current density of 1–5 A / dm². 2 Under certain conditions, electroplating is performed for 5–90 minutes to deposit copper in the mask trenches to form copper conductive grid lines; the mask is then removed to obtain copper conductive grid lines with an arched cross-section.

[0011] Furthermore, the width of the mask trench is 10–50 μm and the depth is 5–20 μm.

[0012] Furthermore, the cross-section of the obtained copper conductive grid wire is arched, and the ratio of its arch height to its base width is 0.3 to 0.8.

[0013] This invention regulates the deposition behavior of copper in mask trenches through the synergistic effect of chloride ions, polyethylene glycol, sodium polydisulfide sulfonate, and methylene violet. Specifically, chloride ions and polyethylene glycol combine to form a dense adsorption layer on the cathode surface, inhibiting copper deposition at the top and sidewalls of the trench; sodium polydisulfide sulfonate acts as an accelerator, preferentially adsorbing at the bottom of the trench, locally accelerating copper deposition; and based on the synergistic effect of these three components, the introduction of methylene violet further enhances the deposition inhibition effect at the top of the trench and promotes preferential growth at the bottom. The four components, aided by chloride ions, produce a synergistic effect, enabling preferential vertical growth of copper in the trench from bottom to top (protrusion deposition), thereby forming an arched copper conductive grid.

[0014] The beneficial effects of this invention are as follows: 1. This invention utilizes the synergistic effect of four components—chloride ions, polyethylene glycol, sodium dipropane sulfonate, and methylene violet—to form a high-curvature arched copper conductive grid line without sharp edges. Under the same bottom width, its oblique light projection area is significantly smaller than that of traditional square or rectangular cross-section grid lines. When sunlight is incident at an oblique angle, the arched structure effectively reduces shading loss. Simultaneously, the arched sidewalls can reflect some of the obliquely incident light to the antireflective layer on the cell surface and couple it into the silicon absorption layer, generating a secondary light absorption effect. This dual optical gain enables the photovoltaic cell to maintain a high photoelectric conversion efficiency even during non-vertical illumination periods of the day (such as morning and afternoon).

[0015] 2. The copper layer deposited by the electroplating solution of this invention has a smooth and dense surface, free of dendrites and pores. In the patterned mask trenches, copper is uniformly filled from bottom to top, without gaps or sidewall voids, ensuring the mechanical strength of the grid lines and their adhesion to the seed layer. The fully filled, dense copper grid lines have low bulk resistance and contact resistance, which helps reduce the series resistance of the battery and improve the fill factor.

[0016] 3. This invention directly electroplats pure copper grid lines, completely replacing silver paste. The resistivity of pure copper (approximately 1.7 μΩ·cm) is only 1 / 3 to 1 / 5 that of silver paste (5–10 μΩ·cm), effectively reducing ohmic losses in the grid lines. Simultaneously, copper is a low-cost material and eliminates the need for high-temperature sintering, further reducing energy consumption and equipment maintenance costs, resulting in significant economic benefits. The copper electroplating system is recyclable, with low wastewater treatment costs and no emissions of harmful substances such as lead or glass powder. Copper resources are abundant, and the supply chain is secure and stable, aligning with the development direction of green manufacturing and a circular economy.

[0017] 4. This invention eliminates the need for complex and expensive equipment such as electron beam evaporation and photolithography; the fabrication of arched grid lines can be achieved simply by changing the plating solution formulation in existing copper electroplating production lines. The electroplating parameter window is wide (current density 1–5 A / dm³). 2The process takes 5–90 minutes, is highly tolerant of process fluctuations, and is easy to scale up for industrial production. This technology is seamlessly compatible with mainstream battery technologies such as PERC, TOPCon, and HJT, requiring no additional modifications to the battery's front-end processes.

[0018] 5. This invention eliminates the need for high-temperature sintering throughout the entire process (traditional silver paste printing requires sintering at approximately 800°C), avoiding potential damage to the silicon wafer's lifespan and passivation film caused by high temperatures, making it particularly suitable for the manufacture of thin-film solar cells. Simultaneously, the electroplating process is carried out at room temperature or low temperature, resulting in low thermal stress and reducing the likelihood of grid lines warping or peeling.

[0019] 6. This invention is the first to realize the direct electroplating of high-curvature arched copper conductive grid lines in photovoltaic mask trenches, breaking through the bottlenecks of existing technologies from multiple dimensions such as optics, electricity, process and economy, and providing a practical solution for improving the photoelectric conversion efficiency of photovoltaic cells and reducing manufacturing costs. Attached Figure Description

[0020] Figure 1 It is a top-view optical image of the mask trench before electroplating and its corresponding cross-sectional profile.

[0021] Figure 2 The image shown is a top-view optical image of a laser confocal scanning microscope and its corresponding cross-sectional profile, as shown in Comparative Example 1.

[0022] Figure 3 The image shown is a top-view optical image of the laser confocal scanning microscope and its corresponding cross-sectional profile, as shown in Comparative Example 2.

[0023] Figure 4 The image shown is a top-view optical image of a laser confocal scanning microscope and its corresponding cross-sectional profile, as shown in Comparative Example 3.

[0024] Figure 5 The image shown is a top-view optical image of the laser confocal scanning microscope and its corresponding cross-sectional profile, as shown in Comparative Example 4.

[0025] Figure 6 This is a top-view optical image of a laser confocal scanning microscope and its corresponding cross-sectional profile, shown in Comparative Scale 5.

[0026] Figure 7 The image shown is a top-view optical image of a laser confocal scanning microscope (scale 6) and its corresponding cross-sectional profile.

[0027] Figure 8 This is a top-view optical image of the laser confocal scanning microscope and its corresponding cross-sectional profile, shown in Comparative Scale 7.

[0028] Figure 9 This is a field emission scanning electron microscope image of the cross-section of the copper grid line in Example 1.

[0029] Figure 10 This is a top-view optical image of the laser confocal scanning microscope of Example 1 and its corresponding cross-sectional profile.

[0030] Figure 11 This is a top-view optical image of the laser confocal scanning microscope of Example 3 and its corresponding cross-sectional profile.

[0031] Figure 12 This is a statistical chart showing the aspect ratio of the copper conductive grid lines prepared by the electroplating solutions of Examples 1 and Comparative Examples 1-7. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0033] Comparative Example 1 Add 160 g of copper sulfate pentahydrate and 45 g of concentrated sulfuric acid to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0034] Comparative Example 2 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, and 100 mg of polyethylene glycol (average molecular weight 8000) to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0035] Comparative Example 3 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, 100 mg of polyethylene glycol (average molecular weight 8000), and 1 mg of sodium dithiodipropane sulfonate to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0036] Comparative Example 4 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, and 1 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0037] Comparative Example 5 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 100 mg of polyethylene glycol (average molecular weight 10000), 1 mg of sodium polydisulfide dipropane sulfonate, and 2 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0038] Comparative Example 6 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, 100 mg of polyoxyethylene dodecyl ether, 1 mg of sodium polydisulfide dipropane sulfonate, and 1 mg of 2-mercaptopyridine to distilled water, mix well, and then dilute to 1 L with distilled water to obtain an electroplating solution.

[0039] Comparative Example 7 Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, 100 mg of polyoxyethylene-polyoxypropylene-polyoxyethylene block polyether, 1 mg of sodium polydisulfide dipropane sulfonate, and 1 mg of 6-dimethyl-2-pyrimidinethiol 2-mercaptopyridine to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution. Example 1

[0040] Add 160 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, 100 mg of polyethylene glycol (average molecular weight 6000), 1 mg of sodium polydisulfide dipropane sulfonate, and 1 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution. Example 2

[0041] Add 180 g of copper sulfate pentahydrate, 45 g of concentrated sulfuric acid, 60 mg of sodium chloride, 100 mg of polyethylene glycol (average molecular weight 8000), 2 mg of sodium polydisulfide dipropane sulfonate, and 2 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution. Example 3

[0042] Add 200 g of copper sulfate pentahydrate, 50 g of concentrated sulfuric acid, 80 mg of sodium chloride, 200 mg of polyethylene glycol (average molecular weight 10000), 5 mg of sodium polydisulfide dipropane sulfonate, and 5 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution. Example 4

[0043] Add 220 g of copper sulfate pentahydrate, 60 g of concentrated sulfuric acid, 100 mg of sodium chloride, 200 mg of polyethylene glycol (average molecular weight 10000), 5 mg of sodium polydisulfide dipropane sulfonate, and 10 mg of methylene violet to distilled water, mix well, and then dilute to 1 L with distilled water to obtain the electroplating solution.

[0044] The electroplating solutions prepared using the above comparative examples and embodiments were used to electroplat copper conductive grid lines on the surface of solar cell silicon wafers after physical vapor deposition of copper seed layers and mask patterning, respectively, according to the following copper electroplating process. The specific experimental results are as follows: The silicon wafer is 150 μm thick, and a copper seed layer with a thickness of 100 nm is sputtered on its surface using physical vapor deposition. Copper seed layers are deposited on both sides of the silicon wafer, and photoresist is coated onto the seed layers for patterning. The front-side gate pattern is designed as follows: main gate width 200 μm, sub-gate width 30 μm, and photoresist thickness 10 μm.

[0045] Pre-treatment process of solar silicon wafers: Immerse one end of the patterned solar silicon wafer covered with photoresist in a 50 ℃, 3 wt% NaOH aqueous solution for 5 min to remove the photoresist from one end and make it conductive. During single-sided electroplating, use insulating varnish to spray on the back side and let it dry to achieve back side insulation. Use electroplating clamps to hold the end with the photoresist removed.

[0046] Copper electroplating process: A solar silicon wafer is held in place by electroplating clamps and immersed in the electroplating solution as the cathode. A phosphorus copper plate containing 5 wt% phosphorus is immersed in the electroplating solution as the anode. A constant current source is connected to both electrodes, with a current density of 2 A / dm³. 2 Electroplating for 30 minutes. After the process, remove the solar silicon wafers, wash them with distilled water, and dry them.

[0047] Depend on Figure 1 As can be seen, the mask trench morphology before electroplating is regular, with a trench width of approximately 30 μm and a trench depth of approximately 10 μm, providing a good patterning basis for subsequent copper electroplating. In Comparative Example 1, electroplating was performed using a base electroplating solution containing only copper sulfate and sulfuric acid. The resulting copper grid lines had a flat cross-section with a slight central depression and a relatively rough surface (e.g., ...). Figure 2 As shown in the figure, this indicates that without any additives, copper deposition growth is uncontrolled, resulting in poor filling effect and inconsistent morphology. Comparative Example 2 added sodium chloride and polyethylene glycol, but not sodium polydisulfide dipropane sulfonate and methylene violet. The resulting copper grid lines had a rougher surface and exhibited obvious uneven deposition morphology (e.g., ...). Figure 3 As shown in the figure, it indicates that sodium chloride and polyethylene glycol additives alone are insufficient to effectively control the smoothness of copper deposition. Comparative Example 3 added sodium chloride, polyethylene glycol, and sodium polydithiopropane sulfonate, but did not add methylene violet. Under these conditions, the copper grid lines obtained had a flat cross-section, but the surface was relatively smooth (as shown in the figure). Figure 4 As shown in the figure, this indicates that the combination of sodium chloride, polyethylene glycol, and sodium dithiodipropane sulfonate can improve the surface smoothness of copper deposition, but still cannot achieve arched deposition. In Comparative Example 4, only methylene violet was added, resulting in a very rough copper grid surface with coarse and extremely uneven grain distribution (as shown in the figure). Figure 5As shown in the figure, this indicates that methylene violet alone cannot effectively suppress the dendrite growth of copper, making it difficult to obtain continuous and dense copper grid lines. Comparative Example 5 added polyethylene glycol, sodium polydithiopropane sulfonate, and methylene violet, but did not add chloride ions. Under these conditions, the surface smoothness of the copper grid lines decreased, and local bumps and depressions appeared (e.g., Figure 6 As shown in the figure, this indicates that the absence of chloride ions weakens the leveling ability of polyethylene glycol, disrupts the synergistic effect between additives, and fails to achieve the desired arched cross-section. Comparative Example 6 added sodium chloride, polyoxyethylene dodecyl ether, sodium polydisulfide dipropane sulfonate, and 2-mercaptopyridine; Comparative Example 7 added sodium chloride, polyoxyethylene-polyoxypropylene-polyoxyethylene block polyether, sodium polydisulfide dipropane sulfonate, and 6-dimethyl-2-pyrimidine mercaptan. The copper grid lines prepared using both plating solutions were unsatisfactory, failing to form a regular arched cross-section (e.g., ...). Figure 7 and Figure 8 (As shown). In Example 1, sodium chloride, polyethylene glycol, sodium polydithiopropane sulfonate, and methylene violet were added simultaneously as additives. The resulting copper grid wire cross-section exhibited a distinct arched deposition morphology and a smooth, dense surface (as shown). Figure 9 (as shown) Figure 10 Further optical images obtained from laser confocal imaging show that the copper grid surface has good reflectivity, and light can be effectively reflected on the grid surface. This indicates that the addition of methylene violet, through the synergistic effect of the additive, regulates the deposition and growth mode of copper, allowing copper to preferentially grow longitudinally in the trenches, thereby achieving the fabrication of high aspect ratio arched copper conductive grids. Figure 11 The copper grid wire in Example 3 also exhibits a typical arched structure and good surface reflectivity. These results demonstrate that, within the formulation range defined in this invention, arched cross-section copper conductive grid wires can be prepared through the synergistic effect of additives.

[0048] A statistical comparison was made of the aspect ratios of the copper grid lines prepared by the electroplating solutions of Examples 1 and 1-7, such as... Figure 12 As shown in the figure. The results show that the copper grid lines prepared using the electroplating solution of Example 1 have the highest aspect ratio, reaching 0.5; while the aspect ratios of the grid lines prepared using the other comparative electroplating solutions are significantly lower than those of Example 1, and some comparative examples even fail to form a regular arched cross-section. This further demonstrates the superiority of the electroplating solution described in this invention in controlling the copper deposition morphology and obtaining a high arched cross-section.

Claims

1. An electroplating solution for copper conductive grid lines with arched cross-sections in photovoltaic silicon panels, characterized in that, Each liter of the electroplating solution comprises: 150–250 g of copper sulfate pentahydrate, 40–70 g of concentrated sulfuric acid, 10–100 mg of soluble chloride, 30–300 mg of polyethylene glycol, 1–20 mg of sodium polydisulfide dipropane sulfonate, 1–30 mg of methylene violet, and the remainder being distilled water.

2. The electroplating solution for arched cross-section copper conductive grid lines in photovoltaic silicon panels according to claim 1, characterized in that, Each liter of the electroplating solution comprises: 160–220 g of copper sulfate pentahydrate, 45–60 g of concentrated sulfuric acid, 60–100 mg of soluble chloride, 100–200 mg of polyethylene glycol, 1–5 mg of sodium dithiopropane sulfonate, 1–10 mg of methylene violet, and the remainder being distilled water.

3. The electroplating solution for arched cross-section copper conductive grid lines in photovoltaic silicon panels according to claim 1, characterized in that, The average molecular weight of the polyethylene glycol is 4000 to 15000.

4. The electroplating solution for arched cross-section copper conductive grid lines in photovoltaic silicon panels according to claim 1, characterized in that, The soluble chloride is one or more of sodium chloride, potassium chloride, and calcium chloride.

5. A method for fabricating arched cross-section copper conductive grid lines on a photovoltaic silicon substrate, characterized in that, A silicon wafer with patterned mask trenches is immersed in the electroplating solution according to any one of claims 1 to 4, wherein a copper seed layer is exposed at the bottom of the mask trenches; the silicon wafer is used as the cathode, and a phosphorus copper plate or an inert electrode is used as the anode, at a current density of 1 to 5 A / dm². 2 Under certain conditions, electroplating is performed for 5–90 minutes to deposit copper in the mask trenches to form copper conductive grid lines; the mask is then removed to obtain copper conductive grid lines with an arched cross-section.

6. The method for preparing arched cross-section copper conductive grid lines on a photovoltaic silicon substrate according to claim 5, characterized in that, The width of the mask trench is 10–50 μm and the depth is 5–20 μm.

7. The method for preparing arched cross-section copper conductive grid lines on a photovoltaic silicon substrate according to claim 5, characterized in that, The cross-section of the resulting copper conductive grid wire is arched, and the ratio of its arch height to its base width is 0.3 to 0.8.

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