Solar cell metal grid line, preparation method thereof, back contact solar cell and photovoltaic module

Abstract

The invention provides a solar cell metal grid line and a preparation method thereof, a back contact solar cell and a photovoltaic module, and relates to the technical field of solar cells, the solar cell metal grid line comprises the steps that a first seed layer, a second seed layer and a conductive layer are sequentially arranged in a silicon substrate area where the grid line is to be formed in a laminated mode; the first seed layer comprises a nickel layer, and the second seed layer comprises a tin-bismuth alloy layer. According to the invention, the problem of high-precision and high-density back wiring of the BC solar cell is solved, and the unification of high efficiency, high reliability and low cost is realized.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and in particular to a solar cell metal grid line, a method for preparing the same, a back-contact solar cell, and a photovoltaic module. Background Technology

[0002] Back contact Back-contact (BC) solar cells are a type of photovoltaic cell based on crystalline silicon materials. They are characterized by high conversion efficiency and low manufacturing cost, and are considered one of the important development directions of the photovoltaic industry in the future. Back-contact solar cells have no grid lines on the front side, and both the main grid and the fine grid lines are on the back side, so they have low requirements for light shading. Among them, the fine grids are evenly and densely arranged in parallel on the surface of the cell to collect the local current on the surface of the cell, while the main grids are evenly arranged with larger spacing to collect the current of the fine grids on both sides.

[0003] Traditional screen-printed silver paste processes have limited linewidth and precision. Silver paste itself has a higher resistivity than copper, and its consumption is relatively high, with data showing a silver consumption of approximately 14 mg / W. Furthermore, the sintering temperature of silver paste is as high as 700℃ or more, resulting in significant energy consumption. The silver paste contains glass powder and organic matter, leading to high bulk resistivity and contact resistance, which limits further improvements in fill factor and conversion efficiency. During photovoltaic module manufacturing, conductive solder strips need to be applied to the main busbar. Traditional high-temperature soldering processes generate significant thermal stress, easily causing microcracks in the silicon wafer and affecting the long-term reliability of the module. Therefore, there is an urgent need for a BC cell metallization interconnection solution that can simultaneously address the issues of cost, efficiency, and reliability.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] One objective of this invention is to provide a metal grid line for solar cells, thereby addressing at least one of the technical problems existing in the prior art. This invention solves the challenge of high-precision, high-density back-side wiring in BC solar cells, while simultaneously achieving a balance between high efficiency, high reliability, and low cost.

[0006] The second objective of this invention is to provide a method for preparing metal grid lines for solar cells.

[0007] The third objective of this invention is to provide a back-contact solar cell.

[0008] The fourth objective of this invention is to provide a photovoltaic module.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a solar cell metal grid line, comprising: sequentially stacking a first seed layer, a second seed layer and a conductive layer in a region of a silicon substrate where the grid line is to be formed; The first seed layer comprises a nickel layer, and the second seed layer comprises a tin-bismuth alloy layer.

[0010] Furthermore, a passivation layer is provided on the surface of the silicon substrate; Preferably, the passivation layer has patterned grooves, and the first seed layer is at least partially located within the grooves and forms an ohmic contact with the silicon substrate; Preferably, the thickness of the first seed layer is 0.2~0.5 μm; Preferably, the thickness of the second seed layer is 2.0~5.0 μm; Preferably, the conductive layer includes a conductive solder strip with a coating on its surface; Preferably, the thickness of the coating is 10~40µm; Preferably, the width of the conductive solder strip is 100~400um; Preferably, the substrate of the conductive solder strip is high-purity oxygen-free copper; purity ≥ 99.99%; Preferably, the surface coating of the conductive solder strip is one or more of tin-bismuth alloy, tin-bismuth-silver alloy, tin-indium alloy, and tin-bismuth-indium alloy; Preferably, the cross-sectional shape of the conductive solder strip is one or more of the following: circular, rectangular, and triangular. Preferably, the thickness of the passivation layer is 80~120 nm; Preferably, the depth of the groove is 80~160nm; the width is 20~100um.

[0011] In a second aspect, the present invention provides a method for preparing metal grid lines for a solar cell, comprising: sequentially stacking a first seed layer, a second seed layer and a conductive layer in a region of a silicon substrate where the grid lines are to be formed; the first seed layer comprising a nickel layer and the second seed layer comprising a tin-bismuth alloy layer.

[0012] Furthermore, the region where the gate line is to be formed is obtained by patterning on a passivation layer on the back side of the silicon substrate; Preferably, the patterning process includes using a laser to create grooves on the passivation layer that expose the surface of the silicon substrate; Preferably, the wavelength of the laser is 355nm; the average power is 1~5W; the repetition frequency is 100~500kHz; and the scanning speed is 1~5m / s.

[0013] Furthermore, the preparation process of the first seed layer includes: The laser-grooved silicon substrate is placed in a nickel electroplating solution, and an electric current is applied to deposit a nickel layer. Preferably, the nickel plating solution comprises: 200-300 g / L nickel sulfate, 30-60 g / L nickel chloride, 35-45 g / L boric acid, 0.05-0.2 g / L sodium dodecyl sulfonate, and the remainder is water; Preferably, the pH of the nickel plating solution is 3.5~4.5; and the temperature is 30~60℃. Preferably, the current density of the deposited nickel layer is 0.3~1.0 A / dm². 2 The deposition time is 30-60 seconds. Preferably, the preparation process of the first seed layer further includes: activating the silicon substrate after laser grooving before depositing the nickel layer; Preferably, the primary activation includes: placing the laser-grooved silicon substrate in a first activation solution for activation; Preferably, the first activation solution comprises: 10-50 mL / L hydrofluoric acid, with the remainder being water; Preferably, the temperature of the first activation solution is 20~30℃; the activation time is 30~120s.

[0014] Furthermore, after preparing the first seed layer and before preparing the second seed layer, the process further includes: annealing; Preferably, the annealing includes: placing the silicon substrate with the first seed layer in a cold or preheated furnace cavity, introducing a protective gas, then heating it to a set peak temperature, holding it at that temperature for a period of time, and then cooling it down. Preferably, the protective gas includes one or more of nitrogen, argon, and a nitrogen-hydrogen mixture with a volume concentration of 3% to 5% hydrogen. Preferably, the heating rate is 10~50℃ / s; Preferably, the peak temperature is 350~450℃; Preferably, the heat preservation time is 60~180s.

[0015] Furthermore, the preparation process of the second seed layer includes: placing the annealed silicon substrate in a tin electroplating solution and applying an electric current to deposit a tin-bismuth alloy layer; Preferably, the tin plating solution comprises: 20-100 g / L tin methanesulfonate, 50-150 mL / L methanesulfonic acid, 2-10 g / L bismuth methanesulfonate, 1-5 mL / L stabilizer, and the remainder is water; Preferably, the stabilizer includes one or more of β-phenolsulfonic acid, citric acid, tartaric acid, catechol, resorcinol, and hydroquinone; Preferably, the temperature of the tin plating solution is 20~40℃; pH < 1.0; Preferably, the current density of the deposited tin-bismuth alloy layer is 0.5~1.0 A / dm².2 The deposition time is 60~180s; Preferably, the preparation process of the second seed layer further includes: performing secondary activation on the annealed silicon substrate before depositing the second seed layer; Preferably, the secondary activation includes: activating the annealed silicon substrate in a second activation solution; Preferably, the second activation solution comprises: 1-10 mL / L of methanesulfonic acid, with the remainder being water; Preferably, the temperature of the second activation solution is 20~30℃; the time for the second activation is 15~60s.

[0016] Furthermore, the conductive layer is applied to the second seed layer by a lay-up method; Preferably, the laying process includes: applying liquid flux to the second seed layer on the back side of the silicon substrate by spraying, foaming or brushing, then using a jig to place the conductive solder strip with a surface coating onto the second seed layer coated with flux, applying pressure to the conductive solder strip, heating and soldering it at the same time, and removing it after cooling. Preferably, the liquid flux includes a no-clean, halogen-free low-temperature flux; Preferably, the welding temperature is 150~200℃; Preferably, the pressure is 1~5N; Preferably, the welding time is 1 to 3 seconds.

[0017] Secondly, the present invention provides a back-contact solar cell, comprising the solar cell metal grid lines prepared by the aforementioned preparation method.

[0018] Thirdly, the present invention provides a photovoltaic module, including the aforementioned back-contact solar cell.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The solar cell metal grid provided by this invention constructs a multilayer metallized structure by sequentially depositing a nickel layer as a first seed layer and a tin-bismuth alloy layer as a second seed layer on a silicon substrate, and then forming a conductive layer on top of these layers. This significantly improves the electrical performance and interface stability of the grid. The nickel layer forms a good ohmic contact with the silicon substrate, effectively reducing the metal-semiconductor contact resistance and minimizing carrier recombination losses at the contact interface, which is beneficial for improving the open-circuit voltage and fill factor of the cell. The tin-bismuth alloy layer, as the second seed layer, not only has excellent solderability and conductivity, but its low melting point also provides a basis for subsequent low-temperature welding, avoiding thermal damage to the silicon wafer caused by high temperatures. This multilayer structure also provides a compatible interface for the direct integration of copper-based conductive solder strips, realizing a low-cost interconnection solution that is all-copper and silver-free. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the structure of the metal grid line of a solar cell provided in an embodiment of the present invention. Detailed Implementation

[0022] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0023] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] like Figure 1 As shown, the first aspect of the present invention provides a solar cell metal grid line, comprising: sequentially stacking a first seed layer, a second seed layer and a conductive layer in a region of a silicon substrate where the grid line is to be formed; the first seed layer comprising a nickel layer and the second seed layer comprising a tin-bismuth alloy layer.

[0025] This invention solves the problem of high-precision, high-density back-side wiring in BC solar cells by using a metallization and interconnection scheme similar to "semiconductorization" with "silver removal" and "low temperature" as the core, while achieving a balance between high efficiency, high reliability and low cost.

[0026] In some preferred embodiments, a passivation layer is provided on the surface of the silicon substrate; Preferably, the passivation layer has patterned grooves, and the first seed layer is at least partially located within the grooves and forms an ohmic contact with the silicon substrate; Preferably, the thickness of the first seed layer is 0.2~0.5um, for example, it can be 0.2um, 0.3um, 0.4um, 0.5um, etc.; Preferably, the thickness of the second seed layer is 2.0~5.0 μm, for example, it can be 2.0 μm, 3.0 μm, 4.0 μm, 5.0 μm, etc.; Preferably, the conductive layer includes a conductive solder strip with a coating on its surface; Preferably, the thickness of the coating is 10~40um, for example, it can be 10um, 15um, 20um, 25um, 30um, 40um, etc.; Preferably, the width of the conductive solder strip is 100~400um, for example, it can be 100um, 200um, 300um, 400um, etc.; Preferably, the substrate of the conductive solder strip is high-purity oxygen-free copper; purity ≥ 99.99%; Preferably, the surface coating of the conductive solder strip is one or more of tin-bismuth alloy, tin-bismuth-silver alloy, tin-indium alloy, and tin-bismuth-indium alloy; Preferably, the cross-sectional shape of the conductive solder strip is one or more of the following: circular, rectangular, and triangular. Preferably, the thickness of the passivation layer is 80~120nm, for example, it can be 80nm, 90nm, 100nm, 110nm, 120nm, etc.; Preferably, the depth of the groove is 80~160nm, for example, it can be 80nm, 100nm, 120nm, 140nm, 160nm, etc.; the width is 20~100um, for example, it can be 20um, 40um, 60um, 80um, 100um, etc.

[0027] A second aspect of the present invention provides a method for fabricating a metal grid line in a solar cell, comprising: sequentially stacking a first seed layer, a second seed layer and a conductive layer in a region of a silicon substrate where the grid line is to be formed; the first seed layer comprising a nickel layer and the second seed layer comprising a tin-bismuth alloy layer.

[0028] This invention employs a process of first electroplating a nickel + tin-bismuth alloy, followed by welding copper solder strips, to completely eliminate the need for silver plating. Firstly, the bulk resistivity of copper is significantly lower than that of silver paste containing glass powder and organic matter, significantly reducing the resistance of the grid lines themselves and directly improving the fill factor of the BC cell. Simultaneously, after annealing, nickel can form excellent ohmic contact with silicon, with a contact resistance far lower than that between silver paste and silicon. Precise laser film opening and metal deposition result in a smaller and more controllable metal-silicon contact area, greatly reducing metal-induced recombination and thus contributing to an increase in the open-circuit voltage of the BC cell. Finally, low-temperature welding significantly reduces the risk of microcracks and cell breakage during welding and lamination, significantly improving the cell manufacturing yield and the long-term reliability of the module for outdoor use.

[0029] In some preferred embodiments, the region where the gate line is to be formed is obtained by patterning on a passivation layer on the back side of the silicon substrate; Preferably, the patterning process includes using a laser to create grooves on the passivation layer that expose the surface of the silicon substrate; Preferably, the wavelength of the laser is 355nm; the average power is 1~5W; the repetition frequency is 100~500kHz; and the scanning speed is 1~5m / s.

[0030] In some preferred embodiments, the preparation process of the first seed layer includes: The laser-grooved silicon substrate is placed in a nickel electroplating solution, and an electric current is applied to deposit a nickel layer. Preferably, the nickel plating solution comprises: 200-300 g / L nickel sulfate, 30-60 g / L nickel chloride, 35-45 g / L boric acid, 0.05-0.2 g / L sodium dodecyl sulfonate, and the remainder is water.

[0031] The nickel plating solution contains nickel sulfate at a concentration of 200-300 g / L, such as 200 g / L, 250 g / L, 300 g / L, etc. The concentration of nickel chloride in the nickel plating solution is 30~60 g / L, for example, it can be 30 g / L, 40 g / L, 50 g / L, 60 g / L, etc. In the nickel electroplating solution, the concentration of boric acid is 35~45 g / L, for example, it can be 35 g / L, 40 g / L, or 45 g / L; The concentration of sodium dodecyl sulfonate in the nickel plating solution is 0.05~0.2g / L, for example, it can be 0.05g / L, 0.1g / L, 0.15g / L, 0.2g / L, etc.

[0032] Preferably, the pH of the nickel plating solution is 3.5~4.5; the temperature is 30~60℃, for example, 30℃, 40℃, 50℃, 60℃, etc. Preferably, the current density of the deposited nickel layer is 0.3~1.0 A / dm². 2 For example, it could be 0.3A / dm 2 0.4A / dm 2 0.5A / dm 2 0.6A / dm 2 0.7A / dm 2 0.8A / dm 2 0.9A / dm 2 1.0 A / dm 2 The deposition time is 30-60 seconds, for example, it can be 30 seconds, 40 seconds, 50 seconds, 60 seconds, etc. Preferably, the preparation process of the first seed layer further includes: activating the silicon substrate after laser grooving before depositing the nickel layer; Preferably, the primary activation includes: placing the laser-grooved silicon substrate in a first activation solution for activation; Preferably, the first activation solution comprises: 10-50 mL / L hydrofluoric acid, with the remainder being water; In the first activation solution, the concentration of hydrofluoric acid is 10~50 mL / L, for example, it can be 10 mL / L, 20 mL / L, 30 mL / L, 40 mL / L, 50 mL / L, etc.

[0033] Preferably, the temperature of the first activation solution is 20~30℃, for example, 20℃, 25℃, 30℃, etc.; the activation time is 30~120s, for example, 30s, 75s, 120s, etc.

[0034] In some preferred embodiments, after preparing the first seed layer and before preparing the second seed layer, the process further includes: annealing; Preferably, the annealing includes: placing the silicon substrate with the first seed layer in a cold or preheated furnace cavity, introducing a protective gas, then heating it to a set peak temperature, holding it at that temperature for a period of time, and then cooling it down. Preferably, the protective gas includes one or more of nitrogen, argon, and a nitrogen-hydrogen mixture with a volume concentration of 3% to 5% hydrogen. Preferably, the heating rate is 10~50℃ / s, for example, it can be 10℃ / s, 20℃ / s, 30℃ / s, 40℃ / s, 50℃ / s, etc.; Preferably, the peak temperature is 350~450℃, for example, it can be 350℃, 400℃, 450℃, etc.; Preferably, the heat preservation time is 60~180s, for example, it can be 60s, 120s, 180s, etc.

[0035] In some preferred embodiments, the preparation process of the second seed layer includes: placing an annealed silicon substrate in a tin plating solution and applying an electric current to deposit a tin-bismuth alloy layer; Preferably, the tin plating solution comprises: 20-100 g / L tin methanesulfonate, 50-150 mL / L methanesulfonic acid, 2-10 g / L bismuth methanesulfonate, 1-5 mL / L stabilizer, and the remainder is water; The concentration of tin methanesulfonate in the tin plating solution is 20~100g / L, for example, it can be 20g / L, 60g / L, 100g / L, etc. The concentration of methanesulfonic acid in the tin plating solution is 50~150mL / L, for example, it can be 50mL / L, 100mL / L, 150mL / L, etc. The concentration of bismuth methanesulfonate in the tin plating solution is 2~10 g / L, for example, it can be 2 g / L, 6 g / L, 10 g / L, etc. The concentration of the stabilizer in the tin plating solution is 1~5 mL / L, for example, it can be 1 mL / L, 3 mL / L, 5 mL / L, etc.

[0036] Preferably, the stabilizer includes one or more of β-phenolsulfonic acid, citric acid, tartaric acid, catechol, resorcinol, and hydroquinone; Preferably, the temperature of the tin plating solution is 20~40℃, for example, 20℃, 30℃, 40℃, etc.; pH < 1.0; Preferably, the current density of the deposited tin-bismuth alloy layer is 0.5~1.0 A / dm². 2 For example, it could be 0.5A / dm 2 0.75A / dm 2 1.0A / dm 2 The deposition time is 60~180s, for example, it can be 60s, 120s, 180s, etc. Preferably, the preparation process of the second seed layer further includes: performing secondary activation on the annealed silicon substrate before depositing the second seed layer; Preferably, the secondary activation includes: activating the annealed silicon substrate in a second activation solution; Preferably, the second activation solution comprises: 1-10 mL / L of methanesulfonic acid, with the remainder being water; In the second activation solution, the concentration of methanesulfonic acid is 1~10 mL / L, for example, it can be 1 mL / L, 5.5 mL / L, 10 mL / L, etc. Preferably, the temperature of the second activation solution is 20~30℃, for example, 20℃, 25℃, 30℃, etc.; the secondary activation time is 15~60s, for example, 15s, 40s, 60s, etc.

[0037] In some preferred embodiments, the conductive layer is applied to the second seed layer by a lay-up method; Preferably, the laying process includes: applying liquid flux to the second seed layer on the back side of the silicon substrate by spraying, foaming or brushing, then using a jig to place the conductive solder strip with a surface coating onto the second seed layer coated with flux, applying pressure to the conductive solder strip, heating and soldering it at the same time, and removing it after cooling. Preferably, the liquid flux includes a no-clean, halogen-free low-temperature flux; Preferably, the welding temperature is 150~200℃, for example, it can be 150℃, 170℃, 200℃, etc.; Preferably, the pressure is 1~5N, for example, it can be 1N, 2N, 3N, 4N, 5N, etc.; Preferably, the welding time is 1 to 3 seconds, for example, 1 second, 2 seconds, 3 seconds, etc.

[0038] A third aspect of the present invention provides a back-contact solar cell, comprising solar cell metal grid lines prepared by the aforementioned preparation method.

[0039] In a preferred embodiment of the present invention, the method for fabricating the back-contact solar cell includes the following steps: Step 1: Provide a BC battery silicon substrate with a pre-fabricated PN junction and back passivation layer; Step 2: First, use laser grooving technology to form a patterned groove on the back of the silicon substrate of the BC cell. Then, deposit the first seed layer (thin nickel layer), form an alloy after rapid thermal annealing, and then deposit the second seed layer (thicker tin-bismuth alloy layer). Finally, lay a conductive layer (conductive solder strip with a coating on the surface) on the second seed layer, apply a certain temperature and pressure, and weld to obtain a back contact solar cell. Step 2.1, the process of depositing the first seed layer is as follows: After laser grooving, the silicon substrate of the BC battery is first activated in an activation solution, washed with water and dried with nitrogen. Then, it is placed face down in a nickel plating solution and a certain current is applied to begin depositing a nickel layer. After the deposition is completed, it is washed with pure water and dried. Step 2.2 Rapid thermal annealing: The BC cell silicon substrate with the first seed layer is placed in a cold or preheated furnace chamber, a protective gas is introduced, and then the temperature is raised to the set temperature at an extremely high rate. After holding at the temperature for a period of time, the temperature is rapidly reduced. Step 2.3, the process of depositing the second seed layer, is as follows: The annealed BC battery silicon substrate was first activated in an activation solution, then placed directly in a tin plating solution and a certain current was applied to begin depositing a tin-bismuth alloy layer. After deposition, it was washed with pure water and dried with nitrogen. Step 2.4, the process of laying the conductive layer, is as follows: First, liquid flux is precisely applied to the second seed layer on the back of the silicon substrate of the BC cell by spraying, foaming or brushing. Then, a high-precision fixture is used to precisely place the conductive solder strip with a coating onto the second seed layer with flux. The hot pressure head of the stringer is precisely lowered to apply a certain pressure to the conductive solder strip and heat it for welding. After cooling, it is removed to obtain the back contact solar cell.

[0040] A fourth aspect of the present invention provides a photovoltaic module, including the aforementioned back-contact solar cell.

[0041] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0042] Example 1 This embodiment provides a back-contact solar cell, the preparation method of which includes the following steps: Step 1: Provide a BC battery silicon substrate with a pre-fabricated PN junction and back passivation layer; The passivation layer has a thickness of 100 nm.

[0043] Step 2: First, use laser grooving technology to form patterned grooves on the back side of the BC cell silicon substrate (wherein, the laser grooving wavelength is 355nm, the average power is 3W, the repetition frequency is 300 kHz, the scanning speed is 3 m / s, the laser grooving depth is 120 nm, and the linewidth is 60 μm). Then, a first seed layer (thin nickel layer) is deposited, which is rapidly thermally annealed to form an alloy. A second seed layer (a thicker tin-bismuth alloy layer) is then deposited. Finally, a conductive layer (conductive solder strip with a coating) is laid on the second seed layer. A certain temperature and pressure are applied, and after welding, a back contact solar cell is obtained. Step 2.1, the process of depositing the first seed layer is as follows: After laser grooving, the silicon substrate of the BC battery is first activated in an activation solution, washed with water and dried with nitrogen. Then, it is placed face down in a nickel plating solution and a certain current is applied to begin depositing a nickel layer. After the deposition is completed, it is washed with pure water and dried. The activation solution consisted of 30 mL / L hydrofluoric acid and the remainder was pure water; the temperature of the activation solution was 25℃ and the activation time was 75 s. The nickel plating solution includes: 250 g / L nickel sulfate, 45 g / L nickel chloride, 40 g / L boric acid, 0.1 g / L sodium dodecyl sulfonate, and the remainder is pure water; the pH of the nickel plating solution is 3.5~4.5, and the temperature is 45℃. The current density for nickel deposition was 0.7 A / dm³. 2 The deposition time was 45 seconds. The thickness of the first seed layer is 0.35 μm; Step 2.2, Rapid thermal annealing: The BC battery silicon substrate with the first seed layer is placed in a preheated furnace chamber, nitrogen gas is introduced, and then the temperature is raised to 400°C at a rate of 30°C / s, held for 120 s, and then rapidly cooled. Step 2.3, the process of depositing the second seed layer, is as follows: The annealed BC battery silicon substrate was first activated in an activation solution, then placed directly in a tin plating solution and a certain current was applied to begin depositing a tin-bismuth alloy layer. After deposition, it was washed with pure water and dried with nitrogen. The activation solution consisted of 6 mL / L methanesulfonic acid and the remainder being pure water; the temperature of the activation solution was 25℃; and the activation time was 40 s. The tin plating solution comprises: 60 g / L tin methanesulfonate, 100 mL / L methanesulfonic acid, 6 g / L bismuth methanesulfonate, 3 mL / L β-phenolsulfonic acid, and the remainder water; the temperature of the tin plating solution is 30℃, and the pH is <1.0. The current density for depositing the tin-bismuth alloy was 0.75 A / dm. 2 The deposition time was 120 s; The thickness of the second seed layer is 3.5 μm.

[0044] Step 2.4, the process of laying the conductive layer, is as follows: First, liquid flux is precisely applied to the second seed layer on the back of the BC cell silicon substrate by brushing. Then, a high-precision fixture is used to precisely place the conductive solder strip with a coating onto the second seed layer with flux. The hot pressure head of the stringer is precisely lowered to apply a certain pressure to the conductive solder strip and heat it for welding. After cooling, it is removed to obtain the back contact solar cell. The liquid flux is a no-clean, halogen-free, low-temperature flux; the substrate of the conductive solder strip is high-purity oxygen-free copper with a purity ≥99.99%; the surface coating of the conductive solder strip is a tin-bismuth alloy; the thickness of the surface coating of the conductive solder strip is 25 μm; the cross-sectional shape of the conductive solder strip is circular; and the width of the conductive solder strip is 300 μm. The welding temperature was 175℃; the pressure of the hot press head was 3N; and the welding time was 2s.

[0045] Example 2 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the activation solution comprises: 10 mL / L hydrofluoric acid, with the remainder being pure water; the activation temperature is 30°C, and the activation time is 30 s; the nickel plating solution comprises: 200 g / L nickel sulfate, 60 g / L nickel chloride, 35 g / L boric acid, 0.2 g / L sodium dodecyl sulfonate, with the remainder being pure water; the nickel plating solution temperature is 30°C; and the current density for nickel deposition is 0.3 A / dm³. 2 The deposition time was 60 s; the thickness of the first seed layer was 0.2 μm. In step 2.2, the temperature is increased to 450℃ at a rate of 10℃ / s, held for 60s, and then rapidly cooled. In step 2.3, the activation solution comprises: 1 mL / L methanesulfonic acid, with the remainder being pure water; the temperature of the activation solution is 30℃; the activation time is 15 s; the tin plating solution comprises: 20 g / L tin methanesulfonate, 150 mL / L methanesulfonic acid, 2 g / L bismuth methanesulfonate, 5 mL / L β-phenolsulfonic acid, and the remainder being water; the temperature of the tin plating solution is 20℃; the current density for depositing the tin-bismuth alloy is 0.5 A / dm³. 2 The deposition time was 180s; the thickness of the second seed layer was 2.0um.

[0046] In step 2.4, the welding temperature is 150℃; the pressure of the hot press head is 5N; and the welding time is 1s.

[0047] Example 3 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the activation solution comprises: 50 mL / L hydrofluoric acid, with the remainder being pure water; the activation temperature is 20°C, and the activation time is 120 s; the nickel plating solution comprises: 300 g / L nickel sulfate, 30 g / L nickel chloride, 45 g / L boric acid, 0.05 g / L sodium dodecyl sulfonate, with the remainder being pure water; the nickel plating solution temperature is 60°C; and the current density for nickel deposition is 1.0 A / dm³. 2 The deposition time was 30 seconds; the thickness of the first seed layer was 0.5 μm. In step 2.2, the temperature is increased to 350℃ at a rate of 50℃ / s, held for 180s, and then rapidly cooled. In step 2.3, the activation solution includes: 10 mL / L methanesulfonic acid, with the remainder being pure water; the temperature of the activation solution is 20℃; the activation time is 60s; the tin plating solution includes: 100g / L tin methanesulfonate, 50mL / L methanesulfonic acid, 10g / L bismuth methanesulfonate, 1 mL / L β-phenolsulfonic acid, and the remainder being water; the temperature of the tin plating solution is 40℃; the current density for depositing the tin-bismuth alloy is 1.0A / dm2, and the deposition time is 60s; the thickness of the second seed layer is 5.0µm.

[0048] In step 2.4, the welding temperature is 200℃; the pressure of the hot press head is 1N; and the welding time is 3s.

[0049] Example 4 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the temperature of the activation solution is 15℃, and the activation time is 150s; In step 2.3, the temperature of the activation solution is 15℃ and the activation time is 70s.

[0050] Example 5 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the temperature of the activation solution is 35℃, and the activation time is 20s; In step 2.3, the temperature of the activation solution is 35℃ and the activation time is 10s.

[0051] Example 6 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the current density for nickel deposition is 0.2 A / dm³. 2 The deposition time was 70 s; the thickness of the first seed layer was 0.15 μm. Example 7 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.1, the current density for nickel deposition is 1.5 A / dm³. 2 The deposition time was 20 s; the thickness of the first seed layer was 0.60 μm. Example 8 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.3, the current density for depositing the tin-bismuth alloy is 0.4 A / dm³. 2 The deposition time was 200s; the thickness of the second seed layer was 5.5um.

[0052] Example 9 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.3, the current density for depositing the tin-bismuth alloy is 1.5 A / dm³. 2 The deposition time was 50 s; the thickness of the second seed layer was 4.5 μm.

[0053] Example 10 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.2, the temperature is raised to 300℃.

[0054] Example 11 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.2, the temperature is raised to 500℃.

[0055] Example 12 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.4, the welding temperature is 100℃ and the welding time is 4s.

[0056] Example 13 This embodiment provides a back-contact solar cell, which differs from Embodiment 1 in that: In step 2.4, the welding temperature is 250℃ and the welding time is 0.5s.

[0057] Comparative Example 1 This comparative example provides a back-contact solar cell, which differs from Example 1 in that: The second layer of tin-bismuth alloy is replaced with a tin layer. The tin plating solution consists of: 50 g / L stannous sulfate, 160 g / L concentrated sulfuric acid, 1 mL / L resorcinol, and the balance being water.

[0058] Comparative Example 2 This comparative example provides a back-contact solar cell, which differs from Example 1 in that: No activation is performed in steps 2.1 and 2.2.

[0059] Comparative Example 3 This comparative example provides a back-contact solar cell, which differs from Example 1 in that: The fine grid and main grid of the back contact battery are printed using screen printing technology, and then sintered at high temperature after baking to form silver electrodes.

[0060] Test case Test samples: The back-contact solar cells prepared in Examples 1-13 and Comparative Examples 1-3 were used as samples for testing.

[0061] Test method: (1) Contact resistance test: The obtained BC electroplated nickel / tin semi-finished battery was cut into cells along the main grid line direction to make test samples. The contact resistance (Ω) of the P region and N region was measured by the four-probe method, and the specific contact resistivity (mΩ·cm) was calculated. 2 ); (2) Electrical performance test (photoelectric conversion efficiency, open circuit voltage, series resistance); specifically, IV test is used, and the test is conducted at a temperature of 25℃ and AM of 1.5G.

[0062] The test results are shown in Table 1.

[0063] Table 1

[0064] As shown in Table 1, the nickel / tin-bismuth alloy laminate structure constructed in this invention has significant advantages in achieving low contact resistance and high electrical performance. The specific contact resistivity of both the P-region and N-region in Examples 1-3 is within the ideal range (0.8~1.0 mΩ·cm). 2 Within this range, the open-circuit voltage reaches 734~738 mV, and the photoelectric conversion efficiency exceeds 26.4%, indicating that the structure can effectively form a good ohmic contact and suppress carrier recombination. Among them, the primary activation treatment (using hydrofluoric acid to remove the natural oxide layer) is crucial to improving the interface quality between nickel and silicon substrate. For example, Comparative Example 2 did not undergo activation, resulting in the nickel layer not completely covering the silicon substrate, and some tin layer directly contacting the silicon substrate, increasing the contact resistivity, thereby deteriorating the battery's electrical performance and making the grid lines prone to detachment. At the same time, the annealing process needs to be controlled within the range of 350~450℃. If it is too low (such as 300℃ in Example 10), it is difficult to form a sufficient nickel-silicon alloy, and if it is too high (such as 500℃ in Example 11), it may cause nickel to diffuse into the passivation layer, destroying the interface stability. Both of these conditions cause it to deviate from the optimal range. In addition, the thickness of the first seed layer (nickel layer) should be moderate (preferably 0.2~0.5 μm). Compared with Example 1, if the nickel layer is too thin (such as in Example 6, 0.15 μm), it is easy for the metal to penetrate into the silicon body during the subsequent heat treatment of the tin-bismuth layer. If the nickel layer is too thick (such as in Example 7), it will increase the resistance and stress, both of which will degrade the electrical performance.

[0065] In Examples 4-5, the activation temperature and time deviated from the preferred range, resulting in insufficient interface cleaning or excessive corrosion, which increased contact resistance and reduced performance. In Examples 8-9, the second seed layer was too thick or too thin, affecting welding consistency and electrical conductivity. In Examples 12-13, the welding temperature was too low or too high, or the time was inappropriate, resulting in poor connection or thermal damage, which significantly increased series resistance and reduced conversion efficiency.

[0066] The use of a tin-bismuth alloy instead of pure tin in the second seed layer (Comparative Example 1) also demonstrates its crucial role: the tin-bismuth alloy has a lower melting point, which is beneficial for low-temperature welding (150~200℃) and avoids high-temperature damage; as an intermediate layer with excellent solderability, it enhances the bonding force and conductive continuity between the copper solder strip and the underlying layer. Although Comparative Example 1 was successfully electroplated, its contact resistance was high, with the series resistance increasing to 0.00235 Ω and the efficiency only 25.68%, indicating that the pure tin layer is inferior to the tin-bismuth alloy in terms of interface matching and stable conduction.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A type of metal grid wire for a solar cell, characterized in that, include: A first seed layer, a second seed layer, and a conductive layer are sequentially stacked in the region of the silicon substrate where the gate line is to be formed; The first seed layer comprises a nickel layer, and the second seed layer comprises a tin-bismuth alloy layer.

2. The solar cell metal grid line according to claim 1, characterized in that, A passivation layer is provided on the surface of the silicon substrate; Preferably, the passivation layer has patterned grooves, and the first seed layer is at least partially located within the grooves and forms an ohmic contact with the silicon substrate; Preferably, the thickness of the first seed layer is 0.2~0.5 μm; Preferably, the thickness of the second seed layer is 2.0~5.0 μm; Preferably, the conductive layer includes a conductive solder strip with a coating on its surface; Preferably, the thickness of the coating is 10~40µm; Preferably, the width of the conductive solder strip is 100~400um; Preferably, the substrate of the conductive solder strip is high-purity oxygen-free copper; Purity ≥ 99.99%; Preferably, the surface coating of the conductive solder strip is one or more of tin-bismuth alloy, tin-bismuth-silver alloy, tin-indium alloy, and tin-bismuth-indium alloy; Preferably, the cross-sectional shape of the conductive solder strip is one or more of the following: circular, rectangular, and triangular. Preferably, the thickness of the passivation layer is 80~120 nm; Preferably, the depth of the groove is 80~160nm; the width is 20~100um.

3. The method for fabricating the metal grid lines of a solar cell as described in claim 1 or 2, characterized in that, include: A first seed layer, a second seed layer, and a conductive layer are sequentially stacked in the region of the silicon substrate where the gate line is to be formed. The first seed layer comprises a nickel layer, and the second seed layer comprises a tin-bismuth alloy layer.

4. The preparation method according to claim 3, characterized in that, The area to be formed as a gate line is obtained by patterning on a passivation layer on the back side of a silicon substrate; Preferably, the patterning process includes using a laser to create grooves on the passivation layer that expose the surface of the silicon substrate; Preferably, the wavelength of the laser is 355nm; the average power is 1~5W; the repetition frequency is 100~500kHz; and the scanning speed is 1~5m / s.

5. The preparation method according to claim 4, characterized in that, The preparation process of the first seed layer includes: The laser-grooved silicon substrate is placed in a nickel electroplating solution, and an electric current is applied to deposit a nickel layer. Preferably, the nickel plating solution comprises: 200-300 g / L nickel sulfate, 30-60 g / L nickel chloride, 35-45 g / L boric acid, 0.05-0.2 g / L sodium dodecyl sulfonate, and the remainder is water; Preferably, the pH of the nickel plating solution is 3.5~4.5; and the temperature is 30~60℃. Preferably, the current density of the deposited nickel layer is 0.3~1.0 A / dm². 2 The deposition time is 30-60 seconds. Preferably, the preparation process of the first seed layer further includes: activating the silicon substrate after laser grooving before depositing the nickel layer; Preferably, the primary activation includes: placing the laser-grooved silicon substrate in a first activation solution for activation; Preferably, the first activation solution comprises: 10-50 mL / L hydrofluoric acid, with the remainder being water; Preferably, the temperature of the first activation solution is 20~30℃; the activation time is 30~120s.

6. The preparation method according to claim 3, characterized in that, After preparing the first seed layer and before preparing the second seed layer, the process further includes: annealing; Preferably, the annealing includes: placing the silicon substrate with the first seed layer in a cold or preheated furnace cavity, introducing a protective gas, then heating it to a set peak temperature, holding it at that temperature for a period of time, and then cooling it down. Preferably, the protective gas includes one or more of nitrogen, argon, and a nitrogen-hydrogen mixture with a volume concentration of 3% to 5% hydrogen. Preferably, the heating rate is 10~50℃ / s; Preferably, the peak temperature is 350~450℃; Preferably, the heat preservation time is 60~180s.

7. The preparation method according to claim 6, characterized in that, The preparation process of the second seed layer includes: placing the annealed silicon substrate in a tin electroplating solution and applying an electric current to deposit a tin-bismuth alloy layer; Preferably, the tin plating solution comprises: 20-100 g / L tin methanesulfonate, 50-150 mL / L methanesulfonic acid, 2-10 g / L bismuth methanesulfonate, 1-5 mL / L stabilizer, and the remainder is water; Preferably, the stabilizer includes one or more of β-phenolsulfonic acid, citric acid, tartaric acid, catechol, resorcinol, and hydroquinone; Preferably, the temperature of the tin plating solution is 20~40℃; pH < 1.0; Preferably, the current density of the deposited tin-bismuth alloy layer is 0.5~1.0 A / dm². 2 The deposition time is 60~180s; Preferably, the preparation process of the second seed layer further includes: performing secondary activation on the annealed silicon substrate before depositing the second seed layer; Preferably, the secondary activation includes: activating the annealed silicon substrate in a second activation solution; Preferably, the second activation solution comprises: 1-10 mL / L of methanesulfonic acid, with the remainder being water; Preferably, the temperature of the second activation solution is 20~30℃; the time for the second activation is 15~60s.

8. The preparation method according to claim 3, characterized in that, The conductive layer is applied to the second seed layer by a lay-up method; Preferably, the laying process includes: applying liquid flux to the second seed layer on the back side of the silicon substrate by spraying, foaming or brushing, then using a jig to place the conductive solder strip with a surface coating onto the second seed layer coated with flux, applying pressure to the conductive solder strip, heating and soldering it at the same time, and removing it after cooling. Preferably, the liquid flux includes a no-clean, halogen-free low-temperature flux; Preferably, the welding temperature is 150~200℃; Preferably, the pressure is 1~5N; Preferably, the welding time is 1 to 3 seconds.

9. A back-contact solar cell, characterized in that, This includes solar cell metal grid lines prepared by the preparation method according to any one of claims 3-8.

10. A photovoltaic module, characterized in that, Including the back-contact solar cell as described in claim 9.

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