Metal electrode for solar cell, preparation method of metal electrode and solar cell

Abstract

The invention relates to the technical field of solar energy, and discloses a metal electrode for a solar cell, a preparation method of the metal electrode and the solar cell. The metal electrode comprises a metal seed layer (which is at least one of a silver seed layer, a nickel seed layer, a zirconium seed layer, a titanium seed layer, a chromium seed layer, a nickel-silicon alloy seed layer and a copper alloy seed layer) arranged in a grid line area on the surface of a silicon substrate, and a base metal grid line (which is at least one of a copper grid line and an aluminum grid line) arranged on the surface of the metal seed layer, and a copper phosphate protection layer (which is at least one of a copper phosphate layer and an organic copper phosphate compound layer) arranged on the surface of the base metal grid line. Through cooperation of the metal seed layer, the base metal grid lines (such as copper grid lines) and the copper phosphate protection layer, the metal electrode can reduce contact resistance, ensure interface bonding force, save cost and improve corrosion resistance of the base metal grid lines, so that stability and reliability of the metal electrode in outdoor long-term use are improved, and attenuation of battery efficiency is slowed down.

Description

Technical Field

[0001] The present invention relates to the field of solar energy technology, and in particular to a metal electrode for a solar cell, a preparation method thereof, and a solar cell. Background Art

[0002] Crystalline silicon solar cells (solar cells for short) generally require the use of metal electrodes (also known as metal grid lines); for example, metals such as silver, copper, and aluminum are used to make metal grid lines on the surface of the cell. These metal grid lines mainly collect and conduct photogenerated current in the cell. Among them, metallic silver has better conductivity, but its price is also higher. Silver accounts for more than 60% of the non-silicon cost of crystalline silicon solar cells and about 15-20% of the total cost of the cell. Currently, about 10% of the world's silver production is used in the solar industry, and it is clear that the production capacity of metallic silver cannot expand rapidly with the expansion of the solar industry. "Silver shortage" will become a problem that the solar industry must face.

[0003] Currently, a more feasible approach is to replace base metals, such as using inexpensive base metals like copper and aluminum instead of silver to make the metal grid lines of solar cells. For example, the price of silver paste (used to make silver grid lines, with a price of 8,000-9,000 yuan / kg) is about 150 times that of copper (which costs around 60 yuan / kg).

[0004] However, although the cost of copper electrodes is lower, the conductivity of copper electrodes is slightly worse than that of silver electrodes (the conductivity of silver electrodes is 62.1×10 6 S / m, while the conductivity of copper electrode is 59.6×10 6 S / m), and copper electrodes have poor bonding properties to silicon surfaces. Therefore, using copper as an electrode increases contact resistance with the silicon of crystalline silicon solar cells, leading to current loss and even possible delamination and shedding, resulting in reduced cell efficiency.

[0005] In this regard, the currently feasible method is partial substitution, such as using silver-clad copper or silver wire copper plating to make metal electrodes. Among them, as shown in publication number CN119593028A, silver wire copper plating is to use methods such as screen printing to prepare a low-height silver wire on the surface of the battery as a contact layer with the silicon surface, and use the high conductivity and low resistance of silver to contact the silicon surface to reduce contact resistance and reduce contact loss; then copper is electroplated on the surface of the silver wire, using copper as a current transmission material. Although this silver wire copper plating can reduce the consumption of metallic silver while ensuring contact as much as possible, copper is more active than silver, so copper has poor corrosion resistance. During the outdoor use of photovoltaic modules, they need to face industrial pollution (sulfur dioxide, nitrogen oxides and other gases emitted from industrial areas), salt spray in coastal areas, EVA film hydrolysis, water vapor penetration, high temperature and high humidity and other outdoor environments. Compared with silver grid lines, copper grid lines are obviously more susceptible to corrosion. Therefore, existing technical solutions such as copper plating of silver wires do not provide a better protection method, and it is difficult to ensure that the copper grid wires can remain stable during the 30-year outdoor use cycle of photovoltaic modules, resulting in a decrease in battery efficiency. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a metal electrode for a solar cell, a preparation method thereof, and a solar cell.

[0007] Based on this, the present invention discloses a metal electrode for a solar cell, comprising a metal seed layer provided in a grid line region on a silicon substrate surface, a base metal grid line provided on a surface of the metal seed layer, and a copper phosphate protective layer provided on a surface of the base metal grid line;

[0008] The metal seed layer is at least one of a silver seed layer, a nickel seed layer, a zirconium seed layer, a titanium seed layer, a chromium seed layer, a nickel silicon alloy seed layer, and a copper alloy seed layer;

[0009] The base metal grid line is at least one of a copper grid line and an aluminum grid line;

[0010] The copper phosphate protective layer is at least one of a copper phosphate layer and an organic copper phosphate composite layer.

[0011] Preferably, the metal seed layer is a silver seed layer, and its thickness is 0.8-1.2 μm.

[0012] Preferably, the base metal grid lines are copper grid lines, and their height is 8-15 μm.

[0013] Preferably, the copper phosphate protective layer is a copper phosphate layer with a thickness of 8-12 nm.

[0014] Preferably, the copper phosphate protective layer is a Cu3(PO4)2·copper phytate complex layer, and its thickness is 8-12 nm.

[0015] Further preferably, a nickel transition layer is provided between the base metal gate line and the copper phosphate protection layer, and the thickness of the nickel transition layer is 8-15 nm.

[0016] Further preferably, the silicon substrate includes: a silicon wafer and a passivation film arranged on the front and / or back of the silicon wafer; the gate line area of ​​the passivation film is provided with an open film area for locally exposing the silicon substrate, and the metal electrode contacts the silicon substrate through the open film area.

[0017] More preferably, the silicon substrate further includes a doped silicon layer disposed between the silicon wafer and the passivation film.

[0018] The present invention also discloses a method for preparing a metal electrode for a solar cell, comprising the following steps:

[0019] Step S1: preparing a silicon substrate, wherein the silicon substrate comprises: a silicon wafer and a passivation film provided on the front and / or back of the silicon wafer;

[0020] Step S2: preparing a metal electrode on the surface of the silicon substrate, which includes:

[0021] Step S21, opening the passivation film in the predetermined gate line area on the surface of the silicon substrate to form an open film area locally exposing the silicon surface;

[0022] Step S22: a metal seed layer, a base metal gate line and a copper phosphate protection layer are sequentially prepared on the silicon surface of the film-opening area to obtain a metal electrode.

[0023] Preferably, in step S22 , the metal seed layer is prepared by, but not limited to, direct sputtering or electroplating; the base metal gate line is prepared by, but not limited to, electroplating.

[0024] Preferably, in step S22, the copper phosphate protective layer is prepared by methods including but not limited to electroplating, solution co-precipitation, hydrothermal method or physical mixing.

[0025] Further preferably, in step S22, when the copper phosphate protective layer is a copper phosphate layer, and the metal seed layer and / or the base metal gate line are made of a copper-containing material, the electroplating process is as follows:

[0026] First, prepare a phosphate electrolyte: it includes 0.2-0.4 mol / L K3PO4, 0.1-0.2 mol / L H3PO4, 0.05-0.1% of the total volume of the phosphate electrolyte oxidant and 0.01-0.02% of the total volume of the phosphate electrolyte corrosion inhibitor, control the pH of the phosphate electrolyte at 3-5, apply a constant voltage of +0.5V to +1.5V, and electroplate for 60-90 seconds at room temperature to obtain a copper phosphate layer.

[0027] More preferably, in step S22, when the copper phosphate protective layer is a Cu3(PO4)2·copper phytate composite layer, the electroplating process is as follows:

[0028] First, prepare copper phosphate electroplating solution: it includes the phosphate electrolyte, phytic acid and Cu 2+ The organic phase is configured in a molar ratio of 1:5-8, the pH value of the electroplating solution is adjusted to 4.5-5.5, a constant voltage of +0.5V to +1.0V is applied, and electroplating is carried out at room temperature for 150-200 seconds to obtain a Cu3(PO4)2·copper phytate complex layer.

[0029] Preferably, in step S22, before forming the copper phosphate protective layer, the method further includes forming a nickel transition layer on the surface of the base metal gate line. The method for forming the nickel transition layer includes but is not limited to electroplating.

[0030] The present invention also discloses a solar cell, which comprises: a silicon substrate, and metal electrodes arranged on the front and / or back of the silicon substrate;

[0031] The silicon substrate comprises: a silicon wafer and a passivation film provided on the front and / or back of the silicon wafer;

[0032] The metal electrode is the metal electrode for solar cells described above in the present invention.

[0033] Compared with the prior art, the present invention has at least the following beneficial effects:

[0034] The present invention utilizes the characteristic of copper phosphate being extremely stable in a humid or weakly acidic environment to prepare a copper phosphate protective layer on the surface of the base metal grid line to improve the corrosion resistance of the base metal grid line, such as the copper grid line. Therefore, the metal electrode of the present invention, through the mutual cooperation of the metal seed layer (especially the silver seed layer), the base metal grid line (especially the copper grid line) and the copper phosphate protective layer, can not only reduce the contact resistance and ensure the interface bonding force to prevent falling off, but also greatly reduce the silver consumption and save costs. It can also improve the corrosion resistance of the base metal grid line, such as the copper grid line, so that the stability of the base metal grid line is improved, and the reliability of the long-term outdoor use of the non-silver metal electrode is improved; thereby, it can effectively slow down the attenuation of the photoelectric conversion efficiency of the battery after the metal electrode is corroded (i.e., the attenuation of the battery efficiency). Moreover, the preparation of the metal electrode of the present invention does not require the additional investment in new equipment, and can be directly implemented on existing equipment (electroplating equipment), which also saves equipment investment costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the cross-sectional structure of a solar cell of the present invention.

[0036] Explanation of the accompanying drawings: silicon substrate 1; silicon wafer 10; p+ emitter 11; aluminum oxide film 12; front silicon nitride film 13; silicon oxide tunneling layer 14; n+ polysilicon layer 15; back silicon nitride film 16; metal electrode 2; metal seed layer 20; base metal gate line 21; nickel transition layer 22; copper phosphate protective layer 23. DETAILED DESCRIPTION

[0037] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] A metal electrode 2 for solar cells of the present invention, see Figure 1 , including a metal seed layer 20 provided in the gate line area on the surface of the silicon substrate 1, a base metal gate line 21 provided on the surface of the metal seed layer 20, and a copper phosphate protective layer 23 provided on the surface of the base metal gate line 21.

[0039] The metal seed layer 20 may be at least one of a silver seed layer, a nickel seed layer, a zirconium seed layer, a titanium seed layer, a chromium seed layer, a nickel-silicon alloy seed layer, and a copper alloy seed layer. The metal seed layer 20 serves two main purposes: first, to enhance adhesion or bonding, allowing the metal grid lines to adhere better to the battery surface and prevent them from falling off; and second, to reduce contact resistance.

[0040] In order to reduce contact resistance in the present invention, the metal seed layer 20 is preferably a silver seed layer with the best conductivity. The thickness of the metal seed layer 20 is 0.8-1.2 μm (eg, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm or 1.2 μm).

[0041] The base metal grid lines 21 are at least one of copper grid lines and aluminum grid lines, preferably copper grid lines, with a height of 8-15 μm (eg, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or 15 μm).

[0042] The copper phosphate protective layer 23 is at least one of a copper phosphate layer and an organic copper phosphate composite layer. For example, the copper phosphate protective layer 23 is a copper phosphate layer having a thickness of 8-12 nm (eg, 8 nm, 9 nm, 10 nm, 11 nm, or 12 nm).

[0043] The acid generated during the use of photovoltaic modules mainly comes from acetic acid produced by the hydrolysis of EVA film. The secondary acid comes from the hydrolysis of sulfides, nitrogen oxides and other substances in the air. Therefore, copper phosphate can be used as an excellent protective layer for the base metal grid lines 21. The main reasons are as follows:

[0044] 1. Copper phosphate is extremely stable in a humid or weakly acidic environment and hardly reacts. Copper phosphate can dissolve in strong acid, but is extremely resistant to weak acid, such as dilute acetic acid.

[0045] 2. Phosphates are chemically inert. Copper phosphate can effectively prevent copper grid lines from reacting with sulfides in the air (such as H2S and SO2) to form Cu2S, which has poor conductivity. It can also effectively prevent low concentrations of nitrogen oxides in the air (such as NO and NO2) from corroding the copper grid lines. Although other copper compounds (such as Cu2S, Cu2O, CuI, CuCl, and CuBr) are slightly more conductive than copper phosphate, copper phosphate can prevent the copper grid lines from converting into other copper compounds, as these other copper compounds have their own hazards. For example, Cu2S has poor stability and will decompose under high temperature and humidity. It also absorbs light and affects the light absorption rate of solar cells. For another example, Cu2O, as an alkaline oxide, is easily corroded by acetic acid, which in turn causes corrosion of the copper grid lines. For another example, CuI (copper halides, including CuI, CuCl, and CuBr) is extremely easy to decompose under light conditions and is unstable. Even with various technical means, its existence time under light conditions is only a few days, which can cause corrosion of the copper grid lines. Furthermore, the copper phosphate layer used as the protective layer is extremely thin (its thickness is only 8-12 nm), and does not affect the conductivity of the copper grid lines that serve as the conductive body of the metal electrode 2. Therefore, compared with other copper compounds (such as Cu2S, Cu2O, CuI, CuCl, CuBr, etc.), the copper phosphate protective layer 23 is preferred as the protective layer.

[0046] 3. The crystal structure of copper phosphate can lock base metal ions (such as copper ions), reduce the risk of migration, and produce an effect similar to field passivation.

[0047] 4. Copper phosphate has weak absorption in the visible light range and will not cause shading loss like Cu2S. Even if a small amount of copper phosphate protective layer 23 is attached to the copper grid line area when preparing the copper phosphate protective layer 23, the copper phosphate protective layer 23 absorbs very little sunlight, which helps to improve the light absorption and light utilization rate of solar cells.

[0048] 5. Copper phosphate material is environmentally friendly and non-toxic, and is easy to prepare.

[0049] Therefore, the present invention utilizes the extremely strong stability of copper phosphate in humid or weakly acidic environments to prepare a copper phosphate protective layer 23 on the surface of the base metal grid line 21 to improve the corrosion resistance of the base metal grid line 21, such as the copper grid line. Therefore, the metal electrode 2 of the present invention, through the interaction of the above-mentioned metal seed layer 20 (especially the silver seed layer), the base metal grid line 21 (especially the copper grid line), and the copper phosphate protective layer 23, can not only reduce contact resistance, but also greatly reduce silver consumption and reduce costs. It can also improve the corrosion resistance of the base metal grid line 21, such as the copper grid line, thereby improving the stability of the base metal grid line 21 and improving the reliability of the non-silver metal electrode 2, thereby effectively slowing down the attenuation of the battery's photoelectric conversion efficiency after the metal electrode 2 corrodes.

[0050] Furthermore, under high temperature and high humidity (85°C / 85% humidity), the copper phosphate layer may slowly hydrolyze to form hydroxy copper phosphate Cu2(OH)PO4 and lose its corrosion resistance. Therefore, the copper phosphate protective layer 23 is preferably an organic copper phosphate complex layer (such as a Cu3(PO4)2·phytate copper complex layer) with a thickness of 8-12 nm to improve its corrosion resistance, adhesion and functionality. The mechanism of action of the organic copper phosphate complex layer is as follows: the hydrophobicity and chelating ability of the organic phosphate can compensate for the instability of copper phosphate in a strong hot and humid environment and reduce the peeling of protective layers such as the copper phosphate layer.

[0051] Taking the Cu3(PO4)2·phytate copper complex layer as an example, the multiple phosphate groups (-PO4 3- ) and Cu on the surface of Cu3(PO4)2 2+ or PO4 3- Vacancy bonding forms a core-shell structure (Cu3(PO4)2·copper phytate complex). At this time, the hydrophobic carbon chains of phytic acid are arranged outward, giving the complex hydrophobicity (contact angle>90°), thereby reducing water vapor accumulation, and ultimately further reducing corrosion and avoiding peeling of the protective layer.

[0052] Furthermore, the copper phosphate protective layer 23 (especially the copper phosphate layer) and the base metal grid lines 21 (such as copper grid lines) have different thermal expansion coefficients (the thermal expansion coefficient of the copper grid lines is 17 ppm / °C; while the thermal expansion coefficient of the copper phosphate layer is lower, ranging from 4-10 ppm / °C). This may cause the copper phosphate layer on the surface of the copper grid lines to crack or peel. Therefore, the present invention preferably provides a nickel transition layer 22 between the base metal grid lines 21 and the copper phosphate protective layer 23 to enhance interfacial bonding. The thickness of the nickel transition layer 22 is 8-15 nm (e.g., 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, or 15 nm).

[0053] The nickel transition layer 22 can enhance the mechanical adhesion of the copper phosphate protective layer 23 to the surface of the base metal grid line 21, such as the copper grid line. This is mainly because: the thermal expansion coefficients of the materials between the copper grid line and the copper phosphate layer are quite different. When the temperature changes, a large amount of mechanical stress will be generated between the copper grid line and the copper phosphate layer, which may cause the copper phosphate layer to fall off. The thermal expansion coefficient of the copper grid line is 17ppm / °C; while the thermal expansion coefficient of the copper phosphate layer is even lower, within the range of 4-10ppm / °C; the thermal expansion coefficient of the C70600 copper alloy (Cu content is 90%, Ni content is 10%) is 16.2ppm / °C, the thermal expansion coefficient of the CuNi34 alloy (Cu content is 57%, Ni content is 34%) is 13.5ppm / °C, and the thermal expansion coefficient of the nickel transition layer 22 is 13.4ppm / °C. The nickel transition layer 22 has a thermal expansion coefficient that lies between the copper grid and the copper phosphate layer, acting as a transition. Furthermore, the nickel transition layer 22 forms chemical bonds with the copper grid and the copper phosphate layer, allowing for chemical bonding. Furthermore, the high micro-roughness of the nickel transition layer 22 increases its contact area with the copper grid and the copper phosphate layer, enhancing its mechanical anchoring to the copper grid and the copper phosphate layer. Therefore, the addition of the nickel transition layer 22 can reduce the generation of mechanical stress during temperature fluctuations and improve the adhesion between the base metal grid 21 (e.g., copper grid) and the copper phosphate protective layer 23.

[0054] The silicon substrate 1 includes a silicon wafer 10 and a passivation film disposed on the front and / or back of the silicon wafer 10. The passivation film has an open film region in the gate line region that partially exposes the silicon substrate 1. The metal electrode 2 contacts the silicon substrate 1 through the open film region. The passivation film is preferably an aluminum oxide film 12 and / or a silicon nitride film.

[0055] Furthermore, the silicon substrate 1 further includes a doped silicon layer disposed between the silicon wafer 10 and the passivation film.

[0056] Take TOPCon battery as an example. Figure 1As shown, the silicon substrate 1 includes: a silicon wafer 10, a front doped silicon layer (such as a p+ emitter 11 provided on the front of the silicon wafer 10) and a front passivation film (such as an aluminum oxide film 12 and a front silicon nitride film 13 provided on the front of the p+ emitter 11), and a back doped silicon layer (such as a silicon oxide tunneling layer 14 and an n+ polysilicon layer 15 provided on the back of the silicon wafer 10) and a back passivation film (such as a back silicon nitride film 16 provided on the back of the n+ polysilicon layer 15) provided on the back of the silicon wafer 10. Moreover, since the TOPCon battery has metal electrodes 2 on both the front and back sides, a front film-opening area for locally exposing the p+ emitter 11 is opened in the gate line area of ​​the front passivation film, so that the front metal electrode 2 contacts the p+ emitter 11 through the front film-opening area; a back film-opening area for locally exposing the n+ polysilicon layer 15 is opened in the gate line area of ​​the back passivation film, so that the back metal electrode 2 contacts the n+ polysilicon layer 15 through the back film-opening area.

[0057] Of course, the metal electrode 2 for solar cells of the present invention can be used not only in TOPCon cells, but also in other types of solar cells in the photovoltaic industry.

[0058] The present invention provides a method for preparing a metal electrode 2 for a solar cell, comprising the following steps:

[0059] Step S1: prepare a silicon substrate 1.

[0060] Step S2, preparing a metal electrode 2 on the surface of the silicon substrate 1, which specifically includes:

[0061] Step S21, opening the passivation film in the predetermined gate line area on the surface of the silicon substrate 1 to form an open film area of ​​the local exposed silicon surface (such as the surface of the p+ emitter 11 and the surface of the n+ polysilicon layer 15);

[0062] Step S22 , sequentially preparing a metal seed layer 20 , a base metal gate line 21 and a copper phosphate protection layer 23 on the silicon surface of the film-opening area, thereby obtaining a metal electrode 2 .

[0063] Specifically, the metal seed layer 20 can be prepared by direct sputtering or electroplating. After the metal seed layer 20 is prepared by electroplating, the silicon substrate 1 is cleaned with water (preferably deionized water) (optimally ultrasonic cleaning) and then the base metal gate lines 21 are prepared.

[0064] Taking the electroplating silver seed layer as an example, the following electroplating solution formula can be used: the mass ratio of silver nitrate, EDTA (ethylenediaminetetraacetic acid), glucose, and deionized water is 1.5-3:20:10:1000, the electroplating temperature is 60-70°C, the electroplating time is 3-8 minutes, and the pH of the electroplating solution is 10-10.5 (pH is adjusted with an alkaline solution such as KOH).

[0065] Specifically, the base metal grid lines 21 may be prepared by, but are not limited to, electroplating. After the base metal grid lines 21 are prepared by electroplating, the silicon substrate 1 is cleaned (optimally, ultrasonic cleaning) with ethanol, dilute hydrochloric acid, and water (preferably deionized water) in sequence to remove grease, oxide layers, and plating solution residues from the surface of the silicon substrate 1.

[0066] The following takes the electroplating of copper grid lines as an example. It is divided into two steps: the first step is pre-plating of neutral copper, and the second step is standard acid copper electroplating:

[0067] The first step is to pre-plating neutral copper. The plating solution formula is as follows: the mass ratio of copper citrate, sodium citrate, boric acid, sodium sulfate, antioxidant, and deionized water is 18-25:90-120:20:10:0.1:1000, the pH is controlled at 6.8-7.2 (adjusted with KOH or H3PO4), the plating temperature is 25-30℃, and the DC pulse current density is 1.0-1.5A / dm 2 , the plating time is 2-3 minutes.

[0068] The second step is standard acid copper electroplating. The plating solution formula is as follows: the mass ratio of copper sulfate, sulfuric acid, potassium chloride, sodium citrate, antioxidant, and deionized water is 180-250:30:0.05:10:0.1:1000, the electroplating temperature is 25-30℃; the current mode is pulse reverse current, and the forward current density is 5-15A / dm 2 , reverse current density is 20-40A / dm 2 , the forward current accounts for 90% and the reverse current accounts for 10%; the electroplating time is 10-12 minutes. After the base metal grid lines 21 of the copper grid lines are prepared, the silicon substrate 1 is ultrasonically cleaned with ethanol, dilute hydrochloric acid (HCl content is 5-10% by volume), and deionized water in sequence to remove grease, oxide layers, and electroplating solution residues from the surface of the silicon substrate 1.

[0069] Specifically, the preparation method of the copper phosphate layer includes but is not limited to electroplating. The following takes the electroplating of the copper phosphate layer as an example to specifically describe its electroplating process: first, prepare a phosphate electrolyte, the basic formula of which is as follows: including 0.2-0.4 mol / L K3PO4 and 0.1-0.2 mol / L H3PO4 (pH 3-5, adjusted with H3PO4 or KOH); then add an oxidant (such as H2O2, H2O2 accounts for 0.05-0.1% of the total volume of the phosphate electrolyte) to accelerate copper dissolution and film formation; then add a corrosion inhibitor (such as benzotriazole, benzotriazole accounts for 0.01-0.02% of the total volume of the phosphate electrolyte) to control the reaction rate and improve the uniformity of the film layer.

[0070] It should be noted that the oxidant is mainly used to maintain the reduction potential of metal ions in the plating solution or electrolyte, promotes metal deposition, and affects the uniformity of the coating. In practice, the oxidant can also have other options, such as persulfate (such as ammonium persulfate). The core of the corrosion inhibitor is to protect the silicon substrate 1 and the metal layer from corrosion by the plating solution or electrolyte, while ensuring that the base metal grid line 21 of the copper grid line has an accurate morphology. The corrosion inhibitor can also have other options, including silicate (such as Na2SiO3), azole compounds (such as mercaptobenzothiazole) or amines (such as hexadecylamine) etc.

[0071] After preparing the phosphate electrolyte, apply a constant voltage of +0.5V to +1.5V using the phosphate electrolyte and electroplate for 60-90 seconds at room temperature. The following reaction is achieved through electroplating:

[0072] 3Cu+2PO4 3- →Cu3(PO4)2+6e - A copper phosphate layer can be electroplated on the surface of a base metal grid line 21 such as a copper grid line. After the reaction is completed, rinse with water (preferably deionized water) and then blow dry with hot nitrogen to complete the preparation of the metal electrode 2, that is, a solar cell.

[0073] Specifically, if the base metal grid line 21 and the metal seed layer 20 are both made of non-copper materials, when preparing the copper phosphate layer on the surface of the base metal grid line 21, copper salts such as copper pyrophosphate need to be added to the electroplating solution used as a copper source. The electroplating solution formula at this time is as follows (taking copper pyrophosphate as the copper source as an example): the mass ratio of copper pyrophosphate, potassium pyrophosphate, dipotassium hydrogen phosphate, potassium citrate, and deionized water is 60-100:240-400:50:20:1000, the pH of the electroplating solution is 8.5-8.8 (adjusted with H3PO4 or KOH), the electroplating temperature is 50-55°C; the cathode current density is 1-5A / dm 2 The electroplating time is 90-100 seconds, and mechanical stirring must be performed during the electroplating process to prevent precipitation and changes in local copper ion concentration.

[0074] Specifically, the preparation method of the organic copper phosphate complex layer includes but is not limited to electroplating, solution coprecipitation, hydrothermal method or physical mixing method. The thickness of the organic copper phosphate complex layer is 8-12nm (such as 8nm, 9nm, 10nm, 11nm or 12nm). The organic copper phosphate complex layer includes but is not limited to a phytate copper complex layer: a Cu3(PO4)2·phytate copper complex layer. The electroplating process is specifically described below using the electroplating of the Cu3(PO4)2·phytate copper complex layer as an example:

[0075] In the above phosphate electrolyte, the following organic phase is additionally added: phytic acid (C6H 18 O24 P6) and Cu 2+ The electroplating solution is prepared in a molar ratio of 1:5-8. The pH of the electroplating solution is adjusted to 4.5-5.5 using KOH to prevent phytic acid precipitation due to excessive acidity. The constant voltage is adjusted to +0.5V to +1.0V, and the electroplating time is increased to 3 minutes (150-200 seconds) at room temperature (to prevent phytic acid precipitation due to excessive acidity of the solution, the acidity of the electroplating solution is reduced, so the time needs to be slightly increased). The phosphate groups of the copper phytate bind to the Cu3(PO4)2 surface through coordination bonds or hydrogen bonds, thereby electroplating a Cu3(PO4)2·copper phytate complex layer on the surface of the base metal grid line 21, such as the copper grid line.

[0076] Furthermore, in step S22, before forming the copper phosphate protective layer 23, the process further includes forming a nickel transition layer 22 on the surface of the base metal gate line 21. The preparation method of the nickel transition layer 22 includes but is not limited to electroplating, and the details are described in the prior art, so it is not described here.

[0077] In practice, the preparation process of the front metal electrode 2 and / or the back metal electrode 2 refers to the above steps S21 - 22 , and thus will not be described in detail here.

[0078] A solar cell of the present invention comprises: a silicon substrate 1, and metal electrodes 2 provided on the front and / or back of the silicon substrate 1 (that is, the metal electrodes 2 include a front metal electrode 2 and / or a back metal electrode 2);

[0079] The metal electrode 2 is the metal electrode 2 for solar cells described above in the present invention.

[0080] The following embodiments take TOPCon battery as an example. Figure 1 As shown, a metal electrode 2 of the present invention, a solar cell having the metal electrode 2 and a preparation method thereof are specifically described:

[0081] Example 1

[0082] A method for preparing a solar cell according to this embodiment is shown in FIG. Figure 1 , which comprises the following preparation steps:

[0083] Step 1, the silicon wafer 10 is processed as follows: the silicon wafer 10 is textured, the front side is diffused to prepare the p+ emitter 11, the back side is cleaned and polished, a passivation contact structure is prepared on the back side (which includes a silicon oxide tunneling layer 14 and an n+ polysilicon layer 15), and a passivation film is plated (for example, the front side passivation film is an aluminum oxide film 12 and a front side silicon nitride film 13 sequentially arranged on the front side of the p+ emitter 11, and the back side passivation film is a back side silicon nitride film 16 arranged on the back side of the n+ polysilicon layer 15) to obtain a silicon substrate 1.

[0084] The above-mentioned processing process of step 1 refers to the existing TOPCon battery preparation technology, so it will not be repeated here.

[0085] Step 2: Prepare a front metal electrode 2 that contacts the p+ emitter 11, and prepare a back metal electrode 2 that contacts the n+ polysilicon layer 15. The specific preparation steps of the front metal electrode 2 and the back metal electrode 2 include:

[0086] Step 21: Use a laser to open the front passivation film in the predetermined gate line area on the front side of the silicon substrate 1 to form a front film-opening area that locally exposes the p+ emitter 11; and use a laser to open the back passivation film in the predetermined gate line area on the back side of the silicon substrate 1 to form a back film-opening area that locally exposes the n+ polysilicon layer 15.

[0087] Step 22: Prepare a front silver seed layer on the surface of the p+ emitter 11 in the front film-opening area by electroplating, and prepare a back silver seed layer on the surface of the n+ polysilicon layer 15 in the back film-opening area by electroplating; the thickness of the front silver seed layer and the back silver seed layer are both 1 μm; and clean the silicon substrate 1 with deionized water.

[0088] In step 22, taking the preparation of the front silver seed layer by electroplating as an example, the electroplating solution formula is as follows: the mass ratio of silver nitrate, EDTA (ethylenediaminetetraacetic acid), glucose, and deionized water is 2:20:10:1000, the electroplating temperature is 60°C, the electroplating time is 5 minutes, and the pH of the electroplating solution is 10.5 (pH is adjusted with KOH).

[0089] The preparation process of the back silver seed layer refers to the electroplating method of the front silver seed layer mentioned above, so it will not be repeated here.

[0090] Step 23: After that, the electroplating solution is replaced, and front copper grid lines and back copper grid lines are respectively electroplated on the surface of the front silver seed layer and the surface of the back silver seed layer; the height of the front copper grid lines and the back copper grid lines are both 10 μm.

[0091] In step 23, taking the electroplating preparation of the front copper grid line as an example, it is divided into two steps, the first step is pre-plating of neutral copper, and the second step is standard acid copper electroplating:

[0092] The first step is to pre-plating neutral copper. The plating solution formula is as follows: the mass ratio of copper citrate, sodium citrate, boric acid, sodium sulfate, antioxidant, and deionized water is 20:100:20:10:0.1:1000, the pH is controlled at 7.0 (adjusted with KOH or H3PO4), the electroplating temperature is 25°C, and the DC pulse current density is 1.0A / dm 2 , the plating time is 150 seconds.

[0093] The second step is standard acid copper electroplating. The plating solution formula is as follows: the mass ratio of copper sulfate, sulfuric acid, potassium chloride, sodium citrate, antioxidant, and deionized water is 200:30:0.05:10:0.1:1000. The electroplating temperature is 25°C; the current mode is pulse reverse current, and the forward current density is 10A / dm 2 , reverse current density is 20A / dm 2 The forward current accounts for 90% and the reverse current accounts for 10%. The electroplating time is 10 minutes. After the front and back copper grid lines are prepared, the silicon substrate 1 is ultrasonically cleaned using ethanol, dilute hydrochloric acid (HCl content is 5% by volume), and deionized water to remove grease, oxide layers, and electroplating solution residues from the surface of the silicon substrate 1.

[0094] The preparation process of the back copper grid line refers to the electroplating method of the front copper grid line, so it is not described in detail here.

[0095] It should be noted that there are many ways to prepare the silver seed layer in step 22 and the copper grid lines in step 23, and the electroplating solutions and electroplating processes used to electroplate the silver seed layer and copper grid lines are also various. In this embodiment, steps 22-23 above only illustrate one example of the electroplating process for preparing the silver seed layer and copper grid lines. Of course, other existing processes for preparing the silver seed layer and copper grid lines are also feasible.

[0096] Step 24: Use the following process to electroplate a front copper phosphate layer and a back copper phosphate layer on the front copper grid line surface and the back copper grid line surface respectively; the thickness of the front copper phosphate layer and the back copper phosphate layer are both 10 nm.

[0097] In step 24, the electroplating process of preparing the front copper phosphate layer is described in detail by taking the electroplating process as an example:

[0098] First, prepare a phosphate electrolyte with the following basic formula: 0.2 mol / L K3PO4 and 0.1 mol / L H3PO4 (pH 3.5, adjusted with H3PO4 or KOH). Then, add an oxidizing agent (such as H2O2, H2O2 accounts for 0.05% of the total volume of the phosphate electrolyte) to accelerate copper dissolution and film formation. Then, add a corrosion inhibitor (such as benzotriazole, benzotriazole accounts for 0.01% of the total volume of the phosphate electrolyte) to control the reaction rate and improve the uniformity of the film layer. Then, use this phosphate electrolyte, apply a constant voltage of +1.0V, and electroplate at room temperature for 1 minute. The following reaction is achieved through electroplating:

[0099] 3Cu+2PO4 3- →Cu3(PO4)2+6e - ; The front copper phosphate layer can be prepared by electroplating on the surface of the front copper grid line.

[0100] The preparation process of the back copper phosphate layer refers to the electroplating method of the front copper phosphate layer, so it will not be repeated here. After the reaction is completed, rinse with deionized water and blow dry with hot nitrogen; through the above steps 21-24, the preparation of a metal electrode 2 of this embodiment can be completed, that is, the solar cell of this embodiment (which is a TOPCon cell, the TOPCon cell structure is shown in FIG. Figure 1 ).

[0101] See also Figure 1 A metal electrode 2 of this embodiment includes a silver seed layer contacting a doped silicon layer (such as a p+ emitter 11 or an n+ polysilicon layer 15), a copper gate line contacting the silver seed layer, and a copper phosphate layer contacting the copper gate line.

[0102] See also Figure 1 A solar cell according to this embodiment includes a silicon substrate 1, a front metal electrode 2 contacting the front surface of the silicon substrate 1, and a back metal electrode 2 contacting the back surface of the silicon substrate 1. The silicon substrate 1 includes: a silicon wafer 10, a p+ emitter 11 and a front passivation film (the front passivation film is an aluminum oxide film 12 and a front silicon nitride film 13 sequentially disposed on the front surface of the silicon wafer 10), and a silicon oxide tunneling layer 14, an n+ polysilicon layer 15, and a back passivation film (such as a back silicon nitride film 16 disposed on the back surface of the n+ polysilicon layer 15) sequentially disposed on the back surface of the silicon wafer 10. After passing through the back passivation film, the back metal electrode 2 contacts the n+ polysilicon layer 15; and after passing through the front passivation film, the front metal electrode 2 contacts the p+ emitter 11.

[0103] Example 2

[0104] The metal electrode 2, the solar cell having the metal electrode 2, and the manufacturing method thereof of this embodiment are all based on the embodiment 1. The difference between the embodiment 1 and the embodiment 1 is as follows:

[0105] In this embodiment, after step 23, see Figure 1 First, a 10 nm thick front nickel transition layer 22 is electroplated on the surface of the front copper grid line, and a 10 nm thick back nickel transition layer 22 is electroplated on the surface of the back copper grid line to enhance the interface bonding strength; then, according to step 24 of Example 1, a front copper phosphate layer and a back copper phosphate layer are electroplated on the surface of the front nickel transition layer 22 and the surface of the back nickel transition layer 22 respectively, and the thickness of the front copper phosphate layer and the back copper phosphate layer also refer to step 24 of Example 1.

[0106] Example 3

[0107] The metal electrode, solar cell having the metal electrode and preparation method thereof of this embodiment are all based on Example 1. The difference between this embodiment and Example 1 is that:

[0108] Step 24 of this embodiment is changed to prepare a Cu3(PO4)2·phytate copper complex layer (which correspondingly includes: a front Cu3(PO4)2·phytate copper complex layer and a back Cu3(PO4)2·phytate copper complex layer) on the copper grid line surface (which includes: a front copper grid line surface and a back copper grid line surface) to replace the copper phosphate layer (which includes: a front copper phosphate layer and a back copper phosphate layer) in step 24 of embodiment 1 to improve its corrosion resistance, adhesion and functionality; and the thickness of the Cu3(PO4)2·phytate copper complex layer refers to the thickness of the copper phosphate layer in step 24 of embodiment 1. Taking the preparation of the front Cu3(PO4)2·phytate copper complex layer by electroplating as an example, the specific operation is as follows:

[0109] In the phosphate electrolyte of step 24 of Example 1, the following organic phase was additionally added: phytic acid (C6H 18 O 24 P6) and Cu 2+ The electroplating solution was prepared in a 1:6 molar ratio. The pH of the electroplating solution was adjusted to 5 using KOH to prevent phytic acid precipitation due to excessive acidity. The constant voltage was adjusted to +1.0V, and the electroplating time was increased to 3 minutes at room temperature. This allowed the phosphate groups of the copper phytate to bind to the Cu3(PO4)2 surface through coordination or hydrogen bonding, thereby electroplating a front-side Cu3(PO4)2·copper phytate complex layer on the front copper grid line surface.

[0110] The preparation process of the back Cu3(PO4)2·phytate copper composite layer refers to the electroplating method of the front Cu3(PO4)2·phytate copper composite layer, so it is not repeated here.

[0111] Comparative Example 1

[0112] A metal electrode, a solar cell having the metal electrode, and a preparation method thereof in this comparative example are all based on Example 2. The difference between the comparative example and Example 2 is that:

[0113] In this comparative example, the step of preparing the front copper phosphate layer and the back copper phosphate layer by electroplating in step 24 of Example 2 is omitted.

[0114] Performance Testing

[0115] 1. The corrosion resistance test of the metal electrodes (including the front metal electrode and the back metal electrode) of the solar cells of Example 2 and Comparative Example 1 is as follows:

[0116] A solution was prepared according to the weight ratio of potassium chloride: pure water: acetic acid = 125g: 199.4g: 0.6g. The solution and solar cells were placed in a 10L sealed container and heated to 85°C for 8 hours to perform an acetic acid corrosion test.

[0117] The anti-corrosion copper electroplating TOPCon cell sheet of Example 2 (the metal electrodes on the front and back surfaces of which are silver seed layer + copper grid line + nickel transition layer + copper phosphate layer) and the conventional copper electroplating TOPCon cell sheet of Comparative Example 1 (the metal electrodes on the front and back surfaces of which are silver seed layer + copper grid line + nickel transition layer) are each taken 5 pieces and placed in the above-mentioned sealed container to perform acetic acid corrosion; then the cell photoelectric conversion efficiency before and after corrosion is tested, and the test results are shown in Table 1 below.

[0118] In the table, the data before corrosion is the cell photoelectric conversion efficiency before corrosion of the metal electrode, the data after corrosion is the cell photoelectric conversion efficiency after corrosion of the metal electrode, the attenuation data is the cell photoelectric conversion efficiency attenuation after corrosion of the metal electrode, and the attenuation rate is the cell photoelectric conversion efficiency attenuation rate after corrosion of the metal electrode.

[0119] Table 1

[0120]

[0121] As can be seen from the test results in Table 1, the attenuation rate of the cell photoelectric conversion efficiency of the metal electrode of Example 2 after acetic acid corrosion is 24.39%, while the attenuation rate of the cell photoelectric conversion efficiency of the metal electrode of Comparative Example 1 after acetic acid corrosion is 25.79%. Compared with Comparative Example 1, the corrosion resistance of the metal electrode of Example 2 is improved, and the attenuation rate of the cell photoelectric conversion efficiency of the metal electrode after acetic acid corrosion is reduced by 1.40%.

[0122] 2, the corrosion resistance test results of the metal electrodes of Example 2 and Example 3 are shown in Table 2 below:

[0123] Table 2

[0124]

[0125] When the metal electrode is combined with a copper phosphate layer (such as Example 2), the attenuation rate of the cell photoelectric conversion efficiency of the metal electrode after acetic acid corrosion is 24.39%; when the copper phosphate layer is replaced by a Cu3(PO4)2· phytic acid copper composite layer (such as Example 3), the attenuation rate of the cell photoelectric conversion efficiency of the metal electrode after acetic acid corrosion is 23.86%. It can be seen that when the metal electrode is combined with an organic copper phosphate composite layer such as a Cu3(PO4)2· phytic acid copper composite layer, the corrosion resistance of the metal electrode is further improved, and the cell photoelectric conversion efficiency of the metal electrode after acetic acid corrosion is further improved by 0.53%.

[0126] Although the preferred embodiments of the embodiments of the present application have been described, those skilled in the art can make further changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all changes and modifications falling within the scope of the embodiments of the present application.

[0127] The technical solution provided by the present invention is introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A metal electrode for a solar cell, characterized in that: The invention comprises a metal seed layer provided in the gate line region on the surface of the silicon substrate, a base metal gate line provided on the surface of the metal seed layer, and a copper phosphate protective layer provided on the surface of the base metal gate line; The metal seed layer is at least one of a silver seed layer, a nickel seed layer, a zirconium seed layer, a titanium seed layer, a chromium seed layer, a nickel silicon alloy seed layer, and a copper alloy seed layer; The base metal grid line is at least one of a copper grid line and an aluminum grid line; The copper phosphate protective layer is at least one of a copper phosphate layer and an organic copper phosphate composite layer.

2. The metal electrode for solar cells according to claim 1, characterized in that: The metal seed layer is a silver seed layer, and its thickness is 0.8-1.2 μm.

3. The metal electrode for a solar cell according to claim 1, characterized in that: The base metal grid lines are copper grid lines, and their height is 8-15 μm.

4. The metal electrode for a solar cell according to claim 1, wherein: The copper phosphate protective layer is a copper phosphate layer or a Cu3(PO4)2·copper phytate composite layer; the thickness of the copper phosphate protective layer is 8-12nm.

5. A metal electrode for a solar cell according to any one of claims 1 to 4, characterized in that: A nickel transition layer is further provided between the base metal gate line and the copper phosphate protection layer, and the thickness of the nickel transition layer is 8-15 nm.

6. The method for preparing a metal electrode for a solar cell according to any one of claims 1 to 5, characterized in that: The method comprises the following preparation steps: Step S1: preparing a silicon substrate, wherein the silicon substrate comprises: a silicon wafer and a passivation film provided on the front and / or back of the silicon wafer; Step S2: preparing a metal electrode on the surface of the silicon substrate, which includes: Step S21, opening the passivation film in the predetermined gate line area on the surface of the silicon substrate to form an open film area locally exposing the silicon surface; Step S22: a metal seed layer, a base metal gate line and a copper phosphate protection layer are sequentially prepared on the silicon surface of the film-opening area to obtain a metal electrode.

7. The method for preparing a metal electrode for a solar cell according to claim 6, characterized in that: In step S22, the copper phosphate protective layer is prepared by electroplating, solution co-precipitation, hydrothermal method or physical mixing; When the copper phosphate protective layer is a copper phosphate layer, and the metal seed layer and / or the base metal gate line are made of a copper-containing material, the electroplating process is as follows: First, prepare a phosphate electrolyte: it includes 0.2-0.4 mol / L K3PO4, 0.1-0.2 mol / L H3PO4, 0.05-0.1% of the total volume of the phosphate electrolyte oxidant and 0.01-0.02% of the total volume of the phosphate electrolyte corrosion inhibitor, control the pH of the phosphate electrolyte at 3-5, apply a constant voltage of +0.5V to +1.5V, and electroplate for 60-90 seconds at room temperature to obtain a copper phosphate layer.

8. The method for preparing a metal electrode for a solar cell according to claim 7, characterized in that: In step S22, when the copper phosphate protective layer is a Cu3(PO4)2·copper phytate composite layer, the electroplating process is as follows: First, prepare copper phosphate electroplating solution: it includes the phosphate electrolyte, phytic acid and Cu 2+ The organic phase is configured in a molar ratio of 1:5-8, the pH value of the electroplating solution is adjusted to 4.5-5.5, a constant voltage of +0.5V to +1.0V is applied, and electroplating is carried out at room temperature for 150-200 seconds to obtain a Cu3(PO4)2·copper phytate complex layer.

9. The method for preparing a metal electrode for a solar cell according to claim 6, wherein: In step S22, before forming the copper phosphate protective layer, the method further includes: forming a nickel transition layer on the surface of the base metal gate line.

10. A solar cell, characterized in that: It includes: A silicon substrate, and metal electrodes provided on the front and / or back side of the silicon substrate; The silicon substrate comprises: a silicon wafer and a passivation film provided on the front and / or back of the silicon wafer; The metal electrode is a metal electrode for a solar cell according to any one of claims 1 to 5.

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