Photovoltaic copper paste, preparation method thereof and application of photovoltaic copper paste in preparation of solar cell

Abstract

The invention provides photovoltaic copper paste, a preparation method thereof and application of the photovoltaic copper paste in preparation of solar cells, and relates to the technical field of photovoltaic cell manufacturing, and the photovoltaic copper paste comprises gradient micron-sized copper powder, a nano conductive reinforcing material, an organic binder, glass powder, a reinforcer and an optional additive; the reinforcing agent comprises an antioxidant, a nano interface reinforcing agent, a passivating agent and an optional corrosion inhibitor; wherein the nanometer interface reinforcing agent comprises one or more of aluminum oxide nanoparticles, antimony nanoparticles and graphene quantum dots; the passivating agent is prepared from benzotriazole and a silane coupling agent. According to the improved copper paste formula, the contact resistance performance and oxidation resistance can be improved, and the uniformity of grid lines is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic cell manufacturing, and in particular to a photovoltaic copper paste, a preparation method thereof, and an application thereof in the preparation of solar cells. Background Art

[0002] The electrode material of photovoltaic cells is a key factor influencing cell efficiency and cost. Traditional processes primarily use silver paste as the electrode material, but the high cost of silver has prompted the industry to turn to copper alternatives. To reduce solar cell costs and improve cell efficiency, copper-in-silver technology offers the greatest potential for cost reduction, as it allows for low or no silver paste consumption.

[0003] In the current copper manufacturing process for photovoltaic cells, a seed layer of metals such as nickel, titanium, silver, tungsten, and their alloys is typically first plated at the slots of the cell to prevent copper diffusion. Copper and tin are then plated as the conductive and protective layers to create double-sided electroplated solar cells. For example, some researchers have screen-printed a thin silver layer as a seed layer, followed by electroplating copper as the main conductive layer. However, this method does not significantly reduce the cost of silver. Further proposals have been made to use electroplating to prepare the front and back electrodes. Typically, nickel layers are first applied as seed layers on both sides, followed by electroplating copper on both sides as the conductive layer, and finally, electroplating tin on both sides as the protective layer, ultimately creating a double-sided electroplated solar cell. While this method completely replaces the use of silver, the copper plating process is time-consuming. Even the currently popular horizontal double-sided plating method still suffers from complex equipment, uneven coating thickness, and uneven current. This not only affects the conductivity efficiency, but also results in high internal stress in the coating, leading to poor adhesion between the coating and the silicon substrate. Therefore, it is particularly important to develop new copper grid line preparation technologies while ensuring that copper replaces silver.

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

[0005] One of the objectives of the present invention is to provide a photovoltaic copper paste that solves at least one of the technical problems existing in the prior art. The improved copper paste formula of the present invention can improve contact resistance and oxidation resistance, and enhance the uniformity of the grid lines.

[0006] A second object of the present invention is to provide a method for preparing photovoltaic copper paste.

[0007] A third object of the present invention is to provide a photovoltaic copper paste or an application of the photovoltaic copper paste prepared by the above preparation method in the preparation of solar cells.

[0008] A fourth object of the present invention is to provide a solar cell.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0010] In a first aspect, the present invention provides a photovoltaic copper paste, the components of which include: gradient micron-sized copper powder, nano-conductive reinforcing material, organic binder, glass powder, reinforcing agent and optional additives;

[0011] The strengthening agent includes an antioxidant, a nano interface enhancer, a passivator and an optional corrosion inhibitor;

[0012] The nano-interface enhancer includes one or more of aluminum oxide nanoparticles, antimony nanoparticles and graphene quantum dots; and the passivator includes benzotriazole and a silane coupling agent.

[0013] Furthermore, the photovoltaic copper paste comprises the following components by mass percentage:

[0014] 0.5%-1% nano-conductive reinforcing material, 10%-12% organic binder, 1%-5% glass powder, 0.3%-0.5% antioxidant, 0.5%-1% nano-interface enhancer, 0.5%-1% corrosion inhibitor, 0.3%-0.8% passivator and the balance gradient micron-grade copper powder;

[0015] Preferably, the photovoltaic copper paste further comprises 0.5%-1% additives;

[0016] Preferably, the nano-conductive reinforcing material comprises nano-copper wires and carbon nanotubes; the nano-copper wires account for 50%-70% of the total mass of the nano-conductive reinforcing material; the carbon nanotubes account for 30%-50% of the total mass of the nano-conductive reinforcing material;

[0017] Preferably, the antioxidant comprises magnesium and / or copper oxide nanoparticles; the magnesium accounts for 65%-85% of the total mass of the antioxidant; the copper oxide nanoparticles account for 15%-35% of the total mass of the antioxidant;

[0018] Preferably, the mass ratio of the benzotriazole to the silane coupling agent is 1-1.5:1.

[0019] Furthermore, the gradient micron-grade copper powder includes micron-grade copper powders of several different particle sizes; the particle size of the micron-grade copper powder is 0.1-3 μm; the proportion of each particle size of the micron-grade copper powder is 15%-50%;

[0020] Preferably, the organic binder comprises one or more of terpineol, ethyl cellulose, dibutyl phthalate, ethylene glycol ethyl ether acetate, tributyl citrate, dibutyl phthalate and hydrogenated castor oil;

[0021] Preferably, the glass powder comprises a PbO-B2O3-SiO2 system; the components of the glass powder include one or more of PbO, B2O3, Li2O, Bi2O3, ZnO, SiO2, CuO, MnO2, TiO2, Cr2O3, NiO, TeO2 and K2O;

[0022] Preferably, the corrosion inhibitor comprises fluorinated polyimide;

[0023] Preferably, the additives include one or more of a thickener, a surfactant, a leveling agent and a lubricant.

[0024] In a second aspect, the present invention provides a method for preparing a photovoltaic copper paste, comprising: mixing a formulated amount of gradient micron-grade copper powder, a nano-conductive reinforcing material, an organic adhesive, a glass powder, a reinforcing agent and optional additives to obtain the photovoltaic copper paste.

[0025] Furthermore, the preparation method includes:

[0026] (a) mixing gradient micron-sized copper powder, anhydrous ethanol, and dilute sulfuric acid, and filtering to obtain copper powder;

[0027] (b) adding a nano-conductive reinforcing material, an antioxidant, and a nano-interface enhancer to a mixture of copper powder and a passivating agent to obtain a slurry mixture;

[0028] (c) adding an organic binder, glass powder, an optional corrosion inhibitor, and an optional additive to the slurry mixture to obtain the photovoltaic copper slurry;

[0029] Preferably, the mass fraction of the anhydrous ethanol is 20%-40%; the mass fraction of the dilute sulfuric acid is 15%-20%;

[0030] Preferably, in step (a), during the mixing process of the gradient micron-sized copper powder, anhydrous ethanol and dilute sulfuric acid, the water bath heating time is 1-3 hours; the water bath heating temperature is 60-90° C.;

[0031] Preferably, in step (b), the copper powder and the passivating agent are mixed under stirring conditions; the stirring time is 20-40 minutes.

[0032] Furthermore, the glass powder is prepared by the following steps: mixing, melting, ball milling, drying and sieving the glass powder raw materials in the prescribed amount in sequence to obtain the glass powder;

[0033] Preferably, the melting temperature is 900-1400°C;

[0034] Preferably, the ball milling time is 0.5-5h;

[0035] Preferably, the drying time is 0.5-2 hours.

[0036] In a third aspect, the present invention provides a photovoltaic copper paste or an application of the photovoltaic copper paste prepared by the preparation method in the preparation of solar cells.

[0037] In a fourth aspect, the present invention provides a solar cell, wherein the preparation method comprises: forming patterned grooves on the front and back surfaces of a silicon substrate, and then sequentially forming a nickel layer, a copper layer, and a tin layer in the patterned grooves;

[0038] Wherein, the copper layer is prepared by using the photovoltaic copper paste or the photovoltaic copper paste prepared by the preparation method.

[0039] Furthermore, the width of the patterned groove is 10-50 μm; the depth of the patterned groove is 80-100 nm;

[0040] Preferably, after patterned grooves are formed on the front and back sides of the silicon substrate, pickling and annealing are sequentially performed before forming the metal layer;

[0041] Preferably, the pickling solution used for pickling includes hydrofluoric acid; the concentration of the hydrofluoric acid is 10-20 mL / L; the temperature of the hydrofluoric acid is 25-38° C.; and the pickling time is 5-50 s;

[0042] Preferably, the annealing temperature is 50-800°C.

[0043] Furthermore, the thickness of the nickel layer is 0.1-2 μm; the thickness of the copper layer is 9-12 μm; and the thickness of the tin layer is 1-2 μm.

[0044] Preferably, the nickel-plated battery cell is pickled; the pickling solution used for pickling includes a sulfuric acid solution; the concentration of the sulfuric acid solution is 15-20%;

[0045] Preferably, the copper layer is prepared by screen printing; the printed cell is subjected to a curing treatment; and the curing temperature is 400-600°C.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The photovoltaic copper paste provided by the present invention has a gradient micron-sized copper powder added to its components. The gradient distribution can effectively alleviate the problem of insufficient mixing, promote stable current transmission, enhance the mechanical bite of the copper paste and the seed layer by particle size matching, reduce the contact resistance, and improve the uniformity of the grid line; the components are added with a nano-conductive reinforcing material, which can achieve efficient connection of the conductive network, reduce the copper content requirement in the paste, and improve printing uniformity; the components are added with a nano-interface enhancer, and the addition of aluminum oxide nanoparticles + antimony (Sb) nanoparticles or graphene quantum dots can improve the adhesion between the paste and the seed layer and improve the contact resistance; the passivator composed of a compound of benzotriazole and a silane coupling agent can form a chemical bond with each other to form a semi-permeable polymer complex, thereby providing stronger chemical bonding between the matrixes, effectively improving the corrosion resistance, and cooperating with the antioxidant to further enhance the oxidation resistance. In addition, the photovoltaic copper paste formula also includes organic adhesives and glass powder. The organic adhesive has the function of bonding and auxiliary molding. The glass powder can reduce the sintering temperature and promote the bonding of other components. It can also fill the gaps between materials and improve the density, strength and corrosion resistance of the product.

[0048] The photovoltaic copper paste provided by the present invention is used to prepare solar cells, which can improve the uniformity of the grid lines, improve the contact resistance performance and enhance the oxidation resistance of the product. DETAILED DESCRIPTION

[0049] Unless otherwise defined herein, scientific and technical terms used in conjunction with the present invention shall have the meanings commonly understood by those of ordinary skill in the art. The meaning and scope of the terms should be clear; however, in the event of any potential ambiguity, the definitions provided herein take precedence over any dictionary or external definitions. In this application, the use of "or" means "and / or" unless otherwise stated. In addition, the use of the term "including" and other forms is non-limiting.

[0050] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0051] The first aspect of the present invention provides a photovoltaic copper paste, the components of which include: gradient micron-sized copper powder, nano-conductive reinforcing material, organic binder, glass powder, reinforcing agent and optional additives;

[0052] The strengthening agent includes an antioxidant, a nano interface enhancer, a passivator and an optional corrosion inhibitor;

[0053] The nano-interface enhancer includes one or more of aluminum oxide nanoparticles, antimony nanoparticles and graphene quantum dots; and the passivator includes benzotriazole and a silane coupling agent.

[0054] In the present invention, a combination of aluminum oxide nanoparticles (particle size <50 nm) + antimony (Sb) nanoparticles (particle size <50 nm) or graphene quantum dots (particle size <50 nm) is preferably used, which can improve the adhesion between the slurry and the seed layer and optimize the contact resistance.

[0055] In some preferred embodiments, the photovoltaic copper paste comprises the following components by mass percentage:

[0056] 0.5%-1% nano-conductive reinforcing material, 10%-12% organic binder, 1%-5% glass powder, 0.3%-0.5% antioxidant, 0.5%-1% nano-interface enhancer, 0.5%-1% corrosion inhibitor, 0.3%-0.8% passivator and the balance gradient micron-grade copper powder;

[0057] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the nano-conductive reinforcing material is 0.5%-1%, for example, 0.5%, 0.75%, 1%, etc.;

[0058] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the organic binder is 10%-12%, for example, 10%, 11%, 12%, etc.;

[0059] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the glass powder is 1%-5%, for example, 1%, 2%, 3%, 5%, 5%, etc.;

[0060] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the antioxidant is 0.3%-0.5%, for example, 0.3%, 0.4%, 0.5%, etc.;

[0061] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the nano interface enhancer is 0.5%-1%, for example, 0.5%, 0.75%, 1%, etc.;

[0062] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the corrosion inhibitor is 0.5%-1%, for example, 0.5%, 0.75%, 1%, etc.;

[0063] Based on the total mass of the photovoltaic copper paste as 100%, the added amount of the passivator is 0.3%-0.8%, for example, it can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0064] Preferably, the photovoltaic copper paste further includes 0.5%-1% additives, for example, 0.5%, 0.75%, 1%, etc.

[0065] Preferably, the nano-conductive reinforcing material comprises nano-copper wires and carbon nanotubes; the nano-copper wires account for 50%-70% of the total mass of the nano-conductive reinforcing material; and the carbon nanotubes account for 30%-50% of the total mass of the nano-conductive reinforcing material.

[0066] In the present invention, a combination of nano copper wires (diameter <50nm) and carbon nanotubes (diameter <50nm) is used to achieve efficient connection of the conductive network through microstructure regulation, reduce the copper content requirement in the slurry, and improve printing uniformity.

[0067] Based on the total mass of the nano-conductive reinforcing material being 100%, the amount of the nano-copper wire added is 50%-70%, for example, 50%, 55%, 60%, 65%, 70%, etc.;

[0068] Taking the total mass of the nano-conductive reinforcing material as 100%, the added amount of the carbon nanotubes is 30%-50%, for example, 30%, 35%, 40%, 45%, 50%, etc.

[0069] Preferably, the antioxidant comprises magnesium and / or copper oxide nanoparticles; the magnesium accounts for 65%-85% of the total mass of the antioxidant; and the copper oxide nanoparticles account for 15%-35% of the total mass of the antioxidant.

[0070] In the present invention, magnesium and copper oxide nanoparticles (particle size <50 nm) are preferably used in combination as dynamic antioxidants. During sintering, a Mg+CuO→MgO+Cu reaction occurs, consuming residual oxygen in the slurry system.

[0071] Based on the total mass of the antioxidant as 100%, the added amount of magnesium is 65%-85%, for example, 65%, 70%, 75%, 80%, 85%, etc.;

[0072] Based on the total mass of the antioxidant being 100%, the added amount of the copper oxide nanoparticles is 15%-35%, for example, 15%, 20%, 25%, 30%, 35%, etc.

[0073] Preferably, the mass ratio of the benzotriazole to the silane coupling agent is 1-1.5:1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc. The concentration of the silane coupling agent is 1%-5%.

[0074] In the present invention, the silane coupling agent preferably includes 3-mercaptopropyltrimethoxysilane and / or 3-aminopropyltriethoxysilane. The passivator in the present invention is a self-repairing passivator, comprising benzotriazole and a silane coupling agent. The two interact to form a chemical bond, forming a semi-permeable polymer complex, thereby providing stronger chemical bonding between the substrates, effectively improving corrosion resistance, and repairing microcracks that appear in the grid lines during battery operation. In addition, the interaction with a dynamic antioxidant can further enhance its antioxidant properties.

[0075] In some preferred embodiments, the gradient micron-sized copper powder includes micron-sized copper powders of several different particle sizes; the particle size of the micron-sized copper powder is 0.1-3 μm, for example, it can be 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, etc.; the proportion of each particle size of micron-sized copper powder is 15%-50%, for example, it can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc.

[0076] In the present invention, gradient micron-sized copper powder is used as the conductive phase. The gradient distribution can effectively alleviate the problem of insufficient mixing, promote stable current transmission, enhance the mechanical bite between the copper slurry and the seed layer through particle size matching, and reduce contact resistance. The gradient distribution refers to the different particle sizes of the gradient copper powder. Preferably, the content of copper powder with a particle size of 0.1μm accounts for 15%, the content of copper powder with a particle size of 0.5μm accounts for 15%, the content of copper powder with a particle size of 1μm accounts for 20%, and the content of copper powder with a particle size of 2μm accounts for 50%, forming a gradient distribution. The total mass of the gradient micron-sized copper powder is 100%.

[0077] Preferably, the organic binder comprises one or more of terpineol, ethyl cellulose, dibutyl phthalate, ethylene glycol ethyl ether acetate, tributyl citrate, dibutyl phthalate and hydrogenated castor oil;

[0078] Preferably, the glass powder includes a PbO-B2O3-SiO2 system; the components of the glass powder include one or more of PbO, B2O3, Li2O, Bi2O3, ZnO, SiO2, CuO, MnO2, TiO2, Cr2O3, NiO, TeO2 and K2O.

[0079] Preferably, the glass powder comprises, by mass percentage, the following: PbO: 40-60%, B2O3: 15-25%, Li2O: 1-5%, Bi2O3: 0.5-5%, ZnO: 1-8%, SiO2: 10-20%, CuO: 0.1-3%, MnO2: 0.1-2%, TiO2: 0.5-5%, Cr2O3: 0.1-2%, NiO: 0.1-1%, TeO2: 0-3% and K2O: 0.5-4%.

[0080] In the present invention, the glass powder is prepared by sequentially melting, ball milling, drying and screening several of the above raw material components to complete the preparation of the entire glass powder.

[0081] Preferably, the corrosion inhibitor includes fluorinated polyimide; using fluorinated polyimide as a corrosion inhibitor, after curing, a hydrophobic surface with a contact angle >150° is formed, and a corrosion inhibitor (such as molybdate) is released, thereby increasing the life in a wet and hot environment and improving the acid resistance and wet and hot resistance of copper.

[0082] Preferably, the additives include one or more of a thickener, a surfactant, a leveling agent and a lubricant.

[0083] Among them, various additives are preferably:

[0084] Thickener: 0.5%-1% (hydroxyethyl cellulose), surfactant: 0.1%-0.5% (sodium dodecylbenzene sulfonate), leveling agent: 0.1%-0.5% (polyether modified polydimethylsiloxane) and lubricant: 0.5%-2% (stearic acid amide).

[0085] The second aspect of the present invention provides a method for preparing a photovoltaic copper paste, comprising: mixing a formulated amount of gradient micron-grade copper powder, a nano-conductive reinforcing material, an organic adhesive, glass powder, a reinforcing agent and optional additives to obtain the photovoltaic copper paste.

[0086] In some preferred embodiments, the method for preparing the photovoltaic copper paste includes:

[0087] (a) mixing gradient micron-sized copper powder, anhydrous ethanol, and dilute sulfuric acid, and filtering to obtain copper powder;

[0088] (b) adding a nano-conductive reinforcing material, an antioxidant, and a nano-interface enhancer to a mixture of copper powder and a passivating agent to obtain a slurry mixture;

[0089] (c) adding an organic binder, glass powder, an optional corrosion inhibitor, and an optional additive to the slurry mixture to obtain the photovoltaic copper slurry;

[0090] Preferably, the mixed solution of anhydrous ethanol and dilute sulfuric acid includes: anhydrous ethanol, sulfuric acid and the balance water; the mass fraction of anhydrous ethanol is 20%-40%, for example, it can be 20%, 30%, 40%, etc.; the mass fraction of dilute sulfuric acid is 15%-20%, for example, it can be 15%, 18%, 20%, etc.

[0091] In the present invention, the addition of dilute sulfuric acid can promote the full fusion of copper powder and self-repairing passivator; it can also effectively avoid the problem of copper oxidation during the preparation of conductive copper slurry. At the same time, the presence of dynamic antioxidant ensures the use of subsequent slurry, and in the subsequent preparation process, there is no need to additionally supplement benzotriazole or silane coupling agent.

[0092] Preferably, in step (a), during the mixing process of the gradient micron-sized copper powder, anhydrous ethanol and dilute sulfuric acid, the water bath heating time is 1-3 hours, for example, 1 hour, 2 hours, 3 hours, etc.; the water bath heating temperature is 60-90° C., for example, 60° C., 75° C., 90° C., etc.;

[0093] Preferably, in step (b), the copper powder and the passivating agent are mixed under stirring conditions; the stirring time is 20-40 minutes, for example, 20 minutes, 30 minutes, 40 minutes, etc.

[0094] In some preferred embodiments, the glass powder is prepared by the following steps: mixing, melting, ball milling, drying and sieving glass powder raw materials in a prescribed amount in sequence to obtain the glass powder;

[0095] Preferably, the melting temperature is 900-1400°C, for example, 900°C, 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, etc.

[0096] Preferably, the ball milling time is 0.5-5h, for example, 0.5h, 1h, 2h, 3h, 4h, 5h, etc.;

[0097] Preferably, the drying time is 0.5-2 hours, for example, 0.5 hours, 1 hour, 1.5 hours, 2 hours, etc.

[0098] A third aspect of the present invention provides a photovoltaic copper paste or an application of the photovoltaic copper paste prepared by the preparation method in the preparation of solar cells.

[0099] A fourth aspect of the present invention provides a solar cell, the preparation method of which comprises: forming patterned grooves on the front and back surfaces of a silicon substrate, and then sequentially forming a nickel layer, a copper layer, and a tin layer in the patterned grooves;

[0100] Wherein, the copper layer is prepared by using the photovoltaic copper paste or the photovoltaic copper paste prepared by the preparation method.

[0101] In some preferred embodiments, the width of the patterned groove is 10-50 μm, for example, 10 μm, 30 μm, 50 μm, etc.; the depth of the patterned groove is 80-100 nm, for example, 80 nm, 90 nm, 100 nm, etc.;

[0102] Preferably, after patterned grooves are formed on the front and back sides of the silicon substrate, pickling and annealing are sequentially performed before forming the metal layer;

[0103] Preferably, the pickling solution used for pickling includes hydrofluoric acid; the concentration of the hydrofluoric acid is 10-20 mL / L, for example, 10 mL / L, 15 mL / L, 20 mL / L, etc.; the temperature of the hydrofluoric acid is 25-38° C., for example, 25° C., 30° C., 35° C., 38° C., etc.; the pickling time is 5-50 s, for example, 5 s, 10 s, 20 s, 30 s, 40 s, 50 s, etc.;

[0104] Preferably, the annealing temperature is 50-800°C, for example, 50°C, 425°C, 800°C, etc.

[0105] In some preferred embodiments, the double-sided nickel layer is electroplated by constant current electroplating or pulse electroplating or constant current light induced electroplating or pulse light induced electroplating, and the thickness of the nickel layer is 0.1-2um, for example, 0.1um, 0.5um, 1um, 1.5um, 2um, etc.; the thickness of the copper layer is 9-12um, for example, 9um, 10um, 11um, 12um, etc.; the thickness of the tin layer is 1-2um, for example, 1um, 1.5um, 2um, etc.;

[0106] Preferably, the nickel-plated battery cell is pickled; the pickling solution used for pickling includes a sulfuric acid solution; the concentration of the sulfuric acid solution is 15%-20%, for example, 15%, 18%, 20%, etc.;

[0107] Preferably, the copper layer is prepared by screen printing; the printed cell is cured; the curing temperature is 400-600°C, for example, 400°C, 500°C, 600°C, etc.;

[0108] Preferably, the double-sided tin layer is electroplated by constant current plating or pulse plating or constant current light induced plating or pulse light induced plating.

[0109] The photovoltaic copper paste provided by the present invention is used to reduce the cost of traditional silver paste, improve the conductivity and oxidation resistance of the grid line and the low production capacity under the conditions of electroplating process. The formula of the photovoltaic copper paste adopts gradient copper powder, and the gradient distribution can effectively alleviate the problem of insufficient mixing, promote the stability of current transmission, enhance the mechanical bite of the copper paste and the seed layer by particle size matching, reduce the contact resistance, improve the uniformity of the grid line, and increase the production efficiency of the slurry screen printing process compared with traditional electroplating. The present invention improves the copper paste formula, adopts nano copper wire + carbon nanotube, realizes the efficient connection of the conductive network by microstructure regulation, reduces the copper content demand in the slurry, and improves the printing uniformity; adopts aluminum oxide nanoparticles + antimony (Sb) nanoparticles or graphene quantum dots, improves the adhesion of the slurry to the seed layer and improves the contact resistance; adopts benzotriazole and silane coupling agent compound, the two cooperate with each other to form a chemical bond, forming a semi-permeable polymer complex, thereby providing a stronger chemical bonding force between the matrix, effectively improving the corrosion resistance, and repairing the microcracks that appear in the grid line during battery operation. In addition, its antioxidant properties can be further enhanced by working with dynamic antioxidants.

[0110] In addition, the photovoltaic copper paste provided by the present invention has high stability, and the additives in the formula are not easy to decompose, which solves the technical problems of the existing traditional plating solution in which the additives are easy to decompose, resulting in the accumulation of by-products, resulting in a short plating solution life, frequent replacement, and high cost.

[0111] The use of the photovoltaic copper paste provided by the present invention to prepare solar cells can improve the uniformity of their grid lines, increase production efficiency, improve contact resistance performance and oxidation resistance, and improve the resistance of electrodes to microcracks easily generated under thermal cycles or mechanical stress, thereby enhancing self-repair capabilities.

[0112] The present invention is further described below by way of examples. Unless otherwise specified, the materials in the examples were prepared according to existing methods or directly purchased from the market.

[0113] Example 1

[0114] This embodiment provides a photovoltaic copper paste, which includes the following components by mass percentage:

[0115] 0.8% nano-conductive reinforcing material, 11% organic binder, 3% glass powder, 0.4% antioxidant, 0.8% nano-interface enhancer, 0.8% corrosion inhibitor, 0.5% passivator and the balance gradient micron-grade copper powder;

[0116] The gradient micron-grade copper powder includes, by mass percentage, 15% of copper powder with a particle size of 0.1 μm, 15% of copper powder with a particle size of 0.5 μm, 20% of copper powder with a particle size of 1 μm, and 50% of copper powder with a particle size of 2 μm.

[0117] The nano-conductive reinforcing material comprises, by mass percentage, 60% nano-copper wires (diameter < 50 nm) and 40% carbon nanotubes (diameter < 50 nm);

[0118] Wherein, the organic binder is terpineol;

[0119] The glass powder is a PbO-B2O3-SiO2 system, and its specific components, by mass percentage, include: 20% B2O3, 3% Li2O, 3% Bi2O3, 5% ZnO, 15% SiO2, 2% CuO, 1% MnO2, 3% TiO2, 1% Cr2O3, 0.5% NiO, 1.5% TeO2, 2% K2O and the balance PbO;

[0120] The antioxidant comprises, by mass percentage, 75% magnesium and 25% copper oxide nanoparticles (particle size <50 nm);

[0121] The nano-interface enhancer comprises aluminum oxide nanoparticles (particle size <50 nm) and antimony (Sb) nanoparticles (particle size <50 nm) in a mass ratio of 1:1;

[0122] Wherein, the corrosion inhibitor is fluorinated polyimide;

[0123] The passivating agent includes benzotriazole and a silane coupling agent (specifically 3-mercaptopropyltrimethoxysilane, with a concentration of 3%) in a mass ratio of 1.2:1;

[0124] The preparation process of the photovoltaic copper paste includes the following steps (prepared according to the above formula):

[0125] (1) Mix gradient micron-sized copper powder and a mixed aqueous solution of anhydrous ethanol and dilute sulfuric acid (wherein, the mass fraction of anhydrous ethanol in the mixed aqueous solution of anhydrous ethanol and dilute sulfuric acid is 30% and the mass fraction of dilute sulfuric acid is 18%) in a three-necked flask, heat in a water bath at 70°C for 2 hours, stir thoroughly and let stand, filter out the copper powder, then place the copper powder in a container containing a passivating agent and stir in a magnetic stirrer for 30 minutes;

[0126] (2) mixing, stirring, grinding and dispersing the copper powder mixed with the passivation agent, the nano copper wires + carbon nanotubes, the magnesium + copper oxide nanoparticles, and the aluminum oxide nanoparticles + antimony nanoparticles to obtain a slurry mixture;

[0127] (3) Mixing and stirring the slurry mixture, corrosion inhibitor, organic binder, glass powder and other remaining additives to obtain photovoltaic copper slurry.

[0128] The glass powder is prepared by the following steps:

[0129] The glass powder raw materials in the formulated amounts were sequentially mixed, melted (melting temperature was 1200° C.), ball-milled (ball milled for 3 h), dried (drying time was 1.5 h), and sieved to obtain glass powder.

[0130] Example 2

[0131] This embodiment provides a photovoltaic copper paste, which differs from Example 1 in that the nano-interface enhancer includes aluminum oxide nanoparticles (particle size <50 nm), antimony (Sb) nanoparticles (particle size <50 nm) and graphene quantum dots (particle size <50 nm) in a mass ratio of 1:1:1, and the rest is consistent with Example 1.

[0132] Example 3

[0133] This embodiment provides a photovoltaic copper paste, which differs from embodiment 1 in that:

[0134] 0.5% nano-conductive reinforcing material, 12% organic binder, 1% glass powder, 0.5% antioxidant, 0.5% nano-interface enhancer, 1% corrosion inhibitor, 0.3% passivator and the balance gradient micron-grade copper powder;

[0135] The gradient micron-grade copper powder includes, by mass percentage, 15% copper powder with a particle size of 0.1 μm, 20% copper powder with a particle size of 0.5 μm, 20% copper powder with a particle size of 1 μm, 25% copper powder with a particle size of 2 μm, and 20% copper powder with a particle size of 3 μm.

[0136] The nano-conductive reinforcing material comprises, by mass percentage, 50% nano-copper wires (diameter < 50 nm) and 50% carbon nanotubes (diameter < 50 nm);

[0137] The antioxidant comprises, by mass percentage, 65% magnesium and 35% copper oxide nanoparticles (particle size <50 nm);

[0138] The passivating agent comprises benzotriazole and a silane coupling agent (specifically 3-mercaptopropyltrimethoxysilane) in a mass ratio of 1:1;

[0139] During the preparation of the photovoltaic copper paste, the mass fraction of anhydrous ethanol is 20% and the mass fraction of dilute sulfuric acid is 20%;

[0140] The rest is consistent with Example 1.

[0141] Example 4

[0142] This embodiment provides a photovoltaic copper paste, which differs from embodiment 1 in that:

[0143] 1% nano-conductive reinforcing material, 10% organic binder, 5% glass powder, 0.3% antioxidant, 1% nano-interface enhancer, 0.5% corrosion inhibitor, 0.8% passivator and the balance gradient micron-grade copper powder;

[0144] The gradient micron-grade copper powder includes, by mass percentage, 15% copper powder with a particle size of 0.1 μm, 20% copper powder with a particle size of 0.5 μm, 15% copper powder with a particle size of 1 μm, 20% copper powder with a particle size of 2 μm, and 30% copper powder with a particle size of 3 μm.

[0145] The nano-conductive reinforcing material comprises, by mass percentage, 70% nano-copper wires (diameter < 50 nm) and 30% carbon nanotubes (diameter < 50 nm);

[0146] The antioxidant comprises, by mass percentage, 85% magnesium and 15% copper oxide nanoparticles (particle size <50 nm);

[0147] The passivating agent comprises benzotriazole and a silane coupling agent (specifically 3-mercaptopropyltrimethoxysilane) in a mass ratio of 1.5:1;

[0148] During the preparation of the photovoltaic copper paste, the mass fraction of anhydrous ethanol is 40% and the mass fraction of dilute sulfuric acid is 15%;

[0149] The rest is consistent with Example 1.

[0150] Example 5

[0151] This embodiment provides a photovoltaic copper paste, which differs from embodiment 1 in that:

[0152] 0.4% nano-conductive reinforcing material, 13% organic binder, 0.5% glass powder, 0.6% antioxidant, 0.4% nano-interface enhancer, 1.5% corrosion inhibitor, 0.1% passivator and the balance gradient micron-grade copper powder;

[0153] The gradient micron-grade copper powder includes, by mass percentage, 70% of copper powder with a particle size of 4 μm, 10% of copper powder with a particle size of 5 μm, 10% of copper powder with a particle size of 6 μm, and 10% of copper powder with a particle size of 7 μm.

[0154] The nano-conductive reinforcing material comprises, by mass percentage, 40% nano-copper wires (diameter < 50 nm) and 60% carbon nanotubes (diameter < 50 nm);

[0155] The antioxidant comprises, by mass percentage, 90% magnesium and 10% copper oxide nanoparticles (particle size <50 nm);

[0156] The passivating agent comprises benzotriazole and a silane coupling agent (specifically 3-mercaptopropyltrimethoxysilane) in a mass ratio of 0.9:1;

[0157] The rest is consistent with Example 1.

[0158] Example 6

[0159] This embodiment provides a photovoltaic copper paste, which differs from embodiment 1 in that:

[0160] 1.1% nano-conductive reinforcing material, 9% organic binder, 6% glass powder, 0.2% antioxidant, 1.5% nano-interface enhancer, 0.4% corrosion inhibitor, 1% passivator and the balance gradient micron-grade copper powder;

[0161] The gradient micron-grade copper powder includes, by mass percentage, 70% of copper powder with a particle size of 4 μm, 10% of copper powder with a particle size of 5 μm, 10% of copper powder with a particle size of 6 μm, and 10% of copper powder with a particle size of 7 μm.

[0162] The nano-conductive reinforcing material comprises, by mass percentage, 80% nano-copper wires (diameter < 50 nm) and 20% carbon nanotubes (diameter < 50 nm);

[0163] The antioxidant comprises, by mass percentage, 60% magnesium and 40% copper oxide nanoparticles (particle size <50 nm);

[0164] The passivating agent comprises benzotriazole and a silane coupling agent (specifically 3-mercaptopropyltrimethoxysilane) in a mass ratio of 1.6:1;

[0165] The rest is consistent with Example 1.

[0166] Example 7

[0167] This embodiment provides a photovoltaic copper paste, which is different from the embodiment 1 in that the nano-conductive reinforcing material does not contain carbon nanotubes, and the rest is the same as the embodiment 1.

[0168] Example 8

[0169] This embodiment provides a photovoltaic copper paste, which is different from Example 1 in that the antioxidant does not contain copper oxide nanoparticles, and the rest is the same as Example 1.

[0170] Example 9

[0171] This embodiment provides a photovoltaic copper paste, which is different from the embodiment 1 in that the nano-interface enhancer is only aluminum oxide nanoparticles and does not contain antimony (Sb) nanoparticles. The rest is the same as the embodiment 1.

[0172] Example 10

[0173] This embodiment provides a photovoltaic copper paste, which is different from Example 1 in that dilute sulfuric acid is not added during the preparation of the photovoltaic copper paste, and the rest is the same as Example 1.

[0174] Example 11

[0175] This embodiment provides a photovoltaic copper paste, which is different from Example 1 in that the photovoltaic copper paste components also include: thickener: 0.75% (hydroxyethyl cellulose), surfactant: 0.75% (sodium dodecylbenzene sulfonate), leveling agent: 0.75% (polyether modified polydimethylsiloxane) and lubricant: 1% (stearic acid amide).

[0176] Application Examples 1-11

[0177] Application Examples 1-11 provide a solar cell. Application Examples 1-11 respectively use the photovoltaic copper paste of Examples 1-11. The preparation method thereof includes the following steps:

[0178] Step 1: Providing a solar cell silicon substrate;

[0179] Step 2: Patterned grooves are formed on the front and back sides of the solar cell silicon substrate by laser cutting. The width of the patterned grooves is 30 μm and the depth is 90 nm.

[0180] Step 3: The opened film portion of the solar cell is pickled and dried. The pickling solution used contains hydrofluoric acid with a concentration of 15 mL / L. The temperature of the pickling solution is 30°C, and the silicon substrate is floated in the pickling solution for 30 seconds.

[0181] Step 4: After pickling and drying, the cells are annealed at high temperature at 780°C, with a sintering furnace conveyor speed of 4m / min and nitrogen as the protective gas.

[0182] Step 5: Double-sided nickel plating is performed by constant current electroplating, and the thickness of the nickel plating layer is 1 μm; the nickel-plated battery cell is washed with deionized water and dried; then pickled for 80 seconds, and the pickling solution is 18% sulfuric acid solution;

[0183] Step 6: Using the photovoltaic copper paste of Examples 1-11, the dried nickel-plated solar cells were screen-printed to evenly coat the copper paste. The thickness of the copper layer was 11 μm. The printed solar cells were dried and cured in an annealing furnace (temperature 500° C.).

[0184] Step 7: Double-sided tin plating is performed by constant current electroplating, and the thickness of the tin plating layer is 1.5um to obtain a finished battery cell.

[0185] Application Example 12

[0186] This application example provides a solar cell, which uses the photovoltaic copper paste of Example 1. The preparation method thereof includes the following steps:

[0187] Step 1: Providing a solar cell silicon substrate;

[0188] Step 2: Patterned grooves are formed on the front and back sides of the solar cell silicon substrate by laser cutting. The width of the patterned grooves is 10 μm and the depth is 100 nm.

[0189] Step 3: The opened film portion of the solar cell is pickled and dried. The pickling solution used contains hydrofluoric acid with a concentration of 10 mL / L. The temperature of the pickling solution is 38°C, and the silicon substrate is floated in the pickling solution for 5 seconds.

[0190] Step 4: After pickling and drying, the cells are annealed at high temperature at 750°C, with a sintering furnace conveyor speed of 5m / min and nitrogen as the protective gas.

[0191] Step 5: Double-sided nickel plating is performed by constant current electroplating, and the thickness of the nickel plating layer is 0.1 μm; the nickel-plated battery cell is washed with deionized water and dried; then pickled for 70 seconds, and the pickling solution is 20% sulfuric acid solution;

[0192] Step 6: The dried nickel-plated cell is evenly coated with copper paste by screen printing, with a copper layer thickness of 9 μm; the printed cell is dried and solidified in an annealing furnace (temperature 600°C);

[0193] Step 7: Double-sided tin plating is performed by constant current electroplating, and the thickness of the tin plating layer is 1um to obtain a finished battery cell.

[0194] Application Example 13

[0195] This application example provides a solar cell, which uses the photovoltaic copper paste of Example 1. The preparation method thereof includes the following steps:

[0196] Step 1: Providing a solar cell silicon substrate;

[0197] Step 2: Patterned grooves are formed on the front and back sides of the solar cell silicon substrate by laser cutting. The width of the patterned grooves is 50 μm and the depth is 80 nm.

[0198] Step 3: The opened film portion of the solar cell is pickled and dried. The pickling solution used contains hydrofluoric acid with a concentration of 20 mL / L. The temperature of the pickling solution is 25°C, and the silicon substrate is floated in the pickling solution for 50 seconds.

[0199] Step 4: After pickling and drying, the cells are annealed at high temperature at 800°C, with a sintering furnace conveyor speed of 3m / min and nitrogen as the protective gas.

[0200] Step 5: Electroplating nickel on both sides by constant current electroplating, with a thickness of 2 μm; washing and drying the nickel-plated battery cell with deionized water; and then pickling for 90 seconds using a 15% sulfuric acid solution.

[0201] Step 6: The dried nickel-plated cell is evenly coated with copper paste by screen printing, with a copper layer thickness of 12 μm; the printed cell is dried and solidified in an annealing furnace (temperature 400°C);

[0202] Step 7: Double-sided tin plating is performed by constant current electroplating, and the thickness of the tin plating layer is 2um to obtain a finished battery cell.

[0203] Comparative Example 1

[0204] This comparative example provides a photovoltaic copper paste, which is different from Example 1 in that non-gradient micron-sized copper powder is used, and the particle size of the micron-sized copper powder is 1 μm. The rest is consistent with Example 1.

[0205] Comparative Example 2

[0206] This comparative example provides a photovoltaic copper paste, which is different from Example 1 in that the passivating agent is only a silane coupling agent and does not contain benzotriazole, and the rest is consistent with Example 1.

[0207] Comparative Example 3

[0208] This comparative example provides an existing photovoltaic copper paste, which has the following formula: copper powder: 88.0%, glass powder (the formula of the glass powder is consistent with that of Example 1): 5.0%, organic carrier (the organic carrier includes 90% terpineol and 10% ethyl cellulose): 6.5%, and additives: 0.5% (including antioxidants, trace wetting agents, etc.).

[0209] Comparative Application Examples 1-3

[0210] Comparative Application Examples 1-3 provide a solar cell. Comparative Application Examples 1-3 respectively use the photovoltaic copper paste of Comparative Examples 1-3, and the preparation steps are consistent with those of Application Example 1.

[0211] Test Case

[0212] Test samples: The solar cells prepared in Application Examples 1-13 and Comparative Application Examples 1-3 were used as samples for testing.

[0213] Test method:

[0214] (1) Electrical performance test: Offline solar IV tester;

[0215] (2) Welding tension under low temperature conditions: tensile force meter;

[0216] (3) Resistivity test: Use a probe resistance tester to test the initial resistivity and the resistivity after 72 hours of oxidation;

[0217] (4) Grid line uniformity: 3D microscope is used to observe the surface flatness of the grid lines and whether the aspect ratio is uniform.

[0218] The test results are shown in Table 1.

[0219] Table 1

[0220]

[0221]

[0222]

[0223] As can be seen from the data in Table 1, by comparing Application Example 1 with Application Examples 5-6, it can be seen that the photovoltaic copper paste with components within a specific range is used to prepare solar cells, and the welding tension of the battery is large, the resistance is small (wherein, the resistance can reflect the self-repair function of improving microcracks, the more microcracks, the worse the conductivity, and the greater the resistance, and Application Example 1 uses raw material components within a specific range, and its self-repair function is stronger), the battery efficiency is high, and the oxidation resistance is good. It can be seen from Application Example 1, Application Example 2 and Application Example 9 that the presence of nano-interface enhancers optimizes the resistance performance of photovoltaic cells, and the solar cells prepared by using the copper paste formula with the addition of multi-component composite nano-interface enhancers have better comprehensive performance; it can be seen from Application Example 1, Application Example 8 and Comparative Application Example 2 that passivators and antioxidants can improve the oxidation resistance of grid lines, that is, they still have a relatively reduced resistance after undergoing oxidation treatment. The improved copper paste formula in the present invention can improve contact resistance performance and oxidation resistance, and enhance the uniformity of grid lines.

[0224] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 photovoltaic copper paste, characterized in that: The components include: gradient micron-sized copper powder, nano-conductive reinforcing material, organic binder, glass powder, reinforcing agent and optional additives; The strengthening agent includes an antioxidant, a nano interface enhancer, a passivator and an optional corrosion inhibitor; The nano-interface enhancer includes one or more of aluminum oxide nanoparticles, antimony nanoparticles and graphene quantum dots; and the passivator includes benzotriazole and a silane coupling agent.

2. The photovoltaic copper paste according to claim 1, characterized in that The photovoltaic copper paste comprises the following components by mass percentage: 0.5%-1% nano-conductive reinforcing material, 10%-12% organic binder, 1%-5% glass powder, 0.3%-0.5% antioxidant, 0.5%-1% nano-interface enhancer, 0.5%-1% corrosion inhibitor, 0.3%-0.8% passivator and the balance gradient micron-grade copper powder; Preferably, the photovoltaic copper paste further comprises 0.5%-1% additives; Preferably, the nano-conductive reinforcing material comprises nano-copper wires and carbon nanotubes; the nano-copper wires account for 50%-70% of the total mass of the nano-conductive reinforcing material; the carbon nanotubes account for 30%-50% of the total mass of the nano-conductive reinforcing material; Preferably, the antioxidant comprises magnesium and / or copper oxide nanoparticles; the magnesium accounts for 65%-85% of the total mass of the antioxidant; the copper oxide nanoparticles account for 15%-35% of the total mass of the antioxidant; Preferably, the mass ratio of the benzotriazole to the silane coupling agent is 1-1.5:

1.

3. The photovoltaic copper paste according to claim 1, characterized in that The gradient micron-grade copper powder includes micron-grade copper powders of several different particle sizes; the particle size of the micron-grade copper powder is 0.1-3 μm; the proportion of each particle size of the micron-grade copper powder is 15%-50%; Preferably, the organic binder comprises one or more of terpineol, ethyl cellulose, dibutyl phthalate, ethylene glycol ethyl ether acetate, tributyl citrate, dibutyl phthalate and hydrogenated castor oil; Preferably, the glass powder comprises a PbO-B2O3-SiO2 system; the components of the glass powder include one or more of PbO, B2O3, Li2O, Bi2O3, ZnO, SiO2, CuO, MnO2, TiO2, Cr2O3, NiO, TeO2 and K2O; Preferably, the corrosion inhibitor comprises fluorinated polyimide; Preferably, the additives include one or more of a thickener, a surfactant, a leveling agent and a lubricant.

4. The method for preparing the photovoltaic copper paste according to any one of claims 1 to 3, wherein: The method comprises the following steps: mixing gradient micron-grade copper powder, nano-conductive reinforcing material, organic adhesive, glass powder, reinforcing agent and optional additives in a prescribed amount to obtain the photovoltaic copper paste.

5. The preparation method according to claim 4, characterized in that include: (a) mixing gradient micron-sized copper powder, anhydrous ethanol, and dilute sulfuric acid, and filtering to obtain copper powder; (b) adding a nano-conductive reinforcing material, an antioxidant, and a nano-interface enhancer to a mixture of copper powder and a passivating agent to obtain a slurry mixture; (c) adding an organic binder, glass powder, an optional corrosion inhibitor, and an optional additive to the slurry mixture to obtain the photovoltaic copper slurry; Preferably, the mass fraction of the anhydrous ethanol is 20%-40%; the mass fraction of the dilute sulfuric acid is 15%-20%; Preferably, in step (a), during the mixing process of the gradient micron-sized copper powder, anhydrous ethanol and dilute sulfuric acid, the water bath heating time is 1-3 hours; the water bath heating temperature is 60-90° C.; Preferably, in step (b), the copper powder and the passivating agent are mixed under stirring conditions; the stirring time is 20-40 minutes.

6. The preparation method according to claim 4, characterized in that The glass powder is prepared by the following steps: mixing, melting, ball milling, drying and sieving the glass powder raw materials in the prescribed amount in sequence to obtain the glass powder; Preferably, the melting temperature is 900-1400°C; Preferably, the ball milling time is 0.5-5h; Preferably, the drying time is 0.5-2 hours.

7. Use of the photovoltaic copper paste according to any one of claims 1 to 3 or the photovoltaic copper paste prepared by the preparation method according to any one of claims 4 to 6 in the preparation of solar cells.

8. A solar cell, characterized in that: The preparation method includes: preparing patterned grooves on the front and back of a silicon substrate, and then sequentially preparing a nickel layer, a copper layer and a tin layer in the patterned grooves; Wherein, the copper layer is prepared by using the photovoltaic copper paste described in any one of claims 1-3 or the photovoltaic copper paste prepared by the preparation method described in any one of claims 4-6.

9. The solar cell according to claim 8, characterized in that The width of the patterned groove is 10-50 μm; the depth of the patterned groove is 80-100 nm; Preferably, after patterned grooves are formed on the front and back sides of the silicon substrate, pickling and annealing are sequentially performed before forming the metal layer; Preferably, the pickling solution used for pickling includes hydrofluoric acid; the concentration of the hydrofluoric acid is 10-20 mL / L; the temperature of the hydrofluoric acid is 25-38° C.; and the pickling time is 5-50 s; Preferably, the annealing temperature is 50-800°C.

10. The solar cell according to claim 8, wherein The thickness of the nickel layer is 0.1-2 μm; the thickness of the copper layer is 9-12 μm; the thickness of the tin layer is 1-2 μm; Preferably, the nickel-plated battery cell is pickled; the pickling solution used for pickling includes a sulfuric acid solution; the concentration of the sulfuric acid solution is 15-20%; Preferably, the copper layer is prepared by screen printing; the printed cell is subjected to a curing treatment; and the curing temperature is 400-600°C.

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