Conductive paste, electrode prepared therefrom, and crystalline silicon solar cell comprising said electrode

Abstract

The present invention relates to an electroconductive paste comprising: a) electroconductive metal particles, b) glass frit, c) a reducing additive, and d) an organic vehicle, where the reducing additive is represented by formula (I): ZrxCuyAlz, in formula (I), x = 10-90 wt%, y = 5-50 wt%, z = 5-20 wt%, the weight percentages being based on the weight of the reducing additive. The invention further relates to an electrode formed by sintering the conductive paste and a crystalline silicon solar cell comprising a substrate and the electrode combined on the substrate.

Description

Technical Field

[0001] The present invention relates to a conductive paste, an electrode prepared therefrom, and a crystalline silicon solar cell including the electrode. Background Art

[0002] Solar energy is an attractive green energy source because it is sustainable and produces only non-polluting by-products. Therefore, solar cells that utilize the photovoltaic effect to convert solar energy into electrical energy have been developed. Solar cells are generally made of a semiconductive material, such as silicon material appropriately doped. When light shines on the solar cell, a part of the incident light is reflected by the surface, and the remaining incident light is transmitted into the solar cell. The transmitted photons are absorbed by the solar cell, thereby exciting electrons in the semiconductive material to generate electron-hole pairs. The electrons and holes are respectively guided by electrodes located on the front side (i.e., the side irradiated by light) and the back side (i.e., the side not irradiated by light) of the substrate to form an electric current, thereby obtaining electrical energy.

[0003] The front electrode of a solar cell is generally arranged in two sets of vertical straight lines, respectively called "grid lines" and "bus bars". The grid lines form electrical contact with the front side, and the bus bars connect these grid lines, thereby allowing charge to be effectively extracted into an external circuit. For this arrangement of the grid lines and bus bars, it is usually applied in the form of a conductive paste, which is then baked to form a solid electrode body. The back electrode of a solar cell is also usually applied in the form of a conductive paste, which is then baked to obtain a solid electrode body.

[0004] The quality of the conductive paste will directly affect the performance of the electrode material, such as the contact resistance. A typical conductive paste includes conductive metal particles, glass powder, and an organic carrier. The conductive metal particles form a metal-semiconductor ohmic contact with the underlying silicon and are directly responsible for the transmission of current from the silicon emitter region to the grid lines. Therefore, whether a low contact resistance between the conductive paste and the silicon emitter region can be obtained has an important impact on the performance of the solar cell.

[0005] CN101609849A discloses a silver conductor paste for a front electrode of a solar cell and its preparation process. The silver paste for the front electrode of a solar cell selects silver powders with different particle size ranges as the conductive functional phase. The mixed use of two spherical silver powders with different particle size ranges enables the small-particle-size silver powders to fill the voids of the large-particle-size silver powders. After forming the electrode, a more compact conductive network can be formed, thereby reducing the contact resistance of the battery and improving the electrical performance and conversion efficiency. In addition, by adding BaO and CaO powders, the glass phase is microcrystallized during the sintering process, the supersaturation of silver dissolved in the glass phase increases, and more crystalline silver precipitates at the Ag-Si interface, forming a good ohmic contact, thereby reducing the contact resistance. However, the results in Table 1 of CN101609849A show that the contact resistance of its examples is higher than that of the comparative examples.

[0006] Therefore, there is still a need to further find a method to reduce the contact resistance of solar cells in order to improve the efficiency of solar cells. SUMMARY OF THE INVENTION

[0007] In order to reduce the contact resistance of solar cells, the present invention provides a conductive paste, which comprises:

[0008] a) Conductive metal particles,

[0009] b) Glass powder,

[0010] c) Reducing additive, and

[0011] d) Organic carrier,

[0012] wherein the reducing additive is represented by formula (I):

[0013] Zr x Cu y Al z (I)

[0014] In formula (I), x = 10-90 wt%, y = 5-50 wt%, z = 5-20 wt%, and the weight percentages are based on the weight of the reducing additive.

[0015] The present invention also provides an electrode for a crystalline silicon solar cell, which is formed by sintering the conductive paste.

[0016] The present invention also provides a crystalline silicon solar cell, which comprises a substrate and the electrode bonded to the substrate.

[0017] The applicant has found that by adding a reducing additive to the conductive paste, the contact resistance of the solar cell can be reduced, and thus the solar cell prepared using the conductive paste can improve the efficiency of the solar cell. DETAILED DESCRIPTION OF THE INVENTION

[0018] Conductive paste

[0019] According to the present invention, the conductive paste is applied to the surface of a solar cell wafer and forms a solid electrode body in electrical contact with the surface during baking. According to the present invention, the conductive paste comprises:

[0020] a) Conductive metal particles,

[0021] b) Glass powder,

[0022] c) Reducing additive, and

[0023] d) Organic carrier, wherein the reducing additive is represented by formula (I):

[0024] Zr x Cu y Al z (I)

[0025] In formula (I), x = 10 - 90 wt%, y = 5 - 50 wt%, z = 5 - 20 wt%, and the weight percentages are based on the weight of the reducing additive.

[0026] In a preferred embodiment, the conductive paste comprises:

[0027] a) Conductive metal particles,

[0028] b) Glass powder,

[0029] c) Reducing additive, and

[0030] d) Organic carrier, wherein the reducing additive is represented by formula (I):

[0031] Zr x Cu y Al z (I)

[0032] In formula (I), x = 40 - 90 wt%, y = 5 - 20 wt%, z = 5 - 20 wt%, and the weight percentages are based on the weight of the reducing additive.

[0033] a) Conductive metal particles

[0034] The conductive metal particles present in the conductive paste provide metallic conductivity to the solid electrode formed during sintering of the conductive paste. Metal particles that are conducive to effective sintering and result in an electrode with high electrical conductivity and low contact resistance are preferred. All metal particles known to those skilled in the art and considered suitable for the context of the present invention can be used as the conductive metal particles in the conductive paste.

[0035] According to the present invention, preferred conductive metal particles are metals, alloys, mixtures of at least two metals, mixtures of at least two alloys, or mixtures of at least one metal and at least one alloy.

[0036] According to the present invention, the conductive metal particles may comprise Ag, Al, Cu, Zn, Pd, Ni, Pb, Au, or combinations thereof, preferably Ag, Al, Cu, or their alloys, and more preferably Ag. In a preferred embodiment of the present invention, the conductive metal particles are silver particles (silver powder).

[0037] According to the present invention, when the conductive metal particles are an alloy, the conductive metal particles can be crystalline metal particles or amorphous metal particles, preferably amorphous metal particles.

[0038] According to the present invention, the conductive metal particles can have various shapes, surfaces, sizes, surface area to volume ratios, oxygen contents, and oxide layers. Many shapes are known to those skilled in the art. Some examples are spherical, angular, elongated (rod-shaped or needle-shaped), and flattened (sheet-shaped). The metal particles can also exist as a combination of different shaped particles. According to the present invention, metal particles having one shape or a combination of shapes that are advantageous for the favorable sintering, electrical contact, adhesiveness, and electrical conductivity of the resulting electrodes are preferred. Without considering surface characteristics, one way to characterize such shapes is by means of the parameters length, width, and thickness. For the purposes of the present invention, the length of the particle is given by the length of the longest spatial displacement vector whose two end points are contained within the particle. The width of the particle is given by the length of the longest spatial displacement vector that is perpendicular to the length vector defined above and whose two end points are contained within the particle. The thickness of the particle is given by the length of the longest spatial displacement vector that is perpendicular to the length vector and width vector defined above and whose two end points are contained within the particle. In one embodiment of the present invention, metal particles having as uniform a shape as possible are preferred, i.e., a shape in which the ratios involving length, width, and thickness are as close to 1 as possible, preferably all ratios are in the range of 0.7 - 1.5, more preferably 0.8 - 1.3, and most preferably 0.9 - 1.2. In one embodiment of the present invention, preferred shape examples of the conductive metal particles are spherical and cubic or a combination thereof, or a combination of one or more of them with other shapes. In one embodiment of the present invention, the conductive metal particles in the conductive paste are spherical.

[0039] The particle size D50 is a well-known particle characteristic to those skilled in the art. The determination of the particle size D50 is well-known to those skilled in the art. According to the present invention, it is preferred that the particle size D50 of the conductive metal particles is 0.5 - 10 μm, preferably 1 - 5 μm.

[0040] In one embodiment of the present invention, the conductive metal particles are silver particles (silver powder) having a particle size D50 of 1 - 4 μm, preferably 2 - 3.5 μm, more preferably 2.8 - 3.2 μm.

[0041] In another embodiment of the present invention, the conductive metal particles are aluminum particles (aluminum powder) having a particle size D50 of 1 - 5 μm, preferably 2 - 4 μm, more preferably 2.5 - 3.5 μm.

[0042] In yet another embodiment of the present invention, the conductive metal particles are copper particles (copper powder) having a particle size D50 of 1 - 6 μm, preferably 2 - 4 μm, more preferably 2 - 3 μm.

[0043] In one embodiment of the present invention, the conductive metal particles are present in the conductive paste in a proportion greater than 50% by weight, preferably greater than 70% by weight, and most preferably greater than 80% by weight.

[0044] In a preferred embodiment of the present invention, the conductive metal particles are present in the conductive paste in a proportion of 50 - 95% by weight, such as 50% by weight, 60% by weight, 70% by weight, 80% by weight, 90% by weight.

[0045] b) Glass powder

[0046] According to the present invention, glass powder is present in the conductive paste to cause etching and sintering. For the purposes of the present invention, it is preferred that the glass powder is an amorphous or partially crystalline solid having a low glass transition temperature Tg. The glass transition temperature Tg is the temperature at which, upon heating, a rigid solid transforms into a partially flowing supercooled melt. The method for measuring the glass transition temperature Tg is well known to those skilled in the art. The etching and sintering caused by the glass powder occur at a temperature higher than the glass transition temperature Tg of the glass powder, and preferably the glass transition temperature Tg is lower than the required peak firing temperature.

[0047] All glass powders known to those skilled in the art and considered suitable for the context of the present invention can be used as the glass powder in the conductive paste. For the purposes of the present invention, the glass powder present in the conductive paste preferably comprises elements, their oxides, compounds that produce oxides upon heating, or mixtures thereof. In this regard, preferred elements are Si, B, Al, Bi, Li, Na, K, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, Cr, or combinations thereof. For the purposes of the present invention, the preferred oxides that the glass powder can comprise are alkali metal oxides, alkaline earth metal oxides, rare earth oxides, oxides of Group V and VI elements, other oxides, or combinations thereof. In this regard, preferred alkali metal oxides are sodium oxide, lithium oxide, potassium oxide, rubidium oxide, cesium oxide, or combinations thereof. In this regard, preferred alkaline earth metal oxides are beryllium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, or combinations thereof. In this regard, preferred oxides of Group V elements are phosphorus oxides such as P2O5; bismuth oxides such as Bi2O3; or combinations thereof. In this regard, preferred oxides of Group VI elements are tellurium oxides such as TeO2 or TeO3; selenium oxides such as SeO2; or combinations thereof. Preferred rare earth oxides are cerium oxides such as CeO2; and lanthanum oxides such as La2O3. In this regard, other preferred oxides are silicon oxides such as SiO2; zinc oxides such as ZnO; aluminum oxides such as Al2O3; germanium oxides such as GeO2; vanadium oxides such as V2O5; niobium oxides such as Nb2O5; boron oxides such as B2O3; tungsten oxides such as WO3; molybdenum oxides such as MoO3; indium oxides such as In2O3; other oxides of the elements listed above as preferred elements; or combinations thereof. Preferred oxides are also mixed oxides comprising at least two elements listed as preferred elemental components of the glass powder, or mixed oxides formed by heating at least one of the above-mentioned oxides with at least one of the above-mentioned metals. For the purposes of the present invention, mixtures of at least two of the above-listed oxides and mixed oxides are also preferred.

[0048] In one embodiment of the present invention, the glass powder comprises oxides of elements selected from Si, B, Al, Bi, Li, Na, K, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, Cr, Te, P, and combinations thereof, preferably comprising lead oxide, boron oxide, and silicon oxide.

[0049] In one embodiment of the present invention, the glass powder can comprise:

[0050] 30 - 50 mol% of PbO;

[0051] 5 - 15 mol% of B2O3; and

[0052] 40 - 60 mol% of SiO2,

[0053] wherein the mole percentages are based on the total moles of all oxides.

[0054] According to the present invention, preferably, the glass powder has a glass transition temperature Tg lower than the required firing temperature of the conductive paste. In one embodiment of the present invention, preferably, the glass powder has a glass transition temperature Tg of 300 - 600 °C, more preferably 300 - 500 °C, and most preferably 320 - 450 °C.

[0055] According to the present invention, the glass powder particles can have various shapes, surface properties, sizes, surface area to volume ratios, and coatings. Many shapes of glass powder particles are known to those skilled in the art. Some examples are spherical, angular, elongated (rod - shaped or needle - shaped), and flat (sheet - shaped). The glass powder particles can also exist as a combination of different - shaped particles. According to the present invention, it is preferred that the glass powder has one shape or a combination of shapes that is beneficial for the favorable sintering, adhesiveness, electrical contact, and conductivity of the fabricated electrodes.

[0056] According to the present invention, preferably, the particle size D50 of the glass powder is 0.1 - 10 μm, such as 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, more preferably 0.2 - 7 μm, and most preferably 0.5 - 5 μm. The determination of the particle size D50 is well - known to those skilled in the art.

[0057] In one embodiment of the present invention, the glass powder has a particle size D50 of 0.1 - 3 μm, preferably 0.5 - 2 μm, and more preferably 0.8 - 1.5 μm.

[0058] In one embodiment of the present invention, the glass powder is present in an amount of 0.1 - 15 wt% of the conductive paste, preferably 3 - 10 wt%.

[0059] In one embodiment of the present invention, the glass powder of the present invention can be prepared by the following method and then used to prepare the conductive paste of the present invention: combining all components of the glass powder to obtain a composition, melting the composition to obtain a glass frit, quenching in deionized water, and finally forming the product into particles with the desired particle size to obtain the glass powder of the present invention.

[0060] Preferably, the "melting" is carried out by loading the composition into a crucible, placing the crucible in a muffle furnace and melting the composition at a high temperature; the "water quenching" is carried out by removing the molten glass from the muffle furnace and pouring it into a bucket filled with deionized water; the "making into particles with a desired particle size" is carried out by grinding the water-quenched glass cullet with a ball mill to obtain glass powder with a desired particle size D50.

[0061] In the above method, the temperature of the muffle furnace is high enough for melting the components of the glass powder, and the melting time is long enough for the components to be uniformly mixed.

[0062] More preferably, in the preparation of the glass powder, the temperature of the muffle furnace is 800 - 1500 °C, preferably 900 - 1200 °C, and the melting time of the mixture is 15 minutes to 2 hours, preferably 30 minutes to 1 hour.

[0063] In another embodiment of the present invention, the glass powder of the present invention can be prepared by the following method and then used to prepare the conductive paste of the present invention: combining a part of the components of the glass powder to obtain a first composition, melting the first composition to obtain glass, water quenching in deionized water, and finally making the product into first particles with a desired particle size; combining the remaining components of the glass powder to obtain a second composition, melting the second composition to obtain glass, water quenching in deionized water, and finally making the product into second particles with a desired particle size; combining the first particles and the second particles to obtain the glass powder of the present invention.

[0064] The glass powder of the present invention can be divided into more parts, and the glass powder of the present invention can be obtained by corresponding methods.

[0065] For example, in yet another embodiment of the present invention, the glass powder of the present invention can be prepared by the following method and then used to prepare the conductive paste of the present invention: combining the first part of the components of the glass powder to obtain a first composition, melting the first composition to obtain glass, water quenching in deionized water, and finally making the product into first particles with a desired particle size; combining the second part of the components of the glass powder to obtain a second composition, melting the second composition to obtain glass, water quenching in deionized water, and finally making the product into second particles with a desired particle size; combining the remaining components of the glass powder to obtain a third composition, melting the third composition to obtain glass, water quenching in deionized water, and finally making the product into third particles with a desired particle size; combining the first particles, the second particles and the third particles to obtain the glass powder of the present invention.

[0066] Obviously, the above first particles, second particles and / or third particles, etc. can be pre-prepared particles.

[0067] c) Reducing additive

[0068] According to the present invention, the reducing additive is represented by formula (I):

[0069] Zr x Cu y Al z (I)

[0070] In formula (I), x = 10 - 90 wt%, y = 5 - 50 wt%, z = 5 - 20 wt%, and the weight percentages are based on the weight of the reducing additive.

[0071] In one embodiment of the present invention, in formula (I), x = 10 - 90 wt%, preferably 40 - 90 wt%, more preferably 60 - 80 wt%, and the weight percentages are based on the weight of the reducing additive.

[0072] In one embodiment of the present invention, in formula (I), y = 5 - 50 wt%, preferably 5 - 20 wt% or 10 - 30 wt%, and the weight percentages are based on the weight of the reducing additive.

[0073] In one embodiment of the present invention, in formula (I), z = 5 - 20 wt%, preferably 5 - 20 wt%, more preferably 5 - 15 wt%, and the weight percentages are based on the weight of the reducing additive.

[0074] In one embodiment of the present invention, the reducing additive is amorphous (non - crystalline) metal alloy particles.

[0075] In one embodiment of the present invention, the reducing additive is amorphous metal alloy particles having a particle size D50 of 0.1 - 8 μm. In a preferred embodiment of the present invention, the reducing additive is amorphous metal alloy particles having a particle size D50 of 1 - 5 μm. The determination of the particle size D50 is well - known to those skilled in the art.

[0076] In one embodiment of the present invention, the reducing additive is present in an amount of 0.1 - 10 wt% of the conductive paste, preferably 0.1 - 5 wt%, more preferably 0.1 - 1 wt%.

[0077] In one embodiment of the present invention, the reducing additive is prepared by a jet - flow powder preparation method, which includes the following steps:

[0078] (1) Mix and disperse evenly the high - purity metal powders for preparing the reducing additive according to the formula ratio;

[0079] (2) Heat and melt the obtained metal powder mixture in a crucible (such as a copper pressure crucible lined with quartz) at a temperature above 1000 °C to obtain a liquid master alloy;

[0080] (3) Under the action of pressure, directly spray the liquid master alloy into ice water mixed with NaCl to obtain amorphous LTP alloy powder.

[0081] According to an embodiment of the present invention, the high-purity metal powder in step (1) has a purity of, for example, more than 99%.

[0082] According to an embodiment of the present invention, the temperature in step (2) is preferably 1050 - 1500 °C, such as 1100 °C, 1200 °C, 1300 °C, 1400 °C.

[0083] According to an embodiment of the present invention, the pressure in step (3) is preferably 20 - 100 PSI, such as 30 PSI, 40 PSI, 50 PSI, 60 PSI, 70 PSI, 80 PSI, 90 PSI.

[0084] According to an embodiment of the present invention, the NaCl concentration in step (3) is preferably 1 - 10 wt%, such as 2 - 5 wt% or 6 - 9 wt%.

[0085] According to the present invention, by adjusting the temperature, pressure and spraying speed, LTP powders with different particle sizes and morphologies can be obtained.

[0086] d) Organic carrier

[0087] In an embodiment of the present invention, the conductive paste contains an organic carrier commonly used in the art. The preferred organic carrier is an organic carrier that provides the best stability of the components in the conductive paste and imparts a viscosity to the conductive paste that allows effective printability.

[0088] In one embodiment, the amount of the organic carrier can be 2 - 20 wt%, more preferably 5 - 15 wt%, and most preferably 6 - 10 wt%, based on the total weight of the conductive paste.

[0089] In one embodiment, the organic carrier includes a solvent, a binder (such as an organic binder, such as a polymer, resin), a surfactant, an additive or any combination thereof, preferably an organic binder and a solvent. The additives include a thixotropic agent, a viscosity modifier, a stabilizer, a thickener, an emulsifier, a dispersant, a slip agent (such as an alkyl-modified silicone oil) or a pH regulator and any combination thereof. For example, in one embodiment, the organic carrier includes one or more binders in an organic solvent.

[0090] The binder may be present in an amount of 0.1 - 10% by weight, preferably 0.1 - 8% by weight, more preferably 0.5 - 7% by weight, based on the total weight of the organic carrier. Preferred binders are those that promote the formation of a conductive paste having favorable stability, printability, tackiness, and sintering properties. Preferred binders (which generally fall within the class referred to as "resins") are polymeric binders, monomeric binders, and binders that are a combination of polymers and monomers. The polymeric binder may also be a copolymer.

[0091] Preferred polymeric binders include those having functional groups carried on the polymer backbone, those having functional groups carried outside the backbone, and those having functional groups carried both within and outside the backbone. Preferred polymers having functional groups on the backbone include, for example, polyesters, substituted polyesters, polycarbonates, substituted polycarbonates, polymers having cyclic groups on the backbone, polysaccharides, substituted polysaccharides, polyurethanes, substituted polyurethanes, polyamides, substituted polyamides, phenolic resins, substituted phenolic resins, copolymers of monomers of one or more of the above polymers (optionally, with other comonomers), or combinations of at least two of them.

[0092] Preferred polymers having cyclic groups on the backbone include, for example, polyvinyl butyral (PVB) and its derivatives, and polyterpene alcohols and their derivatives or mixtures thereof. Preferred polysaccharides include, for example, cellulose, methylcellulose, ethylcellulose, hydroxyethylcellulose, propylcellulose, hydroxypropylcellulose, butylcellulose, their derivatives, and mixtures of at least two of them. Other preferred polymers include, for example, cellulose ester resins such as cellulose acetate propionate, cellulose acetate butyrate, and any combination thereof. Other preferred polymers are those disclosed in U.S. Patent Application Publication No. 2013 / 0180583, which is incorporated herein by reference.

[0093] Preferred polymers having functional groups outside the polymer backbone are polymers having amide groups, polymers having acid and / or ester groups (commonly referred to as acrylic resins), or polymers having a combination of the above functional groups, or combinations thereof. Preferred polymers having amide groups outside the backbone include, for example, polyvinylpyrrolidone (PVP) and its derivatives. Preferred polymers having acid and / or ester groups outside the backbone include, for example, polyacrylic acid and its derivatives, polymethyl methacrylate (PMMA) and its derivatives, or mixtures thereof.

[0094] Preferred monomeric binders include, for example, ethylene glycol-based monomeric binders. Preferred ethylene glycol-based monomeric binders are binders having multiple ether groups, multiple ester groups, or a binder having one ether group and one ester group. Preferred ether groups are methyl, ethyl, propyl, butyl, pentyl, hexyl, and higher alkyl ethers. Preferred ester groups are acetate and its alkyl ether derivatives. Preferred is ethylene glycol monobutyl ether monoacetate or mixtures thereof.

[0095] Preferred binders in the present invention are, for example, alkyl celluloses (preferably ethyl cellulose), their derivatives, and mixtures thereof with other binders listed from the previous binders.

[0096] The amount of the organic solvent can be 40 - 90% by weight, more preferably 35 - 85% by weight, based on the total weight of the organic carrier.

[0097] Preferred solvents are solvents that allow the formation of a conductive paste having favorable viscosity, printability, stability, and sintering properties. All solvents known in the art and considered suitable in the present invention can be used as solvents in the organic carrier. According to the present invention, preferred solvents are solvents that allow the achievement of a preferably high level of printability of the conductive paste as described above. Preferred solvents according to the present invention are solvents that exist in liquid form at standard ambient temperature and pressure (SATP) (25 °C, 100 kPa), preferably having a boiling point above 90 °C and a glass transition temperature Tg above -20 °C.

[0098] Preferred solvents are polar or non - polar, protic or aprotic, aromatic or non - aromatic. Preferred solvents include, for example, monoalcohols, diols, polyols, monoesters, diesters, polyesters, monoethers, diethers, polyethers, solvents including at least one or more of the functional groups in these categories, optionally including other categories of functional groups, and mixtures of two or more of the above solvents, such as diethylene glycol butyl ether acetate.

[0099] The organic carrier may also include a surfactant. The amount of the surfactant can be 0 - 10% by weight, preferably 0 - 8% by weight, more preferably 0.01 - 6% by weight, based on the total weight of the organic carrier. Preferred surfactants in the present invention are surfactants that promote the formation of a conductive paste having favorable stability, printability, viscosity, and sintering properties. All surfactants known in the art and considered suitable in the present invention can be used as surfactants in the organic carrier. Preferred surfactants can have non - ionic, anionic, cationic, amphiphilic, or zwitterionic heads. Preferred surfactants are polymeric and monomeric or mixtures thereof.

[0100] According to the present invention, the conductive paste optionally contains additives commonly used in the art. Preferred conductive paste additives are components added to the conductive paste in addition to the components already explicitly mentioned, which are used to promote higher performance of the conductive paste, the electrodes made therefrom, or the resulting crystalline silicon solar cell. All additives known in the art and considered suitable in the present invention can be used as conductive paste additives. Preferred additives are thixotropic agents, viscosity modifiers, stabilizers, thickeners, emulsifiers, dispersants, slip agents, or pH regulators and any combination thereof. Preferred thixotropic agents herein are carboxylic acid derivatives, preferably fatty acid derivatives or combinations thereof. Preferred fatty acid derivatives are C9H19 COOH (capric acid), C 11 H 23 COOH (lauric acid), C 13 H 27 COOH (myristic acid), C 15 H 31 COOH (palmitic acid), C 17 H 35 COOH (stearic acid), C 18 H 34 O2 (oleic acid), C 18 H 32 O2 (linoleic acid), castor oil, hydrogenated castor oil, or a combination thereof.

[0101] In one embodiment of the present invention, to form a conductive paste, the glass powder can be combined with conductive metal particles, reducing additives, organic carriers, and optionally conductive paste additives using any method known in the art for preparing pastes. The details of the preparation method are not critical as long as it produces a homogeneously dispersed paste. The components can be mixed, for example, with a mixer and then made into a uniformly dispersed paste by, for example, a three-roll mill.

[0102] For the electrodes of crystalline silicon solar cells

[0103] The electrode for a crystalline silicon solar cell according to the present invention is formed by sintering the above-mentioned conductive paste.

[0104] In one embodiment of the present invention, the temperature for sintering the conductive paste is 700 - 850 °C, preferably 750 - 800 °C.

[0105] Crystalline silicon solar cell

[0106] The present invention also relates to a crystalline silicon solar cell, which includes a substrate and the electrode bonded to the substrate.

[0107] In one embodiment of the present invention, the preferred crystalline silicon solar cell according to the present invention is a crystalline silicon solar cell with high efficiency in terms of the proportion of the total energy of incident light converted into electrical energy output. A lightweight and durable crystalline silicon solar cell is also preferred. The crystalline silicon solar cell includes at least: (i) a front electrode, (ii) a front doping layer, (iii) a p-n junction boundary, (iv) a back doping layer, (v) a back electrode, and (vi) a passivation layer. The crystalline silicon solar cell may also include additional layers for chemical / mechanical protection.

[0108] In one embodiment of the present invention, the substrate of the crystalline silicon solar cell of the present invention is a substrate well-known to those skilled in the art for crystalline silicon solar cells.

[0109] The crystalline silicon solar cell of the present invention basically has an electrode formed by sintering treatment of the conductive paste of the present invention bonded to the substrate.

[0110] In one embodiment of the present invention, the conductive paste of the present invention is applied to a substrate, such as a semiconductor substrate (e.g., a crystalline silicon wafer), to form a printed electrode.

[0111] The conductive paste of the present invention can be applied to the substrate by any method known in the art and considered applicable in the present invention. Examples of such methods include, but are not limited to, dipping, impregnating, casting, dropping, injecting, spraying, doctor blading, curtain coating, brush coating, or printing or a combination of at least two thereof. Preferred printing techniques are inkjet printing, screen printing, flexographic printing, offset printing, letterpress printing, or stencil printing or a combination of at least two thereof. According to the present invention, it is preferred to apply the conductive paste of the present invention by printing, preferably by screen printing.

[0112] Firing is required to sinter the printed electrode to form a solid conductor. Firing is well known in the art and can be achieved in any manner recognized as appropriate in the present invention. Preferably, firing is carried out at a temperature higher than the Tg of the glass powder material.

[0113] Outside the region occupied by the electrode, the substrate of the present invention, preferably a crystalline silicon wafer, has a region where light can be efficiently absorbed to generate electron-hole pairs and the holes and electrons can be efficiently separated across a boundary, preferably across a p-n junction boundary.

[0114] The p-n junction boundary is located at the position where the front doping layer and the back doping layer of the wafer meet. In an N-type solar cell, the back doping layer is doped with an n-type dopant and the front doping layer is doped with a p-type dopant. In a P-type solar cell, the back doping layer is doped with a p-type dopant and the front doping layer is doped with an n-type dopant. According to a preferred embodiment of the present invention, a wafer having a p-n junction boundary is prepared by first providing a doped silicon substrate and then applying a doped layer of the opposite type to one surface of the substrate.

[0115] The above dopants are preferably dopants that form the p-n junction boundary by introducing electrons or holes into the energy band structure when added to the crystalline silicon wafer. According to the present invention, it is preferred to specifically select the types and concentrations of these dopants to adjust the energy band structure profile of the p-n junction and set the light absorption rate and conductivity profile as required. The preferred p-type dopants according to the present invention are dopants that add holes to the energy band structure of the crystalline silicon wafer. All dopants known in the art and considered applicable in the present invention can be used as p-type dopants. The preferred p-type dopants according to the present invention are trivalent elements, especially trivalent elements of Group 13 of the periodic table. The preferred Group 13 elements of the periodic table herein include, but are not limited to, boron, aluminum, gallium, indium, thallium, or a combination of at least two thereof, with boron being particularly preferred.

[0116] The preferred n-type dopants according to the present invention are dopants that add electrons to the energy band structure of the crystalline silicon wafer. All dopants known in the art and considered applicable in the present invention can be used as n-type dopants. The preferred n-type dopants according to the present invention are elements of Group V of the periodic table. The preferred Group V elements of the periodic table herein include nitrogen, phosphorus, arsenic, antimony, bismuth, or a combination of at least two thereof, with phosphorus being particularly preferred.

[0117] In one embodiment of the present invention, according to the present invention, the anti-reflection layer can be applied as an outer layer before the electrodes are applied to the front side of the crystalline silicon solar cell. The preferred anti-reflection layer according to the present invention is an anti-reflection layer that reduces the proportion of incident light reflected by the front side and increases the proportion of incident light that will be absorbed by the wafer across the front side. The anti-reflection layer that produces a favorable absorption ratio / reflection ratio is vulnerable to etching by the conductive paste. Additionally, an anti-reflection layer that is resistant to the temperature required for firing the conductive paste and does not promote greater recombination of electrons and holes near the electrode interface is preferred. All anti-reflection layers known in the art and considered applicable in the present invention can be employed. The preferred anti-reflection layer according to the present invention is silicon nitride, silicon dioxide, aluminum oxide, titanium dioxide, or a mixture of at least two thereof and / or a combination of at least two layers thereof. According to a preferred embodiment, the anti-reflection layer is silicon nitride, i.e., Si x N y , especially when using a crystalline silicon wafer, where x is 2 - 4 and y is 3 - 5.

[0118] In one embodiment of the present invention, one or more passivation layers can be applied to a substrate, preferably to the front side and / or the back side of a crystalline silicon wafer as an outer layer. The passivation layer can be applied before forming the front electrode or before applying an antireflection layer (if one exists). Preferably, the passivation layer is a passivation layer that reduces the electron / hole recombination rate near the electrode interface. Any passivation layer known in the art and considered applicable in the present invention can be used. According to the present invention, the passivation layer can be silicon nitride, aluminum oxide, silicon dioxide, and titanium dioxide. According to the most preferred embodiment, aluminum oxide is used. Preferably, the passivation layer has a thickness of 0.1 nm to 2 μm, more preferably 1 nm to 1 μm, and most preferably 1 nm to 200 nm.

[0119] In one embodiment of the present invention, in addition to the above layers that directly promote the main function of the crystalline silicon solar cell, other layers can be added for mechanical and chemical protection.

[0120] The cell can be encapsulated to provide chemical protection. Encapsulation is well known in the art and any encapsulation suitable for the present invention can be used. According to a preferred embodiment, a transparent polymer (commonly referred to as a transparent thermoplastic resin) is used as the encapsulation material, provided that such an encapsulation exists. Preferably, the transparent polymer herein is silicone rubber and ethylene vinyl acetate (EVA).

[0121] A transparent glass sheet can also be added to the front side of the crystalline silicon solar cell to provide mechanical protection. Transparent glass sheets are well known in the art and any transparent glass sheet applicable in the present invention can be used.

[0122] A backside protection material can be added to the back side of the crystalline silicon solar cell to provide mechanical protection. Backside protection materials are well known in the art and any backside protection material considered applicable in the present invention can be used. According to the present invention, the preferred backside protection material is a backside protection material having good mechanical properties and weather resistance. According to the present invention, the preferred backside protection material is polyethylene terephthalate having a polytetrafluoroethylene layer (such as a PTFE layer). According to the present invention, it is preferred that the backside protection material is present below the encapsulation layer (in the case where a backside protection layer and encapsulation exist).

[0123] A frame material can be added to the outside of the crystalline silicon solar cell to provide mechanical support. Frame materials are well known in the art and any frame material considered applicable in the present invention can be used. According to the present invention, the preferred frame structure is aluminum.

[0124] In a preferred embodiment of the present invention, the conductive paste of the present invention is used to prepare an N-type solar cell, particularly a TOPCon solar cell, and the conductive paste comprises:

[0125] a) 50 - 95 wt% of conductive metal particles,

[0126] b) 0.1 - 15 wt% of glass powder,

[0127] c) 0.1 - 10 wt% of reducing additive, and

[0128] d) 2 - 20 wt% of organic carrier, where the weight percentages are based on the total weight of the conductive paste.

[0129] Those skilled in the art can more easily understand the present invention according to the following embodiments:

[0130] Embodiment 1. A conductive paste, comprising:

[0131] a) Conductive metal particles,

[0132] b) Glass powder,

[0133] c) Reducing additive, and

[0134] d) Organic carrier, where the reducing additive is represented by formula (I):

[0135] Zr x Cu y Al z (I)

[0136] In formula (I), x = 10 - 90 wt%, y = 5 - 50 wt%, z = 5 - 20 wt%, and the weight percentages are based on the weight of the reducing additive.

[0137] Embodiment 2. The conductive paste according to Embodiment 1, where in formula (I), x = 40 - 90 wt% and y = 5 - 20 wt%.

[0138] Embodiment 3. The conductive paste according to Embodiment 1 or 2, where the reducing additive is amorphous metal alloy particles.

[0139] Embodiment 4. The conductive paste according to any one of Embodiments 1 - 3, where the reducing additive has a particle size D50 of 0.1 - 8 μm, preferably 1 - 5 μm.

[0140] Embodiment 5. The conductive paste according to any one of Embodiments 1 - 4, where the conductive metal particles include Ag, Al, Cu, Zn, Pd, Ni, Pb, Au, or a combination thereof, preferably Ag, Al, Cu, or their alloys, more preferably Ag.

[0141] Embodiment 6. The conductive paste according to any one of Embodiments 1 - 5, where the conductive metal particles have a particle size D50 of 0.5 - 10 μm, preferably 1 - 5 μm.

[0142] Embodiment 7. The conductive paste according to any one of Embodiments 1-6, wherein the glass powder comprises oxides of elements selected from Si, B, Al, Bi, Li, Na, K, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, Cr, Te, P and combinations thereof, preferably comprising lead oxide, boron oxide and silicon oxide.

[0143] Embodiment 8. The conductive paste according to any one of Embodiments 1-7, wherein the glass powder has a glass transition temperature Tg of 300-600 °C, preferably 300-500 °C.

[0144] Embodiment 9. The conductive paste according to any one of Embodiments 1-8, wherein the glass powder has a particle size D50 of 0.1-10 μm, preferably 0.2-7 μm, more preferably 0.5-5 μm.

[0145] Embodiment 10. The conductive paste according to any one of Embodiments 1-9, wherein the conductive paste comprises:

[0146] a) 50-95 wt% of conductive metal particles,

[0147] b) 0.1-15 wt% of glass powder,

[0148] c) 0.1-10 wt% of reducing additive, and

[0149] d) 2-20 wt% of organic carrier,

[0150] The weight percentages are based on the total weight of the conductive paste.

[0151] Embodiment 11. An electrode for a crystalline silicon solar cell, the electrode being formed by sintering the conductive paste according to any one of Embodiments 1-10.

[0152] Embodiment 12. The electrode according to Embodiment 11, wherein the sintering treatment is carried out at a temperature of 700-850 °C, preferably 750-800 °C.

[0153] Embodiment 13. A crystalline silicon solar cell, which comprises a substrate and an electrode according to any one of Embodiments 11-12 bonded to the substrate, wherein the crystalline silicon solar cell is an N-type crystalline silicon solar cell, preferably a TOPCon solar cell.

[0154] Example

[0155] The present invention will be illustrated by the following examples. However, it should be understood that the following examples are non-limiting and are not intended to limit the scope of protection of the present invention.

[0156] Raw material

[0157] PbO, B2O3, and SiO2 are 4N-grade chemical reagents.

[0158] The silver powder (Ag) and aluminum powder (Al) are spherical powders with a D50 particle size of 2 μm.

[0159] The silicon wafer is an N-type silicon wafer with a silicon nitride and aluminum oxide passivation layer and a size of 182 mm.

[0160] The organic carrier (V1) is composed as follows:

[0161] Diethylene glycol butyl ether acetate: 5.6 parts by weight;

[0162] Cellulose acetate butyrate: 0.6 parts by weight;

[0163] Oleic acid: 0.6 parts by weight;

[0164] Hydrogenated castor oil: 0.6 parts by weight;

[0165] Alkyl-modified silicone oil: 0.6 parts by weight.

[0166] The glass powder (G1) is composed of 40 mol% PbO, 10 mol% B2O3, and 50 mol% SiO2.

[0167] The composition of the reducing additives (AM01 - AM03) is as shown in Table 1 below:

[0168] Table 1 Composition of the reducing additives

[0169] wt% AM01 AM02 AM03 Zr 67 69 70 Cu 25 20 16 Al 8 11 14

[0170] Test method

[0171] The IV experiment of the cell is carried out using a commercial IV tester "cetisPV-Celltest4-BF" from Halm Elektronik GmbH to measure the cell conversion efficiency (Eta), open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF), and series resistance (Rs).

[0172] Preparation of glass powder

[0173] Weigh the components of the glass powder G1 and combine them to obtain the composition corresponding to the glass powder G1;

[0174] The obtained composition was loaded into an alumina crucible, placed in a muffle furnace and kept at 1100 °C for 60 minutes;

[0175] The alumina crucible containing the molten glass was removed from the muffle furnace, and the molten glass was poured into a bucket filled with deionized water for water quenching;

[0176] The water-quenched glass slag was ground into a powder with a D50 particle size of about 1.5 μm by a ball mill to obtain glass powder G1.

[0177] Preparation of reducing additive

[0178] The reducing additive was prepared by the jet powder preparation method, which included the following steps:

[0179] (1) Mix and disperse evenly the high-purity metal powders (purity 99%) used for preparing the reducing additive according to the formulation ratio in Table 1;

[0180] (2) Heat and melt the obtained metal powder mixture in a copper pressure crucible lined with quartz at a temperature of 1100 °C to obtain a liquid master alloy;

[0181] (3) Directly inject the liquid master alloy under a pressure of 50 PSI into ice water mixed with 2 wt% NaCl to obtain amorphous LTP alloy powder.

[0182] Preparation of conductive paste (Examples 1 - 3 / Comparative Examples 1 - 2)

[0183] Weigh silver powder, reducing additive or aluminum powder, glass powder and organic carrier respectively according to the ratio shown in Table 2, combine them, mix with a planetary mixer, and then mix with a three-roll mill to prepare the conductive pastes of Examples 1-3 / Comparative Examples 1-2.

[0184] Table 2 Composition of the conductive paste

[0185]

[0186] Preparation of solar cell substrate with electrodes (Sample 1 - 3 of the present invention / Comparative Sample 1 - 2)

[0187] The conductive pastes of Examples 1-3 were printed onto N-type silicon wafers by screen printing (430-11-15-3.5-14-9BB screen), and then rapidly sintered under the conditions of a peak temperature of 800 °C and a time from room temperature to peak temperature of 16 seconds to obtain solar cell substrates with electrodes (Samples 1-3 of the present invention), and the electrical properties were tested. The results are shown in Table 3.

[0188] The conductive pastes of Comparative Examples 1-2 were respectively printed onto N-type silicon wafers by screen printing (430-11-15-3.5-14-9BB screen), and then rapidly sintered under the conditions of a peak temperature of 800 °C and a time from room temperature to the peak temperature of 16 seconds to obtain solar cell substrates with electrodes (Comparative Samples 1-2), and electrical performance tests were carried out. The results are shown in Table 3.

[0189] Table 3 Performance of Solar Cells

[0190]

[0191]

[0192] The results in Table 3 show that, compared with the comparative samples using aluminum powder, for the samples of the present invention using the reducing additive of the present invention, the cell conversion efficiency (Eta), fill factor (FF), and resistance (Rs) of the solar cells of the present invention have all been improved. The decrease in Rs indicates a decrease in the contact resistance of the solar cells of the present invention.

[0193] Preparation of conductive paste (Examples 4 - 6)

[0194] Silver powder, reducing additive, glass powder, and organic carrier were weighed respectively according to the ratios shown in Table 4, combined, mixed with a planetary mixer, and then mixed with a three-roll mill to prepare the conductive pastes of Examples 4-6.

[0195] Table 4 Composition of Conductive Paste

[0196]

[0197] Preparation of solar cell substrate with electrodes (Sample 4 - 6 of the present invention)

[0198] The conductive pastes of Examples 4-6 were respectively printed onto N-type silicon wafers by screen printing (430-11-15-3.5-14-9BB screen), and then rapidly sintered under the conditions of a peak temperature of 800 °C and a time from room temperature to the peak temperature of 16 seconds to obtain solar cell substrates with electrodes (Samples 4-6 of the present invention), and electrical performance tests were carried out. The results are shown in Table 5.

[0199] Table 5 Performance of Solar Cells

[0200] Performance Comparative Sample 2 Sample 4 of the present invention Sample 5 of the present invention Sample 6 of the present invention Eta (%) Reference +0.04％ +0.1％ +0.07％ Voc (V) Reference +0.002 +0.005 +0.002 Isc (mA) Reference -0.01 -0.01 -0.01 FF (%) Reference +0.05％ +0.08％ +0.07％ Rs (mOhm) Reference -0.001 -0.002 -0.002

[0201] The results in Table 5 show that, compared with the comparative samples using aluminum powder, for the samples of the present invention using different contents of the reducing additive of the present invention, the cell conversion efficiency (Eta), fill factor (FF), and resistance (Rs) of the solar cells of the present invention have all been improved. The decrease in Rs indicates a decrease in the contact resistance of the solar cells of the present invention.

[0202] Preparation of conductive paste (Examples 7 - 9)

[0203] Weigh silver powder, reducing additive, glass powder and organic carrier respectively according to the ratios shown in Table 6, combine them, mix with a planetary mixer, and then mix with a three-roll mill to obtain the conductive pastes of Examples 7-9.

[0204] Table 6 Composition of Conductive Paste

[0205]

[0206] Preparation of solar cell substrate with electrodes (Sample 7 - 9 of the present invention)

[0207] Print the conductive pastes of Examples 7-9 onto N-type silicon wafers respectively by screen printing (430-11-15-3.5-14-9BB screen), and then sinter them rapidly under the conditions of peak temperatures of 780 °C, 770 °C, 750 °C respectively and a time from room temperature to peak temperature of 16 seconds to obtain solar cell substrates with electrodes (Samples 7-9 of the present invention), and conduct electrical performance tests.

[0208] The results of the performance tests show that for the samples of the present invention with different contents of the reducing additive of the present invention and different sintering temperatures, compared with the comparative samples using aluminum powder, the cell conversion efficiency (Eta), fill factor (FF) and resistance (Rs) of the solar cells of the present invention have all been improved. Among them, the cell conversion efficiency (Eta) has increased by at least 0.04%, and even increased by up to 0.12%, the fill factor (FF) has increased by at least 0.03%, and even increased by up to 0.12%, and the resistance (Rs) has also been reduced. The reduction of the Rs indicates the reduction of the contact resistance of the solar cells of the present invention.

Claims

1. A conductive paste, which comprises: a) Conductive metal particles, b) Glass powder, c) Reduction additive, and d) Organic carrier, wherein the reduction additive is represented by formula (I): Zr x Cu y Al z (I) In formula (I), x = 10 - 90 wt%, y = 5 - 50 wt%, z = 5 - 20 wt%, and the weight percentages are based on the weight of the reduction additive.

2. The conductive paste according to claim 1, wherein in formula (I), x = 40 - 90 wt% and y = 5 - 20 wt%.

3. The conductive paste according to claim 1 or 2, wherein the reduction additive is amorphous metal alloy particles.

4. The conductive paste according to any one of claims 1 - 3, wherein the reduction additive has a particle size D50 of 0.1 - 8 μm, preferably 1 - 5 μm.

5. The conductive paste according to any one of claims 1 - 4, wherein the conductive metal particles comprise Ag, Al, Cu, Zn, Pd, Ni, Pb, Au or a combination thereof, preferably Ag, Al, Cu or an alloy thereof, more preferably Ag.

6. The conductive paste according to any one of claims 1 - 5, wherein the conductive metal particles have a particle size D50 of 0.5 - 10 μm, preferably 1 - 5 μm.

7. The conductive paste according to any one of claims 1 - 6, wherein the glass powder comprises oxides of elements selected from Si, B, Al, Bi, Li, Na, K, Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba, Cr, Te, P and combinations thereof, preferably comprising lead oxide, boron oxide and silicon oxide.

8. The conductive paste according to any one of claims 1 - 7, wherein the glass powder has a glass transition temperature Tg of 300 - 600 °C, preferably 300 - 500 °C.

9. The conductive paste according to any one of claims 1 - 8, wherein the glass powder has a particle size D50 of 0.1 - 10 μm, preferably 0.2 - 7 μm, more preferably 0.5 - 5 μm.

10. The conductive paste according to any one of claims 1 - 9, wherein the conductive paste comprises: a) 50 - 95 wt% of conductive metal particles, b) 0.1 - 15 wt% of glass powder, c) 0.1 - 10 wt% of reduction additive, and d) 2 - 20 wt% of organic carrier, The weight percentages are based on the total weight of the conductive paste.

11. An electrode for a crystalline silicon solar cell, which is formed by sintering the conductive paste according to any one of claims 1 - 10.

12. The electrode according to claim 11, wherein the sintering treatment is carried out at a temperature of 700 - 850 °C, preferably 750 - 800 °C.

13. A crystalline silicon solar cell, which comprises a substrate and the electrode according to any one of claims 11 - 12 bonded to the substrate, wherein the crystalline silicon solar cell is an N - type crystalline silicon solar cell, preferably a TOPCon solar cell.

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