Copper paste, copper electrode, and solar cell

Abstract

The present application provides a copper paste. The copper paste has a shear viscosity of 0.1-10 Pa•s at a temperature of 20-25°C and a shear rate of 200-500 s-1. Therefore, the present application provides a copper paste, which has a high storage stability before printing and which is capable reducing the viscosity to improve the fluidity at a high shear rate during the printing process, and exhibits a stable and excellent continuous printability, thereby providing a high-quality, low-resistance copper electrode.

Description

Copper paste, copper electrodes, and solar cells

[0001] This application claims priority to Chinese invention patent application No. 202510045723.1, filed on January 10, 2025, entitled "Copper Paste, Copper Electrode and Solar Cell", and Chinese invention patent application No. 202511821140.X, filed on December 3, 2025, entitled "Copper Paste, Copper Electrode and Solar Cell". Technical Field

[0002] This application relates to the field of batteries, and more specifically to a copper paste and a copper electrode prepared therefrom. Background Technology

[0003] Conductive paste is a crucial component of solar cells, significantly impacting their photoelectric conversion efficiency and cost per kilowatt-hour. Traditional conductive pastes primarily utilize silver paste, known for its high conductivity and stability; however, silver's high price drastically increases the cost of solar cells. Therefore, finding a conductive paste that offers good conductivity, low cost, and easy availability is urgently needed. Copper, one of the earliest metals used by humankind, possesses electrical properties similar to silver. Due to its low cost, copper paste can significantly reduce the cost of solar cells by replacing silver paste.

[0004] In the fabrication of solar cells, the rheological or thixotropic properties of the paste have a significant impact on processes such as printing, coating, and deposition. Therefore, controlling the rheological or thixotropic properties of the paste is crucial. Poor printability of the paste can lead to a series of problems such as incomplete grid lines, grid breakage, and extrusion, which in turn affect electrical performance and cell efficiency. Copper paste differs in composition from silver paste, and the unavoidable presence of small amounts of copper ions in copper paste can also affect the rheological and thixotropic properties of the organic carrier. Therefore, in order to fabricate high-quality, low-cost electrode grid lines on solar cells, it is necessary to find a copper paste with excellent rheological or thixotropic properties. Summary of the Invention

[0005] The inventors of this application discovered that the excellent screen printing performance of copper paste is closely related to its rheological or thixotropic properties. Based on this, this application proposes a stable and excellent continuously printable copper paste, thereby completing this application.

[0006] In a first aspect of this application, a copper paste is provided, wherein the copper paste is heated at 20-25°C for 200-500 seconds. -1 The shear viscosity at any shear rate is 0.1-10 Pa·s.

[0007] In a second aspect of this application, a copper electrode is provided, which is prepared from the copper paste described in the first aspect.

[0008] In a third aspect of this application, a solar cell is provided, which includes the copper electrode described in the second aspect.

[0009] This application discloses a copper paste, which is subjected to a temperature of 20-25°C and a reaction time of 200-500 seconds. -1 The shear viscosity at any shear rate is 0.1-10 Pa·s. Therefore, the copper paste provided in this application can reduce viscosity and improve paste flowability during printing at high shear rates, resulting in uniform paste dispersion, easy screen printing with good continuity, and easy deposition onto the substrate. Stable and excellent continuous printability of the copper paste can be obtained, thereby producing high-quality, low-resistance copper electrodes. Attached Figure Description

[0010] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0011] Figure 1 shows a three-segment viscosity curve of slurry 1 prepared according to an embodiment of this application.

[0012] Figure 2 shows the three-segment viscosity curve of slurry 2 prepared according to the embodiment of this application.

[0013] Figure 3 shows the three-segment viscosity curve of the slurry 3 prepared according to the embodiment of this application.

[0014] Figure 4 shows the second segment of a three-segment viscosity curve with a shear rate of 200 s⁻¹ for slurry 1 prepared according to the embodiment of this application. -1 The linear fit plot.

[0015] Figure 5 shows the slurry 2 prepared according to the embodiment of this application at a shear rate of 200 s in the second segment of a three-segment viscosity curve. -1 The linear fit plot.

[0016] Figure 6 shows the slurry 3 prepared according to the embodiment of this application at a shear rate of 200 s in the second segment of a three-segment viscosity curve. -1 The linear fit plot. Detailed Implementation

[0017] The present application will be described in detail below. It should be understood that the following description is merely illustrative and is not intended to limit the scope of the application; the scope of protection of the present application is determined by the appended claims. Furthermore, those skilled in the art will understand that modifications can be made to the technical solutions of the present application without departing from its spirit and intent. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter of this application pertains. Before a detailed description of this application, the following definitions are provided to better understand it.

[0019] In cases where numerical ranges are provided, such as concentration ranges, percentage ranges, or ratio ranges, it should be understood that, unless the context explicitly specifies otherwise, all intermediate values ​​between the upper and lower limits of the range, up to one-tenth of the lower limit unit, and any other values ​​or intermediate values ​​within the range are included in the subject matter. The upper and lower limits of these smaller ranges may be independently included in the smaller ranges, and such embodiments are also included in the subject matter, limited by any specific excluded limit values ​​within the range. Where the range includes one or two limit values, the range excluding any one or both of those included limit values ​​is also included in the subject matter.

[0020] In the context of this application, many embodiments use the expressions "comprising," "including," or "basically / mainly composed of...". The expressions "comprising," "including," or "basically / mainly composed of..." are generally understood as open-ended expressions, indicating that they include not only the elements, components, parts, or method steps specifically listed after the expression, but also other elements, components, parts, or method steps. However, in this document, the expressions "comprising," "including," or "basically / mainly composed of..." can also be understood as closed-ended expressions in certain situations, indicating that they only include the elements, components, parts, or method steps specifically listed after the expression, and exclude any other elements, components, parts, or method steps. In this case, the expression is equivalent to the expression "composed of...".

[0021] As mentioned above, this application aims to provide a stable and superior continuous printable copper paste.

[0022] Rheology is the science that studies the deformation and flow of matter, combining the properties of solids and liquids as a whole. Matter is mainly divided into two types in rheology: Newtonian fluids and non-Newtonian fluids. Newtonian fluids are those whose viscosity does not change under different shear rates; many fluids in nature are Newtonian fluids. Non-Newtonian fluids are those whose viscosity increases or decreases with increasing shear rate. The essence of rheology is the characteristic of the destruction and recovery of the structure of viscoelastic pastes under the action of different forces. Pastes are non-Newtonian fluids; their viscosity decreases with increasing shear rate. Copper paste, as a new type of low-temperature conductive paste, can replace silver paste and significantly reduce the cost of solar cells. However, the printing process of copper paste still suffers from poor thixotropic properties, leading to adhesion to the printing plate or low-quality grid lines with large grid line widths. Because the conductive metal surface activities of copper paste and silver paste are different, it is necessary to develop rheological properties suitable for printing copper paste. However, existing technologies focus more on the shear viscosity of copper paste at low shear rates and less on the shear viscosity of copper paste at high shear rates. High shear rates correspond to the state of copper paste during printing by the squeegee. Therefore, existing technologies have not effectively solved the problems in copper paste printing, especially the inability to prepare fine grid lines with low width.

[0023] In a first aspect of this application, a copper paste is provided, wherein the copper paste is heated at 20-25°C for 200-500 seconds. -1 The shear viscosity at any shear rate is 0.1-10 Pa·s. For example, at 20-25℃, the shear viscosity at 200 s is... -1 The shear viscosity of the copper paste at the shear rate is 0.1-10 Pa·s, or at 20-25℃ and 500 s⁻¹. -1 The shear viscosity of the copper paste at the shear rate is 0.1-10 Pa·s. Applying a shear rate of 200-500 s to the copper paste of this application... -1 At any shear rate within the range, the shear viscosity of the copper paste can be reduced to 0.1-10 Pa·s.

[0024] In the context of this application, shear viscosity is obtained by measuring with a rotational rheometer at 20-25°C for 10-30 seconds, for example, 200-500 seconds. -1 Tested at a shear rate of 10s, at 1s -1 The test was conducted for 30 seconds at the specified shear rate.

[0025] The inventors have discovered that copper paste with a shear viscosity range of 0.1-10 Pa·s, measured under high shear rate conditions, exhibits a rapid decrease in viscosity at high shear rates, strong shear thinning ability, uniform dispersion of the printing paste, and ease of screen printing. When the shear viscosity of the copper paste measured under these high shear rate conditions is too high, printing becomes difficult, eventually leading to defects such as breakpoints and grid breaks. Conversely, when the shear viscosity of the copper paste measured under these high shear rate conditions is too low, the printed copper paste is not easily formed quickly, resulting in blurred and irregular grid lines, or a problem where the thickness of the lower edge is greater than that of the upper edge.

[0026] In one specific implementation, the copper paste is heated at 20-25°C for 200-500 seconds. -1 The shear viscosity at any shear rate can be 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 or 10.0 Pa·s, or a range of any two of these values.

[0027] In a further specific embodiment, the copper paste is heated at 20-25°C for 200-400 seconds. -1 The shear viscosity at any shear rate is 1-8 Pa·s, preferably 1-4 Pa·s. Copper pastes within this high shear rate range have better shear thinning ability, better printability, and are more conducive to preparing low-width electrode grid lines.

[0028] In another specific implementation, the copper paste meets the following condition: when the copper paste is subjected to a three-stage thixotropic property test at 20-25℃, the shear rate of the second stage is set to 200s. -1 -400s -1 The second viscosity curve has time (s) on the x-axis and shear viscosity (Pa·s) on the y-axis. A linear fit is performed on the second viscosity curve, and the slope of the resulting straight line is -2.5 to -0.8.

[0029] The excellent screen printing properties of the paste (i.e., the film properties of the paste that can be obtained by screen printing with strictly controllable geometry and thickness) are closely related to its rheological or thixotropic properties.

[0030] The three-stage thixotropic property (3ITT curve) test corresponds to different application stages of the copper paste. The first stage, low shear, corresponds to the state of the copper paste before printing; the second stage, high shear, corresponds to the state of the copper paste during printing; and the third stage, low shear, corresponds to the recovery of the copper paste on the screen after printing. Copper paste with excellent rheological properties has a higher viscosity under low shear, which is beneficial for stable storage. Under high shear, the viscosity of the copper paste decreases rapidly, with strong shear thinning ability, resulting in smooth feeding and uniform printing during screen printing. Moreover, the viscosity change over time after high shear reflects the rate at which the internal structure of the copper paste is destroyed. An appropriate shear rate setting can ensure that the viscosity change of the copper paste matches the printing rate of the squeegee, achieving good printing lines. Under the third stage of low shear, the viscosity of the copper paste increases rapidly and recovers to the initial value, thus preventing the rheological properties of the copper paste from decreasing due to excessive shear flow, ensuring the consistency and stability of screen printing. Based on this, the inventors discovered that when the copper paste is subjected to a three-stage thixotropic property test at 20-25℃, the shear rate of the second stage is set to 200s. -1 -400s -1 The horizontal axis of the viscosity curve of the second segment is time (s), and the vertical axis is shear viscosity (Pa·s). When the slope of the straight line obtained by linear fitting the viscosity curve of the second segment is in the range of -2.5 to -0.8, the viscosity of the copper paste is suitable for printing. It does not stick to the screen when printed on the machine, and the paste utilization rate is high, and the linearity is smooth and full.

[0031] As an example, the above three-stage thixotropic performance testing method can be implemented using the following steps:

[0032] 1) After centrifuging and stirring the copper paste, let it stand for 30 minutes to 2 hours;

[0033] 2) The shear rates of the three stages are fixed at 1 s. -1 200s -1 1s -1 The rotor was calibrated by testing for 30-60s, 5-10s, and 30-60s respectively.

[0034] 3) Take an appropriate amount of copper paste on the sample stage, run the rotor, conduct the test, and plot the three-segment viscosity curve.

[0035] In a further specific implementation, in the above three-segment thixotropic performance test, the slope of the straight line obtained by linear fitting the viscosity curve of the second segment can be -2.5, -2.4, -2.3, -2.2, -2.1, -2.0, -1.9, -1.8, -1.7, -1.6, -1.5, -1.4, -1.3, -1.2, -1.1, -1.0, -0.9, or -0.8, or a range consisting of any two of these values.

[0036] In yet another specific implementation, the copper paste is heated at 20-25°C for 1 second. -1 The shear viscosity at the shear rate is 200-1500 Pa·s.

[0037] The inventors have discovered that copper paste with a shear viscosity range of 200-1500 Pa·s measured under the above conditions exhibits good storage stability before printing, thus avoiding the formation of large aggregated particles. It is understood that large aggregated particles in the copper paste would have difficulty passing through the printing screen during printing.

[0038] In a further specific embodiment, the copper paste is heated at 20-25°C for 1 second. -1 The shear viscosity at the shear rate can be 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450 or 1500 Pa·s, or a range of any two of these values.

[0039] In yet another specific implementation, the thixotropic index of the copper paste is 20-500 at 20-25°C, and the thixotropic index is defined as the coefficient of performance in 1 second. -1 The shear viscosity at the shear rate and at 200 s -1 The ratio of the shear viscosity at a given shear rate. That is, the thixotropic index = the ratio of the shear rate at 1 s⁻¹. -1 The shear viscosity / shear rate at that time was 200 s. -1 The shear viscosity at that time.

[0040] Thixotropic properties refer to the ability of viscosity to return to its original viscosity after a decrease (or increase) due to stress, and are usually expressed by the thixotropic index. However, the thixotropic index in current technologies is typically calculated using a shear rate of 10 s⁻¹. -1 The viscosity at 100s -1 The ratio of the viscosity to the viscosity of the paste. Furthermore, existing techniques typically only use this method to characterize paste viscosity without relating it to the printability of the paste.

[0041] The inventors have discovered that when using the thixotropic index calculation method of this application, copper paste with a thixotropic index in the range of 20-500 has a viscosity that prevents copper powder from settling at low shear rates. This means that the copper paste has high storage stability before printing, and at the same time, it can reduce viscosity at high shear rates during printing, thereby improving the fluidity of the paste and obtaining stable and excellent continuous printability, thus obtaining high-quality, low-resistance electrodes.

[0042] In a further specific embodiment, the thixotropic index of the copper paste at 20-25°C can be 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450 or 500, or a range of any two of these values.

[0043] In yet another specific embodiment, the copper paste comprises, by weight, 80%-95% copper powder and 5%-20% dispersion medium.

[0044] In a further specific embodiment, the copper paste may comprise copper powder in a range of 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, or 95% by weight, or any two of these values. In a further specific embodiment, the copper paste may comprise a dispersion medium in a range of 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20% by weight, or any two of these values.

[0045] In yet another specific embodiment, the dispersion medium may comprise a resin, a curing agent, a dispersant, and a solvent.

[0046] In a more specific embodiment, the resin may be an epoxy resin, phenolic resin, phenoxy resin, acrylic resin, aldehyde-ketone resin, polyurethane, polyester, etc. Preferably, the resin includes epoxy resin. Further, the epoxy resin may be a thermosetting resin, such as one or more selected from bisphenol A type epoxy resin, bisphenol F epoxy resin, hydrogenated bisphenol A type epoxy resin, polyurethane modified epoxy resin, dimer acid modified epoxy resin, siloxane modified epoxy resin, phenolic type epoxy resin, polyol glycidyl ether type epoxy resin, and polyacid glycidyl ester type epoxy resin. Of course, other resins known in the art that can be used in this application may also be selected, and this application does not further limit them.

[0047] In a more specific embodiment, the curing agent may be selected from one or more of dicyandiamide curing agents, tertiary amine curing agents, isocyanates, imidazole curing agents, acid anhydride curing agents, and latent imidazole curing agents, such as triethanolamine, blocked isocyanates, 1-butyl-3-methylimidazolium dibutyl phosphate, and Busington (Lanxess) 7982. The isocyanate is selected from one or more of Trixene BI 7982 (a blocked isocyanate based on HDI, i.e., blocked HDI isocyanate), MF-K60X (a blocked polyisocyanate HDI curing agent), ketoxime-terminated isocyanates, hexamethylene hexamethylene diisocyanate-terminated, and dodecyl mercaptan-terminated diphenyl diisocyanate. Of course, other curing agents known in the art that can be used in this application may also be selected, and this application does not further limit them.

[0048] In a preferred embodiment, the curing agent comprises a blocked isocyanate, preferably a blocked HDI isocyanate.

[0049] In a more specific embodiment, the resin comprises an epoxy resin, and the curing agent comprises a blocked isocyanate, preferably a blocked HDI isocyanate.

[0050] This application allows for the adjustment of the shear viscosity of copper paste at both low and high shear rates by selecting the type of resin and the type of curing agent.

[0051] In a more specific embodiment, the dispersant may be an ether, amine, carboxylic acid, or a dispersant having an amino group or a polar group such as a hydroxyl group with 16-20 carbon atoms, for example (Tween, OP series, oleic acid, Span). Of course, other dispersants known in the art that can be used in this application may also be selected, and this application does not further limit them.

[0052] In a more specific embodiment, the solvent is selected from esters, ethers, ketones, and alcohols, and may include, for example, sec-amyl acetate, cyclohexanone, amyl propionate, isopropyl lactate, divalent esters (DBE), amyl acetate, diethylene glycol acetate, diethylene glycol butyl ether acetate, ethylene glycol carbonate, propylene glycol carbonate, tributyl borate, triphenyl phosphate, tricresyl phosphate, butyl acetate, diethyl carbonate, ethylene glycol ethyl ether acetate, ethylene glycol butyl ether acetate, γ-butyrolactone, butyl carbitol acetate, and ethyl carbitol acetate. The solvents used include: diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol butyl ether, ethylene glycol diethyl ether, diethylene glycol butyl ether, ethylene glycol monoisopropyl ether, dipropylene glycol methyl ether, dipropylene glycol ethyl ether, dipropylene glycol propyl ether, dipropylene glycol butyl ether, dipropylene glycol ether, ethylene glycol dimethyl ether, tripropylene glycol methyl ether, tripropylene glycol dimethyl ether, diethylene glycol monobutyl ether, 4-heptanone, sec-pentanol, ethylene glycol, propylene glycol, pentanol, hexanol, heptanol, octanol, methylpentanol, butyl carbitol, terpineol, and dihydroterpineol. Of course, other solvents known in the art that can be used in this application may also be selected, and this application does not further limit them.

[0053] In a preferred embodiment, the copper paste may comprise 80-95 parts by weight of copper powder, 1-15 parts by weight of resin, 0-5 parts by weight of curing agent, 1-10 parts by weight of dispersant, and 2-15 parts by weight of solvent.

[0054] In yet another specific implementation, the copper powder includes spherical copper powder and flake copper powder.

[0055] In the context of this application, the term "flaky copper powder" refers to copper powder with an aspect ratio (shortest diameter / thickness) greater than or equal to 2 in a scanning electron microscope (SEM); the term "spherical copper powder" refers to copper powder with an aspect ratio (shortest diameter / thickness) less than 2 in a scanning electron microscope (SEM).

[0056] In a further preferred embodiment, the ratio of the longest diameter to the shortest diameter of the flake copper powder is 1-10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, or a range consisting of any two of these values.

[0057] In a preferred embodiment, based on the total mass of the copper powder, the mass percentage of the flake copper powder is 35%-60%, and the mass percentage of the spherical copper powder is 40%-65%.

[0058] Specifically, in the copper powder, the mass percentage of the flake copper powder can be 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or 60%, or a range consisting of any two of these values.

[0059] Specifically, in the copper powder, the mass percentage of the spherical copper powder can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, or 65%, or a range consisting of any two of these values.

[0060] In a further preferred embodiment, the D of the flake copper powder 50 The particle size is 2μm-8μm, and the median thickness is 100nm-500nm. Specifically, the D of the flake-shaped copper powder... 50 The particle size can be 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, or 8μm, or a range consisting of any two of these values. More specifically, the median thickness of the flake copper powder can be 100nm, 120nm, 150nm, 180nm, 200nm, 220nm, 250nm, 280nm, 300nm, 320nm, 350nm, 380nm, 400nm, 420nm, 450nm, 480nm, or 500nm, or a range consisting of any two of these values.

[0061] In a further preferred embodiment, the tap density of the flake copper powder is 4.4–5.2 g / ml, and the tap density of the spherical copper powder is 3.5–5.5 g / ml. Specifically, the tap density of the flake copper powder can be 4.4 g / ml, 4.5 g / ml, 4.6 g / ml, 4.7 g / ml, 4.8 g / ml, 4.9 g / ml, 5.0 g / ml, 5.1 g / ml, or 5.2 g / ml, or a range consisting of any two of these values. Specifically, the tap density of the spherical copper powder can be 3.5 g / ml, 3.6 g / ml, 3.7 g / ml, 3.8 g / ml, 3.9 g / ml, 4.0 g / ml, 4.1 g / ml, 4.2 g / ml, 4.3 g / ml, 4.4 g / ml, 4.5 g / ml, 4.6 g / ml, 4.7 g / ml, 4.8 g / ml, 4.9 g / ml, 5.0 g / ml, 5.1 g / ml, 5.2 g / ml, 5.3 g / ml, 5.4 g / ml, or 5.5 g / ml, or a range consisting of any two of these values. When the tap density of the flake-shaped and spherical copper powders is within this range, the copper electrode formed by the copper powder has a high density and a low porosity, thereby exhibiting a low resistivity.

[0062] In the context of this application, the term "tap density" refers to the bulk density of powder after it has been tapped. It is the density of powder after it has been packed into a specific container and the container has been vibrated to break down the voids in the powder, resulting in a tightly packed state. The tap density can be used to determine the flowability and porosity of the powder. It can be calculated by measuring the volume after 1000 vibrations using a BT-301 vibrator.

[0063] The specific surface area of ​​the flaky copper powder is 0.4-0.7 m². 2 / g, the specific surface area of ​​the spherical copper powder is 0.1-3.0m². 2 / g. Specifically, the specific surface area of ​​the flake-shaped copper powder can be 0.4m². 2 / g, 0.5m 2 / g, 0.6m 2 / g or 0.7m 2 / g, or a range consisting of any two of these values. Specifically, the specific surface area of ​​spherical copper powder can be 0.1m². 2 / g, 0.3m 2 / g, 0.5m 2 / g, 0.7m 2 / g, 0.9m 2 / g, 1.1m 2 / g, 1.3m 2 / g, 1.5m 2 / g, 1.7m 2 / g, 1.9m2 / g、2.1m 2 / g, 2.3m 2 / g, 2.5m 2 / g, 2.7m 2 / g, 2.9m 2 / g or 3.0m 2 / g, or a range consisting of any two of these values. In the context of this application, the term "specific surface area" refers to the surface area per unit mass of a porous solid material, commonly measured in m³. 2 / g. It can be tested using conventional methods, such as the determination of the specific surface area of ​​copper powder according to GB / T13390-2008.

[0064] The inventors have discovered that when the specific surface area of ​​flake copper powder and spherical copper powder is within the above-mentioned range, it can ensure that the copper powder has good oxidation resistance. The larger the specific surface area, the higher the surface activity and the easier it is to be oxidized. The smaller the specific surface area, the larger the particle size of the copper powder, which will affect the low-temperature sintering properties of the copper paste prepared from the copper powder, and may cause insufficient sintering at low temperature or poor bonding between the copper powder and the substrate after sintering.

[0065] In this application, by combining flake copper powder and spherical copper powder in terms of shape and / or quality, such as specific surface area, particle size, and tap density, agglomeration of copper paste at low shear rates can be avoided, giving it good storage stability. At the same time, during the printing process at high shear rates, the viscosity of the copper paste can be reduced to the aforementioned range, resulting in good printability. Furthermore, after printing, copper powder particles of various sizes and shapes are tightly packed, making the conductive channels between the copper powder particles more unobstructed.

[0066] In a second aspect of this application, a copper electrode is provided, which is prepared from the copper paste described in the first aspect of this application.

[0067] As an example, the method for preparing the copper electrode of this application may include the following steps: placing the aforementioned copper paste in a nitrogen atmosphere for heating and curing to obtain the electrode. The curing temperature can be 100-220℃, for example, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, etc., and the curing time can be 10min-60min, for example, 10min, 15min, 20min, 25min, 30min, 35min, 40min, 45min, 50min, 55min, 60min, etc.

[0068] For example, the method for preparing the copper electrode of this application may also include the following steps: pre-prepared copper paste is screen-printed onto a solar cell (e.g., HJT / HBC) with a grid line width of 40μm-120μm, and then cured by heating in a nitrogen oven at a temperature of 100-220℃ for 10min-60min to form the electrode grid line.

[0069] In a third aspect of this application, a solar cell is provided, which includes the copper electrode described in the second aspect of this application.

[0070] In yet another specific embodiment, the surface of the solar cell may have a textured surface, with the copper electrode located on top of the textured surface of the solar cell.

[0071] In one specific implementation, the solar cell may be selected from BC cells, HJT cells, or perovskite / crystalline silicon tandem cells.

[0072] In yet another specific implementation, the solar cell may be a crystalline silicon cell, and a conductive barrier layer exists between the copper electrode and the crystalline silicon cell.

[0073] Example

[0074] The following embodiments illustrate the preparation method and characterization of the related properties of the product of this application. Unless otherwise specified, all test methods used are conventional methods, and all test materials used in the following embodiments were purchased from conventional reagent stores. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0075] It should be noted that the terminology used in this application's specification is for the purpose of describing specific embodiments only and is not intended to limit the application. The foregoing summary section and the following detailed description are for the purpose of illustrative purposes only and are not intended to limit the invention in any way. The scope of this application is defined by the appended claims without departing from the spirit and intent of the invention.

[0076] Example 1

[0077] Take 0.3g of bisphenol A epoxy resin and the curing agent Lanxess. 0.1g of BI 7982, 0.1g of triethanolamine, 0.1g of dispersant oleic acid, 0.1g of solvent diethylene glycol butyl ether acetate, and 0.2g of terpineol were stirred evenly in a glass dish to obtain a mother liquor. 4g of flake copper powder and 4g of spherical copper powder were poured into the mother liquor, stirred, and then poured into a three-roll mill for rolling and grinding to obtain homogeneous slurry 1.

[0078] Example 2

[0079] Take 0.3g of bisphenol A epoxy resin, 0.1g of polyurethane resin, and Lanxess curing agent. 0.1g of BI 7982, 0.05g of triethanolamine, 0.1g of dispersant oleic acid, 0.2g of solvent diethylene glycol butyl ether acetate, and 0.2g of terpineol were stirred evenly in a glass dish to obtain a mother liquor. 4g of flake copper powder and 4g of spherical copper powder were poured into the mother liquor, stirred, and then poured into a three-roll mill for rolling and grinding to obtain homogeneous slurry 2.

[0080] Example 3

[0081] Take 0.3g of bisphenol A epoxy resin and the curing agent Lanxess. 0.1g of BI 7982, 0.1g of triethanolamine, 0.1g of dispersant oleic acid, 0.1g of solvent diethylene glycol butyl ether acetate, and 0.2g of terpineol were stirred evenly in a glass dish to obtain a mother liquor. 4g of flake copper powder and 4g of spherical copper powder (which have different specific surface areas compared to Example 1) were added to the mother liquor. After stirring, the mixture was poured into a three-roll mill for rolling and grinding to obtain homogeneous slurry 3.

[0082] Example 4

[0083] Take 0.25g of bisphenol A epoxy resin, 0.05g of phenoxy resin, 0.05g of curing agent dicyandiamide, 0.1g of 2-ethyl-4-methylimidazolium, 0.1g of dispersant oleic acid, 0.1g of solvent diethylene glycol butyl ether acetate, and 0.2g of terpineol and stir evenly in a glass dish to obtain a mother liquor. Add 4g of flake copper powder and 4g of spherical copper powder to the mother liquor, stir, and then pour into a three-roll mill for rolling and grinding to obtain homogeneous slurry 4.

[0084] Example 5

[0085] In this embodiment, a rheometer was used to test the viscosity of slurries 1, 2, 3, and 4 prepared in Examples 1-4 at different shear rates, based on a shear rate of 1 s. -1 and 200s -1 The thixotropic index was calculated from the viscosity data, and the results are shown in Table 1 below.

[0086] The thixotropic properties of slurries 1, 2, and 3 prepared in Examples 1-3 were tested using a three-stage thixotropic property testing method. The specific steps are as follows:

[0087] 1) After centrifuging and mixing the slurry, let it stand for 30 minutes;

[0088] 2) Fix the shear rates of the first, second, and third stages to 1 s. -1 200s -1 1s -1The rotor was calibrated for test times of 30s, 5s, and 60s.

[0089] 3) Take about 1g of slurry onto the sample stage, run the rotor to remove excess slurry, and then perform the test.

[0090] The rheometer is a Thermo Scientific HAAKE Mars40; temperature 20-25℃, humidity 30-45%.

[0091] Based on the formula viscosity = stress / shear rate, three-segment viscosity curves were plotted for slurry 1, slurry 2, and slurry 3, respectively. The results are shown in Figures 1 to 3. For the three-segment viscosity curves of slurry 1, slurry 2, and slurry 3, the second segment was plotted at a shear rate of 200 s. -1 The viscosity curves at different times were linearly fitted, and the results are shown in Figures 4 to 6 and Table 1, respectively.

[0092] Table 1: Rheological and thixotropic properties of slurries 1-4

[0093] Example 5:

[0094] Copper films were prepared by screen printing of pastes 1-4, cured at 170℃ with nitrogen for 30 min, and the resistivity was tested by the four-probe method. The results are shown in Table 2.

[0095] The pastes 1-4 were screen-printed onto the solar cell as grid lines, cured at 170℃ under nitrogen for 30 minutes, and the grid line width was 60μm. The initial contact resistance was tested. Then, the solar cell was irradiated at 185℃ with light of wavelength 760-1000nm. After irradiation, the solar cell was placed in a constant temperature and humidity chamber and allowed to stand at 85℃ and 85% humidity for 5 hours. The contact resistance was then tested and defined as the post-irradiation contact resistance. The contact resistance was tested using TLM (Total Contact Membrane Analyzer).

[0096] Table 2: Resistance-related properties of copper films and grid lines prepared from pastes 1-4

[0097] As shown in Figures 1 to 3 and Tables 1 and 2, slurry 1 at a shear rate of 400 s⁻¹ -1 The shear viscosity at that time was less than 10 Pa·s, and the viscosity of slurry 2 and slurry 3 was within 200 s. -1 and 400s -1 The shear viscosity at 400 s is less than 10 Pa·s, making it easy to pass through the screen during printing, resulting in continuous and regular linear shapes. The prepared electrodes all exhibit low resistivity, and the contact resistance does not show a significant increase after light exposure and treatment at 85°C and 85% humidity. Furthermore, compared to paste 1, pastes 2 and 3 show lower resistance at 400 s. -1The shear viscosity of the slurry was less than 4 Pa·s, resulting in higher printing quality and lower resistivity of the prepared electrode. Compared to slurry 2, slurry 3 had a smaller specific surface area of ​​spherical powder, and the increased contact resistance of the prepared electrode after light irradiation and treatment at 85°C and 85% humidity was less. Slurry 4 showed a lower shear viscosity at 200 s. -1 and 400s -1 The shear viscosity at that time was greater than 10 Pa·s, which caused problems such as printing difficulties and grid breakage. As a result, the resistivity of the electrode prepared was much higher than that of the paste 1-3. Moreover, after being exposed to light and treated at 85°C and 85% humidity, the contact resistance increased significantly, even more than double the original contact resistance.

[0098] As shown in Figures 4 to 6 and Table 1, the absolute slope of the fitted curves in the second segment of the three-segment thixotropic property curves for pastes 1 and 4 is greater than 2.5. This will cause pastes 1 and 4 to stick together during printing, resulting in insufficient line fullness. In contrast, at a shear rate of 200s... -1 and 400s -1 Slurries 2 and 3, with lower shear viscosity and an absolute slope of less than 2.5, produce smoother and fuller lines during printing, with no sticking to the printing plate and higher slurry utilization.

[0099] It should be noted that the terminology used in this application's specification is for the purpose of describing specific embodiments only and is not intended to limit the application. The foregoing summary section and the following detailed description are for illustrative purposes only and are not intended to limit the application in any way. Without departing from the spirit and intent of this application, the scope of this application is defined by the appended claims.

Claims

1. A copper paste, wherein the copper paste is heated at 20-25°C for 200-500 seconds. -1 The shear viscosity at any shear rate is 0.1-10 Pa·s.

2. The copper paste according to claim 1, wherein, The copper paste is heated at 20-25°C for 200-400 seconds. -1 The shear viscosity at any shear rate is 1-8 Pa·s.

3. The copper paste according to claim 1, wherein, The copper paste is heated at 20-25°C for 200-400 seconds. -1 The shear viscosity at any shear rate is 1-4 Pa·s.

4. The copper paste according to claim 1, wherein, The copper paste meets the following conditions: when the copper paste is subjected to a three-stage thixotropic property test at 20-25℃, the shear rate of the second stage is set to 200s. -1 -400s -1 The second viscosity curve has time (s) on the x-axis and shear viscosity (Pa·s) on the y-axis. A linear fit is performed on the second viscosity curve, and the slope of the resulting straight line is -2.5 to -0.

8.

5. The copper paste according to any one of claims 1-4, wherein, The copper paste is heated at 20-25°C for 1 second. -1 The shear viscosity at the shear rate is 200-1500 Pa·s.

6. The copper paste according to any one of claims 1-5, wherein, The thixotropic index of the copper paste is 20-500 at 20-25℃, and the thixotropic index is defined as the coefficient of performance in 1 second. -1 The shear viscosity at the shear rate and at 200 s -1 The ratio of shear viscosity to the viscosity of the target material.

7. The copper paste according to any one of claims 1-6, wherein, The copper paste contains 80%-95% copper powder and 5%-20% dispersion medium.

8. The copper paste according to claim 7, wherein, The dispersion medium comprises resin, curing agent, dispersant, and solvent.

9. The copper paste according to claim 8, wherein, The resin is selected from epoxy resin, and / or the curing agent is selected from blocked isocyanate, preferably blocked HDI isocyanate.

10. The copper paste according to claim 8 or 9, wherein, The copper paste contains 80-95 parts by weight of copper powder, 1-15 parts by weight of resin, 0-5 parts by weight of curing agent, 1-10 parts by weight of dispersant, and 2-15 parts by weight of solvent.

11. The copper paste according to any one of claims 7-10, wherein, The copper powder includes spherical copper powder and flake copper powder.

12. The copper paste according to claim 11, wherein, Based on the total mass of the copper powder, the mass percentage of the flake copper powder is 35%-60%, and the mass percentage of the spherical copper powder is 40%-65%.

13. The copper paste according to claim 11 or 12, wherein, The D of the flake copper powder 50 The particle size is 2μm-8μm, and the median thickness is 100nm-500nm; the D of the spherical copper powder 50 The particle size is 100nm-500nm.

14. The copper paste according to any one of claims 11-13, wherein, The tap density of the flake copper powder is 4.4-5.2 g / ml, and the tap density of the spherical copper powder is 3.5-5.5 g / ml.

15. The copper paste according to any one of claims 11-14, wherein, The specific surface area of ​​the flaky copper powder is 0.4-0.7 m². 2 / g, the specific surface area of ​​the spherical copper powder is 0.1-3.0m². 2 / g.

16. A copper electrode, which is prepared from the copper paste according to any one of claims 1-15.

17. A solar cell comprising the copper electrode of claim 16.

18. The solar cell according to claim 17, wherein the solar cell is selected from BC cells, HJT cells, and perovskite / crystalline silicon tandem cells.

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