Micron-sized flaky silver powder for HJT solar cells and preparation method thereof

Abstract

The present invention relates to a micron-sized flaky silver powder for HJT solar cells and a preparation method thereof, comprising the following steps: S1, reacting a silver nitrate solution and a sodium carbonate solution to obtain a solution containing silver carbonate, dropwise adding the solution containing silver carbonate to a mixed solution of a reducing agent and a fatty acid for reaction, washing, and drying to obtain silver powder particles; S2, uniformly mixing the silver powder particles, a grinding aid, and an organic solvent to obtain a slurry; S3, ball-milling the slurry, and then separating, washing, drying, crushing, and sieving to obtain micron-sized flaky silver powder for HJT solar cells. The preparation method of the present invention effectively controls the morphology, particle size, and impurity content of the flaky silver powder. The prepared flaky silver powder has the characteristics of high degree of flaking, good dispersibility, low specific surface area, and high purity. The method is simple to operate and suitable for large-scale rapid production.

Description

Technical Field

[0001] The present invention belongs to the field of precious metal powders, and in particular relates to micron-sized flaky silver powder for HJT solar cells and a preparation method thereof. Background Art

[0002] Low-temperature conductive silver paste and its functional material, flake silver powder, have broad application prospects in the photovoltaic and electronics industries. As an important conductive functional material in low-temperature conductive silver paste, the physical and chemical properties of flake silver powder, such as specific surface area, tap density, flake diameter and thickness, dispersibility, and impurity content, significantly influence the conductive performance of the low-temperature conductive paste.

[0003] HJT (Heterojunction with Intrinsic Thin-layer, crystalline silicon heterojunction, abbreviated as HJT) solar cells have many advantages such as high conversion efficiency, low temperature coefficient, bifacial power generation, and low process temperature. The low-temperature conductive silver paste it uses uses silver powder as the main raw material, and has high requirements for the silver powder's flake diameter, dispersion, and impurity content.

[0004] At present, the commonly used methods for preparing flaky silver powder at home and abroad are mainly chemical method and mechanical ball milling method. The chemical method can be further divided into liquid phase reduction method, template method, photoinduction method, microwave radiation method, etc. How to avoid excessive organic residues in the liquid phase reduction method while improving dispersibility, which leads to problems such as reduced product conductivity, poor process stability and low yield. The flaky silver powder prepared by mechanical ball milling has a bright color, large specific surface area and good conductivity. However, silver powder with too large specific surface area absorbs a lot of oil, resulting in high viscosity and poor fluidity of the subsequently prepared silver paste system, which affects the subsequent printing and feeding. In order to improve the dispersibility of silver powder, the silver powder is usually surface modified by adding multiple dispersants or even emulsifiers, which makes post-processing difficult and the impurity content of the obtained silver powder is high. Therefore, there are currently problems such as stable control of the particle size of silver powder for HJT solar cells, elimination of impurities, and surface modification. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned technical deficiencies and provide a micron-sized flaky silver powder for HJT solar cells and a preparation method thereof, so as to solve the technical problems that the silver powder prepared in the prior art has a large specific surface area and is prone to introduce more impurities, and cannot meet the requirements of silver powder for HJT solar cells.

[0006] In order to achieve the above technical objectives, the technical solution provided by the present invention is:

[0007] In a first aspect, the present invention provides a method for preparing micron-sized flaky silver powder for HJT solar cells, comprising the following steps: S1, reacting a silver nitrate solution and a sodium carbonate solution to obtain a solution containing silver carbonate, dropwise adding the solution containing silver carbonate to a mixed solution of a reducing agent and a fatty acid for reaction, and washing and drying to obtain silver powder particles; S2, uniformly mixing the silver powder particles, a grinding aid, and an organic solvent to obtain a slurry; S3, ball milling the slurry, and then separating, washing, drying, crushing, and sieving to obtain micron-sized flaky silver powder for HJT solar cells.

[0008] In a second aspect, the present invention provides a micron-sized flaky silver powder for HJT solar cells produced by the above-mentioned preparation method.

[0009] Compared with the prior art, the present invention has the following beneficial effects:

[0010] In the preparation method of the present invention, a fatty acid is added as a dispersant to the silver carbonate reduction reaction system to make the resulting silver powder particles uniform, which helps improve the performance of the final flaky silver powder. The silver powder particles are then used as raw material for ball milling with the aid of external mechanical force, a grinding aid, and an organic solvent, effectively controlling the morphology, particle size, and impurity content of the flaky silver powder. The resulting flaky silver powder exhibits a high degree of flakeness, good dispersibility, a low specific surface area, and high purity. This method is simple to operate, has good process continuity, a large production batch size, and high reaction efficiency, making it suitable for rapid, large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a scanning electron microscope image of the flaky silver powder prepared in Example 1;

[0012] Figure 2 This is a scanning electron microscope image of the flaky silver powder prepared in Example 2. DETAILED DESCRIPTION

[0013] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0014] The low-temperature conductive silver paste used in HJT solar cells uses silver powder as the main raw material, and has high requirements for the flake size, dispersibility, and impurity content of the silver powder. Existing silver powder preparation methods mainly use chemical methods and mechanical ball milling methods. The liquid phase reduction method improves dispersibility while avoiding excessive organic residues, which leads to reduced product conductivity, poor process stability, and low yield. The flaky silver powder prepared by the mechanical ball milling method has a bright color, large specific surface area, and good conductivity. However, silver powder with a large specific surface area absorbs a lot of oil, resulting in high viscosity and poor fluidity in the subsequent silver paste system, which affects subsequent printing and cutting. Therefore, there are currently problems in the stable control of silver powder particle size, elimination of impurities, and surface modification in the low-temperature conductive silver paste used in HJT solar cells. This makes its morphology difficult to control, the impurity content is high, and mass production is impossible.

[0015] The present invention provides a micron-sized flaky silver powder for HJT solar cells and a preparation method thereof, thereby effectively solving technical problems such as batch production, morphology control, particle size control, and high impurity levels of flaky silver powder. The flaky silver powder prepared by the present invention can stably have a size of 1μm to 6μm and is suitable for the production of low-temperature silver paste for HJT solar cells.

[0016] Furthermore, the liquid-phase chemical reduction method for preparing silver powder uses a variety of oxidants, including silver nitrate, silver carbonate, silver oxide, silver ammonium ion, and silver halide. The order of ionization strength of the silver salt intermediates is: silver iodide < silver bromide (equivalent to silver sulfide) < silver ammonium ion < silver chloride < silver oxide < silver carbonate. Powders produced from silver nitrate and ascorbic acid (VC) exhibit narrow particle size distribution, high sphericity, smooth surface, and high tap density. However, flaky silver powders produced using horizontal mechanical ball milling as the initial silver powder exhibit overly regular and uniform morphology, but often suffer from defects such as high viscosity, poor thixotropy, and stringiness during application. Therefore, the present invention primarily utilizes silver carbonate, which has the highest ionization strength, as the silver source to prepare a flocculent, spherical initial silver powder, effectively addressing these issues.

[0017] In a first aspect, the present invention provides a method for preparing micron-sized flaky silver powder for HJT solar cells, comprising the following steps:

[0018] S1, reacting a silver nitrate solution with a sodium carbonate solution to obtain a solution containing silver carbonate, adding the solution containing silver carbonate dropwise to a mixed solution of a reducing agent and a fatty acid to react, and washing and drying to obtain silver powder particles;

[0019] S2, mixing the silver powder particles, the grinding aid and the organic solvent to obtain a slurry;

[0020] S3, ball milling the slurry, and then separating, washing, drying, crushing and screening to obtain micron-sized flaky silver powder for HJT solar cells.

[0021] Preferably, in step S1, the molar ratio of silver nitrate to sodium carbonate is 1:(0.48-0.50); ​​the molar mass ratio of silver nitrate, reducing agent and fatty acid is 1:(0.60-0.62):(0.003-0.006).

[0022] Preferably, in step S1, the fatty acid includes one or more of lauric acid, myristic acid, oleic acid, palmitic acid and stearic acid; and the reducing agent includes ascorbic acid.

[0023] Preferably, in step S1, the concentration of the silver nitrate solution is 120-160 g / L; the concentration of the sodium carbonate solution is 35-50 g / L; and the mixed solution of the reducing agent and the fatty acid is obtained by mixing the reducing agent solution and the fatty acid solution, wherein the concentration of the reducing agent solution is 35-50 g / L and the concentration of the fatty acid is 48-55 g / L.

[0024] Preferably, in step S1, washing is performed with distilled water and anhydrous ethanol, respectively. The present invention further reduces the content of organic matter through water washing and alcohol washing, and avoids excessive retention of dispersant, which in turn affects the impurity content of subsequent flaky silver powder.

[0025] Preferably, in step S1, the drying is performed at 80-90° C. for 10-14 hours.

[0026] Preferably, in step S1, the average particle size of the silver powder particles is 0.8 μm to 1.0 μm.

[0027] Preferably, in step S2, the grinding aid includes oleic acid; and the organic solvent includes anhydrous ethanol or acetone.

[0028] Preferably, in step S2, the mass ratio of the silver powder particles, the grinding aid and the organic solvent is 100: (0.75-1.5): (45-55).

[0029] In the present invention, by adopting a relatively high amount of grinding aid and organic solvent, it is ensured that the outer surface of the silver powder particles is fully adsorbed and coated with the organic shell layer, thereby avoiding the occurrence of undesirable phenomena such as cold welding, agglomeration, and clumping in the subsequent ball milling process, which lead to undesirable phenomena such as wide powder particle size distribution, large D100, and low first-time qualified rate.

[0030] Preferably, in step S2, the silver powder particles, the grinding aid and the organic solvent are mixed and stirred for 5 to 40 minutes using a high-speed disperser with a rotation speed of 800 to 1200 r / min to obtain a slurry.

[0031] Preferably, in step S3, the process conditions adopted for ball milling are: the diameter of the zirconia balls is 1.5-2.5 mm, the ball-to-material ratio is (2.8-3.2):1, the ball milling temperature is 35-42° C., the ball milling speed is 40-60 r / min, and the ball milling time is 5-10 h.

[0032] At the same time, compared with the defects of wall sticking or long ball milling time caused by using other substances as dispersants, for example, using ricinoleic acid as a dispersant will cause wall sticking, and using benzophenone oxime acetic acid benzophenone oxime ester ethanol solution as a dispersant has the defect of slow infiltration rate. It is necessary to first ball mill at a low speed (30r / min) for 120min in the early stage of ball milling before the speed can be increased to 60r / min for formal ball milling; this application uses oleic acid as a grinding aid, increases its dosage, and increases the amount of organic solvent used during ball milling, so that a slurry can be obtained by high-speed dispersion, and then high-speed ball milling is performed without wall sticking, and the desired target product can be quickly obtained, the ball milling time is short, the ball milling efficiency is effectively improved, and the specific surface area of ​​the obtained silver powder is low; it is more economical and environmentally friendly, and the organic dispersant used can be easily removed by post-treatment to obtain flaky silver powder with higher purity.

[0033] Preferably, in step S3, the separation is to separate the slurry and zirconia balls after ball milling using a sieve to obtain slurry A, and the zirconia balls are washed to obtain slurry B; slurry A and slurry B are combined and then filtered under reduced pressure to obtain flaky silver powder; and the washing is to wash the flaky silver powder with deionized water until the conductivity is ≤20μS / cm.

[0034] Preferably, in step S3, the drying is carried out by keeping the temperature at 50°C to 80°C for 8 hours to 24 hours; and the sieving is carried out using a 200-mesh sieve.

[0035] In a second aspect, the present invention provides a micron-sized flaky silver powder for HJT solar cells produced by the above-mentioned preparation method.

[0036] Preferably, the diameter D50 of the micron-sized flaky silver powder for HJT solar cells is 1 to 6 μm, more preferably 2 to 4 μm; the bulk density is 2.75 to 3.6 g / cm 3 , the tap density is 4.5~5.5g / cm 3 , the specific surface area is 0.33~0.52m 2 / g.

[0037] The main mechanism of action and advantages of the present invention:

[0038] The present invention uses silver carbonate, which has the highest degree of ionization, as a silver source to prepare flocculent, spherical, ultrafine initial silver powder particles. During the preparation process, long straight-chain fatty acids with moderate molecular weight and high shrinkage during low-temperature curing are added as dispersants, particularly lauric acid and oleic acid. This allows the silver powder to grow uniformly during the growth process, which helps improve the performance of the final flaky silver powder. The ultrafine spherical silver powder particles are then used as raw materials. Through the use of external mechanical force, with the help of grinding balls and grinding aids, the materials are hammered, ball-milled, and refined through frequent collision, friction, and extrusion between the grinding balls, ball milling jars, and the spherical silver powder particles. Ultimately, the surface morphology and particle size distribution of the silver powder particles are completely changed under the ball milling action for a relatively short period of time, forming a product that meets the expected technical indicators. The present invention comprises the steps of preparing the spherical silver powder particles, dispersing them, ball milling, washing and filtering, drying and crushing, etc., adding an organic solvent as a ball milling medium during the dispersion process, and adding a grinding aid. According to a certain ball-to-material ratio, the target flaky silver powder is prepared by a mechanical horizontal ball milling method.

[0039] The flaky silver powder prepared by the present invention has the characteristics of high degree of flakeness, good dispersibility, low specific surface area, high purity, etc. The use of the flaky silver powder in preparing low-temperature conductive silver paste is beneficial to improving the fluidity of the silver paste. The flaky silver powder has high bulk density and tap density. The conductive film obtained after slurry preparation and sintering has few and small voids, which is beneficial to improving conductivity.

[0040] The present invention is further described in detail below through specific examples.

[0041] Example 1

[0042] A method for preparing micron-sized flaky silver powder for HJT solar cells comprises the following steps:

[0043] S1, 10.5 kg (61.8 mol) of silver nitrate is added to 75 kg of distilled water to prepare a silver nitrate solution; 3.2 kg (30.2 mol) of sodium carbonate is added to 75 kg of water to prepare a sodium carbonate solution; 6.6 kg (37.5 mol) of ascorbic acid is added to 150 kg of distilled water to prepare a reducing agent solution; 64 g (0.32 mol) of lauric acid is added to 1.25 kg of distilled water to prepare a fatty acid solution, and the fatty acid and reducing agent solution are mixed to obtain a mixed solution;

[0044] A silver nitrate solution and a sodium carbonate solution are mixed and reacted to obtain a solution containing silver carbonate, which is then added dropwise to a mixed solution of ascorbic acid and lauric acid while stirring until the reaction terminates. The reaction product is washed three times with distilled water and three times with anhydrous ethanol, and then dried at 85°C for 12 hours to obtain silver powder particles.

[0045] S2, basic silver powder dispersion: 15 kg of silver powder particles, 120 g of oleic acid and 7.5 kg of anhydrous ethanol were mixed thoroughly at a stirring speed of 1000 r / min in a high-speed disperser for 30 min to obtain a mixed slurry;

[0046] S3, transfer the slurry to a ball mill for ball milling, add 45 kg of 2 mm diameter zirconia balls (i.e., a ball-to-material ratio of 3:1) into the horizontal ball mill, turn on the gas source, make the ball mill in a horizontal position, set the ball mill speed to 50 r / min, turn on the cooling system, maintain the system temperature at 40 ° C for mixing and ball milling, and take samples for testing regularly.

[0047] After 6 hours of ball milling, the flake powder produced meets the technical specifications. Ball milling is stopped, and the silver powder slurry and zirconia balls are separated through a sieve. The zirconia balls are then cleaned to obtain a slurry of flaky silver powder. The slurry is then poured into a polyester filter bag in a stainless steel filter. Solid-liquid separation is achieved by vacuum filtration to obtain flaky silver powder. The silver powder in the filter bag is rinsed with deionized water multiple times until the conductivity is ≤20μS / cm.

[0048] The flaky silver powder was dried at 65° C. for 12 hours in an electric blast drying oven to obtain dried silver blocks. The dried flaky silver powder was crushed by a mixer and all of it was sieved through a 200-mesh sieve to obtain micron-sized flaky silver powder for HJT solar cells.

[0049] Example 2

[0050] The difference from Example 1 is that: sampling is performed after ball milling for 10 hours, and the sample is obtained by washing, filtering, and drying. The other steps and conditions are the same as those in Example 1.

[0051] Performance Testing

[0052] One gram of the prepared silver powder sample was ultrasonically dispersed in 50 mL of anhydrous ethanol for 5 minutes. The powder particle size and distribution were measured using a particle size analyzer. Particle morphology and size were observed using a scanning electron microscope. The tap density and bulk density of the powder were measured using a tap density tester. The moisture and ash contents of the powder were measured using a fully automatic moisture and ash analyzer. The specific surface area of ​​the powder was measured using a specific surface area analyzer.

[0053] Scanning electron microscopy revealed that the silver powder particles obtained in step S1 of Example 1 were spherical silver powders with an average particle size of 0.8 μm to 1.0 μm.

[0054] The results of the flaky properties test at different ball milling times of Examples 1 and 2 are shown in Table 1 below.

[0055] Table 1 Flaky properties test results of different ball milling times in Examples 1 to 2

[0056]

[0057] From the data in Table 1 above, it can be seen that the process of the present invention can produce flaky silver powder with a diameter of 1 μm to 6 μm. The average D50 of the flaky silver powder in Example 1 is 2.0 μm, and the tap density is 5.5 g / cm 3 The average D50 of the silver flake powder in Example 2 is 4.0 μm, and the tap density is 6.0 g / cm 3 The prepared flaky silver powder has the characteristics of high degree of flakeness, good dispersibility, low specific surface area (low oil absorption, the viscosity of the slurry system is not too high, and does not affect subsequent printing and fluidity, etc.), high tap density, and high purity. It is very suitable for low-temperature slurry for HJT solar cells.

[0058] Depend on Figure 1 and Figure 2 It can be seen that the micron-sized flaky silver powder for the HJT solar cells prepared in Examples 1 and 2 of the present invention is evenly distributed, and from Table 1 and Figure 1-Figure 2 It can be seen that as the ball milling time increases, the flake diameter of the obtained flaky silver powder will increase and the specific surface area will increase. If the specific surface area increases further, it will affect subsequent use; therefore, the preferred ball milling time of the present invention is 5 to 10 hours.

[0059] X-ray diffraction tests show that the micron-sized flaky silver powder for HJT solar cells prepared in Examples 1 and 2 of the present invention has no impurity diffraction peaks, indicating that it has high purity.

[0060] Comparative Example 1

[0061] The difference from Example 1 is that the amount of the dispersant lauric acid in step S1 is 0 (ie, the dispersant is removed); the other steps and conditions are the same as those in Example 1.

[0062] The results showed that the obtained silver powder particles agglomerated, resulting in a wide particle size distribution of the final flaky silver powder, an increase in specific surface area, and a decrease in yield.

[0063] Comparative Example 2

[0064] The difference from Example 1 is that the amount of anhydrous ethanol in step S2 is adjusted to 6 kg; the other steps and conditions are the same as those in Example 1.

[0065] As a result, it was found that the specific surface area of ​​the obtained flaky silver powder was increased.

[0066] Comparative Example 3

[0067] The difference from Example 1 is that the oleic acid in step S3 is adjusted to benzophenone oxime acetic acid benzophenone oxime ester; the other steps and conditions are the same as those in Example 1.

[0068] The results showed that the powder adhered to the wall, the yield was reduced, and the purity of the obtained flaky silver powder was reduced, indicating that it was difficult to remove by washing.

[0069] As can be seen from the comparison of Examples 1-2 and Comparative Examples 1-3, the present invention adds a long straight-chain fatty acid as a dispersant during the preparation of silver powder particles, so that the prepared silver powder particles are uniform, which is beneficial to improving the performance of the final flaky silver powder. The silver powder particles are then used as raw materials, and high-speed ball milling is performed with the help of external mechanical force, a high amount of grinding aids and an organic solvent. There is no wall sticking phenomenon, the ball milling time is short, and the ball milling efficiency is effectively improved. The obtained flaky silver powder has a uniform and moderate flake diameter, a small specific surface area, a high purity, and a large bulk density and tap density.

[0070] The present invention provides micron-sized flaky silver powder for HJT solar cells and a preparation method thereof. The method uses silver carbonate as a silver source, and ultrafine spherical silver powder particles obtained by reduction with a reducing agent as a raw material. An organic solvent and an organic grinding aid are added, and the powder is mechanically ball-milled. The powder is then filtered, washed, and dried to obtain the target product. This method solves the technical problems of inconsistent particle size and morphology, high impurity content, and the need for surface modification in traditional flaky silver powder preparation processes. The resulting flaky silver powder has a stable average particle size of 1 to 6 μm, is easy to operate, and can be mass-produced. It is particularly suitable for the production of silver powder for low-temperature silver pastes.

[0071] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing micron-sized flaky silver powder for HJT solar cells, characterized in that: The following steps are involved: S1, reacting a silver nitrate solution with a sodium carbonate solution to obtain a solution containing silver carbonate, adding the solution containing silver carbonate dropwise to a mixed solution of a reducing agent and a fatty acid to react, and washing and drying to obtain silver powder particles; S2, uniformly mixing silver powder particles, a grinding aid, and an organic solvent to obtain a slurry; the grinding aid includes oleic acid; the organic solvent includes anhydrous ethanol or acetone; the mass ratio of the silver powder particles, the grinding aid, and the organic solvent is 100: (0.75-1.5): (45-55); S3, ball milling the slurry, the process conditions adopted for ball milling are: the diameter of the zirconia ball is 1.5-2.5 mm, the ball-to-material ratio is (2.8-3.2):1, the ball milling temperature is 35-42°C, the ball milling speed is 40-60 r / min, and the ball milling time is 5-10 hours; then, the slurry is separated, washed, dried, crushed and sieved to obtain micron-sized flaky silver powder for HJT solar cells.

2. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 1, characterized in that: The following steps are involved: In step S1, the fatty acid includes one or more of lauric acid, myristic acid, oleic acid, palmitic acid and stearic acid; and the reducing agent includes ascorbic acid.

3. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 1, characterized in that: In step S1, the molar ratio of silver nitrate to sodium carbonate is 1:(0.48-0.50); ​​the molar mass ratio of silver nitrate, reducing agent and fatty acid is 1:(0.60-0.62):(0.003-0.006).

4. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 3, characterized in that: In step S1, the concentration of the silver nitrate solution is 120-160 g / L; the concentration of the sodium carbonate solution is 35-50 g / L; and the mixed solution of the reducing agent and the fatty acid is obtained by mixing the reducing agent solution and the fatty acid solution, wherein the concentration of the reducing agent solution is 35-50 g / L and the concentration of the fatty acid is 48-55 g / L.

5. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 1, characterized in that: In step S1, washing is performed with distilled water and anhydrous ethanol respectively; Drying is done at 80-90°C for 10-14h; The average particle size of the silver powder particles is 0.8 μm to 1.0 μm.

6. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 1, characterized in that: In step S2, the silver powder particles, the grinding aid and the organic solvent are mixed and stirred for 5 to 40 minutes using a high-speed disperser with a rotation speed of 800 to 1200 r / min to obtain a slurry.

7. The method for preparing micron-sized flaky silver powder for HJT solar cells according to claim 1, characterized in that: In step S3, the slurry and the zirconia balls after ball milling are separated by a sieve to obtain slurry A, and the zirconia balls are washed to obtain slurry B; slurry A and slurry B are combined and filtered under reduced pressure to obtain flaky silver powder; Washing is to use deionized water to wash the flaky silver powder until the conductivity is ≤20μS / cm; The drying is carried out at 50℃~80℃ for 8h~24h; the sieving is carried out using a 200-mesh screen.

8. Micron-sized flaky silver powder for HJT solar cells prepared by the preparation method according to any one of claims 1 to 7.

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