A spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells

Abstract

The present disclosure provides a spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells, belonging to the technical field of conductive silver powder. Among them, the spherical silver powder includes a plurality of stacked thin sheet layers, and a plurality of gaps between the plurality of stacked thin sheet layers form a plurality of holes inside the spherical silver powder. The spherical silver powder of the present disclosure is formed by stacking a plurality of thin sheet layers, and the gaps between the thin sheet layers form a uniform porous structure. The spherical silver powder with this structure has a large specific surface area, a large tapped density, and a high sintering activity.

Description

Technical Field

[0001] The present disclosure belongs to the technical field of conductive silver powder, and particularly relates to a spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells. Background Art

[0002] With the rapid development of photovoltaic solar cells, the application market of electronic pastes as raw materials in the upstream has also expanded rapidly. Among them, metallic silver powder has the best thermal and electrical conductivity and relatively stable chemical properties, and is currently the most widely used and largest consumption of a precious metal powder material.

[0003] As the main raw material of electronic paste, silver powder accounts for the main cost expenditure. The morphology and size of silver powder directly affect the actual application function of the silver paste. Finding a silver powder with high performance and low cost is an inevitable trend in the development of silver paste. To ensure the performance of the paste and reduce the silver content of the paste is the first choice to reduce costs. Therefore, the performance requirements for silver powder are getting higher and higher. As a conductive filler, the microscopic morphology and internal structure of silver powder are the main factors affecting the performance of silver paste. Compared with ordinary spherical silver micropowders, microcrystalline silver powder with a hollow structure has a higher specific surface area and sintering activity, which is beneficial to obtaining a higher sintering density, can effectively reduce the film resistivity, and improve the electrical properties of the paste. At the same time, due to its special spatial structure, hollow structure silver powder is beneficial to reducing the silver consumption in the electronic paste and improving the rheological properties of the paste.

[0004] There are many methods for preparing silver powder at home and abroad, and the preparation technologies are roughly divided into physical methods and chemical methods. Since the physical method has a complex preparation process and the ultrafine silver powder prepared cannot meet the application requirements of conductive paste, the chemical method for preparing ultrafine silver powder is more common. Among them, the liquid-phase chemical reduction method in the chemical method has the advantages of simple equipment, low preparation cost, low energy consumption, and easy control of the morphology and particle size of silver powder by adjusting process parameters such as temperature, reaction time, pH, dropping method, reactant dosage and type during the reaction process, and the process is simple, so it is easy to realize industrial large-scale production.

[0005] At present, the most commonly used method for preparing hollow silver powder is synthesis by introducing bubbles, micelles, etc. For example, in Chinese Patent Application CN106041123A, one of the prior arts, nano-bubbles are used as carriers to adsorb silver nitrate and gum arabic powder on the surface, and ascorbic acid is used to reduce silver nitrate to prepare hollow-structured silver powder. Another example is Chinese Patent CN110355382B, one of the prior arts. After synthesizing highly crystalline microcrystalline silver powder, gas is generated by reacting hydrazine hydrate with the acid in the system, and the gas wrapped by an emulsifier is used as the core to generate microcrystalline silver powder containing a hollow structure. It can be seen that the above preparation methods all require the introduction of gas to form silver powder with a hollow structure. The reaction conditions are relatively harsh, the production process is complex, and the requirements for equipment are also relatively high, making large-scale production difficult. Moreover, the hollow silver powder formed above mostly has a single hollow structure inside, or the pore sizes inside the powder are uneven. Such silver powder has a relatively small specific surface area, low sintering activity, and a large residual space during sintering, resulting in a significant increase in resistance. Summary of the Invention

[0006] The present disclosure aims to at least solve one of the technical problems existing in the prior art, and provides a spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells.

[0007] On the one hand, the present disclosure provides a spherical silver powder, which includes a plurality of stacked thin layers; wherein,

[0008] A plurality of gaps between the plurality of stacked thin layers form a plurality of holes inside the spherical silver powder.

[0009] Optionally, the diameter range of the holes is 3 to 15 nm; and / or,

[0010] The D50 particle size range of the spherical silver powder is 1.5 to 2.0 μm; and / or,

[0011] The tapped density range of the spherical silver powder is 6.0 to 6.4 g / cm 3 ; and / or,

[0012] The specific surface area of the spherical silver powder is 0.7 to 1.4 m 2 / g.

[0013] On the other hand, the present disclosure provides a preparation method of the spherical silver powder as described above, and the preparation method includes:

[0014] Adding a first silver salt solution to a reducing solution, and stirring and reacting for 1 to 2 h to obtain silver crystal nuclei;

[0015] Adding a solution of a high molecular compound to a second silver salt solution, and stirring for 2 to 3 h to obtain a first reaction solution containing a silver source;

[0016] Mix the dispersant solution with the reducing agent solution, and add the solution of the inorganic salt mixture and the silver crystal nuclei in sequence under the conditions of a temperature of 15 - 25°C and a pH value of 3.5 - 4.5, and stir for 1 - 1.5 h to obtain a second reaction solution;

[0017] Drop the first reaction solution into the second reaction solution to obtain a silver powder turbid liquid;

[0018] Perform solid-liquid separation, washing, modification, drying, surface polishing, and sieving on the silver powder turbid liquid to obtain spherical silver powder.

[0019] Optionally, the polymer compound is a pyrrolidone-type polymer compound.

[0020] Optionally, the pyrrolidone-type polymer compound is polyvinylpyrrolidone.

[0021] Optionally, the average molecular weight of polyvinylpyrrolidone is above 80,000.

[0022] Optionally, the inorganic salt mixture is a mixture of potassium sulfate, potassium carbonate, and trisodium citrate.

[0023] Optionally, the ratio of potassium sulfate, potassium carbonate, and trisodium citrate is (1 - 2):(2 - 4):(6 - 8).

[0024] Optionally, the content of the polymer compound is 1 - 3 wt% of the mass of the silver salt in the second silver salt solution; and / or,

[0025] the content of the silver crystal nuclei is 1 - 10 wt‰ of the mass of the silver salt in the second silver salt solution.

[0026] Optionally, the concentration of the second silver salt solution is 0.3 - 0.7 M;

[0027] the concentration of the polymer compound solution is 1 - 3 mM;

[0028] the concentration of the reducing agent solution is 0.45 - 0.75;

[0029] the concentration of the dispersant solution is 0.15 - 0.45 M;

[0030] the concentration of the inorganic salt mixture solution is 0.8 - 1.5 mM.

[0031] Optionally, adding the first silver salt solution to the reducing solution and stirring for 1 - 2 h to obtain silver crystal nuclei includes:

[0032] Dissolve the silver salt in deionized water to form a first silver salt solution;

[0033] Dissolve the dispersant and reducing agent in deionized water, stir evenly, and adjust the pH value of the solution to 13-14 to form a reducing solution;

[0034] Mix the first silver salt solution and the reducing solution according to a volume ratio of 1:(19-21), stir and react to obtain silver crystal nuclei.

[0035] On the other hand, the present disclosure proposes a silver paste for crystalline silicon solar cells, which includes a conductive filler, an organic carrier, glass powder, an organic solvent, and an auxiliary agent; wherein,

[0036] The conductive filler includes the spherical silver powder described above.

[0037] Optionally, the silver paste for crystalline silicon solar cells is a front silver paste or a back silver paste for solar cells.

[0038] The present disclosure proposes a spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells. Among them, the spherical silver powder includes a plurality of stacked thin sheet layers; wherein, a plurality of gaps between the plurality of stacked thin sheet layers form a plurality of holes inside the spherical silver powder. Without introducing foreign gases, the present disclosure only controls the silver particles to grow regularly in multiple directions in the shape of small thin sheets along the crystal nuclei under the action of components such as inorganic salts and dispersants, so that the spherical silver powder is formed by stacking a plurality of thin sheet layers, and gaps are formed between the thin sheet layers, and these gaps form a porous structure evenly distributed inside the silver powder. The spherical silver powder has a large specific surface area, a high tapped density, and a high sintering activity. Description of the Drawings

[0039] Figure 1 It is a flowchart of a preparation method of spherical silver powder according to an embodiment of the present disclosure;

[0040] Figure 2 It is a scanning electron microscope image of the spherical silver powder of Example 1 of the present disclosure;

[0041] Figure 3 It is a cross-sectional scanning electron microscope image of the spherical silver powder of Example 1 of the present disclosure;

[0042] Figure 4 It is a cross-sectional scanning electron microscope image of the spherical silver powder of Example 1 of the present disclosure;

[0043] Figure 5 It is a scanning electron microscope image of the silver powder of Comparative Example 1 of the present disclosure;

[0044] Figure 6 It is a cross-sectional scanning electron microscope image of the silver powder of Comparative Example 1 of the present disclosure;

[0045] Figure 7 It is a scanning electron microscope image of the silver powder of Comparative Example 2 of the present disclosure;

[0046] Figure 8 SEM image of internal cutting of the silver powder of Comparative Example 2 of the present disclosure. Detailed implementation manners

[0047] To enable those skilled in the art to better understand the technical solutions of the present disclosure, the present disclosure will be further described in detail below with reference to the accompanying drawings and specific implementation manners. Obviously, the described embodiments are a part of the embodiments of the present disclosure, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present disclosure without creative efforts shall fall within the protection scope of the present disclosure.

[0048] Unless otherwise specifically stated, the technical terms or scientific terms used in the present disclosure should have the ordinary meanings understood by those with ordinary skills in the field to which the present disclosure pertains. The "including" or "comprising" etc. used in the present disclosure neither limit the mentioned shapes, numbers, steps, operations and / or their groups, nor exclude the occurrence or addition of one or more other different shapes, numbers, steps, operations and / or their groups.

[0049] As Figures 2 to 4 shown, on the one hand, the present disclosure provides a spherical silver powder, which includes a plurality of stacked thin sheet layers; wherein, a plurality of gaps between the plurality of stacked thin sheet layers form a plurality of uniformly distributed hole structures inside the spherical silver powder, so that the inside of the spherical silver powder has a uniformly distributed porous structure.

[0050] It should be understood that the spherical silver powder of this embodiment includes a plurality of small thin-sheet-shaped silver solids, and these small thin-sheet-shaped silver solids grow by stacking to form a plurality of thin sheet layers, and the plurality of thin sheet layers are stacked to form a spherical silver powder, so that the surface of the spherical silver powder is a sheet-like structure. At the same time, it is worth noting that when adjacent small thin-sheet-shaped silver solids grow by stacking, they are not completely adhered, but there are many gaps between the thin sheets, so that a plurality of smaller holes are generated inside the spherical silver powder to form an internal multi-hole structure, and then a spherical silver powder with a hollow structure is formed.

[0051] In this embodiment, the spherical silver powder is formed by stacking a plurality of thin sheet layers, so that the inside of the silver powder has a plurality of fine hole structures, which can increase its internal specific surface area and at the same time improve the tap density; in addition, based on the fact that the surface of the spherical silver powder has a plurality of sheet-like structures, its outer surface also has a large specific surface area, so that the spherical silver powder has a high sintering activity, which is beneficial to reducing the sintering temperature, obtaining a higher sintering density, further effectively reducing the film resistivity, and improving the electrical properties of the paste.

[0052] In some preferred embodiments, the multiple pore structures inside the spherical silver powder are evenly distributed. Further preferably, the diameter of each pore structure is between 3 and 15 nm, so as to form multiple uniformly distributed fine pores inside the spherical silver powder. During sintering, the silver powder can be melted more evenly and has higher density.

[0053] In some other preferred embodiments, the D10 particle size range of the spherical silver powder is 0.8 - 1.3 μm, the D50 particle size range is 1.5 - 2.0 μm, and the D90 particle size range is 2.3 - 3.3 μm.

[0054] In some other preferred embodiments, the tap density range of the spherical silver powder is 6.0 - 6.4 g / cm 3 .

[0055] In some other preferred embodiments, the specific surface area of the spherical silver powder is 0.7 - 1.4 m 2 / g.

[0056] As Figure 1 shown, on the other hand, the present disclosure provides a preparation method S100 of spherical silver powder, which specifically includes the following steps S110 - S150:

[0057] S110. Add the first silver salt solution to the reducing solution, and stir and react for 1 - 2 h to obtain silver crystal nuclei.

[0058] Specifically, dissolve the silver salt in deionized water to form a first silver salt solution with a concentration range of 1 - 3 g / L; dissolve the dispersant and the reducing agent in the same deionized water, stir evenly to dissolve, and then adjust the pH value of the solution to 13 - 14 to form a reducing solution; mix the first silver salt solution and the reducing solution according to a volume ratio of 1:(19 - 21), and stir and react for 1 - 2 h to obtain silver crystal nuclei.

[0059] It should be noted that the prepared first silver salt solution can be directly mixed and reacted with the reducing solution, or it can be filled into a brown reagent bottle as a standby solution for subsequent use, and no specific limitation is made thereto.

[0060] Furthermore, it should be noted that the present embodiment does not specifically limit the mixing method and mixing ratio of the first silver salt solution and the reducing solution, which can be adjusted according to actual needs. For example, directly pour the first silver salt solution into the reducing solution to quickly mix and react them to obtain a large number of fine silver crystal nuclei; for example, the ratio of the first silver salt solution to the reducing solution is 1:19.

[0061] It should still be noted that the present embodiment also does not specifically limit the type of silver salt used in the first silver salt solution, as well as the types and concentrations of the dispersant and the reducing agent. For those skilled in the art, they can be selected according to actual needs.

[0062] In some preferred embodiments, the silver salt in the first silver salt solution includes at least one of water-soluble silver salts such as silver nitrate, silver fluoride, silver sulfate, and silver perchlorate.

[0063] As a further preferred solution, the silver salt in the first silver salt solution is a nitrate.

[0064] In some other preferred embodiments, when the dispersant is dissolved in deionized water, its concentration is controlled at 40 - 60 g / L, and when the reducing agent is dissolved in deionized water, its concentration is controlled at 5 - 10 g / L, that is, the amount of the dispersant is more than that of the reducing agent. While forming fine silver crystal nuclei, the dispersibility of the silver crystal nuclei is increased.

[0065] In some other preferred embodiments, the reducing agent includes at least one of ascorbic acid, sodium ascorbate, formaldehyde, sodium borohydride, hydrazine hydrate, and glucose.

[0066] In some other preferred embodiments, the dispersant is at least one of polyvinylpyrrolidone, gum arabic, gelatin, and polyethylene glycol.

[0067] S120. Add a solution of a high molecular compound to the second silver salt solution and stir for 2 - 3 h to obtain a first reaction solution containing a silver source.

[0068] It should be noted that in this embodiment, the concentration and type of the second silver salt solution are not specifically limited either. A silver salt solution of the same type as in step S110 can be used, or a silver salt solution of a different type can be used.

[0069] In some preferred embodiments, at least one silver salt such as silver nitrate, silver fluoride, silver sulfate, and silver perchlorate is dissolved in deionized water to form a second silver salt solution.

[0070] In some other preferred embodiments, the concentration of the second silver salt solution is preferably 0.3 - 0.7 M. For example, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, etc.

[0071] It should be further noted that in this embodiment, the addition time of the high molecular compound and the stirring time are not specifically limited. For example, when the silver salt is dissolved in deionized water to form a second silver salt solution, the temperature of this solution is controlled at 20 - 25 °C. After the silver salt is completely dissolved and homogenized, the high molecular compound is added. Then, after the high molecular compound is completely dissolved, timing starts, and stirring continues until the solution becomes turbid and fine silver particles are formed, that is, a first reaction solution containing a silver source is obtained.

[0072] It should still be noted that the concentration and type of the polymer compound solution are not specifically limited in this embodiment. For those skilled in the art, they can be selected according to actual needs.

[0073] In some other preferred embodiments, the polymer compound is a pyrrolidone-based polymer compound. Based on the reducibility of this type of polymer compound, fine silver particles are formed as crystal seeds by reacting with silver salts.

[0074] As a further preferred solution, the pyrrolidone-based polymer compound is polyvinyl pyrrolidone (PVP).

[0075] As an even further preferred solution, the average molecular weight of polyvinyl pyrrolidone is above 80,000. Higher molecular weight PVP has a certain reducibility. Based on the weak reducibility of PVP, it reacts with silver salts to form extremely fine silver particles as crystal seeds.

[0076] It should still be noted that the addition method and concentration of the polymer compound are not specifically limited in this embodiment. For example, the solid can be directly added to the second silver salt solution. Of course, in order to improve the reaction efficiency, generally, it can be added to the second silver salt solution in the form of a solution, that is, the polymer compound is dissolved in a solvent to form a solution of the polymer compound, and this solution is added to the second silver salt solution.

[0077] In some preferred embodiments, the solution concentration of the polymer compound is 1 - 3 mM, for example, 1 mmol / L, 2 mmol / L, 3 mmol / L, etc.

[0078] In some other preferred embodiments, the content of the polymer compound is 1 - 3 wt% of the mass of the silver salt in the second silver salt solution. For example, the content of the polymer compound is 1 wt%, 2 wt%, or 3 wt% of the mass of the silver salt.

[0079] S130. Mix the dispersant solution and the reducing agent solution, and under the conditions of a temperature of 15 - 25°C and a pH value of 3.5 - 4.5, after the dispersant and the reducing agent are completely dissolved, add the solution of the inorganic salt mixture, and then add the silver crystal nuclei, and stir well for 1 - 1.5 h to obtain the second reaction solution.

[0080] It should be noted that the reaction temperature, pH value, and reaction time are not specifically limited in this embodiment. The spherical silver powder formed within the above temperature range, pH range, and reaction time range has a relatively large particle size, tap density, and specific surface area.

[0081] In some preferred embodiments, the reaction temperature is preferably 15°C, 20°C, 25°C, the pH value is preferably 3.5, 4, 4.5, and the reaction time is preferably 1 h, 1.2 h, 1.5 h.

[0082] Further, it should be noted that in this embodiment, all components are added in the form of solutions. For example, components such as dispersants, reducing agents, and inorganic salt mixtures are first dissolved in a solvent to form corresponding solutions, and then the solutions are mixed. Of course, no specific limitation is imposed on the above-mentioned solvent, as long as it can dissolve the corresponding components, such as deionized water and the like.

[0083] It should still be noted that in this embodiment, no specific limitation is imposed on the concentration and type of the dispersant, reducing agent, and inorganic salt. Among them, the dispersant and the reducing agent can be the same substances as those in step S110, or different dispersants and reducing agents can be used, and no specific limitation is imposed on this. As long as the inorganic salt can control the growth direction of the silver powder.

[0084] In some preferred embodiments, the dispersant includes at least one of polyvinylpyrrolidone, gum arabic, gelatin, and polyethylene glycol.

[0085] In some other preferred embodiments, the concentration range of the dispersant solution is 0.15 - 0.45 M, for example, 0.15 mol / L, 0.25 mol / L, 0.35 mol / L, 0.45 mol / L, etc.

[0086] It is worth noting that there are certain differences between the polyvinylpyrrolidone included in the dispersant in this embodiment and the polyvinylpyrrolidone used in the high molecular compound. Among them, no requirement is imposed on the molecular weight of the polyvinylpyrrolidone in the dispersant, which is generally polyvinylpyrrolidone with a small molecular weight and only plays a dispersing role, while the polyvinylpyrrolidone in the high molecular compound has a relatively large molecular weight and has a certain reducing effect.

[0087] In some other preferred embodiments, the reducing agent includes at least one of ascorbic acid, sodium ascorbate, formaldehyde, sodium borohydride, hydrazine hydrate, and glucose. No specific limitation is imposed on this, and those skilled in the art can select according to actual needs.

[0088] In some other preferred embodiments, the concentration range of the reducing agent solution is 0.45 - 0.75 M, for example, 0.45 mol / L, 0.55 mol / L, 0.65 mol / L, 0.75 mol / L, etc.

[0089] In some other preferred embodiments, the inorganic salt mixture is a mixture of potassium sulfate, potassium carbonate, and trisodium citrate. This inorganic salt mixture plays a role in controlling the growth direction of the silver powder, making the silver particles grow into small flake shapes.

[0090] In some other preferred embodiments, the solution concentration of the inorganic salt mixture is controlled between 0.8 and 1.5 mM. For example, 0.8 mmol / L, 1 mmol / L, 1.2 mmol / L, 1.5 mmol / L, etc.

[0091] In some other preferred embodiments, the ratio of potassium sulfate, potassium carbonate, and trisodium citrate is preferably (1 - 2):(2 - 4):(6 - 8).

[0092] As a further preferred solution, the ratio of potassium sulfate, potassium carbonate, and trisodium citrate is preferably 1:3:7.

[0093] In some other preferred embodiments, in step S130, the content of silver crystal nuclei is 1 - 10 wt‰ of the mass of the silver salt. For example, the amount of silver crystal nuclei added in step S130 is 1 wt‰, 2 wt‰, 3 wt‰, 4 wt‰, 5 wt‰, 6 wt‰, 8 wt‰, 9 wt‰, 10 wt‰, etc. of the silver salt content required in the second silver salt solution. Of course, other contents of silver crystal nuclei can also be selected here, and specific settings can be made according to the number of silver crystal nuclei per unit volume.

[0094] S140. Drop the first reaction solution into the second reaction solution to obtain a silver powder turbid liquid.

[0095] It should be noted that the present embodiment does not specifically limit the manner of dropping the first reaction solution into the second reaction solution. For example, a flow pump or a peristaltic pump is used to slowly mix the two solutions so that the small flake silver particles are slowly stacked and grown along a specific growth direction.

[0096] In some preferred embodiments, the first reaction solution is slowly dropped into the second reaction solution at a speed of 0.05 - 0.4 L / min through a flow pump to allow the silver particles to grow slowly. After the dropping is completed, a silver powder turbid liquid can be obtained.

[0097] S150. Perform solid-liquid separation, washing, modification, drying, surface polishing, and sieving on the silver powder turbid liquid to obtain spherical silver powder with multiple uniform pores.

[0098] It should be noted that the modification treatment of the silver powder in step S150 is not specifically limited. For example, a surfactant is added to the silver powder solution to coat a layer of surfactant on the surface of the silver powder to increase the dispersibility between the silver powders and avoid agglomeration.

[0099] Furthermore, it should be noted that the present embodiment does not specifically limit the type of surfactant. For example, it can be a mixture of one or more of stearic acid, myristic acid, oleic acid, and lauric acid.

[0100] In this embodiment, by the method of uniformly dropping the silver source solution, the produced silver particles grow slowly and regularly in multiple directions along the crystal nuclei in the form of small flakes under the control of various inorganic salts. When the adjacent small flake-shaped silver solids grow by stacking, they are not completely adhered to each other, forming multiple stacked thin-film layers. There are many gaps between the thin films, and these gaps generate a plurality of pore structures with uniform distribution inside the silver powder. The preparation process is simple, and spherical silver powder with a hollow structure can be obtained without adding complex processes such as bubbles, which is easy for large-scale industrial production.

[0101] On the other hand, the present disclosure provides a silver paste for crystalline silicon solar cells, which includes a conductive filler, an organic carrier, glass powder, an organic solvent, and an additive; wherein, the conductive filler includes the spherical silver powder described above. For the structure, size, and preparation method of the spherical silver powder, please refer to the above description and will not be elaborated here.

[0102] It should be understood that the spherical silver powder of this embodiment, as a conductive functional material, can be applied in the electronics industry, mainly used in solar cells such as TOPCon and PERC. For example, the spherical silver powder is used in the front silver paste of crystalline silicon solar cells, and the spherical silver powder can also be used in the back silver paste of crystalline silicon solar cells, without specific limitation. Correspondingly, according to the different application environments of the spherical silver powder, the silver paste for crystalline silicon solar cells is the front silver paste or the back silver paste.

[0103] In this embodiment, based on the fact that the spherical silver powder has a uniform porous structure inside, when it is used as a conductive filler to form a silver paste, the amount of silver powder can be reduced, and at the same time, it helps to reduce the sintering temperature.

[0104] The following will further illustrate the spherical silver powder and its preparation method with several specific examples:

[0105] Example 1

[0106] The preparation method of the spherical silver powder in this example includes the following steps:

[0107] S1. Dissolve nitrate in deionized water to form a silver nitrate solution with a concentration of 2 g / L. Dissolve a dispersant and a reducing agent in deionized water to form a reducing solution, wherein the concentration of the dispersant is controlled at 50 g / L, and the concentration of the reducing agent is controlled at 7.5 g / L. Then adjust the pH of the solution to 13; then add the silver nitrate solution to the reducing solution, and the volume ratio of the silver nitrate solution to the reducing solution is 1:19. After the two are rapidly mixed and reacted, stir for 1.5 h to obtain silver crystal nuclei.

[0108] S2. Dissolve the silver salt in deionized water to form a second silver salt solution with a concentration of 0.5 mol / L. After all the second silver salt is dissolved, add a solution of a polymer compound accounting for 2% of the mass of the silver salt, and stir for 2.5 h until the solution becomes turbid and fine silver particles are formed, obtaining a first reaction solution containing a silver source.

[0109] Among them, the concentration of the polymer compound solution is 2 mM.

[0110] S3. Dissolve the dispersant and reducing agent in deionized water. Under the conditions of a temperature of 15 °C and a pH value of 3.5, add an inorganic salt mixture (the mass ratio of potassium sulfate, potassium carbonate, and trisodium citrate is 1:3:7). Then, add silver crystal nuclei and stir for 1.3 h to obtain a second reaction solution.

[0111] Among them, the concentration of the dispersant dissolved in water is 0.3 M, the concentration of the reducing agent dissolved in water is 0.6 M, and the concentration of the inorganic salt mixture dissolved in water is 1.1 mM.

[0112] S4. Slowly drip the first reaction solution into the second reaction solution at a rate of 0.2 L / min through a flow pump to obtain a silver powder suspension.

[0113] S5. Separate the solid and liquid of the silver powder suspension, wash, modify, dry, polish the surface, and sieve to obtain spherical silver powder with multiple uniform pores.

[0114] As Figures 2 to 4 shown, the spherical silver powder obtained in Example 1 is formed by stacking multiple thin layers. The diameter of the spherical silver powder is 1.4 - 2.5 μm, and it has multiple uniformly distributed small pore structures inside, with the diameter of the pore structures being 3 - 15 nm.

[0115] Furthermore, as shown in Table 1, D10 of the spherical silver powder is 1.268 μm, D50 is 1.934 μm, D90 is 2.645 μm, the tapped density is 6.24 g / cm 3 , and the specific surface area is 0.95 m 2 / g.

[0116] Example 2

[0117] The preparation method of the spherical silver powder in this example is the same as that in Example 1, only changing the reaction temperature in step S3 to 20 °C and the pH value of the solution to 4.0.

[0118] As shown in Table 1, D10 of the spherical silver powder is 1.018 μm, D50 is 1.735 μm, D90 is 2.583 μm, the tapped density is 6.18 g / cm 3 , and the specific surface area is 1.17 m 2 / g.

[0119] Example 3

[0120] The preparation method of the spherical silver powder in this example is the same as that in Example 1, only changing the reaction temperature in step S3 to 25 °C and the pH value of the solution to 4.5.

[0121] As shown in Table 1, D10 of the spherical silver powder is 0.945 μm, D50 is 1.546 μm, D90 is 2.376 μm, and the tapped density is 6.05 g / cm 3 , and the specific surface area is 1.35 m 2 / g.

[0122] Comparative Example 1

[0123] The preparation method of the spherical silver powder in this example is the same as that in Example 1, only changing the reaction temperature in step S3 to 0 °C and the pH value of the solution to 1.0.

[0124] As Figure 5 and Figure 6 shown, when the reaction temperature is 0 °C and the solution pH is 1, the formed silver powder is an irregular polygon, and its surface is smooth. The interior is basically a dense structure, and only one or two particles have a single hollow structure inside, approximating a fully solid silver powder.

[0125] Furthermore, as shown in Table 1, D10 of the silver powder is 1.524 μm, D50 is 2.471 μm, D90 is 3.764 μm, and the tapped density is 6.05 g / cm 3 , and the specific surface area is 0.38 m 2 / g.

[0126] In summary, compared with Examples 1-3, when the reaction temperature is 0 °C and the solution pH is 1, the formed silver powder has a larger particle size, a lower specific surface area, and poorer sintering activity.

[0127] Comparative Example 2

[0128] The preparation method of the spherical silver powder in this example is the same as that in Example 1, only changing the reaction temperature in step S3 to 50 °C and the pH value of the solution to 10.

[0129] As Figure 7 and Figure 8 shown, when the reaction temperature is 50 °C and the solution pH is 10, the formed silver powder is spherical and has a flaky structure, but the flake layer is not obvious. It has a smaller particle size, a lower tapped density, and is basically a single hollow structure inside. The size of this hollow structure is relatively large, which can increase the specific surface area of the silver powder to a certain extent, but there will be a relatively large space remaining after sintering, resulting in poor densification and an increase in resistance.

[0130] Further, as shown in Table 1, the D10 of the silver powder is 0.705 μm, D50 is 1.103 μm, D90 is 1.831 μm, and the tap density is 5.65 g / cm 3 , and the specific surface area is 1.64 m 2 / g.

[0131] In summary, compared with Examples 1-3, when the reaction temperature is 50 °C and the solution pH is 10, the formed silver powder has the smallest particle size, the lowest tap density, and the largest specific surface area. This shows that although too high temperature and pH value can increase the specific surface area of silver powder to a certain extent, its tap density and particle size are relatively small, and it is easy to aggregate together when forming the silver powder into a slurry, forming an uneven microstructure and having poor fluidity, thus affecting the electrical conductivity of the sintered film.

[0132] Generally speaking, the optimal range of the reaction temperature is 15-25 °C, and the optimal range of the pH value of the solution is 3.5-4.5. Within this range, the formed spherical silver powder has a moderate particle size, relatively high tap density and specific surface area. The spherical silver powder particles are more likely to form a uniformly dispersed state in the slurry, reducing the aggregation between silver powder particles, thus obtaining a better microstructure. When preparing a thick film, the uniform dispersion of silver powder particles can make the film layer more dense, reduce the porosity, and at the same time improve the sintering activity. During the sintering process, the spherical silver powder can interact better with other materials to form a more dense sintered body and improve the electrical conductivity.

[0133] Table 1 Particle size, tap density and specific surface area data of spherical silver powder

[0134]

[0135] The present disclosure provides a spherical silver powder, a preparation method thereof, and a silver paste for crystalline silicon solar cells, which have the following beneficial effects compared with the prior art:

[0136] First, compared with silver powder having a single hollow structure or a dense structure, the spherical silver powder of the present disclosure has many uniform small holes inside, and at the same time has a higher specific surface area and tap density. The silver powder can obtain a larger active specific surface when expanding during sintering, has higher sintering activity, and the silver powder with uniform pores inside can melt more uniformly during sintering, with higher densification, which can effectively reduce the film resistivity and improve the electrical performance of the paste;

[0137] Second, the preparation method of the present disclosure uses the liquid-phase reduction method. Without introducing foreign gases, only by the action of inorganic salts and dispersants, etc., the growth of silver particles along the crystal nucleus is controlled to stack regularly in multiple directions in the form of small flakes, and gaps are formed between the flake layers to form a spherical silver powder with a uniform porous structure. The process is simple and easy to produce and prepare industrially.

[0138] It is understood that the above embodiments are merely exemplary embodiments adopted to illustrate the principles of the present disclosure. However, the present disclosure is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present disclosure, and these modifications and improvements are also regarded as the protection scope of the present disclosure.

Claims

1. A method for preparing spherical silver powder, characterized in that, The preparation method includes: Adding a first silver salt solution to a reducing solution, stirring and reacting for 1 - 2 h to obtain silver crystal nuclei; Adding a solution of a polymer compound to a second silver salt solution, stirring for 2 - 3 h, and reacting the polymer compound with the second silver salt solution to form fine silver particle seeds, obtaining a first reaction solution containing seeds; the polymer compound is polyvinylpyrrolidone with a reducing effect and an average molecular weight of more than 80,000; Mixing a dispersant solution and a reducing agent solution, and sequentially adding a solution of an inorganic salt mixture and the silver crystal nuclei under the conditions of a temperature of 15 - 25 °C and a pH value of 3.5 - 4.5, stirring for 1 - 1.5 h to obtain a second reaction solution; the inorganic salt mixture is a mixture of potassium sulfate, potassium carbonate, and trisodium citrate; Adding the first reaction solution dropwise to the second reaction solution to obtain a silver powder turbid liquid; Separating the solid and liquid of the silver powder turbid liquid, washing, modifying, drying, surface polishing, and sieving to obtain spherical silver powder, the spherical silver powder includes multiple stacked thin sheet layers; wherein, Multiple gaps between the multiple stacked thin sheet layers form multiple pores inside the spherical silver powder.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the potassium sulfate, the potassium carbonate, and the trisodium citrate is (1 - 2):(2 - 4):(6 - 8).

3. The preparation method according to claim 1 or 2, characterized in that, The concentration of the second silver salt solution is 0.3 - 0.7 M; The concentration of the solution of the polymer compound is 1 - 3 mM; The concentration of the reducing agent solution is 0.45 - 0.75; The concentration of the dispersant solution is 0.15 - 0.45 M; The concentration of the solution of the inorganic salt mixture is 0.8 - 1.5 mM.

4. The preparation method according to claim 1 or 2, characterized in that, The adding the first silver salt solution to the reducing solution, stirring and reacting for 1 - 2 h to obtain silver crystal nuclei includes: Dissolving a silver salt in deionized water to form a first silver salt solution; Dissolving a dispersant and a reducing agent in deionized water, stirring evenly, and adjusting the pH value of the solution to 13 - 14 to form a reducing solution; Mixing the first silver salt solution and the reducing solution in a volume ratio of 1:(19 - 21), stirring and reacting to obtain silver crystal nuclei.

5. A spherical silver powder, characterized in that, The spherical silver powder is prepared by the preparation method according to any one of claims 1 - 4, wherein, The diameter range of the pores is 3 - 15 nm; and / or, The D50 particle size range of the spherical silver powder is 1.5 - 2.0 μm; and / or, The tap density range of the spherical silver powder is 6.0~6.4 g / cm 3 ; and / or, The specific surface area of the spherical silver powder is 0.7 to 1.4 m 2 / g.

6. A silver paste for crystalline silicon solar cells, characterized in that, The silver paste includes a conductive filler, an organic carrier, glass powder, an organic solvent, and an additive; wherein, The conductive filler includes the spherical silver powder according to claim 5.

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