Conductive silver paste for solar cell and application thereof

Abstract

The invention discloses conductive silver paste for a solar cell and application of the conductive silver paste, and belongs to the technical field of conductive silver paste. According to the invention, the prepared silver paste is printed on a silicon wafer cell in a silk-screen printing manner, so that the good rheological property of printing can be obtained. And a rheometer is introduced for testing, and an SEM scanning electron microscope is introduced for checking the surface appearance of the silver grid line. The invention discloses a TOPCon silicon wafer for a silicon solar cell, which comprises the following components in percentage by weight: 82-90% of silver powder, 20-30% of glass powder and 20-30% of organic carrier. 2-5% of glass powder; and 8-13% of an organic carrier. The conductive silver paste with good rheological property is prepared by using a mixture of polyamide wax and hydrogenated castor oil in a mass ratio of 1: 1 as a thixotropic agent. The main grid silver paste for the silicon solar cell provided by the invention can be well matched with step-by-step printing of a cell piece, different proportions of the binder and the thixotropic agent are used, the photoelectric conversion efficiency is high, the peeling strength of the main grid is high, and the step-by-step printing main grid paste requirements of current different types of paste and cell pieces can be well met.

Description

Technical Field

[0001] This invention relates to the field of conductive silver paste technology, and more particularly to a conductive silver paste for solar cells. Background Technology

[0002] Solar energy, as a renewable and clean energy source, produces almost no pollution to the environment during use. Currently, crystalline silicon solar cells are the most mature and widely used type, and considering both the sourcing of materials and photoelectric conversion efficiency, future development will still focus on crystalline silicon solar cells. Crystalline silicon solar cells possess characteristics such as high conversion efficiency, mature technology, long lifespan, and simple structure, while also offering marketable manufacturing technology and the use of all-material raw materials.

[0003] Conductive silver paste for solar cells mainly consists of three parts: a conductive phase, a binder phase, and an organic carrier. The conductive phase, namely silver powder, primarily conducts electricity. The binder phase, namely glass powder, acts as a binder between the conductive phase and the substrate, and is generally composed of glass, oxide crystals, or a mixture of both. The organic carrier acts as a carrier for both the conductive and binder phases. It is a solution formed by dissolving various polymers in an organic solvent, which disperses the conductive and binder phases and ensures thorough mixing for screen printing. The organic carrier, as the medium for dispersing the silver and glass powders, also has a crucial impact on the printability of the paste. The organic carrier includes organic solvents, anti-settling agents, surfactants, and thickeners, and each component consists of one or more materials.

[0004] Many factors influence the performance of conductive pastes, with the composition and content of the organic carrier being the most critical. During sintering, the organic carrier is easily decomposed. Rheology is a complex and important fluid property inherent in electronic pastes, serving as a crucial indicator for evaluating their screen printing performance. The rheology of the printing paste is closely related to the quality of high-precision screen printing. Generally, the printability of the paste is closely related to yield stress, shear rate, relevant viscosity, and thixotropy. Furthermore, the paste should possess high yield stress, shear thinning characteristics, and suitable viscoelasticity. Therefore, finding a conductive silver paste with good rheology and a high aspect ratio is essential. Summary of the Invention

[0005] To address the issues of poor rheological properties and high aspect ratio in conductive silver paste during screen printing, this invention aims to provide a conductive silver paste for monocrystalline silicon solar cells. By improving the organic carrier, the rheological properties and high aspect ratio of the silver paste are enhanced. The conductive silver paste comprises silver powder, glass powder, and an organic carrier, with the following mass percentage content: silver powder 82-90%; glass powder 2-5%; organic carrier 8-13%. The organic carrier is composed of an organic solvent, binder, thixotropic agent, surfactant, and coupling agent. The thixotropic agent is a mixture of hydrogenated castor oil and polyamide wax in a 1:1 mass ratio.

[0006] Preferably, the mass percentage content of each component of the organic carrier of the present invention is as follows: organic solvent 78-88 wt%, binder 8-12 wt%, thixotropic agent 0.5-3 wt%, surfactant 0.5-4 wt%, and coupling agent 1-4 wt%. Preferably, the organic solvent of the present invention is one or a mixture of dodecyl alcohol ester, butyl carbitol, butyl carbitol acetate, tributyl citrate, terpineol, dibutyl phthalate, glycerol, lavender oil, anise oil, tributyl citrate, isopropanol, and dimethyl succinate. The organic solvent selected in the present invention can adjust the evaporation rate of the organic carrier, making the evaporation of the organic carrier hierarchical and avoiding the appearance of pores on the surface of the silver grid lines after sintering. Moreover, the solvent is an important factor in adjusting the storage stability of the silver paste.

[0007] The silver powder and glass powder mentioned in this invention are the commonly used silver powder and glass powder in existing conductive silver pastes. The key to this invention lies in the optimization and improvement of the organic carrier.

[0008] Preferably, the binder of the present invention is one or a mixture of ethyl cellulose, polymethyl styrene, polyvinyl alcohol, nitrocellulose, and phenolic resin. The binder of the present invention is a solid powder at room temperature, typically a polymeric substance, in a network or chain-like form, containing strong polar groups, and can be dissolved by organic solvents; the binder is a major component of the organic carrier; the binder selected in the present invention gives the conductive silver paste a viscosity suitable for screen printing, facilitating screen printing.

[0009] Preferably, the surfactant used in this invention is one or a mixture of triethanolamine, lecithin, xylene, ethanol, and Span 85. The surfactant used in this invention reduces the surface tension at the interface between the organic carrier and the silver powder, allowing the organic carrier to fully wet the surface of the silver powder. The addition of the surfactant enables the silver powder particles to be uniformly and stably dispersed in the organic carrier without affecting other properties of the conductive silver paste.

[0010] Preferably, the coupling agent of the present invention is one or a mixture of silane coupling agents KH570, KH560, and KH550. The coupling agent selected in the present invention can improve the adhesion strength between the organic carrier and the silver powder and glass powder particles, forming silver grid lines with high silver powder density, and enabling the silver powder particles and silicon wafer to make full contact.

[0011] The thixotropic agent described in this invention can increase the thixotropic properties of conductive silver paste, resulting in finer and taller silver grid lines for the printed front electrode. This ensures a smaller light-shielding area while maintaining a larger cross-sectional area of ​​the grid lines, thus minimizing their line resistance. The thixotropic agent of this invention forms a weak three-dimensional network structure in a static state. When the electronic paste is extruded through screen printing, it exhibits low viscosity under high shear stress, but should exhibit high viscosity after settling to ensure good morphology and aspect ratio after printing.

[0012] The method for preparing conductive silver paste for solar cells according to the present invention specifically includes the following steps: (1) The specific preparation process of the organic carrier is as follows: weigh the following by mass percentage: organic solvent, binder, thixotropic agent, surfactant and coupling agent; wherein, the organic solvent is 78~88wt%, the binder is 8~12wt%, the thixotropic agent is 0.5~3wt%, the surfactant is 0.5~4wt% and the coupling agent is 1~4wt%; first, place the organic solvent in a beaker, heat it at 70℃ with a magnetic stirrer, and then add the weighed binder, thixotropic agent, surfactant and coupling agent in sequence, and stir magnetically at 450rpm to obtain the organic carrier.

[0013] (2) Preparation of conductive silver paste: Weigh 82-90 wt% silver powder, 2-5 wt% glass powder and 8-13 wt% organic carrier, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5 μm to obtain conductive silver paste.

[0014] The method for preparing the conductive silver paste described in this invention is a conventional method. Glass powder and silver powder are weighed in the required proportions and mixed thoroughly and evenly. This is because glass powder and silver powder are easier to grind evenly in powder form, and soft agglomerated lumps of silver powder can be well dispersed. Next, an organic carrier is weighed and slowly added to a mortar and mixed evenly with the solid powder until it completely wets the solid powder to obtain the conductive silver paste. The preparation of the silver powder and glass powder can refer to existing conductive silver paste preparation processes and will not be specifically described here.

[0015] During use, the conductive silver paste can be rolled to obtain a suitable fineness according to actual needs. The specific method is as follows: the prepared conductive silver paste is placed in a three-roll mill and rolled 3 to 5 times until the front gap of the three-roll mill is adjusted to 3μm and the rear gap is adjusted to 5μm; finally, a portion of the rolled paste is taken out for fineness testing. When the fineness is less than 8μm, a conductive silver paste with suitable fineness is obtained.

[0016] Another objective of this invention is to provide the application of conductive silver paste for solar cells in the screen printing fabrication of solar cells, specifically screen printing followed by high-temperature sintering. During the screen printing process, the conductive silver paste exhibits good stability and consistency throughout the entire production process, not only improving the viscosity of the organic carrier but also ensuring the stability of the printed paste on the crystalline silicon solar cell substrate and improving the aspect ratio.

[0017] The beneficial effects of this invention are: The conductive silver paste of this invention utilizes different thixotropic agents and binders to achieve high viscosity in a static state. However, when shear force is applied during screen printing, this structure is disrupted, resulting in a significant decrease in viscosity. Once the shear force stops, the structure gradually returns to high viscosity. The polyamide wax is produced by polymerizing and dehydrating fatty acid amides and organic amines. It contains non-polar aliphatic hydrocarbons and polar amide groups, exhibiting reversible thixotropy and the advantage of being able to withstand repeated shear cycles.

[0018] The conductive silver paste of this invention improves the rheological properties and aspect ratio of the silver paste by modifying the thixotropic agent in the organic carrier, thus solving the problems of poor silver paste stability and reduced printing yield, and improving photoelectric conversion efficiency to meet the requirements of screen printing. The conductive silver paste of this invention can effectively improve the rheological properties of each component of the silver paste, improve the viscosity and rheological properties of the silver paste, and solve the problems of uneven silver paste leveling and broken lines and grids in printing. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the silver grid lines of the conductive silver paste prepared in Example 1 of the present invention under a scanning electron microscope (SEM).

[0020] Figure 2 This is a schematic diagram of the conductive silver paste prepared in Comparative Example 2 of the present invention under a scanning electron microscope (SEM) showing the silver grid lines.

[0021] Figure 3 This is a schematic diagram of the conductive silver paste prepared in Comparative Example 3 of the present invention under a scanning electron microscope (SEM) showing the silver grid lines. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0023] Example 1 The conductive silver paste for solar cells described in this embodiment includes silver powder, glass powder, and an organic carrier. The organic carrier is composed of an organic solvent, a binder, a thixotropic agent, a surfactant, and a coupling agent. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 8 wt% of ethyl cellulose STD10, 86 wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 1 wt% of Span 85, 2 wt% of lecithin, 2 wt% of silane coupling agent, 0.5 wt% of polyamide wax and 0.5 wt% of polyamide wax hydrogenated castor oil. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, thixotropic agent, surfactant and coupling agent in sequence. Stir magnetically at 450 rpm to obtain the organic carrier, denoted as Oc1.

[0024] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 88wt% silver powder, 3wt% glass powder and 9wt% organic carrier Oc1, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0025] Example 2 The conductive silver paste for solar cells described in this embodiment includes silver powder, glass powder, and an organic carrier. The organic carrier is composed of an organic solvent, a binder, a thixotropic agent, a surfactant, and a coupling agent. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 10 wt% of ethyl cellulose STD10, 88 wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 0.25 wt% of Span 85, 0.25 wt% of lecithin, 1 wt% of silane coupling agent, 0.25 wt% of polyamide wax and 0.25 wt% of polyamide wax hydrogenated castor oil. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, thixotropic agent, surfactant and coupling agent in sequence. Stir magnetically at 450 rpm to obtain the organic carrier, denoted as Oc2.

[0026] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 90wt% silver powder, 2wt% glass powder and 8wt% organic carrier Oc2, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0027] Example 3 The conductive silver paste for solar cells described in this embodiment includes silver powder, glass powder, and an organic carrier. The organic carrier is composed of an organic solvent, a binder, a thixotropic agent, a surfactant, and a coupling agent. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 12wt% of ethyl cellulose STD10, 78wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 2wt% of Span 85, 2wt% of lecithin, 4wt% of silane coupling agent, 1wt% of polyamide wax, and 1wt% of hydrogenated castor oil. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, thixotropic agent, surfactant, and coupling agent in sequence. After magnetic stirring at 450 rpm, the organic carrier is obtained and denoted as Oc3.

[0028] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 82wt% silver powder, 5wt% glass powder and 13wt% organic carrier Oc3, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0029] Comparative Example 1 This comparative example is the same as Example 1, except that no thixotropic agent was added and the silane coupling agent was 3 wt%. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 10 wt% of ethyl cellulose STD10, 83 wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 1 wt% of Span 85, 1 wt% of lecithin, 3 wt% of silane coupling agent, and 2 wt% of polyamide wax. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, thixotropic agent, surfactant, and coupling agent in sequence. After magnetic stirring at 450 rpm, the organic carrier is obtained and denoted as Oc4.

[0030] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 80wt% silver powder, 5wt% glass powder and 15wt% organic carrier Oc4, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0031] Comparative Example 2 This comparative example is the same as Example 1, except that the thixotropic agent is hydrogenated castor oil. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 12wt% of ethyl cellulose STD20, 78wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 2wt% of Span 85, 2wt% of lecithin, 4wt% of silane coupling agent, and 2wt% of hydrogenated castor oil. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, thixotropic agent, surfactant, and coupling agent in sequence. After magnetic stirring at 450 rpm, the organic carrier is obtained and denoted as Oc5.

[0032] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 85wt% silver powder, 4wt% glass powder and 11wt% organic carrier Oc5, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0033] Comparative Example 3 This comparative example is the same as Example 1, except that the thixotropic agent is polyamide wax. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 12wt% of ethyl cellulose STD20, 78wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 2wt% of Span 85, 2wt% of lecithin, 3wt% of silane coupling agent, 1.5wt% of polyamide wax, and 1.5wt% of hydrogenated castor oil. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70℃. Then, add the weighed binder, thixotropic agent, surfactant, and coupling agent in sequence. Stir magnetically at 450rpm to obtain the organic carrier, denoted as Oc6.

[0034] (2) The composition of the conductive silver paste is shown in Table 2. Weigh 90wt% silver powder, 2wt% glass powder and 8wt% organic carrier Oc6, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0035] Comparative Example 4 This comparative example is the same as Example 1, except that the thixotropic agent is hydrogenated castor oil and polyamide wax, and the mass ratio of hydrogenated castor oil to polyamide wax is 1:3. The specific steps are as follows: (1) The composition of the organic carrier is shown in Table 1. The specific preparation process is as follows: Weigh 10 wt% of ethyl cellulose STD10, 84 wt% of organic solvent (the mass ratio of alcohol ester dodecyl: butyl carbitol: butyl carbitol acetate: dibutyl phthalate is 3:4:2:1), 1 wt% of Span 85, 1 wt% of lecithin, and 4 wt% of silane coupling agent. First, place the prepared organic solvent in a beaker, place it in a magnetic stirrer and heat it at 70°C. Then, add the weighed binder, surfactant, and coupling agent in sequence. After magnetic stirring at 450 rpm, the organic carrier is obtained and denoted as Oc7.

[0036] (2) The composition of the conductive silver paste is shown in Table 2. The specific preparation process is as follows: weigh 90wt% silver powder, 2wt% glass powder and 8wt% organic carrier Oc7, mix them evenly in an agate mortar, and grind them with a three-roll mill until the fineness is less than 5μm to obtain conductive silver paste.

[0037] The specific formulations of the organic carriers in Examples 1-3 and Comparative Examples 1-4 of the present invention are shown in Table 1.

[0038] Table 1 The specific formulations of the conductive silver pastes in Examples 1-3 and Comparative Examples 1-4 of this invention are shown in Table 2: Table 2 The 3ITT test of conductive silver paste is as follows: Figure 3 As shown in Table 3, the response rates at 60s, 90s, 120s, and 150s were calculated according to the response rate formula. Table 3 The conductive silver pastes prepared in Examples 1-3 were tested using a rheometer and scanning electron microscope. An organic solvent was prepared by mixing dodecyl alcohol ester, butyl carbitol, butyl carbitol acetate, and dibutyl phthalate in a mass ratio of 3:4:2:1. Organic carriers with different thixotropic agent ratios were prepared using this organic solvent. A three-stage variable shear test was conducted to compare the recovery rates of different thixotropic agent contents within a specified time. When the thixotropic agent in the organic carrier was hydrogenated castor oil and polyamide wax in a mass ratio of 1:1, the prepared conductive silver paste exhibited excellent thixotropic properties. SEM examination of the silver grid lines showed that the silver grid lines in Example 1 exhibited superior printing rheological properties. Figure 1 The viscosity is moderate, and the continuity is good, without any broken lines or inconsistent shades. Comparative Example 1, without the addition of a thixotropic agent, produced a conductive silver paste that was too thin. Comparative Examples 2-4, which did not use the thixotropic agent ratio described in this invention, resulted in "sagging" or edge diffusion during printing. The conductive silver pastes prepared in Comparative Examples 2 and 3 were analyzed using SEM scanning electron microscopy. Figure 2 and Figure 3 The silver paste lines in the image are irregular, with obvious rough edges and undulations. The line widths are inconsistent, and there is silver paste accumulation or absence in some areas. The conductive silver paste has poor rheological properties and stability, which is not conducive to screen printing.

[0039] In summary, conductive silver paste prepared from thixotropic paste exhibits excellent rheological properties. Therefore, selecting appropriate thixotropic agents and organic carriers plays a crucial role in the rheological properties of silver grid lines printed from conductive silver paste.

[0040] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A conductive silver paste for solar cells, characterized in that: The conductive silver paste comprises silver powder, glass powder, and an organic carrier, with the following mass percentage content: silver powder 82-90%, glass powder 2-5%, and organic carrier 8-13%; the organic carrier is composed of organic solvent, binder, thixotropic agent, surfactant, and coupling agent; the thixotropic agent is a mixture of hydrogenated castor oil and polyamide wax in a mass ratio of 1:

1.

2. The conductive silver paste for solar cells according to claim 1, characterized in that: The mass percentage content of each component of the organic carrier is as follows: organic solvent 78~88wt%, binder 8~12wt%, thixotropic agent 0.5~3wt%, surfactant 0.5~4wt%, and coupling agent 1~4wt%.

3. The conductive silver paste for solar cells according to claim 1, characterized in that: The organic solvent is one or more of the following: butyl carbitol, butyl carbitol acetate, tributyl citrate, terpineol, dibutyl phthalate, glycerol, lavender oil, anise oil, tributyl citrate, isopropanol, and dimethyl succinate.

4. The conductive silver paste for solar cells according to claim 1, characterized in that: The binder is one or a mixture of ethyl cellulose, polymethyl styrene, polyvinyl alcohol, nitrocellulose, and phenolic resin.

5. The conductive silver paste for solar cells according to claim 1, characterized in that: The surfactant is one or a mixture of triethanolamine, lecithin, xylene, ethanol, and Span 85.

6. The conductive silver paste for solar cells according to claim 1, characterized in that: The coupling agent is one or a mixture of silane coupling agents KH570, KH560, and KH550.

7. The application of the conductive silver paste for solar cells according to claims 1 to 6 in the screen printing preparation of solar cells.

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