Microstructure and electrical mechanism of Sn–xAg–Cu PV-ribbon for solar cells

• This study uses SAC105 and SAC305 alloys to estimate the feasible application for PV ribbon.
• With a greater Ag content gradient between solder and Ag electrode, the consumption rate of Ag electrode is faster.
• The growth mechanism of IMC is found in each interface, which is related to thermal diffusion behavior.
• Applying high-Ag-content solders to PV ribbon can avoid the Ag layer to be over-consuming.

The microstructure, fusion current, and series resistance of photovoltaic (PV) ribbon containing SAC105 and SAC305 alloys are investigated. After reflow, the interfacial microstructures of solder/Cu and solder/Ag were observed and an electrical current test was conducted to clarify the effect of the interface transition on the resistance of the structure. The area of the eutectic region of SAC305 solder is larger than that of SAC105 solder, and thus the resistance of the former is lower than latter. However, SAC105 solder has a higher fusion current density than that of SAC305 solder. After reflow, the thickness of the residual Ag film of SAC105 is smaller than that of the residual Ag film of SAC305 because of the Ag concentration gradient between SAC105 and the Ag electrode is larger. Therefore, the series resistance of a SAC105 PV ribbon structure is larger than that of a SAC305 PV ribbon structure. After the electrical current test, part of the Ag film formed Ag3Sn compounds. The Ag3Sn layer thickness increased with increasing current duration. The Ag3Sn thickness of the SAC105 PV ribbon structure is nearly equal to that of the SAC305 PV ribbon structure, but the residual Ag film (electrode) of SAC105 is smaller, which leads to higher resistance. Applying high-Ag-content solders to PV ribbon can avoid the Ag layer to be over-consuming, which can decrease series resistance.

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