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摘要
This paper presents the degradation analysis of monocrystalline silicon modules (SM55, produced by Siemens Solar company in 1992) installed for 18 years in Shenzhen, China, in hot-humid climatic conditions, less than 500 m from the sea. From the measurement of the electrical performance, the average power degradation and the median power degradation of the modules were 24.62% and 24.38%, respectively, or 1.56% per year in average. The reduction of power output is mainly attributed to the decrease of the fill factor. Through electroluminescence imaging, the degradation of the fill factor was found to be due to an increase in series resistance of the modules caused by solder bond failure and the corrosion of the Ag electrodes. From material analysis, scanning electron microscope inspection and analysis by energy dispersive X-ray spectroscopy showed the presence of acetate at the surface of the cells. The observation of needles in the silver metallization is a hint that corrosion of the metal occurred over time, probably caused by acetic acid, which can be a product of hydrolysis within ethylene vinyl acetate. It was found that the chemical reaction rate of acetate exhibits strong temperature dependence, following a typical Arrhenius-type dependence with temperature, which can result over long-time exposure in the field in corrosion of Ag metal fingers with predominance in the center of the cells operating at a higher temperature. Electron microscope imaging of cross section of cells also showed the presence of cracks between the ribbons and the Ag fingers, which can be caused by the large mismatch in coefficient of thermal expansion between the Si wafers and the Cu ribbons. Over time, the formation of intermetallic compounds in the solder layer may have made the solder more brittle and more susceptible to cracks.
