Solder Bond Degradation of Fielded PV Modules: Correlation between Performance, Series Resistance and Electroluminescence Imaging

One of the most dominant degradation modes affecting the degradation rate and lifetime of field deployed modules is the degradation of interconnect-metallization system. The effect of solder bond degradation on the performance of three 10-year-old modules installed at a Florida site has been quantified through the cell series resistance (R
_s
) acquired from dark current-voltage (I-V) curves, which has been correlated with performance loss and electroluminescence (EL) intensity. The regions of poor solder joints are identified in EL image through high luminescence spots at interconnect ribbons because of current crowding. Each individual cell of the tested modules was accessed by cutting off a small segment of backsheet and soldering at cell interconnect. A spatial distribution of R
_s
in the modules is mapped, which demonstrated a good correlation with fill factor and output power as identified by two parallel trendlines. This implies that the performance loss of these fielded modules is significantly dictated by the solder bond degradation. Further, the ultraviolet fluorescence images showed an identical encapsulant browning pattern over the cells, indicating considerable current loss in all cells of the module.