Corrosion-Induced AC Impedance Elevation in Front Electrodes of Crystalline Silicon Photovoltaic Cells Within Field-Aged Photovoltaic Modules

To identify the common degradation mechanism(s) between field-aged photovoltaic (PV) modules and PV cells/modules exposed to artificial corrosive stress, the ac impedance associated with the corrosion in the front electrodes was analyzed in the individual PV cells laminated within the PV modules, fielded for 21 years. By evaluating the ac impedance together with the PV performance for these PV cells, we found that the short-circuit current (I_sc) remarkably decreased, depending on the expression and elevation of the ac impedance relevant to the front-electrode corrosion. This I_sc-reduction can be explained by the formation of rectifying contacts to restrict the current conduction between the front electrodes and the associated silicon wafer. Since the I_sc-reduction was primarily observed in the PV cells with extensive loss of fill factor, we suggest that the power-loss is induced by the gap formation underneath the front electrodes, followed by the emergence of rectifying contacts, even in the PV cells/modules degraded in field. The ac impedance parameters of the PV cells/modules could be a valuable predictor of the service lifetime of PV modules, as our findings in this study mean the equivalency of the failure attained by extended damp-heat testing with that observed in the field.