Crystalline Si solar cells that exhibit potential-induced degradation of the shunting type (PID-s) are investigated on a microstructural level. Cell pieces with PID-shunts are imaged by SEM using the EBIC technique in order to investigate PID-s positions with high lateral resolution. ToF-SIMS depth profiles reveal Na accumulation localized at these shunt positions. Subsequently, cross-sectional FIB-lamellas of individual PID-shunts have been prepared. TEM is applied to a number of PID-s defects. TEM/EDX measurements reveal that stacking faults crossing the p-n junction are decorated with Na causing PID-s. These defects are further characterized by high resolution STEM methods down to the atomic scale. A model for the shunting mechanism in PID-s affected solar cells is developed. The results are discussed with respect to different shunting mechanisms. (C) 2013 Elsevier B.V. All rights reserved.