Within the last years the effect of potential-induced degradation (PID) has gained a lot of interest because of its strong impact on the reliability of PV installations.Solutions to overcome PID have been found on system,module and cell level.Detailed investigations on solar module and cell level revealed a number of factors leading to PID.Namely the field-induced migration of Na ions and their accumulation at the SiN/Si interface plays a crucial role.In this contribution the physical mechanisms of PID shunt formation within the solar cell will be investigated on a fundamental basis.A novel PID test setup is applied to mono and multi-crystalline Si solar cells,which allows subsequent high-resolution investigations at PID defect sites.Individual PID shunts are studied by scanning electron microscopy (SEM) with electron beam induced current (EBIC) measurement.A local accumulation of Na at shunt sites is analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS).Transmission electron microscopy (TEM) including elemental analysis reveals μm-sized crystal defects (stacking faults) at the p-n junction,which are decorated by Na.Based on these findings a model of PID shunting in solar cells is introduced relying on quasi-metallic,Na decorated Si stacking faults.Furthermore,the implications for PID degradation mechanisms and PID resistance of solar modules will be discussed.