Nanoscopic studies of 2D-extended defects in silicon that cause shunting of Si-solar cells

Potential-induced degradation (PID) can cause massive power losses in photovoltaic power plants with serially connected modules when high electrical potential between solar cells and the module front glass surface leads to shunting of Si solar cells. The shunting is reversible to a large extend through elevated temperatures and/or by application of reversed voltages. It was previously shown that PID-related shunting (PID-s) of the p-n junction occurs due to Na-decorated stacking faults in {111} planes. Defects at former PID-shunt positions that have been thermally ‘healed' (recovered) do not show a Na decoration anymore.

In this contribution we investigate stacking faults with Na decoration and with vanished Na decoration after thermal recovery. HAADF STEM imaging is used for principal investigation of the atomic structure of PID-s defects. The Na decorated stacking fault appears as a dark region in the HAADF STEM image. By contrast, the atomic structure of the recovered stacking fault can be clearly imaged. The orientation of Si dumbbells indicates an extrinsic stacking fault that remains after recovery by Na out-diffusion. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)