Mechanisms of plasma damage caused by sputtering of transparent conductive oxide (TCO) layers in silicon heterojunction (SHJ) solar cells have been investigated. It is shown that a buffer layer at the amorphous/crystalline silicon (a-Si/c-Si) interface can play an essential role in mitigating the sputter damage. More than 9%_abs. loss in the conversion efficiency is observed for rear emitter SHJ cells with nanocrystalline silicon n-layer when the underlying buffer layer changes from amorphous silicon carbide to amorphous silicon. It is revealed that the anomalous efficiency loss is mostly related to breaking Si-H bonds by NUV photons at the a-Si/c-Si interface during the TCO sputtering. Illuminated annealing of these cells at elevated temperature using a distributed light source based on light emitting diodes (LEDs) recovers the anomalous efficiency loss by more than 7%_abs. Other possible mechanisms of sputter damage and mitigation strategies are also discussed. Keywords: magnetron sputtering, transparent conductive oxide, a-Si/c-Si interface, surface passivation, conversion efficiency.