Cu₂ZnSnS₄ from oxide precursors grown by pulsed laser deposition for monolithic CZTS/Si tandem solar cells

Cu₂ZnSnS₄ (CZTS) is a promising earth-abundant and non-toxic material as the top cell absorber in silicon (Si)-based tandem solar cells. One of the challenges for enabling high-efficiency kesterite–Si tandem devices is to realize a barrier layer that can efficiently combine the two subcells while preventing Si degradation during high temperature sulfurization of CZTS, which can occur severely upon diffusion of Cu into Si. Here, we tackle this issue by introducing a barrier layer against Cu diffusion consisting of an ultrathin layer of Al between two TiN films in a TiN/Al/TiN configuration. Next, we compare tandem devices using CZTS top cells produced from oxide and sulfide precursors grown by pulsed laser deposition (PLD) and we show that the nature of the precursors can influence the chemical composition of the barrier layer after tandem cell processing. In particular, the presence of a high content of oxygen in the case of oxide precursors leads to a more pronounced oxidation of the barrier layer which can ultimately increase the optical transparency and the effectiveness of preventing diffusion of Cu atoms into Si. Indeed, we experimentally demonstrate that the CZTS/Si tandem solar cells based on oxide precursors exhibit a superior performance as compared to the sulfide counterpart. The two-terminal (2-T) CZTS–Si tandem devices based on sulfide precursors have reached an efficiency of 1.7%, while for the oxide route an efficiency of 4.8% was achieved (open circuit voltage of 1.03 V), which is the highest value reported for a CZTS–Si tandem cell up to date. Our studies provide useful insights that would assist further development of high-efficiency kesterite–silicon tandem solar cells.