Single-junction and Tandem Cu₂BaSnS₄ Solar Cells with a TaS₂ hole contact

Solar cells based on the wide band-gap Cu₂BaSnS₄ (CBTS) absorber have achieved open circuit voltages up to 1.1 V over a short development period, making CBTS an attractive material for tandem photovoltaic and photoelectrochemical cells. In this work, we explore an alternative CBTS growth route based on sulfurization of reactively sputtered oxide precursors, and we propose TaS₂ as an alternative back contact material. Compared to direct deposition of CBTS films from ceramic targets, reactive sputtering of oxide precursors can ultimately achieve a higher throughput and a lower cost, with the additional advantage that sulfur contamination of vacuum systems is avoided. The TaS₂ compound is selected as a prospective hole-selective contact due to its high work function and its metallic conductivity, which could prevent the losses associated with carrier transport across the semiconducting MoS₂ layer. By comparing CBTS solar cells with Mo and TaS₂ back contacts, the latter shows a significantly lower series resistance, resulting in a 10% relative efficiency improvement. Finally, we fabricate a proof-of-concept monolithic CBTS/Si tandem cell using a thin Ti(O,N) interlayer intended both as a diffusion barrier and as a recombination layer between the two subcells.