TaS₂ back contact improving oxide-converted Cu₂BaSnS₄ solar cells

Solar cells based on the wide band-gap Cu₂BaSnS₄ (CBTS) photoabsorber 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 oxide precursors, and we propose TaS₂ as an alternative back contact material to the commonly used Mo/MoS₂. The oxide precursor route does not require higher sulfurization temperatures than other more common fabrication routes, and it yields CBTS films with negligible Stokes shift between photoluminescence maximum and band gap energy, while at the same time avoiding sulfur contamination of vacuum systems. The high work-function metallic TaS₂ compound is selected as a prospective hole-selective contact, which could also 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.