Crystallinity and optical properties of Cu₂FeSn₃S₈thin films prepared by pulsed laser deposition

The rapid development of tandem photovoltaics is propelled by advances in the halide perovskite top cells. However, their widespread industrial implementation is hindered by practical challenges, such as material scarcity, toxicity, and limited chemical and structural instability. In this context, certain poly-chalcogenide compounds have emerged as potential alternatives, comprising abundant, non-toxic elements and exhibiting overall higher stability. Here, we report the synthesis and characterization of quaternary chalcogenide Cu₂FeSn₃S₈thin films prepared by pulsed laser deposition on soda-lime glass substrates, followed by post-deposition annealing. X-ray diffraction (XRD) and Raman spectroscopy demonstrate that post-deposition annealing at 400 °C for 1 h in N₂atmosphere triggers conversion of as-deposited amorphous films into partially crystalline films described by thiospinel structure (Fd3¯mspace group) within a micron-size vicinity confirmed by Raman mapping. Notably, as confirmed by EDS compositional analysis and XRD Rietveld refinement, the cubic thiospinel phase remains stable within a range of Cu/Fe/Sn molar ratios, achieved by varying the target composition, accompanied by a corresponding shift in the lattice parameter a. The optoelectronic measurements revealed a finite carrier mobility, photosensitivity, and the optical bandgap within the range of 1.46-1.52 eV, along with enhanced optical absorption coefficient upon annealing. These results highlight Cu₂FeSn₃S₈as a promising, non-toxic, abundant, and efficient candidate for absorbers in solar cell applications.