Enhancing Visible Light Absorption for Ferroelectric Sn₂P₂S₆ by Se Anion Substitution

Ferroelectric (FE) semiconductors that simultaneously exhibit spectrally suitable band gaps and room temperature stable FE polarizations necessary for separation of photo-excited carriers have been extensively investigated for FE-photovoltaic (PV) applications. Di tin hexathiohypodiphosphate, Sn₂P₂S₆, is one of the rare semiconducting FE materials. However, owing to the indirect nature of its energy band gap and relatively large direct band gap, FE Sn₂P₂S₆ absorbs only a small portion of the visible light spectrum. In the current work, we propose substitution of S by Se anion as an experimental feasible route to enhance the visible light absorption for FE Sn₂P₂S₆. Using first-principles calculations, we demonstrate that the effective "band gap engineering" with respect to Se substitution concentration can be achieved in Sn₂P₂S_6(1-x)Se_6x solid solutions. Especially, Sn₂P₂S_4.5Se_1.5 compound is predicted to be an indirect band gap semiconductor, with a direct band gap at Γ point even lower than that of BiFeO₃, and meanwhile exhibits a stable ferroelectricity comparable with single-phase Sn₂P₂S₆. As a result, the improved visible light absorption can be achieved in Sn₂P₂S_4.5Se_1.5 compound, making Se-doped FE Sn₂P₂S₆ more suitable for PV applications.