Screening Mixed-Metal Sn₂M(III)Ch₂X₃ Chalcohalides for Photovoltaic Applications

Quaternary mixed-metal chalcohalides (Sn₂M(III)Ch₂X₃) are emerging as promising lead-free, perovskite-inspired photovoltaic absorbers. Motivated by recent developments of a first Sn₂SbS₂I₃-based device, we used density functional theory to identify lead-free Sn₂M(III)Ch₂X₃ materials that are structurally and energetically stable within Cmcm, Cmc2₁, and P2₁/c space groups and have a band gap in the range of 0.7-2.0 eV to cover outdoor and indoor photovoltaic applications. A total of 27 Sn₂M(III)Ch₂X₃ materials were studied, including Sb, Bi, and In for the M(III)-site, S, Se, and Te for the Ch-site, and Cl, Br, and I for the X-site. We identified 12 materials with a direct band gap that meet our requirements, namely, Sn₂InS₂Br₃, Sn₂InS₂I₃, Sn₂InSe₂Cl₃, Sn₂InSe₂Br₃, Sn₂InTe₂Br₃, Sn₂InTe₂Cl₃, Sn₂SbS₂I₃, Sn₂SbSe₂Cl₃, Sn₂SbSe₂I₃, Sn₂SbTe₂Cl₃, Sn₂BiS₂I₃, and Sn₂BiTe₂Cl₃. A database scan reveals that 9 of 12 are new compositions. For all 27 materials, P2₁/c is the thermodynamically preferred structure, followed by Cmc2₁. In Cmcm and Cmc2₁, mainly direct gaps occur, whereas indirect gaps occur in P2₁/c. To open up the possibility of band gap tuning in the future, we identified 12 promising Sn₂M(III)_1-aM(III)′_aCh_2-bCh′_bX_3-cX′_c alloys, which fulfill our requirements, and an additional 69 materials by combining direct and indirect band gap compounds.