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

Quaternary mixed-metal chalcohalides (Sn2M(III)Ch(2)X(3)) 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)Ch(2)X(3) materials that are structurally and energetically stable within Cmcm, Cmc2(1), and P(2)1/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)Ch(2)X(3) 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, P2(1)/c is the thermodynamically preferred structure, followed by Cmc2(1). In Cmcm and Cmc2(1), mainly direct gaps occur, whereas indirect gaps occur in P2(1)/c. To open up the possibility of band gap tuning in the future, we identified 12 promising Sn2M(III)(1-a)M(III)'(a)Ch(2-b)Ch'bX(3-c)X(c)' alloys, which fulfill our requirements, and an additional 69 materials by combining direct and indirect band gap compounds.