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)′aCh2–bCh′bX3–cX′c alloys, which fulfill our requirements, and an additional 69 materials by combining direct and indirect band gap compounds.