Design rules for optimizing quaternary mixed-metal chalcohalides

Quaternary mixed-metal M(II)2M(III)Ch2X3 chalcohalides are an emerging material class for photovoltaic absorbers that combines the beneficial optoelectronic properties of lead-based halide perovskites with the stability of metal chalcogenides. Inspired by the recent discovery of lead-free mixed-metal chalcohalides (MMCHs) materials, we utilized a combination of density functional theory and machine learning to determine compositional trends and chemical design rules in the lead-free and lead-based materials spaces. We explored a total of 54 M(II)2M(III)Ch2X3 materials with M(II) = Sn, Pb; M(III) = In, Sb, Bi; Ch = S, Se, Te; and X = Cl, Br, I per phase (Cmcm, Cmc21, and P21/c). The P21/c phase is the equilibrium phase at low temperatures, followed by Cmc21 and Cmcm. The fundamental band gaps in Cmcm and Cmc21 are smaller than those in P21/c, but direct band gaps are more common in Cmcm and Cmc21. The effective electron masses in P21/c are significantly larger than in Cmcm and Cmc21, while the effective hole masses are nearly the same across all three phases. Using random forest regression, we found that the two electron acceptor sites [Ch and X] are crucial in shaping the properties of MMCH compounds. Furthermore, the electron donor sites [M(II) and M(III)] can be used to fine-Tune the material properties to desired applications. These design rules enable precise tailoring of MMCH compounds for a variety of applications.