Aqueous stability of lead-free ASnI3 and AYbI3 perovskites from implicit-solvation DFT

Moisture and lead toxicity are the twin barriers to halide-perovskite photovoltaics. With iodine fixed, how much of the modeled water response is set by the B-site metal alone? Across six lead-free ABI 3 absorbers on one iodide sublattice—three tin iodides (MASnI 3 , CsSnI 3 , RbSnI 3 ) and three hypothetical ytterbium iodides (CsYbI 3 , RbYbI 3 , KYbI 3 )—we model that response with PBEsol DFT and VASPsol implicit solvation, benchmarked against MAPbI 3 , so that every difference reflects Sn 2+ versus Yb 2+ . Immersion barely shifts the optical edge (< 5%), and the metals part on two concordant descriptors: the bond-resolved solvation free energy of tin (|ΔG sol | = 0.86–0.91 eV) is 1.4–1.7× that of ytterbium (0.55–0.62 eV), screening of an intact lattice rather than a driving force for dissolution, and the B I stretch is ≈1% against ≈2.4%. Dielectric screening orders the families the same way (ε 0 = 22.1–24.5 vs ≈19). Against AI + BI2, all three tin iodides lie below their binary products (ΔEdecomp = −0.19 to − 0.10 eV per formula unit) and far below MAPbI 3 (−0.003 eV); CsSnI 3 is the deepest and the least distorted by water. All six lie inside the Goldschmidt window ( t = 0.83–0.92). Hybrid-functional gaps from a second code bracket the tin-iodide values without reproducing experiment, and place the ytterbium iodides at 4.2–4.5 eV: closed-shell and wide-gap, on a scale the two frameworks do not yet share. The covalent Sn 5s 2 –iodine bond is the most economical account of the concordance. Explicit water, the YbI 2 reference and spin–orbit-corrected gaps remain to be computed.