General trends for the effect of ionic substitution on the band gap of MAPbX3

Perovskites are diverse materials to explore for advanced functional applications, but experimental or computational characterization of individual perovskite compositions is inefficient considering nearly endless possible combinations of the constituent ions. We analyze the band edges of a semiconducting perovskite by constructing electronic wavefunctions based on orbital symmetries, and then extract such information as electron wavevectors, band-edge transition, and chemical bonding. Using MAPbX3 (MA = methylammonium; X = Br, I), CsCdBr3, CsCaBr3, and TMASn(N3)3 (TMA = tetramethylammonium) as prototypical perovskites, we propose a set of trends on whether ionic substitution changes MAPbX3 from a direct band gap to an indirect one. Compositions containing an s2 metal cation and a (pseudo)halide are found to exhibit a direct band gap as MAPbX3 does. The broad applicability of these trends, verified by an extensive range of perovskite compositions, indicates that pseudohalide perovskites should be explored for novel functional materials, and that substitution of Pb2+ in MAPbI3 by non-s2 metal cations will probably deteriorate the optoelectronic properties of MAPbI3.