Upward band gap bowing and negative mixing enthalpy in multi-component cubic halide perovskite alloys

Semiconductor compounds are often alloyed to obtain target physical properties that are absent in the individual components. Conventional tetrahedral semiconductors generally have lower alloy gaps than the composition average gap of the constituents (``downward bowing''). We designed via DFT multi-component halide perovskite alloys that have significant upward bowing. Such alloys have a rather low mixing enthalpy, suggesting stability towards phase separation. The enabling idea is to mix perovskites with B atoms that have low lying s-orbitals in the valence band, with a compound that has IB atoms (e.g., Cd) with s-orbitals in the conduction band. The ensuing s-s repulsion opens the alloy gap with respect to the constituents' gap.