Tin oxynitride-based ferroelectric semiconductors for solar energy conversion applications

Lead halide perovskites have emerged as a promising class of semiconductors; however, they suffer from issues related to lead toxicity and instability. We report results of a first-principles-based design of heavy-metal-based oxynitrides as alternatives to lead halide perovskites. We have used density functional theory calculations to search a vast composition space of ABO₂N and ABON₂ compounds, where B is a p-block cation and A is an alkaline, alkali-earth, rare-earth, or transition metal cation, and identify 10 new ABO₂N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with A³⁺SnO₂N stoichiometry (A = Y, Eu, La, In, and Sc) in the LuMnO₃-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 to 3.3 eV and a sizable electric polarization up to 17 μC/cm², which is predicted to be switchable by an external electric field through a nonpolar phase. With their unique combination of polarization, low carrier effective mass, and band gaps spanning the entire visible spectrum, we expect ASnO₂N ferroelectric semiconductors will find useful applications as photovoltaics and photocatalysts as well as for optoelectronics.