Defect Tolerance of Mixed B-Site Organic–Inorganic Halide Perovskites

Further improvements in the photovoltaic performance of B-site alloyed organic–inorganic halide perovskites (OIHPs) will rely on accurate modeling of defect properties and passivation strategies. Herein, we report that B-site alloying results in defect behaviors distinct from those of pure OIHPs, a finding obtained by uniting first-principles calculations with experimental measurements. We identify from computational studies a defect-tolerant region spanning a Sn content of 30–70% in mixed Pb-Sn perovskites and experimentally observe notably longer carrier lifetimes in 50% Sn mixed perovskite films than at other Sn contents. We discuss a strategy of applying defect-tolerant 50% Pb-Sn perovskites in ideal-bandgap (1.3–1.4 eV) active layer materials which conventionally rely on 25–30% Sn compositions. The composition (FA0.75Cs0.25Pb0.5Sn0.5(I0.9Br0.1)3) achieves increased carrier lifetimes of >1 μs. This work reveals a general trend in defect tolerance for B-site alloying: a higher valence band maximum (lower conduction band minimum), along with strengthened ionic bonding, can potentially contribute to improved photovoltaic performance.