Antagonism between Spin-Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH₃NH₃Sn_1-xPb_xI₃

Halide perovskite solar cells are a recent ground-breaking development achieving power conversion efficiencies exceeding 18%. This has become possible owing to the remarkable properties of the AMX₃ perovskites, which exhibit unique semiconducting properties. The most efficient solar cells utilize the CH₃NH₃PbI₃ perovskite whose band gap, E_g, is 1.55 eV. Even higher efficiencies are anticipated, however, if the band gap of the perovskite can be pushed deeper in the near-infrared region, as in the case of CH₃NH₃SnI₃ (E_g = 1.3 eV). A remarkable way to improve further comes from the CH₃NH₃Sn_1-xPb_xI₃ solid solution, which displays an anomalous trend in the evolution of the band gap with the compositions approaching x = 0.5 displaying lower band gaps (E_g ≈ 1.1 eV) than that of the lowest of the end member, CH₃NH₃SnI₃. Here we use first-principles calculations to show that the competition between the spin-orbit coupling (SOC) and the lattice distortion is responsible for the anomalous behavior of the band gap in CH₃NH₃Sn_1-xPb_xI₃. SOC causes a linear reduction as x increases, while the lattice distortion causes a nonlinear increase due to a composition-induced phase transition near x = 0.5. Our results suggest that electronic structure engineering can have a crucial role in optimizing the photovoltaic performance.