Antagonism between Spin-Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH3NH3Sn1-xPbxI3

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, Eg, 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 (Eg = 1.3 eV). A remarkable way to improve further comes from the CH3NH3Sn1-xPbxI3 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 (Eg ≈ 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 CH3NH3Sn1-xPbxI3. 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.