Effect of atomic configuration on band gap behaviour in CH3NH3SnxPb1-xI3 perovskites

The atomic structure and electronic properties of Sn/Pb mixed organometal halide perovskites were investigated by using first-principles method. The structure-dependent band gap behaviour of tetragonal and pseudocubic MASn_xPb_1-xI₃ (0 ⩽ x ⩽ 1) was emphasized. The configurations with two or more adjacent Sn octahedra in the same layer inducing less tilting of neighbouring octahedra have the lower formation energy. For tetragonal or pseudocubic MASn_0.5Pb_0.5I₃, the configuration of Sn octahedra layer alternating with Pb octahedra layer leads to elongated octahedra along the stacking direction, which stimulates the remarkable band splitting due to anomalous hybridization of Pb or Sn orbitals and I orbitals. This arrangement mode of Sn octahedra produces the narrowest band gap of Sn/Pb binary perovskite, whereas the separated and dispersed Sn octahedra significantly broaden the band gap. The results are helpful to understand the abnormal band gap behaviour in Sn/Pb binary perovskite experiments.