Band Gap Variation and Structural Disorder in the Two-Dimensional Mixed-Halide Hybrid Lead Perovskites

The two-dimensional (2D) layered halide perovskites are emergent materials for solar devices. The ability to tune their band gaps would add considerable utility for potential applications. Here we report an experimental and computational study of how the optical band gaps of the linear alkyl mixed halide perovskites, (CnH2n+1NH3)2PbCl4(1–y)Br4y and (CnH2n+1NH3)2PbBr4(1–y)I4y (n = 4, 8), change with halide composition. Optical absorption measurements showed the mixed halide perovskites to be direct band gap semiconductors with the magnitude of the optical band gap exhibiting a linear shift to lower values with increasing content of the heavier halide. Disordered special quasi-random structures of the halide-substituted perovskites were considered for computing the optical band gap. Band structure calculations for these structures were able to capture the observed linear variation of the band gap with composition and with computed magnitudes comparable to experiment. The shift in band gap values with halide composition was shown to arise from the energy offsets of the ns and np orbitals of the two halide ions and the linear variation because the contribution of these orbitals to the conduction and valence band is directly proportional to the stoichiometric ratio of the two halides at that composition of the mixed halide perovskite.