Anharmonicity and Octahedral Tilting in Hybrid Vacancy-Ordered Double Perovskites

Abstract

The advantageous performance of hybrid organic–inorganic perovskite halide semiconductors in optoelectronic applications motivates studies of their fundamental crystal chemistry. In particular, recent studies have sought to understand how dipolar, dynamic, and organic cations such as methylammonium (CH3NH3+) and formamidinium (CH(NH2)2+) affect physical properties such as light absorption and charge transport. To probe the influence of organic–inorganic coupling on charge transport, we prepared the series of vacancy-ordered double perovskite derivatives A2SnI6, where A = Cs+, CH3NH3+, and CH(NH2)2+. Despite nearly identical cubic structures by powder X-ray diffraction, replacement of Cs+ with CH3NH3+ or CH(NH2)2+ reduces conductivity through a reduction in both carrier concentration and carrier mobility. We attribute the trends in electronic behavior to anharmonic lattice dynamics from the formation of hydrogen bonds that yield coupled organic–inorganic dynamics. This anharmonicity manifests as asymmetry of the interoctahedral I–I pair correlations in the X-ray pair distribution function of the hybrid compounds, which can be modeled by large atomistic ensembles with random rotations of rigid [SnI6] octahedral units. The presence of soft, anharmonic lattice dynamics holds implications for electron–phonon interactions, as supported by calculation of electron–phonon coupling strength that indicates the formation of more tightly bound polarons and reduced electron mobilities with increasing cation size. By exploiting the relatively decoupled nature of the octahedral units in these defect-ordered perovskite variants, we interrogated the impact of organic–inorganic coupling and lattice anharmonicity on the charge transport behavior of hybrid perovskite halide semiconductors.