Hybrid main-group halides that adopt the perovskite crystal structure have attracted immense research attention in recent years for their promise in photovoltaic and other optoelectronic applications. While technological progress has been rapid in devices based on CH3NH3PbI3 and HC(NH2)2PbI3, fundamental understanding of the origins of the startling properties observed in these systems lags behind. Further, the composition space of chemically- and structurally-related materials remains largely unexplored, and the relationships between composition, structure, and properties in this complex family of hybrid crystals are minimally understood. In pursuit of a more detailed picture of the structural chemistry of the main-group halide perovskites AMX3 (A = CH3NH3, HC(NH2)2; M = Sn, Pb; X = Br, I), we study the evolution of structure and disorder with temperature and composition via a range of x-ray and neutron total scattering experiments, dielectric spectroscopy, and thermodynamic measurements. Local deviations from long-range order have proven to be important determinants of functional behavior in many crystalline materials, including colossal magnetoresistance manganites, group IV-chalcogenide thermoelectrics, and relaxor ferroelectrics. Here we probe the role of local structure via pair distribution function analysis of x-ray and neutron scattering data, and by comparison to the heavily-studied isostructural oxide perovskites, elucidate the chemical and structural differences due to the size, valence, and polarizability of the heavy halide anions. Additionally, the organic molecular cations present in these hybrid compositions introduce significant dynamic disorder akin to that seen in many molecular crystals. The evolution of this disorder with temperature, its role in structural phase transitions, and its impact on dielectric and thermodynamic properties is explored. This detailed study of structure and disorder in these scientifically rich and technologically promising phases provides important new information that will aid in the search for the elusive high-performance, solution-processable, lead-free photovoltaic absorber.

Hybrid main-group halide perovskites: Local structure and disorder

251st National Meeting (2016)

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