Hybrid perovskite halide semiconductors have taken the solid-state chemistry and solar energy communities by storm, owing to their unique optoelectronic properties that contribute to photovoltaic efficiencies as high as 23%. However, the excellent photovoltaic performance of perovskite halide semiconductors raises questions regarding the fundamental crystal-chemical relationships that contribute to their advantageous properties. Hybrid perovskite halides (ABX3) are comprised of an inorganic framework of corner-sharing BX6 octahedra surrounding a small organic cation such as methylammonium (CH3NH3+) or formamidinium (CH(NH2)2+) in the A-site. In contrast to diamond-like semiconductors, hybrid perovskite halides are topologically under-constrained, and thus exhibit a complex potential energy landscape due to coupled, yet frustrated, interactions between the organic and inorganic sublattices. While dynamic organic species have been shown to couple to the inorganic framework, the extent to which these ions may direct structural and electronic behavior remains an active area of research. Vacancy-ordered double perovskites present an ideal structural framework to study organic-inorganic coupling on the structure and optoelectronic properties of perovskite halide semiconductors. Vacancy-ordered double perovskites are a family of perovskite derivatives of the general formula A2BX6 comprised of a face-centered lattice of isolated BX6 octahedral units bridged by A-site cations. By removing interoctahedral connectivity yet retaining the close-packed halide sublattice, vacancy-ordered double perovskites exhibit stronger organic-inorganic coupling within a close-packed framework that promotes mobile charge carriers. Our work illustrates the impact of the A-site cation upon the structural and optoelectronic behavior of vacancy-ordered double perovskite semiconductors. Through preparation of the A2SnI6 series (A = Rb+, Cs+, CH3NH3+, CH(NH2)2+), we find that the smaller Rb+ cation promotes cooperative octahedral tilting, while the organic CH3NH3+ and CH(NH2)2+ cations drive unique structural topologies brought about by competing dipolar interactions and organic-inorganic coupling via hydrogen bonding. The absence of interoctahedral connectivity in vacancy-ordered double perovskites provides a unique framework to study the influence of organic-inorganic coupling on the properties of perovskite halide semiconductors.

Competing interactions: Octahedral tilting, organic-inorganic coupling, and charge transport in vacancy-ordered double perovskites

2019 Fall National Meeting

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