The hybrid organometallic halide perovskites have recently attracted much attention because of their large and growing photovoltaic power conversion efficiencies. However, the underlying mechanism for their high performance remains poorly understood, and many aspects of their structural properties remain under debate. In particular, because of the presence of disorder, the orientations of the molecular dipoles and the nature of their interactions with the surrounding inorganic lattice are still uncertain. To address these issues, we have performed an extensive density functional theory (DFT) study of the structural energy landscape of the hybrid perovskites. We have generated an unbiased structure-energy database by randomizing the orientations of the molecular dipoles in methylammonium lead iodide. Analysis of this database shows that the low-energy structures are heavily dominated by a few hydrogen bonding patterns and inorganic lattice distortions. Using these motifs to progressively refine our database, we deduce a set of simple combinatorial rules to explain the structure of the hybrid perovskites. We explain the observed symmetries of the low temperature orthorhombic phase and the room temperature tetragonal phase. We address the question of disorder by performing Monte Carlo simulations at experimentally relevant length scales. Correlation analysis shows the coexistence of local domains and glassy behavior. Electronic structure calculations of the resulting disordered structures show that spin-orbit coupling is essential for understanding the long carrier lifetime in these materials.

Extensive first-principles study of the energy landscape of hybrid organometallic halide perovskites

251st National Meeting (2016)

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