Publication date: 14th January 2021
Lead halide perovskites have attracted great interest due to their possible application in solar cells, providing excellent power conversion efficiencies (PCE) of currently up to 25.5 % combined with simple and low-cost production procedures. Recent developments in the field of high efficient perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI3, while preserving its excellent optoelectronic properties. While compositional engineering of e.g. MA or Br mixed into FAPbI3 results in the desired effects, detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA) is still limited, but crucial for their further development. Here, we elucidate the microscopic distribution of MA and FA in mixed perovskites MA1-xFAxPbI3, and FA0.85MA0.15PbI2.55Br0.45, by combining high-resolution double-quantum 1H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first principles molecular dynamics (MD) simulations. We show that on the scale of a few nearest-neighbor coordinations, partial MA and FA clustering takes place over the whole MA/FA compositional range. While similar MA and FA ordering as in the MA1˗xFAxPbI3 systems was observed for FA0.85MA0.15PbI2.55Br0.45, the average cation-cation interaction strength increased significantly indicating a restriction or partial immobilization of the cation dynamics upon Br- incorporation. Our results shed light on the heterogeneities in composition due to the cation microstructure of mixed halide perovskites, helping to further exploit their full optoelectronic potential.
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