Correlated nanoimaging of structure and dynamics of cation-polaron coupling in hybrid perovskites

Hybrid organic-inorganic perovskites exhibit high photovoltaic performance and other novel photonic functions. While polaron formation is believed to facilitate efficient carrier transport, the elementary processes of the underlying electron-lattice coupling are yet poorly understood because of the multi-scale chemical and structural heterogeneities. Here, we resolve in the combined ground and excited state spatio-spectral ultrafast nanoimaging how structural characteristics are related to both molecular cation and polaron dynamics. We observe nano-scale spatial variations of the formamidinium (FA) cation transient vibrational blue shifts used as a local probe of the non-local polaron-cation coupling. From the correlation with nano-movies of the polaron dynamics, we then infer how a softer more polarizable lattice supports stabile polarons and longer-lived residual carriers. This, together with a relative intra-grain homogeneity in contrast to high inter-grain heterogeneity thus suggests pathways for improved synthesis and device engineering, and that perovskite photonics performance is still far from any fundamental limits.

Methods
This dataset was obtained by using a heterodyne pump-probe infrared scanning-scattering near field optical microscopy technique (HPP IR s-SNOM) as established in prior works [Nishida J. et al., Nature Communications 13, 1083 (2022)] to resolve on nanometer spatial and ps temporal scales interactions between carriers and lattice vibrations in perovskite films, providing a way to optimize materials synthesis efforts by controlling material properties that lead to enhanced photovoltaic performance.