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.