  In recent years, metal halide perovskites have generated tremendous interest
for optoelectronic applications and their underlying fundamental properties.
Due to the large electron-phonon coupling characteristic of soft lattices,
self-trapping phenomena are expected to dominate hybrid perovskite
photoexcitation dynamics. Yet, while the photogeneration of small polarons was
proven in low dimensional perovskites, the nature of polaron excitations in
technologically relevant 3D perovskites, and their influence on charge carrier
transport, remain elusive. In this study, we used a combination of first
principle calculations and advanced spectroscopy techniques spanning the entire
optical frequency range to pin down polaron features in 3D metal halide
perovskites. Mid-infrared photoinduced absorption shows the photogeneration of
states associated to low energy intragap electronic transitions with lifetime
up to the ms time scale, and vibrational mode renormalization in both frequency
and amplitude. Density functional theory supports the assignment of the
spectroscopic features to large polarons leading to new intra gap transitions,
hardening of phonon mode frequency, and renormalization of the oscillator
strength. Theory provides quantitative estimates of the charge carrier masses
and mobilities increase upon polaron formation, confirming experimental
results. Overall, this work contributes to complete the scenario of elementary
photoexcitations in metal halide perovskites and highlights the importance of
polaronic transport in perovskite-based optoelectronic devices.
