Ultrafast exciton-phonon coupled dynamics in lead-free perovskite-inspired materials studied by transient optical spectroscopy

In the search for novel materials that can beat the state-of-art silicon-based technology, perovskites have gained attention thanks to their ease of processability and wide tunability of the optoelectronic properties. In particular, lead-free perovskites are emerging due to their low toxicity and high environmental stability under ambient conditions. The performance of perovskites is dictated by the interaction of the photoexcited charges and the electron-hole pairs (excitons) with the soft, polar lattice. This interaction rules both the coherent excitation regime on the sub-picosecond time scale as well as the thermalization dynamics up to time scales relevant for optoelectronic applications. In this Perspective, we discuss the recent advances of transient optical spectroscopy in addressing charge/exciton-phonon coupling and the relevant ultrafast dynamics in a set of lead-free perovskite-inspired materials. We show that exciton-phonon coupling enable coherent modulation of the excitonic resonance on the phonon dephasing time scale. Polaron formation and self-trapping of excitons govern the nonequilibrium behavior on the picosecond up to several nanoseconds timescale. We highlight the key parameters that regulate the energetics and time scales of polaron formation, and discuss the opportunities that control of charge/exciton-phonon coupling enables for next-generation optoelectronic technologies.