Direct Evidence of Polaron Formation in Halide Perovskites via Carrier Effective Mass Measurements*

Carrier effective mass is a central parameter in solid-state physics. It is a measure of the strength of the coupling between a carrier and excitations arising from its surrounding medium, and features prominently in transport and optical calculations. Experimental techniques employed to determine it are steady-state ones, and so are unable to detect any change in the effective mass after a strong perturbation to the system, e.g. strong optical excitation. By combining time-resolved terahertz spectroscopy and transient absorption spectroscopy, on a mixed-cation mixed-halide perovskite thin film, we observed a large and long-lived photoinduced enhancement of the carrier effective mass, and from it deduced a two-fold increase of the carrier-phonon coupling constant, giving direct evidence of polaron formation. Our work demonstrated a new approach to track the strength and ultrafast lifetimes of photoinduced carrier-boson interactions down to picosecond timescales, that can be applied to a wide range of solid-state systems.