Long-Lived Hot Carriers in Formamidinium Lead Iodide Nanocrystals

The efficient harvesting of hot carrier energy in semiconductors is typically inhibited by their ultrafast thermalization process. Recently, highly promising experiments reported on the slowdown of the intraband relaxation in hybrid metal halide perovskites. In this work, we report on the presence of long-lived hot carriers in weakly confined colloidal nanocrystals (NCs) of formamidinium lead iodide perovskite (FAPbI₃). The effect is apparent from the excitation-dependent lengthening of the rise time and broadening of the high-energy tail of the transient absorption bleaching signal, yielding a retardation of the carrier relaxation by 2 orders of magnitude compared to typical time scales in colloidal semiconductor NCs. Three distinct cooling stages are observed, occurring at sub-picosecond, ∼5 ps, and ∼40 ps time scales, which we attribute to scattering from LO-phonons, contribution from a hot phonon bottleneck effect and Auger heating, respectively. Thermalization appears also influenced by the FAPbI₃ NCs purity, with trapping at unreacted precursor impurities further reducing the carrier energy loss rate.