Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs₃Bi₂I₉ Perovskite Single Crystal

In this report, we study the hot carrier cooling mechanism of the Cs₃Bi₂I₉ single crystal by using femtosecond transient reflectance (fs-TR) spectroscopy. We find an unusual slow hot carrier cooling associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. We posit the interplay between the hot-carriers and the LO and CLA phonons in subpicosecond to subnanosecond time scales, respectively, by analyzing the TR kinetics upon perturbation with excess energy. Furthermore, we measured the CLAPs propagation velocity in Cs₃Bi₂I_9, the crystal, ranging from 1820 to 2000 ms^-1. The elastic constants and frequency of Brillouin oscillations were estimated as 20.08 GPa and 14.66 GHz, respectively. Our discovery delivers new physical insights into how the hot carriers in Cs₃Bi₂I₉ single crystal are coupled with a crystal lattice that controls the hot carrier dynamics.