Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free\nCs₃Bi₂I₉ Perovskite Single Crystal

Abstract

In\nthis report, we study the hot carrier cooling mechanism of the\nCs₃Bi₂I₉ single crystal by using\nfemtosecond transient reflectance (fs-TR) spectroscopy. We find an\nunusual slow hot carrier cooling associated with longitudinal optical\n(LO) and coherent longitudinal acoustic phonons (CLAPs) emission during\nthe deexcitation of the hot carriers. We posit the interplay between\nthe hot-carriers and the LO and CLA phonons in subpicosecond to subnanosecond\ntime scales, respectively, by analyzing the TR kinetics upon perturbation\nwith excess energy. Furthermore, we measured the CLAPs propagation\nvelocity in Cs₃Bi₂I_9, the crystal,\nranging from 1820 to 2000 ms^–1. The elastic constants\nand frequency of Brillouin oscillations were estimated as 20.08 GPa\nand 14.66 GHz, respectively. Our discovery delivers new physical insights\ninto how the hot carriers in Cs₃Bi₂I₉ single crystal are coupled with a crystal lattice that controls\nthe hot carrier dynamics.