Strong Coupling of Self-Trapped Excitons to Acoustic Phonons in Bismuth Perovskite Cs₃Bi₂I₉

To assess the potential optoelectronic applications of metal-halide perovskites, it is critical to have a detailed understanding of the nature and dynamics of interactions between carriers and the polar lattices. Here, the electronic and structural dynamics of bismuth-based perovskite Cs3Bi2I9 are revealed by transient reflectivity and ultrafast electron diffraction. A cross-examination of these results combined with theoretical analyses allows the identification of the major carrier-phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into Cs3Bi2I9 form self-trapped excitons on an ultrafast time scale. However, they retain most of their energy, and their coupling to Frohlich-type optical phonons is limited at early times. Instead, the long-lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub-ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye-Waller random atomic motions on longer times. The current results suggest the potential use of bismuth-based perovskites for applications other than photovoltaics to take advantage of the carriers' stronger self-trapping and long lifetimes.