Strong coupling of carriers to acoustic phonons in bismuth perovskite Cs₃Bi₂I₉ revealed by optical transient reflectivity and ultrafast electron diffraction

In this talk, we present the electronic and structural dynamics of bismuth-based perovskite Cs₃Bi₂I₉. A cross-examination of the experimental results from time-resolved optical and diffraction measurements 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, most of their energy is retained and their coupling to Fröhlich-type optical phonons is limited at initial times. 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 carriers’ stronger self-trapping and long lifetime.