Pronounced exciton and coherent phonon dynamics in BiI₃

Trivalent bismuth compounds featuring octahedral halide coordination, such as BiI3, double-perovskite structures and vacancy-ordered perovskite derivatives, have recently attracted considerable attention as potential environmentally benign thin film materials for solar light-harvesting and optoelectronic applications aiming to replace toxic lead-based perovskites. Yet, their photovoltaic performance still lags considerably behind their lead-counterparts. Improving their performance requires a detailed knowledge of the photoinduced carrier dynamics and kinetics in these systems. In the current study, we present a detailed investigation of layered BiI3. The compound exhibits pronounced exciton dynamics which is associated with a characteristic second-derivative type feature in the transient absorption spectra. Accurate time constants are assigned to carrier cooling and recombination processes. Pronounced coherent oscillations are observed in the kinetic traces indicating strong electron-phonon coupling. Analysis of the coherences recovers the steady-state Raman spectrum of BiI3 with a dominant mode of Ag symmetry at 114 cm-1, associated with the zone center C(Γ) phonon. The line position is well reproduced by DFT calculations. In terms of the carrier lifetime, high absorption coefficient and band-gap position, BiI3 is a promising candidate as a photovoltaic material but challenges posed are the difficulty to split the strongly bound excitons which are confined in the 2D layers and the efficient extraction of electrons and holes at n- and p-selective contacts, beside well-known issues regarding high resistivity and the intrinsic softness of this material.