Exciton-Polaron in a Quasi-One-Dimensional Chain of Hexyl-Diammonium-BiI₅ Octahedra

Lower-dimensional organic-inorganic hybrid perovskite materials promise to revolutionize the optoelectronics industry due to the tremendous possibilities of exotic control on excitonic properties driven via quantum confinement. Flexible organic cations acting as spacers and stabilizers enhance electron-phonon couplings, further amplifying the potential for modular light-matter interactions in these materials. Herein we unravel the nature of excitons in a quasi-1D chain of corner-sharing bismuth iodide octahedra with an intrinsic quantum well structure stabilized by a hexyl-diammonium cation. Using broadband femtosecond impulsive Raman spectroscopy and detailed electronic structure calculations, we directly quantify the exciton lifetime along with the electron-phonon coupling constants to fully describe the excitation as an exciton-polaron. We find ∼30 times larger electron-phonon couplings beyond the standard 2D-hybrid perovskite materials along with picosecond time-scale decoherences, thereby shedding light for the first time on the immense potential of these 1D perovskite analogues for developing novel materials for efficient light-conversion technologies.