Ultrafast Carrier Self-Trapping Driven by Strong Exciton-Phonon Coupling in 2D MA₃Bi₂I₆Cl₃ Perovskite

Two-dimensional (2D) hybrid bismuth halide perovskites have emerged as promising lead-free materials for optoelectronic applications due to their solution processability and tunable structures. Here, we investigate 2D layered hybrid perovskite MA₃Bi₂I₆Cl₃ using temperature-dependent photoluminescence (PL) and femtosecond transient absorption spectroscopy. Our results reveal strong coupling between excitons and phonons, evidenced by giant Huang-Rhys factors, coherent phonon oscillations, and ultrafast carrier self-trapping into small-polaron and self-trapped exciton (STE) states. These processes appear as time-dependent ground-state bleach and photoinduced absorption features, highlighting the influence of the lattice in carrier dynamics. Wavelength- and time-resolved measurements reveal that PL emission is dominated by STEs, while free exciton emission is weak and short-lived. By comparing 2D MA₃Bi₂I₆Cl₃ with 0D MA₃Bi₂I₉, which contains molecularly isolated [BiI₆]^3- octahedra and 2D MA₃Bi₂Br₉ perovskites, we demonstrate how halide composition and structural dimensionality influence the balance between free exciton populations and carrier localization. These insights uncover the intrinsic kinetic landscape of photoexcited states in MA₃Bi₂I₆Cl₃. Overall, our study contributes to a mechanistic understanding of exciton-phonon interactions in lead-free 2D perovskites.