Intricate carrier dynamics of bismuth halide perovskites: localized excitons and polarons†

The interaction between carriers and photons in halide perovskites gives rise to intriguing phenomena in their excited states. In particular, bismuth halide perovskites exhibit behavior that extends beyond free carriers, involving excitons and polarons. Here, we report the steady state and excited state dynamics in the lead-free A₃Bi₂I₉ [A = FA (formamidinium), MA (methylammonium), Cs (cesium)] perovskite derivatives. The A₃Bi₂I₉ system exhibits strong excitonic peaks in the absorption spectra because of defect-related direct-bound excitons. The emission from self-trapped excitons influenced by carrier-phonon coupling and exciton–exciton interactions results in broad photoluminescence spectra. The low-energy photo-induced absorption (PIA-L) band below the bandgap energy is attributed to band gap renormalization (BGR) and the formation of self-trapped excitons (STSs) through electron-acoustic phonon coupling. Hot carrier cooling results in a transient absorption response and the occupation of modified band edge states. The interplay between BGR and polaron formation plays a crucial role in determining the amplitude of PIA-L during the cooling process. We observe that the carrier dynamics in the A₃Bi₂I₉ system are mostly dominated by localized excitons and small polarons. This study enhances our understanding of the fundamental processes governing their optoelectronic behavior and paves the way for their further utilization in advanced device applications.