Elucidating Polaron Dynamics in Cs 2 AgBiBr 6 Double Perovskite.

Lead-free Cs 2 AgBiBr 6 double perovskites have recently emerged as a possible alternative to lead-based halide perovskites for photovoltaic and optoelectronic applications. Significant research efforts have been devoted toward device engineering to enhance the performance of Cs 2 AgBiBr 6 double-perovskite-based photovoltaic and optoelectronic devices; however, less attention has been paid to address their intrinsic photophysical properties. In this work, we have shown that the small polaron formation under photoexcitation and polaron localization limits the carrier dynamics in Cs 2 AgBiBr 6 double halide perovskites. Furthermore, temperature-dependent ac conductivity measurement reveals that single polaron hopping is the dominant conduction mechanism. Ultrafast transient absorption spectroscopy reveals that a deformed lattice under photoexcitation leads to the formation of small polarons which act as self-trapped states (STSs) and lead to the ultrafast trapping of charge carriers. Our findings provide an in-depth understanding of intrinsic limitations of Cs 2 AgBiBr 6 perovskites, which can be applicable for other bismuth-based semiconductors.