Heterogeneity and Population Branching Govern Exciton Dynamics in Two-Dimensional Cs2AgBiBr6 Nanoplatelets

Abstract Low-dimensional halide perovskites are promising materials because of their excellent optoelectronic and photovoltaic properties, rendering them a propitious choice for device applications. Here we elucidate the photophysics and ultrafast relaxation dynamics in two-dimensional Cs2AgBiBr6 double-perovskite nanoplatelets. At room temperature, a photoluminescence band higher in energy than that of the three-dimensional counterpart is observed Surprisingly, we observe that the emissive state can be directly photoexcited. Another interesting observation is that at 77 K, the photoluminescence spectrum shows a diminished higher-energy band and a prominent lower-energy band similar to that in three-dimensional systems. Based on these findings, we rationalize that the population from the initially photoexcited state branches into the ground state and a state responsible for the higher-energy band, and that the dynamics is governed by heterogeneity. Thus, the present work underscores the importance of understanding the microscopic processes governing the excited-state relaxation dynamics of two-dimensional perovskites.