Time-Resolved Optical Assessment of Exciton Formation in Mixed Two-Dimensional Perovskite Films

The formation and relaxation of excitons have significant impacts on the carrier transport and recombination in low-dimensional semiconductors, whose excitonic properties are prominent due to the strong confinement effects. The exciton binding energy sets the critical barrier for initiating these dynamics. Here, we report the observation of exciton formation from cooled band-edge free carriers in the mixed two-dimensional hybrid organic–inorganic perovskites (F-PEA)₂(MA)_n−1Pb_nI_3n+1 using femtosecond transient absorption spectroscopy. By monitoring the changes of the bleach signal following the variance of photon energy required for exciting different electronic states, we identified the process of exciton formation and extracted the values of exciton binding energy in each two-dimensional phase. With injected carrier density reaching the Mott criterion, the subpicosecond duration of exciton formation persists while the carrier transfer rate is enhanced by one order of magnitude. Our findings set up a phenomenological approach to quantify the excitonic properties, bringing a helpful understanding for applications.