Observation of the Molecular Passivation Effect upon Ultrafast Fermi Population Process in Perovskite Films

This study employs visible pump–probe femtosecond transient absorption (TA) spectroscopy to analyze the carrier dynamics of perovskite films in their excited states, with a particular focus on how passivation affects the quasi-Fermi level distribution. By fitting the TA spectra using the Fermi–Dirac distribution model, we investigate the contributions of band gap renormalization (BGR), free-carrier Stark (FCS) effects, and the Burstein–Moss (BM) effect under various excitation states. Results show that increasing the concentration of the passivator aids in elevating the quasi-Fermi level and retarding hot-carrier relaxation. We find an interplay among those effects and highlight the positive impact of the BM effect on maintaining the quasi-Fermi level and hot carriers. Our observation provides new insights into the underlying mechanics of the passivation effect on ultrafast time scales.