Ultrafast Exciton Dynamics in Shape-Controlled Methylammonium Lead Bromide Perovskite Nanostructures: Effect of Quantum Confinement on Charge Carrier Recombination

We report ultrafast spectroscopy investigations of photo induced exciton dynamics in three novel CH3NH3PbBr3 perovskite nanostructures: nanocrystals (OD), nanowires (1D), and nanoplatelets (2D). Aided by analysis of UV visible absorption and photoluminescence spectra, features in the transient absorption (TA) spectra are assigned to different charge carrier processes, time constants of which are determined in fitting the transient kinetics. Immediately after photoexcitation, the charge carrier thermalization process occurs within the instrument response function time (fwhm similar to 350 fs) and results in a quasi-equilibrium distribution of charge carriers. It is followed by charge carrier cooling on a sub-picosecond time scale (tau(c) similar to 300 fs). The ensuing charge carrier recombination process obeys the rate law of a second-order reaction, which suggests that it occurs mainly via the bimolecular nongeminate recombination process. Dependence of the charge recombination rate constant (k(r)) on the initial charge carrier density ([n(0)]) has also been investigated in fluence dependence measurements. Although in general a linear relationship is observed, the sensitivity of kr to [n(0)] is strikingly different for the three perovskite nanostructures studied in the present work, signifying the strong impact of quantum confinement on exciton dynamics. Absence of monomolecular geminate recombination and strong dependence of the bimolecular nongeminate recombination on the charge carrier density are attributed to formation of strong-coupling polarons and self-trapping of electrons. In addition to state filling, stimulated emission, and photoinduced intraband absorption signals, TA spectra are further complicated by band gap renormalization. A spectroscopic and kinetic model has been developed for global fitting of TA spectra in both the frequency and time domains.