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

Abstract

We report ultrafast spectroscopy investigations of photoinduced exciton dynamics in three novel CH 3 NH 3 PbBr 3 perovskite nanostructures: nanocrystals (0D), 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 ∼ 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 (τ c ∼ 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 k r 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.

Effect of Anisotropic Confinement on Electronic Structure and Dynamics of Band Edge Excitons in Inorganic Perovskite Nanowires

“…This has also been seen in methylammonium lead bromide, and was attributed to optical phonon-assisted sub-gab absorption. 42 We come to the same conclusion, after ruling out two possible alternatives. One alternative possibility would be that there is decreased scattering in the excited state, but the negative contrast signal in our stroboSCAT measurements indicates that excitons in CsPbBr 3 make the material more polarizable, not less.…”

Polaron and Spin Dynamics in Organic–Inorganic Lead Halide Perovskite Nanocrystals

“…[ 41,42 ] A similar derivative feature is not observed for near bandedge excitation of 700 nm (Figure S3, Supporting Information), which confirms that this predominantly arises due to the presence of hot carriers. [ 43,44 ] Concurrent to the JDOS shift, pump excitation also redshifts the polaron absorption (details in Supporting Information and Figure S2, Supporting Information). As time progresses, Δ t ≥ 1 ps, a strong bleach replaces the derivative lineshape.…”

Twin Domains in Organometallic Halide Perovskite Thin-Films

“…E.g., larger and smaller crystal grains may have similar photophysical behaviors. However, on the other hand, if we reduce the grain size to the nano scale or lower its dimension to nanowire and two-dimensional layers, the photophysical behavior will be significantly different [86,87]. Therefore, if the variation on photoproducts is found, it would represent significant change in morphology.…”