Lead-halide perovskites have attracted tremendous attention, initially for their performance in thin film photovoltaics, and more recently for a variety of remarkable optical properties. Defect tolerance through polaron formation within the ionic lattice is a key aspect of these materials. Polaron formation arises from the dynamical coupling of atomic fluctuations to electronic states. Measuring the properties of these fluctuations is therefore essential in light of potential optoelectronic applications. Here we apply two-dimensional electronic spectroscopy (2DES) to probe the timescale and amplitude of the electronic gap correlations in CsPbI3 perovskite nanocrystals via homogeneous lineshape dynamics. The 2DES data reveal irreversible, diffusive dynamics that are qualitatively inconsistent with the coherent dynamics in covalent solids such as CdSe quantum dots. In contrast, these dynamics are consistent with liquid-like structural dynamics on the 100 femtosecond timescale. These dynamics are assigned to the optical signature of polaron formation, the conceptual solid-state analogue of solvation.
Semiconductor nanocrystals are known to have properties of bulk semiconductors as well as molecules. Two rules that govern molecules are that there is no dual emission (Kasha) and there is no spectrum to the emission quantum yield (Vavilov). We show that the latter rule of molecular spectroscopy is generally violated in semiconductor nanocrystals. Through experiments and theory on CdSe and perovskite nanocrystals, these violations are shown to arise via hot carrier effects. Experiments and simple phenomenology reveal that quantum yield spectra arise because of enhanced hot carrier trapping rates. A semiclassical electron-transfer theory rationalizes a microscopic picture of the carrier kinetics. These effects are especially significant when quantifying syntheses of bright emitters such as perovskite nanocrystals. These effects are also a general approach to simple steady-state measurements of the action of hot carrier kinetics.
We monitor the time-resolved photoluminescence (t-PL) from CsPbBr3 perovskite nanocrystals with a time resolution of 3 ps, fast enough to resolve emission from potential multiexcitonic states. Being 15 nm in length and twice the Bohr length, these nanocrystals are either weakly confined or bulk-like. In contrast to this expectation of weak confinement, emission from multiexcitons is observed with binding energies consistent with strongly confined quantum dots. In addition to emission from biexcitons, emission from triexcitons is observed. The triexciton emission includes both S and P recombination channels. Excitation with different amounts of excess energy yields the same PL spectral dynamics, indicating that there are no hot carrier effects, and the electronic structure of the absorbing states are the same. The kinetics of the multiexciton populations are presented in two ways. The kinetics are first shown in a spectrally integrated form, showing faster t-PL at higher fluences, independent of excitation excess energy. Both excess energies show the same saturation response. In the second way of presenting the kinetics, the multiexciton populations are decomposed and presented as transients and saturation curves. These decomposed spectra into exciton and biexciton and triexciton populations enables further insight into their kinetics and fluence dependence.
Semiconductor perovskites are known to be strongly coupled to a dynamically disordered lattice. Two key phenomena to consider are phase transitions and phonon contributions to homogeneous linewidths. Each of these parameters will be highly dependent on material parameters and would benefit from a simple measure. Here, we perform temperaturedependent photoluminescence spectroscopy on bulk CsPbBr 3 perovskite nanocrystals. The data reveals a discontinuity in the energy gap, suggesting a phase transition between orthorhombic and cubic phases. The temperature dependence of the linewidth reveals negligible coupling to the ligands but strong coupling to polar optical phonons via the Froḧlich coupling. This temperature dependence suggests that homogeneous line broadening dominates the observed linewidths.
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