Light Emission from CsPbBr₃ Metal Halide Perovskite Nanocrystals Arises from Dual Emitting States with Distinct Lattice Couplings

Metal halide perovskite nanocrystals are under intense investigation for their outstanding optical and electronic properties. The presence of higher fine structure states, let alone nonequilibrium processes within the fine structure, and multiexcitonic fine structure remains poorly understood due to a lack of experimental probes. Here, we use time-resolved photoluminescence (t-PL) spectroscopy with an improvement from 100 to 3 ps resolution which reveals previously unobserved spectral dynamics from excitons to multiexcitons in 15 nm CsPbBr₃ nanocrystals. The simple and immediate observation from temperature dependence is a previously unobserved fine structure to the multiexcitons. Further insight is gleaned from t-PL spectral bandwidth trajectories at extrema in temperature and exciton density. The bandwidth trajectories reveal the presence of a previously unobserved fine structure in excitons as well as multiexcitons. The bandwidth trajectories reveal a complex history, from multiexciton recombination to exciton thermalization to Auger heating to lattice thermalization. Whereas the amplitude of these spectral effects is large, ∼60 meV, modeling suggests that the spectral effects are mostly phonon based illustrating the importance of the lattice on light emission from metal halide perovskite nanocrystals.