Impact of bright-dark exciton thermal population mixing on the brightness of CsPbBr_3 nanocrystals
arxiv(2024)
摘要
Understanding the interplay between bright and dark exciton states is crucial
for deciphering the luminescence properties of low-dimensional materials. The
origin of the outstanding brightness of lead halide perovskites remains
elusive. Here, we analyse temperature-dependent time-resolved photoluminescence
to investigate the population mixing between bright and dark exciton sublevels
in individual CsPbBr3 nanocrystals in the intermediate confinement regime. We
extract bright and dark exciton decay rates, and show quantitatively that the
decay dynamics can only be reproduced with second-order phonon transitions.
Furthermore, we find that any exciton sublevel ordering is compatible with the
most likely population transfer mechanism. The remarkable brightness of lead
halide perovskite nanocrystals rather stems from a reduced asymmetry between
bright-to-dark and dark-to-bright conversion originating from the peculiar
second-order phonon-assisted transitions that freeze bright-dark conversion at
low temperature together with the very fast radiative recombination and
favourable degeneracy of the bright exciton state.
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