posted on 2017-05-30, 00:00authored byVladimir S. Chirvony, Soranyel González-Carrero, Isaac Suárez, Raquel E. Galian, Michele Sessolo, Henk J. Bolink, Juan P. Martínez-Pastor, Julia Pérez-Prieto
The
mechanism responsible for the extremely long photoluminescence (PL)
lifetimes observed in many lead halide perovskites is still under
debate. While the presence of trap states is widely accepted, the
process of electron detrapping back to the emissive state has been
mostly ignored, especially from deep traps as these are typically
associated with nonradiative recombination. Here, we study the photophysics
of methylammonium lead bromide perovskite nanocrystals (PNCs) with
a photoluminescence quantum yield close to unity. We show that the
lifetime of the spontaneous radiative recombination in PNCs is as
short as 2 ns, which is expected considering the direct bandgap character
of perovskites. All longer (up to microseconds) PL decay components
result from the rapid reversible processes of multiple trapping and
detrapping of carriers with a slow release of the excitation energy
through the spontaneous emission channel. As our modeling shows, the
trap (dark) and excitonic states are coupled by the trapping–detrapping
processes so that they follow the same population decay kinetics,
while a majority of excited carriers are in the dark state. The lifetime
of the PNCs delayed luminescence is found to be determined by the
depth of the trap states, lying from a few tens to hundreds meV below
the emitting excitonic state. The delayed luminescence model proposed
in this work can serve as a basis for the interpretation of other
photoinduced transient phenomena observed in lead halide perovskites.