Delayed Luminescence in Lead Halide Perovskite Nanocrystals

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.