Delayed Luminescence in Lead Halide Perovskite Nanocrystals

Abstract

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.

Tuning Charge Carrier Dynamics and Surface Passivation in Organolead Halide Perovskites with Capping Ligands and Metal Oxide Interfaces

“…For MAPBr 3 films, a characteristic energy of 41 meV was reported based on the analysis of tr‐PL data . Trap states were determined to be 60–180 meV below the emissive states for MAPbBr 3 NPs and used to explain the observed delayed luminescence . The characteristic energy of the exponential tail of states we determined is also similar to the 47 meV activation energy for recombination recently determined for MAPbI 3 perovskite films .…”

“…The incorporation of capping alkylammonium ligands to form nanoparticulate perovskites also results in well‐terminated surfaces (rich in methylammonium, poor in Pb), leading to surface trap state passivation ( Figure A). In fact, it has recently been reported that well‐passivated adamantylammonium‐bromide‐capped MAPbBr 3 NPs exhibit a PL quantum yield close to unity, which is consistent with the absence of nonradiative deactivation channels . Moreover, a reversible process of trapping/detrapping of carriers was proposed to explain the delayed photoluminescence of the NPs.…”

Incorporation of potassium halides in the mechanosynthesis of inorganic perovskites: feasibility and limitations of ion-replacement and trap passivation

“…37 In this second hypothesis, it is possible that an exchange of a part of Cs atoms by K decreases the relative contribution of the PL emission from free electrons at 522 nm and, respectively, increases contribution of the emission from trap states at 540 nm. Following the delayed luminescence model, 38 trapassisted luminescence should be longer lived than the emission of free electrons from the conducting band, as is indeed observed in Fig. 1i.…”

Highly Efficient and Water‐Stable Lead Halide Perovskite Quantum Dots Using Superhydrophobic Aerogel Inorganic Matrix for White Light‐Emitting Diodes

“…The PL decay curves of the AeroPQDs were fitted using double exponential function or monoexponential function; [56] the fitted fast decay time t 1 , slow decay time t 2 , and the respective amplitudes a 1 and a 2 are listed in Table 1. For PQDs, the electron transitions associated with surface states lead to a longer PL decay time, [57] indicating that the PQD powders obtained by drying the solutions possess more surface traps. For PQDs, the electron transitions associated with surface states lead to a longer PL decay time, [57] indicating that the PQD powders obtained by drying the solutions possess more surface traps.…”