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.

The photophysical properties of the lowest excited singlet states, S1(π,π*), of two porphyrin diacids have been investigated. The diacids are H4TPP2+ and H4OEP2+, the diprotonated forms of free base tetraphenylporphyrin (H2TPP) and octaethylporphyrin (H2OEP), respectively. Both diacids exhibit perturbed static and dynamic characteristics relative to the parent neutral complexes in solution at room temperature. These properties include enhanced yields of S1 → S0 radiationless deactivation (internal conversion), which increase from ∼0.1 for H2TPP and H2OEP to 0.4 for H4OEP2+ and 0.6 for H4TPP2+. The fluorescence lifetimes of both diacids are strongly temperature dependent, with an activation enthalpy of ∼1400 cm-1 for S1-state deactivation. The enhanced nonradiative decays and many other photophysical consequences of diacid formation are attributed primarily to nonplanar macrocycle distortions. Both H4TPP2+ and H4OEP2+ have been shown previously by X-ray crystallography to adopt saddle-shaped conformations, and the magnitudes of the perturbed properties for the two diacids in solution correlate with the extent of the deviations from planarity in the crystals. A model is proposed to explain the nonradiative decay behavior of the porphyrin diacids that is relevant to nonplanar porphyrins in general. The model includes the existence of decay funnels on the S1(π,π*)-state energy surface that are separated from the equilibrium conformation and other minima by activation barriers. It is suggested that these funnels involve configurations at which the potential-energy surfaces of the ground and excited states approach more closely than at the equilibrium excited-state structure(s) from which steady-state fluorescence occurs. Possible contributions to the relevant nuclear coordinates are discussed.

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