Blinking Beats Bleaching: The Control of Superoxide Generation by Photo-ionized Perovskite Nanocrystals.

Moisture- or oxidation-induced degradation is a major challenge in the advancement of perovskites-based technology. The oxidation is caused by electron transfer from a photo-excited perovskite nanocrystal to oxygen and the formation of superoxide that disintegrates the perovskite structure. In air, the emission intensity of a methylammonium lead iodide (MAPbI ) perovskite nanocrystal continuously decreases, whereas a nanocrystal in argon or a polymer shows exceptionally stable emission intensity. Surprisingly, in air, the emission intensity of a nanocrystal with long-lived OFF states completely recovers after the OFF state. This property, along with the rate of non-radiative relaxation that exceeds the rate of electron transfer suggest that the perovskite nanocrystals produce and react with superoxide in the excited neutral state, but not in the ionized state. In other words, the ultrafast non-radiative relaxation in the ionized state hinders electron transfer to oxygen and prevents oxidation of perovskites.