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

Abstract

Moisture-or oxidation-induced degradation is am ajor challenge in the advancement of perovskites-based technology.T he oxidation is caused by electron transfer from ap hoto-excited perovskite nanocrystal to oxygen and the formation of superoxide that disintegrates the perovskite structure.I na ir,t he emission intensity of am ethylammonium lead iodide (MAPbI 3 )p erovskite nanocrystal continuously decreases,whereas ananocrystal in argon or apolymer shows exceptionally stable emission intensity.Surprisingly,inair,the emission intensity of ananocrystal with long-lived OFF states completely recovers after the OFF state.T his property,a long with the rate of non-radiative relaxation that exceeds the rate of electron transfer suggest that the perovskiten anocrystals produce and react with superoxidei nt he excited neutral state,b ut not in the ionizeds tate.I no ther words,t he ultrafast non-radiative relaxation in the ionizeds tate hinders electron transfer to oxygen and prevents oxidation of perovskites.Halide perovskites are emerging into ac lass of costeffective semiconductor materials with promising optical and electronic properties [1] for electroluminescent [2] and photovoltaic devices. [3] Despite the exciting power conversion efficiency and the attractive luminescence of perovskites,the major limitation hampering the practical applications of these materials is moisture-and oxygen-sensitive degradation. [4] While investigations are progressing to stabilize perovskites in the humid atmosphere,t he roles of oxygen on the photoluminescence (PL) properties and the structure of perovskites remain controversial.Recently,H aque and co-workers observed oxidative disintegration of aM APbI 3 film into PbI 2 ,C H 3 NH 2 ,I 2 ,a nd H 2 O, [4a] which is based on their original work on superoxide generation by MAPbI 3 . [4b,c] By employing TiO 2 ,they successfully extracted electrons from MAPbI 3 ,m inimized electron transfer to oxygen, and prevented the oxidation of MAPbI 3 . These observations are correlated with the ab initio studies on superoxide generation by MAPbI 3 and the successive reaction of superoxide with the surface Pb ion to disintegrate the leadiodide octahedral structure. [4d] Conversely,superoxide enhances the PL quantum efficiencies of perovskites,which is by the passivation of iodide vacancies and trap states. [5] Forexample, Scheblykin and co-workers reported the photochemical reaction of oxygen to trap states in MAPbI 3 ,r esulting in an enormous enhancement of PL quantum efficiency. [5c,d] Later, Stranks and co-workers [5b] and He and co-workers [5a] independently reported the reaction of superoxide with MAPbI 3 , the passivation of defects by oxides,a nd an enhancement of the PL quantum efficiency.These contradicting reports on the roles of oxygen on the degradation, defect passivation, and quenching as well as enhancement of PL quantum efficiencies of perovskites attract us to investigate the roles of oxygen and electron transfer on the photooxidation of MAPbI 3 nanocrystals at the ensemble a...

Presence of Maximal Characteristic Time in Photoluminescence Blinking of MAPbI3 Perovskite

“…To cover a broad range of frequencies, we measured PL traces as long as 75 min with 30 ms time resolution resulting in 150 000 data points covering at least one order of magnitude larger frequency range than the currently available data for MHPs. [26][27][28][29][30][31][32] One can see that PL fluctuations are very complex consisting of jumps between multiple intensity levels and semigradual undulations (Figure 1a-f and Figure S2.1 and Note S2, Supporting Information). Often the lowest PL intensity level is not close to zero.…”

“…[16,18] It is interesting that PL blinking of large crystals [21,24] was reported even before the same phenomenon in MHP colloidal quantum dots (QDs) was observed. [25][26][27][28][29][30][31][32][33] The nature of these fluctuations lay in the presence of strong metastable nonradiative recombination centers with a very large recombination cross section (so-called supertraps). [19,21] Thus, PL yield depends on the actual number of metastable nonradiative recombination centers per crystal which fluctuates in time and depends on other conditions.…”

“…PL blinking in MHPs is very complex and only very seldomly resembles two-level switching. [26][27][28][29][30][31][32] Even though there are several studies on large nano-or micro-crystals of MHPs where some aspects of PL fluctuation kinetics were discussed, thorough statistical data and analysis of PL fluctuations in these systems are missing. [17][18][19][20][21]36] In the current contribution we investigated properties of PL fluctuations in MAPbI 3 submicrometer crystals of 100-500 nm in size resembling grains in a standard MAPbI 3 polycrystalline film used for solar cell fabrication.…”

Underestimated Effect of a Polymer Matrix on the Light Emission of Single CsPbBr3 Nanocrystals

“…At room temperature, APbX 3 NCs have been shown to emit a regular stream of single photons with very high single photon purity, as evidenced by the antibunching in the second-order correlation function [ g 2 (0) < 0.1]. − These NCs are presently nearly unmatched in terms of spectral tunability and facile solution synthesis and processability. However, despite their great potential, single perovskite NCs still suffer from insufficient optical stability in ambient conditions. ,− Moisture-assisted photodegradation is widely regarded as a major cause of instability, in addition to the thermal and solvent lability of these materials. − At the single particle level, the dynamic blueshift of the PL emission and a concomitant PL quenching were indeed attributed to the photoaccelerated reaction with atmospheric water …”

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time‐Resolved Microspectroscopy

“…[48,53] Using the same techniques of single-molecule spectroscopy and time-resolved spectroscopy, the fluorescence blinking has recently been investigated in MAPbBr 3 QDs, [57][58][59][60] CsPbBr 3 QDs, [61][62][63][64] 0D Cs 4 PbBr 6 , [65] MAPbI 3 QDs. [66] Mandal et al [67] observed blinking in inorganic perovskite CsPbBr 3 QDs. As shown in Figure 2A,B, different ON/OFF power laws are confirmed for different excitation wavelengths of 405, 463, and 488 nm.…”