Spatially Correlated Blinking of Perovskite Micro-crystals: Deciphering Effective Modes of Communication between Distal Photoexcited Carriers

Recent observations on spatially correlated photoluminescence (PL) intermittency (blinking or flickering) of bulk perovskite crystals have invoked considerable interest regarding their origins, as the ensemble averaging effect should result in heterogeneous intensity fluctuations over micron length scales. This intriguing phenomenon can only be explained by considering (i) photogeneration of few transient nonradiative traps which act as highly efficient quenchers for photoexcited charge carriers and (ii) long-range communication (or correlation) between a large number of spatially segregated (∼microns) charge carriers photogenerated simultaneously. In light of spatially synchronous blinking in systems with a length scale beyond diffusion parameters in microcrystalline thin films, we investigated the modes of excitation energy migration which augment intracrystal communication. Here, we used spectrally resolved wide-field epi-fluorescence microscopy with optional confocal (local) excitation to probe excited energy migration modes. We identified the waveguide effect and its assistance to produce secondary excitons through photon recycling in MAPbBr3 microcrystals (MCs). Upon formation of nonradiative trap/s within the excitation domain, these secondary modes of carrier migration help in collective quenching of photogenerated carriers and redistribution of emission throughout the MC. This report discusses a method to investigate excitation migration in spatially extended systems and provides insights into the carrier communication process in bulk perovskites, which results in the PL blinking of entire individual MCs

Spatially Correlated Blinking of Perovskite Micro-crystals: Deciphering Effective Modes of Communication between Distal Photoexcited Carriers

Recent observations on spatially correlated photoluminescence (PL) intermittency (blinking or flickering) of bulk perovskite crystals have invoked considerable interest regarding their origins, as the ensemble averaging effect should result in heterogeneous intensity fluctuations over micron length scales. This intriguing phenomenon can only be explained by considering (i) photogeneration of few transient nonradiative traps which act as highly efficient quenchers for photoexcited charge carriers and (ii) long-range communication (or correlation) between a large number of spatially segregated (∼microns) charge carriers photogenerated simultaneously. In light of spatially synchronous blinking in systems with a length scale beyond diffusion parameters in microcrystalline thin films, we investigated the modes of excitation energy migration which augment intracrystal communication. Here, we used spectrally resolved wide-field epi-fluorescence microscopy with optional confocal (local) excitation to probe excited energy migration modes. We identified the waveguide effect and its assistance to produce secondary excitons through photon recycling in MAPbBr3 microcrystals (MCs). Upon formation of nonradiative trap/s within the excitation domain, these secondary modes of carrier migration help in collective quenching of photogenerated carriers and redistribution of emission throughout the MC. This report discusses a method to investigate excitation migration in spatially extended systems and provides insights into the carrier communication process in bulk perovskites, which results in the PL blinking of entire individual MCs

Investigating Spatiotemporal Correlation of Multi-state Photoluminescence Intermittency in Organo-Lead Bromide Microcrystal Films

Recent observations of spatially correlated photoluminescence intermittency of spatially extended organo-lead halide perovskite microcrystals (MCs) has been explained via involvement of highly efficient, transient non-radiative traps and long-range communication between photogenerated carriers; however, the nature/origins of these quenchers as well as factors determining carrier communication remain obscure. Here, we studied the effect of excitation powers, energies, and crystal dimensions on the observed multi-state intermittency of methylammonium lead bromide (MAPbBr3) MCs. Our results support a model of involvement of a few quenchers in each crystal and effective diffusion of a subpopulation of charge carriers over entire crystals. Furthermore, we developed comprehensive analytical methods to quantify the extent of spatiotemporal blinking correlation, which establish a remarkably high (>0.9) correlation in flickering of spatially segregated MC grains. Surprisingly, however, we observe a transient yet dramatic loss of synchronicity in blinking for some fused MC grains, which point out to intermittent lack and re-establishment of inter-grain diffusion of photogenerated carriers, likely owing to a slow time-varying reversible change in the potential energy landscape in fused crystal grains. Finally, we report yet another intriguing phenomenon of transiently correlated and anti-correlated emissivity fluctuations between fused crystals, suggestive of directional excitation energy migration between adjacent grains

Investigating Spatiotemporal Correlation of Multi-state Photoluminescence Intermittency in Organo-Lead Bromide Microcrystal Films

Recent observations of spatially correlated photoluminescence intermittency of spatially extended organo-lead halide perovskite microcrystals (MCs) has been explained via involvement of highly efficient, transient non-radiative traps and long-range communication between photogenerated carriers; however, the nature/origins of these quenchers as well as factors determining carrier communication remain obscure. Here, we studied the effect of excitation powers, energies, and crystal dimensions on the observed multi-state intermittency of methylammonium lead bromide (MAPbBr3) MCs. Our results support a model of involvement of a few quenchers in each crystal and effective diffusion of a subpopulation of charge carriers over entire crystals. Furthermore, we developed comprehensive analytical methods to quantify the extent of spatiotemporal blinking correlation, which establish a remarkably high (>0.9) correlation in flickering of spatially segregated MC grains. Surprisingly, however, we observe a transient yet dramatic loss of synchronicity in blinking for some fused MC grains, which point out to intermittent lack and re-establishment of inter-grain diffusion of photogenerated carriers, likely owing to a slow time-varying reversible change in the potential energy landscape in fused crystal grains. Finally, we report yet another intriguing phenomenon of transiently correlated and anti-correlated emissivity fluctuations between fused crystals, suggestive of directional excitation energy migration between adjacent grains

Investigating Spatiotemporal Correlation of Multi-state Photoluminescence Intermittency in Organo-Lead Bromide Microcrystal Films

Recent observations of spatially correlated photoluminescence intermittency of spatially extended organo-lead halide perovskite microcrystals (MCs) has been explained via involvement of highly efficient, transient non-radiative traps and long-range communication between photogenerated carriers; however, the nature/origins of these quenchers as well as factors determining carrier communication remain obscure. Here, we studied the effect of excitation powers, energies, and crystal dimensions on the observed multi-state intermittency of methylammonium lead bromide (MAPbBr3) MCs. Our results support a model of involvement of a few quenchers in each crystal and effective diffusion of a subpopulation of charge carriers over entire crystals. Furthermore, we developed comprehensive analytical methods to quantify the extent of spatiotemporal blinking correlation, which establish a remarkably high (>0.9) correlation in flickering of spatially segregated MC grains. Surprisingly, however, we observe a transient yet dramatic loss of synchronicity in blinking for some fused MC grains, which point out to intermittent lack and re-establishment of inter-grain diffusion of photogenerated carriers, likely owing to a slow time-varying reversible change in the potential energy landscape in fused crystal grains. Finally, we report yet another intriguing phenomenon of transiently correlated and anti-correlated emissivity fluctuations between fused crystals, suggestive of directional excitation energy migration between adjacent grains

Photoluminescence Flickering of Micron-Sized Crystals of Methylammonium Lead Bromide: Effect of Ambience and Light Exposure

Recent reports on temporal photoluminescence (PL) intensity fluctuations (<i>blinking</i>) within localized domains of organo-metal lead halide (hybrid) perovskite microcrystals have invoked considerable interest to understand their origins. Using PL microscopy, we have investigated the effect of atmospheric constituents and photoillumination on spatially extended intensity fluctuations in methylammonium lead bromide (MAPbBr₃) perovskite materials, explicitly for micrometer (ca. 1–2 μm)-sized crystals. Increase in the relative humidity of the ambience results in progressive reduction in the PL intensity, and beyond a threshold value, individual microcrystalline grains exhibit multistate PL intermittency (<i>flickering</i>), which is characteristically different from quasi two-state blinking observed in nanocrystals. Such flickering disappears upon removal of moisture, accompanied by considerable enhancement of the overall PL efficiency. We hypothesize that initiation of moisture-induced degradation marked by the lowering of PL intensity correlates with the appearance of PL flickering, and such processes further accelerate in the presence of oxygen as opposed to an inert (nitrogen) environment. We find that the intrinsic defects not only increase the threshold level of ambient moisture needed to initiate flickering but also modulate the nature of PL intermittency. Our results therefore establish a strong correlation between initiation of material degradation and PL flickering of hybrid perovskite microcrystals, induced by transient defects formed via interaction with the ambience