Deciphering modes of long-range energy transfer in perovskite crystals using confocal excitation and wide-field fluorescence spectral imaging.

Excitation energy migration beyond mesoscale is of contemporary interest for both solar photovoltaic and light-emissive devices, especially in context of organometal halide perovskites (OMHPs) which have been shown to have very long (charge carrier) diffusion lengths. While understanding the energy propagation pathways in OMHPs is crucial for further advancement of material design and improvement of opto-electronic features, the simultaneous existence of multiple processes like carrier diffusion, photon recycling, and photon transport makes it often complex to differentiate them. In this study, we unravel the diverse yet dominant excitation energy transfer mode(s) in crystalline MAPbBr3micron-sized 1D rods and plates by localized (confocal) laser excitation coupled with spectrally-resolved wide-field fluorescence imaging. While rarely used, this technique can efficiently probe excitation migration beyond the diffraction limit and can be realized by simple modification of existing epifluorescence microscopy setups. We find that in rods of length below ∼2 microns, carrier diffusion dominates amongst various energy transfer processes. However, the transient non-radiative defects severely inhibit the extent of carrier migration and also temporarily affect the radiative recombination dynamics of the photo-carriers. For MAPbBr3plates of several tens of micrometers, we find that the photoluminescence (PL) spectral characteristics remain unaltered at short distances (< ∼3µm) while at a larger distance, the spectral profile is gradually red-shifted. This implies that carrier diffusion dominates over small distances, while photon recycling,i.e., repeated re-absorption and re-emission of photons, propagates excitation energy transfer over extended length scales with assistance from wave-guided photon transport. Our findings can potentially be used for future studies on the characterization of energy transport mechanisms in semiconductor solids as well as for organic (molecular) self-assembled microstructures.

Molecular Intercalation and Electronic Two Dimensionality in Layered Hybrid Perovskites.

In layered hybrid perovskites, such as (BA)2 PbI4 (BA=C4 H9 NH3 ), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb-I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials.

Fluorescence Blinking Beyond Nanoconfinement: Spatially Synchronous Intermittency of Entire Perovskite Microcrystals.

Abrupt fluorescence intermittency or blinking is long recognized to be characteristic of single nano-emitters. Extended quantum-confined nanostructures also undergo spatially heterogeneous blinking; however, there is no such precedent in dimensionally unconfined (bulk) materials. Herein, we report multi-level blinking of entire individual organo-lead bromide perovskite microcrystals (volume=0.1-3 µm3 ) under ambient conditions. Extremely high spatiotemporal correlation (>0.9) in intracrystal emission intensity fluctuations signifies effective communication amongst photogenerated carriers at distal locations (up to ca. 4 µm) within each crystal. Fused polycrystalline grains also exhibit this intriguing phenomenon, which is rationalized by correlated and efficient migration of carriers to a few transient nonradiative traps, the nature and population of which determine blinking propensity. Observation of spatiotemporally correlated emission intermittency in bulk semiconductor crystals opens the possibility of designing novel devices involving long-range (mesoscopic) electronic communication.