Several Orders of Magnitude Difference in Charge-Transfer Kinetics Induced by Localized Trapped Charges on Mixed-Halide Perovskites.

Abstract

Partial halide substitution in organolead halide perovskites MAPbX3 (MA = CH3NH3+, X = Cl-, Br-, or I-) leads to semiconductor heterostructures with precisely tuned band-gap energies, which facilitates efficient charge extraction or separation for high-performance solar cells and optoelectronic devices. In this study, partially iodide-substituted MAPbBr3 perovskites were prepared through a halide-exchange reaction in the liquid phase, and in situ space- and time-resolved photoluminescence profiles were acquired by means of confocal microscopy. The rates of charge transfer from the bulk MAPbBr3 to the surface MAPbBr3- xI x domains, which are widely distributed over a single crystal, were found to greatly depend on the excitation-power density. In particular, an abnormally slow charge-transfer process, lasting a few nanoseconds, was observed at higher excitation density. To explain the dependence of this rate on the excitation density, and its correlation with the charge-trapping rate in the bulk MAPbBr3, we propose a plausible mechanism in which trap filling associated with surface-trapped holes induces band bending within the space charge region. This band bending modulates carrier dynamics near the surface, thereby leading to efficient charge extraction from the bulk. To validate the mechanism, the carrier dynamics was numerically simulated using a diffusion model that includes the effect of the localized electric field. Our findings provide significantly deeper insight into the carrier dynamics within heterostructured perovskites with nanoscale heterogeneities, and a robust route for manipulating the photogenerated charges in various types of perovskite devices.