ACS applied materials & interfaces | 10 Oct 2019
M Ahmadi, L Collins, K Higgins, D Kim, E Lukosi and SV Kalinin
Despite the rapid progress in organic-inorganic halide perovskites (OIHPs) for applications such as high-power conversion efficiency solar cells, knowledge of coupling between electronic and ionic charge carrier dynamics is so far limited. While the presence of dual conduction channels is widely accepted, the precise physical mechanisms governing the impact of electronic (e.g. electrons and holes) and ionic conduction, and especially interface phenomena, remains uncertain. The lack of understanding stems largely from the lack of appropriate tools to capture the electrochemical dynamics on the length scales of the local inhomogeneities present (e.g. interfaces, grain boundaries, space charge regions) and time scales over which the coupled dynamics take place. Here, we implement Kelvin probe force microscopy (KPFM) to explore the charge carrier dynamics at the methylammonium lead tribromide (MAPbBr3) single crystal-gold electrode interface. In this work, the temporal dynamics of the electric field and charge carrier distribution at the electrode interface is spatially visualized by time resolved KPFM mapping. The results demonstrate an interplay of several phenomena, including charge injection, recombination and ion migration, leading to an unbalanced charge dynamic in MAPbBr3 single crystal- electrode interface under forward and reverse bias condition, explaining the origin of the current-voltage hysteresis in these devices. We contrast the bias assisted charge dynamics under both illuminated and dark conditions, providing a comprehensive picture of overall carrier dynamics and interface properties in a MAPbBr3 single crystal with lateral symmetric Au electrodes. Remarkably, illumination leads to formation of a wider space charge region due to accumulation of negative charges (both electrons and halide ions) at the positive electrode, which can effectively screen the external electric field leading to lower charge collection efficiency. The results suggest that the choice of contact or interfacial engineering can control the performance of OIHP devices without requiring modification of the material’s bulk properties.
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