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Ultralong Perovskite Microrods: One- versus Two-Step Synthesis and Enhancement of Hole-Transfer During Light Soaking

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journal contribution
posted on 20.05.2016 by Xiangyang Wu, Jin Wang, Edwin K. L. Yeow
In this work, a two-step synthesis of CH3NH3PbI3 perovskite microrods involving first the production of PbI2 microrods by the evaporation of a dimethylformamide (DMF) solution of PbI2 is introduced. The PbI2 rods, acting as a shape template, yield rod-like perovskites of similar lengths and diameters when immersed in a solution of CH3NH3I. When comparable amounts of PbI2 are used, the two-step synthesis produces perovskite microrods that are shorter and more uniform in length than the ones prepared by a one-step synthesis via the evaporation of a DMF mixture of PbI2 and CH3NH3I. This is due to an uncontrolled crystal growth reaction in the one-step synthesis versus a nucleation dominated one in the two-step synthesis. The hole-transfer dynamics from the as-prepared perovskite rods to spiro-OMeTAD is also examined. Both the photoluminescence (PL) intensity and lifetime of the perovskite in the absence of spiro-OMeTAD are found to be enhanced as the CH3NH3PbI3 rods are continuously exposed to the excitation light. However, in the presence of spiro-OMeTAD, the quenched PL is gradually reduced as light irradiation progresses, and an increase in the hole-transfer (HT) rate between the two moieties is observed. We propose that during the light soaking process, trapping of free charge carriers by electronic traps improve the overall electronic coupling between the perovskite and spiro-OMeTAD or/and facilitate the diffusion of free holes to the perovskite/spiro-OMeTAD interface. The phenomenon seen here has important implications as it offers a plausible explanation for the discrepancies in reported HT rates between perovskites and spiro-OMeTAD.