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Ionic transport in organometal halide perovskites is of vital importance because it dominates anomalous phenomena in perovskite solar cells,from hysteresis to switchable photovoltaic effects.However,excited state ionic transport under illumination has remained elusive,although it is essential for understanding the unusual light-induced effects (light-induced self-poling,photo-induced halide segregation and slow photoconductivity response) in organometal halide perovskites for optoelectronic applications.Here,we quantitatively demonstrate light-enhanced ionic transport in CH3NH3Pbl3 over a wide temperature range of 17-295 K,which reveals a reduction in ionic transport activation energy by approximately a factor of five (from 0.82 to 0.15 eV) under illumination.The pure ionic conductance is obtained by separating it from the electronic contribution in cryogenic galvanostatic and voltage-current measurements.On the basis of these findings,we design a novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and Pbl2 stacking layers in sequential deposition perovskite synthesis.X-ray diffraction pattes indicate a significant reduction of Pbl2 residue in the optimized CH3NH3Pbl3 thin film produced via lightassisted sequential deposition,and the resulting solar cell efficiency is increased by over 100％ (7.5％-15.7％) with little Pbl2 residue.This new method enables fine control of the reaction depth in perovskite synthesis and,in tu,supports light-enhanced ionic transport.