Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3-dimethyl-3-imidazolium hexafluorophosphate (DMIMPF6) ionic liquid (IL) is adopted to passivate the surface of a formamidinium-cesium lead iodide perovskite (Cs0.08FA0.92PbI3) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb-cluster and Pb-I antisite defects can be effectively passivated by [DMIM]+ bonding with the Pb2+ ion on the perovskite surface, leading to significantly suppressed non-radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF6-treated perovskite device demonstrated long-term stability because the hydrophobic DMIMPF6 layer blocked moisture permeation.
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