Ionic Liquid-Assisted Crystallization and Defect Passivation for Efficient Perovskite Solar Cells with Enhanced Open-Circuit Voltage

Perovskite materials have demonstrated many excellent properties in next-generation photovoltaic devices, but the intrinsic defects and the quality of perovskite film still limit the performance and stability of PSCs. Here, 1,3-dimethylimidazolium iodide (DMII) ionic liquid was employed as an additive to passivate the various defects and produce the high-quality perovskite film with enlarged grain sizes. DMII could act as an "ionic stabilizer" for passivating the point defects including the vacancies defects of organic cations and halogen anions of perovskite. At the same time, the extra problematic PbI2 on surfaces and at grain boundaries of the perovskite film could also be reacted by DMII, leading to the reduction of recombination centers and trap states. On the other hand, the DMII ionic liquid with a "Ostwald ripening effect" could retard the crystallization process of perovskite crystals and yield better film quality with higher crystallinity, smoother morphology and larger grains. As a result, the optimal device achieved a champion power conversion efficiency (PCE) of 20.4 %. Particularly, the modified devices demonstrated a significant elevation in open-circuit voltage from 1.03 to 1.10 V. The hydrophobicity of perovskite films modified by DMII was enhanced and the un-encapsulated DMII devices retained 91 % of their initial PCE after aging 60 days under 15 +/- 5 % relative humidity.