Ionic liquid-mediated reconstruction of perovskite surface for highly efficient photovoltaics

• 1-Butylimidazolium hexafluorophosphate (BMIMPF 6 ) ionic liquid was used to passivate the surface of perovskite. • After post-treatment of BMIMPF 6 , the power conversion efficiency of perovskite photovoltaics was improved from 20.1% to 23.2%. • Uncontrolled usage of ionic liquid was found to degrade perovskite by penetrating into active layer. • Importance of selective reconstruction of perovskite surface during ionic liquid treatment was demonstrated. To achieve efficient harvesting of light energy, perovskite photovoltaics require a surface passivation process of the perovskite layer. In this study, 1-butylmethylimidazolium hexafluorophosphate ionic liquid was adopted as a surface modifier to passivate a perovskite layer through post-treatment. Results revealed that the ionic liquid effectively reduces surface traps by inducing simultaneous interactions from both cation and anion components. Consequently, the surface treatment with the ionic liquid provided a ∼ 15% improvement in the efficiency of the perovskite photovoltaics. However, the formation of perovskite–ionic liquid complex, leading to inefficient charge transport and extraction, was observed when an excess amount of the ionic liquid was used owing to the penetration of the ionic liquid into the photo-active layer. These observations suggest that the ionic liquid should be selectively exposed to the perovskite surface to optimize the efficiency of perovskite photovoltaics treated with ionic liquids.