Over 20% Efficiency in Printable Mesoscopic Perovskite Solar Cells with Enhanced Open‐Circuit Voltage via a Multifunctional Ionic Liquid

The large open‐circuit voltage ( V OC ) losses limit the enhancement of power conversion efficiency (PCE) in printable mesoscopic perovskite solar cells (p‐MPSCs). These losses primarily result from the high defect density at perovskite grain boundaries within the mesoporous scaffold, which promotes non‐radiative recombination. In this study, the crystallization improvement and defect modulation of perovskite is promoted by adopting a multifunctional ionic liquid, 1‐butyl‐2,3‐dimethylimidazolium trifluoromethanesulfonate (BMMIm[OTF]). The imidazolium ions in BMMIm[OTF] form hydrogen bonds with the PbI 6 4− framework and coordinate with under‐coordinated lead ions through S═O bonds. These interactions synergistically improve the crystallinity of perovskite films and optimize energy level alignment at the perovskite/carbon electrode interface. This improved interface facilitates more efficient charge transfer and extraction while reducing non‐radiative recombination. As a result, the champion p‐MPSCs incorporating BMMIm[OTF] achieve a PCE of 20.02% and a V OC of 1.055 V, significantly outperforming control devices with a V OC of 0.965 V. Furthermore, the hydrophobic nature of BMMIm[OTF] enhances device stability. This research provides a practical strategy for developing efficient and durable p‐MPSCs.