Multifunctional In Situ Polymerizable Imidazolium Ionic Liquid for High-Performance Perovskite Solar Cells

Currently, most in situ polymerization additives used in perovskite solar cells (PSCs) employ bulky or conjugated monomers. Although such monomers exhibit negligible vapor pressure that prevents volatilization during annealing, their excessive steric bulk may restrict their multifunctionality. Herein, we report a series of imidazolium-based ionic liquids (EMIBF 4, VEIBF 4, and AVIBF 4 ) as additives for perovskite precursor solutions, which feature small steric bulk and negligible vapor pressure. Triggered by the thermal annealing of the perovskite film, the vinyl-functionalized VEIBF 4 and AVIBF 4 initiate an in situ cross-linking polymerization. Perovskite films incorporating polymerized VEIBF 4 (pVEIBF 4 ) exhibit significantly enhanced crystallization quality, reduced defect densities, and effectively suppressed non-radiative recombination. Consequently, pVEIBF 4 -based PSCs deliver an excellent power conversion efficiency (PCE) of 25.38% (0.071 cm 2 ), with the open-circuit voltage ( V oc ) reaching 1.197 V. Moreover, the unsealed cells exhibited exceptional thermal robustness and storage stability.