Precision Grain-Boundary Filling with Ionic Liquids for High-Performance Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) still face several critical challenges that hinder their practical application, among which grain boundary defects remain a key limiting factor for device performance and long-term stability. These defects arise from anisotropic grain growth, which can significantly accelerate the degradation of the perovskite films. Here, we develop a liquefied-ion precision filling (LIPF) strategy to selectively fill the grain boundaries of perovskite films using 1-methylimidazolium tetrafluoroborate (MBF₄). Owing to the solvation effect between the ionic liquid and isopropanol (IPA), MBF₄ preferentially accumulates at grain boundaries. Furthermore, its low melting point enables MBF₄ to liquefy during the annealing process, thereby facilitating the formation of F···NH and Pb···NH chemical interactions between MBF4and the perovskite at the grain boundaries. As a result, high-quality perovskite films with enlarged grain sizes and reduced grain boundary densities are obtained. Devices based on this strategy achieve an efficiency of 21.5%, which is significantly higher than that of the control device (19.6%). Besides, the devices also exhibit enhanced stability, maintaining 90% of their initial efficiencies over 840 and 590 h under the ISOS-D-1 and ISOS-L-2I protocols, respectively.