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.