Balancing Lattice Strain by Embedded Ionic Liquid for the Stabilization of Formamidinium-Based Perovskite Solar Cells

Formamidinium (FA)-based perovskites remained state-of-the-art in the field of perovskite solar cells (PSCs) owing to the exceptional absorption and carrier transport properties, while the transition from photoactive (alpha-) to photoinactive (delta-FAPbI(3)) phase is the impediment that causes performance degradation and thus limits the deployment of FA-based PSCs. The unfavorable phase transition originates from tensile strain in the FAPbI(3) crystal lattice, which undergoes structural reorganization for lattice strain balancing. In this work, we found that the ionic liquid (IL) could be used as the strain coordinator to balance the lattice strain for stability improvement of FAPbI(3) perovskite. We theoretically studied the electronic coupling between IL and FAPbI(3) and unraveled the originality of the IL-induced compressive strain. The strain-relaxed alpha-FAPbI(3) by IL showed robust stability against environmental factors, which can withstand ambient aging for 40 days without any phase transition or decomposition. Moreover, the strain-relaxed perovskite films showed a lower trap density and resulted in conversion efficiency improvement from 18.27 to 19.88%. Based on this novel strain engineering strategy, the unencapsulated PSCs maintained 90% of their initial efficiency under ambient-air aging for 50 days.