Strain Regulation via Pseudo Halide-Based Ionic Liquid toward Efficient and Stable alpha-FAPbI(3) Inverted Perovskite Solar Cells

Inverted (p-i-n) perovskite solar cells have drawn great attention due to their outstanding stability and low-temperature processibility. However, their power conversion efficiency (PCE) still lags behind conventional (n-i-p) devices mainly due to the lack of strategies to stabilize alpha-FAPbI(3) without changing the bandgap. In this work, a facile and effective strategy is reported to regulate the residual strain via pseudo halide-based ionic liquids incorporation to stabilize alpha-FAPbI(3) perovskite in inverted perovskite solar cells (PVSCs). The employment of methylamine formate (MAFa) ionic liquid enables a homogenously stronger compressive strain to restrain the transition of shared-corner PbI6 octahedron into shared-face delta-FAPbI(3), as well as affecting the dynamic behavior of carriers and defects to achieve a record PCE (24.08%) among the reported inverted FAPbI(3) perovskite solar cells up to now. In addition, the MAFa incorporation results in enhanced device stability, unencapsulated PVSC retains over 90% of its initial efficiency after stored in ambient environment (RH:30 +/- 5%) for 1000 h. This work provides an efficient strategy to realize efficient and stable alpha-FAPbI(3) based inverted PVSCs to further catch up with the conventional ones.