Crystalline engineering of FAPbI pyrrolidinium ionic liquid for stable perovskite solar cells with 21.72% efficiency.

Improving the crystallinity of formamidinium triiodide (FAPbI) perovskite layer is one of the most effective approaches to increase the photovoltaic performance and stability of FAPbI-based solar cells (FSCs). In the current study, FAPbI layers were fabricated through a sequential deposition method. The morphology and crystalline properties of the FAPbI layers were modified by controlling the lead iodide (PbI) precursor by adding pyrrolidinium (Pyr) material into the PbI layer and modulating the FAPbI crystallization. The Pyr contributed to obtain (001)-preferred FAPbI orientation with no yellow photo-inactive phase. Subsequently, it reduced the unreacted PbI phase in the perovskite layer and suppressed the defect density, resulting in extended carrier lifetimes and improved ambient air and illumination stabilities. The Pyr-mediated FSCs recorded a champion efficiency of 21.72%, which is higher than that of control FSCs with a maximum efficiency of 19.08%. The developed Pyr-mediated method offers a practical and effective approach to fabricate stable and efficient FSCs.