A multifunctional flame retardant enabling efficient and stable formamidine-cesium perovskite solar cells

The defects concentrated at grain boundaries and on film surfaces acts as trap states of charge carriers and trigger points of degradation of perovskite, greatly hampering the efficiency and stability of perovskite solar cells (PSCs). It is highly desirable to fabricate high-quality perovskite polycrystalline films with low grain boundary density and defect density. Herein, a flame retardant phosphaphenanthrene is introduced into the precursor to simultaneously modulate perovskite crystallization and passivate defects of perovskite films. The crystallinity and stability of perovskite films are greatly enhanced owing to strong coordination between the phosphaphenanthrene and Pb2+ and intermolecular interactions. The efficiency of FA0.85Cs0.15PbI3 (FA, formamidine and Cs, cesium) PSC with the active area of 0.255 cm2 is significantly increased from 17.95% to 21.13%, along with negligible hysteresis. This facile strategy of employing multifunctional molecules has shown great promise for pursuing efficient and stable FACs-based PSCs.