Surface Formamidine Cation Immobilization for Efficient FA-Based Perovskites Solar Cells

State-of-the-art perovskite solar cells (PSCs) continue to encounter stability challenges throughout their current commercialization process, primarily due to the instable organic components. Especially, surface (interface) imperfections, like the undercoordinated Pb2+ and halide sites, further compromise the confinement of organic cations at the surface (interface) and provide a rapid pathway for ion migration and volatilization, decreasing stability and efficiency. Herein, the study has developed a surface Formamidine (FA) cation immobilization strategy through hydrogen bond effect, achieved by a post-treatment of piperazine dihydrochloride (PDCl2), to obtain stable FA-based perovskites. The piperazine can immobilize surface FA(+) cation through hydrogen bond. Moreover, the post-treatment of PDCl2 can induce surface Cl- doping to establish strong coordinating bond with the uncoordinated Pb2+, reducing the imperfections of surface octahedral cage. Such a synergistic effect effectively constrains surface FA(+) cations, simultaneously alleviates surface lattice stress. Because of improved surface properties, the resultant perovskite demonstrates not only outstanding light/thermal stability, but also more pronounced n-type characteristics and uniform potential distribution for improving charge transfer dynamics. Finally, the champion PSCs exhibit a significantly enhanced efficiency from 23.15% to 25.52%. Moreover, these PSCs exhibit excellent stability: retain 91% of their initial efficiency after over 1000 h maximum power point test.