Fluorinated Epoxy for Tailoring PbI ₂ Residue in Perovskite Films to Realize Stable Perovskite Solar Cells with Fill Factor over 87%

Abstract The residual/excess PbI ₂ at the buried interface is directly related to undesirable charge transport loss and poor device stability, although empirical device optimization shows that a moderate excess PbI ₂ in the starting precursor solution benefits the film crystallization. Herein, the feasibility of fabricating a porous and rough PbI ₂ layer is demonstrated by incorporating a perfluorinated oxirane (TFHO) additive, thereby facilitating better penetration of the organic amine salts and promoting the crystallization process of perovskite films, as well as enhancing the resistance of water erosion. Surprisingly, the distribution of residual PbI ₂ in perovskite films is precisely regulated, and the oversized PbI ₂ clusters at the bottom surface are completely diminished. Consequently, TFHO‐modified perovskite solar cells (PSCs) achieve a champion efficiency of 25.24% with an ultra‐high fill factor of 87.61%. The enhanced light stability enables these cells to retain 80% of their initial efficiency after 1200 h of continuous 1‐sun illumination. Moreover, by incorporating TFHO into all‐air‐processed PSCs, a champion efficiency of 23.65% is achieved offering a promising prospect for promoting the commercialization of PSCs.