Rationally Tailored Passivator with Multisite Surface-anchors for Suppressing Ion Migration toward Air-Stable Perovskite Solar Cells

Trap states in perovskite films fabricated from solution-method can capture photo-generated carriers, expedite ion migration, and contribute to decomposition of perovskite layer, thereby emerging as a major threat to the commercialization of perovskite solar cells (PSCs). To address these issues, passivation of surface traps on perovskite films via molecules with functional groups has been proved to be one of the most effective tactics for obtaining high-performance PSCs. Herein, potassium nonafluoro-1-butanesulfonate (KNFBS) molecules with multiple chemical bonds including multisite F atoms, sulfonic acid group and K ions were introduced as surface-anchoring passivators to improve the film quality as well as passivate trap states. Based on the in situ conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) results, it was found that undercoordinated Pb and I vacancy defects on the surface and grain boundaries (GBs) of perovskite films can be synergistically curtailed via multiple chemical interaction including Lewis acid-base, hydrogen and ionic bonds, respectively. Moreover, the influence of the varied ligands on defect and the halide ion migration in perovskites as well as the mechanism behind it have been extensively explored. Therefore, the KNFBS-treated perovskite films with a more homogeneous surface potential distribution, significantly reduced point, vacancy defect and dangling bond density, and facilitated charge transfer, resulted in an optimized power conversion efficiency (PCE) of 20.88% and enhanced air stability for the PSCs fabricated and stored in fully open-air condition. The work has not only elucidated the fundamental mechanisms of ion migration and multisite passivation at surface and GBs of perovskites, but also probes into the ligand design strategies for further improving the performance of perovskite photovoltaics.