Rationally tailored passivator with multisite surface-anchors for suppressing ion migration toward air-stable...

Trap states in perovskite films fabricated using the solution method can capture photo-generated carriers, expedite ion migration, and contribute to decomposition of the perovskite layer, thereby emerging as a major threat to the commercialization of perovskite solar cells (PSCs). To address these issues, passivation of the surface traps on perovskite films via molecules with functional groups is proven 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 groups and K ions, were introduced as surface-anchoring passivators to improve the film quality and passivate trap states. Based on 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 interactions, including Lewis acid-base, hydrogen and ionic bonds. Moreover, the influence of varied ligands on defects and halide ion migration in perovskites as well as the mechanism behind it were extensively explored. Therefore, the KNFBS-treated perovskite films with a more homogeneous surface potential distribution significantly reduced point and vacancy defects and dangling bond density, facilitated charge transfer, exhibited an optimized power conversion efficiency (PCE) of 20.88% and enhanced air stability for the PSCs fabricated and stored in fully open-air conditions. The work has not only elucidated the fundamental mechanisms of ion migration and multisite passivation at the surface and GBs of perovskites but also probes into ligand design strategies for further improving the performance of perovskite photovoltaics.