Suppressing Halide Defect Formation Through Fluorinated Piperidine Surface Termination Toward Efficient and Stable Perovskite/Silicon Tandem Photovoltaics

ABSTRACT Wide‐bandgap (WBG) perovskites are indispensable for high‐efficiency perovskite/silicon tandem solar cells, yet their operational stability is severely compromised by halide‐defect‐assisted ion migration and the resulting phase segregation. Here, we demonstrate that molecularly engineered surface termination via fluorinated piperidine ligands provides an effective route to stabilize WBG perovskites. Through a series of fluorinated piperidine ligands including monofluoro, gem‐difluoro, and trifluoromethyl substitutions, we reveal a strong fluorination dependence, where increasing fluorination progressively strengthens ligand‐perovskite interactions without altering the perovskite crystal structure, thereby elevating the formation energies of halide‐related defects and effectively reducing defect densities. Meanwhile, the strengthened ligand‐perovskite interactions enable effective surface termination by stabilizing surface and near‐surface regions, increasing the activation barrier for defect‐assisted ion migration and mitigating macroscopic phase‐segregation‐induced degradation. Consequently, the surface‐terminated monolithic perovskite/silicon tandem solar cells deliver a certified efficiency of 33.03% with a stabilized efficiency of 32.76%, maintaining 97% of their initial efficiency after 800 h of maximum power point tracking. This work establishes surface termination by suppressing halide defect formation as a strategy to simultaneously regulate defect chemistry and halide migration in WBG perovskites.