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

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.