Bifunctional Ligand Anchoring Modulates Colloidal Nucleation and Intermediate-Phase Crystallization for δ-Phase-Suppressed High-Efficiency Perovskite Solar Cells

The α-formamidinium lead triiodide (α-FAPbI₃) perovskite is a leading candidate for high-performance perovskite photovoltaics owing to its narrow bandgap and better thermal stability. However, solution-processed perovskite films tend to form optically inactive δ-FAPbI₃ impurities during nonideal crystallization, which compromise device performance. We introduce 3-(pentafluorophenyl)propionic acid (PFPA) to modulate the perovskite precursor solution. PFPA promotes uniform colloid formation and solvent evaporation, reducing the nucleation barrier and enabling the growth of perovskite nanocrystals. This results in dense, pinhole-free films with improved phase purity by suppressing amorphous or mixed phases. The resultant devices achieve power conversion efficiency (PCE) values of 26.41% (0.09 cm²) and 24.38% (1 cm²) with negligible hysteresis. The devices maintain >90% initial efficiency after 1500 h of continuous maximum power point tracking under AM1.5G illumination, and >85% after 1300 h under damp-heat conditions (85 °C and 85% RH).