Transient Regulation of Crystallization via Volatile Additives to Obtain High-Quality Perovskite Films

Exact regulation of nucleation and crystallization kinetics is essential for fabricating high-performance perovskite solar cells (PSCs). However, regulating the fast assembly of precursors without incorporating residual impurities presents a significant difficulty. We present an approach of a "transient kinetic modulation" strategy employing octafluorotoluene (OCF) to control the crystallization of formamidinium (FA)-based perovskites. Distinct from conventional additives that focus on cations or lead centers, OCF utilizes its electron-deficient perfluorinated aromatic ring to form robust anion-π interactions with iodide ions (I^-). This interaction effectively sequesters iodide species in the precursor solution, kinetically inhibiting the formation of PbI₃ ^- intermediates and reducing the disordered precipitation of phases. The resultant delayed nucleation and synchronized crystal growth provide perovskite films characterized by markedly increased grain size, alleviated lattice strain, and reduced defect density. Significantly, OCF totally volatilizes during annealing, guaranteeing a pure, residue-free film. Consequently, the optimized devices achieve a champion power conversion efficiency (PCE) of 25.44% with a high fill factor of 85.27%, alongside exceptional long-term stability, retaining 93% of the initial PCE after 1800 h of ambient aging. This study establishes volatile additive modulation as an effective, non-invasive method for crystallization management.