Passivating Defects via Retarding the Reaction Rate of FAI and PbI₂ Enables Stable Perovskite Solar Cells

The two-step sequential deposition strategy has garnered widespread usage in the fabrication of high-performance perovskite solar cells based on FAPbI₃. However, the rapid reaction between FAI and PbI₂ during preparation often leads to incomplete reactions, reducing the device efficiency and stability. Herein, we introduced a multifunctional additive, 2-thiophenyl trifluoroacetone (TTA), into the FAI precursor. The incorporation of TTA has proven to be highly effective in slowing the reaction rate between FAI and PbI₂, resulting in increased perovskite formation and improved efficiency and stability of the devices. TTA's CF₃ groups interact with FAI via hydrogen bonding, effectively suppressing FA⁺ defects. The S and C═O groups share lone pair electrons with uncoordinated Pb²⁺, leading to a reduction in perovskite film defects and suppressing nonradiative recombination. Additionally, the CF₃ groups impart hydrophobicity, protecting the perovskite film from moisture-induced erosion. As a result, the TTA-modified perovskite film achieves a Champion efficiency of 23.42% compared to the control's 21.52, with 20.58% efficiency for a 25 cm² solar module. Remarkably, the unencapsulated Champion device retains 86% of its initial PCE after 1080 h under dark conditions (60 ± 5 °C, 35 ± 5% RH), indicating enhanced long-term stability. These findings offer a promising and cost-effective tactic for high-quality perovskite film fabrication.