Anion-π interactions suppress phase impurities in FAPbI 3 solar cells.

Achieving both high efficiency and long-term stability is the key to the commercialization of perovskite solar cells (PSCs) 1,2 . However, the diversity of perovskite (ABX 3 ) compositions and phases makes it challenging to fabricate high-quality films 3-5 . Perovskite formation relies on the reaction between AX and BX 2 , whereas most conventional methods for film-growth regulation are based solely on the interaction with the BX 2 component. Herein, we demonstrate an alternative approach to modulate reaction kinetics by anion-π interaction between AX and hexafluorobenzene (HFB). Notably, these two approaches are independent but work together to establish 'dual-site regulation', which achieves a delicate control over the reaction between AX and BX 2 without unwanted intermediates. The resultant formamidinium lead halides (FAPbI 3 ) films exhibit fewer defects, redshifted absorption and high phase purity without detectable nanoscale δ phase. Consequently, we achieved PSCs with power conversion efficiency (PCE) up to 26.07% for a 0.08-cm 2 device (25.8% certified) and 24.63% for a 1-cm 2 device. The device also kept 94% of its initial PCE after maximum power point (MPP) tracking for 1,258 h under full-spectrum AM 1.5 G sunlight at 50 ± 5 °C. This method expands the range of chemical interactions that occur in perovskite precursors by exploring anion-π interactions and highlights the importance of the AX component as a new and effective working site to improved photovoltaic devices with high quality and phase purity.