Highly‐Stable CsPbI3 Perovskite Solar Cells with an Efficiency of 21.11% via Fluorinated 4‐Amino‐Benzoate Cesium Bifacial Passivation

Abstract The poor interface quality between cesium lead triiodide (CsPbI 3 ) perovskite and the electron transport layer limits the stability and efficiency of CsPbI 3 perovskite solar cells (PSCs). Herein, a 4‐amino‐2,3,5,6‐tetrafluorobenzoate cesium (ATFC) is designed as a bifacial defect passivator to tailor the perovskite/TiO 2 interface. The comprehensive experiments demonstrate that ATFC can not only optimize the conductivity, electron mobility, and energy band structure of the TiO 2 layer by passivation of the undercoordinated Ti 4+ , oxygen vacancy ( V O ), and free OH defects but also promote the yield of high‐quality CsPbI 3 film by synergistic passivation of undercoordinated Pb 2+ defects with the CO group and F atom, and limiting I − migration via F···I interaction. Benefiting from the above interactions, the ATFC‐modified CsPbI 3 device yields a champion power conversion efficiency (PCE) of 21.11% and an excellent open‐circuit voltage ( V OC ) of 1.24 V. Meanwhile, the optimized CsPbI 3 PSC maintains 92.74% of its initial efficiency after aging 800 h in air atmosphere, and has almost no efficiency attenuation after tracking at maximum power point for 350 h.