Stable 24.29%‐Efficiency FA 0.85 MA 0.15 PbI 3 Perovskite Solar Cells Enabled by Methyl Haloacetate‐Lead Dimer Complex

Abstract Formamidinium methylammonium lead iodide (FAMAPbI 3 ) perovskite has been intensively investigated as a potential photovoltaic material because it has higher phase stability than its pure FAPbI 3 perovskite counterpart. However, its power conversion efficiency (PCE) is significantly inferior due to its high density of surface detects and mismatched energy level with electrodes. Herein, a bifunctional passivator, methyl haloacetate (methyl chloroacetate, (MClA), methyl bromoacetate (MBrA)), is designed to reduce defect density, to tune the energy levels and to improve interfacial charge extraction in the FAMAPbI 3 perovskite cell by synergistic passivation of both CO groups and halogen anions. As predicted by modeling undercoordinated Pb 2+ , the MBrA shows a very strong interaction with Pb 2+ by forming a dimer complex ([C 6 H 10 Br 2 O 4 Pb] 2+ ), which effectively reduces the defect density of the perovskite and suppresses non‐radiative recombination. Meanwhile, the Br − in MBrA passivates iodine‐deficient defects. Consequently, the MBrA‐modified device presents an excellent PCE of 24.29%, an open‐circuit voltage ( V oc ) of 1.18 V ( V oc loss ≈ 0.38 V), which is one of the highest PCEs among all FAMAPbI 3 ‐based perovskite solar cells reported to date. Furthermore, the MBrA‐modified devices without any encapsulation exhibit remarkable long‐term stability with only 9% of PCE loss after exposure to ambient air for 1440 h.