Stable 24.29%-Efficiency FA0.85MA0.15PbI3 Perovskite Solar Cells Enabled by Methyl Haloacetate-Lead Dimer Complex

Formamidinium methylammonium lead iodide (FAMAPbI3) perovskite has been intensively investigated as a potential photovoltaic material because it has higher phase stability than its pure FAPbI3 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 FAMAPbI3 perovskite cell by synergistic passivation of both C-O groups and halogen anions. As predicted by modeling undercoordinated Pb2+, the MBrA shows a very strong interaction with Pb2+ by forming a dimer complex ([C6H10Br2O4Pb]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 (Voc) of 1.18 V (Voc loss ≈ 0.38 V), which is one of the highest PCEs among all FAMAPbI3-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.