Although buckminsterfullerene (C60) is usually the electron transport layer (ETL) in inverted perovskite solar cells (PSCs), its molecular nature leads to weak interfaces that result in nonideal interfacial electronic and mechanical degradation. In this study, we synthesized an ionic salt from C60, 4-(1',5'-dihydro-1'-methyl-2'H-[5,6] fullereno-C60-Ih-[1,9-c]pyrrol-2'-yl) phenylmethanaminium chloride (CPMAC), and used it as the electron shuttle in inverted PSCs. The CH2-NH3+ head group in the CPMA cation improved the ETL interface, and the ionic nature enhanced the packing, leading to a ~threefold increase in the interfacial toughness compared with that of C60. Using CPMAC, we obtained ~26% power conversion efficiencies (PCEs) with ~2% degradation after 2100 hours of 1-sun operation at 65°C. For minimodules (four subcells, 6 square centimeters), we achieved a PCE of ~23% with <9% degradation after 2200 hours of operation at 55°C.
