Single-isomer bis(pyrrolidino)fullerenes as electron-transporting materials for tin halide perovskite solar cells

Although fullerene bisadducts are promising electron-transporting materials for tin halide perovskite solar cells, they are generally synthesized as a mixture of isomeric products that require a complicated separation process. Here, we introduce a phenylene-bridged bis(pyrrolidino)fullerene, Bis-PC, which forms only a single isomer due to geometrical restriction. When used in a tin perovskite solar cell with a PEA_0.15FA_0.85SnI₃ (PEA: phenylethylammonium and FA: formamidinium) light absorption layer, the resulting open-circuit voltage (V _OC) was 0.78 V, a value higher than that of fullerene monoadducts and comparable to that of the commonly used indene-C₆₀ bisadduct (ICBA). The performance could be further improved by the composition engineering of perovskite, where the PEA_0.15(FA_0.87MA_0.13)_0.85SnI₃ based device (MA: methylammonium) exhibited a photoelectric conversion efficiency of 12.3% with a V _OC of 0.86 V. The device with single-isomer Bis-PC shows superior stability to that with mixed-isomer ICBA, retaining its initial performance after 3000 h storage under an inert atmosphere.