Raising the LUMO level of fullerene derivatives alleviates the output voltage loss in tin halide perovskite solar cells

Two fullerene derivatives C₆₀-MP and C₆₀-ETPA were designed and synthesized for tin-halide perovskite solar cells. The introduction of an electron-withdrawing group and an alkyl chain reduced the electron affinity, resulting in a shallower lowest unoccupied molecular orbital (LUMO) energy level than that of common phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). Moreover, the smaller energy level offset provided efficient carrier transport and suppressed charge recombination. As a result, the devices with C₆₀-ETPA achieved an open-circuit voltage of 0.76 V and a power conversion efficiency of over 10%, which were substantially higher than 0.63 V and 8.25% for the control devices with PC₆₁BM, respectively.