Bis-adducts of benzocyclopentane- and acenaphthene-C60 superior to mono-adducts as electron acceptors in polymer solar cells

• Geometrical structures and molecular packing of well-defined fullerene derivatives were identified by X-ray crystallography.
• Bis-adducts of fullerenes have higher LUMO energy levels than mono-adducts.
• High open-circuit voltages and short-circuit current density benefit from high LUMO level and proper packing of fullerene molecules.

Four fullerene derivatives, benzocyclopentane-C60 mono-adduct (BPCMA) and its bis-adduct (BPCBA), acenaphthene-C60 mono-adduct (ACMA) and its bis-adduct (ACBA), were synthesized by cyclopropanation reaction. Geometrical structures and molecular packing of the mono-adduct compounds (BPCMA and ACMA) were identified by X-ray crystallography. As measured by cyclic voltammetry, LUMO energy levels of BPCBA and ACBA are about 0.08 and 0.09 eV higher than their corresponding mono-adducts. Even though the power conversion efficiency remains to be improved, the polymer solar cells (PSCs) incorporating P3HT as donor and BPCBA (or ACBA) as acceptor exhibit open-circuit voltage (Voc) of 0.67 V (or 0.68 V), which is about 0.22 V (or 0.23 V) higher than the PSCs with the mono-adduct BPCMA (or ACMA) as electron acceptor. In addition, the short-circuit current density (Jsc) of the PSCs involving bis-adducts (BPCBA and ACBA) are obviously higher than those based on BPCMA and ACMA. The photovoltaic difference between mono- and bis-adducts can be rationalized by their different behaviors in LUMO energy and molecular packing.

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