High LUMO energy level C60(OCH3)4 derivatives: Electronic acceptors for photovoltaic cells with higher open-circuit voltage

Two regioselective C60(OCH3)4 derivatives, C60(OCH3)4-PCBM (a methanofullerene derivative of C60(OCH3)4, PCBM=[6,6]-phenyl-C61-butyric acid methyl ester) and C60(OCH3)4-APCBM (an aziridinofullerene derivative of C60(OCH3)4) were synthesized from C60Cl6 and used as acceptor for polymer solar cells. Revealed by cyclic voltammetry, the LUMO energy levels of C60(OCH3)4-APCBM and C60(OCH3)4-PCBM are 0.2 and 0.3 eV higher than that of PCBM, respectively. For the polymer photovoltaic cells with fullerene (PCBM, C60(OCH3)4-APCBM or C60(OCH3)4-PCBM) acceptor in combination with poly(3-hexylthiophene) (P3HT) donor, the open-circuit voltage is increased from 0.58 V (for PCBM) to 0.63 V (for C60(OCH3)4-APCBM) and 0.72 V (for C60(OCH3)4-PCBM). The higher open-circuit voltages are reasonably attributed to the higher LUMO levels of the C60(OCH3)4 derivatives because of four electron-donating methoxy groups attached. The photovoltaic performance of C60(OCH3)4-PCBM-based device is higher than that involving C60(OCH3)4-APCBM, largely due to the structural changeability of C60(OCH3)4-APCBM resulting from the rotatable N–C bond bridge therein. This work demonstrates that fullerene derivatives with higher LUMO level can be functionalized from multi-addition of electron-donating groups, and exemplifies that photovoltaic performances of fullerene-based solar cells are sensitive even to trivial bridge between functional group and fullerene core.