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

Two regioselective C-60(OCH3)(4) derivatives, C-60(OCH3)(4)-PCBM (a methanofullerene derivative of C-60(OCH3)(4), PCBM=[6,6]-phenyl-C-61-butyric acid methyl ester) and C-60(OCH3)(4)-APCBM (an aziridinofullerene derivative of C-60(OCH3)(4)) were synthesized from C60Cl6 and used as acceptor for polymer solar cells. Revealed by cyclic voltammetry, the LUMO energy levels of C-60(OCH3)4-APCBM and C-60(OCH3)(4)-PCBM are 0.2 and 0.3 eV higher than that of PCBM, respectively. For the polymer photovoltaic cells with fullerene (PCBM, C-60(OCH3)(4)-APCBM or C-60(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 C-60(OCH3)(4)-APCBm) and 0.72 V (for C-60(OCH3)(4)-PCBM). The higher open-circuit voltages are reasonably attributed to the higher LUMO levels of the C-60(OCH3)(4) derivatives because of four electron-donating methoxy groups attached. The photovoltaic performance of C-60(OCH3)(4)-PCBMbased device is higher than that involving C-60(OCH3)(4)-APCBM, largely due to the structural changeability of C-60(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 multiaddition 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. (C) 2012 Elsevier B.V. All rights reserved .