▎ 摘　　要

The efficiency of polymer solar cells can be essentially enhanced by improving the performance of electron acceptor materials especially by increasing its lowest unoccupied molecular orbital (LUMO) level. To this aim, the electronic properties of an extremely pi-extended system, graphene flake and functionalized graphene flake with phenyl-C-butyric methyl ester (PCBM) group, were investigated and compared with those of C-60 ones. Several properties of phenyl-C-61-butyric methyl ester ([C-60]PCBM) derivatives have been calculated and compared against the experimental and other theoretical results. All geometrical and electronic structures calculations were carried out by using the density functional theory (DFT) method at the B3LYP level of theory. The results show that the phenyl ring of [C-60]PCBM was substituted with the methoxy groups to increase the LUMO level of the respective system which is in agreement with other studies. We found that graphene flake functionalized by methoxy-PCBM group offer significant increase in the value of the LUMO energy level in comparison with C-60 counterpart. Furthermore, the electrophilicity of this compound is lower than that of the other counterparts, which results in higher open circuit voltage (V-oc) value in the photovoltaic device. These findings could provide fundamental insights in improving the V-oc value as well as raising the LUMO levels of electron acceptor materials and may also serve as an alternative strategy to increase open circuit voltage in polymer solar cells. (C) 2016 Elsevier B.V. All rights reserved.