Abstract
As electronic acceptor materials in heterojunction, fullerene derivatives (FDs) can significantly affect the power conversion efficiency of organic solar cells (OSCs). Here, in order to investigate the number and size effects of fullerene-cage in FDs for OSCs, the geometries, electronic structures and related properties, as well as photovoltaic parameters of several FDs, including MP, PCBM, BP, TP and PC71BM, were analyzed based upon density functional theory (DFT) and time dependent DFT calculations. The results indicate that the fullerene-size is more important than the number of fullerene-unit to affect the local geometrical parameters, energy level alignments, hyper-polarizabilities, and optical absorptions in visible region. Furthermore, the lowest unoccupied molecular orbital (LUMO) energies are almost same for these FDs, and the orbital energies near frontier molecular orbitals for BP and TP exhibit two- and three-fold quasi-degenerate, respectively. The transition configurations and molecular orbitals reveal the absorption bands in visible region of MP, PCBM, PC71BM and TP are local excitations, and that of BP are charge transfer excitations. The similar open-circuit voltages and fill factors of OSCs based upon P3HT/MP, P3HT/PCBM, P3HT/BP, P3HT/TP, and P3HT/PC71BM blend films result from the similar LUMO energy levels of these FDs.
Published Version
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