First Principle Calculations of the Electronic Properties of the Fullerene Derivative as an Electron Acceptor in Organic Solar Cells

The electronic parameters of fullerenes are essential for their potentials used as active layers in organic solar cells. Two isomeric forms of the C 60 ([5,6] fulleroid and [6,6] methanofullerene), named [60] PCBM (phenyl-C 61 -butyric acid methyl ester) clusters, were calculated using the B3LYP method with the 6-31G(d) basis set. It has been found that the contraction of C 6−6 double bonds is favorable for addition. The first adiabatic electron affinity (AEA) for [60] PCBM is similar to that for C 60 . The energy gaps between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of [60] PCBM have been reduced compared with C 60 . PCBM derivatives show an increased level of LUMO of fullerenes. From the natural charge populations, it has been found that adding a PCBM unit onto the C 60 cages does not change the charge populations remarkably; attaching a PCBM has no effect on the electronic structures of C 60 . The results of theoretical calculation suppose that PCBM is not involved in the process of photoelectric conversion, but it plays a key role in adjusting the level of HOMO−LUMO for increasing photoelectric conversion efficiencies.