Mechanism of charge transfer and separation in polymer/nonfullerene acceptor organic solar cells

Recently, in the research fields on the organic solar cells (OSCs), it has been reported that it succeeded in synthesizing polymer/nonfullerene acceptor OSCs, and showed the power conversion efficiency (PCE) higher than polymer/fullerene type. Also, it was reported that nonfullerene acceptors were successfully fluorinated and showed higher PCE than non-fluorinated nonfullerene types. In this study, from the viewpoint of electronic structure, absorption spectrum, and HOMO–LUMO gap, we consider the mechanism of charge transfer and separation in polymer/nonfullerene acceptor OSCs, using time dependent density functional theory method.
We compared the absorption spectra and HOMO–LUMO gap of the PTB7/ITIC complex with those of the fullerene PTB7/PC₇₁BM complex, with those of the non-fluorinated nonfullerene PTB7/IT-4F complex. It turns out that nonfullerene type has an absorption band in the longer wavelength region compared to the fullerene type. Moreover, fluorination causes the absorption band to be slightly red shifted. In the nonfullerene type, Voc is reduced, but the PCE is increased. Therefore, the improvement in PCE is due to the Jsc improvement due to the increase in light absorption of nonfullerene acceptor.