Mechanism of charge transfer in polymer/fullerene-free type organic solar cell

Recently, in the research fields on the organic solar cells, it has been reported that it succeeded in synthesizing polymer / fullerene-free (donor/acceptor) type organic solar cell, and showed the light conversion efficiency higher than polymer/fullerene-free type. In this study, focusing on polymer / fullerene-free type PTB7 / ITIC complex, we obtain the electronic structure of complex and absorption spectrum at donor / acceptor interface using time dependent density functional theory method. In addition, from the viewpoint of electronic structure, absorption spectrum, and HOMO–LUMO gap, we consider the mechanism of charge transfer in polymer / fullerene-free type organic solar cell.
We compared the amounts of D/A transition contribution in absorption spectra, charge transfer (CT) and CT distance of the PTB7/ITIC complex with those of the fullerene PTB7/PC71BM complex. It turns out that any of these quantities were larger for the former. In addition, PCE is reported to be larger for fullerene-free OSC than for fullerene OSC and to be higher when CT distance becomes longer in fullerene OSC. Therefore, we have found that the CT distance is a descriptor for assessing PCE in the fullerene-free type OCS as well.