Enhancement of Organic Photovoltaic Efficiency via Nanomorphology Control using Conjugated Polymers Incorporating Fullerene Compatible Side-Chains

We present controls of nanomorphology of polymer:fullerene BHJ films via synthesis of a series of push–pull-type copolymers with various molar percentages of side chains terminated with o-dichlorobenzyl (DCBZ) groups. As the molar percentage of the DCBZ-containing repeat units increases, the miscibility of the polymers in the series with PC71BM increases with respect to that of the polymer (PTBBO) that does not contain DCBZ. In the optimal film morphology, which consists of a polymer containing 25 mol % DCBZ-terminated side chains in the electron-pull unit (PTBDCB25), the polymer/PC 71 BM interface area is sufficiently large for efficient charge separation and percolated pathways is present for efficient charge carrier transport. In contrast, the BHJ film prepared from PTBBO has smaller interfaces and larger PC 71 BM aggregates. Furthermore, the intermolecular interaction between PC 71 BM and DCBZ induced changes in the PC 71 BM’s electronic structure at the polymer:PC 71 BM interface, resulting in an increase of the PC 71 BM’s LUMO level and thereby the improved open-circuit voltage. As a result, the BHJ solar cell device fabricated with PTBDCB25 exhibits significantly improved performance with a PCE of 8.30%, whereas the PCE of PTBBO is 6.90%.