A theorectical design of performant chlorinated benzothiadiazole-based polymers as donor for organic photovoltaic devices

Chlorination is often adopted in the synthesis of donor materials considering the obvious improvement on efficiency of organic photovoltaic (OPV) device. Thus the chlorination effect was firstly probed based on density functional theory (DFT) calculation, which shows that the chlorinated benzothiadiazole connected with two thiophene rings (BTClTT) fragment although has larger optical bandgap and relatively blue-shifted spectrum, its deeper frontier molecular orbital (FMO) energy level, larger charge transfer distance (DCT) and much more transferred charges (CT), obvious negative natural population analysis (NPA) charges, larger twisted dihedral angles and smaller value of BLA indicate its favorable properties for the application in OPVs. Then, we designed a series of donor materials 2–7 adopting two strategies based on the experimental polymer 1 which contains BTClTT fragment. The calculated physical parameters characterizing OPV performance such as open-circuit voltage (VOC) and absorption-properties manifest that the design through side chains modifications for 2–5 may have slight influence on their cell performance while the backbone replacements for 6 and 7 exhibit prominent enhancement whether on VOC and photon-absorption property or on the charge separation ability. Therefore, our calculations suggest feasible strategies of designing donor materials for the chlorinated BT-based donor materials in OPVs. We hope our work can provide some guidelines for the future study on chlorinated donor materials.