Modulating the molecular orientation of linear benzodifuran-based isomeric polymers by exchanging the positions of chlorine and fluorine atoms

The molecular orientation and intermolecular interaction of photovoltaic polymers play a crucial role in determining the blend film morphology, improving the exciton dissociation and carries transport in bulk-heterojunction organic solar cells (BHJ-OSCs). However, there is still a lack of simple and effective methods to adjust the molecular orientation, which tremendously limit the development of new photovoltaic polymers. Here, we utilize the linear D-π-A type polymers to study this topic, where benzodifurans (BDF), thienothiophene (TT) and benzotriazole (BTA) are utilized as D, π and A units respectively. By exchanging the substitution position of the fluorine and chlorine atoms, the polymer orientation can be successfully transferred from edge-on for PE24 (chlorination on D unit) to face-on for PE25 (chlorination on A unit). Notably, when blended with three classic acceptors Y6, ITIC, and F-BTA5 with different bandgaps, PE25-based devices fabricated from THF always exhibit greatly enhanced performance over PE24. In addition, energy loss analysis manifests that higher EQEEL occured in all PE25-based devices, ultimately resulting in smaller total energy losses and larger VOCs than PE24-based devices. Excitingly, PE25: F-BTA5-based device realizes the highest VOC of 1.14 V with a PCE of 11.3%, which are among the best results for high-voltage OSCs. Our results not only provide a feasible method to tune the molecular orientation, but also suggest that chlorinated A unit may be more worth trying than classic fluorinated A unit in some polymers.