Tailoring Microstructure and Morphology via Sequential Fluorination to Enhance the Photovoltaic Performance of Low-Cost Polymer Donors for Organic Solar Cells

For utilizing organic solar cells (OSCs) for commercial applications, reducing the overall cost of the photo absorbent materials is also very crucial. Herein, such a challenge is addressed by synergistically controlling the amount of fluorine (F)-substituents (n = 2, 4) on a low-cost wide-bandgap molecular design involving alternate fluorinated-thienyl benzodithiophene donor and 2,5-difluoro benzene (2FBn) or 2,3,5,6 tetrafluorobenzene (4FBn) to form two new polymer donors PBDT-2FBn and PBDT-4FBn, respectively. As expected, sequential fluorination causes a lowering of the frontier energy levels and planarization of polymer backbone via F center dot center dot center dot S and C-H center dot center dot center dot F noncovalent molecular locks, which results in more pronounced molecular packing and enhanced crystallinity from PBDT-2FBn to PBDT-4FBn. By mixing with IT-4F acceptor, PBDT-2FBn:IT-4F-based blend demonstrates favorable molecular orientation with shorter pi-pi stacking distance, higher carrier mobilities and desirable nanoscale morphology, hence delivering a higher power conversion efficiency (PCE) of 9.3% than PBDT-2FBn:IT-4F counterpart (8.6%). Furthermore, pairing PBDT-2FBn with BTP-BO-4Cl acceptor further improved absorption range and promoted privileged morphology for efficient exciton dissociation and charge transport, resulting in further improvement of PCE to 10.2% with remarkably low energy loss of 0.46 eV. Consequently, this study provides valuable guidelines for designing efficient and low-cost polymer donors for OSC applications.