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···S and C-H···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 π-π 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.
Keywords: fluorine effect; high open-circuit voltage; low photon energy loss; low-cost polymer donor; non-fullerene solar cells; wide-bandgap donors.
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