Utilizing Difluorinated Thiophene Units To Improve the Performance of Polymer Solar Cells

While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and dFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while dFT-HTAZ utilizes difluorinated thiophene (dFT) linkers. These polymers serve as the model system to explore the impact of dFT units in OSCs; additionally, this is the first publication to investigate polymers containing dFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the dFT units maintained the optical properties while lowering the energy levels by ∼0.4 eV, which allowed for a much improved V_oc value of ∼1 V. Importantly, when compared with the appropriate non-fullerene acceptor, dFT-HTAZ:ITIC-Th1 blends reached an efficiency of ∼10%, which is nearly 3× that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using dFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety.