Effect of Fluorine Substitution Number in Naphthobisthiadiazole‐Based Polymers on the Performance of Nonfullerene Organic Photovoltaic Cells

Incorporation of electronegative fluorine atoms into the polymer backbone is an effective strategy to lower the highest occupied molecular orbital (HOMO) energy levels and to enhance backbone coplanarity through nonbonding intramolecular interactions, which are essential for improving the performance of the organic photovoltaic cells (OPVs). In this work, we synthesized a series of new naphtobisthiadiazole (NTz)-based semiconducting polymers by incorporating a bithiophene-phenyl-bithiophene unit bearing zero, two, or four fluorine atoms on the phenyl ring, namely, PNT4TB, PNT4TB-F2, and PNT4TB-F4, respectively, for application in nonfullerene acceptor (NFA) OPVs. Fluorination effectively lowered the HOMO energy levels of the polymers, leading to increased open-circuit voltages in the order PNT4TB < PNT4TB-F2 < PNT4TB-F4. In addition, fluorination enhanced backbone coplanarity; however, excessive aggregation was observed in the NFA blend films. As a result, the cells based on PNT4TB-F2, which achieved a favorable balance of HOMO energy level, backbone coplanarity, and thin film morphology, exhibited the highest power conversion efficiency of up to 11%, compared with those based on PNT4TB (9.2%) and PNT4TB-F4 (2.4%). This simple yet effective molecular design strategy provides useful guidelines for developing versatile p-type semiconducting polymers for high-performance OPVs.