Ester-Functionalized Naphthobispyrazine as an Acceptor Building Unit for Semiconducting Polymers: Synthesis, Properties, and Photovoltaic Performance

Strongly electron-deficient π-conjugated systems are key building units for semiconducting polymers that are used in organic electronic devices, such as organic photovoltaic (OPV) cells. Here, we designed and synthesized a naphthobispyrazine derivative bearing four ester groups ( e NPz) as a new electron-deficient building unit and three e NPz-based semiconducting polymers with different donor units, that is, bithiophene ( P e NPz2T ), terthiophene ( P e NPz3T ), and quaterthiophene ( P e NPz4T ). These new polymers have relatively deep lowest unoccupied molecular orbital (LUMO) energy levels of around −3.5 eV, along with narrow optical band gaps of around 1.5 eV. The LUMO energy levels and the optical band gaps are significantly deeper and narrower than those of a polymer based on alkylated naphthobispyrazine. The results indicate that e NPz has a strong electron deficiency. The polymers show reasonably high power conversion efficiency of more than 6% in OPV cells in combination with a fullerene derivative. This study demonstrates that e NPz can be a useful building unit for high-performance semiconducting polymers.