Naphthobispyrazine bisimide-based semiconducting polymers as electron acceptors for all-polymer photovoltaic cells

In this work, we investigated the feasibility of using strongly electron-deficient naphthobispyrazine bisimide (NPI) as a building unit for n-type semiconducting polymers used in organic photovoltaics. We synthesized new n-type semiconducting polymers based on NPI, in which thienothiophene and thiophene were used as counits. The polymers exhibited deep lowest unoccupied molecular orbital energy levels of ~−3.7 eV and narrow optical bandgaps of less than 1.5 eV, which originated from the high electron deficiency of NPI. We found that decreasing the counit length improved the solubilities and reduced the crystallinities of the NPI-based polymers. When blended with the benchmark p-type polymer PTB7-Th, the NPI-based polymers afforded power conversion efficiencies of up to 1.6% and exhibited clear photoresponses at the absorption band of NPI-based polymers. This study shows that NPI-based polymers have good potential as n-type materials for use in organic photovoltaic cells.