2020, Vol.93, No.8


Incorporation of an electron-deficient building unit based on a large π-system into the polymer backbone is a powerful strategy for the development of high-performance π-conjugated polymers. The naphthobispyrazine (NPz) tetracyclic π-electron system is a promising electron-deficient building unit but its use in π-conjugated polymers has seldom been investigated. Here, we synthesized four NPz-based π-conjugated polymers with different donor units, namely thiophene (P1), bithiophene (P2), terthiophene (P3), and quaterthiophene (P4), and systematically studied their structure-property relationships. The absorption range was significantly red-shifted when the spacer length was longer than bithiophene due to the decreased steric hindrance between the NPz moieties. Furthermore, the crystallinity in the polymer thin films was dramatically improved when the spacer length was longer than terthiophene. These results clearly affect the efficiency of polymer/fullerene-based solar cells, with the maximum efficiency of 4.4% obtained using P3. This study may be helpful for the design of NPz-based π-conjugated polymers with improved performance.