Design and synthesis of the quinacridone-based donor polymers for application to organic solar cells

Abstract

D–π–A type quinacridone-based polymers were designed with different acceptor moieties and successfully synthesized via a Suzuki coupling reaction. Depending on the introduction of benzothiadiazole and quinoxaline moieties, the synthesized polymers, poly(quinacridone-thienothiphene-benzothiadiazole) (PQCfTB) and poly(quinacridone-thienothiphene-quinoxaline) (PQCfTQx), showed different physical, optical and electrochemical properties. PQCfTB had a more extended π-conjugation length and a higher number average molecular weight (Mn, 365.1 kDa) than that of PQCfTQx (167.5 kDa). Furthermore, each polymer showed different optical behavior and optical band gaps (Egopt: 1.91 eV for PQCfTB; 1.87 eV for PQCfTQx) in the film state. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PQCfTB and PQCfTQx were calculated as −5.20 eV and −5.22 eV, as well as −3.29 eV and −3.35 eV, respectively. The XRD measurement results showed that the polymers for PQCfTB and PQCfTQx had bimodal and face-on structures, respectively. Finally, organic solar cells for both polymers were fabricated and optimized with respect to the acceptor. In summary, the PQCfTB-based devices showed higher power conversion efficiencies (w/PC71BM 3.9%; w/IT-M 5.2%; w/PM6:Y6-BO-4Cl 11.0%) than PQcfTQx (2.3%, 4.7%, and 10.0%).