Ester-functionalized quinoxaline-based polymers for application in organic photovoltaics

Abstract

Quinoxaline (Qx) is an interesting building unit for π-conjugated polymers as it allows us to easily control the electronic property by selecting the substituents. In this study, we synthesized ester-functionalized Qx (QE) and its polymers with octyl (PTQE-C8) and 2-ethylhexyl groups (PTQE-EH). Due to the electron-withdrawing property of the ester group, QE had strong electron-deficient nature and thereby the polymers had relatively low-lying HOMO energy levels of around −5.5 eV and relatively small optical bandgaps of narrower than 1.6 eV. It was found that PTQE-C8 consistently exhibited higher photovoltaic performance than PTQE-EH in both cells based on fullerene (PC61BM) and non-fullerene (Y6), in which the efficiencies were 5.8% and 9.2%, respectively. We ascribed the higher performance of PTQE-C8 compared to PTQE-EH to the higher crystallinity of the materials in the blend films. Interestingly, we also found that the polymers functioned as n-type materials. This work demonstrates that QE can be an interesting building unit for both p-type and n-type polymers.