Novel narrow bandgap polymer donors based on ester-substituted quinoxaline unit for organic photovoltaic application

Two narrow bandgap conjugated polymers, PTT-EFQX and PT-DFBT-T-EFQX, comprising a novel bis(2-alkyl) 5,8-dibromo-6,7-difluoroquinoxaline-2,3-dicarboxylate (EF-Qx) unit, are designed and synthesized through palladium-catalyzed Stille coupling reaction. The ester groups and difluorine substituents are introduced into the EF-Qx unit to improve the electron affinity. The polymers containing EF-Qx unit exhibit a relatively narrow bandgap of around 1.6 eV. To optimize the effects of solubility and molecular packing, D-A structure and D-A1-D-A2 structure synthesis strategies are adopted in this work and the structure-property relationship is systematically studied. The results morphology characterizations indicate that the active layer based on D-A structure polymer PTT-EFQX has a superior morphology compared to active layer based on D-A1-D-A2 structure polymer PT-DFBT-T-EFQX, which helps PTT-EFQX-based polymer solar cells (PSCs) to obtain better PCEs of up to 5.4%. Our work demonstrates that the effective design strategy for development of quinoxaline-based polymer donor is necessary for controlling the morphology and thus achieving high-performance PSCs.