Wide Bandgap Copolymers Based on Quinoxalino[6,5-f]quinoxaline for Highly Efficient Nonfullerene Polymer Solar Cells

Two novel wide bandgap copolymers based on quinoxalino[6,5-f]quinoxaline (NQx) acceptor block, PBDT-NQx and PBDTS-NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low-lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS-NQx to -5.31 eV, contributing to a high open-circuit voltage, V-oc, of 0.91 V. Conjugated n-octylthienyl side chains attached on the NQx skeletons also significantly improve the pi-pi* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short-circuit current density, J(sc), value of 15.62 mA cm(-2). The sulfur atom insertion also enhances the crystallinity of PBDTS-NQx and presents its blend film with a favorable nano-phase separation, resulting in improved J(sc) and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high-performance wide bandgap copolymers but also provides the NQx-based copolymers for achieving highly efficient nonfullerene PSCs.