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

Two novel wide bandgap copolymers based on quinoxalino6,5‐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 π–π* 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)‐dithieno2,3‐d:2′,3′‐d′]‐s‐indaceno1,2‐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 nanophase 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.