Highly Efficient Non‐Fullerene Polymer Solar Cells Enabled by Wide Bandgap Copolymers With Conjugated Selenyl Side Chains

In this work, the authors design and synthesize two novel wide bandgap copolymers based on selenophene substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTSe) as the donor unit and fluorinated benzotriazole as the acceptor unit for high performance non‐fullerene polymer solar cells (NF‐PSCs). A larger maximum molar extinction coefficient ( ϵ ) of 8.54 × 10 4 M −1 cm −1 is achieved when introducing sulfur atom onto the two‐dimensional (2D) BDTSe units, which should realize the better complementary absorption with ITIC as the acceptor, leading to a higher J sc of 19.51 mA cm −2 . Furthermore, a lower highest occupied molecular orbital (HOMO) energy level with almost no change in bandgap can be also achieved after inserting the sulfur atoms, thus resulting in an enhanced open‐circuit voltage ( V oc ) of 0.84 V without sacrificing the short‐current density ( J sc ). In addition, the higher crystallinity and optimized morphology are found to be beneficial to more efficient exciton dissociation and charge transport, giving rise to a higher fill factor (FF) of 75.1% and an elevated power conversion efficiency (PCE) of 12.31%. The results indicate that the strategy of alkylthioselenyl side chains on the BDT unit for constructing the donor‐acceptor (D‐A) copolymer donor materials is an excellent approach for realizing highly efficient NF‐PSCs.