NIR-Absorbing Electron Acceptor Based on a Selenium-Heterocyclic Core Attaching to Phenylalkyl Side Chains for Polymer Solar Cells with 17.3% Efficiency.

Selenium-heterocyclic and side-chain strategies for developing near-infrared (NIR) small fused-ring acceptors (FRAs) to further obtain short-circuit current density (Jsc) have proven advantageous in the top-performing polymer solar cells (PSCs). Herein, a new electron-rich central selenium-containing heterocycle core (BTSe) attaching alkyl side chains with a terminal phenyl group was coupled with a difluorinated and dichlorinated electron-accepting terminal 1,1-dicyanomethylene-3-indanone (IC) to afford two types of new FRAs, BTSe-IC2F and BTSe-IC2Cl. Interestingly, in spite of the weaker intramolecular charge transfer, BTSe-IC2F shows a stronger NIR response because of the smaller bandgap (Egopt) up to 1.26 eV, benefiting from the stronger ordered molecular packing in comparison to BTSe-IC2Cl with an Egopt of 1.30 eV. Additionally, thermal annealing induced an evident red shift by ∼50 nm in the absorption of D18:BTSe-IC2F blend films. Such a phenomenon may be attributed to the synergistic impact of the formation of inward constriction toward the molecular backbone because of the combination of bulky side chains and fluorinated IC as well as the reduced aromaticity of the selenium heterocycle. Consequently, the thermally annealed device based on BTSe-IC2F/D18 achieves a champion power conversion efficiency (PCE) of 17.3% with a high fill factor (FF) of 77.22%, which is among the highest reported PCE values for selenium-heterocyclic FRAs in binary PSCs. The improved Jsc and FF values of the D18:BTSe-IC2F film are simultaneously achieved mainly because of the preferred face-on orientations, the well-balanced electron/hole mobility, and the favorable blend morphology compared to D18:BTSe-IC2Cl. This work suggests that the selenium-heterocyclic fused-ring core (with proper side chains) combined with fluorinated terminal groups is an effective strategy for obtaining highly efficient NIR-responsive FRAs.