Over 18% Efficiency from Halogen-Free Solvent-Processed Polymer Solar Cells Enabled by Asymmetric Small Molecule Acceptors with Fluoro-Thienyl Extended Terminal

The potential impact of end-group (EG) in non-fullerene acceptor (NFA) on enabling green solvent-processable polymer solar cells (PSCs) remains underexplored, offering opportunities for advancements in environmentally friendly PSC development. Herein, the EG of 1 ',1 '-dicyanomethylene-4-fluoro-5-thienyl-3-indanone (IC-FT) is developed by modifying the state-of-the-art of Y6 derivative NFA, BTP-4F, resulting in two novel NFAs, namely BTP-FT and BTP-2FT. Distinctively, this study reveals that it is the noncovalent F<middle dot><middle dot><middle dot>S interaction, other than the commonly believed strong hydrogen bonding of F<middle dot><middle dot><middle dot>H that plays a key role in determining the final molecular conformation, as confirmed by means of 2D NMR study and Gibbs free energy calculations. The asymmetric BTP-FT possesses an upshifted lowest unoccupied molecular orbital level and enhances solubility in toluene. Consequently, it can mitigate phase separation, promote the formation of nanofibrillar morphology, facilitate exciton dissociation, and ultimately enhance the performance of the PSCs, achieving a high open circuit voltage of 0.900 V and a power conversion efficiency (PCE) of 17.56%. Furthermore, the ternary blend PM6:BTP-FT:BTP-4F achieves an enhance PCE of 18.39% in devices processed from toluene. This study offers a novel perspective on NFA design for high-efficiency and eco-friendly processable PSCs by enriching the array of electron-withdrawing EGs on NFA molecules.