Preventing isomerization of the fused-ring core by introducing a methyl group for efficient non-fullerene acceptors

Abstract

Different cyclization reaction sites on the benzene ring will produce isomers during the cyclization process for the synthesis of small-molecule acceptors. Herein, we developed a structural unit (named BTMe) that can prevent isomerization during cyclization by introducing a methyl into a benzo[b]benzo[4,5]thieno[2,3-d]thiophene (BTBT) unit. Two fused ring electron acceptors (BTMe-C6-2F and BTMe-C8-2F) with the BTMe core were synthesized by changing the length of the side chain. When they were blended with donor PM6, the active layer PM6:BTMe-C8-2F exhibited a more excellent charge transport, phase morphology and π–π stacking, which are beneficial to improve the short-circuit current density (J_sc) and fill factor (FF). Compared with BTMe-C6-2F, the organic solar cell (OSC) based on BTMe-C8-2F exhibited a higher power conversion efficiency (PCE) of 12.72% with a V_oc of 0.97 V, a J_sc of 19.65 mA cm⁻² and a FF of 66.76%. These results proved that it is a promising approach to achieve a high-performance small-molecule acceptor by changing the length of the side chain and introducing methyl groups to prevent the isomerization of the fused ring core owing to special reaction sites.