Medium-Band-Gap, Fully Nonfused Electron Acceptors with High <i>V</i>_oc for High-Efficiency Binary and Ternary Organic Solar Cells

Fully nonfused electron acceptors (FNEAs) have shown a huge potential for organic solar cells (OSCs). Herein, two medium-band-gap FNEAs, namely, <b>2T-BO-1</b> and <b>2T-BO-2</b>, are developed based on the "benzene-dithiophene-benzene" skeleton, with the assistance of alkoxyl side chains to form S···O conformational locks. Two FNEAs exhibit medium optical gaps (<i>E</i>_g^opt ≈1.70 eV) coupled with high lowest unoccupied molecular orbital (LUMO) levels (∼ -3.71 eV), contributing to enhanced open-circuit voltage (<i>V</i>_oc) for OSCs. Side chain engineering is applied to the regulation of molecular crystallinity, active layer morphology, and molecular orientation in films. Compared to <b>2T-BO-1</b>, the <b>2T-BO-2</b> blend film displays homogeneous morphology, suppresses the bimolecular recombination, and has high and balanced charge mobility with wide-band-gap polymer <b>reg-PThE</b> as a donor. As a result, the <b>2T-BO-2</b>-based device can achieve a higher power conversion efficiency (PCE) of 10.19% with a high <i>V</i>_oc of 1.00 V. Subsequently, <b>2T-BO-2</b> as a third component is employed to fabricate ternary OSCs. A <b>D18</b>:<b>L8-BO</b>:<b>2T-BO-2</b> ternary device can accomplish an impressive PCE of 19.37%. The research provides a rational molecular design strategy for high-efficiency, medium-band-gap FNEAs.