Medium-Band-Gap, Fully Nonfused Electron Acceptors with High Voc 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, 2T-BO-1 and 2T-BO-2, 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 (Egopt ≈1.70 eV) coupled with high lowest unoccupied molecular orbital (LUMO) levels (∼ -3.71 eV), contributing to enhanced open-circuit voltage (Voc) for OSCs. Side chain engineering is applied to the regulation of molecular crystallinity, active layer morphology, and molecular orientation in films. Compared to 2T-BO-1, the 2T-BO-2 blend film displays homogeneous morphology, suppresses the bimolecular recombination, and has high and balanced charge mobility with wide-band-gap polymer reg-PThE as a donor. As a result, the 2T-BO-2-based device can achieve a higher power conversion efficiency (PCE) of 10.19% with a high Voc of 1.00 V. Subsequently, 2T-BO-2 as a third component is employed to fabricate ternary OSCs. A D18:L8-BO:2T-BO-2 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.