A medium-bandgap acceptor incorporating a novel electron-rich building block enables efficient organic photovoltaics.

Organic photovoltaics (OPVs) have emerged as a highly promising renewable energy technology due to their solution-processability, mechanical flexibility, and potential for low-cost manufacturing. Despite remarkable progress, further improving the power conversion efficiencies (PCEs) remains a critical challenge for their commercial applications. The incorporation of medium-bandgap non-fullerene acceptors (NFAs) as third components in ternary OPV devices has proven particularly effective in enhancing device performance. In this work, we designed a moderately electron-rich building block, 3-octylbenzo[b]thiophene (3-OBT), and utilized it to develop a medium-bandgap NFA (2Z,2'Z)-4,10-bis(4-hexylphenyl)-6,12-di(5-[2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile]-4-octylthiophen-2-yl)indaceno[1,2-b:5,6-b']dithiophene (X1). Binary OPV devices based on X1 exhibited a high open-circuit voltage of 1.03 V. Furthermore, incorporating X1 as a third component in ternary blends increased the PCE from 18.5 % to 19.7 %, highlighting its effectiveness as a high-performance third compound in ternary OPV systems. The successful development of the moderately electron-rich building block 3-OBT provides a valuable molecular design strategy for constructing future medium-bandgap NFAs. Moreover, the successful establishment of the synthetic route for X1 provides practical guidance for the development of other medium-bandgap NFAs.