A thieno[3,4-b]thiophene linker enables a low-bandgap fluorene-cored molecular acceptor for efficient non-fullerene solar cells

A low-bandgap small-molecule acceptor NFTI with a fluorene central core, 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propane-dinitrile end groups and thieno[3,4-b]thiophene linkers was designed and synthesized for bulk-heterojunction organic solar cell applications. NFTI exhibits broad absorption with a low optical bandgap of approximately 1.57 eV in the thin film state. An optimized power conversion efficiency (PCE) of 9.02% with a high short-circuit current of 17.8 mA cm (2) was achieved with diphenyl ether (DPE) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) binary-processing additives. According to the detailed morphological investigation, we found that binary-processing additives helped to reduce the size of nanocrystals and enhance the intermolecular interaction, which led to improved charge separation and transport in the BHJ thin film, and thus achieved high device performance.