Ladder-Type Fused Benzodithiophene Extended along the Short-Axis Direction as a New Donor Building Block for Efficient Organic Solar Cells

Ladder-type fused aromatic systems are important core structures of small molecule acceptors for organic solar cells (OSCs). In this study, a new ladder-type donor building block, based on the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit where the 3,7 positions of the BDT thiophene rings and the 3′ position of the thiophene rings of the vertical BDT were fused to construct a seven-ring core structure named f-DTBDT, was investigated. In the f-DTBDT structure, the fusion of the BDT core and the thiophene rings at the 4,8 positions of BDT constrains all of the aromatic rings in a coplanar structure. The newly designed f-DTBDT was successfully employed as a core donor building block and conjugated with three electron-withdrawing acceptors (2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2HIC), 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC), and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2ClIC)) as acceptor–donor–acceptor (A–D–A)-type acceptor materials for OSCs. Characterization results showed that the three synthesized A–D–A acceptors of DTBDT-IC, DTBDT-4F, and DTBDT-4Cl have high absorption behavior in the vis–NIR region as result of an intramolecular charge transfer interaction engendered by f-DTBDT and the ending group. The absorption regions of the acceptors were complementary with that of polymer PM6. Also, the frontier orbital energy levels of the new acceptors and wide-band-gap PM6 are well matched. Bulk heterojunction OSCs were fabricated using PM6 and the acceptors, and the highest power conversion efficiency (PCE) of 10.15% was obtained when using PM6:DTBDT-4F as the active layer.