Medium-Bandgap (Acceptor′–Donor)₂Acceptor-Type Small-Molecule Donors Based on an Asymmetric Thieno[3,2-c]isochromene Building Block for Organic Solar Cells with High Efficiency and Voltage

Small-molecule donors have a critical effect on the properties of the small-molecule-based organic solar cells (SM-OSCs). In order to develop novel small-molecule donors, two (A′–D)2A-type (A = acceptor; D = donor) small molecules (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT are designed and synthesized, in which 5,6-difluoro-2,1,3-benzothiadiazole (DFBT) and novel asymmetry electron-rich thieno[3,2-c] isochromene (TiC) are introduced as the central electron-accepting (A) and the armed electron-donating (D) units, respectively. Also, their photoelectronic properties are studied. A significant effect of the TiC and electron-accepting (A′) end units on these properties is observed. A deep HOMO energy level of −5.45 eV with a medium bandgap of 1.77 eV is exhibited. A significantly improved power conversion efficiency (PCE) of 7.55% is received in the (IDO-TiC-T)2-DFBT/PC71BM-based OSCs with a high open circuit voltage (VOC) of 0.9 V, which is 1.38 times higher than that in the (Rh-TiC-T)2-DFBT/PC71BM-based cells. Our results illuminate that the asymmetrical TiC unit has a great potential to build up small-molecule donors for OSCs with high PCE and VOC.