Wide-Band Gap Small-Molecule Donors with Diester-Terthiophene Bridged Units for High-Efficiency All-Small-Molecule Organic Solar Cells

It is crucial to maintain both an ordered intermolecular packing and deep highest occupied molecule orbital energy levels (EHOMO) for cutting-edge organic solar cells (OSCs) with a high open-circuit voltage (VOC). Although halogen substitution has become the most popular strategy to lower the EHOMO of donor materials, there still remains great room for exploring new approaches to construct wide-band gap small-molecule donors (SMDs) with a deeper EHOMO. For this purpose, in this work, we primarily took a strategy of diester-terthiophene as a π-bridged unit to construct a type of wide-band gap SMDs named BER6 and BECN with an A−π–D−π–A framework using alkyl-thienyl-substituted benzo-dithiophene as a central electron-donating (D) unit and rhodanine or cyanoacetate derivatives as a terminal electron-accepting (A) unit. It is found that both molecules have almost coincident coplanarity and dipole interaction for the diester-terthiophene unit. As a result, an ordered molecular stacking and a deep EHOMO of about −5.40 eV with an optical band gap of 1.85–1.91 eV are presented for both molecules. A high power conversion efficiency of 9.03% with a high VOC of 0.97 eV was obtained in the BER6/IDIC-based all-small-molecule OSCs (ASM-OSCs). Our results demonstrate that diester-terthiophene is a promising strategy to develop wide-band gap SMDs applied in high-performance ASM-OSCs.