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 (E-HOMO) for cutting-edge organic solar cells (OSCs) with a high open-circuit voltage (V-OC). Although halogen substitution has become the most popular strategy to lower the E-HOMO of donor materials, there still remains great room for exploring new approaches to construct wide-band gap small-molecule donors (SMDs) with a deeper E-HOMO. For this purpose, in this work, we primarily took a strategy of diester-terthiophene as a pi-bridged unit to construct a type of wide-band gap SMDs named BER6 and BECN with an A-pi-D-pi-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 E-HOMO 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 V-OC 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.