Nonfullerene Acceptors based on Octyl-Substituted Dithienonaphthobisthiadiazole as a Fused π-Extended Electron-Accepting Central Unit

Abstract

The development of nonfullerene acceptors (NFAs) is critical for advancing organic solar cells (OSCs) and organic photo detectors (OPDs). Modular-type non-fused NFAs-constructed by linking electron-donating and electron-accepting unitshave attracted interest due to their synthetic simplicity and structural tunability. However, the repertoire of available central electron-accepting units remains limited. Here, we synthesized an octylthiophene-fused naphthobisthiadiazole (TNTz) as a new electron-accepting unit and developed a TNTz-containing NFA, named TNTz-DCI. The TNTz framework was constructed through the formation of fused thiophene rings via intramolecular cyclization reactions. Introduction of alkyl groups on the thiophene rings enhanced the solubility of TNTz-DCI compared to a reference compound NTz-DCI. Theoretical calculations using dimer models indicated that TNTz-DCI shows stronger intermolecular interactions than NTz-DCI. Due to the electrondonating nature of the fused thiophene rings, TNTz-DCI exhibited a shallower lowest unoccupied molecular orbital energy level compared to NTz-DCI. The exciton binding energies (E_b) of TNTz-DCI and NTz-DCI were estimated to be 0.29 and 0.31 eV, respectively. OSCs based on TNTz-DCI with PBDB-T as the donor exhibited a higher power conversion efficiency than those based on NTz-DCI. The PBDB-T:TNTz-DCI-based films also functioned as OPDs. The PBDB-T:TNTz-DCI films exhibited smooth surface morphology and favorable molecular orientation at the donor-acceptor interfaces. These results highlight the potentials of TNTz as a versatile electron-accepting unit for non-fused NFAs in OSCs and OPDs.