Optimization of Film Morphology and Photovoltaic Performance by Employing Asymmetric π-Bridges in Dithienobenzodithiophene-Based Polymer Donors

Understanding the impact of molecular structure on the molecular packing arrangement and aggregation behaviors of organic semiconductor materials is crucial for investigating their properties in multiple organic photoelectrical applications. In this study, a high-performance polymer donor based on dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (FBT) unit, named PDTBDT-Cl-TFBT, was designed and synthesized by introducing an asymmetric 3-octylthiophene π-bridge between the donor and acceptor segment. The density functional theory (DFT) calculation reveals that the asymmetric π-bridge increases the average dipole moment of the repeating units as well as the configurational disorder in polymer PDTBDT-Cl-TFBT, resulting in the overall diminished self-aggregation and crystallinity, which leads to higher miscibility with the nonfullerene acceptor Y6 than that of the symmetric π-bridge-modified polymer PDTBDT-Cl-DTFBT. This feature leads to a suitable phase separation in the PDTBDT-Cl-TFBT:Y6 blend and contributes to better photovoltaic performance. As a result, organic solar cells (OSCs) based on PDTBDT-Cl-TFBT:Y6 achieve a notably higher power conversion efficiency (PCE) of 14.13%, surpassing the performance of that based on PDTBDT-Cl-DTFBT:Y6 (9.63%). Detailed analyses indicate that the performance enhancement is primarily attributed to the reduced trap density, mitigated energetic disorder, improved charge transport, and suppressed charge recombination. This research uncovers an effective strategy for optimizing the film morphology and photovoltaic performance of DTBDT-based polymer donors.