Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film Nonfullerene Solar Cells.

A nonfullerene acceptor, IDTT-OB, employing indacenodithieno[3,2- b]thiophene (IDTT) decorated with asymmetric substituents as the core, is designedly prepared. In comparison with the analogue IDT-OB, extending the five-heterocyclic indacenodithiophene (IDT) core to seven-heterocyclic fused ring endows IDTT-OB with more broad absorption and elevated highest occupied molecular orbital energy level. In addition, IDTT-OB shows a more intense molecular packing and a higher crystalline behavior with a strong face-on orientation in the neat film and the PBDB-T:IDTT-OB blend film. Furthermore, an ideal nanomorphology with a domain size of 19 nm can be obtained, which is in favor of exciton diffusion and charge separation. Accordingly, PBDB-T:IDTT-OB-based polymer solar cells demonstrate a maximum power conversion efficiency (PCEmax) of 11.19% with an impressive fill factor of 0.74, comparable to the state-of-the-art acceptors with similar molecular backbones. More importantly, IDTT-OB-based devices show good tolerance to the film thickness, which maintain a high PCE of 10.20% with a 250 nm thick active layer, demonstrating that the asymmetric acceptor is profound for fabricating high-efficiency thick-film nonfullerene solar cells.