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

A non-fullerene acceptor, IDTT-OB, employing indacenodithieno[3,2-b]thiophene as the core with asymmetric side chains, is designed and synthesized. In comparison with the analogue IDT-OB, extending the five-heterocyclic indacenodithiophene core to seven-heterocyclic fused ring endows IDTT-OB with more broad absorption and elevated HOMO energy level. In addition, IDTT-OB shows a more intense molecular packing and a higher crystallinity with a strong face-on orientation in neat film and the PBDB-T:IDTT-OB blend film. Furthermore, an ideal nanoscale phase separation with appropriate domain size of 19 nm can be obtained, which is beneficial for the 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 FF 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 the active-layer thickness of 250 nm. The above results suggest that the asymmetric acceptor is profound for designing high performance thick film non-fullerene solar cells.