Fine-Tuning Energy Levels and Molecular Packing of Asymmetric End Group Non-Fullerene Acceptor for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have emerged as promising photovoltaic technology because of low-cost, lightweight, flexibility, and transparency. Recently, the power conversion efficiencies (PCEs) of OSCs have exceeded 18% with the development of non-fullerene acceptors (NFAs). Reported Y6 which has an A-DA’D-A structure, various Y-series derivatives have been designed. However, a trade-off between an open-circuit voltage (<i>V</i>_OC) and a short-circuit current density (<i>J</i>_SC) remains a problem even though the PCEs of OSCs based on the Y-series derivatives have increased rapidly. Hence, it is crucial to finely tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of photoactive materials for balance between <i>V</i>_OC and <i>J</i>_SC. In this work, we report efficient and stable OSCs based on a new asymmetric NFA, IPC-BEH-IC2F. This asymmetric NFA is comprised of a weak electron-donating core dithienothiophen[3,2-<i>b</i>]-pyrrolobenzothiadiazole (BEH) and two kinds of end groups having strong electron-accepting ability 9<i>H</i>-indeno[1,2-<i>b</i>]pyrazine-2,3-dicarbonitrile (IPC) with tricyclic fused ring and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1<i>H</i>-inden-1-ylidene)malononitrile (IC2F) which used as an end group in Y6. The IC2F-BEH-IC2F (or Y6) and IPC-BEH-IPC having symmetric structures were also characterized to compare with asymmetric IPC-BEH-IC2F. The IPC moiety significantly affects the optical properties and electronic structures of the NFAs. The asymmetric IPC-BEH-IC2F shows the highest extinction coefficient among the three NFAs due to its strong dipole moment and highly crystalline feature. Its HOMO and LUMO energy levels are successfully tuned between those of the IC2F-BEH-IC2F and IPC-BEH-IPC through the asymmetric structure. Besides, the IPC moiety promotes strong intermolecular π-π stacking due to their tricyclic fused system, which is beneficial for charge transport. Inverted type OSCs based on polymer:NFA blends were fabricated. Among all these polymer:NFA based OSCs, the asymmetric IPC-BEH-IC2F based OSC blended with PBDB-T showed a balance between <i>V</i>_OC and <i>J</i>_SC, leading to the highest PCE of 12.70% with a high <i>V</i>_OC of 0.85 V and a <i>J</i>_SC of 22.29 mA cm^-2. In addition, the PBDB-T:IPC-BEH-IC2F based OSC also exhibited the best long-term stability under ambient conditions due to the strongly interacting IPC moiety, which forms the densely packed morphology and prevents penetration of Ag atom into the photoactive layer. These results demonstrate that using asymmetric NFAs can be useful for improving the photovoltaic performance and stability of OSCs.