High-efficiency ternary nonfullerene organic solar cells with record long-term thermal stability

Achieving high-efficiency organic solar cells (OSCs) with long-term thermal stability is a major challenge toward commercialization. In the present study, a ternary blend strategy was employed to improve the thermal stability of OSCs with a novel small molecular acceptor ITC6-2F serving as the second acceptor. Compared with its parent compound ITC6-IC, the introduction of fluoro substituents at the two end groups can reduce the π-π stacking distance and enhance the intermolecular interactions in films. The PBDB-T:IDT-PDOT-C6:ITC6-2F-based ternary blend film exhibits a reduced lamellar distance and an increased crystalline coherence length, compared to the corresponding binary blend films. Photo-physics and device physics analyses demonstrate that the charge-transfer state energy is dominated by PBDB-T and IDT-PDOT-C6 in the ternary OSCs. Also, ITC6-2F can facilitate the photonic energy transfer to IDT-PDOT-C6 and promote more excitons to reach the donor/acceptor (D/A) interfaces to achieve high-efficiency photoelectric conversion. More importantly, the enhanced intermolecular interactions and good miscibility between the two acceptors help to "freeze"the film morphology, leading to a significantly improved long-term thermal stability. The device efficiency remained at 80.3% of its initial value after 137 days of continuous heating at 75 °C in a nitrogen-filled glove box, which is a record result for high-efficiency OSCs reported so far.