Resolving the molecular diffusion model based on butterfly-shaped non-fused ring electron acceptors for efficient ternary organic photovoltaics with improved stability

Achieving both high efficiency and stability is essential for the commercial applications of organic photovoltaics (OPVs). However, the molecular diffusion behaviour of small molecule acceptors (SMAs) under light or thermal stress is detrimental to device stability. Herein, we developed two butterfly-shaped non-fused ring electron acceptors (NFREAs), X7-D and X8-D, featuring four outstretched terminal groups that are fluorinated in X7-D and chlorinated in X8-D, aimed at developing thermodynamically stable systems. These NFREAs were subsequently incorporated as the third component into the D18:Y6-based binary OPV for higher efficiency and better stability. Based on this system, the molecular diffusion model was quantitatively resolved, establishing a correlation between the diffusion coefficient and thermal characteristics, expressed by the equation D85 = 5.7 x 108e(-0.15Tg). The fluorinated X7-D not only serves as an effective morphology stabilizer, suppressing molecular diffusion to maintain a robust morphology, but also facilitates faster charge separation and enhances intermolecular interactions for efficient charge transport. Consequently, the efficiency increased to 18.80%, accompanied by improved photo stability and thermal stability in D18:Y6:X7-D-based ternary OPVs. Our work underscores the potential of butterfly-shaped NFREAs in achieving high-performance and stable OPVs. Additionally, it provides invaluable insights into prescreening the stability of emerging materials through simple thermal characteristics, thus holding great promise for accelerating the development of cost-effective and durable OPV technologies.