Achieving an Efficient and Stable Morphology in Organic Solar Cells Via Fine-Tuning the Side Chains of Small-Molecule Acceptors

Both the efficiency and stability of low-cost organic solar cells are central components for meeting the requirements of commercialization for organic photovoltaics (OPV). Furthermore, the relationship between the chemical structure of an active material and morphology and its effects on efficiency and stability is still largely undetermined. Additionally, both the kinetic and thermodynamic morphology states of an active layer can have a huge impact on efficiency and stability, even when the chemical structures of materials applied in the active layer are especially the same or similar. Here, using two series of acceptor–donor–acceptor (A–D–A)-type small-molecule acceptors (SMAs) with similar backbone structures, we demonstrate the relevance of fine-tuned chemical structures with their solution and solid-state properties, further leading to significantly different behavior in terms of both device efficiency and stability. This is also partially due to the different morphology states caused by such fine chemical structure tuning. Our results indicate that a delicate balance of molecular aggregation and ordered stacking morphology is required to achieve and lead to high efficiency and stability. Thus, among the two series of molecules, UF-EH-2F, with both optimal length and steric hindrance of side chains, achieves the preponderant morphology in its corresponding device, where its morphologies “efficient state” and “stable state” are almost overlapped, and thus lead to both the highest efficiency (power conversion efficiency, PCE = 13.56%) and the best stability. Our results indicate that it is highly possible to achieve the morphology state required for both high efficiency and stability simultaneously by fine-tuning the chemical structure of active materials for organic solar cells.