Low temperature aggregation transitions in N3 and Y6 acceptors enable double-annealing method that yields hierarchical morphology and superior efficiency in nonfullerene organic solar cells

Thermal transition of OSCs constituent materials are often insufficiently researched, resulting in trial-and-error rather than rational approaches to post-casting processing strategies to improve aggregation to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the high-performance acceptors. Here, by using an optical method, we discover that the acceptor N3 has a clear solid-state aggregation transition at 82 °C. The transition informs and enables a double-annealing method that can fine‐tune aggregation and the device morphology. Compared with 16.6% efficiency for the control devices, higher efficiency of 17.6% is obtained through the improved protocol. Design of high-performance acceptors with yet lower aggregation transitions might be required to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.