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 organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial-and-error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern high-performance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid-state aggregation transition at 82 degrees C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double-annealing method that can fine-tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC71BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x-ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double-annealing method is illustrated and easily transferred and validated with Y6-based devices, using the transition of Y6 at 102 degrees C. As a result, the PCE improved from 16.0% to 16.8%. Design of high-performance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.