Record Carrier Diffusion Lengths in Large, Dense Nonfullerene Electron Acceptor Crystals Grown from Polar Aromatic Solvents.

Semicrystalline films of nonfullerene acceptors (NFAs) attract considerable interest for optoelectronic applications. Organic single crystals promise superior optoelectronic properties to films; however, NFA crystals have been little used in applications because of their challenging growth and micrometer-nanometer sizes. Here, phenol and acetophenone are used as aromatic, halogen-free recrystallization solvents, resulting in the growth of millimeter-sized Y6 crystals of varying polymorphisms─a crystallization technique which generalizes to other NFA molecules. The crystals show a novel structure which is denser than any previously reported for Y6. We use power-dependent fluorescence and fluorescence microscopy to demonstrate enhanced exciton diffusion lengths (70 ± 15 nm) and novel photophysical responses in these crystals compared to thin films, and compare these properties to quantum chemical calculations. This work demonstrates how material processing can improve future optoelectronics and emphasizes the critical role of molecular packing in current high-performance NFA devices.