Shaken, not stirred—or rather phase-separated? Photophysics of pentacene-based binary blends and the relevance of local structure

The electronic and optical properties of organic semiconductors are strongly determined by their structural properties. Combining two or more semiconductor compounds in multicomponent blends is commonly exploited to tune the properties and meet the requirements for high-performance applications such as organic solar cells, light-emitting diodes, and transistors. Here, we discuss the structural and optical properties characteristic of binary systems containing the archetypal organic semiconductor pentacene. Typical examples are shown for the formation of a solid solution, a co-crystal, and phase separation. They not only allow the elucidation of the mixing behavior in more complicated binary systems but also broaden the understanding of binary blends in general. We highlight the importance of the local occupation configuration geometry and environment beyond the global thermodynamic mean-field considerations of mixing vs demixing. A thorough understanding of mixing in small organic molecule binary blends on the nanoscale is highly beneficial for capitalizing on the full power of organic semiconductors.