Discerning Variable Extents of Interdomain Orientational and Structural Heterogeneity in Solution-Cast Polycrystalline Organic Semiconducting Thin Films

By spatially resolving the polarized ultrafast optical transient absorption within several tens of individual domains in solution-processed polycrystalline small-molecule organic semiconducting films, we infer the domains’ extents of structural and orientational heterogeneity. As metrics, we observe variations in the time scales of ultrafast excited state dynamics and in the relative strength of competing resonant probe transitions. We find that films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) exhibit a much higher degree of both structural and orientational heterogeneity among their domains than do films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn), despite the apparent structural similarity between these two small molecules. Since both molecules feature prominently in solution-processed organic transistors, correlating the extent of heterogeneity to bulk transport using our approach will be highly valuable toward determining the underlying design principles for creating high-performing devices. Furthermore, our ability to characterize such variation in heterogeneity will enable fundamental studies of the interplay between molecular dynamics and driving forces in controlling emergent unequilibrated structures.