Advanced self-assembly control of rod-shaped organic semiconductors

Numerous rod-like organic molecules self-organize into layered structures, as demonstrated in various systems, including smectic liquid crystals, micelles, and lipid bilayers, owing to side-by-side intermolecular interactions. These layered structures are of interest to another class of layered molecular assemblies in the solid state, namely, small-molecule organic semiconductors (OSCs). Many OSCs are composed of various π-electron cores substituted with flexible side chains. Layered-structure formation is important in OSC systems because it enables the production of aligned molecular layers interfaced with gate dielectric layers, which are used to fabricate high-performance organic thin-film transistors (OTFTs). Using these rod-shaped OSCs, we developed a technique for fabricating single-crystal thin films of uniform thickness at the molecular level by introducing a geometric frustration effect between the layers. We also selectively produced crystalline polymorphs with distinct herringbone packing motifs. By producing these layer-controlled films on the trap-minimized surface of gate insulators, we fabricated OTFTs exhibiting sharp on/off switching characteristics approaching the Boltzmann limit. In this paper, we introduce recent techniques for the rational design of organic semiconductors.