Structurally ordered organic semicondoctor nanoparticles, which form a stable nanoparticle dispersion in an appropriate liquid, are generated from regioregular poly(3,3"'-didodecylquarterthiophene) (PQT-12). This dispersion enables facile solution fabrication of high-performance semiconductor layers composed of extensive crystalline domains of lamellar π-π stacks (see Figure), yielding organic thin-film transistors with excellent field-effect-transistor properties.