The first example for thienoacene derivatives with selective growth of different crystal polymorphs is simply achieved by solution-phase self-assembly. Compared with platelet-shaped a-phase crystals, organic field-effect transistors (OFETs) based on microribbon-shaped beta-phase crystals show a hole mobility up to 18.9 cm(2) V-1 s(-1), which is one of the highest values for p-type organic semiconductors measured under ambient conditions.