Simple solvent-vapor annealing is used to fabricate single crystals of dioctylbenzothienobenzothiophene on a polymer dielectric surface. By involving self-organized phase separation, the crystal length is enhanced and a good semiconductor/ insulator interface is obtained. The field-effect transistors (FETs) exhibit an average p-type FET mobility of 3.0 cm(2) V-1 s(-1), with a highest value of 9.1 cm(2) V-1 s(-1). The FET mobility increases as temperature decreases, which suggests intrinsic bandlike transport.