Poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-2-yl)benzo[1,2- b:4,5- b ‘]dithiophene) 1 represents a new class of polymer semiconductors which self-assemble into higher structural orders without thermal annealing and provide excellent field-effect transistor performance with mobility up to 0.25 cm 2 V -1 s -1 when used as a solution-processed thin-film semiconductor in thin-film transistors.