Poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiop hene) 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.