Three copolymers of benzo[1,2-b:4,5-b ']dithiophene and 3,3'-bis(alkyl)-5,5'-bithiophene (dodecyl, tetradecyl and hexadecyl side chains) have been synthesized through Stille copolymerization. The polymers have number-average molecular weights over 20 kg/mol, are well-packed in the bulk and thin film, and possess an ionization potential of -5.1 eV in thin film, which offers stability versus oxidation in environmental conditions. The thin film packing of the polymer with dodecyl side chains leads to an excimeric emission upon excitation, which is not observed for longer side chain lengths. The presence of the dimers responsible for this excimer formation results in a device performance improvement as well. Field-effect transistors fabricated from these copolymers have On/Off ratios >10(7), equal saturation and linear hole mobilities above 10(-2) cm(2)/Vs, almost no hysteresis and turn-on voltages around 0 V in bottom-contact devices. (C) 2010 Elsevier Ltd. All rights reserved.