The performance of a regioregular polythiophene semiconductor for solution-processed organic thin-film transistors (OTFTs) is critically dependent on its ability to self-organize from solution and its stability against p-doping by oxygen. Structural features that promote lamellar ordering and delicately curtail the /spl pi/-conjugation of regioregular polythiophene system would enable achievement of high field-effect mobility in air. Molecular design principles and structure-property correlation studies that lead to the design of a solution-processed, all-round high-performance polythiophene semiconductor system (PQT) for OTFTs are discussed. Mobility to 0.14 cm/sup 2/ V/sup -1/ s/sup -1/ and current on/off ratio of 10/sup 7/, together with other desirable thin-film transistor (TFT) properties have been obtained with this class of organic semiconductors in OTFTs under ambient fabrication conditions.