A donor-acceptor polymer semiconductor, PDQT, comprising diketopyrrolopyrrole (DPP) and beta-unsubstituted quaterthiophene (QT) for organic thin film transistors (OTFTs) is reported. This polymer forms ordered layer-by-layer lamellar packing with an edge-on orientation in thin films even without thermal annealing. The strong intermolecular interactions arising from the fused aromatic DPP moiety and the DPP-QT donor-acceptor interaction facilitate the spontaneous self-assembly of the polymer chains into close proximity and form a large pi-pi overlap, which are favorable for intermolecular charge hopping. The well-interconnected crystalline grains form efficient intergranular charge transport pathways. The desirable chemical, electronic, and morphological structures of PDQT bring about high hole mobility of up to 0.97 cm(2)/(V.s) in OTFTs with polymer thin films annealed at a mild temperature of 100 degrees C and similarly high mobility of 0.89 cm(2)/(V.s) for polymer thin films even without thermal annealing.