A donor-acceptor (D-A) semiconducting copolymer, PDPP-TVT-29, comprising a diketopyrrolopyrrole (DPP) derivative with long, linear, space-separated alkyl side-chains and thiophene vinylene thiophene (TVT) for organic field-effect transistors (OFETs) can form highly -conjugated structures with an edge-on molecular orientation in an as-spun film. In particular, the layer-like conjugated film morphologies can be developed via short-term thermal annealing above 150 degrees C for 10 min. The strong intermolecular interaction, originating from the fused DPP and D-A interaction, leads to the spontaneous self-assembly of polymer chains within close proximity (with -overlap distance of 3.55 angstrom) and forms unexpectedly long-range -conjugation, which is favorable for both intra- and intermolecular charge transport. Unlike intergranular nanorods in the as-spun film, well-conjugated layers in the 200 degrees C-annealed film can yield more efficient charge-transport pathways. The granular morphology of the as-spun PDPP-TVT-29 film produces a field-effect mobility ((FET)) of 1.39 cm(2) V-1 s(-1) in an OFET based on a polymer-treated SiO2 dielectric, while the 27-angstrom-step layered morphology in the 200 degrees C-annealed films shows high (FET) values of up to 3.7 cm(2) V-1 s(-1).