A novel diketopyrrolopyrrole-based π-conjugated copolymer P(DPP-alt-DTBSe), 5, and a known copolymer P(DPP-alt-QT), 4, have been synthesized in 80-90% yield using the Stille coupling reaction. The molecular weights of 4 and 5 are 58,781 and 19,271 g/mol, respectively, with polydispersity values of 3.25-3.35. A relatively small band gap of 1.32-1.39 eV and excellent solubility in organic solvents were achieved in the two polymers. Thin-film transistors made of 5 exhibit outstanding performance (e.g., μ > 1.0-1.5 cm(2)·V(-1)·s(-1), I(on)/I(off) > 10(5)-10(6)) with a conventional n-octyltrichlorosilane-SiO(2) gate dielectric.