This study demonstrates that thin-film transistors with double-coated poly(3-hexylthiophene) (P3HT) channel layers (DPTFTs), which are formed by coating a second P3HT layer on top of the N2-annealed first P3HT layer, show a higher on/off current ratio and better subthreshold swing, compared to single-layer P3HT transistors (PTFTs). Characteristics of DPTFTs were investigated by varying the thicknesses of the first P3HT layer and the second P3HT layer. Moreover, DPTFTs with an as-prepared first layer, i.e., without N2 annealing, were also prepared for comparison. A thin gate oxide was incorporated into the DPTFTs, which resulted in an impressive subthreshold swing (smaller than 1 V/decade). Furthermore, the study proposes new DPTFTs with an N2-annealed first layer and a functionalized single-wall carbon nanotube (F-SWNCT)-doped P3HT second layer. Significant improvements are observed not only in subthreshold swing and on/off current ratio but also in mobility induced by the innovative channel structure and doping of F-SWCNTs, respectively.