Presented here is a novel and efficient method used to improve carrier mobilities of poly(3-hexylthiophene) (P3HT)-based organic field effect transistors by means of nanowire formation. The treatment, termed solvation, consists of depositing a small quantity of a solvent directly on top of a previously deposited semiconducting film, and allowing the solvent to evaporate slowly. Such treatment results in an increase of the saturation mobility by more than one order of magnitude, from 1.3 × 10−3 up to 3.4 × 10−2 cm2/Vs, while devices preserve their high ON/OFF ratio of ∼104. The atomic force and scanning electron microscopy studies show that solvated P3HT layers develop a network of nanowires, which exhibit increased degree of structural order, as demonstrated by micro Raman spectroscopy. The time-of-flight photoconductivity studies suggest that higher hole mobility stems from a reduced energy disorder of the transporting states in these structures. Display omitted •Improvement in carrier mobilities, in P3HT-based organic transistors, by means of solvation treatment.•Improvement in carrier mobilities is over one order of magnitude.•Time of flight analysis also reveals improvement in mobilities.•AFM and SEM analyses reveal improvement is due formation of nanowires.