We report on the improvement of field effect transistors based on Poly(3-hexylthiophene-2,5-diyl) (P3HT) as channel semiconductor and crosslinked poly(vinyl alcohol) (cr-PVA) as gate insulator, through the treatment of the cr-PVA film surface before P3HT deposition. We treated the cr-PVA either with hydrochloric acid (HCl) or with a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), aiming at the passivation of the hole traps at the cr-PVA/P3HT interface. The treatment with HCl leads to an excessive increase in the transistor leakage current and unstable electrical characteristics, despite implying in an increase of the gate capacitance. The treatment with CTAB leads to transistors with c.a. 50% higher specific capacitance and a tenfold increase in the charge carrier field-effect mobility, when compared to devices based on untreated cr-PVA.