The effect of annealing on the microstructure and electrical characteristics of poly(3-hexylthiophene)(P3HT)films doped with very small amounts of the electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F4-TCNQ)is studied.X-ray diffraction and UV-vis spectrum studies show that unlike the pure P3HT film,the thermal treatment on the doped films under an Ar atmosphere can effectively enhance the crystalline order of P3HT films,as well as successfully facilitate the orientation of the polymer chains.This improvement is attributed to the electrostatic force between P3HT and F4-TCNQ molecules.This force induces the polymer chains to crystallize and orient during the annealing process.As a result,annealing significantly improves performance,especially for the Ion/Ioff ratio of the TFTs based on the doped P3HT films.