NATURE OF IMPURITIES IN PI-CONJUGATED POLYMERS PREPARED BY FERRIC-CHLORIDE AND THEIR EFFECT ON THE ELECTRICAL-PROPERTIES OF METAL-INSULATOR-SEMICONDUCTOR STRUCTURES

Poly(3-hexylthiophene), prepared by oxidative coupling using ferric chloride, has been purified to different extents so as to yield a series of samples with identical chemical structure and molecular weight but containing varying levels of residual iron impurity (0.05-3.2 wt % Fe). The polymers were analyzed by Mossbauer, UV-vis, IR, and EPR spectroscopies. The impurity is an octahedral iron(III) complex. The electrical response of thin polymer films was investigated in the form of metal-insulator-semiconductor (MIS) structures. Analysis of MIS field effect transistors (MISFETs) shows that both field effect mobility and bulk conductivity decrease as a function of impurity concentration. Capacitance-voltage (C-V) measurements performed on MIS structures show that the concentration of immobile, and ionized, electron-acceptor impurities increases with concentration of residual iron complexes. Evidence is provided for the formation of a relatively ''thick'' accumulation layer at the polymer/insulator interface. The thickness of the accumulation layer was found to be field dependent.