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 ch loride, 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 spectr oscopies. The impurity is an octahedral iron(III) complex. The electri cal response of thin polymer films was investigated in the form of met al-insulator-semiconductor (MIS) structures. Analysis of MIS field eff ect transistors (MISFETs) shows that both field effect mobility and bu lk conductivity decrease as a function of impurity concentration. Capa citance-voltage (C-V) measurements performed on MIS structures show th at the concentration of immobile, and ionized, electron-acceptor impur ities increases with concentration of residual iron complexes. Evidenc e is provided for the formation of a relatively ''thick'' accumulation layer at the polymer/insulator interface. The thickness of the accumu lation layer was found to be field dependent.