The electrical properties of junctions between aluminium and doped polypyrrole

Polypyrrole (PPy) doped with small anions like ClO4- or large polymeric anions such as poly(styrenesulphonate) (PSS-) forms a conducting organic solid. The electrical properties of metal contacts to this material were evaluated by current-voltage characteristics and impedance spectroscopy measurements. The I-V characteristics of the PSS--doped device was symmetrical but non-ohmic, while that of the ClO4--doped diode was rectifying. The complex impedance spectra of PSS--doped structures showed two partially overlapping semi-circles, which revealed the existence of two distinct regions at the metal/doped PPy junctions. They were modelled by an equivalent circuit consisting of two parallel RC circuits in series representing a thin insulating interfacial layer and a depletion region. The impedance spectra of ClO4--doped devices manifested a single semi-circle whose diameter depended on the bias voltage. In this case, the thin interfacial non-conducting layer was absent.