The electrical characteristics of the systems Al/poly(4,4'-dipentoxy-2,2'-bithiophene)/ITO and of Al/SiO/poly(4,4'-dipentoxy-2,2'-bithiophene)/ITO were investigated in the dark and under white light illumination. It was found that a thin layer (similar to 0.4 nm) of silicon monoxide, at the interface between the conducting polymer and the aluminium electrode, is able to reduce the ion conductivity, induced by the formation of an aluminium hydroxide layer, in the structure Al/poly(4,4'-dipentoxy-2,2'-bithiophene)/ITO. The photovoltaic parameters of the junction, hidden by photoelectrochemical processes in the case without the SiO layer, were observed and discussed. A photosensitivity of the order of 10(-3) A W-1 was obtained under a reverse bias of -2 V, at low incident power irradiation.