The electrical characteristics of Al/poly(4,4'-dipentoxy-2,2'bithiophene)/ITO and of Al/poly(3-butylthiophene)/ITO Schottky diodes were investigated in the dark and under white light illumination, In both cases a battery was found superimposed to the rectifying junctions, leading to a negative current for positive bias applied in the dark. Under illumination, the build-up of a photoelectrochemical cell was demonstrated, which prevents the photovoltaic parameters inherent to the Schottky junctions being observed. In the case of the Al/poly(4,4'-dipentoxy-2,2 'bithiophene)/ITO system a dependence of the photocurrent on the relative environmental humidity was revealed. All these findings were explained on the basis of the ion conductivity induced by the water absorbed by the polymers and by little amounts of ions included in them. The formation of an aluminium hydroxide layer, electronic insulator but ion conductor, at the Al/polymer interface was hypothesised to justify that both a battery in the dark and a photoelectrochemical cell can be established.