Thin polymer films on semiconductors: From their protection to the realization of molecular electronic devices

When homogeneously grafted as thin films on small band-gap inorganic semiconductors (n-GaAs, n-CdS), organic conjugated polymers such as poly-methylthiophene afford interesting new structures. In their doped conducting state, these polymer films bring a long-term protection of these semiconductors against their photocorrosion in aqueous medium, and the subsequent inclusion of metallic aggregates in these films allows increased catalytic activity for achieving photoelectrochemical reactions. In their undoped semiconducting state, they lead to the realization of “organic-on-inorganic” p-n junctions which show very low leakage current and high allowed current density. As photovoltaic cell, an energy conversion efficiency of 17.5% has been obtained under 100 mW.cm⁻² irradiation with a p-PMeT/n-GaAs cell. The larger number of work already devoted to organic conjugated polymers such as polyacetylene, polypyrrole, polythiophene, has shown that these compounds can be switched between a doped oxidized state, with a nearly metallic conductivity, σ ∼ 10²-10³S.cm⁻¹, and an undoped neutral state which presents semiconducting properties, σ ∼ 10⁻⁷ -10⁻⁹ S. cm⁻¹. Either as free standing film or grafted on an electrode, these polymers exhibit interesting organic electrode properties which have been widely characterized. On the other hand much less is known on their behavior when deposited on inorganic semiconductors. In the following, this area will be discussed on two examples, the protection of narrow band-gap semiconductors against their photo-degradation in aqueous solution, and the new junction properties presented by “organic-on-inorganic” electronic devices.