Optical and electrooptical properties of porous-silicon/conjugated-polymer composite structures

We report on the fabrication and characterization of porous-silicon/conjugated-polymer hybrids, created by combining a host columnar matrix of mesoporous silicon and a network of organic nanowires made from poly(N-vinylcarbazole) (PVK). A uniform and homogeneous filling of the pores by the polymers was accomplished by electrochemical polymerization of organic monomers inside the pores by using cyclic voltammetry. Spectroscopic measurements showed that polymerization inside the confined environment of the nanometric pores results in a tighter and denser packing of the polymer due to a change of the polymerization process from the vinyl groups to the conjugated carbazole groups, giving rise to a redshift of the absorption spectra and better electrical conductivity. Current-voltage characterization of the hybrids under dark conditions and under illumination were investigated. We demonstrate a simple method to control the band alignment between the organic polymer and the porous silicon, altering it from a type-I to a type-II interface by changing the doping polarity of the silicon substrate (from p-type to n-type, respectively). An efficient photoinduced charge separation was observed for the type-II interface (n-type porous-silicon-polymer interface), while no such effect was observed for the type-I organic-inorganic interface. An organic-inorganic hybrid structure consisting of porous silicon and nanowires of poly(N-vinylcarbazole) (PVK) exhibits optical behavior of the PVK nanowires, which is unique when compared with the bulk material. Furthermore, a simple method to control the band alignment between type-I and type-II is demonstrated. Our results enable a variety of applications and electrooptical characteristics.