Metal oxide nanostructures for the fabrication of efficient DSC photoanodes

The role of the nanostructured oxide in dye sensitized solar cell photoanode is crucial, towards the development of efficient photoelectrochemical solar harvesters. In fact, the transition from a porous metal oxide film to a nanocrystalline mesoporous one represented the real breakthrough in the development of the first example of dye sensitized solar cell in 1991. Several criteria have to be fulfilled in the optimization of the morphology of the nanostructured material for an efficient light capturing and charge transport at the photoelectrode. The exposed surface for dye sensitization plays a crucial role, since the planar dimension of the dye molecule is significantly higher than its optical cross section and a great number of light capturing centers are necessary to generate a high number of carriers. The electrons injected on the conduction band of the wide band gap semiconductor have to draw a tortuous path when moving on a random assembly of nanoparticles and the possibility to have a direct pathway for the photogenerated charges can reduce significantly the electron recombination. 1D or quasi 1D nanostructures can guarantee a fast and direct charge transport, but usually pay a lot in terms of surface area, being it significantly lower than in nanocrystalline network. Thus, material with a 3D branched nanomorphology with size and spacing on the order of the exciton diffusion length could represent the equilibrium point between the different requirements. In this presentation we give an overview on the application of different metal oxide nanostructures (e.g. TiO2 nanotubes and spongelike ZnO) in the fabrication of efficient DSC photoanodes. TiO2 nanotubes produced by anodic oxidation of Ti foil probably represent the most interesting quasi 1D nanostructure for application in DSC photoanodes. The material synthesis and its integration in front-side illuminated DSC will be illustrated. Moreover, we will report on the synthesis of coral-shaped Zn nanostructured film by radio frequency magnetron sputtering and its treatment in oxidizing atmosphere, leading to the fabrication of spongelike ZnO photoelectrodes with high light conversion efficiency. DSCs were fabricated using both a reversible microfluidic architecture and a standard thermoplastic irreversible sealing. For both materials, particular emphasis will be dedicated to the evaluation of the charge transport properties, showing the reduced charge recombination behavior with respect to standard TiO2 nanoparticle network.