In DSC, the sensitizer injects an electron in the conduction band of the oxide and is regenerated by hole injection into an electrolyte or hole transporting material. The injected electrons flow through the semiconductor network to the front and back contacts of the cell, where they are collected as electric current. The nanocrystalline morphology of the oxide semiconductor film is essential for the highpower conversion efficiency of DSC.

Various theoretical approaches to the description of the semiconductor have been explored, which can be gathered into two families, i.e. cluster versus periodic approaches. Both approaches have their merits and should both be regarded as valuable tools in describing semiconductor surfaces or nanoparticles. A problem with periodic calculations is that excited states within TDDFT are not generally implemented and in any case standard continuum solvation models are not applicable due to the impossibility of defining an infinite solvation cavity. Therefore, we have focused our attention in cluster approaches of TiO2 and ZnO, in which a portion of a surface slab is properly extracted from the periodic system. A clear drawback of cluster calculations is that large dimensions are necessary to converge the cluster properties compared to that of the infinite solid, border effects dominating small clusters. Moreover, the convergence size vary when studying different semiconductors.



We believe that a good trade-off between accuracy and computer resources is the use of non-hybrid functionals in the structural determination, including molecular dynamics simulations by means of the Car-Parrinello method, followed by property calculations performed by hybrid functionals in localized basis sets coupled to the use of continuum solvation models and calculation of the excited states by TDDFT. This integrated computational strategy has been successfully employed in various studies on the optical properties of nanoscale systems.

Another line of investigation that we have undertaken is the characterization of the structural, electronic and adsorption properties of single-walled semiconductor nanotubes. In particular we studied the adsorption of simple species containing a carboxylic group, i.e., formic acid (HCOOH), on TiO2 sidewalls nanotubes with the aim to model the interaction of the photosensitizers anchored onto the oxide semiconductor surface in DSC.