Core-shell nanoporous electrode for dye sensitized solar cells: The effect of shell characteristics on the electronic properties of the electrode

Nanoporous TiO₂ electrodes coated with a thin layer of various wide band gap materials were tested in dye sensitized solar cells (DSSCs). Using Nb₂O₅, ZnO, SrTiO₃, ZrO₂, Al ₂O₃ and SnO₂ as shell materials, we find that the mechanism by which the shell affects the electrode properties depends on the coating material. In the exceptional case of Nb₂O₅, the coating forms a surface energy barrier, which slows the recombination reactions. The other shell materials each form a surface dipole layer that shifts the conduction band potential of the core TiO₂. The shift direction and magnitude depend on the dipole parameters which are induced by the properties of the two materials at the core-shell interface. The results show that either the shell acidity or the electron affinity of the shell are the shift controlling parameters, although the former seems more likely. This new tool for the modification of the electronic properties of the nanoporous electrodes allows for optimization towards a wide range of applications.