Origin of efficiency enhancement in Nb2O5 coated titanium dioxide nanorod based dye sensitized solar cells

Ordered one-dimensional metal oxides are considered promising architectures for dye-sensitized solar cells (DSC). Here we explore the properties determining the photovoltaic performance of DSC based on titanium dioxide nanorods prepared by a hydrothermal method. Ultrathin conformal coating with other oxides such as Al2O3 or Nb2O5 has been widely used in the literature to improve the conversion efficiency. Usually the effects attributed to the coating are either to prevent recombination by barrier effect or to change the position of titanium dioxide conduction band. Here we show that coating TiO2 nanorods with Nb2O5 increases the photocurrent and conversion efficiency of the DSC. However, impedance spectroscopy results indicate that neither recombination is reduced by the coating nor the conduction band position is moved. The improvement in the performance of the cell has been attributed to an enhancement in the charge injection efficiency promoted by Nb2O5.