Photoelectrochemical properties of a porous Nb2O5 electrode sensitized by a ruthenium dye

In studying the photoelectrochemical properties of TiO2, Nb2O5, ZnO, SnO2, In2O3, WO3, Ta2O5, and ZrO2 porous semiconductor films sensitized by the ruthenium(II) cis-bis-(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylic acid) complex, it was found that the Nb2O5 semiconductor cell had the next highest incident of monochromatic photon-to-current efficiency (IPCE = 18%) compared to the TiO2 cell and showed the highest open-circuit photovoltage (V-oc) among them. The V-oc of the dye-sensitized cell was proved to be related to the flatband potential of the semiconductor electrode. The Ru dye adhered to the Nb2O5 surface mainly through an ester-like linkage. It is speculated that electrons are transferred mainly through the conjugated orbitals of the ester Linkage and semiconductor conduction band and that the TiO2 and Nb2O5 conduction bands consisting of d-orbitals are more advantageous than those of s-orbitals in attaining the desired IPCE. The IPCE of the Nb2O5 cell was markedly improved by treating the Nb2O5 electrode with Nb alkoxide, and the maximum IPCE was 32% at 548 nm. The overall solar-to-electric energy conversion efficiency was about 2% (AM-1.5, 100 mW/cm(2)).