Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells

To improve the conversion efficiency of dye-sensitized solar cells (DSSCs) it is necessary to understand the electronic structure of the TiO2–dye–electrolyte interface in detail. A sturdy junction at the interface can be provided by modifying the electronic structure of the TiO2 electrode with Nb doping. The Nb-doped TiO2 was prepared by a sol–gel method followed by a hydrothermal treatment; the Nb content was varied from 0.5 to 3.0 mol%. The X-ray photoelectron spectroscopy showed that the Fermi level of TiO2 electrode shifted away from the conduction band minimum (CBM) when the Nb content is low (≤1.5 mol%) and shifted toward the CBM when the Nb content is high (≥2.5 mol%). The shift of Fermi level with low Nb doping was due to the passivation of the oxygen vacancies at the TiO2 nanoparticle surface. Intraband states were formed when dopant content was 1.5 and 2.5 mol%. We have found that the photovoltaic parameters of DSSCs based on doped TiO2 sensitized with a cis-[Ru(dcbpyH)2(NCS)2](NBu4)2, N719 dye, are closely related to the electronic structure of the Nb-doped TiO2 electrode. The changes of short circuit current and open circuit voltage of DSSCs were explained in relation to the electronic structure of the TiO2 electrode. The best efficiency of 8.0% was demonstrated by DSSCs with 2.5 mol% Nb-doped TiO2.