Design of conduction band structure of TiO2 electrode using Nb doping for highly efficient dye‐sensitized solar cells

A barrier layer of undoped TiO2 was deposited on the Nb-doped TiO2 electrode to suppress the recombination at the Nb-doped TiO2/dye–electrolyte interface for highly efficient dye-sensitized solar cells (DSCs). The Nb content in TiO2 was varied in a range of 0.7–3.5 mol% to modify the TiO2 energy-band structure. Nb-doped TiO2/dye interfaces were characterized by a combination of ultraviolet photoemission spectroscopy and optical absorption spectroscopy measurements, allowing the determination of the conduction band minimum (CBM) of the TiO2 electrode and the lowest unoccupied molecular orbital of the N719 dye. The lowering of TiO2 CBM by Nb doping induced the increase in short-circuit current of DSCs. However, open-circuit voltage and fill factor are decreased, and this result was ascribed to the enhanced recombination at the Nb-doped TiO2/dye–electrolyte interface. The effect of doping on charge transport in DSCs was analyzed using electrochemical impedance spectroscopy. We have shown that by introducing of TiO2 barrier layer, the Nb doping content, which results in DSC highest efficiency, can be increased because of the suppression of the dopant-induced recombination. The energy conversion efficiency of the solar cells increased from 7.8% to 9.0% when undoped TiO2 electrode is replaced with electrode doped with 2.7 mol% of Nb because of the improvement of the electron injection and collection efficiencies. The correlation between the electronic structure of the TiO2 electrode, charge transfer characteristics, and photovoltaic parameters of DSCs is discussed. Copyright © 2012 John Wiley & Sons, Ltd.