Nb-doped TiO2nanoparticles for organic dye-sensitized solar cells

Abstract

Nb-doped anatase TiO₂ nanoparticles were prepared by the sol–gel process followed by a hydrothermal treatment and successfully used as the photoanodes in organic dye-sensitized solar cells (DSSCs). Phase identification of the TiO₂ samples was confirmed by X-ray diffraction and Raman shift spectroscopy. The electronic structure and Nb doping in the TiO₂ lattice were confirmed by X-ray photoelectron spectroscopy and energy-disperse X-ray spectroscopy, respectively. Also, the conduction band edge (CB) shift due to the Nb-doping in the TiO₂ lattice by UV-vis diffuse reflectance spectroscopy and the effect of Nb doping on the charge transporting and recombination behaviors of the DSSCs by electrochemical impedance spectroscopy (EIS) analysis were investigated. The Nb-doped TiO₂ exhibited a positive shift of the conduction band edge (CB) compared to the undoped TiO₂. Consequently, the increased driving force for electron injection, that is, the difference between the CB of TiO₂ and the lowest unoccupied molecular orbital (LUMO) energy level of the dye, could correspondingly contribute to the enhancement of the electron injection efficiency, but the V_oc has the opposite behavior. The V_oc drop in the DSSCs based on the Nb-doped TiO₂ could be prevented using a multi-functional HC-A as a coadsorbent instead of DCA. As expected, a PCE of 7.41% was obtained for the NKX2677/HC-A-sensitized DSSC based on the 2.5 mol% Nb-doped TiO₂, which was an improvement of 11% relative to that of the DSSC based on the undoped TiO₂.