Nb-doped TiO2 nanoparticles for organic dye-sensitized solar cells

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.