Nb2O5 Photoelectrodes for Dye-Sensitized Solar Cells: Choice of the Polymorph

Nanowires of Nb2O5 were developed in three polymorphic forms, namely, pseudo-hexagonal, orthorhombic, and monoclinic, by electrospinning a polymeric solution and by subsequent annealing. The materials were characterized by X-ray and electron diffraction, scanning and transmission electron microscopy, BET surface area measurements, absorption spectrometry, and cyclic voltammometry. These characterizations indicate that the monoclinic phase has a higher conduction band edge compared with the other two and is likely to display higher open circuit voltage in solar cells. Dye-sensitized solar cells were fabricated using the above polymorphs, and the device performances were studied. The pseudo-hexagonal phase showed higher device performance owing to its higher specific surface area compared with the others. However, when normalized, the device performances with respect to the dye-loading, the monoclinic phase gave superior performance. Studies on the charge transport properties using electrochemical impedance spectroscopy and open circuit voltage decay indicate that the monoclinic phase has high resistance against charge recombination and improved electron lifetime compared with the other phases and conventional TiO2 nanostructures. The monoclinic Nb2O5 is likely to be a material of choice as photoelectrode in dye-sensitized solar cells if its mesoporous particles with large surface area could be synthesized.