Designed structure of bilayer TiO₂–Nb₂O₅ photoanode for increasing the performance of dye-sensitized solar cells

Here, we investigate the effect of crystalline structure and design configuration of photoanode semiconductor on physical properties and electron transport parameters of dye-sensitized solar cells based on niobium pentoxide (Nb₂O₅) composition. At the first step, the effect of calcination on structural properties of Nb₂O₅ has been studied. Phase transformation from mixed monoclinic/orthorhombic to pure monoclinic phase has been observed. The performance of cells is then investigated for different designs of photoanode, including pure Nb₂O₅, pure TiO₂ and bilayer structure composed of TiO₂/calcined Nb₂O₅. The highest performance belonged to the cell prepared bilayer photoanode. Bilayer structure facilitated charge extraction which leads to the drastically improvement of photoconversion efficiency. Remarkable increase in J_SC is reported comparing to cells without TiO₂ layer, from about 0.08 to around 5.7 mA cm⁻², leading to conversion efficiency as high as 1.48% in the best cell. Results of dye loading measurement and impedance spectroscopy analysis revealed that this higher photocurrent is attributed to both more injected electrons from dye to photoanode and higher electron lifetime. Furthermore, inhibition of electron back-transfer from the composite layer to the electrolyte as well as low recombination resistance plays critical roles in such dramatically photoconversion efficiency.