Effect of morphology evolution from nanotubes to concatenated nanoparticles of hierarchical TiO2 nanostructures on power conversion efficiency of dye-sensitized solar cells

Abstract

Compared with TiO2 nanoparticles (TNPs) or TiO2 nanotubes (TNTs), the hierarchical TiO2 nanostructures with high porosity and direct charge pathway are desired for the fabrication of efficient TiO2-based devices for the dye-sensitized solar cells (DSSCs). The preparation methods applied so far to synthesize hierarchical TiO2 nanostructures are multi-step processes involving the control of many parameters and producing uneven nanoparticles and agglomeration on the decorated nanotubes. Herein, we report on the formation and morphology evolution of the hierarchical TiO2 nanostructures with tailored porosity in the absence of any template. The different hierarchical TiO2 nanostructures were obtained by thermal annealing at 450 °C for 1, 2, and 4 h of the free-standing TNT layers prepared by anodization. The hierarchical TiO2 nanostructures with pores formed on the TNT walls were obtained when the annealing time was set to 1 h, and a further increase in the annealing time led to the formation of larger concatenated nanoparticles of TiO2. The resulting hierarchically structured TiO2 nanotubes annealed for 1 h were subsequently exploited as a photoanode in the fabrication of the DSSCs, markedly enhancing the power conversion efficiency (PCE) of 7.24%. A 71.1% increase in the PCE was achieved in comparison to the device prepared by using pure anatase-TiO2 nanotubes (4.25%) due to the synergistic effect of higher capacity for dye loading because of a larger specific surface area, fast charge transport, and superior light scattering ability.