Hybrid TiO₂–SnO₂ Nanotube Arrays for Dye-Sensitized Solar Cells

Tin oxide (SnO₂) is a promising wide band gap semiconductor material for dye-sensitized solar cells (DSCs) because of its high bulk electron mobility. Employing vertically ordered 1-D nanostructures of SnO₂ as the photoanode may overcome the limit of current DSCs by using new redox mediators with faster kinetics than currently used ones. Synthesizing such nanostructures and integrating them into DSCs, however, has been proven challenging. Here, we demonstrate that, by using ZnO nanowires as a sacrificial template, vertically aligned SnO₂ nanotube arrays may be feasibly synthesized through a liquid-phase conversion process, and the synthesized SnO₂ nanotubes can be further coated with a thin layer of TiO₂ to form hybrid TiO₂–SnO₂ nanotube arrays. Both the resulting SnO₂ and hybrid TiO₂–SnO₂ nanotube arrays are used to fabricate DSCs, and the best performing cell delivers a promising efficiency of 3.53%. Transient photovoltage measurements indicate that the electron recombination lifetime in hybrid TiO₂–SnO₂ nanotubes is significantly larger than those in TiO₂ nanotubes, ZnO nanowires, and films of sintered TiO₂ nanoparticles, suggesting promise of the TiO₂-coated SnO₂ nanotubes for further improvement of DSCs.