Realization of ultra-long columnar single crystals in TiO2 nanotube arrays as fast electron transport channels for high efficiency dye-sensitized solar cells

Abstract

A balance between strong light-harvesting and efficient charge collection is not only important but also challenging for photoelectrochemical applications. We developed here TiO₂ nanotube arrays (TNTAs) with ultra-long single crystals on the nanotube walls and along the axial direction. Those oriented nanotubes show ultra-fast 1D electron transport due to the reduction of grain boundaries that scatter electrons. Based on the TNTAs with ultra-long crystals, we further developed an electronically heterogeneous hierarchical architecture for dye-sensitized solar cell (DSSC) application. In this crafted structure, the columnar single crystals on the nanotube walls provide well-aligned pathways with high electron mobility for rapid electron transport, while a mesoporous TiO₂ network welded on the nanotube walls provides sufficient surface area for dye loading. The significant mismatch in electron mobility between the TNTA scaffold and mesoporous TiO₂ network drains the electrons from the TiO₂ network to the TNTAs, enabling high charge collection efficiency exceeding 90% within a ∼30 μm film. As a result, an impressively high power conversion efficiency (PEC) exceeding 10% was achieved for the resulting DSSCs with such a TNTA based hierarchical structure. The presented technology provides a promising prospect for constructing multi-hierarchical structures for a wide range of applications, such as photocatalysis, batteries and gas sensors.