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