High efficiency Solid-state Dye-sensitized solar cells assembled with hierarchical anatase pine Tree-like TiO₂ nanotubes

A facile and effective method to prepare hierarchical pine tree-like TiO₂ nanotube (PTT) arrays with an anatase phase directly grown on a transparent conducting oxide substrate via a one-step hydrothermal reaction. The PTT arrays consist of a vertically oriented long nanotube (NT) stem and a large number of short nanorod (NR) branches. Various PTT morphologies are obtained by adjusting the water/diethylene glycol ratio. The diameter of the NTs and the size of the NR branches decreases from 300 to100 nm and from 430 to 230 nm, respectively, with increasing water content. The length of the PTT arrays could be increased up to 19 μm to significantly improve the charge transport and specific surface area. The solid-state dye-sensitized solar cells (ssDSSC) assembled with the 19 μm long PTT arrays exhibit an outstanding energy-conversion efficiency of 8.0% at 100 mW/cm², which is two-fold higher than that of commercially available paste (4.0%) and one of the highest values obtained for N719 dye-based ssDSSCs. The high performance is attributed to the larger surface area, improved electron transport, and reduced electrolyte/electrode interfacial resistance, resulting from the one-dimensional, well-aligned structure with a high porosity and large pores. Hierarchical, 19 μm long, anatase, pine tree-like TiO₂ nanotube arrays directly grown on a transparent conducting oxide via a one-step hydrothermal reaction result in high-efficiency, solid-state dye-sensitized solar cells. The efficiency is 8.0% at 100 mW/cm², one of the highest values observed for the N719 dye.