Influence of TiCl ₄ post-treatment on TiO ₂ nanotube arrays for dye-sensitized solar cells

Two types of TiO ₂ nanotubes, conventional TiO ₂ nanotube (Type I) and nanoporous layer-covered TiO ₂ nanotube (Type II), were synthesized by electrochemical anodization for dye-sensitized solar cells (DSSCs). Those surfaces were treated with various concentrations of aqueous TiCl ₄ (5-50mM). DSSCs with the TiCl ₄-treated Types I and II exhibited the optimized efficiencies in the 20 and 5mM TiCl ₄ treatments for the Types I and II nanotubes, respectively. The DSSC with 5mM TiCl ₄-treated type II exhibited much higher cell performance (5.5%) than that (4.1%) with 20mM TiCl ₄-treated Type I, indicating that the Type II nanotubes is more effective for the TiCl ₄ surface treatment. The TiCl ₄ treatment on the Type II nanotubes having fewer surface defect states led to significantly enhanced electron lifetime and electron transport, in addition to slightly increased amount of the adsorbed dyes, resulting in significantly improved cell efficiency. As the TiCl ₄ concentration is further increased over the optimized concentration, the cell performances were gradually reduced for both of the Types I and II, due to the blocked voids and narrowed pores which facilitated the recombination reaction and shortened electron lifetime.