Influence of TiCl4 Post-Treatment on TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells

Abstract

Two types of TiO2 nanotubes, conventional TiO2 nanotube (Type I) and nanoporous layer-covered TiO2 nanotube (Type II), were synthesized by electrochemical anodization for dye-sensitized solar cells (DSSCs). Those surfaces were treated with various concentrations of aqueous TiCl4 (5-50 mM). DSSCs with the TiCl4-treated Types I and II exhibited the optimized efficiencies in the 20 and 5 mM TiCl4 treatments for the Types I and II nanotubes, respectively. The DSSC with 5 mM TiCl4-treated type II exhibited much higher cell performance (5.5%) than that (4.1%) with 20 mM TiCl4-treated Type I, indicating that the Type II nanotubes is more effective for the TiCl4 surface treatment. The TiCl4 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 TiCl4 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.