Double-layer TiO2 nanotube arrays by two-step anodization: Used in back and front-side illuminated dye-sensitized solar cells

Double-layer TiO2 nanotube arrays were fabricated by a two-step anodization process on Ti foils. The first TiO2 nanotube layer was annealed after anodization and then exposed to the second anodization to grow the second TiO2 nanotube layer beneath it. The crystallized upper layer acts as a protective layer against chemical etching of the lower layer tubes top by electrolyte that leads to growing of thick layers with open-top-tubes beneath the upper one. The effect of different anodization parameters on the final geometry of the nanotubes, grown beneath the protective layer was investigated. The upper TiO2 layer was detached as an intact membrane at the end of the two-step anodization process and used in front-side illuminated dye-sensitized solar cells (DSSCs) with high conversion efficiency up to 8.66%. Also, the remained TiO2 nanotubes on the substrate with different diameters were used in the back-side illuminted DSSCs.