Anodic TiO2 nanotube arrays with high surface area for dye-sensitized solar cells
Date of Issue2013
School of Mechanical and Aerospace Engineering
Dye-sensitized solar cells (DSSCs) have received worldwide research interest as a promising and competitive candidate for low-cost photovoltaics. Highly-ordered TiO2 nanotube arrays prepared by electrochemical anodization have been developed as an intriguing photoanode owing to vertical tube alignment for one-dimensional electron transport and available inner and outer tube surface for dye attachment. For nanotubes based DSSCs, one main efficiency limitation factor is the relatively low tube surface area due to accessible large diameter (usually ~ 100 nm). This project aims to enlarge tube surface area for high dye loading amount and in turn enhance conversion efficiency of DSSC devices. Two approaches are adopted to increase the surface area of nanotubes. One method is synthesis of nanotube/nanoparticle composite by a facile hydrothermal treatment. The other one is fabrication of long nanotubes of small diameter by an optimized two-step anodization. The results show that hydrothermal treatment of anodic nanotubes is facile and effective to create TiO2 nanoparticles on tube walls. With optimized treatment duration for a particular tube length, an optimal nanotube/nanoparticle composite structure is obtained. The composite electrode greatly improves the conversion efficiency of DSSCs, due to the combined advantages of high surface area from nanoparticles and vertical electron transport from nanotubes. Two-step anodization by adjusting voltage from high to low produces double-layered nanotubes. In the double-layered structure, the top layer sacrifices itself to protect the bottom layer from significant chemical dissolution during growth of the bottom nanotubes, resulting in longer tubes. Furthermore, the second layer nanotubes are tailored to seed from upper tube bottoms by interim treatment to facilitate ion diffusion. Subsequently, the two-step anodization is optimized and long enough nanotubes of small diameter are achieved, that is, with tube length of 31 μm and diameter of ~35 nm. This overcomes tube length limitation challenge of small diameter tubes in conventional one-step anodization. Small diameter tubes greatly enlarge the dye loading amount and thus increase solar cell efficiency, in contrast to large diameter tubes. The small diameter tubes with a length of 23 μm yield a high conversion efficiency of 5.02% in dye-sensitized solar cells.