Heterojunction Quantum Dot Solar Cells Based on Vertically Growth TiO₂ Anatase Nanorod Arrays with Improved Charge Collection Property

The Quantum dot (QD) solar cells have been under active research due to their high light harvesting efficiencies and low fabrication cost. In spite of these advantages, there have been some problems on the charge collection due to the limitation of the diffusion length. The modification of advanced nanostructure is capable of solving the charge collection problem by increasing diffusion length of electron. One dimensional nanomaterials such as nanorods, nanowires, and nanotubes may enhance charge collection efficiency in QD solar cells. In this study, we synthesized $TiO_2$ anatase nanorod arrays with length of 200 nm by two-step sol-gel method. The morphology and crystal structure for the nanorod were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The anatase nanorods are single-crystalline and possess preferred orientation along with (001) direction. The photovoltaic properties for the heterojunction structure QD solar cells based on the anatase nanorod were also characterized. Compared with conventional $TiO_2$ nanoparticle based QD solar cells, these nanostructure solar cells exhibited better charge collection properties due to long life time measured by transient open circuit studies. Our findings demonstrate that the single crystalline anatase nanorod arrays are promising charge transport semiconductors for heterojunction QD solar cells.