Design and synthesis of TiO2 nanorod assemblies and their application for photovoltaic devices

We have developed a new method to fabricate an array of TiO2 nanorod assemblies on a flat TiO2 surface. The TiO2 nanostructures with a height of approximately 40 nm were synthesized via a low temperature sol-gel reaction in a reversed micelle system. Cross-sectional transmission electron microscopy observation revealed that each nanostructure is an assembly of nanorods with a diameter and length of approximately 4 nm and approximately 40 nm, respectively. The fabricated array of TiO2 nanorods was applied to construct photovoltaic devices combined with a semiconducting polymer. When the nanostructured TiO2 substrate is used, photovoltaic performance reaches a power conversion efficiency of 0.39% and a fill factor of 0.47 under AM 1.5 illumination. Dependence of external quantum efficiency on polymer thickness showed the maximum efficiency in a thicker polymer film condition in the presence of the TiO2 nanorod assembly, indicating the nanostructure functions as an efficient exciton collector.