Nanowire-Based Three-Dimensional Transparent Conducting Oxide Electrodes for Extremely Fast Charge Collection

A 3D transparent conducting oxide (3D-TCO) has been fabricated by growing Sn-doped indium oxide (ITO) nanowire arrays on glass substrates via a vapor transport method. The 3D TCO charge-collection properties have been compared to those of conventional two-dimensional TCO (2D-TCO) thin films. For use as a photoelectrode in dye-sensitized solar cells, ITO-TiO2 core-shell nanowire arrays were prepared by depositing a 45 nm-thick mesoporous TiO2 shell layer consisting of similar to 6 nm anatase nanoparticles using TiCl4 treatments. Dye-sensitized solar cells fabricated using these ITO-TiO2 core-shell nanowire arrays show extremely fast charge collection owing to the shorter electron paths across the 45 nm-thick TiO2 shell compared to the 2D TCO. Interestingly, the charge-collection time does not increase with the overall electrode thickness, which is counterintuitive to conventional diffusion models. This result implies that, in principle, maximum light harvesting can be achieved without hindering the charge collection. The proposed new 3D TCO should also be attractive for other photovoltaic applications where the active layer thickness is limited by poor charge collection.