Investigation on the dynamics of electron transport and recombination in TiO2nanotube/nanoparticle composite electrodes for dye-sensitized solar cells

In this work, we report on fabrication and characterization of dye-sensitized solar cells based on TiO2 nanotube/nanoparticle (NT/NP) composite electrodes. TiO2 nanotubes were prepared by anodization of Ti foil in an organic electrolyte. The nanotubes were chemically separated from the foil, ground and added to a TiO2 nanoparticle paste, from which composite NT/NP electrodes were fabricated. In the composite TiO2 films the nanotubes existed in bundles with a length of a few micrometres. By optimizing the amount of NT in the paste, dye-sensitized solar cells with an efficiency of 5.6% were obtained, a 10% improvement in comparison to solar cells with pure NP electrodes. By increasing the fraction of NT in the electrode the current density increased by 20% (from 11.1 to 13.3 mA cm−2), but the open circuit voltage decreased from 0.78 to 0.73 V. Electron transport, lifetime and extraction studies were performed to investigate this behavior. A higher fraction of NT in the paste led to more and deeper traps in the resulting composite electrodes. Nevertheless, faster electron transport under short-circuit conditions was found with increased NT content, but the electron lifetime was not improved. The electron diffusion length calculated for short-circuit conditions was increased 3-fold in composite electrodes with an optimized NT fraction. The charge collection efficiency was more than 90% over a wide range of light intensities, leading to improved solar cell performance.