Observation of Annealing-Induced Doping in TiO2 Mesoporous Single Crystals for Use in Solid State Dye Sensitized Solar Cells

Mesoporous single crystals (MSCs) of TiO2 are promising materials for more efficient dye sensitized solar cells and other energy conversion or storage devices, since they combine high surface area with large crystalline domain size. In this work, we investigate the charge transport properties of TiO2 MSCs after annealing them within a confining template at temperatures from 500 to 850 degrees C. We observe that higher anneal temperatures do not change the crystal phase, as in nanocrystalline TiO2, but do influence the MSC absorption spectrum in a manner consistent with the signature of increased oxygen-vacancy defects. By comparing MSC film conductivity in vacuum and in air, we infer that these anneal-induced defects increase the background charge density in TiO2. Subsequently, we measure higher effective mobility in annealed MSCs using transient mobility spectroscopy (TMS), consistent with higher anneal temperatures filling sub-bandgap trap states by n-doping TiO2. Finally, we measure faster charge transport rates in solid-state dye sensitized solar cells as well as increased open-circuit voltages at low light intensity with increasing MSC anneal temperature. This study leverages the fixed geometry and crystal phase of MSCs under thermal treatment to identify and isolate the doping effect of annealing at high temperature, previously inaccessible for mesoporous anatase TiO2. The results offer insight into the influence of doping on charge transport in TiO2-based solar cells and the tunability of MSCs for use in enhancing device performance.