Enhanced Performance of Dye-Sensitized Solar Cells by Utilization of an External, Bifunctional Layer Consisting of Uniform β-NaYF<sub>4</sub>:Er<sup>3+</sup>/Yb<sup>3+</sup> Nanoplatelets

Uniform β-NaYF<sub>4</sub>:Er<sup>3+</sup>/Yb<sup>3+</sup> hexagonal nanoplatelets were synthesized via a modified hydrothermal route, and the nanoplatelets were applied as an external, bifunctional layer in a novel DSC configuration consisting of only one internal TiO<sub>2</sub> transparent layer. Approximately 10% enhancements of photocurrent and overall DSC efficiency are demonstrated by the addition of the external layer, which exhibits two functions of light reflecting and near-infrared (NIR) light harvesting. The novel DSC configuration not only simplifies the DSC fabrication process but also eliminates charge recombination induced by the conducting up-converting nanocrystals when used internally thus opening the path for other more efficient up-converting nanocrystals to be designed and applied.