Upconversion enhancement of lanthanide-doped NaYF₄ for quantum dot-sensitized solar cells

As an upconverter, lanthanide-doped NaYF₄ (Ln-NaYF₄) has been integrated into the photoanodes for quantum-dot sensitized solar cells. Compared with the solar cells with pure TiO₂, the upconverter based solar cells exhibited a 17.6% increase in the photocurrent and a 20% increase in the power conversion efficiency. Furthermore, when the integrated Ln-NaYF₄ is pretreated by annealing, the resulting photoanode demonstrates an obvious improvement in the photocurrent generation and the power conversion efficiency due to the annealing-induced enhancement of the upconversion emission as well as the elimination of the defects allowing for efficiently reducing charge recombination. The best photovoltaic performance with a short-circuit current of 14.36 mA/cm² and a power conversion efficiency of 4.37% has been obtained with the solar cells containing the annealed upconverters. Both the solar cells integrated with the Ln-NaYF₄ pretreated by annealing or not demonstrate obvious photocurrent responses under the illumination of the NIR light. This finding demonstrates the feasibility of the combination of the up-converting materials with quantum dot-sensitized solar cells to extend the exploitation of the solar light to a wider range thus enabling the fabrication of highly efficient solar cells.