Enhanced Near-Infrared to Visible Upconversion Nanoparticles of Ho3+-Yb3+-F- Tri-Doped TiO2 and Its Application in Dye-Sensitized Solar Cells with 37% Improvement in Power Conversion Efficiency

New near-infrared (NIR)-to-green upconversion nano-particles of Ho3+-Yb3+-F- tridoped TiO2 (UC-F-TiO2) were designed and fabricated via the hydrosol hydrothermal method. Under 980 nm NIR excitation, UC-F-TiO2 emit strong green upconversion fluorescence with three emission bands at 543, 644, and 751 nm and convert the NIR light in situ to the dye-sensitive visible light that could effectively reduce the distance between upconversion materials and sensitizers; thus, they minimize the loss of the converted light. Our results show that this UC-F-TiO2 offers excellent opportunities for the other types of solar cells applications, such as organic solar cells, c-Si solar cells, multijunction solar cells, and so on. When integrating the UC-F-TiO2 into dye-sensitized solar cells (DSSCs), superior total energy conversion efficiency was achieved. Under AM1.5G light, open-circuit voltage reached 0.77 +/- 0.01 V, short-circuit current density reached 21.00 +/- 0.69 mA cm(-2), which resulted in an impressive overall energy conversion efficiency of 9.91 +/- 0.30%, a 37% enhancement compared to DSSCs with pristine TiO2 photoanode.