Enhanced near-infrared to visible upconversion nanoparticles of Ho ³⁺-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 nanoparticles of Ho ³⁺-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-TiO ₂ 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.