Constructing a “Native” Oxyfluoride Layer on Fluoride Particles for Enhanced Upconversion Luminescence

The efficiency and tenability of upconversion fluorescence are correlated closely to the structure of host materials. Herein, an oxyfluoride layer grown in situ on fluoride-based upconversion crystals through a high-temperature air annealing is employed to synthesize stable lattice-mismatch NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺ core/shell architecture with greatly enhanced upconversion luminescence. Such a “native” oxyfluoride layer suppresses the surface quenching and modulates the phonon energy of host materials. Consequentially, the upconversion emitting intensity of NaYF₄:Yb³⁺/Er³⁺ increases over ≈25 times after covering a “native” YOF layer. The luminescence red/green ratio is manipulated from ≈1.3 to ≈11.2. By incorporating NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺ into the TiO₂ photoanode of dye-sensitized solar cells (DSSCs), the photon–electron conversion efficiency of DSSCs increases by ≈17.1% due to the improved near-infrared photon harvest. This work provides a novel core/shell construction route toward high-efficiency upconversion systems.