Efficient Down- and Up-Conversion Luminescence in Er³⁺–Yb³⁺ Co-doped Y₇O₆F₉ for Photovoltaics

Samples of Er 3+ –Yb 3+ co-doped Y 7 O 6 F 9 are synthesized by solid state reaction method, and their down- and up-conversion luminescence are studied. Upon excitation into the 4 F 7/2 level of Er 3+ at 490 nm, effective energy transfer from Er 3+ to Yb 3+ via cross-relaxation Er 3+ ( 4 S 3/2 ) + Yb 3+ ( 2 F 7/2 ) → Er 3+ ( 4 I 13/2 ) + Yb 3+ ( 2 F 5/2 ) occurs, splitting an absorbed high-energy photon (λ = 490 nm) into two photons (λ = 1000 and 1540 nm). Based on the changing of the lifetime of Er 3+ 4 S 3/2 → 4 I 15/2 emission without and with Yb 3+, the efficiency of Er 3+ → Yb 3+ energy transfer in Y 7 O 6 F 9:1%Er 3+,9%Yb 3+ has been evaluated to 58%. Upon excitation at 980 nm, as the Yb 3+ doping concentration increases, not only an overall increase in intensity but also an enhancement of the red ( 4 F 9/2 → 4 I 15/2 ) emission with respect to the green ( 4 S 3/2 → 4 I 15/2 ) emission were observed. Under 980 nm laser excitation with power of 20 mW, the overall integrated emission intensity in the 500–700 nm of Y 7 O 6 F 9:1%Er 3+,5%Yb 3+ is 1.8 times higher than that of β-NaYF 4:2%Er 3+,18%Yb 3+ that is to date the most efficient up-conversion phosphor. In view of the efficient down-/up-conversion in Er 3+ -Yb 3+ co-doped Y 7 O 6 F 9, this material is expected to be used to enhance the efficiency of solar cells.