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

Samples of Er3+–Yb3+ co-doped Y7O6F9 are synthesized by solid state reaction method, and their down- and up-conversion luminescence are studied. Upon excitation into the 4F7/2 level of Er3+ at 490 nm, effective energy transfer from Er3+ to Yb3+ via cross-relaxation Er3+ (4S3/2) + Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2) 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 Er3+4S3/2 → 4I15/2 emission without and with Yb3+, the efficiency of Er3+ → Yb3+ energy transfer in Y7O6F9:1%Er3+,9%Yb3+ has been evaluated to 58%. Upon excitation at 980 nm, as the Yb3+ doping concentration increases, not only an overall increase in intensity but also an enhancement of the red (4F9/2 → 4I15/2) emission with respect to the green (4S3/2 → 4I15/2) emission were observed. Under 980 nm laser excitation with power of 20 mW, the overall integrated emission intensity in the 500–700 nm of Y7O6F9:1%Er3+,5%Yb3+ is 1.8 times higher than that of β-NaYF4:2%Er3+,18%Yb3+ that is to date the most efficient up-conversion phosphor. In view of the efficient down-/up-conversion in Er3+-Yb3+ co-doped Y7O6F9, this material is expected to be used to enhance the efficiency of solar cells.