Ultraviolet to near infrared down-conversion in CaF2:Nd3+/Yb3+/Li+ phosphors

Highlights: • Down-conversion (DC) process is confirmed in CaF₂:Nd/Yb/Li from UV to NIR region. • DC process is carried out through Nd^{3+ 4}D_3/2 level (353 nm). • DC efficiency values over 100% confirm DC process. • Li⁺ increase NIR emission due to DC process through a charge compensation effect in the CaF₂ structure originated by the trivalent lanthanide ions doping, but does not have a direct intervention in DC process itself. • Effective quantum yield measurements present higher values for UV excitation than for Visible excitation, confirming DC process. Down conversion (DC) in rare-earth-doped optical materials is a process of great interest for the possibility of a substantial increase of the efficiency of silicon solar cells. Here we report the structural and photoluminescence properties of co-doped CaF₂ phosphors obtained by hydrothermal synthesis. In particular, the DC photoluminescence characteristics for UV (353 nm) excitation of Nd³⁺/Yb³⁺ co-doped CaF₂ phosphors are discussed, underlining the effects due to the co-doping with Li⁺. The photoluminescence emission is dominated by the near-infrared (NIR) Yb³⁺ emission peaked at 975 nm, although the excitation spectrum corresponds to the characteristic peaks associated with Nd³⁺ excitation at both the UV and visible wavelength ranges. The Nd³⁺ to Yb³⁺ energy transfer mechanisms were determined from a detailed analysis of the excitation spectra characteristics of Nd³⁺ and Nd³⁺/Yb³⁺/Li ⁺ doped CaF₂ phosphors. DC photoluminescence for UV excitation was confirmed by both the analysis of DC quantum yield efficiency and effective quantum yield measurements. In the first case, an efficiency up to 150% was found, while the effective quantum yield measurements, carried out for the UV (DC) and visible (downshift) excitation and NIR photoluminescence emission, give values of 81 ± 10% and 37 ± 5% for excitation with 353 nm and 577 nm light, respectively.