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

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 CaF phosphors obtained by hydrothermal synthesis. In particular, the DC photoluminescence characteristics for UV (353 nm) excitation of Nd/Yb co-doped CaF phosphors are discussed, underlining the effects due to the co-doping with Li. The photoluminescence emission is dominated by the near-infrared (NIR) Yb emission peaked at 975 nm, although the excitation spectrum corresponds to the characteristic peaks associated with Nd excitation at both the UV and visible wavelength ranges. The Nd to Yb energy transfer mechanisms were determined from a detailed analysis of the excitation spectra characteristics of Nd and Nd/Yb/Li doped CaF 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.