Down-conversion of YOF: Pr3+, Yb3+ phosphor

Abstract

Abstract The photoluminescence (PL) of yttrium oxyfluoride (YOF) samples co-doped with praseodymium (Pr3+) and ytterbium (Yb3+) was investigated for possible down-conversion (DC) applications for c-Si solar cells (SCs). The optimum infrared (IR) emission obtained through DC was recorded with a Yb3+ content of 2% and a Pr3+ content of 0.3% and this was within the effective response range for Si SCs. YOF:Pr3+, Yb3+ samples were synthesized by the pyrolysis method with trifluoroacetate as precursor. The X-ray diffraction patterns indicated a crystalline phase of stoichiometric rhombohedral YOF (space group: R 3 ‾ m (166)) after annealing at 900 °C. X-ray photoemission spectroscopy's high resolution peak fits for the high concentration YOF: 0.3 mol % Pr3+, 6 mol % Yb3+ co-doped sample revealed two Pr oxidation states, Pr3+ and Pr4+. The presence of Pr4+ was due to the annealing in ambient air. The Pr3+ visible (VIS) emission's excitation spectra showed an intense 4f-5d band of Pr3+ at 250 nm accompanied with weaker 4f-4f peaks at 456, 470 and 483 nm. The VIS green PL emission was due to the 4f-4f [3P0 → 3H4] and [3P0 → 3F2] transitions at 498 nm and 659 nm, respectively. Quenching of the Pr3+ green emission was due to the energy transfer to Yb3+ ions through the cross-relaxation mechanism with a dipole-dipole interaction. The Yb3+ IR emission's excitation spectrum revealed a new band at 225 nm that overlapped with the 4f-5d band of Pr3+. The band at 225 nm was ascribed to the charge transfer band of Yb3+ due to electrons transferred from the O2− 3p6 level to the 4f13 level of Yb3+. Excitation with 250 nm showed multi-narrow peaks in the IR range between 885 nm and 1120 nm that corresponded to the Pr3+ 3P0 → 1G4 and 1D2 → 3F3, 3F4 transitions and to the 2F7/2 → 2F5/2 transition of Yb3+. Upon excitation of 225 nm, the IR emission showed only emission of Yb3+ transitions with almost no traces of Pr3+ emission. The decay times for VIS and IR emissions were calculated to be in the microseconds range.