Photoluminescence studies of a YOF phosphor synthesized by the pyrolysis method

Abstract

Photoluminescence (PL) studies of praseodymium (Pr3+) doped yttrium oxyfluoride (YOF) samples were investigated for solar cell applications. The PL emission results showed green Pr3+ emission during excitation of a broad band at 250 nm at room temperature. YOF:Pr3+ samples were synthesized by the pyrolysis method with trifluoroacetate as precursor. The X-ray diffraction patterns exhibited a crystalline phase of stoichiometric rhombohedral YOF (space group: R3‾mm (166)) after annealing at 900 °C. The crystallite sizes decreased with an increase in Pr3+ doping concentration. During thermal decomposition from trifluoroacetate to YOF (600 °C to 900 °C), the scanning electron spectroscopy images showed an agglomeration of small particles (<100 nm) that started to melt and agglomerate to form bigger particles with sizes > 500 nm. X-ray photoemission spectroscopy high resolution peak fits for the high Pr3+ doped sample (YOF:0.5% Pr3+) revealed two Pr oxidation states, Pr3+ and Pr4+. Annealing in air caused the formation of a small amount of Pr4+. The PL excitation spectra showed an intense band at 250 nm with weaker bands at 456, 470 and 483 nm. The weaker bands were ascribed to the 4f-4f 3H4–3P2, 3H4–1I6, 3P1 and 3H4–3P0 transition bands of the Pr3+ ion, respectively. The green Pr3+ PL emission was ascribed to the 4f-4f [3P0–3H4] and [3P0–3F2] transitions at 498 nm and 659 nm, respectively. A YOF:Ce3+ sample was synthesized in order to predict the value of the Pr3+ 4f-5d level in the YOF host. The PL excitation and emission results obtained showed that the lowest 4f-5d excitation of Pr3+ in this host has to peak around 250 nm. The 250 nm band was therefore ascribed to the 4f-5d band of Pr3+ in the YOF host. The optimum Pr3+ concentration for the PL emission was recorded for the sample doped with 0.3% of Pr3+. Concentration quenching occurred through a cross relaxation process due to dipole-quadrupole interactions. Near infra-red emission for the 0.3% Pr3+ doped sample during excitation of 250 nm showed multi narrow peaks in the range between 885 nm and 1120 nm that corresponded to the 3P0 → 1G4 and the 1D2 → 3F3, 3F4 transitions. The decay lifetimes were calculated to be in the μs range.