Luminescent properties of YOF phosphor for solar cell application
Saeed, Nadir Azhari Mustafa
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The luminescent properties of yttrium oxyfluoride (YOF) phosphor doped and co-doped with different ions (praseodymium (Pr3+), cerium (Ce3+), ytterbium (Yb3+), bismuth (Bi3+) and holmium (Ho3+)) were investigated for c-Si solar cell application. The pyrolysis method with trifluoroacetate (CF3COO) as precursor was used to synthesize all the powders. Investigations were done on the crystal structure, the surface morphology, surface and optical properties. The X-ray diffraction patterns exhibited a crystalline phase of stoichiometric rhombohedral YOF (space group: R3 ̅m (166)) after annealing at 900 0C. The crystallite sizes for the YOF:Pr3+ sample, decreased with an increase in Pr3+ doping concentrations. During thermal decomposition from CF3COO to YOF (600 0C to 900 0C), the scanning electron spectroscopy (SEM) 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 (XPS) 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 photoluminescent (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 (NIR) 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. YOF:Pr3+, Yb3+ samples were investigated for down-conversion applications for c-Si solar cells. The SEM images showed an agglomeration and melting of small particles to form bigger particles. XPS’s high-resolution peak fits for the high Pr3+ co-doped Yb3+ sample (YOF: 0.3 % Pr3+, 6 % Yb3+) revealed two Pr oxidation states, Pr3+ and Pr4+. The presence of Pr4+ was due to the annealing in ambient air. The deconvolution of the Yb 4d peak showed only the 4d5/2 peak that was ascribed to Yb3+. 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. These weaker 4f-4f peaks were ascribed to the 3H4-3P2, 3H4-1I6, 3P1 and 3H4-3P0 transition of Pr3+, respectively. 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 electron transfers from the O2- 3p6 level to the 4f13 level of Yb3+. Excitation with 225 nm showed the typical Pr3+ VIS emission and this confirmed the nature of the 225 nm band as a charge transfer band. Excitation with 250 nm showed multinarrow 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 optimum IR emission for both excitations was recorded with a Yb3+ content of 2 % and a constant Pr3+ content of 0.3 %. The decay times for VIS and IR emissions were calculated to be in the microseconds range. The investigations of YOF:Bi3+ were done at room temperature. Auger electron spectroscopy results showed that Bi was homogeneously distributed on the surface of the sample. XPS showed two doublet peaks for the Bi 4f region which were attributed to Bi3+ ions and Bi metal. PL studies revealed an asymmetric broad ultraviolet (UV) emission band centered at 314 nm that originated from the 3P1 → 1S0 A band of Bi3+. The excitation band was a symmetric broad band centered at 267 nm and corresponded to the 1S0 → 3P1 A band with a Stokes shift of 47 nm. This excitation overlapped with a metal to metal charge transfer band of Bi3+. The optimum concentration for maximum luminescence intensity was 0.4 mol % of Bi3+ and the quantum yield of this sample was about 60 %. The decay curves of the prepared samples were also investigated and the lifetimes were found to be in the microsecond range. Bi3+ was investigated as sensitizer for Ho3+ emission in the YOF:Bi3+, Ho3+ phosphor in the VIS and IR regions. The PL studies of YOF:Bi3+, Ho3+ were investigated for energy transfer possibilities for IR emission. The morphology investigations revealed agglomerations of small particles into bigger particles that developed during annealing. XPS high resolution peak fits for the high concentration co-doped sample (YOF: 0.4 mol % Bi3+, 5 mol % Ho3+) revealed overlapping of Bi 4f and Ho 4d peaks with the Y 3d peak. PL excitation spectra showed an intense 1S0 → 3P1 Bi3+ band at 265 nm that dominated the 4f-4f Ho3+ peaks at 360 nm and 449 nm. VIS PL emission was done upon 449 nm and 265 nm excitation. The UV emission that peaked at 314 nm upon 265 nm excitation was ascribed to the 3P1 → 1S0 transition of Bi3+. The emissions at 538 nm and 753 nm were ascribed to the 5F4,5S2 → 5I8 and 5F4,5S2 → 5I7 transitions of Ho3+ with the optimum concentration of Ho3+ obtained at 2 mol %. IR emission occurred for both the 449 nm and 265 nm excitations, respectively. The IR emissions that peaked at 1014 nm and 1200 with weak emission at 1400 nm were ascribed to the 5F4,5S2 → 5I6, 5I6 → 5I8 and 5F4,5S2 → 5I5 transitions of Ho3+, respectively. The decay times for both Bi3+ and Ho3+ in the VIS and IR regions were calculated to be in the microseconds range. A great enhancement of the emission in the IR and VIS regions have been achieved. Despite the efficient energy conversion in the Bi3+, Ho3+ system, enhancement of the energy flow from Bi3+ to Ho3+ needs to be considered. This will reduce the dissipated energy into many unnecessary manifolds of Ho3+ and direct it to the range of spectral absorption of Si-SC. Tunable emission was achieved through Ho3+ co-doped Bi3+ doped YOF for optoelectronic applications. Cathodoluminescent (CL) studies were investigated under electron beam irradiation (5 keV) for both YOF: x mol % Bi3+ (x = 0.3, 0.4, 0.5, 0.6, 0.8) and YOF: 0.4 mol % Bi3+, x mol % Ho3+ (x = 0.8, 1.4, 2, 3, 4, 5) samples. The morphology investigations revealed agglomerations of small particles into bigger particles that developed during annealing. XPS’s surveys for the YOF: 5 mol % Bi3+ and the YOF: 0.4 mol % Bi3+, 5 mol % Ho3+ samples revealed the presence of all elemental compositions. UV CL emission for the YOF:Bi3+ samples were ascribed to the 3P1 → 1S0 transition of Bi3+ at 314 nm and the VIS emission at 624 nm to the 2P3/2 (1) → 2P1/2 transition of Bi2+. Bi2+ were created during CL excitation as a result of ionization. The CL emission of the YOF:Bi3+, Ho3+ samples in the UV and VIS regions peaked at 316 nm, 540 nm and 624 nm and were ascribed to the 3P1 → 1S0, 5F4,5S2 → 5I8 and 2P3/2 (1) → 2P1/2 transitions of Bi3+, Ho3+ and Bi2+, respectively. The fitted International commission on Illumination (CIE) coordinates of the YOF:Bi3+, Ho3+ samples showed a tunable CL emission from green to yellow and orange.