Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "Coetsee, E."

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    Effect of broadband excitation ions in the luminescence of Ln.³+ doped SrF₂ nanophosphor for solar cell application
    (University of the Free State, 2015-06) Yagoub, Mubarak Yagoub Adam; Coetsee, E.; Swart, H. C.
    SrF2:Pr3+-Yb3+ phosphor powder was previously investigated for down-conversion application in solar cells. The rst surface, structural and optical characterization results indicated that the Pr3+-Yb3+ couple requires a sensitizer for effective enhancement in energy conversion. Broadband excitation ions of Ce3+ and Eu2+, that could be used as sensitizers, were therefore doped and co-doped in the SrF2 crystal. Detailed characterizations and investigations were then done on the surface, structure and optical aspects to see the effect on the energy conversion. Initially, the influence of different synthesis techniques on the surface, structure and concentration quenching of Pr3+ doped SrF2 was studied. The singly doped SrF2:Pr3+ was prepared by the hydrothermal and combustion methods. Scanning electron microscope (SEM) images showed different morphologies which was an indication that the morphology of the SrF2:Pr3+ phosphor strongly depended on the synthesis procedure. Both the SrF2:Pr3+ samples exhibited blue-red emission under a 439 nm excitation wavelength at room temperature. The emission intensity of Pr3+ was also found to be dependent on the synthesis procedure. The dipole-dipole interaction was found to be responsible for the concentration quenching effects at high Pr3+ concentration in both methods. SrF2:Eu nano-phosphors were successfully synthesized by the hydrothermal method. The crystalline size of the phosphors was found to be in the nanometre scale. The photoluminescence and high resolution x-ray photoelectron spectroscopy (XPS) results indicated that the Eu was in both Eu2+ and Eu3+ valance states. The presence of Eu2+ and Eu3+ in the system largely enhanced the response of the Eu3+ under ultraviolet excitation. Time of flight secondary ion mass spectrometry (tof-SIMS) results suggested that the energy transfer between these two ions was likely occurred. The relative photoluminescence intensity of the Eu2+ rapidly decreased with an increasing laser beam irradiating time. This result would make the current Eu2+ doped SrF2 samples unsuitable candidates for several applications, such as white light-emitting diodes and wavelength conversion films for silicon photovoltaic cells. The effect of Ce3+ ions on the SrF2:Eu nano-phosphor was also studied. Ce3+ largely enhanced the Eu3+ emission intensity via energy transfer mechanism. The calculated energy transfer efficiency was relatively effcient at high Eu concentration. The results suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Eu ions in SrF2 host. Eu2+ co-doped Pr3+, Yb3+ and Pr3+-Yb3+ couple in SrF2 were successfully prepared. XPS confirmed that all Eu contents were in Eu2+ oxidation states. Initially, Eu2+ co-doped SrF2:Pr3+ was studied. From PL and decay curve results, an efficient energy transfer was demonstrated in SrF2:Eu2+, Pr3+ phosphors. The energy transfer process was effective until a concentration quenching between Pr3+ ions occurred. The results proposed that Eu2+ could be a good sensitizer for absorbing the UV photons and hence efficiently enhancing the Pr3+ emission intensity. SrF2:Eu2+ (1.5 mol%) co-doped with Na+ (0.5 mol%) and various concentrations of Yb3+ were also investigated. XRD results showed a mixture of the cubic SrF2 and NaYbF4 phases. The NaYbF4 phase gradually formed with increasing Yb3+ doping concentration. Emission spectra and the fluorescence decay curve measurements were utilized to demonstrate the cooperative energy transfer. Energy transfer occurred subsequently from Eu2+ to Yb3+ followed by intense NIR emission. The energy transfer was completed at high concentrations but the Yb3+ emission intensity was reduced as a result of concentration quenching. In addition, from the photoluminescence data it was evident that Na+ induced significant change to NIR emission. The possibility of using the broadband absorption of Eu2+ to sensitize the Pr3+-Yb3+ down-conversion couple in SrF2 matrix was also investigated. The energy transfer process was demonstrated by the decrease of Eu2+ and Pr3+ related photoluminescence and lifetime with increasing Yb3+ concentration. Upon 325 nm excitation into the 5d levels of Eu2+, the samples yield intense near infrared emission corresponding to Pr3+:4f-4f and Yb3+:4f-4f transition. Yb3+ emission was clearly observed only at high Yb3+ concentrations after the emission intensity of Pr3+ was quenched. The PL lifetime results of Eu2+ confirmed the the second-order cooperative energy transfer also occurred between Eu2+ and Yb3+ ions.
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    ItemOpen Access
    Luminescence properties of Y2O3:Bi3+ as powder and thin film phosphor for solar cell application
    (University of the Free State, 2015-07) Rasha, Mohmmed Jafer; Coetsee, E.; Swart, H. C.
    The luminescent properties of the bismuth doped yttrium oxide (Y2-xO3:Bix) phosphor material was investigated as a powder and as thin films for possible application as a down-conversion material for solar cells. The goal of this investigation is to improve the energy conversion efficiency of photovoltaics (PV) by using the solar spectral conversion principle. A downconversion (DC) material converts a high-energy ultraviolet photon to two less energetic redemitting photons to improve the spectral response of Si solar cells. The luminescent properties of Y2-xO3:Bix=0.2% phosphor powder were investigated and the fluorescence spectra show that the luminescence was stimulated by the emission from two types of centers. These two types of centers were associated with the substitution of the Y3+ ion with the Bi3+ ion in two different sites in the crystal lattice of Y2O3 (with point symmetries C2 and S6). The emission of Bi3+ in the S6 site caused blue luminescence with maxima at 360 nm and 407 nm, and in the C2 site it gave green luminescence with the maxima at 495 nm. Both these emissions are related to the 3P1→1S0 transition in Bi3+. The diffuse reflectance was measured for Y2O3 and Y2-xO3:Bix=0.2%. No change in the band gap, when 0.2 mol% of Bi was doped in the Y2O3 host, was observed. X-ray photoelectron spectroscopy (XPS) results provided proof for the blue and green emission of Bi3+ in the Y2O3:Bi3+ phosphor powder. The Y2O3:Bi3+ phosphor was successfully prepared by the combustion process during the investigation of DC materials for Si solar cell application. The X-ray diffraction (XRD) patterns indicated that a single phase cubic crystal structure with the Ia3 space group was formed. XPS showed that the Bi3+ ion replaces the Y3+ ion in two different coordination sites in the Y2O3 crystal structure. The O 1s peak shows 5 peaks, two which correlate with the O2- ion in Y2O3 in the two different sites, two which correlate with O2- in Bi2O3 in the two different sites and the remaining peak relates to hydroxide. The Y 3d spectrum shows two peaks for the Y3+ ion in the Y2O3 structure in two different sites and the Bi 4f spectrum shows the Bi3+ ion in the two different sites in Bi2O3. The photoluminescence (PL) results showed three broad emission bands in the blue and green regions under ultraviolet excitation, which were also present for panchromatic cathodoluminescence (CL) results. These three peaks have maxima at ~ 365, 412 and 490 nm. The PL emission ~ 407 nm (blue emission) showed two excitation bands centered at ~ 338 and 370 nm while the PL emission at ~ 495.0 nm (green emission) showed a broad excitation band from ~ 310 to 365 nm. The panchromatic CL images were obtained for selected wavelengths at (415 ± 10.5) nm (for blue emission) and (530.0 ± 12.5) nm (for green emission). These luminescence results correlate with the XPS results that show that there are two different Bi3+ sites in the host lattice. The effect of different annealing temperatures on the PL properties of Y2-xO3:Bix phosphor powders were then investigated. Y2-xO3:Bix was synthesized by the combustion method with varying the Bi3+ dopant concentrations (x = 0.08, 0.1, 0.2, 0.3 and 0.5 mol%). The minimum PL emission intensity was observed for the high dopant concentration of 0.5 mol% and can be ascribed to concentration quenching. The effect of different annealing temperatures (800, 1000, 1200, 1400 and 1600 °C) were investigated for this sample in order to increase the emission intensity. Results showed that the emission intensity did increase with an increase in the annealing temperature up to 1400 °C. The increased intensities were attributed to two factors. The first one is the improvement of the Y2O3 crystal structure and second one is the segregation of Bi3+ ions from the bulk to populate the particles’ surfaces. The intensity increase up to 1200 °C is due to the segregation of Bi3+ ions from the bulk to populate the particles’ surfaces as a result of the increased temperature. Temperatures higher than 1200 °C resulted in a Bi3+ deficiency from the sample’s surface and therefore leading to a decrease in the dopant concentration. The decrease in the dopant concentration is creating the second factor, which is the further increase in intensity to 1400 °C due to a lower dopant concentration (then the effect of concentration quenching is lower). A further increase in the annealing temperature up to 1600 °C resulted in a decrease in the intensity because the majority of the Bi3+ ions evaporated from the sample’s surface as volatile species. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and XPS confirmed the segregation of Bi3+ ions to the particles surface with an increase in annealing temperature. These results concluded that the luminescence properties of Y2-xO3:Bix can be affected by different annealing temperatures and different dopant concentrations, Y2O3:Bi3+ phosphor thin films were prepared by PLD in the presence of oxygen (O2) gas. The microstructures and PL of these films were found to be highly dependent on the substrate temperature. XRD analysis showed that the Y2O3:Bi3+ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C the cubic phase became dominant. The crystallinity of the thin films therefore increased with increasing substrate temperatures. Surface morphology results obtained by scanning electron microscope (SEM) and atomic force microscopy (AFM) showed a decrease in the surface roughness. The increase in the PL intensities was attributed to the increase in the crystallinity and to the decrease in the surface roughness. The thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift in the main peak position to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to the change in the Bi3+ ions’ environment in the monoclinic and cubic phases. The reactive radio-frequency (RF) magnetron sputtering and spin coating fabrication techniques were also used to fabricate Y2-xO3:Bix=0.5% phosphor thin films. The two techniques were analyzed and compared as part of investigations being done on the application of DC materials for a Si solar cell. The morphology, structural and optical properties of these thin films are comparatively investigated. The XRD results of the thin films fabricated by both techniques showed cubic structures with different space groups. The optical properties showed different results because the Bi3+ ion is very sensitive towards it’s environment. The luminescence results for the thin film fabricated by the spin coating technique is very similar to the luminescence observed in the powder form. It showed three obvious emission bands in the blue and green regions centered at about 360, 420 and 495 nm. These emissions were related to the 3P1→1S0 transition of the Bi3+ ion situated in the two different sites of Y2O3 matrix with I a- 3(206) space group. Whereas the thin film fabricated by the RF magnetron technique shows a broad single emission band in the blue region centered at about 416 nm. This was assigned to the 3P1→1S0 transition of the Bi3+ ion situated in one of the Y2O3 matrix’s sites with a Fm-3 (225) space group. The spin coating fabrication technique is suggested to be the best technique to fabricate the Y2O3:Bi3+ phosphor thin films.