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Browsing Physics by Author "Abdelrehman, Mogahid Hassan Mohammed"

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    Luminescence properties of Gd2O3:Bi3+ co-doped Ln3+ as powder and thin films phosphor for solar cell application
    (University of the Free State, 2021-11) Abdelrehman, Mogahid Hassan Mohammed; Swart, H. C.; Kroon, R. E.
    The main aim of this project is to study the luminescence properties of Gd2O3:Bi3+ co-doped Ln3+(Yb3+ and Er3+) phosphor powder and thin films to improve the energy conversion efficiency of photovoltaics (PV) by using the solar spectral conversion principle for use in solar cell applications. The luminescence properties of Gd2O3:Bi3+ single doped was investigated. The effect of different annealing temperatures and different concentrations of Bi3+ on the luminescence properties were investigated. The combustion method was used to synthesize all the powders samples. The optimum Bi3+ doping concentration was found to be x = 0.003 and the optimum annealing temperature was found to be 1000 °C (2 h). An increase in the average crystallite size with increasing annealing temperature and a decrease with an increase in Bi3+ doping concentration were obtained. The morphological studies showed small particles (less than 100 nm) that started to agglomerate to form bigger particles with sizes up to close to 400 nm during thermal treatment at higher temperature. Diffuse reflectance measurements of the pure host material showed absorption bands at 250, 275 and 315 nm that were attributed to the 4f-4f transitions of the Gd3+ ions. The bandgap was found to be influenced by the annealing temperature, which was determined to be 5.09 eV for the as prepared Gd2O3 host and then increased with increasing temperature. For all the doped samples the strong band at about 227 nm was observed corresponding to inter-band transitions of the Gd2O3 matrix in addition to three bands located around 260, 335, and 375 nm corresponding to the excitation transitions of Bi3+ ions into the different sites (C2 and S6). The luminescent properties of Gd2-xO3:Bix phosphor powder were investigated and the fluorescence spectra show that the luminescence was stimulated by the emission from two types of centers which exhibited efficient blue-green emission bands. The excitation and emissions that correspond to the 1S0→3P1 transitions in the Bi3+ ion depend on the presence of Bi3+ in the two different sites (S6 and C2) of Gd2O3. Significant changes in the thermoluminescence (TL) intensity for the different annealing temperatures and different dopant concentration in the Gd2O3 were observed. The TL glow curves of the UV-irradiated samples showed a low temperature peak at about 364 K and a high temperature peak at 443 K for all the samples. Auger electron spectroscopy (AES) was employed to analyse the surface chemical composition of the powder at a vacuum base pressure of 1.3 × 10−8 Torr and after back-filling with O2 to a pressure of 1.0 × 10−7 Torr. Simultaneous monitoring of the cathodoluminescence (CL) and AES peak-to-peak heights (APPHs) during prolonged electron bombardment in vacuum and O2 over time for 40 h was done. The CL emission of Gd2O3:Bi powder was found to be stable under electron irradiation. Thin films were successfully prepared using the pulsed laser deposition technique. Gd2-xO3:Bix=0.003 phosphor that was optimized for blue luminescence was deposited on Si (100) substrates in vacuum and an oxygen atmosphere at different substrate temperatures. The background atmosphere and substrate temperature were found to significantly affect the microstructure and luminescence of the thin films. The thicknesses for the thin films were relatively constant around 100 nm. The CL emission intensity degradation in a vacuum and an oxygen atmosphere was checked synchronously with the APPHs using the same electron beam for both measurements. The effect of the electron bombardment on the surface state of the samples was studied by using AES and X-ray photoelectron spectroscopy (XPS). All major elements (gadolinium and oxygen) were located, with additional carbon and chlorine that were removed during the early stages of electron bombardment. The CL intensity of the thin films with a blue-green emission was stable under electron bombardment. The Gd2O3 materials based on the Bi3+,Yb3+ phosphor powders were investigated for possible improvement of the photovoltaic conversion efficiency via spectral modification, utilizing the down-conversion (DC) process. The optical bandgap of Gd2O3 increased with additional doping. The DC emission was obtained successfully from Yb3+ co-doped Gd2-xO3:Bix=0.003 samples. The visible emission has blue and green bands related to the Bi3+ in the S6 and C2 symmetry of Gd2O3. The near-infrared (NIR) of DC emissions were centered at 976 nm and less intense peaks centered around 950 nm, 1025 nm and 1065 nm caused by the crystal field Stark splitting of the 2F5/2 and 2F7/2 energy levels corresponding to the Yb3+: 2F5/2 →2F7/2 transitions after absorption of a single UV photon. It was observed that the visible Bi3+ emission gradually decreased with the addition of Yb3+ ion concentration while the NIR emission increased due to energy transfer from Bi3+ to Yb3+ ions. There was a strong increase in DC emissions with doping by adding of Bi3+ ions compared with the samples doped by Yb3+ ions. Er3+ co-doped Gd2-xO3:Bix=0.003 was investigated for up-conversion (UC) processes for the possible use in applications for c-Si SCs. The 980 nm infrared excitation was successfully converted into visible (green-orange-red) emissions and was confirmed for Er3+ doped Gd2O3 samples with and without the presence of Bi3+. There was an increase in the UC emissions with the addition of Bi3+ compared to the sample doped with single Er3+ ions due to the enhancement in Er3+ emission. The UC emissions were observed at 520, 537, 560, 670 and 870 nm and were assigned to the 2H11/2, 4S3/2, 4F9/2 and 4I9/2→ 4I15/2 transitions of the Er3+ ion. The results showed that the presence of Bi3+ ions improved the UC emission of the Er3+ ions. The effect of different concentrations of Yb3+ on the luminescence of Gd2O3:Bi3+, Er3+ phosphor powder was investigated successfully. The structure and morphology of the surfaces revealed that the phosphors were affected by an increasing concentration of the Yb3+ ions. There was a decrease in the crystallite size with an increase in the Yb3+ doping concentration. The successful UC emission spectra of Gd1.977-xO3:Bi0.003, Er0.02, Ybx (x = 0.0, 0.01, 0.03, 0.06, 0.09 and 0.12) were studied under 980 nm infrared excitation. The UC visible emission spectra consisted of a series of green, red and NIR emission bands at 520, 537, 550, 560 and 670 nm and are ascribed to the 4F5/2, 4F7/2, 2H11/2, 4S3/2 and 4F9/2→ 4I15/2 transitions of the Er3+ ions, respectively. The intensity of the UC emission of the green, red and NIR emission bands of the Gd2O3:Er3+ was dependent on the introduction of the Bi3+ and Yb3+ into the matrix. Co-doping by Bi3+ ions strongly enhancing the UC green emission, while co-doping with Yb3+ ions enhanced the UC red emission intensity. The intensity for both the visible emission spectra of Er3+ monitored by using a 379 nm excitation wavelength and CL at 5 keV exhibited a reduction in the intensity with an increase in the Yb3+ concentration due to the energy transfers from Er3+ to Yb3+ by cross-relaxation in contrast to the UC emission results. Under the excitation at 375 nm the emissions were centered at 418 nm corresponding to the 3P1 → 1S0 transition of the Bi3+ ions and in addition the peaks mentioned earlier corresponding to the transitions of the Er3+ ions. Our results indicated that the obtained phosphors might be possibly used for applications in displays, lighting and as a luminescence layer used to modify the solar spectrum with the aim of improving the efficiency of solar cells.