Luminescence properties of Y2O3:Bi3+ as powder and thin film phosphor for solar cell application
Rasha, Mohmmed Jafer
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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.