Synthesis and luminescence properties of Bi3+, Yb3+ co-doped Y2O3 phosphor powder and thin film for application in solar cells
Solar cells based on Si are currently the most widely studied and adopted form of photovoltaic cells used to convert solar energy into electrical energy. However, Si solar cells are known for its poor conversion efficiencies due to the spectral mismatch between the solar spectrum and the absorption spectrum of the Si solar cell. This study focuses on synthesising the Y2O3:Bi3+,Yb3+ phosphor powder using the coprecipitation technique. Various parameters such as: varying the pH levels, Bi3+ and Yb3+ concentrations during preparation in order to so study their effect on the structural and luminescence properties of the phosphor. After optimisation of the above mentioned parameters, Y2O3:Bi3+,Yb3+ thin films were prepared using the spin coating and pulsed laser deposition techniques. The X-ray diffraction patterns showed that the Y2O3:Bi3+,Yb3+ phosphor powders all crystallised as a single phase cubic structure even at high doping concentrations. While in the thin films the monoclinic phase of Y2O3 was present in addition to the single phase cubic structure. The results from the diffraction patterns also revealed that the crystallite size of the phosphor powders was mostly dependent on the pH during the synthesis process rather than the concentration of the dopants present in the host. Using a scanning electron microscope, it was found that the surface morphology of the thin films varied significantly between the two preparation techniques. The spin coating technique yielded smooth films but at higher molarities and with an increased number of coatings the films started cracking and peeling due to the poor adhesion between the substrate and the film. With the pulsed laser deposition technique, the films adhered to the substrate very well but were significantly rougher. Films prepared using the KrF laser had only some particulates present on the films, while the films prepared using the Nd:YAG laser were covered with particulates. X-ray photoelectron spectroscopy and energy dispersive spectroscopy results provided proof that the dopants Bi3+ and Yb3+ were successfully incorporated into the host material and that they were homogenously spread throughout the material. The photoluminescence spectra showed and confirmed that the dopants may occupy two sites within the host material namely, the S6 and C2 sites. With an increase in the Bi3+ and Yb3+ ion concentration an increase in the visible and infrared emission intensity, respectively, was observed. Both the visible and infrared emission intensities increased up to a certain molarity (Bi3+ = 2.0 mol% and Yb3+ = 10.0 mol%) before decreasing dramatically due to concentration quenching. For the cathodoluminescence spectra the results showed that with an increase in the Bi3+ concentration a decrease in the emission intensity ratio between the S6 and C2 sites occurred due to the limited available S6 sites. However, by introducing the Yb3+ ions some of the Bi3+ ions were forced to occupy some of the unoccupied S6 sites leading to an increase in the Bi3+ emission intensity originating from the S6 site. The photoluminescence of the thin films was also studied and found to be similar to that obtained from the bulk powder samples. With an increase in the molarity and an increase in the number of coatings, the emission intensity prepared using the spin coating also increased due to more material being present on the substrate. As for the films prepared using pulsed laser deposition the film that was prepared at a high substrate temperature had a significantly lower emission intensity than the film prepared at a lower substrate temperature. Both the spin coating and pulsed laser deposition prepared thin films exhibit visible and more importantly infrared emission, which may be used to modify the solar spectrum with the aim of improving the efficiency of solar cells.