Synthesis and luminescence properties of Bi3+, Yb3+ co-doped Y2O3 phosphor powder and thin film for application in solar cells

[摘要] Solar cells based on Si are currently the most widely studied and adopted form of photovoltaiccells used to convert solar energy into electrical energy. However, Si solar cells are known forits poor conversion efficiencies due to the spectral mismatch between the solar spectrum andthe absorption spectrum of the Si solar cell.This study focuses on synthesising the Y2O3:Bi3+,Yb3+ phosphor powder using the coprecipitationtechnique. Various parameters such as: varying the pH levels, Bi3+ and Yb3+concentrations during preparation in order to so study their effect on the structural andluminescence properties of the phosphor. After optimisation of the above mentionedparameters, Y2O3:Bi3+,Yb3+ thin films were prepared using the spin coating and pulsed laserdeposition techniques.The X-ray diffraction patterns showed that the Y2O3:Bi3+,Yb3+ phosphor powders allcrystallised as a single phase cubic structure even at high doping concentrations. While in thethin films the monoclinic phase of Y2O3 was present in addition to the single phase cubicstructure. The results from the diffraction patterns also revealed that the crystallite size of thephosphor 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, itwas found that the surface morphology of the thin films varied significantly between the twopreparation techniques. The spin coating technique yielded smooth films but at highermolarities and with an increased number of coatings the films started cracking and peeling dueto the poor adhesion between the substrate and the film. With the pulsed laser depositiontechnique, the films adhered to the substrate very well but were significantly rougher. Filmsprepared using the KrF laser had only some particulates present on the films, while the filmsprepared using the Nd:YAG laser were covered with particulates.X-ray photoelectron spectroscopy and energy dispersive spectroscopy results provided proofthat the dopants Bi3+ and Yb3+ were successfully incorporated into the host material and thatthey were homogenously spread throughout the material.The photoluminescence spectra showed and confirmed that the dopants may occupy two siteswithin the host material namely, the S6 and C2 sites. With an increase in the Bi3+ and Yb3+ ionconcentration an increase in the visible and infrared emission intensity, respectively, wasobserved. 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 concentrationquenching. For the cathodoluminescence spectra the results showed that with an increase in theBi3+ concentration a decrease in the emission intensity ratio between the S6 and C2 sitesoccurred due to the limited available S6 sites. However, by introducing the Yb3+ ions some ofthe Bi3+ ions were forced to occupy some of the unoccupied S6 sites leading to an increase inthe Bi3+ emission intensity originating from the S6 site. The photoluminescence of the thinfilms 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 emissionintensity prepared using the spin coating also increased due to more material being present onthe substrate. As for the films prepared using pulsed laser deposition the film that was preparedat a high substrate temperature had a significantly lower emission intensity than the filmprepared at a lower substrate temperature. Both the spin coating and pulsed laser depositionprepared thin films exhibit visible and more importantly infrared emission, which may be usedto modify the solar spectrum with the aim of improving the efficiency of solar cells.

[发布机构] University of the Free State