| With the development of society, energy shortage and environmental pollution have become a bottleneck restricting the sustainable development of society. Therefore, renewable energy sources such as solar energy are paid more and more attention. Crystalline silicon cells still occupy the dominant position in the solar cell market currently. However, the application of solar cells have not been used widely in our daily life because all the solar cells have faults of the high cost and low photoelectric conversion efficiency. Photoelectric conversion efficiency of silicon solar cells is low, because the bandgap of c-Si (Eg=1.12eV) does not match the solar spectrum. For the incident photons with energies below the bandgap, the energies are not be absorbed, whereas for the photons with energies above the bandgap, the excess energy is wasted as heat within the solar cell. It becomes the current focus by modulating spectrum to match spectral absorption characteristics of solar cells. Quantum cutting, converting one high-energy ultraviolet photon into two or more low-energy photons, is likely to improve conversion efficiency of solar cells.For the purpose of getting down-conversion materials applied to solar cells, we use high-temperature solid-state method to synthesize CaWCH:Pr3+ luminescent powders, CaWO4:Pr3+, Yb3+and CaWO4:Yb3+down-conversion phosphors in this article. Furthermore, the samples were characterized and analyzed in their phases and luminescence properties, and the mechanism of energy transfer between rare earth ions were analyzed. The main conclusions were drawn as follows:1.CaWO4:Yb3+down-conversion phosphors were synthesized by high temperature solid-phase method.There are emission peaks in the visible light and near infrared light region upon the excitation of256-nm light. With the increase of Yb3+ions concentration, the emission intensity at428nm is gradually weakened, but the emission intensity of Yb3+is increased firstly then decreased, which is caused by concentration quenching. We use the light of652-nm light and998-nm light to monitor the excitation of the samples and find that there is an overlap from250nm to300nm, which suggests energy transfer exists from WO42-to Yb3+.2.CaWO4:Pr3+phosphors were prepared by the solid state reaction method. The structures and luminescence properties of the samples were characterized by X-ray diffractometer and fluorescence spectrometer. It shows that all of samples are pure scheelite-typed CaWO4. We use the light of652nm to monitor the excitation of the samples and find that there are a excitation broadband (230-300nm) and three sharp excitation peaks, which correspond to excitation of WO42-and3H4â†'3PJ transition of Pr3+(J=0,1,2), respectively. It is found that energy transfer exists between WO42-and Pr3+. Upon the excitation of263-nm light, the emission intensity at430nm is gradually weakened and the intensity of Pr3652nm is increased with the increase of Pr3+concentration, which is attributed to energy transfer from WO42-to Pr3+. The best doping concentration of Pr3+is1%.3. Near-infrared quantum cutting CaWCH:Pr3+, Yb3+phosphors were prepared by the solid state reaction method. Three near-infrared emission peaks located at1001,1030and1048nm can be found in the emission spectra upon the excitation of453-nm light, which correspond to2F5/2â†'2F7/2transition of Yb3+and1D2â†'>3F4transition of Pr3+, respectively. The luminescence lifetime decay curve of Pr3+(652nm) with different Yb3+concentrations are measured for analyzing the energy transfer from Pr3+to Yb3+. It can be seen that lifetime of Pr3+is gradually shortened with the increase of Yb3+ions concentration, which suggests energy transfer efficiency and quantum efficiency can be improved with the increase of Yb3+ions concentration. The highest quantum efficiency can be calculated to be166.9%. Therefore, the material may be applied in the silicon solar cells to improve its photoelectric conversion efficiency. |
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