| Solar energy has been considered as a renewable source owe to its stable source, low cost and environment friendly. The photovoltaic conversion process can be completed for solar cell by absorbing specific wavelengths of sunlight, however the efficiency of this process is quite low because of the narrow absorption wavelength, in which it is not full to absorb the ultraviolet light, as well the infrared light. The improvement of the conversion efficiency of solar cell becomes necessary and the spectrum cutting can be used as an effective way. CaF2 appears as a very attractive phosphor host because of its low phonon energy and high damage threshold. Eu2+ can be used as activator on account of strong broadband absorption. Yb3+ has the potential to serve as the sensitizer due to its unique and simple energy level with only one excited level of 2F5/2. On the other hand, Er3+ has been used as up-conversion activator of luminescent materials due to its long lived excited states and excellent resonance with transition of Yb3+.In this article, CaF2 phosphor and glass-ceramics were synthesized by solid state reaction and one-step crystalline method. The effects of activator sensitizer, dopants and the sintering process on the luminescent properties of samples have been investigated. The composition, structure and spectroscopic properties of the samples are characterized via XRD, SEM and PL measurements. The results show that:1. The Eu2+/Er3+/Yb3+co-doped CaF2 phosphor has been synthesized by conventional solid state reaction. The excitation spectra consist of three peaks at 340nm,355nm, and 398nm which attributed to 4f?5d transition of Eu2+. Emission spectra located at 423nm, corresponding to the 5d?4f transition of Eu2+. While doped with 10% SiO2 can make the luminous intensity increased by 186%, and the energy transfer critical distance can be 1.612nm with dipole-electric quadrupole energy transfer; doped with 15% SrO can make the luminous intensity increased by 101% and the critical distance isl.4nm with electric quadrupole-electric quadrupole energy transfer.2. Transparent glass-ceramic with composition (in mol %) 40SiO2-20Al2O3-10CaO-15CaF2-10NaF-5B2O3was fabricated via traditional melting technique with one step method. X-ray diffraction (XRD) results of as-prepared glass-ceramics confirmed the existence of CaF2 phase. Scanning electron microscopy (SEM) observation showed the CaF2 crystalline phase had a size of 200nm and dispersed in the amorphous phase. The intensity ratio of 5D0-7F2 transition to the 5D0-7F1 transition and ?2 of Judd-Ofelt parameters were used to measure the rare earth ion site symmetry. The data have revealed that Eu3+ions are embedded into Ca-F environment from Si-O environment during the crystallization process.3. Under a 980nm laser diode pump, the as-prepared Eu2+/Er3+/Yb3+ co-doped CaF2 phosphors exhibited red and green emission which corresponding to the transition of Er3+:2H11/2?4I15/24S3/2,4S3/2?-4I15/2 and 4F9/2?4I15/2. The optimal concentration of Er3+ is 1.0 mol% and with the increasing of Eu2+, the intensity decreased. Yb3+acts as the sensitizer can transfer energy to Er3+ to improve the luminous intensity. The up-conversion spectra of as-prepared glass ceramic consist of three peaks located at 522nm, 545nm and 650nm which corresponding to the transition of Er3+: 2H11/2?4I15/24S3/2,4S3/2?4I15/2 and 4F9/2?4I15/2- With increasing of Er3+ concentration, the luminous intensity of samples increased before and later decreased. |
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