| In the present work, RE3+-doped hexagonal NaYF4 up-conversion phosphor and hexagonal Gd2O2S up-conversion phosphor were prepared successfully by hydrothermal method and solid phase method, respectively. Crystal structure, morphology and optical properties of these phosphors and their dependence on phosphor compositions were studied via XRD, SEM, absorption spectra, luminescence spectra (emission spectra under 980nm laser excitation) and so on. Experimental results show that the up-conversion emission of Er3+ ions in the host of Yb3+doped NaYF4 is much enhanced under 980nm laser excitation and the up-conversion emission intensity increases with the concentration of Yb3+ions increasing from 5% to 20%. On the other hand, Dy3+doping can increase the absorption of NaYF4:Er3+ phosphor in the near-infrared region accompanied by two new absorption peaks appearing in 1115nm and 1300nm, as a result of electron transitions of Dy3+ion from ground state to 6F9/2 and 6F11/2 states, respectively. In the case of Gd2O2S:Er3+phosphor, because of the lattice crystal field splitting, three emission peaks split into multiple overlapping bands under 980nm laser excitation. They are caused by transitions of Er3+ion from 4F9/2 to 4I15/2 (660nm), 4S3/2 to 4I15/2 (546nm), and 4H11/2 to 4I15/2 (524nm), respectively. The up-conversion emission intensity of Gd2O2S:Er3+significantly increases after Yb3+ions co-doping with the optimal Er3+/Yb3+ doping ratio of 0.02:0.05. When co-doped with Dy3+ions, Gd2O2S:Er3+exhibits two new absorption peaks in 1115nm and 1300nm as a result of transitions of Dy3+ion from ground state to 6F9/2 and 6F11/2 states, respectively. When the concentration of Er3+ion is a constant while the concentration of Dy3+ions increases from 0.1% to 5%, the up-conversion emission intensity of phosphor increases gradually. Broadening of absorption spectrum and increases in emission intensity have made the present phosphors potential for application in solar cells.
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