Abstract
Na5Lu9F32: Yb3+, Tm3+ single crystals prepared by an improved Bridgman method are expected to be incorporated into solar cells as an upconversion layer to expand the absorption range. Upconversion (UC) with weak emission of 488 nm blue, 704 nm red and strong 805 nm near-infrared fluorescence upon 964 nm excitation as well as its pump power dependence on UC emission were obtained in the single crystals. Furthermore, a high external quantum efficiency (EQE) of 2.65% is obtained for Na5Lu9F32: 8%Yb3+, 1%Tm3+ under 1 W/cm2 980 nm excitation. Meanwhile, the temperature-dependent UC emission spectra is studied as a function of temperature in the range of 250–370 K. The cubic dependence of pump power on UC emission reveals that three-photon UC processes are mainly responsible for the 488 nm, 704 nm emissions and the quadratic dependence indicates two-photon process for the 805 nm emission in Tm3+/Yb3+ co-doped Na5Lu9F32 single crystals, respectively. The energy transfer processes were studied based on energy level diagram, and the results indicated that the cross-relaxation (CR) between Tm3+ and Tm3+ affect the luminous intensity. Moreover, it can infer that the energy transfer from Tm3+ to Yb3+ exists from the decay curves of Tm3+ at 805 nm under excitation by 964 nm.
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