Visible/near-infrared luminescence and concentration effects of Pr3+-doped Sr2Al2GeO7 downconversion phosphors
The Pr 3+ ion has been widely doped into various materials as a red and near-infrared (NIR) emitting center for applications in lighting and solar spectrum downconversion. Herein, the preparation of a new library of Pr 3+ -doped Sr 2 Al 2 GeO 7 phosphors was proved by powder x-ray diffraction patterns and Rietveld refinements and characterized by a scanning electron microscope with energy-dispersive x-ray spectrometry. The Sr 2 Al 2 GeO 7 :Pr 3+ sample strongly absorbs blue photons over 420–500 nm and yields intense visible emissions with dominant peaks around 490 nm from the Pr 3+ 3 P 0 → 3 H 4 transition, as well as robust NIR emission bands over 800–1200 nm. In addition to the typical transitions of 1 D 2 → 3 F 2 at 880 nm, 1 G 4 → 3 H 4 at 1000 nm, and 1 D 2 → 3 F 3,4 at 1070 nm, the distinguishable NIR emission at 929 nm was demonstrated from the 3 P 0 → 1 G 4 transition via static and dynamic spectroscopic analysis. Most interestingly, for the 3 P 0 blue-excited state, a considerably elevated concentration of about 10%Pr 3+ was optimal for the visible/NIR emissions, in stark contrast to the diluted optimal 1%Pr 3+ for the 1 D 2 state. The relevant cross-relaxation from the 3 P 0 and 1 D 2 states between Pr 3+ was comprehensively treated by theoretical speculations and experimental results. Such concentrated Pr 3+ blue activators would significantly facilitate the blue-to-NIR downconversion through a desired two-step sequential transition from the 3 P 0 initial state to the 1 G 4 intermediate level for quantum efficiency exceeding unity. The current results would consolidate the basis of concentrated Pr 3+ donors to promote the novel Pr 3+ /Yb 3+ codoping downconversion for greatly increasing Si solar cell efficiency.
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