Enhancement of\nYb<sup>3+</sup> Emission in Bi<sub>3</sub>TeBO<sub>9</sub> through\nEfficient Energy Transfer from Bi<sup>3+</sup> Ions

An efficient near-infrared emission of Yb<sup>3+</sup> ions sensitized\nby Bi<sup>3+</sup> ions was revealed in Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> microcrystalline powders. Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> doped with different concentrations of Yb<sup>3+</sup> ions (0.5, 1.0, 2.5, 4.0, and 7.5 at %) were synthesized\nby means of the modified Pechini method. The structure, morphology,\nand elemental composition of samples were investigated using X-ray\ndiffraction, scanning electron microscopy, and energy-dispersive X-ray\nspectroscopy techniques. The μ-Raman spectroscopic measurements\nof Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> allowed us to determine\nthe energy vibrations of characteristic molecular groups BO<sub>3</sub> and TeO<sub>6</sub> present in the Bi<sub>3</sub>TeBO<sub>9</sub> matrix. Emissions of Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> powders were measured upon excitation at 330 nm by Bi<sup>3+</sup> ions (the <sup>1</sup>S<sub>0</sub> → <sup>3</sup>P<sub>1</sub> transition) in VIS (Bi<sup>3+</sup> ions emission assigned to the <sup>3</sup>P<sub>0</sub> → <sup>1</sup>S<sub>0</sub> transition)\nand NIR (Yb<sup>3+</sup> ions emission assigned to the <sup>2</sup>F<sub>5/2</sub> → <sup>2</sup>F<sub>7/2</sub>) spectral ranges.\nIn particular, the strongest emission band of Yb<sup>3+</sup> ions\nat 975 nm was observed in the spectrum of Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> doped with 4.0 at % of Yb<sup>3+</sup> ions. This\nsuggests that Bi<sup>3+</sup> ions present in the Bi<sub>3</sub>TeBO<sub>9</sub> matrix can be used as a UV absorption sensitizer for Yb<sup>3+</sup> ions in the considered systems. Moreover, the decay times\nof <sup>3</sup>P<sub>0</sub> → <sup>1</sup>S<sub>0</sub> transitions\nof Bi<sup>3+</sup> ions (emission in VIS with maximum at 550 nm) and <sup>2</sup>F<sub>5/2</sub> → <sup>2</sup>F<sub>7/2</sub> transitions\nof Yb<sup>3+</sup> ions (emission in NIR at 975 nm) upon excitation\nat 330 nm were measured. The calculated energy transfer efficiency\nand quantum efficiency coefficients confirmed the effective energy\ntransfer from Bi<sup>3+</sup> to Yb<sup>3+</sup> ions in the Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> system. The obtained results\nshow that Bi<sub>3</sub>TeBO<sub>9</sub>:Yb<sup>3+</sup> powders,\nas efficient UV to NIR spectral converters, can be potentially used\nin c-Si solar cell technology, enhancing the photovoltaic effect.