Broadband downconversion in bi ³⁺, yb ³⁺-co-doped yvo ₄ phosphor

Yttrium vanadate phosphors co-doped with Bi ³⁺ and Yb ³⁺ ions have been prepared via the solidstate reaction. The phosphors were characterized by various methods including X-ray diffraction, photoluminescence excitation and photoluminescence spectra. Upon ultraviolet (UV) light excitation, an intense near-infrared (NIR) emission of Yb ³⁺ corresponding to the transition of ²F _5/2 → ²F _7/2 peaking at 985 nm was observed as a result of energy transfer from O ²?-V ⁵⁺ or Bi ³⁺-V ⁵⁺ charge transfer state (CTS) to Yb ³⁺. A broad excitation band ranging from 250 to 375 nm was recorded when the Yb3+ emission was monitored, which suggests an efficient energy transfer from CTS to Yb ³⁺ ions. The dependence of Yb ³⁺ doping concentration on the visible emission, the NIR emission and decay lifetime has been investigated. The results of visible and NIR spectral evolution with temperature indicate that the mechanism for the NIR-emission is mainly phonon-assisted energy transfer at room temperature, while the mechanism is mainly cooperative energy transfer at low temperature. The YVO ⁴:Bi ³⁺, Yb ³⁺ phosphor has prospects for realizing high efficiency crystalline Si solar cells by converting broadband UV energy into NIR light.