Broadband downconversion in Bi3+, Yb3+-Co-doped YVO4 phosphor.

Yttrium vanadate phosphors co-doped with Bi3+ and Yb3+ ions have been prepared via the solid-state 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 Yb3+ corresponding to the transition of 2F(5/2) --> 2F(7/2) peaking at 985 nm was observed as a result of energy transfer from O2(-)-V5+ or Bi3+-V5+ charge transfer state (CTS) to Yb3+. 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 Yb3+ ions. The dependence of Yb3+ 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 YVO4:Bi3+, Yb3+ phosphor has prospects for realizing high efficiency crystalline Si solar cells by converting broadband UV energy into NIR light.