Investigation of the Energy-Transfer Mechanism in Ho- and Yb-Codoped LuO Phosphor with Efficient Near-Infrared Downconversion.

A high-temperature solid-state method was used to synthesize the Ho- and Yb-codoped cubic LuO powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho ions and Yb ions was verified by the steady-state spectra including visible and near-infrared (NIR) regions. Beyond that, the decay curves were also measured to certify the existence of the ET process. The downconversion phenomena appeared when the samples were excited by 446 nm wavelength corresponding to the transition of Ho: I→G/F. On the basis of the analysis of the relationship between the initial transfer rate of Ho: F level and the Yb doping concentration, it indicates that the ET from F state of Ho ions to F state of Yb ions is mainly through a two-step ET process, not the long-accepted cooperative ET process. In addition, a 62% ET efficiency can be achieved in LuO: 1% Ho/30% Yb. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb, the sensitizers Ho also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the I→I transition and F/S→I transition of Ho as well as the F→F transition of Yb match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that LuO: Ho/Yb is a promising material to improve conversion efficiency of crystalline silicon solar cell.