Investigation of the Energy-Transfer Mechanism in Ho3+- and Yb3+-Codoped Lu2O3 Phosphor with Efficient Near-Infrared Downconversion

A high-temperature solid-state method was used to synthesize the Ho3+- and Yb3+-codoped cubic Lu2O3 powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho3+ ions and Yb3+ 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 Ho3+: I-5(8)-> G(6)/F-5(1). On the basis of the analysis of the relationship between the initial transfer rate of Ho3+: F-5(3) level and the Yb3+ doping concentration, it indicates that the ET from F-5(3) state of Ho3+ ions to F-2(5/2) state of Yb3+ 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 Lu2O3: 1% Ho3+/30% Yb3+. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb3+, the sensitizers Ho3+ also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the I-5(5)-> I-5(8) transition and F-5(4)/S-5(2)-> I-5(6) transition of Ho3+- as well as the F-2(5/2)-> E-2(7/2) transition of Yb3+ match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that Lu2O3: Ho3+/Yb3+ is a promising material to improve conversion efficiency of crystalline silicon solar cell.