The energy transfer mechanism in Pr3+ and Yb3+ codoped β-NaLuF4 nanocrystals

The Pr3+ and Yb3+ codoped beta-NaLuF4 hexagonal nanoplates with a size of 250 nm x 110 nm were synthesized by a solvothermal process. X-Ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near infrared spectra as well as the decay curves of Pr3+:P-3(0) level were used to demonstrate the energy transfer from Pr3+ ions to Yb3+ ions. The downconversion phenomenon has been observed under the direct excitation of the P-3(2) level of Pr3+. According to the analysis of the dependence of the initial transfer rate upon Yb3+ ion concentration, it indicates that the ET from Pr3+ ions to Yb3+ ions is only by a two-step ET process when the Yb3+ concentration is very low; however, with the increase of the Yb3+ concentration, a cooperative ET process occurs and gradually increases; when the Yb3+ ion concentration increases to 20 mol%, the ET from Pr3+ ions to Yb3+ ions occurs only by the cooperative ET process. When the doping concentration of Yb3+ ions reaches 20 mol% at a fixed concentration of Pr3+ ions (1 mol%), the theoretical quantum efficiency is 192.2%, close to the limit of 200%. The current research has great potential in improving the conversion efficiency of silicon solar cells.