Efficient Quantum Cutting in Tb3+/Yb3+ Codoped α-NaYF4 Single Crystals Grown by Bridgman Method Using KF Flux for Solar Photovoltaic

Tb3+/Yb3+ codoped alpha-NaYF4 single crystals with various Yb3+ concentrations and similar to 0.40 mol% Tb3+ are grown by Bridgman method using KF (Potassium fluoride) as flux with a temperature gradient of 70 degrees C/cm-90 degrees C/cm across the solid-liquid interface. The effect and mechanisms of KF in the growing process are studied. The crystal structure is characterized by means of XRD. The high transmission from 300 to 7350 nm and maximum phonon energy about 390 cm(-1) for the alpha-NaYF4 single crystal are obtained from the measured transmission and Raman spectra, respectively. The luminescent properties of the crystals are investigated through excitation, emission spectra, and decay curves. Downconversion with emission of two near-infrared photons about 1000 nm for each blue photon at 486-nm absorption is obtained in Tb3+/Yb3+ codoped alpha-NaYF4 single crystals. Moreover, the energy transfer processes is studied based on the Inokuti-Hirayama model from the measured luminescent decay curves, and the results indicated that the interaction between Tb3+ and Yb3+ is electric dipole-dipole. The maximum quantum cutting efficiency approached 174.6% in alpha-NaYF4 single crystal with 0.42 mol% Tb3+ and 9.98 mol% Yb3+, making it a potential candidate for applications in silicon-based solar cells.