Optical absorption, luminescence, and energy transfer processes studies for Dy3+/Tb3+-codoped borate glasses for solid-state lighting applications

By using melt quenching technique, good optical quality singly doped Dy3+ or Tb3+ and Dy3+/Tb3+-codoped borate glasses were synthesized and studied by optical absorption, excitation, emission and decay lifetimes curve analysis. Following the absorption spectrum, the evaluated Judd-Ofelt (J-O) intensity parameters (Omega(lambda) (lambda = 2, 4 and 6)) were used to calculate the transition probability (A(R)), the branching ratio (beta(R)), and the radiative lifetime (tau(R)) for different luminescent transitions such as I-4(15/2) -> H-6(15/2), F-4(9/2) -> H-6(15/2), F-4(9/2) -> H-6(13/2), F-4(9/2) -> H-6(11/2) and F-4(9/2) -> H-6(9/2), F-6(11/2) for the 0.5 mol % singly Dy3+-doped glass. The beta(R) calculated (65%) indicates that for lasing applications, F-4(9/2) -> H-6(13/2) emission transition is highly suitable. For all the Dy3+/Tb3+-codoped glasses, Tb3+: D-5(3) -> F-7(6) emission decay lifetime curves are found to be non-exponential in nature for different concentrations of Dy3+ codoping. Using the Inokuti-Hirayama model, these nonexponential decay curves were analyzed to identify the nature of the energy transfer (ET) processes and here the electric dipole-dipole interaction is dominant for the ET. Based on the excitation and emission spectra and decay lifetimes curve analysis, the cross relaxation and ET processes between Dy3+ and Tb3+ were confirmed. For the 0.5 mol % Tb3+ and 2.0 mol % Dy3+-codoped glass, the evaluated Tb3+ -> Dy3+ ET efficiency (eta) is found to be 45% under 369 nm excitation. Further, for Tb3+/Dy3+-codoped glasses, an enhancement of Tb3+ green emission is observed up to 1.5 mol % Dy3+ codoping, and this is due to the non-radiative resonant ET from Dy3+ to Tb3+ upon 395 nm excitation. For singly 0.5 mol % Dy3+ or 0.5 mol % Tb3+-doped glass, the calculated color coordinates (x,y) and correlated color temperatures (CCT) represent the neutral white or warm white light regions, whereas Dy3+/Tb3+-codoped glasses (x,y) and CCT values fall in the yellowish green region with respect to the different Dy3+ concentrations under 350 and 395 nm excitations. Following the analyzed optical data, the singly Dy3+ or Tb3+-doped and Dy3+/Tb3+-codoped glasses could be suggested as promising materials for their applications in solid state light emitting diodes and luminescent display devices. (C) 2017 Elsevier B.V. All rights reserved.