Quenching of the red Mn^(4+) luminescence in Mn^(4+)-doped fluoride LED phosphors

摘要

Red-emitting Mn^(4+)-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes(w-LEDs).For w-LEDs,the luminescence quenching temperature is very important,but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn^(4+)-doped fluorides.Furthermore,concentration quenching of the Mn^(4+) luminescence can be an issue but detailed investigations are lacking.In this work,we study thermal quenching and concentration quenching in Mn^(4+)-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn^(4+) between 4 and 600 K and for Mn^(4+) concentrations from 0.01%to 15.7%.Temperature-dependent measurements on K2TiF6:Mn^(4+) and other Mn^(4+)-doped phosphors show that quenching occurs through thermally activated crossover between the ^(4)T_(2) excited state and ^(4)A_(2) ground state.The quenching temperature can be optimized by designing host lattices in which Mn^(4+) has a high 4T2 state energy.Concentration-dependent studies reveal that concentration quenching effects are limited in K_(2)TiF_(6):Mn^(4+)up to 5%Mn^(4+).This is important,as high Mn^(4+)concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn^(4+)d–d transitions.At even higher Mn^(4+) concentrations(>10%),the quantum efficiency decreases,mostly due to direct energy transfer to quenching sites(defects and impurity ions).Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn^(4+) phosphors.The present systematic study provides detailed insights into temperature and concentration quenching of Mn^(4+) emission and can be used to realize superior narrow-band red Mn^(4+) phosphors for w-LEDs.