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

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 K(2)TiF(6):Mn(4+) between 4 and 600 K and for Mn(4+) concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K(2)TiF(6):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 (4)T(2) 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.