Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors

Abstract

Red-emitting Mn4+-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 Mn4+-doped fluorides. Furthermore, concentration quenching of the Mn4+ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn4+-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn4+ between 4 and 600 K and for Mn4+ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K2TiF6:Mn4+ and other Mn4+-doped phosphors show that quenching occurs through thermally activated crossover between the 4T2 excited state and 4A2 ground state. The quenching temperature can be optimized by designing host lattices in which Mn4+ has a high 4T2 state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K2TiF6:Mn4+ up to 5% Mn4+. This is important, as high Mn4+ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn4+ d–d transitions. At even higher Mn4+ 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 Mn4+ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn4+ emission and can be used to realize superior narrow-band red Mn4+ phosphors for w-LEDs.

Li2TiO3:Mn4+ Deep‐Red Phosphor for the Lifetime‐Based Luminescence Thermometry

“…As shown in Figure (a), the temperature dependence of the emission decay time, , perfectly fits to the cross‐over model: …”

“…As shown in Figure 4(a), the temperature dependence of the emission decay time, t T ð Þ, perfectly fits to the cross-over model: [42,43]…”

“…It is interesting to note that by controlling the cross‐over energy (for example by controlling the energy of the 4 T 2g level), the energy of phonon coupling to the 2 E g → 4 A 2g transition, and the non‐radiative decay rate through the choice of host lattice, it is feasible to engineer the Mn 4+ activated luminescent thermometry probes with very high sensitivity in the desired temperature region. Figure displays changes in the lifetime and relative sensitivity temperature dependences when varying (Figures (a) and (d)), (Figures (b) and (e)), and (Figures (c) and (f)) around fitted values.…”

Co-Precipitation Synthesis and Optical Properties of Mn4+-Doped Hexafluoroaluminate w-LED Phosphors

“…The decrease in PL intensity between 298 K and 423 K for M 3 AlF 6 :Mn 4+ we ascribe to an increase of the energy transfer from Mn 4+ ions to quenching sites (defects and impurities) with temperature [48]. The rapid quenching of the Mn 4+ luminescence above 430 K is attributed to thermally activated crossing of the Mn 4+ 4 T 2 excited state and 4 A 2 ground state, as is explained in [49]. …”

Synthesis and photoluminescence properties of a novel red phosphor SrLaGaO4:Mn4+

“…Much beyond our expectation, the PL intensity decreases sharply and remains 3.6% of the initial one at 298 K when temperature is at 423 K (150°C, the working temperature for high‐power LEDs), suggesting that SrLaGaO 4 :Mn 4+ phosphors have a poor thermal stability. For Mn 4+ , the mechanism behind the thermal quenching is systematically investigated by Merjerink et al As presented in Figure C, upon excitation from 4 A 2 to 4 T 1 , 4 T 2 , the excited electron relaxes rapidly to excited state 2 E. The radiative transition 2 E‐ 4 A 2 gives off far‐red emission. While at certain temperature, with the thermal phonon, the electron in 2 E state relaxes to ground state 4 A 2 via crossover of 4 T 2 .…”