Mechanisms of Li⁺Ions in the Emission Enhancement of KMg₄(PO₄)₃:Eu²⁺for White Light Emitting Diodes

Abstract

Codoping with Li is a prevalent strategy to improve the optical efficiency of luminescent materials, while the mechanisms of enhancement are still ambiguous. Herein, we delineate the major ways by which Li enhanced the emission of orthophosphate phosphor KMg(PO):Eu and quantify the relative contributions of each mechanism. Results from X-ray diffraction, scanning electron microscopy, and steady-state and time-resolved fluorescence spectroscopies show that the 3.8-fold increase in emission intensity caused by optimized Li doping was attributed to flux effect (∼30.84%), crystal-field splitting (∼5.30%), and a reduction in concentration quenching (∼63.86%), respectively. The as-synthesized materials also show excellent thermal stability and an increased internal quantum efficiency of 84.02% compared with 53.13% of nondoped phosphors. The white light emitting diodes employing KMg(PO):Eu,Li as a blue-emitting component exhibit superior electroluminescence properties. The above results demonstrate that introducing Li ions can obviously enhance the luminescence efficiency of KMg(PO):Eu phosphor.

Effect of codoping Ce3+ on the luminescence properties of Sr9Mg1.5(PO4)7:Eu2+ orange–yellow phosphor

“…When more Eu 3+ ion are reduced to Eu 2+ ion, the concentration of Eu 3+ in the phosphors is decreased, leading to an decrease of the emission lines of Eu 3+ . Ce 3+ ions substitute the position of Sr 2+ and dissociate the Eu 2+ ions, which in turn weaken the energy transfer between Eu 2+ and Eu 2+ and subdue the nonradiative concentration quenching . Therefore, the enhancement of Eu 2+ emission by Ce 3+ codoping is reasonable to be explained by the reduction of concentration quenching effect.…”

Discovery of novel solid solution Ca3Si3−x O3+x N4−2x : Eu2+ phosphors: structural evolution and photoluminescence tuning

“…As the particles grow smaller, the concentration of surface activator ions steadily increases. With the crystallite enlargement, more and more activator ions are located in the interior of the crystals, rather than at the near surface where energy can rapidly transfer to surface defects and then be consumed by high vibrational energies 34 . That is to say, the enlargement of particles can effectively suppress energy transfer to the crystal surface to result in efficient luminescence by confining the ions in the interior core of the grains 35 .

Figure 5Granulometric distribution of selected Ca 2.97 Si 3− x O 3+ x N 4−2 x : 0.03Eu 2+ phosphors ( x  = 0, 0.2, 1, 1.2, 1.4 and 1.5).

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“…8(b), the excitation peaks at 270 nm show a blue-shift while the other two peaks present a red-shift with decreasing x values. Hence, the entire excitation bands are broadened due to the crystal field splitting effect 34 . When the x values decrease, the nitrogen-rich phosphors are obtained and Eu 2+ ions have opportunities to coordinate with more nitrogen atoms.…”

“…It has been reported that the energy of an excited Eu 2+ ion is likely to be consumed by another neighboring Eu 2+ or the traps caused by the defects if the distances between these are close enough via a non-radiative way, resulting in a lifetime reduction 43,45 . With the crystallite enlargement, more and more activator ions are located in the interior of the crystals, rather than at the near surface where energy can rapidly transfer to surface defects and then be consumed by high vibrational energies 34 .

Figure 11Room temperature decay curves of Ca 2.97 Si 3− x O 3+ x N 4−2 x : 0.03Eu 2+ ( x  = 0, 0.5, 0.8, 1, 1.5) phosphors.

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Luminescence properties and energy transfer behavior of colour-tunable white-emitting Sr4Al14O25 phosphors with co-doping of Eu2+, Eu3+ and Mn4+

“…The consecutive increasing of the ET probabilities with the various Mn 4+ content manifests an enhanced ET process between Eu 2+ and Mn 4+ because of the decrease in the average distance between the ions. 34 Furthermore, the energy transfer efficiency h from a donor Eu 2+ to acceptor Mn 4+ would be estimated according to the expression as follows:…”