Blue-Emitting K₂Al₂B₂O₇:Eu²⁺ Phosphor with High Thermal Stability and High Color Purity for Near-UV-Pumped White Light-Emitting Diodes

Novel blue-emitting K₂Al₂B₂O₇:Eu²⁺ (KAB:Eu²⁺) phosphor was synthesized by solid state reaction. The crystal structural and photoluminescence (PL) properties of KAB:Eu²⁺ phosphor, as well as its thermal properties of the photoluminescence, were investigated. The KAB:Eu²⁺ phosphor exhibits broad excitation spectra ranging from 230 to 420 nm, and an intense asymmetric blue emission band centered at 450 nm under λ_ex = 325 nm. Two different Eu²⁺ emission centers in KAB:Eu²⁺ phosphor were confirmed via their fluorescence decay lifetimes. The optimal concentration of Eu²⁺ ions in K_2–<i>x</i>Eu_<i>x</i>Al₂B₂O₇ was determined to be <i>x</i> = 0.04 (2 mol %), and the corresponding concentration quenching mechanism was verified to be the electric dipole–dipole interactions. The PL intensity of the nonoptimized KAB:0.04Eu²⁺ phosphor was measured to be ∼58% that of the commercial blue-emitting BaMgAl₁₀O₁₇:Eu²⁺ phosphor, and this phosphor has high color purity with the CIE coordinate (0.147, 0.051). When heated up to 150 °C, the KAB:0.04Eu²⁺ phosphor still has 82% of the initial PL intensity at room temperature, indicating its high thermal stability. These results suggest that the KAB:Eu²⁺ is a promising candidate as a blue-emitting n-UV convertible phosphor for application in white light emitting diodes

Low-Concentration Eu²⁺-Doped SrAlSi₄N₇: Ce³⁺ Yellow Phosphor for wLEDs with Improved Color-Rendering Index

Luminescence property of low-concentration Eu²⁺-doped SrAlSi₄N₇:Ce³⁺ yellow phosphor is reported in this paper. Three optical centers Ce1, Ce2, and Eu2 are observed in the phosphor. Deconvolution of emission spectrum confirms the three centers to be green (530 nm), yellow (580 nm), and red (630 nm), respectively. This property promises considerable improvement of color-rendering property of a white light-emitting diode (wLED). For example, color-rendering index (CRI) of wLED fabricated by combining a blue LED chip and SrAlSi₄N₇:0.05Ce³⁺,0.01Eu²⁺ phosphor reaches 88. A competitive energy transfer process between Ce1–Ce2 and Ce1–Eu2 is confirmed based on Inokuti–Hirayama formula. Ratio of energy transfer rate between Ce1–Ce2 and Ce1–Eu2 (<i>W</i>_Ce1–Eu2/<i>W</i>_Ce1–Ce2) is calculated to be 2.0. This result reveals the effect of Eu²⁺ concentration on quantity of green and red components in SrAlSi₄N₇:Ce³⁺,Eu²⁺ phosphor

Importance of Suppression of Yb³⁺ De-Excitation to Upconversion Enhancement in β‑NaYF₄: Yb³⁺/Er³⁺@β-NaYF₄ Sandwiched Structure Nanocrystals

Nanosized Yb³⁺ and Er³⁺ co-doped β-NaYF₄ cores coated with multiple β-NaYF₄ shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb³⁺ ions. We also explored the population of the Er³⁺⁴F_9/2 level. The results reveal that energy transfer from the lower intermediate Er³⁺⁴I_13/2 level is dominant for populating the Er³⁺⁴F_9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er³⁺⁴S_3/2 level for populating the Er³⁺⁴F_9/2 level, which mainly comes from the cross relaxation energy transfer from Er³⁺ ions to Yb³⁺ ions followed by energy back transfer within the same Er³⁺–Yb³⁺ pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er³⁺–Yb³⁺ pairs. This population route is in good agreement with that in nanomaterials and bulk materials

Improvement of Green Upconversion Monochromaticity by Doping Eu³⁺ in Lu₂O₃:Yb³⁺/Ho³⁺ Powders with Detailed Investigation of the Energy Transfer Mechanism

The monochromaticity improvement of green upconversion (UC) in Lu₂O₃:Yb³⁺/Ho³⁺ powders has been successfully realized by tridoping Eu³⁺. The integral area ratio of green emission to red emission of Ho³⁺ increases 4.3 times with increasing Eu³⁺ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb³⁺/Ho³⁺/Eu³⁺ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho³⁺ ⁵F₄/⁵S₂ level tothe Eu³⁺ ⁵D₀ level and ET from the Ho³⁺ ⁵I₆ level to the Eu³⁺ ⁷F₆ level. In addition, the population routes of the red-emitting Ho³⁺ ⁵F₅ level in the Yb³⁺/Ho³⁺ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb³⁺ ²F_5/2 level to the Ho³⁺ ⁵I₇ level and the cross-relaxation process between two nearby Ho³⁺ ions in the ⁵F₄/⁵S₂ level and ⁵I₇ level, respectively, have been demonstrated to be the dominant approaches for populating the Ho³⁺ ⁵F₅ level. The multiphonon relaxation process originating from the Ho³⁺ ⁵F₄/⁵S₂ level is useless to populate the Ho³⁺ ⁵F₅ level. As the energy level gap between the Ho³⁺ ⁵I₇ level and Ho³⁺ ⁵I₈ level matches well with that between Eu³⁺ ⁷F₆ level and Eu³⁺ ⁷F₀ level, the energy of the Ho³⁺ ⁵I₇ level can be easily transferred to the Eu³⁺ ⁷F₆ level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho³⁺ ⁵F₄/⁵S₂ level to the Eu³⁺ ⁵D₀ level as well as the ET from the Ho³⁺ ⁵I₆ level to the Eu³⁺ ⁷F₆ level, the integral area ratio of green emission to red emission of Ho³⁺ has been improved significantly

Enhancement of Eu³⁺ Red Upconversion in Lu₂O₃: Yb³⁺/Eu³⁺ Powders under the Assistance of Bridging Function Originated from Ho³⁺ Tridoping

The red upconversion (UC) emission of Eu³⁺ ions in Lu₂O₃: Yb³⁺/Eu³⁺ powders was successfully enhanced by tridoping Ho³⁺ ions in the matrix, which is due to the bridging function of Ho³⁺ ions. The experiment data manifest that, in Yb³⁺/Eu³⁺/Ho³⁺ tridoped system, the Ho³⁺ ions are first populated to the green emitting level ⁵F₄/⁵S₂ through the energy transfer (ET) processes from the excited Yb³⁺ ions. Subsequently, the Ho³⁺ ions at ⁵F₄/⁵S₂ level can transfer their energy to the Eu³⁺ ions at the ground state, resulting in the population of Eu³⁺ ⁵D₀ level. With the assistance of the bridging function of Ho³⁺ ion, this ET process is more efficient than the cooperative sensitization process between Yb³⁺ ion and Eu³⁺ ion. Compared with Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺, the UC intensity of Eu³⁺ ⁵D₀→⁷F₂ transition in Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺/0.5 mol % Ho³⁺ is increased by a factor of 8

Phonon Energy Dependent Energy Transfer Upconversion for the Red Emission in the Er³⁺/Yb³⁺ System

The red emission through upconversion (UC) upon 980 nm excitation based on Er³⁺/Yb³⁺ combination is very attractive for bioimaging applications. The intensity of the red emission is observed to be strongly dependent on the host materials. However, the origin of the behavior and the quantitative dependence remain unclear. Here, the effectiveness of the second step UC excitation from the Er³⁺ intermediate state ⁴I_13/2 to the ⁴F_9/2 level by energy transfer from Yb³⁺ is studied for three popular hosts (β-NaYF₄, Ba₅Gd₈Zn₄O₂₁, and Y₂O₃) that have different phonon energies. Their emission efficiencies of the red emitting state are calculated, and the radiative lifetime of the ⁴F_9/2 level in Ba₅Gd₈Zn₄O₂₁ is reported for the first time. We present a spectroscopic method to evaluate the relative energy transfer coefficients for the three hosts and find the coefficient increases markedly with the increase of phonon energy, reflecting the nature of phonon-assisted energy transfer. The coefficient for β-NaYF₄ is 89 and 408 times smaller than that for the other two oxide hosts, well revealing the origin of the green emission governed UC in β-NaYF₄ and the red emission in the other two. Accordingly, a comprehensive analysis of the luminescence dynamical processes shows that selecting material with appropriate phonon energy is essential for both effective excitation and efficient emission of the red level

Efficient Triplet Application in Exciplex Delayed-Fluorescence OLEDs Using a Reverse Intersystem Crossing Mechanism Based on a Δ<i>E</i>_S–T of around Zero

We demonstrate highly efficient exciplex delayed-fluorescence organic light-emitting diodes (OLEDs) in which 4,4′,4″-tris­[3-methylphenyl­(phenyl)­aminotriphenylamine (m-MTDATA) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were selected as donor and acceptor components, respectively. Our m-MTDATA:Bphen exciplex electroluminescence (EL) mechanism is based on reverse intersystem crossing (RISC) from the triplet to singlet excited states. As a result, an external quantum efficiency (EQE) of 7.79% at 10 mA/cm² was observed, which increases by 3.2 and 1.5 times over that reported in <i>Nat. Photonics</i> <b>2012</b>, <i>6</i>, 253 and <i>Appl. Phys. Lett.</i> <b>2012</b>, <i>101</i>, 023306, respectively. The high EQE would be attributed to a very easy RISC process because the energy difference between the singlet and triplet excited states is almost around zero. The verdict was proven by photoluminescence (PL) rate analysis at different temperatures and time-resolved spectral analysis. Besides, the study of the transient PL process indicates that the presence of an unbalanced charge in exciplex EL devices is responsible for the low EQE and high-efficiency roll-off. When the exciplex devices were placed in a 100 mT magnetic field, the permanently positive magnetoelectroluminescence and magnetoconductivity were observed. The magnetic properties confirm that the efficient exciplex EL only originates from delayed fluorescence via RISC processes but is not related to the triplet–triplet annihilation process