Wide color-range tunable and low roll-off fluorescent organic light emitting devices based on double undoped ultrathin emitters

We demonstrated a novel wide color-range tunable, highly efficient and low efficiency roll-off fluorescent organic light-emitting diode (OLED) using two undoped ultrathin emitters having complementary colors and an interlayer between them. The OLED can be tuned to emit sky blue (0.22, 0.30), cold white (0.29, 0.33), warm white (0.43, 0.42) and yellow (0.40, 0.45) according to the Commission Internationale de L’Eclairage (CIE) 1931 (x, y) chromaticity diagram. The device fabrication was simplified by eliminating doping process in the emission layers. The influence of interlayer thickness on luminous efficiency, efficiency roll-off and color tuning mechanism is thoroughly studied. The recombination zone is greatly broadened in the optimized device, which contributes to stable energy transfer to both emitters and suppressed concentration quenching. With a threshold voltage of 2.82 V, the color tunable organic light emitting diode (CT-OLED) shows a maximum luminance of 39,810 cd/m², a peak external quantum efficiency (EQE) 6% and the efficiency roll-off as low as 11.1% at the luminance from 500 cd/m² to 5000 cd/m². This structure of CT-OLED has great advantages of easy fabrication and low reagent consumption. The fabricated CT-OLEDs are tunable from cold white (0.30, 0.36) to warm white (0.43, 0.42) with correlated color temperature (CCT) 6932 K and 3072 K, respectively, demonstrating that our proposed approach helps to meet the need for lighting with various CCTs.     

Effect of energy transfer and local crystal field perturbation on the thermometric sensitivity of Ga-Tb-Eu ternary emission system

A series of optical thermometers based on Eu³⁺/Tb³⁺ doped Y₃Ga₅O₁₂ with self-excited GaO₆ group phosphors were designed through controllable energy transfer and local crystal field perturbation simulated using the density-functional theory approach and related structures. Color-tunable properties of the phosphors could be achieved through controllable energy transfer. In addition, the thermometers exhibited superb temperature sensitive properties. Over the entire temperature range (298.15–598.15 K), maximum values of the absolute sensitivity and relative sensitivity are 0.028 K^?1 and 7.03 %K^?1, respectively. Meanwhile, the thermometer has outstanding resolution (ΔT = 0.0043 K) and repeatability (98.37%).     

Deep blue,cyan, orange-red,and white multicolor emissions generated by Bi3+/Eu3+ activated KBaYSi2O7 luminescent materials for white light-emitting diodes

A variety of Bi³⁺ and/or Eu³⁺ doped KBaYSi₂O₇ phosphors with deep blue, cyan, orange-red, and white light multicolor emissions have been fabricated by a Pechini sol-gel (PSG) method. The KBaYSi₂O₇:Bi³⁺ phosphors exhibit an intense cyan emission or a unique narrow deep blue emission when excited by different wavelengths, which may bridge the cyan gap or act as a promising deep blue phosphor for white light-emitting diodes (WLEDs). The tunable multicolor emissions can be achieved by changing the Bi³⁺ doping concentrations. The Bi³⁺/Eu³⁺ co-doped KBaYSi₂O₇ phosphors display intrinsic emissions of Bi³⁺ and Eu³⁺ and an energy transfer process between Bi³⁺ and Eu³⁺ can be detected. The luminescence colors of KBaYSi₂O₇:Bi³⁺,Eu³⁺ regularly shift from blue, through cold and warm white, finally toward orange-red by adjusting the relative doping concentrations of Bi³⁺ and Eu³⁺. The single-phase white light-emitting material can be generated in both cold and warm white regions by simply varying the Eu³⁺ doping concentrations. Furthermore, three kinds of WLEDs devices are fabricated by KBaYSi₂O₇:Bi³⁺ or KBaYSi₂O₇:Bi³⁺,Eu³⁺ phosphors, which can exhibit dazzling white light emissions with eminent CIE coordinates, correlated color temperature, and color rendering index. The result offers direct evidence that the as-synthesized phosphors may be potentially applied in WLEDs and solid-state lighting.     

Energy transfer and cross-relaxation induced multicolor upconversion emissions in Er~(3+)/Tm~(3+)/Yb~(3+) doped double perovskite La_2ZnTiO_6 phosphors

Investigation on the bright and stable upconversion(UC) phosphors with multicolor emissions is fundamental and significant for the frontier applications of display and tempe rature probe.He re,dive rse emitting colors with blue,cyan and yellowish green,which are caused by the energy transfer and crossrelaxation processes,are obtained by altering Er~(3+),Tm~(3+)and Yb~(3+) concentrations in Er~(3+)singly,Er~(3+)-Tm~(3+)-Yb~(3+)co-and tri-doped double perovskite La_2 ZnTiO_6(LZT) phosphors synthesized by a simple solid-state reaction.In addition,excellent infrared emission at 801 nm located at "first biological windo w" is collected in Tm~(3+)-Yb~(3+)co-doped phosphors.Meanwhile,the temperature sensing properties based on the thermally coupled levels(~2 H_(11/2)/~4S_(3/2)) of Er~(3+) ions were analyzed from 298 to 573 K of LZT:0.15 Er~(3+)/0.10 Yb~(3+)phosphor,demonstrating that the maximal sensitivity value is about56×10-4 K~(1-) at 448 K.All these results imply that this kind of UC material has potential applications in display,bioimaging and optical device.     

Synthesis and photoluminescence properties of color-tunable BaLa2WO7:Eu phosphor

Color-tunable phosphors BaLa_2−xEu_xWO₇ were synthesized via a solid-state reaction. The absorption, excitation, emission and decay curves were obtained to study the luminescence properties. The experimental results indicate that BaLa_2−xEu_xWO₇ phosphors have two regions in the excitation spectra: one is assigned to the charge-transfer state (CTS) band at about 338 nm, and the other is assigned to the intra-4f transitions at 360-600 nm. The emission spectra of BaLa_2−xEu_xWO₇ phosphors excited at 395 nm exhibit a series of sharp peaks, which are attributed to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions. Luminescence from higher excited states, such as ⁵D₁, ⁵D₂, and ⁵D₃, were also observed at low Eu³⁺ concentration. The optimal emission intensity of ⁵D₀ → ⁷F₂ red emission is at x = 0.4 (BaLa_1.6Eu_0.4WO₇). The chromaticity coordinates of BaLa_2−xEu_xWO₇ phosphors vary with Eu³⁺ content from white, orange-red, to red, making it a candidate for a white-light-emitting phosphor in UV-LEDs.     

Study on the color tunability and energy transfer mechanism in Tm3+/Dy3+ co-doped LiLaSiO4 phosphors

Many color-tunable LiLaSiO₄: αTm³⁺, βDy³⁺ phosphors were prepared using the traditional high-temperature solid-phase method. The phase composition, surface morphology, fluorescence spectrum, fluorescence lifetime, energy transfer mechanism, and color coordinates of the samples were analyzed using XRD, SEM, TEM and fluorescence spectrometer. The results show that LiLaSiO4: αTm3+ phosphor emits high intensity blue light at 460 nm and the concentration quenching point of Tm³⁺, α = 0.015 mol. In LiLaSiO₄: αTm³⁺, βDy³⁺ phosphors, as the doped Dy³⁺ doping increases, the luminescence intensity of Tm³⁺ gradually decreases, the luminescence intensity of Dy³⁺ first increases and then decreases. The concentration quenching point of Dy³⁺, β = 0.015 mol. Adjustable light-emitting color can be obtained by changing the doping molar mass of Dy³⁺ or the wavelength of the excitation light. There is an effective energy transfer between Tm³⁺→Dy³⁺. The energy transfer mechanism is the electric dipole-electric dipole interaction, and the energy transfer efficiency reaches 90.11% when β = 0.06 mol. The quantum yield of LiLaSiO₄:0.015 Tm³⁺,0.015Dy³⁺ was 39.0%. The single-matrix white light LiLaSiO₄: αTm³⁺, βDy³⁺ phosphors with excellent performance is a prospective material for of white LEDs.     

Color-tunable properties based on complex anion substitution in Eu2+ doped Ca8Sc2(PO4)6-y(SiO4)1+y phosphor

We synthesized and investigated the effect of Eu²⁺ ions doping in a novel phosphor-silicate Ca₈Sc₂(PO₄)₆(SiO₄) phosphor. The structure and photoluminescence properties were determined by X-ray powder diffraction Rietveld refinement, diffuse reflection spectra, emission-excitation spectra, decay curves and temperature dependence spectra. The phosphors showed an asymmetric broad-band blue emission (Eu²⁺) with peak at 470?nm. Furthermore, we presented the Ca_7.96Sc₂(PO₄)_6-y(SiO₄)_1+y:0.04Eu²⁺ phosphors by co-substituting [Eu²⁺-Si⁴⁺] for [Ca²⁺-P⁵⁺], and different behaviors of luminescence evolution in response to structural variation were verified among the series of phosphors. The results were attributed to the presence of multi Ca²⁺ sites, resulting in the mixing of blue and green emissions for Eu²⁺ ions. The complex anion substitution of [PO₄]^3- by [SiO₄]^4- induced an increased crystal field splitting of the Eu²⁺ ions, which caused a decrease in emission energy from the 5d excited state to the 4f ground state and a resultant red-shift from 470?nm to 520?nm. All the properties indicated that the Ca₈Sc₂(PO₄)₆(SiO₄):Eu²⁺ phosphors have potential application for color-tunable WLEDs.     

Synthesis,structure and luminescent properties of Eu3+ doped Ca3LiMgV3O12 color-tunable phosphor

A new vanadate Ca₃LiMgV₃O₁₂ and its Eu³⁺-doped counterparts were synthesized. Rietveld confinement result of Ca₃LiMgV₃O₁₂ host indicates that it belongs to cubic space group Ia-3d with parameters of a =?12.4300?Å, V =?1920.49?ų, Z?=?8. Under UV excitation, pure Ca₃LiMgV₃O₁₂ exhibits a bluish-green broadband emission at 490?nm, while Eu³⁺ doped Ca₃LiMgV₃O₁₂ shows one bluish-green broad band with a series of red sharp peaks, which originate from the V⁵⁺-O^2- charge transfer and the Eu³⁺ intra-4f transitions, respectively. The occurrence of VO₄→Eu³⁺ energy transfer is confirmed by decay lifetime analysis and time-resolved emission spectra. It is found that emitting color varies from bluish-green to orange-red with increasing Eu³⁺ concentration. VO₄ bluish-green and Eu³⁺ red emission shows different thermal quenching response with increasing temperature, due to their different activation energy.     

Color-tunable electrophosphorescent device fabricated by a photo-bleaching method

We demonstrated an efficient color-tunable electrophosphorescent device fabricated by a photo-bleaching method. Electroluminescence studies indicate that excellent device performance can be achieved through efficient Förster energy transfer from the conjugated polymer to the iridium complexes by improving their miscibility. The use of a very low concentration of red phosphorescent dye and the easy degradation characteristics of conjugated structure of the red dopant enable color-tuning from red to green emission by a simple UV-irradiation process without a sacrifice of luminescent properties.tp     

Color-tunable up-conversion luminescence of Zn(AlxGa1-x)2O4:Yb3+,Tm3+,Er3+ powders

Color-tunable up-conversion powder phosphors Zn(Al_xGa_1-x)₂O₄: Yb³⁺,Tm³⁺,Er³⁺ were synthesized via high temperature solid-state reaction. Also, the morphological and structural characterization, up-conversion luminescent properties were all investigated in this paper. In brief, under the excitation of a 980?nm laser, all powders have same emission peaks containing blue emission at 477?nm (attributed to ¹G₄→³H₆ transition of Tm³⁺ ions), green emission at 526?nm and 549?nm (attributed to ²H_11/2→ ⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions respectively), red emission at about 659?nm and 694?nm (attributed to ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ions and ³F₃→³H₆ transition of Tm³⁺ ions, respectively), which are not changed after the doping of Al³⁺ ions. However, the doping of Al³⁺ ions can enhance the up-conversion luminescent intensity and efficiency, while the emission color of as-prepared powder phosphors can be tunable by controlling the doping amount of Al³⁺ ions. Taking Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er as the cut-off value, the emissions have clear blue-shift firstly and then show obvious red-shift with the increasing doping of Al³⁺ ions. Stated thus, pink emission in ZnAl₂O₄:Yb,Tm,Er, purplish pink emission in ZnGa₂O₄:Yb,Tm,Er and Zn(Al_0.9Ga_0.1)₂O₄:Yb,Tm,Er, purple emission in Zn(Al_0.1Ga_0.9)₂O₄:Yb,Tm,Er and Zn(Al_0.3Ga_0.7)₂O₄:Yb,Tm,Er, purplish blue emission in Zn(Al_0.7Ga_0.3)₂O₄:Yb,Tm,Er, blue emission in Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er can be observed, which confirm the potential applications of as-prepared Zn(Al_xGa_1-x)₂O₄:Yb³⁺,Tm³⁺,Er³⁺ powder phosphors in luminous paint, infrared detection and so on.