Enhanced near-infrared quantum cutting in CaMoO4:Yb phosphors induced by doping with Li ions for improving solar cells efficiency

The near-infrared (NIR) quantum cutting (QC) CaMoO₄:Yb³⁺ phosphors co-doped with Li⁺ ions were synthesized by the sol–gel methods. The dependence of the structure, morphology, photoluminescence (PL) and downconversion (DC) quantum efficiency on the Li⁺ doping concentration have been investigated in details. It was demonstrated that the CaMoO₄:Yb³⁺ phosphors with appropriate concentration of the Li⁺ ions show improved crystallinity and remarkable increase of the NIR emission from the Yb³⁺ ions, if compared with the Li⁺-free samples. The enhancement of the NIR emission and DC quantum efficiency was suggested to be a consequence of the improved crystallinity and stronger absorption of the host due to the [MoO₄] tetrahedra distortion induced by the Li⁺ ions. Cooperative energy transfer (CET) from the host to the Yb³⁺ ions is discussed as a possible mechanism for the NIR emission enhancement. Excellent luminescence properties of the Li⁺ doped CaMoO₄:Yb³⁺ phosphor demonstrate its potential application as a better QC layer to increase the energy conversion efficiency of the Si-based solar cells.     

On the Ce3+ → Cr3+ energy transfer in Lu3Al5O12 garnets

Two series of Lu₃Al₅O₁₂:Cr³⁺ and Lu₃Al₅O₁₂:0.5% Ce³⁺,Cr³⁺ luminescent materials were prepared by a sol–gel combustion method. All samples were characterized by powder X-ray diffraction (XRD), infrared (IR) and photoluminescence (PL) measurements. Moreover, luminous efficacies (LE), CIE 1931 colour points, and quantum efficiencies (QE) were calculated and discussed. Luminescence measurements indicated that Ce³⁺ ions located at Lu³⁺ site transfers absorbed energy to Cr³⁺ ions located at Al³⁺ site. However, with increasing Cr³⁺ concentration the total light output of Lu₃Al₅O₁₂:0.5% Ce³⁺,Cr³⁺ phosphors decrease.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Effect of Al ions on fluorescence properties of YPO4: Eu phosphors

Different concentrations of Al³⁺-doped YPO₄:Eu_0.05 powder phosphors have been synthesized by the conventional solid state reaction method and are characterized by X-ray diffraction (XRD), field emission scanning electronic microscopy (FESEM), photoluminescence excitation (PLE) and emission (PL) investigations. The influence of Al³⁺-doping on crystallinity, grain size and PL intensity of the YPO₄:Eu phosphors has been investigated. These characteristics are found improved with increase in concentration of Al³⁺ ions from 0.00 to 0.10 mol and then decreased for higher concentrations. The results are discussed in comparison with earlier reported similar works.     

Preparation and photoluminescence properties of Y2O3:Eu, Bi phosphors by molten salt synthesis for white light-emitting diodes

Y₂O₃:Eu, Bi red phosphors were first prepared by molten salt synthesis (MSS) method at a low temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM), and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The results show that as-obtained Y₂O₃:Eu, Bi phosphors have good cubic crystallinity, presenting octahedral morphology with smooth surface and relatively uniform particle size. Bi³⁺ ion as a sensitizer plays a significant effect on the emission intensity of Y₂O₃:Eu, Bi by energy transfer from Bi³⁺ to Eu³⁺ and the optimal concentration of Bi³⁺ is 1.5 mol%. Y₂O₃:Eu, Bi emits excellent red light as the excitation wavelength is between 330 and 420 nm or excited by 254, 466 nm. Meanwhile, its emission intensity is as strong as that of sample prepared by solid-state reaction. So the as-fabricated red phosphors by MSS method would have a promising application in the area of white light-emitting diodes.     

Preparation and characterization of boron oxide-based red-emitting phosphors using Eu,Al and Ca additives

New red-emitting phosphors have been produced and studied by investigating the effects of various additives such as europium (Eu), aluminum (Al), and calcium (Ca) on the photoluminescence performance of boron oxide-based phosphors. When both Eu and Ca ions were added, Ca₃B₂O₆:Eu phosphors could be synthesized via a straightforward process using a liquid precursor. The photoluminescent spectra of Ca₃B₂O₆:Eu phosphors exhibited a red emission at 611 nm under 254 nm excitation, and the internal quantum efficiency reached 41.5% when the concentration of Eu ion was fixed at an optimal condition of 0.13 M.     

Color tunable Sr2SiO4:Eu2+ phosphors through the modification of crystal structure

Eu²⁺ doped Sr₂SiO₄ phosphors were prepared through a solid-state reaction method. The phase-composition and photoluminescence of the obtained phosphors were systematically studied in terms of calcination temperature, Eu and Ba doping. High calcination temperature promoted the phase transformation from α′–Sr₂SiO₄ (orthorhombic) to β–Sr₂SiO₄ (monoclinic), while the doping of Eu or Ba ions could stabilize α′–Sr₂SiO₄ phase due to their long bond length with oxygen. Small amount of Eu/Ba doping prefers to occupy Sr(I) sites in the crystal lattice of Sr₂SiO₄, acting as nucleation sites for both α′– and β–Sr₂SiO₄ phases. After nucleation, Eu²⁺ ions distribute equally in the two sites. Through structural modification, the Sr₂SiO₄:Eu²⁺ phosphors could be controlled to emit different colors in a wide range, from blue to yellow, making them good candidates for tuning the chromaticity in application.     

Photoluminescence properties of NaGd(MoO4)2:Eu nanophosphors prepared by sol-gel method

NaGd(MoO₄)₂:Eu³⁺ (hereafter NGM:Eu) phosphors have been prepared by sol-gel method. The properties of the resulting phosphors are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curve. The excitation spectra of NGM:Eu phosphors are mainly attributed to O → Mo charge-transfer (CT) band at about 282 nm and some sharp lines of Eu³⁺ f-f transitions in near-UV and visible regions with two strong peaks at 395 and 465 nm, respectively. Under the 395 and 465 nm excitation, intense red emission peaked at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺ are observed for 35 at.% NGM:Eu phosphors as the optimal doping concentration. The luminescence properties suggest that NGM:Eu phosphor may be regarded as a potential red phosphor candidate for near-UV and blue light-emitting diodes (LEDs).     

Photoluminescence properties of red emitting BaGdB9O16:Eu phosphor

An investigation is reported of the photoluminescence properties of the BaGdB₉O₁₆:Eu phosphors. Under VUV excitation, BaGdB₉O₁₆:Eu exhibited a bright red luminescence with CIE chromaticity coordinates of (0.66, 0.34). The brightness was 80% of the commercial (Y,Gd)BO₃:Eu under identical excitation conditions. The PL spectrum showed a low field splitting of the ⁷F_J levels of Eu, indicating the Eu ions experience local distortion. It also showed a vibrational mode associated with the Eu-O vibration. The physical mechanism responsible for Eu excitation was investigated by PLE and time-resolved spectroscopy. The PLE spectrum showed sharp lines due to the ⁸S_7/2 – ⁶I_J and ⁶P_J transitions of Gd³⁺ in addition to the f-f transitions of Eu³⁺. The charge transfer band of Eu was observed at 233 nm and the host absorption band was detected at 159 nm. The time-resolved spectroscopy showed that the Eu ions decay purely exponentially with a lifetime of 2.1 ms. Under excitation into the Gd ⁶I_J level located below the fundamental absorption edge, the Eu luminescence was excited through the energy transfer process from Gd to Eu. The energy transfer rate was estimated to be 6000 s^–1.     

Hydrothermal synthesis and luminescent properties of GdVO4 : Eu3+ nanophosphors

Eu³⁺-doped GdVO₄ has been synthesised via hydrothermal method by altering the hydrothermal temperature, reaction time and surfactant. The microstructure and morphology information of the phosphors were investigated via the techniques of X-ray powder diffraction and scanning electronic microscopy, which show that the phosphors wear tetragonal phase and the products present various regular morphologies under different reaction conditions such as bulk and nanoparticle. Moreover, the morphologies of the products have been controlled by altering reaction temperature. In addition, the surfactant was also included to control the morphologies of the products and the phosphors present different morphologies. All the phosphors exhibit the characteristic fluorescence of Eu ion (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₂). The electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ is dominant indicating that most sites of Eu³⁺ ions in GdVO₄ have no inversion centre. Furthermore, we found that the reaction time and the morphologies have great influence on optical properties.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.     

Luminescent properties of YAl3(BO3)4:Eu3+ phosphors

YAL₃(BO₃)₄:Eu³⁺ phosphors were fabricated by the sol-gel method. The structure properties were measured by x-ray diffraction (XRD) and infrared spectra (IR). Doping concentration of Eu³⁺ ions in YAL₃(BO₃)₄:Eu³⁺ phosphors of 0, 1, 3, 4, and 5 mol% were studied. The excitation spectra and emission spectra of YAL₃(BO₃)₄:Eu³⁺ phosphors were examined by fluorescent divide spectroscopy (FDS). The luminescent properties of YAL₃(BO₃)₄:Eu³⁺ phosphors are discussion. The optimal doping concentration of Eu³⁺ ions in YAL₃(BO₃)₄:Eu³⁺ phosphors was found to be approximately 3 mol%.     

The comparison: photoluminescence and afterglow behavior in CaSnO<Subscript>3</Subscript>:Dy<Superscript>3+</Superscript> and Ca<Subscript>2</Subscript>SnO<Subscript>4</Subscript>:Dy<Superscript>3+</Superscript> phosphors

This paper reports the comparison of photoluminescence and afterglow behavior of Dy³⁺ in CaSnO₃ and Ca₂SnO₄ phosphors. The samples containing CaSnO₃ and Ca₂SnO₄ were prepared via solid-state reaction. The properties have been characterized and analyzed by utilizing X-ray diffraction (XRD), photoluminescence spectroscope (PLS), X-ray photoelectron spectroscopy (XPS), afterglow spectroscopy (AS) and thermal luminescence spectroscope (TLS). The emission spectra revealed that CaSnO₃:Dy³⁺ and Ca₂SnO₄:Dy³⁺ phosphors showed different photoluminescence. The Ca₂SnO₄:Dy³⁺ phosphor showed a typical ⁴F_9/2 to ⁶H_j energy transition of Dy³⁺ ions, with three significant emissions centering around 482, 572 and 670 nm. However, the CaSnO₃:Dy³⁺ phosphor revealed a broad T₁ → S₀ transitions of Sn²⁺ ions. The XPS demonstrate the existence of Sn²⁺ ions in CaSnO₃ phosphor caused by the doping of Dy³⁺ ions. Both the CaSnO₃:Dy³⁺ and Ca₂SnO₄:Dy³⁺ phosphors showed a typical triple-exponential afterglow when the UV source switched off. Thermal simulated luminescence study indicated that the persistent afterglow of CaSnO₃:Dy³⁺ and Ca₂SnO₄:Dy³⁺ phosphors was generated by the suitable electron or hole traps which were resulted from the doping the calcium stannate host with rare-earth ions (Dy³⁺).     

Visible quantum cutting in Tb<Superscript>3+</Superscript> doped BaGdF<Subscript>5</Subscript> phosphor for plasma display panel

Visible quantum cutting (QC) via down-conversion and enhancement in photoluminescence properties has been observed in terbium (Tb³⁺) doped BaGdF₅ phosphor. This phosphor was synthesized by varying molar concentration of Tb³⁺ ions via co-precipitation method. The prepared phosphor was characterized through X-ray diffraction technique. The photoluminescence spectra of BaGdF₅:Tb³⁺ phosphor measured under vacuum ultraviolet or UV excitation. The QC process was observed in prepared phosphor due to cross relaxation and direct energy transfer between Tb³⁺ and Tb³⁺ or Tb³⁺ and Gd³⁺ ions depending on the excitation wavelength. The maximum quantum efficiencies were found to be 162, 174 and 177 %, under the excitation of 172, 187 and 240 nm respectively. The green emission of 544 nm was observed at excitation of 172 and 187 nm. Hence this phosphor may be prime candidate for application in plasma display panels and mercury free fluorescent lamps.     

Molten salt synthesis and photoluminescence properties of novel red emitting phosphors Ba5(VO4)3Cl:Eu3+,K+

A series of Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors have been synthesized by the molten salt synthesis method. The crystalline structure, morphology, photoluminescence properties and lifetimes were characterized using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and photoluminescence spectroscopy, respectively. XRD indicates that the Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors are synthesized successfully via molten salt method. SEM image demonstrates that the obtained phosphors have hexagonal polyhedron morphology. The photoluminescence spectra reveal that the as-prepared phosphors exhibit a bright red emission under the excitation of blue or near ultraviolet light. The concentration quenching was also investigated, and the dipole–dipole interaction is responsible for the concentration quenching of fluorescence emission of Eu³⁺ ions in Ba₅(VO₄)₃Cl phosphor. The present work suggests that the Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors would be a potential candidate for light emitting devices.     

Tunable luminescence and energy transfer properties in YVO<Subscript>4</Subscript>:Bi<Superscript>3+</Superscript>,Ln<Superscript>3+</Superscript> (Ln = Dy,Sm, Eu) phosphors prepared by microwave sintering method

YVO₄:Bi³⁺,Ln³⁺ (Ln?=?Dy, Sm, Eu) phosphors were successful synthesized by microwave sintering method, and characterized by X-ray powder diffraction, scanning electron microscope, photoluminescence spectra, lifetime, quantum efficiency and general structure analysis system structure refinement. Refinement results indicated that the introduced ions occupy the sites of Y³⁺. Under 275 nm excitation, the luminescent intensity of YVO₄:Bi³⁺ samples reach the maximum when Bi³⁺ concentration is 0.02, the broad excitation spectrum of YVO₄:Bi³⁺ has a strongest peak at near 343 nm. Doped Bi³⁺ can effectively improve the emission intensity of YVO₄:Ln³⁺. The energy transfer mechanism of Bi³⁺?→?Ln³⁺ was dipole-quadrupole mechanism of electric multipole interaction. The critical distance (R_c) between Ln³⁺ and Bi³⁺ were calculated by concentration quenching method. Emitting color of YVO₄:Bi³⁺,Ln³⁺ phosphors were tunable by adjusting Ln³⁺ content. In a word, the material has a good application prospects on light emitting diodes.     

Investigation on photoluminescence properties and defect chemistry of GdAlO3:Dy3+ Ba2+ phosphors

GdAlO₃:Dy³⁺ Ba²⁺ phosphors are synthesized by citrate-based sol-gel method. Photoluminescence and positron annihilation studies are used to investigate the emission and defect chemistry of the phosphors respectively. The strong yellow (Dy³⁺) emission properties of phosphors are discussed for various concentrations of Dy³⁺ ions. Upon the addition of Ba²⁺ ion, an enhancement in emission intensity is observed due to the lattice distortions around Dy³⁺ ion. The positron studies indicate the presence of defects at crystallite boundaries, vacancy clusters and large voids in the materials. The influence of Ba²⁺ ion on the photoluminescence and lattice distortion around Dy³⁺ is also explored.     

Precipitation based synthesis and luminescence of Ln (Eu,Ce, Dy,Sm, Tb) activated BaCa2Si3O9-Walstromite cyclosilicate phosphors

A series of new phosphors of BaCa₂Si₃O₉:Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) were synthesized by precipitation based method. Good crystallinity was achieved after annealing the sample at 750 °C for 1 h in air. X-ray powder diffraction (XRD) result confirmed the formation of desired BaCa₂Si₃O₉ host. The photoluminescent excitation and emission properties of Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) activated BaCa₂Si₃O₉ were investigated in detail. The photoluminescence (PL) analysis of individual Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) activated BaCa₂Si₃O₉ phosphors exhibits interesting characteristic emission properties in their respective regions. From the measured emission spectra, Judd–Ofelt (J–O) intensity parameters (Ω₂, Ω₄, Ω₆) have been calculated and using these J–O parameters various radiative parameters such as radiative transition probabilities (A_rad), radiative lifetimes (τ_R), branching ratios (β_R) and relative quantum yield have been calculated for the studied ions.     

Luminescence properties of monodispersed spherical BaWO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> microphosphors for white light-emitting diodes

Monodispersed spheres (1–4 μm in diameter) of BaWO₄:Eu³⁺ (hereafter BWO:Eu) red-phosphor exhibiting intense emission at 615 nm were synthesized via a mild hydrothermal method. X-ray diffraction, scanning electron microscope, photoluminescence excitation and emission spectra, and decay curve were used to characterize the properties of BWO:Eu phosphors. An intense red emission was obtained by exciting either into the ⁵L₆ state with 394 nm or the ⁵D₂ state with 465 nm, that correspond to two popular emission lines from near-UV and blue LED chips, respectively. The values of Ω _2,4 experimental intensity parameters (13.8 × 10⁻²⁰ and 8.2 × 10⁻²⁰ cm²) are determined. The high-emission quantum efficiency of the BWO:Eu phosphor suggests this material could be promising red phosphors for generating white light in phosphor-converted white light-emitting diodes.     

Synthesis and optical properties of KCaPO4:Eu phosphor

A series of Eu²⁺ doped KCaPO₄ phosphors were prepared by high temperature solid state reaction and an efficient blue-green emission was observed. The photoluminescence (PL) spectrum of the phosphor appeared one asymmetric peak under near-ultraviolet (n-UV) excitation and two emission bands at 480 nm and 540 nm were obtained using Gaussian fit, which was because Eu²⁺ ions inhabited two different Ca²⁺ sites: Eu(I) and Eu(II) in the host lattice, respectively. The excitation spectrum was a broadband extending from 250 to 450 nm, which matched well with the emission of ultraviolet light-emitting diodes (UV LEDs). The effect of Eu²⁺ concentration on the emission intensity of KCaPO₄:Eu²⁺ phosphor was investigated in detail.