Pc-white LEDs Research Articles

A new Milarite type KMLS:Eu3+ orange-red-emitting phosphor for pc-white LEDs and Forensic applications

In this article, we present the synthesis and characterization of Eu3+-activated KMLS:Eu3+ phosphor for pc-WLEDs, latent fingerprint detection, and security ink applications. The structural phase, luminescent property, and colour purity of the as-prepared phosphor were investigated. XRD profiles and Rietveld refinement analysis confirm the hexagonal crystal system of the as-prepared phosphor. Concentration quenching effect was not observed up to 17 mol% of Eu3+. The site occupancy preferences of Eu3+ were investigated using the bond-valence model. The CIE-coordinate of optimal Eu3+ is estimated to be close to the standard NTSC value, and color purity is found to be above 90% for all the Eu3+ concentrations. Both Judd-Ofelt (J-O) analysis and asymmetric ratios revealed that the local environment around Eu3+ is asymmetric in nature. The radiative properties, including the stimulated emission cross section and optical gain parameters, of the highest concentration of Eu3+ (17mol%) were determined. Further, the results of thermal quenching analysis show that 91% of the luminescence intensity can be maintained at 150 °C, demonstrating good thermal stability. The multilevel ridge characteristics of the latent fingerprint stained with as-prepared phosphor could be clearly visualized under 254 nm UV excitation. All of the above results indicate that as-prepared phosphor materials can be suitable for use in forensic applications as well as solid state lighting applications.

Photoluminescence properties of wet-chemically synthesized Tb3+ and Sm3+doped K3Ca2(SO4)3Cl phosphor

In this study, we are successfully synthesized terbium and samarium (Tb3+/Sm3+) doped K3Ca2(SO4)3Cl phosphors by wet chemical method, and synthesized phosphors were characterized by XRD, SEM, and Photoluminescence (PL) techniques, and studied in detail. Photoluminescence analysis, we can see that under the Near-UV (NUV) excitation of 380nm, the Tb3+ activated samples showed a prominent green emission peak at 546nm attributed to the 5D4→7F5 transition of Tb3+ions. While the PL emission spectra of 404 nm light excited K3Ca2(SO4)3Cl:Sm3+phosphorshowedthree emission lines at 565nm, 598nm and 645nm attributed to 4G5/2→6H5/2, 4G5/2→6H7/2, and4G5/2→6H9/2 transitions of Sm3+ ions, respectively. The CIE coordinates were calculated to be (0.263, 0.515) for K3Ca2(SO4)3Cl:Tb3+and (0.532, 0.462) for K3Ca2(SO4)3Cl: 1.5mol%Sm3+, which liedin the green and orange regions, respectively. The entire photoluminescence results indicated that phosphor K3Ca2(SO4)3Cl doped with Tb3+ and Sm 3+may turn out to be an important component of pc-white LEDs and display devices.

Dynamic Optical Power Measurements and Modeling of Light-Emitting Diodes Based on a Photodetector System and Photo-Electro-Thermal Theory

Optical measurements based on integrating sphere are primary suitable for steady-state measurements and unsuitable for dynamic real-time measurements. This paper presents an alternative method using the combination of a photodetector system and the dynamic photo-electro-thermal theory for real-time optical measurements. It is demonstrated that the voltage output of a photodetector can be correlated to the optical power in real time. Such method has been successfully applied to analyze a phosphor-converted white light-emitting diode (PC white LED) using an extended dynamic model with separate calculations of optical power from the blue LED chips and phosphor layer. Both calculated and practical results are included to confirm the validity of the new method.

The sensitized luminescence and tunable color of single-component Sr2MgSi2O7:Bi3+/Sm3+/Tb3+ phosphor via energy transfer for white-light emitting diodes

A series of single-phase blue-to-red tunable-color emitting and excellent green-emission Sr2MgSi2O7:Bi3+, Sm3+/Tb3+ phosphors have been synthesized by high temperature solid state reaction. The crystal structure of Sr2MgSi2O7 and the crystallographic sites of Bi3+, Sm3+ and Tb3+ ions in the host have been identified through the crystallographic data from Rietveld refinements. The luminescent properties of Sr2MgSi2O7:Bi3+/Sm3+/Tb3+ phosphors were investigated and the Bi3+, Sm3+ or Tb3+ single doped samples exhibit blue, orange-red and green emissions, originating from the 3P1→1S0 transition of Bi3+, the 4G5/2 to 6HJ (J = 5/2, 7/2, 9/2) transitions of Sm3+ and the 5D4→7FJ (J = 6, 5, 4, 3) multiplet transitions of Tb3+, respectively. Meanwhile, the energy migration from Bi3+ to Sm3+ and Tb3+ in co-doping samples was confirmed by emission/excitation spectra and luminescent decay behaviors. The mechanism of energy transfer from Bi3+ to Sm3+ has been demonstrated to be the resonant type via a dipole–dipole interaction and the critical distances between Bi3+ and Sm3+ were calculated to be 22.28 Å. Additionally, the temperature-dependent photoluminescence for as-prepared phosphors have been investigated in detail. The luminescence intensity of Sm3+ and Tb3+ ions can be sensitized to enhance by Bi3+ ion. The blue and orange-red colors can be tuned by adjusting the concentration ratio of Bi3+/Sm3+ base on the energy transfer from Bi3+ to Sm3+ ions, and the phosphors Sr2MgSi2O7:Bi3+, Tb3+ develop excellent green-emission properties, which indicate that the Sr2MgSi2O7:Bi3+, Sm3+/Tb3+ phosphors with tunable multicolor and green emissions may be of potential application in pc-white LEDs.

Structural and photoluminescent elucidation of the efficient green emitting erbium doped BaY2ZnO5 nanophosphor for light emitting materials

Erbium doped BaY2ZnO5 nanophosphor was synthesized by solution combustion approach using urea as an organic fuel. The spherical shaped phosphors in the nano dimension (40–70 nm) were characterized by X-ray diffraction, transmission electron microscopy and photoluminescence spectroscopy. XRD studies reveal that in BaY1.92Er0.08ZnO5 nano phosphor substitution of Y3+ ion by Er3+ ion does not lead to any major structural changes in the host lattice and confirms single phased nanophosphor crystallized in orthorhombic lattice with Pbnm (62) space group. The excitation spectrum depicts that this nanophosphor can be effectively excited by NUV light, resulting characteristic emissions from 2H9/2, 2H11/2, 4S3/2 to 4I15/2 transition level with predominant green emission at 549 nm. Concentration dependent luminescence tendency reveals optimized concentration for maximum luminescence as 4 mol%. A larger value of critical distance (14.38 A) is found responsible for non-radiative energy transfer occurring via dipole–quadrupole (d–q) interactions leading to concentration quenching. Luminescence decay kinetics and color coordinates (0.283, 0.616) has also been evaluated. The findings of the study support the application of this nanophosphor in the field of NUV excited tricolor based PC-white LED systems.

Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI>80, CCT~3000K and 5000K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.

Studies on the photoluminescent properties of a single phase white light emitting phosphor CaLa1−xNbMoO8: x Dy3+ for pc-white LED applications

Dy3+ doped CaLa1−xNbMoO8 (x=0.02, 0.025, 0.03, 0.035, 0.05, 0.075) phosphors were synthesized using a high temperature solid state method. Powder XRD results confirm the tetragonal phase and detailed investigations on the photoluminescence properties indicated that Dy3+ doped CaLaNbMoO8 phosphors has a strong absorption in near UV region and two sharp emission peaks in the visible region. The yellow (575nm) and blue emissions (487nm) upon UV excitation resulted from the forced electric dipole transition of 4F9/2→6H13/2 and the magnetic dipole transition of 4F9/2→6H15/2 respectively. The emission colors of all the doped samples are located in the white light region. Therefore, Dy3+ doped CaLaNbMoO8 can serve as a promising candidate for single-phase white-light emitting phosphor under near UV excitation.

Tunable color and energy transfer in single-phase white-emitting Ca20Al26Mg3Si3O68:Ce3+,Dy3+ phosphors for UV white light-emitting diodes

Ce3+ and/or Dy3+ activated Ca20Al26Mg3Si3O68 phosphors were synthesized by high temperature solid state reaction and their luminescent properties were studied. There are two emissions peaking at 407 and 577nm in the emission spectra of Ca20Al26Mg3Si3O68:Ce3+, Dy3+, which are due to the transitions of Ce3+ and Dy3+ ions, respectively. More importantly, the effective energy transfer from Ce3+ to Dy3+ ions has been confirmed and investigated by emission/excitation spectra and luminescent decay behaviors. Furthermore, the energy level scheme and mechanism of energy transfer were investigated and it was demonstrated to be resonant type via dipole–dipole reaction. Under the excitation of 345nm, the emitting color can change from blue to white by adjusting the relative doping concentration of Ce3+ and Dy3+ ions, indicating that the phosphors Ca20Al26Mg3Si3O68:Ce3+, Dy3+ are promising single-phase white-emitting phosphors for application in pc-white LEDs.

Synthesis And Photoluminescence Property Of RE3+ Activated Na2CaP2O7 Phosphor

As host materials, phosphate compounds offer great potential for lanthanides to display luminescence characteristics. In this work luminescence behaviour of Na2CaP < sub>2O7:RE 3+ are presented for the first time. Na2CaP < sub>2O7 novel phosphors activated with the trivalent rare earth ions ((RE = Ce, Eu, Tb, Sm) were synthesized by solid state diffusion method. Phosphors were characterized for phase purity and luminescent properties. The emission and excitation spectra were followed to explore the luminescence attributes. The as prepared powders of Ce 3+ , Eu 3+ ,Tb 3+ and Sm 3+ doped Na2CaP < sub>2O7 emit near-uv, red, green and orange reddish light as result of f-d and f-f transitions respectively. The study is novel as no such luminescence data are available for this compound. The results are promising in view of the requirement for pc-white LEDs for solid state lighting applications.

Prognostics of Chromaticity State for Phosphor-Converted White Light Emitting Diodes Using an Unscented Kalman Filter Approach

Phosphor-converted white light-emitting diodes (pc-white LEDs) utilize a blue LED chip converted by the phosphor to obtain white light emission. Pc-white LEDs have become one of the most popular white LEDs. The reliability concerns of pc-white LEDs involve both lumen maintenance and chromaticity state. However, previous research on the health of LEDs has not taken chromaticity state shift into consideration. Therefore, this paper investigates the chromaticity state shift of pc-white LEDs during an aging test using a data-driven prognostic approach. The chromaticity coordinates (u', v') in the CIE 1976 color space are used to define the states of chromaticity. The Euclidean distance measures between two different chromaticity states represent the chromaticity state shift of LED after aging. A nonlinear dual-exponential model is selected to describe the chromaticity state shift process. Usually, the LED industry used an extrapolating approach to project future states of LED lighting sources, which relies on the nonlinear least square method to fit the obtained data and extrapolates the fitting curve to get the future state. In this paper, a recursive nonlinear filter (an unscented Kalman filter) is used to track the future chromaticity state. The result shows that the unscented Kalman filter approach can improve the prognostic accuracy more compared with the conventional extrapolating approach.

Luminescence properties and energy transfer of Ca2Mg0.5AlSi1.5O7:Ce3 +, Eu2 + phosphors for UV-excited white LEDs

Ce3+ and/or Eu2+ activated Ca2Mg0.5AlSi1.5O7 phosphors have been synthesized by high temperature solid state reaction. The luminescence of Ca2Mg0.5AlSi1.5O7:Ce3+, Eu2+ is studied as potential ultraviolet light-emitting diodes (UV-LEDs). There are two emissions peaked at 409 and 525nm which come from d–f transitions of Ce3+ and Eu2+ ions. The energy transfer from the Ce3+ to Eu2+ has been discussed and it is demonstrated to be a resonant type via a dipole–dipole mechanism. The critical distance of Ce3+ to Eu2+ in Ca2Mg0.5AlSi1.5O7 has been calculated to be 26.71Å by spectral overlap method. Furthermore, the Ca2Mg0.5AlSi1.5O7:Ce3+, Eu2+ phosphors can emit a tunable blue-to-green light under the excitation of 350nm by appropriately tuning the relative proportion of Ce3+/Eu2+, indicating that the phosphors Ca2Mg0.5AlSi1.5O7:Ce3+, Eu2+ are promising blue-to-green emitting phosphors for application in pc-white LEDs.

Photoluminescent properties of tunable green-emitting oxynitride (Ba3−xSrx)Si6O12N2:Eu2+ phosphor and its application in white LEDs

Green emitting Eu2+-doped (Ba3−xSrx)Si6O12N2 solid solutions were synthesized through solid state reaction at 1350 °C for 10 h under a N2/H2 atmosphere. The XRD patterns revealed that the solid solution series of (Ba3−x−ySrx)Si6O12N2:yEu2+ with x value ranging from 0–0.6 were established. An efficient and intense tunable green light was observed by varying the cation Sr/Ba ratio. The emission spectra exhibited an entire shift towards long wavelength with increasing of x value, which was caused by large crystal field splitting and Stokes shift. The x value dependence of emission intensity was discovered and explained by the enhanced probability of electron from excited 4f state to 5d ground state via nonradioactive transition. Highly thermal stability and feasible color coordinates were verified. White LEDs with excellent photochromic properties were fabricated by packing GaN based blue chips and (Ba,Sr)3Si6O12N2:Eu2+ phosphors. All results indicated that the (Ba3−xSrx)Si6O12N2:Eu2+ phosphors were confirmed to be a promising candidate for pc-white LEDs in solid state lighting.

Blue Emitting Ca8La2(PO4)6O2:Ce3+/Eu2+ Phosphors with High Color Purity and Brightness for White LED: Soft-Chemical Synthesis, Luminescence, and Energy Transfer Properties

Ce3+ and/or Eu2+ activated Ca8La2(PO4)6O2 (CLPA) oxyapatite blue phosphors have been prepared via a Pechini-type sol–gel process. X-ray diffraction (XRD), photoluminescence (PL) spectra, absolute quantum yield, as well as lifetimes were utilized to characterize samples. The emission of Ce3+ and Eu2+ ions at different lattice sites has been identified and discussed. The CLPA:0.04Ce3+ phosphor exhibits bright blue emission with higher quantum yield (67%) and excellent CIE coordinates (x = 0.160, y = 0.115) under UV excitation, and the CLPA:0.05Eu2+ phosphor also exhibits blue emission with CIE coordinates (0.187, 0.164). The energy transfer from Ce3+ to Eu2+ in CLPA:Ce3+/Eu2+ phosphors has been validated and demonstrated to be a resonant type via a dipole–dipole mechanism. The critical distance (Rc) of Ce3+ to Eu2+ ions in CLPA was calculated (by the spectral overlap method) to be 26.67 Å. The quantum yields of Ce3+ and Eu2+ coactivated CLPA phosphors are enhanced compared with that of Eu2+ activated samples due to energy transfer. The CIE coordinates of CLPA:0.04Ce3+, 0.02Eu2+ are (0.179, 0.169). The corresponding luminescence and energy transfer mechanisms have been proposed in detail. These blue phosphors might be promising for use in pc-white LEDs.

Photoluminescence Properties and Energy Transfer in Eu2+-Yb2+ Codoped SrSi2O2N2 Oxynitride Phosphor

Eu2+-Yb2+ codoped SrSi2O2N2 phosphors were prepared and their photoluminescence properties were investigated in comparison with the single Eu2+-doped and the single Yb2+-doped samples. Their photoluminescence properties were investigated as the factors of Yb2+ ion concentration, excitation wavelength, and temperature. Compared to the single Yb2+-doped sample, the Eu2+-Yb2+ codoped sample showed improved emission intensity, fluorescence lifetime, quantum efficiency, and thermal stability. An efficient energy transfer process from Eu2+ ions to Yb2+ ions was proposed to be responsible for the enhancement. The highest energy transfer efficiency was estimated to be 91.9%. A broadband yellow emission with full width at half maximum of ∼170 nm was also observed in the Eu2+-Yb2+ codoped sample, which could be assigned to the co-excitation of Eu2+ and Yb2+ ions. In view of too low quantum efficiency and the considerable thermal quenching of the emission at the operation temperature of high-power LED (>1 W/mm2), the SrSi2O2N2:Eu2+/Yb2+ phosphor might be only potential for low-power high Ra pc-white LED.

Effect of Phosphor Particle Size on Luminous Efficacy of Phosphor-Converted White LED

In this paper, the influence of YAG:Ce phosphor particle sizes on the lumen output and the conversion efficiency of both in-cup phosphor and top remote phosphor LED packages are investigated with 3-D ray-tracing simulations. The lumen output and the conversion efficiency of both types of phosphor-converted (pc) white LED packages are dependent on the size of YAG:Ce particles. The lumen output and conversion efficiency of both types of pc-white LED packages are minimal at the phosphor particle size with the size parameter of around one and are highest at the particle size in micron size. The simulation results show that both in-cup and top remote phosphor packages have the highest lumen output and the highest conversion efficiency at the particle size of around 20 mum.

A novel blue-emitting silica-coated KBaPO 4:Eu 2+ phosphor under vacuum ultraviolet and ultraviolet excitation

An intense blue-emitting phosphor KBaPO 4:Eu 2+ was synthesized by a solid-state reaction method. The crystal structure of KBaPO 4:Eu 2+ was refined using neutron diffraction data by Rietveld refinement for the first time. KBa 0.99Eu 0.01PO 4 composition shows intense blue emission (peak wavelength, 420 nm) with relative intensity vs commercial BaMgAl 10O 17:Eu 2+ blue phosphor (Nichia) 65%, 122% and 108% under 147, 254 and 365 nm excitation, respectively. In addition, by applying a nano-sized silica coating on the phosphor surface, excellent moisture resistance of KBP:Eu 2+ was obtained. Due to its intense blue emission and excellent color purity (0.1606, 0.0271), it is very potent as a blue-emitting phosphor for PDPs, CCFLs, and pc-white LEDs based on near UV LED.