Anant Achyut Setlur

Eu2+–Mn2+ phosphor saturation in 5mm light emitting diode lamps

This letter reports on phosphor quenching by saturation, a sublinear phosphor response with excitation intensity, in light emitting diode (LED) packages using 405nm LEDs and Ca5(PO4)3Cl:Eu2+,Mn2+ phosphors. This saturation is due to the high light flux incident on the phosphors in these LED packages and the slow radiative relaxation rate of Mn2+. Apart from known saturation processes of Mn2+ ground state depletion and energy transfer between excited Mn2+ ions, an additional quenching pathway, Eu2+→Mn2+ (excited) energy transfer, is taken into account to quantitatively fit both the efficiency under pulsed operation and the time-resolved luminescence of Ca5(PO4)3Cl:Eu2+,Mn2+.

White light with UV LEDs

The advantages of near UV LED chips with phosphors for white light generation are discussed. Recently developed UV LED excitable phosphor blends are presented. Monte Carlo simulations suggest low color point variation (entirely within the first MacAdam oval) for the standard LED chip bin (400-410 nm), compared to high color point variation (outside the fourth MacAdam oval) for the standard bin (460-470 nm) and typical phosphors, modeled as Gaussians of realistic spectral width and targeting the 3000K ANSI color point (x=0.440, y=0.403). A discussion of the full LED package performance is also offered.

Chip to System Levels Thermal Needs and Alternative Thermal Technologies for High Brightness LEDS

Light emitting diodes (LEDs) historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors has allowed them to penetrate specialty and general illumination applications. The increased electrical currents used to drive the LEDs have resulted in higher heat fluxes than those for average silicon integrated circuits (i.e., ICs). This has created a need to focus more attention on the thermal management engineering of LED power packages. The output of a typical commercial high brightness, 1 mm 2 , LED has exceeded 100 1m at drive levels approaching 3 W. This corresponds to a heat fiux of up to 300 W/cm 2 . Novel thermal solutions need to address system architectures, packaging, phosphors for light color conversion, and encapsuianfs and fillers for optical extraction. In this paper, the effect of thermal management on packaging architectures, phosphors, encapsulants, and system design tire discussed. Additionally, discussions of microscopic defects due to packaging problems as well as chip active layer defects are presented through experimental and computational findings.

Charge creation, trapping, and long phosphorescence in Sr2MgSi2O7:Eu2+, RE3+

This report describes experiments that elucidate the phosphorescence mechanism in Sr2MgSi2O7:Eu2+, RE3+ phosphors. The first step of phosphorescence, the storage of carriers in traps, is traced to thermally assisted photoionization of Eu2+ ions, supported by thermal quenching studies that give the thermal ionization threshold of Eu2+. The trapping of these delocalized electrons has been detected using the spectroscopic signature of Sm2+ ions created by electron capture by Sm3+ in samples doped with Eu2+ and Sm3+ ions. The final steps for phosphorescence, liberation of trapped carriers and recombination with the luminescence center, have also been studied by monitoring the Eu2+ luminescence induced by optical excitation into the Sm2+ 4f6→4f55d1 absorption band. These results show that the phosphorescence in this system starts with the promotion of electrons into the conduction band and subsequent reduction of RE3+ ions, not from hole formation and RE3+ oxidation, as has been reported for similar systems. F...

Nature of luminescent centers in cerium-activated materials with the CaFe2O4 structure

Abstract The cerium luminescence in SrY 2 O 4 shows two bands emitting in the blue and green regions of the optical spectrum. Based on standard emission and excitation spectroscopy combined with time-resolved emission studies, we can attribute the blue emission to a Ce 3+ 5d–4f transition and tentatively assign the green emission to a “Ce 4+ –O 2− ”charge transfer transition.

Solid Solution Formation and Ce3 + Luminescence in Silicate Garnets

In this article, we discuss the crystal chemistry and luminescence properties of Ce 3+ activated silicate garnets. Our results indicate that the two previously reported end members of this family, Lu 2 CaMg 2 Si 3 O 12 and Ca 3 Sc 2 Si 3 O 12 , form a complete solid solution, and Sc 3+ primarily substitutes for Mg 2+ in the octahedral sites with charge compensation by Ca 2+ substitution in the dodecahedral sites. Within this solid solution, there is a continuous shift in the lowest energy Ce 3+ 4f → 5d excitation band and the Ce 3+ 5d → 4f emission band toward higher energy as the composition is varied between Lu 2 CaMg 2 Si 3 O 12 and Ca 3 Sc 2 Si 3 O 12 .

Ce3+-based phosphors for blue LED excitation

The luminescence of Ce3+ in host lattices based on Y3Al5O12 garnets that can be used in blue LED based solid state lighting sources is discussed. Specifically, the effect of Ga3+ and Tb3+ substitution for Al3+ and Y3+, respectively, on the emission color and thermal quenching of these phosphors is described with the initial implications for white light devices.

High CRI phosphor blends for near-UV LED lamps

Currently, the highest color rendering index (CRI) value obtained in commercially available LED devices is around 90. This falls short of the CRI values typical for incandescent lamps (defined at 100). Similarly, the commercially available LEDs for higher color temperature have CRI values of 65-85, well below the theoretical maximum of 100. New phosphor blends are proposed for use with LED chips emitting in the 350-450 nm range. The application of such blends can afford CRI values greater than 95, over the entire range of color temperatures of interest for general illumination (2500K - 8000K). In some cases, the CRI values approach the theoretical maximum of 100. LED based lamps with a steady state performance of 23 LPW and 25 lumens per chip at 3000K, with a general CRI (Ra) of 97 and a mean CRI (R1-R14) of 96 are demonstrated.

Synthesis and Luminescence Properties of Green Oxynitride Phosphor

Recently, nitride and oxy nitride based phosphors are of significant interest to use in white light emitting diodes. They exhibit good thermal and chemical stability with high quantum efficiencies. In particular, silicon based nitrides such as M2Si5N8:Eu (M = Ca,Sr,Ba) (1), MSi2O2N2: Eu (M= Ca, Sr, Ba) (2), CaAlSiN3 (3), Sialon:Eu (4) have been studied extensively as phosphors. In this article, we discuss synthesis and luminescence properties of Sr doped barium silicon oxynitride phosphor (5,6). The Eu activated material shows efficient green emission peak around ~525 nm under 405/450nm excitation. Our results indicate continuous redshift up to 30nm in Eu luminescence by ‘Sr’ doping (Figure 1). The effect of dopant concentration on the crystal structure, luminescence properties and thermal quenching of these materials will also be discussed.

Optimized Phosphors for Warm White LED Light Engines

The objective of this program is to develop phosphor systems and LED light engines that have steady-state LED efficacies (using LEDs with a 60% wall-plug efficiency) of 105–120 lm/W with correlated color temperatures (CCT) ~3000 K, color rendering indices (CRI) >85, <0.003 distance from the blackbody curve (dbb), and <2% loss in phosphor efficiency under high temperature, high humidity conditions. In order to reach these goals, this involves the composition and processing optimization of phosphors previously developed by GE in combination with light engine package modification.