Anthony Parmentier

Selecting Conversion Phosphors for White Light-Emitting Diodes

Light emitting diodes (LEDs) are on the verge of a breakthrough in general lighting, due to their rapidly improving efficiency. Currently, white LEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. This Review first describes how to quantify the quality of a light source, discussing the color rendering index (CRI) and alternative color quality indices. Then, six main criteria are identified and discussed, which should be fulfilled by a phosphor candidate to be considered for actual application in LEDs. These criteria deal with the shape and position of the emission and the excitation spectra, the thermal quenching behavior, the quantum efficiency, the chemical and thermal stability and finally with the occurrence of saturation effects. Based on these criteria, the most common dopant ions (broad-band emitting Eu 2+ , Ce 3+ and Mn 2+ , line-emitting rare earth ions,...) and host compounds (garnets, sulfides, (oxy)nitrides,...) are evaluated. Although many phosphor materials have been proposed in literature in recent years, the number of phosphors effectively fulfilling all six requirements is relatively small.

Structure and luminescence of (Ca,Sr)2SiS4?:?Eu2+ phosphors

Sr2SiS4 : Eu2+, Ca2SiS4 : Eu2+ and the solid solution of both, europium-doped (Ca,Sr)2SiS4, were investigated as UV–VIS excitable green to red powder phosphors. Sr2SiS4 : Eu2+ shows two emission bands, peaking at 480 and 550 nm. By changing the ratio between Ca2+ and Sr2+, the photoluminescent emission spectrum can be tuned. Using x-ray diffraction, the phase composition and lattice parameters of the thiosilicate compounds were determined. The material forms a single, monoclinic Sr2SiS4-like phase up to 40% substitution of Sr2+ by Ca2+. From 50% to 90% of substitution by Ca2+, phase separation was observed, leading to more complex emission spectra. These spectra were studied in detail using photoluminescence spectroscopy, cathodoluminescence microscopy and temperature dependent optical measurements. The thermal quenching temperature decreases from 470 K in Ca2SiS4 : Eu2+ to 380 K in Sr2SiS4 : Eu2+ upon increasing substitution of Ca2+ by Sr2+. The possibilities of these materials as wavelength converters for white light emitting diodes were evaluated.

A XAS study of the luminescent Eu centers in thiosilicate phosphors

Due to its bright yellow-to-red emission, europium doped Ca2SiS4 is a very interesting material for phosphor converted light emitting diodes. The emission spectrum is highly dependent on the Eu concentration and can consist of more than one emission band. We combined X-ray absorption fine structure and photoluminescence measurements to analyze the structure of europium centers in (Ca,Eu)2SiS4 luminescent powders. This paper provides an explanation for the concentration dependency of the emission spectra. We find that at low dopant concentrations a large fraction of trivalent europium ions is unexpectedly present in the powders. These trivalent europium ions tend to form defect clusters in the luminescent powders. Furthermore we observe a preferential substitution of the europium ions over the two different substitutional Ca sites, which changes upon increasing the dopant concentration. At high dopant concentration, the powder crystallizes in the monoclinic Eu2SiS4 structure. Once more a preferential substitution of the europium ions is observed. Summarizing, the influence of the concentration on the emission spectrum is explained by a difference in preferential occupation of the Eu ions in the lattice.

Broadband Luminescence in Rare Earth Doped Sr2SiS4: Relating Energy Levels of Ce3+ and Eu2+

Sr2SiS4:Ce3+ is an efficient blue-emitting (460 nm) phosphor, excitable with light of wavelengths up to 420 nm. From the excitation spectrum, we construct the energy level scheme and use it to check the predictive power of the Dorenbos model, relating the positions of the Ce3+ energy levels with those of Eu2+ in the same host. For strontium thiosilicate, this method gives excellent results and allows us to determine which of two available crystallographic sites is occupied by cerium. We use the Dorenbos method for extracting information on the coordination of Ce3+ from the observed crystal field splitting.