Andreas Tuecks

Materials design and properties of nitride phosphors for LEDs

We have studied structure-property relations of Eu(II) doped nitridosilicates M2Si5N8 and MSi7N10 (M = Sr, Ba). For both systems that are described as being efficient LED phosphors, we show that a detailed examination of the local activator environment in combination with net positive charge calculations for Eu with the EHTB-MO method allows a qualitative prediction of the luminescence properties of nitridosilicate LED phosphors. The non-linear shift of the amber to red emission of solid solutions Ba2-xSrxSi5N8:Eu is explained by a non-statistical distribution of Eu over the available lattice sites. The Stokes shift differences between the two available Eu sites are significantly larger for Ba2Si5N8:Eu than for Sr2Si5N8:Eu. In contradiction to literature data, BaSi7N10:Eu shows emission in the cyan spectral region and a large Stokes shift, in accordance with the host lattice geometry and the electronic structure calculations that were carried out. SiAlON formation as an additional design tool to tune nitridosilicate phosphor emission properties is demonstrated for Sr2Si5-xAlxOxN8-x:Eu. (Al,O) incorporation leads to anisotropic changes of lattice constants and a red shift and broadening of the Eu emission.

Layered oxonitrido silicate (SiON) phosphors for high power LEDs

In our contribution we discuss structure-luminescence property relations of MSi2O2N2:Eu (M = Ba, Sr, Ca) phosphors to explain the differences in excitability, emission band position and width. The differences in Eu2+ site coordination, number and size of sites lead to a shift of emission from M = Ba over M = Sr to M = Ca from cyan to yellow accompanied by an increased Stokes shift. Because of its favourable emission properties with a peak at ~ 538 nm SrSi2O2N2:Eu was selected and optimized as down-conversion material for green pcLEDs. pcLEDs built with LUXEONR thin-film flip chip (TFFC) LEDs show stable color points under a wide range of drive conditions (I ≤ 1A, T ≤ 150°C) as a consequence of the very high conversion efficiency of optimized SrSi2O2N2:Eu color converters. Although cutbacks in color purity have to be made because of the broad band phosphor emission spectrum, efficacies of the discussed green pcLEDs are significantly higher compared to direct green emitting InGaN LEDs.