Fully phosphor-converted LEDs (FpcLeds) with saturated emission have been realized in the green and amber spectral
region. With the Lumiramic phosphor technology it is possible to achieve high package efficiency with minimum
transmission of blue light from the primary LED source. This is done by keeping the scattering properties of the
phosphor layer low while the phosphor thickness is chosen to fully convert all blue LED emission. It is shown that this
can be done not only for optically isotropic Lumiramic materials like garnets, but also for oxonitridosilicate materials
like the green emitting Europium doped SrSi2O2N2, crystallizing in a triclinic lattice with three optical axes. The
scattering power of the Lumiramic can be decreased to acceptable levels by increasing the size of the crystallites in the
densely sintered ceramics. Light propagation is found to be described well with Mie scattering of mono-sized SrSi2O2N2
spheres with refraction index differing by 0.07 to the refractive index of a SrSi2O2N2 matrix material. Using this
technology, the green-yellow gap of visible light emitting LEDs can be bridged and color tunable lamps with the
efficiency and flux of today's white phosphor-converted LEDs become feasible.
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.
A new phosphor technology for phosphor converted light-emitting diodes (pcLEDs) is presented. A polycrystalline
ceramic plate (LumiramicTM) of Ce (III) doped yttrium gadolinium aluminum garnet (Y,GdAG:Ce) is combined with a
blue LED to produce white light in the range of 5000 K correlated color temperature. Scattering and light extraction
means of the Lumiramic ceramic color converter plates enable production of reliable and efficient white pcLEDs.
Measurement of the optical properties of the Lumiramic plates before the final LED assembly allows pick and place
packaging with exact targeting of the desired white color point of the LED. Combination with a red phosphor powder
layer, coated onto the Lumiramic plate, results in high quality white pcLEDs with any color temperature required for the
general lighting market.
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.
Subwavelength silica particle layers have been applied between glass and thin film luminescent layers of Alq3 and a polymer MEH-PPV layer, respectively. The layers acted as a randomised two-dimensional diffraction lattice, which increased the fraction of emitted power from thin film organic layers into air. In contrast to perfectly ordered structures strong interference emission patterns did not occur. Still, an optical feedback of the particle layer on the emission spectrum could be observed, which can be used to improve colour saturation for blue and green emission and to increase the lumen efficiency for red emission, without changing the colour point of the red emitter.
In photoluminescence experiments a gain factor of 3 and 2.5 in light outcoupling was realized for Alq3 and MEH-PPV layers, respectively. With a MEH-PPV polymer OLED device an efficiency gain of 30 to 40 % has been realized in electroluminescence.