Regina Mueller-Mach

High-power phosphor-converted light-emitting diodes based on III-Nitrides

Phosphor conversion of light-emitting diode light for white light sources and some monochrome applications requires particular phosphor properties and has to take into account specific issues if aimed at high-power output. Limitations and solutions will be discussed, giving special considerations to drive and temperature dependencies. Efficiencies of 32 lm/W for white with good color rendering at 4600 K and 35 lm/W for green (535 nm) have been demonstrated.

Phosphor materials and combinations for illumination-grade white pcLEDs

Blue III-nitride based power LEDs have opened the way for Light Emitting Diodes to penetrate the illumination market. Most wanted seem to be warm white, high color rendering devices. Phosphor-converted LEDs, which meet these requirements, have been recently announced and sampled; they will be put into the context of possible other solutions and for the first time discussed in some detail.

Illumination-grade white LEDs

Phosphor conversion of blue radiation from InGaN LEDs has recently gone to a new quality: white light, which is illumination grade in the sense of high color rendering and color temperatures below 4000°K. At the same time power packages taking inputs of 1 Watt or even 5 W and surpassing in efficiency small size halogen lamps have been introduced. After a short review of alternative solutions state of the art devices are described and open questions outlined.

Green phosphor-converted LED

Green phosphor-converted LEDs using a blue pump InGaN diodes have advantages over the direct green InGaN LED with regards to color stability with drive and/or temperature. Added manufacturing steps are outweighed by higher color yield, as a range of pump colors can be used without changing the final chromaticity. The conversion losses can be smaller than the decrease in wall-plug efficiency from blue towards green, which has been reported by many sources. A distinct disadvantage of the concept is due to only one color and phosphor proven - SrGa2S4:Eu2+ and 535 nm peak wavelength.

Photoluminescence and cathodoluminescence properties of green emitting SrGa2{S}4 : Eu2+ thin film

Photoluminescence and cathodoluminescence properties of SrGa2S4 : Eu2+ thin films prepared by reactive RF magnetron sputtering are investigated. Luminescence performances of the phosphor in the thin film form are compared to those of powder samples: the brightness efficiency of thin films is found to be about 30% of the efficiency of powder at low current density. A ratio higher than 40% is expected at higher current density. Thin film screens for FEDs will become a positive alternative to powder screens provided that film quality and light extraction could be improved by optimization of thickness and deposition parameters.

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.

Nitridosilicates: a new family of phosphors for color conversion of LEDs

Phosphor-converted Light Emitting Diodes (pc-LEDs) to generate white light from blue or UV emitting diodes can be made using Eu2+ doped nitridosilicates - M2Si5N8- and/or oxo nitridosilicates - MSi2O2N2 with M=alkaline earth. Luminescence properties of some out of this new class of color converters have been investigated. As expected from strong absorption the decay times of the internally excited Eu2+ is short (around 1 microsec) and depends on the cation sharing the unit cell. Time resolved spectroscopy illustrates the behavior even more clearly. Laboratory pcLEDs using 2 of the nitride phosphors show excellent drive and temperature stability of all color properties as expected.

ELECTROLUMINESCENCE FROM CAS:TMF3 FILM PREPARED BY RADIO FREQUENCY MAGNETRON SPUTTERING

Blue electroluminescence is reported from CaS:TmF3 thin films prepared by radio frequency magnetron sputtering method. The dependences of brightness on substrate temperature, concentration of TmF3, and the applied voltage were investigated on alternating current thin-film electroluminescent devices. The obtained maximum brightness was above the level of ZnS:TmF3 devices. The equivalent average energy of the excited electrons contributed to the electroluminescence in CaS:TmF3 alternating current thin-film electroluminescent devices and was estimated at around 4.23 eV by comparing the ratio of infrared to blue peak in electroluminescent spectra with that in photoluminescent spectra excited by different photon energy. The excitation processes of electroluminescence in CaS:TmF3 thin films were assumed to be energy transfers from the conduction band edge to Tm3+ centers.

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.

Phosphor-Converted High Power LEDs

Phosphor converted Light Emitting Diodes (pcLED) have undergone many changes of design over the last 11 years. For advanced applications, however, besides other binning criteria a closely binned color point appears to be a major issue. Manufacturing yield is closely related to this point. Philips Lumileds Lighting (PLL) introduced very recently a new compact solid state color converter - Lumiramic - which allows much closer color control by selecting perfect matches between blue pump LEDs and previously characterized converter tiles in a pick-and-place process. Lumiramic phosphor technology utilizes a ceramic phosphor plate and PLLs new Thin Film Flip Chip (TFFC) technology. A first application in Automotive Forward Lighting and some of the design considerations are outlined in the paper