Agenda

There is an increasing need for high power density light sources, e.g. for the next generation of car headlights, diode laser pumped white light sources and projection devices. However, saturation and droop at high excitation densities limit the light output in high power devices. Excited state absorption and long excited state lifetimes play a role, but the relation between light output and excitation power is poorly understood and is a complex interplay of a variety of quenching processes including ground state depletion, Auger quenching, reabsorption and color center formation. The development of superior materials is crucial and relies on a better understanding of droop processes and the relation with the nature and processing condition of light conversion materials.  In this presentation a variety of quenching mechanisms will be evaluated and illustrated. Experiments are reported that elucidate the physics of photosaturation in common LED phosphors. By measuring emission intensity and reflectivity of a phosphor material under a variety of (modulated) excitation schemes, we distinguish between the different possible photosaturation mechanisms. Implications of our results and possible ways to mitigate phosphor saturation will be discussed. Finally, new experimental and theoretical capabilities will be outlined that may help to acquire new insights in what limits the light output in current and future high brightness light sources.
 

Line emitters (FWHM <5 meV) have an enormous potential for use in LEDs, because they allow both improvement of color rendering while maintaining high lm/W for white LEDs. However, conventional line emitters (i.e. trivalent lanthanide doped phosphors) lack absorption in the blue spectral region, thus limiting their use as color converters on blue LEDs. This bottleneck can be overcome by our nanoscale engineering approach, based on inter particle energy transfer. Making use of inter particle energy transfer allows tuning of absorption and emission wavelengths over a wide range of the spectrum. When the initial challenge of preparing high quality nanoparticles has been overcome, the nanoscale brings in various additional advantages, such as solution processability and a gamut of material treatment possibilities borrowed from the well developed field of semiconductor quantum dots. This talk will discuss the relevant energy transfer process in more detail and provide experimental evidence for this new sensitization mechanism paving the road towards blue excitable line emitters.