Boston Chapter of the IEEE Photonics Society

Abstract:  LED technology is on its way to replace the conventional lighting in the future.  Whereas the LED lighting market share is around 10% in the year 2010, it is projected to increase well above 50% in the years 2016 to 2020.  Currently, 75% of that LED market is based on phosphor conversion technology where a blue InGaN LED is combined with a luminescent material to generate white light or other unsaturated colours.  In this presentation, we will discuss various materials and methods used in this technology of hybrid light sources.

Most commonly, the blue emission from the InGaN LEDs is converted to targeted light using inorganic phosphors.  A typical phosphor consists of an inorganic host activated by impurity ions for luminescence, the most common activators being Ce3+ and Eu2+ for LED application.  These rare earth ions emit in a broad band through 4f-to-5d transitions.  Due to the nephelauxetic effect and the crystal-field splitting of the excited 5d states, the emission wavelength depends on the host lattice.  The concentration of the activator ions also leads to minor shifts in the peak wavelength due to overlapping emission and absorption bands.  Although YAG:Ce3+ is the most popular yellow emitting phosphor used for converting the blue emission from the LEDs to white light, there is a wide variety of luminescent materials as well as application methods available in the industry for designing solid state lights for specific applications.

The phosphor blends in suitable resins were the first application method for designing hybrid lamps.  Conversion ceramics have recently emerged as a second option for LED light conversion.  The material basis and luminescence process of these ceramics are comparable to the above mentioned regular phosphors. Nevertheless the ceramics as inorganic solid pieces enable performance advantages over the regular phosphors in some applications.  This is especially true for highest power and high luminance LEDs.

Another class of material, the light emitting quantum dot (QD), is fast becoming an alternate choice as a potential future LED conversion material because of the chemically benign environment of the solid state lighting sources, significantly less light scattering compared to the micron sized phosphor particles and, more importantly, the availability of Cd-free QDs with comparable efficiency to Cd-based QD.  The QDs are semiconductor nanocrystals whose emission can be tuned through quantum confinement by varying the particle size.  The use of QDs can be especially beneficial for applications in projection technology, LCD backlighting and even general lighting, which require narrow band emission to achieve large color gamuts.

Biography:  Joerg Strauss graduated and received his Ph.D. in Chemistry at the University of Regensburg, Germany. In 2002 he joined OSRAM Opto Semiconductors in Regensburg Germany in the Backend Technology department. In 2006 he became head of the Backend Technology Materials department. In 2008 he joined OSRAM GmbH as head of the Phosphor Development department in Schwabmuenchen Germany. Since 2011, Joerg Strauss is Director of Research SSL at OSRAM Sylvania in Beverly, Massachusetts USA.

Location:  MIT Lincoln Laboratory