Development of Advanced LED Phosphors by Spray-based Processes for Solid State Lighting

Abstract

The overarching goal of the project was to develop luminescent materials using aerosol processes for making improved LED devices for solid state lighting. In essence this means improving white light emitting phosphor based LEDs by improvement of the phosphor and phosphor layer. The structure of these types of light sources, displayed in Figure 1, comprises of a blue or UV LED under a phosphor layer that converts the blue or UV light to a broad visible (white) light. Traditionally, this is done with a blue emitting diode combined with a blue absorbing, broadly yellow emitting phosphor such as Y{sub 3}Al{sub 5}O{sub 12}:Ce (YAG). A similar result may be achieved by combining a UV emitting diode and at least three different UV absorbing phosphors: red, green, and blue emitting. These emitted colors mix to make white light. The efficiency of these LEDs is based on the combined efficiency of the LED, phosphor, and the interaction between the two. The Cabot SSL project attempted to improve the over all efficiency of the LED light source be improving the efficiency of the phosphor and the interaction between the LED light and the phosphor. Cabot's spray based process for producing phosphor powders is able to improve the brightness of the powder itself by increasing the activator (the species that emits the light) concentration without adverse quenching effects compared to conventional synthesis. This will allow less phosphor powder to be used, and will decrease the cost of the light source; thus lowering the barrier of entry to the lighting market. Cabot's process also allows for chemical flexibility of the phosphor particles, which may result in tunable emission spectra and so light sources with improved color rendering. Another benefit of Cabot's process is the resulting spherical morphology of the particles. Less light scattering results when spherical particles are used in the phosphor layer (Figure 1) compared to when conventional, irregular shaped phosphor particles are used. This spherical morphology will result in better light extraction and so an improvement of efficiency in the overall device. Cabot is a 2.5 billion dollar company that makes specialized materials using propriety spray based technologies. It is a core competency of Cabot's to exploit the spray based technology and resulting material/morphology advantages. Once a business opportunity is clearly identified, Cabot is positioned to increase the scale of the production to meet opportunity's need. Cabot has demonstrated the capability to make spherical morphology micron-sized phosphor powders by spray based routes for PDP and CRT applications, but the value proposition is still unproven for LED applications. Cabot believes that the improvements in phosphor powders yielded by their process will result in a commercial advantage over existing technologies. Through the SSL project, Cabot has produced a number of different compositions in a spherical morphology that may be useful for solid state lights, as well as demonstrated processes that are able to produce particles from 10 nanometers to 3 micrometers. Towards the end of the project we demonstrated that our process produces YAG:Ce powder that has both higher internal quantum efficiency (0.6 compared to 0.45) and external quantum efficiency (0.85 compared to 0.6) than the commercial standard (see section 3.4.4.3). We, however, only produced these highly bright materials in research and development quantities, and were never able to produce high quantum efficiency materials in a reproducible manner at a commercial scale.