Composite Luminescent Materials and Light-Harvesting Systems

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A. Photovoltaics. Advances in photovoltaic energy systems require better use of the solar spectrum than today's silicon cells achieve. Silicon solar panels are inherently optimized for efficient harvesting of red and near-infrared wavelengths but are less efficient over other parts of the visible spectrum due to an energy loss mechanism called thermalization. In addition, longer wave infrared radiation is not harvested at all. Our approach to better harvest visible light uses fluorescence-based solar cell systems. Fluorescence based solar cells use phosphors or quantum dots to absorb light and then re-radiate the light at selected wavelengths. During the absorption and re-radiation process, the light is randomized in direction and it can be wave-guided in a collection plate and directed to the edges of the plate where small, high efficiency solar cells can achieve high conversion efficiency. To minimize re-absorption of light in the collector plate the distribution and density of phosphors or quantum dots may be optimized or spatially patterned. We are investigating the use of spatially patterned phosphors to allow concentrated sunlight to reach phosphors while maintaining an efficient waveguiding system to allow the fluorescence to reach the edges of the collector plate. For the harvesting of long wave infrared radiation we are developing GaSb (gallium antimonide) thin film solar cells. GaSb absorbs photons having energies from as low as 0.7 electron volts (eV) which allows the efficient harvesting of 0.7-1.1 eV photons. We are sputtering GaSb from bulk targets that allow the stoichiometry (composition) to be varied thereby enabling the optimization of thin film semiconductor properties. B. Luminescence. Electroluminescence in powder materials is an attractive approach to producing flexible sheets of light. Here a powder may be sandwiched between two electrodes (one of which is transparent) and upon the application of an AC voltage, luminescence is observed from the powder. Traditional electroluminescent powders are made from sulphides, however we are developing oxide powders such as zinc silicate powder. Activation of these materials requires the incorporation of a second conductive phase. Whereas light emitting diodes have proven themselves in the rapidly growing solid state lighting market to replace incandescent and fluorescent lamps, they are inherently point sources and therefore do not lend themselves to applications where thin, flexible sheets of light are needed. The goal of this work is to improve the brightness and stability of flexible light sheets for applications in lighting, signage and displays. One specific use of electroluminescence is for glowing tapes that can mark emergency exits or edges of furniture or equipment in a dark environment. A larger future market is for roll-up televisions.