Research Project Summary Information
Bulk Downconverters for High E(ST10975-1)
United Semiconductors, LLC
Phosphor is one of the key elements in a phosphor-converted (PC) white light-emitting diode (LED). For white LEDs to reach 200 lm/Watt by 2020 (the U.S. Department of Energy's goal), improved phosphor (downconversion) materials are needed. United Semiconductors, LLC (USLLC), a small, woman-owned business with research and manufacturing facilities in Rensselaer, New York, is teaming with Rensselaer Polytechnic’s Lighting Research Center (LRC) to develop a YAG:Ce-based bulk downconverter (BDC) that could result in a new white LED that is 30% more efficacious than the state-of-the-art white LED on the market. Several factors affect the efficiency of traditional powered phosphors: the downconversion loss; phosphor efficiency loss due to crystal defects; photons trapped and absorbed within the LED package; and the heat-induced efficiency loss due to the close vicinity of the semiconductor chip.
To synthesize bulk rods of YAG:Ce, dice and polish the rods to form lenses of suitable shapes and dimensions for making the white LED. The work called for these YAG:Ce BDC lenses to be placed above a high-power blue LED chip in a package to create the novel high efficacy white LED. The goal was to develop a white LED that is at least 30% more efficacious than the state-of-the-art LED on the market. Specific project tasks included: synthesis of YAG:Ce rods; wafer preparation of YAG:Ce rods; characterization of YAG:Ce wafers for optical properties (LRC work); designing of YAG:Ce BDC optics (LRC work); prototyping a white LED (LRC work); design of a commercialization market survey; and preparation of a final commercialization plan.
The project could create a white light LED that is more efficient, provides higher light output with improved and more consistent color properties, and is less expensive to produce. This could significantly reduce the cost of LEDs, increase their market penetration in New York State, and lead to dramatic energy savings for New York rate payers. Specific project metrics will include amount of phosphor product sold by USLLC, number of SSL products using this phosphor technology installed in New York, commensurate energy savings and demand reductions associated with these installations, and jobs created in New York State.
The Contractor explored the crystal growth of YAG:Ce from NaMoO4 flux, which has demonstrated very high luminescence in the YAG:Ce grown at temperature as low as 1,100 oC. Based on initially encouraging results, the activities focused on increasing the crystal size by increasing the volume of the flux and molten material during the synthesis, reducing the crystal growth rate by increasing the growth duration, and increasing the temperature gradient in the growth chamber necessary for directional solidification process.
YAG:Ce synthesis using various flux resulted in the formation of small yellow micro crystals using low temperature solutions (1,100 -1,350 oC) but the size, yield and luminescence was not significant. Use of new flux compositions containing ammonium chloride, barium oxide and boron oxide at lower growth temperatures did not improve the size of the YAG:Ce crystals. Growth of longer length crystals will require very long duration of production (two to three months for a one-cm-long crystal) and hence does not seem to be feasible for large-scale LED applications (due to economic reasons).
United Semiconductors, LLC
Attn: Geeta Rajagopalan 2 Eagles Glen
Clifton Park, NY 12065
Rensselaer Polytechnic Institute
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