Bathing the Earth with enough energy in one hour to meet human needs for an entire year, the sun represents the ultimate source of clean, green sustainable energy. Credit: courtesy of NASA
By combining designer quantum dot light-emitters with spectrally matched photonic mirrors, a team of scientists with the Lawrence Berkeley National Laboratory and the University of Illinois “achieved a luminescent concentration ratio greater than 30 with an optical efficiency of 82% for blue photons,” the highest luminescent concentration factor ever recorded. This breakthrough paves the way for the future development of low-cost solar cells that efficiently utilize the high-energy part of the solar spectrum.
The solar energy industry in the United States is soaring with the number of photovoltaic installations having grown from generating 1.2 gigawatts of electricity in 2008 to generating >20 gigawatts today, according to the U.S. Department of Energy (DOE). Still, nearly 70% of the electricity generated in this country continues to come from fossil fuels. Low-cost alternatives to today’s photovoltaic solar panels are needed for the immense advantages of solar power to be fully realized. One promising alternative has been luminescent solar concentrators (LSCs).
Unlike conventional solar cells that directly absorb sunlight and convert it into electricity, an LSC absorbs the light on a plate embedded with highly efficient light-emitters called “lumophores” that then re-emit the absorbed light at longer wavelengths called Stokes shift. This re-emitted light is directed to a micro-solar cell for conversion to electricity. Because the plate is much larger than the micro-solar cell, the solar energy hitting the cell is highly concentrated.
With a sufficient concentration factor, only small amounts of expensive III-V photovoltaic materials are needed to collect light from an inexpensive luminescent waveguide. But the concentration factor and collection efficiency of the molecular dyes that up until now have been used as lumophores are limited by non-unity quantum yields of the lumophores, imperfect light trapping within the waveguide, and reabsorption and scattering of propagating photons.
“We replaced the molecular dyes in previous LSC systems with core/shell nanoparticles composed of cadmium selenide (CdSe) cores and cadmium sulfide (CdS) shells that increase the Stokes shift while reducing photon re-absorption,” says Bronstein.
“The CdSe/CdS nanoparticles enabled us to decouple absorption from emission energy and volume, which in turn allowed us to balance absorption and scattering to obtain the optimum nanoparticle,” he says. “Our use of photonic mirrors that are carefully matched to the narrow bandwidth of our quantum dot lumophores allowed us to achieve waveguide efficiency exceeding the limit imposed by total internal reflection.”
http://newscenter.lbl.gov/2015/09/01/made-from-solar-concentrate/
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