Annual Report 2007

U Boosting the Efficiency of Solar Cells nique three-dimensional solar cells that capture nearly all of the light that strikes them could boost the efficiency of photovoltaic (PV) systems while reducing their size, weight and mechanical complexity. GTRI researchers have developed the new 3D solar cells, which capture photons from sunlight using an array of miniature “tower” structures that resemble high-rise buildings in a city street grid. The cells could find nearterm applications for powering spacecraft, and by enabling efficiency improvements in photovoltaic materials, they could also change the way solar cells are designed for a broad range of applications. “Our goal is to harvest every last photon that is available to our cells,” said Jud Ready, a GTRI senior research engineer. “By capturing more of the light in our 3D structures, we can use much smaller photovoltaic arrays. On a satellite or other spacecraft, that would mean less weight and less space taken up with the PV system.” The 3D design was described in the journal JOM. The research has been supported by the Air Force Office of Scientific Research, the Air Force Research Laboratory, NewCyte, Inc., and Intellectual Property Partners, LLC. The GTRI photovoltaic cells trap light between tower structures that are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart – and built from arrays containing millions of vertically aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the amount of energy they absorb. Because the tower structures can trap and absorb light received from many different angles, the new cells remain efficient even when the sun is not directly overhead. That could allow them to be used on spacecraft without the mechanical aiming systems that maintain a constant orientation to the sun, reducing weight and complexity – and improving reliability. The ability of the 3D cells to absorb virtually all the light that strikes them could also enable improvements in the efficiency with which the cells convert the photons they absorb into electrical current. Fabrication of the cells begins with a silicon wafer, which serves as the bottom junction. Using photolithography, the wafer is then coated with a thin layer of iron in a grid pattern. Carbon nanotubes are then grown atop the pattern. Once the nanotubes are in place, researchers use molecular beam epitaxy to coat them with cadmium telluride and cadmium sulfide, which serve as the p-type and n-type photovoltaic layers. Atop that, a thin coating of indium tin oxide, a clear conducting material, is added to serve as the cell’s top electrode. Three-dimensional solar cells that capture nearly all of the light that strikes them could boost the efficiency of photovoltaic systems. 5 www.gtri.gatech.edu http://www.gtri.gatech.edu

Table of Contents Feed for the Digital Edition of Annual Report 2007

Annual Report 2007 Contents From the Director Research at GTRI Defense and Security Health and Human Systems Energy and Environment Manufacturing Technologies Information and Communication Technologies GTRI Around the World Interdisciplinary Research Centers The Laboratories of GTRI Georgia Tech Ireland GTRI by the Numbers GTRI and Georgia Tech Supporting Breakthrough Research GTRI Professional Education The People of GTRI

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