太阳能电池－燃料电池研究进展系列专贴

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Nano-Manhattan' 3D solar cells boost efficiency (Update)

Unique three-dimensional solar cells that capture nearly all of thelight that strikes them could boost the efficiency of photovoltaic (PV)systems while reducing their size, weight and mechanical complexity.

The new 3D solar cells capture photons from sunlight using an array ofminiature "tower" structures that resemble high-rise buildings in acity street grid. The cells could find near-term applications forpowering spacecraft, and by enabling efficiency improvements inphotovoltaic coating materials, could also change the way solar cellsare designed for a broad range of applications.

"Our goal is to harvest every last photon that is available to ourcells," said Jud Ready, a senior research engineer in theElectro-Optical Systems Laboratory at the Georgia Tech ResearchInstitute (GTRI). "By capturing more of the light in our 3D structures,we can use much smaller photovoltaic arrays. On a satellite or otherspacecraft, that would mean less weight and less space taken up withthe PV system."

The 3D design was described in the March 2007 issue of the journal JOM,published by The Minerals, Metals and Materials Society. The researchhas been sponsored by the Air Force Office of Scientific Research, theAir Force Research Laboratory, NewCyte Inc., and Intellectual PropertyPartners, LLC. A global patent application has been filed for thetechnology.

The GTRI photovoltaic cells trap light between their tower structures,which are about 100 microns tall, 40 microns by 40 microns square, 10microns apart -- and built from arrays containing millions ofvertically-aligned carbon nanotubes. Conventional flat solar cellsreflect a significant portion of the light that strikes them, reducingthe amount of energy they absorb.

Because the tower structures can trap and absorb light received frommany different angles, the new cells remain efficient even when the sunis not directly overhead. That could allow them to be used onspacecraft without the mechanical aiming systems that maintain aconstant orientation to the sun, reducing weight and complexity – andimproving reliability.

"The efficiency of our cells increases as the sunlight goes away fromperpendicular, so we may not need mechanical arrays to rotate ourcells," Ready noted.

The ability of the 3D cells to absorb virtually all of the light thatstrikes them could also enable improvements in the efficiency withwhich the cells convert the photons they absorb into electrical current.

In conventional flat solar cells, the photovoltaic coatings must bethick enough to capture the photons, whose energy then liberateselectrons from the photovoltaic materials to create electrical current.However, each mobile electron leaves behind a "hole" in the atomicmatrix of the coating. The longer it takes electrons to exit the PVmaterial, the more likely it is that they will recombine with a hole --reducing the electrical current.

Because the 3D cells absorb more of the photons than conventionalcells, their coatings can be made thinner, allowing the electrons toexit more quickly, reducing the likelihood that recombination will takeplace. That boosts the "quantum efficiency" – the rate at whichabsorbed photons are converted to electrons – of the 3D cells.
Fabrication of the cells begins with a silicon wafer, which can alsoserve as the solar cell’s bottom junction. The researchers first coatthe wafer with a thin layer of iron using a photolithography processthat can create a wide variety of patterns. The patterned wafer is thenplaced into a furnace heated to 780 degrees Celsius. Hydrocarbon gasesare then flowed into furnace, where the carbon and hydrogen separate.In a process known as chemical vapor deposition, the carbon growsarrays of multi-walled carbon nanotubes atop the iron patterns.

Once the carbon nanotube towers have been grown, the researchers use aprocess known as molecular beam epitaxy to coat them with cadmiumtelluride (CdTe) and cadmium sulfide (CdS) which serve as the p-typeand n-type photovoltaic layers. Atop that, a thin coating of indium tinoxide, a clear conducting material, is added to serve as the cell’s topelectrode.

In the finished cells, the carbon nanotube arrays serve both as supportfor the 3D arrays and as a conductor connecting the photovoltaicmaterials to the silicon wafer.

The researchers chose to make their prototypes cells from the cadmiummaterials because they were familiar with them from other research.However, a broad range of other photovoltaic materials could also beused, and selecting the best material for specific applications will bea goal of future research.

Ready also wants to study the optimal heights and spacing for thetowers, and to determine the trade-offs between spacing and the angleat which the light hits the structures.

The new cells face several hurdles before they can be commerciallyproduced. Testing must verify their ability to survive launch andoperation in space, for instance. And production techniques will haveto scaled up from the current two-inch laboratory prototypes.

"We have demonstrated that we can extract electrons using thisapproach," Ready said. "Now we need to get a good baseline to see wherewe compare to existing materials, how to optimize this and what’sneeded to advance this technology."

Intellectual Property Partners of Atlanta holds the rights to the 3Dsolar cell design and is seeking partners to commercialize thetechnology.

Another commercialization path is being followed by an Ohio company,NewCyte, which is partnering with GTRI to use the 3D approach forterrestrial solar cells. The Air Force Office of Scientific Researchhas awarded the company a Small Business Technology Transfer (STTR)grant to develop the technology.

"NewCyte has patent pending, low cost technology for depositingsemiconductor layers directly on individual fullerenes," explainedDennis J. Flood, NewCyte’s president and CTO. "We are using ourtechnology to grow the same semiconductor layers on the carbon nanotubetowers that GTRI has already demonstrated. Our goal is to achieveperformance and cost levels that will make solar cells using the GTRI3D cell structure competitive in the broader terrestrial solar cellmarket."

On the Net:
http://www-stage.gatech.edu/news-room/flash/CNTpv.html
Source: Georgia Institute of Technology