Towards nanowire solar cells with a 65-percent efficiency

Eindhoven University of Technology (TU/) researchers want to develop solar cells with an efficiency of over 65 percent by means of nanotechnology. In Southern Europe and North Africa these new solar cells can generate a substantial portion of the European demand for electricity. The Dutch government reserves EUR 1.2 million for the research.

The current thin-film solar cells (type III/V) have an efficiency that lies around 40 percent, but they are very expensive and can only be applied as solar panels on satellites. By using mirror systems that focus one thousand times they can now also be deployed on earth in a cost-effective manner. The TU/ researchers expect that in ten years their nano-structured solar cells can attain an efficiency of more than 65 percent. Jos Haverkort: "If the Netherlands wants to timely participate in a commercial exploitation of nanowire solar cells, there is a great urgency to get on board now."

The research is conducted together with Philips MiPlaza.

They think that nanotechnology, in combination with the use of concentrated sunlight through mirror systems, has the potential to lead to the world's most efficient solar cell system with a cost price lower than 50 cent per Watt peak. In comparison: for the present generation of solar cells that cost price is 1.50 euro per Watt peak.
Stacking Nanowires make it possible to stack a number of subcells (junctions). In this process each subcell converts one color of sunlight optimally to electricity. The highest yield reported until now in a nanowire solar cell is 8.4 percent. Haverkort: "We expect that a protective shell around the nanowires is the critical step towards attaining the same efficiency with nanowire solar cells as with thin-film cells." Haverkort thinks that at 5 to 10 junctions he will arrive at an efficiency of 65 percent.

Scarcity of raw materials In addition, the researchers expect considerable savings can be made on production costs, because nanowires grow on a cheap silicon substrate and also grow faster, which results in a lower cost of ownership of the growth equipment. What is more, the combination of the mirror systems with nanotechnology will imply an acceptable use of the scarce and hence expensive metals gallium and indium.

An agency of the Ministry of Economic Affairs, will grant the EUR 1.2 million to researchers dr. Jos Haverkort, dr. Erik Bakkers en dr. ir. Geert Verbong for their research into nanowire solar cells. It is their expectation that, when combined with mirror systems, these solar cells can generate a sizeable portion of the European electricity demand in Southern Europe and North Africa.

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Nanocoax solves solar cell thick and thin dilemma

A nano-scale solar cell inspired by the coaxial cable offers greater efficiency than any previously designed nanotech thin film solar cell by resolving the "thick & thin" challenge inherent to capturing light and extracting current for solar power, Boston College researchers report in the current online edition of the journal Physica Status Solidi.

The quest for high power conversion efficiency in most thin film solar cells has been hampered by competing optical and electronic constraints. A cell must be thick enough to collect a sufficient amount of light, yet it needs to be thin enough to extract current.

Physicists at Boston College found a way to resolve the "thick & thin" challenge through a nanoscale solar architecture based on the coaxial cable, a radio technology concept that dates back to the first trans-Atlantic communications lines laid in the mid 1800s.

"Many groups around the world are working on nanowire-type solar cells, most using crystalline semiconductors," said co-author Michael Naughton, a professor of physics at Boston College. "This nanocoax cell architecture, on the other hand, does not require crystalline materials, and therefore offers promise for lower-cost solar power with ultrathin absorbers. With continued optimization, efficiencies beyond anything achieved in conventional planar architectures may be possible, while using smaller quantities of less costly material."

Optically, the so-called nanocoax stands thick enough to capture light, yet its architecture makes it thin enough to allow a more efficient extraction of current, the researchers report in PSS's Rapid Research Letters. This makes the nanocoax, invented at Boston College in 2005 and patented last year, a new platform for low cost, high efficiency solar power.

Constructed with amorphous silicon, the nanocoax cells yielded power conversion efficiency in excess of 8 percent, which is higher than any nanostructured thin film solar cell to date, the team reported.

The ultra-thin nature of the cells reduces the Staebler-Wronski light-induced degradation effect, a major problem with conventional solar cells of this type, according to the team, which included Boston College Professors of Physics Krzysztof Kempa and Zhifeng Ren, as well as BC students and collaborators from Solasta Inc., of Newton, Mass., and École Polytechnique Fédérale de Lausanne, Institute of Microengineering in Switzerland.

The research was funded in part by a Technology Incubator grant from the Department of Energy.

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