28 June 2010


RoseStreet Lab demos multi-band cell in single-layer thin-film PV device

RoseStreet Labs Energy Inc (RSLE) of Phoenix, AZ, USA has announced a laboratory demonstration of what is claimed to be the first multi-band photovoltaic device featuring three distinct light absorption regions integrated into a single-layer thin-film device.

RSLE was formed in October 2006 as a 50:50 joint venture between parent firm RoseStreet Labs LLC (a privately held supplier of products and services for the renewable energy, semiconductor and life science markets) and Tokyo-based Sumitomo Chemical Co Ltd to develop both thin-film and multi-junction high-efficiency solar cells.

In 2005 RoseStreet Labs agreed exclusive patent licenses with both the US Department of Energy's Lawrence Berkeley National Laboratory (LBNL) (inter-band, or ‘I-band’, technology) as well as Cornell University (an indium nitride-based solar cell) for devices that use a larger fraction of the solar spectrum compared to conventional products. The firm subsequently began commercializing LBNL’s multi-band technology (developed by RSLE’s chief technology officer Wladek Walukiewicz and Kin Man Yu at LBNL), which uses oxygen impurities to split the conduction band of zinc manganese tellurium (ZnMnTe), enabling devices to achieve the efficiencies of a triple-junction device with the manufacturing cost and simplicity of a single-junction device, it is claimed.

The demonstrated device is based on RSLE’s IBand technology and is the first known intermediate-band solar cell reduced to practice in a laboratory demonstration. The firm says that the technology shows promise for thin-film PV efficiencies above 35% by potentially capturing the full spectrum of the sunlight.  

Efficient solar cells require optimized utilization of the whole solar spectrum, and currently this is achieved in a complex and expensive technology in which several solar cells with different bandgaps are connected in series, says RSLE. A much simpler approach in which a single semiconductor has several different bandgaps sensitive to different parts of the solar spectrum has been proposed but never realized, the firm adds.

The intermediate-band solar cell developed by RSLE is a thin-film technology based on the highly mismatched alloys. The three-bandgap, one-junction device has the potential of much improved solar light absorption and higher power output than the III-V triple-junction compound semiconductor devices that hold the existing PV efficiency record. RSLE’s demonstration device was fabricated using high-volume CVD technology, validating the potential for commercialization, the firm claims.  

“Although we are three to four years away from high-volume production with the IBand product, this development opens up a new class of semiconductor devices for photovoltaic conversion and other advanced semiconductor applications,” says RSLE’s CEO Bob Forcier. “It fits seamlessly with our hybrid PV commercialization.”

“This demonstration is a major breakthrough in our photovoltaic semiconductor roadmap which will allow us to go to the next step in our PV research at an accelerated pace,” says Walukiewicz. “The IBand technology is synergistic with our thin-film nitride hybrid product development and will allow upside potential for higher solar conversion efficiencies compared to conventional technologies.”

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