The Key to Low-Cost Solar Cells: Thinner Than a Human Hair?


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While thin film technologies receive worldwide attention with their potential to lower the cost of solar energy, some researchers are thinking outside the box and looking into different approaches--resulting in cost reductions for photovoltaic technologies. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

One of these approaches comes with the collaboration between the Department of Engineering Physics at McMaster University, Cleanfield Energy and the Ontario Centers of Excellence (OCE). These organizations have formed a partnership to pursue the commercialization of nanowire technology in the production of solar cells.

"One of the biggest obstacles to widespread use of solar cells as a clean source of energy is cost," said Ray LaPierre, assistant professor of engineering physics at McMaster University and project leader for the collaboration. "Our work with nanowire fabrication at this stage shows the potential for greater energy efficiency with less costly materials." (Source: Eureka)

Semiconducting nanowires (e.g., Si, InP, GaN, etc.) exhibit aspect ratios (length-to-width ratios) of 1,000 or more. As such, these nanowires are often referred to as one-dimensional structures with controlled lengths of one to five microns and diameters of 10 to 100 nanometers--thousands of times thinner than a human hair. Some of the advantages they offer over thin film and crystalline silicon technologies (both currently used in solar cell production) include: low material utilization; use of low-cost substrates; defect-free materials, with high conversion efficiency; and strong light trapping and absorption.

The exceptional properties of nanowires--not seen in bulk or 3-D materials--are due to the lateral quantum confinement of electrons. Nanowires are excellent at trapping light, very efficient at absorbing the sun's energy and also allow for greater electrical output per unit of surface area.

How are Nanowires Grown?

A common technique for creating a nanowire is the vapor-liquid-solid (VLS) synthesis method. In a simplified explanation, tiny balls of gold or aluminum are planted on a surface that is exposed to a feed gas (e.g. silane, gallium arsenide gases). The gas atoms are sucked up by the gold to form a layer. As each layer is added, the nanowire begins to develop. The process is repeated until a desired length and thickness is reached.

Researchers at McMaster University are now exploring different ways of growing nanowires on a variety of surfaces, known as substrates, that include silicon, glass, flexible metal foils and even a kind of high-tech fabric made of carbon nanotubes. The researchers are also looking at ways of harvesting nanowires grown and scraped from one material and later embedded in flexible plastics.

According to LaPierre, the aim is to achieve 20 per cent efficiency in the next five years.

Professor LaPierre will be reporting on the synthesis of coaxial compound semiconductor III-V nanowires and fabrication of nanowire-based solar cells, as well as their enhanced carrier extraction, light trapping effects and light absorption at the IDTechEx conference "Photovoltaics: Beyond Conventional Silicon" in Denver, CO, June 17-18, 2008.

For more information on the conference, please visit www.idtechex.com./pvUSA.

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