The key to low-cost solar cells: Is it thinner than a human hair?

While thin film technologies are receiving worldwide attention with their potential to lower the cost of solar energy, there are researchers who are thinking outside the box and are looking into different approaches that will result in cost reductions for photovoltaic technologies.

 

One of those approaches comes with the collaboration of the Department of Engineering Physics at McMaster University, Cleanfield Energy and the Ontario Centers of Excellence (OCE), which 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."¹

 

Semiconducting nanowires (e.g., Si, InP, GaN, etc) exhibit aspect ratios (length-to-width ratio) of 1000 or more. As such they are often referred to as one-dimensional structures with controlled lengths of one to five microns and diameters of 10 to 100 nanometres (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:

The exceptional properties of nanowires that are 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 allow for greater electrical output per unit surface area.

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. They are also looking at ways of harvesting nanowires that are grown and scraped from one material and later embedded in flexible plastics.

 

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

 

Professor Ray 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 USA 2008 in Denver, Colorado, on June 17-18, 2008.

 

For further information also read Printed and Thin Film Photovoltaics and Batteries.