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Nanotechnology paves way for exciting 2011 Solar Cell Innovations

Polymers used in solar power cells are attracting great interest and being promoted widely as an alternative to thin silicon and have the advantages of being inexpensive, flexible, light, and cheap to produce. The drawback is that they suffer from very low conversion efficiencies: less than 5% until very recently. However, several research groups are aiming to increase solar energy conversion efficiency through the use of nanomaterials and nanoscale morphologies. For example, a group at the University of California, Los Angeles (UCLA), reported an efficiency of 4.5% from a polymer solar cell with a nanocrystalline surface in 2009. Other research groups, including the joint venture between DelSolar and IBM, have experimented with polymers incorporating nanomaterials such as zinc oxide quantum dots, multi-walled carbon nanotubes, cadmium selenide and titanium dioxide nanoparticles and indium phosphide nanowires.

Early 2010 saw what some industry commentators view as a major breakthrough: a polymer-based PV cell with a conversion efficiency of 7.9%. Developed by start-up Solarmer Energy and exploiting technology developed by UCLA and the University of Chicago, these cells use a narrow-band-gap polymer combined with carbon nanostructures that acts as the transport medium for the electrons to the external circuitry. Solarmer believes it can achieve an efficiency of 10% by the end of this year and that figures of 15 to 20% may ultimately be possible. While groups are combining the nanomaterial concept with silicon technology and have experimented with silicon nanowires on doped silicon substrates, cadmium telluride (CdTe) thin films appear to offer greater promise.

The benefits of CdTe technology are relatively high conversion efficiency (currently about 11%) and lower material and processing costs than silicon. The use of CdTe in photovoltaics has been studied for many years, but the road to commercialisation has been plagued by many corporate failures. However, First Solar is now exploiting the technology very successfully. Founded in 1999, the company generated an income of over $2 billion in 2009, making it the world’s number-one PV manufacturer by revenue. Despite this success, research into improved CdTe photovoltaics continues and, in addition to the development of improved fabrication techniques, some groups are again investigating the role of nanotechnology.

In 2009, researchers at the University of California, Berkeley, reported a PV cell comprising 500 nm-high pillars of cadmium sulphide embedded in a thin film of CdTe on an aluminium foil substrate. This design has the benefit of being flexible and inexpensive to produce and although the conversion efficiency was only about 6%, this is likely to improve as the technology is developed further.

Nanotechnology also has the potential to play a very different (but nevertheless important) role in reducing the cost of conventional silicon PV. Currently, the silicon ingots (“boules”) used are sawn into 200 µm-thick slices with a 120 µm-diameter wire. During the process, there is “kerf”, (material loss caused by sawing), which depends on the diameter of the wire and the particle size of the abrasive slurry used in the cut. Further, the surface of the wafer is damaged by the abrasion to a depth of about 11 µm, so this thickness needs to be removed from both sides of the wafer in an etching process. As a result, only 2,793 wafers of 200 µm-thickness can be cut from a one metre-long silicon boule. Now, Nanosteel Company, Inc. is developing a proprietary type of super-hard, high-strength steel based on nanotechnology that would allow a 60 µm-diameter wire to be used in the cutting process. The reduced kerf would allow 3,356 wafers to be produced from a 1-metre boule, resulting in a 20% increase in yield.

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