14 April 2010
Corporate and academic research groups from around the world are rushing to develop new and improved photovoltaic (PV) technologies. Key objectives are to improve the conversion efficiency and reduce material and manufacturing costs with the ultimate aim of yielding PV technologies that are a cost-effective alternative to fossil fuel-based power generation. Other efforts aim to create flexible rather than rigid PV arrays that will open up all manner of new applications.
“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.”
Conversion efficiency is clearly a critical factor. Most of today’s commercial solar cells are first-generation devices made from single-crystal silicon that convert up to 18% of the incident solar radiation into electrical power. Developments are rapidly gaining pace, and it seems that hardly a week passes without some research group claiming a new efficiency record. However, these figures can be misleading and often relate to a very specific type of device and, therefore, do not always compare “like with like”. For instance, in late 2009, Sanyo Electric reported an impressive sounding figure of 22.8%, but this was from an ultra-thin (98 µm) cell, based on a technique dubbed HIT (heterojunction with intrinsic thin layer), a hybrid technology that combines a crystalline silicon substrate with an amorphous silicon thin film. Then, in early 2010, Mitsubishi Electric announced a seemingly lower world record value of 19.3%, but this was for a 15 cm × 15 cm × 200 µm polycrystalline silicon PV cell and not an HIT device.
Sanyo developed an ultra-thin cell that is less than half thickness
of its highest efficiency cell (23.0%, on the left),
but achieves almost the same efficiency (22.8%, right)
As part of the effort to reduce the cost of crystalline silicon cells, manufacturers are aggressively pursuing the use of thinner silicon wafers. Aside from being cheaper, thinner cells are also lighter and more flexible. However, reducing wafer thickness also tends to reduce the optical absorption and therefore the conversion efficiency. Thus, Mitsubishi announced another world record in 2009, namely an efficiency figure of 18.1% from a polycrystalline silicon cell with a thickness of 100 µm rather than 200 µm.
Polymers and nanotechnology
Polymers 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 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 cell with a nanocrystalline surface in 2009. Other research groups 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.
Solamer Energy has developed a polymer-based PV cell
with a record-breaking conversion efficiency of 7.9%
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.
However, cadmium poses a significant environmental and health threat and is one of the six most toxic materials controlled by the EU’s RoHS (Restriction of Hazardous Substances) Directive. These issues have understandably been raised, but according to First Solar, their panels are not subject to RoHS restrictions or to the WEEE (Waste Electrical and Electronic Equipment) regulations. Tests on the company’s units by the Brookhaven National Laboratory showed that the glass plates that sandwich the CdTe seal during a fire and do not allow any cadmium release. Health and Environmental Risks researchers from Brookhaven concluded that the large-scale use of CdTe PV modules does not present any risks to health and the environment and recycling modules at the end of their useful life completely resolves any environmental concerns. In recognition of potential waste management issues when panels reach the end of their life, First Solar operates a pre-funded collection and recycling programme for all panels sold, regardless of geographical legislation. During normal operation, CdTe modules do not produce any pollutants and furthermore, by displacing fossil fuels, they offer great environmental benefits.
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 2,793 200 µm-thick wafers 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.
Nanotechnology has already exerted a demonstrable impact on optoelectronic devices such as optical sensors, sources and detectors. The evidence suggests strongly that nanotechnology will also play a vital role here. Although it remains unclear exactly how it will be used, all manner of intriguing possibilities are being investigated.
Written by Rob Bogue, Contributing Editor - UK