Solar Cells Info

by David Kay, Cosmos Online, 18 April 2007
source: CosmosOnline
http://www.cosmosmagazine.com/node/1224

The expense of photovoltaic cells has prevented their widespread use, but a raft of new technologies is pushing their prices down. One of them is solar slivers.  The usual weather conditions in Scotland – cold, overcast and damp – are hardly inspiring for scientists trying to figure out how to cheaply capture energy from the sun.

But in May 2000, physicist Andrew Blakers and electrical engineer Klaus Weber from the Australian National University in Canberra traveled to Glasgow for a professional conference. To their surprise, in two weeks they experienced rain just once; the long, sixteen-hour days were bright and sunny. Perhaps that helps to explain what happened there.

Blakers enjoys attending professional conferences. “There are lots of good ideas,” he explains, “There is no teaching, no administration, none of the ordinary day-to-day stuff.” On the floor of the conference hall, he came up with an idea of his own. “I was very much excited,” says Blakers, “I thought it was possibly at the top of all the ideas I’d ever had.”

Electricity from sunlight is produced by photovoltaic cells. At the heart of each cell is a single wafer of highly refined and expensive silicon that contributes around 75 per cent of the total cost. The process that converts light to electricity occurs only on the surface of the wafer, which means that most of the silicon is wasted. So the key to producing low-cost solar cells is to reduce the amount of silicon required.

Novel approach

There have been many attempts and many different approaches to the problem. At the time of the conference Blakers and Weber were working on a technology called Epilift, which proved too expensive. It involved growing a 100-micron-thick silicon layer on a silicon substrate and then peeling it off. Another approach, called thin-film solar cells, uses a layer of silicon just a few microns thick deposited onto a supporting material such as glass. But these cells have only around half the efficiency of conventional photovoltaic cells.

Blakers’ idea at the conference was simple in principle. He suggested, half joking, that they could slice a silicon wafer like a loaf of bread, and lay the slices on their side. This would substantially increase the surface area for electricity production.  As Blakers and Weber talked, they realised it wasn’t such a joke after all. They slept on it and the next morning boarded a train to Edinburgh. There, as they walked around tourist sites such as Edinburgh Castle, they continued their discussion.

“It just seemed like everything was falling into place,” says Blakers, “One of the reasons it was so exciting was that for every potential problem there was a potential solution – there didn’t appear to be any show-stoppers.” They returned to Australia confident that the approach had promise, and re-directed their research resources accordingly.

Silicon slicing

Along with physicist Vernie Everett and their team at the ANU’s Centre for Sustainable Energy Systems in Canberra, the pair faced two major issues: how to slice the silicon and how to handle hundreds of tiny slices produced from each wafer.  Their approach was to slice the wafers using existing techniques for etching silicon. Parallel grooves are etched through the wafer leaving a series of slivers only around 50 microns wide – about the width of a human hair. These are detached and wired together to create a photovoltaic cell.

Amazingly, this process reduces by 90 per cent the amount of silicon used in a cell. A single wafer 15 cm in diameter can be used to cover an area of one square metre. Blakers is optimistic about the effect this could have on the price of solar panels. “We think that taking current knowledge would knock off three-quarters of the cost,” he claims.

Origin Energy, a gas and electricity company based in Sydney, is taking the technology seriously and has invested around A$30 million (US$25 million) in it. Most of this has gone into a South Australian pilot plant to manufacture cells, which is currently producing prototype panels.

“We have demonstrated the feasibility of the manufacturing process,” says Origin spokesperson Natali Bennett. “We need a larger facility – to get product out as quickly as possible.” Even under the best circumstances, commercial quantities of the cells are unlikely to be available for at least two years.

Dedicated machines

“It’s a smart way to get more cells out of a silicon wafer,” says Armin Aberle with the Australian Research Council’s Photovoltaics Centre of Excellence at the University of New South Wales in Sydney.  But it’s not faultless, he adds. “The biggest problem is that they have to connect billions of slivers,” which will require dedicated machines, possibly even new robots, says Aberle.

This more complicated production process could mean that the cost of electricity from sliver cells may in fact only be about 10 per cent lower than conventional cells. Abarle argues that the sliver technology might be overtaken as others improve – such as the thin-film solar cells his own team is working on.

Sliver cells have a number of properties that make them suitable for novel applications, however. They are flexible so they can be mounted on curved surfaces and, unlike conventional cells, they can produce relatively high voltages in just a small area. This makes them ideal for use in devices such as mobile phones and iPods, eliminating the need to recharge batteries.

They are also translucent and can be mounted in glass. With a high power to weight ratio, other possible applications include recharging hybrid cars and even solar powered aircraft.

Because of their unique characteristics, “there will always be [at least] a niche market,” says Aberle.

All about potential

Whatever the technology, the consequences of low-cost solar cells are considerable. It currently takes 10 to 15 years to recover the costs of photovoltaic solar panels installed in homes. If this can be reduced to around five years, then solar electricity becomes competitive with existing sources.

“Photovoltaic solar panels are currently in the hands of enthusiasts, those totally committed to reducing greenhouse gases,” says Artur Zawadski, chair of the Australian and New Zealand Solar Energy Society, a renewable energy advocacy group. “When it starts to reach the same level of costs as the electricity grid, it puts them in the hands of the everyday householder.”

Zawadski – also a businessman at Wizard Power, a solar technology company in Canberra – says that adoption of solar electricity in homes could reduce the demands on the grid by between 30 and 70 per cent, with a corresponding reduction in greenhouse gas production. “Widespread use of solar electricity would make an enormous impact, not just on electricity generation but also on [grid] infrastructure.”

Cheaper solar power will certainly be part of the solution to the enormous problem the world faces in reducing greenhouse gases.

While sliver cell technology is promising, we can’t start making plans to knock down our coal-fired power plants just yet. New technology can take a long time to come to fruition – and initially solar panels made with sliver cells may be no cheaper than conventional panels, says Origin’s Bennet. “It’s all about potential.”