May 3, 2007

Silver Bullish: New Solar Cells See the Light

New Solar Cells See the Light

By John Simpson
ScienceNOW Daily News
24 April 2007

A team of Australian researchers has cleared an important hurdle in the development of cheap solar power. The scientists have devised a new manufacturing technique that could boost the efficiency of an inexpensive type of solar cell by up to 50%, making the technology an economically feasible alternative to fossil fuels in the search for sustainable energy.

The high cost of solar cells means that the energy they generate is about five times more expensive than conventional power, keeping the technology off the roofs of the average citizen. Much of the expense comes from the silicon in the wafers that make up the cells. As a result, researchers have focused on improving so-called thin-film cells, which minimize the use of silicon. The downside to these one- or two-micron-thick films is that they can only convert about 5% to 10% of incoming sunlight into electricity, versus up to 25% for thicker silicon wafers. That's because a process called etching, which is used to maximize light absorption by the cell, reduces the cell's current, voltage, and overall output.

So researchers led by doctoral candidate Supriya Pillai of the University of New South Wales in Sydney, Australia, took etching out of the equation. Their new approach involves depositing a thin film of silver (measuring about 10 nanometers thick) onto a solar cell surface and heating it to 200° Celsius. That breaks the film into flattened spheres, called islands, which are about 100 nanometers in diameter. When struck by light, these islands achieve the same feat as etching by a natural but complex process. Incoming light, as an oscillating electric and magnetic field, strikes the silver nanoparticles, which also oscillates the metal's free electrons back and forth. These oscillating electrons, known as surface plasmons, reradiate light into the underlying silicon, which increases light absorption into the cell. Plasmons have been used in wafer-type cells, but Pillai's group is the first to experiment with them in thin films.

Most thin-film solar cells are about 8% to 10% efficient, says Kylie Catchpole, a co-author of the study, but this technique could increase the value to between 13% and 15%. That's an important advance, she says: "If they're below 10% efficient, then you can't really afford to install them, because it would take up too much of your roof area, for example, to power your house." Once the technology approaches 15% efficiency, says Catchpole, it becomes commercially viable. An average house could be powered for a one-time investment of $10,000, including installation of panels that cover eight square meters, she says. Further research and development should bring prices even lower, the team reports in an upcoming issue of the Journal of Applied Physics.

The use of plasmons to enhance solar cell technology is promising, says materials scientist Mark Brongersma of Stanford University in California. "There's a huge opportunity ... to manipulate light in ways that semiconductors and insulators never have been able to do," he says.

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