Energy

Cheaper, More Efficient Solar Cells

A new type of material could allow solar cells to harvest far more light.

Much more efficient solar cells may soon be possible as a result of technology that more efficiently captures and uses light. StarSolar, a startup based in Cambridge, MA, aims to capture and use photons that ordinarily pass through solar cells without generating electricity. The company, which is licensing technology developed at MIT, claims that its designs could make it possible to cut the cost of solar cells in half while maintaining high efficiency. This would make solar power about as cheap as electricity from the electric grid.

Better solar: In conventional solar cells (a), light (dashed line) enters an antireflective layer (yellow) and then a layer of silicon (green) in which much of the light is converted into electricity. But some of the light (solid arrows) reflects off an aluminum backing, returns through the silicon, and exits without generating electricity. A new material (represented by the dots in [b]) makes it possible to convert more of this light into electricity. Instead of reflecting back out of the solar cell, the light is diffracted by one layer of the material (larger dots). This causes the light to reenter the silicon at a low angle, at which point it bounces around until it is absorbed. The light that makes it through the first layer is reflected by the second layer of material (smaller dots) before being diffracted into the silicon.

The effort uses a type of material called a photonic crystal that makes it possible to “do things with light that have never been done before,” says John Joannopoulos, a professor of physics at MIT who heads the lab where the new designs for solar applications were developed. Photonic crystals, which can be engineered to reflect and diffract all the photons in specific wavelengths of light, have long been attractive for optical communications, in which the materials can be used to direct and sort light-borne data. Now new manufacturing processes could make the photonic crystals practical for much-larger-scale applications such as photovoltaics.

StarSolar’s approach addresses a long-standing challenge in photovoltaics. Silicon, the active material that is used in most solar cells today, has to do double duty. It both absorbs incoming light and converts it into electricity. Solar cells could be cheaper if they used less silicon. If the silicon is made thinner than it is now, it may still retain its ability to convert the photons it absorbs into electricity. But fewer photons will be absorbed, decreasing the efficiency of the cell.

MIT researchers developed sophisticated computer simulations to understand how thin layers of photonic crystal could be engineered to capture and recycle the photons that slip through thin layers of silicon. Silicon easily absorbs blue light, but not red and infrared light. The researchers found that by creating a specific pattern of microscopic spheres of glass within a precisely designed photonic crystal, and then applying this pattern in a thin layer at the back of a solar cell, they could redirect unabsorbed photons back into the silicon.

Today’s solar cells already reflect some of the light that passes through the silicon. But the photonic crystal has distinct advantages. Conventional solar cells are backed with a sheet of aluminum. The photonic crystal reflects more light than the aluminum does, especially once the aluminum oxidizes. And the photonic crystal diffracts the light so that it reenters the silicon at a low angle. The low angle prevents the light from escaping the silicon. Instead, it bounces around inside; this increases the chances of the light being absorbed and converted into electricity.

As a result, the photonic crystal can increase the efficiency of solar cells by up to 37 percent, says Peter Bermel, CTO and a cofounder of StarSolar. This makes it possible to use many times less silicon, he says, cutting costs enough to compete with electricity from the grid in many markets. The savings would be especially large now, since a current shortage in refined silicon is keeping solar-cell prices high and slowing the growth of solar-cell production.

The company plans to work with existing solar-cell makers, applying its photonic crystals with a machine added to the solar-cell makers’ assembly lines, Bermel says. But StarSolar needs to choose a large-scale manufacturing technique that will allow it to produce the photon crystals inexpensively. What’s needed is a way to cheaply arrange two materials in an orderly three-dimensional pattern. For example, microscopic spheres of glass would be arranged in rows and columns inside silicon. Currently, techniques such as e-beam lithography can be used, but that’s too slow for large-scale manufacturing.

Shawn-Yu Lin, professor of physics at Rensselaer Polytechnic Institute, has developed a method for manufacturing eight-inch disks of photonic crystal–a measurement considerably larger than what can be done with conventional techniques. The method, which employs optical lithography similar to that used in the semiconductor industry, works best for a type of solar cell that concentrates light onto a small chunk of expensive semiconductor material. Such a device would require a relatively small amount of photonic crystal compared with conventional solar cells. Lin says the technique could be applied for more-conventional solar panels, although it would be expensive.

Another potentially less-expensive method, called interference lithography, creates orderly patterns in the photonic-crystal materials. The method is fast and uses machines that are far less expensive than those used for conventional optical lithography. It also requires fewer steps than Lin’s existing process, so he says it could be far cheaper. Such methods have been developed by Henry Smith, professor of electrical engineering at MIT, who was not involved with the StarSolar-related work. Smith says his interference-lithography method could be used to build templates for imprinting photonic-crystal patterns on large areas.

Another promising technique is self-assembly, in which the chemical and physical properties of material building blocks are engineered so that they arrange themselves in orderly patterns on a surface. For example, Chekesha Liddell, professor of materials science and engineering at Cornell University, has engineered building blocks in the shape of peanuts and the caps of mushrooms that line up in rows because of the way they fit together and the tug of short-range forces between them. She says this could be useful for assembling photonic crystals for solar cells.

With such approaches available, Bermel says that StarSolar hopes to have a prototype solar cell within a year and a pilot manufacturing line operating in 2008.

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