Cheaper, More Efficient Solar Cells

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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.