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Climate change and energy

Silicon Nanocrystals for Superefficient Solar Cells

Research shows that silicon can wring two electrons from each photon of incoming light.
August 15, 2007

A typical solar cell generates only one electron per photon of incoming sunlight. Some exotic materials are thought to produce multiple electrons per photon, but for the first time, the same effect has been seen in silicon. Researchers at the National Renewable Energy Laboratory (NREL), in Golden, CO, showed that silicon nanocrystals can produce two or three electrons per photon of high-energy sunlight. The effect, they say, could lead to a new type of solar cell that is both cheap and more than twice as efficient as today’s typical photovoltaics.

Souped-up silicon: A micrograph of a seven-nanometer chunk of crystalline silicon, called a nanocrystal or quantum dot. Such structures could dramatically increase the efficiency of solar cells.

As in earlier work with other materials, the extra electrons come from photons of blue and ultraviolet light, which have much more energy than those from the rest of the solar spectrum, especially red and infrared light. In most solar cells, the extra energy in blue and ultraviolet light is wasted as heat. But the small size of nanoscale crystals, also called quantum dots, leads to novel quantum-mechanical effects that convert this energy into electrons instead.

By generating multiple electrons from high-energy photons, solar cells made of silicon nanocrystals could theoretically convert more than 40 percent of the energy in light into electrical power, says Arthur Nozik, a senior research fellow at NREL. In contrast, today’s flat rooftop solar panels are at best just over 20 percent efficient and are theoretically limited to about 30 percent efficiency. Concentrating sunlight with mirrors or lenses could raise that figure to about 40 percent, but the same approach could boost the efficiency of a silicon-nanocrystal solar cell to well over 60 percent, Nozik says.

What’s more, solar cells made of silicon nanocrystals could prove to be cheap, giving them a significant advantage over other approaches to high-efficiency solar cells. For example, advanced “multijunction” cells have shown efficiencies of more than 40 percent. But these require complicated manufacturing processes that combine expensive semiconductors optimized for different parts of the solar spectrum. Silicon nanocrystals, in contrast, are relatively easy to make, even compared with the material in conventional solar cells, the best of which are made of very large, single crystals of silicon.

Silicon nanocrystals also have marked advantages over the other nanocrystal materials that have shown the multielectron effect. Some of these materials contain toxic elements such as lead or cadmium, and others rely on elements such as indium that are in limited supply. But silicon is both safe and abundant. It’s also well studied, says Christiana Honsberg, professor of electrical and computer engineering at the University of Delaware, so engineers know how to work with it to make solar cells. Indeed, for many of the same reasons, silicon is by far the most common material in solar cells today, and it’s attractive as the basis for broader deployment of photovoltaics in the future.

Before the NREL work, researchers had believed that silicon crystals small enough to produce the multielectron effect would be impractical as a photovoltaic material. At the nanoscale, the optical properties of silicon change so that it converts less light from the red end of the spectrum into electrons. As a result, any gains from more efficiently converting blue and ultraviolet light would be offset. Nozik and his colleagues found that the nanocrystals did not have to be as small as was previously thought, skirting this problem.

To be sure, the NREL work is only a first step. Making solar cells that take advantage of multielectron generation is a challenge. That’s because the extra electrons are very short-lived, making it difficult to extract them from the nanocrystals to generate an electrical current. Indeed, this has proved so difficult that evidence of the effect has come from indirect methods such as spectroscopy rather than from current generated by a solar cell. The use of the indirect measures has led some prominent experts to question whether the extra electrons are actually being produced, although Nozik says that the effect has been confirmed using multiple techniques. Nozik and his colleagues are now working to make solar cells out of silicon nanocrystals–they’re exploring a number of novel designs–and he says they’ve recently made direct measurements indicating that their cells are releasing multiple electrons per photon absorbed. (Their results have yet to be published.)

Honsberg is cautiously optimistic, calling the finding of the multiple-electron effect in silicon nanocrystals a breakthrough, but only “one breakthrough out of maybe three or four” needed to produce cheap, superefficient solar cells.

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