catching more rays

Nanoscale crystals can make solar cells with unheard-of efficiency.

By Jeffrey Winters

Almost anyone with a calculator knows the advantages of photovoltaic cells: They generate electricity quietly with little or no maintenance. PV works so well in applications that draw little power that frequently one doesn't mind paying several dollars per kilowatt-hour.

To get more widely deployed, the costs of PV-generated electricity obviously must come down. That means either bringing down the manufacturing costs of solar cells or boosting their efficiency at converting sunlight into electricity. A group at the University of Toronto recently made a breakthrough in the efficiency department, developing a new type of plastic cell that can harness some 30 percent of the energy falling on it, up from an industry standard of 6 percent. The key is a thin layer of nanoparticles.

Although ubiquitous in niche applications, solar cells are quite sophisticated. The photovoltaic effect is a subtle bit of quantum mechanics: An incoming photon hits a semiconducting material, such as doped silicon crystals, and knocks loose electrons that can be used in a current. Like all quantum effects, it is an all-or-nothing affair. If the incoming photon has too little energy, it won't excite the electron; too much, and any excess energy is wasted. Since the energy of a photon is proportional to its wavelength, that means PV cells usually miss out on red and infrared light and waste light in the blue end of the spectrum.

Until recently, photovoltaic cells were derived from silicon semiconductor technology, but much recent research into improving the efficiency of PV cells has gone into polymer materials. Unlike crystals, plastic semiconductors are relatively inexpensive and highly flexible. Konarka Technologies of Lowell, Mass., and the Ecole Polytechnique Fédérale de Lausanne in Switzerland, for instance, are developing plastic solar cells that can be woven into the fabric of tents, backpacks or even clothing.

But for all their advantages, polymer PV cells are inefficient, converting less than 6 percent of the energy landing on them. That's about a third less efficient than top-of-the-line silicon-based solar cells. One of the problems has been finding a polymer material that is sensitive to light in the red and infrared ends of the spectrum.

Instead of continuing the search for a polymer that can gobble up the red, engineer Edward Sargent and his colleagues at the University of Toronto opted to mate the polymer with another material that can absorb longer wavelengths. The hope was that the hybrid material would be sensitive to a broad spectrum of light.

Plastic solar cells already on the market are useful in niche applications, such as recharging cell phones.

Sargent's team began looking into quantum dots—nanoscale semiconductor crystals that can confine electrons in all three dimensions (see "Points of Light," September 2004). Because of that property, their sensitivity to light is tied directly to their size: The larger the dot, the longer the wavelength it absorbs. Nanodots of this sort are being investigated for use in many applications, including lasers and computing.

The team chose to use nanocrystals made of lead sulfide, which can be "tuned" to absorb wavelengths from 800 to 2,000 nanometers—that is, from the red to the deep infrared. Combined with a polymer sensitive to green and blue light, nanocrystals can convert red and infrared light to energy the polymer can turn into electrical current.

To test the idea, Sargent and his colleagues sandwiched a 100-nanometer layer of nanocrystals blended with a polymer semiconductor between two metal layers. When it was exposed to infrared light, the experimental cell generated about 1,000 times more electricity than a non-blended polymer PV—an efficiency of about 3 percent.

According to Sargent, with the right mix of nanocrystals and polymers, a photovoltaic cell made from this material could achieve overall efficiencies of 30 percent, as measured in the amount of radiant energy converted into electricity. This makes the technology at least equal to the best conventional solar cells.

And the nanocrystal-polymer blend has other advantages. The material literally could be spray-painted on surfaces, driving down the cost of mounting PV cells to the sides and roofs of buildings. It could also be incorporated in fabrics, such as those used for tents and awnings.



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