Wednesday, November 01, 2006

Silicon and Sun

Continued from page 2

By Kevin Bullis

Solar Dreams

The crystalline-silicon solar cells that currently dominate the photovoltaic market are expensive--so expensive that the energy they produce costs several times as much as energy generated by fossil fuels. One reason is the high price of their raw materials. Silicon is extremely abundant on earth, but it doesn't exist as a pure element; instead, it's bound up with oxygen and other elements--in sand, for example. Making pure silicon requires a lot of energy.

To lower the costs of solar cells, researchers have looked for ways to cut down on the amount of silicon they use. Some have turned to less expensive thin films made from cadmium telluride or copper indium diselenide. Extremely thin layers of these new semiconductors can absorb the same amount of light as thicker slabs of crystalline silicon. Morse's fabrication technique could be an inexpensive way to make such thin films; in addition, the nanostructure that his method produces is particularly well suited for absorbing light and converting it into power.

A challenge in designing solar cells is making sure that the electrons dislodged when light hits a semiconductor create a current. When a photon strikes a solar-cell material, the result is both a free electron and its positive counterpart, called a hole. If these can be pulled apart quickly to opposite electrodes, an electrical current results. However, the difficulty of separating them before they recombine and dissipate energy as heat is "one of the major roadblocks for higher-efficiency solar cells," says Aravinda Kini, program manager for biomolecular materials research at the U.S. Department of Energy.

Morse's structures could surmount this roadblock. The network of crystalline projections could be immersed in a transparent solid or liquid electrode. Light would pass through the electrode, where it would be absorbed by the crystal. Because the surface area of the structured thin film is high (in one material, 90 to 100 times that of a traditional thin film), many of the electron-hole pairs generated by the light would be near the electrode interface; as a result, they could quickly separate, with one charge carrier moving into the transparent electrode and the other carrier traveling through the crystal to exit at the opposite electrode.

Already, Morse and colleagues have made more than 30 types of semiconductor thin films and tested their photovoltaic properties. They are now working to incorporate the semiconductors into functional solar cells. At the same time, Morse continues to develop new biologically inspired methods for assembling materials, with an eye to additional applications, including semiconductor devices for safer, higher-power-density batteries and smaller memory chips; he is also interested in creating laminated fibers for ultrastrong building materials.

But excited though he is by the potential applications of his work, Morse remains at heart a molecular biologist. Even as he talks about how his research could lead to better solar cells, he gazes out the window at the dolphins frolicking in the harbor. And he's still devoted to understanding the mechanism behind the complexity of the sponge. Once again he examines the exquisite skeleton of the Venus's flower basket, though he's no doubt seen it thousands of times. "This was made of glass, by a living creature," he exclaims. "It's incredible!"

Kevin Bullis is Technology Review's nanotechnology and materials science editor.