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More Efficient Solar Cells

Researchers are using layers of silicon quantum dots to create ultra-efficient silicon solar cells.
October 26, 2006

Scientists who work with solar cells are constantly looking for ever-more-efficient materials and engineering approaches that are better able to convert sunlight into electricity. Now, researchers at the University of New South Wales, in Sydney, Australia, have completed the preliminary steps in making an all-silicon structure that can, in theory, eke out nearly twice the electricity that traditional silicon solar cells–the industry’s mainstay–can.

To maximize efficiency, Martin Green, lead researcher on the project and professor of photovoltaic- and renewable-energy engineering, and his team are developing a multilayered silicon system that converts varying wavelengths of sunlight into electricity. Each layer is tuned to collect light at a certain wavelength, says Green.

Multilayered systems–commonly used by NASA to power satellites–have been studied for years. Currently, these systems can operate at an efficiency of about 30 percent. By comparison, the best single-layer silicon cells in research labs have an efficiency of 25 percent. However, the multilayer cells are often made from stacks of exotic semiconductor materials composed of gallium, indium, phosphorous, and arsenic. These materials, Green notes, are expensive and not practical to mass-produce on the scale that’s necessary for solar power to compete with other sources of power generation.

So, Green and his team turned to silicon–inexpensive and abundant, but traditionally bypassed when making multilayered solar cells–to see if they could control the material’s electronic and optical properties so that it could absorb different wavelengths of light. They used a method that mixes small amounts of silicon with silicon dioxide, silicon carbide, or silicon nitride. When heated, the silicon precipitated out into quantum dots–tiny crystals that absorb different wavelengths of light, depending on their size. The researchers then showed that they could make the silicon quantum dots, which are sandwiched within the layers, in different sizes, depending on the thickness of the layer.

The researchers tuned the dots to absorb light at wavelengths from about 1,100 nanometers (infrared light) to roughly 600 nanometers (red light). For the complete solar cell to work, the layers of quantum dots would need to be stacked according to their size. The top layer would contain the smallest dots, which absorb the shortest wavelength. The rest of the light passes through to the layers below, which would contain subsequently larger dots. Green’s proposed scheme contains three of these quantum-dot layers.

There is still much work to be done, however, to turn the results and underlying concept into a viable solar cell. When light is absorbed by the quantum dots, electrons are generated, but the researchers still can’t control how those electrons travel through the layers. The team is working on modifying the system so that the electrons can be transported from the quantum dots to metal contacts to generate electricity. The team expects to have worked out the majority of the remaining challenges and to have a working cell within two years.

The “all-silicon approach” is more amenable to large-scale manufacturing than is using the more exotic materials for multilayered cells, says Ryne Raffaelle, professor of physics and director of the NanoPower Research Labs at the Rochester Institute of Technology, in NY. For many years, Raffaelle adds, researchers who have been looking toward the future of solar technology have been exploring inexpensive thin films and new materials. But, he says, if Green is successful, “silicon may rise once again to the pinnacle of [solar-cell] conversion efficiency.”

In an arena of technology that is filled with new approaches, solar-cell experts are taking a wait-and-see attitude about whether the initial optical experimental results will hold up in an actual device. “It will take a lot of work to realize the predicted high efficiencies” of the multilayered silicon quantum-dot cell, says Arthur Nozik, senior research fellow at the U.S. DOE National Renewable Energy Laboratory. But, he says, the concept is promising, and it’s a “very worthwhile research effort.”

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