Vladimir Bulovic: Solar Booster
Peering through a glass panel into a 13-foot-long steel vacuum tube, Vladimir Bulovic seems to see another world. In chambers branching off from the custom-built tube, he and his students make films that may one day provide an inexpensive boost to the efficiency of silicon solar cells. “If we could provide cheap energy for everyone, we could uplift the world in ways we can’t foresee,” says Bulovic, associate professor of electrical engineering and computer science.
Bulovic builds devices from electrically and optically active thin films of precisely placed molecules. These films can consist of semiconducting crystals called quantum dots (only 10 atoms across) or organic semiconducting dyes with names like copper stellacyanin. Bulovic keeps the films inside a vacuum during the manufacturing process; even the layer or two of water molecules from the air that would accumulate on a film if he carried it across the room would totally change its properties. But the vacuum is as sterile as outer space.
Bulovic compares his devices to cakes with layers of variously flavored icing and pastry. A typical device starts with a piece of glass coated with a transparent electrode. On this are deposited layers of quantum dots and other semiconducting molecules. When light shines on the film, photons hit the quantum dots and molecules, imparting their energy to electrons. These “excited” electrons move farther away from their base atoms; some break free and travel through the layers of the film to the electrode. The resulting current could power, say, a light bulb attached to the device. Bulovic’s lab works closely with chemistry professor Moungi Bawendi, a pioneer in quantum-dot research, and his students.
Bulovic’s solar-cell research has its roots in his work on light-emitting devices. “I learned how to take electrons, put them into molecules, and get out a photon,” he says. But putting in photons and getting out electrons is more difficult. Bulovic says commercially available silicon-based solar cells typically convert just 8 to 12 percent of sunlight into electricity. The organic photovoltaics he made before turning to quantum dots did no better than 3 percent.
Nanostructured materials will probably supplement, not replace, the silicon in solar cells, says Bulovic. Silicon is inefficient at converting ultraviolet light into electricity; but nanomaterial films could turn it into green light that silicon can absorb. Likewise, silicon can’t absorb infrared light, and “a good chunk of the sun’s light is there,” Bulovic says. “You can engineer a second solar cell in tandem with silicon to absorb these wavelengths.”
Bulovic imagines that nano solar-cell films could be printed like newspapers. “There is nothing different about it except for the need for precision in placement of the nanomaterials,” he says. “We may not need high-efficiency [solar] cells if the process is cheap enough.” (He notes, however, that less efficient devices would have to cover more area.) Though the idea of harnessing solar power is old, the costs of implementing it have made it impractical for most people. Tandem silicon and nanomaterial solar cells that tap the infrared spectrum might generate about 5 percent more energy than silicon alone, Bulovic says.
“We receive 10,000 times more energy from the sun than we spend,” he says. “If we can capture that, we can power the world many times over.”