Charles Leiber and colleagues at Harvard University describe silicon nanowire they devised that can convert light into electrical energy. Virtually invisible to the naked eye, a single strand can crank out up to 200 picowatts. The nanowire is not made of metal but of silicon with three different types of conductivity arranged as layered shells.
Incoming light generates electrons in the outer shell, which are then swept into the second layer and the inner core along micropores.
The nanowire, which resembles a miniscule coaxial cable, is made of layers of silicon (Image: Nature)
Although the proof-of-concept device only converts about 3% of light into electricity, Lieber says it “allows us to study a fundamentally different geometry for photovoltaic cells, which may be attractive for improving the efficiency.”
He also believes it may be possible to boost the nanowire’s efficiency by getting rid of defects in the crystal. “Our goal is to get in the 15% [efficiency] range,” Lieber says.”
Lieber’s new nanowire functions as a complete solar cell. At its core is a rod-shaped crystal of silicon, about 100 nanometres across, doped with boron. Layers of polycrystalline silicon are added to wrap it in a 50-nm-thick layer of undoped silicon and a 50-nm-thick outer coating of silicon doped with phosphorus.
Harry Atwater, a physicist at Caltech, called the Harvard research “an important first experimental step forward.” Atwater recently wrote a theoretical paper that suggested it may be possible to get the efficiency of such a nanowire above the 20 to 25 percent seen in highly ordered crystalline silicon. Lieber sees no reason that the efficiency can’t be improved to at least 10 or 15 percent. At that point, he says, the lower costs that his production process entails might make large arrays of nanowires competitive with macroscale solar cells.
Since the materials are thin, the chances of an electron being trapped by a defect before escaping from one layer to the next are low, so it’s possible to use cheaper materials with more defects.
Lieber has tested only small numbers of nanowire solar cells. For large-scale applications, the nanowires would need to be chemically grown in dense arrays. Atwater and Lewis recently took steps in this direction, publishing in the past month two papers in which they describe growing dense arrays of microscopic wires, but wires without the multiple layers that Lieber’s have. Paired with a liquid electrolyte, the wires generated electricity from light. Since it may prove easier to manufacture solid-state solar cells such as Lieber’s, however, Lewis and Atwater are working to produce arrays of wires with multiple layers.
Even with the potential advantage of cheaper materials, wire-based solar cells would probably need to be about 10 percent efficient if they were to compete with existing technology. The researchers’ next steps include finding ways to make more dense arrays of wires to absorb more light and, in Lieber’s case, to find ways to generate increased voltage from nanowire solar cells.