Researchers at Harvard apply a new twist to inorganic nanoscale photovoltaics

A team led by Charles M. Lieber of Harvard has developed a nanowire which is both conductive and photovoltaic. The wire, comprised of silicons, may be ideal for powering various nanoscale devices.

While nanoscale photovoltaics are not a new invention, Lieber's nanowires are both more efficient and more durable than others. The nanowires can convert up to 3.4 percent of received sunlight into electricity, and almost 5 percent in concentrated light. The wires also do not degrade under these conditions the way organic photovoltaics do.

The wire itself, like traditional amorphous silicon solar cells, uses three types of silicon. A core doped with B2H6 is first grown, then covered with a layer of pure silicon. A third layer, doped with PH3 follows, after which the wire is coated with a protective mask. This layering creates an electric field between the core and the outer silicon layer, with the pure silicon acting as a resistor. Photons striking the wire create electricity and holes in in the neutral layer, which then separate into the core and outer layer.

While the core of the wire is a single crystal, the outer layers are nanocrystalline. Lieber believes the nanocrystalline structure is the key to the the wire's enhanced absorption properties over that of single-crystal materials.

To gather electricity from the wires, the outer layers are first etched away to expose the core. Then, using lithography, metallic contacts are laid down and attached to the exposed wire.

The cells, Lieber says, are not ready for commercial application. Typical commercial solar cells have an efficiency of 12 to 18 percent, compared to the nanowire's 3.4 percent. Lieber plans to research ways to boost the efficiency of the wire, targeting a 10 to 15 percent yield. Even at these levels, the low production cost could make them viable in larger applications.

“It will have to be unique to be an economically viable application, some place where you want durability and flexibility, where if it gets destroyed, people don't care,” said Lieber.

The nanowires have already been used to power various electronics, including a nanoelectronic pH sensor and a cancer detector.

"It's okay. The scenarios aren't that clear. But it's good looking. [Steve Jobs] does good design, and [the iPad] is absolutely a good example of that." -- Bill Gates on the Apple iPad

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