Solar Cells That Work All Day

On the surface of a new photovoltaic prototype, microscopic nanotube towers perform best when they catch light on their sides.

By David Talbot

3-D solar: Jud Ready, a senior research engineer at the Georgia Tech Research Institute, holds up a prototype photovoltaic material that is efficient at generating electricity when sunlight strikes it from many different angles. The surface is covered with thousands of microscopic tower structures that are 100 micrometers tall, 40 micrometers wide, and spaced 10 micrometers apart.

Solar cells generally crank out the most power at noon, when the sun is at its highest point and can strike the cell at a 90-degree angle. Before and after noon, efficiencies drop off. But researchers Georgia Tech Research Institute have come up with a prototype that does the opposite. Their solar cell, whose surface consists of hundreds of thousands of 100-micrometer-high towers, catches light at many angles and actually works best in the morning and afternoon.

"It may be intuitive: when the light goes straight down, the only interaction is with the tops of towers and the 'streets' below," says Jud Ready, senior research engineer at the institute's Electro-Optical Systems Laboratory. "But at an angle, the light has an opportunity to reflect off the sides of the towers." When the sun is at a 90-degree angle, the prototype delivers only 3.5 percent efficiency. But it delivers better efficiencies at many other angles and is actually at its peak efficiency--7 percent--when light comes in at a 45-degree angle. That means the device operates at relatively high efficiencies during much of the day and has two efficiency peaks: one before noon, and one after noon.

While those efficiencies are too low for commercialization, Ready is working on optimizing the size and spacing of his towers as well as their chemical composition. As a first application, his sights are set on powering spacecraft and satellites, which could benefit from solar cells that don't require a mechanical means of moving the orientation of the cell to keep it facing the sun. "Anytime you have anything mechanical, it breaks," says Ready. "In space, that is fabulously difficult to try and repair."

Construction of the towers begins with a foundation of silicon wafers coated with a patterned layer of iron. The iron-coated areas become a seedbed for carbon nanotubes, which are grown using standard chemical vapor deposition; the carbon--separated from hydrocarbon gases in a furnace--assembles into nanotubes on the iron areas. The finished towers, each made of arrays of nanotubes, are 100 micrometers tall, 40 micrometers wide, and 10 micrometers apart.

Once the carbon-nanotube towers are complete, they are coated with cadmium-telluride and cadmium-sulfide semiconductors, which do the work of electron generation. Finally, a thin coating of indium tin oxide is deposited to serve as an electrode. In the finished cells, as with some other nanosolar approaches, the nanotubes serve both as a scaffold for the photovoltaic material and also as a conductor to help move electrons to the electrodes. (See "Cheap Nano Solar Cells.") In Ready's technology, each square centimeter of the finished solar cell contains 40,000 towers, and each tower consists of millions of vertically aligned carbon nanotubes.

Ready says that over the next two years, he will scale up the prototypes and test them to ensure that they can survive a rocket launch and the harsh environment of space. He is also trying to make the technology work with semiconductors other than cadmium telluride, which is considered too toxic for widespread commercial use. If all goes well, some version of the technology could be commercialized in five to ten years, Ready says.