Hello,

We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not an Insider? Subscribe now for unlimited access to online articles.

Sustainable Energy

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.

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.

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.

“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.

Want to go ad free? No ad blockers needed.

Become an Insider
Already an Insider? Log in.
More from Sustainable Energy

Can we sustainably provide food, water, and energy to a growing population during a climate crisis?

Want more award-winning journalism? Subscribe and become an Insider.
  • Insider Plus {! insider.prices.plus !}* Best Value

    {! insider.display.menuOptionsLabel !}

    Everything included in Insider Basic, plus the digital magazine, extensive archive, ad-free web experience, and discounts to partner offerings and MIT Technology Review events.

    See details+

    Print + Digital Magazine (6 bi-monthly issues)

    Unlimited online access including all articles, multimedia, and more

    The Download newsletter with top tech stories delivered daily to your inbox

    Technology Review PDF magazine archive, including articles, images, and covers dating back to 1899

    10% Discount to MIT Technology Review events and MIT Press

    Ad-free website experience

  • Insider Basic {! insider.prices.basic !}*

    {! insider.display.menuOptionsLabel !}

    Six issues of our award winning print magazine, unlimited online access plus The Download with the top tech stories delivered daily to your inbox.

    See details+

    Print Magazine (6 bi-monthly issues)

    Unlimited online access including all articles, multimedia, and more

    The Download newsletter with top tech stories delivered daily to your inbox

  • Insider Online Only {! insider.prices.online !}*

    {! insider.display.menuOptionsLabel !}

    Unlimited online access including articles and video, plus The Download with the top tech stories delivered daily to your inbox.

    See details+

    Unlimited online access including all articles, multimedia, and more

    The Download newsletter with top tech stories delivered daily to your inbox

/3
You've read of three free articles this month. for unlimited online access. You've read of three free articles this month. for unlimited online access. This is your last free article this month. for unlimited online access. You've read all your free articles this month. for unlimited online access. You've read of three free articles this month. for more, or for unlimited online access. for two more free articles, or for unlimited online access.