One disadvantage of solar cells is that they take up a lot of room - and not every building has enough space.
But, using carbon nanotubes, MIT chemical engineers have found a way to concentrate solar energy 100 times more than a standard photovoltaic cell. They can function as antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.
"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says research team leader Michael Strano.
The devices could also be useful for other applications that require light to be concentrated, such as night-vision goggles or telescopes.
The antenna consists of a fibrous rope about 10 micrometers long and four micrometers thick, containing about 30 million carbon nanotubes. The fiber is made of two layers of nanotubes with different bandgaps.
In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.
The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap.
When light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.
While it's an expensive solution at the moment, the cost of carbon nanotubes is coming down.
"At some point in the near future, carbon nanotubes will likely be sold for pennies per pound, as polymers are sold," says Strano. "With this cost, the addition to a solar cell might be negligible compared to the fabrication and raw material cost of the cell itself, just as coatings and polymer components are small parts of the cost of a photovoltaic cell."
Strano’s team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. At the moment, the nanotube bundles lose about 13 percent of the energy they absorb, but the team is working on a new version that would lose only one percent.