A group of chemical engineers at MIT have devised a way to collect solar energy 100 times more concentrated than a traditional photovoltaic cell. If their ‘solar funnel’ venture proves to be a success, it could drastically alter how solar energy is collected in the future — there will no longer be a need for massive solar arrays or extensive space to generate significant and sufficient amounts of power. The engineers’ research has determined that carbon nanotubes — hollow tubes made up of carbon atoms — will be the primary instrument in capturing and focusing light energy, allowing for not just smaller, but more powerful solar arrays.
In the Sept. 12 online edition of the journal Nature Materials, Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team said, “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.” Their work is being funded by a National Science Foundation Career Award, a Sloan Fellowship, the MIT-Dupont Alliance and the Korea Research Foundation.
The antenna itself is incredibly small – it consists of a fibrous rope about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes. The prototype made by Strano’s team consisted of a fiber made of two layers of nanotubes, each with different electrical properties.
When a photon strikes the surface of the solar funnel, it excites an electron to a higher energy level, which is specific to the material. The relationship 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. Excitons like to flow from high to low energy, and in the solar funnel’s case means they can flow from the outer layer to the inner layer where they can exist in a lower energy state. When light strikes the antenna, all of the excitons flow to the center of the antenna where they are concentrated and the photons are converted to an electrical current. Like with all solar cells however, its efficiency depends on the materials utilized for the electrode.
Strano’s team is the first to construct nanotube fibers in which the properties of different layers can be controlled — an achievement made possible by recent advances in separating nanotubes with different properties. It is not just the higher rate of concentrated energy that makes the solar funnels a breakthrough — by utilizing carbon nanotubes, solar cells can be constructed at a lower-cost than traditional silicon-based solar cells.
While the cost of carbon nanotubes was once prohibitive, it has come down in recent years as chemical companies build up their manufacturing capacity. “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.”
In theory, with this new technology, not only could the solar funnels be used to generate power, but they could be used in applications where light needs to be concentrated — such as telescopes or night-vision goggles. The design behind the solar funnel is quite innovative, by capturing the light in a tube, Strano’s solar funnel, also know as an nanotube antenna, boosts the number of photons that can be transformed into energy, but in a similar process to that of tradition solar cells.
Strano’s team is now reportedly working on ways to minimize the energy lost as excitons flow through the fiber, as well as new antennas that would lose only 1 percent of the energy they absorb versus the standard 13 percent.