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.