carbon nanotube  (Source:
Nanotubes concentrate solar energy 100 times more and can be used for a variety of applications

MIT chemical engineers have developed a way to concentrate solar energy 100 times more than a traditional photovoltaic cell through the use of carbon nanotubes. 

Jae-Hee Han, postdoctoral associate and lead author; Geraldine Paulus, graduate student and lead author; and Michael Strano, leader of the research team have devised a way to use carbon nanotubes (hollow tubes of carbon atoms) to form antennas that capture and focus light energy, resulting in more powerful and smaller solar arrays. 

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antenna's that would drive photons into them," said Strano. 

Traditional solar panels convert photons into an electric current to generate electricity, but with the use of the nanotube antenna, the number of photons being captured increases and light is transformed into energy that can be funneled into the solar cell. 

These new antenna's are called "solar funnels," and can be used in various other applications such as telescopes or night-vision goggles, where light needs to be concentrated. They contain a fibrous rope that is 10 micrometers long and four micrometers thick, and consist of 30 million carbon nanotubes. The fiber is made up of two layers of nanotubes with different bandgaps, which is the difference in energy levels between an electron and the hole it leaves behind. When photons strike a surface, this excites the electron to a higher degree depending on the material, and interactions between the electron and the hole it leaves behind is an exciton.

The outer layer of the nanotubes have a higher bandgap while the inner layer has a lower bandgap, and the excitons flow from the higher to lower energy. When light strikes the material, the excitons become more concentrated, flowing to the center of the fiber. 

What makes this study such a significant advancement is that it's the first to construct nanotube fibers where the properties of different layers can be controlled. Costs of carbon nanotubes originally prohibited this kind of experimentation, but prices have fallen and made the nanotubes more accessible. 

The next step is to build a photovoltaic device using the antenna, where the antenna would concentrate photons before they are converted into an electrical current by the photovoltaic cell. The antenna would be constructed around a core of semiconducting material and the system would generate electricity by separating the electron from the hole and collecting electrons at one electrode on the inner semiconductor and collecting holes at the other electrode touching the nanotubes. 

The research team is also looking to increase the number of excitons per photon, and to decrease the energy lost as excitons "flow through the fiber."

This study was published in Nature Materials on September 12. 

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