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.

—Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering

Their new antennas might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes. The work was funded by a National Science Foundation Career Award, a Sloan Fellowship, the MIT-Dupont Alliance and the Korea Research Foundation.

The antenna—which boosts the number of photons that can be captured—consists of a fiber about 10 micrometers long and four micrometers thick, containing about 30 million carbon nanotubes. Strano’s team built, for the first time, a fiber made of two layers of nanotubes with different electrical properties—specifically, different bandgaps. Strano’s team is the first to construct nanotube fibers in which they can control the properties of different layers, an achievement made possible by recent advances in separating nanotubes with different properties.

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. That’s important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated. Strano and his team have not yet built a photovoltaic device using the antenna, but they plan to. In such a device, the antenna would concentrate photons before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.

The interface between the semiconductor and the nanotubes would separate the electron from the hole, with electrons being collected at one electrode touching the inner semiconductor, and holes collected at an electrode touching the nanotubes. This system would then generate electric current. The efficiency of such a solar cell would depend on the materials used for the electrode, according to the researchers.

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. The nanotube bundles described in the Nature Materials paper lose about 13% of the energy they absorb, but the team is working on new antennas that would lose only 1%.

Resources

• Jae-Hee Han, Geraldine L. C. Paulus, Ryuichiro Maruyama, Daniel A. Heller, Woo-Jae Kim, Paul W. Barone, Chang Young Lee, Jong Hyun Choi, Moon-Ho Ham, Changsik Song, C. Fantini & Michael S. Strano (2010) Exciton antennas and concentrators from core–shell and corrugated carbon nanotube filaments of homogeneous composition. Nature Materials. doi: 10.1038/nmat2832