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  • Virus improves solar-cell efficiency

BioPhotonics
Jul 2011
Compiled by BioPhotonics staff

CAMBRIDGE, Mass. – In the body, a virus might knock out your lights, but in a solar cell, it just might be the key to improved efficiency.

In solar cells, sunlight hits a light-harvesting material, causing the cells to release electrons that can be harnessed to produce an electric current. Scientists at MIT have made significant improvements to the power-conversion efficiency of solar cells by incorporating a genetically modified virus into the design. Their research is based on findings that carbon nanotubes can enhance electron collection efficiency from a solar cell’s surface.

In the past, two problems thwarted scientists trying to use nanotubes for solar cells. First, when making carbon nanotubes, they usually produce a mix of two types – some act as semiconductors, while others act as metals. The new research shows that the semiconducting nanotubes can enhance the performance of solar cells but that the metallic ones have the opposite effect. Second, nanotubes tend to clump together, reducing their effectiveness.

To the rescue? The genetically engineered virus called M13. The researchers found that, although this virus normally infects bacteria, it can be used to control the arrangement of the nanotubes on a surface, keeping the tubes separate so they don’t short out the circuit or clump together.


In this diagram, the M13 virus consists of a strand of DNA (the figure-eight coil on the right) attached to a bundle of proteins called peptides – the virus coats proteins (the corkscrew shapes in the center), which attach to the carbon nanotubes (gray cylinders) and hold them in place. A coating of TiO2 (yellow spheres) attached to dye molecules (pink spheres) surrounds the bundle. More of the viruses with their coatings are scattered across the background. Courtesy of Matt Klug, Biomolecular Materials Group.


The system they tested used lightweight, inexpensive dye-sensitized solar cells, where the active layer is composed of titanium dioxide (TiO2), rather than the silicon used in conventional solar cells. They said the same technique could be applied to other types of cells, including quantum dot and organic solar. By adding the virus-built structures, they enhanced the power conversion efficiency to 10.6 percent from 8 percent – almost a one-third improvement.

The improvement comes even though the viruses and the nanotubes make up only 0.1 percent by weight of the finished cell.

The viruses help improve one step in the process of converting sunlight to electricity. In a solar cell, the first step is for the energy of the light to knock electrons loose from the solar-cell material (usually silicon). Those electrons then must be funneled toward a collector, from which they can form a current that flows to charge a battery or power a device. After that, they return to the original material, where the cycle begins again. The system is intended to enhance the efficiency of the second step, helping the electrons find their way.

In this process, the viruses perform two functions. First, they possess peptides that bind tightly to the carbon nanotubes, holding them in place and keeping them separated from one another. Second, the M13 virus was engineered to produce a coating of TiO2 over each of the nanotubes, putting the TiO2 in proximity to the wirelike nanotubes that carry the electrons.

Both functions are carried out in succession by the same virus, whose activity is “switched” from one function to the next by changing the acidity of its environment – a discovery demonstrated for the first time.

Because the process would add only one simple step to standard solar cell production, it should be an easy adaptation to existing production facilities and make for rapid implementation, the researchers say. They believe that they will be able to ramp up the efficiency even further.

Their findings appeared online April 24, 2011, in Nature Nanotechnology (doi: 10.1038/nnano.2011.50).


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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