This is really quiteunbelievable. We are harnessing a virusto properly manufacture at the nanometer level. Imagine me giving you an resultant product and asking you to explain howit was made?
Obviously we have a brave newprotocol for tackling nano engineering.
We are still a long way todeveloping a really attractive solar manufacturing protocol that iseconomically satisfying, but I think I am now glimpsing the road forward forthe first time. I have grown weary ofincremental improvements while climbing an exponential curve. This method may allow us to do really niftyassembly other that what is traditional photo masking manufacture.
Solar power goes viral
by David L. Chandler
for MIT News Office
In this diagram, the M13 virus consists of a strand of DNA (thefigure-8 coil on the right) attached to a bundle of proteins called peptides -the virus coat proteins (the corkscrew shapes in the center) which attach tothe carbon nanotubes(graycylinders) and hold them in place. A coating of titanium dioxide (yellow spheres)attached to dye molecules (pink spheres) surrounds the bundle. More of theviruses with their coatings are scattered across the background. Image: MattKlug, Biomolecular Materials Group
Researchers at MIT have found a way to make significant improvements to thepower-conversion efficiency of solar cells by enlisting the services of tinyviruses to perform detailed assembly work at the microscopic level.
In a solar cell, sunlight hits a light-harvesting material, causing itto release electronsthat can be harnessed to produce an electric current. The new MIT research,published online this week in the journal Nature Nanotechnology,is based on findings that carbon nanotubes -microscopic, hollow cylinders of pure carbon - can enhance the efficiency ofelectron collection from a solar cell's surface.
Previous attempts to use the nanotubes, however, had been thwarted bytwo problems.
First, the making of carbon nanotubes generally produces a mix of twotypes, some of which act as semiconductors (sometimes allowing an electriccurrent to flow, sometimes not) or metals (which act like wires, allowingcurrent to flow easily).
The new research, for the first time, showed that the effects of thesetwo types tend to be different, because the semiconducting nanotubes canenhance the performance of solar cells, but the metallic ones have the oppositeeffect. Second, nanotubes tend to clump together, which reduces theireffectiveness.
And that's where viruses come to the rescue. Graduate students XiangnanDang and Hyunjung Yi - working with Angela Belcher, the W. M. Keck Professor ofEnergy, and several other researchers - found that a genetically engineeredversion of a virus called M13, which normally infects bacteria, can be used tocontrol the arrangement of the nanotubes on a surface, keeping the tubesseparate so they can't short out the circuits, and keeping the tubes apart sothey don't clump.
The system the researchers tested used a type of solar cell known asdye-sensitized solar cells, a lightweight and inexpensive type where the activelayer is composed of titanium dioxide, rather than the silicon used inconventional solar cells. But the same technique could be applied to othertypes as well, including quantum-dot and organic solar cells, the researcherssay. In their tests, adding the virus-built structures enhanced the powerconversion efficiency to 10.6 percent from 8 percent - almost a one-thirdimprovement.
This dramatic improvement takes place even though the viruses and thenanotubes make up only 0.1 percent by weight of the finished cell. "Alittle biology goes a long way," Belcher says. With further work, theresearchers think they can ramp up the efficiency even further.
The viruses are used to help improve one particular step in the processof converting sunlight to electricity. In a solar cell, the first step is forthe energy of the light to knock electrons loose from the solar-cell material(usually silicon); then, those electrons need to be funneled toward a collector,from which they can form a current that flows to charge a battery or power adevice.
After that, they return to the original material, where the cycle canstart again. The new system is intended to enhance the efficiency of the secondstep, helping the electrons find their way: Adding the carbon nanotubes to thecell "provides a more direct path to the current collector," Belchersays.
The viruses actually perform two different functions in this process.First, they possess short proteins called peptides that can bind tightly to thecarbon nanotubes, holding them in place and keeping them separated from eachother. Each virus can hold five to 10 nanotubes, each of which is held firmlyin place by about 300 of the virus's peptide molecules.
In addition, the virus was engineered to produce a coating of titaniumdioxide (TiO2), a key ingredient for dye-sensitized solar cells, over each ofthe nanotubes, putting the titanium dioxide in close proximity to the wire-likenanotubes that carry the electrons.
The two functions are carried out in succession by the same virus,whose activity is "switched" from one function to the next bychanging the acidity of its environment. This switching feature is an importantnew capability that has been demonstrated for the first time in this research,Belcher says.
In addition, the viruses make the nanotubes soluble in water, whichmakes it possible to incorporate the nanotubes into the solar cell using awater-based process that works at room temperature.
Prashant Kamat, a professor of chemistry and biochemistry at Notre DameUniversity who has done extensive work on dye-sensitized solar cells, says thatwhile others have attempted to use carbon nanotubes to improve solar cellefficiency, "the improvements observed in earlier studies weremarginal," while the improvements by the MIT team using the virus assemblymethod are "impressive."
"It is likely that the virus template assembly has enabled theresearchers to establish a better contact between the TiO2 nanoparticles andcarbon nanotubes. Such close contact with TiO2 nanoparticles is essential todrive away the photo-generated electrons quickly and transport it efficientlyto the collecting electrode surface."
Kamat thinks the process could well lead to a viable commercialproduct: "Dye-sensitized solar cells have already been commercialized in Japan , Korea and Taiwan
Belcher and her colleagues have previously used differently engineeredversions of the same virus to enhance the performance of batteries and otherdevices, but the method used to enhance solar cell performance is quitedifferent, she says.
Because the process would just add one simple step to a standardsolar-cell manufacturing process, it should be quite easy to adapt existingproduction facilities and thus should be possible to implement relativelyrapidly, Belcher says.
The research team also included Paula Hammond, the Bayer Professor ofChemical Engineering; Michael Strano, the Charles (1951) and Hilda RoddeyCareer Development Associate Professor of Chemical Engineering; and four othergraduate students and postdoctoral researchers. The work was funded by the Italiancompany Eni, through the MIT Energy Initiative's Solar Futures Program.
No comments:
Post a Comment