PV Gap

Vanessa Said:

Carbon nanotubes as solar cells?

We Answered:

Carbon nanotubes--tiny strawlike cylinders of pure carbon--have interesting electrical properties. Indeed, they have already been used to manufacture tiny transistors and nanowires. Now scientists say the minuscule cylinders may one day find their way into solar cells. Researchers report in the latest issue of the journal Angewandte Chemie International Edition that they have succeeded in tweaking the tubes so that they supply electrons when exposed to visible light.

To make the altered nanotubes, Dirk M. Guldi of the University of Notre Dame and his colleagues attached molecules of ferrocene to their walls. Made up of two flat rings of five carbon atoms each that sandwich an iron atom, ferrocene is known for its tendency to give up its electrons. The scientists determined that when they exposed their altered nanotubes to light in the visible range, carbon atoms in the walls of the tubes accept the electrons originally associated with the ferrocene molecules. "This separation of charge is sufficiently long-lived for us to divert and use the electrons," Guldi notes.

This is the first time hybrid nanotube complexes have been shown to undergo such photoinduced electron transfer. And it didn't take much: the researchers added just one ferrocene group for every 100 carbon atoms in the nanotubes. The results, Guldi remarks, meet "the first criteria for the development of solar cells based on modified carbon nanotubes."

The new 3D solar cells capture photons from sunlight using an array of miniature “tower” structures that resemble high-rise buildings in a city street grid. The cells could find near-term applications for powering spacecraft, and by enabling efficiency improvements in photovoltaic coating materials, could also change the way solar cells are designed for a broad range of applications.
“Our goal is to harvest every last photon that is available to our cells,” said Jud Ready, a senior research engineer in the Electro-Optical Systems Laboratory at the Georgia Tech Research Institute (GTRI). “By capturing more of the light in our 3D structures, we can use much smaller photovoltaic arrays. On a satellite or other spacecraft, that would mean less weight and less space taken up with the PV system.”
The 3D design was described in the March 2007 issue of the journal JOM, published by The Minerals, Metals and Materials Society. The research has been sponsored by the Air Force Office of Scientific Research, the Air Force Research Laboratory, NewCyte Inc., and Intellectual Property Partners, LLC. A global patent application has been filed for the technology.
The GTRI photovoltaic cells trap light between their tower structures, which are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart—and built from arrays containing millions of vertically-aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the amount of energy they absorb.
Because the tower structures can trap and absorb light received from many different angles, the new cells remain efficient even when the sun is not directly overhead. That could allow them to be used on spacecraft without the mechanical aiming systems that maintain a constant orientation to the sun, reducing weight and complexity – and improving reliability.
“The efficiency of our cells increases as the sunlight goes away from perpendicular, so we may not need mechanical arrays to rotate our cells,” Ready noted.
The ability of the 3D cells to absorb virtually all of the light that strikes them could also enable improvements in the efficiency with which the cells convert the photons they absorb into electrical current.
In conventional flat solar cells, the photovoltaic coatings must be thick enough to capture the photons, whose energy then liberates electrons from the photovoltaic materials to create electrical current. However, each mobile electron leaves behind a “hole” in the atomic matrix of the coating. The longer it takes electrons to exit the PV material, the more likely it is that they will recombine with a hole—reducing the electrical current.
Because the 3D cells absorb more of the photons than conventional cells, their coatings can be made thinner, allowing the electrons to exit more quickly, reducing the likelihood that recombination will take place. That boosts the “quantum efficiency” – the rate at which absorbed photons are converted to electrons – of the 3D cells.
Fabrication of the cells begins with a silicon wafer, which can also serve as the solar cell’s bottom junction. The researchers first coat the wafer with a thin layer of iron using a photolithography process that can create a wide variety of patterns. The patterned wafer is then placed into a furnace heated to 780 degrees Celsius. Hydrocarbon gases are then flowed into furnace, where the carbon and hydrogen separate. In a process known as chemical vapor deposition, the carbon grows arrays of multi-w

bhagyashree said:

what are the disadvantages of it???