Around 40% of solar energy that reaches the Earth’s surface is located within the near infrared area of the spectrum. This energy is not able to be harnessed by normal silicon based solar cells. However, MIT researchers have developed a new all carbon type of solar cell that could utilise that unused energy which would open up combination solar cells including both new all carbon solar cells and silicon based ones as being a possibility. This would mean almost the whole range of the sunlight’s energy could be made use of.
Michael Strano who is the Charles and Hilda Roddey Professor of Chemical Engineering at MIT and is also the senior author of a document that described this new device in the Advanced Materials journal recently stated “It’s a fundamentally new kind of photovoltaic cell,“
C60 and carbon nanotubes make up the new solar cell that is otherwise referred to as buckyballs. This has been possible as a result of new developments in producing purified carbon nanotubes on a large scale.
Professor Strano said: “It has only been within the last few years or so that it has been possible to hand someone a vial of just one type of carbon nanotube.”
So that the new cells work nanotubes have to be uniform, very pure, single-walled and all have to have only one of the two possible nanotubes symmetrical configurations.
Photovoltaic (PV) cells have previously been made by other groups that have used carbon nanotubes but this has only been achieved by utilising a polymer layer that holds the nanotubes in place and then collects electrons that have been loosened when absorbing sunlight. Unfortunately that adds further steps within the production and also needs further coatings to stop degradation with air exposure. However the all carbon PV cell would appear stable within the air.
This new all carbon based PV cell is at its most effective when capturing the sunlight within the near-infrared area. These cells may be overlaid onto normal solar cells as the material is transparent to light that is visible. This would create a tandem device enabling the harnessing of the majority of the sunlight’s energy. Apparently the carbon cells shall need to be refined as the proof-of-concept devices only achieve around 0.1% energy conversion efficiency.
Rishabh Jain, who is a graduate student and lead author of the paper said: “we are very much on the path to making very high efficiency near-infrared solar cells.”
Rishabh Jain added: “One of the really nice things about carbon nanotubes is that their light absorption is very high, so you don’t need a lot of material to absorb a lot of light.”
Jain says: “It’s pretty clear to us the kinds of things that need to happen to increase the efficiency.” For example, the MIT researchers are looking at controlling more precisely the exact thickness and shape of the produced material layers.
It is hoped that other people will join the research team to help improve the system as Jain said: “It’s very much a model system and other groups will help to increase the efficiency.”
Strano pointed out that as the near-infrared area of the solar spectrum has remained unused by normal solar cells even a cell that is of low efficiency, as long as costs are low, could be beneficial.
Michael Arnold who has not been involved in the research and who is an assistant professor of materials science and engineering at the University of Wisconsin at Madison said: “Carbon nanotubes offer tantalizing possibilities for increasing the efficiency of solar cells and are kind of like photovoltaic polymers on steroids.”
He further added that the work: “is exciting because it demonstrates photovoltaic power conversion using an active layer that is entirely made from carbon. This seems like a very promising direction that will eventually allow for nanotubes’ promise to be more fully harnessed.”
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