Nanotechnology seems to help a lot in solar cells improvement. Recent research in solar cell technology shows that a film of carbon nanotubes could replace two of the layers normally used in a solar cell, with improved performance and reduced costs. There is a surprising way the researchers found out to give the nanotubes the properties they need: add them some imprefections.
The current solar cells (aka dye-sensitized solar cells) have an oxide transparent film, is applied to glass. That dye-sensitized transparent film conducts electricity. In addition, another separate film made of platinum acts as a catalyst to speed up the chemical reactions that occur in the making of electricity. The oxide films have the disadvantage that they can’t easily be applied to flexible materials: they perform much better on a rigid and heat resistant substrate like glass. This increases the production costs and limits the kinds of products the solar cells can be applied to. The second drawback is that expensive equipment is necessary to create the platinum films.
Jessika Trancik of the Santa Fe Institute, Scott Calabrese Barton of Michigan State University and James Hone of Columbia University decided to use nanotechnology in the making of some new type of solar cells. The carbon nanotubes they use create a single layer that performs the functions of both the oxide and platinum layers. The carbon nanotubes needs to have three properties: to be transparent, to conduct electricity, and to be a catalyst of the electricity-producing reaction.
Regular carbon nanotubes films have these properties, but not enough of each one. The way of improving one, though, sacrifices one of the others. For example, making the film thicker makes it a better catalyst, but then it’s less transparent.
Previous theory had suggested that materials may function better as catalysts when they have tiny defects, impurities, providing spots for chemicals to attach. The researchers then tried exposing the carbon nanotubes to ozone, which roughs them up a bit. Very thin films, they found, became dramatically better catalysts, with more than ten-fold improvement. In fact, the performance of the impurified nanotubes gets close to that of platinum. “That’s remarkable,” Trancik says, “because platinum is considered pretty much the best catalyst there is.”
In order to address the trade-off between transparency and conductivity, the researchers tried another trick on a bottom layer of tubes: they created longer carbon nanotubes. This improved both conductivity and transparency.
The carbon nanotube films can be used in fuel cells and batteries as well.
“This study is an example of using nanostructuring of materials – changing things like defect density and tube length at very small scales – to shift trade-offs between materials properties and get more performance out of a given material,” Trancik says. “Making inexpensive materials behave in advanced ways is critical for achieving low-carbon emissions and low cost energy technologies.”
The impurification of materials is not a new idea: in semiconductors, for example, impurification has been for decades used to create them. Without it, we wouldn’t have laptops, radios, phones, not even electronic hand watches.
So “Oldies but goldies” applies in electronics, too…