22 May 2012
In their quest for more efficient photovoltaic materials, researchers at the US Department of Energy’s Brookhaven National Laboratory (BNL) in Upton, New York (US) explored atomic-level details that may be key to one organic material’s superior ability to convert sunlight into electricity. The material’s double-layer carbon backbone may allow charges to move more efficiently, and offer clues to the design of a new generation of solar cells.
Past studies of this material, PCDTBT, have not revealed the structure at this level of detail. Benjamin Ocko, a researcher at BNL’s Condensed Matter Physics and Materials Science Department, is glad he and his team took a closer look. “When PCDTBT, an electron donor material, is mixed with a buckyball (fullerene) derivative containing electron acceptor molecule, this blend is one of the better performing organic photovoltaic materials,” he says, able to convert sunlight to electricity with an efficiency as high as 7.2%.
As stated by BNL, conjugated polymers like PCDTBT are made up of a conjugated backbone that absorbs light and generates electrons and alkyl chains that dangle off the conjugated backbone that allow the polymer to be dissolved in a solvent. The ability to dissolve conjugated polymers in a solvent means that they can be printed or painted onto a substrate and lowers the cost of manufacturing significantly. However, this ease of processing comes at a cost to performance because alkyl chains are insulators. Most common conjugated polymers have a large number of side-chains, which results in a layered structure of alternating alkyl and conjugated layers.
The difference between PCDTBT and most conjugated polymers, Ocko claims, is in the alkyl chains, which are “attached at distant points along the carbon polymer backbone.” Ocko goes on to say, “The volume ratio of the conjugated backbone to the alkyl chains is relatively large in PCDTBT compared with other conjugated polymers, and it is the conjugated backbone that provides the excellent electrical performance.” What’s unique about PCDTBT is that the backbones can rotate around their long axes in such a way that the alkyl part comes off on either side, giving “rise to the unusual bilayer backbone structure deduced from our x-ray diffraction patterns,” Ocko says.
As the researcher explains, this is the first known conjugated polymer with such high efficiency that forms a bilayer structure rather than the more common monolayer structure. The outstanding question now is, why the bilayer structure makes PCDTBT one of the more efficient converters of sunlight to electricity. “Our hypothesis is that the relatively low abundance of alkyl groups enables the close pairing of the conducting backbones of the polymer, and that this minimizes the insulating effects of the alkyl groups,” Ocko says, adding that further work is needed to prove his hypothesis. In the next phase of this research work, he and his team “would like to look at simple systems where you could go between the bilayer and the monolayer by varying the length and position of the alkyl chains and see how it affects the performance.”
Written by Sandra Henderson, Research Editor, Solar Novus Today