The use of polymer-based photovoltaic cells is on the rise, mostly because their advantages over semiconductor-based cells are becoming increasingly obvious. Their efficiency and low cost of manufacture as a source of renewable energy is of commercial interest and, with a great deal of research going on, accurate accounts of the morphology involved are increasingly necessary.
A new study, by scientists at the University of Delaware and the National Institute of Standards and Technology, is providing this type of understanding. One approach they have taken is to use a light-absorbing polymer in combination with a derivative of a sixty-carbon fullerene molecule, also known as a buckyball. In order to achieve the greatest efficiency, thin layers of each material must be present near opposite electrodes, as most analytical methods are not able to sufficiently distinguish between polymer and the buckyball to characterize the plastic solar cell film.
The Mackay lab at the University of Delaware has been working on polymer thin films, and their focus on polymer photovoltaics has led them to explore neutron scattering as a potential way of characterizing these devices. The morphology of the two components studied, a polymer and a fullerene derivative, is crucial as all charge transfer happens on the nanometer scale; however, characterizing the morphology was not easy as the materials are mostly carbon.
Their study, published in the Journal of Chemical Physics [Kiel et al, J. Chem. Phys. (2010) DOI: 10.1063/1.3471583], investigates a way to analyze the reflection of neutrons to locate the buckyballs within the composite material. Because the polymer and the buckyball are mostly made up of carbon and their positions have to be defined within a few nanometers, the usual techniques have not provided sufficient resolution to describe their location.
However, because neutrons interact with the polymer and the buckyball derivative very differently, a high degree of contrast can be observed between the two components. The researchers showed how neutrons can be used to investigate polymer-based solar cells, and that a properly executed neutron scattering experiment can very accurately define these locations.
Neutron scattering has so far not been widely applied to this class of materials. Brian Kirby, a co-author of the paper, said “With this paper we are providing an instruction book for researchers who want to use neutrons to study polymer photovoltaics.”
It is expected that the study of polymer photovoltaics will progress using new and more efficient materials that require high quality and precise measurements. As new materials become available, their corresponding devices will be optimized much more quickly and efficiently with high quality and precise measurements that neutron scattering can provide.

Laurie Donaldson