Much research activity is presently devoted to organic photovoltaic devices (OPV), in particular ones comprising polymers as donors and a variety of C60 fullerenes with organic molecules attached as acceptors. Now, a group of scientists collaborating from several research institutions, namely the Georgetown University, Washington DC, Luna Innovations Inc., Virginia, the Friedrich-Alexander-Universität, Erlangen, Germany, the National Renewable Energy Laboratory, Colorado, and the University of Santa Barbara have developed a novel fullerene species for this application [Ross, et al., Nature Materials (2009), doi:10.1038/NMAT2379]
“We believe that our discovery is a significant contribution to the improvement in conversion efficiencies of organic solar cells,” says Martin Drees, corresponding author. In contrast to the acceptor materials utilized to date, Drees and his colleagues used fullerenes large enough to incarcerate trimetallic nitrides (therefore called trimetallic nitride endohedral fullerenes, or TNEFs) and filled them with Lu3N. The main advantage over the presently used empty C60 molecules and their derivatives is the higher open circuit voltage. Drees and his group found values of about 890 mV (in comparison to 630 mV for present state-of-the-art C60 devices), in fact the highest reported for any fullerene OPV. The reason for the low voltage output of the C60 devices is the orbital mismatch of the donor polymer and the fullerene acceptors, a situation which the researchers could significantly improve by incorporating Lu3N-ions in the bigger fullerenes.
Since the solubility of pristine TNEFs is not nearly as high as solution processing of OPV devices would require, Drees and his group created several much more soluble variations by exohedral functionalization, i.e., attaching organic molecules to the outside, as has been done previously with the smaller molecules. “The processability of our materials is similar to the current C60 acceptor,” explains Drees. “Combined with the improved open circuit voltage that our materials offer they are an ideal replacement since they can be easily integrated into already existing manufacturing processes.”
This new material looks promising, since neither the material itself nor its production are more intricate than current state-of-the-art acceptor materials. So far, these compounds were studied only in combination with a well-tried, widely used polymer. In order to obtain higher efficiencies, Drees and his group are going to investigate combinations of TNEFs and new polymers with broader absorption spectra. “Our modeling shows that efficiencies of more than ten percent should be achievable with some already published donor polymer systems,” hopes Drees.