In the world of solar cells, those fabricated from plastic are regarded as an attractive alternative to their more costly cousins made from inorganic materials. But solar cells with one set of strengths usually are saddled with a weakness as well, and the Achilles’ heel of plastic cells is a big one. When converting sunlight into usable electricity, their performance is rather paltry by comparison.A processing additive provided a degree of control over the morphology of bulk heterojunction materials. Reprinted with permission of the Journal of the American Chemical Society. A team of researchers from the University of California, Santa Barbara, and from Konarka Technologies Austria in Linz reports that it has improved this critical area of performance by chemically controlling the morphology of the bulk heterojunction materials that form the building blocks of plastic solar cells.Principal investigator Alan J. Heeger of the university, a co-recipient of the Nobel Prize in Chemistry in 2000 for research on conducting polymers, said that the semiconducting polymers and fullerenes that are used to fabricate plastic solar cells are promising. However, manipulating the nanoscale morphology of these materials to enhance their performance has not been well understood.The investigators found that the use of processing additives gave them a degree of control over the morphology of the bulk heterojunction materials. Two criteria were identified: selective (differential) solubility of the fullerene component and a higher boiling point than the host solvent. Using these criteria, the 1,8-di(R)octanes with various functional groups (R) were investigated as processing additives for bulk heterojunction solar cells. The best results were obtained with R = iodine (i); the conversion efficiency of the plastic solar cells increased from 3.4 to 5.1 percent.Details of the research appear in the March 19 issue of the Journal of the American Chemical Society.