Organic solar cells could be made more efficient if the orientation of their constituent molecules gets more attention. Exciton transfer is the key to efficient organic solar cells: A team from North Carolina State University and the University of North Carolina at Chapel Hill has found that the positions of differently shaped donor and acceptor molecules affect the way they transfer excitons to each other. An artist’s rendering of molecules in face-on orientation inside an organic solar cell. Courtesy of Peter Allen, North Carolina State University. Using advanced soft x-ray techniques to study the orientation of molecules within donors and acceptors, and manipulating this orientation in different solar cell polymers, they showed that a face-on alignment between donor and acceptor was much more efficient in generating power than an edge-on alignment. "A face-on orientation is thought to allow favorable interactions for charge transfer and inhibit recombination, or charge loss, in organic solar cells, though precisely what happens on the molecular level is still unclear,” said NCSU physicist Harald Ade. "This research gives us a method for measuring this molecular orientation, and will allow us to find out what the effects of orientation are and how orientation can be fine-tuned or controlled." The research focused on fullerene-based bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT–DTBT, PBnDT–DTBT or PBnDT–TAZ. The work is published in Nature Photonics (doi:10.1038/nphoton.2014.55). For more information, visit: www.ncsu.edu