29 April 2012
Solar researchers have long hypothesized that the alignment at the interface between an electron donor and electron acceptor molecule holds the key to more efficient organic solar cells. However, until now, no one has been able to observe what happens at the molecular level. In what could be a critical advancement for the solar industry, for the first time ever, North Carolina State University (NCSU) researchers were able to do just that with an innovative X-ray technique they discovered.
The novel method uses X-rays that are resonant with atomic bonds in the molecules used in organic electronics. “In this way, the technique is individually sensitive to each molecular species and can probe how the materials organize themselves,” says Brian A. Collins, a postdoctoral associate at NCSU’s Department of Physics. “The molecular bonds are also highly directional and only resonate if the polarized X-rays are aligned with them.” This phenomenon has been used in microscopy, but with much less precise results. The new scattering technique achieves “high resolution and full morphological statistics on an entire device in one shot with high sensitivity,” Dr. Collin says.
During this project, which was done in collaboration with the University of California Santa Barbara (UCSB), Dr. Collins and his fellow researchers made another exciting discovery: Their new X-ray technique works well regardless of material crystallinity. “Until now, most of the investigations in the field of organic electronics have centered on crystals in the material using X-ray or electron diffraction techniques, but many of these materials are not highly crystalline,” the NCSU physicist explains.
The X-ray technique could deliver major new insights into how organic polymers can be used in organic solar cells; specifically, how molecular alignment fundamentally affects light harvesting. “Solar researchers have been struggling to fully understand the nature of this interaction, but know well that it is highly directional,” Dr. Collins says. “Importantly, this alignment is occurring in primarily non-crystalline molecules and therefore cannot be measured with traditional techniques.” With this new knowledge, engineers could make more informed decisions considering appropriate materials and processing techniques. “Manipulating this alignment may be a key step in improving device efficiency, and the X-ray technique we’ve developed is currently the only way to measure this alignment,” the NCSU scientist explains.
Dr. Collins sees his research success as a starting point. “We report the discovery of such an ordering of the materials. We don’t yet know the impact this ordering will have in solar harvesting,” he says. “The next steps will be to manipulate the alignment and investigate how that affects the generation of charges.” The researcher holds high hopes for the future of this new X-ray technique. “It would be wonderful to see it answer some important questions on how these devices really work. And after that, I would hope it could be helpful in identifying synthesis routes for new polymers or new processing steps for more efficient devices.”
Written by Sandra Henderson, Research Editor, Solar Novus Today
Image: Rendering by Peter Allen, UCSB, showing an X-ray beam penetrating the polymer sample and the scattered light hitting a detector in the background.