For Highly Efficient Organic PV Cells, Size Matters
RALEIGH, N.C., and BERKELEY, Calif., Jan. 8, 2013 — For highly efficient polymer-based organic photovoltaic cells — which are far less expensive to manufacture than silicon-based solar cells — size matters.
To compete with energy from fossil fuels, photovoltaic (PV) devices must convert sunlight to electricity with a certain measure of efficiency. For polymer-based organic PV cells, scientists have long believed that the key to high efficiencies rests in the purity of the polymer/organic cell’s two domains — acceptor and donor. Now, however, an alternate and possibly easier route forward has been discovered.
Working at Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS), a premier source of x-ray and ultraviolet light beams for research, an international team of scientists, led by physicist Harald Ade of North Carolina State University, found that for highly efficient polymer/organic PV cells, size matters. The team used ALS beams to simultaneously measure, for the first time, the domain size, composition and crystallinity of an organic solar cell.
A molecular view of polymer/fullerene solar film showing an interface between acceptor and donor domains. Red dots are PC71BM molecules, and blue lines represent PTB7 chains. Excitons are shown as yellow dots, purple dots are electrons, and green dots represent holes. Courtesy of Harald Ade, NC State University.
“We’ve shown that impure domains if made sufficiently small can also lead to improved performances in polymer-based organic photovoltaic cells,” said Ade, a longtime user of the ALS. “There seems to be a happy medium, a sweet spot of sorts, between purity and domain size that should be much easier to achieve than ultrahigh purity.”
Solar cell conversion efficiency in these cells hinges on excitons — electron/hole pairs energized by sunlight — getting to the interfaces of the donor and acceptor domains quickly so as to minimize energy lost as heat. Conventional wisdom has held that the greater the purity of the domains, the fewer the impedances and the faster the exciton journey.
Ade and colleagues were the first to measure simultaneously the domain size, composition and crystallinity by using a trifecta of ALS beamlines — 188.8.131.52, a resonant soft x-ray scattering facility; 7.3.3, a small- and wide-angle x-ray scattering end station; and 5.3.2, an end station for scanning transmission x-ray microscopy.
“The combination of these three ALS beamlines enabled us to obtain comprehensive pictures of polymer-based organic photovoltaic film morphology from the nano- to the meso-scales,” said Brian Collins of the university’s department of physics. “Until now, this information has been unattainable.”
The three beams were used to study the polymer/fullerene blend PTB7:PC71BM in thin films made from chlorobenzene solution with and without the addition (3 percent by volume) of the solvent diiodooctane. The films were composed of dropletlike dispersions in which the dominant acceptor domain size without the additive was about 177 nm. The addition of the solvent shrank the acceptor domain size down to about 34 nm while preserving the film’s composition and crystallinity, resulting in an efficiency gain of 42 percent.
“In showing for the first time just how pure and how large the acceptor domains in organic solar devices actually are, as well as what the interface with the donor domain looks like, we’ve demonstrated that the impact of the solvents and additives on device performance can be dramatic and can be systematically studied,” Ade said. “In the future, our technique should help advance the rational design of polymer-based organic photovoltaic films.”
The research, supported primarily by the Department of Energy Office of Science, is detailed in Advanced Energy Materials (doi: 10.1002/aenm.201200377).
For more information, visit: www.ncsu.edu or www.lbl.gov
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