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New path to more efficient organic solar cells is uncovered

09 January 2013

For polymer-based organic photovoltaic 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.

A molecular view of the polymer/fullerene solar film showing an interface between acceptor and donor domains. Red dots are PC71BM molecules and blue lines represent PTB7 chains

Now, however, an alternate and possibly easier route forward has been shown. Working at Berkeley Lab's Advanced Light Source (ALS) (a source of X-ray and ultraviolet light beams for research) an international team of scientists found that for highly efficient polymer/organic photovoltaic cells, size does indeed matter.

"We've shown that impure domains if made sufficiently small can also lead to improved performances in polymer-based organic photovoltaic cells," says Harald Ade, a physicist at North Carolina State University, who led this research. "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 ultra-high purity." Ade is author of a paper describing this work in the journal, Advanced Energy Materials.

Solar cell conversion efficiency in polymer/organic photovoltaic cells hinges on 'excitons' (electron/hole pairs energised by sunlight) getting to the interfaces of the donor and acceptor domains quickly so as to minimise energy lost as heat. Conventional wisdom held that the greater the purity of the domains, the fewer the impedances and the faster the exciton journey.

Ade and his co-authors became the first to simultaneously measure the domain size, composition and crystallinity of an organic solar cell. This feat was made possible by a combination of three ALS beamlines, which enabled the researchers to obtain comprehensive pictures of polymer-based organic photovoltaic film morphology from the nano- to the meso-scales.

The international team used the trifecta of ALS beams to study the polymer/fullerence blend PTB7:PC71BM in thin films made from chlorobenzene solution with and without the addition (three-percent by volume) of the solvent diiodooctane. The films were composed of droplet-like dispersions in which the dominant acceptor domain size without the additive was about 177 nanometres. The addition of the solvent shrank the acceptor domain size down to about 34 nanometres while preserving the film's composition and crystallinity. This resulted 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 solvents and additives on device performance can be dramatic and can be systematically studied," Ade says. "In the future, our technique should help advance the rational design of polymer-based organic photovoltaic films."

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