Organic Solar Cells One Step Closer To Success
June 15, 2012

Organic Solar Cells One Step Closer To Success

Brett Smith for

Organic solar cells have long been a promising alternative to conventional solar cells, but their low efficiency, low stability, and low strength have kept them from widespread commercial use.

Senior chemist Lin Chen of the U.S. Department of Energy´s (DOE) Argonne National Laboratory hopes to change that by focusing on a key component of the cells, the exciton.

According to Chen, excitons are considered a sort of “quasiparticle,” because they exhibit certain exceptional behaviors. When the two charged areas of the exciton — the electron and a region known as the “hole” — are bound together, they are difficult to pry apart. However, if the charges are separated, it allows for the possibility of generating current and electricity.

“The closer the hole and the electron regions are inside an exciton, the more likely they are to recombine without generating electricity,” Chen said. “But if they are already ℠pre-separated,´ or polarized, the more likely they are to escape from this potential trap and become effective charge carriers.”

In the laboratory´s report published in the Journal of the American Chemical Society, the research team described how four different molecules in the polymer layer in the middle of a solar cell generated different exciton dynamics. They also used a transparent silica coating to achieve a 36 percent increase in relative efficiency versus uncoated cells. The coating allowed for more heavily polarized excitons, which yielded more efficient polymer-based solar cells.

“If the conventional exciton, right after it is generated, contains the hole and electron in almost the same location, these new materials are generating an exciton that is much more polarized at the beginning,” Chen said.

Chen's work on organic solar cells represents one of several efforts to better understand and improve the efficiency of solar energy at the laboratory. This research is part of the Argonne-Northwestern Solar Energy Research Center (ANSER), a collaborative endeavor between Argonne and Northwestern University. ANSER is one of 46 Energy Frontier Research Centers established in 2009 by DOE´s Office of Science to advance basic research on energy.

Despite the ongoing research at ANSER, organic solar cells still have a large gap between their efficiency and that of their inorganic, silicon-based competitors. However, their development is very attractive from a cost perspective because most of the materials used in their production are relatively cheap.

The progress made by Chen and the Argonne research team comes as a new report published this week said the American solar industry, powered by conventional solar cells, had its second-best quarter ever in terms of installations.

During the first quarter of 2012, despite a global oversupply and a possible trade war with China, the U.S. solar industry had 506 megawatts worth of installations, enough to power around 350,000 homes. This number was beaten only by the fourth quarter of last year, which saw a 708 megawatts worth of solar units installed. The report also predicts that installed U.S. solar power will increase 75 percent this year from 4.4 gigawatts that are currently installed in the country to around 7.7 gigawatts.