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US team resolves hysteresis problem in perovskite solar cells

Fullerenes double efficiency by filling up electron "˜traps'

Engineers at the University of Nebraska-Lincoln (UNL) have doubled the efficiency of CH3NH3PbI3 perovskite solar cells by unraveling the origins of a hysteresis problem that hinders performance, stability and evaluation. Their findings were published in the December 15 edition of Nature Communications.

Perovskite has gained significant interest among solar cell manufacturers and researchers. However, a certain class of perovskite-based cell, (organometal trihalide perovskites) exhibits photocurrent hysteresis - undesirable shifts in electrical conductivity that arise when applying increasing or decreasing amounts of voltage to a cell for the purpose of measuring its photocurrent.

Led by Jinsong Huang, associate professor of mechanical and materials engineering, the research group determined that this hysteresis results from the presence of 'traps' that can ensnare electrons and consequently reduce electric current. These traps also diminish the lifespan of perovskite-based devices and complicate measurements of how efficiently such devices convert sunlight into electricity.

"It makes measurements of efficiency much more difficult and much less accurate. It can actually lead to false information about how efficient a device is," Huang said. "That matters a lot in the field of photovoltaics, because even a one-percent increase is a big deal."

Huang's team identified the sites of the traps, which reside both at perovskite's surface and along the boundaries of microscopic grains that form its foundation, and then turned their attention to resolving the hysteresis problem.

The answer was more straightforward than expected. When the researchers fabricated and deposited a thin film of fullerenes onto the perovskite, they found that it effectively filled a large number of the traps and subsequently made the solar cell two times more efficient.

"There's been a strong debate in the solar cell community about how to get rid of hysteresis," Huang said. "What makes this very exciting is that we did it just by using molecules - fullerenes - that are already used frequently in organic solar cells.

"And it's a simple method that's really easy for us and anyone else who would want to replicate it."

'Origin and elimination of photocurrent hysteresis by fullerene passivation in  CH3NH3PbI3 planar heterojunction solar cells' by Yuchuan Shao et al, appeared in Nature Communications 5, Article number: 5784  doi:10.1038/ncomms6784

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