Fullerene-free organic photovoltaic (OPV) multilayer stacks developed by the Belgian nanoelectronics research institution Imec have achieved a record power conversion efficiency of 8.4%. The researchers attribute the performance boost to a novel combination of donor and acceptor materials that improves light absorption and a unique architecture that aids charge transfer.
Imec’s thin-film solar cell consists of a three-component organic active layer with one donor compound and two acceptors, sandwiched between commonly used contact and electrode layers. Traditionally, only one acceptor is used in organic solar cells. “The use of three compounds in the active layer, with energy levels that are well-positioned, relative to each other, allow for a cascade effect that ensures that excitons — bound states generated upon excitation by light — can be efficiently separated into free charges,” explains Tom Aernouts, R&D manager for Imec's organic solar cell activities.
Until now, fullerenes were the only known acceptor compounds for use in organic solar cells that showed very fast and efficient charge transfer, according to Imec. But fullerenes have a limited absorption spectrum. “With this novel structure, we open up the library of materials to be used as acceptors,” says Aernouts. Experimenting with many different combinations of donor and acceptor materials allowed the team to improve the absorption of light and thus the organic solar cell’s overall power conversion efficiency.
The record achievement of 8.4% efficiency is breaking new ground in OPV. “It is — to our knowledge — the first fullerene-free organic solar cell reported with such high efficiency,” Aernouts says. Previous fullerene-free cells merely reached efficiencies around 2–3%.
This leap in efficiency could hold promises for the future of OPV. Aernouts believes that a broader range of materials will further improve performance parameters of organic solar cells and could eventually enable power conversion beyond 15%, with this structure. The organic photovoltaics group leader says the innovation that enabled the remarkable performance, the multilayer stacks’ cascade effect, “has been realised by an accurate combination of materials with energy levels for the excited states of the electronic charges well-positioned relative to each other.” Thus, charges can minimise their energy by transferring to the neighbouring material, generating the photocurrent.
After an “intensive materials study,” Aernouts and his colleagues are “excited that we have been able to open up the library of acceptor materials beyond the mostly used compounds like fullerenes,” he says. His team will continue to investigate other materials and material combinations to further optimise performance. They also want to study the multilayer architecture deeper to be able to upscale module processing, he says, adding that initial result indicated “no bottlenecks” thus far. “Therefore, it should find its way easily into mainstream organic solar cell activities,” Aernouts concludes.
The findings are presented the article “8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer,” published in Nature Communications.
Written by Sandra Henderson, Research Editor Solar Novus Today