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Solar Power

Organic solar cell smashes performance record

With help from computer modeling, researchers designed and built an organic photovoltaic device nearly as efficient as commercial silicon cells

by Katherine Bourzac, special to C&EN
August 19, 2018 | A version of this story appeared in Volume 96, Issue 33


Structures of O6T-4F and F-M.
These two electron-accepting materials helped an organic solar cell set a new efficiency record.

A new solar cell made from organic materials converts 17.3% of the energy in sunlight into electricity, far surpassing the previous record of 14% for organic cells. The device’s developers say the potential of organic solar cells has been underestimated and that these cells might one day outperform conventional silicon ones—if the researchers can solve problems with long-term stability (Science 2018, DOI: 10.1126/science.aat2612).

Organic solar cells offer advantages over silicon devices. They’re lightweight and flexible and, similar to newsprint, can be printed over large areas. But solar cell researchers have long assumed that the performance of organic cells would be limited, and experts have debated whether the devices could ever break past 15% efficiency, says Yongsheng Chen, a chemist at Nankai University.

Researchers developing new organic solar cells often rely on computer models to point them to the best materials to use. Either the models predict device performance from fundamental physics and materials properties, Chen says, or they extrapolate performance based on experimental data. Chen saw the potential to gain new insights by making a model that does a bit of each, using what he calls a semiempirical approach.

Chen’s group used its models to explore materials for a multilayer, or tandem, organic solar cell. Organic solar cells use a pair of organic molecules, one that absorbs light and then jettisons an electron and another that grabs that electron. In tandem cells, each layer uses a different combination of organic materials to soak up different bandwidths of sunlight, in theory maximizing overall performance. But past tandem organic cells haven’t outperformed single-layer ones because the choice of materials for the different layers has been limited. One major weakness, Chen says, has been in developing materials that can take advantage of ample solar energy in the near-infrared range.

Chen’s model suggested a new electron-grabbing material called O6T-4F, which works better at infrared energies. The researchers then used their model to pair a layer containing that material with one that had well matched electrical properties and could absorb visible light. That second layer used a relatively new electron acceptor, called F-M for short, which Chen and colleagues previously developed.

The resulting cell has a maximum power conversion efficiency of 17.3%, nearly in range of commercial silicon solar cells, which have efficiencies between 18 and 22%. Christoph Brabec, a materials scientist at Friedrich-Alexander University, says “this record will hold for years because it’s amazingly high.”

However, he says tackling greater power conversion efficiency is not the main challenge facing those working on organic photovoltaics. Brabec was an executive at the now-folded organic solar company Konarka Technologies and has dealt firsthand with commercializing this technology. These kinds of organic materials are expensive to make on a larger scale, and they are not stable enough, he says. “Efficiency is not the bottleneck; it’s the overall cost and lifetime,” he says.



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