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 out-perform 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, 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. “Organic solar cells have been studied for many years, but they’re still relatively young compared to silicon,” he says. “We still don’t understand their device performance very well.”
Researchers developing new organic solar cells often rely on computer models to point them to the best materials to build a device from. 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 their 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%. “There’s no reason why an organic solar cell can’t have similar or higher performance to silicon or perovskites,” Chen says. Christoph Brabec, a materials scientist at Friedrich-Alexander University, says getting above 15% efficiency represents a major breakthrough. “This record will hold for years because it’s amazingly high,” he says.
However, he says tackling greater power conversion efficiency is not the main challenge facing those working on organic photovoltaics. Brabec was an executive at now-folded organic solar company Konarka Technologiesand 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.
Chen says his lab’s cells maintained their performance for 165 days. He agrees stability is a hurdle and hopes further modeling studies will help get over it.