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Research & Development

Photon recycling promises more perovskite efficiency gains

29 Mar 2016

Finding published in Science suggests simple way to generate 'huge gains' in energy efficiency.

A collaborative effort by scientists at the UK universities of Oxford and Cambridge has discovered a phenomenon that promises to further boost the efficiency of perovskite-based solar cells.

The team, which includes the much-cited Henry Snaith from Oxford’s Clarendon Laboratory and the spin-out company Oxford Photovoltaics, along with Richard Friend from the Cavendish Laboratory in Cambridge, found that their prototype lead halide solar cells “recycled” infrared photons.

While in most solar cells energy is wasted when the charge carriers created by light absorption recombine, in perovskite cells it appears that the recombination process generates a photon that can then be re-absorbed and create even more charge carriers.

“The fact that we were able to show photon recycling happening in our own cell, which had not been optimized to produce energy, is extremely promising,” said Friend. “If we can harness this it would lead to huge gains in terms of energy efficiency.”

“Scientifically remarkable”: Yablonovitch
Commenting on the development, published in the latest issue of the journal Science, photonic crystal expert Eli Yablonovitch from the University of California, Berkeley, wrote that the work shows that perovskite material behaves in a similar way to gallium arsenide (GaAs), and could therefore become a “top-performing” type of cell – and also find wider use in photonics.

“This is scientifically remarkable, because these compounds are the first high-quality halide semiconductors,” noted Yablonovitch. “The materials show promise for photovoltaics, LEDs, laser refrigeration, thermophotonics, and a host of other major optoelectronic applications.”

Discovered less than five years ago, there has been a rush of interest in perovskite photovoltaics, with conversion efficiencies of research-grade rapidly improved to just over 22 per cent – close to the performance of conventional silicon cells but still some way short of compound semiconductors like GaAs and the multi-junction cells in which such materials are used.

Perovskite cells are yet to made at the kind of size that would be useful in energy generation, while other issues such as their incorporation of lead must also be addressed. However, the clay-like raw material on which such cells could be based promises the longed-for combination of high performance and low cost – with the prospect of enhancing conventional silicon cells with a layer of perovskite in tandem designs also becoming a hot topic.

Uncomplicated fabrication
According to the research team, the ability to recycle photons could be exploited with relative ease to create cells capable of pushing the limits of energy efficiency in solar panels.

Felix Deschler, one of the authors of the study from the Cavendish Laboratory in Cambridge, said: “It’s a massive demonstration of the quality of this material and opens the door to maximizing the efficiency of solar cells.

“The fabrication methods that would be required to exploit this phenomenon are not complicated, and that should boost the efficiency of this technology significantly beyond what we have been able to achieve until now.”

In the paper lead author Luis Miguel Pazos Outón explained how the photon recycling phenomenon was revealed. In tests with lasers, the team mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites.

“We observed light emission at distances of more than 50 µm and found that the peak of the internal photon spectrum red-shifts from 765 to [above] 800 nm,” he wrote in the paper’s abstract.

Outón added: “The low-energy component enables charges to be transported over a long distance, but the high-energy component could not exist unless photons were being recycled.”

The latest result comes shortly after the US National Renewable Energy Laboratory confirmed that a perovskite cell made by researchers in Korea had set a new record conversion efficiency of 22.1 per cent. That figure matches the best-performing thin-film cadmium telluride cell made by US PV giant First Solar, and is hot on the heels of the latest 22.3 per cent thin-film CIGS cell from Japan’s Solar Frontier.

Meanwhile development teams at CSEM in Switzerland, the Wuhan National Laboratory for Optoelectronics in China, and IMEC in Belgium are among those working to fabricate larger and more practical cells and panels based on the material.

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