Light 'upconverted' for solar cell capture

Australian and US scientists managed to ‘upconvert’ low-energy light into high-energy light that can be captured by solar cells — with oxygen the surprise secret ingredient. Their breakthrough has been published in the journal Nature Photonics, and could have potential future implications for solar photovoltaics, biomedical imaging, drug delivery and photocatalysis.

As explained by senior author Professor Tim Schmidt, from UNSW Science, the energy from the sun is not just visible light.

“The spectrum is broad, including infrared light which gives us heat and ultraviolet which can burn our skin,” Prof Schmidt said. “Most solar cells, charge-coupled device (CCD) cameras and photodiodes (a semiconductor that converts light into electrical current) are made from silicon, which cannot respond to light less energetic than the near infrared. This means that some parts of the light spectrum are going unused by many of our current devices and technologies.”

To extend the range of sensitivity of these devices, and potentially increase the efficiency of solar cells, one strategy is to ‘upconvert’ light, turning low-energy light into more energetic, visible light that can excite silicon.

“One way of doing this is to capture multiple smaller energy photons of light and glue them together,” Prof Schmidt said. “This can be done by interacting the excitons — bound states of electrons and electron holes that can transport energy without transporting net electric charge — in organic molecules.”

Until now, this had never been achieved beyond the silicon band gap, which is the minimum energy that is required to excite an electron in silicon up to a state where it can participate in conduction. Researchers from the ARC Centre of Excellence in Exciton Science researchers, based at UNSW Sydney, transformed oxygen into an unlikely ally to achieve their goal.

Together with collaborators at RMIT University and the University of Kentucky, the researchers used semiconductor quantum dots (nanoscale man-made crystals) to absorb the low-energy light, and molecular oxygen to transfer light to organic molecules. Usually oxygen is detrimental to molecular excitons, but at such low energies its role changes and it can mediate energy transfer, allowing the organic molecules to emit visible light above the silicon band gap.

“What’s interesting is that often without oxygen, lots of things work well. And as soon as you allow oxygen in, they stop working,” said contributing author Professor Jared Cole from RMIT University. “It was the Achilles heel that ruined all our plans but now, not only have we found a way around it, suddenly it helps us.”

The efficiencies are still very low, but the scientists have strategies to improve this in the near future. Lead author Elham Gholizadeh, from UNSW Sydney, is particularly optimistic about the potential of the work to make a rapid positive impact on the research field.

“As this is the first time we’ve been successful with this method, we will face some challenges,” she said. “But I’m very hopeful and think that we can improve the efficiency quickly. I think it’s quite exciting for everyone.”

Image caption: The Molecular Photonics Laboratories at UNSW. Image credit: UNSW Sydney/Exciton Science.

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