Oxygen molecule has been used by scientists to upconvert low energy light to high energy above the silicon band gap.

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Scientists use oxygen to improve energy transmission in Solar Cells

Oxygen molecule has been used by scientists to upconvert low energy light to high energy above the silicon band gap.
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A team of scientists from Australia and the United States has upconverted low energy light into high energy with the help of oxygen molecule. This upconversion has wider application ramifications from biological imaging and drug delivery to photovoltaics and photocatalysis.

The research was done by a team of scientists from ARC Center for Excellence in Exciton Science, UNSW Sydney, RMIT University and Kentucky University. According to senior author of the study, Professor Tim Schmidt from the ARC Center of Excellence in Exciton Science and UNSW Sydney, the research is still nascent, and a lot needs to be done, but still, its implications are far-reaching.

solar cells, CCD, photovoltaics,

Lead author Elham Gholizadeh from UNSW Sydney

"The energy from the sun is not just visible light," Prof. Schmidt explains.

"The spectrum is broad, including infrared light which gives us heat and ultraviolet which can burn our skin.

"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."

"One way of doing this is to capture multiple smaller energy photons of light and glue them together," Prof. Schmidt says.

"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."

Upconverting light can potentially increase the range of sensitivity of solar cells. Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, which can excite silicon.

Until now, this had never been achieved beyond the silicon bandgap. This is the minimum energy required to make an electron in silicon participate in conduction.

However, the research team resolved this challenge by using oxygen. 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.

Oxygen is known to hinder molecular excitons, but at low energy, it changes roles and becomes a facilitator of energy transfer and allows the organic molecules to transmit visible light above the silicon bandgap.

Co-author Professor Jared Cole of RMIT University says: "What's interesting is that often without oxygen, lots of things work well. And as soon as you allow oxygen in, they stop working.

"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 oxygen is able to achieve manageable efficiencies but the scientists hope to improve on that in the future."This is only an early demonstration, and there's quite a lot of materials development needed to make commercial solar cells, but this shows us it's possible," Prof. Schmidt said.

Lead author Elham Gholizadeh, from UNSW Sydney, is optimistic about the research impact.

"As this is the first time we've been successful with this method, we will face some challenges," she says.

"But I'm very hopeful and think that we can improve efficiency quickly. I think it's quite exciting for everyone. It's a good method to use oxygen to transfer energy.

"Violanthrone doesn't have the perfect photoluminescence quantum yield so the next step will be to look for an even better molecule."

The results are published in Nature Photonics.

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