New titanium solar cell is more powerful and less toxic

10th February 2020

QUT materials scientist Associate Professor Ziqi Sun is part of an international team that has developed a new solar cell that’s more efficient, cheaper and safer to produce than silicon.

Commercial multicrystalline silicon solar cells have an efficiency conversion of 14-19 per cent. However, silicon is expensive, and the refining process is highly toxic. 

“Titanium dioxide (TiO2) is an alternative that’s environmentally friendly and safer to produce,” says Professor Sun. “It’s also abundant in the earth’s crust—there are lots of advantages compared to silicon.” 

Professor Sun and his team are rethinking the structure of quantum dot (QD) photovoltaic solar cells made from TiO2 to maximise energy output. Their prototype has surpassed the efficiency of silicon solar cells. 

What is a quantum dot?

“Quantum dots are a type of solar cell: a solar conversion device” explains Professor Sun. “It’s a small particle, only two nanometres. It captures photons and passes them through a layer of TiO2 nanocrystals, which generates an electrical current.”

Standard QD photovoltaic devices are not as efficient as silicon solar cells, reaching only 8-11 per cent efficiency in the lab. 

“This type of solar cell is not on the market yet because its efficiency is too low. But it would be much cheaper if we can make it work,” says Professor Sun.

A new approach to QD devices

In standard QD photovoltaic devices, photons are trapped by the QDs, stimulating the electrons. These electrons then pass through a layer of TiO2 nanocrystals to create an electrical current. However, only some of the electrons make it through—many become trapped or ‘lost’ in the tiny spaces, or interfaces, between the nanocrystals in a process called interface trapping.

“If we can remove this disordered interface, we can improve efficiency,” says Professor Sun.
“Our nanowire design eliminates the interface inside the TiO2 band, as it’s just a single layer of QD-coated TiO2.

“This means that more of the electrons can contribute to generating a more powerful electric current. We recorded 24 per cent efficiency from the device.”

That 24 per cent efficiency is much closer to the theoretical 33 per cent conversion efficiency of QD sensitised solar cells. The device they’ve devised has almost tripled the energy output from standard QD photovoltaic devices.

Research team member Hui Dong assembled the nanowire crystal in China where he could access an advanced transmission electron microscope that was sensitive enough to test and observe the technology. 

“Hui was a visiting student at my lab here at QUT in 2016, and I mentored him for six months,” says Professor Sun, who is a known expert in nanocrystal technology. “This is collaborative work, and our team has been working on it for more than four years.”

The bigger picture

The research will lead to cleaner, more efficient ways of producing solar power. Professor Sun hopes this will in turn contribute to less reliance on fossil fuels, and cheaper solar power.

“It’s all part of our ongoing research to optimise the harvest and conversion of solar energy,” says Professor Sun. 

“This is a much more effective use for solar cells. This principle we’ve developed can now be expanded on to fabricate more efficient solar panels.” 

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