Photovoltaic researchers at UNSW demonstrate best-ever results for emerging solar cell material antimony chalcogenide.
UNSW engineers have made a major step forward in the development of a new type of solar cell that could help make future solar panels cheaper, more efficient and more durable.
The research team has improved the performance of solar cells made from antimony chalcogenide, which is an emerging photovoltaic material regarded as a strong candidate for next-generation solar technology.
Their work, published in Nature Energy, has resulted in a certified efficiency of 10.7% — the highest independently verified performance for this material anywhere in the world to date.
This result earned antimony chalcogenide its first ever inclusion in the international Solar Cell Efficiency Tables which track record-setting results worldwide.
And just as importantly, the team say they have discovered the fundamental chemical mechanism underlying the hydrothermal deposition process. It explains why earlier versions of the material underperformed.
And that knowledge could accelerate the development of antimony chalcogenide even faster and further moving forward.
Professor Xiaojing Hao, from UNSW's School of Photovoltaic and Renewable Energy Engineering, led the research and says: "The next generation of technology for solar panels is tandem cells, which is where two or more solar cells are stacked on top of each other.
"Each layer absorbs different parts of the sunlight to make more electricity. What researchers around the world are trying to work out is what material is best to use as the top cell, in partnership with a traditional silicon cell.
"Each material has its own pros and cons, and I don't think there is an ideal top cell candidate yet. We need more top cell candidates that can partner with silicon cell. Antimony chalcogenide is one of those and very positive, especially given its distinct properties."
Antimony chalcogenide has several advantages that make it attractive for use as that top solar cell.
Firstly, it is made from abundant elements that cost relatively little to produce, unlike some high-performance solar materials that rely on scarce or expensive materials.
Secondly, it is inorganic, which means it is inherently more stable than some newer solar materials that can degrade over time.
Thirdly, its high light absorption coefficient means a layer only 300 nanometres thick — about one-thousandth the thickness of a human hair — is enough to harvest sunlight efficiently.
Another benefit is the fact the material can be deposited at low temperatures, reducing energy usage during manufacturing and opening the door to large-scale, low-cost production.
Energy barrier
Despite these advantages, the efficiency of antimony chalcogenide had not progressed beyond 10% since 2020 as developments frustratingly stalled.
But during their latest research, the UNSW team found the major problem was being caused by the elements that make up the material — sulfur and selenium — not being distributing evenly as it was being produced.
This uneven distribution created a so-called 'energy barrier' which was making it harder for the electrical charge generated by the sunlight to move through the solar cell.
Dr Chen Qian, the first author of the paper, said: "It was like driving a car up a steep slope. If you do that, you need to use more fuel to get to the end, whereas if the road is flat it's more efficient to reach there.
"When the distribution of the elements inside the cell is more even, then the charge can move more easily through the absorber rather than being trapped before they are collected, which means more sunlight is converted into electricity."
The solution to the problem was shown to be the addition of a small amount of sodium sulfide during the manufacturing process which stabilises the chemical reactions that form the solar-absorbing layer.
The improved antimony chalcogenide solar cells reached a power conversion efficiency of 11.02% in UNSW laboratory, with an independently certified value of 10.7% by CSIRO, one of nine internationally recognised independent photovoltaic measurement centres.
The UNSW team, which also includes Dr Jialiang Huang, acknowledge that further work is required to reduce defects inside the material. They are confident that can be achieved through chemical treatments known as passivation.
Besides tandem solar panels, more application scenarios are available for antimony chalcogenide. Its ultrathin, semi-transparent and high bifaciality of 0.86 make it suitable for futuristic see-through solar windows. And a spinout company, Sydney Solar, is upscaling solar "stickers" on window.
In addition, the material's bandgap matches well with the indoor light spectrum, making it a strong candidate for indoor solar applications, such as smart badges, e-paper displays, self-powered sensors and internet-connected devices, where safety, stability and performance under low light is more important than maximum efficiency.
"In the next few years we will continue to work on reducing the defects in this material via that passivation process," said Dr Qian.
"We believe an achievable aim is to increase the efficiency up to 12% in the near future by addressing the challenges that still remain, one step at a time."
