Thin-film solar cells are a type of photovoltaic technology that, as you might guess from its name, are much thinner than traditional solar cells, able to be used in a far broader range of applications. The problem with traditional thin-film solar technology is that the components used to make them are both highly toxic and incredibly rare, making them unsuitable for more widespread use. This UNSW team is well on the way to changing that, having developed some of the most efficient, non-toxic thin-film cells to date.
The new cell is known as CZTS, an acronym for the materials it is comprised of — copper, zinc, tin and sulphur. All of these are abundant materials, with none of the toxicity problems of its competitors. The most widely used types of thin-film cells are CdTe (cadmium-telluride) and CIGS (copper-indium-gallium-selenide). From these options, cadmium and selenium are toxic even in very small doses, while tellurium and indium are incredibly rare.
Of course, the more sustainable and less toxic material comes at a price — the CZTE cells aren't nearly as efficient as its more widely used counterparts, which achieve efficiency of over 20%. This is set to change, with a UNSW team led by by Dr Xiaojing Hao of the Australian Centre for Advanced Photovoltaics achieving efficiency of 7.6% in a one square centimetre CZTS cell this month — the highest recorded efficiency in the world for a CZTS cell. They're not stopping there, either.
"This is the first step on CZTS’s road to beyond 20% efficiency, and marks a milestone in its journey from the lab to commercial product,” said Hao, “there is still a lot of work needed to catch up with CdTe and CIGS, in both efficiency and cell size, but we are well on the way."
The applications of this technology are many and varied, especially when thin-film cells can potentially be flexible, able to be layered on top of many different types of surfaces. Other types of thin-film cells are currently used in large-scale solar installations, though the cadmium toxicity of CdTe cells makes them unsuitable for residential installations. This toxicity has also made the construction industry wary of adopting existing thin-film technology — mindful of their history with asbestos. CZTS cells have no such problems.
"In addition to its elements being more commonplace and environmentally benign, we’re interested in these higher bandgap CZTS cells for two reasons," said Professor Martin Green, a mentor of Dr Hao, "they can be deposited directly onto materials as thin layers that are 50 times thinner than a human hair, so there's no need to manufacture silicon 'wafer' cells and interconnect them separately. They also respond better than silicon to blue wavelengths of light, and can be stacked as a thin-film on top of silicon cells to ultimately improve the overall performance.”
Even though the team has not yet achieved 20% efficiency, UNSW is already collaborating with a number of companies, working on applications for these solar cells. The first iterations of this technology will probably see commercial use within the next few years, Hao estimates.