Let there be light! And the research to harness it

Solar science is still far from peaking. Rather, in striving for energy security on our climate-challenged planet, many research organisations are evolving our understanding of light and its potential to interact with materials. Similar to the way in which research for space travel has brought us the improved satellite navigation system known as GPS and the cordless vacuum cleaner, research on how to best convert solar energy into electricity is spinning off some unexpected technologies.

pv magazine recently spoke with some of the researchers at the Australian Research Council Centre of Excellence in Exciton Science, to try to extend our gaze beyond the now venerable and entrenched silicon solar cell. The Centre’s work with excitons, which form when light is absorbed by a material, resulting in free charges — or the clean electric power we all crave — is forcing new thinking … about materials, about excitability, about the absorption and reflection of light throughout the spectrum.

How could we harness the inherent instability of perovskites?

Professor Jacek Jasieniak, Professor of Materials Science and Engineering at Monash University and a principal researcher with the Exciton Science ARC, has for the past four years been working with perovskites as a material class that shows promise for the next wave of solar generation. If you shine a light on a perovskite, he says, “that light will be absorbed, and in the absorption process, an electron is promoted into an excited state; at that point the electron can be collected in a solar cell, or the charges it generates can be recombined”.

Jasieniak and his team worked with that property — the potential for recombination — and the fact that halide substances are present in some chemical warfare agents and pesticides, and in perovskite crystals, to develop a light-emitting detection system for such toxins. 

“Quite simply,” says Jasieniak, “if we have a perovskite with a given halide, and we expose it to whatever analite we’re looking to detect, provided there’s a difference in that halide between the two species we’re going to see a big change in the colour that’s emitted.” 

The process, which offers sensitivity to these toxins that is two orders of magnitude greater than current capability (potentially below 10 parts per billion sensitivity compared to the current 1000+ parts per billion detection capability) has just been patented. The Exciton Science ARC is now working with the Department of Defence on the prototype for a portable detection system, likely deploying single-use cartridges based on perovskite crystals, for use in the field where soldiers’ lives depend on rapid detection of such deadly agents. 

The system also has applications in checking produce imported into Australia for potentially harmful pesticides. The use of chemicals such as methyl bromide and methyl iodide on crops such as strawberries and garlic is banned in some countries, but in others it isn’t. The ARC’s light-emitting alarm system lets Australian border forces screen for their presence in imports in an easy, mobile way.

Jasieniak muses over the need to see the qualities of materials through different lenses. “Perovskites are super materials for solar cells, and although they have inherently high efficiency and at the moment probably lack the stability that’s necessary to compete directly with silicon; it’s actually that instability that gives you the real sensitivity applicable to other technologies.”

Overt and machine-readable infrared-active nanomaterials will secure confidence in Australian currency

Australia has some of the most beautiful and secure — as in difficult to counterfeit — currencies in the world, but would-be forgers are constantly catching up given improved access to low-cost technologies that can produce excellent mimics of our polymer banknotes.

Once again, a kind of reverse thinking in the realm of light-based reactions is driving the development of anti-counterfeiting technology at the Exciton Science ARC. Centre Director, Paul Mulvaney says, “Unlike solar energy, where we want to make everything highly scalable and reproducible, the Reserve Bank has the exact opposite requirement: it wants materials that are exciting and new, but completely unable to be reproduced by others — specifically counterfeiters.”

Without giving too much away, the ARC will run two trials this year incorporating infrared-active nanomaterials in inks used to print Australian currency. “The overarching goal,” says the CRC Centre of Excellence in Exciton Science Annual Report 2020  is to produce banknote security features that are difficult to counterfeit and simple to verify.” The security technology must be cost-effective, durable, printable and safe to handle.

Efficiency boosting overlays achieve “upconversion” of light for absorption by silicon solar cells

While this ARC Centre of Excellence works to develop the next generation of manufacture-ready solar-to-energy conversion materials — might be perovskites, might be organic solar cells, might be something as yet unidentified — some emerging technologies could be retrofitted to boost the performance of solar cells now in production.

Although silicon absorbs pretty much all the visible part of the light spectrum, “about 40% of the solar energy that hits your roof goes straight through the silicon solar cell,” says Mulvaney. A number of ARC Exciton Science groups around the country are working on materials that effectively harvest the part of that unused bounty known as infrared energy or heat. 

Retrofitting such materials to silicon solar cells “could make them much more efficient”, says Mulvaney.

In 2019 one international multi-university ARC research team published a paper in Nature Photonic, examining the key factors in upconversion performance.

The strategy of this group was to ‘upconvert light’, turning low energy light into more energetic, visible light which can excite silicon.

It concluded that there were still issues to be resolved, but that the novel hybrid nanosystem design offers “exciting opportunities for applications such as solar photovoltaic devices, deep-tissue biomedical imaging, optogenetics and nanomedicine among others.” 

Again, the knock-on applications of this technology go beyond energy production, but for solar upconversion it has been proven in principle, says Mulvaney, and he hopes to partner with a solar cell manufacturer on developing a prototype and perhaps a test bed. 

“A lot of people see solar as kind of ‘done’”, says Mulvaney. “We think that as the government hopefully, eventually, swings behind renewables in a stronger way that we’ll see a rollout of all sorts of different applications of photovoltaics.”