As the world seeks to decarbonise rapidly to counter the worst effects of climate disruption, the search for better materials to harness electricity from the sun is being scaled up. One stand-out material has shown remarkable promise, yet the reasons for its efficiency have proved to be elusive until recently.
The qualities of perovskites, which in various combinations make up a high-performance solar cell material that turns more of the sun’s rays into electricity, have been demonstrated repeatedly.
But despite a decade of laboratory breakthroughs, it wasn't clear why the material performed as well as it did until a team at the University of Cambridge, led by young Irish nanoscientist Kyle Frohna, provided answers.
He was lead author on a paper recently published in Nature Nanotechnology on their properties. It prompted a flurry of coverage throughout the world because of its obvious implications for an imminent energy transition where renewable energy replaces fossil fuels.
Frohna hails from Clontarf in Dublin, attended St Andrew's College in Booterstown, graduated in nanoscience from Trinity College Dublin and is currently doing his PhD at Trinity College Cambridge.
Their discovery explains a long-standing paradox by uncovering the structure of this “magical” material, with potential to radically transform everything from solar energy harvesting to communication speeds. A 2017 study suggested it could boost computing and internet speeds by up to 1,000 times.
The mineral made of lead salts is regarded as key to creating low-cost, durable, strong solar cells, as it allows the technology to be printed and to last up to 25 years. It has excellent light absorption and “charge-carrier mobilities”, which determine how quickly it can be switched on and off.
Frohna with colleagues found two forms of disorder or structures happening in parallel within perovskites; an electronic disorder and a spatial-chemical disorder.
‘Good’ disorder
The former reduces performance of the solar panel and then the latter improves it, Frohna says. The “good” disorder mitigates the “bad” disorder by funnelling the charge carriers away, in the way a ball rolls down a slope, so negatively charged electrons, which absorb light strongly, move very fast and generate electricity before relaxing. With bad disorder, electrons lose energy in the form of heat.
The material’s variegated chemical structure actually guards against the electronic pitfalls associated with similar materials – and could help point the way to improving both it and other materials being developed for green technologies. All this emerged from surveying its chemical, structural and electronic disorder by using different microscopes.
The observations are set to accelerate perovskite’s development, allowing tweaks to its composition while integrating the material into commercial solar arrays – and are predicted to assist energy research using other materials.
It is now possible to explain why perovskite is very good, he explains. To exceed performance standards, solar cell materials typically need to be highly uniform. Crystalline silicon, the material currently used in most commercial solar arrays, is almost perfectly homogenous.
On the other hand, perovskite’s ability to absorb a wide spectrum of solar wavelengths and turn them into electricity had previously confounded scientists, primarily because it is not homogenous. The chemical composition of perovskite is more complicated compared to silicon.
“There’s a lot more stuff that goes into them,” Frohna says. “We effectively dissolve all these chemicals into a solvent and then print them out onto a substrate, so you end up with a very complicated landscape.”
Significantly, this relatively imprecise production process is much more energy efficient, cheaper and easier to scale, which is why scientists expect perovskite to eventually supplant crystalline silicon in solar cell devices.
Some companies are at an advanced stage of manufacturing; his team enable realtime feedback on performance.
Record efficiency
Scientists from Technical University Berlin have achieved a new world record in solar cell efficiency with perovskite, beating the silicon record of 26.7 per cent with an efficiency of 29.8 per cent. There is strong indication this will improve even further.
Solar cell materials work best when their electrons can move freely, without becoming trapped by material defects.
“When there are impurities or defects in the material, they can trap these electrons and cause them to stay there for too long and lose their energy, generating heat instead of electricity,” Frohna says.
To learn why perovskite’s incompatibilities and resulting electron traps don’t seem to be problematic, they used the microscopes to map more precisely the material’s structural, chemical and electronic properties.
"First, we used an optical microscope to measure the material's optical quality, or electronic efficiency," co-worker Miguel Anaya, a physicist and research fellow at Cambridge, explains. "For a high-quality, efficient material, we expect to see close to one photon emitted for every photon absorbed."
Then they used the Diamond Light Source synchrotron facility located nearby to look at the material’s chemical composition and structure. It works like a giant microscope, harnessing the power of electrons to produce bright light – which is made up of X-rays – that scientists can use to study anything from fossils to jet engines to viruses and vaccines.
The Cambridge team found perovskite’s chemical disorder creates structural gradients that cause electrons to gravitate toward the parts of the material that have the fewest electronic defects. Basically, perovskite’s weakness is also a strength.
“Defects are everywhere, but by inducing the electron to find a place in the material that is high quality, electronically speaking, the chemical disorder masks the effects of the electronic disorder,” Anaya adds.
“Rather than fumbling around in the dark, maybe now we can make more rational decisions about how to modulate these heterogeneous landscapes to suit our purposes even better,” Frohna points out.
Computational shortcuts
They plan to repeat their work by using fully assembled solar cells, as opposed to a naked perovskite substrate, and to find ways to identify analytical and computational shortcuts to allow them measure the interplay between electronic and chemical disorder without the painstaking process of acquiring and comparing the results from multiple microscopes.
It may be possible to use nanoscale observations like theirs for algorithms that could predict how materials will perform inside solar cells or batteries.
Prof László Forró of Ecole Polytechnique Fédérale de Lausanne in Switzerland has acknowledged the solar energy-to-electricity conversion of perovskite solar cells "is unbelievably high", but noted an associated environmental issue: "Their central element is lead, which is a poison; if the solar panel fails, it can wash out into the soil, get into the food chain and cause serious diseases."
His research team has found that adding a transparent phosphate salt that does not interfere with light-conversion efficiency prevents lead from seeping into the soil in cases of solar panel failure.
When details were published last July, Forró concluded: “This is an extremely important study – I would say, a central one – for large-scale commercialisation of perovskite-based solar cells.” Frohna says lead capture technology is essential and enables easier recycling.
Last September, the US Department of Energy released a study showing solar energy could provide at least 37 per cent of the country’s electricity by 2035 and 44 per cent by 2050, versus 3 per cent today.
A dramatic ramp-up of solar energy will require new technologies, efficient materials and tools that increase grid flexibility, such as storage and advanced “inverters”, which ramp up to meet demand instead of running full tilt all the time, as well as transmission expansion to tap abundant, lowest-cost resources, it found.
That potential exists in many parts of the world. In Ireland, solar is set to be a much more significant part of the State's energy mix in the coming decade. Perovskite has the ability to be the great facilitator on all fronts. As Frohna sees it, "perovskite is currently the obvious candidate" to accelerate solar scale up.
