Irish scientists shine in solar-cell development

A team of Irish scientists has helped shatter the world record for ultrathin solar cells. The research was led partially by two Irish friends in London, who met while studying chemistry and physics at Trinity College Dublin.

"We lead our own independent groups," explains Prof Aron Walsh at Imperial College London in South Kensington, "though David is over at the bad side of London, in University College London" – teasing his collaborator Prof David Scanlon. Both of them work to improve solar-energy materials.

The most familiar solar cells today are made from silicon. These can be seen on rooftops or powering inexpensive consumer items such as calculators and garden lights. Surprisingly, silicon is a metal that is not ideal as a solar material, and the technology has not changed much in 50 years.

"A big research challenge is in trying to replace silicon, which is a material that doesn't absorb light very strongly," Walsh says. "You need to have a thick film of silicon to absorb enough sunlight to generate a useful electrical current." There are also questions around its environmental credentials, with most silicon manufactured cheaply in China, with subsidies, using energy from coal power plants.

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There are more than 100 elements in the periodic table, and scientists have been hunting for the right combinations of atoms to best absorb sunlight and convert it into electricity. This is where a new study from the Trinity graduates enters the picture. They reported on an environmentally friendly material that achieved record performance with cells that are just 30 nanometers thick.

Harvesting

“That’s about 100 times thinner than the active layer in a typical silicon solar cell,” says Walsh. “One thousand times thinner than a human hair. Yet the material can absorb enough light to convert a good fraction of sunlight into electricity.” This opens myriad new opportunities for harvesting solar energy, such as by incorporating thin coatings into window glass or throughout a building or on to car surfaces or even floors.

The material comprises nanocrystals made from two metal atoms – one silver, one bismuth – and two sulphur atoms. Bismuth is a heavy metal used a few pharmaceuticals, while sulphur is the fifth most abundant element on earth and present in minerals such as pyrite and gypsum. Silver is a precious metal, great at conducting electricity.

The team needed to tap into some of the biggest supercomputers in the world to reveal the right configuration

Combined they form a non-toxic crystalline material, which was used by a group in Barcelona to make thin films of solar cells. They reported efficiencies above 6 per cent – 6 per cent of light converted to electricity – in the journal Nature Photonics in 2016.

But the lab in Spain hit a roadblock. They couldn't improve much beyond this 6 per cent figure. This is where Irish student Seán Kavanagh, also a Trinity chemistry graduate, stepped in. He began training in London for a PhD jointly supervised by Walsh and Scanlon in late 2019.

Filling the gaps

The Barcelona lab could probe the material in lab experiments, but they could not determine where precisely the silver and bismuth atoms were positioned. The Irish researchers put their shoulders to the wheel by filling in the gaps using computer calculations. "We tried to predict the behaviour of the atoms," says Kavanagh. He moved back to Ratoath in Meath in early 2020, where he could log on to some of the world's most power supercomputers from his laptop. "I moved back home for a year and a half, so saved a lot of money on London rent," he says.

The computer models revealed that an even distribution of silver and bismuth in the material was crucial. The name of the game was setting the right conditions to stop the bismuth and silver atoms from clumping. “The highest efficiency was when you had an even distribution of the metals,” says Walsh, “rather than bunched up in different regions of the crystal.” With an even spread, “you can very efficiently get these transfers of electrons from the silver to the bismuth, which is how the material absorbs the light. And then we extract those electrons to turn it into electricity.”

Temperature

Once the right arrangement of atoms was revealed by the Irish researchers, the experimentalists in Barcelona were able to adjust how they synthesised their devices. Mostly, this involved getting the temperature just right while creating the thin layers on a layer of glass. This boosted sunlight-to-electricity efficiency to 9.17 per cent.

The team needed to tap into some of the biggest supercomputers in the world to reveal the right configuration. Kavanagh ran his models on supercomputers in the UK and elsewhere in Europe – each equal in computing power to about 100,000 desk top computers.

He explains the reason why such computer heft was required. “We understand the theory and the maths behind the quantum mechanics and the chemistry of how atoms behave and bond with each other,” says Kavanagh, but each atom in the system interacts with others, creating an extremely complex maths problem. “It means you must solve millions and millions of equations, simultaneously, to figure out what each atom is going to do next,” he explains.

Both Walsh and Scanlon credit their PhD student Seán Kavanagh for his significant work on the project

The material generated was thin and flexible, as well as extremely light and potentially very cheap. Scanlon at University College London says that 10 per cent efficiency is considered the critical tipping point, where such ultrathin solar cells start to become commercially viable. “If we have a low energy coating that can generate that much electricity, then you could see companies using the technology,” he says. “We’re very close to that.” The results were reported in Nature Photonics.

Silicon over a period of 60 years has managed to get to about 25 per cent efficiency, but this comes at an environmental cost. “The industrial silicon manufacturing plants are often powered by coal, which drive a process that runs at 2,000 degrees,” says Walsh. “That’s very energy intensive, so there can be a lot of embedded energy in a solar panel that you buy from your roof, depending on how it was manufactured.”

‘Many possibilities’

A dream material is one more akin to their thin film made from abundant and non-toxic materials. “You could think about coating jackets or tents with these very thin-film solar cells or having a very thin layer of glass that would absorb sunlight and power your internal lighting. There are so many possibilities that this can be a step-change in how we use solar power,” Walsh adds.

Both Walsh and Scanlon credit their PhD student Seán Kavanagh for his significant work on the project. A master with numbers, Kavanagh achieved 100 per cent in his Leaving Cert maths exam. He is in no rush to leave his current position, however, which officially is funded for four years.

He has published more than 10 research papers, so he could write up his PhD and finish early. “But I’m not too worried about that. I really enjoy doing it – it doesn’t feel like work at all.” Right now, he has his eyes on a possible move to a research university either in the United States or Japan.