Looking at perovskite in a new light

Australian researchers may have overcome a significant hurdle in the global quest to develop next-generation perovskite solar cells.

A chance observation and fast follow-up has allowed a team from the ARC Centre of Excellence in Exciton Science to resolve a fundamental stability problem caused when the material is exposed to sunlight.

Their breakthrough, reported in a paper in the journal Nature Materials, means the material can remain stable without the need to change the ideal composition.

There is a real buzz around perovskites because their distinctive crystal structure can easily be synthesised from a diverse number of elements. Scientists have designed perovskite crystals with a range of intriguing properties, useful in technologies such as ultrasound machines, sensors, lasers and memory chips.

Solar cells made from metal-halide perovskite crystals are particularly promising. They are 500 times thinner than cells that use silicon – the material that solar technology has relied on since the 1950s – meaning that production costs can be kept low.

They are also more flexible and more efficient. In the mere decade that scientists have been working on them, their conversion efficiency has skyrocketed to around 25%. In comparison, it took 40 years for silicon-based cells to reach a similar efficiency.

But it’s not all smooth sailing. “The different halides tend to spatially separate when the sun is shining on these materials, which obviously reduces their usefulness,” explains Asaph Widmer-Cooper, materials scientist from the University of Sydney and co-author of the new paper.

This disruption of the carefully arranged composition interferes with the material’s absorption of certain wavelengths and affects its efficiency and charge-carrier conduction. This is a critical issue for a material that will be constantly exposed to sunlight.

The solution? In their paper, the researchers describe how higher-intensity light can actually undo the disruption caused by lower-intensity light. The effect was accidentally observed while performing an unrelated measurement, but the team quickly realised they had stumbled across something important.

Computational modelling led by Stefano Bernardi from the University of Sydney helped illuminate the significance of this surprise observation.

“What we found is that as you increase the excitation intensity, the local strains in the ionic lattice – which were the original cause of segregation – start to merge together,” he explains.

This causes the local deformations that disrupt the crystal’s composition to disappear.

“On a normal sunny day, the intensity is so low that these deformations are still localised,” Bernardi says. “But if you find a way to increase the excitation above a certain threshold, for example by using a solar concentrator, then segregation disappears.”

Other research groups have attempted to solve this problem by tweaking the composition of metal-halide perovskites or changing the material’s dimensions. The key to the new work is that the composition doesn’t have to change.

“What we’ve shown is that you can actually use the material in the state that you want to use it, for a solar cell,” says co-author Chris Hall from the University of Melbourne. “All you need to do is focus more light onto it.”

This research has resolved a critical problem facing perovskite-based solar research, but the real test will be in its practical application.

Windmer-Cooper adds: “There’s still a lot of work to be done, to not only test our ideas for stabilising mixed-halide perovskites in solar cells, but also to scale up the printing process.”

This, he says, is a problem that the ARC Centre of Excellence in Exciton Science is working on with Australia’s CSIRO.