Overcoming atomic level perovskite defects

A group of Australian and Chinese researchers have found tiny defects – and ways to resolve them – in the atomic structures of perovskite crystals.

Perovskites have gained attention in the past five years as materials that could potentially improve the efficiency of solar cells.

“They might be used as a replacement entirely for silicon,” explains Joanne Etheridge, a professor of materials science and engineering at Monash University and corresponding author on a paper describing the research, published in Nature Energy.

“A more popular and promising avenue is that they can be used in what they call tandem solar cells, where the perovskite layer is added to the silicon on top.”

The perovskite material, which can be tuned to absorb different wavelengths of light, would absorb light from parts of the spectrum that silicon cannot, leaving the rest of the light for silicon and resulting in added efficiency to the solar panel.

Perovskites are only just emerging as commercial products, and there are a few roadblocks that need to be addressed before they’re as widespread as silicon solar panels. “They’re very delicate, they don’t last very long in air, and they can suffer from a problem known as current-voltage hysteresis,” says Etheridge.

It was previously thought that this problem was mostly due to how perovskites were installed in a device, with research being done on the materials placed around the perovskite in a solar cell.

“But we’ve just seen some defects inside the perovskite material itself, which will also affect performance,” says Etheridge.

“These defects are at the level of atoms, the way the atoms are arranged. They’re not arranged perfectly as you would hope in a crystal, where things repeat regularly. Every so often, the repetition is broken,” she adds.

“There’s some evidence that this is actually causing some of the limits to performance that have been seen in perovskites.”

The defects are known as intragrain planar defects. The researchers were able to control for these defects by adjusting the chemical composition of their perovskites. Compositions without defects had the highest performance in solar cells.

The team examined specific perovskite crystals at the Monash Centre for Electron Microscopy. They were able to examine the crystals at a level of atomic detail that has previously been very difficult to do.

“Electron microscopy is a very important way to look at the material, because it can show us the arrangement of atoms,” says Etheridge.

But things that can convert sunlight into electrons can be highly sensitive to electrons. This makes electron microscopy – which uses a beam of electrons – a risky practice with perovskites.

“Electrons can damage the material very easily,” says Etheridge. “So something that we’ve been developing at Monash since this became a material of interest are ways to look at it with electrons that don’t damage the structure.”

The researchers hope this information has removed a hurdle to getting perovskites into commercial solar panels.

“To make a good solar cell, a material must be able to transform sunlight into electricity efficiently and do so outdoors for many decades,” says Wei Li, co-lead author on the paper and a researcher at the Wuhan Institute of Technology, China.

Li adds that the movement of photons and electrons within a material, and thus the performance of the cell, “strongly depends on the crystallographic properties of the material.”

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