How defects destabilise next-generation solar cells


Rebecca Pool

Thursday, November 28, 2019 - 07:15
This image of a perovskite surface shows the shifting of ions across the surface, and the vacancies causing these movements. [OIST]
A team of Japan- and US-based researchers has characterised the structural defects that destabilise perovskites, materials critical for the next generation solar cells.
Using scanning tunnelling microscopy, Professor Yabing Qi from OIST, and colleagues, imaged the movement of individual ions across the perovskite surface, pinpointing defects along the way.
As Collin Stecker from OIST points out: ““For a long time, scientists have known structural defects exist, but didn’t understand their precise chemical nature... Our study delves into fundamental characteristics of perovskite materials to help device engineers further improve them.”
Versatile compounds called perovskites are valued for their application in next generation solar energy technologies.
But despite being cheap and efficient, perovskite devices often contain atomic-level structural defects that can disrupt charge transfer across the solar cell, hindering performance and stability.
To better understand the electronic and dynamic properties of these perovskite defects, the OIST researchers used STM to image the dynamic behaviour and electronic properties of intrinsic defects in organic-inorganic hybrid perovskites.
As the researchers report in ACS Nano, they were able to study the vacancy-assisted transport of individual ions as well as the existence of vacancy defect clusters at the perovskite surface.
Left to right: Collin Stecker, Dr Jeremy Hieulle, Dr Luis K. Ono, Professor Yabing Qi of the Energy Materials and Surface Sciences Unit, OIST. [OIST]
According to the researchers, they noted that pairs of bromide ions on the perovskite surfaces were shifting and changing direction.
“We combine these observations with density functional theory (DFT) calculations to identify the mechanisms for this ion motion and show that ion transport energy barriers, as well as transport mechanisms, at the surface depend on crystal direction,” explains Stecker. 
The researchers concluded that the surface vacancies were likely causing these ions to move across the perovskite materials.
Understanding this mechanism of ion movement may later help scientists and engineers mitigate the structural and functional consequences of these defects.
“These perovskite surfaces are much more dynamic than we previously anticipated,” says Stecker. “Now, with these new findings, we hope engineers can better account for the effect of defects and their motion in order to improve devices.”
Research is published in ACS Nano.
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