Australian scientists shed light on perovskite problem


From pv magazine Australia

Australian researchers have shed light on a major obstacle to the widespread uptake of next-generation mixed-halide perovskite solar cells – light-induced phase segregation. This is a troublesome issue, whereby illumination, such as sunlight, disrupts the delicately arranged composition of elements within mixed-halide perovskites. Of course, an allergy to sunlight is not a great feature for a solar material.

Thankfully, members of the Australian Research Council’s Centre of Excellence in Exciton Science have discovered that by actually increasing the intensity of light illuminating the hybrid perovskite material seems to settle its elements and maximise efficiency. Mixed-halide perovskites are a hybrid organic-inorganic single crystal that can provide a cheap and flexible way of increasing solar efficiency, if that is, they can be smoothed for commercial use.

Indeed, a solution seems to be on the cards, as the introduction of high-intensity life works to undo the disruption caused by lower-intensity light. It seems that mixed-halide perovskites are a nervous wreck backstage, but when pushed in front of the intense glare of the spotlight, they recover themselves and their bandgap.

Like the scientists behind the microwave, Viagra, and vulcanized rubber, Chris Hall of the University of Melbourne, and Wenxin Mao of Monash University, made their discovery while studying something else.

“It was one of those unusual discoveries that you sometimes hear about in science,” said Hall. “We were performing a measurement, looking for something else, and then we came across this process that at the time seemed quite strange. However, we quickly realized it was an important observation.”

Having clocked the potential of this observation, the two researchers brought in Stefano Bernardi from the University of Sydney to lead the computational modeling work for this new and surprising solution to light-induced phase segregation.

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Scanning confocal microscope image of a single mixed-halide perovskite crystal showing emission from mixed (green) and segregated (red) regions. The central region is exposed to intense light, which causes the halide-ions in this region to mix, generating green (540-570 nm) fluorescence. The red emission (>660 nm) is from phase-segregated perovskite driven by the low-intensity confocal microscope scanning laser.

“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,” explained Stefano. “When this happens, the local deformations that drove segregation disappear.”

This is to say that on a regular day of sunshine, the light’s intensity is so low that it causes local deformities, but when exposed to a solar concentrator, the excitation is increased to such a level that the segregation vanishes in a glare.

Unlike some other famous discoveries, these researchers are not unsure about the significance of their findings. A solution to light-induced phase segregation means that mixed-halide perovskites can now retain their optimal composition when exposed to light, allowing them to be useful in solar cells.

It is, in some sense, a lesson in the utility of daring. Whereas many people had previously thought to solve the problem of light-induced disorder by suppressing light, a rational enough hypothesis, but sometimes it is better to walk through the fire rather than away from it.

“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 – all you need to do is focus more light onto it. We’ve done the fundamental work and the next step is to put it into a device.” That device could be concentrator and tandem solar cells, or even in high-power light-emissive devices and optical memory applications.

The scientists recently published their findings in Nature Materials.

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