Shedding light on dark traps

However, the current limitation of perovskite materials is the presence of a “deep trap” caused by a certain type of defect, or minor blemish, in the material. These are areas in the material where energised charge carriers can get stuck and recombine, losing their energy to heat, rather than converting it into useful electricity or light. This undesirable recombination process can have a significant impact on the efficiency and stability of solar panels and LEDs.

Until now, very little was known about the cause of these traps, in part because they appear to behave rather differently to traps in traditional solar cell materials.

In 2015, Dr Stranks and colleagues published a Science paper looking at the luminescence of perovskites, which reveals how good they are at absorbing or emitting light. “We found that the material was very heterogeneous; you had quite large regions that were bright and luminescent, and other regions that were really dark,” says Stranks. “These dark regions correspond to power losses in solar cells or LEDs. But what was causing the power loss was always a mystery, especially because perovskites are otherwise so defect-tolerant.”

Due to limitations of standard imaging techniques, the group couldn’t tell if the darker areas were caused by one, large trap site, or many smaller traps, making it difficult to establish why they were forming only in certain regions.

Later on in 2017, Professor Keshav Dani’s group at OIST published a paper in Nature Nanotechnology, where they made a movie of how electrons behave in semiconductors after absorbing light. “You can learn a lot from being able to see how charges move in a material or device after shining light. For example, you can see where they might be getting trapped,” says Dani. “However, these charges are hard to visualize as they move very fast – on the timescale of a millionth of a billionth of a second; and over very short distances – on the length scale of a billionth of a metre.”

On hearing of Dani’s work, Dr Stranks reached out to see if they could work together to address the problem visualising the dark regions in perovskites.

The team at OIST used a technique called photoemission electron microscopy (PEEM) for the first time on perovskites, where they probed the material with ultraviolet light and built up an image based on how the emitted electrons scattered.

When they looked at the material, they found that the dark regions contained traps, around 10-100 nanometers in length, which were clusters of smaller atomic-sized trap sites. These trap clusters were spread unevenly throughout the perovskite material, explaining the heterogeneous luminescence seen in Stranks’s earlier research.

Intriguingly, when the researchers overlaid images of the trap sites onto images that showed the crystal grains of the perovskite material, they found that the trap clusters only formed at specific places, at the boundaries between certain grains.

To understand why this only occurred at certain grain boundaries, the groups worked together with Professor Paul Midgley’s team from Cambridge University’s Department of Materials Science and Metallurgy using a technique called scanning electron diffraction to create detailed images of the perovskite crystal structure. The project team made use of the electron microscopy setup at the ePSIC facility at the Diamond Light Source Synchrotron, which has specialized equipment for imaging beam-sensitive materials, like perovskites.

“Because these materials are very beam-sensitive, typical techniques that you would use to probe local crystal structure on these length scales will quite quickly change the material as you're looking at it, which can make interpreting the data very difficult” explains Tiarnan Doherty, a PhD student in Stranks’s group and co-lead author of the study. “Instead, we were able to use very low exposure doses and therefore prevent damage.

“From the work at OIST, we knew where the trap clusters were located, and at ePSIC, we scanned around those same areas to see the local structure. We were then able to quickly pinpoint unexpected variations in the crystal structure around the trap clusters.”

The group discovered that the trap clusters only formed at junctions where an area of the material with slightly distorted structure met an area with pristine structure.

“In perovskites, we have these regular mosaic grains of material and most of the grains are nice and pristine – the structure we would expect,” says Stranks. “But every now and again, you get a grain that's slightly distorted and the chemistry of that grain is inhomogeneous. What was really interesting and which initially confused us, was that it's not the distorted grain that's the trap but where that grain meets a pristine grain; it's at that junction that the traps cluster.”