Scientists at the University of Cambridge and Okinawa Institute of Science and Technology Graduate University (OIST), have identified the source of "deep traps", a known limitation of perovskite materials caused by a defect, or minor blemish, in the material.
"Deep traps" are areas in the material where energized charge carriers can get stuck and recombine, losing their energy to heat, rather than converting it into useful electricity or light. This recombination process can have a significant impact on the efficiency and stability of solar panels and LEDs.
The team says that until now, limited information was available on the cause of these traps because of their different behavior while trapping in traditional solar cell materials.
Using a technique called photoemission electron microscopy (PEEM), scientists probed the material with ultraviolet light and built up an image based on how the emitted electrons scattered.
They observed the material and found that the dark regions contained traps, around 10-100 nanometers in length, which were clusters of smaller atomic-sized trap sites. These traps clusters were spread unevenly throughout the perovskite material.
Scientists then overlaid the 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 find out the reason why this only occurred at certain grain boundaries, the scientists used a scanning electron diffraction technique to create detailed images of the perovskite crystal structure. They also used 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.
Tiarnan Doherty, a Ph.D. student in Stranks’s group and co-lead author of the study said, “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”. “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.”
Scientists determined that the trap clusters only formed at junctions where an area of the material with slightly distorted structure met an area with pristine structure.
With this understanding of the nature of the traps, the team at OIST also used the custom-built PEEM instrumentation to visualize the dynamics of the charge carrier trapping process happening in the perovskite material.
“This was possible as one of the unique features of our PEEM setup is that it can image ultrafast processes – as short as femtoseconds,” said Andrew Winchester, a Ph.D. student in Dani’s Unit, and co-lead author of this study. “We found that the trapping process was dominated by charge carriers diffusing to the trap clusters.”
Dr. Sam Stranks’s group at Cambridge’s Department of Chemical Engineering and Biotechnology said, “We still don’t know exactly why the traps are clustering there, but we now know that they do form there, and seemingly only there. That’s exciting because it means we now know what to target to bring up the performances of perovskites. We need to target those inhomogeneous phases or get rid of these junctions in some way.”
“The fact that charge carriers must first diffuse to the traps could also suggest other strategies to improve this device. Maybe we could alter or control the arrangement of the trap clusters, without necessarily changing their average number, such that charge carriers are less likely to reach these defect sites.”
The teams’ research focused on one particular perovskite structure. The scientists will now be investigating whether the cause of these trapping clusters is universal across other perovskite materials.