̽��ֱ��

However, perovskites��still��show��significant��performance losses and instabilities, particularly in the specific materials that promise the highest ultimate efficiency.��Most��research to date has focused on ways to remove��these losses, but��their actual physical causes remain unknown.������

Now, in a published��in��Nature, researchers from Dr��Sam�����ٰ����԰��’s��group at Cambridge’s��Department of Chemical Engineering and Biotechnology��and Cavendish Laboratory,��and��Professor��Keshav Dani’s��Femtosecond Spectroscopy Unit��at��OIST��in Japan,��identify the source of the problem. Their discovery could��streamline��efforts to increase��the��efficiency��of perovskites, bringing��them��closer to mass-market production.��������

Perovskite materials are much more tolerant of defects in their structure than silicon solar cells, and previous research carried out by�����ٰ����԰��’s��group found that to a certain extent,��some heterogeneity in their��composition��actually improves their��performance as solar cells and light-emitters.����

However, the current limitation of perovskite materials is the presence of a 'deep trap'��caused by a 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 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��differently to traps in traditional solar cell materials.����

In 2015,��Stranks��and colleagues��published��a paper in����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,”��said��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�����dzܱ����’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.����

In 2017,��Dani’s group at OIST��made��a�� 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,”��said��Dani.��“However,��these��charges��are hard to visualise 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,��Stranks��reached out to see if they could��work together to��address��the problem��visualising��the dark regions in��perovskites.����

̽��ֱ��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�����ٰ����԰��’s��earlier research.��

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’s��Department of Materials Science and Metallurgy��using��a technique called��scanning electron��diffraction��to��create detailed images of the perovskite crystal structure.�� ̽��ֱ��project��team made use��of��the��electron��microscopy setup at the��ePSIC��facility��at the Diamond Light Source Synchrotron,��which has��specialised��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,” said��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 regular mosaic grains of material and most of the grains are nice and pristine – the structure we would expect,”��said��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.”����

With this understanding of the nature of the traps,��the team��at OIST��also used��the��custom-built��PEEM��instrumentation��to��visualise 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 PhD 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.”��

These��discoveries��represent��a breakthrough in the quest to bring perovskites to the solar energy market.����

“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,”��said��Stranks.��“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.”��

“ ̽��ֱ��fact that��charge carriers must first diffuse to the traps��could also suggest other strategies to improve these devices,”��said��Dani. “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.�� ̽��ֱ��scientists��will now be investigating whether the cause of these trapping clusters is universal across��other��perovskite materials.����

“Most of the progress in device��performance has been��trial and error��and so far,��this has been quite an inefficient process,”��said��Stranks. “To date,��it really hasn't been driven by��knowing a specific cause and��systematically��targeting that.��This is one of the first breakthroughs��that��will help us to use the fundamental science to��engineer more efficient devices.”��

Reference:
Tiarnan A.S. Doherty et al. '.' Nature (2020). DOI:��10.1038/s41586-020-2184-1

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