̽��ֱ�� of Cambridge - Paul Midgley

Templating approach stabilises ‘ideal’ material for alternative solar cells

erh68 — Thu, 23 Dec 2021 19:00:00 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, used an organic molecule as a ‘template’ to guide perovskite films into the desired phase as they form. Their results are reported in the journal Science.

Perovskite materials offer a cheaper alternative to silicon for producing optoelectronic devices such as solar cells and LEDs.

There are many different perovskites, resulting from different combinations of elements, but one of the most promising to emerge in recent years is the formamidinium (FA)-based FAPbI3 crystal.

̽��ֱ��compound is thermally stable and its inherent ‘bandgap’ – the property most closely linked to the energy output of the device – is not far off ideal for photovoltaic applications.

For these reasons, it has been the focus of efforts to develop commercially available perovskite solar cells. However, the compound can exist in two slightly different phases, with one phase leading to excellent photovoltaic performance, and the other resulting in very little energy output.

“A big problem with FAPbI3 is that the phase that you want is only stable at temperatures above 150 degrees Celsius,” said Tiarnan Doherty from Cambridge’s Cavendish Laboratory, the paper's first author. “At room temperature, it transitions into another phase, which is really bad for photovoltaics.”

Recent solutions to keep the material in its desired phase at lower temperatures have involved adding different positive and negative ions into the compound.

“That's been successful and has led to record photovoltaic devices but there are still local power losses that occur,” said Doherty, who is also affiliated with the Department of Chemical Engineering and Biotechnology. “You end up with local regions in the film that aren’t in the right phase.”

Little was known about why the additions of these ions improved stability overall, or even what the resulting perovskite structure looked like.

“There was this common consensus that when people stabilise these materials, they’re an ideal cubic structure,” said Doherty. “But what we’ve shown is that by adding all these other things, they're not cubic at all, they’re very slightly distorted. There’s a very subtle structural distortion that gives some inherent stability at room temperature.”

̽��ֱ��distortion is so minor that it had previously gone undetected, until Doherty and colleagues used sensitive structural measurement techniques that have not been widely used on perovskite materials.

̽��ֱ��team used scanning electron diffraction, nano-X-ray diffraction and nuclear magnetic resonance to see, for the first time, what this stable phase really looked like.

“Once we figured out that it was the slight structural distortion giving this stability, we looked for ways to achieve this in the film preparation without adding any other elements into the mix.”

Co-author Satyawan Nagane used an organic molecule called Ethylenediaminetetraacetic acid (EDTA) as an additive in the perovskite precursor solution, which acts as a templating agent, guiding the perovskite into the desired phase as it forms. ̽��ֱ��EDTA binds to the FAPbI3 surface to give a structure-directing effect, but does not incorporate into the FAPbI3 structure itself.

“With this method, we can achieve that desired band gap because we’re not adding anything extra into the material, it’s just a template to guide the formation of a film with the distorted structure – and the resulting film is extremely stable,” said Nagane.

“In this way, you can create this slightly distorted structure in just the pristine FAPbI3 compound, without modifying the other electronic properties of what is essentially a near-perfect compound for perovskite photovoltaics,” said co-author Dominik Kubicki from the Cavendish Laboratory, who is now based at the ̽��ֱ�� of Warwick.

̽��ֱ��researchers hope this fundamental study will help improve perovskite stability and performance. Their own future work will involve integrating this approach into prototype devices to explore how this technique may help them achieve the perfect perovskite photovoltaic cells.

“These findings change our optimisation strategy and manufacturing guidelines for these materials,” said senior author Dr Sam Stranks from Cambridge’s Department of Chemical Engineering & Biotechnology. “Even small pockets that aren’t slightly distorted will lead to performance losses, and so manufacturing lines will need to have very precise control of how and where the different components and ‘distorting’ additives are deposited. This will ensure the small distortion is uniform everywhere – with no exceptions.”

̽��ֱ��work was a collaboration with the groups of Paul Midgley in the Materials Science Department and Clare Grey in the Yusuf Hamied Department of Chemistry at Cambridge, the Diamond Light Source and the electron Physical Science Imaging Centre (ePSIC), Imperial College London, Yonsei ̽��ֱ��, Wageningen ̽��ֱ�� and Research, and the ̽��ֱ�� of Leeds.

Reference:
Tiarnan A. S. Doherty et al. ‘Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases.’ Science (2021). DOI: 10.1126/science.abl4890

Researchers have developed a method to stabilise a promising material known as perovskite for cheap solar cells, without compromising its near-perfect performance.

Tiarnan Doherty

Artist's impression of formamidinium (FA)-based crystal

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Shedding light on dark traps

erh68 — Thu, 16 Apr 2020 09:27:22 +0000

A multi-institutional collaboration, co-led by scientists at the ̽��ֱ�� of Cambridge and Okinawa Institute of Science and Technology Graduate ̽��ֱ�� (OIST), has identified the source of efficiency-limiting defects in potential materials for next-generation solar cells and LEDs.

In the last decade, perovskites – a diverse range of materials with a specific crystal structure – have emerged as promising alternatives to silicon solar cells, as they are cheaper and greener to manufacture, while achieving a comparable level of efficiency.

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 paper published in Nature, researchers from Dr Sam Stranks’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 Stranks’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 Science 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 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.

In 2017, Dani’s group at OIST 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,” 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 Stranks’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. 'Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites.' Nature (2020). DOI: 10.1038/s41586-020-2184-1

Researchers pinpoint the origin of defects that sap the performance of next-generation solar technology.

We now know what to target to bring up the performances of perovskites.

Samuel Stranks

Andrew Winchester

Perovskites

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