̽��ֱ�� of Cambridge - Okinawa Institute of Science and Technology Graduate ̽��ֱ�� (OIST)

Secret to treating ‘Achilles’ heel’ of alternatives to silicon solar panels revealed

erh68 — Tue, 24 May 2022 15:00:00 +0000

̽��ֱ��researchers used a combination of techniques to mimic the process of aging under sunlight and observe changes in the materials at the nanoscale, helping them gain new insights into the materials, which also show potential for optoelectronic applications such as energy-efficient LEDs and X-ray detectors, but are limited in their longevity.

Their results, reported in the journal Nature, could significantly accelerate the development of long-lasting, commercially available perovskite photovoltaics.

Perovksites are abundant and much cheaper to process than crystalline silicon. They can be prepared in liquid ink that is simply printed to produce a thin film of the material.

While the overall energy output of perovskite solar cells can often meet or – in the case of multi-layered ‘tandem’ devices – exceed that achievable with traditional silicon photovoltaics, the limited longevity of the devices is a key barrier to their commercial viability.

A typical silicon solar panel, like those you might see on the roof of a house, typically lasts about 20-25 years without significant performance losses.

Because perovskite devices are much cheaper to produce, they may not need to have as long a lifetime as their silicon counterparts to enter some markets. But to fulfil their ultimate potential in realising widespread decarbonisation, cells will need to operate for at least a decade or more. Researchers and manufacturers have yet to develop a perovskite device with similar stability to silicon cells.

Now, researchers at the ̽��ֱ�� of Cambridge and the Okinawa Institute of Science and Technology (OIST) in Japan, have discovered the secret to treating the ‘Achilles heel’ of perovskites.

Using high spatial-resolution techniques, in collaboration with the Diamond Light Source synchrotron facility and its electron Physical Sciences Imaging Centre (ePSIC) in Oxfordshire, and the Department of Materials Science and Metallurgy in Cambridge, the team was able to observe the nanoscale properties of these thin films and how they change over time under solar illumination.

Previous work by the team using similar techniques has shone light on the defects that cause deficiencies in the performance of perovskite photovoltaics – so-called carrier traps.

“Illuminating the perovskite films over time, simulating the aging of solar cell devices, we find that the most interesting dynamics are occurring at these nanoscopic trap clusters,” said co-author Dr Stuart Macpherson from Cambridge’s Cavendish Laboratory.

“We now know that the changes we see are related to photodegradation of the films. As a result, efficiency-limiting carrier traps can now be directly linked to the equally crucial issue of solar cell longevity.”

“It’s pretty exciting,” said co-author Dr Tiarnan Doherty, from Cambridge’s Department of Chemical Engineering and Biotechnology, and Murray Edwards College, “because it suggests that if you can address the formation of these surface traps, then you will simultaneously improve performance and the stability of the devices over time.”

By tuning the chemical composition, and how the perovskite film forms, in preparing the devices, the researchers have shown that it’s possible to control how many of these detrimental phases form and, by extension, how long the device will last.

“ ̽��ֱ��most stable devices seem to be serendipitously lowering the density of detrimental phases through subtle compositional and structural modifications,” said Doherty. “We’re hoping that this paper reveals a more rational, targeted approach for doing this and achieving the highest performing devices operating with maximal stability.”

̽��ֱ��group is optimistic that their latest findings will bring us closer still to the first commercially available perovskite photovoltaic devices.

“Perovskite solar cells are on the cusp of commercialisation, with the first production lines already producing modules,” said Dr Sam Stranks from Cambridge’s Department of Chemical Engineering and Biotechnology, who led the research.

“We now understand that any residual unwanted phases – even tiny nanoscale pockets remaining from the processing of the cells – will be bad news for the longevity of perovskite solar cells. ̽��ֱ��manufacturing processes need to incorporate careful tuning of the structure and composition across a large area to eliminate any trace of these unwanted phases – even more careful control than is widely thought for these materials. This is a great example of fundamental science directly guiding scaled manufacturing.”

“It has been very satisfying to see the approaches that we've developed at OIST and Cambridge over the past several years provide direct visuals of these tiny residual unwanted phases, and how they change over time,” said co-author Dr Keshav Dani of OIST’s Femtosecond Spectroscopy Unit. “ ̽��ֱ��hope remains that these techniques will continue to reveal the performance limiting aspects of photovoltaic devices, as we work towards studying operational devices.”

“Another strength of perovskite devices is that they can be made in countries where there’s no existing infrastructure for processing monocrystalline silicon,” said Macpherson. “Silicon solar cells are cheap in the long term but require a substantial initial capital outlay to begin processing. But for perovskites, because they can be solution-processed and printed so easily, using far less material, you remove that initial cost. They offer a viable option for low- and middle-income countries looking to transition to solar energy.”

Reference:
Samuel Stranks et al. 'Local Nanoscale Phase Impurities are Degradation Sites in Halide Perovskites.' Nature (2022). DOI: 10.1038/s41586-022-04872-1

A team of researchers from the UK and Japan has found that the tiny defects which limit the efficiency of perovskites – cheaper alternative materials for solar cells – are also responsible for structural changes in the material that lead to degradation.

Perovskites offer a viable option for low- and middle-income countries looking to transition to solar energy

Stuart MacPherson

Alachua County

Solar panels

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Mystery of high-performing solar cell materials revealed in stunning clarity

erh68 — Mon, 22 Nov 2021 15:38:18 +0000

̽��ֱ��most commonly used material for producing solar panels is crystalline silicon, but achieving efficient energy conversion requires an energy-intensive and time-consuming production process to create a highly ordered wafer structure.

In the last decade, perovskite materials have emerged as promising alternatives to silicon.

̽��ֱ��lead salts used to make perovskites are much more abundant and cheaper to produce than crystalline silicon, and they can be prepared in liquid ink that is simply printed to produce a film of the material. They also show great potential for other applications, such as energy-efficient light-emitting diodes (LEDs) and X-ray detectors.

̽��ֱ��performance of perovskites is surprising. ̽��ֱ��typical model for an excellent semiconductor is a highly ordered structure, but the array of different chemical elements in perovskites creates a much ‘messier’ landscape.

This messiness causes defects in the material that lead to tiny ‘traps’, which typically reduce performance. But despite the presence of these defects, perovskite materials still show efficiency levels comparable to their silicon alternatives.

In fact, earlier research by the same team behind the current work showed the disordered structure can actually increase the performance of perovskite optoelectronics, and their latest work seeks to explain why.

Combining a series of new microscopy techniques, the group present a complete picture of the nanoscale chemical, structural and optoelectronic landscape of these materials, that reveals the complex interactions between these competing factors and ultimately, shows which comes out on top.

“What we see is that we have two forms of disorder happening in parallel,” said first author Kyle Frohna from Cambridge’s Department of Chemical Engineering and Biotechnology (CEB). “ ̽��ֱ��electronic disorder associated with the defects that reduce performance, and then the spatial chemical disorder that seems to improve it.

“And what we’ve found is that the chemical disorder – the ‘good’ disorder in this case – mitigates the ‘bad’ disorder from the defects by funnelling the charge carriers away from these traps that they might otherwise get caught in.”

In collaboration with researchers from the Cavendish Laboratory, the Diamond Light Source synchrotron facility in Didcot, and the Okinawa Institute of Science and Technology in Japan, the researchers used several different microscopic techniques to look at the same regions in the perovskite film. They could then compare the results from all these methods to present the full picture of what’s happening at a nanoscale level in these materials.

̽��ֱ��findings will allow researchers to further refine how perovskite solar cells are made in order to maximise efficiency.

“We have visualised and given reasons why we can call these materials defect tolerant,” said co-author Miguel Anaya, also from CEB. “This methodology enables new routes to optimise them at the nanoscale to perform better for a targeted application. Now, we can look at other types of perovskites that are not only good for solar cells but also for LEDs or detectors and understand their working principles.”

“Through these visualisations, we now much better understand the nanoscale landscape in these fascinating semiconductors – the good, the bad and the ugly,” said Dr Sam Stranks from CEB, who led the research. “These results explain how the empirical optimisation of these materials by the field has driven these mixed composition perovskites to such high performances. But it has also revealed blueprints for design of new semiconductors that may have similar attributes – where disorder can be exploited to tailor performance.”

Reference:
Kyle Frohna et al ‘Nanoscale chemical heterogeneity dominates the optoelectronic response of alloyed perovskite solar cells.’ Nature Nanotechnology (2021) DOI: 10.1038/s41565-021-01019-7

Researchers have visualised, for the first time, why perovskites – materials which could replace silicon in next-generation solar cells - are seemingly so tolerant of defects in their structure. ̽��ֱ��findings, led by researchers from the ̽��ֱ�� of Cambridge, are published in the journal Nature Nanotechnology.

We now much better understand the nanoscale landscape in these fascinating semiconductors – the good, the bad and the ugly

Sam Stranks

Alex T at Ella Maru Studios

Artistic representation of electrons funneling into high quality areas of perovskite material

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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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