̽��ֱ�� of Cambridge - Sam Stranks

̽��ֱ��cost of solar power: how low can we go?

sc604 — Tue, 08 Oct 2024 14:27:07 +0000

Professor Sam Stranks is developing next-generation solar cell technology, which could drive down renewable energy prices even further.

A simple ‘twist’ improves the engine of clean fuel generation

sc604 — Wed, 24 Apr 2024 14:31:37 +0000

̽��ֱ��researchers, led by the ̽��ֱ�� of Cambridge, are developing low-cost light-harvesting semiconductors that power devices for converting water into clean hydrogen fuel, using just the power of the sun. These semiconducting materials, known as copper oxides, are cheap, abundant and non-toxic, but their performance does not come close to silicon, which dominates the semiconductor market.

However, the researchers found that by growing the copper oxide crystals in a specific orientation so that electric charges move through the crystals at a diagonal, the charges move much faster and further, greatly improving performance. Tests of a copper oxide light harvester, or photocathode, based on this fabrication technique showed a 70% improvement over existing state-of-the-art oxide photocathodes, while also showing greatly improved stability.

̽��ֱ��researchers say their results, reported in the journal Nature, show how low-cost materials could be fine-tuned to power the transition away from fossil fuels and toward clean, sustainable fuels that can be stored and used with existing energy infrastructure.

Copper (I) oxide, or cuprous oxide, has been touted as a cheap potential replacement for silicon for years, since it is reasonably effective at capturing sunlight and converting it into electric charge. However, much of that charge tends to get lost, limiting the material’s performance.

“Like other oxide semiconductors, cuprous oxide has its intrinsic challenges,” said co-first author Dr Linfeng Pan from Cambridge’s Department of Chemical Engineering and Biotechnology. “One of those challenges is the mismatch between how deep light is absorbed and how far the charges travel within the material, so most of the oxide below the top layer of material is essentially dead space.”

“For most solar cell materials, it’s defects on the surface of the material that cause a reduction in performance, but with these oxide materials, it’s the other way round: the surface is largely fine, but something about the bulk leads to losses,” said Professor Sam Stranks, who led the research. “This means the way the crystals are grown is vital to their performance.”

To develop cuprous oxides to the point where they can be a credible contender to established photovoltaic materials, they need to be optimised so they can efficiently generate and move electric charges – made of an electron and a positively-charged electron ‘hole’ – when sunlight hits them.

One potential optimisation approach is single-crystal thin films – very thin slices of material with a highly-ordered crystal structure, which are often used in electronics. However, making these films is normally a complex and time-consuming process.

Using thin film deposition techniques, the researchers were able to grow high-quality cuprous oxide films at ambient pressure and room temperature. By precisely controlling growth and flow rates in the chamber, they were able to ‘shift’ the crystals into a particular orientation. Then, using high temporal resolution spectroscopic techniques, they were able to observe how the orientation of the crystals affected how efficiently electric charges moved through the material.

“These crystals are basically cubes, and we found that when the electrons move through the cube at a body diagonal, rather than along the face or edge of the cube, they move an order of magnitude further,” said Pan. “ ̽��ֱ��further the electrons move, the better the performance.”

“Something about that diagonal direction in these materials is magic,” said Stranks. “We need to carry out further work to fully understand why and optimise it further, but it has so far resulted in a huge jump in performance.” Tests of a cuprous oxide photocathode made using this technique showed an increase in performance of more than 70% over existing state-of-the-art electrodeposited oxide photocathodes.

“In addition to the improved performance, we found that the orientation makes the films much more stable, but factors beyond the bulk properties may be at play,” said Pan.

̽��ֱ��researchers say that much more research and development is still needed, but this and related families of materials could have a vital role in the energy transition.

“There’s still a long way to go, but we’re on an exciting trajectory,” said Stranks. “There’s a lot of interesting science to come from these materials, and it’s interesting for me to connect the physics of these materials with their growth, how they form, and ultimately how they perform.”

̽��ֱ��research was a collaboration with École Polytechnique Fédérale de Lausanne, Nankai ̽��ֱ�� and Uppsala ̽��ֱ��. ̽��ֱ��research was supported in part by the European Research Council, the Swiss National Science Foundation, and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Sam Stranks is Professor of Optoelectronics in the Department of Chemical Engineering and Biotechnology, and a Fellow of Clare College, Cambridge.

Reference:
Linfeng Pan, Linjie Dai et al. ‘High carrier mobility along the [111] orientation in Cu2O photoelectrodes.’ Nature (2024). DOI: 10.1038/s41586-024-07273-8

For more information on energy-related research in Cambridge, please visit the Energy IRC, which brings together Cambridge’s research knowledge and expertise, in collaboration with global partners, to create solutions for a sustainable and resilient energy landscape for generations to come.

Researchers have found a way to super-charge the ‘engine’ of sustainable fuel generation – by giving the materials a little twist.

orange via Getty Images

Abstract orange swirls

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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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

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Licence type:

Public Domain

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

Yes

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

Yes

‘Messy’ production of perovskite material increases solar cell efficiency

erh68 — Mon, 11 Nov 2019 16:00:00 +0000

Scientists at the ̽��ֱ�� of Cambridge studying perovskite materials for next-generation solar cells and flexible LEDs have discovered that they can be more efficient when their chemical compositions are less ordered, vastly simplifying production processes and lowering cost.

̽��ֱ��surprising findings, published in Nature Photonics, are the result of a collaborative project, led by Dr Felix Deschler and Dr Sam Stranks.

̽��ֱ��most commonly used material for producing solar panels is crystalline silicon, but to achieve efficient energy conversion requires an expensive and time-consuming production process. ̽��ֱ��silicon material needs to have a highly ordered wafer structure and is very sensitive to any impurities, such as dust, so has to be made in a cleanroom.

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

̽��ֱ��lead salts used to make them are much more abundant and cheaper to produce than crystalline silicon, and they can be prepared in a liquid ink that is simply printed to produce a film of the material.

̽��ֱ��components used to make the perovskite can be changed to give the materials different colours and structural properties, for example, making the films emit different colours or collect sunlight more efficiently.

You only need a very thin film of this perovskite material – around one thousand times thinner than a human hair – to achieve similar efficiencies to the silicon wafers currently used, opening up the possibility of incorporating them into windows or flexible, ultra-lightweight smartphone screens.

“This is the new class of semiconductors that could actually revolutionise all these technologies,” said Sascha Feldmann, a PhD student at Cambridge’s Cavendish Laboratory.

“These materials show very efficient emission when you excite them with energy sources like light or apply a voltage to run an LED.

“This is really useful but it remained unclear why these materials that we process in our labs so much more crudely than these clean-room, high-purity silicon wafers, are performing so well.”

Scientists had assumed that, like with silicon materials, the more ordered they could make the materials, the more efficient they would be. But Feldmann and his co-lead author Stuart MacPherson were surprised to find the opposite to be true.

“ ̽��ֱ��discovery was a big surprise really,” said Deschler, who is now leading an Emmy-Noether research group at TU Munich. “We do a lot of spectroscopy to explore the working mechanisms of our materials, and were wondering why these really quite chemically messy films were performing so exceptionally well.”

“It was fascinating to see how much light we could get from these materials in a scenario where we’d expect them to be quite dark,” said MacPherson, a PhD student in the Cavendish Laboratory. “Perhaps we shouldn’t be surprised considering that perovskites have re-written the rule book on performance in the presence of defects and disorder.”

̽��ֱ��researchers discovered that their rough, multi-component alloyed preparations were actually improving the efficiency of the materials by creating lots of areas with different compositions that could trap the energised charge carriers, either from sunlight in a solar cell, or an electrical current in an LED.

“It is actually because of this crude processing and subsequent de-mixing of the chemical components that you create these valleys and mountains in energy that charges can funnel down and concentrate in,” said Feldmann. “This makes them easier to extract for your solar cell, and it’s more efficient to produce light from these hotspots in an LED.”

Their findings could have a huge impact on the manufacturing success of these materials.

“Companies looking to make bigger fabrication lines for perovskites have been trying to solve the problem of how to make the films more homogenous, but now we can show them that actually a simple inkjet printing process could do a better job,” said Feldmann. “ ̽��ֱ��beauty of the study really lies in the counterintuitive discovery that easy to make does not mean the material will be worse, but can actually be better.”

“It is now an exciting challenge to find fabrication conditions which create the optimum disorder in the materials to achieve maximum efficiency, while still retaining the structural properties needed for specific applications,” said Deschler.

“If we can learn to control the disorder even more precisely, we could expect further LED or solar cell performance improvements – and even push well beyond silicon with tailored tandem solar cells comprising two different colour perovskite layers that together can harvest even more power from the sun than one layer alone,” said Dr Sam Stranks, ̽��ֱ�� Lecturer in Energy at the Cambridge Department of Chemical Engineering and Biotechnology and the Cavendish Laboratory.

Another limitation of perovskite materials is their sensitivity to moisture, so the groups are also investigating ways to improve their stability.

“There’s still work to do to make them last on rooftops the way silicon can – but I’m optimistic,” said Stranks.

Reference:
Sascha Feldmann et al. ‘Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence.’ Nature Photonics (2019). DOI: 10.1038/s41566-019-0546-8

A bold response to the world’s greatest challenge
̽��ֱ�� ̽��ֱ�� of Cambridge is building on its existing research and launching an ambitious new environment and climate change initiative. Cambridge Zero is not just about developing greener technologies. It will harness the full power of the ̽��ֱ��’s research and policy expertise, developing solutions that work for our lives, our society and our biosphere.

Discovery means simpler and cheaper manufacturing methods are actually beneficial for the material’s use in next-generation solar cells or LED lighting.

̽��ֱ��beauty of the study really lies in the counterintuitive discovery that easy to make does not mean the material will be worse, but can actually be better

Sascha Feldmann

Ella Maru Studio

Artist's impression of perovskite structures

Yes

Potassium gives perovskite-based solar cells an efficiency boost

sc604 — Wed, 21 Mar 2018 18:00:00 +0000

An international team of researchers led by the ̽��ֱ�� of Cambridge found that the addition of potassium iodide ‘healed’ the defects and immobilised ion movement, which to date have limited the efficiency of cheap perovskite solar cells. These next-generation solar cells could be used as an efficiency-boosting layer on top of existing silicon-based solar cells, or be made into stand-alone solar cells or coloured LEDs. ̽��ֱ��results are reported in the journal Nature.

̽��ֱ��solar cells in the study are based on metal halide perovskites – a promising group of ionic semiconductor materials that in just a few short years of development now rival commercial thin film photovoltaic technologies in terms of their efficiency in converting sunlight into electricity. Perovskites are cheap and easy to produce at low temperatures, which makes them attractive for next-generation solar cells and lighting.

Despite the potential of perovskites, some limitations have hampered their efficiency and consistency. Tiny defects in the crystalline structure of perovskites, called traps, can cause electrons to get ‘stuck’ before their energy can be harnessed. ̽��ֱ��easier that electrons can move around in a solar cell material, the more efficient that material will be at converting photons, particles of light, into electricity. Another issue is that ions can move around in the solar cell when illuminated, which can cause a change in the bandgap – the colour of light the material absorbs.

“So far, we haven’t been able to make these materials stable with the bandgap we need, so we’ve been trying to immobilise the ion movement by tweaking the chemical composition of the perovskite layers,” said Dr Sam Stranks from Cambridge’s Cavendish Laboratory, who led the research. “This would enable perovskites to be used as versatile solar cells or as coloured LEDs, which are essentially solar cells run in reverse.”

In the study, the researchers altered the chemical composition of the perovskite layers by adding potassium iodide to perovskite inks, which then self-assemble into thin films. ̽��ֱ��technique is compatible with roll-to-roll processes, which means it is scalable and inexpensive. ̽��ֱ��potassium iodide formed a ‘decorative’ layer on top of the perovskite which had the effect of ‘healing’ the traps so that the electrons could move more freely, as well as immobilising the ion movement, which makes the material more stable at the desired bandgap.

̽��ֱ��researchers demonstrated promising performance with the perovskite bandgaps ideal for layering on top of a silicon solar cell or with another perovskite layer – so-called tandem solar cells. Silicon tandem solar cells are the most likely first widespread application of perovskites. By adding a perovskite layer, light can be more efficiently harvested from a wider range of the solar spectrum.

“Potassium stabilises the perovskite bandgaps we want for tandem solar cells and makes them more luminescent, which means more efficient solar cells,” said Stranks, whose research is funded by the European Union and the European Research Council’s Horizon 2020 Programme. “It almost entirely manages the ions and defects in perovskites.”

“We’ve found that perovskites are very tolerant to additives – you can add new components and they’ll perform better,” said first author Mojtaba Abdi-Jalebi, a PhD candidate at the Cavendish Laboratory who is funded by Nava Technology Limited. “Unlike other photovoltaic technologies, we don’t need to add an additional layer to improve performance, the additive is simply mixed in with the perovskite ink.”

̽��ֱ��perovskite and potassium devices showed good stability in tests, and were 21.5% efficient at converting light into electricity, which is similar to the best perovskite-based solar cells and not far below the practical efficiency limit of silicon-based solar cells, which is (29%). Tandem cells made of two perovskite layers with ideal bandgaps have a theoretical efficiency limit of 45% and a practical limit of 35% - both of which are higher than the current practical efficiency limits for silicon. “You get more power for your money,” said Stranks.

̽��ֱ��research has also been supported in part by the Royal Society and the Engineering and Physical Sciences Research Council. ̽��ֱ��international team included researchers from Cambridge, Sheffield ̽��ֱ��, Uppsala ̽��ֱ�� in Sweden and Delft ̽��ֱ�� of Technology in the Netherlands.

Reference:
Mojtaba Abdi-Jalebi et al. ‘Maximising and Stabilising Luminescence from Halide Perovskites with Potassium Passivation.’ Nature (2018). DOI: 10.1038/nature25989

A simple potassium solution could boost the efficiency of next-generation solar cells, by enabling them to convert more sunlight into electricity.

Perovskites are very tolerant to additives – you can add new components and they’ll perform better.

Mojtaba Abdi-Jalebi

Matt Klug

Atomic scale view of perovskite crystal formation

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Defects in next-generation solar cells can be healed with light

sc604 — Wed, 06 Sep 2017 15:48:00 +0000

̽��ֱ��international team of researchers demonstrated in 2016 that defects in the crystalline structure of perovskites could be healed by exposing them to light, but the effects were temporary.

Now, an expanded team, from Cambridge, MIT, Oxford, Bath and Delft, have shown that these defects can be permanently healed, which could further accelerate the development of cheap, high-performance perovskite-based solar cells that rival the efficiency of silicon. Their results are reported in the inaugural edition of the journal Joule, published by Cell Press.

Most solar cells on the market today are silicon-based, but since they are expensive and energy-intensive to produce, researchers have been searching for alternative materials for solar cells and other photovoltaics. Perovskites are perhaps the most promising of these alternatives: they are cheap and easy to produce, and in just a few short years of development, perovskites have become almost as efficient as silicon at converting sunlight into electricity.

Despite the potential of perovskites, some limitations have hampered their efficiency and consistency. Tiny defects in the crystalline structure of perovskites, called traps, can cause electrons to get “stuck” before their energy can be harnessed. ̽��ֱ��easier that electrons can move around in a solar cell material, the more efficient that material will be at converting photons, particles of light, into electricity.

“In perovskite solar cells and LEDs, you tend to lose a lot of efficiency through defects,” said Dr Sam Stranks, who led the research while he was a Marie Curie Fellow jointly at MIT and Cambridge. “We want to know the origins of the defects so that we can eliminate them and make perovskites more efficient.”

In a 2016 paper, Stranks and his colleagues found that when perovskites were exposed to illumination, iodide ions – atoms stripped of an electron so that they carry an electric charge – migrated away from the illuminated region, and in the process swept away most of the defects in that region along with them. However, these effects, while promising, were temporary because the ions migrated back to similar positions when the light was removed.

In the new study, the team made a perovskite-based device, printed using techniques compatible with scalable roll-to-roll processes, but before the device was completed, they exposed it to light, oxygen and humidity. Perovskites often start to degrade when exposed to humidity, but the team found that when humidity levels were between 40 and 50 percent, and the exposure was limited to 30 minutes, degradation did not occur. Once the exposure was complete, the remaining layers were deposited to finish the device.

When the light was applied, electrons bound with oxygen, forming a superoxide that could very effectively bind to electron traps and prevent these traps from hindering electrons. In the accompanying presence of water, the perovskite surface also gets converted to a protective shell. ̽��ֱ��shell coating removes traps from the surfaces but also locks in the superoxide, meaning that the performance improvements in the perovskites are now long-lived.

“It’s counter-intuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells,” said Stranks, who is now based at Cambridge’s Cavendish Laboratory. “We’ve seen an increase in luminescence efficiency from one percent to 89 percent, and we think we could get it all the way to 100 percent, which means we could have no voltage loss – but there’s still a lot of work to be done.”

̽��ֱ��research was funded by the European Union, the National Science Foundation, and the Engineering and Physical Sciences Research Council.

Reference:
Roberto Brenes et al. ‘Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals.’ Joule (2017). DOI: 10.1016/j.joule.2017.08.006

Researchers have shown that defects in the molecular structure of perovskites – a material which could revolutionise the solar cell industry – can be “healed” by exposing it to light and just the right amount of humidity.

We want to know the origins of the defects so that we can eliminate them and make perovskites more efficient.

Sam Stranks

Dr Matthew T Klug

' ̽��ֱ��concoction of light with water and oxygen molecules leads to substantial defect-healing in metal halide perovskite semiconductors

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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