̽��ֱ�� of Cambridge - Stuart MacPherson

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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‘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

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