̽��ֱ�� of Cambridge - Felix Deschler

‘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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Eight Cambridge researchers awarded major European starter grants

cjb250 — Tue, 03 Sep 2019 13:35:55 +0000

̽��ֱ��European Research Council (ERC) Starting Grants have been awarded to 408 researchers from across Europe. ̽��ֱ��awards will help individual researchers to build their own teams and conduct world-leading research across all disciplines, creating an estimated 2,500 jobs for postdoctoral fellows, PhD students and other staff at the host institutions.

̽��ֱ��successful Cambridge researchers are:

Roland Bauerschmidt - Renormalisation, dynamics, and hyperbolic symmetry
Quentin Berthet - Computational Trade-offs and Algorithms in Statistics
Felix Deschler - Twisted Perovskites: Control of Spin and Chirality in Highly-luminescent Metal-halide Perovskites
Lorenzo Di Michele - A DNA NANOtechology toolkit for artificial CELL design
Louise Hirst - Gliding epitaxy for inorganic space-power sheets
Sertac Sehlikoglu - Imaginative Landscapes of Islamist Politics Across the Balkan-to-Bengal Complex
Blake Sherwin - CMB Lensing at Sub-Percent Precision: A New Probe of Cosmology and Fundamental Physics
Margherita Turco - Human Placental Development and the Uterine Microenvironment

Commenting on the awards, Dr Peter Hedges, Head of the ̽��ֱ�� Research Office at the ̽��ֱ�� of Cambridge, said: “ ̽��ֱ��success of UK researchers, and in particular Cambridge researchers, demonstrates the world-leading position that our country holds in research and innovation. This is a position we have will have to fight hard to maintain in the face of competition from other nations across Europe, the USA and China.

“Six of our successful researchers are non-UK nationals, showing once again that Cambridge has the ability to attract the very best talent from around the world to carry out research at its world class facilities.”

̽��ֱ��ERC-funded research will be carried out in 24 countries, with institutions from Germany (73), the UK (64) and the Netherlands (53) to host the highest number of projects. ̽��ֱ��grants, worth in total €621 million (£560 million), are part of the EU Research and Innovation programme, Horizon 2020.

Carlos Moedas, European Commissioner for Research, Science and Innovation, said: “Researchers need freedom and support to follow their scientific curiosity if we are to find answers to the most difficult challenges of our age and our future. This is the strength of the grants that the EU provides through the European Research Council: an opportunity for outstanding scientists to pursue their most daring ideas.”

President of the ERC, Professor Jean-Pierre Bourguignon, added: “In this year’s ERC Starting Grant competition, early-career researchers of 51 nationalities are among the winners – a record. It reminds us that science knows no borders and that talent is to be found everywhere. It is essential that, for its future successful development, the European Union keeps attracting and supporting outstanding researchers from around the world. At the ERC we are proud to contribute to this goal by supporting some of the most daring creative scientific talent.”

Eight Cambridge researchers are among the latest recipients of European Union awards given to early-career researchers from over 50 countries.

Six of our successful researchers are non-UK nationals, showing once again that Cambridge has the ability to attract the very best talent from around the world to carry out research at its world class facilities

Peter Hedges

Géry PARENT

Naturally-occurring perovskite

Researcher profile: Dr Margherita Turco

Among this year’s successful awardees is Dr Margherita Turco from Cambridge’s Centre for Trophoblast Research (CTR).

Margherita began her career studying the development of embryos in domestic animals during her studies for Veterinary Biotechnology at the ̽��ֱ�� of Bologna, in Italy. During her PhD in Molecular Medicine at the European Institute of Oncology in Milano, she became interested in how early cell lineage decisions are made and began using various stem cells models to address this question.

This led Margherita to come to Cambridge in 2012 to carry out her postdoctoral work on human trophoblast stem cells at the CTR. Her goal is to understand how the human placenta grows and develops during pregnancy.

“ ̽��ֱ��placenta is a remarkable organ that is formed early in pregnancy. It plays the crucial role of nourishing and protecting the baby throughout its development before birth,” she says. However, there is a lot that can go wrong during this period.

“Complications occurring during pregnancy, such as pre-eclampsia, fetal growth restriction, stillbirth, miscarriage and premature birth, are principally due to defective placental function. These conditions, which collectively affect around one in five pregnancies, can pose a risk to both the baby and mother’s health. Understanding early placental development is the key to understanding successful pregnancy.”

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10 quadrillionths of a second to extraction: Researchers set time limit for ultrafast perovskite solar cells

tdk25 — Thu, 21 Sep 2017 10:00:16 +0000

̽��ֱ��study, which investigated photovoltaic devices based on a type of materials called perovskites, suggests that these could achieve unprecedented levels of super-efficiency. But to do so, they will need to turn sunlight into electrons and then extract these as electrical charge within just quadrillionths of a second – a few “femtoseconds”, to give them their scientific name.

Moving electrons at this ultrafast rate would enable the creation of “hot carrier” cells. These are solar cells which can generate electricity more efficiently by making use of the added kinetic energy that electrons have for a brief moment just after they are created, while they are moving at high speed.

̽��ֱ��amount of electrical energy that can be extracted from a hot carrier cell, relative to the amount of light absorbed, could potentially match or even break an energy efficiency rate of 30%. In rough terms, this is the maximum energy efficiency that solar cells can conceivably achieve – although standard silicon cells typically have efficiencies closer to 20% in practice.

Despite the minuscule fractions of time involved, the authors of the new paper say that it is possible that perovskites could ultimately push this efficiency barrier.

̽��ֱ��study, published in the journal Nature Communications, was carried out by academics in Italy and the UK. ̽��ֱ��British team involved researchers in the Cavendish Laboratory’s Optoelectronics research group of Professor Sir Richard Friend, a Fellow of St John’s College, Cambridge. ̽��ֱ��Italian team are based at the Politecnico di Milano in the group of Professor Guilio Cerullo.

Johannes Richter, a PhD student in the Optoelectronics group and the paper’s lead author, said: “ ̽��ֱ��timescale that we calculated is now the time limit that we have to operate within if we want to create super-efficient, hot carrier solar devices. We would need to get electrons out before this tiny amount of time elapses.”

“We are talking about doing this extremely quickly, but it’s not impossible that it could happen. Perovskite cells are very thin and this gives us hope, because the distance that the electrons have to cover is therefore very short.”

Perovskites are a class of materials which could before long replace silicon as the material of choice for many photovoltaic devices. Although perovskite solar cells have only been developed within the past few years, they are already almost as energy-efficient as silicon.

Partly because they are considerably thinner, they are much cheaper to make. While silicon cells are about a millimetre thick, perovskite equivalents have a thickness of approximately one micrometre, about 100 times thinner than a human hair. They are also very flexible, meaning that in addition to being used to power buildings and machines, perovskite cells could eventually be incorporated into things like tents, or even clothing.

In the new study, the researchers wanted to know for how long the electrons produced by these cells retain their highest possible levels of energy. When sunlight hits the cell, light particles (or photons), are converted into electrons. These can be drawn out through an electrode to harvest electrical charge.

For a brief moment after they are created, the electrons are moving very quickly. However, they then start to collide, and lose energy. Electrons which retain their speed, prior to collision, are known as “hot” and their added kinetic energy means that they have the potential to produce more charge.

“Imagine if you had a pool table and each ball was moving at the same speed,” Richter explained. “After a certain amount of time, they are going to hit each other, which causes them to slow down and change direction. We wanted to know how long we have to extract the electrons before this happens.”

̽��ֱ��Cambridge team took advantage of a method developed by their colleagues in Milan called two dimensional spectroscopy. This involves pumping light from two lasers on to samples of lead iodide perovskite cell in order to simulate sunlight, and then using a third “probe” laser to measure how much light is being absorbed.

Once the electrons have collided and slowed down, and are thus starting to take up space in the cell, the amount of light being absorbed changes. ̽��ֱ��time it took for this to happen in the study effectively allowed the researchers to establish how much time is available to extract electrons while they are still “hot”.

̽��ֱ��study found that electron collision events started to happen between 10 and 100 femtoseconds after light was initially absorbed by the cell. To maximise energy efficiency, the electrons would thus need to reach the electrode in as little as 10 quadrillionths of a second.

̽��ֱ��researchers are nonetheless optimistic that this might be possible. As well as taking advantage of the intrinsic thinness of perovskite, they believe that nanostructures could be created within the cells to reduce further the distance that the electrons need to travel.

“That approach is just an idea for now, but it is the sort of thing that we would require in order to overcome the very small timescales that we have measured,” Richter added.

̽��ֱ��paper, Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy, is published in Nature Communications.

Researchers have quantified the astonishingly high speeds at which future solar cells would have to operate in order to stretch what are presently seen as natural limits on their energy conversion efficiency.

̽��ֱ��timescale that we calculated is now the time limit that we have to operate within if we want to create super-efficient, hot carrier solar devices.

Johannes Richter

Credit: James Cridland, via Flickr

̽��ֱ��GLA building’s solar panels. Perovskite solar cells are already challenging the energy-efficiency of silicon cells such as these.

̽��ֱ��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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Solar cell material can recycle light to boost efficiency

tdk25 — Thu, 24 Mar 2016 18:00:18 +0000

Scientists have discovered that a highly promising group of materials known as hybrid lead halide perovskites can recycle light – a finding that they believe could lead to large gains in the efficiency of solar cells.

Hybrid lead halide perovskites are a particular group of synthetic materials which have been the subject of intensive scientific research, as they appear to promise a revolution in the field of solar energy. As well as being cheap and easy to produce, perovskite solar cells have, in the space of a few years, become almost as energy-efficient as silicon – the material currently used in most household solar panels.

By showing that they can also be optimised to recycle light, the new study suggests that this could just be the beginning. Solar cells work by absorbing photons from the sun to create electrical charges, but the process also works in reverse, because when the electrical charges recombine, they can create a photon. ̽��ֱ��research shows that perovskite cells have the extra ability to re-absorb these regenerated photons – a process known as “photon recycling”. This creates a concentration effect inside the cell, as if a lens has been used to focus lots of light in a single spot.

According to the researchers, this ability to recycle photons could be exploited with relative ease to create cells capable of pushing the limits of energy efficiency in solar panels.

̽��ֱ��study builds on an established collaboration, focusing on the use of these materials not only in solar cells but also in light-emitting diodes, and was carried out in the group of Richard Friend, Cavendish Professor of Physics and Fellow of St John’s College at the ̽��ֱ�� of Cambridge. ̽��ֱ��research was undertaken in partnership with the team of Henry Snaith at the ̽��ֱ�� of Oxford and Bruno Ehrler at the FOM Institute, AMOLF, Amsterdam.

Felix Deschler, who is one of the corresponding authors of the study and works with a team studying perovskites at the Cavendish Laboratory, said: “It’s a massive demonstration of the quality of this material and opens the door to maximising the efficiency of solar cells. ̽��ֱ��fabrication methods that would be required to exploit this phenomenon are not complicated, and that should boost the efficiency of this technology significantly beyond what we have been able to achieve until now.”

Perovskite-based solar cells were first tested in 2012, and were so successful that in 2013, Science Magazine rated them one of the breakthroughs of the year.

Since then, researchers have made rapid progress in improving the efficiency with which these cells convert light into electrical energy. Recent experiments have produced power conversion efficiencies of around 20% - a figure already comparable with silicon cells.

By showing that perovskite-based cells can also recycle photons, the new research suggests that they could reach efficiencies well beyond this.

̽��ֱ��study, which is reported in Science, involved shining a laser on to one part of a 500 nanometre-thick sample of lead-iodide perovskite. Perovskites emit light when they come into contact with it, so the team was able to measure photon activity inside the sample based on the light it emitted.

Close to where the laser light had shone on to the film, the researchers detected a near-infrared light emission. Crucially, however, this emission was also detected further away from the point where the laser hit the sample, together with a second emission composed of lower-energy photons.

“ ̽��ֱ��low-energy component enables charges to be transported over a long distance, but the high-energy component could not exist unless photons were being recycled,” Luis Miguel Pazos Outón, lead author on the study, said. “Recycling is a quality that materials like silicon simply don’t have. This effect concentrates a lot of charges within a very small volume. These are produced by a combination of incoming photons and those being made within the material itself, and that’s what enhances its energy efficiency.”

As part of the study, Pazos Outón also manufactured the first demonstration of a perovskite-based back-contact solar cell. This single cell proved capable of transporting an electrical current more than 50 micrometres away from the contact point with the laser; a distance far greater than the researchers had predicted, and a direct result of multiple photon recycling events taking place within the sample.

̽��ֱ��researchers now believe that perovskite solar cells, may be able to reach considerably higher efficiencies than they have to date. “ ̽��ֱ��fact that we were able to show photon recycling happening in our own cell, which had not been optimised to produce energy, is extremely promising,” Richard Friend, a corresponding author, said. “If we can harness this it would lead to huge gains in terms of energy efficiency.”

Reference:
Luis M. Pazos-Outón et al. 'Photon recycling in lead iodide perovskite solar cells.' Science (2016). DOI: 10.1126/science.aaf1168

Perovskite materials can recycle light particles – a finding which could lead to a new generation of affordable, high-performance solar cells.

It’s a massive demonstration of the quality of this material and opens the door to maximising the efficiency of solar cells

Felix Deschler

Criss Hohmann

Depiction of photon recycling inside the crystalline structure of perovskite.

̽��ֱ��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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