Atom swapping could lead to ultra-bright, flexible next generation LEDs

sc604 — Mon, 07 Jun 2021 15:25:23 +0000

̽��ֱ��researchers, led by the ̽��ֱ�� of Cambridge and the Technical ̽��ֱ�� of Munich, found that by swapping one out of every 1,000 atoms of one material for another, they were able to triple the luminescence of a new material class of light emitters known as halide perovskites.

This ‘atom swapping’, or doping, causes the charge carriers to get stuck in a specific part of the material’s crystal structure, where they recombine and emit light. ̽��ֱ��results, reported in the Journal of the American Chemical Society, could be useful for low-cost printable and flexible LED lighting, displays for smartphones or cheap lasers.

Many everyday applications now use light-emitting devices (LEDs), such as domestic and commercial lighting, TV screens, smartphones and laptops. ̽��ֱ��main advantage of LEDs is they consume far less energy than older technologies.

Ultimately, also the entirety of our worldwide communication via the internet is driven by optical signals from very bright light sources that within optical fibres carry information at the speed of light across the globe.

̽��ֱ��team studied a new class of semiconductors called halide perovskites in the form of nanocrystals which measure only about a ten-thousandth of the thickness of a human hair. These ‘quantum dots’ are highly luminescent materials: the first high-brilliance QLED TVs incorporating quantum dots recently came onto the market.

̽��ֱ��Cambridge researchers, working with Daniel Congreve’s group at Harvard, who are experts in the fabrication of quantum dots, have now greatly improved the light emission from these nanocrystals. They substituted one out of every one thousand atoms with another – swapping lead for manganese ions – and found the luminescence of the quantum dots tripled.

A detailed investigation using laser spectroscopy revealed the origin of this observation. “We found that the charges collect together in the regions of the crystals that we doped,” said Sascha Feldmann from Cambridge’s Cavendish Laboratory, the study’s first author. “Once localised, those energetic charges can meet each other and recombine to emit light in a very efficient manner.”

“We hope this fascinating discovery: that even smallest changes to the chemical composition can greatly enhance the material properties, will pave the way to cheap and ultrabright LED displays and lasers in the near future,” said senior author Felix Deschler, who is jointly affiliated at the Cavendish and the Walter Schottky Institute at the Technical ̽��ֱ�� of Munich.

In the future, the researchers hope to identify even more efficient dopants which will help make these advanced light technologies accessible to every part of the world.

Reference:
Sascha Feldmann et al. ‘Charge carrier localization in doped perovskite nanocrystals enhances radiative recombination.’, Journal of the American Chemical Society (2021). DOI: 10.1021/jacs.1c01567

An international group of researchers has developed a new technique that could be used to make more efficient low-cost light-emitting materials that are flexible and can be printed using ink-jet techniques.

Ella Maru Studio

Artist’s impression of glowing halide perovskite nanocrystals

̽��ֱ��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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‘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

̽��ֱ�� of Cambridge - Sascha Feldmann

Atom swapping could lead to ultra-bright, flexible next generation LEDs

‘Messy’ production of perovskite material increases solar cell efficiency