Inexpensive perovskite LEDs now rival the efficiency if not the stability – yet – of much more expensive organic LEDs

via University of Cambridge

Researchers have set a new efficiency record for LEDs based on perovskite semiconductors, rivalling that of the best organic LEDs (OLEDs).

Compared to OLEDs, which are widely used in high-end consumer electronics, the perovskite-based LEDs, developed by researchers at the University of Cambridge, can be made at much lower costs, and can be tuned to emit light across the visible and near-infrared spectra with high colour purity.

The researchers have engineered the perovskite layer in the LEDs to show close to 100% internal luminescence efficiency, opening up future applications in display, lighting and communications, as well as next-generation solar cells.

These perovskite materials are of the same type as those found to make highly efficient solar cells that could one day replace commercial silicon solar cells. While perovskite-based LEDs have already been developed, they have not been nearly as efficient as conventional OLEDs at converting electricity into light.

Earlier hybrid perovskite LEDs, first developed by Professor Sir Richard Friend’s group at the University’s Cavendish Laboratory four years ago, were promising, but losses from the perovskite layer, caused by tiny defects in the crystal structure, limited their light-emission efficiency.

Now, Cambridge researchers from the same group and their collaborators have shown that by forming a composite layer of the perovskites together with a polymer, it is possible to achieve much higher light-emission efficiencies, close to the theoretical efficiency limit of thin-film OLEDs. Their results are reported in the journal Nature Photonics.

“This perovskite-polymer structure effectively eliminates non-emissive losses, the first time this has been achieved in a perovskite-based device,” said Dr Dawei Di from Cambridge’s Cavendish Laboratory, one of the corresponding authors of the paper. “By blending the two, we can basically prevent the electrons and positive charges from recombining via the defects in the perovskite structure.”

The perovskite-polymer blend used in the LED devices, known as a bulk heterostructure, is made of two-dimensional and three-dimensional perovskite components and an insulating polymer. When an ultra-fast laser is shone on the structures, pairs of electric charges that carry energy move from the 2D regions to the 3D regions in a trillionth of a second: much faster than earlier layered perovskite structures used in LEDs. Separated charges in the 3D regions then recombine and emit light extremely efficiently.

“Since the energy migration from 2D regions to 3D regions happens so quickly, and the charges in the 3D regions are isolated from the defects by the polymer, these mechanisms prevent the defects from getting involved, thereby preventing energy loss,” said Di.

“The best external quantum efficiencies of these devices are higher than 20% at current densities relevant to display applications, setting a new record for perovskite LEDs, which is a similar efficiency value to the best OLEDs on the market today,” said Baodan Zhao, the paper’s first author.

While perovskite-based LEDs are beginning to rival OLEDs in terms of efficiency, they still need better stability if they are to be adopted in consumer electronics. When perovskite-based LEDs were first developed, they had a lifetime of just a few seconds. The LEDs developed in the current research have a half-life close to 50 hours, which is a huge improvement in just four years, but still nowhere near the lifetimes required for commercial applications, which will require an extensive industrial development programme. “Understanding the degradation mechanisms of the LEDs is a key to future improvements,” said Di.

Learn more: New efficiency record set for perovskite LEDs

 

 

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Organic light-emitting devices and printed electronics can now be connected to a socket in the wall

Deyu Tu , informationskodning, 2017

Organic light-emitting devices and printed electronics can be connected to a socket in the wall by way of a small, inexpensive organic converter, developed in a collaboration between Linköping University and Umeå University.

Printed electronics and organic light-emitting devices now perform at levels sufficient for a number of eco-friendly, energy-efficient applications. Previously the idea has been to drive the organic electronics using solar cells, batteries or wireless transformers, which works well in many cases. But for fixed installations like lighting, signage or UV-blocking windows, it is convenient to use a wall socket. Until now this has not been possible, because the high voltage damages the electronics.

Thin-film transistors

Docent Deyu Tu from LiU’s Division of Information Coding has led a project where colleagues at Umeå University joined forces to find a solution to this problem. And they have now been able to demonstrate an organic converter that makes it possible to drive organic light-emitting devices with high luminescence, and to charge supercapacitors, both using electricity from an ordinary wall socket.

The converter consists of diode-connected organic thin-film transistors, operated at high voltages up to 325 V, with the capacity to transform high alternating current (AC) to a selected direct current (DC).

“For the first time in the world we have been able to demonstrate an AC/DC converter in organic electronics that functions at voltages above 300 V,” says Deyu Tu.

“Our converter paves the way for a wave of flexible, thin, cost-effective and eco-friendly solutions for the electronics of the future.”

A pioneer work

This is a pioneer work of organic AC/DC converters, a first stage to prove the concept of organic power electronics. To be used in real products, the power conversion efficiency needs to be improved.

”We have initiated the follow-up work to deal with this issue”, says Deyu Tu.

Learn more: Organic electronics can use power from socket

 

 

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Towards a dramatically better television, tablet and phone screen and there’s more

Harvard University researchers have designed more than 1,000 new blue-light emitting molecules for organic light-emitting diodes (OLEDs) that could dramatically improve displays for televisions, phones, tablets and more (Image courtesy of Samsung)

Harvard University researchers have designed more than 1,000 new blue-light emitting molecules for organic light-emitting diodes (OLEDs) that could dramatically improve displays for televisions, phones, tablets and more (Image courtesy of Samsung)

New molecules promise cheaper, more efficient OLED displays

Harvard University researchers have designed more than 1,000 new blue-light emitting molecules for organic light-emitting diodes (OLEDs) that could dramatically improve displays for televisions, phones, tablets and more.

OLED screens use organic molecules that emit light when an electric current is applied. Unlike ubiquitous liquid crystal displays (LCDs), OLED screens don’t require a backlight, meaning the display can be as thin and flexible as a sheet of plastic. Individual pixels can be switched on or entirely off, dramatically improving the screen’s color contrast and energy consumption. OLEDs are already replacing LCDs in high-end consumer devices but a lack of stable and efficient blue materials has made them less competitive in large displays such as televisions.

The interdisciplinary team of Harvard researchers, in collaboration with MIT and Samsung, developed a large-scale, computer-driven screening process, called the Molecular Space Shuttle, that incorporates theoretical and experimental chemistry, machine learning and cheminformatics to quickly identify new OLED molecules that perform as well as, or better than, industry standards.

“People once believed that this family of organic light-emitting molecules was restricted to a small region of molecular space,” said Alán Aspuru-Guzik, Professor of Chemistry and Chemical Biology, who led the research. “But by developing a sophisticated molecular builder, using state-of-the art machine learning, and drawing on the expertise of experimentalists, we discovered a large set of high-performing blue OLED materials.”

The research is described in the current issue of Nature Materials.

The biggest challenge in manufacturing affordable OLEDs is emission of the color blue.

Like LCDs, OLEDs rely on green, red and blue subpixels to produce every color on screen.  But it has been difficult to find organic molecules that efficiently emit blue light. To improve efficiency, OLED producers have created organometallic molecules with expensive transition metals like iridium to enhance the molecule through phosphorescence. This solution is expensive and it has yet to achieve a stable blue color.

Aspuru-Guzik and his team sought to replace these organometallic systems with entirely organic molecules.

The team began by building libraries of more than 1.6 million candidate molecules. Then, to narrow the field, a team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), led by Ryan Adams, Assistant Professor of Computer Science, developed new machine learning algorithms to predict which molecules were likely to have good outcomes, and prioritize those to be virtually tested. This effectively reduced the computational cost of the search by at least a factor of ten.

“This was a natural collaboration between chemistry and machine learning,” said David Duvenaud, a postdoctoral fellow in the Adams lab and coauthor of the paper. “Since the early stages of our chemical design process starts with millions of possible candidates, there’s no way for a human to evaluate and prioritize all of them. So, we used neural networks to quickly prioritize the candidates based on all the molecules already evaluated.”

“Machine learning tools are really coming of age and starting to see applications in a lot of scientific domains,” said Adams.  “This collaboration was a wonderful opportunity to push the state of the art in computer science, while also developing completely new materials with many practical applications. It was incredibly rewarding to see these designs go from machine learning predictions to devices that you can hold in your hand.”

“We were able to model these molecules in a way that was really predictive,” said Rafael Gómez-Bombarelli, a postdoctoral fellow in the Aspuru-Guzik lab and first author of the paper.  “We could predict the color and the brightness of the molecules from a simple quantum chemical calculation and about 12 hours of computing per molecule. We were charting chemical space and finding the frontier of what a molecule can do by running virtual experiments.”

“Molecules are like athletes,” Aspuru-Guzik said. “It’s easy to find a runner, it’s easy to find a swimmer, it’s easy to find a cyclist but it’s hard to find all three. Our molecules have to be triathletes. They have to be blue, stable and bright.”

But finding these super molecules takes more than computing power — it takes human intuition, said Tim Hirzel, a senior software engineer in the Department of Chemistry and Chemical Biology and coauthor of the paper.

To help bridge the gap between theoretical modeling and experimental practice, Hirzel and the team built a web application for collaborators to explore the results of more than half a million quantum chemistry simulations.

Every month, Gómez-Bombarelli and coauthor Jorge Aguilera-Iparraguirre, also a postdoctoral fellow in the Aspuru-Guzik lab, selected the most promising molecules and used their software to create “baseball cards,” profiles containing important information about each molecule. This process identified 2500 molecules worth a closer look.  The team’s experimental collaborators at Samsung and MIT then voted on which molecules were most promising for application. The team nicknamed the voting tool “molecular Tinder” after the popular online dating app.

“We facilitated the social aspect of the science in a very deliberate way,” said Hirzel.

“The computer models do a lot but the spark of genius is still coming from people,” said Gómez-Bombarelli.

“The success of this effort stems from its multidisciplinary nature,” said Aspuru-Guzik. “Our collaborators at MIT and Samsung provided critical feedback regarding the requirements for the molecular structures.”

“The high throughput screening technique pioneered by the Harvard team significantly reduced the need for synthesis, experimental characterization, and optimization,” said Marc Baldo, Professor of Electrical Engineering and Computer Science at MIT and coauthor of the paper. “It shows the industry how to advance OLED technology faster and more efficiently.”

After this accelerated design cycle, the team was left with hundreds of molecules that perform as well as, if not better than, state-of-the-art metal-free OLEDs.

Applications of this type of molecular screening also extend far beyond OLEDs.

“This research is an intermediate stop in a trajectory towards more and more advanced organic molecules that could be used in flow batteries, solar cells, organic lasers, and more,” said Aspuru-Guzik. “The future of accelerated molecular design is really, really exciting.”

Learn more: Towards a better screen

 

 

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Organic lights and solar cells straight from the printer

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Organic lights and solar cells straight from the printer

Flickering façades, curved monitors, flashing clothing, fluorescent wallpaper, flexible solar cells – and all printable.

This is no make-believe vision of the future; it will soon be possible using a new printing process for organic light-emitting diodes.

Time is slowly running out for bulky television sets, boxy neon signs and the square-edged backlit displays we all know from shops and airports. It won’t be long before families gathering together to watch television at home will be calling out: “Unroll the screen, dear, the film’s about to start!” And members of the public may soon encounter screens everywhere they go, as almost any surface can be made into a display. “These may just be ideas at the moment, but they have every chance of becoming reality,” says Dr. Armin Wedel, head of division at the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam-Golm. The first curved screens were on display at this year’s consumer electronics trade show (IFA) in Berlin. The technology behind it all? OLEDs: flexible, organic, light-emitting diodes.

Molecule solutions as ink

But the potential offered by this technology extends beyond screens and displays for consumer electronics, according to Wedel. He believes OLEDs are also ideally suited to all kinds of lighting and to digital signage applications – that is to say, advertising and information systems such as electronic posters, advertisements, large image projections, road signs and traffic management systems. The scientists worked together with mechanical engineering company MBRAUN to develop a production facility able to create OLEDs as well as organic solar cells on an industrial scale. The innovative part is that it is now possible to print OLEDs and solar cells from solutions containing luminescent organic molecules and absorptive molecules respectively, which makes printing them onto a carrier film very straightforward. Usually, printing them involves vaporizing small molecules in a high vacuum, making it a very expensive process.

Scientists had previously only ever used various printing technologies to design components on a laboratory scale. They can now produce larger sample series – and this is particularly advantageous for the applications that the IAP has in mind, as large illuminated surfaces and information systems require tailored solutions produced in relatively small numbers. “We’re now able to produce organic components under close-to-real-life manufacturing conditions with relative ease. Now for the first time it will be possible to translate new ideas into commercial products,” Wedel says.

At the heart of the pilot plant is a robot that controls different printers that basically act like an inkjet printing system. OLEDs are applied to the carrier material one layer at a time using a variety of starting materials. This produces a very homogenous surface that creates a perfect lighting layer. “We’re able to service upscale niche markets by offering tailored solutions, as we can apply the organic electronic system to customers’ specifications, just like in digital printing,” explains Wedel.

Industry experts estimate that printed OLEDs hold out the promise of becoming a billion-dollar market. “The focus in Germany and Europe is on OLED lighting because this is the home market for large companies such as Osram and Philips,” explains Wedel. “The manufacturing facility will help secure competitive advantages in this particular segment of the market. It strengthens the German research community, and also demonstrates the capabilities of German plant engineering,” says Dr. Martin Reinelt, CEO of MBRAUN in Garching.

OLEDs have several advantages over conventional display technologies. Unlike liquid crystal displays they do not require backlighting, which means they consume less energy. As it is the diodes themselves that emit colored light, contrast and color reproduction are better. The electroluminescent displays also offer a large viewing angle of almost 180 degrees. And because they require no backlighting, they can be very thin, making it possible to create entirely new shapes.

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Cheaper, more efficient OLEDs – just add chlorine

A team of researchers from the University of Toronto has developed a new technique to produce OLED devices that they say will accelerate the adoption of OLED technology into mainstream flat-panel displays and other lighting technologies.

The process involves engineering a one-atom thick sheet of chlorine onto the surface of an indium tin oxide (ITO) material, which is used as a standard electrode material in today’s flat panel displays. The end result is an OLED device that is not only more efficient, but also simpler and cheaper to produce.

The one-atom thick layer of chlorine is applied to the surface of ITO using a UV light assisted process that negates the need for chlorine gas, which the researcher point out makes the process safe and reliable. The resultant chlorinated ITO material allows for the efficient transport of electrons, while eliminating the need for several costly layers found in conventional OLED devices.

In tests that pitted their green-emitting “Cl-OLED” against a conventional OLED, the researchers found that the efficiency of their device was more than double that of conventional OLEDs at very high brightness. Although OLEDs are recognized for their high-efficiency, which is much greater than LCD, as the brightness is increased, the efficiency drops off dramatically. With the chlorinated ITO, the researchers were able to prevent this drop off and achieve a record efficiency of 50 percent at 10,000 cd/m2, which is more than twice as efficient as conventional OLEDs.

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