̽��ֱ�� of Cambridge - Dawei Di

New efficiency record set for perovskite LEDs

sc604 — Mon, 05 Nov 2018 16:00:56 +0000

Compared to OLEDs, which are widely used in high-end consumer electronics, the perovskite-based LEDs, developed by researchers at the ̽��ֱ�� 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.

̽��ֱ��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 ̽��ֱ��’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.”

̽��ֱ��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.

“ ̽��ֱ��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. ̽��ֱ��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.

̽��ֱ��research was funded by the Engineering and Physical Sciences Research Council (EPSRC) and the European Research Council (ERC).

Reference:
Baodan Zhao et al. ‘High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes.’ Nature Photonics (2018). DOI: 10.1038/s41566-018-0283-4

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̽��ֱ�� ̽��ֱ�� 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.

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

Artistic impression of the perovskite-polymer heterostructure used in LEDs

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Rotating molecules create a brighter future

ps748 — Thu, 30 Mar 2017 18:30:00 +0000

Writing in Science this week, the team, from the ̽��ֱ�� of Cambridge, the ̽��ֱ�� of East Anglia and the ̽��ֱ�� of Eastern Finland, describes how it developed a new type of material that uses rotatable molecules to emit light faster than has ever been achieved before. It could lead to televisions, smart-phone displays and room lights which are more power-efficient, brighter and longer lasting than those currently on the market.

Corresponding author, Dr Dan Credgington, of the ̽��ֱ�� of Cambridge’s Cavendish Laboratory, says:

“It’s amazing that the very first demonstration of this new kind of material already beats the performance of technologies which have taken decades to develop. If the effect we have discovered can be harnessed across the spectrum, it could change the way we generate light.”

Molecular materials are the driving force behind modern organic light-emitting diodes (OLEDs). Invented in the 1980s, these devices emit light when electricity is applied to the organic (carbon based) molecules in them. OLED lighting is now widely used in televisions, computers and mobile phones. However it has to overcome a fundamental issue which has limited efficiency when it comes to converting electrical energy into light.

Passing an electric current through these molecules puts them into an excited state, but only 25% of these are ‘bright’ states that can emit light rapidly. ̽��ֱ��remaining 75% are ‘dark’ states that usually waste their energy as heat limiting the efficiency of the OLED device. This mode of operation produces more heat than light just like in an old fashioned filament light bulb. ̽��ֱ��underlying reason is a quantum property called ‘spin’ and the dark states have the wrong type.

One approach to tackle this problem is to use rare elements, such as iridium, which help the dark states to emit light by allowing them to change their spin. ̽��ֱ��problem is this process takes too long, so the energy tied up in the dark states can build up to damaging levels and make the OLED unstable. This effect is such a problem for blue emitting materials (blue light has the highest energy of all the colours) that, in practice, the approach can’t be used.

Dr Le Yang holding one of the most efficient OLED devices, developed in Cambridge

Chemists at the ̽��ֱ�� of East Anglia have now developed a new type of material where two different organic molecules are joined together by an atom of copper or gold. ̽��ֱ��resulting structure looks a bit like a propeller. ̽��ֱ��compounds, which can be made by a simple one-pot procedure from readily available materials, were found to be surprisingly luminescent. By rotating their “propeller”, dark states formed on these materials become twisted, which allows them to change their spin quickly. ̽��ֱ��process significantly increases the rate at which electrical energy is converted into light achieving an efficiency of almost 100% and preventing the damaging build-up of dark states.

Dr Dawei Di and Dr Le Yang, from Cambridge, were co-lead authors long with Dr Alexander Romanov, from the UEA. He says:

“Our discovery that simple compounds of copper and gold can be used as bright and efficient materials for OLEDs demonstrates how chemistry can bring tangible benefits to society. All previous attempts to build OLEDs based on these metals have led to only mediocre success. ̽��ֱ��problem is that those materials required the sophisticated organic molecules to be bound with copper but has not met industrial standards. Our results address an on-going research and development challenge which can bring affordable high-tech OLED products to every home.”

Computational modelling played a major role in uncovering this novel way of harnessing intramolecular twisting motions for energy conversion.

Professor Mikko Linnolahti, of the ̽��ֱ�� of Eastern Finland, where this was done, comments:

“This work forms the case study for how we can explain the principles behind the functioning of these new materials and their application in OLEDS.”

̽��ֱ��next step is to design new molecules that take full advantage of this mechanism, with the ultimate goal of removing the need for rare elements entirely. This would solve the longest standing problem in the field – how to make OLEDs without having to trade-off between efficiency and stability.

Co-lead author, Dr Dawei Di, of the Cavendish Laboratory, says:

“Our work shows that excited-state spin and molecular motion can work together to strongly impact the performance of OLEDs. This is an excellent demonstration of how quantum mechanics, an important branch of fundamental science, can have direct consequences for a commercial application which has a massive global market.”

Reference:

Dawei Di et al: “High-performance light-emitting diodes based on carbene-metal-amides” is published in Science 30th March 2017

DOI 10.1126/science.aah4345

Scientists have discovered a group of materials which could pave the way for a new generation of high-efficiency lighting, solving a quandary which has inhibited the performance of display technology for decades. ̽��ֱ��development of energy saving concepts in display and lighting applications is a major focus of research, since a fifth of the world’s electricity is used for generating light.

If the effect we have discovered can be harnessed across the spectrum, it could change the way we generate light

Dr Dan Credgington

Molecules in a test tube giving off light

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