A team, from the University of Cambridge, the University of East Anglia and the University of Eastern Finland, have 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. The work published in Science (Dawei Di et al: “High-performance light-emitting diodes based on carbene-metal-amides”) was led by Dr Credgington, at the Cavendish Laboratory with Winton Scholar, Johannes Richter one of the co-authors.
Molecular materials are the driving force behind modern organic light-emitting diodes (OLEDs). 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. The 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. The underlying reason is a quantum property called ‘spin’ and the dark states have the wrong type.
The new type of material has two different organic molecules that are joined together by an atom of copper or gold. The resulting structure looks a bit like a propeller. These 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. The 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.
The 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.
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