A group of materials has paved the way for brighter future, by bringing a new generation of high-efficiency lighting closer to the reality.
The 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.
The team, from the University of Cambridge, the University of East Anglia and the University 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 Dan Credgington, said: "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 percent of these are 'bright' states that can emit light rapidly. The remaining 75 percent 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.
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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. The 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.
Chemists at the University 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. The resulting structure looks a bit like a propeller. The 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. The process significantly increases the rate at which electrical energy is converted into light achieving an efficiency of almost 100 percent and preventing the damaging build-up of dark states.
The study appears in Science.