Brighter, cheaper blue light could revolutionise screen technology

blue light vial

Molecular structure of the OLED molecule with solution glowing blue. Credit: Petri Murto.

Researchers have found a new way to simplify the structure of high-efficiency blue organic light-emitting diodes (OLEDs), which could lead to longer-lasting and higher definition television screens.

OLEDs are a class of organic electronics that are already found commercially in smartphones and displays and can be more efficient than competing technologies.

Although OLED television screens have vivid picture quality, they also have drawbacks such as high cost and comparatively short lifespans. Another issue is blue light stability.

The screen pixels used in OLED displays are composed of three different coloured subpixels—red, green and blue—that light up at different intensities to create different colours.

However, the subpixels that emit blue light are the least stable and can be susceptible to screen "burn-in," which can discolour the screen and ruin viewing quality.

In a paper published in Nature Materials, a team of researchers from Loughborough, Cambridge, Northumbria, and Imperial universities describe a new design that overcomes these issues and may lead to simpler, less expensive systems with purer and more stable blue light.

The findings may pave the way for TV and smartphone screens that consume less energy, making them more efficient and sustainable.

An OLED Sandwich

An OLED is built like a sandwich, with organic semiconductor layers between two electrodes. In the middle of the stack is the emissive layer, which lights up when powered with electricity. Electrical energy goes into the molecules , which then release this extra energy as light.

An ideal OLED turns most of the electric energy into light, but sometimes the energy gets diverted and degrades the structure of the OLED. This is especially a problem with blue light, and reduces the OLED efficiency and lifetime. The design of efficient OLEDs comes down to managing how the structure of an OLED can channel energy constructively.

A new molecule

To solve this problem, the research team designed a new light emitter molecule that blocks the destructive energy pathways. The imaginative leap was to add shields to the emitter, which controls how the molecules interact.

Loughborough University's Dr Felix Plasser used computational models to explain the remarkable properties of the central light-emitting molecule.

"We were particularly interested in understanding why light emission occurred within the specific energy range required for clear blue emission", said Dr Plasser.

"Our computational models underlined the idea that the molecule in its normal, stable state - called the 'ground state' - is already in a state that makes it ready or 'pre-relaxed' to transition to an excited state when energy is applied.

"This readiness in the ground state helps explain the molecule's unique properties."

Video of the new OLED molecule glowing blue under UV light, credit Petri Murto.

This better understanding of how efficient a molecule in an OLED can be will inform how materials are designed and used in future, supporting the push towards higher device performance.

Cambridge University's Dr Daniel Congrave, who led the material design and synthetic work alongside Professor Hugo Bronstein, said, "OLED screens have great picture quality and carry a high premium. However, OLED TVs don't last as long as other screens.

"Pixels that emit blue light are essential for a practical display but are also where the problems lie. We've designed a molecule that's allowed us to simplify the emissive layer of the blue pixel to only two components, while maintaining high efficiency, which could help to drive down cost.

"The molecule we describe in this paper is also one of the narrowest emitting blue molecules out there, which is very useful for screens because it allows for high colour purity."

The paper, titled 'Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs', can be read on the Nature Materials webpage.