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Long-lasting blue LEDs promise better OLED display longevity

Image credit: Dreamstime

Japanese researchers have cracked the longstanding problem of developing blue LEDs which match the performance and longevity of green and blue ones.

While organic LEDs are known for their ability to produce vibrant colours and form thin and even flexible devices, blue OLEDs have proved a bottleneck in terms of efficiency and stability, and often have a shorter lifespan.

Unlike LCD technologies which employ liquid crystals to selectively block emissions from a filtered backlight covering many pixels, the separate red, green, and blue emitting pixels of an OLED display can be switched on and off individually, producing deeper blacks and reducing power consumption.

“A growing number of options exist for red and green OLEDs with excellent performance, but devices emitting high-energy blue light are more of a challenge, with tradeoffs almost always occurring among efficiency, colour purity, cost, and lifetime,” said Dr Chin-Yiu Chan, a researcher at Kyushu University in Japan.

The prototype LEDs

Using a new combination of emitter molecules, they have demonstrated blue LEDs that match the performance of red and green LEDs.

By splitting energy conversion and emission processes between two molecules, the researchers achieved devices that produce pure-blue emission with high efficiency, maintain brightness for relatively long times, and lack any expensive metal atoms: a combination of properties that has so far been difficult to obtain simultaneously.

They use an alternative method of light emission based on the process of thermally activated delayed fluorescence (TADF) which can achieve excellent efficiency without the metal atom. It uses a two-molecule approach, which has been termed hyperfluorescence, which enables longer operational lifetimes at higher brightness than previously reported for highly efficient OLEDs with a similar colour purity.

“That this kind of approach can extend the lifetime of pure-blue emission from a molecule we previously developed is really exciting,” said researcher Takuji Hatakeyama.

Adopting a tandem structure that basically stacks two devices on top of each other to effectively double the emission for the same electrical current, lifetime was nearly doubled at high brightness, and the researchers estimated that devices could maintain 50 per cent of their brightness for over 10,000 hours at more moderate intensities.

“Though this is still too short for practical applications, stricter control of fabrication conditions often leads to even longer lifetimes, so these initial results point to a very promising future for this approach to finally obtain an efficient and stable pure-blue OLED,” Professor Chihaya Adachi said.

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