TIT Develops a Better Blue LED

Tuckerfan · Sep 24, 2023

That's the Tokyo Institute of Technology.

In a first, scientists light up blue LED with an AA battery

Scientists at the Tokyo Institute of Technology (TIT), in collaboration with other universities in Japan, have developed a novel organic light-emitting diode (LED) that emits blue fluorescence but has a low turn-on voltage, a press release said.

LEDs are crucial components of modern electronic displays, whether in pocket-sized devices such as smartphones or in giant-sized screens used for advertising on billboards. An RGB LED module can produce any color for the display by using three colors: red, green, and blue. While red and green LEDs work well, the blue LED has been tricky from an energy efficiency perspective.

Conventionally used blue LEDs have a high turn-on voltage of 4V for a luminance of 100 cd per square meter (cd/m2). This might not sound very high, but at the industrial level, it brings about issues since the voltage is beyond what can be supplied by a typical lithium-ion battery.

The search for new blue LEDs

Researchers around the world have been looking to solve this problem but the collaboration between TIT and other Japanese institutes, such as the University of Toyama, Shizuoka University, and the Institute for Molecular Science, led to novel blue LED.

In their approach, the team used NDI-HF (2,7-di(9H-fluoren-2-yl)benzo[lmn][3,8]-phenanthroline-1,3,6,8(2H,7H)-tetraone) as the acceptor, 1,2-ADN (9-(naphthalen-1-yl)-10-(naphthalen-2-yl)anthracene) as the donor, and TbPe (2,5,8,11-tetra-tert-butylperylene) as the fluorescent dopant.

When compared to conventional LEDs, this novel LED works via a mechanism called upconversion.
What is upconversion? How does it help?

For a conventional LED to work, charge carriers – holes and electrons – move toward the donor/acceptor (D/A) interface or PN junction and recombine to release light.

In the case of the novel LED though the recombination leads to the formation of a charged state (CT). The energy of the charged state is then selectively transferred to the emitter's low-energy first triplet excited states. This results in the formation of the first singlet excited state by triplet-triplet annihilation (TTA) and results in blue light emission.

"As the energy of the CT state is much lower than the emitter's bandgap energy, the UC mechanism with TTA significantly decreases the applied voltage required for exciting the emitter. As a result, this UC-OLED reaches a luminance of 100 cd/m2, equivalent to that of a commercial display, at just 1.97 V," said Seiichiro Izawa, an associate professor at TIT and Osaka University, in the press release.

The blue light emission can also be reached at a voltage as low as 1.47 V, effectively making it possible for even a single AA battery to light up the LED. However, commercial-scale luminance needs a slightly higher voltage of 1.97V, which is less than half of what conventional LEDs require.

The fluorescent emitter used by the researchers is widely used in commercial displays, further confirming that the technology can be rapidly adopted at commercial scales and does not require major changes in manufacturing to be adopted.

The findings also highlight the importance of the design of the D/A interface that could be used to further improve other devices such as photovoltaics.
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TIT Develops a Better Blue LED

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TIT Develops a Better Blue LED

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