Seiichiro Izawa and Team
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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.
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-DNA (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.
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.
The research findings were published in the journal Nature Communications.
Led Display Board Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device with a typical blue-fluorescent emitter that is widely utilized in a commercial display. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons.