Scientists light up novel blue OLED with an AA battery

Blue light is vital for light-emitting diodes, lighting applications and large screen displays. However, it can be challenging to develop efficient blue organic light-emitting diodes (OLEDs) due to the high applied voltage required for their function. Conventional blue OLEDs require around 4 V for a luminance of 100 cd/m2; this is higher than the industrial target of 3.7 V — the voltage of lithium-ion batteries typically used in smartphones. Therefore, there is a need to develop novel blue OLEDs that can operate at lower voltages.

Now, Associate Professor Seiichiro Izawa from the Tokyo Institute of Technology, in collaboration with researchers from the University of Toyama, Shizuoka University and the Institute for Molecular Science, has developed a novel OLED device with an ultralow voltage of 1.47 V for blue emission and a peak wavelength at 462 nm (2.68 eV). The research findings will be published in Nature Communications.

The materials used in this OLED influences its turn-on voltage. The device utilises 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.

The OLED operates via a mechanism called upconversion (UC), in which holes and electrons are injected into donor (emitter) and acceptor (electron transport) layers, respectively. They recombine at the donor–receptor (D/A) interface to form a charge transfer (CT) state. Izawa said the intermolecular interactions at the D/A interface play a significant role in CT state formation, with stronger interactions yielding superior results.

Subsequently, the energy of the CT state is selectively transferred to the low-energy first triplet excited states of the emitter, which results in blue light emission through the formation of a high-energy first singlet excited state by triplet-triplet annihilation (TTA). “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,” Izawa said.

The researchers have efficiently produced a novel OLED, with blue light emission at an ultralow turn-on voltage, using a typical fluorescent emitter utilised in commercial displays, this marking a step toward meeting the commercial requirements for blue OLEDs. It emphasised the importance of optimising the design of the D/A interface for controlling excitonic processes and holds promise for OLEDs and for organic photovoltaics, along with other organic electronic devices.

Image caption: Lighting up a blue organic LED with a single AA battery. Image credit: Associate Professor Izawa and the member and authors of this research team.