Novel OLED Has Ultralow Turn-on Voltage
Researchers at Tokyo Tech just circumvented an existing high-voltage requirement for blue OLEDs. Their OLED, based on a blue-fluorescence emitter, achieves emission at an ultralow turn-on voltage of 1.47 V. Target applications for the OLEDs include smartphones and large screen displays.
Blue light is necessary for light-emitting devices, lighting applications, smartphone screens, and large-screen displays. The difficulty is in developing efficient blue organic light-emitting diodes (OLEDs) given their need for high applied voltage to function, requiring around 4 V for a luminance of 100 cd/m2. This voltage is higher than the industrial target of 3.7 V, the voltage of lithium-ion batteries used in smartphones.
In collaboration with several others, the researchers presented a novel OLED device with an ultralow turn-on voltage of 1.47 V for blue emission and a peak wavelength at 462 nm (2.68 eV). Their work will be published in Nature Communications.
The materials they used in the OLED influenced its turn-on voltage, including 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 upconversion (UC), injecting holes and electrons into the emitter and acceptor (electron transport) layers, respectively. They recombine at the donor/acceptor (D/A) interface to form a charge transfer (CT) state. The energy of the CT state is selectively transferred to the low-energy first triplet excited states of the emitter, resulting in blue light emission by forming a high-energy first singlet excited state by triplet-triplet annihilation (TTA). Given that the energy of the CT state is much lower than the emitter’s bandgap energy, the UC mechanism with TTA decreases the applied voltage required for exciting the emitter, and the UC-OLED reaches a luminance of 100 cd/m2 at just 1.97 V.
