Blue light is vital for light-emitting devices, lighting applications, as well as smartphone screens and large screen displays. However, it is challenging to develop efficient blue organic light-emitting diodes (OLEDs), owing to the high applied voltage required for their function. Conventional blue OLEDs typically 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 commonly used in smartphones. Therefore, there is an urgent need to develop novel blue OLEDs that can operate at lower voltages.
The choice of materials used in this OLED significantly influences its turn-on voltage. The device utilizes NDI-HF benzo[lmn][3,8]-phenanthroline-1,3,6,8-tetraone) as the acceptor, 1,2-ADN -10-anthracene) as the donor, and TbPe as the fluorescent dopant. This OLED operates via a mechanism called upconversion . Herein, holes and electrons are injected into donor and acceptor layers, respectively. They recombine at the donor/acceptor interface to form a charge transfer state.
An upconversion organic light-emitting diode based on a typical blue-fluorescence emitter achieves emission at an ultralow turn-on voltage of 1.47 V, as demonstrated by researchers from Tokyo Tech. Their technology circumvents the traditional high voltage requirement for blue OLEDs, leading to potential advancements in commercial smartphone and large screen displays.
Subsequently, the energy of the CT state is selectively transferred to the low-energy first triplet excited states of the emitter, which results inemission through the formation of a high-energy first singlet excited state by triplet-triplet annihilation . "As the energy of the CT state is much lower than the emitter's bandgap energy, the UC mechanism with TTA significantly decreases the, equivalent to that of a commercial display, at just 1.97 V," explains Dr. Izawa.
In effect, this study efficiently produces a novel OLED, with blue light emission at an ultralow turn-on voltage, using a typical fluorescent emitter widely utilized in commercial displays, thus marking a significant step toward meeting the commercial requirements for blue OLEDs.
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