Researchers develop efficient and stable blue OLEDs
Organic light-emitting diodes, OLEDs for short, are energy-efficient and flexible. They are commonly used in smartphones, tablets and large TV screens. They can also be produced at low cost by thin-film technology and also work on flexible carrier materials, thus enabling bendable displays and variable room illumination solutions. However, it is still difficult to produce blue OLEDs, as they lack luminance and stability.
Researchers from the Karlsruhe Institute of Technology (KIT) and Shanghai University have developed a strategy for the production of efficient deep-blue OLEDs: electronic excitation of a specially developed new molecule that results in a dual-channel intra-intermolecular exciplex emission, and therefore, deep-blue electroluminescence. The research findings are published in Science Advances.
An OLED consists of two electrodes, of which at least one is transparent. Thin layers of organic semiconducting materials are located between them. Their light results from electroluminescence. When applying an electrical field, electrons from the cathode and holes (positive charges) from the anode are injected into the organic materials that act as emitters. There, electrons and holes meet and form electron-hole pairs. When these pairs collapse into their initial state again, they release energy that is used by the organic materials to emit light. All colours can be obtained by mixing blue, green and red.
So far, phosphorescent OLEDs in red and green have been available for commercial applications only. Blue light can be produced by fluorescent OLEDs for a short time, as blue OLEDs struggle to combine a high efficiency with a high luminescence and a long service life. Blue pixels are also fainter or faint faster than green and red pixels. In cooperation with researchers from Shanghai University, researchers from KIT have produced a new type of molecule, in which carbazole and triazine fragments are linked by a silicon atom (CzSiTrz). When the molecules assemble to form nanoparticles, electronic excitation causes intramolecular charge transfer emission and intermolecular exciplex luminescence, resulting in a dual-channel intra-/intermolecular exciplex emission.
An exciplex is an electronically excited molecule complex. Its emission differs from the emissions of excited single molecules. “The exciplex strategy allows researchers to achieve deep-blue electroluminescence, because the energy levels of the electron-donating carbazole fragments and electron-accepting triazine fragments can be adjusted independently of each other,” Professor Stefan Bräse said.
The researchers succeeded in producing deep-blue OLEDs with an external quantum efficiency of 20.35%. The external quantum efficiency is the ratio between the radiation output and power input. These OLEDs also reached a high luminance of 5000 candela per square metre (cd/m2). The perceivable blue has the coordinates of 0.157/0.076 on the chromaticity diagram of the International Commission on Illumination (CIE). “Easy synthesis of the molecule and production of the components pave the way for a new generation of efficient and long-lived deep-blue OLEDs,” Bräse said.
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