(Nanowerk News) Organic light-emitting diodes are used in smartphones, tablets and large TV screens. Since they do not require additional background lighting, they are energy efficient. It can be produced at low cost with thin film technology and also works on flexible carrier materials, enabling bendable displays and a variety of room lighting solutions.
OLED consists of two electrodes, of which at least one is transparent. A thin layer of organic semiconducting material lies between them. Their light results from electroluminescence. When applying an electric field, electrons from the cathode and holes (positive charge) from the anode are injected into the organic material which acts as an emitter. There, electrons and holes meet and form electron-hole pairs. When these pairs collapse to their original state again, they release energy which is used by organic matter to emit light. All colors can be obtained by mixing blue, green, and red.
Why Blue Is Hard
So far, OLEDs fluorescent in red and green have only been available for commercial applications. Blue light can be produced by fluorescent OLEDs for only a short time. Blue OLEDs have the difficulty of combining high efficiency with high luminosity and long lifetime. Blue pixels dim or dim faster than green and red pixels.
In collaboration with researchers from Shanghai University, researchers from KIT’s Institute of Organic Chemistry (IOC) and Institute of Biology and Chemical Systems – Functional Molecular Systems (IBCS-FMS) have now developed a new strategy for highly efficient and stable internal production. blue organic light emitting diode (Science Advances, “Efficient dark blue luminescence based on two-channel intra/intermolecular exciton”).
The researchers produced a new type of molecule, in which the carbazole and triazine fragments are linked by silicon atoms (CzSiTrz). When molecules assemble to form nanoparticles, electronic excitation causes intramolecular charge transfer emission and intermolecular excitation luminescence, resulting in multiple channel intra-/intermolecular excitation emission.
Exciplex is an electronically excited molecular complex. Its emission is different from that of an excited single molecule. “The exciplex strategy makes it possible to achieve deep blue electroluminescence, because the energy levels of the electron-donating carbazole fragment and the electron-accepting triazine fragment can be adjusted independently of each other,” says Professor Stefan Bräse from the IOC and IBCS-FMS.
New OLED Achieves High External Quantum Efficiency and High Luminance
The team succeeded in producing a dark blue OLED with a record-breaking external quantum efficiency of 20.35 percent. External quantum efficiency is the ratio between the radiation output and the power input. In addition, this OLED achieves a high luminance of 5000 candela per square meter (cd/m2). The visible blue color has coordinates 0.157/0.076 on the International Commission on Illumination (CIE) chromaticity diagram.
“The easy synthesis of molecules and production of components paved the way for a new generation of efficient and long-lived dark blue OLEDs,” explained Bräse.