Organic light-emitting diodes: the blue shines brighter and longer

Thanks to a new type of molecule, blue OLEDs should shine brighter in the future and fade less quickly.
Photo Credit: Markus Breig, KIT

Two-channel intra / intermolecular exciplex emission enables efficient deep blue electroluminescence.

Organic LEDs, or OLEDs for short, are characterized by energy efficiency and flexibility. But one challenge lies in the production of blue OLEDs - these have so far lacked luminance and stability. Researchers at the Karlsruhe Institute of Technology (KIT) and at Shanghai University have now developed a new strategy for producing efficient deep blue OLEDs: A specially produced novel molecule enables two-channel intra / intermolecular exciplex emission with electronic excitation, thereby allowing deep blue electroluminescence. The researchers report in the journal Science Advances.

Organic LEDs are already in many smartphones, tablets and large-scale TVs. They do not require additional backlighting and are therefore energy-efficient, can be produced inexpensively using thin-film technology and also work on flexible substrates, which enables flexible displays and variable room lighting solutions. An OLED (stands for: organic light-emitting diode) consists of two electrodes, at least one of which is transparent. In between are thin layers of organic semiconducting materials. The lighting is created by electroluminescence. When creating an electric field, electrons from the cathode and holes (positive charges) from the anode are injected into the organic materials that act as emitters. Electrons and holes meet there and form electron-hole pairs. These then disintegrate into their initial state and release energy that the organic materials use to emit light. All colors are created by mixing the three colors blue, green and red.

Emission within the molecules and between the molecules makes deep blue OLEDS shine efficiently and stably.
Graphic Credit: Zhen Zhang

Why the color blue is difficult

So far, only the colors red and green have been available for commercial applications as phosphorescent, i.e. longer luminous OLEDs. The color blue is available as fluorescent OLEDs that only light up briefly. With blue OLEDs, it is difficult to reconcile high efficiency, high luminescence and a long service life - the blue pixels shine weaker or fade faster than the green and red pixels. Researchers at the Institute for Organic Chemistry (IOC) and at the Institute for Biological and Chemical Systems - Functional Molecular Systems (IBCS-FMS) at KIT now have a new strategy for the production of highly efficient and stable deep blue organic ones together with scientists at Shanghai University Light emitting diodes developed.

The researchers produced a novel molecule consisting of carbazole and triazine fragments that are connected by a silicon atom (CzSiTrz). When the molecules accumulate into nanoparticles, intramolecular charge transfer emission and intermolecular exciplex luminescence occur with electronic excitation, which leads to two-channel intra / intermolecular exciplex emission. An exciplex is an electronically excited molecular complex; its emission differs from the emissions of the excited individual molecules. "The Exciplex strategy enables deep blue electroluminescence to be achieved because the energy levels of the electron-giving carbazole fragments and the electron-absorbing triazine fragments can be set independently of one another," explains Professor Stefan Bräse from the IOC and IBCS-FMS of KIT

New OLEDs achieve high external quantum efficiency and high luminance

The team succeeded in producing deep blue OLEDs with a record-breaking external quantum efficiency of 20.35 percent. The external quantum efficiency indicates the relationship between the radiation power effected and the electrical power supplied. In addition, these OLEDs achieve a high luminance of 5,000 candela per square meter (cd / m2). The perceptible blue has the coordinates 0.157 / 0.076 on the standard color chart of the International Lighting Commission (CIE). "The light synthesis of the molecule and the simple manufacture of the components pave the way for a new generation of efficient and durable deep blue OLEDs," says Bräse.

Published in journal: Science Advances

Authors: Zhen Zhang, Dehai Dou, Rongrong Xia, Peng Wu, Eduard Spuling, Ke Wang, Jin Cao, Bin Wei, Xifeng Li, Jianhua Zhang, Stefan Bräse, and Zixing Wang

Source/Credit: Karlsruhe Institute of Technology

Reference Number: tn060623_01

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