Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz, Germany, have developed a new organic light-emitting diode (OLED) prototype that consists of just a single layer yet enables higher luminosity and efficiency compared to current commercially available OLEDs.

The single-layer OLED is supplied with electricity via two electrodes. This simplifies the production of OLEDs and paves the way for printable displays with an inkjet printer.

With their first prototype, the scientists were able to show that they can generate a brightness of emitted light of 10,000 candela/square meter at just 2.9 volts — this corresponds to about 100 times the luminosity of modern screens. They claim that achieving such high luminosity at this low voltage is a record for current OLEDs. The researchers were also able to measure an external efficiency of 19%, which means that 19% of the electrical energy supplied is converted into light that comes out in direction of the viewer. With this value, the OLED prototype can compete with current OLEDs consisting of five or even more layers.

In continuous operation, the researchers were able to measure a so-called LT50 lifetime of almost 2000 hours at a brightness equivalent to ten times that of modern displays. Within this time, the initial luminosity drops to 50% of its value.

Single-layer Printable OLED

The first prototype of the OLED developed in by MPI-P in Mainz (Image: Max Planck Institute for Polymer Research)

Organic light-emitting diodes no longer consist of compounds containing the semiconducting material gallium, but of organic compounds in which carbon is a main component. Compared to conventional light-emitting diodes, however, the luminosity and lifetime of OLEDs are currently lower, which is why they represent a current field of research. Current OLEDs consist of various wafer-thin layers. Some layers are used to transport charges, while others are used to efficiently introduce electrons into the active layer in which light is generated. Thus, current OLEDs can easily consist of five to seven layers.

The German scientists are using a light-emitting layer based on so-called ‘thermally activated delayed fluorescence’ (TADF). This physical principle has been known for several decades but became the focus of OLED research about 10 years ago, when an efficient conversion of electrical energy into light was demonstrated in Japan. Since then, researchers have been working to produce TADF-based OLEDs, as these do not require expensive molecular complexes containing rare-earth metals that are being used in current OLEDs.

The MPI-P scientists, led by group leader Gert-Jan Wetzelaer, have published the results of their research in Nature Photonics in a paper, ‘Efficient and stable single-layer organic light-emitting diodes based on thermally activated delayed fluorescence’.