Merck Concludes NEMO Research on OLED Materials
After 3 years of intensive research in the field of new solution-processable materials for OLEDs (organic light-emitting diodes), Merck and ten other partners from industry and academia have successfully concluded the “new materials for OLEDs from solutions” (NEMO) project co-funded by the German Federal Ministry of Education and Research (BMBF). Merck, a global pharmaceutical and chemical company, led the consortium of 11 partners and the overall project budget of €29 million.
The newly developed materials can now be integrated into large-surface OLED components, e.g. for televisions, signage applications, and illumination for objects or rooms. They are especially suited for printing processes which, thanks to their high material utilization rates in comparison with conventional vacuum evaporation processes, permit more economical production of OLEDs.
In this project, Merck developed and tested new phosphorescent materials for red, green and blue applications. For example, it was possible to increase the lifetime extrapolated to 50% of initial brightness (indicating stability in use) of green triplet emitter materials from 10,000 hours to more than 200,000 hours, while at the same time increasing the efficiency of these materials from 30 cd/A up to more than 70 cd/A (candela/ampere) at a brightness of 1000 cd/m².
“The success of the project is an enormous and important step for printable material systems with very good performance data,” says Udo Heider, head of the OLED unit at Merck. “We are enabling our customers to use cost-efficient manufacturing processes, which thanks to their low material losses in production, will ultimately also benefit the environment.”
The scope of the project extended from soluble light-emitting materials, to charge transport materials and new adhesives for reliable encapsulation of each OLED component. In addition, physical tests were performed on the materials and on the OLED components in order to gain more in-depth knowledge for future material developments.
Apart from Merck, the following companies and institutes were involved in the project:
- Humboldt University of Berlin: Researchers used modular synthesis strategies to produce and test new electron transport materials.
- DELO Industrie Klebstoffe: The company worked on the development of adhesives with low water vapor permeation for flat encapsulation. A main focus of the work was on optimizing the compatibility of the adhesive with the OLED materials. Suitable adhesive systems were identified, and a significant reduction in component defects was achieved. The developed systems were extensively characterized.
- Enthone GmbH (formerly Ormecon):
- Scientists at Enthone Nano Science Centre developed dispersions of polyaniline, an electrically conductive polymer, from which charge carrier layers for OLEDs were produced. These displays show electrical properties equivalent to those of the previously used material.
- For OLED component characterization, impedance spectroscopy was used to investigate the OLEDs prepared by Merck. It was possible to identify unstable areas, which are responsible for the short lifetimes of OLEDs. Additionally, the impedance measurements were used to predict the lifetime of displays.
- Fraunhofer Institute for Applied Polymer Research (IAP): The lab developed polymer-based phosphorescent systems for green and red Merck emitters. Suitable charge transport molecules were bonded as a side group to a main polymer chain. It was possible to demonstrate that this leads to comparable or even better performance parameters and lifetimes of OLEDs in comparison with solution-processable small molecules. For “green,” energy efficiencies of 61 cd/A and lifetimes of 66,000 h @ 1000 cd/m² were achieved
- Heraeus Precious Metals GmbH & Co. KG (formerly H.C. Starck Clevios GmbH):
- The company developed new materials for the intermediate layers, which will improve the charge carrier injection from the anode into the OLED emitter layer and help to increase the lifetime of the components. The work function of the hole injection layers can be set to a specific target value within a wide range of 4.8 – 6.1 eV. Water-soluble, polymer counterions have been developed, which have helped to realize dehydrated PEDOT materials for the first time.
- In parallel to this, work was conducted on transparent electrodes that can be separated from solution and are expected to lower the costs of OLEDs. The conductivity of the PEDOT:PSS films was further increased. Initial ITO-free OLED lamps have been realized. In combination with screen-printed silver lines, this enables the production of OLEDs for lighting application without any identifiable decrease in luminance from the edge to the center of the component.
- University of Potsdam: Researchers studied physical properties such as charge carrier transport and excitation dynamics in newly synthesized materials and in the finished component. In combination with stationary and transient simulations, information was obtained on what processes restrict the efficiency of light emitting diodes and which ones impact component aging.
- University of Regensburg (represented by two chairs):
- The working group led by Professor Yersin developed new emitter classes with both strong and weak metal-metal interactions that show the singlet harvesting effect discovered in Regensburg. It is thus possible to realize highly efficient emitters for OLEDs based on highly economical copper clusters. This work on singlet harvesting with newly developed emitters made from copper clusters was recognized in April 2012 with an innovation prize at the international SPIE Organic Photonics conference in Brussels.
- The working group of Professor König synthesized emitter libraries in accordance with a simple, combinatorial protocol. A screening system was developed for the rapid and virtually automated identification and characterization of individual emitters as well as photostability testing thereof. This made it possible to investigate the degradation behavior of many substances and to draw conclusions on various degradation mechanisms.
- University of Tübingen (represented by two chairs): The groups from Tübingen provided new metallorganic cluster compounds that can be used as luminescent molecules in OLEDs. In chemical synthesis, coordination compounds of the metals rhodium, iridium, palladium, platinum, copper, silver and gold were presented and characterized, giving rise to new, highly promising lead structures for emitter materials.
Merck is a global pharmaceutical and chemical company with total revenues of €10.3 billion in 2011, a history that began in 1668, and a future shaped by more than 40,000 employees in 67 countries. Its success is characterized by innovations from entrepreneurial employees. Merck’s operating activities come under the umbrella of Merck KGaA, in which the Merck family holds an approximately 70% interest and free shareholders own the remaining approximately 30%. In 1917 the US subsidiary Merck & Co. was expropriated and has been an independent company ever since.
Caption: Bettina Schröder, does research in an OLED laboratory © Merck KGaA, Darmstadt Germany