September 25, 2012 - A three-year European project to research solution-processable materials for OLEDs has concluded, with newly developed materials that can be integrated into large-surface OLED components and are suited for printing processes.
The NEMO (NEw Materials for OLEDs from solutions) project, a consortium of 11 companies led by Merck, was formed in Nov. 2009 with backing from the German Federal Ministry of Education and Research (BMBF), to explore a variety of OLED materials and capabilities: soluble light-emitting materials, charge transport materials, new adhesives for reliable encapsulation of each OLED component. Physical tests were included to understand more about the materials for future development work. The project’s total budget was €29M (roughly US $38M).
"The success of the project is an enormous and important step for printable material systems with very good performance data," stated Dr. 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."
Results of the project include Merck’s development of a new phosphorescent materials for red, green and blue applications — increasing lifetime extrapolated to 50% of initial brightness (i.e., stability in use) of green triplet emitter materials from 10,000 hours to more than 200,000 hours, and increasing the efficiency of these materials from 30 cd/A up to more than 70 cd/A (candela/ampere) at a brightness of 1000cd/m2.
Here’s a list of other results achieved by the NEMO project’s four industry companies and seven research institute/academic groups:
Humboldt University of Berlin: Modular synthesis strategies were used to produce and test new electron transport materials.
DELO Industrie Klebstoffe: 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): 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): 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): 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: 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: One working group, led by Professor Yersin, developed new emitter classes with both strong and weak metal-metal interactions that show a singlet harvesting effect. 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.
Another working group (led by 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: Two 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.