A major European project has investigated OLED materials for printing processes, which should lead to lower-cost production compared with competing methods.
A consortium of eleven partners led by Merck has concluded a three-year research project into new solution-processable materials for OLEDs.
The NEMO – or "New materials for OLEDs from solutions" – project had an overall budget of EUR 29 million (around $37 million) and was co-funded by the German Federal Ministry of Education and Research (BMBF).
The focus has been on materials that are especially suited for printing processes. Thanks to their high material-utilization rates in comparison with conventional vacuum evaporation processes, printing processes should permit more economical production of OLEDs.
The scope of the project extended from soluble light-emitting materials and charge-transport materials to 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.
Merck’s main role was to develop and test new phosphorescent materials for red, green and blue applications. The lifetime (extrapolated to 50% of initial brightness) of green triplet emitter materials was increased from 10,000 hours to more than 200,000 hours. At the same time, the efficiency of these materials was increased from 30 cd/A to more than 70 cd/A 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.“
Work by project partners
At Humboldt University of Berlin, modular synthesis strategies were used to produce and test new electron transport materials.
DELO Industrie Klebstoffe 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) 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.
The Fraunhofer Institute for Applied Polymer Research in Potsdam (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 demonstrated that this leads to comparable or even better performance parameters and lifetimes of OLEDs in comparison with solution-processable small molecules. For green OLEDs, energy efficiencies of 61 cd/A and lifetimes of 66,000h at 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 counter-ions were 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.
Researchers from the 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 OLEDs and which ones impact component aging.
The University of Regensburg 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.
Furthermore, Professor König’s group at the University of Regensburg 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.
Two groups from the University of Tübingen provided new metal-lorganic 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.
