Organic light-emitting diodes made from small molecular weight organic semiconductors are composed of multiple functional layers. These layers are in most cases amorphous assemblies of the molecular building blocks. In our joint paper with our collaborators of the Group of Nanomaterials and Microsystems (GNaM) at the Universitat Autònoma de Barcelona entitled ‘High-performance organic light-emitting diodes comprising ultrastable glass layers‘, we shed some light on the growth of these layers. The work is published in Science Advances. The molecules making up the amorphous layers do not necessarily fall into place perfectly so that over time (but these are very long times beyond the lifespan of an OLED) the molecules wiggle into a more compact assembly. This settling can be accelerated greatly if the molecules on the surface are given some extra energy to migrate. Providing excess thermal energy through an elevated substrate temperature, much more stable morphologies – called ultrastable glasses – are formed. The optimum condition for this growth is around 85% of the materials glass transition temperature.
In our study we have tested this growth condition for four different phosphorescent emitters in one common device stack and found that both the external quantum efficiencies and device lifetimes significantly increased. The illustration below summarizes our work graphically.
The full citation is: J. Ràfols-Ribé, P.-A. Will, C. Hänisch, M. González-Silveira, S. Lenk, J. Rodríguez-Viejo, S. Reineke, High-performance organic light-emitting diodes comprising ultrastable glass layers. Sci. Adv. 4, eaar8332 (2018). DOI: 10.1126/sciadv.aar8332.
Ramon Springer joined the group of Prof. Jang Hyuk Kwon (Department of Information Display, Kyung Hee University, South Korea) to carry out a Master thesis topic within the international Masters course Organic and Molecular Electronics (OME) at the TU Dresden. His thesis task was to develop a white-light emitting, multiple OLED stack based on blue and yellow units to be used in AMOLED displays. Here, aside from the optimization of device efficiency, the color quality and angular stability were parameters to be optimized. His work led to a recent publication in Optics Express entitled “Cool white light-emitting three stack OLED structures for AMOLED display applications“. Congratulations to a very successful research stay abroad.
A couple of impressions from our tutorial connected to the MRS Spring 2016 Symposium EP1: Organic Excitonic Materials and Devices: In short, the room has been packed with more attendees than chairs in the room. Chihaya Adachi started out giving us a complete overview on a class of materials for organic light-emitting diodes, he pioneered, namely thermally activated delayed fluorescence (TADF)-type emitters. He reflected on his initial reports on TADF in 2009, when nobody in the community cared about this concept, lacking competitive efficiency values at the time. Nowadays, these materials are TADF on par with phosphorescence in respect to device efficiency. Listening between the lines, we learned that up to date, the Adachi Lab counts more than 200 TADF molecules made – impressive.
After very limited coffee supply during the break, Stéphane Kéna-Cohan switched gears and gave us a very insightful introduction to the world of organic based lasers. Beginning with the very basics needed for understanding the concepts of lasers in general, we ended up learning the very current developments connected with laser research based on organic materials. The key potential here is seen for its high degree of possible integration, which is for instance important for bio-applications. When discussing the challenges remaining towards the demonstration of an electrically pumped organic laser, Stéphane Kéna-Cohan motivated where the threshold for electrical pumping roughly is using a back-on-an-envelope calculation, which actually fits on an envelope, as the picture below proves.
With the tutorial done, we are looking forward to the upcoming four days of regular symposium program. The sessions will reconvene at a different room: PCC North, 200 Level, Room 227 A. See you tomorrow.
In a new review article (Recent advances in light outcoupling from white organic light-emitting diodes) in the Journal of Photonics for Energy, my colleague Malte C. Gather (University of St. Andrews) and I summarize the recent efforts on the development of concepts for light outcoupling in white organic light-emitting diodes (OLEDs). Especially with respect to the complementary inorganic LED technology, it is extremely important to unlock higher external quantum efficiencies in OLEDs to stay competitive. Here, improving the outcoupling efficiency by far offers the largest margin (standard structures are at 20-30% of the internal efficiency maximum). The current status reveals that a universal solution with satisfying enhancement still waits to be found.