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.
In our recent publication entitled ‘Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters‘ published in the Journal of Physical Chemistry C, we discuss how the population of triplet excitons in emitters which sport efficient phosphorescence at room temperature influence the overall luminescence properties. An important emphasis here is on the exciton dynamics of the fast fluorescence (nanoseconds) and the slow phosphorescence (milliseconds), which span over six orders of magnitude in excited state lifetimes, depending on the respective sample composition. All of these results are obtained at room temperature.
We acknowledge the funding from the German excellence cluster cfaed (TU Dresden) and from European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 679213).
Our recent paper together with our colleagues at the Leibniz Institute of Polymer Research Dresden (IPF), is featured in Advanced Science News. This work discusses a novel way to achieve thermally activated delayed fluorescence in polymers through an extension of the HOMO conjugation, which ultimately leads to a smaller splitting between singlet and triplet excited charge transfer states. The paper can be found here: Conjugation-Induced Thermally Activated Delayed Fluorescence (TADF): From Conventional Non-TADF Units to TADF-Active Polymers.
In a recent collaboration with our colleagues at the Leibniz Institute of Polymer Research Dresden (IPF), we have developed polymers that show thermally activated delayed fluorescence (TADF) properties with high efficiency. This work has now been published in Advanced Functional Materials under the title: Conjugation-Induced Thermally Activated Delayed Fluorescence (TADF): From Conventional Non-TADF Units to TADF-Active Polymers. Interestingly, the monomer building block does not show TADF but rather only phosphorescence. Hence, the TADF property is induced as a consequence of increased conjugation during polymer formation. Ultimately, the singlet-triplet splitting is reduced in the polymer to allow for TADF. The emitter shows sky-blue emission with roughly 70% PLQY. This report includes the synthesis of the monomer and polymer materials, quantum chemical calculations and a detailed photo-physical characterization.
In this paper entitled ‘Transparent and color-tunable organic light-emitting diodes with highly balanced emission to both sides‘ we demonstrate transparent, two-color, stacked OLEDs that allow for balanced top- and bottom-emission. Making use of ultra thin, composite metal electrodes, this design avoids the use of ITO, such that this architecture can be transferred to flexible substrates. Careful optical design made it possible that the luminance of the device is virtually identical to both viewing directions, which is a great improvement over many earlier device layouts.
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.
Our new paper entitled “Adjustable white-light emission from a photo-structured micro-OLED array” published in Light: Science & Applications discusses an approach towards micro-OLED arrays made of differently emitting sub pixels without non-emissive areas. This is achieved using orthogonal lithography techniques in a way that only the first OLED unit is structured while the next one to follow is made in a “fill-the-gap” approach. In this conceptual demonstration, we pair blue and yellow OLEDs in a stripe layout, which can be addressed individually for complete color tunability. Feature sizes of the stripes are down to 20 micrometer.