Images courtesy of Adv. Funct. Mater., 25, 1955-1971, 2015.
A deeper picture of blue pixels
Organic electronics devices could have a major role to play in addressing contemporary social challenges like reducing CO2 emissions, yet some technical challenges remain before they can be more widely applied. We spoke to Dr Denis Andrienko about the MOSTOPHOS project’s work in investigating the issues which limit the stability of blue phosphorescent materials A relatively small
amount of material is required to produce AMOLED electronic displays, which are made of thin organic layers, making them an attractive option in certain electronic devices, including mobile phones, laptops and televisions. They also have some other beneficial attributes, as Dr Denis Andrienko of the MOSTOPHOS project, an EC-funded initiative bringing together both academic and commercial partners, explains. “AMOLED displays can be tuned fairly easily, by tuning the chemical structure of the emitter, or the entire pixel. Also, the power consumption, which is very critical for the display, is fairly low for an OLED. As a result, they don’t heat up much,” he outlines. However, there are also some limitations to consider. “If you look at a typical OLED display on the market, whether it’s in a TV or a mobile phone, you will see that there are three different pixels – red, green and blue. The green and red pixels, because of their lower excitation energy, are already phosphorescent. They harvest both triplet and singlet electronic states, which boosts their efficiency” says Dr Andrienko.
Blue pixels The same efficiency gains could potentially be achieved with blue pixels, yet in practice this is not currently the case, as blue light has a significantly higher excitation energy and hence there is a higher chance that an organic material will be damaged by the excitation. One of the key challenges in the organic electronics field is providing a high efficiency of blue pixels in a display, while at the same time ensuring that the lifetime is long enough to be used in a device, a topic central to the work of the MOSTOPHOS project. “That basically is the key point. The project is about figuring out what limits the stability of blue phosphorescent materials in OLEDs and providing chemical design rules for more stable deep blue emitters,” says Dr Andrienko. This work centres on developing a simulation framework, which will allow Dr Andrienko and his colleagues to look deeper into these stability-limiting mechanisms in phosphorescent OLEDs. “We’re working on a simulation platform,
which can in principle be used not only for blue phosphorescent OLEDs, but also to simulate other organic semiconducting devices” he continues. “We aim to provide feedback for organic chemists, helping them to synthesise structures with properties relevant to devices.” A good example could be a situation where a manufacturer wants to display a sky-blue colour in a device, or another that maybe wants their device to have a lifetime of 40,000 hours. Determining a material’s suitability in these terms can be a complex and expensive process, so Dr Andrienko and his colleagues aim to develop an efficient method of effectively pre-screening them. “The idea is basically to have a database of compounds, and to pre-screen those compounds before synthesising them. Synthesising and characterising materials for OLED applications is very time-consuming and expensive, so if we can reduce that cost using a computer simulation, then that would be a real asset to a company working on the design of this material,” he
An example of a complex blue phosporescent OLED design targeted by the consortium.
explains. There are a number of parameters to consider in characterising a material. “There are microscopic properties such as energetic disorder, electronic coupling elements, and reorganization energies,” outlines Dr Andrienko. “We are interested in relating them to exciton diffusion, exciton tolerance, and triplet-triplet and tripletpolaron interactions.” This also provides the foundation for researchers to simulate the entire device, which in the long term could help scientists to improve reliability and efficiency, and also to extend their lifetimes. While certain simulation techniques are effective at certain time and length scales, on their own they are not sufficient to simulate the entire device. “For example, quantum mechanics can deal with wave functions and atoms up to length scales of let’s say 10 nanometres. However, a device may be micrometres thick, so you
cannot use just that single technique to obtain the properties of the entire device,” points out Dr Andrienko. A multi-scale approach is therefore required in order to build a more complete picture. “So we use more techniques to complete the description, such as the
Synthesising and characterising materials for OLED applications is very time-consuming and expensive, so if we can reduce that cost using a computer simulation, then that would be a real asset to a company working on the design of this material master equation approach and continuous drift-diffusion equations. They cover different scales,” explains Dr Andrienko. “We are trying to effectively transfer model parameters from a fine-scale resolution to a more large-scale resolution, to simulate the device.”
Meeting of the consortium members in Dresden on 1st December 2017.
The project’s research in this area relies on a deep knowledge of fundamental processes occurring on OLEDs. For example, two triplet states annihilate at a certain rate; these rates can be obtained through quantum mechanical calculations. “These rates are then used in the so-called master equation, where there are only rates and events - the overall system is simulated on the level of these rates and events. Once this has been done, we can then transfer the information we have obtained from that scale to the drift-diffusion equations. Then, at that scale we can add light outcoupling, and simulate the entire device,” explains Dr Andrienko. This multi-scale approach to modelling OLEDs will enable researchers to analyse the underlying mechanisms behind the particular characteristics of a device. “We will be able to
pinpoint the factors behind the degradation, for example it might be because there is an imbalance in electron and hole mobilities,” says Dr Andrienko.
Efficiency roll-off The wider goal in this research is to understand the degradation process of OLEDs and the socalled efficiency roll-off, which can essentially be described as the reduction in efficiency of an OLED as the voltage is increased. This is an issue which currently limits the application of OLEDs. “The lower efficiency of this blue pixel means that either a higher current is required, or you need to have a larger area of blue pixels in the OLED. That has a negative impact on the battery lifetime,” explains Dr Andrienko. The software suite that has been developed could help companies identify the right materials for
Mostophos Modeling Stability of Organic Phosphorescent Light Emitting Diodes In-silico OLED design requires addressing all scales, from Angstroms (quantum mechanics) to micrometers (drift-diffusion and master equations).
The objectives of this project are to integrate various levels of theoretical materials characterization into a single software package, to streamline the research workflows in order for the calculations to be truly usable by OLED materials developers, complementary to experimental measurements.
H2020-EU.22.214.171.124. - Cross-cutting and enabling materials technologies.
• Max Planck Institute for Polymer Research (Coordinator), Germany • Consiglio Nazionale delle Ricerche, Italy • Universidad del País Vasco, Donostia, Spain • University Rome “Tor Vergata”, Italy • COSMOLogic GmbH, Germany • Technische Universiteit Eindhoven, Netherlands • Technische Unversität Dresden, Germany • FLUXiM AG, Switzerland • CYNORA GmbH, Germany
OLED applications. “You need to pre-screen a material before you synthesize it. That’s what this software could potentially help in doing,” outlines Dr Andrienko. “A second important point is the optimisation of the efficiency of an entire OLED, which depends on the orientation of emitter molecules, light outcoupling, and other issues.” This research is a core element of the overall agenda within MOSTOPHOS, yet Dr Andrienko says there are other strands of research in the project. Alongside investigating blue phosphorescent emitter lifetimes, researchers are also developing a package to obtain OLED characteristics from scratch, from the chemical structure, while there are also other avenues of exploration. “We are also looking at other concepts. CYNORA, a company that is joining the MOSTOPHOS project consortium is particularly interested in a different way of harvesting triplet states in an OLED device, namely thermally
activated delayed fluorescence, or TADF,” says Dr Andrienko. This type of work could hold the key to improving the efficiency and performance of OLEDs, which holds important implications for several area of industry; with this in mind, Dr Andrienko is keen to maintain strong links with the commercial sector, with a view to pursuing further research in future. “Discussions with company representatives are currently ongoing,” he says. The project itself has a clearly defined funding term, but Dr Andrienko says the conclusion of the project will not mark the end of this research. With global competition in the OLED field growing increasingly intense, continued research is essential if European companies are to maintain a strong presence in the market. “This project has helped us to develop a set of clearly defined goals, and to get funding for specific target applications. But the overall research will not stop with the end of the project,” stresses Dr Andrienko.
Project Coordinator, Dr Denis Andrienko Max Planck Institute for Polymer Research Ackermannweg 10 55128 Mainz Germany T: +49 (0)6131 379147 E: email@example.com W: http://www.mostophos-project.eu/wp/ W: www.mpip-mainz.mpg.de/~andrienk/ P. Kordt, J. M. van der Holst, M. Al Helwi, W. Kowalsky, F. May, A. Badinski, C. Lennartz, D. Andrienko, Modeling of organic light emitting diodes: from molecular to device properties, Adv. Funct. Mater., 25, 1955-1971, 2015 P. Kordt, P. Bobbert, R. Coehoorn, F. May, C. Lennartz, D. Andrienko, Simulations of organic light emitting diodes, In “Handbook of Optoelectronic Device Modeling and imulation”, Vol. 1, 473-522, 2017 https://www.taylorfrancis.com/books/e/9781498749473
Dr Denis Andrienko
Dr Denis Andrienko obtained his PhD in optics/structural transitions in liquid crystals from the Institute of Physics, Ukraine in 2000. In 1999 he joined the group of Prof. M. P. Allen at Bristol, UK, where he obtained a second PhD on computer simulations of complex fluids (2001). He then moved to Max Planck Institute for Polymer Research as a Humboldt Fellow doing theoretical studies of slippage effect and nematic colloids. In 2004 he joined the group of Prof. K. Kremer as a postdoctoral fellow working on multiscale simulations of polycarbonate melts. Since 2005 he is a group leader working on the development of multiscale simulation techniques for organic semiconductors.
Published on Mar 14, 2018
Published on Mar 14, 2018
Organic electronics devices could play a major role in addressing contemporary social challenges. We spoke to Dr Denis Andrienko about the M...