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Strengthening and survey beyond the knowledge of the TADF emitters as materials for superefficient OLED devices

Periodic Reporting for period 1 - TADFORCE (Strengthening and survey beyond the knowledge of the TADF emitters as materials for superefficient OLED devices)

Reporting period: 2015-05-01 to 2017-04-30

The OLED worldwide market is growing rapidly and Europe needs experts possessing a comprehensive knowledge and practical experience in this technology. OLED technology is used in small devices such as smart phones and tablets but also in high-end TVs and lighting, as OLEDs are still relatively expensive compared to LCD. But with research progressing towards lower cost and longer lifetime, together with a growing trend to use flexible displays in smartphones etc., the OLED market is growing fast. Europe is a huge supplier of the materials for OLED displays, taking into account that electronic market is changing very fast and companies are searching for new cheaper materials the input of research in this area is needed. The flexible OLED display market is predicted to quadruple next year, with predicted global market revenue for flexible OLEDs to increase from $21.9 million in 2013 to $12 billion by 2020 and this project will help to maintain Europe place as a major supplier of OLED materials.
The high demand for flexible OLEDs will increase the need for very expensive and rare iridium. The TADFORCE project aims to explore exciplex emitters and thermally activated delayed fluorescence (TADF) in OLEDs i) to replace currently used Ir complexes, and ii) to show how to easily tune emitter, resulting in reduced production cost, especially blue, where Ir-based emitters fail.
The main research goal of the multi-disciplinary TADFORCE project is to explore CT and exciplex emitters and their application in OLED devices by training Experienced Researcher through joint research in chemical, physical and material science in both academia and industry. The ER will gain experience in conducting research in a multidisciplinary environment to produce important data in this new OLED field to enable the development, modeling, and tailoring of TADF OLED devices, which are at the forefront of new OLED research and development.
In my project I was exploring the use of charge transfer and exciplex based emitters with 100% efficiency through the use of thermally activated delayed fluorescence (TADF) in OLEDs. These stable emitters remove the need to use iridium in OLED devices. This was not a trivial matter due to the way an OLED works. Electron–hole recombination in the organic emitter layer creates 25% singlet excitons that decay radiatively, producing light directly, and 75% non-radiative triplet excitons. Thus, to make an efficient OLED, these triplets must in some way be converted into singlets. Heavy metal complexes have efficiently intermixed singlet and triplet states, via effective heavy atom spin orbit coupling, yielding near 100% light emission by phosphorescence. Without this ‘triplet harvesting’, a fluorescent OLED would be limited to a maximum 25% internal quantum efficiency. My ideas were to use exciplex phenomena, which was previously discovered in solution and never used in solid state emitters. Exciplex effects were considered as undesirable by many scientists, viewed exclusively as a loss process diminishing OLED efficiency. However, thanks to my investigations, exciplexes have now been shown to provide possibilities to make a wide range of emitters. This is due to the fact that the wavelength of the emission in such systems is not dependent on the band-gap value of a single compound, but the HOMO-LUMO offset between donor and acceptor molecules. This means, I was able to design a wide range of stable exciplex emitters in a wide range of emitting wavelengths.
In this project I was responsible for designing, fabricating and characterization of OLED devices, as well as searching for new compounds and new exciplex systems for analysis. Thanks to my wide knowledge of electrochemical processes and spectroscopic analysis I was able to use this in a search for new co-compounds for exciplex layers. In my project I was able to design and form exciplex based OLED devices, which had 100% interconversion. I was able to demonstrate that an exciplex emitter can yield efficient OLEDs, >19% EQE (almost 100% efficiency). In conclusion, I was able to form several highly efficient, heavy atom free OLED devices based on TADF and exciplex phenomena.
The use of TADF and exciplex emitters will decrease the price of OLED based screens and lighting panels by avoiding very rare and expensive iridium. Also such technology will allow for the production of better lighting panels through the facile design of emitters to produce desired wavelengths; safer for direct visualisation. If there is a possibility to produce cheaper, low energy, flexible OLED lighting, it could help the poorest nations, where energy is very expensive. One of my latest findings was to apply the TADF emitters as pressure and temperature sensors. One of recently developed phenazine based compounds have mechanochromic properties which allow to change their emission depending on acting force. Such behaviour will allow to form a lighting panel changing the emission in danger condition without using additional sensors and converters. In normal life at this point such application is quite limited but in high-tech industrial products like submarines or aerospace they could find use.
Small sun in the lab