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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HyperOLED (Development of high-performance, hyperfluorescence OLEDs for use in display applications and solid state lighting)

Teaser

Summary

Organic light emitting diodes (OLEDs) have entered the mass market some years ago and can be found in various display applications from smartphones to TVs, where they now dominate the high-end market. Performance of OLED based displays has been improved steadily over the last years and is now on an excellent level. However, one issue not yet solved and strongly requested by the display industry is highly efficient blue.

The HyperOLED project targets this demand by developing blue emitting materials and matching OLED stacks which combine thermally activated delayed fluorescence (TADF) and fluorescent emitters adapted to this so-called hyperfluorescent device architecture. High performance will be achieved by novel molecular design aimed at avoiding energy loss-mechanisms, maximising desired energy transfer and tuning of molecular alignment.
Our approach will enable simpler device structures with less layers by eliminating the need for stacked OLEDs for white emission which are state-of the art in TV applications and lighting. Apart from less energy consumption during usage, the project will thus contribute to simpler display fabrication which saves resources in the production stage. Compared to the â€“ not yet realised â€“ alternative technology of using blue emitting metal-complexes, the hyperfluorescent approach leads to less waste and energy consumption during production of the active materials due to simpler chemistry. An additional benefit of avoiding stacked OLEDs is low drive voltage, making the devices compatible with low voltage CMOS back plane technology, allowing us to demonstrate the conceptâ€™s feasibility in high-brightness, full-colour OLED micro displays.

To develop the hyperfluorescence OLEDs, clear scientific and technical objectives have been defined. These include the development of the necessary chemistry to make the materials, but also the techniques to characterise the highly complex emission system. Photo physical and optical measurement combined with theoretical investigations are carried out to understand the underlying physics and guide the design of new TADF materials and fluorescent emitters. After synthesis and purification of materials, they are tested in prototype OLEDs. The results are used to judge performance but are also important to gain further understanding which is fed back to material development. Building on the knowledge from the different characterisation methods, a white stack is designed which will be used in the fabrication of a hyperfluorescent micro display demonstrator.

Work performed

Since the beginning of the HyperOLED project, all partners made significant advancements in their respective work packages. To speed up the design of new fluorescent emitters, a tool was developed to visualise the effect of different chemical side groups in new molecules. The tool not only helps the chemists in the design of new materials but is also used to screen new ideas to identify the most promising targets. Significant computational effort was invested to calculate the emission colour of a huge number of core structures. Based on this and further analysis, the most promising core was chosen and is now established as reference material in the project. Numerous photo physical, optical and device measurements have been carried out already and more work in this direction steadily adds to our understanding.
Derivatives of well known, so-called donor-acceptor-donor TADF molecules from the University of Durham have been synthesised. From extensive characterisation by the consortium, design rules could be inferred on how to adapt the materials to hyperfluorescence. Matching the TADF to the host material present in the emission layer was identified as a key target for the 2nd project half to improve performance.
The complex nature of the involved physical processes proves to be a scientific challenge. Information can be gained from well-known measurement techniques like transient photoluminescence, but it was found that a new way of interpreting the data had to be established. The result is a theoretical model for the emission layer which can explain a lot of observations and yields reliable values for important parameters like the rate of reverse intersystem crossing. Nevertheless, energy loss mechanisms cannot be characterised fully yet as they are hard to identify from emission measurements, so transient absorption equipment was installed and optimised to high sensitivity. The data from this so far fits very well into the developed theory and will deliver valuable information about energy loss mechanisms in the 2nd project half.
Apart from developing a highly sensitive method to determine absorption (refractive index) anisotropy, applying it to the project materials and determining emitter orientation for different material combinations, the theory of emission in photo excited emission layers was carefully re-examined. As it turns out, energy transfer processes in hyperfluorescent OLEDs influence the traditional photoluminescence measurement for emitter orientation in unexpected ways, but it was also found that the measurement can yield more information than expected. These findings will complement the other methods and will be a valuable tool to improve our understanding of hyperfluorescence during the 2nd half of the project.
The micro display demonstrator is a joint work in the project, and so we analysed in detail its specifications and the technical requirements to carry out the various steps during production like wafer coating with the necessary precision, transportation between facilities etc. Technical equipment for this was specified and partly manufactured. We also started to design the white stack for the demonstrator. As materials evolve during the 2nd project half, it will be adjusted and further optimised.

Final results

HyperOLED is one of very few research initiatives in thermally activated delayed fluorescence (TADF) and is building the current state-of-the-art in the development of hyperfluorescent OLEDs. All partners are leading experts in their respective domain of expertise and the HyperOLED project is helping them to multiply their impact by regular scientific exchange beyond their core field of knowledge. This synergy is explored and already led to highly relevant results beyond what is known like a theoretical model for the emission layer, the connection of the optical emitter orientation measurements to energy transfer mechanisms or the link between photo physical effects and device performance. The project thus not only delivers excellent science, but science that thinks out of the box.
Materials, methods and investigation results have and will be published in high impact journals and presented during conferences to provide the community with new tools and methodologies to speed up progress in the investigation of hyperfluorescence. By providing world-class research, the project establishes Europe as leader in the understanding driven development of highly efficient blue emission for OLEDs, thus keeping jobs and creating new ones in Europe linked to the high-tech display industry.
The manufacturing of a hyperfluorescent micro display demonstrator will show the potential of the device stacks and materials developed in the project. We thus ensure attention of prospective customers and stakeholders. Where appropriate, the results are protected by patents to ensure exploitation by the project partners.