Making use of extensive prior experience on organic materials, we were able to identify a list of suitable organic materials, covering different organic material classes such as fluorescent emitters, solar cell absorbers, charge transport layers, and carbon nanomaterials. We have found suitable candidates for strong coupling in all these material classes, covering the whole visible spectral range and into the NIR. This enabled a strong foundation for the following works.
We were quickly able to translate the idea of dispersion management into simple metal-organic-metal filters. By careful tuning of the thicknesses involved, we could convincingly demonstrate narrowband polariton filters with an ultralow angle-dependent spectral shift that is less than the resonance linewidth. These polariton filters surpass conventional metal-dielectric metal filters in all relevant performance metrics. As we realised the application potential of such filters, we focused more strongly on further developing the concept also to high-quality distributed Bragg reflectors (DBR) with incorporated organic layers, leading to a strong coupling of the DBR sidebands with the material excitons and in turn an angle-independent stopband with high rejection rates of optical density 6 and higher. For this purpose, we also employed a computational optimization routine for optimizing fabrication parameters, leading to filters with great application potential, for example as fluorescence filters to use with green fluorescent protein biomarkers.
Within the project, we were able to further develop polariton-based organic light-emitting diodes (OLEDs), utilising an assistant strong coupling layer, with high quality factor (narrowband emission) and a highly angle-stable emission spectrum. We could furthermore translate these OLEDs into the ultra-strong coupling regime using a coumarin dye C545T, that is compatible with a high-efficiency OLED architecture. With our concept we could increase the efficiency of polariton OLEDs to 10%, an order of magnitude higher and a brightness above 100,000 cd/m2, two orders of magnitude higher than previously reported in scientific literature. These devices are highly relevant for display applications as narrowband and angle-independent emission is a key ingredient to reach high colour purities as demanded by the new display standard BT2020.
We were able to utilise a substrateless architecture comprising ALD-nanolaminates and parylene-C layers as encapsulation and quasi substrate for both filter and OLED devices developed in this work. While simple architectures with only metal and organic films were straight-forward to implement, we were able to utilise even oxide films for ultrahigh-Q DBRs by reducing the stress in the most brittle layers through stack engineering. Our flexible OLEDs and filters are on a thickness range between 10 and 20 micrometers and thus comparable to commercial cling film, ultraflexible and show comparable performance to solid reference devices. This opens up different application pathways but is also interesting for large-scale fabrication, e.g. as roll-to-roll process.
The dissemination of project work was highly successful, leading to a total of 4 publications (with a further 6 publications submitted or in preparation), and 5 presentations at international conferences. The application-relevant nature also involved a continued patent application with further in preparation. The work was disseminated to a broad public audience via Twitter (estimated audience of 75,000 via Altmetric), ResearchGate, the University of Cologne website and several scientific news blogs.
