Design rules for the development of sublimable Iridium emitter complexes with horizontal dipole orientation and improved light outcoupling in OLEDs

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Organic light-emitting diodes (OLEDs) are meanwhile an established display technology which is commercially used in smartphones or TV screens. Unique selling points are their high image quality and low energy consumption, which originate from the fact that these displays are emissive and, thus, do not require any backlight. To this end, phosphorescent emitters with their strong spin-orbit coupling provide decisive advantages, as they enable almost 100% internal quantum efficiency because phosphorescence from the triplet state to the ground state is an allowed optical transition. Nevertheless, the achieved external quantum efficiency is limited by light outcoupling, which amounts to only about 20-25% in conventional OLED structures since the majority of the produced photons is trapped inside an OLED. A powerful tool to improve light outcoupling is the use of emitter molecules having an inherent tendency to align horizontally during film growth such that their optical transition dipoles radiate preferentially toward the substrate, i.e. in the viewing direction of an observer. However, it remains challenging to achieve high degrees of alignment with phosphorescent Iridium emitter complexes, since their three ligands have an octahedral coordination around the central Iridium atom so that their shape anisotropy is relatively low. Thus, further progress of materials design requires detailed understanding of structure-property relations in this class of OLED emitters. The main goal of this transfer project is to improve existing methods for predicting and analyzing emitter orientation in evaporated guest-host systems with Iridium emitters, such that design rules can be established and verified. Further, we plan to study both intermolecular interactions between emitter and host molecules, as well as intramolecular energy transfer processes among the different ligands, which will allow better control of the resulting emitter orientation. Overall, we plan to develop tools for targeted materials design for the industrial partner (Merck) as well as to create scientific knowledge for the academic partner (University of Augsburg).