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

有机发光二极管（OLED）同时是一种成熟的显示技术，在智能手机或电视屏幕中得到商业应用。独特的卖点是其高图像质量和低能耗，这源于这些显示器是发射型的，因此不需要任何背光。为此，具有强自旋-轨道耦合的磷光发光体提供了决定性的优势，因为它们能够实现几乎100%的内部量子效率，因为从三重态到基态的磷光是允许的光学跃迁。然而，所实现的外部量子效率受到光外耦合的限制，光外耦合在常规OLED结构中仅为约20-25%，因为大部分产生的光子被捕获在OLED内部。改善光外耦合的有力工具是使用具有在膜生长期间水平对准的固有趋势的发射器分子，使得它们的光学跃迁偶极子优先朝向衬底（即，在观察者的观察方向上）辐射。然而，实现与磷光铱发射体络合物的高度对准仍然具有挑战性，因为它们的三个配体在中心铱原子周围具有八面体配位，使得它们的形状各向异性相对较低。因此，材料设计的进一步进展需要详细了解这类OLED发光体的结构-性能关系。该转移项目的主要目标是改进现有的方法，预测和分析蒸发的客人-主系统与铱发射器的发射器的方向，这样的设计规则可以建立和验证。此外，我们计划研究发射体和宿主分子之间的分子间相互作用，以及不同配体之间的分子内能量转移过程，这将允许更好地控制所产生的发射体取向。总体而言，我们计划为工业合作伙伴（默克）开发针对性材料设计的工具，并为学术合作伙伴（奥格斯堡大学）创造科学知识。

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).