Effect of Host Anisotropy on Phosphorescent Emitter Orientation and Light Outcoupling in OLEDs

ABSTRACT Preferential alignment of the emitting molecules' transition dipole moments (TDMs) is an established method to boost light outcoupling from organic light–emitting diodes (OLEDs). Key factors to achieve this are shape and/or chemical anisotropy of the emitter itself, and the glass transition temperature of the host in light–emitting guest‐host systems, if the layers in the OLED are prepared by vacuum deposition. Here we demonstrate that the optical anisotropy of the host material plays a decisive role for tuning the orientation of phosphorescent emitters as well. We find that the TDM orientation of a cage‐like metal–organic Iridium complex, which is derived from the well‐known Ir(ppy), is correlated with the orientation order parameter of the host material (and its birefringence ). Specifically, strong horizontal TDM alignment is achieved for lying host molecules having negative (and ). However, the actual increase in OLED efficiency by a more horizontal emitter orientation is less than expected. This can be explained by the effect of birefringence of the emission layer on optical wave propagation and the coupling to lossy modes in an OLED. Furthermore, since the used (co)host materials are two‐component mixtures of electron‐ respectively hole‐transporting species, the balance between both and its impact on the location of the emission zone are critical as well. Overall, we present a comprehensive treatment of the different effects of host anisotropy on the efficiency of OLEDs.