From Molecule to Device: Prediction and Validation of the Optical Orientation of Iridium Phosphors in Organic Light-Emitting Diodes

Due to their thin amourphous structure, unique electrical properties, and the associated variety of possible applications, OLEDs can now be found in smartphones, TVs, laptops, and wearables. While already big steps have been made in optimizing and understanding the properties influencing the external quantum efficiency (EQE), there is still room for improvement, especially when it comes to finding design principles for new emitter complexes. One contributer to the EQE here is the molecular orientation of the emitter in a given host matrix. In this work we study the viability of using molecular modeling approaches in sampling these emitter orientations for a set of already published homoleptic Ir carbene emitters and a set of emitter materials synthesized at Merck KGaA, Darmstadt, Germany, comprising both homoleptic and heteroleptic Ir(ppy)(3) derivatives. We combine these simulations with different measurements for the orientation parameter and EQE, all performed with the same material stack under the same conditions. We observe a good agreement between simulation and experiment and find that the horizontal orientation of emitter molecules seems to be the main factor contributing to a higher EQE.