The molecular orientation is an important factor affecting electrical and optical properties of the organic semiconducting layers. In organic light emitting diodes (OLEDs), the outcoupling efficiency of light significantly relies on the emitting dipole orientation (EDO). The horizontally aligned dipole moment embedded in a 2-dimensional microcavity structure contributes to larger far-field emission than the vertically aligned dipole moment. Therefore, employing the horizontally oriented emitter is one of the effective methods to enhancing the outcoupling efficiency of OLEDs. Ir complexes are excellent phosphorescent dyes that have been used in OLEDs for decades because they have high photoluminescence quantum yield and allow electroluminescence from triplet excited states. However, it is only recent years to attract attention on the EDO of Ir complexes as doped in the emissive layers because their iridium-centered octahedral structures and the amorphous surrounding nature in the emissive layers are far from having strong molecular alignments.[1-5] Recently, many Ir complexes have been known to have horizontal EDOs and expected to increase external quantum efficiencies (EQEs) of OLEDs.
Here, we reveal the origin of the molecular alignment of Ir complexes by simulation of vacuum deposition using molecular dynamics along with quantum chemical characterization. The molecular alignment of the dye varies largely depending on the type of the host materials. Close observation of the molecules on the film surface by the vacuum deposition simulation indicates that the interactions between the phosphor and nearest host molecules on the surface, minimizing the non-bonded van der Waals and electrostatic interaction energies determines the molecular alignment during the vacuum deposition. Parallel alignment of the main cyclometalating ligands in the molecular complex due to host interactions rather than the ancillary ligand orienting to vacuum leads to the horizontal emitting dipole orientation.