Unraveling the orientation of phosphors doped in organic semiconducting layers

Emitting dipole orientation is an important issue of emitting materials in organic light-emitting diodes for an increase of outcoupling efficiency of light. The origin of preferred orientation of emitting dipole of iridium-based heteroleptic phosphorescent dyes doped in organic layers is revealed by simulation of vacuum deposition using molecular dynamics along with quantum mechanical characterization of the phosphors. Consideration of both the electronic transitions in a molecular frame and the orientation of the molecules at the vacuum/molecular film interface allows quantitative analyses of the emitting dipole orientation depending on host molecules and dopant structures. 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.Iridium-based phosphors show high photoluminescence quantum yield in organic light-emitting diodes. Here, Moon et al. reveal the mechanism responsible for the preferred orientation of iridium complexes in an organic host and highlight the interaction between phosphor and host molecules at play.