  Emitting dipole orientation (EDO) is an important issue of emitting materials
in organic light-emitting diodes (OLEDs) for an increase of outcoupling
efficiency of light. The origin of preferred orientation of emitting dipole of
spherically shaped iridium-based heteroleptic phosphorescent dyes doped in
organic layers is revealed by simulation of vacuum deposition using molecular
dynamics (MD) along with quantum mechanical (QM) characterization of the
phosphors for a direct comparison with experimental observations of EDO.
Consideration of both the electronic transitions in a molecular frame and the
orientation of the molecules interacting with the environment at the
vacuum/molecular film interface allows quantitative analyses of the EDO
depending on host molecules and dopant structures. Interaction between the
phosphor and nearest host molecules on the surface, minimizing the non-bonding
energy 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 EDO.
