Unraveling the orientation of phosphors doped in organic semiconducting layers

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.