Abstract
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.
Highlights
Emitting dipole orientation is an important issue of emitting materials in organic light-emitting diodes for an increase of outcoupling efficiency of light
In organic light-emitting diodes, the molecular orientation of emitter embedded in the emissive layer has been an issue to enhance the outcoupling efficiency of light pursuing the horizontal alignment of the emitting dipole moment[3, 12,13,14,15,16,17,18,19,20,21]
A few mechanisms have been proposed to explain the preferred molecular orientation of the Ir complexes doped in vacuum-deposited organic semiconducting layers: molecular aggregation of the dopants leading to randomizing their orientation by suppressing the intermolecular interaction between the dopant and host molecules[22], strong intermolecular interactions between electro-positive sides of the dopant, and the electro-negative host molecules promoting parallel alignment of the N-heterocycles of Ir complexes by forming host-dopant–host pseudo-complex mainly participating in 3MLCT transition[16, 23], and π–π interactions between the dopant and host molecules on the organic surface bringing alignment of aliphatic ligands to the vacuum side[20, 24]
Summary
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. A few mechanisms have been proposed to explain the preferred molecular orientation of the Ir complexes doped in vacuum-deposited organic semiconducting layers: molecular aggregation of the dopants leading to randomizing their orientation by suppressing the intermolecular interaction between the dopant and host molecules[22], strong intermolecular interactions between electro-positive sides of the dopant, and the electro-negative host molecules promoting parallel alignment of the N-heterocycles of Ir complexes by forming host-dopant–host pseudo-complex mainly participating in 3MLCT transition[16, 23], and π–π interactions between the dopant and host molecules on the organic surface bringing alignment of aliphatic ligands to the vacuum side[20, 24] It is not very clear which mechanism most comprehensively describes the origin of the preferred EDO of the heteroleptic iridium phosphors. Close observation of the simulation results indicates that non-bonded interaction energy has a critical influence on the molecular orientation during the deposition
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