Theoretical Insight into Molecular Orientation for Thermally Activated Delayed Fluorescence Emitters in Vacuum Deposition

The molecular orientation of thermally activated delayed fluorescence (TADF) emitters in vapor-deposited organic semiconductor films significantly affects the light out-coupling efficiency and the photoelectric performance of organic light-emitting diodes. Herein, the vacuum deposition of indolocarbazole-isomer derivatives, as highly emissive emitters for TADF-OLEDs, was simulated by use of all atomistic molecular dynamics (MD) simulations and quantum mechanical analysis. It is shown from the results that the molecular orientation in vacuum deposition is determined by the intermolecular pi - pi stacking. There are the strong van der Waals interactions (mainly dispersion) between emitters and substrate due to the intermolecular pi-pi stacking. The van der Waals forces lead to the formation of an energetically stable molecular conformation, which makes emitters as stable as possible at the horizontal orientation relative to substrate. The compact pi - pi stacking of emitters on the substrate is responsible for emitter-substrate weak interactions. Furthermore, theoretical optical properties of indolocarbazole-isomer derivatives are in good accordance with experiments. The present work provides a theoretical perspective for understanding molecular orientation in vacuum deposition.

Automated summary

The molecular orientation in vacuum deposition is determined by the intermolecular pi - pi stacking, with strong van der Waals interactions leading to energetically stable molecular conformation. The compact pi - pi stacking on the substrate is responsible for weak emitter-substrate interactions. The theoretical optical properties of indolocarbazole-isomer derivatives match well with experiments, providing a theoretical basis for understanding molecular orientation in vacuum deposition.