All-atom simulation of molecular orientation in vapor-deposited organic light-emitting diodes

Molecular orientation in vapor-deposited organic semiconductor films is known to improve the optical and electrical efficiencies of organic light-emitting diodes, but atomistic understanding is still incomplete. In this study, using all-atom simulation of vapor deposition, we theoretically investigate how the molecular orientation depends on various factors such as the substrate temperature, molecular shape, and material composition. The simulation results are in good agreement with experiment, indicating that the all-atom simulation can predict the molecular orientation reliably. From the detailed analysis of the dynamics of molecules, we suggest that the kinetics of molecules near the surface mainly determines the orientation of the deposited film. In addition, the oriented films have higher density and thermal stability than randomly oriented films. We also show that higher mobility of laterally oriented films can be explained in terms of the site-energy correlation.