Predictions of molecular orientation and charge mobility in organic vacuum-deposited thin films by multiscale simulation

In organic semiconductors, elucidation of amorphous structures in the aggregates is important because it determines crucial factors for device performance. The amorphous structures determine densities of states, electronic couplings, and reorganization energies, all of which affect current and light-emitting characteristics of devices. However, due to the amorphous nature, the detailed molecular-level structure, especially the distribution, has not been well characterized. In this study, to reproduce the experimentally obtained amorphous structure in the vacuum-deposited thin films, we fabricated organic amorphous thin films by molecular dynamic (MD) simulations mimicking the experimental deposition process. The simulation clearly exhibited that the molecules are oriented with a broad distribution with respect to the substrate; the average orientation successfully reproduced the experiments quantitatively. We also conducted charge transport simulations. The horizontal molecular orientation resulted in an increase in hole mobility as in the experiment. The origin of the increased mobility in horizontally oriented systems is found to be narrower distribution of site energy.