Molecular dynamics simulation of molecular orientation transformation of pentacene on a-SiO2

The molecular orientation has a great impact on the performance of organic thin film transistors (OTFTs) and undesired orientation also appears. There is a critical size n_c of the orientation transformation from lateral to normal for pentacene (5A) on the a-SiO₂ surface during the vapor-phase deposition process. Molecular dynamics (MD) simulations are performed to get the critical size and gain insight into the transformation mechanism. The results suggest that the delicate interplay between the interaction of molecule-molecule and the interaction of molecule-substrate appears to govern the growth and morphology of pentacene. When n<;n_c, the 5A molecules prefer to form lateral oriented cluster with (1-10) surface parallel to the substrate driven by the interaction of molecule-substrate. For n>n_c the normal orientation with (001) surface parallel to the substrate becomes stable because the interaction of molecule-molecule holds the dominant position. And a competitive factor Δ between the interaction of molecule-molecule and the interaction of molecule-substrate is established to characterize the results.