Unraveling the standing-up orientation transition of π-conjugated molecules on graphene templates

Abstract

The molecular orientation of π-conjugated organic semiconducting (OSC) thin films is critical for optimizing the performance of organic optoelectronic devices. Graphene templates have been shown to facilitate high crystallinity and large grain sizes in OSC films through π–π interactions. However, controlling the molecular orientation on graphene templates remains challenging, particularly in achieving the energetically unfavorable “standing-up” configuration of π-conjugated molecules. In this study, we systematically investigate the effects of sub-nanometer scale surface roughness and mechanical strain in graphene on the orientation of pentacene molecules in thin films. Our findings, supported by both experimental observations and density functional theory (DFT) calculations, indicate that surface roughness and strain destabilize lying-down pentacene molecules. In contrast, standing-up pentacene molecules are not affected by the changes in the physical properties of the underlying graphene templates, facilitating their transition to a standing-up orientation. This effect is further enhanced by intermolecular interactions between pentacene molecules. We believe our findings offer a novel strategy for the precise control of molecular orientation in OSC thin films.