Orientation Control of Selected Organic Semiconductor Crystals Achieved by Monolayer Graphene Templates

Abstract

Crystal orientation in organic thin films is one of the key parameters that determine absorption cross‐section, interfacial energetics and excitonic states, and free charge properties. In this work, monolayer graphene is used to direct the crystal orientation of selected planar organic molecules. Lying‐down orientation with π‐stacking normal to the surface is achieved with graphene templating. The absorption spectra of the graphene‐templated films are correlated to molecular orientation. The same set of absorption features with or without graphene suggests that no vibronic states are forbidden or newly introduced. However, the light absorption with graphene templating is enhanced due to the altered relative orientation between the transition dipole moment of the constituent molecules and the electric field of incident light. The energy level of the highest occupied molecular orbital (HOMO) of graphene templated p‐type films is observed at a deeper‐lying value (relative to vacuum) compared to untemplated diindenoperylene films grown on indium tin oxide. In contrast, the HOMO energy levels of graphene templated n‐type films are observed at higher‐lying energy levels (relative to vacuum) compared to the respective untemplated films due to surface dipoles. Such a change can potentially increase the theoretical V oc expected for photovoltaic devices incorporating these templated films.