Molecular level alignment at the interface with electrodes can strongly influence the performance of organic-based electronics. For this reason, interfacial band engineering, through charge transfer and band alignment, is crucial for potential applications of graphene/organic semiconductors heterostructures. On the other hand, graphene and other 2D materials are very promising candidates as functional substrates for organic semiconductor thin films [1,2]. Especially in the case of graphene, it’s unique density of states and exceptionally low electrical noise allow for graphene-based field effect devices to be utilized as extremely sensitive potentiometers for probing charge transfer with adsorbed species.
In this study, we examine charge transfer between graphene and two molecular semiconductors, parahexaphenyl and C60. Through in situ measurements of the current in the hot wall epitaxy setup under high-vacuum conditions, we directly probe the charge transfer as the interfacial dipole is formed. It is shown that adsorbed molecules do not affect electron scattering rates in graphene, indicating that charge transfer is the main mechanism governing the molecular level alignment at the interface with graphene. From the amount of transferred charge and the molecular coverage of the grown films, the charge transfer occurring per adsorbed molecule is estimated. In both cases, p-type doping with only ~10-3 electrons transferred from graphene per molecule is observed, indicating very weak interaction.