In this study, several experimental techniques were employed to gain insights into the molecular interactions at the interfaces in organic field-effect transistors (OFETs). A self-assembled monolayer (SAM) was applied to the gate insulator to influence the interactions at the interface. Macroscopic electrical measurements, microscopic potentiometry, absorption/luminescence spectroscopy, and ellipsometry are combined to study top-contact OFETs based on an air-stable n-channel perylene derivative. They show clearly the advantage and capability of local electrical investigations compared to DC electrical characterization. The DC electrical measurements provide an improvement of the electron mobilities within the channel and unchanged contact resistances (within the error bars). Only the potentiometry allowed a deeper insight: It reveals a small decrease of the source contact resistance and a drain bias dependence of the mobility, which decreases due to prior hydrophobization of the SiO2 gate insulator. This is attributed to different charge carrier trap concentrations and injection barriers in the organic semiconductor near the dielectric interface caused by diffused water which is discussed in detail. The optical absorption/luminescence spectroscopy and ellipsometry indicate an unchanged ordered molecular arrangement with a molecular tilt angle of (33 ± 4)° with respect to the dielectric surface and rather weak intermolecular interaction for the devices with treated and untreated SiO2.