Effect of the organic semiconductor side groups on the structural and electronic properties of their interface with dopants

Two derivatives of [1]benzothieno[3,2-b][1]benzothiophene (BTBT), namely, 2,7-dioctyl-BTBT (C8-BTBT) and 2,7-diphenyl-BTBT (DPh-BTBT), belonging to one of the best performing organic semiconductor (OSC) families, have been employed to investigate the influence of the substitutional side groups on the properties of the interface created when they are in contact with dopant molecules. As a molecular p-dopant, the fluorinated fullerene C60F48 is used because of its adequate electronic levels and its bulky molecular structure. Despite the dissimilarity introduced by the OSC film termination, dopant thin films grown on top adopt the same (111)-oriented FCC crystalline structure in the two cases. However, the early stage distribution of the dopant on each OSC film surface is dramatically influenced by the group side, leading to distinct host–dopant interfacial morphologies that strongly affect the nanoscale local work function. In this context, Kelvin probe force microscopy and photoelectron emission spectroscopy provide a comprehensive picture of the interfacial electronic properties. The extent of charge transfer and energy level alignment between OSCs and dopant are debated in light of the differences in the ionization potential of the OSC in the films, the interface nanomorphology, and the electronic coupling with the substrate.This work has been funded by the Spanish Government through projects PID2019-110907GB-I00, MAT2016-77852-C2-1-R (AEI/FEDER, UE), and the “Severo Ochoa” Programme for Centers of Excellence in R&D (CEX2019-000917-S). We thank the 2017-SGR668 recognition. A.B. thanks the Spanish Government financial support through BES-2016-077519 FPI fellowship. F.S. acknowledges the European Union’s Horizon 2020 Research and Innovation Program under grant agreement no. 730872 and the Marie Skłodowska-Curie grant agreement no. 722651 (SEPOMO project). This work has been carried out within the framework of the doctoral program (PhD) of Material Science (Department of Physics) of the Universitat Autònoma de Barcelona (UAB). O.S. acknowledges the support of the Israel Ministry of Science, the Center for Absorption in Science of the Ministry of Immigrant Absorption, the Committee for Planning and budgeting of the Council for Higher Education under the framework of the KAMEA Program. Y.H.G. is thankful to the Belgian National Fund for Scientific Research (FNRS) for financial support through research projects BTBT no. 2.4565.11, Phasetrans no. T.0058.14, Pi-Fast no. T.0072.18, and 2Dto3D no. 30489208. Financial supports from the French Community of Belgian (ARC no. 20061) are also acknowledged. A.R. acknowledges doctoral fellowship support from the FRIA. We thank the European Synchrotron Radiation Facility (ESRF) for providing access to ID03 for the time of the experiment. We are indebted to G. Sauthier from the ICN2 Photoemission Spectroscopy Facility.Peer reviewe