title: Temperature and layer thickness dependentin situinvestigations on epindolidione organic thin-film transistors.
abstract: We report onin situperformance evaluations as a function of layer thickness and substrate temperature for bottom-gate, bottom-gold contact epindolidione organic thin-film transistors on various gate dielectrics. Experiments were carried out under ultra-high vacuum conditions, enabling quasi-simultaneous electrical and surface analysis. Auger electron spectroscopy and thermal desorption spectroscopy (TDS) were applied to characterize the quality of the substrate surface and the thermal stability of the organic films.Ex situatomic force microscopy (AFM) was used to gain additional information on the layer formation and surface morphology of the hydrogen-bonded organic pigment. The examined gate dielectrics included SiO2, in its untreated and sputtered forms, as well as the spin-coated organic capping layers poly(vinyl-cinnamate) (PVCi) and poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE, from the class of polynorbornenes). TDS and AFM revealed Volmer-Weber island growth dominated film formation with no evidence of a subjacent wetting layer. This growth mode is responsible for the comparably high coverage required for transistor behavior at 90-95% of a monolayer composed of standing molecules. Surface sputtering and an increased sample temperature during epindolidione deposition augmented the surface diffusion of adsorbing molecules and therefore led to a lower number of better-ordered islands. Consequently, while the onset of charge transport was delayed, higher saturation mobility was obtained. The highest, bottom-contact configuration, mobilities of approximately 2.5 × 10-3cm2/Vs were found for high coverages (50 nm) on sputtered samples. The coverage dependence of the mobility showed very different characteristics for the different gate dielectrics, while the change of the threshold voltage with coverage was approximately the same for all systems. An apparent decrease of the mobility with increasing coverage on the less polar PNDPE was attributed to a change in molecular orientation from upright standing in the thin-film phase to tilted in the bulk phase. From temperature-dependent mobility measurements we calculated activation barriers for the charge transport between 110 meV and 160 meV, depending on the dielectric configuration.
authors.affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.
authors.forename: R
authors.initials: R
authors.lastname: Lassnig
authors.affiliation: Materials-Institute for Surface Technologies and Photonics, Joanneum Research Forschungsgesellschaft mbH, Franz-Pichler-Straße 30, A-8160 Weiz, Austria.
authors.forename: B
authors.initials: B
authors.lastname: Striedinger
authors.affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.
authors.forename: A O F
authors.initials: AO
authors.lastname: Jones
authors.affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.
authors.forename: B
authors.initials: B
authors.lastname: Scherwitzl
authors.affiliation: Materials-Institute for Surface Technologies and Photonics, Joanneum Research Forschungsgesellschaft mbH, Franz-Pichler-Straße 30, A-8160 Weiz, Austria.
authors.forename: A
authors.initials: A
authors.lastname: Fian
authors.affiliation: Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, Altenbergerstraße 69, A-4040 Linz, Austria.
authors.forename: E D
authors.initials: ED
authors.lastname: Głowacl
authors.affiliation: Materials-Institute for Surface Technologies and Photonics, Joanneum Research Forschungsgesellschaft mbH, Franz-Pichler-Straße 30, A-8160 Weiz, Austria.
authors.forename: B
authors.initials: B
authors.lastname: Stadlober
authors.affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.
authors.forename: A
authors.initials: A
authors.lastname: Winkler
keywords: Bottom-contact; Charge carrier mobility; Epindolidione; Hydrogen-bonding; Organic thin-film transistor