Temperature and layer thickness dependent in situ investigations on epindolidione organic thin-film transistors

• Published in: Synthetic metals Vol. 218; pp. 64 - 74
• Main Authors: Lassnig, R., Striedinger, B., Jones, A.O.F., Scherwitzl, B., Fian, A., Głowacki, E.D., Stadlober, B., Winkler, A.
• Format: Journal Article
• Language: English
• Published: Switzerland Elsevier B.V 01-08-2016
• Subjects: Atomic force microscopy; Bottom-contact; Charge carrier mobility; Charge transport; Dielectrics; Epindolidione; Formations; Gates; Hydrogen-bonding; Organic thin-film transistor; Phase transformations; Semiconductor devices; Thickness; Thin films; Epindolidione; Hydrogen-bonding; Bottom-contact; Organic thin-film transistor; Charge carrier mobility
• ISSN: 0379-6779; 1879-3290
• DOI: 10.1016/j.synthmet.2016.05.003

•Epindolidione OTFTs were fabricated in UHV on SiO2, PVCi and PNDPE.•Evaluations of bottom-contact, hydrogen-bonded semiconductors.•Coverage and temperature dependent mobility and AFM investigations were performed.•Volmer-Weber island growth was observed for all investigated substrate configurations.•The thermal activation energies for charge transport and desorption were calculated.
We report on in situ performance 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 situ atomic 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 (50nm) 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 110meV and 160meV, depending on the dielectric configuration.