Solution Processable Semiconductor Thin Films: Correlation Between Morphological, Structural, Optical and Charge Transport Properties

Cette these de doctorat est le resultat d’un travail de recherche multidisciplinaire reunissant divers concepts fondamentaux comme l'ingenierie et la caracterisation des couches minces, l'electrochimie ainsi que la physique des dispositifs a base de couches minces semiconductrices. L’objet des experiences menees lors du travail de these porte sur les couches minces semiconductrices et leurs interfaces avec des materiaux electroniques ou des couches dielectriques. Le but ultime de la these est de mettre en correlation la morphologie, la structure cristalline et la structure electronique des couches minces, ainsi que leurs proprietes fonctionnelles, avec le fonctionnement des dispositifs electroniques de type transistors utilisant ces memes couches minces en tant que materiau actif. En outre, de nouvelles strategies fondees sur des phenomenes ayant lieu aux interfaces electrolyte/couche mince semiconductrice ont ete explorees afin de controler la conductivite electrique au sein des couches minces. Trois principaux systemes chimiques ont fait l'objet d’etudes approfondies durant cette these: deux types de semiconducteurs organiques (oligomere et polymeres derives d’azomethinethiophenes et derives solubles de pentacene) et un semi-conducteur metal-oxyde (le trioxyde de tungstene, WO3). Dans le but d’explorer les proprietes morphologiques de couches minces a base de semiconducteurs organiques, les microscopies a force atomique et a fluorescence en mode hyperspectrale ont ete employees. Grâce aux observations realisees par ces techniques, des hypotheses de correlation ont ete formulees entre les caracteristiques morphologiques et les proprietes de transport des porteurs de charges au sein des couches minces. La diffraction par rayons X en configuration glancing angle (GIXRD, utilisant de la lumiere de synchrotron) a ete utilisee pour examiner la structure cristalline des couches mais aussi pour en apprendre plus sur l'organisation moleculaire au sein de ces dernieres. Pour les transistors en couches minces, une configuration de type bottom contact a ete choisie. Les caracteristiques de transfert et de sortie ont ete utilisees pour calculer la mobilite des porteurs de charges, le voltage de seuil du dispositif ainsi que le ratio ION/IOFF.----------Abstract This Ph.D. thesis is a result of multidisciplinary research bringing together fundamental concepts in thin film engineering, materials science, materials processing and characterization, electrochemistry, microfabrication, and device physics. Experiments were conducted by tackling scientific problems in the field of thin films and interfaces, with the aim to correlate the morphology, crystalline structure, electronic structure of thin films with the functional properties of the films and the performances of electronic devices based thereon. Furthermore, novel strategies based on interfacial phenomena at electrolyte/thin film interfaces were explored and exploited to control the electrical conductivity of the thin films. Three main chemical systems were the object of the studies performed during this Ph.D., two types of organic semiconductors (azomethine-based oligomers and polymers and soluble pentacene derivatives) and one metal oxide semiconductor (tungsten trioxide, WO3). To explore the morphological properties of the thin films, atomic force microscopy was employed. The morphological properties were further investigated by hyperspectral fluorescence microscopy and tentatively correlated to the charge transport properties of the films. X-ray diffraction (Grazing incidence XRD, GIXRD) was used to investigate the crystallinity of the film and the effect of the heat treatment on such crystallinity, as well as to understand the molecular arrangement of the organic molecules in the thin film. The charge transport properties of the films were evaluated in thin film transistor configuration. For electrolyte gated thin film transistors, time dependent transient measurements were conducted, in parallel to more conventional transistor characterizations, to explore the specific effects played on the gating by the anion and cation constituting the electrolyte. The capacitances of the electrical double layers at the electrolyte/WO3 interface were obtained from electrochemical impedance spectroscopy. In the context of ARTICLE 1, thin film transistors based on soluble pentacene derivatives (prepared by the research group directed by Professor J. Anthony, at the University of Kentucky) were fabricated and characterized. GIXRD results performed on the thin films suggested a molecular arrangement favorable to charge transport in the source-drain direction, with the π-π

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