CARBON NANOTUBE ARRAY ELECTRODES FOR ORGANIC THIN FILM TRANSISTORS

Depuis environ une vingtaine d’annees, les transistors organiques a base de couches minces (acronyme en anglais, OTFT) se sont developpes de facon impressionnante, passant de l'echelle de recherche au laboratoire a l'echelle de commercialisation. Les caracteristiques les plus interessantes des OTFT sont leur compatibilite avec des substrats flexibles et leur methode de fabrication, basee sur des techniques en solution. Les performances des OTFT sont influencees tres fortement par les caracteristiques de l’interface semiconducteur organique/electrode metallique. L'interface semiconducteur organique/electrode metallique influence le processus d'injection de charge. En general, l'efficacite d'injection de charge dans les OTFT est limitee, parce que les niveaux electroniques du semiconducteur organique ne sont pas alignes avec le niveau de Fermi du metal de l’electrode. Dans ce memoire de maitrise, nous avons focalise l'attention sur le processus d'injection de charge dans les OTFT utilisant comme materiau d'electrode des rangees de nanotubes de carbone monoparois (acronyme en anglais, SWCNT). Nous avons compare les performances des OTFT bases sur des electrodes de type rangee de SWCNT avec les performances des OTFT utilisant des materiaux d'electrode plus conventionnels, tel que l’Au. Les OTFTs ont ete fabriques sur une couche de SiO2, obtenue par oxydation thermique d’une tranche de Si (100) fortement dopee (type n). La formation des rangees de SWCNT a ete effectuee par la methode de filtration sous vide des reseaux de SWCNT au-dessus de la surface de SiO2. La geometrie concentrique d'electrode a ete realisee par photolithographie et par developpement. Nous avons observe des resultats tres encourageants dans les cas de trois semiconducteurs organiques differents, tels que le poly-3 (hexyl) thiophene (P3HT), le derive du fullerene PCBM, et la ftalocyanine de tytanil (TiOPc). ----------Abstract Over the past two decades, organic thin film transistors (OTFTs) have developed from the research laboratory scale to the scale of commercially feasible technology. The most attractive features of OTFTs are their low-cost manufacturing procedures and their compatibility with flexible substrates. The characteristics of the organic semiconductor/metal electrode interface dramatically affect the performance of OTFTs. For instance, charge carrier injection efficiency can be limited by the offset between the HOMO and LUMO energy levels of the organic semiconductor and the Fermi level of the metal electrode, since this offset leads to the formation of an energy barrier for charge carrier injection. This MSc work focuses on the use of single walled carbon nanotube (SWCNT) array electrodes, to improve the injection efficiency in OTFTs. A comparative approach has been adopted, based on the systematic comparison between SWCNT array electrode-based OTFTs with their, well investigated, Au electrode-based counterparts. An improvement in the injection characteristics of OTFTs making use of SWCNT array electrodes has been previously demonstrated in OTFTs based on organic semiconductors such as (p-type) pentacene and copper phthalocyanine and (n-type) phenyl-C61-butyric acid methyl ester (PCBM). In these previous studies, it has been hypothesized that the improvement of the charge carrier injection characteristics is mainly due to the one dimensional (1D) structure of SWCNTs, which favors tunneling injection across the injection barrier. To assess the general validity of this hypothesis, a large number of experiments need to be carried out with different organic semiconductors, belonging to different classes of materials (e.g., polymers and small molecules) and differently processed (e.g. solution processed and vacuum processed). In this project, the performance of OTFTs making use of SWCNT array electrodes has been investigated for thin films of Titanyl-phthalocyanine (TiOPc), Poly-3 (hexylthiophene) (P3HT), and P3HT/PCBM blends. TiOPc belongs to metal phthalocyanines (MPcs), which, due to their low solubility in organic solvents, are typically deposited by vacuum-based techniques. P3HT is