Scanning probe microscopy study of electronic properties of oligothiophenes thin films

Organic semiconductors have been extensively studied due to their potential applications in cheap, flexible, and large area electronic devices. Among potential organic semiconductors, alkyl-substituted oligothiophenes, that are able to self-assemble in close-packed crystalline structures, have shown promising high field-effect mobilities. The objectives of this thesis were to microscopically study the electronic properties of three semiconducting alkyl-substituted oligothiophenes, α,α’-dioctylterthiophene (DOTT), α,α'-dihexyl¬quaterthiophene (DH4T) and α,7-dimethyldienyloctyl-α'dodecyl¬quaterthiophene (ddOD4T), as well as the morphology and electronic transport in the channel of organic thin film transistors (OTFT) based on these molecules. Scanning tunneling microscopy investigations were realized on monolayers adsorbed on MoS2 at the liquid/solid interface or under dry conditions. Our results indicate that DOTT and DH4T form well-ordered monolayers at large scales on MoS2, with interdigitated packing while not having preferred adsorption sites and orientations. The electronic properties of the self-assembled structures were also probed with scanning tunneling spectroscopy. The longest oligothiophene, ddOD4T, did not adsorb on MoS2, perhaps due to its liquid-crystalline properties at room temperature. Then, using Kelvin probe force microscopy (KPFM), we studied charge transport and charge trapping mechanisms in the channel of OTFT based on these three molecules, without or with surface modification (silanization) of the gate silicon oxide. Smaller and more densely packed crystals were obtained on the silanized devices. Inter-grain contact resistances, charge trapping and hysteresis effects appeared to be less important in the devices with silanized gate oxide compared to devices with native gate oxide. Finally, OTFTs based on DH4T spin-coated on a composite gate dielectrics (polyvinylalcohol, PVA, charged with SiO2 nanoparticles), were fabricated and characterized. These devices may present ambipolar behaviors and the best performances were obtained for the devices with 5% SiO2 nanoparticles in PVA.