Ultra-thin organic films for field effect transistors

Organic field effect transistors FETs have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of rigid conjugated sexithiophene (6T) blocks and flexible poly(ethylene oxide) (PEO) blocks have been deposited onto Si/SiO₂ substrates with a surface modified with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10^-2 - 10^-3 cm²/Vs and ON-OFF current ratio of 10³ - 10⁵. Film microstructure, orientation of ordered domains and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.