Low-operating-voltage organic transistors made of bifunctional self-assembled monolayers

Self-assembled monolayers (SAMs) are molecular assemblies that spontaneously form on an appropriate substrate dipped into a solution of an active surfactant in an organic solvent. Organic field-effect transistors are described, built on an SAM made of bifunctional molecules comprising a short alkyl chain linked to an oligothiophene moiety that acts as the active semiconductor. The SAM is deposited on a thin oxide layer (alumina or silica) that serves as a gate insulator. Platinum - titanium source and drain electrodes (either top- or bottom-contact configuration) are patterned by using electron-beam (e-beam) lithography, with a channel length ranging between 20 and 1000 nm. In most cases, ill-defined current-voltage (I-V) curves are recorded, attributed to a poor electrical contact between platinum and the oligothiophene moiety. However, a few devices offer well-defined curves with a clear saturation, thus allowing an estimation of the mobility: 0.0035 cm² V^-1 s^-1 for quaterthiophene and 8×10^-4 cm² V^-1 s^-1 for terthiophene. In the first case, the on-off ratio reaches 1800 at a gate voltage of -2 V. Interestingly, the device operates at room temperature and very low bias, which may open the way to applications where low consumption is required.