Assembly, structure, and performance of an ultra-thin film organic field-effect transistor (OFET) based on substituted oligothiophenes

We report on the improved assembly and characterization of a small molecule organic field‐effect transistor (OFET). Novel α,ω‐dicyano substituted β,β′‐dibutylquaterthiophene molecules (DCNDBQT) were synthesized and characterized by UV–Vis spectroscopy, differential scanning calorimetry, thermal gravimetric analysis and cyclic voltammetry. The ultra‐thin organic film formation on TiO2 templates was effectively promoted through the specifically designed bifunctional self assembly molecules (SAM) 5‐cyano‐2‐(butyl‐4‐phosphonic acid)‐3‐butylthiophene (CNBTPA). Excellent structural properties were found for up to 9 DCNDBQT molecule thick films prepared through UHV vacuum sublimation as investigated with UHV non‐contact atomic force microscopy (nc‐AFM) and X‐ray diffraction. Both X‐ray and nc‐AFM data indicate that the DCNDBQT molecules form a well‐ordered terraced structure exhibiting step heights of 1.5 nm to 2.0 nm layers. Hence, the DCNDBQTmolecules are linked to the functional SAM interface layer by H‐bond interactions (see structure model) standing quasi perpendicular to the TiO2 template, and thus providing optimal orbital overlap neigh‐bouring thiophene rings. The vacuum sublimated DCNDBQT molecules form a closed packed and dense molecular layer that was used to construct and operate a nanoscopic OFET‐structure. The resulting field mobilities of 10–5 cm2 V–1 s–1 reflect a high current density in our ultrathin but highly ordered structure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)