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

Abstract

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)

Self-assembled monolayers and titanium dioxide: From surface patterning to potential applications

“…As an example, a self-assembled monolayer of 5-cyano-2-(butyl-4-phosphonic acid)-3-butylthiophene (CNBTPA) was formed on TiO 2 by using its phosphonic acid group as a binding group. Once chemisorbed, the monolayer served to attach molecules of α,ω-dicyano substituted β,β'-dibutylquaterthiophene (DCNDBQT) molecules by forming a hydrogen bond between the cyano group of CNBTPA and a hydrogen on the thiophene ring of DCNDBQT, and by forming a hydrogen bond between the cyano group of DCNDBQT and a hydrogen on the thiophene ring of CNBTPA [ 90 ]. A quasi-perpendicular structure of the CNBTPA–DCNDBQT layer relative to the TiO 2 surface was inferred, suggesting optimal orbital overlap between neighboring thiophene rings.…”

Electronic transport properties through thiophenes on switchable domains

“…The investigation of organic semiconductors, which consist of special organic molecules or polymers, has become more and more attractive in recent years 1,2 . From the experimental point of view, the main focus was directed to the fabrication and characterization of suitable organic devices such as organic lightemitting diodes (OLEDs) and organic field-effect transistors (OFETs) 3,4,5 . Theoretical effort was linked to the description of the physical background, which includes the investigation of the underlying transport mechanisms.…”

Current without external bias and diode effect in shuttling transport of nanoshafts

“…This could be understood as a transport channel which exists additionally besides the hopping. The performance of OFETs based on oligothiophene [3] shows characteristic features, e.g. the current starts at a certain threshold of gate voltage and reaches a saturation value for certain drain voltages.…”

“…For exploratory reasons, we restrict ourselves to 11 nanoshafts of length 20l 0 and have arranged them at a distance of 3l 0 . These are typical parameters of quarterthiophene used in recent experiments [3]. First, we charge all shafts equivalently and start the process of shuttling by bending the leftmost tube to the left contact, allowing transfer of the first charge to the left lead.…”