Al2O3 as gate dielectric for organic transistors: Charge transport phenomena in poly-(3-hexylthiophene) based devices

We studied organic thin film transistors based on poly-(3-hexylthiophene) having as gate dielectric Al2O3, which was prepared by atomic layer deposition (ALD) technique, that provides films with very good electrical properties, roughness below 1 nm and compatibility with virtually any type of substrate, including polymeric ones. High-k gate oxides such as Al2O3 are advantageous since they enable a reduction of operating voltages, but when used in conjunction with organic semiconductors, they induce worse transport properties if compared to low-k dielectrics. To address this issue, we focused on the interface between the gate dielectric and the active material and we explored the effects of functionalizing the Al2O3 surface by means of self-assembled monolayers (SAM). We studied and compared a set of SAMs differing in the ligand groups and in the chain lengths. We show that the most important parameter is the SAM chain length, the longer SAM being more effective in giving better charge carrier mobility. The microscopical origins of this finding are analyzed exploiting and comparing the dependence of the mobility on the gate voltage in the transistors based on the variously functionalized Al2O3. With n-octadecyltrichlorosilane (OTS) the mobility reaches the value of 10(-2) cm(2)/V s, comparable to the mobility on transistors with properly functionalized SiO2 gate dielectric. (c) 2007 Elsevier B.V. All rights reserved.