Pentacene-based organic thin film transistors have been fabricated with and without the presence of a modifying monolayer at the interface between the silicon dioxide insulator and the organic semiconductor. The monolayer consists of poly(dimethyldiallylammonium chloride) (PDDA), and is deposited by electrostatic self-assembly. The interface modification by the PDDA monolayer has resulted in improved device performance, including 33% higher effective hole mobility, 42% lower threshold voltage, about 50% lower subthreshold slope, and 100% higher on/off ratio. Scanning electron microscopy (SEM) profiles reveal the morphology of pentacene is affected by the insertion of the PDDA monolayer. The presence of this monolayer appears to improve the interface characteristics of the deposited pentacene layer.. resulting in better thin film transistors. (c) 2005 Elsevier Ltd. All rights reserved.