We investigated the electrical performances of pentacene-based thin-film transistors with a thin LiF film as a modification layer between the source/drain electrodes and the active layer. Au, Pt, and Pd were employed as the source/drain (S/D) electrodes of the organic thin-film transistors (OTFTs). The thickness of the LiF layer was varied to optimize the electrical performances of the OTFTs. We found that the electrical performances of the devices with LiF layer are better than those without inserting the LiF layer, regardless of which one metal used for the S/D electrodes. The electrical performances of the devices in terms of drain current, on resistance, threshold voltage, on/off current ratio and mobility have been optimized when the thickness of the LiF buffer layer was at 1.0 nm. These improvements are attributed to work function of S/D contact materials were modified by the dipole of the inserted LiF layer and an ultrathin LiF layer provides the tunneling condition to enhance the carrier injection efficiently. Additionally, LiF layer with optimal thickness (1.0 nm) can modify surface roughness of pentacene film and partly enhance the carrier injection. (C) 2011 Elsevier Ltd. All rights reserved.