Pentacene phototransistors (PTs) prepared with a thin film transistor (TFT) configuration were investigated as a red light sensor. The pentacene TFTs showed an efficient photo-current response under various intensities of both modulated or continuous red light. The pentacene PTs showed a reliable high responsivity of similar to 1 A/W and the ratio of photocurrent to dark current (I-Ph/I-Dark) was 9000 under visible light at a wavelength of 650 nm due to the relatively small band gap of pentacene. (c) 2007 Elsevier Ltd. All rights reserved.