A simple UV-treatment process for poly͑methyl methacrylate͒ ͑PMMA͒ dielectric is proposed to enhance photoresponsivity of pentacene-based organic thin-film transistors. The UV treatment creates excess negatively charged sites on the PMMA dielectric, which makes the device exhibit a large photoinduced current and prolongs persistent conductance recovery. In order to describe time-dependent photoinduced current, double-time constant equations are proposed. Based on time-constant fittings, slow-varied responses are found to be influenced by the UV treatment. The rapid-varied response is independent of gate bias and UV treatment. A plausible model for spatial carrier distribution is discussed and proposed to describe this observed phenomenon. © 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2936224͔ All rights reserved. Organic thin-film transistors ͑OTFTs͒ have received intense attention due to their low cost, light weight, and low-temperature processing compatible with flexible substrates. The integration of OTFTs with other organic devices such as organic light emitting devices and organic solar cells also opens up the field of flexible optoelectronics.1,2 In this field, OTFTs not only act as field-effect transistors, but their application in phototransistors is also important for light detection and photoswitching. However, only a small number of researchers have published works on organic phototransistors. Researchers have observed reversible photocurrent and lightinduced threshold voltage shift.3-5 Pentacene-based OTFTs with Ta 2 O 5 gate dielectrics were also proposed to obtain high photoresponsivity. 4 In this paper, an approach to enhance photoresponsivity of pentacene-based OTFTs is proposed. By using poly͑methyl methacrylate͒ ͑PMMA͒ as the dielectric and applying UVlight treatment, photoresponsivity of OTFTs can be significantly enhanced. By analyzing the response lifetime, the underlying mechanism is investigated carefully. The proposed approach is simple and compatible with the development of all-organic electronics. ExperimentalFirst, the Ni/Pd bilayer was deposited on a glass substrate as a gate electrode. The PMMA was dissolved with a 95 wt % in anisole and was spun onto the gate electrodes. The following was the curing process. The glass substrate was heated with PMMA dielectric on a hot plate at 90°C for 30 min at atmosphere. The solvent was removed and the PMMA was solidified to serve as a gate dielectric. The thickness of the PMMA dielectric was about 360-370 nm. Then, some samples were exposed to UV light ͑Jelight Company, GLS-144 UV Lamp͒, with a wavelength of 175-285 nm and irradiation of 0.043 mW/cm 2 for 90 s. Both the UV-treated PMMA and the untreated ͑standard͒ PMMA dielectrics were transferred into a vacuum chamber for the deposition of 100 nm thick pentacene film and 100 nm thick gold ͑Au͒ electrodes. The device channel width ͑W͒ and length ͑L͒ were 1000 and 100 m, respectively. The illumination system was a combination of a probing bench ͑Cascade microtech, RHM-06͒ and a halogen lamp...