The switch-on voltage for disordered organic field-effect transistors is defined as the flatband voltage, and is used as a characterization parameter. The transfer characteristics of the solution processed organic semiconductors pentacene, poly͑2,5-thienylene vinylene͒ and poly͑3-hexyl thiophene͒ are modeled as a function of temperature and gate voltage with a hopping model in an exponential density of states. The data can be described with reasonable values for the switch-on voltage, which is independent of temperature. This result also demonstrates that the large threshold voltage shifts as a function of temperature reported in the literature constitute a fit parameter without a clear physical basis. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1479210͔The charge transport in organic field-effect transistors has been a subject of research for several years. It has become clear that disorder severely influences the charge transport in these transistors.1-3 Studies on the effect of molecular order ultimately resulted in the observation of band transport in high quality organic single crystals. 4 The electrical transport in these crystals is well described by monocrystalline inorganic semiconductor physics.4,5 However, devices envisaged for low-cost integrated circuit technology are typically deposited from solution, 6,7 resulting in amorphous or polycrystalline films. In these solution-processed organic transistors the disorder in the films dominates the charge transport, due to the localization of the charge carriers. The disorder is observed experimentally through the thermally activated field-effect mobility and its gate voltage dependency. [8][9][10][11][12] A further common feature of disordered organic field-effect transistors is the temperature dependence of the threshold voltage, V th , 11,12 which is addressed in this letter. It is argued that V th , as used in literature, is a fit parameter with no clear physical basis. Instead, a switch-on voltage, V so , is defined for the transistor at flatband. We model the experimental data obtained on solution-processed pentacene, poly͑2,5-thienylene vinylene͒ ͑PTV͒ and poly͑3-hexyl thiophene͒ ͑P3HT͒, with a disorder model of variable-range hopping in an exponential density of states. 9 The modeling shows that good agreement with the experiment can be obtained with reasonable values for the switch-on voltage, which is independent of temperature.The device geometry and the sample fabrication used in the experiments have been described previously. 13 The films of PTV are truly amorphous whereas the pentacene and P3HT films are polycrystalline. We do not observe any hysteresis in the current-voltage characteristics and the curves are stable with time ͑in vacuum͒. The field-effect mobilities in the devices have been estimated from the transconductance 8 at V g ϭϪ19 V at room temperature and are given in Table I. For the P3HT transistor described here the processing conditions were not optimized to give the high mobilities reported in literature.
1The difficu...