Influence of environmental conditions on the electrical performance and stability of pentacene thin film transistors

The influence of environmental conditions on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) were investigated. Electrical in-situ and ex-situ measurements of staggered pentacene TFTs were carried out to study the influence of dry oxygen and moisture on the device operation. The transistors were fabricated by Organic Molecular Beam Deposition on thermal oxide dielectrics. Oxygen exposure of the pentacene films lead to the creation of acceptorlike states in the bandgap. The acceptorlike states cause a shift of the onset of the drain current towards positive gate voltages. The shift of the onset voltage is influenced by the structural properties of the pentacene film and the device geometry of the transistor. The charge carrier mobility and the on/off ratio of the transistor are not affected by the acceptorlike states. Furthermore, the acceptorlike states have an influence on the stability of the TFTs. Devices exposed to oxygen exhibit a shift of the threshold voltage due to prolonged biasing. Transistors characterized under vacuum conditions ( no oxygen exposure) do not exhibit a shift of the threshold voltage ( bias stress effect) as a consequence of prolonged biasing. The experimental results show a clear correlation between the device behavior upon oxygen exposure and the stability of the devices. A shift of the onset voltage upon oxygen exposure is correlated with the shift of the threshold voltage upon prolonged bias. The influence of dry oxygen on the threshold voltage, onset voltage and subthreshold slope will be described. Furthermore, the influence of environmental conditions on the operation of organic circuits like active matrix addressed displays will be discussed.